HOT-LAMINATED STEEL SHEET AND METHOD FOR PRODUCING THE SAME

专利摘要:
Patent specification: "Hot-rolled steel plate and method for producing it". The present invention relates to a hot rolled steel plate which the average pole density of the {100}? to {223} <110> orientation group is 1.0 to 5.0 and the pole density of the crystal orientation {332} <113> is 1.0 to 4.0. In addition, hot-rolled steel sheet includes, as a metallographic structure, area%, ferrite and bainite from 30% to 99% in total and martensite from 1% to 70%. In addition, hot-rolled steel plate meets the following expressions 1 and 2 when the martensite area fraction is defined as fm in% area units, the average martensite size is defined as day in units of? m , the average distance between the martensite is defined as dis in units of? m, and the tensile strength of the steel plate is defined as ts in units of mpa. day ? 13? M ... expression 1 ts / fm? dis / day? 500 ... expression 2 ws / docs / sda p200983 / draft / 19801511v1
公开号:BR112013029839B1
申请号:R112013029839-1
申请日:2012-05-24
公开日:2019-06-25
发明作者:Kohichi Sano；Kunio Hayashi；Kazuaki Nakano；Riki Okamoto；Nobuhiro Fujita
申请人:Nippon Steel & Sumitomo Metal Corporation；
IPC主号:

专利说明:

Description of the Invention Patent for "HOT-LAMINATED STEEL SHEET AND METHOD FOR PRODUCING THE SAME".
The present invention relates to a high strength hot rolled steel sheet which is excellent in uniform deformation capacity contributing to the drawability, printing capability, and the like, and is excellent in local deformation contributing to the folding ability, drawability of flanging capability, deburring conformability, or the like, and relates to a method for producing the same. Particularly, the present invention relates to a steel sheet including a Double Phase (DP) structure.
Priority is claimed over Japanese Patent Application No. 2011-117432, filed May 25, 2011, the content of which is hereby incorporated by reference.
Background of the Invention [003] To suppress the emission of carbon dioxide gas from a vehicle, the weight reduction of a car chassis has been attempted by the use of a high strength steel plate. In addition, from the point of view of ensuring the safety of a passenger, the use of high-strength steel sheet for the car chassis has been attempted in addition to a mild steel plate. However, in order to also improve the weight reduction of the automobile chassis in the future, the usable strength level of the high strength sheet should be increased compared to the conventional one. Furthermore, in order to utilize the high strength steel sheet for suspension parts or the like of the automobile chassis, the local deformability which contributes to the conformability of the deburring or the like must also be improved in addition to the unilateral deformation capacity - for me.
In general, however, when the strength of the sheet steel is increased, the forming capacity (deformability) is decreased. For example, Non-Patent Document 1 describes that uniform elongation, which is important for stamping or drawing, is diminished by the reinforcement of the steel sheet. [005] In contrast, Non-Patent Document 2 describes a method which ensures uniform elongation by the composition of the metallographic structure of the steel sheet even when the strength is the same.
In addition, Non-Patent Document 3 describes a method of controlling the metallographic structure which enhances local ductility by representing the folding capacity, the bore capacity, or the conformability in the deburring by controlling the inclusions, which controls the microstructure for single phase, and decreases the hardness difference between the microstructures. In Non-Patent Document 3, the microstructure of the steel sheet is controlled for single phase by control of the microstructure, and thus the local deformability which contributes to the hole expansion capacity or the like is improved. However, to control the microstructure for single phase, a heat treatment from the single austenite phase is a base method of production as described in Non-Patent Document 4.
In addition, Non-Patent Document 4 describes a technique which satisfies both the strength and the ductility of the sheet of steel by control the cooling after hot rolling to control the metallographic structure, specifically to obtain the desired morphologies of precipitates and transformation structures and to obtain a suitable fraction of ferrite and bainite. However, all techniques as described above are methods of improvement for the local deformation capacity which depends on the control of the microstructure, and are greatly influenced by the formation of the microstructure of a base.
In addition, a method which improves the properties of the sheet material by increasing the reduction in a continuous hot rolling to refine the beans is known as the prior art. For example, Non-Patent Document 5 describes a technique that improves the strength and toughness of the sheet of steel by conducting a large reduction lamination over a comparatively lower temperature range within an austenite range to refine the ferrite grains which is the main phase of a product by the transformation of hand-recrystallized austenite into ferrite. However, in Non-Patent Document 5, the method for improving the local deformation capacity to be solved in the present invention is not considered at all.
Non-Patent Document 1 Kishida: Nippon Steel Technical Report No. 371 (1999), pg.13.
[0010] Non-Patent Document 2 O. Matsumura et al .: Trans. ISIJ vol.27 (1987), p.570.
[Non-Patent Document 3 Katoh et al .: Steelmanufacturing studies vol.312 (1984), pg. 41.
[0012] Non-Patent Document 4 K. Sugimoto et al .: vol. 40 (2000), p. 920.
[0013] Non-Patent Document 5 NFG product introduction of NAKAYAMA STEEL WORKS, LTD.
SUMMARY OF THE INVENTION Technical Problem [0014] As described above, it is a fact that the technique that satisfies the high strength and both properties of uniform deformation capacity and local deformation capacity is not discovered. For example, to improve the local deformability of high strength steel sheet, it is necessary to conduct control of the microstructure including inclusions. However, since the improvement depends on the control of the microstructure, it is necessary to control the fraction or morphology of the microstructure such as the precipitates, the ferrite, or the bainite, and therefore the base metallographic structure is limited. Since the metallographic structure of the base is restricted, it is difficult not only to improve the local deformation capacity, but also to improve both the strength and the local deformation capability.
An object of the present invention is to provide a hot-rolled steel sheet having high strength, excellent uniform deformation capability, excellent local deformation ability, and low reliance on the orientation (anisotropy) of the forming ability by the texture control and by controlling the size or morphology of the grains in addition to the metallographic structure of the base, and is to provide a method for producing the same. Here, in the present invention, the strength mainly represents the tensile strength, and the high strength indicates the strength of 440 MPa or more in the tensile strength. In addition, in the present invention, the satisfaction of high strength, excellent uniform deformation capacity, and excellent local deformation capability indicates a case of simultaneously satisfying all conditions of TS> 440 (unit: MPa), TS χ u- (Unit: MPa%), ed / RmC> 1 (without unit) by the use of characteristic values of tensile strength (TS), uniform elongation ( u-EL), the bore expansion ratio (λ), and d / RmC, which is the ratio of the thickness d for the minimum bend radius RmC to a direction C.
Solution to Problem [0016] In relative techniques, as described above, the improvement in local deformability contributing to bore expansion capacity, bending capacity, or the like was attempted by controlling inclusions by refining the precipitates , by homogenizing the microstructure, by controlling the microstructure to the single phase, by reducing the hardness difference between the microstructures, or similar. However, only by the techniques described above, the main constituent of the microstructure must be restricted. In addition when an element that greatly contributes to an increase in strength, such as representatively Nb or Ti, is added for high reinforcement, the anisotropy can be significantly increased. Consequently, other factors for forming ability must be abandoned or instructions for withdrawing a sample prior to conformation should be limited, and as a result, the application is restricted. On the other hand, the uniform deformation ability can be improved by dispersing the hard phases, such as martensite, in the metallographic structure.
In order to obtain the high strength and improve both the uniform deformation capability, which contributes to the drawability or the like, as to the local deformability, which contributes to the bore expansion capacity, the folding ability or the like , the inventors recently focused on influences of the texture of the sheet of steel in addition to controlling the fraction of the morphology of the metallographic structures of the sheet of steel, and investigated and investigated the operation and its effects in detail. As a result, the inventors have discovered that by controlling the chemical composition, the metallographic structure, and the texture represented by the densities pole of each orientation of a group of orientations of specific crystal of the sheet steel, the high strength is obtained, the capacity of local deformation is remarkably improved due to the balance of the Lankford values (r values) in the direction of rolling, in one direction (direction C) which makes an angle of 90 ° with the direction of rolling, in a direction which makes an angle of 30 ° in the direction of rolling, or in a direction which makes an angle of 60 ° with the direction of rolling, and the uniform deformation capacity is also guaranteed due to the dispersion of hard phases such as martensite. One aspect of the present invention employs the following: A hot-rolled steel sheet according to one aspect of the present invention includes, as a chemical composition, in mass%, C: 0.01% to 0.4% , Si: 0.001% to 2.5%, Mn: 0.001% to 4.0%, Al: 0.001% to 2.0%, P: limited to 0.15% or less. S: limited to 0.03% or less, N: limited to 0.01% or less, O: limited to 0.01% or less, and the balance consisting of Fe and the inevitable impurities, wherein the average group of orientations from {100} to {223} <110>, which is the density polo represented by the arithmetic mean of the densities pole of each crystal orientation {100} <110>, <116> <110>, { 114 is 110 to 110, and 223 to 110 is 1.0 to 5.0 and the polar density of the crystal orientation 332 is from 1.0 to 4, 0 in a central portion of the thickness which is the thickness range of 5/8 to 3/8 based on the surface of the steel sheet; the steel sheet includes as a metallographic structure various grains and includes in% by area a ferrite and a bainite of from 30% to 99% in total and a martensite of 1% to 70% and when the fraction of area of martensite is defined as fM in units of% area, the mean martensite size is defined as day in units of pm, the mean distance between martensite is defined as dis in units of pm, and the tensile strength of the sheet of steel is defined as TS in units of MPa, Expression 1 below and Expression 2 below are satisfied. day <13 pm ... Expression 1 TS / fM χ dis / day> 500 ... Expression 2 [0020] The hot-rolled steel sheet according to item (1) may also include, as a chemical composition, in mass%, at least one element selected from the group consisting of Mo: 0.001% to 1.0%, Cr: 0.001% to 2.0%, Ni: 0.001% to 2.0%, Cu: 0.001% to 2.0%, B: 0.0001% to 0.005%, Nb: 0.001% to 0.2%, Ti: 0.001% to 0.2%, V: 0.001% to 1.0%, W: 0.001% to 1.0%, Ca: O, 0.01%, Mg: 0.0001% to 0.01%, Zr: 0.0001% to 0.2%, Rare Earth Metal (REM): 0.0001% to 0.1%, As : 0.0001% to 0.5%, Co: 0.0001% to 1.0%, Sn: 0.0001% to 0.2%, Pb: 0.0001% to 0.2%, Y: 0 , 0001% to 0.2%, and Hf: 0.0001% to 0.2%.
In the hot-rolled steel sheet according to item (1) or (2), the average grain diameter may be 5 pm to 30 pm.
In the steel plate according to item (1) or (2), the average pole density of the orientation group from (100) <221 to (223) <110> may be 1.0 to 4.0, and the polar density of the crystal orientation 332 can be 1.0 to 3.0.
In hot-rolled steel sheet according to any of items (1) to (4), when the major axis of martensite is defined as La, and the minor axis of martensite is defined as Lb, the area fraction of martensite which satisfies Expression 3 below may be 50% to 100% as compared to the fraction of the martensite fM area.
La / Lb <5.0 ... Expression 3 In hot-rolled steel sheet according to any one of items (1) to (5), the steel sheet may include as a% , ferrite from 30% to 99%.
In the hot-rolled steel sheet according to any one of items (1) to (6), the steel sheet may include, as a metallographic structure, in% by area, the bainite of 5% to 80%.
In the hot-rolled steel sheet according to any one of items (1) to (7), the steel sheet may include a martensite which is welded in the martensite.
In the hot-rolled steel sheet according to any one of items (1) to (8), the area fraction of raw grains having a grain size of more than 35 μm may be 0% to 10% between the grains in the metallographic structure of the sheet steel.
In the hot-rolled steel plate according to any of items (1) to (9), the hardness H of the ferrite may satisfy Expression 4 below. H + 200 χ [Si] + 21 χ [Mn] + 270 χ [P] + 78 χ [Nb] 1/2 + 108 χ [Ti] 1/2 ... (Expression 4) hot rolled steel sheet according to any one of tens (a) to (10), when the hardness of the ferrite or bainite, which is the main phase is measured at 100 points or more, the value that divides the standard deviation of the hardness by the average hardness can be 0.2 or less.
A method for producing a hot-rolled steel sheet according to one aspect of the present invention includes: a first hot rolling of a steel in a temperature range of 1000 ° C to 1200 ° C under conditions such that at least one which is reduced by 40% by weight or more is included so as to control the average grain size of an austenite in steel at 200 æm or less, wherein the steel comprises, as a chemical composition, in mass%, C: 0 , 0.1% to 0.4%, Si: 0.001% to 2.5%, Mn: 0.001% to 4.0%, Al: 0.001% to 2.0%, P: S: limited to 0.03% or less, N: limited to 0.01% or less, O: limited to 0.01% or less, and the balance consisting of Fe and the inevitable impurities; under conditions such that when the temperature calculated by Expression 5 below is defined as T1 in units of ° C and the ferritic transformation temperature calculated by Expression 6 below is defined as Ara in units of ° C, a large reduction pass whose reduction is 30% or more in a temperature range from T1 + 30 ° C to T1 + 200 ° C is included, the cumulative reduction in the temperature range from T1 + 30 ° C to T1 + 200 ° C is 50% or more, cumulative reduction over a range of temperatures of Ara unless T1 + 30 ° C is limited to 30% or less, and the end temperature of the lamination is Ara or greater; a first cooling in the steel under conditions such that when the waiting time from the end of the final pass in the large reduction pass to the beginning of the cooling is set to t in units of seconds, the waiting time t satisfies Expression 7 a then the average cooling rate is 50 ° C / s or faster, the change in the cooling temperature which is the difference between the temperature of the steel at the beginning of the cooling and the temperature of the steel at the end of the cooling is 40 ° C 140 ° C, and the temperature of the cooling Np steel is T1 + 100 ° C or lower; a second cooling of the steel to a temperature range of 600 ° C to 800 ° C under an average cooling rate of 15 ° C / s at 300 ° C / s after the completion of the second hot rolling; keep steel in the temperature range of 600 ° C to 800 ° C for 1 second to 15 seconds; a third cooling of the steel to a temperature range from ambient temperature to 350 ° C with an average cooling rate of 50 ° C / s at 300 ° C / s after the end of the retention; coil the steel in the temperature range from room temperature to 350 ° C. [[[T1 T1 T1 T1 T1 T1 T1 T1 T1 T1 T1 T1 T1 T1 T1 T1 T1 T1 T1 T1 T1 T1 T1 T1 T1 T1 T1 T1 T1 T1 T1 T1 T1 T1 T1 T1 T1 T1 T1 T1 T1 T1 T1 T1 T1 T1 T1 T1 T1 T1 T1 [ and Mn respectively.
Ara = 879.4 - 516.1 χ [C] - 65.7 χ [Mn] + 38.0 χ [Si] + 274.7 χ [P] ... Expression 6 Here, in Expression 6 , [C], [Mn], [Si] and [P] represent mass percentages of C, Mn, Si, and P respectively. t <2.5 χ t1 ... Expression 7 [0033] Here, t1 is represented by Expression 8 below.
[0034] t1 = 0.001 x ((Tf-T1) χ P1 / 100) 2 - 0.109 x ((Tf-T1) x P1 / 100) + 3.1 ... Expression 8 Here, temperature Celsius of the steel at the end of the final pass, and P1 represents the percentage reduction in the final pass.
In the method for producing the hot-rolled sheet according to item (12), the steel may also comprise, as a chemical composition, in mass%, at least one element selected from the group consisting of Mo: 0.001% a 1.0%, Cr: 0.001% to 2.0%, Ni: 0.001% to 2.0%, Cu: 0.001% to 2.0%, B: 0.0001% to 0.005%, Nb: 0.001% a 0.2%, Ti: 0.001% to 0.2%, V: 0.001% to 1.0%, W: 0.001% to 1.0%, Ca: 0.0001% to 0.01%, Mg: , 0.001% to 0.01%, Zr: 0.0001% to 0.2%, Rare Earth Metal (REM): 0.0001% to 0.1%, As: 0.0001% to 0.5% Co: 0.0001% to 1.0%, Sn: 0.0001% to 0.2%, Pb: 0.0001% to 0.2%, Y: 0.0001% to 0.2%, and Hf : 0.0001% to 0.2%, wherein the temperature calculated by Expression 9 below can be replaced by the temperature calculated by Expression 5 as T1. [[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[+ (C), [N], [Mn], [Nb], [Ti], [B], [Cr], [Mo], and [V] ] represent mass percentages of C, N, Mn, Nb, Ti, B, Cr, Mo, and V respectively.
In the method for producing the hot-rolled steel sheet according to item (12) or (13), the holding time t may also satisfy Expression 10 below. In the method for producing the hot-rolled steel sheet according to item (12) or (13), the holding time t may also satisfy the expression 11a below. In the method for producing the hot-rolled steel sheet according to any one of items (12) to (15), in the first hot rolling, it is to be conducted at least two laminations whose reduction is 40% or more and the average grain size of the austenite can be controlled at 100 æm or less.
In the method for producing hot rolled steel sheet according to any of items (12) to (16), the second cooling can start within 3 seconds after the end of the second hot rolling.
In the method for producing the hot-rolled steel sheet according to any one of items (12) to (17), in the second hot rolling the increase in the temperature of the steel between biases may be 18 ° C or less.
In the method for producing the hot-rolled sheet according to any one of items (12) to (18), the final pass of the laminations at a temperature range of T1 + 30 ° C to T1 + 200 ° C may be the pass of great reduction.
In the method for producing hot rolled steel sheet according to any one of items (12) to (19), in the retention, the steel can be maintained in a temperature range of 600 ° C to 680 ° C for 3 seconds to 15 seconds.
In the method for producing hot rolled steel sheet according to any one of items (12) to (20), the first cooling can be conducted in a gap between the laminating chairs.
Advantageous Effects of the Invention In accordance with the above aspects of the present invention, it is possible to obtain a hot-rolled steel sheet having high strength, excellent uniform deformation ability, excellent local deformation capability, and small anisotropy even when a element such as Nb or Ti is added.
Brief Description of Drawings FIG. 1 shows the relationship between the average pole density D1 of a group of orientations from {100} <221 to <223 <110> and d / RmC (thickness of minimum folding radius RmC).
FIG. 2 shows the relationship between the D2 pole density of a crystal orientation {332} <113> and d / RmC.
Detailed Description of the Preferred Embodiments Hereinafter, the hot-rolled steel sheet according to one embodiment of the present invention will be described in detail. Initially, the polar density of a crystal orientation of the hot-rolled steel sheet will be described.
[0050] Polar Orientation D1 Density of Crystal Orientation: 1.0 to 5.0 [0051] Density Polo D2 of Crystal Orientation: 1.0 to 4.0 In the hot-rolled steel sheet according to the embodiment , as the polar densities of two types of crystal orientation, the average pole density D1 of a group of orientations from {100} to {223} <110> (hereinafter referred to as "pole density") and the D2 pole density of a crystal orientation (332) in a central portion of the thickness, which is the thickness range of 5/8 to 3/8 (a range which is a distance of 5/8 to 3 / 8 from the surface of the sheet of steel along the normal direction (direction of the depth of the steel sheet) are controlled in relation to the thickness cross-section (its normal vector corresponds to the normal direction) which is parallel to the rolling direction.
In the embodiment, the average pole density D1 is a particularly important feature integrating the texture's orientation and degree of texture development (crystal orientation of the grains in the metallographic structure). Here, the average pole density D1 is the density pole which is represented by the arithmetic mean of the densities pole of each crystal orientation {100} <116>, <116> <110>, <114> <110>, < 110>, and {223} <110>.
The intensity ratio of the electron diffraction intensity or X-ray diffraction intensity of each orientation to that of a random sample is obtained by conducting Electron Back Scattering Diffraction (EBSD) or X-ray diffraction at cross section above in the central portion of the thickness which is the thickness range of 5/8 to 3/8, and the mean pole density D1 of the orientation group from {100} <221> to 223 <110> can be obtained from each intensity ratio.
When the average pole density D1 of the orientation group from (100) <221 to (223) <110> is 5.0 or less, it is satisfied that d / RmC (a parameter in which the thickness d is divided by the minimum bend radius (folding in the C direction)) is 1.0 or more, which is minimally required to work suspension parts or frame parts. In particular, the condition is a requirement that the TS tensile strength, λ bore expansion capacity, and total elongation EL preferably satisfy TS χ λ> 30000 and TS χ EL> 14000 which are two necessary conditions for the suspension parts of the car chassis.
In addition, when the average pole density D1 is 4.0 or less, the ratio (Rm45 / RmC) of the minimum folding radius R45 of the folding direction at 45ø to the minimum folding radius RmC of folding in the direction C is decreased in which the ratio is a dependence parameter of the isotropy of the forming capacity and the excellent local deformability that is independent of the folding direction can be guaranteed. As described above, the average pole density D1 may be 5.0 or less, and may be preferably 4.0 or less. In a case where also excellent bore expansion capacity or small critical folding properties are required, the average pole density D1 may be more preferably less than 3.5, and may most preferably be less than 3.0.
When the average pole density D1 of the orientation group from {100} To {223} <110> is greater than 5.0, the anisotropy of the mechanical properties of the steel sheet is significantly increased. As a result, although the local deformation capacity in only one specific direction is improved, the local deformation capacity in a direction other than the specific direction is significantly decreased. Therefore, in this case, the sheet steel can not satisfy d / RmC> 1.0.
[0058] On the other hand, when the average pole density D1 is less than 1.0, the local deformation capacity can be decreased. Accordingly, the average pole density D1 may preferably be 1.0 or more.
In addition, from similar ratios, the density D2 of the crystal orientation {332} <113> in the central portion of the thickness which is the thickness range of 5/8 to 3/8 may be 4.0 or less. The condition is a requirement for the steel plate to satisfy d / RmC> 1.0, and in particular that the tensile strength TS, the bore expansion ratio λ, and the total elongation EL preferably satisfy TS χ λ> 30000 and TS χ EL> 14000 which are two conditions required for suspension parts.
Further, when the density pole D2 is 3.0 or less, TS χ λ or d / RmC can also be improved. The D2 pole density may be preferably 2.5 or less, and may be more preferably 2.0 or less. When the pole density D2 is greater than 4.0, the anisotropy of the mechanical properties of the sheet steel is significantly increased. As a result, although the local deformation capacity in only one specific direction is improved, the local deformation capacity in a direction other than the specific direction is significantly decreased. Therefore, in this case, the sheet steel can not sufficiently satisfy d / RmC> 1.0.
[0061] On the other hand, when the average pole density D2 is less than 1.0, the local deformation capacity can be decreased. Consequently, the D2 polarity density of the crystal orientation 332 may be preferably 1.0 or more.
The pole density is a synonym of random intensity ratio of X-rays. The random intensity ratio of X-rays can be obtained as follows. The diffraction intensity (X-rays or electron) of a standard sample which lacks a texture for a specific orientation and the diffraction intensity of a test material is measured by an X-ray diffraction method under the same conditions. The ratio of random intensity of X-rays is obtained by dividing the diffraction intensity of the test material by the diffraction intensity of the standard sample. The pole density can be measured using X-ray diffraction, Electron Back Scattering Diffraction (EBSD), or Electron Channeling Pattern (ECP). For example, the mean pole density D1 of the orientation group from {100} to {223} <110> can be obtained as follows. The pole densities of each orientation 110, 110, 114, 110, 112 and 110 are obtained from a three-dimensional texture ( ODF: Guidance Distribution Functions) that is computed by a series of expansion methods using multiple values at the pole values of {110}, {100}, {211}, and {310} measured by the above methods. The average pole density D1 is obtained by calculating the arithmetic mean of the densities pole.
With respect to samples which are supplied for X-ray diffraction, EBSD, and ECP, the thickness of the steel sheet can be reduced to a predetermined thickness by mechanical polishing or the like, the tension can be removed by polishing, electrolytic polishing, or the like, the samples may be adjusted so that a suitable surface including the thickness range of 5/8 to 3/8 is the measuring surface, and then the densities polo can be measured by the above methods . With respect to a transverse direction, it is preferred that the samples are collected in the vicinity of the 1/4 and 3/4 position of the thickness (a position which is at a distance of 1/4 of the width of the steel plate from the edge of the sheet steel).
When the above pole densities are satisfied in many other thickness portions of the sheet of steel in addition to the central portion of the thickness, the local deformability is also improved. However, since the texture in the central portion of the thickness significantly influences the anisotropy of the sheet steel, the properties of the central material of the thickness roughly represent the properties of the material of the entire sheet of steel. Consequently, the mean pole density D1 of the orientation group from {100} to {223} <110> and the density D2 of the crystal orientation {332} <113> in the central portion of the thickness of 5/8 to 3/8 are prescribed.
Here, {hkl} <uvw> indicates that the normal direction of the plate surface is parallel to <hkl> and the rolling direction is parallel to <uvw> when the sample is collected by the method described above. In addition, generally in orientation of the crystal, an orientation perpendicular to the surface of the sheet is represented by (hkl) or {hkl} and a orientation parallel to the rolling direction is represented by [uvw] or <uvw>. {hkl} <uvw> collectively indicates equivalent planes, and (hkl) [uvw] indicates each crystal plane. Specifically, since the embodiment aims at a centered cube structure (bcc), for example, planes 111, 111, 11-11, 11-11, (-11-1), (1-11), and (-1-1-1) are equivalent and can not be classified. In this case, the orientation is collectively called {111}. Since ODF expression is also used for orientation expressions of other crystal structures having low symmetry, generally each orientation is represented by (hkl) [uvwj in the ODF expression. However, in the embodiment, {hkl} <uvw> and (hkl) [uvwj are synonyms.
The metallographic structure of the hot-rolled steel sheet according to the embodiment will now be described.
The metallographic structure of the hot rolled steel sheet according to the embodiment is fundamentally a double phase (DP) structure which includes various grains, includes ferrite and / or bainite as the main phase, and includes martensite as the secondary phase. The strength and uniform deformability can be increased by the dispersion of the martensite which is the secondary phase and the hard phase for the ferrite or bainite which is the main phase and has excellent deformation capacity. The improvement in the uniform deformation capacity is derived from an increase in the hardening rate of the work by the fine dispersion of martensite which is the hard phase in the metallographic structure. Furthermore, the ferrite or bainite here include polygonal ferrite and bainitic ferrite.
The hot-rolled steel sheet according to the embodiment includes residual austenite, perlite, cementite, various inclusions, or the like as microstructures other than ferrite, bainite and martensite. It is preferred that the microstructures other than ferrite, bainite and martensite are limited, in% by area, to 0% to 10%. In addition, when austenite is retained in the microstructure, the embrittlement of the secondary work or the property of delayed fracture deteriorates. Consequently, except for the residual austenite of approximately 5% in fraction of area that inevitably exists, it is preferred that the residual austenite is not substantially included.
[0069] Ferrite and Bainite Area Fractions which are the Major Phase: 30% to less than 99% The ferrite and bainite which are the main phase are comparatively soft, and have excellent deformability. When the ferrite and bainite area fraction is 30% or more in total, both the properties of uniform deformation capacity and local deformation capacity of the hot-rolled steel sheet according to the embodiment are satisfied. More preferably, the ferrite and bainite may be, in area%, 50% or more in total. On the other hand, when the ferrite and bainite area fraction is 99% or more in total, the strength and the uniform deformation capacity of the sheet steel are decreased.
Preferably, the ferrite area fraction, which is the major phase, may be from 30% to 99%. By controlling the ferrite area fraction which is comparatively excellent in deformation capacity to 30% to 99%, it is possible to preferably increase the ductility (deformability) in an equilibrium between the strength and the ductility (deformability) of the ferrite. Stainless steels. Particularly, ferrite contributes to the improvement of the uniform deformation capacity.
Alternatively, the fraction of the area of the bainite which is the main phase may be 5% to 80%. By controlling the fraction of bainite area which is comparatively excellent in the resistance to 5% to 80%, it is possible to preferably increase the resistance in the balance between the strength and the ductility, (deformability) of the steel sheet. By increasing the fraction of bainite area which is a harder phase than ferrite, the strength of the sheet steel is improved. In addition, bainite, which has a lower hardness difference from martensite compared to ferrite, suppresses the beginning of the spans at the interface between the soft phase and the hard phase, and improves the hole expansion capacity.
Martensite fM Area Fraction: 1% to 70% By dispersing the martensite, which is the secondary phase and is the hard phase, in the metallographic structure, it is possible to improve the strength and the uniform deformation capacity. When the martensite area fraction is less than 1%, the hard phase dispersion is insufficient, the work hardening rate is decreased, and the uniform deformation capacity is decreased. Preferably, the martensite fraction of area may be 3% or more. On the other hand, when the martensite fraction is greater than 70%, the hard-phase area fraction is excessive, and the deformation capacity of the steel sheet is significantly decreased. According to the balance between strength and deformability, the martensite fraction of area may be 50% or less. Preferably, the martensite fraction of area may be 30% or less. More preferably, the martensite fraction of area may be 20% or less.
When the average martensite size is greater than 13 pm, the uniform deformation capacity of the sheet steel can be decreased, and the local deformation capacity can be reduced. be diminished. It is considered that the uniform elongation is decreased due to the fact that the contribution to the hardening of the work is decreased when the average martensite size is crude, and that the local deformation capacity is decreased due to the fact that the spans start easily in the vicinity of the crude martensite. Preferably, the average martensite size may be less than 10 Âμm. Most preferably, the average martensite size may be 7 æm or less. TS / fM χ dis / day ratio: 500 or more Further, as a result of the detailed investigation by the inventors, it has been found that when tensile strength is defined as TS (tensile strength) in units of MPa , the martensite fraction is defined as fM (Martensite fraction) in units of%, the mean distance between the martensite grains is defined as dis (distance) in units of pm, and the average martensite grain size is defined as day (diameter) in units of pm, the uniform deformability of the steel sheet is improved in a case where the ratio of TS, fM, dis, and day satisfies Expression 1 below. TS / fM χ dis / day> 500 ... Expression 1 When the TS / fM χ dis / day ratio is less than 500, the uniform deformation capacity of the steel sheet can be significantly decreased. The physical meaning of Expression 1 was not clear. However, it is considered that the work hardening occurs more effectively as the mean distance dis between the martensite grains is decreased and as the average grain size martensite day is increased. Moreover, the TS / fM χ dis / day ratio does not particularly have an upper limit. However, from the industrial point of view, since the TS / fM χ dis / day ratio rarely exceeds 10000, the upper limit may be 10,000 or less. Martensite fraction having 5.0 or less in Greater Axis to Minor Axis Ratio: 50% or more In addition, when the major axis of the martensite grain is defined as La in units of pm, and the minor axis of the martensite grain is defined as Lb in units of pm, the local deformability may preferably be improved in a case where the martensite grain area fraction satisfies Expression 2 below is 59% to 100 % as compared to the martensite fraction fM area.
La / Lb <5.0 ... Expression 2 The detailed reason why the effect is obtained was unclear. However, it is considered that the local deformation capacity is improved due to the fact that the martensite form varies from an acicular form to a spherical shape and that an excessive stress concentration on ferrite or bainite near martensite is alleviated. Preferably the area fraction of the martensite grain having La / Lb of 3.0 or less may be 50% or more compared to fM. More preferably, the area fraction of the martensite grain having La / Lb of 2.0 or less may be 50% or more as compared to fM. Moreover, when the fraction of equiaxed martensite is less than 50%, compared to fM, the local deformation capacity may deteriorate. In addition, the lower bound of Expression 2 can be 1.0. Further, all martensite or part thereof may be martensite tempered. When martensite is welded martensite, although the strength of the sheet steel is decreased, the borehole expansion capacity of the sheet steel is improved by the decrease of the hardness difference between the main phase and the secondary phase. According to the balance between the required strength and the required deformation capacity, the fraction of area of the annealed martensite can be controlled as compared to the fraction of the martensite fM area. The metallographic structure such as ferrite, bainite, or martensite as described above can be observed by a field-emission electronic scanning microscope (FE-SEM) in a thickness range of 1/8 to 3/8 (the thickness range in which the 1/4 position of the thickness is the center). The above characteristic values can be determined from the microphotographs that are obtained by observation. In addition, the characteristic values can also be determined by the EBSD as described below. For FE-SEM observation, samples are collected so that the observed section is the cross-section of the thickness (its normal vector corresponds to the normal direction) that is parallel to the rolling direction of the steel plate, and the observed section is polished and causticated with nital. Furthermore, in the thickness direction, the metallographic (constituent) structure of the steel sheet may be significantly different between the vicinity of the steel sheet surface and the vicinity of the center of the sheet steel due to decarburization and Mn segregation. Accordingly, in the embodiment, the metallographic structure based on 1/4 of the thickness is observed.
Medium Grain Diameter: 5 pm to 30 pm In addition, to also improve the deformability, the grain size in the metallographic structure, particularly the average diameter, can be refined. Furthermore, the fatigue properties (fatigue limit ratio) required for a steel sheet for automobile or the like are also improved by the refining of the median diameter. Since the number of coarse grains significantly influences the deformation capacity when compared to the number of fine grains, the deformation capacity correlates significantly with the mean diameter calculated by the mean volume compared to the average diameter. Accordingly, to achieve the above effect, the average diameter may be 5 Âμm to 30 Âμm, may be more preferably 5 Âμm to 20 Âμm, and may be still more preferably 5 Âμm to 10 Âμm.
In addition, it is considered that when the mean diameter is decreased, the local stress concentration occurring in the micro-order is suppressed, the stress can be dispersed during local deformation, and the elongation, particularly the uniform elongation, is improved. In addition, when the mean diameter is decreased, the grain boundary acting as a displacement barrier can be adequately controlled, the grain boundary can affect the repetitive plastic deformation (fatigue phenomenon) derived from the displacement, and thus the properties of fatigue can be improved.
Further, as described below, the diameter of each grain (unit of grain) can be determined. Perlite is identified by a metallographic observation by an optical microscope. In addition, the grain units of ferrite, austenite, bainite, and martensite are identified by EBSD. If the crystal structure of an area measured by EBSD is a cubic face face structure (fcc structure), the area is considered as austenite. In addition, if the crystal structure of an area measured by EBSD is a cubic structure of a centered body (bcc structure), the area is considered as any between ferrite, bainite and martensite. Ferrite, bainite and martensite can be identified using a Kernel Mean Disorientation (KAM) method that is added in an Electron Back Scatter Diffraction Pattern (EBSP-OIM). In the KAM method, in relation to the first approximation (total 7 pixels) using a regular hexagonal pixel (center pixel) in the measurement data and 6 pixels adjacent to the central pixel, a second approximation (total 19 pixels) using 12 pixels also out of the 6 pixels above, or a third approximation (total 37 pixels) using 18 pixels also outside the 12 pixels above, the average of the disorientation between each pixel is calculated, and the above operation is performed on all pixels. The calculation by the KAM method is performed so as not to exceed the edge of the grain, and a map representing the rotation of the intergranular crystal can be obtained. The map shows the voltage distribution based on local intergranular crystal rotation. In the embodiment, the disorientation between adjacent pixels is calculated using the third approximation in EBSP-IOM (trademark). For example, the measurement of the orientation described above is conducted by a measurement step of 0.5 Âμm or less at a magnification of 1500 times, the position at which the disorientation between adjacent measurement points is greater than 15Â ° is considered as the edge (the edge of the grain is not always a common grain boundary), the equivalent diameter of the circle is calculated, and thus the grain sizes of ferrite, bainite, martensite, and austenite are obtained. When perlite is included in the metallographic structure, the grain size of the perlite may be calculated by applying an image processing method such as binarization processing or intersecting method to a photomicrograph obtained by the optical microscope.
In the grain (grain unit) defined as described above, when the radius of the equivalent circle (half of the equivalent circle diameter value) is defined as r, the volume of each grain is obtained by 4 χ π χ r3 / 3, and the mean diameter can be obtained by the heavy volume average. In addition, the area fraction of the gross grains described below can be obtained by dividing the area fraction of the gross grains obtained using the method by the measured area. Furthermore, except for the mean diameter, the equivalent circle diameter of the grain size obtained by binarization processing, the intersection method, or the like is used, for example, as the average grain size day of the martensite.
The mean dis distance between the martensite grains can be determined by using the frontier between the martensite grain and the different martensite grain obtained by the EBSD method (however, FE-SEM in which the EBSD can be conducted) in addition to the FE-SEM observation method.
[0089] In addition, in order to also improve the local deformation capacity, in relation to all the constituents of the metallographic structure, the area fraction (gross grain area fraction) that is occupied by grains (grain grains) having a grain size of more than 35 μm per unit area may be limited to be 0% to 10%. When the grains having large size are increased, the tensile strength can be decreased, and the local deformation capacity can also be decreased. Accordingly, a grain refining is preferable. In addition, since the local deformation capacity is improved by tensing all grains uniformly and equivalently, the local grain tension can be suppressed by limiting the grain fraction.
[0091] In addition, in order to also improve local deformability such as folding capacity, drawability of flanging capacity, the ability to conformation, or bore expansion capability, it is preferred that the martensite which is the hard phase be dispersed in the metallographic structure. Therefore, it is preferred that the standard deviation of the mean distance dis between the martensite grains is 0 æm to 5 æm. In this case, the mean distance dis and its standard deviation can be obtained by measuring the distance between the martensite grains by 100 points or more.
Ferrite Hardness H: It is preferable to satisfy Expression 3 below. Ferrite which is the primary phase and the soft phase contributes to the improvement in the deformability of the sheet of steel. Accordingly, it is preferred that the average hardness H of the ferrite satisfies Expression 3 below. When the ferrite which is harder than Expression 3 below is contained, the effects of improving the deformability of the sheet steel can not be obtained. In addition, the average hardness H of the ferrite is obtained by measuring the hardness of the ferrite at 100 points or more under a load of 1 mN in a nanopenetrator. H <200 + 30 χ [Si] + 21 χ [Mn] + 270 χ [P] + 78 χ [Nb] 1/2 + 108 χ [Ti] 1/2 ... Expression 3 Here, Si], [Μη], [P], [Nb], and [Ti] represent mass percentages of Si, Μη, P, Nb, and Ti respectively.
As a result of the investigation which is focused on the homogeneity of ferrite or bainite which is the main phase by the inventors, it is found that when the homogeneity of the main phase is high in the microstructure, the balance between the uniform deformation capacity and the local deformation capacity can preferably be improved. Specifically, when a value in which the standard deviation of ferrite hardness divided by the average ferrite hardness is 0.2 or less, effects may preferably be obtained. Further, when a value in which the standard deviation of the bainite hardness divided by the average bainite hardness is 0.2 or less, the effects may preferably be obtained. Homogeneity can be obtained by measuring the hardness of the ferrite or bainite which is the primary phase at 100 points or more under a load of 1 mN in the nanopetter and using the mean obtained and the standard deviation obtained. Specifically, homogeneity increases with the decrease in the standard deviation value of the hardness / average hardness, and the effects can be obtained when the value is 0.2 or less. In the non-penetrator (for example, UMIS-2000 produced by CSIRO Corporation), using a penetrator smaller than the grain size, the hardness of a single grain, which does not include the grain edge, can be measured.
The chemical composition of the hot-rolled steel sheet according to the embodiment will now be described.
Hereinafter, the base elements of the hot-rolled sheet according to the embodiment and the limiting range and the reasons for the limitation will be given. In addition,% in the description represents% by mass. C: 0.01% to 0.4% [00100] C (carbon) is an element that increases the strength of the sheet steel, and is an essential element to obtain the fraction of martensite area. The lower limit of the C content should be 0.01% to obtain the martensite of 1% or more in% by area. On the other hand, when the C content is greater than 0.40%, the deformability of the sheet steel is decreased, and the welding ability of the sheet steel also deteriorates. Preferably, the C content may be 0.30% or less.
Si: 0.001% to 2.5% Si (silicon) is a deoxidizing element of the steel and is an element which is effective in increasing the mechanical strength of the sheet steel. In addition, Si is an element that stabilizes the ferrite during temperature control after hot rolling and suppresses the precipitation of cementite during the bainitic transformation. However, when the Si content is greater than 2.5%, the deformability of the sheet steel is decreased, and dents tend to be made on the surface of the sheet steel. On the other hand, when the Si content is less than 0.001%, the effects are difficult to obtain.
Mn: 0.001% to 4.0% Mn (manganese) is an element that is effective in increasing the mechanical strength of the sheet steel. However, when the Mn content is higher than 4.0%, the deformation capacity of the steel sheet is decreased. More preferably, the Mn content may be 3.0% or less. On the other hand, when the Mn content is less than 0.001%, the effects are difficult to obtain. In addition, Mn is also an element that suppresses fractures during hot rolling by fixing the S (sulfur) on the steel. When elements such as Ti which suppress the occurrence of fractures due to S during hot rolling are not added sufficiently except by Mn, it is preferred that the Mn content and the S content meet Mn / S> 20 in% by weight.
Al: 0.001% to 2.0% Al (aluminum) is a deoxidizing element of the steel. In addition, Al is an element that stabilizes ferrite during temperature control after hot rolling and suppresses precipitation of cementite during bainitic transformation. To obtain the effects, the Al content should be 0.001% or more. However, when the Al content is higher than 2.0%, the welding capacity deteriorates. In addition, although it is difficult to quantitatively show the effects, Al is an element that significantly increases the temperature Ara in which the transformation starts from γ (austenite) to α (ferrite) in the cooling of the steel. Accordingly, the steel's steel can be controlled by the Al content. The hot-rolled steel sheet according to the embodiment includes the inevitable impurities in addition to the base elements described above. Here, the inevitable impurities indicate elements such as P, S, N, O, Cd, Zn, or Sb which are inevitably mixed from auxiliary raw materials such as scrap or by production processes. In the elements, P, S, N, and O are limited to the following to obtain preferably the effects. It is preferred that unavoidable impurities other than P, S, N, and O are individually limited to 0.02% or less. In addition, even when impurities of 0.02% or less are included, the effects are not affected. The impurities limitation range includes 0%, however it is industrially difficult to be stably 0%. Here, the described is% by mass. Ρ: 0.15% or less P (phosphorus) is an impurity, and an element contributing to fracture during hot rolling or cold rolling when the steel content is excessive. In addition, P is an element that impairs the ductility or weldability of the sheet steel. Accordingly, the P content is limited to 0.15% or less. Preferably, the P content may be limited to 0.05% or less. In addition, since P acts as a reinforcing element for the solid solution and is inevitably included in steel, it is not particularly necessary to prescribe the lower limit of the P content. The lower limit of the P content may be 0%. In addition, considering the current general refining (including secondary refining), the lower limit of the P content may be 0.0005%. S: 0.03% or less S (sulfur) is an impurity, and an element that deteriorates the deformability of the sheet of steel by the formation of MnS drawn by hot rolling when the content in the steel is excessive. Accordingly, the S content is limited to 0.03% or less. Moreover, since S is inevitably included in steel, it is not particularly necessary to prescribe the lower limit of the S content. The lower limit of the S content may be 0%. In addition, considering the current general refining (including secondary refining), the lower limit of the P content may be 0.0005%. N: 0.01% or less N (nitrogen) is an impurity, and an element that impairs the deformability of the sheet steel. Accordingly, the N content is limited to 0.01% or less. Furthermore, since N is inevitably included in steel, it is not particularly necessary to prescribe the lower limit of the N content. The lower limit of the N content may be 0%. In addition, considering the current general refining (including secondary refining), the lower limit of the N content may be 0.0005%. Ο: 0.01% or less O (oxygen) is an impurity, and an element that deteriorates the deformability of the sheet steel. Accordingly, the O content is limited to 0.01% or less. Moreover, since O is inevitably included in steel, it is not particularly necessary to prescribe a lower limit of O content. The lower limit of O content may be 0%. Furthermore, considering the current general refining (including secondary refining), the lower limit of the O content may be 0.0005%. The chemical elements are base components of the steel in the embodiment, and the chemical composition in which the base elements are controlled (included or limited) and the balance consists of Fe and the inevitable impurities, is the base composition of the implementation. However, in addition to the base elements (instead of a part of the Fe which is the balance), in the embodiment, the following chemical elements (optional elements) may be further included in the steel if necessary. Furthermore, even when the optional elements are inevitably included in the steel (e.g., smaller amounts than the lower limit of each optional element), the effects in the embodiment are not diminished.
Specifically, the hot-rolled steel sheet according to the embodiment may also include, as an optional element, at least one element selected from the group consisting of Mo, Cr, Ni, Cu, B, Nb, Ti, V, W , Ca, Mg, Zr, REM, As, Co, Sn, Pb, Y, and Hf in addition to the base elements and the impurity elements. Hereinafter, the numerical ranges of the limitation and the reasons for the limitation of the optional elements will be described. Here, the percentage described is% by mass.
Ti: 0.001% to 0.2% Nb: 0.001% to 0.2% Β: 0.001% to 0.005% Ti (titanium), Nb (niobium), and B (boron) are optional elements that form carbonitrides thin by the fixation of carbon and nitrogen in steel, and which have effects such as precipitation reinforcement, microstructure control, or grain refining reinforcement for steel. Accordingly, if necessary, at least one element between Ti, Nb and B may be added to the steel. To obtain the effects, preferably the Ti content may be 0.001% or more, the Nb content may be 0.001% or more, and the B content may be 0.0001% or more. However, when optional elements are added excessively to the steel, the effects may be saturated, crystal orientation control may be difficult due to the suppression of recrystallization after hot rolling, and the working ability (deformability) of the sheet may deteriorate. Accordingly, preferably, the Ti content may be 0.2% or less, the Nb content may be 0.2% or less, and the B content may be 0.005% or less. Further, even when optional elements having an amount less than the lower limit are included in the steel, the effects in the embodiment are not diminished. In addition, since it is not necessary to add optional elements to the steel intentionally to reduce connection costs, the lower limits of the quantities of the optional elements may be 0%.
Mg: 0.0001% to 0.01% REM: 0.0001% to 0.1% Ca: 0.0001% to 0.01% Ma (magnesium), REM (rare earth metal), and Ca (calcium) are the optional elements that are important for controlling inclusions to be harmless forms and for improving the local deformation capacity of the sheet steel. Accordingly, if necessary, at least one element between Mg, REM, and Ca may be added to the steel. To obtain the effects, preferably the Mg content may be 0.0001% or more, the REM content may be 0.0001% or more, and the Ca content may be 0.0001% or more. On the other hand, when the optional elements are added excessively to the steel, inclusions having drawn shapes can be formed, and the deformability of the steel sheet can be decreased. Accordingly, preferably the Mg content may be 0.01% or less, the REM content may be 0.1% or less, and the Ca content may be 0.01% or less. Furthermore, even when optional elements having amounts smaller than the lower limit are included in the steel, the effects in the embodiment are not diminished. In addition, since it is not necessary to add the optional elements to the steel intentionally to reduce connection costs, the lower limits of the quantities of the optional elements may be 0%.
In addition, herein, REM collectively represents the total of 16 elements which are 15 lanthanum elements with atomic number 57, the lutetium with atomic number 71 in addition to the scandium with atomic number 21. In general, REM is provided in the state of misch metal which is a mixture of elements, and is added to steel.
Mo: 0.001% to 1.0% Cr: 0.001% to 2.0% Ni: 0.001% to 2.0% W: 0.001% to 1.0% Zr: 0.0001% to 0.2% As: 0.0001% to 0.5% Mo (molybdenum) Cr (chromium), Ni (nickel), W (tungsten), Zr (zirconium), and As (arsenic) are optional elements that increase the mechanical strength of the sheet steel. Accordingly, if necessary, at least one element between Mo, Cr, Ni, W, Zr, and As can be added to the steel. To obtain the effects, preferably the Mo content may be 0.001% or more, the Cr content may be 0.001% or more, the Ni content may be 0.001% or more, the W content may be 0.001% or more , the Zr content may be 0.0001% or more, and the As content may be 0.0001% or more. However, when the optional elements are added excessively to the steel, the deformability of the steel sheet can be decreased. Accordingly, preferably, the Mo content may be 1.0% or less, the Cr content may be 2.0% or less, the Ni content may be 2.0% or less, the W content may be 1% , 0% or less, the Zr content may be 0.2% or less, and the As content may be 0.5% or less. Further, even when optional elements having an amount less than the lower limit are included in the steel, the effects in the embodiment are not diminished. In addition, since it is not necessary to add the optional elements to the steel intentionally to reduce connection costs, the lower limits of the optional elements may be 0%. V: 0.001% 1.0% Cu: 0.001% to 2.0% V (vanadium) and Cu (copper) are optional elements which are similar to Nb, Ti or the like, and which have the effect of enhancing precipitation. In addition, the decrease in local deformation capacity due to addition of V and Cu is small compared to that of addition of Nb, Ti or the like. Accordingly, in order to obtain high strength and also increase local deformability such as bore expansion capacity or bending capacity, V and Cu are optional elements more effective than Nb, Ti or the like. Therefore, if necessary, at least one element between V and Cu can be added to the steel. To obtain the effects, preferably, the V content may be 0.001% or less and the Cu content may be 0.001% or less. However, when the optional elements are added excessively, the deformability of the steel sheet can be decreased. Accordingly, preferably, the V content may be 1.0% or less and the Cu content may be 2.0% or less. Further, even when optional elements having less than the lower limit are included in the steel, the effects in the embodiment are not diminished. In addition, since it is not necessary to add the optional elements to the steel intentionally to reduce the connection costs, the lower limits of the optional elements may be 0%.
Co: 0.0001% to 1.0% Although it is difficult to quantitatively show the effects, Co (cobalt) is the optional element that significantly increases the temperature at which Ara begins to transform γ (austenite) to α ( ferrite) in the cooling of the steel. Accordingly, the steel can be controlled by the content of Co. In addition, Co is the optional element which improves the strength of the sheet steel. To obtain the effect, preferably the Co content may be 0.0001% or more. However, when Co is added excessively to the steel, the weldability of the sheet steel can deteriorate, and the deformability of the sheet steel can be decreased. Accordingly, preferably, the Co content may be 1.0% or less. Further, even when the optional member having an amount less than the lower limit is included in the steel, the effects in the embodiment are not diminished. In addition, since it is not necessary to add the optional element to the steel intentionally to reduce the connection costs, the lower limit of the amount of optional element may be 0%.
Sn: 0.0001% to 0.2% Pb: 0.0001% to 0.2% Sn (tin) and Pb (lead) are optional elements which are effective in improving the wettability of the coating and adhesion of the coating. Accordingly, if necessary, at least one element between Sn and Pb may be added to the steel. To obtain the effects, preferably, the Sn content may be 0.0001% or more and the P content may be 0.0001% or more. However, when optional elements are added excessively to steel, fractures may occur during hot work due to brittleness at high temperature, and surface dents tend to be produced in the sheet steel. Accordingly, preferably, the Sn content may be 0.2% or less and the Pb content may be 0.2% or less. Further, even when optional elements having amounts smaller than the lower limit are included in the steel, the effects in the embodiment are not diminished. In addition, since it is not necessary to add the optional elements to the steel intentionally to reduce the connection costs, the lower limits of the quantities of the optional elements may be 0%. Y: 0.0001% to 0.2% Hf: 0.0001% to 0.2% Y (yttrium) and Hf (hafnium) are optional elements that are effective in improving the corrosion resistance of the sheet steel . Accordingly, if necessary, at least one element between Y and Hf may be added to the steel. To obtain the effect, preferably the Y content may be 0.0001% or more and the Hf content may be 0.0001% or more. However, when the optional elements are added excessively to the steel, the local deformability such as the hole expansion capacity can be decreased. Accordingly, preferably, the Y content may be 0.20% or less and the Hf content may be 0.20% or less. In addition, Y has the effect of forming oxides in steel and of absorbing hydrogen in steel. Consequently, the diffusible hydrogen in the steel is decreased, and an improvement in the properties of resistance to the embrittlement by the hydrogen in the sheet of steel can be expected. The effect can also be obtained within the range described above of the Y content. Furthermore, even when optional elements having amounts smaller than the lower limit are included in the steel, the effects in the embodiment are decreased. In addition, since it is not necessary to add the optional elements to the steel intentionally to reduce connection costs, the lower limits of the quantities of the optional elements may be 0%.
As described above, the hot-rolled steel sheet according to the embodiment has a chemical composition which includes the base elements described above and the balance consisting of Fe and the inevitable impurities, or has a chemical composition comprising the described base elements above, at least one element selected from the group consisting of the optional elements described above, and the balance consisting of Fe and the inevitable impurities.
In addition, a surface treatment may be conducted on the hot-rolled steel sheet according to the embodiment. For example, surface treatment such as electrocoating, hot dip coating, evaporative coating, bonding treatment after coating, organic film formation, film lamination, treatment with organic salt and inorganic salt, or treatment without chromium ( treatment can be applied, and thus the hot-rolled steel sheet may include various types of films (film or coating). For example, a galvanized layer or a galvanized layer may be disposed on the surface of the hot-rolled steel sheet. Even if the hot-rolled steel sheet includes the above-described coating, the sheet steel can achieve high strength and can sufficiently warrant uniform deformation capacity and local deformability.
In addition, in the embodiment, the thickness of the hot-rolled steel sheet is not particularly limited. However, for example, the thickness may be 1.5 mm to 10 mm, and may be 2.0 mm to 10 mm. In addition, the strength of the hot-rolled steel sheet is not particularly limited, and, for example, the tensile strength may be 440 MPa at 1500 MPa.
The hot-rolled steel sheet according to the embodiment can be applied for general use to the high-strength sheet, and has an excellent uniform deformation capacity and a markedly improved local deformation capacity such as folding working capacity or Hole expansion capacity of high strength steel sheet.
In addition, since the directions in which the folding is conducted to the hot-rolled sheet differ in the parts being folded, the direction is not particularly limited. In the hot-rolled steel sheet according to the embodiment, similar properties can be obtained in any folding direction, and the hot-rolled steel sheet can be subjected to the composite forming including working modes such as folding, drawing or stamping.
A method for producing hot rolled steel sheet according to one embodiment of the present invention will now be described. To produce hot rolled steel sheet having high strength, excellent uniform deformation capacity, and excellent local deformability, it is important to control the chemical composition of the steel, the metallographic structure, and the texture that is represented by the densities polo of each orientation of a group of specific crystal orientations. Details will be described below.
The production process prior to hot rolling is not particularly limited. For example, steel (molten steel) can be obtained by conducting a smelting and refining using a blast furnace, an electric furnace, a converter, or the like, and subsequently carrying out various types of secondary refining for melt the steel that satisfies the chemical composition. Subsequently, to obtain a steel piece or a plate from the steel, for example, the steel can be cast by a casting process such as a continuous casting process, a conventional casting process, or a casting process of thin plates in general. In the case of continuous casting, the steel may be subjected to hot rolling after the steel has been cooled once to a lower temperature (eg ambient temperature) and reheated, or the steel (lin-dropleted plate) can be continuously subjected to the hot rolling immediately after the steel is cast. In addition, scrap can be used as the raw material for steel (cast steel).
In order to obtain a high strength steel plate having high strength, excellent uniform deformation capacity, and excellent local deformation ability, the following conditions can be satisfied. In addition, henceforth "steel" and "steel plate" are synonyms.
First Hot Rolling Process In the first hot rolling process, using the cast and cast steel part, a rolling pass whose reduction is 40% or more is conducted at least once in a temperature range 1000Â ° C to 1200Â ° C (preferably 1150Â ° C or less). By conducting the first hot rolling under these conditions, the average grain size of the austenite of the sheet steel after the first rolling process is controlled at 200 æm or less, which contributes to the improvement of the uniform deformation capacity and to the local deformation capacity of the hot-rolled sheet finally obtained.
The austenite grains are refined with an increase in reduction and an increase in the frequency of the rolling. For example, in the first lamination process, leading at least twice (two shifts) of the lamination whose reduction is 40% or more per pass, the average grain size of the austenite of the austenite may preferably be controlled at 100 μl or less . In addition, in the first hot rolling, the reduction being reduced to 70% or less per pass, or by limiting the rolling frequency (the number of times the deviations) to 10 times or less, the falling sheet temperature of steel or excessive rosewood formation can be reduced. Consequently, in the roughing lamination, the pass reduction can be 70% or less, and the frequency of the lamination (number of times of the deviations) can be 10 times or less.
As described above, by refining the austenite grains after the first hot rolling process, it is preferred that the austenite grains may also be refined by the subsequent processes, and the ferrite, the bainite, and the martensite transformed from of the austenite in later processes can be finely and uniformly dispersed. As a result, the anisotropy and local deformation capacity of the sheet steel are improved because the texture is controlled, and the uniform deformation capacity and local deformation capacity (particularly the uniform deformation capacity) of the sheet are improved due to the fact that the metallographic structure is refined. Furthermore, it appears that the edge of the austenite grain refined by the first lamination process acts as one of the recrystallization cores during the second hot rolling process which is the subsequent process.
In order to inspect the average grain size of austenite after the first hot rolling process, it is preferred that the steel sheet after the first hot rolling process is cooled rapidly at a cooling rate as fast as possible. For example, the sheet steel is cooled under an average cooling rate of 10 ° C / s or faster. Subsequently, the cross section of the sheet which is taken from the sheet of steel obtained by the cooling is quenched to make visible the austenite grain, and the edge of the austenite grain in the microstructure is observed by an optical microscope. At that time, 20 or more visual fields are observed at a magnification of 50 times or more, the grain size of the austenite is measured by image analysis of the intersection method, and the mean grain size of the austenite is obtained by taking measurements of the austenite grains measured in each of the visual fields.
After the first hot rolling process, the bars may be joined, and the second hot rolling process, which is the subsequent process, may be continuously driven. At that time, the sheets may be joined after the blank is temporarily coiled in a coil form, stored in a cover having a heater if necessary, and rewound. Second Hot Lamination Process In the second hot lamination process, when the temperature calculated by Expression 4 below is defined as T1 in units of ° C, the sheet of steel after the first hot rolling process is subjected to lamination under conditions such that a large reduction pass whose reduction is 30% or more in a temperature range from T1 + 30 ° C to T1 + 200 ° C is included, the cumulative reduction in the temperature range of T1 + 30 ° C to T1 + 200 ° C is 50%, the cumulative reduction in a temperature range of Ar 3 ° C unless T1 + 30 ° C is limited to 30% or less, and the end-of-lamination temperature is Ara ° C or more.
As one of the conditions for controlling the average pole density D1 of the orientation group from {100} To {223} <110> and the density D2 of the crystal orientation {332} <113> in the center portion - the thickness is the range of 5/8 to 3/8 for the strips described above, in the second lamination process, the lamination is controlled based on the temperature T1 (unit: ° C) which is determined by Expression 4 a chemical composition (unit:% by mass) of the steel. [[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[+ (C), [N], [Mn], [Nb], [Ti], [B], [Cr], [Mo] , and [V] represent percentages by mass of C, N, Mn, Nb, Ti, B, Cr, Mo, and V respectively.
The amount of the chemical element that is included in Expression 4 but not included in the steel is considered as 0% for the calculation. Therefore, in the case of the chemical composition in which the steel includes only the base elements, Expression 5 below can be used instead of Expression 4. T1 = 850 + 10 χ ([C] + [N]) χ [Mn] In addition, in the chemical composition in which the steel includes the optional elements, the temperature calculated by Expression 4 can be used for T1 (unit: ° C) instead of the temperature calculated by Expression 5 .
In the process of the second hot rolling, on the basis of temperature 1 (unit: ° C) obtained by Expression 4 or 5, the large reduction is included in the temperature range from T1 + 30 ° C to T1 + 200 ° C (preferably in a temperature range of T1 + 50 ° C to T1 + 100 ° C), and reduction is limited to a small range (including 0%) in the range of temperatures of ° C to less than T1 + 30 ° C. By conducting the process of the second hot rolling in addition to the first hot rolling process, the uniform deformation capacity and the local deformability of the sheet of steel are preferably improved. Particularly, including the large reduction in the temperature range from T1 + 30 ° C to T1 + 200 ° C and limiting the reduction in the temperature range from Ar3 ° C to T1 + 30 ° C, the mean density D1 of the orientation group from {100} to {223} <110> and the density D2 of the crystal orientation {332} <113> in the central portion of the thickness which is the thickness range of 5 / 8 to 3/8 are sufficiently controlled, and as a result, the anisotropy and local deformability of the sheet steel are remarkably improved.
The temperature T1 is obtained empirically. It is empirically discovered by the inventors through experiments that the range of temperatures at which the recrystallization of the austenite strip from each steel is promoted can be determined based on the temperature T1. In order to obtain the excellent uniform deformation capacity and the excellent local deformation capacity, it is important to accumulate a large amount of tension by the lamination and to obtain the fine recrystallization grains. Accordingly, the lamination having various deviations is conducted in the temperature range from T1 + 30 ° C to T1 + 200 ° C, and the cumulative reduction should be 50% or more. In addition, to also promote recrystallization by the accumulation of tension, it is preferred that the cumulative reduction be 70% or more. Furthermore, by limiting the upper limit of cumulative reduction, the rolling temperature can be sufficiently maintained, and the rolling load can also be suppressed. Consequently, the cumulative reduction may be 90% or less.
When a lamination having various shifts is conducted in a temperature range of T1 + 30 ° C to T1 + 200 ° C, the tension is accumulated by rolling, and the recrystallization of the austenite occurs in a range between the lamination shifts by a driving force derived from the accumulated tension. Specifically, leading to rolling having various deviations in the temperature range from T1 + 30 ° C to T1 + 200 ° C, recrystallization occurs repeatedly with each pass. Consequently, it is possible to obtain the structure of the recrystallized austenite which is uniform, thin and equiaxial. In the temperature range, dynamic recrystallization did not occur during rolling, the stress is accumulated in the crystal, and static recrystallization occurred in the interval between the lamination deviations by the driving force derived from the accumulated voltage. In general, in the recrystallized dynamic structure, the voltage that is introduced during the work is accumulated in its crystal, and the recrystallized area and the non-recrystallized area are mixed locally. Consequently, the texture is comparatively developed, and thus the anisotropy appears. In addition, metallographic structures may be a duplex grain structure. In the method for producing the hot-rolled steel sheet according to the embodiment, the austenite is recrystallized by the static recrystallization. Accordingly, it is possible to obtain the recrystallized austenite structure which is uniform, fine and equiaxial, and in which the texture development is suppressed.
In order to increase homogeneity, and preferably to increase the uniform deformation capacity and the local deformation capacity of the sheet of steel, the second hot rolling is controlled so as to include at least one large reduction pass whose reduction by pass is 30% or more in the temperature range from T1 + 30 ° C to T1 + 200 ° C. In the second hot rolling mill, in the temperature range from T1 + 30 ° C to T1 + 200 ° C, the lamination whose reduction by pass is 30% or more is driven at least once. Particularly, in view of the cooling process as described below, the final pass reduction in the temperature range may preferably be 25% or more, and may be more preferably 30% or more. Specifically, it is preferred that the final pass in the temperature range is the high pass (the rolling pass with reduction of 30% or more). In the case where also the excellent deformation capacity is required in the sheet steel, it is also preferred that any reduction of the first half of the deviations is less than 30% and the reductions of the two final deviations are individually 20% or more. For a more preferable increase in the homogeneity of the sheet steel, a large reduction pass whose reduction by pass is 40% or more can be driven. In addition, to obtain a more excellent shape of the sheet steel, a large reduction pass whose reduction by pass is 70% or less can be driven. Furthermore, in the rolling in the temperature range from T1 + 30 ° C to T1 + 200 ° C, by suppressing the temperature rise of the sheet of steel between lamination deviations to 18 ° C or less, it is possible to obtain preferably austenite recrystallized which is more uniform.
In order to suppress the development of the texture and to keep the equiaxial structure recrystallized, after rolling in the temperature range from T1 + 30 ° C to T1 + 200 ° C, the amount of work in the temperature range from Ar3 ° C to less that T1 + 30 ° C (preferably T1 less than T1 + 30 ° C) is suppressed to as small as possible. Consequently, the cumulative reduction in the temperature range of Ar3 ° C unless T1 + 30 ° C is limited to 30% or less. In the temperature range, it is preferred that the cumulative reduction be 10% or more to obtain the excellent shape of the sheet of steel, and it is preferred that the cumulative reduction be 10% or less to also improve the anisotropy and local deformability. In that case, the cumulative reduction may be more preferably 0%. Specifically, in the temperature range from Ar 3 ° C to less than T1 + 30 ° C, the rolling can not be conducted, and the cumulative reduction should be 30% or less even when the rolling is conducted.
When the cumulative reduction in the temperature range of Ar 3 ° C unless T1 + 30 ° C is large, the shape of the recrystallized austenite grain in the temperature range from T1 + 30 ° C to T1 + 200 ° C does not must be equiaxial due to the fact that the grain is stretched by the lamination, and the texture is developed again due to the fact that the tension is accumulated by the lamination. Specifically, as a production condition according to the embodiment, the rolling is controlled in both temperature ranges, from T1 + 30 ° C to T1 + 200 ° C, and from Ar3 ° C unless T1 + 30 ° C in the process of second hot rolling. As a result, the austenite is recrystallized to be uniform, fine, and equiaxial, the texture, metallographic structure, and anisotropy of the sheet steel are controlled, and thus the uniform deformation capacity and local deformation capacity can be improved. In addition, the austenite is recrystallized to be uniform, thin and equiaxial, and therefore the ratio of the major axis to the minor axis of martensite, the mean martensite size, the mean distance between the martensite, and the like of the plate hot rolled steel can be controlled.
In the second hot rolling process, when the rolling is conducted in the temperature range less than Ar 3 ° C or the cumulative reduction in the Ar 3 ° C temperature range unless T1 + 30 ° C is excessively large, the austenite texture is developed. As a result, the finely-drawn hot rolled steel sheet does not satisfy at least one of the conditions in which the average pole density D1 of the orientation group from {100} to {223} <110> is 1.0 to 5 , And the condition in which the density D2 of the crystal orientation {332} <113> is 1.0 to 4.0 in the central portion of the thickness. On the other hand, in the process of the second hot rolling, when rolling is conducted in the temperature range of more than T1 + 200 ° C or the cumulative reduction in the temperature range from T1 + 30 ° C to T1 + 200 ° C is excessively small, recrystallization does not occur uniformly and finely, coarse grains or mixed grains may be included in the metallographic structure, and the metallographic structure may be duplex grain structure. Consequently, the area fraction of the mean grain diameter greater than 35 pm is increased.
In addition, when the second hot rolling is finished at a temperature less than Ara (unit: ° C), the steel is rolled over a temperature range from the finishing temperature of the lamination to less than Ara (unit: ° C) which is the range in which two phases of austenite and ferrite exist (two-phase temperature range). Consequently, the texture of the sheet steel is developed, and the anisotropy and local deformation capacity of the sheet steel deteriorate significantly. Here, when the finishing temperature of the lamination of the second lamination is T1 or more, the anisotropy can also be decreased by decreasing the amount of tension in the temperature range less than T1 and as a result the local deformation capacity may also be increased . Therefore, the end temperature of the lamination of the second hot lamination may be T1 or more.
Here, the reduction can be obtained by measuring or calculating from the rolling force. Here, the reduction can be obtained by measurements or calculations from the rolling force, the thickness, or the like. In addition, the rolling temperature (for example, each temperature range above) can be obtained by measurements using a thermometer between the chairs by calculations using a simulation in consideration of the heating of the deformation, line speed, of the reduction, or the like or both (measurements and calculations). In addition, the above reduction per pass is a percentage of a reduced thickness per pass (the difference between the entry thickness before passing through the laminating chair and the outlet thickness after passing through the laminating chair) to the thickness of before you pass the laminating chair. The cumulative reduction is a percentage of cumulatively reduced thickness (the difference between the entry thickness before the first pass in the rolling in each temperature range and the outlet thickness after the final pass in the rolling in each temperature range) for the reference which is the infeed thickness before the first pass in the lamination before each temperature range. Ara, which is the ferritic transformation temperature from the austenite during the cooling, is obtained by Expression 6 and units of ° C. In addition, although it is difficult to quantitatively show the effects as shown above, Al and Co may influence Ara.
Ara = 879.4 - 516.1 x [C] - 65.7 x [Mn] + 38.0 x [Si] + 274.7 x [P] ... Expression 6 In Expression 6, C], [Mn], [Si] and [P] represent mass percentages of C, Mn, Si and P respectively.
First Cooling Process In the first cooling process, after the final pass between the large reduction shifts whose reduction by pass is 30% or more in the temperature range from T1 + 30 ° C to T1 + 200 ° C is terminated , when the waiting time from the end of the final pass to the beginning of the cooling is set to t in units of seconds, the steel plate is subjected to cooling so that the waiting time t satisfies Expression 7 below. Here, t1 in Expression 7 can be obtained from Expression 8 below. In Expression 8, Tf represents the temperature (unit: ° C) of the steel plate at the end of the final pass between the large reduction offsets, and P1 represents the reduction (unit:%) in the final pass between the reduction passages. (Tf - T1) χ P1 / 100) + 3.1 ... Expression 7 t1 = 0.001 x ((Tf - T1) χ P1 / 100) 2 - 0.109 x The first cooling after the final large reduction pass significantly influences the grain size of the finally obtained hot-rolled steel sheet. In addition, by the first cooling, the austenite can be controlled to be a metallographic structure in which the grains are equiaxial and the grains are rarely included (i.e., uniform sizes). Accordingly, the hot-rolled steel sheet finally obtained has the metallographic structure in which the grains are equiaxial and the grains are rarely included (i.e., uniform sizes), and the ratio of the major axis to the minor martensite axis, the average martensite size, the average distance between martensite, and the like may be preferably controlled.
The right hand value (2.5 χ t1) of Expression 7 represents the time at which the recrystallization of the austenite is substantially terminated. When the holding time t is more than the right-hand value (2.5 χ t1) of Expression 7, the recrystallized grains grow significantly, and the grain size is increased. Consequently, the strength, the uniform deformability, the local deformability, the fatigue properties, or the like of the sheet steel are decreased. Therefore, the wait time t should be 2.5 χ t1 seconds or less. In a case where the flowability (for example shape rectification or second cooling control capability) is considered, the first cooling can be conducted between the rolling stands. Also, the lower limit of the wait time t must be 0 seconds or more.
Furthermore, when the holding time t is limited to 0 second and less than t1 seconds so that 0 <t <t1 is satisfied, it may be possible to suppress significantly the growth of the grain. In that case, the average diameter of the finally obtained hot rolled steel sheet can be controlled up to 30 Âμm or less. As a result, even if the recrystallization of the austenite does not progress sufficiently, the properties of the sheet steel, particularly the uniform deformation capacity, the fatigue properties or the like, may preferably be improved.
Furthermore, when the hold time t is limited to t1 seconds to 2.5 χ t1 seconds so that t1 <t <2.5 χ t1 is satisfied, it may be possible to suppress texture development. In this case, although the mean diameter can be increased because the holding time t is prolonged compared to the case where the holding time t is shorter than t1 seconds, the crystal orientation can be randomized because the recrystallization of the austenite progresses enough. As a result, the anisotropy, local deformability, and the like of the sheet steel can preferably be improved.
In addition, the first cooling described above may be conducted in a range between the rolling stands in the temperature range from T1 + 30 ° C to T1 + 200 ° C, or may be conducted after the final rolling chair in the temperature range. Specifically, as long as the wait time satisfies the condition, the lamination whose reduction by pass is 30% or less, the rolling can also be conducted in the temperature range from T1 + 30 ° C to T1 + 200 ° C. Similarly, the first cooling is conducted, provided that the cumulative reduction is 30% or less, the lamination may also be conducted in the temperature range from Ar 3 ° C to T1 + 30 ° C (or Ar 3 ° C to T ° C). As described above, as long as the wait time t after the large reduction pass satisfies the condition, to control the metallographic structure of the finally obtained hot-rolled steel sheet, the first cooling described above can be conducted either in the interval between the chairs or after the laminating chair.
In the first cooling, it is preferred that the change in the cooling temperature which is the difference between the temperature of the steel plate at the start of the cooling and the temperature of the steel plate at the end of cooling is 40 ° C to 140 ° C. When the change in the cooling temperature is 40 ° C or more, the growth of the recrystallized austenite grains may also be suppressed. When the cooling temperature change is 140 ° C or less, the recrystallization may progress far enough, and the pole density may preferably be improved. Furthermore, by limiting the change in the cooling temperature to 140øC or less, in addition to the comparatively easy control of the temperature of the sheet of steel, the variant selection (variant limitation) can be more effectively controlled, and the development of the texture recrystallized can be preferably controlled. Consequently, in this case, the isotropy may also be increased, and the dependence on the orientation of the forming ability may also be decreased. When the change in the cooling temperature is greater than 140 ° C, the progress of recrystallization may be insufficient, the desired texture may not be obtained, the ferrite may not be easily obtained, and the hardness of the obtained ferrite is increased. Accordingly, the uniform deformability and the local deformability of the sheet steel can be decreased. Furthermore, it is preferred that the temperature of the steel plate T2 at the end of the first cooling is T1 + 100 ° C or less. When the temperature of the steel plate T2 at the end of the first cooling is T1 + 100 ° C or less, more sufficient cooling effects are obtained. By the effects of cooling, grain growth can be suppressed, and the growth of austenite grains can also be suppressed.
In addition, it is preferred that the average cooling rate at the first cooling is 50 ° C / s or faster. When the average cooling rate in the first cooling is 50 ° C / s or faster, the growth of the recrystallized austenite grains may also be suppressed. On the other hand, it is not particularly necessary to prescribe an upper limit of the average cooling rate. However, from the plate shape point of view, the average cooling rate may be 300 ° C / s or slower.
Second Cooling Process In the second cooling process, the steel sheet after the second hot rolling and after the first cooling process may preferably be cooled to a temperature range of 600 ° C to 800 ° C at a rate average cooling from 15 ° C / s to 300 ° C / s. When the temperature (unit: ° C) of the sheet steel becomes Are or less by the cooling of the sheet of steel during the second cooling process, the martensite begins to be transformed into ferrite. When the average cooling rate is 15 ° C / sec or faster, the grinding of the austenite grains may preferably be suppressed. It is not particularly necessary to prescribe an upper limit of the average cooling rate. However, from the plate shape point of view, the average cooling rate may be 300 ° C / s or slower. In addition, it is preferable to start the second cooling within 3 seconds after the end of the second hot rolling or after the first cooling process. When the start of the second cooling exceeds 3 seconds, it may occur that the austenite brutalization may occur.
In the retention process, the steel sheet after the second cooling process is maintained in the temperature range of 600 ° C to 800 ° C for 1 second to 15 seconds. If the temperature range is maintained, the transformation from austenite to ferrite proceeds, and therefore the ferrite area fraction can be increased. It is preferred that the steel is retained in a temperature range of 600 ° C to 680 ° C. By conducting the ferritic transformation in the comparatively lower temperature range above, the ferrite structure can be controlled to be thin and uniform. Accordingly, the bainite and martensite which are formed in the subsequent process can be controlled to be fine and uniform in the metallographic structure. In addition, to accelerate the ferritic transformation, the retention time should be 1 second or longer. However, when the retention time is greater than 15 seconds, the ferrite grains may be stiffened, and the cemenite may be precipitated. In a case where the steel is maintained in the comparatively lower temperature range of 600 ° C to 680 ° C, it is preferred that the retention time be from 3 seconds to 15 seconds. Third Cooling Process In the third cooling process, the steel sheet after the retention process is cooled to a temperature range from ambient temperature to 350 ° C under a mean cooling rate of 50 ° C / sec to 300 ° C ° C / s. During the third cooling process, austenite that is not transformed into ferrite even after the retention process is transformed into bainite and martensite. When the third cooling process is interrupted at a temperature greater than 350 ° C, the bainitic transformation proceeds excessively because of the excessively high temperature, and the martensite of 1% or more in% area units can not finally be obtained. Furthermore, it is not particularly necessary to prescribe the lower limit of the quenching temperature of the cooling of the third cooling process. However, in a case where water cooling is conducted, the lower limit may be room temperature. In addition, when the average cooling rate is slower than 50 ° C / s, the pearlitic transformation may occur during cooling. In addition, it is not particularly necessary to prescribe a higher limit of the average cooling rate in the third cooling process. However, from an industrial point of view, the upper limit may be 300 ° C. By decreasing the average cooling rate within the range described above above the cooling rate described above, the fraction of bainite area can be increased. On the other hand, by increasing the average cooling rate within the above-described range of the average cooling rate, the martensite fraction of area can be increased. In addition, the grain sizes of bainite and martensite are also refined.
According to the properties required for hot-rolled steel sheet, the ferrite and bainite area fractions, which are the major phases, can be controlled, and the martensite fraction, which is the phase secondary, can be controlled. As described above, the ferrite can be mainly controlled in the retention process, and the bainite and the martensite can be controlled primarily in the third cooling process. In addition, the grain sizes or morphologies of ferrite and bainite which are the main phase and the martensite which is the secondary phase depend significantly on the grain size or morphology of the austenite which is the microstructure before transformation. In addition, the grain sizes of the morphologies also depend on the retention process and the third cooling process. Consequently, for example, the TS / fM χ dis / day value, which is the ratio of the martensite fM area fraction, the mean martensite day size, the mean dis distance between the martensite, and the tensile strength TS of steel sheet, can be satisfied by multiplying the production processes described above.
In the winding process, the steel sheet after the third cooling begins to be wound at a temperature from ambient temperature to 350 ° C which is the stop temperature of the winding of the third cooling, and the winding plate steel is cooled to air. As described above, the hot-rolled steel sheet according to the embodiment can be produced.
In addition, if necessary the obtained hot-rolled steel sheet can be subjected to a skinpass lamination. By the skinpass lamination, it may be possible to suppress the draw tension which is formed during the working of the sheet steel, or to rectify the shape of the sheet steel.
In addition, the obtained hot rolled steel sheet can be subjected to a surface treatment. For example, surface treatment such as electrocoating, hot dip coating, evaporative coating, post-coating attachment treatment, organic film forming, film lamination, treatment with organic salt or salt inorganic or the non-chromate treatment can be applied to the obtained hot rolled steel sheet. For example, a galvanized layer or a galvanized layer may be disposed on the surface of the hot-rolled steel sheet. Even if the surface treatment is conducted, the uniform deformation capacity and the local deformation capacity are maintained sufficiently.
In addition, if necessary, the tempering treatment or an aging treatment may be conducted as a reheat treatment. By treatment, Nb, Ti, Zr, V, W, Mo, or the like which is solute-solid in the steel may be precipitated as carbides, and the martensite may be softened as stirred martensite. As a result, the hardness difference between the ferrite and bainite which is the main phase and the martensite which is the secondary phase is decreased, and the local deformation capacity such as the hole expansion capacity or the folding capacity is improved . The effects of the reheat treatment may also be obtained by heating to the hot dip coating, the bond treatment or the like.
Example [00164] Hereinafter, the technical features of the aspect of the present invention will be described in detail with respect to the following examples. However, the condition in the examples is an exemplary condition employed to confirm the operability and the effects of the present invention and therefore the present invention is not limited to the exemplary condition. The present invention may employ various conditions provided that the conditions do not fall outside the scope of the present invention and can achieve the object of the present invention. Steels S1 to S98 include chemical compositions (the balance consisting of Fe and the inevitable impurities) shown in Tables 1 to 6 have been examined, and the results are described. After the steels were cast and cast, or after the steels were cooled once to room temperature, the steels were reheated to a temperature range of 900 ° C to 1300 ° C. Thereafter, hot rolling and temperature control (cooling, retention, or the like) were conducted under production conditions shown in Tables 7 to 14, and hot-rolled steel sheets having thicknesses of 2 to 5 mm were obtained.
In Tables 15 to 22, features such as metallographic structure, texture, or mechanical properties are shown. Furthermore, in the Tables, the mean pole density of the orientation group from {100} <221> to <223 <110> is shown as D1 and the crystal orientation pole density {332} <113> is shown as D2 . In addition, the ferrite, bainite, martensite, perlite, and residual austenite area fractions are shown as F, B, fM, P, and γ respectively. In addition, the mean martensite size is shown as day, and the mean martensite distance is shown as dis. Furthermore, in the Tables, the standard deviation ratio of the hardness represents the value by dividing the standard deviation of the hardness by the average of the hardness with respect to the phase having the largest area fraction between the ferrite and the bainite.
As parameters of the local deformation capacity, the λ bore expansion ratio and critical folding radius (d / RmC) were used by 90 ° V-folding of the final product. The folding test was conducted to a folding direction C. In addition, the tensile test (TS, u-EL and EL measurement), the folding test, and the hole expansion test were conducted respectively on the basis of JIS Z 2241, JIS Z 2248 (90 ° V blocking test) and Japan Iron and Steel Federation Standard JFS T1001. Furthermore, using the EBSD described above, the pole densities were measured by a measuring step of 0.5 μm in the central portion of the thickness which was the range of 5/8 to 3/8 of the thickness cross-section (the its normal vector corresponded to the normal direction) that was parallel to the rolling direction in the 1/4 position of the transverse direction. In addition, the r values (Lankford values) of each direction were measured based on JIS Z 2254 (2008) (ISO 10113 (2006)). In addition, the underlined values in the Tables indicate values out of range of the present invention, and blank columns indicate that no binding element was added intentionally.
The productions Nos. P1, P2, P7, P7, P11, P11, P13, P14, P16 to P19, P21 to P27, P29 to P27, P29 to P31, P33, P34, P36 to P41, P48 to P77, and P141 to P180 are examples that satisfy the conditions of the present invention. In the examples, since all conditions of TS> 440 (unit: MPa), TS χ u - EL> 7000 (unit: MPa-%), TS χ λ> 30000 (unit: MPa-%), ed / RmC > 1 (without unit) were satisfied simultaneously, it can be said that hot-rolled steel sheets have high strength, excellent uniform deformation capacity, and excellent local deformation capacity.
On the other hand, P3 to P6, P8, P9, P12, P15, P20, P22, P28, P32, P35, P42 to P47, and P78 to P140 are comparative examples which do not satisfy the conditions of the present invention. In the comparative examples, at least one condition between TS> 440 (unit: MPa), TS χ u - EL> 7000 (unit: MPa-%), TS χ λ> 30000 (unit: 1 (no drive) was not satisfied.
Referring to the examples and comparative examples, the relationship between D1 and d / RmC is shown in FIG. 1, and the relationship between D2 and d / RmC is shown in FIG. 2. As shown in FIG. 1 and FIG. 2, when D1 is 5.0 or less, and when D2 is 4.0 or less, d / RmC> 1 is satisfied.
Table 1 Table 1 (continued) Tabe a 2 (continued) Tabe a 2 (continued) Tabe a 4 (continued) Tabe a 4 (continued) Table 6 Table 7-1 Table 7-1 (continued) Table 8-2 (continued) Table 8-2 (continued) Table 8-2 (continued) Table 9-1 (continued) Table 9-1 (continued) Table 9-1 Table 9-1 (continued) Table 10-1 (continued) 2 Table 10-2 (continued) Table 10 Table 11 (continued) Table 13 (continued) Table 14 (continued) Table 12 (continued) Table 15-1 (continued) Table 15-1 (continued) Table 15-1 (continued) Table 16-1 (continued) Table 16-1 (continued) 2 Tab she 16-2 (continuation;
Table 17-1 Table 17-1 (continued) Table 17-1 (continued;
Table 17-2 (continued;
Table 18-1 (continued) Table 18-1 (continued) Table 19-1 (continued) Table 19-1 (continued) continued) Table 19-3 Table 19-3 (continued) Table 20-1 Table 20-2 Table 20-1 Table 21-1 Table 21-1 (continued) Tabea21-2 Tabe at 21-2 (continued) Table 21-3 Table 21-3 (continued;
Table 22-1 Table 22-1 (continued) Table 22-3 Table 22-3 (continued;
INDUSTRIAL APPLICABILITY According to the above aspects of the present invention it is possible to obtain hot rolled steel sheet having both high strength, excellent uniform deformation capacity and excellent local deformation capacity. Accordingly, the present invention has significant industrial applicability.

权利要求:
Claims (15)
[1]
Steel sheet which is a hot-rolled steel sheet, characterized in that it comprises, as a chemical composition, in mass%, C: 0.01% to 0.4%, Si: 0.001% to 2, 5%, Mn: 0.001% to 4.0%, Al: 0.001% to 2.0%, P: limited to 0.15% or less, S: limited to 0.03% or less, N: limited to 0 , 01% or less, O: limited to 0.01% or less, optionally at least one element selected from the group consisting of: Mo: 0.001% to 1.0%, Cr: 0.001% to 2.0%, Ni: 0.001% to 2.0%, Cu: 0.001% to 2.0%, B: 0.0001% to 0.005%, Nb: 0.001% to 0.2%, Ti: 0.001% to 0.2% 0.001% to 1.0%, W: 0.001% to 1.0%, Ca: 0.0001% to 0.01%, Mg: 0.0001% to 0.01%, Zr: 0.0001% to 0 , 2%, Rare Earth Metal: 0.0001% to 0.1%, As: 0.0001% to 0.5%, Co: 0.0001% to 1.0%, Sn: 0.0001% to 0 , 2%, Pb: 0.0001% to 0.2%, Y: 0.0001% to 0.2%, Hf: 0.0001% to 0.2%, the balance consisting of Fe and the inevitable impurities, where: the mean pole density of a group of orientations from {100} <221 to <223> <110>, which is the density polo represented by the mean arith ethics of the polar densities of each crystal orientation {100} <116>, <110>, <114> <110>, <112> <110>, and <223> <110>, is 1.0 to 5.0 and the pole density of a crystal orientation 332 is 1.0 to 4.0 in a central portion of the thickness which is the range of thicknesses of 5/8 to 3/8 based on the surface sheet steel; the steel sheet includes, as a metallographic structure, various grains, and includes, in area, ferrite and bainite from 30% to 99% in total and martensite from 1% to 70%; and the different microstructures of ferrite, bainite and martensite are limited, in% by area, to 0% to 10%; when the martensite area fraction is defined as fM in% area units, the mean martensite size is defined as day in units of pm, the mean distance between the martensite grains is defined as dis in units of pm, and the tensile strength is defined as TS in units of MPa, Expression 1a and Expression 2 are satisfied, day <13 pm ... Expression 1, TS / fM x dis / day> 500 ... Expression 2; the average grain diameter is 5 pm to 30 pm; when the major axis of martensite is defined as La, and the minor axis of martensite is defined as Lb, the martensite area fraction that satisfies Expression 3 below is 50% to 100% as compared to the fraction fM area of martensite, La / Lb <5.0 ... Expression 3; the sheet of steel includes the martensite sealed in the martensite; the fraction of gross grain area having grain size of more than 35 μm is 0% to 10% between the grains in the metallographic structure of the steel sheet; and the tensile strength is 440 MPa or more.
[2]
Hot rolled steel sheet according to claim 1, characterized in that the average pole density of the orientation group from (100) <221 to (223) <110> is 1.0 to 4.0, and the polar density of the crystal orientation {332} <113> is 1.0 to 3.0.
[3]
Hot rolled steel sheet according to claim 1, characterized in that the steel sheet includes, as a metallographic structure, in% by area, the ferrite of 30% to 99%.
[4]
Hot rolled steel sheet according to claim 1, characterized in that the steel sheet includes, as a metallographic structure, in% by area, the bainite of 5% to 80%.
[5]
Hot rolled steel sheet according to claim 1, characterized in that the hardness H of the ferrite satisfies Expression 4 below, H <200 + 30x [Si] + 21x [Mn] + 270 χ [P] + 78 x [Nb] 1/2 + 108 x [Ti] 1/2 ... Expression 4.
[6]
Hot-rolled steel sheet according to claim 1, characterized in that, when the hardness of the ferrite or bainite which is the main phase is measured at 100 points or more, the value obtained by dividing the standard deviation of the hardness by the mean hardness is 0.2 or less.
[7]
A method for producing a hot rolled steel sheet, characterized in that it comprises: a first hot rolling of the steel in a temperature range of 1000 ° C to 1200 ° C under conditions such that at least one offset reduction ratio is 40% or more is included so as to control the average grain size of austenite in steel to 200 æm or less, wherein the steel includes, as a chemical composition, in mass%, C: 0.01% to 0 , 4%, Si: 0.001% to 2.5%, Mn: 0.001% to 4.0%, Al: 0.001% to 2.0%, P: limited to 0.15% or less, S: limited to 0 , 03% or less, N: limited to 0.01% or less, O: limited to 0.01% or less, optionally at least one element selected from the group consisting of: Mo: 0.001% to 1.0%, Cr : 0.001% to 2.0%, Ni: 0.001% to 2.0%, Cu: 0.001% to 2.0%, B: 0.0001% to 0.005%, Nb: 0.001% to 0.2%, Ti : 0.001% to 0.2%, V: 0.001% to 1.0%, W: 0.001% to 1.0%, Ca: 0.0001% to 0.01%, Mg: 0.1%, Zr: 0.0001% to 0.2%, Rare Earth Metal: 0.0001% to 0.1%, As: 0.0001% to 0.5% Co: 0.0001% to 1.0%, Sn: 0.0001% to 0.2%, Pb: 0.0001% to 0.2%, Y: 0.0001% to 0.2%, Hf: 0.0001% to 0.2%, the balance consisting of Fe and the inevitable impurities; a second hot rolling of the steel under conditions such that when the temperature calculated by Expression 9 below is defined as T1 in units of ° C and the ferritic transformation temperature calculated by Expression 6 below is defined Ara in units of ° C , a large reduction pass whose reduction is 30% or more in a temperature range from T1 + 30 ° C to T1 + 200 ° C is included, the cumulative reduction in the temperature range from T1 + 30 ° C to T1 + 200 ° C is 50% or more, the cumulative reduction in a temperature range of Ara to less than T1 + 30 ° C is limited to 30% or less, and the end temperature of the lamination is Ara or more; a first cooling of the steel under conditions such that when the waiting time from the end of the final pass in the large reduction pass to the beginning of the cooling is set to t in units of seconds, the waiting time t satisfies Expression 7 a following, the average cooling rate is 50 ° C / s or faster, the change in the cooling temperature which is the difference between the temperature of the steel at the start of the cooling and the temperature of the steel at the end of the cooling is 40 ° C a 140 ° C, and the temperature of the steel at the end of the cooling is T1 + 100 ° C or less; a second cooling of the steel to a temperature range of 600 ° C to 800 ° C under an average cooling rate of 15 ° C / s to 300 ° C / s after the completion of the second hot rolling; keep steel in the temperature range of 600 ° C to 800 ° C for 1 second to 15 seconds; a third cooling of the steel to a temperature range from room temperature to 350 ° C under an average cooling rate of 50 ° C / s at 300 ° C / s after completion of retention; (C C) + [N]) + 350 χ [Nb] + 250 χ [Ti] + 40 x Expression 9, here, [C], [N], [Mn], [Nb], [Ti], PI [B] + 100 x [Mo] + 100 χ [V] , [θΓ]> [Mo], θ M are the mass percentage of C, N, Mn, Nb, Ti, B, Cr, Mo, and V respectively, Ara = 879.4-516.1 χ [C] - Expression 6, here, in Expression 6, [C], [Mn], [Si] and [P] (Tf - T1) χ (t) = t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t t P1 / 100) 2 - 0.109 x ((Tf - T1) χ P1 / 100) + 3.1 ... Expression 8, here, Tf represents the Celsius temperature of the steel at the end of the final pass, and P1 represents the percentage of reduction in the final pass.
[8]
A method for producing hot rolled steel sheet according to claim 7, characterized in that the holding time t also satisfies Expression 10 below, T1 <t <t1 ... Expression 10.
[9]
A method for producing hot rolled steel sheet according to claim 7, characterized in that the holding time t also satisfies Expression 11 below, t1 <t1 x 2.5 Expression 11.
[10]
A method for producing hot rolled steel sheet according to claim 7, characterized in that, in the first hot rolling, at least two rolls of which the reduction is 40% or more are conducted, and mean grain size of the austenite is controlled to 100 æm or less.
[11]
A method for producing hot rolled steel sheet according to claim 7, characterized in that the second cooling starts within 3 seconds after the end of the second hot rolling.
[12]
A method for producing hot rolled steel sheet according to claim 7, characterized in that in the second hot rolling the increase in the steel temperature between deviations is 18 ° C or less.
[13]
A method for producing hot rolled steel sheet according to claim 7, characterized in that the final pass of the laminations in the temperature range from T1 + 30 ° C to T1 + 200 ° C is the pass from reduction with a reduction of 30% or more.
[14]
A method for producing hot rolled steel sheet according to claim 7, characterized in that in the retention the steel is maintained in a temperature range of 600 ° C to 680 ° C for 3 seconds at 15 seconds.
[15]
A method for producing hot rolled steel sheet according to claim 7, characterized in that the first cooling is conducted in a range between laminating chairs.

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法律状态:
2018-10-30| B06A| Notification to applicant to reply to the report for non-patentability or inadequacy of the application according art. 36 industrial patent law|

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2019-04-16| B09A| Decision: intention to grant|

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2019-06-25| B16A| Patent or certificate of addition of invention granted|Free format text: PRAZO DE VALIDADE: 20 (VINTE) ANOS CONTADOS A PARTIR DE 24/05/2012, OBSERVADAS AS CONDICOES LEGAIS. (CO) 20 (VINTE) ANOS CONTADOS A PARTIR DE 24/05/2012, OBSERVADAS AS CONDICOES LEGAIS |

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2019-11-12| B25D| Requested change of name of applicant approved|Owner name: NIPPON STEEL CORPORATION (JP) |

优先权:

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申请号 | 申请日 | 专利标题

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JP2011117432|2011-05-25|

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JP2011-117432|2011-05-25|

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PCT/JP2012/063273|WO2012161248A1|2011-05-25|2012-05-24|Hot-rolled steel sheet and process for producing same|

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