GALVANIZED STEEL SHEET AND MANUFACTURING METHOD

专利摘要:
Patent specification: "Galvanized steel sheet and method of manufacture". The present invention relates to a galvanized steel sheet including a steel sheet and a galvanizing layer on the surface of the steel sheet wherein the steel sheet includes, as a chemical composition of steel, by weight%. : 0.05 to 0.40%, bs: 0.5 to 3.0% and mn: 1.5 to 3.0%, a steel sheet microstructure includes ferrite, bainite, in volume fraction, 30% or more than one mild martensite, and 8% or more than one austenite, and tensile strength of the steel sheet is 980 mpa or more, and the galvanizing layer includes an oxide including at least one chemical selected from itself, mn and al, and when viewed in a cross section including the steel sheet and the galvanizing layer in a direction of plate thickness, a fraction of the oxide projection area is 10% or more. 20868466v1
公开号:BR112014007432B1
申请号:R112014007432-1
申请日:2012-09-28
公开日:2019-04-02
发明作者:Takayuki Nozaki；Manabu Takahashi；Nobuhiro Fujita；Masafumi Azuma；Chisato Wakabayashi
申请人:Nippon Steel & Sumitomo Metal Corporation；
IPC主号:

专利说明:

TECHNICAL FIELD [001] The present invention relates to a galvanized steel sheet having tensile strength (TS) of 980 MPa or more and is excellent in delayed fracture resistance, galvanization adhesion, elongation, and orifice expansion. The galvanized steel sheet according to the present invention is particularly suitable for structural members, reinforcement members, and suspension members for automobiles. Here, the galvanized steel sheet (zinc-coated steel sheet) according to the present invention can be divided into a hot-dip galvanized steel sheet (galvanized steel sheet) and a galvanneal-coated steel sheet .
[002] Priority is claimed for Japanese Patent Application 2011-217811, filed on September 30, 2011, and the contents of which are incorporated herein by reference.
BACKGROUND TECHNIQUE [003] In members such as cross members and side members for automobiles, a reduction in weight has been investigated to respond to a recent trend towards a reduction in fuel consumption, and attempts have been made to increase the strength of a steel plate. point of view of ensuring the collision resistance and safety of automobiles even when a thinner steel plate is used for the members. However, since increasing the strength of the steel plate leads to a deterioration in the formability of the materials, in order to reduce the weight of the members, it is necessary to manufacture a steel sheet that satisfies both the press formability and the high resistance. .
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2/110 [004] Particularly, when the steel sheet is formed as structural members or reinforcement members for automobiles having a complex shape, steel sheet having excellent ductility is required. In recent years, a steel sheet having a tensile strength of 440 MPa or 590 MPa class has been used primarily for automotive structures, and development of a steel sheet having a tensile strength of 980 MPa or more is desired in future to achieve additional weight reduction.
[005] When a 590 MPa grade steel plate is replaced with a 980 MPa class steel plate, the same elongation as the 590 MPa class steel sheet elongation is required on the 980 class steel sheet MPa. Thus, development of a steel sheet that has a tensile strength of 980 MPa or more and has excellent elongation is desired.
[006] As an excellent steel plate in total elongation (El) in a tensile rupture test, there is a steel plate of multiphase structure having a microstructure in which residual austenite as a secondary phase is dispersed in soft ferrite which is a primary phase. In multi-phase steel sheet, ductility is ensured by ferrite and strength is ensured by the martensitic transformation of residual austenite, and the residual austenite is transformed into martensite in plastic processing. There is a steel that is applied in the transformation such as a steel of transformation-induced plasticity (TRIP) and the steel applications of TRIP have been expanded in recent years.
[007] Since TRIP steel has a particularly excellent elongation compared to precipitation-strength steel and dual phase steel (DP) (steel is consisting of ferrite and martensite), TRIP steel applications are strongly desired to be expanded . Although TRIP steel shows excellent strength and ductility, the
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3/110 TRIP steel has an overall low orifice expansion feature.
[008] Also, in order to promote a weight reduction of an automobile chassis in the future, a usable resistance level of a high-strength steel plate should be increased when compared to the conventional one. For example, to use the high strength steel sheet for a hard-to-form member such as a suspension part, formability such as orifice expandability should be improved.
[009] In addition, when a steel plate of 980 MPa or more is applied to the member for an automobile, in addition to the strength and workability properties, delayed fracture resistance is required. The delayed fracture is caused by the tension applied to the steel or hydrogen fragility and is a phenomenon in which a structure is fractured accumulating hydrogen diffused in an area of steel stress concentration used as the structure.
[0010] Specifically, examples of delayed fracture include a suddenly fractured phenomenon that a member, such as a prestressed concrete steel wire (PC) or a screw, is subjected to high stress load under the condition of use.
[0011] It is known that the delayed fracture is strictly related to the hydrogen that penetrates the ambient steel. As for the hydrogen that penetrates steel in the environment, there are several types of hydrogen sources such as the hydrogen that is contained in the atmosphere, hydrogen generated in a corrosive environment. When hydrogen penetrates steel from any of the hydrogen sources, hydrogen can induce delayed fracture.
[0012] For this reason, like the steel usage environment, an environment in the absence of hydrogen is desired. However, when a steel is applied to the structure or to the automobile, the steel is used outdoors and petition 870180125071, of 03/09/2018, p. 13/136
4/110 hydrogen generation cannot be avoided.
[0013] As the stress acting on the steel used as the structure, a stress that is loaded on the structure and a residual stress, that some of the stress generated in the formation remains within the steel, are included. Particularly, in steel used as a member after forming such as a thin steel sheet for an automobile or others, residual stress is a significant problem compared to a thick steel plate or steel bar (for example, a screw) which is a product used as it is, with no deformation being applied. Consequently, when a steel sheet, which delayed fracture is a problem, is formed, it is desirable to form a steel sheet so that the residual stress does not remain.
[0014] For example, in Patent Document 1, a method of forming a hot press of a metal plate is revealed, whose strength is increased by heating a steel sheet to a high temperature and processing the steel sheet and then extinguishing the steel plate using a matrix. In this method of forming a hot press of a metal plate, since the steel plate is processed at a high temperature, the residual stress is relieved by recovering the displacement that causes the residual stress and is introduced to the processing, or causing transformation after processing. Therefore, very little residual stress remains in a formed product. It is possible to improve the delayed fracture resistance of the steel sheet by strengthening the steel sheet using this method. However, in this method, since it is necessary to perform heating before pressing, the energy cost and the cost of the facilities are high compared to cold forming. In addition, since the product formed is extinguished directly at a high temperature of 600 ° C or higher, the properties of the steel sheet (for example,
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5/110 des galvanizing on a galvanized steel sheet) are easily altered and it is difficult to control properties other than strength and delayed fracture resistance.
[0015] In addition, since residual stress is present on a cutting surface in machining such as cutting or drilling, there is a concern of causing delayed fracture. In this way, when a high strength steel sheet having a tensile strength of 980 MPa or more is processed, the steel sheet is cut by a method using a laser or others that are not accompanied by direct machining, and the generation of residual stress is avoided. However, laser cutting is more expensive compared to shearing or drilling.
[0016] Therefore, it is required that the delayed fracture resistance of the steel sheet is ensured not by the method of formation but by the development of materials depending on the required properties.
[0017] In the product categories of a steel bar, a steel rod, and a thick steel plate, a material capable of preventing delayed fracture by improving the resistance of hydrogen embrittlement was developed. For example, in Non-Patent Document 1, a high strength bolt having excellent resistance to embrittlement by hydrogen in which fine precipitates of elements such as Cr, Mo, V and others, which exhibit resistance to softening by temperament, are coherently revealed precipitated in the martensite. In the high strength screw, the steel is extinguished from the simple austenite phase at high temperature to obtain a single phase microstructure of martensite and then the fine precipitates above are coherently precipitated into the martensite by tempering.
[0018] In the high strength bolt, hydrogen penetrating steel is inhibited from being diffused or concentrated in an area such as a
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6/110 starting point of the delayed fracture where the stress is concentrated using the hydrogen penetrated into the steel being captured around the fine precipitates such as VC and others, which are consistently precipitated in the martensite. Conventionally, steel having high strength and excellent in delayed fracture strength was developed using such fine precipitates in the steel.
[0019] To improve the resistance to delayed fracture using the precipitates as hydrogen capture sites such as VC and others, it is necessary to consistently precipitate the precipitates in the martensite structure.
[0020] However, several hours or more of heat treatment is required to precipitate the precipitates, and there is a problem with manufacturability. That is, on a steel sheet manufactured using general-purpose apparatus for a thin steel sheet such as continuous cooking facilities or continuous hot-dip galvanizing facilities, texture control is performed at the most within a short period of time such as several tens of minutes. Thus, when thin steel sheet is manufactured, it is difficult to improve the fracture resistance delayed by precipitates.
[0021] Furthermore, when precipitates that are precipitated in a hot rolling process are used, even if the above precipitates are precipitated in the hot rolling process, an orientation relationship between the precipitates and a base structure ( ferrite and martensite) is lost due to recrystallization during subsequent cold rolling and continuous annealing. That is, in this case, the precipitates are not coherent precipitates. As a result, the delayed fracture resistance of the steel sheet obtained is significantly deteriorated.
[0022] A high-strength steel plate where there is usually a concern for delayed fracture generation has a microstructure
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7/110 tura mainly including martensite. Although martensite can be formed in a low temperature region, precipitates including VC as the hydrogen capture sites in the temperature region cannot be precipitated.
[0023] As a result, when coherent precipitates such as VC are precipitated on the thin steel sheet to improve the resistance to delayed fracture, it is necessary to precipitate the precipitates further by performing heat treatment after the steel microstructure to be formed using the annealing facilities continuous or continuous hot-dip galvanizing installations. This process brings a significant increase in the cost of manufacture.
[0024] In addition, when the above heat treatment is additionally performed mainly on the microstructure including martensite, the martensite is drastically softened. As a result, it is difficult to use coherent precipitates such as VC to improve the delayed fracture strength of the high strength thin steel plate.
[0025] Here, since the steel described in Non-Patent Document 1 is steel including 0.4% or more of C and a large amount of alloying elements, the workability and weldability that are required for the steel sheet fine are deteriorated.
[0026] In Patent Document 2, a thick steel plate is revealed in which hydrogen defects are mainly reduced through oxides including Ti, and Mg. However, in the thick steel plate disclosed in Patent Document 2, only the hydrogen defects that are caused by the hydrogen captured in the steel in manufacture are reduced, and thus resistance to hydrogen fragility (resistance to delayed fracture) is not considered . In addition, the high formability and resistance to hydrogen brittleness that are required for a thin steel sheet are not considered.
[0027] Conventionally, on a thin steel plate, (1) a
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8/110 Since the plate thickness is thin, even when hydrogen penetrates the thin steel plate, hydrogen is released to the outside in a short period of time. Also, (2) since workability is prioritized, a steel sheet having a tensile strength of 900 MPa or more has not been used before. For this reason, the problems of delayed fracture were small. However, since a demand for using high strength steel sheet as a part is rapidly increasing, the development of a high strength steel sheet having excellent resistance to fragility by hydrogen was required.
[0028] As described above, technologies to improve resistance to hydrogen fragility that are mainly related to steel such screws, steel bars, and steel plate have been developed. Steel is barely subjected to formation and is often used at the stress limit or yield stress or less. Therefore, in the related technique, both the workability required for automobile members, such as the cutting capacity or formability of the limbs (compression formability), and resistance to hydrogen embrittlement after processing are not considered.
[0029] In a member after formation, a tension that is referred to as a residual tension remains within the member. Although the residual stress is present at the site, the residual stress has a high value that exceeds the material yield stress in some cases. For this reason, hydrogen embrittlement is not required to generate thin steel sheet under high residual stress.
[0030] Regarding the hydrogen fragility of thin steel sheet, for example, Non-Patent Document 2 reports the worsening of hydrogen fragility due to the stress-induced transformation of residual austenite. In Non-Patent Document 2, training
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9/110 of thin steel plate was considered, but a quantity of the residual austenite is significantly reduced by suppressing the concentration of C in the austenite so as not to cause deterioration in resistance to hydrogen fragility.
[0031] Furthermore, in the technology described in Non-Patent Document 2, since the microstructure of the thin high-strength steel sheet is limited to a very narrow range, only hydrogen fragility that is generated in a relatively short period of time. time is evaluated. Thus, it is difficult to fundamentally solve the problem of hydrogen fragility when the steel sheet is actually used in a limb for an automobile. Also, in the technology described in Non-Patent Document 2, residual austenite cannot be actively used and the application of the steel sheet is limited.
[0032] As described above, when a large amount of residual austenite that easily occurs hydrogen brittleness is included in the steel sheet, it is very difficult to obtain a steel sheet that simultaneously demonstrates high corrosion resistance, high tensile strength, strength to excellent delayed fracture and high ductility.
CITATION LIST
PATENT LITERATURE [0033] [Patent Document 1] Unexamined Japanese Patent Application, First Publication No. 2002-18531 [0034] [Patent Document 2] Unexamined Japanese Patent Application, First Publication No. H11-293383
NON-PATENT DOCUMENTS [0035] [Non-Patent Document 1] New Developments in Elucidation of Hydrogen Embrittlement (Iron and Steel Institute of Japan, January 1997)
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10/110 [0036] [Non-Patent Document 2] CAMP-ISIJ, Vol. 5, No. 6, pages 1839 to 1842, Yamazaki et al., October 1992, issued by the Iron and Steel Institute of Japan.
SUMMARY OF THE INVENTION
PROBLEMS TO BE SOLVED BY THE INVENTION [0037] An objective of the present invention is to provide a galvanized steel sheet (including a hot dip galvanized steel sheet and a galvanneal coated steel sheet) having a tensile strength (TS ) of 980 MPa or more and having excellent delayed fracture resistance, excellent galvanizing adhesion, high elongation and excellent orifice expandability.
MEANS TO SOLVE THE PROBLEM [0038] The inventors investigated. As a result, the inventors have found that when galvanizing capable of improving delayed fracture resistance is performed as a means to improve delayed fracture resistance without influencing steel quality, delayed steel fracture resistance is improved.
[0039] Specifically, when the hydrogen that penetrates from the environment is captured with the oxide dispersing an oxide including at least one chemical element selected from the group consisting of Si, Mn, and Al in a galvanizing layer, it was observed that diffusion of the hydrogen in an area of stress concentration and delayed fracture caused by diffusion of hydrogen in the area of stress concentration can be delayed.
[0040] Furthermore, to achieve both tensile strength (TS) of 980 MPa or more and excellent formability, it has been observed that it is important to form a smooth martensite with a volume fraction of 30% or more and a residual austenite with a fraction volume of 8% or more in a microstructure completely using Si, which is a strengthening element.
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11/110 [0041] That is, the present invention can provide a galvanized steel sheet having tensile strength (TS) of 980 MPa or more and having excellent delayed fracture resistance, excellent galvanizing adhesion, high elongation and orifice expandability excellent, and the essence of the invention is as follows.
[0042] A galvanized steel sheet according to an aspect of the present invention includes: a steel sheet; and a galvanizing layer on a steel sheet surface where the steel sheet includes, as a chemical steel composition, in% by weight, C: 0.05 to 0.40%, Si: 0.5 to 3 , 0%, Mn: 1.5 to 3.0%, P: limited to 0.04% or less, S: limited to 0.01% or less, N: limited to 0.01% or less, Al: limited to 2.0% or less, O: limited to 0.01% or less, and a balance consisting of Fe and unavoidable impurities, a microstructure of the steel plate includes a ferrite, a bainite, in fraction of volume, 30% or more of a soft martensite, 8% or more of an austenite, and limited to 10% or less of a pearlite, where a fraction of the total volume of soft martensite and bainite is 40% or more, and a fraction of the area of grains having a grain size of more than 35 pm occupied per unit area of the microstructure is 10% or less, and a tensile strength of the steel sheet is 980 MPa or more; and a galvanizing metal in the galvanizing layer includes, as a chemical galvanizing composition, limited to 15% by mass or less of Fe, limited to 2% by mass or less of Al, and the balance consisting of Zn and unavoidable impurities, the galvanizing layer includes an oxide, including at least one chemical element selected from Si, Mn, and Al, and when viewed from a cross section including the steel sheet and the galvanizing layer in a thickness direction, a fraction of the area projection obtained by dividing a length in which the oxide is projected on an interface between the galvanizing layer and the steel sheet by a length of the interface between
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12/110 the galvanizing layer and the steel sheet is 10% or more and a coverage of the galvanizing layer for the steel sheet is 99% or more.
[0043] In the galvanized steel sheet according to (1), steel may also include, as the chemical composition of steel, in% by weight, at least one selected from: Mo: 0.01 to 1.0%, Cr: 0.05 to 1.0%, Ni: 0.05 to 1.0%, Cu: 0.05 to 1.0%, Nb: 0.005 to 0.3%, Ti: 0.005 to 0.3% , V: 0.005 to 0.5%, B: 0.0001 to 0.01%, and a total of at least one of the selected elements of Ca, Mg, and REM: 0.0005 to 0.04%.
[0044] In the galvanized steel sheet according to (1) or (2), the galvanizing layer can be a hot dip galvanized layer.
[0045] In the galvanized steel sheet according to (1) or (2), the galvanizing layer can be a layer with a galvanneal type coating.
[0046] In galvanized steel sheet according to any one of (1) to (3), an amount of Fe can be limited to less than 7% by weight in a chemical galvanizing composition.
[0047] In galvanized steel sheet according to any one of (1) to (4), the chemical composition of galvanization can include 7% by mass to 15% by weight of Fe.
[0048] In galvanized steel sheet according to any one of (1) to (6), the chemical composition of galvanization may include more than 0% by mass and 2% by mass or less than Al.
[0049] A method of making a galvanized steel sheet according to another aspect of the present invention, the method includes: a first process of casting a steel which includes, as a chemical composition of steel, in mass%, C: 0 , 05 to 0.40%, Si: 0.5 to 3.0%, Mn: 1.5 to 3.0%, P: limited to 0.04% or less, S: limited to 0.01% or less, N: limited to 0.01% or less, Al: limited to 2.0% or less,
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1/13
O: limited to 0.01% or less, and a balance consisting of Fe and unavoidable impurities; a second process of heating the steel directly or after it has cooled; a third process of hot rolling steel so that hot rolling is completed at a temperature of a transformation point of Ar3 or higher; a fourth process of rolling steel at 300 ° C to 700 ° C; a fifth process of stripping steel; a sixth process of cold rolling steel by a cold rolling mill having a working roll with a roll size of 1,400 mm or less with a cumulative rolling reduction of 30% or more and less than 100%; a seventh process of heating the steel and holding the steel at 550 ° C to 750 ° C for 20 seconds or more; an eighth process of annealing steel at 750 ° C to 900 ° C .; a ninth process of cooling the steel to an intermediate cooling temperature over a temperature range of 500 ° C or higher and less than 750 ° C at an average first cooling rate of 0.1 ° C / s to 30 ° C / if the steel is cooled from the intermediate cooling temperature to a cooling stop temperature of 100 ° C or higher and below 350 ° C at a second average cooling rate, which is equal to or higher than the first average cooling rate ; a tenth process of controlling a steel temperature within a temperature range of one temperature, which is less than a temperature of the plating bath by 40 ° C, or higher and a temperature which is higher than the temperature of the plating bath galvanizing at 40 ° C or lower; an eleventh galvanizing process by immersing the steel in a hot dip galvanizing bath that flows at a flow rate of 10 m / min to 50 m / min; and a twelfth process of cooling the steel to a temperature of less than 100 ° C; where the second average cooling rate is 1 ° C / s to 100 ° C / s, and a time when the steel temperature is within a temperature range of 350 ° C to 500 ° C is 20
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14/110 seconds or more in processes after the ninth process.
[0050] In the method of manufacturing a galvanized steel sheet according to (8), the steel may also include, as the chemical composition of steel, in% by weight, at least one selected tooth Mo: 0.01 to 1, 0%, Cr: 0.05 to 1.0%, Ni: 0.05 to 1.0%, Cu: 0.05 to 1.0%, Nb: 0.005 to 0.3%, Ti: 0.005 to 0 , 3%, V: 0.005 to 0.5%, B: 0.0001 to 0.01%, and a total of at least one of the selected elements of Ca, Mg, and REM: 0.0005 to 0.04% .
[0051] In the method of making a galvanized steel sheet according to (8) or (9), in the ninth process, when the first average cooling rate is equal to the second average cooling rate, the first average cooling rate can be more than 1 ° C / s and 30 ° C / s or less.
[0052] In the method of making a galvanized steel sheet according to any one of (8) to (10), it can additionally include a process of reheating and retaining the steel in the temperature range of 350 ° C to 500 ° C after the tenth process.
[0053] The method of making a galvanized steel sheet according to any of (8) to (11) may additionally include a process of heating the steel to 460 ° C to 600 ° C to perform alloy treatment after the tenth second process.
EFFECTS OF THE INVENTION [0054] In accordance with the above aspects of the present invention, it is possible to supply the galvanized steel sheet (including a hot dip galvanized steel sheet and a galvanneal coated steel sheet) which is suitable for structural members, reinforcement members, and suspension members for automobiles and having a tensile strength of 980 MPa or more, excellent delayed fracture resistance, excellent galvanization adhesion, high elongation and excellent orifice expandability at a low cost.
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1/15
BRIEF DESCRIPTION OF THE DRAWINGS [0055] FIG. 1 is a schematic view illustrating a method of calculating a fraction of the projection area of an oxide on a galvanizing layer of a galvanized steel sheet according to an embodiment of the present invention.
[0056] FIG. 2 is a view illustrating a state in which the oxide is dispersed in the galvanizing layer in a cross section of the galvanized steel sheet (steel sheet with galvanneal type coating) according to the modality.
[0057] FIG. 3 is a schematic vertical cross-sectional view illustrating the galvanized steel sheet according to the modality.
[0058] FIG. 4A is a flow chart illustrating an example of a method of making a galvanized steel sheet according to an embodiment of the present invention.
[0059] FIG. 4B is a flow chart (subsequent to FIG. 4A) illustrating an example of a method of manufacturing a galvanized steel sheet according to an embodiment of the present invention. DESCRIPTION OF THE MODALITIES [0060] The inventors investigated to solve the above problems. As a result, the inventors found that when an oxide including at least one of Si, Mn, and Al is dispersed in a galvanizing layer, the oxide can be used as a hydrogen capture site and the delayed fracture resistance of a sheet. steel (galvanized steel sheet) is improved. In addition, the inventors also found that when the steel sheet is retained at 550 ° C to 750 ° C during annealing heating, and the oxide that includes at least one of Si, Mn, and Al is formed in the outermost layer of the steel sheet, it is possible to obtain a galvanized steel sheet having a galvanizing layer where the oxide is dispersed by
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16/110 subsequent galvanization or by subsequent galvanization and alloy treatment.
[0061] Still, the inventors have found that when oxide on the surface of the steel plate is used, it is easy to control the morphology of the oxide such as the size or numerical density of the oxide. As a method of dispersing the oxide in the galvanizing layer, a method of galvanizing a steel sheet with molten zinc (molten metal) including the oxide is possible, but the method is difficult to use for the following reasons.
[0062] For example, even when the oxide is dispersed in molten zinc, the oxide forms a grouping by the force of Van der Waals and grows to a large oxide having a size of several to several hundred pm. As a result, since the large oxide causes no galvanizing or cracking, it is not preferable to disperse the oxide in a galvanizing bath. In addition, to increase galvanization adhesion, a clean surface is generally obtained by removing an oxide on the surface of the steel sheet before galvanizing, and the oxide is not usually formed on the steel sheet before galvanizing or purpose.
[0063] In general, in the hot dip galvanizing bath, a Zn or Al oxide film floats. Here, the Zn or Al oxide film is called as foam and causes no galvanizing or delay in alloy formation. The inventors have found that when oxide is present on the surface of the steel sheet, it is easy for the foam to adhere to the steel sheet during immersion in the bath, and thus no galvanizing is easily generated.
[0064] In addition, the inventors discovered a problem that the foam that adheres to the steel sheet not only causes non-galvanization, but also delay in alloy formation. This problem becomes significant in a steel plate that includes a large amount
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17/110 of Si and Mn. Although a detailed mechanism is not clear, it was considered that the oxides of Si and Mn formed on the surface of the steel plate react or interact with the foam which is like an oxide, to promote non-galvanization or delay of alloy formation.
[0065] The inventors found that when the molten metal flows in the hot dip galvanizing bath, the reaction or interaction between the oxides is suppressed to inhibit non-galvanization.
[0066] Hereinafter, a galvanized steel sheet according to one embodiment of the present invention will be described in detail.
[0067] The galvanized steel sheet 1 according to the modality (hereinafter referred to as the galvanized steel sheet 1) includes a steel sheet 2, and a galvanizing layer 3 on a surface of the steel sheet 2, as shown in Fig. 3. Here, the galvanized steel sheet 1 can further include several layers of coating such as an organic layer, an inorganic layer, and others on the surface of the galvanizing layer 3. When such a layer of coating film is not formed on the galvanized steel sheet 1, the galvanized steel sheet 1 consists of the steel sheet 2 and the galvanizing layer 3 on the surface of the steel sheet 2. In addition, the galvanizing layer 3 is formed by solidification of molten metal, and the galvanizing layer 3 can be a hot dip galvanized layer (galvanized layer) that is not subjected to an alloy treatment, or it can be a layer with a galvanneal type coating that is subjected to a li treatment ga.
[0068] First, the galvanizing layer 3 will be described.
[0069] The galvanizing layer 3 contains an oxide 3a including at least one chemical element selected from Si, Mn, and Al. It is very important to disperse such an oxide 3a in the galvanizing layer 3. Particularly, when the oxide 3a is dispersed in a region of galvanizing layer 3 within 5 pm of an interface between the
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18/110 steel plate 2 and the galvanizing layer 3, a hydrogen capture effect becomes remarkable.
[0070] Although a detailed mechanism is not clear, oxide 3a includes several defects, and therefore oxide 3a in the galvanizing layer 3 captures the hydrogen that penetrates from the surface of the galvanized steel sheet 1 (for example, hydrogen generated by reaction of corrosion or hydrogen in the atmosphere), and the penetration of hydrogen into steel plate 2 is delayed. As a result, resistance to delayed fracture is considered to be improved.
[0071] Furthermore, since a steel sheet for an automobile is used in an environment where a wet and dry environment is repeated (a wet-dry environment), the hydrogen that is captured by oxide 3a (ie , the oxide 3a above the steel sheet 2) in the galvanizing layer 3 in the wet environment is released to the atmosphere in the dry environment. Therefore, in a current environment where a car is used, it is possible to continuously use a hydrogen capture effect by the above oxide, and the galvanized steel sheet 1 above is considered to have a high effect due to the delayed fracture resistance.
[0072] The effect is notably exhibited by dispersing oxide 3a including at least one chemical element selected from Si, Mn, and Al in the plating layer 3. Particularly, an Si oxide, an Mn oxide, an Al oxide, and a composite oxide of at least two types of chemical elements selected from Si, Mn and Al have a high melting point compared to zinc and are easily dispersed in the galvanizing layer 3 as oxides having a high hydrogen capture effect.
[0073] Oxide 3a in galvanizing layer 3 is an oxide including one or a combination of Si, Mn, and Al (hereinafter, simply referred to as oxide 3a in some cases). However, impurities will inevitably
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19/110 levels that are mixed into a steel during manufacture (eg inevitable oxides including Zn and Al from the galvanizing bath and inevitable oxides including chemical elements (excluding Si, Mn, and Al) due to the chemical composition of the steel sheet 2) can be included in oxide 3a.
[0074] Therefore, for example, oxide 3a may include one or a combination of Si, Mn, and Al (that is, include at least one of them), and an equilibrium consisting of O (oxygen) and unavoidable impurities.
[0075] Here, examples of oxide 3a including one or a combination of Si, Mn, and Al include SiO2, MnO, ALO3, and Mn2SiO4, and oxide 3a preferably includes SiO2 or Mn2SiO4.
[0076] A fraction of the projection area of oxide 3a that oxide 3a is projected onto the surface of steel sheet 2 is 10% or more. The fraction of the projection area is an evident covering of the oxide 3a that a shadow is formed on the surface of the steel sheet 2 when seen the steel sheet 2 from the upper side of the surface of the galvanized steel sheet 1. The greater the fraction of the area of projection of oxide 3a, plus the hydrogen that penetrates from the surface of the galvanized steel sheet 1 can be captured in the galvanizing layer 3. Thus, it is preferable that the oxide 3a is present in the galvanizing layer 3, and on a parallel surface to the surface of the steel plate 2 as much as possible. Here, the fraction of the projection area is adjusted to be 10% or more. The fraction of the projection area is preferably 15% or more and is more preferably 20% or more. In addition, the upper limit of the projection area fraction is not particularly limited and can be 100%. However, to improve galvanization adhesion or to increase the rate of alloy formation, the fraction of the projection area can be 90% or less, and preferably 80% or less.
[0077] When the projection area fraction is 10% or more, the
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20/110 form of oxide 3a is not partially limited. For example, the shape of the oxide 3a can be any of a film shape, a granular shape, and a current shape. Film-shaped oxide can increase the fraction of the projection area of oxide 3a by volume. Therefore, when the fraction of the film-shaped oxide for the total oxide 3a is large, it is possible to increase the fraction of the projection area. Consequently, it is preferable that the oxide form 3a is a film form.
[0078] The fraction of the projection area of oxide 3a can be easily measured by looking at the cross section of the galvanized steel sheet 1 (the cross section including the steel sheet 2 and the galvanizing layer 3 in a thickness direction). For example, as shown in Fig. 1, when oxide 3a is projected vertically on an interface between the galvanizing layer 3 and the steel plate 2 (a linearly approximated interface), the fraction of the projection area A (%) can be evaluated from the ratio of a projected length of the projected oxide 3a (shadow) (for example, a length (Lhhh) in Fig. 1) to an interface length between the galvanizing layer 3 and the steel plate 2 (for example , a length L in Fig. 1). That is, when an example of FIG. 1 is generalized and a length of an i ° (i is a natural number of 1 or more en or less) is assumed to be fixed as h in a case where there are n (n is a natural number) areas on which oxide 3a is not projected (a non-projection area), the fraction of projection area A can be expressed by the following expression using the above measured length of the interface.
EXPRESSION 1 ⁷ Expression 1 [0079] In the mode, the length ratio was measured in cin
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21/110 with visual fields at a magnification of 10000 times, and its average value was defined as the fraction of the projection area.
[0080] The chemical composition and area fraction of oxide 3a can be evaluated by looking at the structure in the cross section of the galvanized steel sheet 1. For example, there is a method that after the galvanized steel sheet 1 is processed into a flake using a focused ion beam working device (FIB working device) to include the galvanizing layer 3 (the cross section of the galvanized steel sheet 1 in the direction of thickness), the surface of the flake is observed using an electron microscope of field emission transmission (FE-TEM) and composition analysis is performed using a distributed energy X-ray detector (EDX).
[0081] For example, in Fig. 2, an observation sample was prepared using FIB, and then oxide 3a was observed using FE-TEM at a magnification of 50,000 times. In addition, it is possible to identify oxide 3a by analyzing oxide 3a using EDX.
[0082] The galvanizing layer 3 includes a galvanizing metal 3b, and the galvanizing metal 3b has a chemical composition (chemical galvanizing composition) that the amount of Fe is limited to 15% by mass or less, the amount of Al is limited to 2% by weight or less and an equilibrium consisting of Zn and unavoidable impurities. When the amount of Fe in the galvanizing metal 3b is more than 15% by mass, the adhesion of the galvanizing layer 3 is deteriorated on the galvanized steel sheet 1, and the galvanizing layer 3 is fractured or separated during forming. When the fractured or detached plating layer 3 adheres to a matrix, a crack is caused during formation. In this way, when the amount of Al in the galvanizing metal 3b is more than 2% by mass, a thick Fe-Al-Zn barrier layer is formed and the
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22/110 adhesion of the galvanizing layer 3 is deteriorated. In this case, a problem has arisen that it is difficult to control the amount of Fe after the alloy treatment.
[0083] In addition, when the galvanizing layer 3 is a layer with a galvanneal type coating, Fe in the steel plate 2 is incorporated into the galvanizing layer 3, and in this way, it is possible to increase spot weldability and paintability. Particularly, when the amount of Fe in the galvanizing metal 3b of the galvanizing layer 3 after the alloy treatment is 7% by mass or more, it is possible to sufficiently increase the spot weldability. Consequently, when the alloy treatment is carried out, the amount of Fe in the galvanizing metal 3b can be 7% by mass to 15% by mass. In addition, when the alloy treatment is carried out, for example, the amount of Al can be 0.05% by mass or more to control the amount of Fe in the galvanizing metal 3b more flexibly by controlling the rate of alloy formation.
[0084] Even when the amount of Fe in the galvanizing metal 3b is less than 7% by mass, the corrosion resistance, formability, and orifice expansion of the galvanized steel sheet 1 is satisfactory. In addition, when galvanizing metal 3b includes Fe, the amount of Fe can be controlled to be more than 0% by mass and 15% by mass or less, and when galvanizing metal 3b includes Al, the amount of Al can be controlled to be more than 0% by mass and 2% by mass or less. In addition, the amount of Zn in the plating metal 3b is, for example, 80% by weight or more and 100% by weight or less.
[0085] Here, as examples of the above unavoidable impurities in the galvanizing metal 3b of the galvanizing layer 3, for example, unavoidable impurities mixed in the manufacture (for example, unavoidable impurities in the galvanizing bath, chemical elements (excl.
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23/110 indo Fe, Al, and Zn) due to the chemical composition of steel sheet 2, and chemical elements (Ni, Cu, and Co) in optional pre-galvanization) are included. Therefore, in addition to Zn, galvanizing metal 3b can include at least one chemical element of Fe and Al as an optional element or an unavoidable impurity and chemical elements such as Mg, Mn, Si, Cr, Ni, Cu and others as unavoidable impurities .
[0086] The quantity of galvanizing layer 3 (quantity of galvanizing) per unit area (1 m ² ) of the surface of steel sheet 2 is not particularly limited, but the quantity of surface galvanizing on one side is preferably 5 g / m ² or more from the point of view of increasing corrosion resistance. In addition, from the point of view of increasing galvanizing adhesion, the amount of surface galvanizing on one side is preferably 100 g / m ² or less. Here, for the purpose of further improving properties such as paintability, weldability and others, coating films formed by various coating film treatments (for example, a top galvanizing layer formed by electroplating or others, a chromate coating film formed by chromate treatment, a phosphate coating film formed by phosphate treatment, a lubricating coating film, and a coating film to improve weldability) can be supplied on the surface of the galvanizing layer 3.
[0087] In addition, to ensure corrosion resistance and resistance to hydrogen fragility when the galvanized steel sheet is used as a structure, a defect (non-galvanizing) that reaches steel sheet 2 in the galvanizing layer 3 is limited. Specifically, when the surface of the galvanized steel sheet 1 (however, a region of 3/8 the width of the sheet to both edges of the central position of the sheet width) is observed in three fields of view or more at a magnification of 100 times using a stereomi
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24/110 croscope, a cover of the galvanizing layer 3 for the steel sheet 2 (a sharing of an area where the outermost surface of the galvanized steel sheet 1 is the galvanizing layer 3, for a surface area of the sheet metal 1) galvanized steel is 99% or more. That is, the sharing of an area (defect rate), where the outermost surface of galvanized steel sheet 1 is steel sheet 2, on the outermost surface of galvanized steel sheet 1 can be limited to less than 1, 0%. The coverage is preferably 100% (that is, the defect rate is preferably 0%). When coverage is 99% or more and less than 100%, for example, a defective area can be trimmed when the galvanized steel sheet is applied to the components.
[0088] The amount of Fe and the amount of Al in the galvanizing layer 3 can be measured by dissolving the galvanizing layer 3 with an acid, removing an undissolved oxide and others, and then performing chemical analysis of a solution obtained. With respect to the galvanized steel sheet, for example, the galvanized steel sheet 1 which is cut to a size of 30 mm x 40 mm is immersed in a 5% aqueous HCl solution to which an inhibitor is added , and while liquidation of the chemical elements in the steel plate 2 is suppressed, it is possible to obtain a solution by dissolving only the galvanizing layer 3. One undissolved oxide and others are removed from the solution obtained, and then, the amount of Fe and the amount of Al can be quantified from the signal strength obtained by analyzing the ICP emission from the solution and a calibration curve prepared from a solution of the known concentration.
[0089] In addition, in this case, the measurement values of at least three samples cut from the same steel sheet with galvanneal type coating in view of the uneven measurement between the
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25/110 respective samples can be weighted.
[0090] Next, the chemical composition of steel plate 2 will be described. Here,% in the chemical composition of steel sheet 2 means% by mass.
C: 0.05 to 0.40% [0091] C is an element that increases the strength of the steel sheet 2. When the amount of C is less than 0.05%, it is difficult to achieve as much tensile strength of 980 MPa or more as workability. In addition, when the amount of C is more than 0.40%, the amounts of martensite and cementite in the microstructure increase, and sufficient elongation and orifice expansion cannot be obtained. Furthermore, in this case, it is difficult to ensure spot weldability. Therefore, the amount of C is adjusted to 0.05 to 0.40%. When the strength of steel sheet 2 is further increased, the amount of C is preferably 0.08% or more, and more preferably 0.10% or more, and even more preferably 0.12% or more. In addition, when the spot weldability of steel sheet 2 is further increased, the amount of C is preferably 0.38% or less, and more preferably 0.35% or less, and even more preferably 0.32% or less .
Si: 0.5 to 3.0% [0092] Si is an important element to improve resistance to hydrogen fragility. When the amount of Si is less than 0.5%, the amount of oxide 3a in the plating layer 3 is insufficient and the resistance to delayed fracture is not improved. Therefore, the lower limit on the amount of Si is set at 0.5%. When the amount of Si is more than 3.0%, the microstructure cannot be controlled due to excessive generation of ferrite, or workability is impaired. Therefore, the amount of Si is adjusted to 0.5 to 3.0%. In addition, Si is an element that increases the resistance of
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26/110 steel sheet 2. Therefore, when the strength of steel sheet 2 is further increased, the amount of Si is preferably 0.6% or more, and more preferably 0.7% or more, and even more preferably 0 , 8% or more. In addition, when the workability of the steel sheet 2 is further increased, the amount of Si is preferably 2.8% or less, and more preferably 2.5% or less, and even more preferably 2.2% or less.
Mn: 1.5 to 3.0% [0093] Mn is an element that forms an oxide, and it is an element that increases the strength of the steel plate 2. When the amount of Mn is less than 1.5%, it is difficult to obtain tensile strength of 980 MPa or more. When a large amount of Mn is included, a common segregation of Mn and P and Mn and S is promoted to deteriorate workability. Therefore, the upper limit of the amount of Mn is adjusted to 3.0%. When the strength of the steel sheet 2 is further increased, the amount of Mn is preferably 1.6% or more, and more preferably 1.8% or more, and even more preferably 2.0% or more. In addition, when the workability of the steel sheet is further increased, the amount of Mn is preferably 2.8% or less, and more preferably 2.7% or less, and even more preferably 2.6% or less.
[0094] In addition, in the chemical composition of steel plate 2, the quantities of the following chemical elements (O, P, S, Al, and N) are limited. Here, all the lower limits of these five types of chemical elements are 0% and are not limited. Therefore, only the upper limits of these five types of chemical elements are limited.
O: 0 to 0.01% [0095] O forms oxides in steel and deteriorates the elongation, flexibility and expandability of the orifice, and is therefore necessary for
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27/110 suppress the amount of O in the steel. In particular, oxides are present as inclusions in many cases and when oxides are present on a perforated edge surface or cutting surface, a notched crack or a thick protrusion is formed on the end surface. The crack or protrusion causes stress concentration during orifice expansion or difficult operation and becomes a starting point for crack generation, and thus, orifice expandability or flexibility is significantly deteriorated.
[0096] When the amount of O is more than 0.01%, the trend above becomes significant, and thus, the upper limit of the amount of O is adjusted to 0.01%. The lower limit on the amount of O is not particularly limited, but when the amount of O is less than 0.0001%, the costs increase excessively. In this way, the lower limit of the amount of O can be adjusted to 0.0001%. In order to further increase the workability of steel sheet 2, the amount of O is preferably limited to 0.008% or less, and more preferably limited to 0.006% or less, and even more preferably limited to 0.005% or less.
P: 0 to 0.04% [0097] P is segregated in the central area of the steel sheet in the direction of thickness and is an element that causes the welded zone to become weak. When P is more than 0.04%, the embrittlement of the welded zone becomes significant, so the upper limit is adjusted to 0.04%. The amount of P is not particularly limited. However, when the amount of P is less than 0.0001%, the costs increase. Thus, the amount of P is preferably 0.0001% or more. In order to further improve the weldability of steel sheet 2, the amount of P is preferably limited to 0.035% or less, and more preferably limited to 0.03% or less, and even more preferably
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28/110 limited to 0.02% or less.
S: 0 to 0.01% [0098] S is an element having a detrimental effect on the weldability and manufacturing capacity of steel sheet 2 in casting and hot rolling. For this reason, the upper limit on the amount of S is adjusted to 0.01%. The lower limit on the amount of S is not particularly limited. However, when the amount of S is less than 0.0001%, the costs increase, and thus, the amount of S is preferably 0.0001% or more. In addition, since the bonds of S to Mn form thick MnS and deteriorate orifice flexibility and expandability, the amount of S has to be reduced as much as possible. In order to further increase the workability of steel sheet 2, the amount of S is preferably limited to 0.008% or less, and more preferably limited to 0.005% or less, and even more preferably limited to 0.004% or less.
Al: 0 to 2.0% [0099] Al is an element that can be used as an oxide to improve resistance to delayed fracture. In addition, Al is an element that can be used as a deoxidizer. However, when an excessive amount of Al is added, the number of thick inclusions based on Al is increased, a deterioration in the expandability of orifice and surface cracks is caused, and thus, the upper limit of the amount of Al is adjusted to 2 , 0%. Although the lower limit on the amount of Al is not particularly limited, it is difficult to adjust the amount of Al to 0.0005% or less. Thus, the lower limit of the amount of Al can be 0.0005%. The amount of Al is preferably 1.8% or less, and more preferably 1.5% or less, and even more preferably 1.2% or less. N: 0 to 0.01% [00100] N forms thick nitrides and is an element that deteriorates the
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29/110 orifice flexibility and expandability. Therefore, the amount of N has to be suppressed. When the amount of N is more than 0.01%, the above trend becomes significant, and thus, the upper limit of the amount of N is adjusted to 0.01%. In addition, a small amount of N is preferable since N generates a bubble during welding. The lower limit of N is not particularly limited. However, when the amount of N is less than 0.0005%, the manufacturing costs increase notably, and in this way, the lower limit of the amount of N can be adjusted by 0.0005%. In order to further improve the weldability of steel sheet 2, the amount of N is preferably limited to 0.008% or less, and more preferably limited to 0.005% or less, and even more preferably limited to 0.004% or less.
[00101] Here, all Al and Si are elements that suppress cementite formation. Therefore, when the total amount of Al and Si is controlled, it is advantageous to control the microstructure that will be described later. When its total amount is 0.5% or more, it is easier to suppress cementite formation. Thus, the total amount of Al and Si is preferably 0.5% or more, and more preferably 0.6% or more, and even more preferably 0.8% or more.
[00102] The chemical elements described above are basic components (basic elements) of steel sheet 2 in the modality, and the chemical composition in which the basic elements are controlled (included or limited) and the balance consisting of Fe and impurities inevitable is a basic composition of steel sheet 2 in the modality. However, in addition to the basic composition (instead of some Fe in equilibrium), in the modality, the following chemical elements (optional elements) can be additionally contained in the steel plate 2 as needed. Furthermore, even when the optional elements are inevitably mixed in the steel sheet 2
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30/110 (for example, the quantity of each optional element being less than its preferred lower limit), the effect of the modality is not affected.
[00103] That is, steel plate 2 can contain at least one of Mo, Cr, Ni, Cu, Nb, Ti, V, B, Ca, Mg, and REM as the optional elements or the inevitable impurities. Here, since the chemical elements are not necessarily added to steel plate 2, the lower limits of the eleven chemical elements are 0% and are not limited. Therefore, only the upper limits of the eleven chemical elements are limited.
Mo: 0 to 1.0% [00104] Mo is a strengthening element and an important element for improving stiffness. In the case where Mo is added to the steel, when the amount of Mo is less than 0.01%, the effect of the addition cannot be obtained and thus, the lower limit of Mo can be 0.01%. When the amount of Mo is more than 1.0%, the manufacturing capacity of steel sheet 2 is impaired in manufacturing and hot rolling, and thus the upper limit of the amount of Mo is adjusted by 1.0% . from the point of view of the manufacturing capacity of the steel plate 2 and the cost, the upper limit of the amount of Mo is preferably 0.8%, and more preferably 0.5%, and even more preferably 0.3%.
Cr: 0 to 1.0% [00105] Cr is a strengthening element and is an important element for improving stiffness. In the case where Cr is added to the steel, when the amount of Cr is less than 0.05%, the effect of the addition cannot be obtained and thus, the lower limit of Cr can be 0.05%. When the amount of Cr is more than 1.0%, the manufacturing capacity of steel sheet 2 is impaired in manufacturing and hot rolling, and thus the upper limit of the amount of Cr
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1/31
Cr is adjusted to 1.0%. from the point of view of the manufacturing capacity of the steel plate 2 and the cost, the upper limit of the amount of Cr is preferably 0.9%, and more preferably 0.8%, and even more preferably 0.5%.
Ni: 0 to 1.0% [00106] Ni is a strengthening element and is an important element for improving stiffness. In the case where Ni is added to steel, when the amount of Ni is less than 0.05%, the effect of the addition cannot be obtained and thus, the lower limit of Ni can be 0.05%. When the amount of Ni is more than 1.0%, the manufacturing capacity of steel sheet 2 is impaired in manufacturing and hot rolling, and thus the upper limit of the amount of Ni is adjusted by 1.0% . In addition, Ni improves the wettability of steel sheet 2 or promotes an alloying reaction. Therefore, the amount of Ni can be 0.2% or more.
[00107] On the other hand, Ni is an element that is not easily oxidized compared to Fe. Thus, in order to flexibly control the size and quantity of oxide 3a in the galvanizing layer 3 or appropriately controlling the galvanizing properties preventing the oxidation of Fe, the upper limit of the amount of Ni can be further limited. For example, the upper limit on the amount of Ni can be 0.9%.
Cu: 0 to 1.0% [00108] Cu is a strengthening element and an important element for improving stiffness. In the case where Cu is added to the steel, when the amount of Cu is less than 0.05%, the effect of the addition cannot be obtained and thus, the lower limit of Cu can be 0.05%. When the amount of Cu is more than 1.0%, the manufacturing capacity of steel plate 2 is impaired in manufacturing and hot rolling, and thus the upper limit of the amount of
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32/110
Cu is adjusted to 1.0%. In addition, Cu improves the wettability of steel sheet 2 or promotes an alloying reaction. Therefore, the amount of Cu can be 0.2% or more. Similar to Ni, Cu is an element that is not easily oxidized compared to Fe. Therefore, the upper limit for the amount of Cu can be 0.9%.
B: 0 to 0.01% [00109] B is an effective element to strengthen a grain limit and improve the strength of the steel plate 2. In the case where B is added to the steel, when the amount of B is less than 0, 0001%, the effect of the addition cannot be obtained and therefore, the lower limit of B can be 0.0001%. On the other hand, when the amount of B is more than 0.01%, not only the effect of the addition is saturated, but also the manufacturing capacity of steel sheet 2 is impaired in manufacturing and hot rolling. In this way, the upper limit of the amount of B is adjusted to 0.01%. from the point of view of the manufacturing capacity of the steel plate 2 and the cost, the upper limit of the amount of B is preferably 0.008%, and more preferably 0.006%, and even more preferably 0.005%.
Ti: 0 to 0.3% [00110] Ti is an element of strengthening. Ti contributes to an increase in the strength of the steel plate 2 through precipitated strengthening, strengthening by refining the grain suppressing the growth of the ferrite grain, and strengthening by displacement through the suppression of recrystallization. In the case where Ti is added to the steel, when the amount of Ti is less than 0.005%, the effect of the addition cannot be obtained and thus, the lower limit of Ti can be 0.005%. On the other hand, when the amount of Ti is more than 0.3%, heavy precipitation from the carbonitrides is caused and the formability deteriorated. In this way, the upper limit of the amount of Ti is adjusted to 0.3%. In order to further increase the formability of the
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33/110 steel plate 2, the upper limit of the amount of Ti is preferably 0.25%, and more preferably 0.20%, and even more preferably 0.15%.
Nb: 0 to 0.3% [00111] Nb is an element of strengthening. Nb contributes to an increase in the strength of the steel plate 2 through precipitated strengthening, strengthening by refining the grain suppressing the growth of the ferrite grain, and strengthening by displacement through the suppression of recrystallization. In the case where Nb is added to the steel, when the amount of Nb is less than 0.005%, the effect of the addition cannot be obtained and thus, the lower limit of the amount of Nb can be 0.005%. On the other hand, when the amount of Nb is more than 0.3%, heavy precipitation of carbonitrides is caused and the formability deteriorated. In this way, the upper limit of the amount of Nb is adjusted to 0.3%. In order to further increase the formability of the steel sheet 2, the upper limit of the amount of Nb is preferably 0.25%, and more preferably 0.20%, and even more preferably 0.15%.
V: 0 to 0.5% [00112] V is an element of strengthening. V contributes to an increase in the strength of the steel plate 2 through precipitated strengthening, strengthening by refining the grain suppressing the growth of the ferrite grain, and strengthening by displacement through the suppression of recrystallization. In the case where V is added to the steel, when the amount of V is less than 0.005%, the effect of the addition cannot be obtained and thus, the lower limit of the amount of V can be 0.005%. On the other hand, when the amount of V is more than 0.5%, heavy precipitation of carbonitrides is caused and the formability deteriorated. In this way, the upper limit of the amount of V is adjusted to 0.5%. In order to additionally increase formabilide
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34/110 of the steel plate 2, the upper limit of the amount of V is preferably 0.4%, and more preferably 0.3%, and even more preferably 0.2%.
Total amount of at least one of Ca, Mg, and REM: 0 to 0.04% [00113] At least one of Ca, Mg, and rare earth metal (REM) can be added to a maximum of 0.04% as a total content of it. Ca, Mg, and REM are elements used for deoxidation, and one, two or three selected types of Ca, Mg, and REM as their total content can be contained 0.0005% or more in steel.
[00114] When the total amount of at least one selected of Ca, Mg, and REM is more than 0.04%, the formability is deteriorated, and thus, the upper limit of the total quantity is adjusted to 0.04%. Here, REM is usually added to steel as a mixed metal. In addition to La and Ce, at least one of the elements of the lantanoid series can be contained in some cases. Steel sheet 2 may contain elements from the lantanoid series other than La and Ce as unavoidable impurities or La metallic and Ce metallic may be added to the steel. In order to further improve the formability of steel sheet 2, the upper limit of the total amount of at least one selected of Ca, Mg, and REM can be preferably 0.03%, and more preferably 0.02%, and even more preferably 0.01%.
[00115] As described above, steel plate 2 includes, as a chemical composition, the basic elements described above, and the balance consisting of Fe and unavoidable impurities, or includes, as a chemical composition, the basic elements described above , at least one selected from the optional elements described above, and the balance consisting of Fe and unavoidable impurities.
[00116] Next, the microstructure of the steel plate 2 which is a material to be galvanized will be described. Here,% in the microstructure of the steel plate 2 means% by volume (volume fraction, that is,%
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35/110 area in the observed cross section). In addition, each structure in the microstructure (six types of soft martensite, austenite, ferrite, bainite, pearlite and martensite) is referred to as a phase for convenience.
[00117] The microstructure of the steel plate 2 includes ferrite, bainite, mild martensite, and residual austenite.
[00118] To achieve ductility and orifice expansion after the tensile strength of 980 Mpa or more is achieved, the amount of the soft martensite is adjusted by 30% or more. Soft martensite can increase tensile strength compared to ferrite and can increase orifice expandability compared to martensite.
[00119] In general, the greater the difference in hardness between the structures, the smaller the orifice expandability. For example, in a steel including ferrite and martensite, since the stress is concentrated at an interface between ferrite and martensite during deformation and voids are generated, the orifice expandability is low. Then, by controlling the amount of soft martensite that is softer than martensite, the generation of voids is suppressed during deformation to improve orifice expandability. Soft martensite is a martensite that includes an iron-based carbide such as cementite and has low strength (tensile strength) and excellent orifice expansibility compared to the extinct martensite (also referred to as fresh martensite) having the same chemical composition.
[00120] When the amount of soft martensite is less than 30%, it is difficult to ensure tensile strength of 980 MPa after ductility and orifice expansion are achieved. In order to further increase the tensile strength, the amount of the soft martensite is preferably 32% or more, and more preferably 35% or more, and even more preferably 38% or more. In this case,
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36/110 it is still preferable that the volume fraction of the soft martensite is greater than the volume fraction of the phases different from the soft martensite. On the other hand, since the microstructure includes 8% or more of austenite, ferrite, and bainite, the amount of smooth martensite can be less than 92% in terms of volume fraction.
[00121] However, since soft martensite includes a large number of displacements, soft martensite has high resistance, but ductility is deteriorated. Here, ductility is improved using the plasticity induced by transformation of residual austenite. When the volume fraction of the residual austenite is less than 8%, sufficient ductility (total elongation El) cannot be obtained. Therefore, the lower limit of the amount of residual austenite is adjusted to 8%. On the other hand, since the microstructure includes (a total of) 40% or more of mild martensite and bainite, and ferrite, the amount of residual austenite can be less than 60% in terms of volume fraction. To ensure higher elongation, the amount of residual austenite is preferably 9% or more, and more preferably 10% or more.
[00122] In addition, the microstructure includes ferrite. Ferrite is effective for increasing the amount of C in austenite. For example, in an embodiment of the manufacturing method that will be described later, ferrite is formed by cooling after dual phase annealing or single phase region annealing to stabilize residual austenite. Here, the higher the volume fraction of the ferrite, the lower the resistance. Therefore, the volume fraction of the ferrite is preferably limited to 30% or less. In addition, the amount of the ferrite can be more than 0%, and it can preferably be 1% or more. For example, when it is necessary to allow austenite to remain in the microstructure after processing in order to increase the impact absorbing capacity when used as a component, increasing the amount
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37/110 of residual austenite and the increase in the amount of C in residual austenite are effective. Therefore, in response to such a request, the volume fraction of the ferrite can be 10% or more, and preferably 20% or more.
[00123] Still, the microstructure includes bainite. Bainite is effective for increasing the amount of C in residual austenite. The amount of bainite is not particularly limited. However, to obtain tensile strength of 980 MPa or more, a total amount of soft martensite and bainite is adjusted by 40% or more. The amount of bainite can be more than 0%, and it can preferably be 1% or more. For example, when it is necessary to allow austenite to remain in the microstructure after processing to increase the impact absorbing capacity when used as a component, increasing the amount of residual austenite and increasing the amount of C in the residual austenite are effective. Therefore, in response to such a request, the volume fraction of the bainite can be 2% or more, and preferably 5% or more. On the other hand, since the microstructure includes 30% or more of mild martensite, ferrite, and 8% or more of austenite, the amount of bainite is less than 62% in terms of volume fraction.
[00124] In addition, the volume fraction of pearlite in the microstructure is limited to 10% or less. Pearl is formed by the transformation of austenite. For this reason, since pearlite reduces the amount of austenite and the amount of C in austenite, strength and ductility are deteriorated. Therefore, it is preferable that the microstructure does not contain pearlite. However, when the volume fraction of the pearlite is limited to 10% or less, it is possible to ensure tensile strength of 980 MPa or more and ductility. In this way, the upper limit of the pearlite quantity is adjusted to 10%. When C is used more effectively, the volume fraction of the pearlite is pre
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38/110 ferably limited to 5% or less. The lower limit of the volume fraction of the pearlite is 0% without limitation.
[00125] To sufficiently ensure elongation and expandability of the orifice, it is preferable that the martensite is not included in the microstructure. Specifically, the volume fraction of the martensite can be limited to 10% or less. In order to further increase the elongation and expandability of the orifice, the volume fraction of the martensite is preferably limited to 8% or less, and more preferably limited to 7% or less, and even more preferably limited to 5% or less. The lower limit of the volume fraction of the martensite is 0% without limitation.
[00126] Consequently, for example, steel plate 2 may have the microstructure including smooth martensite whose volume fraction is 30% or more, austenite (residual austenite) whose volume fraction is 8% or more, pearlite whose volume fraction is limited to 10% or less, the martensite whose volume fraction is limited to 10% or less as needed, and the balance consisting of ferrite and bainite, and the total volume fraction of soft martensite and bainite can be 40% or more.
[00127] Still to improve the orifice expandability, a sharing of an area (coarse grain fraction) whose grains (coarse grains) having a grain size of more than 35 pm occupy per unit area with respect to all constitutional elements ( the respective phases) of the microstructure is limited to 10% or less. When the number of grains whose grain sizes are large increases, the tensile strength is decreased and the local deformability is also deteriorated. Consequently, it is preferable that the grain size is as small as possible. In addition, since orifice expandability is improved for all grains that are stressed evenly and evenly, the local stress on the grain can be
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39/110 suppressed by limiting the amount of coarse grain. Here, at this point, the grain size is assessed as a region surrounded by a grain boundary of 15 ° or more that is measured using an electron backscattering standard (EBSP).
[00128] In addition, each phase (bainite, martensite, soft martensite, residual austenite, ferrite, and pearlite) of the microstructure described above and the memory structure is identified and the positions of existence of each phase are observed to measure a fraction of the area of each phase (corresponding to the volume fraction of each phase). In measurement, a cross section of the steel sheet 2 in a rolling direction or a cross section in the direction of the right angle of the rolling direction was etched using a nital reagent and a reagent disclosed in the Unexamined Japanese Patent Application, First Publication No. S59-219473 and is observed using an optical microscope (at 1000 times magnification), or a scanning or transmission type electron microscope (at 1000 to 100000 times magnification) to quantify each phase. In this case, the fraction of the area of each phase (that is, corresponding to the volume fraction of each phase) can be obtained using a point counting method or using image analysis observing every 20 fields of view or more.
[00129] As described above, by controlling the chemical composition and microstructure of the steel sheet 2, the galvanized steel sheet 1 (steel sheet 2) having tensile strength of 980 MPa, excellent ductility and excellent orifice expandability can be obtained .
[00130] Here, the thickness of the steel plate 2 is not particularly limited, but the upper limit of the thickness can be 6.0 mm. The lower limit of the thickness of the steel sheet 2 can be, for example, 0.5 mm depending on the application.
[00131] Here, when tensile strength is increased, it elongates
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40/110 orifice and expandability of the orifice are generally deteriorated, and thus, elongation and expandability of the orifice are assessed as follows.
[00132] After an elongation index is obtained from a TS tensile strength product (MPa) and El total elongation (%), when the product is 16000 (MPa x%) or more (TS x El> 16000 MPa x% , the elongation is rated to be excellent.When elongation is emphasized, the product (TS x El) is preferably 18000 MPa x% or more, and more preferably 20000 MPa x% or more.
[00133] After an orifice expansion index is obtained from a product of TS tensile strength (MPa) and orifice expansion ratio λ (%), when the product is 40000 (MPa x%) or more (TS x λ > 40000 MPa x%), the orifice expandability is assessed to be excellent. When the orifice expandability is emphasized, the product (TS x λ) is preferably 45,000 MPa x% or more, and more preferably 50,000 MPa x% or more.
[00134] Galvanized steel sheet 1 according to the modality has TS tensile strength of 980 MPa or more and is excellent in delayed fracture resistance, in galvanizing adhesion, in elongation and orifice expansion. The galvanized steel sheet (material) 1 according to the modality adopts a product manufactured through each melting process, steel making (refinement), casting, hot rolling, and cold rolling, which are iron making processes common used in the beginning and can be obtained properly by the method of manufacture according to the modality that will be described later. However, even with a product that is manufactured by omitting part or all of the ironmaking processes, as long as the product meets the conditions of the modality, the effect described in the modality can be obtained. Thus, the galvanized steel sheet 1 according to the modality is not necessarily limited 870180125071, from 03/09/2018, p. 50/136
41/110 by the manufacturing method.
[00135] Furthermore, when the galvanized steel sheet 1 according to the modality is used as a component, for example, a part of the galvanizing layer 3 can be removed to ensure weldability and the galvanized steel sheet can be processed correctly depending on the purpose.
[00136] Next, a method of manufacturing a galvanized steel sheet according to one embodiment of the present invention will be described in detail.
[00137] FIGS. 4A and 4B show a flow chart of an example of the method of manufacturing a galvanized steel sheet according to the modality. As shown in the flowchart, in the modality, a galvanized steel sheet is manufactured by the following processes. That is, steel (slab) is cast (S1), heated (S2), and hot rolled (S3). After hot rolling (S3), the steel (steel plate, hot rolled steel plate) is wound in a coil (S4), pickled (S5), and cold rolled (S6). After cold rolling (S6), the steel (steel sheet, cold rolled steel sheet) is heated to recrystallize the ferrite (S7), and annealing (S8) and controlled cooling (S9) are performed. Then, the temperature is controlled based on a temperature of the galvanizing bath (S10) and hot dip galvanizing is performed (S11). After hot dip galvanizing (511), the steel (steel sheet, galvanized steel sheet) is cooled (512) to obtain a hot dip galvanized steel sheet as a final product. In addition, when steel (steel sheet, galvanized steel sheet) is subjected to alloy formation treatment (S20) after hot dip galvanizing (S11), a steel sheet with a galvanneal type coating is obtained as a final product after cooling (S21). In addition, after controlled cooling (S9), steel (steel plate, cold rolled steel plate or gal steel plate)
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42/110 vanised) can be heated and retained as needed (S30, S31, and S32) in some cases.
[00138] Here, to control the fraction of the projection area of the oxide 3a in the galvanizing layer of the galvanized steel sheet according to the above modality to be limited to 10% or more, and thus, in the modality, at least cold rolling conditions (S6), heating conditions (S7), and hot dip galvanizing conditions (S11) are appropriately controlled as described below.
[00139] From now on, each process of the modality will be described. [00140] In the modality, steel having the chemical composition described in the above modality is manufactured in the usual method and cast (S1).
[00141] After the steel (plate) after the casting is directly or once cooled, the steel is heated (S2) and disposed for hot rolling (S3). Although the heating temperature before hot rolling is not particularly limited, the temperature is preferably 1150 ° C or higher, and more preferably 1200 ° C or higher for more uniform chemical composition in steel. Hot rolling is completed at the temperature of the Ar3 transformation point or higher to prevent the microstructure from being rolled unevenly in a dual phase region. Here, the transformation point of Ar3 (Ar3) and transformation point of Ac3 (Ac3) which will be detailed later in Table 1 can be calculated respectively from Expressions 2 and 3 below using the amount of C (% C), the amount of Mn (% Mn), the amount of Si (% Si), and the amount of Cr (% Cr).
Ar3 = 901-325 x (% C) -92 x (% Mn) + 33 x (% Si) -20 x (% Cr). . . Expression 2
Ac3 = 910-203 x (% C) ^Λ 0.5 + 44.7 x (% Si) - 30 x (% Mn) - 11 x (% Cr). . .
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Expression 3 [00142] When the steel plate does not contain Cr as the optional element, the transformation point of Ar3 and the transformation point of Ac3, respectively, can be calculated from the following Expressions 4 and 5.
Ar3 = 901-325 x (% C) - 92 x (% Mn) + 33 x (% Si). . .
Expression 4
Ac3 = 910-203 x (% C) ^Λ 0.5 + 44.7 x (% Si) - 30 x (% Mn). . .
Expression 5 [00143] Then, the steel (steel sheet, hot rolled steel sheet) after hot rolling is wound in a coil at a rolling temperature of 300 ° C to 700 ° C (S4). When the rolling temperature of the hot rolling mill is more than 700 ° C, the microstructure of the hot rolled steel sheet is a coarse ferrite.pearlite structure and each phase of the final steel sheet microstructure after the subsequent processes (for example , cold rolling, annealing, and plating treatment and alloy formation) become the uneven microstructure. As a result, the coarse grain fraction above cannot be controlled sufficiently and excellent orifice expandability cannot be achieved. In this way, the upper limit of the rolling temperature is set to 700 ° C. The lamination temperature is preferably 650 ° C or lower.
[00144] Although the lower limit of the rolling temperature is not particularly defined, when the rolling temperature is 300 ° C or higher, it is possible to obtain the resistance of the hot rolled steel sheet which is suitable for cold rolling. Therefore, the lamination temperature is preferably 300 ° C or higher.
[00145] The hot rolled steel sheet manufactured in this way is subjected to pickling (S5). Considering that the pickling removes the oxides on the surface of the steel plate, the pickling is imposed
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44/110 to improve the galvanizing properties. The steel sheet can be pickled once or pickled a plurality of times in a split way.
[00146] The pickled hot-rolled steel plate is cold-rolled (S6) by a roller (operating roller) having a roller size of 1400 mm or less under a cumulative rolling reduction of 30% or more and passes through a continuous hot-dip galvanizing pipe. It is possible to promote recrystallization of the ferrite and formation of an oxide which is the result of recrystallization (an oxide necessary to form oxide 3a described above) by cold lamination during heating (retention) in the next process.
[00147] Under a cumulative lamination reduction of less than 30%, since recrystallization is not sufficiently promoted during heating (retention) in the next process, the oxide is not formed sufficiently in the next process and resistance to fragility by sufficient hydrogen cannot be obtained. Therefore, the cumulative lamination reduction (lower limit) is adjusted by 30% or more. Preferably, the cumulative lamination reduction is 40% or more. On the other hand, the upper limit of the cumulative lamination reduction of cold lamination is not particularly defined (less than 100%), but the cumulative lamination reduction is preferably 80% or less to perform cold lamination with suppression of an increase in cold rolling load. Since the plating adhesion, elongation, strength orifice expandability, and resistance to hydrogen brittleness are hardly affected by the number of lamination passages or the reduction of lamination in the respective passages, the number of lamination passages or the reduction of lamination in the respective passages are not particularly defined. Here, when an inlet thickness before an initial pass in the cold rolling mill is set as a reference 870180125071, from 03/09/2018, pg. 54/136
45/110 reference, the cumulative lamination reduction is a percentage of a cumulative lamination reduction amount for this reference (a difference between an inlet thickness before an initial pass in cold rolling and an outlet thickness after a final pass cold rolling).
[00148] In addition, the tension required for recrystallization increases with an increase in a deformation rate (henceforth, an average deformation rate) of the steel sheet by unitary sheet thickness. In this way to obtain sufficiently average strain rate, a roll having a small roll size that a surface area that contacts a material to be laminated and the amount of elastic deformation of the roll on this surface is small, is used. In cold rolling under the cumulative rolling reduction of 30% or more, when a roll having a roll size of 1400 mm or less is used, it is possible to form an oxide that is necessary to obtain resistance to fragility by sufficient hydrogen. The smaller the roll size, the higher the average strain rate above. In this way, a rate of recrystallization can be increased by reducing the time before recrystallization is started, and the amount of the oxide to be formed is also increased. The effect of increasing the recrystallization rate and the effect of oxide formation are caused when the roll size is 1400 mm or less. Therefore, the roll size is adjusted to 1400 mm or less. The size of the roll is preferably 1200 mm or less, and more preferably 1000 mm or less.
[00149] The steel (steel sheet, cold rolled steel sheet) after cold rolling is heated (S7). Since galvanizing adhesion, elongation, strength, orifice expansion, and resistance to hydrogen brittleness are hardly affected by the heating rate (average heating rate) at the time when the sheet
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46/110 of steel passes through the galvanizing pipe, the rate of heating is not particularly defined. When the heating rate is 0.5 ° C / s or more, it is possible to ensure sufficient productivity, and therefore, the heating rate is preferably 0.5 ° C / s or more. When the heating rate is 100 ° C / s or less, the modality can be implemented in a usual facility investment, and therefore, the heating rate is preferably 100 ° C / s from a cost point of view.
[00150] During heating, the steel sheet is held at 550 ° C to 750 ° C for 20 seconds or more. This is because the oxide can be dispersed by retaining the steel sheet in the temperature range. It is considered that the formation of oxide is strictly related to the recrystallization of cold worked ferrite. That is, since Si, Al and Mn that form the oxides are supplied by diffusion (particularly grain boundary diffusion) from inside the steel sheet, the oxide that includes one or a combination of Si, Mn, and Al tends to be formed at the grain boundary of the ferrite on the surface of the steel plate. The fine grain boundary of the ferrite formed by recrystallization as described above is used as an oxide formation site. In addition, as described above, since the oxide is preferably formed at the ferrite grain boundary, the oxide in general has a lattice structure and easily becomes a formation (fraction of the projection area) that is capable of capturing hydrogen effectively.
[00151] Still, in a temperature range of 550 ° C to 750 ° C, a rate of recrystallization of ferrite is higher than a rate of oxide formation. Therefore, when the temperature of the steel plate after cold rolling is controlled within the temperature range, recrystallization is started before the oxide is formed. In this way, it is possible to form a sufficient amount (area) of oxides on the surface of the steel plate.
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47/110 [00152] When the residence temperature is less than 550 ° C, it takes much longer for recrystallization and also, only the ferrite thus worked which is greatly extended is present. Thus, a grain boundary with sufficient quantity (density) to form oxides is not present. In addition, when the residence temperature is more than 750 ° C, the rate of oxide formation is higher than the rate of recrystallization of ferrite and granular oxides are formed at the grain boundary in the middle of the recrystallization and grain growth, or reversible transformation, and thus, it is difficult to form a sufficient amount (area) of oxides on the surface of the steel sheet. Here, the time when the temperature of the steel (steel plate) is within the temperature range of 550 ° C to 750 ° C is controlled. If the time when the steel sheet temperature is within the temperature range of 550 ° C to 750 ° C is less than 20 seconds, a sufficient amount of oxides (particularly, oxides having an advantageous shape in the fraction of the projection area) cannot be obtained and in a final product, the fraction of the projection area of the above oxides is less than 10%. In order to further increase the resistance to hydrogen brittleness of the steel sheet, the dwell time is preferably 30 seconds or more.
[00153] Here, the time when the temperature of the steel plate is within the temperature range of 550 ° C to 750 ° C can be controlled by isothermal retention or controlled by heating (temperature rise). The upper limit of time when the steel sheet temperature is within the temperature range of 550 ° C to 750 ° C is not particularly limited and can be 2000 seconds, or perhaps 1000 seconds.
[00154] On the other hand, in the steel sheet thus cold rolled, the ferrite grain is elongated in one direction of the rolling, the size of the ferrite grain is large, and the amount of the ferrite grain limit
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48/110 is small. As a result, even when the cold rolled steel sheet in which most of the ferrite is non-recrystallized ferrite is annealed, it is difficult to ensure a fraction of the oxide projection area of 10% or more. Therefore, as described above, the grain size of the ferrite can be refined by controlling the time when the steel plate temperature is within the temperature range of 550 ° C to 750 ° C and recrystallizing the ferrite before the formation of oxide.
[00155] Also, the steel plate after recrystallization is annealed at an annealing temperature (highest heating temperature) of 750 ° C to 900 ° C (S8). When the annealing temperature is less than 750 ° C, it takes longer to run the re-solid carbide solution formed during hot rolling, and the carbides remain, and in this way, the rigidity of the steel plate is deteriorated. Therefore, a sufficient amount of soft martensite and austenite cannot be ensured and it is difficult to ensure tensile strength of 980 MPa or more. Therefore, the lower limit of the annealing temperature is 750 ° C.
[00156] Heating in excessive high temperature not only causes an increase in cost, but also difficulties such as deterioration of a sheet form when the steel sheet passes through the galvanizing pipe at a high temperature and a decrease in the life of the roll. Therefore, the upper limit of the annealing temperature is set to 900 ° C. A heat treatment time (annealing time) in the above temperature range (750 ° C to 900 ° C) is not particularly limited but is preferably 10 seconds or more for carbide dissolution.
[00157] To eliminate costs, the heat treatment time is preferably 600 seconds or less. The steel sheet can be annealed by running isothermal maintaining at the highest heating temperature or the steel sheet can be annealed by starting to cool
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49/110 immediately after heating the gradient is performed and the temperature reaches the highest heating temperature. [00158] When an atmosphere in a process of annealing the galvanizing pipe by continuous hot immersion is controlled, it is possible to flexibly control the oxide formed on the surface of the steel plate (oxide including at least one chemical element selected from Si, Mn, and Al) can be controlled. That is, when an H2 concentration and a dew point in the annealing atmosphere are managed, it is possible to control an oxygen potential, which is important for controlling the reaction. For example, the dew point can be adjusted to -20 ° C or higher in the N2 atmosphere with an H2 concentration of 20% by volume or less that is applied under usual annealing conditions. In this case, the amount and shape of the oxide including at least one chemical element selected from Si, Mn, and Al can be more flexibly controlled.
[00159] To ensure a sufficient amount of soft martensite and austenite, it is important to control the cooling conditions so as not to excessively form structures other than soft martensite and austenite (eg ferrite, pearlite, and bainite) in the cooling process after annealing. In particular, it is preferable for austenite to be stabilized by controlling refrigeration conditions (for example, ferrite transformation control and pearlite transformation control) so that the amount of C in austenite can be increased.
[00160] Therefore, the steel sheet after annealing is subjected to controlled cooling by one-stage or two-stage cooling (S9).
[00161] First, when two-stage cooling is performed, the steel sheet is cooled to a predetermined temperature (hereinafter, referred to as an intermediate cooling temperature
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50/110 ria) in a temperature range of 500 ° C or higher to less than 750 ° C after the above annealing is completed at an average cooling rate of 0.1 ° C / s to 30 ° C / s (hereinafter , referred to as a first medium cooling rate) (first cooling step). The first cooling step will be described in detail below.
[00162] To ensure sufficient productivity, the first average cooling rate is 0.1 ° C / s or more. In order to further increase productivity, the first average cooling rate is preferably 0.2 ° C / s or more, and more preferably 0.5 ° C / s or more, and even more preferably 0.8 ° C / s or more. In addition, the first average cooling rate is adjusted to 30 ° C / s or less to form ferrite. To increase the amount of austenite and the stability of austenite by further increasing the amount of ferrite, the first average cooling rate is preferably 25 ° C / s or less, and more preferably 22 ° C / s or less, and still preferably 20 ° C / s or less. Consequently, the first average cooling rate is set at 0.1 to 30 ° C / s. In addition, when the first average cooling rate is 30 ° C / s or less and the intermediate cooling temperature is below 500 ° C, structures other than austenite and martensite (for example, ferrite and bainite) are excessively formed, and therefore, in a final product, 30% or more of mild martensite and 8% or more of austenite (residual austenite) cannot be guaranteed. However, when the first average cooling rate is 0.1 ° C / s to 0.8 ° C / s, the intermediate cooling temperature is preferably Ar3 ° C or higher and less than 750 ° C to ensure productivity and not form pearlite. On the other hand, when the intermediate cooling temperature is 750 ° C or higher, the manufacturing cost increases and ferrite is also not formed in some cases. To form the ferrite more stably, the intermediate cooling temperature is preferably 740 ° C or lower, and most preferred
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51/110 at 730 ° C or lower. Consequently, the intermediate cooling temperature is 500 ° C or higher and less than 750 ° C.
[00163] Then, after the first cooling step is completed, the steel sheet is cooled to a cooling stop temperature of 100 ° C or higher and less than 350 ° C from the intermediate cooling temperature to a average cooling rate of 1 ° C / s to 100 ° C / s (hereinafter referred to as a second average cooling rate) which is higher than the above average first cooling rate (second cooling step). The second cooling step will be described in detail below.
[00164] To ensure the martensite that is required to obtain 30% or more of mild martensite in a final product, the cooling stop temperature is adjusted to less than 350 ° C. To ensure smoother martensite in a final product, the cooling stop temperature is preferably 340 ° C or lower, and more preferably 320 ° C or lower, and even more preferably 300 ° C or lower. In addition, to ensure austenite that is required to obtain 8% or more of austenite (residual austenite) in a final product, the cooling stop temperature is set at 100 ° C or higher. To ensure more austenite in a final product, the cooling stop temperature is preferably 120 ° C or higher, and more preferably 150 ° C or higher, and even more preferably 180 ° C or higher. In particular, the cooling stop temperature is still preferably set to a temperature or higher, and the temperature is less than 100 ° C from a temperature (Ms point) that martensite transformation is initiated. Consequently, the cooling stop temperature is 100 ° C or higher and less than 350 ° C. Controlling the cooling stop temperature in this way, between austenite that immediately came into existence on a steel plate after the completion of the first
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52/110 cooling step, that an appropriate amount of austenite can be transformed into martensite. To ensure martensite, it is necessary to obtain 30% or more of mild martensite in a final product, the second average cooling rate is set at 1 ° C / s or more. When the second average cooling rate is less than 1 ° C / s, not only is productivity deteriorated, but structures other than austenite and martensite are also excessively formed. To ensure a large amount of mild martensite and austenite in a final product, the second average cooling rate is preferably 2 ° C / s or more, and more preferable 5 ° C / s or more, and even more preferably 10 ° C / s or more, and most preferably 20 ° C / s or more. In particular, when the first average cooling rate above is 0.1 ° C / s to 0.8 ° C / s, it is preferable to increase the second average cooling rate as described above. In addition, to sufficiently suppress the manufacturing cost (installation cost), the second average cooling rate is set at 100 ° C / s or less. The second average cooling rate is preferably 80 ° C / s or less and more preferably 50 ° C / s or less. Consequently, the second measured cooling rate is set at 1 ° C / s to 100 ° C / s. In addition, when two-stage cooling is performed to increase productivity and suppress the formation of different phases of austenite and martensite as much as possible, the second average cooling rate may be higher than the first average cooling rate. To increase the amount of C in the austenite after the first cooling step and after the second cooling step and to increase the amount of martensite and austenite after the second cooling step, it is preferable that a difference between the second cooling average and the first average cooling rate is large.
[00165] On the other hand, when cooling a step is performed, for the same reason that the cooling conditions do not cool
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53/110 described above, the steel sheet can be cooled to a cooling stop temperature of 100 ° C or higher and below 350 ° C at an average cooling rate of 1 ° C / s 30 ° C / s. The one-step cooling condition corresponds to a case where the first average cooling rate is equal to the second average cooling rate (in this case, the intermediate cooling temperature is included in the temperature range of 500 ° C or higher and lower at 750 ° C) in conditions above two-stage cooling. The average cooling rate in one-step cooling is preferably more than 10 ° C / s, and more preferably 12 ° C / s or more, and even more preferably 15 ° C / s or more, and most preferably 20 ° C /I'm more.
[00166] In addition to each average cooling rate as described above, it is preferable that a cooling rate during each second satisfies the conditions of the average cooling rates described above.
[00167] Still, after the controlled cooling above, the steel sheet is reheated. Subsequently, the steel sheet is immersed in a hot dip galvanizing bath and then cooled to room temperature. In the processes after the controlled cooling above, the time when the steel sheet temperature is within a temperature range of 350 ° C to 500 ° C is controlled to be 20 seconds or more. By controlling the time in 20 seconds or more, the transformation from austenite to bainite (transformation from bainite) proceeds sufficiently, and in this way, the amount of C in the untransformed austenite can be increased. As a result, the stability of austenite is increased and 8% or more of austenite (residual austenite) can be ensured in a final product. On the other hand, when the time is less than 20 seconds, the transformation from austenite to bainite (transformation from bainite) does not proceed
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54/110 sufficiently, and thus, the stability of austenite is impaired and 8% or more of austenite (residual austenite) cannot be ensured in a final product. In order to further increase the volume fraction of the austenite, the time when the temperature of the steel sheet is within the temperature range of 350 ° C to 500 ° C is preferably controlled in 25 seconds or more, and more preferably controlled in 30 seconds or more. In addition, the upper limit of time when the steel sheet is within the temperature range of 350 ° C to 500 ° C is not particularly limited, and, for example, from the point of view of productivity, the upper limit can be 1000 seconds, or 500 seconds. Here, the temperature range of 350 ° C to 500 ° C is a temperature range where the transformation of bainite is promoted at a sufficient rate. That is, in each process after the second cooling step such as regulating the temperature of the steel sheet before the steel sheet is immersed in the galvanizing bath, immersing the steel sheet in the galvanizing bath, and forming treatment Alloy of the galvanizing layer, the time when the steel sheet temperature is within the temperature range of 350 ° C to 500 ° C can be controlled to be 20 seconds or more in total. To control the time more reliably when the temperature of the steel plate is within the temperature range of 350 to 500 ° C to be 20 seconds or more, a process of retaining the steel plate in the temperature range of 350 ° C to 500 ° C (S30, S31, and S32) can be added after the second cooling step. The time when the steel sheet temperature is maintained in the temperature range of 350 ° C to 500 ° C in the containment process is not particularly limited, but, for example, it can be 20 seconds or more.
[00168] Furthermore, before the steel sheet is immersed in the hot dip galvanizing bath, by re-heating, the
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55/110 steel plate temperature (plate temperature) is controlled within a temperature range of a temperature or higher that is below a temperature of the plating bath at 40 ° C at a temperature or below that is higher than the temperature of the plating bath at 40 ° C (S10). When the temperature of the sheet is below the temperature of the plating bath by 40 ° C or higher, a temperature of molten zinc around the surface of the steel sheet at the time that the steel sheet is immersed in the galvanizing bath decreases significantly and some molten zinc is solidified. Solidification deteriorates the appearance of galvanizing and the plate temperature is re-heated (temperature of the galvanizing bath - 40 ° C). In addition, when the sheet temperature is higher than the plating temperature by 40 ° C or higher, an operational problem arises during plating, and thus the sheet temperature is adjusted (plating bath temperature + 40 ° C).
[00169] After the steel sheet temperature is controlled in this way, the steel is immersed in a hot dip galvanizing bath (galvanizing bath) having molten metal that flows at a flow rate of 10 m / min at 50 m / min and is subjected to hot dip galvanizing (S11).
[00170] By adjusting the flow rate of the molten metal to 10 m / min by 50 m / min, it is possible to form a galvanizing layer including an oxide while non-galvanizing is prevented. When the flow rate of the molten metal is less than 10 m / min, a contact ratio of the molten metal in the galvanizing bath cannot be increased by suppressing an adhesion of the oxide in the galvanizing bath on the surface of the steel plate. In this way, non-galvanizing cannot be prevented and the appearance of the galvanizing layer is deteriorated. On the other hand, when the flow rate of the molten metal is more
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56/110 of 50 m / min, excessive investment of ease is required to obtain such a flow rate and also, a pattern caused by the flow of the molten metal is generated in the galvanizing layer. In this way, the appearance of the galvanizing layer is deteriorated. Consequently, the flow rate of the molten metal is fixed from 10 m / min to 50 m / min. As a result of controlling the flow rate of the molten metal in this way, while suppressing the adhesion of zinc oxide in the galvanizing bath to the surface of the steel sheet, and the zinc oxide has high chemical affinity with the oxide formed on the surface of the steel plate, it is possible to incorporate an oxide that is an easily oxidizable element formed on the surface of the steel plate, in the galvanizing layer. Therefore, it is possible to disperse the oxide in the galvanizing layer having a good appearance.
[00171] In addition, during heating before the aforementioned annealing, since an oxide including at least one chemical element selected from Si, Mn and Al is formed on the surface of the steel sheet, non-galvanizing (a defect in galvanization, a non-galvanized area) easily occurs after the steel sheet is removed from the galvanizing bath. Here, in the galvanizing bath, the molten metal flowed at a flow rate of 10 m / min to 50 m / min. By allowing the molten metal (jet flow) to flow at such a flow rate, non-galvanization can be prevented. In addition, when the oxide is formed on the surface of the steel sheet, in a case where the galvanizing layer is formed from alloy, the formation of alloy is delayed. However, alloying can be promoted by controlling the flow rate above the molten metal. Here, a flow direction of the molten metal is not particularly limited, and only the flow rate of the molten metal can be limited.
[00172] In addition, the molten metal in the galvanizing bath may be pure zinc (zinc and unavoidable impurities) or may contain Al
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57/110 (for example, 2% by mass or less) as the optional element or the unavoidable impurities and chemical elements such as Fe, Mg, Mn, Si, Cr, and others as the unavoidable impurities.
[00173] For example, when steel sheet with galvanneal type coating is manufactured (when the formation of the alloy of the galvanizing layer is carried out), the amount of effective Al in the galvanizing bath is preferably controlled by 0.05% in 0.500 mass by mass to control the properties of the galvanizing layer. Here, the amount of effective Al in the galvanizing bath is a value obtained by subtracting the amount of Fe in the galvanizing bath from the amount of Al in the galvanizing bath.
[00174] When the amount of effective Al is 0.05% by mass to 0.500% by mass, the galvanizing layer having a good appearance can be obtained and productivity can also be sufficiently increased. That is, when the amount of effective Al is 0.05% by mass or more, scrap generation can be suppressed and the galvanizing layer having a good appearance can be obtained. In addition, when the amount of effective Al is 0.500% by mass or less, the formation of the alloy can be effectively carried out and in this way, it is possible to increase productivity.
[00175] In the molten metal, a Zn oxide and an Al oxide are present as the inevitable impurities. It is preferable to remove the oxides as much as possible or to suppress the reaction with the steel plate. However, oxides can inevitably be mixed in the galvanizing layer after galvanizing.
[00176] Also, the steel sheet that is immersed in the galvanizing bath is taken from the galvanizing bath and is rubbed as needed. When the steel sheet is rubbed, it is possible to control the amount of galvanization (amount of deposition of galvanization) adhered to the surface of the steel sheet. Although the amount
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58/110 of galvanizing deposition is not particularly limited, from the point of view of additionally increasing corrosion resistance, on the one hand the amount of galvanizing deposition on a surface is preferably 5 g / m ² or more. In addition, from the point of view of additionally increasing the galvanizing adhesion, the amount of galvanizing deposition on the one side of a surface is preferably 100 g / m ² or less.
[00177] After undergoing hot-dip galvanizing, the steel sheet is cooled to a temperature of less than 100 ° C (for example, room temperature) (S12). The cooling stop temperature on cooling is not particularly limited as long as the microstructure is stabilized, and, for example, the temperature can be 0 ° C or higher (for example, water temperature or room temperature or higher) of a cost point of view.
[00178] After cooling, it is possible to obtain a hot-dip galvanized steel sheet like the galvanized steel sheet. In order to additionally increase spot weldability and paintability of the galvanized steel sheet, the obtained galvanized steel sheet can be subjected to the alloy formation treatment (S20). Since Fe in the steel plate is incorporated in the galvanizing layer by the alloy formation treatment, after cooling (S21), a galvanized steel plate (ie a galvanneal coated steel plate) which is excellent paintability and spot weldability can be achieved.
[00179] In this way, when the alloy formation of the galvanizing layer is carried out, the galvanized steel sheet can be heated to 460 ° C or higher. When the temperature of the alloying treatment (alloying temperature) is 460 ° C or higher, the alloying is effectively carried out at a high alloying rate, and thus it is possible sufficiently to
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59/110 increase productivity. However, when the alloying temperature is higher than 600 ° C, carbides are formed and the volume fraction of austenite is decreased in the steel in a final product. As such, it is difficult to secure 8% or more of austenite. Therefore, the upper limit of the alloying temperature is set at 600 ° C. That is, the highest temperature in the process after the second cooling step can be limited to 600 ° C or less.
[00180] Although the basic configuration of the manufacturing method modality that a galvanized steel sheet has been described above, the addition of the configuration can be done within a range that does not abandon the essence of the present invention. For example, top-layer galvanization (additional galvanization, for example, electroplating) can be performed on galvanized steel sheet for the purpose of improving paintability and weldability, or various treatments (for example, chromate treatment, phosphate treatment, a treatment to improve lubrication and a treatment to improve weldability) can be performed.
[00181] In addition, for example, in order to further improve the adhesion of the galvanization, the steel sheet can be subjected to galvanization including one or a combination of Ni, Cu, Co, and Fe (galvanization including at least one chemical element selected from the elements and unavoidable impurities) between cold rolling and annealing. Galvanizing is carried out on purpose, but the amount of the chemical element mixed in the galvanizing layer by galvanizing is small enough to be determined as an impurity.
[00182] Still, for example, the galvanized steel sheet that is cooled to less than 100 ° C can be subjected to hardening lamination. The cumulative lamination reduction of hardening lamination is preferably 0.1% to 1.5%. When reducing lami
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60/110 cumulative nation is 0.1% or more, it is possible to additionally improve the appearance of the galvanized steel sheet by hardening lamination, and the reduction of cumulative rolling is easily controlled. Therefore, the cumulative lamination reduction is preferably 0.1% or more. When the cumulative lamination reduction is 1.5% or less, sufficient productivity can be ensured, and thus, the cumulative lamination reduction is preferably 1.5% or less. The hardening lamination can be performed online or offline. To achieve a desired cumulative lamination reduction, hardening lamination can be performed once or divided into a plurality of times. Here, when an entry thickness before an initial pass in hardening lamination is fixed as a reference, the cumulative lamination reduction is a percentage of an amount of the cumulative lamination reduction for the reference (a difference between an entrance thickness before an initial pass in hardening lamination and an outlet thickness after a final pass in hardening lamination).
[00183] Here, detailed methods of a process of stripping the steel sheet for a process of immersing the steel sheet in a galvanizing bath are not particularly limited as long as the above conditions are met. For example, such methods as the Sendzimir process of After removing the fat and stripping, heating in a non-oxidizing atmosphere, annealing in the reducing atmosphere containing H2 and N2, then cooling to near the temperature of the galvanizing bath, and immersing in a galvanizing bath, the total reduction furnace method of regulating the atmosphere during annealing, once oxidizes a steel plate surface, then performs reduction of the steel plate surface (here, an oxide of an easily oxidizable element is not reduced) to execute
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61/110 cleaning the steel sheet surface, and then immersing it in a galvanizing bath; the flow process of removing grease and stripping a steel plate, performing flow treatment using ammonium chloride or others, and immersing in a galvanizing bath. can be applied with changes according to each modality process as needed.
EXAMPLES [00184] In the following, examples of the present invention will be described in detail.
[00185] The hot rolled steel sheets obtained by hot rolling the continuously melted plates having chemical compositions (however, a balance includes Fe and unavoidable impurities) shown in Table 1 under the hot rolling conditions shown in Tables 2 and 5 ( in the Tables, plate heating temperature and finishing lamination temperature) were cooled in water in a water cooling zone, and then laminated at the temperature shown in Tables 2 and 5 (in the Tables, lamination temperature). The thickness of the hot-rolled steel sheets was 2 mm to 4.5 mm.
[00186] The hot-rolled steel sheets were stripped and then cold-rolled to a thickness of 1.2 mm after cold rolling under the cold rolling conditions shown in Tables 2 and 5 (in Tables, size of roll and reduction of cold rolling), and thus cold rolled steel sheets were formed. Then, the cold-rolled steel sheets were subjected to various heat treatments and hot dip galvanizing treatment in a continuous galvanneal coating pipe under the conditions shown in Tables 3 (continued from Table
2) and 6 (continuation of Table 5).
[00187] As shown in Tables 3 and 6, in the heat treatment
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62/110 after cold rolling, the cold rolled steel sheets were heated so that the time when the temperature of the cold rolled steel sheets is within a temperature range of 550 ° C to 750 ° C (in Table , tA) is a predetermined time. Then, the cold rolled steel sheets were annealed under predetermined annealing conditions (in the Tables, annealing temperature (however, higher heating temperature), H2 concentration, and dew point). In addition, cold rolled steel sheets were cooled from the annealing temperature in Tables 3 and 6 to a predetermined intermediate cooling temperature at a primary cooling rate, and then cooled to a predetermined cooling stop temperature at a rate of predetermined secondary cooling (one-step or two-step cooling control). In addition, as shown in Tables 4 (continued from Table 3) and 7 (continued from Table 6), as needed, cold rolled steel sheets were reheated to a predetermined temperature range and contained in the temperature range for a predetermined retention time.
[00188] Then, the cold rolled steel sheets controlled at a predetermined temperature (in the Tables, temperature regulated before galvanizing) were immersed in a controlled hot dip galvanizing bath under the predetermined conditions shown in Tables 4 and 7 (in Tables, temperature of the galvanizing bath and flow rate of the galvanizing bath), and the steel sheets obtained (galvanized steel sheets) were cooled to room temperature. The amount of Al in the molten metal (molten zinc) in the plating bath was 0.09% by weight to 0.17% by weight. Some of the steel sheets were subjected to the alloy formation treatment under the respective conditions (in the Tables, alloy formation temperature) after being immersed in the gal bath.
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63/110 hot immersion and the steel sheets obtained were cooled to room temperature. The amount of galvanizing (the amount of galvanizing layer) on both surfaces at that time was approximately 35 g / m ² . Finally, the steel sheets obtained were subjected to hardening lamination under a cumulative rolling reduction of 0.4%. Here, tB in Tables 4 and 7 represents a total time when the temperature of the steel sheets is 350 ° C to 500 ° C after controlled cooling is completed. In addition, with respect to the product plate types in Tables 4 and 7, GI represents a hot-dip galvanized steel sheet (galvanized steel sheet) and GA represents a galvanneal-coated steel sheet.
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Table 1
COMPOSITION N ° CHEMICAL COMPOSITION (% MASS) Ac3 (° C) Ar3 (° C)Ç Si Mn P s Al N B O Si + Al Others THE 0.081 1.12 2.58 0.010 0.0030 0.010 0.0031 0 0.0024 1.13825 674 Example B 0.122 0.61 2.28 0.011 0.0029 0.490 0.0029 0 0.0022 1.10798 672 Example Ç 0.145 0.73 2.51 0.008 0.0033 0.910 0.0028 0 0.0020 1.64790 647 Example D 0.133 1.29 2.03 0.009 0.0028 0.011 0.0026 0 0.0019 1.30 Cr = 0.28 830 708 Example AND 0.128 1.33 1.92 0.011 0.0028 0.013 0.0025 0 0.0029 1.34 Cr = 0.91 829 708 Example F 0.121 1.22 2.16 0.013 0.0032 0.012 0.0022 0 0.0023 1.23 Ni = 0.33 829 703 Example G 0.131 1.41 2.02 0.011 0.0030 0.010 0.0029 0 0.0024 1.42 Ni = 0.88 839 719 Example H 0.153 1.33 2.16 0.015 0.0038 0.013 0.0033 0 0.0025 1.34 Mo = 0.06 825 696 Example I 0.162 1.41 1.96 0.009 0.0030 0.011 0.0029 0 0.0022 1.42 Mo = 0.21 833 715 Example J 0.161 1.32 2.11 0.013 0.0025 0.010 0.0027 0 0.0023 1.33 Nb = 0.12 824 698 Example K 0.160 1.44 2.10 0.013 0.0021 0.009 0.0031 0 0.0027 1.45 Ti = 0.009 830 703 Example L 0.140 1.53 2.11 0.010 0.0029 0.010 0.0030 0 0.0016 1.54 Ti = 0.020 839 712 Example M 0.144 1.45 2.03 0.010 0.0030 0.010 0.0033 0.003 0.0020 1.46 Ti = 0.022 837 715 Example N 0.171 1.21 2.33 0.011 0.0030 0.009 0.0029 0 0.0023 1.22 Ca = 0.0011 810 671 Example O 0.151 1.61 2.22 0.009 0.0023 0.011 0.0023 0 0.0020 1.62 Ni = 0.4 Cu = 0.8 836 701 Example P 0.161 1.39 2.19 0.008 0.0028 0.013 0.0019 0 0.0019 1.40 V = 0.11 825 693 Example Q 0.159 1.55 2.30 0.009 0.0030 0.008 0.0021 0 0.0021 1.56 REM = 0.0013 829 689 Example R 0.188 1.44 2.22 0.010 0.0031 0.010 0.0030 0 0.0036 1.45820 683 Example s 0.171 1.02 2.77 0.013 0.0025 0.012 0.0026 0 0.0023 1.03789 624 Example T 0.200 2.13 2.43 0.010 0.0032 0.011 0.0033 0 0.0021 2.14842 683 Example U 0.280 1.62 1.81 0.011 0.0027 0.010 0.0030 0 0.0031 1.63821 697 Example V 0.310 1.55 1.53 0.007 0.0019 0.051 0.0021 0 0.0019 1.60820 711 Example W 0.010 1.32 2.28 0.008 0.0020 0.020 0.0018 0 0.0022 1.34880 732 Comparative Example X 0.151 0.41 2.41 0.011 0.0029 0.060 0.0017 0 0.0014 0.47777 644 Comparative Example Y 0.188 3.20 2.30 0.009 0.0017 0.010 0.0029 0 0.0017 3.21896 734 Comparative Example Z 0.229 1.55 0.08 0.010 0.0022 0.009 0.0022 0 0.0017 1.56880 870 Comparative Example AA 0.177 1.39 3.21 0.012 0.0029 0.019 0.0019 0 0.0027 1.41790 594 Comparative Example AB 0.411 1.02 2.10 0.009 0.0029 0.031 0.0030 0 0.0013 1.05762 608 Comparative Example
Underlines indicate that the values do not meet the conditions of the present invention
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Table 2
Steel No. Composition No. Plate heating temperature (° C) Finishing lamination temperature (° C) Ar3 (° C) Winding temperature (° C) Roll size (mm) Reduction of cold rolling (%)TO 1 THE 1230 890 674 550 1100 45 Example A-2 THE 1220 930 674 710 1100 55 Comparative Example A-3 THE 1250 950 674 550 1100 55 Comparative Example A-4 THE 1230 890 674 560 1100 50 Comparative Example A-5 THE 1200 890 674 600 1100 60 Comparative Example A-6 THE 1200 910 674 550 1100 45 Comparative Example A-7 THE 1250 930 674 550 1100 55 Comparative Example A-8 THE 1210 930 674 550 1100 55 Comparative Example A-9 THE 1230 940 674 570 1100 45 Example A-10 THE 1260 950 674 580 1100 58 Comparative Example A-11 THE 1170 900 674 640 1700 50 Comparative Example A-12 THE 1200 980 674 600 1100 50 Comparative Example A-13 THE 1210 870 674 520 1100 50 Comparative Example B-1 B 1220 890 672 580 600 55 Example
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Steel No. Composition No. Plate heating temperature (° C) Finishing lamination temperature (° C) Ar3 (° C) Winding temperature (° C) Roll size (mm) Reduction of cold rolling (%)B-2 B 1200 930 672 630 600 45 Example B-3 B 1250 970 672 550 600 45 Comparative Example B-4 B 1210 1020 672 510 600 55 Comparative Example B-5 B 1230 960 672 490 600 58 Example B-6 B 1190 890 672 640 1700 40 Comparative Example B-7 B 1220 920 672 540 800 50 Comparative Example C-1 Ç 1260 960 647 550 800 45 Example C-2 Ç 1220 930 647 510 800 45 Example C-3 Ç 1200 930 647 580 800 45 Comparative Example C-4 Ç 1220 940 647 570 800 45 Comparative Example D-1 D 1250 950 708 580 800 58 Example D-2 D 1230 890 708 580 800 55 Example E-1 AND 1200 890 708 600 800 60 Example E-2 AND 1200 910 708 550 800 45 Example E-3 AND 1250 930 708 730 1700 45 Comparative Example E-4 AND 1210 870 708 550 1100 45 Example F-1 F 1230 1020 703 510 1100 55 Example F-2 F 1210 950 703 580 1100 58 Example G-1 G 1210 890 719 580 1100 55 Example
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Steel No. Composition No. Plate heating temperature (° C) Finishing lamination temperature (° C) Ar3 (° C) Winding temperature (° C) Roll size (mm) Reduction of cold rolling (%)G-2 G 1230 890 719 600 1100 60 Example G-3 G 1220 910 719 550 1100 45 Comparative Example G-4 G 1200 940 719 570 1100 45 Example H-1 H 1230 950 696 580 1100 58 Example H-2 H 1220 910 696 510 1100 55 Example I-1 I 1210 920 715 570 1100 55 Example I-2 I 1250 930 715 550 1100 45 Example I-3 I 1250 920 715 550 1100 50 Example J-1 J 1220 950 698 510 1100 55 Example K-1 K 1230 960 703 540 1100 55 Example K-2 K 1200 960 703 550 1100 45 Example K-3 K 1250 930 703 560 1100 55 Example K-4 K 1250 930 703 560 1100 55 Example
Underlines indicate that the values do not meet the conditions of the present invention
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Table 3
No. of steel tA (S) Annealing temperature (° C) H2 concentration (%) Dew point (° C) Primary cooling rate (° C / s) Intermediate cooling temperature (° C) Secondary cooling rate (° C / s) Cooling stop temperature (° C)TO 1 30 820 5 6 2 700 45 220 Example A-2 25 820 5 3 2 710 45 440 Comparative Example A-3 30 730 4 -2 10 620 20 230 Comparative Example A-4 35 780 8 10 2 480 50 250 Comparative Example A-5 20 880 2 8 20 700 40 190 Comparative Example A-6 25 790 8 7 20 650 80 220 Comparative Example A-7 10 780 8 -8 5 700 20 240 Comparative Example A-8 24 820 10 12 2 650 40 400 Comparative Example A-9 31 830 6 7 5 630 30 180 Example A-10 30 820 7 6 15 650 60 420 Comparative Example A-11 25 810 4 5 2 700 50 220 Comparative Example A-12 350 860 3 8 0.2 680 0.2 250 Comparative Example A-13 400 820 2 1 2 700 20 25 Comparative Example B-1 25 850 2 3 2 720 45 250 Example B-2 35 800 9 -2 5 700 70 225 Example
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No. of steel tA (S) Annealing temperature (° C) H2 concentration (%) Dew point (° C) Primary cooling rate (° C / s) Intermediate cooling temperature (° C) Secondary cooling rate (° C / s) Cooling stop temperature (° C)B-3 25 710 10 4 10 600 30 250 Comparative Example B-4 25 850 12 -1 3 750 40 275 Comparative Example B-5 35 850 14 -6 2 750 50 190 Example B-6 35 800 3 5 30 700 40 200 Comparative Example B-7 120 820 3 5 0.2 720 0.2 200 Comparative Example C-1 30 850 6 7 10 730 50 225 Example C-2 20 850 11 7 4 700 40 250 Example C-3 25 860 2 -8 10 480 30 190 Comparative Example C-4 10 860 7 12 5 720 45 520 Comparative Example D-1 26 830 8 7 20 730 45 225 Example D-2 25 840 6 6 10 660 45 225 Example E-1 32 820 4 -1 10 650 45 210 Example E-2 30 860 3 -6 20 650 60 230 Example E-3 27 850 5 7 5 750 45 210 Comparative Example E-4 24 800 5 6 5 700 50 230 Example F-1 31 830 3 5 2 680 45 220 Example F-2 30 850 4 8 10 700 40 220 Example G-1 25 830 6 1 5 730 30 230 Example G-2 30 790 1 3 20 650 50 225 Example
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No. of steel tA (s) Annealing temperature (° C) H2 concentration (%) Dew point (° C) Primary cooling rate (° C / s) Intermediate cooling temperature (° C) Secondary cooling rate (° C / s) Cooling stop temperature (° C)G-3 20 850 1 -2 5 730 50 200 Comparative Example G-4 25 830 12 4 5 710 20 220 Example H-1 30 830 3 1 5 730 50 230 Example H-2 27 810 3 3 5 710 60 200 Example I-1 27 830 5 -2 5 730 40 190 Example I-2 24 840 3 5 10 700 60 220 Example I-3 30 780 5 7 5 620 50 230 Example J-1 25 800 4 7 15 650 45 225 Example K-1 30 850 6 -8 20 650 45 220 Example K-2 30 880 7 12 10 700 50 230 Example K-3 27 830 8 12 10 710 45 200 Example K-4 27 830 8 12 24 550 24 200 Example
Underlines indicate that the values do not meet the conditions of the present invention.
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Table 4
No. ofsteel Dwelling temperature (° C) Length of stay (s) Temperature regulated before lamination (° C) Lamination bath temperature (° C) Lamination bath flow rate (m / min) Alloy formation temperature(° C) Also(s) Product plate typeTO 1 400 50 480 470 20 - 70 GI Example A-2 380 90 480 460 40 - 110 GI Comparative Example A-3 360 200 470 460 30 - 220 GI Comparative Example A-4 430 175 490 460 35 - 200 GI Comparative Example A-5 300 100 480 460 35 - 13 GI Comparative Example A-6 530 100 480 460 35 - 18 GI Comparative Example A-7 450 10 470 460 20 - 15 GI Comparative Example A-8 400 60 460 460 5 - 85 GI Comparative Example A-9 380 150 450 440 20 470 160 GA Example A-10 420 85 460 450 20 540 95 GA Comparative Example A-11 395 490 470 460 20 620 550 GA Comparative Example A-12 420 100 480 460 20 490 125 GA Comparative Example A-13 375 250 480 460 20 470 275 GA Comparative Example B-1 400 25 470 460 30 - 40 GI Example
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No. ofsteel Dwelling temperature (° C) Length of stay (s) Temperature regulated before lamination (° C) Lamination bath temperature (° C) Lamination bath flow rate (m / min) Alloy formation temperature(° C) Also(s) Product plate typeB-2 420 175 460 460 30 - 225 GI Example B-3 440 200 460 460 20 - 225 GI Comparative Example B-4 380 250 450 460 3 - 270 GI Comparative Example B-5 360 400 460 460 25 520 420 GA Example B-6 360 200 470 450 25 480 210 GA Comparative Example B-7 400 400 460 460 25 470 410 GA Comparative Example C-1 400 300 490 460 20 - 325 GI Example C-2 400 370 480 460 20 520 375 GA Example C-3 420 100 480 460 10 - 125 GI Comparative Example C-4 520 50 470 460 10 - 15 GI Comparative Example D-1 410 175 480 460 30 - 200 GI Example D-2 390 275 420 460 20 480 300 GA Example E-1 400 275 490 460 20 - 300 GI Example E-2 400 200 480 450 30 - 250 GI Example E-3 380 80 480 460 20 - 150 GI Comparative Example E-4 420 150 470 460 30 480 175 GA Example F-1 410 275 460 460 20 - 300 GI Example F-2 410 275 470 460 30 540 280 GA Example
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No. ofsteel Dwelling temperature (° C) Length of stay (s) Temperature regulated before lamination (° C) Lamination bath temperature (° C) Lamination bath flow rate (m / min) Alloy formation temperature(° C) Also(s) Product plate typeG-1 410 375 480 460 20 - 380 GI Example G-2 400 275 480 480 20 - 285 GI Example G-3 340 340 470 460 20 - 18 GI Comparative Example G-4 400 275 460 460 35 520 300 GA Example H-1 420 250 480 460 25 - 275 GI Example H-2 410 375 470 460 25 540 400 GA Example I-1 390 250 460 460 30 - 275 GI Example I-2 390 250 460 460 30 - 275 GI Example I-3 400 250 450 460 30 - 275 GI Example J-1 380 325 460 460 25 530 350 GA Example K-1 410 300 470 450 30 - 350 GI Example K-2 410 275 460 440 35 - 325 GI Example K-3 390 275 490 460 30 540 325 GA Example K-4 390 275 490 460 30 540 300 GA Example
Underlines indicate that the values do not meet the conditions of the present invention.
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Table 5
No. of steel No. of makeup Plate heating temperature (° C) Finishing lamination temperature (° C) Ar3(° C) Winding temperature (° C) Roll size (mm) Reduction of cold rolling (%)L-1 L 1230 940 712 550 1100 45 Example L-2 L 1210 950 712 510 1100 55 Example M-1 M 1230 910 715 490 1100 55 Example M-2 M 1230 940 715 550 1100 45 Example M-3 M 1220 950 715 510 1100 58 Comparative Example N-1 N 1210 930 671 570 1100 55 Example N-2 N 1200 920 671 600 1100 60 Example O-1 O 1250 960 701 520 1100 55 Example P-1 P 1230 960 693 520 1100 55 Example Q-1 Q 1230 930 689 510 1100 45 Example R-1 R 1220 950 683 550 1100 60 Example R-2 R 1250 960 683 580 1100 45 Example R-3 R 1210 1020 683 510 1100 45 Example R-4 R 1230 950 683 710 1100 45 Comparative Example R-5 R 1230 940 683 600 1100 45 Comparative Example R-6 R 1230 950 683 630 1100 58 Comparative Example R-7 R 1200 950 683 570 1100 55 Comparative Example R-8 R 1230 940 683 600 1100 27 Comparative Example S-1 s 1260 890 624 550 1100 55 Example
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No. of steel No. of makeup Plate heating temperature (° C) Finishing lamination temperature (° C) Ar3(° C) Winding temperature (° C) Roll size (mm) Reduction of cold rolling (%)S-2 s 1220 930 624 510 1100 58 Example T-1 T 1250 940 683 580 1100 45 Example T-2 T 1220 950 683 580 1100 45 Example T-3 T 1230 890 683 540 1100 45 Comparative Example T-4 T 1260 930 683 550 1100 45 Example T-5 T 1230 940 683 510 1100 58 Example U-1 U 1230 950 697 570 1100 55 Example U-2 U 1200 950 697 550 1100 45 Example U-3 U 1230 940 697 560 1100 55 Example U-4 U 1200 950 697 580 1100 45 Example V-1 V 1250 940 711 510 1100 45 Example V-2 V 1230 950 711 550 1100 55 Example V-3 V 1210 930 711 510 1100 45 Example V-4 V 1230 920 711 570 1100 45 Example V-5 V 1230 950 711 600 1100 55 Example V-6 V 1220 1020 711 540 1100 55 Example W-1 W 1230 940 732 550 800 55 Comparative Example X-1 X 1200 950 644 510 1100 45 Comparative Example Y-1 Y 1250 890 734 490 800 45 Comparative Example Z-1 Z 1230 1000 870 550 700 45 Comparative Example AA-1 AA 1210 910 594 550 800 55 Example
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No. of steel No. of makeup Plate heating temperature (° C) Finishing lamination temperature (° C) Ar3(° C) Winding temperature (° C) Roll size (mm) Reduction of cold rolling (%)Comparative AB-1 AB 1200 920 608 550 1100 55 Comparative Example
Underlines indicate that the values do not meet the conditions of the present invention.
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Table 6
No. ofsteel tA (S) Annealing temperature (° C) H2 concentration (%) Dew point (° C) Primary cooling rate (° C / s) Intermediate cooling temperature (° C) Secondary cooling rate (° C / s) Cooling stop temperature (° C)L-1 30 870 10 7 10 720 60 190 Example L-2 32 860 5 6 10 700 40 230 Example M-1 30 860 4 -1 15 720 40 220 Example M-2 27 850 4 -6 5 720 45 230 Example M-3 5 860 6 3 10 710 40 210 Comparative Example N-1 30 840 5 -2 15 710 45 225 Example N-2 27 840 7 5 15 710 40 220 Example O-1 27 830 4 7 2 710 40 230 Example P-1 30 850 7 12 4 700 45 220 Example Q-1 27 850 5 7 2 730 50 230 Example R-1 24 880 6 6 2 750 45 225 Example R-2 30 850 1 -1 10 690 80 210 Example R-3 20 850 2 -6 3 710 30 230 Example R-4 30 870 11 8 2 700 45 190 Comparative Example R-5 27 730 5 1 10 580 50 200 Comparative Example R-6 10 840 5 3 5 690 45 200 Comparative Example R-7 30 840 4 -2 10 700 35 280 Comparative Example R-8 30 870 6 4 2 700 45 190 Comparative Example S-1 30 820 4 -1 10 690 50 220 Example
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No. ofsteel tA (S) Annealing temperature (° C) H2 concentration (%) Dew point (° C) Primary cooling rate (° C / s) Intermediate cooling temperature (° C) Secondary cooling rate (° C / s) Cooling stop temperature (° C)S-2 30 850 4 -6 5 700 50 180 Example T-1 32 870 5 5 20 730 50 230 Example T-2 30 880 6 7 10 710 40 240 Example T-3 15 730 4 6 10 600 45 250 Comparative Example T-4 27 870 8 -1 2 750 45 225 Example T-5 25 890 9 -6 4 710 45 220 Example U-1 30 800 9 -1 5 700 50 210 Example U-2 30 780 4 -6 2 710 50 230 Example U-3 27 850 5 8 10 680 55 210 Example U-4 30 760 7 1 2 700 50 200 Example V-1 240 850 6 3 2 750 55 230 Example V-2 30 820 12 12 5 650 50 230 Example V-3 20 800 5 12 5 620 45 260 Example V-4 30 760 10 7 2 690 50 240 Example V-5 30 820 4 6 2 700 50 220 Example V-6 24 840 6 -1 2 700 45 220 Example W-1 30 850 7 -6 5 700 50 220 Comparative Example X-1 30 850 2 1 5 700 45 240 Comparative Example Y-1 30 840 2 3 2 730 45 250 Comparative Example Z-1 27 850 5 12 5 700 45 230 Comparative Example AA-1 25 840 3 12 2 720 50 160 Example
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No. ofsteel tA (s) Annealing temperature (° C) H2 concentration (%) Dew point (° C) Primary cooling rate (° C / s) Intermediate cooling temperature (° C) Secondary cooling rate (° C / s) Cooling stop temperature (° C) Comparative AB-1 30 840 2 5 3 720 40 300 Comparative Example
Underlines indicate that the values do not meet the conditions of the present invention.
79/110
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Table 7
No. ofsteel Dwelling temperature (° C) Lenght of stay(s) Temperature regulated before lamination (° C) Lamination bath temperature (° C) Lamination bath flow rate (m / min) Alloy forming temperature (° C) Also(s) Product plate typeL-1 400 100 480 460 10 - 150 GI Example L-2 420 375 470 460 25 - 400 GI Example M-1 410 250 460 460 25 - 275 GI Example M-2 400 275 480 460 30 - 300 GI Example M-3 420 15 470 460 20 - 18 GI Comparative Example N-1 410 175 460 460 20 - 200 GI Example N-2 400 175 480 460 20 530 200 GA Example O-1 400 200 470 460 30 470 225 GA Example P-1 370 225 480 460 20 - 250 GI Example Q-1 360 200 420 460 10 - 225 GI Example R-1 410 275 490 460 20 - 300 GI Example R-2 400 375 480 460 25 - 400 GI Example R-3 420 300 480 460 35 550 325 GA Example R-4 410 400 470 460 20 - 450 GI Comparative Example R-5 380 250 460 460 30 - 275 GI Comparative Example R-6 400 10 460 460 5 - 14 GI Comparative Example R-7 - - 490 460 20 530 16 GA Comparative Example R-8 410 400 470 450 20 - 425 GI Comparative Example S-1 420 120 480 460 25 - 155 GI Example
80/110
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No. ofsteel Dwelling temperature (° C) Lenght of stay(s) Temperature regulated before lamination (° C) Lamination bath temperature (° C) Lamination bath flow rate (m / min) Alloy forming temperature (° C) Also(s) Product plate typeS-2 400 120 470 460 25 - 150 GI Example T-1 390 250 460 460 20 - 300 GI Example T-2 410 275 480 460 15 - 300 GI Example T-3 400 175 470 460 35 - 200 GI Comparative Example T-4 380 175 480 460 20 540 200 GA Example T-5 400 275 470 460 40 470 300 GA Example U-1 410 250 460 450 20 - 275 GI Example U-2 390 125 480 470 25 - 150 GI Example U-3 400 250 470 460 30 - 275 GI Example U-4 400 125 480 460 35 - 150 GI Example V-1 380 250 470 460 25 - 275 GI Example V-2 400 350 460 460 25 - 400 GI Example V-3 390 250 480 460 35 - 275 GI Example V-4 400 300 470 460 20 - 325 GI Example V-5 400 250 480 450 30 - 275 GI Example V-6 390 250 420 460 30 500 300 GA Example W-1 420 300 490 460 25 - 325 GI Comparative Example X-1 400 250 480 460 25 - 275 GI Comparative Example Y-1 370 100 480 460 30 - 150 GI Comparative Example Z-1 440 300 470 460 35 - 325 GI Comparative Example
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No. ofsteel Dwelling temperature (° C) Lenght of stay(s) Temperature regulated before lamination (° C) Lamination bath temperature (° C) Lamination bath flow rate (m / min) Alloy forming temperature (° C) Also(s) Product plate typeAA-1 400 275 460 460 30 - 300 GI Comparative Example AB-1 400 100 470 460 30 - 125 GI Comparative Example
Underlines indicate that the values do not meet the conditions of the present invention.
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83/110 [00189] In the tensile rupture test, JIS No. 5 test pieces were cut from the steel sheets having a thickness of 1.2 mm in a direction from the right angle of the direction of roll and parallel to the direction of roll to assess elastic properties. Each of the five test pieces was subjected to a tensile strength test according to JIS Z 2241 (2011) and an average value of the respective values (tensile strength, tensile strength, and total elongation of each of the five pieces test) was obtained to calculate the tensile strength (YS), the tensile strength (TS), the total elongation (El), and the yield ratio (YR) of the average value. Here, the yield ratio (YR) can be obtained by dividing the breaking strength (YS) by the tensile strength (TS).
[00190] In addition, an orifice expansion ratio (λ) was defined by an orifice expansion test in accordance with Japan Iron and Steel Federation Standard T 1001.
[00191] Here, when an equilibrium index (TS x El) of the tensile strength (TS) and of the total elongation (El) is greater than 16000 (MPa x%), the elongation was assessed to be excellent. When an equilibrium index (TS x λ) of tensile strength (TS) and the orifice expansion ratio (λ) is greater than 40,000 (MPa x%), the orifice expandability was assessed to be excellent.
[00192] A solution obtained by dissolving the galvanizing layers of the galvanized steel sheets using a 5% aqueous HCl solution to which an inhibitor was added and removing a residue such as an undissolved oxide was subjected to ICP emission analysis for measure the amount of Fe in the galvanizing layers. In the measurement, using three samples, an average value of the amount of Fe in the three samples was adjusted in% Fe of the galvanizing layers.
[00193] In addition, the microstructures of the cross sections of the
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84/110 galvanized steel sheets were observed. Using an optical microscope, a scanning electron microscope, and a transmission electron microscope as needed, each phase of the microstructures was defined and the fraction of the area of each phase and the fraction of the coarse-grained area (a fraction where the grains having a grain size of more than 35 pm per unit area occupied) were measured. Also, using a focused ion beam processing (FIB) apparatus, the surfaces of the steel sheets were processed in the direction of the flake thickness to include the galvanizing layers of the surfaces of the galvanized steel sheets, and then, the oxides in the Galvanization layers of the obtained flakes were observed by a field emission transmission electron microscope (FE-TEM) to perform composition analysis (oxide identification) by a distributed energy X-ray detector (EDX). With FE-TEM, five visual files were observed at a magnification of 10000 to 50000 times and the chemical composition (types of compound) and the fraction of the projection area of the oxides were evaluated from the data obtained by FE-TEM and EDX .
[00194] Then, to assess the resistance to delayed fracture, test pieces were prepared by a U-bend test and were subjected to a test of resistance to delayed fracture through electrolytic charge. The resistance to delayed fracture of galvanized steel sheets obtained using the method described above was evaluated according to a method disclosed in Materia (Bulletin of the Japan Institute of Metals) Vol. 44, No. 3 (2005), p. 254 to 256.
[00195] Specifically, after the steel sheets were subjected to mechanical cutting, the cross sections were subjected to mechanical grinding, and then, the test pieces were subjected to the U-bending test to have a curve radius of 10R. A color measurement was attached to the center of the surface of each piece of
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85/110 test obtained, and both ends of the test pieces were screwed through screws to apply tension to the test pieces. The applied stress was calculated by the monitored color measurement. The applied tension was 0.7 times the tensile strength TS (0.7 x TS). For example, the applied stress is 700 MPa with respect to a steel sheet of class 980 MPa, the applied stress is 840 MPa with respect to a steel sheet of class 1180 MPa, and the applied stress is 925 MPa with respect to a steel sheet of class 1320 MPa.
[00196] The reason for increasing the tensile strength TS as described above is that the residual stress introduced to a steel plate during forming increases as the TS tensile strength of the steel plate increases. Each of the pieces obtained from the U-bend test was immersed in an aqueous solution of ammonium thiocyanate and a current flowed into an electrolytically charged apparatus at a current density of 1.0 mA / cm ² so that the steel (U-bend test piece) was used as a negative electrode and a platinum electrode was used as a positive electrode to conduct an electrolytic charge test for 2 hours.
[00197] The hydrogen generated in the electrolytic charge test penetrates the steel plate and can cause delayed fracture. After the electrolytic load test, the test pieces were removed from the solution and the central area (curved area) of each U-bend test piece was observed to visually inspect for cracks. Since there is a large residual stress in the curved area, if cracks are generated in the curved area, rapid progress is made. Therefore, when cracks are generated, there are large opening cracks in all test pieces and the presence of cracks could be easily determined even visually.
[00198] Using a magnifying glass and a stereomicroscope, the test pieces were carefully observed at the ends and the pre
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86/110 cracking was confirmed again. When there was no longer an opening crack, it was also confirmed that there was no fine crack.
[00199] Here, in the result of the delayed fracture test (resistance to delayed fracture) shown in Tables 10 (the continuation of Table 9) and 13 (the continuation of Table 12), Bom represents that no cracks were generated and Nada Bom represents that cracks have been generated.
[00200] Also, the galvanizing properties (wettability) were evaluated using the stereomicroscope (at a magnification of 100 times). That is, the surface of each galvanized steel sheet (however, a region of 3/8 the sheet width for both edges of the central position of the sheet width) was observed in three or more fields of view and the presence of no galvanizing (a defect that reaches the base material (steel sheet)) has been confirmed. As a result, when the coverage of the galvanizing layer is less than 99% (when a defect ratio is more than 1%), a large number of non-galvanized areas were present and thus, the wettability was assessed as not good. In addition, when the coverage of the galvanizing layer was 100%, the entire surface was galvanized and, therefore, wetting was assessed as Good.
[00201] The measured microstructures were shown in Tables 8 and 11, the elastic properties were shown in Tables 9 (the continuation of Table 8) and 12 (the continuation of Table 11), and the resistance to delayed fracture, galvanizing properties, and% Fe in the galvanizing layers are shown in Tables 10 and 13.
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Table 8
No. of steel Fraction of the volume of the structure (%) Fraction of coarse grain area (%)Tempered Martensite (%) Ferrite (%) Bainite (%) Martensite (%) Pearlite (%) Residual austenite (%) TO 1 45 33 11 0 0 11 5 Example A-2 9 39 35 7 0 10 11 Comparative Example A-3 0 82 11 5 0 2 5 Comparative Example A-4 0 63 10 17 4 6 5 Comparative Example A-5 57 13 8 19 0 3 7 Comparative Example A-6 0 35 8 49 5 3 5 Comparative Example A-7 42 35 7 9 0 7 5 Comparative Example A-8 11 31 48 0 0 10 5 Comparative Example A-9 41 34 15 0 0 10 6 Example A-10 0 34 39 13 0 14 6 Comparative Example A-11 38 27 21 2 12 0 8 Comparative Example A-12 0 52 20 23 0 5 7 Comparative Example
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No. of steel Fraction of the volume of the structure (%) Fraction of coarse grain area (%)Tempered Martensite (%) Ferrite (%) Bainite (%) Martensite (%) Pearlite (%) Residual austenite (%) A-13 68 29 0 0 0 3 4 Comparative Example B-1 48 30 13 0 0 9 6 Example B-2 34 30 24 0 0 12 8 Example B-3 0 100 0 0 0 0 5 Comparative Example B-4 37 35 17 0 0 11 4 Comparative Example B-5 42 30 16 0 0 12 3 Example B-6 38 34 18 0 0 10 8 Comparative Example B-7 0 69 16 12 0 3 5 Comparative Example C-1 37 39 11 0 0 13 5 Example C-2 35 36 19 0 0 10 4 Example C-3 0 72 9 13 0 6 6 Comparative Example C-4 0 32 9 43 8 8 6 Comparative Example D-1 35 37 15 0 0 13 6 Example D-2 36 34 21 0 0 9 6 Example E-1 37 39 11 0 0 13 7 Example
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No. of steel Fraction of the volume of the structure (%) Fraction of coarse grain area (%)Tempered Martensite (%) Ferrite (%) Bainite (%) Martensite (%) Pearlite (%) Residual austenite (%) E-2 33 26 31 0 0 10 5 Example E-3 34 30 27 0 0 9 11 Comparative Example E-4 37 33 20 0 0 10 5 Example F-1 35 30 17 7 0 11 4 Example F-2 39 34 12 6 0 9 6 Example G-1 35 35 21 0 0 9 6 Example G-2 34 36 22 0 0 8 7 Example G-3 35 39 2 17 0 7 5 Comparative Example G-4 39 32 19 0 0 10 6 Example H-1 35 29 24 0 0 12 6 Example H-2 39 30 20 0 0 11 4 Example I-1 40 34 16 0 0 10 6 Example I-2 32 38 20 0 0 10 5 Example I-3 36 34 21 0 0 9 5 Example J-1 37 37 16 0 0 10 4 Example
Underlines indicate that the values do not meet the conditions of the present invention.
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Table 9
No. of steel Traction propertiesYS (MPa) TS (MPa) El. (%) λ(%) TS X El. (MPa X%) TSX λ (MPa X%) YR(-) TO 1 741 1021 23 64 23483 65344 0.73 Example A-2 709 1033 22 32 22726 33056 0.69 Comparative Example A-3 395 683 20 44 13660 30052 0.58 Comparative Example A-4 425 769 22 38 16918 29222 0.55 Comparative Example A-5 801 1099 12 53 13188 58247 0.73 Comparative Example A-6 522 1311 8 9 10488 11799 0.40 Comparative Example A-7 403 1001 11 52 11011 52052 0.40 Comparative Example A-8 690 994 21 25 20874 24850 0.69 Comparative Example A-9 671 981 23 52 22563 51012 0.68 Example A-10 540 1012 23 19 23276 19228 0.53 Comparative Example A-11 701 910 15 41 13650 37310 0.77 Comparative Example A-12 487 981 16 14 15696 13734 0.50 Comparative Example A-13 1021 1137 7 56 7959 63672 0.90 Comparative Example B-1 803 997 19 54 18943 53838 0.81 Example B-2 657 982 23 49 22586 48118 0.67 Example B-3 545 682 22 55 15004 37510 0.80 Comparative Example B-4 678 999 22 45 21978 44955 0.68 Comparative Example B-5 706 1011 22 54 22242 54594 0.70 Example B-6 680 1004 18 40 18072 40160 0.68 Comparative Example B-7 549 1107 13 7 14391 7749 0.50 Comparative Example
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No. of steel Traction propertiesYS (MPa) TS (MPa) El.(%) λ(%) TS X El. (MPa X%) TSX λ (MPa X%) YR(-) C-1 724 1022 23 51 23506 52122 0.71 Example C-2 709 992 21 46 20832 45632 0.71 Example C-3 423 890 16 19 14240 16910 0.48 Comparative Example C-4 639 1371 11 10 15081 13710 0.47 Comparative Example D-1 701 1021 23 53 23483 54113 0.69 Example D-2 669 1000 20 49 20000 49000 0.67 Example E-1 703 1009 24 48 24216 48432 0.70 Example E-2 831 1022 19 67 19418 68474 0.81 Example E-3 723 1017 15 36 15255 36612 0.71 Comparative Example E-4 693 1090 20 46 21800 50140 0.64 Example F-1 721 1033 23 49 23759 50617 0.70 Example F-2 698 999 21 44 20979 43956 0.70 Example G-1 728 1019 19 51 19361 51969 0.71 Example G-2 677 1088 20 44 21760 47872 0.62 Example G-3 780 1188 12 33 14256 39204 0.66 Comparative Example G-4 723 1024 20 46 20480 47104 0.71 Example H-1 681 1060 21 48 22260 50880 0.64 Example H-2 655 1025 20 47 20500 48175 0.64 Example I-1 677 1029 22 55 22638 56595 0.66 Example I-2 649 1051 19 51 19969 53601 0.62 Example I-3 631 1073 19 46 20387 49358 0.59 Example
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No. of steel Traction propertiesYS (MPa) TS (MPa) El.(%) λ(%) TS X El. (MPa X%) TSX λ (MPa X%) YR(-) J-1 644 1099 18 46 19782 50554 0.59 Example
Underlines indicate that the values do not meet the conditions of the present invention.
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Table 10
No. ofsteel Galvanization properties Delayed fracture resistance Product plate typeHumectability Fe concentration in galvanizing (% mass) Oxide included in the galvanizing layer Fraction of the projection area Delayed fracture resistance TO 1 Good 3 SiO2, Mn2SiO4 45 Good GI Example A-2 Good 3 SiO2, Mn2SiO4 38 Good GI Comparative Example A-3 Good 4 SiO2, Mn2SiO4 40 Good GI Comparative Example A-4 Good 4 SiO2, Mn2SiO4 44 Good GI Comparative Example A-5 Good 2 SiO2, Mn2SiO4 50 Good GI Comparative Example A-6 Good 3 SiO2, Mn2SiO4 38 Good GI Comparative Example A-7 Good 4 SiO2 2 Not good GI Comparative Example A-8 Not good 1 SiO2, Mn2SiO4 40 Good GI Comparative Example A-9 Good 11 SiO2, Mn2SiO4 45 Good GA Example A-10 Good 11 SiO2, Mn2SiO4 42 Good GA Comparative Example A-11 Good 18 SiO2, Mn2SiO4 4 Not good GA Comparative Example
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No. ofsteel Galvanization properties Delayed fracture resistance Product plate typeHumectability Fe concentration in galvanizing (% mass) Oxide included in the galvanizing layer Fraction of the projection area Delayed fracture resistance A-12 Good 10 SiO2, Mn2SiO4 77 Good GA Comparative Example A-13 Good 11 SiO2, Mn2SiO4 68 Good GA Comparative Example B-1 Good 3 SiO2, Mn2SiO4, AI2O3 24 Good GI Example B-2 Good 3 SiO2, Mn2SiO4, AI2O3 45 Good GI Example B-3 Good 3 SiO2, Mn2SiO4, AI2O3 49 Good GI Comparative Example B-4 Not good 2 SiO2, Mn2SiO4, AI2O3 38 Good GI Comparative Example B-5 Good 10 SiO2, Mn2SiO4, AI2O3 53 Good GA Example B-6 Good 12 SiO2, Mn2SiO4, AI2O3 3 Not good GA Comparative Example B-7 Good 13 SiO2, Mn2SiO4, Al2O3 60 Good GA Comparative Example C-1 Good 4 Mn2SiO4, Al2O3 42 Good GI Example C-2 Good 11 Mn2SiO4, Al2O3 40 Good GA Example C-3 Good 3 Mn2SiO4, Al2O3 42 Good GI Comparative Example C-4 Good 3 Mn2SiO4, Al2O3 6 Not good GI Comparative Example D-1 Good 5 Mn2SiO4 48 Good GI Example
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No. ofsteel Galvanization properties Delayed fracture resistance Product plate typeHumectability Fe concentration in galvanizing (% mass) Oxide included in the galvanizing layer Fraction of the projection area Delayed fracture resistance D-2 Good 12 Mn2SiO4 52 Good GI Example E-1 Good 4 SiO2, Mn2SiO4 45 Good GI Example e-2 Good 2 SiO2, Mn2SiO4 38 Good GI Example e-3 Good 3 SiO2, Mn2SiO4 3 Not good GI Comparative Example e-4 Good 3 SiO2, Mn2SiO4 27 Good GA Example F-1 Good 2 SiO2, Mn2SiO4 44 Good GI Example F-2 Good 12 SiO2, Mn2SiO4 37 Good GA Example G-1 Good 4 SiO2, Mn2SiO4 56 Good GI Example G-2 Good 2 SiO2, Mn2SiO4 39 Good GI Example G-3 Good 2 SiO2, Mn2SiO4 40 Good GI Comparative Example G-4 Good 11 SiO2, Mn2SiO4 42 Good GA Example H-1 Good 3 SiO2, Mn2SiO4 39 Good GI Example H-2 Good 13 SiO2, Mn2SiO4 40 Good GA Example I-1 Good 4 SiO2, Mn2SiO4 42 Good GI Example I-2 Good 1 SiO2, Mn2SiO4 38 Good GI Example I-3 Good 3 SiO2, Mn2SiO4 30 Good GI Example J-1 Good 2 SiO2, Mn2SiO4 65 Good GA Example
Underlines indicate that the values do not meet the conditions of the present invention.
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Table 11
No. ofsteel Fraction of the volume of the structure (%) Fraction of coarse grain area(%)Tempered Martensite (%) Ferrite(%) Bainite(%) Martensita(%) Pearlite(%) Residual austenite(%) K-1 48 30 11 0 0 11 5 Example K-2 30 36 22 0 0 12 5 Example K-3 46 34 11 0 0 9 5 Example K-4 46 19 21 4 0 10 5 Example L-1 42 33 13 0 0 12 5 Example L-2 33 37 20 0 0 10 4 Example M-1 32 38 20 0 0 10 3 Example M-2 32 42 15 0 0 11 5 Example M-3 31 40 3 20 0 6 4 Comparative Example N-1 42 30 16 0 0 12 6 Example N-2 43 33 13 0 0 11 7 Example O-1 45 37 7 0 0 11 4 Example P-1 39 32 18 0 0 11 4 Example Q-1 42 30 16 0 0 12 4 Example R-1 40 36 12 0 0 12 5 Example R-2 38 32 19 0 0 11 6 Example R-3 40 30 21 0 0 9 4 Example R-4 39 26 23 0 0 12 12 Comparative Example
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No. ofsteel Fraction of the volume of the structure (%) Fraction of coarse grain area(%)Tempered Martensite (%) Ferrite(%) Bainite(%) Martensita(%) Pearlite(%) Residual austenite(%) R-5 0 100 0 0 0 0 7 Comparative Example R-6 34 30 8 22 0 6 8 Comparative Example R-7 31 32 4 29 0 4 6 Comparative Example R-8 37 27 25 0 0 11 7 Comparative Example S-1 43 30 19 0 0 8 5 Example S-2 47 32 11 0 0 10 4 Example T-1 44 33 11 0 0 12 6 Example T-2 52 21 15 0 0 12 6 Example T-3 0 100 0 0 0 0 5 Comparative Example T-4 45 35 9 0 0 11 5 Example T-5 44 33 9 0 0 14 4 Example U-1 45 34 9 0 0 12 6 Example U-2 42 32 15 0 0 11 5 Example U-3 59 28 2 0 0 11 5 Example U-4 42 36 11 0 0 11 6 Example V-1 62 22 4 0 0 12 4 Example V-2 56 28 5 0 0 11 5 Example
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No. ofsteel Fraction of the volume of the structure (%) Fraction of coarse grain area(%)Tempered Martensite (%) Ferrite(%) Bainite(%) Martensita(%) Pearlite(%) Residual austenite(%) V-3 49 32 10 0 0 9 4 Example V-4 32 35 25 0 0 8 6 Example V-5 36 37 15 0 0 12 7 Example V-6 59 24 7 0 0 10 5 Example W-1 0 89 11 0 0 0 5 Comparative Example X-1 20 46 25 0 7 2 4 Comparative Example Y-1 21 59 8 0 0 12 3 Comparative Example Z-1 0 78 0 0 22 0 5 Comparative Example AA-1 67 12 16 2 0 3 5 Comparative Example AB-1 21 15 7 38 11 8 5 Comparative Example
Underlines indicate that the values do not meet the conditions of the present invention.
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Table 12
No. of steel Traction properties YS (MPa) TS (MPa) El.(%) λ(%) TS X El. (MPa X%) TS X λ (MPa X%) YR(-)K-1 698 1071 19 46 20349 49266 0.65 Example K-2 881 1181 18 53 21258 62593 0.75 Example K-3 711 1051 23 48 24173 50448 0.68 Example K-4 700 1121 23 39 25783 43719 0.62 Example L-1 758 1083 22 60 23826 64980 0.70 Example L-2 799 1011 28 53 28308 53583 0.79 Example M-1 811 1101 27 49 29727 53949 0.74 Example M-2 786 1131 21 46 23751 52026 0.69 Example M-3 728 1211 11 19 13321 23009 0.60 Comparative Example N-1 703 1051 24 63 25224 66213 0.67 Example N-2 703 1059 21 59 22239 62481 0.66 Example O-1 681 1022 21 49 21462 50078 0.67 Example P-1 983 1199 17 45 20383 53955 0.82 Example Q-1 827 1051 24 61 25224 64111 0.79 Example R-1 781 1055 24 56 25320 59080 0.74 Example R-2 911 1193 22 55 26246 65615 0.76 Example R-3 799 1181 18 55 21258 64955 0.68 Example R-4 909 1091 19 16 20729 17456 0.83 Comparative Example R-5 499 688 16 26 11008 17888 0.73 Comparative Example R-6 527 1051 15 22 15765 23122 0.50 Comparative Example
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No. of steel Traction properties YS (MPa) TS (MPa) El. (%) λ(%) TS X El. (MPa X%) TS X λ (MPa X%) YR(-)R-7 581 1321 10 18 13210 23778 0.44 Comparative Example R-8 931 1151 13 19 14963 21869 0.81 Comparative Example S-1 694 1083 21 46 22743 49818 0.64 Example S-2 707 1021 23 48 23483 49008 0.69 Example T-1 879 1291 18 56 23238 72296 0.68 Example T-2 910 1391 15 44 20865 61204 0.65 Example T-3 608 783 17 32 13311 25056 0.78 Comparative Example T-4 867 1211 16 42 19376 50862 0.72 Example T-5 887 1234 18 44 22212 54296 0.72 Example U-1 855 1183 21 47 24843 55601 0.72 Example U-2 827 1277 17 46 21709 58742 0.65 Example U-3 749 1211 19 43 23009 52073 0.62 Example U-4 866 1251 21 46 26271 57546 0.69 Example V-1 1053 1362 19 47 25878 64014 0.77 Example V-2 946 1344 17 39 22848 52416 0.70 Example V-3 887 1281 17 47 21777 60207 0.69 Example V-4 964 1211 20 48 24220 58128 0.80 Example V-5 948 1381 20 46 27620 63526 0.69 Example V-6 1038 1349 17 47 22933 63403 0.77 Example W-1 308 422 22 39 9284 16458 0.73 Comparative Example X-1 411 899 17 26 15283 23374 0.46 Comparative Example
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No. of steel Traction properties YS (MPa) TS (MPa) El.(%) λ(%) TS X El. (MPa X%) TS X λ (MPa X%) YR(-)Y-1 785 1239 16 9 19824 11151 0.63 Comparative Example Z-1 516 639 21 23 13419 14697 0.81 Comparative Example AA-1 1024 1401 8 13 11208 18213 0.73 Comparative Example AB-1 722 1488 7 9 10416 13392 0.49 Comparative example
Underlines indicate that the values do not meet the conditions of the present invention.
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Table 13
No. of steel Galvanization properties Delayed fracture resistance Product plate typeHumectability Fe concentration in galvanizing (% mass) Oxide included in the galvanizing layer Projection area fraction (%) Delayed fracture resistance K-1 Good 3 SiO2, Mn2SiO4 67 Good GI Example K-2 Good 2 SiO2, Mn2SiO4 60 Good GI Example K-3 Good 10 SiO2, Mn2SiO4 54 Good GA Example K-4 Good 10 SiO2, Mn2SiO4 48 Good GA Example L-1 Good 2 SiO2, Mn2SiO4 38 Good GI Example L-2 Good 2 SiO2, Mn2SiO4 42 Good GI Example M-1 Good 2 SiO2, Mn2SiO4 39 Good GI Example M-2 Good 5 SiO2, Mn2SiO4 37 Good GI Example M-3 Good 2 SiO2, Mn2SiO4 3 Not good GI Comparative Example N-1 Good 3 SiO2, Mn2SiO4 56 Good GI Example N-2 Good 12 SiO2, Mn2SiO4 52 Good GA Example O-1 Good 11 SiO2, Mn2SiO4 50 Good GA Example P-1 Good 2 SiO2, Mn2SiO4 46 Good GI Example Q-1 Good 3 SiO2, Mn2SiO4 38 Good GI Example R-1 Good 3 SiO2, Mn2SiO4 42 Good GI Example R-2 Good 2 SiO2, Mn2SiO4 50 Good GI Example R-3 Good 12 SiO2, Mn2SiO4 45 Good GA Example
102/110
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No. of steel Galvanization properties Delayed fracture resistance Product plate typeHumectability Fe concentration in galvanizing (% mass) Oxide included in the galvanizing layer Projection area fraction (%) Delayed fracture resistance R-4 Good 2 SiO2, Mn2SiÜ4 41 Good GI Comparative Example R-5 Good 3 SiO2, Mn2SiO4 45 Good GI Comparative Example R-6 Not good 3 SiO2, Mn2SiO4 2 Not good GI Comparative Example R-7 Good 13 SiO2, Mn2SiO4 41 Good GA Comparative Example R-8 Good 2 SiO2, Mn2SiO4 8 Not good GI Comparative Example S-1 Good 4 Mn2SiO4 44 Good GI Example S-2 Good 3 Mn2SiO4 42 Good GI Example T-1 Good 2 SiO2, Mn2SiO4 45 Good GI Example T-2 Good 2 SiO2, Mn2SiO4 33 Good GI Example T-3 Good 5 SiO2, Mn2SiO4 4 Not good GI Comparative Example T-4 Good 12 SiO2, Mn2SiO4 45 Good GA Example T-5 Good 10 SiO2, Mn2SiO4 45 Good GA Example U-1 Good 2 SiO2, Mn2SiO4 35 Good GI Example U-2 Good 2 SiO2, Mn2SiO4 39 Good GI Example U-3 Good 3 SiO2, Mn2SiO4 42 Good GI Example
103/110
Petition 870180125071, of 9/3/2018, p. 113/136
No. of steel Galvanization properties Delayed fracture resistance Product plate typeHumectability Fe concentration in galvanizing (% mass) Oxide included in the galvanizing layer Projection area fraction (%) Delayed fracture resistance U-4 Good 3 SiO2, Mn2SiO4 41 Good GI Example V-1 Good 4 SiO2, Mn2SiO4 28 Good GI Example V-2 Good 3 SiO2, Mn2SiO4 31 Good GI Example V-3 Good 3 SiO2, Mn2SiO4 32 Good GI Example V-4 Good 4 SiO2, Mn2SiO4 29 Good GI Example V-5 Good 2 SiO2, Mn2SiO4 19 Good GI Example V-6 Good 11 SiO2, Mn2SiO4 38 Good GA Example W-1 Good 2 SiO2, Mn2SiO4 37 Good GI Comparative Example X-1 Good 3 SiO2, Mn2SiO4 44 Good GI Comparative Example Y-1 Not good 3 SiO2, Mn2SiO4 79 Not good GI Comparative Example Z-1 Good 2 SiO2 59 Good GI Comparative Example AA-1 Good 2 - 45 Not good GI Comparative Example AB-1 Good 2 SiO2, Mn2SiO4 48 Good GI Comparative Example
Underlines indicate that the values do not meet the conditions of the present invention.
104/110
Petition 870180125071, of 9/3/2018, p. 114/136
105/110 [00202] On all steel Nos. A-1, A-9, B-1, B-2, B-5, C-1, C-2, D-
1, D-2, E-1, E-2, E-4, F-1, F-2, G-1, G-2, G-4, H-1, H-2, I-1, I-2, I-3, J-1, K-1, K-2, K-3, K-4, L-1, L-2, M-1, M-2, N-1, N- 2, O-1, P-1, Q-1, R-1, R-
2, R-3, S-1, S-2, T-1, T-2, T-4, T-5, U-1, U-2, U-3, U-4, V-1, V-2, V-3, V-4, V-5, and V-6, the chemical composition, the microstructure, and the amount of Fe in the galvanizing layer and the oxide were correctly controlled and thus, the resistance to delayed fracture, formability, and plating properties were excellent.
[00203] On steel Nos. A-11, B-6, and E-3, the roll size in cold rolling was more than 1400 mm. In addition, on steel No. R-8, the cumulative rolling reduction in cold rolling was less than 30%. Still, in Nos. A-7, C-4, M-3, R-6, and T-3, during heating for annealing, the time when the steel plate temperature was within the temperature range of 550 ° C to 750 ° C was limit shorter than 20 seconds. Therefore, in Nos. A-7, A-11, B-6, C-4, E-3, M-3, R-6, R-8, and T-3, the fraction of the projection area of the oxides was less than 10% , and the delayed fracture resistance was not enough.
[00204] On steel Nos. A-8, B-4, and R-6, the flow rate of the molten metal in the galvanizing bath was slower than 10 m / min. Therefore, in these steel Nos. A-8, B-4, and R-6, no galvanizing caused by the oxides on the steel sheet surface occurred, and the appearance and durability were deteriorated by this non-galvanizing area (area that was not coated by the galvanizing layer) .
[00205] On steel Nos. A-2, E-3, and R-4, since the rolling temperature was higher than 700 ° C, the microstructure of the hot-rolled steel sheet was a ferrite structure. Thick pearlite and each phase of the microstructure of the final steel sheet after subsequent processes (for example, cold rolling, annealing, and galvanizing and alloying treatment) has been thickened (the fraction of the
Petition 870180125071, of 9/3/2018, p. 115/136
106/110 coarse grain area was more than 30%) to cause inequality in the microstructure. Therefore, in these steel Nos. A-2, E-3, and R-4, at least one of the elongation (TS x El) and orifice expansion (TS x λ) was not enough.
[00206] On steel Nos. A-3, B-3, R-5, and T-3, since the annealing temperature was below 750 ° C, the oxides remained in the ferrite as they were, and the volume fractions of smooth martensite and austenite , and the fraction of total volume of smooth martensite and bainite was not sufficient. Therefore, in Nos. A-3, B-3, R-5, and T-3, the tensile strength (TS) was less than 980 MPa, and the elongation (TS x El) and orifice expandability (TS x λ) were not enough.
[00207] On steel Nos. A-4 and C-3, since the cooling stop temperature of the first cooling stage was below 500 ° C, the ferrite was formed excessively, and the volume fractions of soft martensite and austenite, and the fraction of total volume of mild martensite and bainite were not sufficient. Therefore, in Nos. A-4 and C-3, the tensile strength (TS) was less than 980 MPa, and at least one of the elongation (TS x El) and orifice expansion (TS x λ) was not sufficient.
[00208] On steel Nos. A-12 and B-7, the average cooling rate of the second cooling step was slower than 1 ° C / s. Thus, due to the excessive formation of ferrite and insufficient extinction of the steel plate, the volume fractions of soft martensite and austenite, and the total volume fraction of soft martensite and bainite were not sufficient. Therefore, in Nos. A-12 and B-7, elongation (TS x El) and orifice expansion (TS x λ) were not enough.
[00209] On steel Nos. A-2, A-8, A-10, and C-4, since the cooling stop temperature of the second cooling stage was 350 ° C or higher, the microstructure was not sufficiently extinguished, and the fraction of volume of soft martensite was less than 30%. Per
Petition 870180125071, of 9/3/2018, p. 116/136
107/110 so much, in the nos. A-2, A-8, A-10 and C-4, at least one of the elongation (TS x El) and orifice expansion (TS χ λ) was not enough.
[00210] In steel No. A-13, since the cooling stop temperature of the second cooling stage was below 100 ° C, most of the austenite was transformed into martensite, and the volume fraction of austenite was less than 8%. Therefore, in this steel No. A-13, elongation (TSxEl) was not enough.
[00211] In steel No. A-11, since the temperature of alloy formation after galvanizing was higher than 600 ° C, pearlite was formed. As a result, the volume fraction of pearlite was more than 10%, and the volume fraction of austenite was less than 8%. Therefore, in steel No. A-11, the tensile strength (TS) was less than 980 MPa, and elongation (TS χ El) and orifice expandability (TS χ λ) were not sufficient.
[00212] On steel Nos. A-5, A-6, A-7, C-4, G-3, M-3, R-6, and R-7, the time when the steel plate temperature was within a temperature range of 350 ° C to 500 ° C was shorter than 20 seconds until the final product was obtained after controlled cooling (after the second cooling step). Therefore, in Nos. A-5 and G-3, although the steel plate was kept below 350 ° C, austenite was not sufficiently stabilized, and the volume fraction of austenite was less than 8%. In addition, in Nos. A-6, despite the fact that the steel plate was kept higher than 500 ° C, transformation of bainite was not progressed sufficiently, and the volume fraction of martensite was increased. As a result, the volume fractions of soft martensite and austenite, and the total volume fraction of soft martensite and bainite were not sufficient. In steel No. C-4, since the steel plate was retained immediately after the second cooling step, the volume fraction of mild martensite was less
Petition 870180125071, of 9/3/2018, p. 117/136
108/110 than 30% for the reasons described above. In steels Nos. A-7, M-3, and R-6, the steel sheet was retained within a temperature range of 350 ° C to 500 ° C, but the time when the steel sheet temperature was within a range of temperature from 350 ° C to 500 ° C could not be sufficiently guaranteed. In steel No. R-7, the steel plate was not retained, and the time when the steel plate temperature was within a temperature range of 350 ° C to 500 ° C could not be sufficiently guaranteed. Therefore, in Nos. A-7, M-3, R-6, and R-7, austenite was not sufficiently stabilized, and the volume fraction of austenite was less than 8%. Consequently, in Nos. A-5, A-6, A-7, C-4, G-3, M-3, R-6, and R-7, at least one of the elongation (TS x El) and orifice expansion (TS χ λ) was not enough.
[00213] In steel No. W-1, the amount of C in the steel was less than 0.05%. In steel No. X-1, the amount of Si in the steel was less than 0.5%. Therefore, in these steel Nos. W-1 and X-1, stiffness and stability of austenite (in the case of Si, concentration of C in austenite caused by the formation of ferrite) were not sufficient, and the volume fractions of soft martensite and austenite, and the volume fraction total soft martensite and bainite was not enough. As a result, in Nos. W-1 and X-1, the tensile strength (TS) was less than 980 MPa, and elongation (TS x El) and orifice expandability (TS χ λ) was not enough.
[00214] In steel No. Y-1, since the amount of Si in the steel was more than 3%, ferrite was stabilized and thus excessively formed, the volume fraction of soft martensite was less than 30%, and the fraction of total volume of smooth martensite and bainite was less than 40%. Therefore, in steel No. Y-1, orifice expansion (TS χ λ) was not sufficient. In addition, in steel No. Y-1, since the amount of oxides on the surface of the steel plate was increased, no galvanizing occurred, and delayed fracture resistance was not sufficient.
Petition 870180125071, of 9/3/2018, p. 118/136
109/110 [00215] In steel No. Z-1, since the amount of Mn in steel was less than 1.5%, ferrite was formed excessively due to deterioration in stiffness, and the volume fractions of smooth martensite and austenite, and the fraction of total volume of smooth martensite and bainite was not enough. As a result, in steel No. Z-1, the tensile strength (TS) was less than 980 MPa, and the elongation (TS x El) and orifice expandability (TS x λ) were not sufficient. In addition, in steel No. Z-1, since the formation of pearlite could be suppressed by Mn, the volume fraction of pearlite was more than 10%.
[00216] In steel No. AA-1, since the amount of Mn in steel was more than 3%, stiffness was increased excessively, and thus the majority of austenite was transformed into martensite after the second cooling step. Therefore, in this steel No. AA-1, the volume fraction of austenite was less than 8%, and the elongation (TS x El) and the orifice expandability (TS x λ) were not sufficient.
[00217] In steel No. AB-1, since the amount of C in the steel was more than 0.4%, the volume fraction of cementite was more than 10%. In addition, in this No. AB steel, once rigidity was increased excessively, the volume fraction of soft martensite was less than 30%, and the total volume fraction of martensite and bainite was less than 40%. Therefore, in this steel AB-1, the elongation (TS x El) and the orifice expandability (TS x λ) were not enough.
[00218] Preferred examples of the present invention have been described above. However, the present invention is not limited to these examples. Additions, omissions, substitutions, and other modifications to a configuration can be made without departing from the scope of the present invention. The present invention is not to be considered to be limited by the foregoing description, and is only limited by the scope of the appended claims.
INDUSTRIAL APPLICABILITY
Petition 870180125071, of 9/3/2018, p. 119/136
110/110 [00219] As described above, according to the present invention, it is possible to supply galvanized steel sheet (including hot dip galvanized steel sheet and galvanneal coated steel sheet) which is suitable for structural members, reinforcement members, and suspension members for automobiles, having tensile strength of 980 MPa or more, and excellent in delayed fracture resistance, galvanization adhesion, elongation, and orifice expansion at a low cost. Therefore, since the present invention contributes greatly to a car's weight reduction, industrial applicability is high.
Petition 870180125071, of 9/3/2018, p. 120/136

权利要求:
Claims (10)
[1]
1. Galvanized steel sheet having a tensile strength of 980 MPa or more, characterized by the fact that it comprises:
a steel plate; and a galvanizing layer on a steel sheet surface, where the steel sheet consists of, as a chemical composition of steel, in% by mass,
C: 0.05 to 0.40%,
Si: 0.5 to 3.0%,
Mn: 1.5 to 3.0%,
P: limited to 0.04% or less,
S: limited to 0.01% or less,
N: limited to 0.01% or less,
Al: limited to 2.0% or less,
O: limited to 0.01% or less, and optionally, as the chemical composition of steel, in% by weight, at least one selected from:
Mo: 0.01 to 1.0%,
Cr: 0.05 to 1.0%,
Ni: 0.05 to 1.0%,
Cu: 0.05 to 1.0%,
Nb: 0.005 to 0.3%,
Ti: 0.005 to 0.3%,
V: 0.005 to 0.5%,
B: 0.0001 to 0.01%, and a total of at least one of the selected elements of
Ca, Mg, and REM: 0.0005 to 0.04%, and the balance consisting of Fe and unavoidable impurities, in which a microstructure of the steel plate consists of
Petition 870180125071, of 9/3/2018, p. 121/136
[2]
2/5 a ferrite, a bainite, and in fraction of volume, 30% to 92% of a soft martensite, 8% or more and less than 60% of an austenite, limited to 10% or less of a pearlite, and limited to 10% or less of martensite, where a fraction of the total volume of soft martensite and bainite is 40% or more, and a fraction of the grain area having a grain size of more than 35 pm occupied per unit area of the microstructure is 10% or less, where a galvanizing metal in the galvanizing layer consists of, as a chemical galvanizing composition, limited to 15% by mass or less of Fe, limited to 2% by mass or less of Al, and the balance consisting of Zn and unavoidable impurities, the galvanizing layer includes an oxide including at least one chemical element selected from Si, Mn and Al, and when viewed from a cross section including the steel sheet and the galvanizing layer in one direction thickness, a fraction of the projection area obtained divin of a length that the oxide is projected on an interface between the galvanizing layer and the steel sheet by a length of the interface between the galvanizing layer and the steel sheet is 10% or more and a coverage of the galvanizing layer for the steel sheet is 99% or more.
2. Galvanized steel sheet, according to claim 1, characterized by the fact that the galvanizing layer is a hot-dip galvanized layer.
[3]
3. Galvanized steel sheet, according to claim 1, characterized by the fact that the galvanizing layer is a layer with a galvanneal type coating.
[4]
4. Galvanized steel sheet according to claim 1, characterized by the fact that an amount of Fe is limited
Petition 870180125071, of 9/3/2018, p. 122/136
3/5 less than 7% by weight in the chemical composition of galvanizing.
[5]
5. Galvanized steel sheet, according to claim 1, characterized by the fact that an amount of Fe is 7% by mass to 15% by weight in the chemical composition of galvanization.
[6]
6. Galvanized steel sheet, according to claim 1, characterized by the fact that an amount of Al is greater than 0% by mass and 2% by mass or less in the chemical composition of galvanization.
[7]
7. Method of making a galvanized steel sheet, as defined in claim 1, characterized by the fact that it comprises:
a first process for casting a steel that consists of, as a chemical composition of steel, in% by mass,
C: 0.05 to 0.40%,
Si: 0.5 to 3.0%,
Mn: 1.5 to 3.0%,
P: limited to 0.04% or less,
S: limited to 0.01% or less,
N: limited to 0.01% or less,
Al: limited to 2.0% or less,
O: limited to 0.01% or less, and optionally, as the chemical composition of steel, in% by weight, at least one selected from:
Mo: 0.01 to 1.0%,
Cr: 0.05 to 1.0%,
Ni: 0.05 to 1.0%,
Cu: 0.05 to 1.0%,
Nb: 0.005 to 0.3%,
Ti: 0.005 to 0.3%,
V: 0.005 to 0.5%,
Petition 870180125071, of 9/3/2018, p. 123/136
4/5
B: 0.0001 to 0.01%, and a total of at least one of the selected elements of Ca, Mg, and REM: 0.0005 to 0.04%, and the balance consisting of Fe and unavoidable impurities;
a second process of heating the steel directly or after it has cooled;
a third process of hot rolling steel so that hot rolling is completed at a temperature of a transformation point of Ar3 or higher;
a fourth process of rolling steel at 300 ° C to 700 ° C;
a fifth process of stripping steel;
a sixth process of cold rolling steel by a cold rolling mill having a working roll with a roll size of 1,400 mm or less with a cumulative rolling reduction of 30% or more and less than 100%;
a seventh process of heating the steel and holding the steel at 550 ° C to 750 ° C for 20 seconds or more;
an eighth process of annealing steel at 750 ° C to 900 ° C;
a ninth process of cooling the steel to an intermediate cooling temperature in a temperature range of 500 ° C or higher and less than 750 ° C at an average first cooling rate of 0.1 ° C / s to 30 ° C / if the steel is cooled from the intermediate cooling temperature to a cooling stop temperature of 100 ° C or higher and less than 350 ° C at a second average cooling rate that is equal to or higher than the first average cooling rate;
a tenth process of controlling a steel temperature within a temperature range of a temperature that is less than a temperature of the plating bath by 40 ° C, or higher and a temperature that is higher than the temperature of the gal bath
Petition 870180125071, of 9/3/2018, p. 124/136
5/5 vanization by 40 ° C or lower;
an eleventh process of galvanizing the steel in a hot dip galvanizing bath that flows at a flow rate of 10 m / min to 50 m / min; and a twelfth process of cooling the steel to a temperature of less than 100 ° C;
where the second average cooling rate is 1 ° C / s to 100 ° C / s, and a time when the steel temperature is within a temperature range of 350 ° C to 500 ° C is 20 seconds or more in the processes after the ninth process.
[8]
8. Method of making a galvanized steel sheet, according to claim 7, characterized by the fact that, in the ninth process, when the first average cooling rate is equal to the second average cooling rate, the first average cooling rate is 1 ° C / s or more and 30 ° C / s or less.
[9]
9. Method of manufacturing a galvanized steel sheet, according to claim 7, characterized by the fact that it additionally comprises:
a process of reheating and retaining the steel in the temperature range of 350 ° C to 500 ° C after the tenth process.
[10]
10. Method of making a galvanized steel sheet, according to claim 7, characterized by the fact that it additionally comprises:
a process of heating the steel to 460 ° C to 600 ° C to perform alloy treatment after the twelfth process.

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法律状态:
2018-06-05| B07A| Application suspended after technical examination (opinion) [chapter 7.1 patent gazette]|

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2019-02-05| B09A| Decision: intention to grant [chapter 9.1 patent gazette]|

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2019-04-02| B16A| Patent or certificate of addition of invention granted [chapter 16.1 patent gazette]|Free format text: PRAZO DE VALIDADE: 20 (VINTE) ANOS CONTADOS A PARTIR DE 28/09/2012, OBSERVADAS AS CONDICOES LEGAIS. (CO) 20 (VINTE) ANOS CONTADOS A PARTIR DE 28/09/2012, OBSERVADAS AS CONDICOES LEGAIS |

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2019-11-26| B25D| Requested change of name of applicant approved|Owner name: NIPPON STEEL CORPORATION (JP) |

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2021-08-10| B21F| Lapse acc. art. 78, item iv - on non-payment of the annual fees in time|Free format text: REFERENTE A 9A ANUIDADE. |

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2021-11-30| B24J| Lapse because of non-payment of annual fees (definitively: art 78 iv lpi, resolution 113/2013 art. 12)|Free format text: EM VIRTUDE DA EXTINCAO PUBLICADA NA RPI 2640 DE 10-08-2021 E CONSIDERANDO AUSENCIA DE MANIFESTACAO DENTRO DOS PRAZOS LEGAIS, INFORMO QUE CABE SER MANTIDA A EXTINCAO DA PATENTE E SEUS CERTIFICADOS, CONFORME O DISPOSTO NO ARTIGO 12, DA RESOLUCAO 113/2013. |

优先权:

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申请号 | 申请日 | 专利标题

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JP2011-217811|2011-09-30|

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JP2011217811|2011-09-30|

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PCT/JP2012/075244|WO2013047836A1|2011-09-30|2012-09-28|Galvanized steel sheet and method of manufacturing same|

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