STEEL SHEET, ELEMENT, AND METHODS FOR MANUFACTURING THEM

Abstract

An objective of the present invention is to provide a steel sheet and element having high strength, excellent dimensional uniformity, and excellent resistance to delayed fracture, as well as methods for manufacturing the steel sheet and element. A steel sheet according to the present invention comprises a steel microstructure comprising, by area, martensite: 20% or more and 100% or less, ferrite: 0% or more and 80% or less, and the remainder: 5% or less. A residual stress generated at the transverse center of the steel sheet when the steel sheet is subjected to a V-bending process is 800 MPa or less. A residual stress generated at the transverse end of the steel sheet when the steel sheet is subjected to a V-bending process is 90% or more and 110% or less of the residual stress generated at the transverse center of the steel sheet.The maximum amount of warping of the steel sheet sheared over a length of 1 m in the longitudinal direction is 15 mm or less.

Description

STEEL SHEET, ELEMENT, AND METHODS FOR MANUFACTURING THEM FIELD OF INVENTION The present invention relates to a steel sheet and element used for manufacturing automotive and similar parts, and methods for manufacturing the steel sheet and element. The present invention specifically relates to a steel sheet and element having high strength, excellent dimensional uniformity, and excellent resistance to delayed fracture, and methods for manufacturing the steel sheet and element. BACKGROUND OF THE INVENTION Throughout the automotive industry, efforts have focused on improving fuel efficiency to reduce CO2 emissions and protect the environment worldwide. Since one of the most effective approaches to improving fuel economy is to reduce vehicle weight by thinning automotive parts, the amount of high-strength steel sheets used in automotive components has increased. Many steel sheets have been provided with martensite, a hard phase, to increase their strength. However, when martensite forms, transformation deformation degrades the uniformity of the steel sheet's shape. Since this degradation negatively affects the dimensional accuracy achieved during forming, steel sheets have been corrected by leveling or hardening rolling (tempering) to achieve the intended dimensional accuracy. However, the introduction of deformation through leveling or hardening rolling degrades the dimensional accuracy achieved during forming and makes it impossible to achieve the intended dimensional accuracy.To increase dimensional accuracy, it is necessary to limit the degradation of the uniformity of the shape of a steel sheet that occurs during the transformation into martensite. Several techniques have been proposed. For example, in Patent Literature 1, shape uniformity and delayed fracture resistance are improved by controlling the ferrite and martensite area fractions. Specifically, an ultra-high-strength steel sheet with suitable shape uniformity and delayed fracture resistance is provided by using a multiphase steel with a metallic microstructure that includes, by volume, 50% to 80% quenched martensite phase and 20% to 50% ferrite phase in order to reduce hydrogen ingress. Patent Literature 2 provides a technique in which the degradation of the shape of a steel sheet that occurs due to the transformation of martensite during rapid water quenching is limited by joining the steel sheet with rollers in water. List of appointments Patent Literature PTL 1: Publication of unexamined Japanese patent application no. 2010-90432 PTL 2: Japanese patent no. 6094722 BRIEF DESCRIPTION OF THE INVENTION Lbfrznn / zznz / B / YiAi Technical problem Since the steel sheets used to manufacture car bodies undergo a pressure forming process before use, proper shape uniformity is an essential property. Furthermore, as the amount of high-strength steel sheets used as automotive parts materials has been increasing, it is necessary that delayed fracture strength, which receives particular attention with increasing strength, be at an adequate level. Because automotive parts are manufactured from a steel sheet in such a way that they extend across the entire width of the steel sheet (hereafter also referred to as the “full width of the steel sheet”), it is necessary that delayed fracture strength be consistently excellent across the entire width of the steel sheet.Therefore, these steel sheets must have high strength, excellent uniformity of shape, and excellent resistance to delayed fracture across the entire width of the steel sheet. The techniques described in Patent Literature 1 include a technique in which shape uniformity and delayed fracture resistance are improved by controlling the microstructure. However, the improvement in shape uniformity is considered to be less than that achieved in the present invention, due to transformation expansion that occurs when the martensite transforms, causing shape degradation. The techniques described in Patent Literature 2 include a technique that improves shape uniformity, but do not include any technique that improves delayed fracture resistance to an excellent level. An objective of the present invention is to provide a steel sheet and element having high strength, excellent uniformity of shape, and excellent resistance to delayed fracture, as well as methods for manufacturing the steel sheet and element. Solution to the problem In order to address the foregoing issues, the inventors of the present invention conducted extensive studies of the requirements for a steel sheet having a tensile strength of 750 MPa or more, excellent dimensional uniformity, and excellent delayed fracture resistance. They found that, to achieve excellent delayed fracture resistance, the residual stress generated at the transverse center of the steel sheet must be limited to 800 MPa or less. The inventors of the present invention also discovered that high strength can be achieved by adjusting the volume fraction of martensite to 20% or more by area through rapid quenching. However, martensite transformation occurs unevenly during high-rate water quenching, and the resulting transformation deformation degrades the dimensional uniformity of the steel sheet.Based on the results of studies on reducing the adverse effects of transformation deformation, it was conceived that shape uniformity can be improved by applying a bonding force to the front and back surfaces of the steel sheet during martensite transformation. It was also found that controlling the bonding conditions reduces residual stress fluctuations in the transverse direction and improves delayed fracture resistance across the entire width of the steel sheet. As described above, the inventors of the present invention carried out several studies in order to address the above issues and realize the present invention, therefore Lbfrznn / zznz / B / YiAi found that reducing the residual stress generated at the transverse center of a steel sheet allows the production of a steel sheet that has excellent delayed fracture resistance and controlling the joining conditions of the rolls allows the production of a steel sheet that has excellent shape uniformity and excellent delayed fracture resistance throughout the width of the steel sheet. The summary of the present invention is described below. [1] A steel sheet having a steel microstructure that includes, by area, martensite: 20% or more and 100% or less, ferrite: 0% or more and 80% or less, and the remainder: 5% or less, wherein: a residual stress generated at a transverse center of the steel sheet when the steel sheet is subjected to a V-bending process is 800 MPa or less; a residual stress generated at a transverse end of the steel sheet when the steel sheet is subjected to a V-bending process is equal to or greater than 90% and less than 110% of the residual stress generated at the transverse center of the steel sheet; and a maximum amount of warping of the sheared steel sheet over a length of 1 m in a longitudinal direction of the steel sheet is 15 mm or less. [2] The steel sheet described in point [1], which has a chemical composition containing, by mass, C: 0.05% or more and 0.60% or less, If: 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, To: 0.005% or more and 0.10% or less, and N: 0.010% or less, the remainder being Fe and incidental impurities. [3] The steel sheet described in point [2], wherein the chemical composition further contains at least one selected from, by mass, Cr: 0.01% or more and 0.50% or less, Mo: 0.01% or more and less than 0.15%, and V: 0.001% or more and 0.05% or less. [4] The steel sheet described in point [2] or [3], wherein the chemical composition further contains at least one selected from, by mass, Nb: 0.001% or more and 0.020% or less, and Ti: 0.001% or more and 0.020% or less. [5] The steel sheet described in any of points [2] to [4], wherein the chemical composition further contains at least one selected from, by mass, Cu: 0.001% or more and 0.20% or less, and Ni: 0.001% or more and 0.10% or less. [6] The steel sheet described in any of points [2] to [5], wherein the chemical composition also contains, by mass, B: 0.0001% or more and less than 0.0020%. [7] The steel sheet described in any of points [2] to [6], wherein the composition Lbfrznn / zznz / B / YiAi chemistry also contains at least one selected from, by mass, Sb: 0.002% or more and 0.1% or less, and Sn: 0.002% or more and 0.1% or less. [8] A method for manufacturing a steel sheet, the method includes: a hot rolling step consisting of heating a steel slab having the chemical composition described in any of points [2] to [7] and hot rolling the steel slab; and an annealing step consisting of holding a hot-rolled steel sheet prepared in the hot rolling step at an annealing temperature equal to or greater than a temperature AC1 for 30 seconds or more, subsequently initiating rapid water quenching at a temperature equal to or greater than a temperature Ms, and, after water quenching to 100°C or less, reheating to a temperature of 100°C or more and 300°C or less, wherein, while water quenching for rapid water quenching is being carried out in the annealing step,The steel sheet is joined with two rollers arranged to rest on the front and rear surfaces of the steel sheet so that they are oriented towards each other across the steel sheet when a surface temperature of the steel sheet is equal to or less than (Ms temperature +150° C), and a ratio between a joining pressure applied to a transverse center of the steel sheet and a joining pressure applied to a transverse end of the steel sheet in a position where the steel sheet is joined with the two rollers is 1.05 or more and 2.0 or less. [9] A method for manufacturing a steel sheet, the method includes: a hot rolling step consisting of heating a steel slab having the chemical composition described in any of points [2] to [7] and hot rolling the steel slab; a cold rolling step consisting of cold rolling a hot-rolled steel sheet prepared in the hot rolling step; and an annealing step consisting of holding a cold-rolled steel sheet prepared in the cold rolling step at an annealing temperature equal to or greater than a temperature AC1 for 30 seconds or more, subsequently initiating rapid water quenching at a temperature equal to or greater than a temperature Ms, and, after water quenching to 100°C or less, reheating to a temperature of 100°C or more and 300°C or less, wherein, while water quenching for rapid water quenching is being carried out in the annealing step,The steel sheet is joined with two rollers arranged to rest on the front and rear surfaces of the steel sheet so that they are oriented towards each other across the steel sheet when a surface temperature of the steel sheet is equal to or less than (Ms temperature +150° C), and a ratio between a joining pressure applied to a transverse center of the steel sheet and a joining pressure applied to a transverse end of the steel sheet in a position where the steel sheet is joined with the two rollers is 1.05 or more and 2.0 or less.

[10] An element manufactured by subjecting the steel sheet described in any of points [1] to [7] to at least one of the forming and welding processes. Lbfrznn / zznz / B / YiAi

[11] A method for manufacturing an item, the method includes a step of subjecting a steel sheet manufactured by the method for manufacturing a steel sheet described in point [8] or [9] to at least one of the forming and welding processes. Beneficial effects of the invention According to the present invention, a steel sheet and element possessing high strength, excellent dimensional uniformity, and excellent delayed fracture resistance, along with methods for manufacturing the steel sheet and element, can be provided. Using the steel sheet according to the present invention to manufacture automotive structural elements allows for increased strength and improved delayed fracture resistance of automotive steel sheets. In other words, the present invention improves the performance of automotive bodies. Description of the modalities The following describes one embodiment of the present invention. The present invention is not limited by the following embodiment. The steel sheet according to the present invention has a steel microstructure comprising, by area, martensite: 20% or more and 100% or less, ferrite: 0% or more and 80% or less, and the remainder: 5% or less. A residual stress generated at the transverse center of the steel sheet when the steel sheet is subjected to a V-bending process (hereinafter referred to simply as “residual stress at the transverse center”) is 800 MPa or less. A residual stress generated at the transverse end of the steel sheet when the steel sheet is subjected to a V-bending process is 90% or more and 110% or less of the residual stress generated at the transverse center. The maximum amount of warping of the steel sheet sheared over a length of 1 m in the longitudinal direction is 15 mm or less.Any steel sheet that meets the conditions described above has the beneficial effects described above according to the present invention; the chemical composition of the steel sheet is not limited. The thickness of the steel sheet is preferably 0.2 mm or more and 3.2 mm or less. First, the microstructure of the steel sheet according to the present invention is described. The microstructure of the steel sheet according to the present invention contains, by area, martensite: 20% or more and 100% or less, ferrite: 0% or more and 80% or less, and the remainder: 5% or less. Martensite area fraction: 20% or more and 100% or less The martensite area fraction is limited to 20% or more to achieve a high strength of TS > 750 MPa. If the martensite area fraction is less than 20%, the fractions of ferrite, retained austenite, pearlite, and bainite increase, and consequently, the strength decreases. The martensite area fraction can be 100%. A martensite area fraction of 30% or more is preferred to increase strength. The amount of martensite is the total amount of quenched fresh martensite and tempered martensite. The term “martensite” used herein refers to a hard microstructure formed from austenite at a temperature equal to or lower than the martensite transformation temperature (hereafter referred to simply as the “Ms temperature”). The term “tempered martensite” used herein refers to a microstructure formed by tempering when the material is reheated Lbfrznn / zznz / B / YiAi martensite. Ferrite area fraction: 0% or more and 80% or less The ferrite area fraction is limited to 80% or less to maintain a certain strength in the steel sheet. The area fraction above may be 0%. The term “ferrite” used herein refers to a microstructure transformed from austenite at relatively high temperatures and formed by BCC lattice crystal grains. Fraction of the remaining area: 5% or less The steel microstructure of the steel sheet according to the present invention may include, as a residue other than martensite or ferrite, a metallic phase incidentally included in the steel microstructure. The permissible area fraction of the residue is 5% or less. Examples of the included residue phase are retained austenite, pearlite, and bainite. The term “retained austenite” used herein refers to austenite that does not transform into martensite and remains even after the temperature has been reduced to room temperature. The term “pearlite” used herein refers to a microstructure formed by ferrite and acicular cementite.The term “bainite” used herein refers to a hard microstructure formed from austenite at relatively low temperatures (equal to or above the transformation temperature into martensite), the microstructure includes acicular or plate-shaped ferrite grains and fine carbide grains dispersed within it. The area fractions of the above microstructure components in the steel microstructure are measured using the method described in the Examples below. Residual stress generated at the transverse center of the steel sheet when the steel sheet is subjected to a V-bending process is 800 MPa or less The term “V-bending process” used herein refers to a bending process performed at a 90° bending angle such that the direction of the bending crest line is parallel to the transverse direction of the steel sheet. The steel sheet according to the present invention has excellent resistance to delayed fracture. Specifically, the critical load stress determined by performing the delayed fracture test described in the Examples below is equal to or greater than the yield strength (hereinafter referred to simply as “YS”). The above critical load stress is preferably (YS + 100) MPa or more, and more preferably (YS + 200) MPa or more.To adjust the critical load stress to be equal to or greater than YS, it is necessary to limit the residual stress generated at the transverse center of the steel sheet when it is subjected to a V-bend to 800 MPa or less. To achieve excellent delayed fracture resistance, the above residual stress is preferably 780 MPa or less, more preferably 700 MPa or less, and further preferably 600 MPa or less. The residual stress generated at the transverse end of the steel sheet when the steel sheet is subjected to a V-bending process is equal to or greater than 90% and less than 110% of the residual stress generated at the transverse center The steel sheet according to the present invention has excellent resistance to delayed fracture across its entire width. Specifically, when the steel sheet is subjected to a V-bend process to measure a critical load stress at its center Lbfrznn / zznz / B / YiAi transverse and at the transverse end of the steel sheet, the critical load stress measured at the transverse end is 90% or more and 110% or less, preferably 92% or more and 108% or less, and more preferably 95% or more and 105% or less of the critical load stress measured at the transverse center. To adjust the critical load stress measured at the transverse end to be 90% or more and 110% or less of the critical load stress measured at the transverse center, it is necessary to limit the residual stress generated at the transverse end of the steel sheet when the steel sheet is subjected to a V-bending process to be 90% or more and 110% or less of the residual stress generated at the transverse center.To improve delayed fracture resistance to an excellent level, the residual stress generated at the transverse end of the steel sheet when the steel sheet is subjected to a V-bending process is preferably 92% or more and 108% or less, and more preferably 95% or more and 105% or less of the residual stress generated at the transverse center. The maximum amount of warping of the sheared steel sheet over a length of 1 m in the longitudinal direction is 15 mm or less. The steel sheet according to the present invention has suitable uniformity of shape. Specifically, the maximum amount of warping of the steel sheet sheared to a length of 1 m in the longitudinal direction (the rolling direction) is 15 mm or less. The maximum amount of warping is preferably 13 mm or less, more preferably 12 mm or less, and still more preferably 10 mm or less. The lower limit of the maximum amount of warping has not been established, but is preferably 0 mm. The expression “the maximum amount of warping of the steel sheet sheared to a length of 1 m in the longitudinal direction” used herein refers to the amount determined by shearing the steel sheet to a size that has the same width as the steel sheet and a length of 1 m in the longitudinal direction (the rolling direction) of the steel sheet, by placing the sheared steel sheet on a horizontal platform, and measuring the distance between the horizontal platform and the steel sheet at the position where the amount of space between the horizontal platform and the bottom of the steel sheet is at its maximum. It should be noted that the above distance is measured in a direction (the vertical direction) perpendicular to the horizontal plane of the horizontal platform.The maximum amount of warping is the greater of the amount of warping measured with one surface of the steel sheet facing upwards and the amount of warping measured with the other surface facing upwards. The sheared steel sheet is placed on the horizontal platform so that the number of contact points between the corners of the steel sheet and the horizontal platform is maximized (two or more). The amount of warping is determined by lowering a horizontal plate from a position above the steel sheet until it makes contact with the steel sheet and subtracting the thickness of the steel sheet from the distance between the horizontal platform and the horizontal plate at the point where the horizontal plate is in contact with the steel sheet. The steel sheet according to the present invention has high strength. The term “high strength” used herein refers to a tensile strength of 750 MPa or more, as measured by the method described in the Examples below. The tensile strength of the steel sheet is preferably 950 MPa or more. Lbfrznn / zznz / B / YiAi preferably 1150 MPa or more, and more preferably 1300 MPa or more. Although the upper limit of the tensile strength of the steel sheet is not established, the tensile strength of the steel sheet is preferably 2500 MPa or less to facilitate the balance between strength and other properties. A preferred chemical composition for producing the steel sheet according to the present invention is described below. In the chemical composition description below, the symbol “%” used as a unit of constituent content refers to “% by mass”. C: 0.05% or more and 0.60% or less Carbon (C) is an element that improves hardenability and is necessary to achieve the predetermined area fraction of martensite. C is also necessary to increase and maintain the strength of the martensite. The C content is preferably 0.05% or more to maintain excellent delayed fracture resistance and achieve the predetermined strength. To achieve a tensile strength (TS) > 950 MPa, the lower limit of the C content is preferably 0.11% or more. To further increase tensile strength, the lower limit of the C content is preferably 0.125% or more. However, if the C content exceeds 0.60%, the strength may increase to an excessive level, and it may be difficult to limit the transformation expansion caused by the martensite transformation. Consequently, the uniformity of shape may degrade. Therefore, the C content is preferably 0.60% or less. The C content is more preferably 0.50% or less and is also preferably 0.40% or less. Yes: 0.01% or more and 2.0% or less Silicon (Si) is an element that increases strength by strengthening the solid solution. To sufficiently achieve this advantageous effect, the Si content is preferably 0.01% or more. More preferably, it is 0.02% or more, and further preferably 0.03% or more. However, if the Si content is excessively high, it increases the likelihood of coarse MnS formation along the thickness of the steel sheet, increases the residual stress generated at the transverse center of the steel sheet, and consequently, may degrade the delayed fracture resistance. Therefore, the Si content is preferably 2.0% or less. More preferably, it is 1.7% or less, and further preferably 1.5% or less. Mn: 0.1% or more and 3.2% or less Manganese (Mn) is included in steel to improve its hardenability and achieve a predetermined area fraction of martensite. If the Mn content is less than 0.1%, ferrite can form in the surface layer of the steel sheet, reducing its strength. Therefore, the Mn content is preferably 0.1% or higher. More preferably, it is 0.2% or higher, and even more preferably, 0.3% or higher. Furthermore, Mn is an element that particularly facilitates the formation and thickening of MnS, which is detrimental. Therefore, if the Mn content exceeds 3.2%, the amount of coarse inclusions increases, the residual stress generated at the transverse center of the steel sheet increases, and consequently, the delayed fracture resistance can be degraded. Therefore, the Mn content is preferably 3.2% or lower. More preferably, it is 3.2% or higher.0% or less and is also preferably 2.8% or less. Lbfrznn / zznz / B / YiAi P: 0.050% or less Phosphorus (P) is an element that increases the strength of steel. However, if the P content is high, P segregates at the grain boundaries, increasing the residual stress generated at the transverse center of the steel sheet and consequently degrading its resistance to delayed fracture. Therefore, the P content is preferably 0.050% or less. More preferably, it is 0.030% or less, and further preferably 0.010% or less. There is no lower limit for the P content. The lowest industrially feasible P content is approximately 0.003%. S: 0.0050% or less If the sulfur (S) content is excessively high, inclusions such as MnS, TiS, and Ti(C,S) can form in excessive amounts, degrading delayed fracture strength. To limit the degradation of delayed fracture strength, the S content is preferably 0.0050% or less. More preferably, 0.0020% or less; further preferably, 0.0010% or less; and particularly preferable, 0.0005% or less. There is no lower limit for the S content. The industrially feasible lower limit for S content is approximately 0.0002%. To: 0.005% or more and 0.10% or less Aluminum (Al) is added to steel to achieve sufficient deoxidation and reduce the amount of coarse inclusions present. To sufficiently achieve these beneficial effects, the Al content is preferably 0.005% or more. More preferably, it is 0.010% or more. However, if the Al content exceeds 0.10%, it becomes difficult to dissolve carbides, such as cementite, which form during post-hot-rolling winding after hot rolling, and as a result, coarse inclusions and carbides may form. This reduces the strength of the steel sheet. Furthermore, the residual stress generated at the transverse center of the steel sheet increases, and consequently, the delayed fracture resistance may be degraded. Therefore, the Al content is preferably 0.10% or less. More preferably, it is 0.010% or more.0.08% or less and is also preferably 0.06% or less. N: 0.010% or less Nitrogen (N) is an element that forms coarse nitride and carbonitride inclusions, such as TiN, (Nb,Ti)(C,N), and AlN, in steel. If the N content is excessively high, delayed fracture strength can be degraded due to the formation of coarse inclusions. To avoid degradation of delayed fracture strength, the N content is preferably 0.010% or less. The N content is preferably 0.007% or less, and further preferably 0.005% or less. The lower limit for N content is not established. The industrially feasible lower limit for N content is approximately 0.0006%. It is preferred that the steel sheet according to the present invention include the foregoing constituents as fundamental constituents, with the remainder being Fe (iron) and incidental impurities. The steel sheet according to the present invention may include the following constituents as optional constituents so that the effects according to the present invention are not impaired. In the event that the content of the optional constituents indicated below is less than the lower limits, it is considered that the Lbfrznn / zznz / B / YiAi constituents are included in the incidental impurities. At least one selected from Cr: 0.01% or more and 0.50% or less, Mo: 0.01% or more and less than 0.15%, and V: 0.001% or more and 0.05% or less Cr, Mo, and V can be added to steel to improve its hardenability. To achieve this beneficial effect, the Cr and Mo content is preferably 0.01% or more. The Cr and Mo content is preferably 0.02% or more, and 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 the contents of the above elements are excessively high, carbide fouling occurs, increasing the residual stress generated at the transverse center of the steel sheet and consequently degrading the delayed fracture resistance. 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 content of V is preferably 0.0.05% or less, more preferably 0.04% or less, and more preferably 0.03% or less. 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 Nitrogen (Nb) and titanium (Ti) contribute to increased strength by reducing the size of the preceding γ grains. To achieve this beneficial effect, the Nb and Ti contents are preferably 0.001% or more. More preferably, the Nb and Ti contents are 0.002% or more, and even more preferably, 0.003% or more. However, if the Nb and Ti contents are excessively high, the amounts of coarse Nb precipitates, such as NbN, Nb(C,N), and (Nb,Ti)(C,N), and coarse Ti precipitates, such as TIN, Ti(C,N), Ti(C,S), and TIS, which do not dissolve when the slabs are heated during hot rolling and remain in the steel, increase the residual stress generated at the transverse center of the steel sheet and, consequently, degrade the delayed fracture resistance. Accordingly, the Nb and Ti contents are each preferably 0.020% or less. The Nb and Ti contents are more preferably each 0.020%.015% or less and are each preferably 0.010% or less. 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 Copper (Cu) and nickel (Ni) improve corrosion resistance under automotive service conditions. Furthermore, the corrosion product coats the surface of the steel sheet and reduces the likelihood of hydrogen penetration. To achieve these beneficial effects, the Cu and Ni contents are preferably 0.001% or higher. However, excessively high Cu and Ni contents induce surface defects, consequently degrading coating capacity and chemical conversion. Therefore, the Cu content is preferably 0.20% or lower. More preferably, the Cu content is 0.15% or lower, and further preferably 0.10% or lower. The Ni content is preferably 0.10% or lower. More preferably, the Ni content is 0.08% or lower, and further preferably 0.002%.06% o menos. Lbfrznn / zznz / B / YiAi Β: 0.0001% or more or less than 0.0020% Boron (B) is an element that improves the hardenability of steel. The addition of B allows the formation of the predetermined area fraction of martensite even when the manganese (Mn) content is low. To achieve the beneficial effects of B mentioned above, the B content is preferably 0.000t% or more. The B content is preferably 0.0002% or more, and even more preferably 0.0003% or more. However, if the B content is 0.0020% or more, the rate of cementite dissolution during annealing is reduced, and consequently, carbides containing iron as a major constituent, such as cementite, may remain undissolved. In such a case, the residual stress generated at the transverse center of the steel sheet increases, and consequently, delayed fracture resistance may be degraded. Therefore, the B content is preferably less than 0.0020%. The B content is more preferably 0.0.015% or less and is also preferably 0.0010% or less. At least one selected from Sb: 0.002% or more and 0.1% or less and Sn: 0.002% or more and 0.1% or less Sb and Sn reduce oxidation and nitriding of the surface layer of the steel sheet and limit reductions in C and B content that can be caused by oxidation and nitriding of the surface layer. Furthermore, since reductions in C and B content are limited, ferrite formation in the surface layer of the steel sheet is reduced. This contributes to increased strength. To achieve the above beneficial effects, the Sb and Sn contents are preferably 0.002% or higher. The Sb and Sn contents are preferably each 0.003% or higher, and 0.004% or higher. However, if either the Sb or Sn content exceeds 0.1%, the Sb or Sn may segregate at the grain boundaries of the overlying austenite.In such a case, the residual stress generated at the transverse center of the steel sheet increases and, consequently, the delayed fracture resistance may degrade. Therefore, the Sb and Sn contents are preferably each 0.1% or less. The Sb and Sn contents are more preferably each 0.08% or less and are preferably each 0.06% or less. The steel sheet according to the present invention may include Ta, W, Ca, Mg, Zr, and REM as other elements so that the beneficial effects of the present invention are not impaired. The permissible contents of the aforementioned elements are each 0.1% or less. The following describes a method for manufacturing the steel sheet according to the present invention. The method for manufacturing the steel sheet according to the present invention includes a hot rolling step, a cold rolling step performed as required, and an annealing step. The method for manufacturing the steel sheet according to the present invention includes, for example, a hot rolling step for heating a steel slab having the preferred chemical composition described above and rolling the hot steel slab; a cold rolling step performed as required; and an annealing step in which a hot-rolled steel sheet prepared in the hot rolling step or a cold-rolled steel sheet prepared in the cold rolling step is held at a temperature of Lbfrznn / zznz / B / YiAi annealed at or above temperature AC1 for 30 seconds or more, then water quenching is initiated at a temperature equal to or above the Ms temperature, and, after water quenching to 100°C or less, reheating is performed to a temperature of 100°C or more and 300°C or less. While water quenching for water quenching is being performed in the annealing step, the steel sheet is joined with two rollers arranged to bear on the front and rear surfaces of the steel sheet so that they are oriented relative to each other across the steel sheet when the surface temperature of the steel sheet is equal to or less than (Ms temperature +150°C). The ratio of a joining pressure applied to the transverse center of the steel sheet to a joining pressure applied to the transverse end of the steel sheet in a position where the steel sheet is joined with the two rollers is 1.05 or more and 2.0 or less. Each of the above steps is described below. The term “temperature” used in describing the heating or cooling of a steel slab, steel sheet, or the like refers to the surface temperature of the steel slab, steel sheet, or the like, unless otherwise specified. Hot rolling pass The hot rolling step consists of heating a steel slab with the aforementioned chemical composition and hot rolling it. A steel slab with the chemical composition described above is subjected to hot rolling. The temperature to which the slab is heated is not limited. When the slab's preheating temperature is 1200°C or higher, sulfide dissolution is facilitated and manganese segregation is reduced. Consequently, the amounts of coarse inclusions and carbides, described above, can be reduced. This improves resistance to delayed fracture. Therefore, the slab's preheating temperature is preferably 1200°C or higher, more preferably 1230°C or higher, and further preferably 1250°C or higher. The upper limit of the slab's preheating temperature is preferably, but not limited to, 1400°C or lower. The rate at which the slab is heated is preferably, but not limited to, 5 to 15°C / min.The amount of time during which the iron is soaked in the heating of the iron is preferably, but not limited to, 30 to 100 minutes. The finish rolling temperature is preferably, but not limited to, 840°C or higher. If the finish rolling temperature is lower than 840°C, the time required to reduce the temperature increases. This leads to the formation of coarse inclusions and carbides, which degrade delayed fracture resistance. In addition, the internal quality of the steel sheet may be degraded. Therefore, the finish rolling temperature is preferably 840°C or higher. The finish rolling temperature is preferably 860°C or higher. Although the upper limit for the finish rolling temperature is not established, the finish rolling temperature is preferably 950°C or lower, as otherwise it is difficult to reduce the temperature to the subsequent winding temperature. The finish rolling temperature is preferably 920°C or lower. It is preferable to coil hot-rolled steel sheet, which has been cooled to The winding temperature should be 630°C or lower. If the winding temperature exceeds 630°C, decarbonization of the base steel surface may occur. In such cases, the interior and surface of the steel sheet may have different microstructures, which can lead to inconsistencies in the alloy concentration. Furthermore, ferrite may form in the surface layer as a result of decarbonization. This can reduce tensile strength. Therefore, the winding temperature is preferably 630°C or lower, and more preferably 600°C or lower. Although the lower limit for the winding temperature is not specified, it is preferable to use a winding temperature of 500°C or higher to avoid reducing cold-rolling ease. After winding, the hot-rolled steel sheet can be optionally pickled. There are no restrictions on the pickling conditions. Cold rolling process The cold rolling step consists of cold rolling the hot-rolled steel sheet prepared in the hot rolling step. The rolling reduction used for cold rolling is preferably, but not limited to, 20% or more because if the rolling reduction is less than 20%, the surface flatness may be reduced and the microstructure uniformity degraded. It should be noted that the cold rolling step is not an essential step; it may be omitted when the microstructure and mechanical properties of the steel are within the ranges specified by the present invention. Annealing step The annealing step consists of holding the cold- or hot-rolled steel sheet at an annealing temperature equal to or higher than AC1 for 30 seconds or more, followed by rapid water quenching at a temperature equal to or higher than Ms, and, after water quenching to 100°C or less, reheating to a temperature between 100°C or higher and 300°C or less. While the water quenching for rapid water quenching is being carried out in the annealing step, the steel sheet is joined by two rollers arranged to rest on the front and back surfaces of the steel sheet so that they are oriented relative to each other through the steel sheet when the surface temperature of the steel sheet is equal to or lower than Ms + 150°C.The ratio between a bonding pressure applied to the transverse center of the steel sheet and a bonding pressure applied to the transverse end of the steel sheet in a position where the steel sheet is bonded with the two rollers is 1.05 or more and 2.0 or less. Heating to annealing temperature equal to or higher than AC1 temperature If the annealing temperature is below the AC1 temperature, austenite cannot form. In such a case, it is difficult to produce a steel sheet containing 20% ​​or more martensite, meaning the intended strength may not be achieved. Therefore, the annealing temperature should be equal to or higher than the AC1 temperature and, preferably, equal to or higher than (AC1 temperature + 10°C). Although no upper limit for the annealing temperature is specified, it is preferably 900°C or lower to optimize the temperature at which rapid water quenching is performed and to avoid degradation of dimensional uniformity. The previous AC1 temperature is calculated using the following formula. In the following formula, Lbfrznn / zznz / B / YiAi “(% element symbol)” refers to the content (% by mass) of the element. temperature AC1(°C) = + 723 + 22(% Si) - 18(% Mn) + 17(% Cr) + 4.5(% Mo) + 16(% V) The amount of time (annealing holding time) during which the annealing temperature is held is 30 seconds or more If the aforementioned annealing holding time is less than 30 seconds, the dissolution of the carbides and the transformation of the austenite do not occur to a sufficient degree. In such a case, thickening of the remaining carbides may occur during subsequent heat treatment. This increases the residual stress generated at the transverse center of the steel sheet and, consequently, degrades its resistance to delayed fracture. Therefore, the annealing holding time is limited to 30 seconds or more and is preferably 35 seconds or more. Although the upper limit for the annealing holding time is not established, it is preferably 900 seconds or less to prevent an excessive increase in the diameter of the austenite grains and to limit an increase in the amount of hydrogen ingress. The starting temperature of rapid quenching is equal to or greater than the Ms temperature. The temperature at which rapid quenching begins is an important factor determining the volume fraction of martensite, which in turn controls strength. If the start temperature of rapid quenching is below the Ms temperature, the martensite transformation occurs before the quenching process. In this case, self-hardening of the martensite occurs, consequently degrading the uniformity of shape. Therefore, the start temperature of rapid water quenching is limited to being equal to or greater than the Ms temperature and, preferably, equal to or greater than (Ms temperature + 50°C). There is no fixed upper limit for the start temperature of rapid water quenching; the start temperature can be equal to the annealing temperature. The previous Ms temperature is calculated using the following formula. In the following formula, “(% element symbol)” refers to the content (% by mass) of the element, and “(%VM)” refers to the martensite phase area ratio (unit: %). Temperature Ms (°C) = 550 - 350{(%C) / (%VM) x 100} - 40(%Mn) - 17(%N¡) - 17(%Cr) 21(%Mo) Joining the steel sheet with two rollers positioned to rest on its front and rear surfaces during water quenching for rapid water tempering is an important factor in achieving a shape correction effect. Controlling the joining conditions is crucial for reducing residual stress generated at the transverse center of the steel sheet and stress fluctuations across its entire width.One of the features of the present invention is to improve the shape uniformity of the steel sheet as a result of correcting the transformation deformation that occurs during water quenching by joining the steel sheet, while eliminating the need for correction by leveling or hardening rolling, which increase residual stress and degrade delayed fracture resistance. Since the need for leveling or hardening rolling correction, which is performed to address the degradation of shape uniformity, is eliminated... Lbfrznn / zznz / B / YiAi makes it possible to reduce residual stress. Furthermore, since residual stress fluctuations across the entire width of the steel sheet can be reduced depending on the joining conditions, delayed fracture resistance can be improved across the entire width of the steel sheet. The surface temperature of the steel sheet to which the steel sheet is joined by two rollers arranged to rest on the front and rear surfaces of the steel sheet (hereinafter referred to as the “joining temperature”) is equal to or less than (Ms temperature +150° C) If the aforementioned joining temperature exceeds (Ms temperature +150°C), martensite transformation occurs after the steel sheet is joined. In such a case, it becomes impossible to limit the degradation of dimensional uniformity caused by the expansion of the martensite transformation, and consequently, dimensional uniformity is degraded. Therefore, the joining temperature is limited to being equal to or less than (Ms temperature +150°C), preferably equal to or less than (Ms temperature +100°C), and more preferably equal to or less than (Ms temperature +50°C). Although the lower limit of the joining temperature is not specified, the joining temperature is preferably 0°C or higher, at which water does not freeze. The ratio between the bonding pressure applied to the transverse center of the steel sheet and the bonding pressure applied to the transverse end of the steel sheet in the position where the steel sheet is bonded with two rollers is 1.05 or more and 2.0 or less When the ratio between the bonding pressure applied to the transverse center of the steel sheet and the bonding pressure applied to the transverse end of the steel sheet, where the steel sheet is joined by the two rollers, is limited to 1.05 or greater, the residual stress remaining at the transverse center of the steel sheet can be dissipated in the transverse direction of the steel sheet. This reduces the residual stress generated at the transverse center of the steel sheet and the fluctuations in residual stress across the entire width of the steel sheet, and improves delayed fracture resistance to an excellent level across the entire width of the steel sheet. Consequently, the aforementioned bonding pressure ratio is limited to 1.05 or greater. The bonding pressure ratio is preferably 1.10 or greater. However, if the bonding pressure ratio exceeds 2.At a value of 0, the residual stress generated at the transverse center of the steel sheet increases, and consequently, the delayed fracture resistance degrades. Therefore, the bonding pressure ratio is limited to 2.0 or less. The bonding pressure ratio is preferably 1.7 or less, and more preferably 1.5 or less. Although the bonding pressure ratio must be within the range of the present invention, the bonding pressure is not limited. To achieve the beneficial effects of the present invention more effectively, the bonding pressure (load) per square millimeter of the steel sheet is preferably from 50 to 300 N / mm². Note that the bonding pressure (load) mentioned above is the total of the pressures applied from the front and rear surfaces of the steel sheet to the steel sheet. It is preferable that the bonding pressure applied by the two rollers be applied uniformly from the front and rear surfaces of the steel sheet. There are no limitations on the means of joining the steel sheet while applying greater pressure at the transverse center than at the transverse end. Lbfrznn / zznz / B / YiAi Examples of these methods include adjusting the diameter of a portion of each roller that contacts the transverse center of the steel sheet when the steel sheet is attached to the rollers, so that it is larger than the diameter of a portion of the roller that contacts the transverse end. For example, using rollers that are longer than the width of the steel sheet, with the diameter of a portion of each roller that contacts the transverse center equal to that of a portion of the roller that contacts the transverse center, and adjusting the temperature of the transverse center to be higher than the temperature of the transverse end, allows the diameters of the portions of the rollers that contact the transverse center to be larger due to the difference in the coefficient of thermal expansion.The use of the rollers described above allows the bonding pressure to be applied to the steel sheet in a consistent manner. For another example, rollers prepared such that the diameter of a portion of each roller that comes into contact with the transverse center is larger than the diameter of a portion of the roller that comes into contact with the transverse end can be used while the temperature of the transverse center is adjusted to be equal to the temperature of the transverse end. Another example of the above means consists of dividing the guide rollers that press the rollers, which come into direct contact with the steel sheet, against the steel sheet into portions corresponding to the transverse center and the transverse end of the steel sheet and adjusting the force with which the rollers are pressed by the guide rollers at the transverse center so that it is greater than the force with which the rollers are pressed by the guide rollers at the transverse end. Water cooling is done at 100°C or less If the temperature after water quenching exceeds 100°C, a transformation of the post-water quench martensite occurs to a degree that negatively affects shape uniformity. Therefore, the temperature of the steel sheet exiting the water tank should be 100°C or lower. The temperature after water quenching is preferably 80°C or lower. Reheating is done at 100°C or higher and 300°C or lower After water quenching, reheating is performed to quench the martensite formed during water quenching. This reduces residual stress and thus improves delayed fracture resistance. If the reheating temperature is below 100°C, the aforementioned beneficial effects cannot be achieved. Consequently, the reheating temperature is limited to 100°C or higher. Preferably, the reheating temperature is 130°C or higher. If quenching is performed at a temperature above 300°C, shape uniformity degrades due to transformation shrinkage caused by quenching. Therefore, the reheating temperature is limited to 300°C or lower. Preferably, the reheating temperature is 260°C or lower. The hot-rolled steel sheet prepared in the hot rolling stage can optionally undergo heat treatment to soften its microstructure. After annealing, a final hot rolling stage can be performed to adjust the shape. Optionally, a plating process can be carried out to deposit a coating layer. Lbfrznn / zznz / B / YiAi composed of Zn, Al or similar on the surface of the steel sheet. Assuming that the transverse ends of the steel sheet are removed by trimming in the production of the steel sheet, the transverse center of the steel sheet according to the present invention can be ±200 mm from the position of 1 / 2 of the width of the steel sheet in the transverse direction. The following describes an element according to the present invention and a method for manufacturing the element. The element according to the present invention is manufactured by subjecting the steel sheet according to the present invention to at least one of a forming and welding process. The method for manufacturing the element according to the present invention includes a step of subjecting a steel sheet produced by the method for manufacturing a steel sheet according to the present invention to at least one of a forming and welding process. The steel sheet according to the present invention has high strength, excellent dimensional uniformity, and excellent delayed fracture resistance. Since an element produced using the steel sheet according to the present invention has high strength, adequate dimensional accuracy, and excellent delayed fracture resistance, it can be conveniently used as, for example, an automotive structural element. As a forming process, common working methods such as pressure forming can be used without limitation. As a welding process, common welding processes such as spot welding and arc welding can be used without limitation. EXAMPLES Example 1 The present invention is specifically described with reference to the following examples. 1. Production of evaluation steel sheets Cold-rolled steel sheets with a thickness of 1.4 mm were annealed under the conditions described in Table 1 to produce steel sheets with the properties described in Table 2. The temperature of the steel sheet as it passed through the joining rolls was measured with a contact thermometer attached to the rolls. The joining pressure was calculated using a computer-aided engineering (CAE) analysis with a YU model based on the roll stiffness and shape, penetration length, steel sheet thickness, volume fraction of austenite in transformation, and a stress-strain curve. In the rapid water quenching carried out in the annealing step, a bonding pressure (load) was applied to the steel sheet with two rollers, which were arranged to rest on the front and rear surfaces of the steel sheet so that they were oriented towards each other through the steel sheet, at a uniform pressure so that the bonding pressure (load) per square millimeter of the rolled steel sheet was 50 to 300 N / mm2. In Table 1, item 2, since the steel sheet was not joined with two rollers during water quenching in the annealing step, the “surface temperature of the roller-jointed steel sheet” and the “ratio of joining pressure applied to the transverse center of the steel sheet to the joining pressure applied to the transverse end of the steel sheet at the position where the steel sheet was roller-jointed” are denoted as Lbfrznn / zznz / B / YiAi Table 1 No. Cold Rolled Thickness Annealing Conditions Remarks Rolling Reduction Annealing Temperature Annealing Holding Time Water Quenching Start Temperature Ί *2 Water Quenching Stop Temperature Reheating Temperature % mm °C sec. °C °C % °C °C 1 56 1.4 860 60 775 300 1.20 50 150 Invention Example 2 56 1.4 860 60 782 - - 50 150 Comparative Example 3 56 1.4 860 60 780 300 1.20 50 70 Comparative Example Ί: Surface temperature of the steel sheet joined with rollers Lbfrznn / zznz / B / YiAi *2: Ratio between the bonding pressure applied to the transverse center of the steel sheet and the bonding pressure applied to the transverse end of the steel sheet in the position where it is rolled (bonding pressure at the transverse center / bonding pressure of ^θ1 steel). Each of the steel sheets prepared under the different production conditions above was analyzed to determine the fraction of each component of the microstructure. The steel sheets were subjected to a tensile test to determine their properties, such as tensile strength. The steel sheets were subjected to a delayed fracture test to determine delayed fracture resistance. The shape of each of the steel sheets was measured to determine shape uniformity. The methods used in the above evaluations are as follows. Table 2 lists the results. (Area fractions of the components of the steel microstructure) A test sample was taken from each of the steel sheets in the rolling direction and in a direction perpendicular to the rolling direction. A cross-section of thickness L from the test sample taken parallel to the rolling direction was mirror-polished and treated with a nital solution to expose the microstructure. The microstructure-exposed sample was inspected using a scanning electron microscope. In a 1500x SEM image, a 16 x 15 grid with 4.8 pm intervals was placed over a region with an actual size of 82 pm x 57 pm, and the area fractions of martensite and ferrite were determined by counting the number of points located in the respective phases, i.e., by point counting.The average of three area fractions for each phase, determined from different SEM images obtained at 1500x magnification, was used as the phase area fraction. The above measurement was performed at a position 1 / 4 of the steel sheet thickness. Martensite appears as a white microstructure. Quenched martensite includes fine carbide grains precipitated within the quenched martensite phase. Ferrite appears as a black microstructure. Internal carbide grains may be difficult to visualize depending on the planar orientation of the block grains and the degree of etching. In such cases, confirmation should be made after the etching has been performed to a sufficient degree. The area fraction of the component other than ferrite or martensite, i.e., the remainder, was calculated by subtracting the total area fraction of ferrite and martensite from 100%. (Traction test) A JIS No. 5 test sample with a gauge length of 50 mm and a gauge width of 25 mm was taken from the transverse center of each of the steel sheets in the rolling direction. A tensile test was then performed at a crosshead speed of 10 mm / min according to JIS Z 2241(2011) to measure tensile strength (TS) and yield strength (YS). (Method for measuring residual voltage) Residual stress was measured using X-ray diffraction. Specifically, a sample measuring 100 mm in length in the rolling direction and 30 mm in length in the transverse direction was taken from the transverse center and transverse end of each steel sheet. The sample from each steel sheet was placed in a die at a 90° angle. The steel sheet was then pressed with a punch at a 90° angle so that the direction of the bending crest line was parallel to the transverse direction of the steel sheet, in order to perform a V-bending process. The V-bending process was carried out under the following conditions: punch movement speed: 30 mm / min, load: 15 tons, and holding time to maximum load (penetration time): 5 seconds.Subsequently, the steel sheet (element) that had undergone the bending process was secured using bolts, nuts, and conical washers from both surfaces of the steel sheet. The amount of securing was set at 30 mm. The residual stress of the steel sheet subjected to the bending process was measured at the center of the final surface thickness of the bending crest line. The X-ray irradiation diameter was 150 pm. The measurement direction was set perpendicular to both the thickness direction and the bending crest line. The sample from the transverse end of each steel sheet was taken at a position 0 to 30 mm from the transverse end of the steel sheet in the transverse direction. In the Examples of the present invention, each steel sheet was produced while applying a load to the steel sheet in a manner symmetrical about the transverse center of the steel sheet in the transverse direction. Therefore, the residual stress was measured at only one end of each steel sheet; the residual stress at the other end of the steel sheet was considered equal to the previous residual stress. (Critical load voltage measurement) A critical load stress was measured using a delayed fracture test. Specifically, a sample 100 mm long in the rolling direction and 30 mm long in the transverse direction was taken from the transverse center of each steel sheet. The sample was subjected to a V-bend process, as in the residual stress measurement. Subsequently, the bent steel sheet (element) was secured with bolts, nuts, and conical washers from both surfaces. The relationship between load stress and the amount of fastening was calculated based on a stress-strain curve determined from a tensile test using a CAE analysis with a YU model. Thus, shaped elements with different load stresses were prepared. The shaped elements were immersed in hydrochloric acid with a pH of 1. Lbfrznn / zznz / e / YiAi (25°C). The maximum load stress at which delayed fracture did not occur was determined as the critical load stress. The occurrence of delayed fracture was determined visually or by inspection of a 20x magnified image using a stereomicroscope. When cracking did not occur even after a 96-hour immersion, fracture was considered not to have occurred. Note that the term “cracking” used herein refers to the case where a crack with a length of 200 pm or more formed. (Measuring the shape of the steel sheet) Each of the steel sheets prepared in the Examples was sheared to a size that had the same width as the steel sheet and a length of 1 m in the longitudinal direction (the rolling direction) of the steel sheet. The sheared steel sheet was placed on a horizontal platform. Note that the sheared steel sheet was placed on the horizontal platform so that the number of contact points between the corners of the steel sheet and the horizontal platform was maximized (two or more). The amount of warping was determined by lowering a horizontal plate from a position above the steel sheet until it made contact with the steel sheet and by subtracting the thickness of the steel sheet from the distance between the horizontal platform and the horizontal plate at the position where the horizontal plate was in contact with the steel sheet.The maximum amount of warping was the greater of the amount of warping measured with one surface of the steel sheet facing upwards and the amount of warping measured with the other surface of the steel sheet facing upwards. The clearance of one blade of the shear used to cut the steel sheet in the longitudinal direction was 4% (the upper limit of the control interval was 10%). Lbfrznn / zznz / e / YiAi 3. Evaluation Results Table 2 lists the results of the evaluation. Table 2 No. Microstructure Residual stress Tensile properties Delayed fracture strength Shape Remarks: MF Remainder Transverse center Ί YS TS Critical load stress *2 '3 % % % MPa % MPa MPa MPa % mm 1 97 2 1 350 104 1257 1535 1470 105 6 Invention example 2 97 1 2 1020 88 1238 1522 1120 86 28 Comparative example 3 98 1 1 990 105 1263 1542 1210 104 11 Comparative exampleM: Fraction of martensite area, F: Fraction of ferrite area, Remainder: Fraction of remainder area *1: Ratio between residual stress at the transverse end and residual stress at the transverse center (residual stress at the transverse end / residual stress at the transverse center) '2: Ratio between critical load stress at the transverse end and critical load stress ter^ip¿?| at or above YS, where the ratio between the critical load stress at the transverse center of the steel sheet and the critical load stress at the transverse end, *3: Maximum amount of warping of the steel sheet sheared at a length of 1 m in, , , , of the steel sheet was 90% or more and 110% or less and the maximum amount of warping of the longitudinal direction of the steel sheet sheared at a length of 1 m in the longitudinal direction was 15 mm or less, were evaluated as “approved” and denoted as “Example of invention” in Table 2.Steel sheets that did not meet at least one of the above conditions were evaluated as “Failed” and were denoted as “Comparative Example” in Table 2. Example 2 The present invention is specifically described with reference to the following Examples. 1. Production of evaluation steel sheets Each of the steels with the chemical compositions described in Table 3, the remainder being Fe and incidental impurities, was formed from molten steel in a vacuum melting furnace and then rough-rolled to produce a rough-rolled material 27 mm thick. The rough-rolled material was hot-rolled. For the samples that were to be cold-rolled, the hot-rolled steel sheet was ground and then cold-rolled to one of the roll reductions described in Tables 4 and 5, or to one of the thicknesses described in Tables 4 and 5, to produce a cold-rolled steel sheet. In the case of some samples, cold rolling was not performed after grinding the hot-rolled steel sheet. The symbol in the “Roll Reduction” column of the table indicates that the sample was not cold-rolled.Subsequently, the hot- and cold-rolled steel sheets prepared as described above were annealed under the conditions described in Tables 4 and 5 to produce steel sheets. Blanks in Table 3 indicate that the constituents were not intentionally added to the steel; that is, the steel contained no constituents (0% by mass) or the steel contained the constituents incidentally. In Tables 4 and 5, as in the comparative examples, the “Surface temperature of the roller-jointed steel sheet” and the “Ratio between the joining pressure applied to the transverse center of the steel sheet and the joining pressure applied to the transverse end of the steel sheet in the position where the steel sheet was roller-jointed” are indicated with a symbol. This means that the steel sheet was not joined with two rollers while undergoing rapid water quenching in the annealing step. 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Evaluation methods The microstructure of the steel in each of the steel sheets prepared under the various production conditions mentioned was analyzed to determine the fraction of each microstructural component. The steel sheets were subjected to a tensile test to determine their properties, such as tensile strength. The steel sheets were also subjected to a delayed fracture test to determine their delayed fracture resistance. The shape of each steel sheet was measured to determine its uniformity. The methods used in the above evaluations were the same as those described in Example 1. 3. Evaluation Results Tables 6 and 7 list the results of the evaluation. Lbfrznn / zznz / e / YiAi Table 6 1 dw 8 & £ w 8 & 6 8 £ w 8 & ú w 8 & i 6 8 £ w 8 & d 8 8 & i 8 £ 8 & £ « 8 & 5 c 8 8 & i 5 8 & ¿ 8 ¿l· icw 8 £ i 6 8 & i fi 8 i | jtxxeAJ ac xua= 1 c Λ 8 & £ w 8 i 8 & c 8 8 & É 8 8 5 cw 8 i § 8 i £ W 8 & 1 cw 8 i 1 cw 8 1 cw 8 £ 1 c Λ 8 .ξί· W 8 i 1 c Λ 8 & 1 c Λ 8 & i 8 & í Γ Γ •O Γ* > 11 o 'U m <-. •u 'T •M o. Ό 'MI 4 'U í Ό OI H m •M OI <' 4 ,n 'U OI mm •M Ό I to tn 1 • 4 ·· 11 !n 1' 1-1 ; · V' h X 18 X 18 w 7' 71 w 71 * · ' ' XX 11 71 11 X 71 X 71 11 71 X 71 ·> m £ F £ $ r te te te te te ζ* te ζ' ¿1 te te y te ío 8 $ te te te te te «' 4 < 4 9 y te 1 £ (1 yr te te te te Γi te • yyy 5 y te te «^4 te T te te te $ r; y PP £ te te te te te *•1 te ¡o te 1-4 $ ϊ 8 $ $ y 5 a X $ , t « {· 11 » .n 18 π « 4 71 y te te £ ¡8 te te H rx te »r π < ·Ί -r te te te te te te 18 te te te te te te 1 •í o» i -4 < 4 c> ui OI • 4 < 4 IO « 4 ΊΊ o» OI ' i » 4 r 4 < > ΜΊ r ' • 4 í > < > o> < > • ' o. u IB U lo IJ 18 '4r írí 18 w « yy .1» t < 4 i 1 Ul • 4 o» Ul í ' OI r > OI » -4 í > '4' < 4 í» ··” w M 18 V s !K 18 si til u C) T - -' t 4 O» -r ui 'M t < >1 í > : * * 7» 7 Ul » V n M • -4 H 71 1*^4 MH 11 H P1 M Ai 71 « 18. Lbfrznn / zznz / B / YiAi ι c Λ Η & & C Λ 8 & 1 ύ Λ 8 & 1 Z'MEJKUXC XUB3 | jox&\j ac xu^ i § 8 & Eenx se rvenccr | Ξεπίχ if Rencor | c « 8 & 8 & '.1 :scccr 3e ara xs ^arsrsti - -3car xa=sxs fem ^es: --xacr x ara Trasrasanrasc- raoa er e xrr: Tar^.esa 2 :s0ccr are s :rascr x carga erica er a ecrax rars.ersa ya rascr x caga cica er a cerc :ras.eria censor x carga rtca er a ecrax eras.eres'enscr ce carga erica er a xrv: eras.esa' 2 Caraac wis x aerial x amra x acere estaca a js engrx x ' m er 3=cccr crgtx-a t ·4 -U ; rr Í 4 m •M r Η ri HHHK ri υ ιό H id H fd $ ί5 t !·< Γ' 1 U $ Λ £ Η Λ 8 8 £ rrrrir; C 1 C a C 1 ttri • 'T i 4 t 4 ΜΊ τχ • τχ »r •O oo 1 7' |.| :« π Yes Yes w M Γη i 1 η ι Λ -r ΧΓ 5 ΜΊ '* 9 £ Eemoc de ηνεΥχη Eenroo ουρβέ^λ ic Λ di. UJ i σι e 8 £ di Ul i σι β 8 di Ul ic Λ £ di. UJ É 8 di UJ ic Λ £ di. UJ ii 8 £ di UJ EeToo 'de πνεΥχη icd). UJ Eemoc de πνεΥχη i <n 8 di Uf i c Λ £ di. UJ Ξετοο de πλύχί i c Λ d). UJ i σι β 8 di UJ i c Λ £ di. Ul Ξετρο de πνεΎχη Eenrpo comca^z? Ό 8 co ITJ «0 CO 8 - <o <o rj ID 8 - Ό Ά ID 0 M- m SesE&Tcs 9 s texj’s vanaos Γ' 8 1 · s ΡΊ •Ti co 8 co s ñ t 8 8 8 CO 8 co r · s cg CO ? 8 * 1 a b iS OT QL P uo 5 $ 8 10 B f 8 8 ID 1 P ÍD § ID 8 0 8 <o 8 0 8 8 m $ P Q. r σι Q. 8 «0 X i co X u*j In ID 1 ID 8 $ 8 UO ΰ ΰ P Ό UO t <0 1 0 UO t Ό P « 8 en OJ 0. t 8 ΟΊ 8 a 1 en £ UO Γ4 en co 8 CJ 8 ID CO 8 $ 8 8 ID t 8 B co 1 - $ m s (s .T T P s s 8 8 8 8 8 CO 8 8 8 8 8 8 CJ CO co 8 p 3 | s <n QL $ £ 8 co co § 8 8 8 1 8 iT>8 <0 § 8 8 8 8 8 P CO § 8 cñ Ω OF β β 8 P £ t* JO ω en 8 en 8 co XX m fo •r •r *r I ? *r 8 CO XXX co P s 1 I - ^t· o CJ ID CJ co CJ - - CO - ooo UO UO 11 .Ψ Ό CJ - - cj with CJ with CJ - - CJ ID - - CJ chorus 1 - with 8 X 8 8 8 8 8 8 t »T> in ιΛ iT> ID 8 Ό •O with Ά 8 8 . Distribution / sales / e / YiAi β L Λ ? day. IN the example of β c di. IN β 4! c say. tu β « c Λ ? day. at Eemoo encounter β 4! L of. IN QP m β 8 of. UJ QP σι β 8 di. UJ Q $σι and 8 i. UJ Q $ <n 8 £ di. UJ i σι β 8 £ di. tu 2 P σι β 8 £ di. ur Q c» P 8 ? di. LU Q P m β 8 di. UJ Q P m β 8 ? di. UJ Q £ <n 8 £ di.THE QP<n 8 1 σι 8 gl « ? SI P E 3 £ S 0 íh di di «o t - Ό co P Ό LO co 8 ’í LO O-> ON m <o <o Ό ct τ>fn osss 8 8 fn a ;$ 8 s 8 1 ιΉ 8 8 ai ® * Φ '0 di 'Ί — «i § 8 8 § 8 & δ co § iT> 8 co P<o 8 CO 8 o § 8 Ό Ψ P> £ 8 R 4 β c 1 β a ju ? § 5 Cf « 8 § § 8 CO LO 1 - 8 LO co ti § 8 8 1 41 m Üj íh Ρ fn 1 co ? 8 en en en 8 CO tT) do 1 N ró in 3 8 ID ni 8 § m LO 6 Í gl o jl| the m φ § Lp Λ B ! g¡ B • <n qí Λ 41 « R S 8 ? τ JU 11 Ό σι C a m 0 1 ίτϊ 8 s 8 8 8 8 s s 8 8 8 8 s 8 8 8 8 ΙΟ 3 8 IO 3 8 CO P 3 3 8 cb P iT) $ 8 <O P (T) $ cñ 8 en 41 F Λ (*Ο iT>but 8 in tT» (T) 1 co r tT) r tT) 1 IO in CO 8 s co § 8 ma ? " $ S SI 3 s S 4' R « gl ? sH Λ r' Λι c Si J! iii c- σι 11 di p> S n ¡Η* • s H φ Γ jU íñ Λ Λ f 0 m 0 σι OI σι 3 9 P a r ο — r ο — JO — j or a — o Γ4 mo σι β CD 03 r J IT) r lo — — rj O 8 lT) do ID n Ci co Λ Β 8 8 (T> fn i »T> 8 1 (T) 8 iT> eo 8 co (T) X 8 en iT) in 03 σι 3 σ 1 σ 1 < ι > LU < ι > LU : R? £ Μ for 11 ' Λ I lo CQ •o IT) •o PPPP -r 1 LO 1 . <o 1 r co - en ( 8 co a cLbfrznn / zznz / e / YiAi In the preceding examples, steel sheets having a TS of 750 MPa or more and a critical load stress equal to or greater than YS, where the ratio of the critical load stress at the transverse center of the steel sheet to the critical load stress at the transverse end of the steel sheet was 90% or more and 110% or less, and the maximum amount of warping of the steel sheet sheared over a length of 1 m in the longitudinal direction was 15 mm or less, were evaluated as "passed" and denoted as "Example of Invention" in Tables 6 and 7. Steel sheets that did not meet at least one of the above conditions were evaluated as "Failed" and denoted as "Comparative Example" in Tables 6 and 7.< / o> < / t> < / r> < / t>

Claims

1. A steel sheet comprising a steel microstructure including, by area, martensite: 20% or more and 100% or less, ferrite: 0% or more and 80% or less, and the remainder: 5% or less, characterized in that: a residual stress generated at a transverse center of the steel sheet when the steel sheet is subjected to a V-bending process is 800 MPa or less; a residual stress generated at a transverse end of the steel sheet when the steel sheet is subjected to a V-bending process is 90% or more and 110% or less of the residual stress generated at the transverse center of the steel sheet; and a maximum amount of warping of the sheared steel sheet over a length of 1 m in a longitudinal direction of the steel sheet is 15 mm or less.

2. The steel sheet according to claim 1, further characterized in that it comprises a chemical composition containing, by mass, 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 N: 0.010% or less, the remainder being Fe and incidental impurities.

3. The steel sheet according to claim 2, further characterized in that the chemical composition contains at least one selected from, by mass Cr: 0.01% or more and 0.50% or less, Mo: 0.01% or more or less than 0.15%, and V: 0.001% or more and 0.05% or less.

4. The steel sheet according to claim 2 or 3, further characterized in that the chemical composition further contains at least one selected from, by mass, Nb: 0.001% or more and 0.020% or less, and Ti: 0.001% or more and 0.020% or less.

5. The steel sheet according to any of claims 2 to 4, further characterized in that the chemical composition further contains at least one selected from, by mass, Cu: 0.001% or more and 0.20% or less, and Ni: 0.001% or more and 0.10% or less.

6. The steel sheet according to any of claims 2 to 5, further characterized in that the chemical composition contains, by mass B: 0.0001% or more and less than 0.0020%.

7. The steel sheet according to any of claims 2 to 6, further characterized in that the chemical composition further contains at least one selected from, by mass, Sb: 0.002% or more and 0.1% or less, and Sn: 0.002% or more and 0.1% or less.

8. A method for manufacturing a steel sheet, the method characterized in that it comprises: a hot rolling step consisting of heating a steel slab having the chemical composition Lbfrznn / zznz / e / YiAi according to any one of claims 2 to 7 and rolling the hot steel slab; and an annealing step consisting of holding a hot-rolled steel sheet prepared in the hot rolling step at an annealing temperature equal to or greater than a temperature AC1 for 30 seconds or more, subsequently initiating rapid water quenching at a temperature equal to or greater than a temperature Ms and, after water quenching to 100°C or less, reheating to a temperature of 100°C or more and 300°C or less, wherein, while water quenching for rapid water quenching is being carried out in the annealing step,The steel sheet is joined with two rollers arranged to rest on the front and rear surfaces of the steel sheet so that they are oriented relative to each other across the steel sheet when a surface temperature of the steel sheet is equal to or less than (Ms temperature +150°C), and a ratio between a joining pressure applied to a transverse center of the steel sheet and a joining pressure applied to a transverse end of the steel sheet in a position where the steel sheet is joined with the two rollers is 1.05 or more and 2.0 or less.

9. A method for manufacturing a steel sheet, the method characterized in that it comprises: a hot rolling step consisting of heating a steel slab having the chemical composition according to any one of claims 2 to 7 and rolling the hot steel slab; a cold rolling step consisting of cold rolling a hot-rolled steel sheet prepared in the hot rolling step; and an annealing step consisting of holding a hot-rolled steel sheet prepared in the hot rolling step at an annealing temperature equal to or greater than a temperature AC1 for 30 seconds or more, subsequently performing rapid water quenching at a temperature equal to or greater than a temperature Ms and, after water quenching to 100°C or less, reheating to a temperature of 100°C or more and 300°C or less, wherein,While water quenching for water quenching in the annealing step is being carried out, the steel sheet is joined with two rollers arranged to rest on the front and rear surfaces of the steel sheet so that they are oriented towards each other through the steel sheet when a surface temperature of the steel sheet is equal to or less than (Ms temperature +150°C), and a ratio between a joining pressure applied to a transverse center of the steel sheet and a joining pressure applied to a transverse end of the steel sheet in a position where the steel sheet is joined with the two rollers is 1.05 or more and 2.0 or less.

10. An element manufactured by subjecting the steel sheet according to any of claims 1 to 7 to at least one of the forming and welding processes.

11. A method for manufacturing an element, the method characterized in that it comprises a step of subjecting a steel sheet manufactured by the method for manufacturing a steel sheet according to claim 8 or 9 to at least one of the forming and welding processes.