high strength hot dip galvanized steel sheet and high strength hot dip galvanized steel sheet and me

专利摘要:
"High strength hot dip galvanized steel sheet and high strength alloy hot dip galvanized steel sheet having excellent forming capacity and small anisotropy of material with ultimate tensile strength of 980 mpa or more" and method for its production ". A high strength hot dip galvanized sheet steel is provided having small material anisotropy and excellent forming capacity with a final tensile strength of 980 mpa or more. The hot dip galvanized steel sheet includes a hot dip galvanized layer formed on a surface of a base steel sheet. the base steel plate contains,% by mass, c: 0.1 less than 0.40%, bs: 0.5 to 3.0%, mn: 1.5 to 3.0%, o: limited to 0.006% or less, p: limited to 0.04% or less, s: limited to 0.01% or less, al: limited to 2.0% or less, n: limited to 0.01% or less, and a balance including fe and the inevitable impurities. the microstructure of the base steel plate contains 40% or more ferrite, 8 less than 60% residual austenite in volume fraction, and a balance being either bainite or martensite. In a range of plate thickness from 5/8 to 3/8 from the base steel plate surface, a pole density of the specific crystal orientation is within a predetermined range. The hot dip galvanized layer contains fe: less than 7% by mass and a balance including zn, al and the inevitable impurities.
公开号:BR112014007412B1
申请号:R112014007412-7
申请日:2012-09-28
公开日:2019-01-29
发明作者:Masafumi Azuma；Chisato Wakabayashi；Takayuki Nozaki；Nobuhiro Fujita；Manabu Takahashi
申请人:Nippon Steel & Sumitomo Metal Corporation；
IPC主号:

专利说明:

Report of the Invention Patent for "HIGH RESISTANCE HOT DIP GALVANIZED STEEL PLATE AND HIGH RESISTANCE HOT DIP GALVANIZED STEEL PLATE AND METHOD FOR THEIR PRODUCTION".
Technical Field The present invention relates to a high strength coated steel sheet and a high strength alloy hot dip galvanized steel sheet which has excellent coating adhesion and conformability with an ultimate strength resistance. traction (TS) of 980 Mpa or more that is particularly suitable for a structural member, a reinforcement member, and a car suspension element.
This application claims priority over Japanese Patent Application No. 2011-218040, filed September 30, 2011, the wording of which is incorporated herein by reference.
Background Art The weight reduction of elements such as crossbars and car side elements has been considered to support trends for reducing fuel consumption in recent years. In terms of materials, from the point of view of ensuring strength and safety on impact even when tuned, a higher strength steel plate was made. However, the conformability of materials deteriorates along with their increased strength. To implement a lighter weight of the elements, a steel plate must be produced that satisfies both the press forming capacity and the high strength. In particular, excellent ductility is required in the case of the conformation of the structural element or the reinforcement element of cars which have a complicated shape.
Recently, as a structural element of the automobile, a steel plate having a final tensile strength of 440 MPa or 590 is mainly used. To further reduce weight, the development of a high-grade steel plate is preferred. resistance of 980 MPa or more. In particular, when the 590 MPa grade steel plate is replaced by a 980 MPa class steel plate, it requires an elongation equivalent to the 590 MPa class. Thus it is desired to develop a sheet steel of 980 MPa or more which has excellent elongation.
As steel plate having excellent total elongation (El) in a tensile test, there are complex structure steel plates in which a soft ferrite that serves as the primary phase is used in a steel plate structure to ensure ductility. and residual austenite is dispersed as a second phase to ensure strength.
[006] As steel obtained by dispersion of residual austenite, there is TRIP (Transformation-Induced Plasticity) steel that uses residual austenite martensite transformation at the time of plastic processing, and its applications have recently been expanded.
In particular, TRIP steel has excellent elongation compared to precipitation reinforcing steel and DP steel (steel consisting of ferrite and martensite), and thus expanded application is highly desirable. However, since steel ensures excellent conformability using martensite transformation at the time of conformation, large amounts of residual austenite are required to ensure conformability. To ensure residual austenite, large amounts of Si need to be added. In addition, to ensure strength of 980 MPa or more, alloying elements tend to be added in large quantities and texture develops easily. Particularly, the development of the texture of the {100} To {223} <110> orientation groups or the {332} <113> orientation texture promotes the anisotropy of the material. For example, compared to the total elongation in the case of tensile testing on a steel strip in a direction parallel to the rolling direction, the total elongation in the case of tensile testing in a vertical direction is greatly lower. Accordingly, although the steel plate has characteristics of good elongation in one direction and excellent conformability, it is difficult to apply to an element that has a complicated shape.
On the other hand, a hot dip galvanized steel sheet and a hot dip galvanized steel sheet having excellent corrosion resistance have been known as automotive steel sheets. The steel plate is supplied with a zinc coated layer containing 15% Fe or less on a steel plate surface for excellent corrosion resistance. Hot dip galvanized sheet steel and hot dip galvanized sheet steel alloy are produced in a continuous type hot dip galvanizing line (hereinafter referred to as CGL) in many cases. At CGL, the steel plate is degreased, then annealed by indirect heating with radiant pipes under a reducing atmosphere containing H2 and N2, then cooled to a temperature close to that of the galvanizing bath, and then dipped into a bath. hot dip galvanizing. Subsequently, the steel plate undergoes a coating treatment.
In the case of the production of a hot dip galvanized steel sheet, the steel sheet is dipped in the galvanizing bath and then reheated so that the galvanized layer is subjected to a bonding treatment. At this time, the atmosphere in the furnace is an atmosphere in which Fe is reduced, and steel sheet can be produced without oxidizing Fe so that it is widely used as a galvanized steel sheet production equipment.
However, Si is easily oxidized compared to Fe, and Si oxide is formed on the surface of the steel plate as it passes through the CGL. Si oxide is responsible for galvanization failures due to poor wetting ability with hot dip galvanization. Alternatively, since oxide inhibits a Fe and zinc bonding reaction, there is the problem that galvanized or hot-dip alloy steel sheet cannot be produced.
In connection with this problem, a method of achieving both excellent conformability and coating property, in particular a means of improving the coating property of Si-containing steel in large quantities, is described in Patent Literature 1. in which annealing is performed once, then pickling is performed to remove oxide from the surface of the steel plate, and then hot dip galvanizing treatment is performed once more. However, this method is undesirable because annealing is performed both times, and thus stripping after heat treatment and passing the galvanizing line lead to a significant increase in processing and an increase in cost.
As a means of improving the coating property of Si-containing steel, a method of suppressing the oxides of Si or Mn by making the atmosphere in the furnace a reducing atmosphere of readily oxidizable elements such as Si and Mn or a method of reducing formed oxides is described in Patent Literature 2. In this method, pre-coating or surface polishing is performed on materials having a poor coating property before entering the galvanizing line. However, as the precoating or surface polishing process increases, the cost increases. In addition, since high strength steel sheet generally contains Si and Mn in large quantities, it is difficult to achieve an atmosphere capable of reducing Si in steel sheet containing 0.5 wt% or more Si which is purpose of the present invention, and thus a large investment in equipment is required, resulting in an increase in cost. In addition, since the oxygen released from the reduced Si and Mn oxides changes the atmosphere in the furnace to an Si oxidizing atmosphere, it is difficult to stabilize the atmosphere in case of performing large production. As a result, there are concerns that defects such as coating wetting capacity irregularities or connection irregularity occur in the longitudinal direction or in the width direction of the steel plate.
As a means of achieving both excellent ductility and excellent coating property, Patent Literature 3 describes a method in which cold rolling is performed, and then the surface of the steel plate is pre-treated. Coating with metals such as Ni, Fe, or Co is subjected to a coating treatment while passing through a heat treatment line. This refers to a method of pre-coating metals, which are difficult to oxidize compared to Si and Mn, on a surface layer of the steel plate and to produce a steel plate not containing Si and Mn on the surface layer. of steel sheet. However, even when the surface of the steel plate is pre-coated, these elements diffuse inside the steel plate during heat treatment, and thus a large amount of pre-coating must be performed. Therefore, there is the problem that the cost increases noticeably.
As a means of solving these problems, Patent Literatures 4 to 6 propose a method in which Si oxide is not formed on the steel plate surface but is formed within the steel plate. This may increase the oxygen potential in the furnace and may oxidize Si within the steel plate to suppress Si diffusion on the steel plate surface and Si oxide formation on the surface.
In addition, Patent Literatures 7 and 8 do not refer to TRIP steel, but to galvanize the steel plate and describe a method of adjusting the furnace interior to be a reducing atmosphere in a CGL annealing process. In addition, Patent Literature 9 describes a method of providing a jet stream of a given flow rate in a galvanizing bath to prevent slag galvanizing failures.
However, conventional techniques are extremely difficult to provide simultaneously with corrosion resistance, high strength and ductility.
Prior Art Literature (s) Patent Literature (s) Patent Literature 1 JP 3521851B Patent Literature 2 JP 4-26720A Patent Literature 3 JP 3598087B Patent Literature 4 JP 2004-323970A Patent Literature 5 JP 2004-315960A Literature Patent 6 JP 2008-214752A Patent Literature 7 JP 2011-111674A
Patent Literature 8 JP 2009-030159A Patent Literature 9 JP 2008-163388A
Summary of the Invention Problem (s) To Be Solved By The Invention The present invention should provide a high strength hot dip galvanized steel sheet and a hot dip galvanized steel sheet bonded to the invention. High strength with excellent coating adhesion and conformability with ultimate tensile strength (TS) of 980 MPa or more.
Means to Solve Problem (s) From the results obtained by careful examination to achieve both ultimate tensile strength (TS) of 980 MPa or more and excellent conformability, the present inventors have found that It is important to use Si completely as a reinforcing element and to contain 40% or more of volume fraction ferrite and 8% or more of volume fraction residual austenite. In addition, the inventors have found that even in a cold-rolled steel plate containing a large amount of additive elements, it is possible to produce a steel plate in which the anisotropy of a material is reduced and the conformability is excellent. control of roughing and finishing lamination within a specific range.
On the other hand, the coating and bonding property of steel containing a large amount of Si was ensured by the bonding of molten zinc to flow into a galvanizing bath at 10 to 50 m / min and suppress the reaction between the oxide. zinc (slag) and the steel sheet that is responsible for galvanizing failures. In the event that flux does not occur in the bath, a thin zinc oxide is incorporated into a galvanized layer and the bonding reaction is inhibited.
In addition, the detailed mechanism is unclear, but when there are Si and Mn oxides on the surface of the steel plate, zinc oxide galvanizing failures and bonding delay become more noticeable to have a significant adverse influence on coating property. The suppression of the reaction between the slag and the steel plate that is responsible for galvanizing faults and the delay in bonding also has a significant effect in facilitating the bonding process.
By improving the coating property, a large amount of Si can be added to the hot dip galvanized steel sheet and the hot dip galvanized steel cover.
[0022] The present invention relates to a high strength hot dip galvanized steel sheet and a high strength hot dip galvanized steel sheet having a small anisotropy of material and excellent resilience. tensile strength (TS) of 980 MPa or more and its essence is as follows.
[1] A high strength hot dip galvanized steel sheet having low material anisotropy and excellent forming capacity with a final tensile strength of 980 MPa or more, hot dip galvanized steel sheet comprising a galvanized layer by hot dip formed on the surface of a base steel plate, where the base steel plate contains by weight% C: 0.1 to less than 0.40%;
Si: 0.5 to 3.0%;
Mn: 1.5 to 3.0%; O: limited to 0.006% or less; P: limited to 0.04% or less; S: limited to 0.01% or less;
Al: limited to 2.0% or less; N: limited to 0.01% or less; and the balance including Fe and the inevitable impurities, the microstructure of the base steel plate contains 40% or more ferrite, 8% less than 60% residual austenite in volume fraction, and the balance being bainite and martensite, average value of the pole densities of the {100} to {223} <110> orientation groups represented by each of the crystal orientations {100} <011>, {116} <110>, {114} <110> , {113} <110>, {112} <110>, {335} <110>, and {223} <110> in a sheet thickness range of 5/8 to 3/8 from the sheet surface base steel is 6.5 or less and the crystal orientation pole density {332} <113> is 5.0 or less, and the hot dip galvanized layer contains Fe: less than 7 mass% and the balance including Zn, Al and the inevitable impurities.
[2] High strength hot dip galvanized steel sheet having small material anisotropy and excellent conformability with ultimate tensile strength of 980 MPa or more according to item [1], where the base steel sheet also contains one or two or more elements, by weight, from: Cr: 0,05 to 1,0%;
Mo: 0.01 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%; and B: 0.0001 to 0.01%, [3] High strength hot dip galvanized steel sheet having a small anisotropy of material and excellent conformability with ultimate tensile strength of 980 MPa or more. according to item [1], where the base steel plate also contains, in mass%, 0.0005 to 0.04% in the total of one or more elements selected from Ca, Mg, and REM.
[4] High strength alloy hot-dip galvanized steel sheet having low material anisotropy and excellent forming capacity with a final tensile strength of 980 MPa or more, alloyed hot-dip galvanized steel sheet comprising a hot-dip galvanized bonded layer formed on a surface of the base steel plate, wherein the base steel plate contains by weight%: C: 0,10 less than 0,4%;
Si: 0.5 to 3.0%;
Mn: 1.5 to 3.0%; O: limited to 0.006% or less; P: limited to 0.04% or less; S: limited to 0.01% or less;
Al: limited to 2.0% or less; N: limited to 0.01% or less; and the balance including Fe and the inevitable impurities, the microstructure of the base steel plate contains 40% or more ferrite, 8 less than 60% residual austenite in volume fraction, and the balance being bainite or martensite, the value pole densities of the {100} to {223} <110> orientation groups represented by each of the crystal orientations {100} <011>, {116} <110>, {114} <110>, {113} <110>, {112} <110>, {335} <110>, and {223} <110> in a sheet thickness range from 5/8 to 3/8 from the sheet metal surface base steel is 6.5 or less and the crystal orientation pole density {332} <113> is 5.0 or less, and the hot-dip galvanized bonded layer contains Fe: 7 to 15 mass% and the balance including Zn, Al, and the inevitable impurities.
[5] The high strength alloy hot dip galvanized steel sheet having the small anisotropy of the material and the excellent conformability with the ultimate tensile strength of 980 MPa or more as per item [4], where the base steel also contains one or two or more elements between, by mass%: Cr: 0.05 to 1.0%;
Mo: 0.01 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%; and B: 0,0001 to 0,01%, [6] High strength alloy hot-dip galvanized steel sheet having the small anisotropy of the material and the excellent conformability with the ultimate tensile strength of 980 MPa or more according to item [4], where the base steel plate also contains, in mass%, 0.0005 to 0.04% in total of one or two or more elements selected from Ca, Mg, and REM.
[7] A method of producing a high-strength hot-dip galvanized sheet steel having low material anisotropy and excellent forming capacity with a ultimate tensile strength of 980 MPa or more, the production method comprising: with respect to a steel bar containing by weight% C: 0,10 less than 0,4%;
Si: 0.5 to 3.0%;
Mn: 1.5 to 3.0%; O: limited to 0.006% or less; P: limited to 0.04% or less; S: limited to 0.01% or less;
Al: limited to 2.0% or less; N: limited to 0.01% or less; and the balance including Fe and the inevitable impurities, perform the first hot rolling in which rolling at a reduction ratio of 40% or more is performed once or more over a temperature range of 1000Ό or higher and 1200Ό or lower; adjust the grain diameter of the austenite to 200 μιη or less at first hot rolling; perform the second hot rolling in which rolling at a reduction ratio of 30% or more is performed in one pass at least once in a temperature region of T1 + 30 ° C or more and T1 + 200 ° C or less determined by expression (1) below; adjust the total reduction ratio on the second hot rolling mill to 50% or more; perform the final reduction at a reduction ratio of 30% or more on the second hot rolling and then start cooling before cold rolling such that the wait time t (seconds) satisfies Expression (2) below; adjust the average cooling rate to OO / s or more and the temperature change to be in the range of 40Ό or more and 140Ό or less in the cooling before cold rolling; cool in a temperature region of 700Ό or less; perform cold rolling at a reduction ratio of 40% or more and 80% or less; heat to an annealing temperature of 750 ° or more and 900 ° or less and then anneal in a continuous hot dip galvanizing line; cool to 500 ° C from the annealing temperature at 0.1 to 200 ° / s; and perform hot dip galvanization after holding for 10 to 1000 seconds between 500 and 350Ό. T1 (° C) = 850 + 10 x (C + N) x Mn + 350 x Nb + 250 xTi + 40 x B + 10 x Cr + 100 x Mo + 100 x V ··· Expression (1) where, C , N, Mn, Nb, Ti, B, Cr, Mo, and V each represent the content of each element (mass% of Ti, B, Cr, Mo, and V are calculated as zero when the elements are not). contained). t <2.5 x t1 ··· Expression (2) where t1 is obtained by Expression (3) below. t1 = 0.001 x ((Tf - T1) x P1 / 100) 2 - 0.109 x ((Tf - T1) x P1 / 100) + 3.1 ··· Expression (3) where, in Expression (3) above, Tf represents the temperature of the steel bar obtained after the final reduction at a reduction ratio of 30% or more, and P1 represents the reduction ratio of the final reduction to 30% or more.
[8] The method of production of high strength hot dip galvanized steel sheet having the small anisotropy of the material and excellent conformability with the ultimate tensile strength of 980 MPa or more as per item [7], where the Total reduction ratio in temperature range below T1 + 30 ° C is 30% or less.
[9] The method of production of high-strength hot-dip galvanized steel sheet having small material anisotropy and excellent conformability with ultimate tensile strength of 980 MPa or more according to item [7], where in In a case of heating to annealing temperature in the hot dip continuous galvanizing line, the average heating rate of ambient temperature or greater and 650Ό or less is adjusted to HR1 (“C / s) expressed by expression (4) below , and the average rate of heating from a temperature exceeding 650Ό to annealing temperature [and adjusted for HR2 CC / s) expressed by Expression (5) below. HR1> 0.3 ··· Expression (4) HR2 <0.5 x HR1 ··· Expression (5) [10] The production method of high-strength hot-dip galvanized steel sheet having small material anisotropy. and the excellent conformability with the ultimate tensile strength of 980 MPa or more as per [7], where when hot dip galvanizing is performed, the temperature of the base steel sheet is (galvanizing bath temperature per hot dip - 40) “C or higher e (hot dip galvanizing bath temperature + 50) Ό or lower.
[11] The production method of high strength hot dip galvanized steel sheet having small material anisotropy and excellent conformability with ultimate tensile strength of 980 MPa or more as per item [7], where the rate A flow rate of 10 m / min or faster and 50 m / min or slower is provided in a galvanizing bath when hot dip galvanizing is performed.
[12] A method of producing a high strength alloy hot-dip galvanized steel sheet having small material anisotropy and excellent forming capacity with a ultimate tensile strength of 980 MPa or more, the production method comprising: in ratio to steel bar containing by weight%: C: 0,10 less than 0,4%;
Si: 0.5 to 3.0%;
Mn: 1.5 to 3.0%; O: limited to 0.006% or less; P: limited to 0.04% or less; S: limited to 0.01% or less;
Al: limited to 2.0% or less; N: limited to 0.01% or less; and the balance including Fe and the inevitable impurities, perform the first hot rolling mill in which rolling at 40% or more reduction is performed once or more at a temperature range of 1000Ό or higher and 1200Ό or lower; adjust the austenite grain diameter to 200 μιη or less by the first hot rolling; perform the second hot rolling in which a rolling at a reduction ratio of 30% or more is performed in one pass at least once in a temperature region of T1 + 30 ° C or higher and T1 + 200 ° C or lower determined by Expression (1) below; adjust the total reduction ratio on the second hot rolling mill to 50% or more; perform the final reduction at a reduction ratio of 30% or more on the second hot rolling, and then start cooling before cold rolling such that the holding time t (seconds) satisfies expression (2) below; adjust the average cooling rate to OO / s or more and the temperature change to be in the range of 40Ό or greater and 140Ό or less in the cooling before cold rolling; winding in a temperature region of 700Ό or less; perform cold rolling at a reduction ratio of 40% or more and 80% or less; heat to an annealing temperature of 750Ό or higher and 900Ό or lower and then anneal in a continuous hot dip galvanizing line; cool to 500 ° C from the annealing temperature to 0.1 to 200 ° / s; perform hot dip galvanization after holding for 10 to 1000 seconds between 500 and 350Ό; and perform a bonding treatment at a temperature of 460 ° C or higher, T1 (° C) = 850 + 10 x (C + N) x Mn + 350 x Nb + 250 x Ti + 40 χ B + 10 x Cr + 100 x Mo + 100 χ V ··· Expression (1) where C, N, Mn, Nb, Ti, B, Cr, Mo, and V each represent the content of each element (the mass% of Ti , B, Cr, Mo, and V are calculated as zero when not contained). t <2.5 x t1 ··· Expression (2) where t1 is obtained by Expression (3) below. t1 = 0.001 x ((Tf - T1) χ P1 / 100) 2 - 0.109 x ((Tf - T1) x P1 / 100) + 3.1 ··· Expression (3) where in Expression (3) above, Tf represents the temperature of the steel bar obtained after the final reduction of 30% or more, and P1 represents the reduction ratio of the final reduction to 30% or more.
[13] The method of production of high strength alloy dip galvanized steel sheet having the small anisotropy of the material and excellent conformability with the ultimate tensile strength of 980 MPa or more as per item [12], where the ratio Total reduction in a temperature range below T1 + 30 ° C is 30% or less.
[14] The method of production of high strength alloy hot dip galvanized steel sheet having a small material anisotropy and excellent conformability with ultimate tensile strength of 980 MPa or more according to item [12], where , in a case of heating to annealing temperature in the hot dip continuous galvanizing line, the average heating rate of ambient temperature or greater and 650Ό or less is adjusted to HR1 CC / s) expressed by Expression (4) below , and the rate of heating from a temperature exceeding 650 ° C to annealing temperature is adjusted to HR2 CC / s) expressed by Expression (5) below. HR1> 0.3 ··· Expression (4) HR2 <0.5 x HR1 ··· Expression (5) [15] The production method of high strength alloy hot dip galvanized steel plate having small anisotropy material and excellent conformability with the ultimate tensile strength of 980 MPa or more as per [12], where when hot dip galvanizing is performed, the temperature of the base steel sheet is (bath temperature). hot dip galvanizing - 40) “C or higher and (hot dip galvanizing bath temperature + 50 ° C) or lower.
[16] The method of production of high strength alloy hot dip galvanized steel sheet having the small anisotropy of the material and the excellent conformability with ultimate tensile strength of 980 MPa or more as per item [12], where a flow rate of 10 m / min or faster and 50 m / min or slower is provided in a galvanizing bath when hot dip galvanizing is performed.
Effect (s) of the Invention In accordance with the present invention, the high strength hot dip galvanized steel sheet and the high strength hot dip galvanized steel sheet having the small anisotropy of the material and excellent Conformability with the ultimate tensile strength (TS) of 980 MPa or more, which is suitable for a structural element, a reinforcement element, and a car suspension element, are provided at a low cost.
Brief Description of the Drawing (s) FIG. 1- FIG. 1 is a diagram illustrating the relationship between ΔΕΙ and the average pole density value of the orientation groups {100} <011> to {223} <110>.
FIG. 2 - FIG. 2 is a diagram illustrating the relationship between ΔΕΙ and the pole density of a {332} <113>.
FIG. 3 - FIG. 3 is an explanatory diagram of a continuous hot rolling line.
METHODS OF CARRYING OUT THE INVENTION From the results obtained by careful examination of the hot dip galvanized steel sheet and the hot dip galvanized steel sheet attached to solve the above problems, the present inventors found that for have the ultimate tensile strength of 980 MPa or more and excellent conformability when the primary phase of a base steel plate microstructure is ferrite and residual austenite is contained. In addition, the inventors have found that even in steel plate containing a large amount of Si and Mn, it is possible to produce a cold rolled steel plate having a small anisotropy of the material by controlling hot rolling conditions within a range. specific. In addition, even on a steel plate containing a large amount of Si, the coating's wetting capacity and bonding are ensured by allowing molten zinc to flow into the galvanizing bath. Hereinafter the present invention will be described in detail. (Crystal orientation of base steel plate) The average value of the pole densities of the orientation groups {100} <011> to {223} <110> and the pole density of a crystal orientation {332} <113 >, in a sheet thickness range of 5/8 to 3/8 from the surface of a base steel sheet are particularly important characteristic values in the present invention. As illustrated in FIG. 1 in the case of calculating the pole density of each orientation by performing an x-ray diffraction in the plate thickness range of 5/8 to 3/8 from the base steel plate surface when the average value of the Pole density of the {100} to {223} <110> orientation groups is 6.5 or less, a high strength steel plate having low material anisotropy and excellent conformability is obtained. The average value of the {100} To {223} <110> orientation groups is preferably 4.0 or less.
Guidelines included in guidance groups {100} <011> through {223} <110> are {100} <011>, {116} <110>, {114} <110>, {113} <110> , {112} <110>, {335} <110>, and {223} <110>.
A steel plate having large anisotropy of the material means a steel plate in which ΔΕΙ [= (L-EI) - (C-EI)], which is defined by the difference between the total elongation (L-EI) in the if a tensile test is performed in a direction parallel to the rolling direction and the total elongation (C-EI) if a tensile test is performed in a direction vertical to the rolling direction exceeds 5%. A steel plate containing a large number of alloying elements has a great anisotropy due to texture development and has a small C-EI in particular. As a result, while the L-EI is excellent, it is difficult to apply such a steel plate for elements to be machined in various directions. In the present invention, ο ΔΕΙ was less than 5%, but although the difference in total elongation is less than -5%, the anisotropy of the material becomes large to deviate from the range of the present invention. However, generally the range described above was considered from the fact that texture develops and C-EI deteriorates. Preferably, ο ΔΕΙ is 3% or less.
The pole density is synonymous with the random intensity of x-ray. Pole density (random x-ray intensity ratio) is a numerical value obtained by measuring x-ray intensities of a standard sample having no accumulation in a specific orientation and a test sample using a radius diffraction method. x or similar under the same conditions and dividing the x-ray intensity of the test sample by the x-ray intensity of the standard sample. Pole density is measured using x-ray diffraction, Electron Back Scattering Diffraction (EBSD) or the like. In addition, pole density can be measured either by an EBSP (Electron Back Scattering Pattern) method or an ECP (Electron Channeling Pattern) method. It can be obtained from a three-dimensional texture calculated by a vector method based on a pole figure of {110} or can be obtained from a three-dimensional texture calculated by a series expansion method using a plurality (preferably three or more). ) of pole figures among the figures of {110}, {100}, {211}, and {310}.
For example, for the pole density of each of the crystal orientations, each of the intensities of (001) [1-10], (116) [1-10], (114) [1-10], (113) [1-10], (112) [1-10], (335) [1-10], and (223) [1-10] the cross section φ2 = 45 ° in the three-dimensional texture (ODF) can be used in the state.
The mean value of the pole densities of the orientation groups {100} To {223} <110> is the arithmetic mean of the pole densities of each orientation. When all intensities of these orientations are not obtained, the arithmetic average of the pole density of each orientation {100}, {116} <110>, {114} <110>, {112} <110>, or {223 } <110> can be used as a substitute.
Similarly, as illustrated in FIG. 2, the pole density of the crystal orientation {332} <113> in the sheet thickness range of 5/8 to 3/8 from the base steel plate surface must be 5.0 or less. Preferably the pole density may be 3.0 or less. When the crystal orientation pole density {332} <113> is 5.0 or less, ΔΕΙ is 5% or less and the steel plate to satisfy the ratio of (ultimate tensile strength x total elongation> 16000 MPa x %) is produced.
The sample to be subjected to x-ray diffraction may be measured while adjusting the sample by the method described above such that the steel plate is reduced in thickness from the surface to a predetermined plate thickness by polishing. mechanical or similar, the stress is then removed by chemical polishing or the like, and a suitable plane becomes a measurement plane in the range of sheet thickness from 3/8 to 5/8.
Of course, when the limitation with respect to the x-ray intensity described above is satisfied not only near the central portion of the shell thickness but also in as many portions of thickness as possible, the anisotropy of the material also becomes smaller. . However, the range of 3/8 to 5/8 from the sheet steel surface is measured to make it possible to represent the material properties of the entire sheet steel in general. Thus, 5/8 to 3/8 of the steel sheet thickness is defined as the measuring range.
Furthermore, the crystal orientation represented by {hkl} <uvw> means that the normal direction of a sheet steel plane is parallel to <hkl> and the rolling direction is parallel to <uvw>. With respect to crystal orientation, usually vertical orientations to the sheet steel plane are represented by [hkl] or {hkl} and orientations parallel to the rolling direction are represented by (uvw) or <uvw>. {hkl} and <uvw> are collective terms for equivalent planes, [hkl] and (uvw) represent individual crystal planes. That is, since a centered body cubic structure is applied to the present invention, for example planes (111), (-111), (1-11), (11-1), (-1-11), (-11-1), (1-1-1), and (-1-1-1) are equivalent and cannot be distinguished from each other. In such a case, these guidelines are collectively called {111}. Since an ODF representation is also used to represent orientations of other symmetrical low crystal structures, individual orientations are generally represented by [hkl] (uvw), but in the present invention [hkl] (uvw) and {hkl} < uvw> are synonyms to each other. X-ray crystal orientation measurement is performed according to a method described, for example, in Cullity, Theory of X-ray diffraction (edited 1986, translated by MATSUMURA, Gentaro, published by AGNE Inc.) in pages 274 to 296.
In the present invention, anisotropy was estimated using the total elongation in the tensile test, but the same anisotropy also occurs in the steel plate, in which the texture is developed, in relation to uniform elongation or bendability. . In the steel plate of the present invention, therefore, the anisotropy of bendability or uniform elongation is also small.
In the present invention, excellent conformability means that the steel plate satisfies the ratio of (ultimate tensile strength x total elongation (C-EI)> 16000 MPa%) represented by the product of ultimate elongation tensile strength full in vertical direction to rolling direction. The conformability is preferably 18000 MPa% or more and is preferably 20,000 MPa% or more. (Base steel sheet microstructure) The following describes the base steel sheet microstructure.
In the present invention, a steel plate is provided such that the primary phase is 40% ferrite or more by volume fraction and the residual austenite is dispersed with 8% or more and less than 60% by volume fraction. ensure ultimate tensile strength of 980 MPa or more and excellent conformability. Thus, it is necessary to contain residual austenite. In addition, the ferrite phase may have a form of an acicular ferrite in addition to the polygonal ferrite. Using the primary phase as ferrite, a ferrite having high ductility becomes the primary phase, and thus ductility is improved. Containing residual austenite as a second phase, high reinforcement and also improved ductility are achieved at the same time. When residual austenite is less than 8% by volume, as the effect is difficult to obtain, the lower limit of residual austenite is 8%. The bainite structure is inevitably contained to stabilize the residual austenite. To achieve high reinforcement, the martensite may be contained. In addition, when the volume fraction is less than 10%, the perlite structure may be contained. In addition, each phase of the microstructure such as ferrite, martensite, bainite, austenite, perlite, and residual structures can be identified and their locations and volume fractions can be observed and quantitatively measured using an optical microscope having a magnification. 1000 times and a scanning electron microscope having a magnification of 1000 times to 10000 times after the cross section of the steel plate in the rolling direction or the cross section in the right angle direction to the rolling direction is caused using a Nltal reagent and the reagent as described in JP 59-219473A. The area length of each structure can be obtained by looking at 20 or more fields and applying the dot count method or image analysis. Then, the area fraction obtained is defined as the volume fraction of each structure. (Chemical Composition of Base Steel Sheet) The following will describe the reasons for restricting the quantities of the composition. In addition,% means mass%. In the present invention, the base steel shell contains by weight% C: 0.1 less than 0.40%, Si: 0.5 to 3.0%, and Mn: 1.5 to 3.0. %, O: limited to 0.006% or less, P: limited to 0.04% or less, S: limited to 0.01% or less, Al: limited to 2.0% or less, N: limited to 0, 01% or less, and the balance including Fe and the inevitable impurities.
[0047] C: C is an element that can increase the strength of the steel plate. However, when the grade is less than 0.1%, it is difficult to achieve both tensile strength of 980 MPa or more and working capacity. On the other hand, when its content exceeds 0.40%, it is difficult to guarantee spot welding capability. For this reason, the range is limited to 0.1 to 0.40% or less.
Si: Si is a reinforcing element, and is effective for increasing the strength of the steel plate. Addition is essential for suppressing cementite precipitation and contributing to stabilize residual austenite. However, when the content is less than 0.5%, the high reinforcement effect is small. On the other hand, when the grade exceeds 3.0%, the working capacity is decreased. Consequently, the Si content is limited to the range 0.5 to 3.0%.
[0049] Mn: Mn is a reinforcing element and is effective for increasing the strength of the steel plate. However, when the content is less than 1.5%, it is difficult to obtain tensile strength of 980 MPa or more. In contrast, when the content is large, it facilitates co-segregation with P and S and leads to a noticeable deterioration in working capacity, and thus the upper limit is 3.0%. More preferably the range is 2.0 to 2.7%.
O: O Forms oxides to cause deterioration in bendability and hole expandability, and thus it is necessary to restrict the amount of addition. In particular, oxides often exist in the form of inclusions, and when they exist on a perforated edge or a cut cross section, then notch-like surface defects or rough pits may form on the edge surface. As a result, stress concentration tends to occur during hole expansion or a large deformation process, which can then act as a source for fracture formation; therefore, a dramatic deterioration in bore expandability and foldability occurs. When the O content exceeds 0.006%, then these trends become noticeable, and therefore the upper limit of the O content is 0.006% or less. When the content is less than 0.0001%, the cost increases excessively, and thus is economically undesirable. Consequently, this value is a substantial lower limit.
P: P tends to segregate in the central part of the thickness of the steel plate and makes the welding zone become brittle. When its content exceeds 0.04%, the brittleness of the welding zone becomes noticeable, so the proper range is limited to 0.04% or less. The lower limit value of P is not particularly determined, but when the lower limit is less than 0.0001%, it is economically disadvantageous, so that value is preferably set to the lower limit value. [0052] S: S has an adverse effect on weldability and production capacity at the time of casting and hot rolling. For this reason, the upper limit value was 0.01% or less. The lower limit value of the S content is not particularly determined, but when the lower limit is less than 0.0001%, it is economically disadvantageous, so this value is preferably set as the lower limit value. Since S combines with Mn to form raw MnS, which deteriorates bendability and hole expandability, the S content needs to be reduced as much as possible.
Al: Al promotes the formation of ferrite, which improves ductility, and therefore can be added. In addition, Al may also act as a deoxidizing material. However, an excessive addition increases the number of Al-based gross inclusions, which can cause deterioration of hole expandability as well as surface defects. For this reason, the upper limit for the addition of Al is 2.0%. Preferably, the upper limit is 0.05% or less. The lower limit is not particularly limited, but it is difficult to adjust to be less than 0.0005%, so this value is the substantial lower limit.
N: N forms crude nitrides and causes deterioration of bending capacity and hole expansion capacity, so it is necessary to restrict the amount of addition. This is because when the N content exceeds 0.01%, the above trend becomes noticeable, so the N content is in a range of 0.01% or less. In addition, this causes bubbles to occur at the time of welding, so the less the better. The effect of the present invention is presented without particularly limiting the lower limit, but when the N content is less than 0.0005%, the cost of production increases substantially, so that value is a substantial lower limit.
In the present invention, the base steel plate may also contain any one or two or more of the following elements which are conventionally used to, for example, increase strength. [0056] Mo: Mo is a reinforcing element and is important for improving hardening ability. However, when the content is less than 0.01%, these effects cannot be obtained, so the lower limit value was 0.01%. In contrast, when the grade exceeds 1%, it has an adverse effect on production capacity at the time of production and hot rolling, so the upper limit value was 1%. [0057] Cr: Cr is a reinforcing element and is important for improving hardening ability. However, when the content is less than 0.05%, these effects cannot be obtained, so the lower limit value was 0.05%. In contrast, when the grade exceeds 1%, it has an adverse effect on production capacity at the time of production and hot rolling, so the upper limit value was 1%. [0058] Ni: Ni is a reinforcing element and is important for improving hardening ability. However, when the content is less than 005%, these effects cannot be obtained, so the lower limit value was 0.05%. In contrast, when the grade exceeds 1%, it has an adverse effect on production capacity at the time of production and hot rolling, so the upper limit value was 1%. In addition, it can be added to improve wetting ability and promote bonding reaction. Cu: Cu is a reinforcing element and is important for improving hardening ability. However, when the content is less than 0.05%, these effects cannot be obtained, so the lower limit value was 0.05%. In contrast, when the grade exceeds 1%, it has an adverse effect on production capacity at the time of production and hot rolling, so the upper limit value was 1%. In addition, it can be added to improve wetting ability and promote bonding reaction. B is effective for reinforcing grain edges and reinforcing steel by adding 0.0001 wt% or more, but when its amount of addition exceeds 0.01 wt%, not only the effect of the addition becomes saturated as the production capacity at hot rolling is decreased, so its upper limit was 001%.
Ti: Ti is a reinforcing element. It helps to increase the strength of the steel plate through reinforcement of precipitation, reinforcement of grain refining due to inhibition of growth of ferrite crystal grains, and reinforcement of displacement due to inhibition of recrystallization. When the amount of addition is less than 0.005%, these effects cannot be obtained, so the lower limit value was 0.005%. When the content exceeds 0.3%, the carbonitride precipitation increases and the conformability tends to deteriorate, so the upper limit was 0.3%.
Nb: Nb is a reinforcing element. It helps to increase the strength of the steel plate by reinforcing precipitation, reinforcing grain refining due to inhibition of ferrite crystal grain growth, and reinforcing displacement due to inhibition of recrystallization. When the amount of addition is less than 0.005%, these effects cannot be obtained, so the lower limit value was 0.005%. When the content exceeds 0.3%, the carbonitride precipitation increases and the conformability tends to deteriorate, so the upper limit was 0.3%.
[0063] V: V is a reinforcing element. It helps to increase the strength of the steel plate by reinforcing precipitation, reinforcing grain refining due to inhibition of growth of ferrite crystal grains, and reinforcing displacement due to inhibition of recrystallization. When the amount of addition is less than 0.005%, these effects cannot be obtained, so the lower limit value was 0.005%. When the content exceeds 05%, the carbonitride precipitation increases and the conformability tends to deteriorate, so the upper limit was 0.5%.
One or two or more elements selected from Ca, Mg, and REM may be added by 0.0005 to 0.04% in total. Ca, Mg, and REM are elements used for deoxidation and one or two or more of these elements at 0.0005% or more are preferably contained in total. REM indicates a rare earth metal. However, when the content exceeds 0.04% in total, this may cause deterioration of the forming capacity. Therefore, the total content of the elements is 0.0005 to 0.04%. In addition, the present invention, REM is generally added in a metal misch, which in addition to La and Ce may also contain other elements of the lantanoid series in combination. The effects of the present invention are presented even when elements of the lantanoid series other than La and Ce are contained as unavoidable impurities. However, the effects of the present invention are presented even when metals such as La and Ce are added. (Chemical Composition of Hot-Dip Galvanized Layer and Bonded Hot-Dip Galvanized Layer) In the present invention, a hot-dip galvanized layer formed on the surface of the base steel sheet contains less than 7% by weight. Fe, the balance being Zn, Al and the inevitable impurities. In addition, a hot-dip galvanized bonded layer contains 7 to 15% by weight of Fe, the balance being Z, Al and the inevitable impurities. In addition, when the base steel plate is subjected to hot dip galvanizing treatment by dipping in a hot dip galvanizing bath, a hot dip galvanized layer containing less than 7% by weight of Fe is formed in the base steel plate surface. In addition, after the galvanizing treatment, when the bonding treatment is subsequently performed, a hot-dip galvanized bonded layer containing 7 to 15% by weight of Fe is formed on the surface of the base steel plate.
Depending on the presence or absence of the bonding treatment, the galvanized layer is formed of zinc or Fe-zinc alloy. Zinc oxide may be contained on the surface of the galvanized layer, but when the Fe content (%) contained in the galvanized layer is within the range of the present invention, the effect of the present invention may be obtained. In addition, since the base steel plate of the present invention contains Si, Mn or Al, although the oxide formed during annealing may exist at an edge between the base steel plate and the galvanized layer or exist in the galvanized layer, The effect of the present invention is presented in both cases.
Where spot welding capability and coating property are desired, it is possible to improve these properties by forming a hot-dip galvanized bonded layer containing 7 to 15 mass% Fe on the surface of the sheet. steel base. Specifically, when the base steel plate is bonded after being dipped in the galvanizing bath, Fe is incorporated into the galvanized layer, and thus a high strength alloy hot dip galvanized steel sheet can be obtained having excellent coating property and spot welding capability. When the Fe content after bonding treatment is less than 7% by mass, the spot welding capability becomes insufficient. On the other hand, when the Fe content exceeds 15% by mass, the adhesion of the galvanized layer is impaired, and the galvanized layer is broken and fractured and removed in machining, thus causing scratches when forming by adhering to a mold. Accordingly, the Fe content contained in the galvanized layer during the bonding treatment is within a range of 7 to 15% by mass.
In addition, in a case where the bonding treatment is not performed, even when the Fe content of the galvanized layer is less than 7% by mass, corrosion resistance, conformability and Bore expansions that are effects obtained by bonding are good except spot welding.
In addition, the galvanized layer may contain Al, Mg, Mn, Si, Cr, Ni, Cu or the like in addition to Fe.
To measure the Fe and Al content contained in the galvanized layer, a method of dissolving the galvanized layer with an acid and chemically analyzing the dissolved solution can be used. For example, for the 30mm x 40mm hot-dip galvanized bonded steel sheet cut, only the galvanized layer is dissolved while suppressing the elution of the base steel sheet with a 5% aqueous HCI solution with added inhibitor. . Then the Fe and Al content is quantified using the signal intensities obtained by the ICP emission analysis of the dissolved solution and the calibration curve prepared from solutions of known concentrations. Moreover, in consideration of the measured variation of the samples, an average value obtained by measuring at least three samples which are cut from the same hot-dip galvanized steel sheet is employed.
The coated amount of the coating is not particularly limited, but is preferably 5 g / m2 or more in the coated amount on a single surface of the base steel sheet from the point of view of corrosion resistance. In addition, the coated amount of the single surface is preferably no more than 100 g / m2 from the point of view of ensuring coating adhesion. (Steel Sheet Production Method) To obtain a steel sheet having a small anisotropy of the material of 980 MPa or more in the present invention, it is important to provide a steel sheet in which the formation of a specific texture is suppressed. . Hereinafter, details of the production conditions will be described to simultaneously satisfy these factors.
The production method before hot rolling is not particularly limited. That is, subsequent to melting by a vat furnace, an electric furnace, or the like, secondary refining can be performed in a variety of ways, and then the casting can be performed by normal continuous casting, or by the ingot method, or by casting of thin plates, or similar. In the case of continuous casting, it is possible for a continuous casting plate to be cooled once to a low temperature and then reheated for hot rolling, or it is also possible for the continuous casting plate to be rolled. hot continuously. Scrap can also be used for a steel raw material. (First Hot Rolling) The plate extracted from a heating furnace is subjected to a roughing rolling process, the first hot rolling being the roughing rolling, and thus a rough bar is obtained. The steel plate of the present invention must meet the following requirements. Initially, the diameter of the austenite grain after roughing lamination, ie the diameter of the austenite grain before finishing lamination is important. The austenite grain diameter prior to finishing lamination is desirably small, and an austenite grain diameter of 200 μηι or less greatly contributes to fine-graining the crystal grains and to the homogenization of the crystal grains.
To obtain an austenite grain diameter of 200 μιη or less prior to finishing lamination, lamination must be performed at a reduction ratio of 40% or more once or more in the roughing lamination in a temperature region. from 1000 to 1200 ° C.
The grain diameter of the austenite before finishing lamination is desirably 160 μιη or less or 100 μιη or less, and to obtain this grain diameter, 40% or more twice or more lamination is performed. However, in roughing lamination, when the reduction is greater than 70% or the lamination is performed more than 10 times, there is a concern that the lamination temperature decreases or the scale is excessively generated.
The edge of the austenite grain after rough rolling (i.e. before finishing rolling) is supposed to function as a recrystallization core during finishing rolling. The diameter of the austenite grain after roughing lamination is confirmed so that the sheet steel part before undergoing the finishing lamination is cooled as quickly as possible (ie cooled to 100 / s or more, for example). , and the cross section of the sheet steel part is caused to make the edges of the austenite grains appear, and the edges of the austenite grains are observed by an optical microscope. At this time, at magnifications of 50 times or more, the austenite grain diameter of 20 fields of view is measured by image analysis or the dot count method. (Second Hot Rolling) After the roughing rolling process (first hot rolling) is completed, the finishing rolling process which is the second hot rolling is started. The time between the end of the roughing lamination process and the beginning of the finishing lamination is desirably set to 150 seconds or less.
In the finishing lamination process (second hot rolling), the start temperature of the finishing lamination is desirably set to 1000Ό or higher. When the start temperature of the finishing lamination is less than 1000Ό at each pass of the finishing lamination, the temperature of the lamination to be applied to the blank to be rolled is decreased, the reduction is performed in a region of non-recrystallization temperatures. , texture develops, and so isotropy deteriorates.
Incidentally, the upper limit of the start temperature of the finishing lamination is not limited in particular. However, when it is 1150X3 or larger, a bubble that is the starting point of a scaly-axis scale defect is likely to occur between the base steel plate iron and the surface scale prior to finishing lamination and between passes, and thus the starting temperature of the finishing lamination is desirably lower than 100 ° C.
In finishing lamination, the temperature determined by the chemical composition of the steel plate is set to T1, and in the temperature region of T1 + 30Ό or more and T1 + 200Ό minus 30% or more rolling is performed at one pass at least once. In addition, in finishing lamination, the total reduction ratio is set to 50% or more. Satisfying this condition, in the range 5/8 to 3/8 in plate thickness from the surface of the steel plate, the average value of the pole densities of the orientation groups {100} <011> to {223} < 110> becomes 6.5 minus and the pole density of the crystal orientation {332} <113> becomes 5.0 or less. Thus, the high strength steel plate having small anisotropy of the material can be obtained.
[0082] Here, T1 is the temperature calculated by Expression (1) below. T1 (° C) = 850 + 10 x (C + N) x Mn + 350 x Nb + 250 x Ti + 40 χ B + 10 x Cr + 100 x Mo + 100 χ V ··· Expression (1) [0083 ] C, N, Mn, Nb, Ti, B, Cr, Mo, and V each represent the content of each element (% by mass). Also, Ti, B, Cr, Mo, and V are calculated as zero when not contained.
A heavy reduction in the temperature range of T1 + 30 ° C or higher and T1 + 150 ° C or lower and a slight reduction in a temperature range between T1 and less than T1 + 30Ό further controls the temperature. average value of the pole densities of the orientation groups {100} <011> to {223} <110> and the pole density of the crystal orientation {332} <113> in the range 5/8 to 3/8 in the thickness of the sheet metal from the surface of the steel shell, and thus the anisotropy of the final product material is dramatically improved, as indicated in Tables 2 and 3 of the Example to be described later.
This temperature T1 is obtained empirically. The present inventors have learned empirically from the experiments that recrystallization in the austenite region of each steel is promoted based on the T1 temperature. For better material uniformity, it is important to build up stress through heavy reduction, and the total reduction ratio of 50% or more is essential in finishing lamination. In addition, it is desired to take the reduction of 70% or more and, on the other hand, when the reduction ratio greater than 90% is taken, ensuring the temperature and an excessive rolling load is as the added result.
When the total reduction ratio in the temperature region of T1 + 30Ό or more and T1 + 200Ό or less is less than 50%, the rolling stress to be accumulated during hot rolling is not sufficient, and recrystallization of austenite does not advance sufficiently. Therefore, texture develops and isotropy deteriorates. When the total reduction ratio is 7% or more, sufficient isotropy may be obtained even if variations in temperature fluctuation or the like are considered. On the other hand, when the total reduction ratio exceeds 90%, it becomes difficult to get the temperature region of T1 + 200 ° C at least due to heat generation by the work, and also the rolling load increases to cause the risk. that lamination becomes difficult to perform.
In finishing lamination, to promote uniform recrystallization caused by the release of accumulated stress, lamination at 30% or more is performed at one pass at T1 + 30 ° C or more and T1 + 200 ° C. or less. Incidentally, to accelerate uniform recrystallization by releasing the accumulated voltage, it is necessary to suppress as much work as possible in a temperature range of less than T1 + 30 T1 after heavy reduction to T1 + 30 ° C or more. and T1 + 200 ° C or less. For this reason, the reduction ratio less than T1 + 30Ό is desirably 30% or less. The reduction ratio of 10% or more is desirable in terms of improving the shape of the sheet, but the reduction ratio of 0% is desirable in case the hole expandability is also focused. In addition, when the reduction ratio below T1 + 30Ό is large, the recrystallized austenite grains are expanded, and when the retention time after finishing lamination is short, recrystallization does not proceed sufficiently, and the anisotropy of the material. becomes big. That is, under the production conditions of the present invention, when austenite is uniformly and finely recrystallized in the finishing lamination, the texture of the product is controlled and the anisotropy of the material is improved.
The rolling ratio can be obtained by actual performance or by calculating from the rolling load, sheet thickness measurement, or / and the like. The temperature can actually be measured by a thermometer between the chairs or can be obtained by calculating simulation taking into account the heat generation by the work from the line speed, reduction ratio or the like. Alternatively, it is possible to be obtained by both.
Hot rolling (first hot rolling and second hot rolling) as described above is terminated at a temperature of transformation temperature Ar3 or higher. When the hot rolling is completed at Ar3 or less, the hot rolling becomes a rolling in the two-phase region of austenite and ferrite, and the buildup for the orientation groups {100} <011> to {223} <110 > becomes strong. As a result, anisotropy of the material is promoted. (Cooling before cold rolling) After the final reduction to a reduction ratio of 30% or more is performed on the finishing rolling, cooling before cold rolling is started so that the waiting time t seconds satisfies a Expression (2) below, t <2.5 x t1 ··· Expression (2) [0092] Here, t1 is obtained by Expression (3) below. t1 = 0.001 x ((Tf-T1) x P1 / 100) 2-0.109 x ((Tf-T1) x P1 / 100) + 3.1 ··· Expression (3) [0093] Here, in Expression (3 ) above, Tf represents the temperature of a steel bar obtained after the final reduction at a reduction ratio of 30% or more, and P1 represents the reduction ratio of the final reduction to 30% or more.
Incidentally, the "final reduction at a reduction ratio of 30% or more" indicates the lamination finally performed in lamination processes whose reduction ratio becomes 30% or more among lamination processes in a plurality of passes performed. in finishing lamination. For example, in lamination processes in a plurality of passes performed in the finishing lamination when the lamination reduction ratio performed in the final step is 30% or more, the lamination performed in the final step is the “final reduction to a reduction ratio. 30% or more. ” In addition, in lamination processes in a plurality of passes performed in the finishing lamination, when the lamination reduction ratio performed before the final step is 30% or more and after lamination performed before the final step (lamination at a ratio of 30% or more reduction) is performed, the lamination whose reduction ratio becomes 30% or more is not performed, the lamination performed before the final step (lamination at a reduction ratio of 30% or more) is the “reduction reduction ratio of 30% or more ”.
In finishing lamination, after final reduction to a reduction ratio of 30% or more is performed, the time delay t seconds until cooling before cold rolling begins greatly affects the austenite grain diameter. and strongly affects structure after cold rolling and annealing. When the hold time t exceeds t1 x 2.5, grain stiffness progresses and elongation is markedly reduced.
The wait time t seconds also satisfies Expression (2a) below, thus making it possible to preferentially suppress the growth of the crystal grains. Consequently, although recrystallization does not advance sufficiently, it is possible to sufficiently improve the steel sheet elongation and improve the fatigue property simultaneously, t <t1 ··· Expression (2a) At the same time, the holding time t seconds also satisfies Expression (2b) below, and thus recrystallization proceeds sufficiently and the crystal orientations are random. Therefore, it is possible to sufficiently improve the elongation of the steel sheet and greatly improve the isotropy simultaneously. t1 <t <t1 x 2.5 ··· Expression (2b) Here, as shown in FIG. 3, in a continuous hot rolling line 1, the steel bar (plate) heated to a predetermined temperature in the heating furnace is rolled into a roughing rolling mill 2 and a finishing rolling mill 3 sequentially to be a steel sheet. hot-rolled steel 4 having a predetermined thickness, and hot-rolled steel sheet 4 is conveyed on an exit table 5. In the production method of the present invention, the roughing (first hot rolling) rolling process performed on the rolling mill For roughing 2, rolling at a reduction ratio of 40% or more is performed on the steel bar (plate) once or more in the temperature range of 1000Ό or higher and 1200Ό or lower.
The blank bar rolled to a predetermined thickness in the roughing mill 2 in this manner is then rolled into the finishing lamination (second hot rolling) through a plurality of lamination chairs 6 of the finishing mill 3 to be the sheet. hot-rolled steel 4. Then, in finishing mill 3, 30% or more rolling is performed in one pass at least once in the temperature region of T1 + 30 ° C or higher and T1 + 200Ό or lower. Also, in the finishing mill 3, the total reduction becomes 50% or more.
In addition, in the finishing lamination process, after the final reduction to a reduction ratio of 30% or more is performed, cooling before cold rolling is initiated such that the wait time t seconds satisfies Expression (2) above or one of Expressions (2a) or (2b) above. The initiation of this cooling before cold rolling is performed by cooling nozzles between the chairs 10 arranged between two respective laminating chairs 6 of the finishing mill 3, or cooling nozzles 11 arranged on the outlet table 5.
For example, when the final reduction to a reduction ratio of 30% or more is performed only on the lamination chair 6 arranged in the initial step of the finishing mill 3 (on the left side of FIG. 3, on the front side of lamination) and the lamination whose reduction ratio becomes 30% or more is not performed on the lamination chair 6 arranged in the final step of the finishing laminator 3 (on the right side of FIG. 3, on the rear side of the lamination), when the The start of cooling before cold rolling is performed by the cooling nozzles 11 arranged on the output table 5, in which case the wait time t seconds does not satisfy Expression (2) above or Expressions (2a) and (2b) above is sometimes provoked. In such a case, the cooling before cold rolling is initiated by the cooling nozzles between the chairs 10 disposed between two of the respective laminating chairs 6 of the finishing mill 3. In addition, for example, when the final reduction to a reduction ratio of 30% or more is performed on the lamination chair 6 disposed in the final step of the finishing laminator 3 (on the right side in FIG. 3, on the post-lamination side), although the start of cooling prior to lamination a If the cooling is performed by the cooling nozzles 11 arranged on the exit table 5, there is sometimes the case that the wait time t seconds may satisfy Expression (2) above or Expressions (2a) and (2b) above. In this case, cooling before cold rolling can also be initiated by the cooling nozzles 11 arranged on the outlet table 5. Needless to say, since the final reduction performance at a reduction ratio of 30% or more is In addition, primary cooling prior to cold rolling can also be initiated by the cooling nozzles between the chairs 10 arranged between two of the respective laminating chairs 6 of the finishing mill 3.
Then, in this cooling before cold rolling, cooling is performed at an average cooling rate of OG / s or more, the temperature change (temperature drop) becomes 40Ό or more and 140Ό or less.
When the temperature change is less than 40 ° C, the recrystallized austenite grains grow and the low temperature toughness deteriorates. The temperature change is adjusted to 40Ό or more, thus making it possible to suppress the austenite grain stiffening. When the temperature change is less than 40Ό, the effect cannot be obtained. On the other hand, when the temperature change exceeds 140Ό, recrystallization becomes insufficient to make it difficult to obtain the desired random texture. Moreover, the effective ferrite phase for stretching is not easily obtained either and the hardness of a ferrite phase becomes high and therefore the conformability also deteriorates. In addition, when the temperature change is greater than 140 ° C, an error at / below the temperature of the transformation point Ar 3 is likely to be caused. In this case, even by transformation from recrystallized austenite, as a result of the improved selection of variants, the texture is formed and consequently the isotropy decreases. When the average cooling rate before pre-cold rolling is slower than 60 ° / s, as expected, the recrystallized austenite grains grow and the low temperature toughness deteriorates; The upper limit of the average cooling rate is not determined in particular, but in terms of the shape of the steel sheet, ΣΟΟΌ / β or less is considered to be adequate.
In addition, as previously described, to promote uniform recrystallization, it is preferable that the amount of work in a temperature region of less than T1 + 30 ° C be as small as possible and the reduction ratio in the region of temperatures less than T1 + 30 ° C are 30% or less. For example, in the finishing laminator 3 of the continuous hot rolling line 1 illustrated in FIG. 3, when passing through one or two or more lamination chairs 6 arranged on the front side (left side in FIG. 3, front side of the lamination), the steel sheet is in a temperature region of T1 + 30 ° C or larger and T1 + 200 ° C or lower, and when passing through one, two or more lamination chairs 6 arranged on the rear side (right side in FIG. 3, rear side of the lamination), the steel cover is at a temperature less than T1 + 30 ° C. When passing through one or two or more lamination chairs 6 arranged in the rear step (right side in FIG. 3, rear side of the lamination) the reduction is not performed, or the reduction is performed, the reduction ratio unless T1 + 30Ό is preferably 30% or less in total. In terms of precision of sheet thickness and sheet shape, the reduction ratio less than T1 + 30Ό is preferably 10% or less in total. If more isotropy is required, the reduction ratio in the region of temperatures lower than T1 + 30 ° C is preferably 0%.
In the production method of the present invention, the rolling speed is not limited in particular. However, when the lamination speed on the final chair side of the finishing lamination is less than 400 mpm, γ grains grow to become crude, regions in which ferrite can be precipitated for ductility are reduced, and thus ductility is likely to deteriorate. Although the upper limit of the rolling speed is not limited in particular, the effect of the present invention can be obtained, but it is reasoned that the rolling speed is 1800 mpm or less due to equipment restriction. Therefore, in the finishing lamination process, the lamination speed is preferably 400 mpm or less and 1800 mpm or less. In addition, in hot rolling, the finishing rolling can be performed continuously by joining the bar (rough bar) after the roughing lamination. At this point, the raw bar is reeled once in coil form and is stored in a cover having thermal insulation if necessary. Then the raw bar can be turned on after rewinding. (Coiling) After being obtained in this way, the hot-rolled steel sheet can be coiled at 700Ό or less. When the coiling temperature exceeds 700 ° C, crude ferrite or perlite structure occurs in the hot-rolled structure and the lack of structural homogeneity after annealing increases. As a result, the anisotropy of the final product material is increased. In addition, when the hot-rolled steel plate is coiled at a temperature exceeding 700Ό, the thickness of the oxide formed on the steel plate surface increases excessively and stripping is difficult. Although the lower limit of winding temperature is not defined in particular, the effects of the present invention are presented. However, since it is technically difficult to wind at room temperature or below, room temperature is substantially the lower limit. (Pickling) Pickling is performed on hot-rolled steel plate prepared in this manner. Stripping is an important process for removing oxide on the surface of the base steel plate and for improving the coating property. In addition, stripping can be performed once or several times. (Cold Rolling) [00110] The following cold rolling is performed on the hot rolled steel sheet after pickling at a reduction ratio of 30 to 80%. In the case that the reduction ratio is less than 40%, it is difficult to maintain the flattened shape. Moreover, in this case, since the ductility of the end product is deteriorated, the lower limit of the reduction ratio is 40%. On the other hand, when cold rolling is performed at a reduction ratio exceeding 80%, the cold rolling load is excessively large and cold rolling is difficult to perform. For this reason, the upper limit of the reduction ratio is 80%. More preferably, the reduction ratio is in the range of 45 to 70%. The effects of the present invention may be exhibited without particularly defining the number of lamination passes and the reduction ratio of each pass. (Hot dip galvanizing) After cold rolling, the base steel shell is hot dip galvanized through a continuous hot dip galvanizing (CGL) line. (Annealing) [00112] The steel plate (base steel plate) that has been cold rolled is then heated to the annealing temperature of 750 to 900Ό in the hot dip galvanizing line. When the annealing temperature is below 750 ° C, a carbide re-melt formed during hot rolling requires a long time, all or part of the carbide remains, and thus it is difficult to guarantee resistance of 980MPa or more. For this reason, the lower annealing temperature limit is 750Ό. On the other hand, since heating to an excessive temperature leads to an increase in cost, it is economically unfavorable and the shape of the steel plate becomes poor or the service life of the cylinder is reduced. Therefore, the upper limit of annealing temperature is 900Ό. Retention time at annealing temperature is not limited in particular, but heat treatment is preferably performed for 10 seconds or more to dissolve the carbide. On the other hand, when the heat treatment time becomes longer than 600 seconds, it leads to the economically unfavorable cost increase. The effects of the present invention can be exhibited by performing isothermal retention at annealing temperature from 750 to 900 ° C and even starting to cool immediately after the steel plate reaches maximum temperature by performing gradient heating.
By heating the base steel plate to annealing temperature, the average heating rate from room temperature to 650Ό or less is adjusted to HR1 (Ό / s) expressed by Expression (4) below, and the rate Heating average from temperature exceeding 650Ό to annealing temperature is adjusted to HR2 (O / s) expressed by Expression (5) below. HR1> 0.3 ··· Expression (4) HR2 <0.5 x HR1 ··· Expression (5) [00114] Hot rolling is performed under the condition described above, and also cooling before cold rolling is executed. Thus, both crystal grain refining and the random school of crystal orientations are achieved. However, by cold rolling to be performed later, the strong texture develops and the texture becomes liable to remain in the steel plate. As a result, the isotropy of the steel plate decreases. Thus, it is preferred to make the texture, which has developed by cold rolling, disappear as much as possible by properly performing the heating to be performed after cold rolling. For this reason, it is necessary to divide the average heating rate into two steps expressed by Expressions (4) and (5) above.
The detailed reason why the texture and properties of the base steel plate is improved by this two-step heating is unclear, but this effect is considered to be related to the recovery of displacement and the recrystallization introduced at the time of rolling. cold. That is, the driving force of recrystallization that occurs in the steel plate upon heating is the stress accumulated on the steel plate by cold rolling. When the average heating rate HR1 in the temperature range from room temperature or higher to 650 ° C or lower is small, the displacement introduced by cold rolling is restored and recrystallization does not occur. As a result, the texture that developed at the time of cold rolling remains as is and properties, such as isotropy, deteriorate. When the average heating rate HR1 in the temperature range from room temperature or higher to 650Ό or lower is less than Ο, βΌ / β, the displacement introduced by cold rolling re-establishes, resulting in the strong texture formed in the moment of cold rolling remains. Therefore, it is necessary to adjust the HR1 average heating rate in the temperature range from room temperature or higher to 650Ό or lower to 0.3 CC / s) or higher. When the average heating rate HR1 is 0.3 (C / s) or more, it is possible to recrystallize from the ferrite (recovery of displacement is slow) having a large displacement density, recrystallized grains are formed having different Crystal orientations, texture is random, and thus anisotropy is reduced. In addition, when the heating rate exceeds 100 (C / s), investment in equipment becomes excessive, and thus is economically unfavorable. Therefore, the upper limit of the HR1 average heating rate is substantially 100 (C / s).
On the other hand, when the average HR2 heating rate from temperature exceeding 650C to annealing temperature is large, the ferrite that exists in the steel sheet after cold rolling does not recrystallize and the non-recrystallized ferrite in state of being worked remains. When steel containing more than 0.1% C in particular is heated to a two-phase region of ferrite and austenite, the formed austenite inhibits the growth of recrystallized ferrite, and thus unrecrystallized ferrite becomes more likely to remain. . This unrecrystallized ferrite has a strong texture to adversely affect isotropy, and this unrecrystallized ferrite contains many displacements to dramatically deteriorate ductility. For this reason, in the temperature range exceeding 650C to annealing temperature, the average heating rate HR2 must be 0.5 x HR1 (° C / s) or less. When the average heating rate HR2 exceeds 0.5 x HR1 (° C / s), the carbide becomes austenite before recrystallization, and the formed austenite grains retard the growth of the recrystallized grains. As a result, the texture in a state of being cold rolled remains, and thus anisotropy increases.
From the results obtained by careful investigation of the relationship between production conditions and texture in detail, the inventors found that texture randomness and anisotropy reduction can be achieved when HR1 is twice or more the value of HR2. It is difficult to obtain texture randomness by controlling such a heating rate by a conventional annealing in which the heating rate is constant. (Cooling and Annealing) [00118] After being annealed, the base steel plate is cooled from annealing temperature to 50013 at an average cooling rate of 0.1 to 20013/8. When the average cooling rate is slower than 0.133 / s, productivity is greatly impaired. On the other hand, when the cooling rate increases excessively, the cost of production increases. Consequently, the upper limit of the average cooling rate is 20013/5. In addition, the cooling rate in the temperature region of 650 to 50013 is preferably 3 to 20013 / s. When the cooling rate is too slow, the austenite structure is transformed into perlite structure in the cooling process. Since it is difficult to guarantee austenite of 8% or more per volume fraction, the cooling rate is preferably 313 / s or faster. Examples of a cooling method may include cylinder cooling, air cooling, water cooling, and any combination of these cooling methods. (Temperature Hold) Thereafter the temperature is held between 500 and 35013 for 10 to 1000 seconds. In the process of temperature maintenance the transformation of bainite occurs and the residual austenite is stabilized. The reason why the upper retention temperature limit is 500Ό is because bainite transformation occurs at or below this temperature. On the other hand, when the temperature is kept in the region of temperatures below 350 ° C, it takes a long time for bainite transformation to take place, the installations are excessive, and thus productivity is decreased. Consequently, the holding temperature is 500 to 350Ό. The lower limit of retention time is 10 seconds. The reason is because bainite transformation does not progress sufficiently to a retention of less than 10 seconds, residual austenite is not stabilized, and excellent conformability is not obtained. On the other hand, a hold that exceeds 1000 seconds degrades productivity. In addition, retention not only indicates isothermal retention, but also includes cold removal and heating in that temperature region. (Hot dip galvanization and hot dip galvanization on) [00120] The cold-rolled steel plate (base steel plate) produced in this way is then dipped into a hot dip galvanization bath and subjected to a hot dip galvanizing treatment so that the high strength hot dip galvanized steel sheet of the present invention is produced. In addition, after the galvanizing treatment, when the bonding treatment is subsequently performed, the high strength alloy hot dip galvanized steel sheet of the present invention is produced.
Preferably, the temperature of the base steel plate to be dipped in the hot dip galvanizing bath is in a range from a temperature of less than 40 ° C compared to the temperature of the galvanizing bath to a higher temperature. 50Ό compared to the temperature of the hot dip galvanizing bath. When the temperature of the base steel layer to be dipped is below the “galvanizing bath temperature - 40” (Ό). The heat loss in the dip in the galvanizing bath becomes large and some of the molten zinc is solidified, thus leading to deterioration of the galvanized external appearance in some cases. Before being dipped in the galvanizing bath, the base steel sheet may be dipped by reheating the sheet to a temperature of (galvanizing bath temperature - 40 ° C) or higher. In addition, when the temperature of the base steel sheet is above the “hot dip galvanizing bath temperature + 50) Ό. Operational problems associated with increasing the temperature of the galvanizing bath are induced.
In addition, the bonding treatment of the galvanized layer is performed at 460Ό or more. When the temperature of the binding treatment is less than 460 ° C, the binding progress is slowed and productivity is decreased. The upper limit is not limited in particular, but when the temperature of the bonding treatment is above 600Ό, the carbide is formed and the volume fraction of a hard structure (martensite, bainite, residual austenite) is reduced so that it is difficult to guarantee excellent ductility. Therefore, the upper limit is substantially 600Ό.
To suppress galvanizing faults and promote bonding, it is preferable that a jet stream of 10 m / min or more and 50 m / min or less be provided in the galvanizing bath. The slag, which is a Zn or Al oxide film, floats on the surface of the galvanizing bath. When the oxide film remains on the surface of the base steel plate in large quantities, the slag adheres to the surface of the base steel plate at the time of immersion in the galvanizing bath and galvanizing faults occur easily. In addition, the slag adhering to the steel sheet causes not only galvanization failures but also delays in connection.
[00124] This property is particularly noticeable on steel sheet which contains a lot of Si and Μη. The detailed mechanism is unclear, but it is considered that galvanization failures and bonding delay are facilitated by the reaction between Si oxide and Mn, which is formed on the surface of the base steel plate, and slag as oxide as well. The reason for setting the flow rate to 10 m / min or more and 50 m / min or less is because the effect of suppressing galvanizing faults due to jet stream cannot be obtained at a flow rate of less than 10 m / min The reason for setting the flow rate to 50 m / min or less is because the suppression effect of galvanizing faults is saturated and a high cost due to excessive investment in equipment is also avoided.
The purpose of adjusting the flow rate of molten zinc in the bath to 10 m / min or more and 50 m / min or less is to prevent sludge from adhering to the surface of the base steel plate. For this reason, it is mainly preferable that the flow rate be within the range above to the depth of the base steel plate which is dipped in the galvanizing bath. Meanwhile, the sludge may be deposited on the bottom of the galvanizing bath in some cases. In this case, when molten zinc near the bottom of the bath flows, it is increasingly worrying that sludge will adhere to the surface of the base steel plate by the spatter of the deposited sludge. Thus, the flow rate is preferably adjusted for a region from the surface of the galvanizing bath to the depth of the base plate that is dipped in the galvanizing bath. The size of the galvanizing bath can be any width as long as the base steel plate can be dipped, but the size of the exterior steel plate is generally up to about 2 m wide. The size of the galvanizing bath may be sufficiently larger than the size above. Once the sludge is deposited at the bottom of the galvanizing bath, zinc flows into the bath through the passage plate. And so there is concern that the sludge will adhere to the surface of the base steel plate by the sludge splash. Therefore, the depth of the bath is preferably deep. In addition, the galvanizing bath may contain Fe, Al, Mg, Mn, Si, Cr and the like in addition to pure zinc.
In addition, to also improve coating adhesion, prior to annealing in the continuous hot dip galvanizing line, the base steel sheet may be subjected to coating treatment using materials consisting of single metal or a plurality of metals between Ni, Cu, Co or Fe. In addition, examples of the coating treatment include a Sendzimir method of “degreasing, stripping, and then heating in a non-oxidizing atmosphere, annealing under a reducing atmosphere containing Fk or N2 and then cool to near the temperature of the galvanizing bath, and plunge into the galvanizing bath ”, the total reduction furnace method of“ adjusting the atmosphere at annealing to initially oxidize the surface of the steel plate, then use the reduction to perform cleaning prior to coating, and soaking in a coating bath ”, or a flow method of“ degreasing and stripping the steel sheet, use en ammonium chloride or similar for flow treatment, and then dive into the galvanizing bath. ” However, the present invention may be presented even when the treatment is performed under any conditions.
In addition, in the case of production of the hot-dip galvanized steel sheet, the effective concentration of Al in the galvanizing bath is preferably controlled within the range of 0.05 to 0.500 mass% to control the properties of the layer. galvanized. Here, the effective Al concentration in the galvanizing bath is a value obtained by subtracting the Fe concentration in the galvanizing bath from the Al concentration in the galvanizing bath.
When the effective Al concentration is less than 0.05% by mass, sludge occurs significantly and good appearance cannot be obtained. On the other hand, when the effective Al concentration is greater than 0.500% by mass, binding is retarded and productivity is decreased. For this reason, the upper limit of the effective Al concentration in the galvanizing bath is preferably 0.500% by mass.
In addition, when binding is performed at a low temperature, binding treatment can be used to facilitate transformation of the bainite.
Meanwhile, to improve the coating property and weldability, the surfaces of the hot dip galvanized steel sheet and the hot dip galvanized steel sheet of the present invention are coated with the top layer. and to a variety of treatments, for example, a chromate treatment, a phosphate treatment, an oiliness improvement treatment, a weldability improvement treatment, or the like.
In addition, the hot dip galvanized steel sheet and the hot dip galvanized steel sheet can also be subjected to skin pass rolling. The reduction ratio of skin pass lamination is preferably in a range from 0.1 to 1.5%. When the reduction ratio is less than 0.1%, the effect is small and control is also difficult. When the reduction ratio exceeds 1.5%, productivity is noticeably decreased. Skin pass lamination can be performed on the galvanizing line or outside the galvanizing line. In addition, the desired reduction ratio of skin pass lamination can be performed at one time or several times.
[Example (s)] [00133] The present invention will now be described in detail by way of examples. Incidentally, the conditions of the examples are exemplary conditions employed to confirm the applicability and effects of the present invention, and the present invention is not limited to those exemplary conditions. The present invention may employ various conditions provided that the object of the present invention is achieved without departing from the essence of the present invention. The chemical compositions of the respective steels used in the examples are shown in Table 1. The respective production conditions are illustrated in Tables 2 and 3. In addition, the structural constitutions and mechanical properties of the respective steel types under the production conditions of Table 2 are illustrated in Table 4. Incidentally, the underline in each Table indicates that the numerical value is outside the range of the present invention or is outside the range of a preferred range of the present invention.
Examination results will be described using steels of the invention "A to S" and using comparative steels "aad" having the compositions illustrated in Table 1. Incidentally, in Table 1, each numerical value of the chemical compositions indicates mass%. In Tables 2 to 4, the letters A to U and the letters a to g, which are linked to the steel types, indicate compositions of the steels of the invention A to U and the comparative steels a to g in Table 1, respectively.
These steels (steels of the invention A to S and comparative steels a and d) were heated to 1200 ° C and then hot rolled under the conditions given in Table 2, and thereafter the hot rolling was terminated at a transformation temperature. Ar3 or higher.
In hot rolling, roughing rolling as the first rolling, rolling was performed once or more times at a reduction ratio of 40% or more in a temperature region of 1000Ό or higher and 1200Ό or lower. However, for steel types A2, C2, E2, J2, and 02, in roughing rolling, rolling at a reduction ratio of 40% or more in one pass was not performed. For roughing lamination, the number of times the reduction to a reduction ratio of 40% or more, each reduction ratio (%), and the austenite grain diameter (pm) after the roughing lamination (before the lamination of In addition, the temperature T1 (O) of the respective steel types is shown in Table 2.
After the roughing lamination was completed, the finishing lamination was performed as a second lamination. In finishing lamination, lamination at a reduction ratio of 30% or more was performed in one pass at least once in a temperature region of T1 + 30 ° C or higher and T1 + 200 ° C or lower, and at At a temperature range below T1 + 30 ° C, the total reduction ratio has been adjusted to 30% or less. Incidentally, in finishing lamination, lamination at a reduction ratio of 30% or more on one pass was performed on a final pass in the region of temperatures of T1 + 30 ° C or higher and T1 + 200 ° C or lower.
However, for steel types C3, E3, J3, and 03, rolling at a reduction ratio of 30% or more was not performed in the temperature range T1 + 30 ° C or higher and T1 + 200 ° C or lower. In addition, for steel types A4 and C4, the total reduction ratio in the temperature range below T1 + 30 ° C was greater than 30%.
In addition, in finishing lamination, the total reduction ratio has been adjusted to 50% or more. However, for steel types A3, C3, E3, J3, and 03, the total reduction ratio in the temperature region of T1 + 30 ° C or higher and T1 + 200 ° C or less was less than 50%.
Table 2 indicates the total reduction ratio (%) in the temperature region of T1 + 200 ° C or lower, the temperature (° C) after the final pass reduction in the temperature region of T1 + 30 ° C or greater, and T1 + 30 ° C or higher and T1 + 200 ° C or lower, and P1: the final reduction reduction ratio of 30% or more (the final pass reduction ratio in the T1 + temperature region 30 ° C or higher and T1 + 200 ° C or lower) (%) for finishing lamination. In addition, Table 2 indicates the reduction ratio (%) at the time of reduction in the temperature range below T1 + 30 ° C in the finishing lamination.
After the final reduction in the temperature region of T1 + 30 ° C or higher and T1 + 200 ° C or lower was performed on the finishing lamination, cooling before cold rolling was started before the skew time t ( seconds) pass 2.5 x t1. In cooling prior to cold rolling, the average cooling rate was set to OOOO / s or more. In addition, the change in temperature (amount of cooled temperature) in the cooling before cold rolling has been adjusted to fall within a range of 40 ° C or higher and 140Ό or lower.
However, for steel types A6, C4, E4, J4, and 03, cooling before cold rolling (after hot rolling - finishing rolling - cooling) was started after the waiting time t. (seconds) pass 2.5 x t1 from the final reduction in the temperature region of T1 + 30 ° C or higher and T1 + 200 ° C or lower in the finishing lamination. For steel grades A22, C16, E12, and E13, the temperature change (after hot rolling - finish rolling - cooling amount) in cooling before cold rolling was less than 4013, and compared to steel grades A21, C15, and E11, the temperature change (after hot rolling - finishing rolling - cooling amount) in the cooling before cold rolling was greater than 14013. For steel grades A22, < 316, and E13, the average cooling rate (after hot rolling - finishing rolling - cooling rate) in cooling before cold rolling was slower than 5013 / s.
Table 2 indicates t1 (seconds) of each steel type, the dwell time t (seconds) from the final reduction in the temperature region of T1 + 30 ° C or higher and T1 + 200 ° C or lower until the start of cooling before cold rolling on the finishing lamination, t / t1, the temperature change (cooling amount) (13) in the cooling before cold rolling, and the average cooling rate (13 / s) in cooling before cold rolling.
After cooling before cold rolling, the coiling was performed at 70013 or less, and original hot-rolled steel sheets each having a thickness of 2 to 4.5 mm were obtained.
However, for steel types A7 and <38, the coiling temperature was greater than 70013. For each type of steel, the cooling stop temperature (coiling temperature) (13) of the Cooling before cold rolling was shown in Table 2.
The original hot-rolled steel sheets were then stripped and then cold rolled at a reduction ratio of 40% or more and 80% or less so that the thickness after cold rolling became 1.2 mm. However, for steel types A17, E9, and J15, the cold rolling reduction ratio was less than 40%. In cold rolling, the reduction ratio of each type of steel is shown in Table 3. In addition, when the cold rolling reduction ratio was 80%, the rolling load becomes myth high and thus cold rolling cannot be executed to a predetermined sheet thickness. Therefore, the substantial upper limit of the reduction ratio is about 80%. Thereafter, the cold-rolled plate (base steel plate) was heat-treated and hot-dip galvanized on the hot-dip continuous galvanizing line.
In the hot dip continuous galvanizing line, initially the cold-rolled plate was heated to a temperature region of 750 or higher and 900Ό or less, was held for 10 seconds or more and 600 seconds or less in the region. temperatures, and then underwent annealing treatment. In addition, when heating was performed to the temperature region of 750 to 900 ° C, the average heating rate HR1 (O / s) of room temperature or greater and 650Ό or less was adjusted to 0.3 or more (HR1 > 0.3), and the average heating rate HR2 (O / s) from above 650Ό to 750 to 900Ό was adjusted to 0.5 x HR1 or less (HR2 <0.5 x HR1). Table 3 indicates the heating temperature (annealing temperature), the retention time at the heating temperature (time to start primary cooling after cold rolling) (seconds), and the average heating rates HR1 and HR2 (O / s) of each type of steel.
However, for steel grade A20, the annealing temperature exceeded 900Ό. For steels of types A7, C4, E5, J5, and 04, the annealing temperature was less than 75013. For steel types C3, E6, and J5, retention time was shorter than one second. . For steel grades A18 and C13, the retention time exceeded 600 seconds. In addition, for steel grade C12, the average HR1 coating rate was slower than 0.3 (O / s). For steel types A12, A13, A15, A15, C3, C4, C9, C11, J10, J11, J13, J14, and O10, the average heating rate HR2 (O / s) has exceeded 0.5 x HR1 .
After annealing, cooling was performed from annealing temperature to 500 ° C at an average cooling rate of 0.1 to 200 ° C / s. For steel grades A19 and C13, the average cooling rate was slower than 0.1 “C / s. The average cooling rate (“C / s) of each steel plate is shown in Table 3.
After cooling treatment, retention between 500 and 350Ό was performed for 10 to 1000 seconds. The retention time for each steel plate is shown in Table 3. However, for steel plates A8, C5, J6, and 05, the retention time was less than 10 seconds.
Then, the base steel plate was dipped into the controlled hot dip galvanization bath to a predetermined condition and then cooled to room temperature. The galvanizing bath temperature was administered to 440 to 470 4. In addition, when hot dip galvanizing was performed, the temperature of the base steel plate was (hot dip galvanizing bath temperature - 40 ° C) maior or higher and (hot dip galvanizing bath temperature + 50 ° C). ) ° C or lower. The effective Al concentration in the hot dip galvanizing bath was in the range 0.09 to 0.17 mass%. After being immersed in the hot dip galvanizing bath, a portion of the steel plate was bonded to 460Ό or more and 600Ό or less and then cooled to room temperature. At that time, the weight per unit area was about 35 g / m2 on each surface. Finally, the steel sheet obtained was subjected to skin pass rolling at a reduction ratio of 0.4%.
To suppress the coating and promote bonding, a jet stream of 10 m / min or more and 50 m / min or less was provided in the galvanizing bath. Table 3 indicates the velocity (m / min) of the jet stream delivered to the galvanizing bath and the temperature of the bonding treatment at the time of hot dip galvanizing on each of the steels. However, for steel types A9, C5, C8, E7, J7, and 06, the jet stream velocity provided in the galvanizing bath was slower than 10 m / min. In addition, for steel types A11, C8, E9, J9, and 09, the bonding treatment temperature exceeded 600 ° C.
Table 4 indicates the average pole density values of the {100} To {223} <110> orientation groups and the crystal orientation pole density {332} <113> within a range of the thickness of the 5/8 to 3/8 sheet from the steel plate surface of each type of steel, and the volume fractions (structural fractions) (%) of ferrite, bainite, residual austenite, martensite, and perlite in the metal frame of each type of steel. In addition, each of the volume fractions (structural fractions) was evaluated by the structural fraction prior to skin pass lamination. In addition, Table 4 indicated, as mechanical properties of each type of steel, the tensile strength TS (MPa), the elongation (L-EI), the difference in elongation (ΔΕΙ), the equilibrium (TS x El). resistance (TS) - total elongation (C-EI). In addition, the presence or absence of galvanizing faults, Fe concentration (mass%) of the hot dip galvanized layer, and Fe concentration (mass%) in the hot dip galvanized layer are indicated. .
The tensile test was performed by sampling a specimen from JIS No. 5 of a 1.2 mm thick plate in a vertical direction parallel to the rolling direction to evaluate the tensile properties. The difference (ΔΕΙ) between an elongation (L-EI) when performing the tensile test in the direction parallel to the rolling direction and an elongation (C-EI) when performing the tensile test in the vertical direction to the rolling direction. lamination was calculated from the elongation value obtained. The tensile test was performed on each of the five specimens and a mean value was obtained, so the elongation and the TS were calculated from the mean value. In addition, as for the steel plate having great material anisotropy, there was a tendency that the elongation value was varied. A steel having the strength equilibrium (TS x El) (TS) - total elongation (C-EI) exceeding 16000 (MPa%) was defined as a high strength steel plate having excellent conformability.
The coating property and the binding reaction were evaluated as follows, respectively. O: No galvanizing fault present. Δ: Some galvanizing faults are present. X: Numerous galvanizing faults are present.
The tensile property, the coating property, and the Fe content (%) contained in the galvanized layer that were measured are shown in Table 4. It was understood that all the steel sheets of the present invention were excellent both in capacity. in conformation as in coating property. The underline indicates that the numerical value is outside the range of the present invention. * 1 indicates cases where 40% or more reduction is not performed at 1000X3 or more. The underline indicates that the numerical value is outside the range of the present invention. * 2 indicates cases where binding treatment is not performed. F: ferrite, B: bainite, residual γ: residual austenite, M: martensite, P: perlite * 1: Structure includes ferrite and carbides. However, carbide was counted as ferrite.
Industrial Applicability The present invention should provide high strength galvanized steel sheet having a small anisotropy of the material and excellent conformability with the ultimate tensile strength of 980 MPa or more which is suitable for a structural element, a reinforcement element, and a car suspension element at a low cost. Accordingly, the present invention can be expected to contribute greatly to the lighter weight of automobiles and to have an extremely high effect on the industry.

权利要求:
Claims (16)
[1]
1. High-strength hot-dip galvanized steel plate, characterized in that the base steel plate contains: mass%, C: 0.1 less than 0.40%; Si: 0.5 to 3.0%; Mn: 1.5 to 3.0%; O: limited to 0.006% or less; P: limited to 0.04% or less; S: limited to 0.01% or less; Al: limited to 2.0% or less; N: limited to 0.01% or less; and a balance including Fe and the inevitable impurities, a base steel plate microstructure contains 40% to 55% ferrite, 8 less than 60% residual austenite, in volume fraction, and a balance being bainite or martensite, a average value of the pole densities of the orientation group {100} <011> to {223} <110> represented by each of the crystal orientations {100} <011>, {116} <110>, {114} <110> , {113} <110>, {112} <110>, {335} <110>, and {223} <110> in a sheet thickness range of 5/8 to 3/8 from the sheet surface base steel is 6.5 or less and a pole density of a crystal orientation {332} <113> is 5.0 or less, and the hot dip galvanized layer contains Fe: less than 7 mass% and a balance including Zn, Al and the inevitable impurities.
[2]
High-strength hot-dip galvanized steel plate according to claim 1, characterized in that the base steel plate also contains one or two or more elements from:% by mass, Cr: 0.05 at 1.0%; Mo: 0.01 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%; and B: 0.0001 to 0.01%.
[3]
High-strength hot-dip galvanized steel plate according to claim 1, characterized in that the base steel plate also contains, by weight, 0.0005 to 0.04% in total. or two or more elements selected from Ca, Mg, and REM.
[4]
4. High-strength hot-dip galvanized sheet steel, characterized in that the base steel plate contains:% by mass, C: 0.10 less than 0.4%; Si: 0.5 to 3.0%; Mn: 1.5 to 3.0%; O: limited to 0.006% or less; P: limited to 0.04% or less; S: limited to 0.01% or less; Al: limited to 2.0% or less; N: limited to 0.01% or less; and a balance including Fe and the inevitable impurities, a base steel plate microstructure contains 40% or more ferrite, 8 to less than 60% residual austenite in volume fraction, and a balance being bainite or martensite, a average value of pole densities of orientation groups {100} <011> to {223} <110> represented by each of the crystal orientations {100} <011>, {116} <110>, {114} <110> , {113} <110>, {112} <110>, {335} <110>, and {223} <110> in a sheet thickness range of 5/8 to 3/8 from the sheet surface base steel is 6.5 or less and a pole density of a crystal orientation {332} <113> is 5.0 or less, and the hot-dip galvanized bonded layer contains Fe: 7 to 15% by weight and a balance including Zn, Al, and the inevitable impurities.
[5]
High-strength hot-dip galvanized steel plate according to Claim 4, characterized in that the base steel shell also contains one or two or more elements from:% by mass, Cr: 0 0.05 to 1.0%; Mo: 0.01 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%; and B: 0.0001 to 0.01%.
[6]
High-strength hot-dip galvanized steel plate according to Claim 4, characterized in that the base steel plate also contains, by weight, 0.0005 to 0.04% in total. or two or more elements selected from Ca, Mg, and REM.
[7]
7. Production method of a high-strength hot-dip galvanized sheet steel means the production method, characterized in that it comprises: with respect to a steel bar containing:% by mass, C: 0,10 to less than 0.4%; Si: 0.5 to 3.0%; Mn: 1.5 to 3.0%; O: limited to 0.006% or less; P: limited to 0.04% or less; S: limited to 0.01% or less; Al: limited to 2.0% or less; N: limited to 0.01% or less; and a balance including Fe and the inevitable impurities, initially perform hot rolling in which rolling at a reduction ratio of 40% or more is performed one or more times in a temperature range of 1000Ό or more and 1200Ό or less; adjusting austenite grain diameter to 200 μιη or less by the first hot rolling; perform the second hot rolling in which rolling at a reduction ratio of 30% or more is performed in one pass at least once in a temperature region of T1 + 30 ° C or higher and T1 + 200 ° C or lower determined by Expression (1) below; adjust a total reduction ratio on the second hot rolling mill to 50% or more; perform a final reduction at a reduction ratio of 30% or more on the second hot rolling and then initiate cooling before cold rolling such that a wait time t (seconds) satisfies Expression (2) below; adjusting an average cooling rate to øOO / s or more and a temperature change to be in the range of 40Ό or more and 140Ό or less in cooling prior to cold melting; winding in a temperature region of 700 ° C or less; perform cold rolling at a reduction ratio of 40% or more and 80% or less; heat to an annealing temperature of 750 ° C or higher and 900 ° C or lower and then anneal in a continuous hot dip galvanizing line; cool from annealing temperature to 500Ό to 0.1 to ΣΟΟΌ / β; and perform hot dip galvanization for 10 to 1000 seconds at 500 to 350 ° C, T1 (° C) = 850 + 10 x (C + N) x Mn + 350 x Nb + 250 xTi + 40 x B + 10 x Cr + 100 x Mo + 100 x V ··· Expression (1) where C, N, Mn, Nb, Ti, B, Cr, Mo, and V each represent the content of each element (% in mass of Ti, B, Cr, Mo, and V are calculated as zero when not contained.). t <2.5 x t1 ··· Expression (2) where t1 is obtained by the following Expression (3): t1 = 0.001 x ((Tf - T1) x P1 / 100) 2 - 0.109 x ((Tf - T1) x P1 / 100) + 3.1 ··· Expression (3) where, in Expression (3) above, Tf represents a steel bar temperature obtained after a final reduction at a reduction ratio of 30% or more, and P1 represents a reduction ratio of a final reduction to 30% or more.
[8]
Method of production of the high-strength hot-dip galvanized steel sheet as defined in claim 7, characterized in that the total reduction ratio over a temperature range below T1 + 30 ° C is 30% or any less.
[9]
Method of production of the high-strength hot dip galvanized steel sheet according to claim 7, characterized in that, in the case of heating to annealing temperature in the hot dip galvanizing line, an average heating rate from room temperature or higher to 650Ό or lower is set to HR1 (° C / s) expressed by Expression (4) below, and an average heating rate from temperature exceeding 650Ό to annealing temperature is adjusted for HR2 (° C / s) expressed by Expression (5). HR1> 0.3 ··· Expression (4) HR2 <0.5 x HR1 ··· Expression (5)
[10]
Method of producing the high-strength hot dip galvanized steel sheet according to claim 7, characterized in that when hot dip galvanizing is performed, the temperature of the base steel sheet is (core temperature). hot dip galvanizing bath - 40) Ό or higher and (hot dip galvanizing bath temperature + 50) Ό or lower.
[11]
The method of producing the high-strength hot-dip galvanized steel sheet according to claim 7, characterized in that a flow rate of 10 m / min or faster and 50 m / min or slower It is supplied in a galvanizing bath when hot dip galvanizing is performed.
[12]
12. Production method of a high strength hot-dip galvanized hot-dip galvanized steel sheet means the production method characterized by the fact that it comprises: with respect to a steel bar containing:% by mass, C: 0,10 less than 0.4%; Si: 0.5 to 3.0%; Μη: 1,5 to 3,0%; Ο: limited to 0.006% or less; P: limited to 0.04% or less; S: limited to 0.01% or less; Al: limited to 2.0% or less; N: limited to 0.01% or less; and a balance including Fe and the inevitable impurities, initially hot rolling at a reduction ratio of 40% or more is performed once or more at a temperature range of 1000Ό or more and 1200Ό or less; adjusting austenite grain diameter to 200 μιη or less by the first hot rolling; perform the second hot rolling in which rolling at a reduction ratio of 30% or more is performed in one pass at least once in a temperature region of T1 + 30 ° C or more and T1 + 200 ° C or less determined by Expression (1) below; adjust a total reduction ratio on the second hot rolling mill to 50% or more; perform a final reduction at a reduction ratio of 30% or more on the second hot rolling and then start cooling before cold rolling such that a wait time t (seconds) satisfies Expression (2) below; adjusting an average cooling rate to øOO / s or more and a temperature change to be in the range of 40Ό or more and 140Ό or less in the cooling prior to cold rolling; winding in a temperature region of 700Ό or less; perform cold rolling at a reduction ratio of 40% or more and 80% or less; heat to an annealing temperature of 750Ό or more and 900Ό or less and then anneal in a continuous hot dip galvanizing line; cool to 500 ° C from the annealing temperature of 0.1 to 200 ° / s; perform hot dip galvanization after retention for 10 to 1000 seconds between 500 and 350Ό; and perform a bonding treatment at a temperature of 460Ό or more. T1 (° C) = 850 + 10 x (C + N) x Mn + 350 x Nb + 250 xTi + 40 x B + 10 x Cr + 100 x Mo + 100 x V ··· Expression (1) where, C , N, Mn, Nb, Ti, B, Cr, Mo, and V each represent the content of each element (mass% of Ti, B, Cr, Mo, and V are calculated as zero when not contained. ). t <2.5 x t1 ··· Expression (2) where, t1 is obtained by Expression (3) below. t1 = 0.001 x ((Tf - T1) x P1 / 100) 2 - 0.109 x ((Tf - T1) x P1 / 100) + 3.1 ··· Expression (3) where, in Expression (3) above, Tf represents a steel bar temperature obtained after a final reduction at a reduction ratio of 30% or more, and P1 represents a ratio of a final reduction reduction to 30% or more.
[13]
The method of production of the high-strength hot-dip galvanized steel sheet according to claim 12, characterized in that the total reduction ratio over a temperature range below T1 + 30 ° C is 30 ° C. % or less.
[14]
Method of producing the high-strength hot-dip galvanized sheet steel according to claim 12, characterized in that, in the case of heating to annealing temperature in the hot-dip continuous galvanizing line , an average heating rate from room temperature or above and 650C or less is adjusted as HR1 CC / s) expressed by Expression (4) below, and an average heating rate from a temperature exceeding 650C to an annealing temperature. is set to HR2 (C / s) expressed by Expression (5) below. HR1> 0.3 ··· Expression (4) HR2 <0.5 x HR1 ··· Expression (5)
[15]
The method of producing the high-strength hot-dip galvanized steel sheet according to claim 12, characterized in that when hot-dip galvanizing is performed, a temperature of a base steel sheet is (temperature of hot dip galvanizing bath - 40) ° C or more and (temperature of hot dip galvanizing bath + 50) ° C or less.
[16]
The method of producing the high-strength hot-dip galvanized steel sheet according to claim 12, characterized in that a flow rate of 10 m / min or faster and 50 m / min or slower It is supplied in a galvanizing bath when hot dip galvanizing is performed.

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法律状态:
2018-05-22| B06A| Patent application procedure suspended [chapter 6.1 patent gazette]|

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2018-11-27| B09A| Decision: intention to grant [chapter 9.1 patent gazette]|

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2019-01-29| 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. |

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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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JP2011218040|2011-09-30|

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JP2011-218040|2011-09-30|

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PCT/JP2012/075214|WO2013047819A1|2011-09-30|2012-09-28|High-strength hot dip galvanized steel plate having excellent moldability, weak material anisotropy and ultimate tensile strength of 980 mpa or more, high-strength alloyed hot dip galvanized steel plate and manufacturing method therefor|

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