Steel plates, components and their manufacturing methods

Abstract

To provide a steel sheet and a member subject having high strength, and excellent in shape uniformity and delayed fracture resistance characteristics; and to provide a manufacturing method of them.SOLUTION: In a steel sheet of the present invention having a steel structure having, in terms of the area ratio, martensite: 20% or more and 100% or less, ferrite: 0% or more and 80% or less, and a residue: 5% or less, a residual stress on the sheet width center after performing V-shaped bending is 800 MPa or less; a residual stress on the sheet width edge after performing V-shaped bending is 90% or more and 110% or less to the residual stress on the sheet width center; and the maximum warpage of the steel sheet when shearing with the length of 1 m in a steel sheet longer direction is 15 mm or less.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a steel plate, a member, and a manufacturing method thereof for use in automobile parts, etc. More specifically, the present invention relates to a steel plate, a member, and a manufacturing method thereof that are high in strength and excellent in shape uniformity and delayed fracture resistance. [Background technology]

[0002] In recent years, from the perspective of protecting the global environment, the entire automobile industry has been required to further improve the fuel efficiency of automobiles in order to reduce CO2 emissions. The most effective way to improve fuel efficiency is to reduce the weight of automobiles by making the parts thinner, so in recent years the amount of high-strength steel sheets used as materials for automobile parts has been increasing.

[0003] Many steel sheets utilize martensite, a hard phase, to obtain steel sheet strength. However, when martensite is generated, transformation strain deteriorates the uniformity of the sheet shape. Since the deterioration of the uniformity of the sheet shape has a negative effect on the dimensional accuracy during forming, the sheet has been straightened by leveling and skin pass rolling (temper rolling) to obtain the desired dimensional accuracy. However, if strain is introduced by leveling and skin pass rolling, the dimensional accuracy during forming deteriorates and the desired dimensional accuracy cannot be obtained. In order to improve the dimensional accuracy, it is necessary to suppress the deterioration of the uniformity of the sheet shape during martensite transformation, and various technologies have been proposed so far.

[0004] For example, in Patent Document 1, the area ratios of ferrite and martensite are controlled to improve shape uniformity and delayed fracture resistance. Specifically, the metal structure is made into a dual-phase steel containing 50-80% by volume of tempered martensite phase and 20-50% by volume of ferrite phase, thereby suppressing hydrogen penetration and providing an ultra-high strength steel sheet with good shape uniformity and delayed fracture resistance.

[0005] Moreover, Patent Document 2 provides a technique for suppressing deterioration of the shape of a steel sheet due to martensitic transformation that occurs during water quenching by restraining the steel sheet with rolls in water. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2010-90432 A [Patent Document 2] Patent No. 6094722 Summary of the Invention [Problem to be solved by the invention]

[0007] Since steel plates used in automobile bodies are pressed before use, good shape uniformity is a necessary property. Furthermore, the amount of high-strength steel plates used in recent automobile part materials is increasing, and the delayed fracture resistance, which is a concern as strength increases, must also be good. Since automobile parts are produced over the full width of the steel plate (hereinafter also referred to as the full width of the steel plate), the delayed fracture resistance must be excellent and consistent over the full width of the steel plate. Therefore, the steel plate must have high strength, excellent shape uniformity, and excellent delayed fracture resistance over the full width of the steel plate.

[0008] The technology disclosed in Patent Document 1 provides a technology that achieves excellent shape uniformity and delayed fracture resistance by controlling the structure, but the shape deteriorates due to the transformation expansion that occurs during martensitic transformation, so the improvement effect on shape uniformity is thought to be inferior to that of the present invention.

[0009] The technique disclosed in Patent Document 2 provides a technique for improving shape uniformity, but is not a technique that provides excellent delayed fracture resistance.

[0010] An object of the present invention is to provide a steel plate, a member and a method for manufacturing the same which are high in strength and excellent in shape uniformity and delayed fracture resistance. [Means for solving the problem]

[0011] In order to solve the above problems, the present inventors have conducted extensive research into the requirements for a steel sheet having a tensile strength of 750 MPa or more and excellent shape uniformity and delayed fracture resistance. As a result, they have found that in order to obtain excellent delayed fracture resistance, it is necessary to set the residual stress at the center of the sheet width to 800 MPa or less. The present inventors have also found that high strength can be achieved by setting the martensite fraction to 20% or more in area fraction through rapid cooling. On the other hand, since martensite transformation during water cooling occurs rapidly and non-uniformly, transformation strain deteriorates the shape uniformity of the steel sheet. As a result of investigating how to reduce the adverse effects of transformation strain, they have come up with the idea that shape uniformity can be improved by applying a restraining force from the front and back sides of the steel sheet during martensite transformation. They have also found that controlling the restraining conditions can reduce the residual stress fluctuation in the width direction and improve delayed fracture resistance over the entire width of the steel sheet.

[0012] As described above, the inventors have conducted various investigations to solve the above problems, and as a result have found that a steel sheet having excellent delayed fracture resistance can be obtained by reducing the residual stress at the center of the sheet width, and that a steel sheet having excellent shape uniformity and delayed fracture resistance over the entire width of the steel sheet can be obtained by controlling the roll restraint conditions, and have thus completed the present invention.

[0013] The gist of the present invention is as follows. [1] A steel structure having, in terms of area ratio, martensite: 20% to 100%, ferrite: 0% to 80%, and the balance: 5% or less; The residual stress at the center of the plate width after V-bending is 800 MPa or less. When V-bending, the residual stress at the width end of the plate is 90% to 110% of the residual stress at the center of the plate. A steel plate whose maximum warp amount when sheared at a length of 1m in the longitudinal direction is 15mm or less. [2] In mass percent, C: 0.05% or more and 0.60% or less, Si: 0.01% or more and 2.0% or less, Mn: 0.1% or more and 3.2% or less, P: 0.050% or less, S: 0.0050% or less, Al: 0.005% or more and 0.10% or less, and The steel sheet according to [1], which has a chemical composition containing N: 0.010% or less, with the balance being Fe and unavoidable impurities. [3] The composition further includes, in mass%, Cr: 0.01% or more and 0.50% or less, Mo: 0.01% or more and less than 0.15%, and The steel plate according to [2], containing at least one selected from V: 0.001% or more and 0.05% or less. [4] The composition further comprises, in mass%, Nb: 0.001% or more and 0.020% or less; The steel plate according to [2] or [3], containing at least one selected from Ti: 0.001% or more and 0.020% or less. [5] The composition further comprises, in mass%, Cu: 0.001% or more and 0.20% or less; The steel sheet according to any one of [2] to [4], containing at least one selected from Ni: 0.001% or more and 0.10% or less. [6] The composition further comprises, in mass%, B: The steel sheet according to any one of [2] to [5], containing 0.0001% or more and less than 0.0020%. [7] The composition further comprises, in mass%, Sb: 0.002% or more and 0.1% or less The steel sheet according to any one of [2] to [6], containing at least one kind selected from Sn: 0.002% or more and 0.1% or less. [8] A hot rolling process, which comprises heating a steel slab having a composition according to any one of [2] to [7] and then hot rolling the same; The hot-rolled steel sheet obtained in the hot rolling process is called A C1and an annealing step of holding the annealing temperature at or above the Ms point for 30 seconds or more, then initiating water quenching at or above the Ms point, water cooling to 100°C or less, and then reheating to 100°C or more and 300°C or less. A manufacturing method for a steel plate, in which, during water cooling of the water quenching in the annealing process, in a region where the surface temperature of the steel plate is below (Ms point + 150°C), the steel plate is restrained from the front and back sides by two rolls installed between the steel plate, and the restraint pressure ratio at the center of the plate width to the end of the plate width at the restraint positions of the two rolls is 1.05 or more and 2.0 or less. [9] A hot rolling process, which heats a steel slab having a composition according to any one of [2] to [7] and then hot rolls the steel slab; A cold rolling process for cold rolling the hot-rolled steel sheet obtained in the hot rolling process; The cold-rolled steel sheet obtained in the cold rolling process is called A C1 and an annealing step of holding the annealing temperature at or above the Ms point for 30 seconds or more, then initiating water quenching at or above the Ms point, water cooling to 100°C or less, and then reheating to 100°C or more and 300°C or less. A manufacturing method for a steel plate, in which, during water cooling of the water quenching in the annealing process, in a region where the surface temperature of the steel plate is below (Ms point + 150°C), the steel plate is restrained from the front and back sides by two rolls installed between the steel plate, and the restraint pressure ratio at the center of the plate width to the end of the plate width at the restraint positions of the two rolls is 1.05 or more and 2.0 or less.

[10] A member obtained by subjecting the steel plate according to any one of [1] to [7] to at least one of forming and welding.

[11] A method for manufacturing a component, comprising a step of performing at least one of forming and welding on a steel plate manufactured by the method for manufacturing a steel plate according to [8] or [9]. Effect of the Invention

[0014] According to the present invention, it is possible to provide a steel plate, a component, and a manufacturing method thereof that are high in strength and excellent in shape uniformity and delayed fracture resistance. By applying the steel plate of the present invention to an automobile structural component, it is possible to achieve both high strength and improved delayed fracture resistance of the automobile steel plate. In other words, the present invention improves the performance of automobile bodies. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] Hereinafter, an embodiment of the present invention will be described, however, the present invention is not limited to the following embodiment.

[0016] The steel plate of the present invention has a steel structure with, in terms of area ratio, martensite: 20% to 100%, ferrite: 0% to 80%, and the remainder: 5% or less, the residual stress at the center of the plate width when bent into a V shape (hereinafter also simply referred to as "residual stress at the center of the plate width") is 800 MPa or less, the residual stress at the plate width ends when bent into a V shape is 90% to 110% of the residual stress at the center of the plate width, and the maximum warpage of the steel plate when sheared at a length of 1 m in the longitudinal direction of the steel plate is 15 mm or less. The effect of the present invention can be obtained as long as the steel plate satisfies these conditions, so the component composition of the steel plate is not particularly limited.

[0017] The thickness of the steel plate is preferably 0.2 mm or more and 3.2 mm or less.

[0018] First, the steel structure of the steel plate of the present invention will be described. The steel structure of the steel plate of the present invention has, in terms of area ratio, martensite: 20% to 100%, ferrite: 0% to 80%, and the balance: 5% or less.

[0019] Area ratio of martensite: 20% to 100% In order to obtain a high strength of TS≧750MPa, the area ratio of martensite is 20% or more. If the area ratio of martensite is less than 20%, any one of ferrite, retained austenite, pearlite, and bainite will be large, and the strength will decrease. The area ratio of martensite may be 100% in total. From the viewpoint of improving strength, the area ratio of martensite is preferably 30% or more. Martensite is the sum of fresh martensite that is as quenched and tempered martensite that is tempered. In the present invention, martensite refers to a hard structure generated from austenite at or below the martensite transformation point (also simply referred to as the Ms point), and tempered martensite refers to a structure that is tempered when martensite is reheated.

[0020] Ferrite area ratio: 0% to 80% From the viewpoint of ensuring the strength of the steel sheet, the area ratio of ferrite is 80% or less. The area ratio may be 0%. In the present invention, ferrite is a structure formed by transformation from austenite at a relatively high temperature and composed of crystal grains of a BCC lattice.

[0021] Remaining area ratio: 5% or less The steel structure of the steel plate of the present invention may contain a metallic phase that is unavoidably contained as the remainder other than martensite and ferrite. The area ratio of the remainder is acceptable if it is 5% or less. Phases contained in the remainder include, for example, retained austenite, pearlite, and bainite. The retained austenite in the present invention refers to austenite that remains at room temperature without being transformed into martensite. The pearlite in the present invention is a structure consisting of ferrite and acicular cementite. The bainite in the present invention refers to a hard structure that is formed from austenite at a relatively low temperature (above the martensite transformation point) and in which fine carbides are dispersed in acicular or plate-like ferrite.

[0022] Here, the area ratio of each structure in the steel structure is a value obtained by measuring using the method described in the Examples.

[0023] Residual stress at the center of the plate width after V-bending is 800MPa or less The V-bending process in the present invention means bending the steel sheet at a bending angle of 90° so that the bending ridge direction is parallel to the width direction of the steel sheet. The steel sheet of the present invention has excellent delayed fracture resistance. Specifically, the critical load stress obtained when the delayed fracture test described in the examples is equal to or greater than the yield strength (hereinafter, also simply referred to as YS). The critical load stress is preferably (YS+100) MPa or more, more preferably (YS+200) MPa or more. In order to make the critical load stress equal to or greater than YS, it is necessary to make the residual stress at the center of the sheet width when the steel sheet is V-bended equal to or less than 800 MPa. From the viewpoint of obtaining excellent delayed fracture resistance, the residual stress is preferably 780 MPa or less, more preferably 700 MPa or less, and even more preferably 600 MPa or less.

[0024] When V-bending, the residual stress at the width end of the plate is 90% to 110% of the residual stress at the center of the plate. The steel sheet of the present invention has excellent delayed fracture resistance even over the entire width of the steel sheet. Specifically, when the steel sheet is V-bent and the critical load stress is calculated at the width center and the width end, the critical load stress at the width end is 90% to 110% of the critical load stress at the width center, preferably 92% to 108%, and more preferably 95% to 105%. In order to make the critical load stress at the width end 90% to 110% of the critical load stress at the width center, it is necessary to make the residual stress at the width end 90% to 110% of the residual stress at the width center when the steel sheet is V-bent. From the viewpoint of improving excellent delayed fracture resistance, the residual stress at the width end is preferably 92% to 108%, and more preferably 95% to 105% of the residual stress at the width center when the steel sheet is V-bent.

[0025] The maximum warpage of the steel plate when it is sheared at a length of 1m in the longitudinal direction is 15mm or less. The steel sheet of the present invention has good shape uniformity. Specifically, the maximum warpage of the steel sheet when sheared at a length of 1 m in the longitudinal direction (rolling direction) of the steel sheet is 15 mm or less. The maximum warpage is preferably 13 mm or less, more preferably 12 mm or less, and further preferably 10 mm or less. There is no lower limit for the maximum warpage, and 0 mm is most preferable.

[0026] In the present invention, the "maximum amount of warpage of a steel plate when it is sheared at a length of 1 m in the longitudinal direction of the steel plate" refers to the distance from the horizontal stand to the steel plate at the point where the gap from the horizontal stand to the bottom of the steel plate is maximum after the steel plate is sheared at a length of 1 m in the longitudinal direction (rolling direction) of the steel plate at the original width of the steel plate and the steel plate after shearing is placed on a horizontal stand. The distance here is the distance in the direction perpendicular to the horizontal plane of the horizontal stand (vertical direction). In addition, the amount of warpage is measured with one side of the steel plate facing up, and then the other side of the steel plate facing up, and the maximum value of the measured amounts of warpage is defined as the maximum amount of warpage. In addition, the steel plate after shearing is placed on a horizontal stand so that there are many contact points (two or more points) between the corners of the steel plate and the horizontal stand. The amount of warping is determined by lowering a horizontal plate from a position above the steel plate until it comes into contact with the steel plate, and then subtracting the thickness of the steel plate from the distance between the horizontal stand and the horizontal plate at the point where it comes into contact with the steel plate.

[0027] The steel sheet of the present invention has high strength. In the present invention, high strength refers to a tensile strength of 750 MPa or more measured by the method described in the Examples. The tensile strength of the steel sheet is preferably 950 MPa or more, more preferably 1150 MPa or more, and even more preferably 1300 MPa or more. There is no particular upper limit to the tensile strength of the steel sheet, but it is preferably 2500 MPa or less from the viewpoint of easy balance with other properties.

[0028] A preferred composition for forming the steel sheet of the present invention will be described below. In the following description of the composition, "%" which is the unit of the content of each component means "% by mass".

[0029] C: 0.05% or more and 0.60% or less C is an element that improves hardenability and is necessary to ensure a predetermined area ratio of martensite. C is also necessary from the viewpoint of increasing the strength of martensite and ensuring strength. From the viewpoint of obtaining a predetermined strength while maintaining excellent delayed fracture resistance, the C content is preferably 0.05% or more. From the viewpoint of obtaining TS≧950MPa, the lower limit of the C content is more preferably 0.11% or more. From the viewpoint of further increasing the tensile strength, the lower limit of the C content is even more preferably 0.125% or more. On the other hand, if the C content exceeds 0.60%, the strength tends to become excessive and it tends to be difficult to suppress the transformation expansion due to martensite transformation. Therefore, the shape uniformity tends to deteriorate. Therefore, the C content is preferably 0.60% or less. The C content is more preferably 0.50% or less, and even more preferably 0.40% or less.

[0030] Si: 0.01% or more and 2.0% or less Si is a strengthening element due to solid solution strengthening. To fully obtain such effects, the Si content is preferably 0.01% or more. The Si content is more preferably 0.02% or more, and even more preferably 0.03% or more. On the other hand, if the Si content is too high, coarse MnS is likely to be generated in the sheet thickness direction, and the residual stress at the center of the sheet width increases, which tends to deteriorate the delayed fracture resistance. Therefore, the Si content is preferably 2.0% or less. The Si content is more preferably 1.7% or less, and even more preferably 1.5% or less.

[0031] Mn: 0.1% or more and 3.2% or less Mn is added to improve the hardenability of steel and ensure a predetermined area ratio of martensite. If the Mn content is less than 0.1%, ferrite will be generated in the surface layer of the steel sheet, which tends to reduce the strength. Therefore, the Mn content is preferably 0.1% or more. The Mn content is more preferably 0.2% or more, and even more preferably 0.3% or more. On the other hand, Mn is an element that particularly promotes the generation and coarsening of MnS, and if the Mn content exceeds 3.2%, the delayed fracture resistance tends to deteriorate due to an increase in residual stress at the center of the sheet width caused by an increase in coarse inclusions. Therefore, the Mn content is preferably 3.2% or less. The Mn content is more preferably 3.0% or less, and even more preferably 2.8% or less.

[0032] P:0.050% or less P is an element that strengthens steel, but if the content is high, it segregates at grain boundaries, and the residual stress at the center of the sheet width increases, which tends to deteriorate the delayed fracture resistance. Therefore, the P content is preferably 0.050% or less. The P content is more preferably 0.030% or less, and further preferably 0.010% or less. The lower limit of the P content is not particularly limited, but the lower limit that is currently industrially feasible is about 0.003%.

[0033] S: 0.0050% or less If S is contained in excess, inclusions such as MnS, TiS, Ti(C,S) are formed in excess, and delayed fracture resistance tends to deteriorate. From the viewpoint of suppressing the deterioration of delayed fracture resistance, the S content is preferably 0.0050% or less. The S content is more preferably 0.0020% or less, further preferably 0.0010% or less, and particularly preferably 0.0005% or less. The lower limit of the S content is not particularly limited, but the lower limit that is currently industrially feasible is about 0.0002%.

[0034] Al: 0.005% or more and 0.10% or less Al is added to perform sufficient deoxidation and reduce coarse inclusions in steel. From the viewpoint of obtaining the effect sufficiently, the Al content is preferably 0.005% or more. The Al content is more preferably 0.010% or more. On the other hand, if the Al content exceeds 0.10%, carbides mainly composed of Fe, such as cementite, generated during coiling after hot rolling are less likely to dissolve in the annealing process, and coarse inclusions and carbides tend to be generated. Therefore, not only does the strength of the steel sheet decrease, but the residual stress at the center of the sheet width increases, and the delayed fracture resistance tends to deteriorate. Therefore, the Al content is preferably 0.10% or less. The Al content is more preferably 0.08% or less, and further preferably 0.06% or less.

[0035] N: 0.010% or less N is an element that forms coarse inclusions of nitrides and carbonitrides such as TiN, (Nb, Ti)(C, N), and AlN in steel. If N is contained in excess, the delayed fracture resistance property deteriorates due to the generation of coarse inclusions. In order to prevent the deterioration of the delayed fracture resistance property, the N content is preferably 0.010% or less. The N content is preferably 0.007% or less, and more preferably 0.005% or less. The lower limit of the N content is not particularly limited, but the lower limit that is currently industrially feasible is about 0.0006%.

[0036] The steel sheet of the present invention preferably contains the above-mentioned components as basic components, with the balance being Fe (iron) and unavoidable impurities. The steel sheet of the present invention may contain the following components as optional components within a range that does not impair the effects of the present invention. Note that when the following optional components are contained in an amount less than the lower limit, the components are considered to be included in unavoidable impurities.

[0037] At least one selected from the following: Cr: 0.01% or more and 0.50% or less, Mo: 0.01% or more and less than 0.15%, V: 0.001% or more and 0.05% or less Cr, Mo, and V can be contained for the purpose of improving the hardenability of steel. To obtain such an effect, it is preferable that the Cr content and Mo content are both 0.01% or more. The Cr content and Mo content are more preferably 0.02% or more, and more preferably 0.03% or more. The V content is preferably 0.001% or more, more preferably 0.002% or more, and more preferably 0.003% or more. However, if any of the elements is too large, the residual stress at the center of the plate width increases due to the coarsening of carbides, and the delayed fracture resistance property deteriorates. Therefore, the Cr content is preferably 0.50% or less, and more preferably 0.1% or less. The Mo content is preferably less than 0.15%, and more preferably 0.10% or less. The V content is preferably 0.05% or less, more preferably 0.04% or less, and more preferably 0.03% or less.

[0038] At least one selected from Nb: 0.001% or more and 0.020% or less and Ti: 0.001% or more and 0.020% or less Nb and Ti contribute to high strength through refinement of prior γ grains. In order to obtain such an effect, it is preferable to contain Nb and Ti at 0.001% or more, respectively. The contents of Nb and Ti are more preferably 0.002% or more, and even more preferably 0.003% or more, respectively. On the other hand, if Nb and Ti are contained in a large amount, the amount of Nb-based coarse precipitates such as NbN, Nb(C,N), (Nb,Ti)(C,N) and Ti-based coarse precipitates such as TiN, Ti(C,N), Ti(C,S), TiS, etc. that remain in an insoluble state during slab heating in the hot rolling process increases, and the residual stress at the center of the sheet width increases, deteriorating the delayed fracture resistance. For this reason, the Nb and Ti contents are preferably 0.020% or less, respectively. The Nb and Ti contents are more preferably 0.015% or less, and even more preferably 0.010% or less, respectively.

[0039] At least one selected from Cu: 0.001% or more and 0.20% or less and Ni: 0.001% or more and 0.10% or less Cu and Ni have the effect of improving the corrosion resistance in the environment in which an automobile is used, and also of suppressing hydrogen penetration into the steel sheet by coating the surface of the steel sheet with corrosion products. In order to obtain this effect, it is preferable that Cu and Ni are each contained in an amount of 0.001% or more. The Cu content and Ni content are each more preferably 0.002% or more. However, if the Cu content or Ni content is too high, it will cause surface defects and deteriorate the plating property and chemical conversion property, so the Cu content is preferably 0.20% or less. The Cu content is more preferably 0.15% or less, and even more preferably 0.10% or less. The Ni content is preferably 0.10% or less. The Ni content is more preferably 0.08% or less, and even more preferably 0.06% or less.

[0040] B: 0.0001% or more and less than 0.0020% B is an element that improves the hardenability of steel, and even if the Mn content is small, the inclusion of B provides the effect of forming a predetermined area ratio of martensite. To obtain such an effect of B, it is preferable to set the B content to 0.0001% or more. The B content is preferably 0.0002% or more, and more preferably 0.0003% or more. On the other hand, if the B content is 0.0020% or more, the solid solution rate of cementite during annealing is delayed, and carbides mainly composed of Fe, such as undissolved cementite, remain. This increases the residual stress at the center of the sheet width, deteriorating the delayed fracture resistance. Therefore, the B content is preferably less than 0.0020%. The B content is more preferably 0.0015% or less, and even more preferably 0.0010% or less.

[0041] At least one selected from Sb: 0.002% to 0.1% and Sn: 0.002% to 0.1% Sb and Sn suppress oxidation and nitridation of the surface layer of the steel sheet, and suppress the reduction of C and B due to oxidation and nitridation of the surface layer of the steel sheet. In addition, by suppressing the reduction of C and B, the generation of ferrite in the surface layer of the steel sheet is suppressed, and this contributes to high strength. In order to obtain such effects, the Sb content and the Sn content are preferably 0.002% or more in any case. The Sb content and the Sn content are more preferably 0.003% or more, and further preferably 0.004% or more, respectively. On the other hand, in any case of the Sb content and the Sn content, if the content exceeds 0.1%, Sb and Sn are segregated at the prior γ grain boundary, and the residual stress at the center of the sheet width increases, thereby deteriorating the delayed fracture resistance. For this reason, in any case of the Sb content and the Sn content, it is preferable that the content is 0.1% or less. The Sb content and the Sn content are more preferably 0.08% or less, and further preferably 0.06% or less, respectively.

[0042] The steel sheet of the present invention may contain other elements such as Ta, W, Ca, Mg, Zr, and REM within a range that does not impair the effects of the present invention. The content of each of these elements is permissible if it is 0.1% or less.

[0043] Next, a method for producing a steel sheet according to the present invention will be described.

[0044] The method for producing a steel sheet of the present invention includes a hot rolling step, an optional cold rolling step, and an annealing step. The method for producing a steel sheet of the present invention includes, for example, a hot rolling step in which a steel slab having the above-mentioned preferred composition is heated and then hot rolled, and an optional cold rolling step in which the hot rolled steel sheet obtained in the hot rolling step or the cold rolled steel sheet obtained in the cold rolling step is subjected to a process of A. C1 and an annealing process in which the steel sheet is held at an annealing temperature of not less than the Ms point for 30 seconds or more, then water quenching is initiated at a temperature not less than the Ms point, water-cooled to not more than 100°C, and then reheated to a temperature of not less than 100°C and not more than 300°C. During water-cooling for water quenching in the annealing process, in a region where the surface temperature of the steel sheet is not more than (Ms point + 150°C), the steel sheet is restrained from the front and back sides by two rolls placed between the steel sheet, and the restraining pressure ratio at the center of the sheet width to the end of the sheet width at the restraining positions of the two rolls is 1.05 or more and 2.0 or less.

[0045] Each step will be described below. Note that the temperatures when heating or cooling the steel slab, steel plate, etc. described below refer to the surface temperatures of the steel slab, steel plate, etc., unless otherwise specified.

[0046] Hot Rolling Process The hot rolling process is a process in which a steel slab having the above-mentioned composition is heated and then hot rolled.

[0047] The steel slab having the above-mentioned composition is subjected to hot rolling. The slab heating temperature is not particularly limited, but by setting it to 1200°C or higher, the solid solution of sulfides is promoted and Mn segregation is reduced, the amount of coarse inclusions and carbides is reduced, and the delayed fracture resistance is improved. For this reason, the slab heating temperature is preferably 1200°C or higher. More preferably, it is 1230°C or higher, and further preferably, it is 1250°C or higher. The upper limit of the slab heating temperature is not particularly limited, but it is preferably 1400°C or lower. In addition, the heating rate during slab heating is not particularly limited, but it is preferably 5 to 15°C / min. In addition, the slab soaking time during slab heating is not particularly limited, but it is preferably 30 to 100 minutes.

[0048] The finish rolling temperature is not particularly limited, but is preferably 840°C or higher. If the finish rolling temperature is less than 840°C, it takes a long time for the temperature to drop, and inclusions and coarse carbides are generated, which not only deteriorates the delayed fracture resistance property but also may deteriorate the internal quality of the steel sheet. Therefore, the finish rolling temperature is preferably 840°C or higher. The finish rolling temperature is more preferably 860°C or higher. On the other hand, the upper limit of the finish rolling temperature is not particularly limited, but since it becomes difficult to cool down to the coiling temperature later, the finish rolling temperature is preferably 950°C or lower. The finish rolling temperature is more preferably 920°C or lower.

[0049] The hot-rolled steel sheet cooled to the coiling temperature is preferably coiled at a temperature of 630°C or less. If the coiling temperature exceeds 630°C, the surface of the base steel may be decarburized, which may cause a difference in structure between the inside and the surface of the steel sheet and may cause uneven alloy concentration. In addition, ferrite may be formed in the surface layer due to decarburization, which may reduce the tensile strength. Therefore, the coiling temperature is preferably 630°C or less. More preferably, it is 600°C or less. Although there is no particular lower limit, it is preferably 500°C or more to prevent a decrease in cold rolling property.

[0050] The hot-rolled steel sheet after coiling may be pickled under pickling conditions that are not particularly limited.

[0051] Cold rolling process The cold rolling process is a process of cold rolling the hot-rolled steel sheet obtained in the hot rolling process. The reduction ratio of the cold rolling is not particularly limited, but if the reduction ratio is less than 20%, the surface flatness may be poor and the structure may be non-uniform, so the reduction ratio is preferably 20% or more. Note that the cold rolling process is not an essential process, and may be omitted as long as the steel structure and mechanical properties satisfy the ranges of the present invention.

[0052] Annealing process The annealing process is a process in which cold-rolled or hot-rolled steel sheets are annealed. C1 This is a process in which the steel sheet is held at an annealing temperature above the Ms point for 30 seconds or more, then water quenching begins at a temperature above the Ms point, water cooling to below 100°C, and then reheating to a temperature between 100°C and 300°C. During water cooling for water quenching in the annealing process, in the region where the surface temperature of the steel sheet is below (Ms point + 150°C), the steel sheet is restrained from the front and back sides by two rolls placed between the steel sheet, and the restraint pressure ratio at the widthwise center to the widthwise ends at the restraining positions of the two rolls is 1.05 to 2.0.

[0053] A C1 Heat to annealing temperature above the Annealing temperature is A C1 Below this point, austenite does not form, making it difficult to obtain a steel sheet containing 20% ​​or more martensite, and the desired strength cannot be obtained. C1At least one point is necessary. C1 The upper limit of the annealing temperature is not particularly limited, but from the viewpoint of optimizing the temperature during water quenching and preventing deterioration of shape uniformity, the annealing temperature is preferably 900° C. or less.

[0054] In addition, A here C1 The points are calculated by the following formula: In the formula, (% element symbol) means the content (mass %) of each element. A C1 Point (℃)=723+22(%Si)-18(%Mn)+17(%Cr)+4.5(%Mo)+16(%V) The holding time at the annealing temperature (annealing holding time) is 30 seconds or more. If the annealing holding time is less than 30 seconds, the dissolution of carbides and the austenite transformation do not proceed sufficiently, so that the carbides remaining in the subsequent heat treatment become coarse, and the residual stress at the center of the sheet width increases, deteriorating the delayed fracture resistance. Therefore, the annealing holding time is 30 seconds or more, preferably 35 seconds or more. There is no particular upper limit to the annealing holding time, but from the viewpoint of suppressing the coarsening of the austenite grain size and preventing an increase in the amount of absorbed hydrogen, the annealing holding time is preferably 900 seconds or less.

[0055] Water quenching start temperature is above Ms point The quenching start temperature is an important factor for determining the martensite fraction, which is a controlling factor for strength. If the quenching start temperature is lower than the Ms point, martensite will transform before quenching, causing self-tempering of the martensite and resulting in poor shape uniformity. Therefore, the water quenching start temperature is the Ms point or higher, preferably (Ms point + 50°C) or higher. There is no particular upper limit for the water quenching start temperature, and it may be the annealing temperature.

[0056] The Ms point is calculated by the following formula. In the formula, (% element symbol) represents the content (mass%) of each element, (%V M ) means the martensite area ratio (unit: %). Ms point (℃)=550-350{(%C) / (%V M)×100}-40(%Mn)-17(%Ni)-17(%Cr)-21(%Mo) During water cooling after water quenching, restraining the steel sheet from the front and back sides with two rolls is an important factor for obtaining a shape correction effect, and control of the restraining conditions is an important factor for reducing the residual stress at the center of the sheet width and the residual stress variation across the entire width of the steel sheet. The present invention is characterized in that the transformation strain during water cooling is corrected by restraint, improving the shape uniformity of the steel sheet, and eliminating the need for leveler straightening or skin pass rolling, which increase residual stress and deteriorate delayed fracture resistance. Since leveler processing and skin pass rolling, which are performed when correcting deterioration of shape uniformity, are no longer necessary, it is possible to reduce residual stress. In addition, the residual stress variation across the entire width of the steel sheet is reduced under the restraining conditions, so delayed fracture resistance is improved across the entire width of the steel sheet.

[0057] The surface temperature of the steel plate when it is restrained from the front and back sides by two rolls (hereinafter referred to as the restraining temperature) is (Ms point + 150℃) or less. If the restraint temperature exceeds (Ms point + 150°C), martensite transformation occurs after restraint, and deterioration of shape uniformity due to transformation expansion of martensite transformation cannot be suppressed, resulting in poor shape uniformity. Therefore, the restraint temperature is (Ms point + 150°C) or less, preferably (Ms point + 100°C) or less, and more preferably (Ms point + 50°C) or less. There is no particular lower limit for the restraint temperature, and it is preferably 0°C or more, at which water does not freeze.

[0058] The ratio of the restraining pressure at the strip width center to the strip width end at the restraining position of the two rolls is 1.05 to 2.0. By setting the restraining pressure ratio of the width center to the width ends at the restraining position of the two rolls to 1.05 or more, the residual stress remaining in the width center can be dispersed in the width direction of the steel sheet, the residual stress in the width center and the residual stress fluctuation in the entire width of the steel sheet can be reduced, and it becomes possible to obtain excellent delayed fracture resistance properties in the entire width of the steel sheet. Therefore, the restraining pressure ratio is 1.05 or more. The restraining pressure ratio is preferably 1.10 or more. On the other hand, if the restraining pressure ratio exceeds 2.0, the residual stress value in the width center becomes high, and the delayed fracture resistance properties deteriorate. Therefore, the restraining pressure ratio is 2.0 or less. The restraining pressure ratio is preferably 1.7 or less, more preferably 1.5 or less.

[0059] The confinement pressure is not particularly limited as long as the confinement pressure ratio is within the range of the present invention. However, from the viewpoint of more effectively obtaining the effects of the present invention, 2 The restraining pressure (load) per unit is 50 to 300 N / mm 2 It is preferable that the restraining pressure (applied load) referred to here is the sum of the pressures applied to the steel sheet from the front and back surfaces of the steel sheet. It is also preferable that the restraining pressure applied by the two rolls is uniformly applied to the steel sheet from the front and back surfaces of the steel sheet.

[0060] There is no particular limitation on the means for restraining the steel plate with a greater restraining pressure at the plate width center than at the plate width ends.

[0061] One example of the above means is to make the roll diameter of the part contacting the width center larger than the roll diameter of the part contacting the width end when the steel sheet is restrained by the roll. For example, by using a roll having a body length longer than the width of the steel sheet and a roll diameter of the part contacting the width end and a roll diameter of the part contacting the width center equal to each other, and by making the temperature of the width center higher than that of the width end, the roll diameter of the part contacting the width center can be made larger due to the difference in thermal expansion coefficient. By using such a roll, it is possible to apply a restraining pressure to the steel sheet stably. As another example, a roll may be used in which the temperature of the width center and the width end is constant and the roll diameter of the part contacting the width center is larger than the roll diameter of the part contacting the width end.

[0062] As another example of the above-mentioned means, a guide roll for pressing down the roll that directly contacts the steel sheet may be divided into a center part of the sheet width of the steel sheet and an end part of the sheet width of the steel sheet, and the pressing force from the guide roll may be controlled to be greater at the center part of the sheet width than at the end parts of the sheet width.

[0063] Water cooling to below 100℃ If the temperature after water cooling exceeds 100°C, the martensitic transformation will progress to such an extent that it will adversely affect the shape uniformity. Therefore, the temperature of the steel sheet after it leaves the water tank must be 100°C or less. The temperature after water cooling is preferably 80°C or less.

[0064] Reheat at 100℃ to 300℃ After water cooling, the material is reheated and the martensite formed during water cooling is tempered, which makes it possible to reduce residual stress. This improves delayed fracture resistance. If the reheating temperature is less than 100°C, the above effects cannot be obtained. Therefore, the reheating temperature is 100°C or higher. The reheating temperature is preferably 130°C or higher. On the other hand, tempering at a temperature above 300°C deteriorates shape uniformity due to transformation shrinkage caused by tempering. For the above reasons, the reheating temperature is 300°C or lower. The reheating temperature is preferably 260°C or lower.

[0065] The hot-rolled steel sheet after the hot rolling process may be subjected to a heat treatment for softening the structure. After the annealing process, the hot-rolled steel sheet may be subjected to temper rolling for adjusting the shape. Furthermore, the steel sheet surface may be subjected to a plating process for plating with Zn, Al, or the like.

[0066] In addition, the plate width center in the present invention may be ±200 mm from the half width position of the manufactured steel plate, assuming that the widthwise ends of the steel plate may be removed by trimming during production.

[0067] Next, the member of the present invention and the method for producing the same will be described.

[0068] The member of the present invention is obtained by subjecting the steel plate of the present invention to at least one of forming and welding. Also, the method for manufacturing a member of the present invention includes a step of subjecting the steel plate manufactured by the method for manufacturing a steel plate of the present invention to at least one of forming and welding.

[0069] The steel sheet of the present invention has high strength, excellent shape uniformity and delayed fracture resistance. A member obtained by using the steel sheet of the present invention has high strength, good dimensional accuracy and excellent delayed fracture resistance, and can be suitably used, for example, for automotive structural members.

[0070] The forming process may be performed by any common processing method such as press working without any restrictions, and the welding process may be performed by any common welding method such as spot welding or arc welding without any restrictions. EXAMPLES

[0071] [Example 1] The present invention will now be described in detail with reference to examples.

[0072] 1. Manufacturing of steel sheets for evaluation A cold-rolled steel sheet with a thickness of 1.4 mm was annealed under the conditions shown in Table 1 to produce a steel sheet having the properties shown in Table 2. The temperature when passing through the restraining rolls was measured using a contact thermometer attached to the rolls. The restraining pressure was determined using the rigidity, shape, and push-in amount of the rolls, the thickness of the steel sheet, the austenite fraction at the time of transformation, and the stress-strain curve through CAE (Computer Aided Engineering) analysis using the YU model.

[0073] In addition, in the water quenching process of the annealing process, the roll steel plate 1m 2 The restraining pressure (load) per unit is 50 to 300 N / mm 2 In order to achieve this, two rolls were placed between the steel plate and applied uniform restraining pressure (load) to the steel plate from both the front and back sides.

[0074] In No. 2 of Table 1, since the steel plate was not restrained by two rolls during water cooling after water quenching in the annealing process, "surface temperature of the steel plate when restrained by the rolls" and "restraint pressure ratio at the center of the plate width to the end of the plate width at the roll restraint position" are each recorded as "-".

[0075] [Table 1]

[0076] 2. Evaluation method For the steel sheets obtained under various manufacturing conditions, the microstructure was analyzed to investigate the microstructural fraction, tensile properties such as tensile strength were evaluated by tensile tests, delayed fracture resistance was evaluated by delayed fracture tests, and shape uniformity was evaluated by measuring the steel sheet shape. The methods for each evaluation are as follows. The results are shown in Table 2.

[0077] (area ratio of steel structure) Test pieces were taken from each steel plate in the rolling direction and in the direction perpendicular to the rolling direction, and the plate thickness L cross section parallel to the rolling direction was mirror-polished and the structure was revealed with nital solution. The samples with the revealed structure were observed using a scanning electron microscope, and a 16 x 15 grid with 4.8 μm intervals was placed on an area of ​​82 μm x 57 μm in actual length on a 1500x magnification SEM image, and the area ratio of martensite and ferrite was investigated by the point counting method, which counts the number of points on each phase. The area ratio was the average of three area ratios obtained from separate SEM images with a magnification of 1500x. The measurement location was 1 / 4 of the plate thickness. Martensite has a white structure, and tempered martensite has fine carbides precipitated inside. Ferrite has a black structure. In addition, depending on the plane orientation of the block grains and the degree of etching, it may be difficult to reveal the internal carbides, so in that case it is necessary to perform sufficient etching to confirm. Moreover, the area ratio of the remainder other than ferrite and martensite was calculated by subtracting the total area ratio of ferrite and martensite from 100%.

[0078] (Tensile test) A JIS No. 5 test piece with a gauge length of 50 mm and a gauge width of 25 mm was taken from the center of the width of each steel plate in the rolling direction. A tensile test was conducted at a tension speed of 10 mm / min in accordance with the provisions of JIS Z 2241 (2011) to measure the tensile strength (TS) and yield strength (YS).

[0079] (Method of measuring residual stress) Residual stress was measured by X-ray diffraction. Specifically, samples of 100 mm in the rolling direction and 30 mm in the width direction were taken from the center and end of the width of each steel plate, and the steel plate samples were placed on a die having an angle of 90°, and the steel plate was pressed with a punch having an angle of 90° to perform V-bending so that the bending ridge direction was parallel to the width direction of the steel plate. The V-bending was performed under the following conditions: punch moving speed: 30 mm / min, load: 15 ton, holding time (pressing time) at maximum load: 5 seconds. Next, the steel plate (member) after bending was bolted from both sides of the plate surface using bolts, nuts, and tapered washers. The tightening amount was 30 mm.

[0080] The residual stress in the steel sheet after bending was measured at the center of the sheet thickness at the end face of the bend ridgeline, with the X-ray irradiation diameter set to 150 μm. The measurement direction was perpendicular to the sheet thickness direction and perpendicular to the bend ridgeline direction.

[0081] The widthwise end samples were taken at positions 0 to 30 mm from the widthwise end of the steel plate in the width direction.

[0082] In the examples of the present invention, the steel plates were manufactured by applying loads symmetrically about the center of the plate width in the plate width direction, so the residual stress was measured only at one end, and the residual stress at the other end was considered to be the same value.

[0083] (Measurement of critical load stress) The critical load stress was measured by a delayed fracture test. Specifically, a sample of 100 mm in the rolling direction and 30 mm in the width direction was taken from the center of the width of each steel plate, and the sample was V-bent using the same method as the residual stress measurement method described above. Next, the steel plate (member) after bending was bolted from both sides of the plate surface using bolts, nuts, and tapered washers. The relationship between the load stress and the tightening amount was calculated from the stress-strain curve obtained from the tensile test by CAE analysis using the YU model, and formed members with various load stresses were produced. The formed members were immersed in hydrochloric acid with pH = 1 (25 ° C), and the maximum load stress that did not cause delayed fracture was evaluated as the critical load stress. Delayed fracture was judged by visual inspection and images magnified up to × 20 with a stereo microscope, and if no cracks occurred after immersion for 96 hours, it was considered that there was no fracture. The cracks referred to here refer to the occurrence of cracks with a length of 200 μm or more.

[0084] (Measuring the shape of steel sheets) The steel plates obtained in each example were sheared in the longitudinal direction (rolling direction) of the steel plate at a length of 1 m at the original width of the steel plate, and the sheared steel plate was placed on a horizontal table. The sheared steel plate was placed on a horizontal table so that the corners of the steel plate and the horizontal table had many contact points (two or more points). The amount of warping was calculated by lowering a horizontal plate from a position above the steel plate until it contacted the steel plate, and subtracting the thickness of the steel plate from the distance between the horizontal table and the horizontal plate at the position where the horizontal plate contacted the steel plate. In addition, the amount of warping was measured with one side of the steel plate facing up, and then the amount of warping was measured with the other side of the steel plate facing up, and the maximum value of the measured amounts of warping was taken as the maximum amount of warping. The clearance of the blades of the shearing machine when cutting the steel plate in the longitudinal direction was 4% (the upper limit of the control range was 10%).

[0085] 3. Evaluation results The evaluation results are shown in Table 2.

[0086] [Table 2]

[0087] In this example, steel plates that satisfy TS of 750 MPa or more, critical load stress of YS or more, critical load stress ratio of the width center to the width end of the sheet of 90% to 110%, and maximum warpage of the steel plate of 15 mm or less when sheared at a length of 1 m in the longitudinal direction of the steel plate are deemed to pass, and are shown as invention examples in Table 2. Steel plates that do not satisfy at least one of these criteria are deemed to fail, and are shown as comparative examples in Table 2.

[0088] [Example 2] The present invention will now be described in detail with reference to examples.

[0089] 1. Manufacturing of steel sheets for evaluation Steel having the composition shown in Table 3, the balance being Fe and unavoidable impurities, was melted in a vacuum melting furnace, and then bloomed to obtain a bloomed rolled material having a thickness of 27 mm. The obtained bloomed rolled material was hot rolled. Next, for the samples to be cold rolled, the hot rolled steel sheet was ground and then cold rolled at the rolling reduction shown in Table 4 or 5 to obtain the sheet thickness shown in Table 4 or 5, to produce a cold rolled steel sheet. Note that some samples were not cold rolled after grinding the hot rolled steel sheet. The samples with the rolling reduction "-" in the table mean that they were not cold rolled. Next, the hot rolled steel sheet and cold rolled steel sheet obtained above were annealed under the conditions shown in Table 4 or 5 to produce a steel sheet. Note that the blanks in Table 3 indicate that they were not intentionally added, and include not only the case where they are not contained (0 mass%), but also the case where they are inevitably contained.

[0090] In Tables 4 and 5, the comparative examples in which the "surface temperature of the steel sheet when restrained by the rolls" and the "restraint pressure ratio at the center of the sheet width to the end of the sheet width at the roll restraint position" are recorded as "-" mean that the steel sheet was not restrained by two rolls during water cooling after water quenching in the annealing process.

[0091] [Table 3]

[0092] [Table 4]

[0093] [Table 5]

[0094] 2. Evaluation method For the steel sheets obtained under various manufacturing conditions, the microstructure was analyzed to investigate the microstructure fraction, tensile properties such as tensile strength were evaluated by tensile tests, delayed fracture resistance was evaluated by delayed fracture tests, and shape uniformity was evaluated by measuring the steel sheet shape. The methods of each evaluation were the same as those described in Example 1.

[0095] 3. Evaluation results The evaluation results are shown in Tables 6 and 7.

[0096] [Table 6]

[0097] [Table 7]

[0098] In this example, steel plates that had a TS of 750 MPa or more, a critical load stress of YS or more, a critical load stress ratio of the width center to the width end of the sheet of 90% to 110%, and a maximum warpage of 15 mm or less when sheared at a length of 1 m in the longitudinal direction of the steel plate were deemed to pass, and are shown as invention examples in Tables 6 and 7. Steel plates that did not satisfy at least one of these criteria were deemed to fail, and are shown as comparative examples in Tables 6 and 7.

Claims

1. In mass percent, C: 0.05% or more and 0.60% or less, Si: 0.01% or more and 2.0% or less, Mn: 0.1% or more and 3.2% or less, P: 0.050% or less, Al: 0.005% or more and 0.10% or less, and N: 0.010% or less, the balance being Fe and unavoidable impurities; The steel has a steel structure having, in terms of area ratio, martensite: 20% or more and 100% or less, ferrite: 0% or more and 80% or less, and the balance: 5% or less, The residual stress at the center of the plate width when V-bent is 800 MPa or less, When V-bending is performed, the residual stress at the plate width end is 90% or more and 110% or less of the residual stress at the plate width center, A steel plate in which the maximum warp amount of the steel plate when sheared at a length of 1 m in the longitudinal direction is 15 mm or less.

2. The composition further includes, in mass%, Cr: 0.01% or more and 0.50% or less, Mo: 0.01% or more and less than 0.15%, and The steel sheet according to claim 1, further comprising at least one selected from the group consisting of V: ​​0.001% or more and 0.05% or less.

3. The composition further includes, in mass%, Nb: 0.001% or more and 0.020% or less; The steel sheet according to claim 1 or 2, further comprising at least one selected from the group consisting of Ti: 0.001% to 0.020%.

4. The composition further includes, in mass%, Cu: 0.001% or more and 0.20% or less; The steel sheet according to any one of claims 1 to 3, containing at least one selected from Ni: 0.001% or more and 0.10% or less.

5. The composition further includes, in mass%, The steel plate according to any one of claims 1 to 4, containing B: 0.0001% or more and less than 0.0020%.

6. The composition further includes, in mass%, Sb: 0.002% or more and 0.1% or less; The steel sheet according to any one of claims 1 to 5, containing at least one selected from Sn: 0.002% or more and 0.1% or less.

7. The steel has a steel structure having, in terms of area ratio, martensite: 20% or more and 100% or less, ferrite: 0% or more and 80% or less, and the balance: 5% or less, The residual stress at the center of the plate width when V-bent is 800 MPa or less, When V-bending is performed, the residual stress at the plate width end is 90% or more and 110% or less of the residual stress at the plate width center, A manufacturing method of a steel plate in which the maximum warpage of the steel plate when sheared at a length of 1 m in the longitudinal direction of the steel plate is 15 mm or less, A hot rolling process in which a steel slab having the component composition according to any one of claims 1 to 6 is heated and then hot rolled; The hot-rolled steel sheet obtained in the hot rolling step is Ｃ１ and an annealing step of holding the annealing temperature at or above the Ms point for 30 seconds or more, then starting water quenching at or above the Ms point, water cooling to 100°C or less, and then heating again at 100°C or more and 300°C or less. A method for producing a steel plate, in which, during water cooling of the water quenching in the annealing step, in a region where the surface temperature of the steel plate is equal to or lower than (Ms point + 150°C), the steel plate is restrained from the front and back sides of the steel plate by two rolls installed between the steel plate, and a restraining pressure ratio at the center of the plate width to the end of the plate width at the restraining positions of the two rolls is 1.05 or more and 2.0 or less.

8. The steel has a steel structure having, in terms of area ratio, martensite: 20% or more and 100% or less, ferrite: 0% or more and 80% or less, and the balance: 5% or less, The residual stress at the center of the plate width when V-bent is 800 MPa or less, When V-bending is performed, the residual stress at the plate width end is 90% or more and 110% or less of the residual stress at the plate width center, A method for producing a steel plate in which the maximum warpage of the steel plate when sheared at a length of 1 m in the longitudinal direction of the steel plate is 15 mm or less, A hot rolling process in which a steel slab having the component composition according to any one of claims 1 to 6 is heated and then hot rolled; A cold rolling process for cold rolling the hot-rolled steel sheet obtained in the hot rolling process; The cold-rolled steel sheet obtained in the cold rolling step is Ｃ１ and an annealing step of holding the annealing temperature at or above the Ms point for 30 seconds or more, then starting water quenching at or above the Ms point, water cooling to 100°C or less, and then heating again at 100°C or more and 300°C or less. A method for producing a steel plate, in which, during water cooling of the water quenching in the annealing step, in a region where the surface temperature of the steel plate is equal to or lower than (Ms point + 150°C), the steel plate is restrained from the front and back sides of the steel plate by two rolls installed between the steel plate, and a restraining pressure ratio at the center of the plate width to the end of the plate width at the restraining positions of the two rolls is 1.05 or more and 2.0 or less.

9. A member obtained by subjecting the steel plate according to any one of claims 1 to 6 to at least one of forming and welding.

10. A method for manufacturing a member, comprising a step of performing at least one of forming and welding on a steel plate manufactured by the method for manufacturing a steel plate according to claim 7 or 8.

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