Steel plate, member, manufacturing method thereof, manufacturing method of hot-rolled steel plate for cold-rolled steel plate, and manufacturing method of cold-rolled steel plate

Abstract

Provided are: a steel sheet having a tensile strength (TS) of 780 MPa or more and having excellent collision characteristics; a member; and methods for manufacturing the steel sheet and the member.　This steel sheet has a component composition having a carbon equivalent (CE) of 0.46% or more, and has a specific steel structure. In the steel sheet, the average crystalline particle size of ferrite is 25 μm or less, and variation coefficient (CV) of ferrite grain size × carbon equivalent (CE) equals 0.28 or less. When the steel sheet is bent by 90° in a rolling (L) direction by using the width (C) direction as an axis at curvature radius / sheet thickness of 4.2, and then bend restoration is performed on the steel sheet so as to be flat again, the ratio (NFvoid / NF) of ferrite grains having voids at interfaces with respect to all ferrite grains is 15% or less in an L cross-section in a 0-50 μm range from the steel sheet surface on the compression-tensile deformed side. The steel sheet has a tensile strength of 780 MPa or more.

Description

[Technical Field]

[0001] The present invention relates to a high-strength steel sheet and a member having excellent crashworthiness, a method for manufacturing the same, a method for manufacturing a hot-rolled steel sheet for cold-rolled steel sheet, and a method for manufacturing a cold-rolled steel sheet. The steel sheet of the present invention can be suitably used mainly as an automotive steel sheet. [Background technology]

[0002] From the perspective of protecting the global environment, reducing the weight of automobile bodies while maintaining their strength and improving automobile fuel efficiency in order to reduce CO2 emissions have always been important challenges in the automotive industry. To achieve this while maintaining the strength of automobile bodies, it is effective to reduce the thickness of steel sheets used as raw materials for automobile parts by increasing their strength. On the other hand, the premise of automobile parts made from steel sheets is to ensure the safety of people inside the vehicle in the event of a collision. Therefore, high-strength steel sheets used as raw materials for automobile parts are required to have not only the desired strength but also excellent crashworthiness.

[0003] In recent years, the use of high-strength steel sheets with a tensile strength (TS) of 780 MPa or higher in automobile bodies has been expanding. From the perspective of crashworthiness, automotive components can be broadly divided into non-deformable components such as pillars and bumpers and energy-absorbing components such as side members. Each component requires specific crashworthiness to ensure the safety of occupants in the event of a collision while the vehicle is in motion. Progress has been made in increasing the strength of non-deformable components, and high-strength steel sheets with a tensile strength (TS) of 780 MPa or higher have already been put into practical use. However, when applied to energy-absorbing components, high-strength steel sheets with a tensile strength of 590 MPa or lower are often used, as they are prone to fracture during a collision at the primary processing point (i.e., forming) and therefore lack the ability to consistently absorb impact energy. Therefore, there is potential for reducing component fracture during a collision and ensuring high energy absorption, thereby ensuring crash safety, while contributing to environmental conservation through weight reduction. Therefore, it is necessary to use high-strength steel sheets with a TS of 780 MPa or higher, which offer excellent crashworthiness, for energy-absorbing components.

[0004] In response to such demands, for example, Patent Document 1 discloses a technology relating to an ultra-high strength steel sheet having excellent formability and impact resistance and a TS of 1200 MPa or more. Also, Patent Document 2 discloses a technology relating to a high strength steel sheet having a maximum tensile strength of 780 MPa or more and applicable to impact absorbing members in the event of a collision. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2012-31462 [Patent Document 2] Japanese Patent Application Laid-Open No. 2015-175061 Summary of the Invention [Problem to be solved by the invention]

[0006] However, although Patent Document 1 examines collision characteristics, it examines impact resistance on the assumption that no breakage of components occurs during a collision, and does not examine collision characteristics from the perspective of breakage of resistant components.

[0007] Furthermore, in Patent Document 2, a crack assessment was performed on a hat material in a dynamic axial crushing test using a falling weight, and the fracture resistance characteristics of a TS exceeding 780 MPa were evaluated. However, the assessment of cracks after crushing does not allow for evaluation of the process from the occurrence of cracks during crushing to fracture, which is important for crash performance. This is because if a crack occurs early in the crushing process, even a minor crack that does not penetrate the plate thickness may reduce the absorbed energy. Furthermore, if a crack occurs late in the crushing process, even a large crack that penetrates the plate thickness may have little effect on the absorbed energy. Therefore, it is considered that the assessment of cracks after crushing alone is insufficient for evaluating fracture resistance characteristics.

[0008] The present invention has been made in view of the above circumstances, and aims to provide a steel plate and component having a tensile strength (TS) of 780 MPa or more and excellent collision characteristics, which are suitable for use in energy absorbing components of automobiles, as well as a method for manufacturing the same. [Means for solving the problem]

[0009] The present inventors have conducted extensive research to solve the above problems and have discovered the following.

[0010] A steel plate has a chemical composition that satisfies a carbon equivalent (CE) of 0.46% or more, and a steel structure that has, in area ratios, ferrite: 10 to 50%, tempered martensite and bainite: 30% or more in total, retained austenite: 3 to 20%, fresh martensite: 15% or less, and ferrite, tempered martensite, bainite, retained austenite and fresh martensite: 85% or more in total, the average grain size of ferrite: 25 μm or less, the coefficient of variation (CV) of ferrite grain size x carbon equivalent (CE) is 0.28 or less, and when the steel plate is bent 90° in the rolling (L) direction with the width (C) direction as the axis at a curvature radius / plate thickness of 4.2 and then bent back flat again, the ratio of ferrite grains with voids at the interface (NF) to all ferrite grains is 0.50, in the L cross section within a region of 0 to 50 μm from the steel plate surface on the compression-tensile deformation side. void The tensile strength of the steel sheet is 780 MPa or more, and the tensile strength is 15% or less. These results show that the steel sheet has high strength and excellent crashworthiness.

[0011] The present invention was made based on these findings, and the gist of the present invention is as follows. [1] A component composition with a carbon equivalent (CE) of 0.46% or more; The steel structure has, in terms of area ratio, ferrite: 10 to 50%, the total of tempered martensite and bainite: 30% or more, retained austenite: 3 to 20%, fresh martensite: 15% or less, and the total of ferrite, tempered martensite, bainite, retained austenite and fresh martensite: 90% or more, Average grain size of ferrite: 25 μm or less, The coefficient of variation (CV) of ferrite grain size × carbon equivalent (CE) is 0.28 or less, When the specimen is bent 90° in the rolling (L) direction with the width (C) direction as the axis at a curvature radius / plate thickness of 4.2, and then bent back flat, the ratio of ferrite grains with voids at the interface to all ferrite grains in the L cross section within the 0 to 50 μm region from the steel plate surface on the compression-tensile deformation side (NF void / NF) is 15% or less, Steel plate with a tensile strength of 780 MPa or more. [2] The component composition is, in mass%, C: 0.07~0.20%, Si: 0.10 to 2.00%, Mn: 1.5-4.0% P: 0.100% or less, S: 0.050% or less, Sol.Al: 0.005 to 0.100%, and The steel sheet according to [1], containing N: 0.0100% or less, with the balance consisting of Fe and unavoidable impurities. [3] The component composition further includes, in mass%, The component composition further includes, in mass%, Cr: 1.000% or less, Mo: 0.500% or less V: 0.500% or less, Ti: 0.500% or less, Nb: 0.500% or less, B: 0.0050% or less, Ni: 1.000% or less, Cu: 1.000% or less, Sb: 1.000% or less, Sn: 1.000% or less, As: 1.000% or less, Ca: 0.0050% or less, W: 0.500% or less, Ta: 0.100% or less, Mg: 0.050% or less, Zr: 0.050% or less, and The steel sheet according to [2], containing at least one selected from REM: 0.005% or less. [4] A steel sheet according to any one of [1] to [3], which has an electrogalvanized layer, a hot-dip galvanized layer, or an alloyed hot-dip galvanized layer on the surface of the steel sheet. [5] A member obtained by subjecting the steel plate according to any one of [1] to [4] to at least one of forming and welding. [6] A hot rolling process in which a steel slab having a carbon equivalent (CE) of 0.46 or more and having the composition described in [2] or [3] is heated to a temperature range of 1100 to 1300°C, hot rolled at a finish rolling temperature of 800 to 950°C, with a cumulative reduction of 60% or more in the finish rolling, and in the cooling process from the finish rolling exit side to coiling, the residence time in a temperature range of 750 to 600°C is 10 seconds or less, and the coiling temperature is 600°C or less; a cold rolling step of pickling the hot-rolled steel sheet obtained in the hot rolling step and cold-rolling it at a cumulative reduction rate of 20% or more; An annealing step in which the cold-rolled steel sheet obtained in the cold rolling step is heated to an annealing temperature of 750 to 880 ° C. and held for 30 seconds or more; After the annealing step, a quenching step of cooling to a cooling stop temperature of (Ms-250°C) to (Ms-50°C); After the quenching process, a tempering process is performed in which the steel is reheated to a temperature of 300 to 500 ° C. and held for 20 seconds or more. A method for manufacturing a steel plate comprising: [7] A method for manufacturing a hot-rolled steel sheet for cold-rolled steel sheet, comprising a hot-rolling step of heating a steel slab having a carbon equivalent (CE) of 0.46 or more and having the chemical composition described in [2] or [3] to a temperature range of 1100 to 1300°C, hot-rolling the slab at a finish rolling exit temperature of 800 to 950°C, setting the cumulative reduction in the finish rolling to 60% or more, setting the residence time in a temperature range of 750 to 600°C to 10 seconds or less in the cooling process from the finish rolling exit to coiling, and coiling the slab at a coiling temperature of 600°C or less, to produce a hot-rolled steel sheet having a structure in which, in terms of area ratios of the hot-rolled steel sheet structure, the total of ferrite: 20% or less and fresh martensite and bainite: 80% or more. [8] A method for producing a cold-rolled steel sheet, comprising a cold-rolling step of pickling the hot-rolled steel sheet obtained by the method according to [7] and cold-rolling it at a cumulative reduction of 20% or more. [9] The method for producing a steel sheet according to [6], further comprising a plating step of applying electrogalvanizing, hot-dip galvanizing, or alloyed hot-dip galvanizing to the surface of the steel sheet after the annealing step and before the quenching step, or after the tempering step.

[10] The method for producing a steel sheet according to [9], wherein the plating step after the annealing step and before the quenching step includes a step of holding the steel sheet in a temperature range of 300 to 500°C for 0 to 300 seconds before plating.

[11] 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 [6], [9] or

[10] . [Effects of the Invention]

[0012] According to the present invention, a steel plate having a tensile strength (TS) of 780 MPa or more and excellent crashworthiness can be obtained. Components obtained by subjecting the steel plate of the present invention to forming, welding, etc. can be suitably used as energy absorbing components in the automotive field. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 10 is a diagram for explaining 90° bending (primary bending) in the bending-orthogonal bending test of the example. [Figure 2] FIG. 10 is a diagram for explaining orthogonal bending (secondary bending) in the bending-orthogonal bending test of the example. [Figure 3] FIG. 1 is a perspective view showing a test piece that has been subjected to a 90° bending process (primary bending process). [Figure 4] FIG. 1 is a perspective view showing a test piece subjected to orthogonal bending (secondary bending). [Figure 5] FIG. 1 is a front view of a test member manufactured for conducting an axial crushing test of an example, in which a hat-shaped member and a steel plate are spot-welded together. [Figure 6] FIG. 6 is a perspective view of the test member shown in FIG. 5. [Figure 7] FIG. 1 is a schematic diagram for explaining an axial crushing test of an example. DETAILED DESCRIPTION OF THE INVENTION

[0014] The present invention will be described in detail below.

[0015] The steel sheet of the present invention has a chemical composition satisfying a carbon equivalent (CE) of 0.46% or more, and a steel structure in which, in terms of area ratio, ferrite: 10 to 50%, the total of tempered martensite and bainite: 30% or more, retained austenite: 3 to 20%, fresh martensite: 15% or less, and the total of ferrite, tempered martensite, bainite, retained austenite and fresh martensite: 90% or more, the average grain size of ferrite: 25 μm or less, the coefficient of variation (CV) of ferrite grain size × carbon equivalent (CE) is 0.28 or less, and when the steel sheet is bent 90° in the rolling (L) direction with the width (C) direction as the axis at a curvature radius / sheet thickness of 4.2 and then bent back flat again, the ratio of the number of ferrite grains having voids at the interface (NF) to the total number of ferrite grains is 0.50, in the L cross section within a region of 0 to 50 μm from the steel sheet surface on the compression-tensile deformation side. void / NF) is 15% or less, and the tensile strength is 780 MPa or more.

[0016] Carbon equivalent (CE): 0.46% or more The carbon equivalent CE is an index of the strength of steel, converting the influence of elements other than C into the amount of C. By setting the carbon equivalent CE to 0.46% or more, the area fraction of each metal structure, such as ferrite, described below, can be controlled within the range of the present invention, thereby achieving the tensile strength (780 MPa or more) and impact resistance of the present invention. The carbon equivalent CE is preferably set to 0.48% or more. There is no particular upper limit, but in consideration of the balance between weldability and formability, the carbon equivalent CE is preferably set to 0.85% or less, more preferably 0.82% or less.

[0017] The carbon equivalent CE can be calculated using the following formula (1). Carbon equivalent CE=[C%]+([Si%] / 24)+([Mn%] / 6)+([Ni%] / 40)+([Cr%] / 5)+([Mo%] / 4)+([V%] / 14) ···(1) In the above formula, the [element symbol %] represents the content (mass %) of each element, and elements that are not contained are set to 0.

[0018] Ferrite area ratio: 10 to 50% If the ferrite area ratio exceeds 50%, it becomes difficult to achieve both a tensile strength (TS) of 780 MPa or more and crashworthiness. If the ferrite area ratio is less than 10%, stress may concentrate on the ferrite during deformation, which may promote void generation at the interface. Therefore, the ferrite area ratio is 10 to 50%. The ferrite area ratio is preferably 15% or more. Furthermore, the ferrite area ratio is preferably 45% or less.

[0019] Total area ratio of tempered martensite and bainite: 30% or more Tempered martensite is effective in improving collision characteristics by suppressing component fracture during collision deformation, while also improving energy absorption and strength during a collision. If the total area ratio of tempered martensite and bainite is less than 30%, these effects cannot be fully achieved. Therefore, the total area ratio is 30% or more, preferably 40% or more. There is no upper limit to the total area ratio, but in consideration of the balance with other structures, the total area ratio is preferably 80% or less.

[0020] Area ratio of retained austenite: 3 to 20% Retained austenite is effective in delaying the occurrence of cracks during a collision and improving collision characteristics. Although the mechanism is unclear, it is thought to be as follows: Retained austenite work-hardens during collision deformation, increasing the radius of curvature during bending deformation, thereby dispersing strain in the bent portion. Dispersing strain alleviates stress concentration in areas where voids have formed due to primary processing, resulting in improved collision characteristics. If the area fraction of retained austenite is less than 3%, this effect cannot be obtained. Therefore, the area fraction of retained austenite is 3% or more, and preferably 5% or more. On the other hand, if the area fraction of retained austenite exceeds 20%, fresh martensite formed by stress-induced transformation may reduce fracture resistance during a collision. Therefore, the area fraction of retained austenite is 20% or less, and preferably 15% or less.

[0021] Fresh martensite: 15% or less Fresh martensite is effective for increasing strength. However, voids are likely to form at the grain boundaries with the soft phase, and if the area fraction of fresh martensite exceeds 15%, the generation of voids at the interface with ferrite is promoted, which may result in a deterioration of impact properties. Therefore, the area fraction of fresh martensite is 15% or less, preferably 10% or less, and more preferably 5% or less. The lower limit of the area fraction of fresh martensite may be 0%.

[0022] Total area ratio of ferrite, tempered martensite, bainite, retained austenite and fresh martensite: 90% or more If the total area ratio of ferrite, tempered martensite, bainite, retained austenite, and fresh martensite is less than 90%, the area ratio of phases other than the above will be high, making it difficult to achieve both strength and crashworthiness. Examples of phases other than the above include pearlite and cementite. If these phases increase, they may become the starting point for void generation during crash deformation, resulting in reduced crashworthiness. Furthermore, if pearlite or cementite increases, strength may decrease. As long as the total area ratio is 90% or more, high strength and crashworthiness can be obtained regardless of the type and area ratio of the remaining phases. The total area ratio is preferably 95% or more. The total area ratio may be 100%. The total area ratio of pearlite and cementite, which constitute the remaining structure, is 10% or less. Preferably, the total area ratio of this remaining structure is 7% or less, more preferably 5% or less, and even more preferably 3% or less.

[0023] The area ratio of each structure refers to the ratio of the area of each phase to the observed area. The area ratio of each structure is measured as follows: After polishing the cross-section of a steel plate cut perpendicular to the rolling direction, the plate is etched with 3% by volume of nital. Three fields of view are photographed at 1 / 4 of the plate thickness using a scanning electron microscope (SEM) at 1500x magnification. The area ratio of each structure is determined from the obtained image data using Image-Pro manufactured by Media Cybernetics. The average value of the area ratios of the three fields of view is defined as the area ratio of each structure in the present invention. In the image data, ferrite is black, bainite is black containing island-shaped retained austenite or gray containing aligned carbides, tempered martensite is light gray containing fine, misaligned carbides, and retained austenite is white. Here, fresh martensite also appears white, making it difficult to distinguish between fresh martensite and retained austenite in SEM images. Therefore, the area ratio of fresh martensite is determined by subtracting the area ratio of retained austenite determined by the method described below from the total area ratio of fresh martensite and retained austenite.

[0024] In the present invention, the volume fraction of retained austenite was determined by measuring the X-ray diffraction intensity, and this volume fraction was considered to be the area fraction of retained austenite. The volume fraction of retained austenite was determined by the ratio of the integrated X-ray diffraction intensities of the (200), (220), and (311) planes of fcc iron to the integrated X-ray diffraction intensities of the (200), (211), and (220) planes of bcc iron in the quarter-thickness plane.

[0025] Average grain size of ferrite: 25 μm or less In the steel sheet of the present invention, high impact resistance can be achieved by setting the average ferrite grain size to 25 μm or less. While the mechanism is unclear, it is believed to be as follows: The fracture during impact, which causes degradation of impact resistance, is initiated by the initiation and propagation of cracks. It is believed that cracks are more likely to occur due to a decrease in work hardening capacity and the generation and connection of voids in the high hardness difference region. Furthermore, during impact, the actual part undergoes primary processing during forming, where it is deformed by being bent back in a direction perpendicular to the primary processing. If voids are generated in the high hardness difference region of the primary processing, stress concentrates around the voids, promoting the initiation and propagation of cracks and ultimately leading to fracture. Voids are generated in the high hardness difference region because the soft phase is more deformed than the hard phase. Therefore, by refining the ferrite, the amount of deformation is reduced, suppressing the generation and propagation of voids in the primary processing region and the resulting component fracture, resulting in high fracture resistance. Therefore, the average ferrite grain size is 25 μm or less, preferably 20 μm or less. Although there is no particular lower limit for the average grain size of ferrite, it is preferably 3 μm or more.

[0026] The average grain size of ferrite is measured by photographing at least 10 fields of view of a 40 μm × 50 μm area at 1 / 4 the thickness position with a SEM (scanning electron microscope) at 2000x magnification, and then calculating the circle-equivalent diameter from the area ratio of each ferrite grain using the above-mentioned Image-Pro from the obtained image data and averaging them. Furthermore, the standard deviation of the ferrite grain size, which will be described later, can be calculated from the ferrite grain size determined using the above-mentioned Image-Pro.

[0027] Coefficient of variation of ferrite grain size (CV) x carbon equivalent (CE): 0.28 or less In the steel sheet of the present invention, high crashworthiness can be obtained by setting CV×CE to 0.28 or less. The mechanism behind this is unclear, but it is thought to be as follows: The generation and propagation of voids in the primary processed portion, which is the origin of fracture during a collision, is promoted by local stress concentration. To suppress this, it is thought that reducing the variation in ferrite grain size in the steel structure and softening the hard phase are effective. Therefore, CV is used as an indicator of the former (reducing the variation in ferrite grain size in the steel structure) and CE is used as an indicator of the latter (softening the hard phase), and high fracture resistance can be obtained by setting CV×CE to 0.28 or less. It is preferably 0.25 or less.

[0028] The coefficient of variation CV of the ferrite grain size can be calculated by the following formula (2).

[0029]

number

[0030] The carbon equivalent CE can be calculated using the following formula (1). CE=[C%]+([Si%] / 24)+([Mn%] / 6)+([Ni%] / 40)+([Cr%] / 5)+([Mo%] / 4)+([V%] / 14) ···(1) In the above formula, the [element symbol %] represents the content (mass %) of each element, and elements that are not contained are set to 0.

[0031] The desired average ferrite grain size and CV×CE can be obtained by controlling the reduction ratio in finish rolling during hot rolling, the cooling process from the finish rolling exit side to coiling, and the coiling temperature, as described below, to form a hot-rolled structure mainly composed of fresh martensite and bainite. If the hot-rolled structure is mainly composed of fine fresh martensite and bainite, the number of nucleation sites for ferrite formation in the annealing process and the cooling process after annealing increases, resulting in a structure in which uniform and fine ferrite grains are dispersed.

[0032] When the specimen is bent 90° in the rolling (L) direction with the width (C) direction as the axis at a curvature radius / plate thickness of 4.2, and then bent back flat, the ratio of the number of ferrite grains with voids at the interface to the total number of ferrite grains in the L cross section within the 0 to 50 μm region from the steel plate surface on the compression-tensile deformation side (NF void / NF): 15% or less In the steel sheet of the present invention, NF void High impact resistance can be achieved by keeping the / NF at 15% or less. (NF is the number of all ferrite grains in the L cross section within the 0-50 μm region from the surface of the steel sheet on the compression-tensile deformation side when the steel sheet is bent 90° in the rolling (L) direction with the width (C) direction as the axis at a curvature radius / sheet thickness of 4.2 and then bent back flat. NF void is the number of ferrite grains with voids at the interface in the L cross section within a region of 0 to 50 μm from the steel sheet surface on the compression-tensile deformation side when the steel sheet is bent 90° in the rolling (L) direction with the width (C) direction as the axis at a curvature radius / sheet thickness of 4.2 and then bent back flat. While the mechanism is unclear, it is thought to be as follows: The fracture during a collision, which causes the degradation of impact properties, begins with the initiation and propagation of cracks. It is believed that cracks are more likely to occur due to a decrease in work hardening capacity and the generation and connection of voids in areas with high hardness differences. Furthermore, when an actual component collides, the area deformed during forming (primary processing) undergoes secondary deformation during the collision, and the deformation history of the fracture initiation point is thought to be a location that has undergone compressive deformation due to the primary processing and secondary deformation, followed by tensile deformation. In compressively and tensilely deformed areas, when voids occur in areas with high hardness differences, stress concentrates around the voids, promoting the initiation and propagation of cracks, ultimately leading to fracture. Therefore, by reducing the high hardness difference area with tempered martensite and bainite, and further utilizing retained austenite as necessary to suppress macroscopic stress concentration in the primary processed area during deformation, and by controlling the grain size of ferrite to suppress microscopic stress concentration in coarse ferrite grains, void initiation and propagation in the primary processed area and the resulting component fracture are suppressed, resulting in high fracture resistance. Therefore, to achieve these effects, NF void / NF is set to 15% or less, preferably 10% or less. NF void / NF is set to 1 or more as the industrially obtainable lower limit.

[0033] There are no restrictions on the processing method as long as the primary bending conditions (curvature radius / plate thickness: 4.2, bending 90° in the rolling (L) direction with the width (C) direction as the axis) are met. Examples of primary bending methods include bending using the V-block method and bending using draw forming. Examples of unbending methods include press processing using a flat jig.

[0034] NF void The method for measuring / NF is as follows. The steel sheet is bent 90° in the rolling (L) direction with the width (C) direction as the axis at a curvature radius / sheet thickness of 4.2, and then bent back flat again. The cross section of the sheet is polished, and the L cross section within the 0 to 50 μm region from the steel sheet surface on the compression-tension side is observed. The L cross section is photographed in three fields at 2000x magnification using an SEM (scanning electron microscope), and the number of all ferrite grains in the field and the number of ferrite grains with voids at the interface are counted from the obtained image data using Image-Pro manufactured by Media Cybernetics, and the percentage is calculated. The average value of the three fields is taken as NF void Voids are darker black than ferrite and can be clearly distinguished from other structures.

[0035] In the present invention, performing a 90° bending process in the rolling (L) direction around the width (C) direction as an axis refers to bending the steel sheet by pressing from one side of the steel sheet surface in a direction perpendicular to the width direction and the rolling direction (see symbols D1 and D2 in FIG. 1 ) so that the distance between both ends becomes shorter when the steel sheet is viewed in the width (C) direction (see symbol D1 in FIG. 1 ) (as viewed from the steel sheet in the width direction (as viewed in a cross section perpendicular to the width direction)), and pressing until the angle formed by the flat portions that have not been bent at both ends becomes 90°. The steel sheet surface on the compressive-tensile deformation side refers to the steel sheet surface on one side that is pressed (the steel sheet surface that comes into contact with the pressing part of the punch or the like that applies the pressure). The L-shaped cross section after bending back refers to a cross section formed by cutting the steel sheet parallel to the direction of deformation due to bending and perpendicular to the surface of the steel sheet, and is perpendicular to the width direction.

[0036] The measurement position of ferrite grains after bending back is a region including a corner formed by bending and extending in the width (C) direction (see symbol D1 in Fig. 1). More specifically, the number of ferrite grains is measured within a region of 0 to 50 µm in the thickness direction in the region that is the lowest in the direction perpendicular to the width direction and the rolling direction (the pressing direction of the pressing part of the punch or the like) due to bending.

[0037] The steel sheet of the present invention may have an electrogalvanized layer, a hot-dip galvanized layer, or a galvannealed layer on the surface of the steel sheet.

[0038] The tensile strength (TS) of the steel sheet of the present invention is 780 MPa or more. High strength in the present invention means a tensile strength (TS) of 780 MPa or more. There is no particular upper limit to the tensile strength (TS), but from the viewpoint of harmony with other properties, it is preferably 1470 MPa or less. The tensile strength (TS) is measured by taking a JIS No. 5 tensile test piece (JIS Z2201) from the steel sheet in the direction perpendicular to the rolling direction, and straining the test piece at a strain rate of 10 -3 A tensile test is carried out in accordance with the provisions of JIS Z2241 (2011) at / s to determine the tensile strength (TS).

[0039] The thickness of the steel sheet of the present invention is preferably 0.2 mm or more from the viewpoint of effectively obtaining the effects of the present invention, and is preferably 3.2 mm or less from the viewpoint of effectively obtaining the effects of the present invention.

[0040] The steel sheet of the present invention has excellent impact properties. In the present invention, "excellent impact properties" means that the fracture resistance properties are good and that the absorbed energy is good. In the present invention, "good fracture resistance properties" means that the average value ΔS of the stroke at the point where the load is reduced by 50% from the maximum load when the bending-orthogonal bending test described below is carried out. 50 In the present invention, "good collision characteristics" means that the average value F of the area in the stroke range of 0 to 100 mm in the stroke-load graph at the time of crushing is 29 mm or more.ave This means that the load is 38,000N or more.

[0041] The bending-orthogonal bending test is carried out as follows. First, the steel sheet is bent 90° in the rolling (L) direction with a curvature radius / thickness ratio of 4.2 around the width (C) direction, and then flattened again (primary bending) to prepare a test specimen. In the 90° bending (primary bending), as shown in Figure 1, a punch B1 is pressed into the steel sheet placed on a die A1 with a V-groove to obtain test specimen T1. Next, as shown in Figure 2, a punch B2 is pressed into test specimen T1 placed on a support roll A2 so that the bending direction is perpendicular to the rolling direction, performing an orthogonal bending (secondary bending). In Figures 1 and 2, D1 indicates the width (C) direction, and D2 indicates the rolling (L) direction.

[0042] Test piece T1, which was obtained by bending the steel plate 90 degrees (primary bending), is shown in Figure 3. Test piece T2, which was obtained by bending test piece T1 orthogonally (secondary bending), is shown in Figure 4. The position indicated by the dashed line on test piece T2 in Figure 4 corresponds to the position indicated by the dashed line on test piece T1 in Figure 3 before orthogonal bending.

[0043] The conditions for orthogonal bending are as follows: [Orthogonal bending condition] Test method: Roll support, punch indentation Roll diameter: φ30mm Punch tip R: 0.4 mm Distance between rolls: (plate thickness x 2) + 1.5 mm Stroke speed: 20mm / min Test piece size: 60mm x 60mm Bending direction: perpendicular to rolling direction

[0044] In the stroke-load curve obtained when the above orthogonal bending is performed, the stroke at the point where the load has decreased by 50% from the maximum load is determined. The average value of the stroke at the point where the load has decreased by 50% from the maximum load when the above bending-orthogonal bending test is performed three times is called ΔS 50 Let's say.

[0045] The axial crushing test is carried out as follows. First, considering the influence of plate thickness, all axial crushing tests were performed using steel plates with a plate thickness of 1.2 mm. A steel plate was cut out and formed (bent) to a depth of 40 mm using a die with a punch shoulder radius of 5.0 mm and a die shoulder radius of 5.0 mm to produce the hat-shaped component 10 shown in Figures 5 and 6. The steel plate used as the raw material for the hat-shaped component was also cut out separately to a size of 200 mm x 80 mm. The cut-out steel plate 20 was then spot-welded to the hat-shaped component 10 to produce the test component 30 shown in Figures 5 and 6. Figure 5 is a front view of the test component 30 produced by spot-welding the hat-shaped component 10 and the steel plate 20. Figure 6 is a perspective view of the test component 30. The spot welds 40 were positioned so that the distance between the edge of the steel plate and the weld was 10 mm and the distance between the welds was 45 mm, as shown in Figure 6. Next, as shown in FIG. 7, the test member 30 is joined to the base plate 50 by TIG welding to prepare a sample for an axial crushing test. Next, an impactor 60 is caused to collide with the prepared sample for an axial crushing test at a constant velocity of 10 m / s, and the sample for an axial crushing test is crushed by 100 mm. As shown in FIG. 7, the crushing direction D3 is parallel to the longitudinal direction of the test member 30. In the stroke-load graph at the time of crushing, the area in the stroke range of 0 to 100 mm is calculated, and the average value of this area when the test is performed three times is taken as the absorbed energy (F ave )

[0046] Next, the preferred range of the chemical composition of the steel sheet will be described. Note that "%" representing the content of the chemical element means "mass %" unless otherwise specified.

[0047] C: 0.07 to 0.20% C is an element necessary for improving strength because it facilitates the formation of phases other than ferrite and also forms alloy compounds with Nb, Ti, etc. If the C content is less than 0.07%, the desired strength may not be achieved even if the manufacturing conditions are optimized. Therefore, the C content is preferably 0.07% or more, and more preferably 0.10% or more. On the other hand, if the C content exceeds 0.20%, the strength of martensite increases excessively, and the impact properties of the present invention may not be achieved even if the manufacturing conditions are optimized. Therefore, the C content is preferably 0.20% or less, and more preferably 0.18% or less.

[0048] Si: 0.10 to 2.00% Si is a ferrite-forming element and also a solid-solution strengthening element. Therefore, it contributes to improving the balance between strength and ductility. To achieve this effect, the Si content is preferably 0.10% or more, more preferably 0.20% or more. On the other hand, if the Si content exceeds 2.00%, it may cause a decrease in zinc plating adhesion and deterioration of surface properties. Therefore, the Si content is preferably 2.00% or less, more preferably 1.50% or less.

[0049] Mn: 1.5 to 4.0% Mn is a martensite-forming element and also a solution strengthening element. It also contributes to the stabilization of retained austenite. To achieve these effects, the Mn content is preferably 1.5% or more. The Mn content is more preferably 2.0% or more. On the other hand, if the Mn content exceeds 4.0%, the fraction of retained austenite increases, which may result in a deterioration in impact properties. Therefore, the Mn content is preferably 4.0% or less, more preferably 3.5% or less.

[0050] P:0.100% or less P is an element effective in strengthening steel. However, if the P content exceeds 0.100%, the alloying rate may be significantly delayed. Furthermore, if P is contained in excess of 0.100%, grain boundary segregation may cause embrittlement, which may deteriorate the fracture resistance during collision even if the steel structure of the present invention is satisfied. Therefore, the P content is 0.100% or less, preferably 0.050% or less. There is no particular lower limit for the P content, but the lower limit currently industrially feasible is about 0.002%, and a content of 0.002% or more is preferable.

[0051] S: 0.050% or less S becomes inclusions such as MnS, which can cause cracks along the metal flow path of welds, and can reduce impact resistance even if the steel structure meets the requirements of the present invention. Therefore, the S content should be as low as possible, but from the perspective of production costs, the S content is preferably 0.050% or less. The S content is more preferably 0.010% or less. There is no particular lower limit for the S content, but the lower limit currently industrially feasible is about 0.0002%, and a content of 0.0002% or more is preferable.

[0052] Sol.Al: 0.005~0.100% Al acts as a deoxidizer and also acts as a solid-solution strengthening element. If the sol. Al content is less than 0.005%, these effects may not be obtained, and even if the steel structure of the present invention is satisfied, strength may decrease. Therefore, the sol. Al content is preferably 0.005% or more. On the other hand, if the sol. Al content exceeds 0.100%, the quality of the slab during steelmaking deteriorates. Therefore, the sol. Al content is preferably 0.100% or less, and more preferably 0.04% or less.

[0053] N: 0.0100% or less N forms coarse nitride and carbonitride inclusions such as TiN, (Nb, Ti)(C, N), and AlN in steel, degrading impact properties, so its content must be kept low. Since impact properties tend to deteriorate when the N content exceeds 0.0100%, the N content is preferably 0.0100% or less. The N content is more preferably 0.007% or less, and even more preferably 0.005% or less. While there are no particular restrictions on the lower limit of the N content, the currently industrially feasible lower limit is approximately 0.0003%, and a content of 0.0003% or more is preferred.

[0054] The steel sheet of the present invention has a composition containing the above-mentioned components with the balance including Fe (iron) and unavoidable impurities. In particular, the steel sheet according to one embodiment of the present invention preferably has a composition containing the above-mentioned components with the balance consisting of Fe and unavoidable impurities.

[0055] The steel sheet of the present invention may contain the following components (optional elements) as appropriate depending on the desired properties.

[0056] At least one selected from Cr: 1.000% or less, Mo: 0.500% or less, V: 0.500% or less, Ti: 0.500% or less, Nb: 0.500% or less, B: 0.0050% or less, Ni: 1.000% or less, Cu: 1.000% or less, Sb: 1.000% or less, Sn: 1.000% or less, As: 1.000% or less, Ca: 0.0050% or less, W: 0.500% or less, Ta: 0.100% or less, Mg: 0.050% or less, Zr: 0.050% or less, and REM: 0.005% or less Cr, Mo, and V are elements effective in improving hardenability and strengthening steel. However, if added in excess of 1.000% Cr, 0.500% Mo, and 0.500% V, the above effects saturate and raw material costs increase. Furthermore, the secondary phase fraction may become excessive, deteriorating fracture resistance during a collision. Therefore, when any of Cr, Mo, and V is contained, the Cr content is preferably 1.000% or less, the Mo content is preferably 0.500% or less, and the V content is preferably 0.500% or less. More preferably, the Cr content is 0.800% or less, the Mo content is 0.400% or less, and the V content is 0.400% or less. Since the effects of the present invention can be obtained even with small contents of Cr, Mo, and V, there are no particular lower limits for the respective contents. To more effectively obtain the hardenability effect, the Cr, Mo, and V contents are preferably 0.005% or more.

[0057] Ti and Nb are elements effective for precipitation strengthening of steel. However, if the Ti content or Nb content exceeds 0.500%, respectively, it may deteriorate the fracture resistance characteristics during a collision. Therefore, when either Ti or Nb is contained, the Ti content and Nb content are preferably 0.500% or less, respectively. More preferably, the Ti content and Nb content are each 0.400% or less. Since the effects of the present invention can be obtained even with small contents of Ti and Nb, there are no particular restrictions on the lower limits of the respective contents. In order to more effectively obtain the effect of precipitation strengthening of steel, the Ti content and Nb content are preferably 0.005% or more, respectively.

[0058] B contributes to improving hardenability by suppressing the formation and growth of ferrite from austenite grain boundaries, and can be added as needed. However, if the B content exceeds 0.0050%, it may deteriorate the fracture resistance during a collision. Therefore, when B is contained, the B content is preferably 0.0050% or less. More preferably, the B content is 0.0040% or less. Since the effects of the present invention can be obtained even with a small B content, there is no particular lower limit for the B content. To more effectively obtain the effect of improving hardenability, the B content is preferably 0.0003% or more.

[0059] Ni and Cu are elements effective in strengthening steel. However, if the Ni and Cu contents exceed 1.000% each, the fracture resistance during a collision may deteriorate. Therefore, when either Ni or Cu is contained, the Ni and Cu contents are preferably 1.000% or less. More preferably, the Ni content and Cu content are each 0.800% or less. Since the effects of the present invention can be obtained even with small Ni and Cu contents, there are no particular restrictions on the lower limits of the respective contents. To more effectively obtain the effect of strengthening steel, the Ni content and Cu content are preferably 0.005% or more each.

[0060] Sb and Sn can be added as needed to suppress nitriding and oxidation of the steel sheet surface and decarburization in the region near the steel sheet surface. Suppressing such nitriding and oxidation prevents a decrease in the amount of martensite formed on the steel sheet surface, thereby improving impact resistance. However, if the Sb and Sn contents exceed 1.000%, the impact resistance may be reduced due to grain boundary embrittlement. Therefore, when either Sb or Sn is contained, the Sb content and Sn content are preferably 1.000% or less. More preferably, the Sb content and Sn content are each 0.800% or less. Since the effects of the present invention can be obtained even with small Sb and Sn contents, the lower limits of the respective contents are not particularly limited. To more effectively obtain the effect of improving impact resistance, the Sb content and Sn content are preferably 0.005% or more.

[0061] As is an element that segregates at grain boundaries and is contained as an impurity in raw material scrap. From the viewpoint of suppressing grain boundary embrittlement, the As content is preferably 1.000% or less. More preferably, the As content is 0.800% or less. The lower the As content, the better, and although there is no particular lower limit for the As content, from the viewpoint of refining costs, it is preferably 0.005% or more.

[0062] Ca is an effective element for improving workability by controlling the morphology of sulfides. However, if the Ca content exceeds 0.0050%, it may adversely affect the cleanliness of the steel and result in a deterioration in properties. Therefore, when Ca is contained, the Ca content is preferably 0.0050% or less. More preferably, the Ca content is 0.0040% or less. Since the effects of the present invention can be obtained even with a small Ca content, there is no particular lower limit for the content. To more effectively obtain the effect of improving workability, the Ca content is preferably 0.0010% or more.

[0063] W forms fine carbides, nitrides, or carbonitrides during hot rolling or annealing, and is useful for precipitation strengthening of steel. If the W content exceeds 0.500%, workability decreases. Therefore, if W is contained, it is set to 0.500% or less. If W is contained, it is preferably 0.005% or more, more preferably 0.050% or more. If W is contained, it is preferably 0.400% or less, more preferably 0.300% or less.

[0064] Ta forms fine carbides, nitrides, or carbonitrides during hot rolling or annealing, and is useful for precipitation strengthening of steel. If the Ta content exceeds 0.100%, workability decreases. Therefore, if Ta is contained, it is set to 0.100% or less. If Ta is contained, it is preferably 0.001% or more, more preferably 0.010% or more. If Ta is contained, it is preferably 0.08% or less, more preferably 0.060% or less.

[0065] Mg is an element effective in improving workability by controlling the morphology of inclusions. On the other hand, if the Mg content exceeds 0.050%, it may adversely affect the cleanliness of the steel. Therefore, if Mg is contained, the Mg content is set to 0.050% or less. If Mg is contained, it is preferably 0.0005% or more, more preferably 0.001% or more. If Mg is contained, it is preferably 0.040% or less, more preferably 0.030% or less.

[0066] Zr is an effective element for improving workability by controlling the morphology of inclusions. On the other hand, if the Zr content exceeds 0.050%, it may adversely affect the cleanliness of the steel. Therefore, if Zr is contained, the Zr content is set to 0.050% or less. If Zr is contained, it is preferably 0.0005% or more, more preferably 0.001% or more. If Zr is contained, it is preferably 0.040% or less, more preferably 0.030% or less.

[0067] REM is an element effective in improving workability by controlling the morphology of sulfides. However, if the REM content exceeds 0.005%, it may adversely affect the cleanliness of the steel and result in a deterioration of its properties. Therefore, when any of the REMs is contained, the REM content is preferably 0.005% or less. More preferably, the REM content is 0.004% or less. Since the effects of the present invention can be obtained even with a small REM content, there is no particular lower limit for the content of each. To more effectively obtain the effect of improving workability, the REM content is preferably 0.001% or more.

[0068] Furthermore, when the above-mentioned optional elements are contained in amounts less than the above-mentioned preferable lower limit, the elements are considered to be contained as unavoidable impurities.

[0069] An embodiment of the method for producing a steel sheet according to the present invention will be described in detail below. Note that the temperatures at which a steel slab (steel material), a steel sheet, etc. are heated or cooled refer to the surface temperatures of the steel slab (steel material), a steel sheet, etc., unless otherwise specified.

[0070] 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-described chemical composition is heated to a temperature range of 1100 to 1300°C, the finish rolling temperature (finish rolling exit temperature) is set to 800 to 950°C, the finish rolling reduction is set to 60% or more, and in the cooling process from the finish rolling exit to coiling, the residence time in the temperature range of 750 to 600°C is set to 10 seconds or less, and the coiling temperature is set to 600°C or less, followed by coiling. After coiling, the hot-rolled steel sheet may have a structure in which, in terms of area ratio, ferrite is 20% or less and the total of fresh martensite and bainite is 80% or more. The method also includes a cold rolling step of pickling the hot-rolled steel sheet after the hot rolling step and cold rolling it at a cumulative reduction of 20% or more. The method also includes an annealing process in which the cold-rolled steel sheet after the cold rolling process is heated to an annealing temperature of 750 to 880°C and held for 30 seconds or more, a quenching process in which the cold-rolled steel sheet is cooled to a cooling stop temperature of (Ms-250°C) to (Ms-50°C) after the annealing process, and a tempering process in which the cold-rolled steel sheet is heated to a reheating temperature of 300 to 500°C after the quenching process and held for 20 seconds or more. The method for producing a steel sheet of the present invention may further include a plating step of applying hot-dip galvanizing or hot-dip galvannealing to the surface of the steel sheet before the quenching step or after the tempering step.

[0071] First, the conditions of the hot rolling process will be explained.

[0072] Finishing rolling temperature: 800~950℃ If the finish rolling temperature (finish rolling delivery temperature) is less than 800°C, ferrite transformation occurs during rolling, and the hot-rolled structure of the present invention may not be obtained. Therefore, the finish rolling temperature is 800°C or higher, preferably 850°C or higher, and more preferably 880°C or higher. On the other hand, if the finish rolling temperature exceeds 950°C, the crystal grains become coarse, and non-uniform ferrite grains may be formed after annealing. Therefore, the finish rolling temperature is 950°C or lower, preferably 930°C or lower.

[0073] Cumulative reduction rate of finishing rolling: 60% or more By setting the cumulative reduction rate of finish rolling to 60% or more, the recrystallization rate during hot rolling increases, resulting in a fine hot-rolled structure. Furthermore, by controlling the cooling process from the finish rolling exit side to coiling and the coiling temperature, the generation of ferrite is suppressed, resulting in a fine hot-rolled structure mainly composed of fresh martensite and bainite. This is thought to increase the number of ferrite nucleation sites in the annealing process, resulting in uniform and fine ferrite grains. If the cumulative reduction rate of finish rolling is less than 60%, these effects cannot be obtained. Therefore, the cumulative reduction rate of finish rolling is 60% or more, preferably 70% or more. There is no particular upper limit, but in consideration of the balance with the reduction rate during cold rolling, the cumulative reduction rate of finish rolling is preferably 99% or less, more preferably 96% or less.

[0074] Residence time in the temperature range of 750 to 600°C during the cooling process from the finish rolling exit to coiling: 10 seconds or less If the residence time in the temperature range of 750 to 600°C exceeds 10 seconds during the cooling process from the finish rolling exit to coiling, ferrite transformation may proceed and the hot-rolled structure of the present invention may not be obtained. Therefore, the residence time in the temperature range of 750 to 600°C is 10 seconds or less, and preferably 8 seconds or less. Although there is no particular lower limit, in consideration of production costs, the residence time is preferably 1 second or more, more preferably 3 seconds or more.

[0075] Winding temperature: 600℃ or less If the coiling temperature exceeds 600°C, ferrite transformation may proceed after coiling, making it impossible to obtain the hot-rolled structure of the present invention. Furthermore, carbides in the hot-rolled steel sheet may become coarse, and these coarse carbides may not be completely dissolved during soaking during annealing, making it impossible to obtain the required strength. Therefore, the coiling temperature is 600°C or lower, and preferably 580°C or lower. While there are no particular restrictions on the lower limit of the coiling temperature, it is preferable to set the coiling temperature to 400°C or higher in order to make it less likely that the steel sheet will have a defective shape and to prevent the steel sheet from becoming excessively hard.

[0076] Ferrite area ratio of hot-rolled steel sheet: 20% or less In the steel sheet of the present invention, controlling the structure of the hot-rolled steel sheet (hot-rolled sheet) is important for obtaining ferrite with an average grain size of 25 μm or less and a CV × CE of 0.28 or less in the final structure. In the hot-rolling process, by controlling the reduction rate during finish rolling, the cooling process from the finish-rolling exit side to coiling, and the coiling temperature, ferrite formation is suppressed, and the ferrite area ratio in the hot-rolled steel sheet structure is set to 20% or less. This is thought to result in a fine hot-rolled structure containing 80% or more of fresh martensite and bainite, as described below. In the annealing process, the number of ferrite nucleation sites increases, resulting in uniform and fine ferrite grains. Therefore, the ferrite area ratio of the hot-rolled steel sheet is 20% or less, preferably 15% or less. The ferrite area ratio of the hot-rolled steel sheet may be 0%.

[0077] Total area ratio of fresh martensite and bainite in hot-rolled steel sheet: 80% or more In the present invention, controlling the structure of the hot-rolled steel sheet to be mainly composed of fresh martensite and bainite is important for the same reasons as above in order to obtain ferrite with an average grain size of 25 μm or less and a CV×CE of 0.28 or less in the final structure. Therefore, the total area ratio of fresh martensite and bainite in the hot-rolled steel sheet is 80% or more, preferably 85% or more. The total area ratio of fresh martensite and bainite in the hot-rolled steel sheet may be 100%.

[0078] As long as the area ratio of ferrite in the hot-rolled steel sheet (hot-rolled sheet) is 20% or less and the total area ratio of fresh martensite and bainite in the hot-rolled sheet is 80% or more, the structure, strength, and crash properties of the present invention can be obtained regardless of the type and area ratio of phases other than those mentioned above. Examples of phases other than those mentioned above include pearlite and cementite. If these phases increase excessively and the total area ratio of fresh martensite and bainite in the hot-rolled steel sheet becomes less than 80%, non-uniform ferrite grains may form during annealing, which may serve as starting points for void generation during crash deformation and reduce crash properties. It is preferable that the area ratio of these phases is 15% or less.

[0079] The hot-rolled steel sheet obtained by the hot rolling process is subjected to pre-treatment such as pickling and degreasing by a commonly known method, and then subjected to cold rolling as necessary. The conditions of the cold rolling process when cold rolling is performed will be described below.

[0080] Cumulative reduction rate of cold rolling: 20% or more If the cumulative reduction rate of cold rolling is less than 20%, the recrystallization of ferrite is not promoted, unrecrystallized ferrite remains, and the steel structure of the present invention may not be obtained. Therefore, the cumulative reduction rate of cold rolling is 20% or more, and preferably 30% or more.

[0081] Next, the conditions of the annealing step when annealing the cold-rolled steel sheet obtained by the cold rolling step will be described.

[0082] Annealing temperature: 750-880℃, holding time: 30 seconds or more If the annealing temperature is less than 750°C, the formation of austenite will be insufficient and excessive ferrite will be formed, making it impossible to obtain the steel structure of the present invention. Therefore, the annealing temperature is set to 750°C or higher. If the annealing temperature exceeds 880°C, the formation of austenite will be excessive and the formation of ferrite may be insufficient. Therefore, the annealing temperature is set to 880°C or lower. Furthermore, if the holding time is less than 30 seconds, the formation of austenite will be insufficient and excessive ferrite will be formed, making it impossible to obtain the steel structure of the present invention. Therefore, the holding time is 30 seconds or more, preferably 60 seconds or more. There is no particular upper limit to the holding time, but in order not to impair productivity, it is preferable to set the holding time to 600 seconds or less.

[0083] After the annealing process, the steel is quenched. The conditions for the quenching process will be explained below.

[0084] Cooling stop temperature: (Ms-250℃)~(Ms-50℃) If the cooling stop temperature exceeds (Ms - 50°C), the formation of tempered martensite is insufficient, and the steel structure of the present invention cannot be obtained. Therefore, the cooling stop temperature is (Ms - 50°C) or less, and preferably (Ms - 100°C) or less. On the other hand, if the cooling stop temperature is less than (Ms - 250°C), the formation of tempered martensite may become excessive, and the formation of retained austenite may be insufficient. Therefore, the cooling stop temperature is (Ms - 250°C) or more, and preferably (Ms - 200°C) or more.

[0085] Ms can be calculated using the following formula (3). Ms(℃)=539-423×{[C%]×100 / (100-[α area%])}-30×[Mn%]-12×[Cr%]-18×[Ni%]-8×[Mo%] ···(3) In the above formula, each element symbol represents the content (mass %) of each element, and elements that are not contained are represented as 0. Furthermore, [α area %] is the ferrite area ratio after annealing. The ferrite area ratio after annealing is determined in advance by simulating the heating rate, annealing temperature, and holding time during annealing using a thermal expansion measuring device. [α area %] is treated as the same as the area ratio of ferrite contained in the steel sheet finally obtained after annealing and the quenching and tempering processes of the present invention.

[0086] After the quenching process, tempering is carried out. The conditions for the tempering process will be described below.

[0087] Tempering temperature (reheating temperature): 300~500℃, holding time: 20 seconds or more At temperatures below 300°C, tempering of martensite is insufficient, resulting in a large difference in hardness between ferrite and tempered martensite. This means that the tempered martensite does not deform following the ferrite during primary processing, making voids more likely to occur at the interface with the ferrite, and is thought to result in poor crashworthiness. Furthermore, bainite transformation may be insufficient, making it impossible to obtain the steel structure and fracture resistance properties of the present invention. Therefore, the tempering temperature (reheating temperature) is 300°C or higher, preferably 350°C or higher. On the other hand, if the tempering temperature (reheating temperature) exceeds 500°C, excessive ferrite is formed, making it impossible to obtain the steel structure and fracture resistance properties of the present invention. Furthermore, bainite transformation may be insufficient, making it impossible to obtain the steel structure and fracture resistance properties of the present invention. Therefore, the tempering temperature (reheating temperature) is 500°C or lower, preferably 450°C or lower. Furthermore, if the holding time is less than 20 seconds, the tempering of martensite will be insufficient, and the fracture resistance properties of the present invention will not be obtained. Furthermore, the bainite transformation will be insufficient, and the steel structure and fracture resistance properties of the present invention may not be obtained. Therefore, the holding time is 20 seconds or more, and preferably 30 seconds or more. While there is no particular upper limit to the holding time, from the viewpoints of productivity and suppression of excessive bainite transformation, it is preferable to set the holding time to 500 seconds or less.

[0088] Next, the conditions for the plating process will be described.

[0089] In the method for producing a steel sheet of the present invention, the surface of the steel sheet may be subjected to electrogalvanization, hot-dip galvanization, or galvannealing after the annealing step and before the quenching step, or after the tempering step.

[0090] The plating step after the annealing step and before the quenching step preferably includes a step of holding the steel sheet in a temperature range of 300 to 500° C. for 0 to 300 seconds before plating. If the temperature range is below 300°C, martensitic transformation may occur, and excessive C concentration in untransformed austenite may cause decomposition during plating or plating alloying, resulting in a decrease in retained austenite. On the other hand, if the temperature range is above 500°C, ferrite may be formed, and the steel structure of the present invention may not be obtained. Furthermore, if the holding time exceeds 300 seconds, the bainite transformation may proceed excessively, making it impossible to obtain the steel structure and fracture resistance properties of the present invention. Therefore, in the present invention, the plating step after the annealing step and before the quenching step preferably includes a step of holding the steel sheet in a temperature range of 300 to 500°C for 0 to 300 seconds before plating.

[0091] The electrogalvanizing treatment is preferably carried out by immersing the workpiece in a zinc solution at 50 to 60°C while passing an electric current through it. The hot-dip galvanizing treatment is preferably carried out by immersing the steel sheet obtained as described above in a galvanizing bath at a temperature of 440°C to 500°C. The coating weight is then preferably adjusted by gas wiping or the like. An alloying step may be carried out after the hot-dip galvanizing treatment. When alloying the galvanized steel sheet, it is preferable to hold the steel sheet at a temperature of 450°C to 580°C for 1 second to 180 seconds to alloy the steel sheet.

[0092] Steel sheets after hot-dip galvanizing or galvannealed hot-dip galvanizing can be subjected to temper rolling for the purposes of shape correction, adjustment of surface roughness, etc. However, if the temper rolling tempering rate exceeds 0.5%, bendability may deteriorate due to surface hardening, so the tempering rate is preferably 0.5% or less, and more preferably 0.3% or less. In addition, various painting treatments such as resin or oil coating can also be applied.

[0093] The other conditions for the production method are not particularly limited, but the following conditions are preferred.

[0094] Slabs are preferably produced by continuous casting to prevent macrosegregation, but can also be produced by ingot casting or thin slab casting. To hot-roll a slab, the slab may be cooled to room temperature and then reheated before hot-rolling. Alternatively, the slab may be loaded into a heating furnace without being cooled to room temperature and then hot-rolled. An energy-saving process can also be applied in which the slab is hot-rolled immediately after a short period of heat retention. When heating a slab, it is preferable to heat it to 1100°C or higher to prevent an increase in rolling load and to dissolve carbides. Furthermore, the heating temperature of the slab is preferably 1300°C or lower to prevent an increase in scale loss.

[0095] When hot rolling a slab, the rough bar after rough rolling can be heated to prevent problems during rolling when the slab heating temperature is low. Alternatively, the rough bars can be joined together and finish rolling can be performed continuously, a so-called continuous rolling process. Furthermore, to reduce the rolling load and achieve uniform shape and material quality, it is preferable to perform lubricated rolling with a friction coefficient of 0.10 to 0.25 in all or some passes of the finish rolling.

[0096] After coiling, the steel sheet may be subjected to scale removal by pickling, etc. After pickling, the steel sheet is subjected to cold rolling, annealing, and galvanization under the above-mentioned conditions.

[0097] Next, the member of the present invention and the method for manufacturing the same will be described.

[0098] 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.

[0099] The steel sheet of the present invention has high strength and excellent collision properties. Therefore, a member obtained using the steel sheet of the present invention also has high strength and excellent collision properties, and is less likely to break during deformation due to collision. Therefore, the member of the present invention can be suitably used as an energy absorbing member in an automobile part.

[0100] The forming process can be performed by any common processing method such as press working, etc., without any restrictions. The welding process can be performed by any common welding method such as spot welding or arc welding, without any restrictions. [Example]

[0101] The present invention will be specifically described with reference to examples, but the scope of the present invention is not limited to the following examples.

[0102] [Example 1] Steels having the chemical compositions shown in Table 1 were melted in a vacuum melting furnace and then bloomed to form steel slabs. These steel slabs were heated to 1100 to 1300°C and subjected to hot rolling, cold rolling, annealing, quenching, tempering, and heat treatment under the conditions shown in Table 2 to produce steel sheets. When producing the steel sheets under the conditions shown in Table 2, some of the steel sheets were subjected to a plating treatment before the quenching process or after the tempering process. In the hot-dip galvanizing treatment, the steel sheets were immersed in a plating bath and a coating weight of 10 to 100 g / m was applied. 2 In the galvannealed coating, a hot-dip galvanized layer (GI) was formed on the steel sheet. In the galvannealed coating, a hot-dip galvanized layer was formed on the steel sheet, followed by alloying treatment to form a galvannealed layer (GA). The final thickness of each steel sheet was 1.2 mm.

[0103] [Table 1]

[0104] [Table 2]

[0105] The obtained steel sheets were subjected to skin-pass rolling at a rolling reduction of 0.2%, and then the area fractions of ferrite (F), bainite (B), fresh martensite (FM), tempered martensite (TM), and retained austenite (RA) were determined according to the following method. Furthermore, according to the above-mentioned method, the steel sheets were bent 90° in the rolling (L) direction with the width (C) direction as the axis at a curvature radius / thickness ratio of 4.2, and then bent back flat again. The ratio of ferrite grains with voids at the interface (NF) to all ferrite grains was determined in the L cross section within a region of 0 to 50 μm from the steel sheet surface on the compression-tensile deformation side. void / NF) was also measured.

[0106] NF void The method for measuring / NF is as follows. The steel sheet was bent 90° in the rolling (L) direction with the width (C) direction as the axis at a curvature radius / sheet thickness of 4.2, and then bent back flat again. The cross section of the sheet thickness was polished, and the L cross section within the 0 to 50 μm region from the steel sheet surface on the compression-tension side was observed. The L cross section was photographed in three fields at 2000x magnification using an SEM (scanning electron microscope), and the number of all ferrite grains in the field and the number of ferrite grains with voids at the interface were counted from the obtained image data using Image-Pro manufactured by Media Cybernetics, and the percentage was calculated. The average value of the three fields was taken as NF. void The voids are darker black than the ferrite and can be clearly distinguished from the other structures. The measurement position of the ferrite grains after the bending back process was a region including a corner formed by the bending process and extending in the width (C) direction (see symbol D1 in Figure 1). More specifically, the number of ferrite grains was measured within a region of 0 to 50 μm in the thickness direction in the region that was the lowest in the direction perpendicular to the width direction and the rolling direction (the pressing direction of the pressing part of the punch or the like) due to the bending process.

[0107] The area fraction of each microstructure was measured as follows. A cross-section of the steel plate cut perpendicular to the rolling direction was polished and then etched with 3% by volume of nital. Three fields of view were photographed at 1 / 4 of the plate thickness using a scanning electron microscope (SEM) at 1500x magnification. The area fraction of each microstructure was determined from the obtained image data using Image-Pro (Media Cybernetics). The average of the area fractions of the three fields of view was used to determine the area fraction of each microstructure in the present invention. In the image data, ferrite was distinguished as black, bainite as black containing island-like retained austenite or gray containing aligned carbides, tempered martensite as light gray containing fine, misoriented carbides, and retained austenite as white. Fresh martensite also exhibited a white color, making it difficult to distinguish between fresh martensite and retained austenite in SEM images. Therefore, the area fraction of fresh martensite was determined by subtracting the area fraction of retained austenite, determined by the method described below, from the total area fraction of fresh martensite and retained austenite. Although not shown in Table 3, the remaining structure was determined by subtracting the total area ratio of ferrite (F), tempered martensite (TM), bainite (B), retained austenite (RA), and fresh martensite (FM) from 100%, and this remaining structure was determined to be pearlite and / or cementite.

[0108] The volume fraction of retained austenite was calculated by measuring the X-ray diffraction intensity, and this volume fraction was considered to be the area fraction of retained austenite. The volume fraction of retained austenite was calculated as the ratio of the integrated X-ray diffraction intensities of the (200), (220), and (311) planes of fcc iron to the integrated X-ray diffraction intensities of the (200), (211), and (220) planes of bcc iron at 1 / 4 of the plate thickness.

[0109] The tensile properties and crash properties were determined according to the following test methods, and the results are shown in Table 3.

[0110] <Tensile test> JIS No. 5 tensile test pieces (JIS Z2201) were taken from each of the obtained steel plates in the direction perpendicular to the rolling direction, and the strain rate was increased to 10-3 A tensile test was conducted in accordance with the JIS Z2241 (2011) standard, which specifies a tensile strength (TS) of 780 MPa or more.

[0111] <Bending - Orthogonal bending test> The resulting steel sheet was bent 90° in the rolling (L) direction around the width (C) direction with a curvature radius / thickness ratio of 4.2, and then flattened again (primary bending) to prepare a test specimen. For the 90° bending (primary bending), as shown in Figure 1, a punch B1 was pressed into the steel sheet placed on a die A1 with a V-groove to obtain test specimen T1. Next, as shown in Figure 2, a punch B2 was pressed into test specimen T1 placed on a support roll A2 so that the bending direction was perpendicular to the rolling direction, thereby performing an orthogonal bending (secondary bending). In Figures 1 and 2, D1 indicates the width (C) direction, and D2 indicates the rolling (L) direction.

[0112] Test piece T1, which was obtained by bending the steel plate 90 degrees (primary bending), is shown in Figure 3. Test piece T2, which was obtained by bending test piece T1 orthogonally (secondary bending), is shown in Figure 4. The position indicated by the dashed line on test piece T2 in Figure 4 corresponds to the position indicated by the dashed line on test piece T1 in Figure 3 before orthogonal bending.

[0113] The conditions for orthogonal bending are as follows: [Orthogonal bending condition] Test method: Roll support, punch indentation Roll diameter: φ30mm Punch tip R: 0.4 mm Distance between rolls: (plate thickness x 2) + 1.5 mm Stroke speed: 20mm / min Test piece size: 60mm x 60mm Bending direction: perpendicular to rolling direction

[0114] In the stroke-load curve obtained when the orthogonal bending test was performed, the stroke at the point where the load decreased by 50% from the maximum load was determined. The average value of the stroke at the point where the load decreased by 50% from the maximum load when the bending-orthogonal bending test was performed three times was defined as ΔS 50 ΔS 50 The fracture resistance was evaluated as good when the ΔS was 29 mm or more. 50 When calculating the load, which determines the stroke amount, is important in evaluating fracture resistance characteristics. Fracture during axial crushing deformation occurs when cracks that occur in the primary processed part of the component grow larger and penetrate the plate thickness, resulting in fracture. In bending-orthogonal bending tests, cracks appear in the test piece near the maximum load, and as the cracks grow larger, the cross-sectional area of the bent part decreases and the load decreases. In other words, the percentage decrease in load from the maximum load indicates how much the cracks have grown with deformation. Stroke ΔS at the point where the load has decreased by 50% from the maximum load 50 By setting ΔS to 29 mm or more, fracture can be suppressed even when considering variations in actual crushing deformation. 50 When the fracture resistance was 29 mm or more, the fracture resistance was evaluated as good.

[0115] <Axial crushing test> Considering the influence of plate thickness, all axial crushing tests were conducted using steel plates with a plate thickness of 1.2 mm. The steel plates obtained by the above manufacturing process were cut out and formed (bent) to a depth of 40 mm using a mold with a punch shoulder radius of 5.0 mm and a die shoulder radius of 5.0 mm to produce hat-shaped components 10 shown in FIGS. 5 and 6. The steel plates used as the raw materials for the hat-shaped components were also cut out to a size of 200 mm x 80 mm. The cut-out steel plates 20 were then spot-welded to the hat-shaped components 10 to produce test components 30 as shown in FIGS. 5 and 6. FIG. 5 is a front view of the test component 30 produced by spot-welding the hat-shaped components 10 and the steel plates 20. FIG. 6 is a perspective view of the test component 30. The spot welds 40 were positioned so that the distance between the edge of the steel plate and the weld was 10 mm and the distance between the welds was 45 mm, as shown in FIG. 6. Next, as shown in FIG. 7, a sample for an axial crushing test was prepared by joining the test member 30 to the base plate 50 by TIG welding. Next, an impactor 60 was made to collide with the prepared sample for the axial crushing test at a constant velocity of 10 m / s, and the sample for the axial crushing test was crushed by 100 mm. As shown in FIG. 7, the crushing direction D3 was set parallel to the longitudinal direction of the test member 30. In the graph of stroke-load at the time of crushing, the area in the stroke range of 0 to 100 mm was calculated, and the average value of the area obtained from three tests was used as the absorbed energy (F ave ) F ave The energy absorption was evaluated as good when the force was 38,000 N or more. In addition, when both the breaking resistance and the energy absorption were good, the impact resistance was evaluated as good.

[0116] [Table 3]

[0117] The steel sheets of the invention examples had a TS of 780 MPa or more and were excellent in impact resistance, whereas the steel sheets of the comparative examples had a TS of less than 780 MPa or were poor in impact resistance.

[0118] [Example 2] Steel plate No. 1 (Example of the Invention) in Table 3 of Example 1 was formed by press working to produce an Example of the Invention member. Furthermore, steel plate No. 1 in Table 3 of Example 1 and steel plate No. 30 (Example of the Invention) in Table 3 of Example 1 were joined by spot welding to produce an Example of the Invention member. The Example of the Invention member produced using the steel plate of the invention had excellent collision characteristics and high strength, and it was confirmed that all of the member produced by forming steel plate No. 1 (Example of the Invention) in Table 3 of Example 1 and the member produced by spot welding steel plate No. 1 in Table 3 of Example 1 and steel plate No. 30 (Example of the Invention) in Table 3 of Example 1 could be suitably used for automotive frame parts and the like.

[0119] [Example 3] The galvanized steel sheet No. 1 (Example of the Invention) in Table 3 of Example 1 was formed by press working to produce an Example of a member of the Invention. Furthermore, the galvanized steel sheet No. 1 in Table 3 of Example 1 and the galvanized steel sheet No. 30 (Example of the Invention) in Table 3 of Example 1 were joined by spot welding to produce an Example of a member of the Invention. The Example of the Invention members produced using the steel sheet of the Invention had excellent collision characteristics and high strength, and it was confirmed that all of the member produced by forming the steel sheet No. 1 (Example of the Invention) in Table 3 of Example 1 and the member produced by spot welding the steel sheet No. 1 in Table 3 of Example 1 and the steel sheet No. 30 (Example of the Invention) in Table 3 of Example 1 could be suitably used for automotive frame parts and the like. [Explanation of symbols]

[0120] 10 Hat-shaped member 20 steel plate 30 Test components 40 Spot welds 50 Main plate 60 Impactor A1 Die A2 Support Roll B1 Punch B2 Punch D1 width (C) direction D2 Rolling (L) direction D3 Crushing direction T1 specimen T2 specimen [Industrial Applicability]

[0121] According to the present invention, a steel sheet having a TS of 780 MPa or more and excellent crashworthiness can be obtained. If a member obtained from the steel sheet of the present invention is used as an automobile part, it can contribute to reducing the weight of the automobile and greatly contribute to improving the performance of the automobile body.

Claims

1. In mass %, C: 0.07-0.20%, Si: 0.10-2.00%, Mn: 1.5-4.0%, P: 0.100% or less, S: 0.050% or less, Sol. Al: 0.005 to 0.100%, and N: 0.0100% or less; Carbon equivalent (CE) is 0.46% or more, The balance is composed of Fe and unavoidable impurities; and a steel structure having, in area ratios, ferrite: 10 to 50%, a total of tempered martensite and bainite: 30% or more, retained austenite: 3 to 20%, fresh martensite: 15% or less, and a total of ferrite, tempered martensite, bainite, retained austenite and fresh martensite: 90% or more, Average grain size of ferrite: 25 μm or less, The coefficient of variation (CV) of ferrite grain size × carbon equivalent (CE) is 0.28 or less, When the steel sheet is bent 90° in the rolling (L) direction with the width (C) direction as the axis at a curvature radius / thickness of 4.2 and then bent back flat again, the ratio of the number of ferrite grains having voids at the interface to the total number of ferrite grains (NF) in the L cross section within a region of 0 to 50 μm from the steel sheet surface on the compression-tensile deformation side is ｖｏｉｄ / NF) is 15% or less, A steel plate having a tensile strength of 780 MPa or more.

2. The component composition further includes, in mass %, Cr: 1.000% or less, Mo: 0.500% or less, V: 0.500% or less, Ti: 0.500% or less, Nb: 0.500% or less, B: 0.0050% or less, Ni: 1.000% or less, Cu: 1.000% or less, Sb: 1.000% or less, Sn: 1.000% or less, As: 1.000% or less, Ca: 0.0050% or less, W: 0.500% or less, Ta: 0.100% or less, Mg: 0.050% or less, Zr: 0.050% or less, and 2. The steel sheet according to claim 1, further comprising at least one selected from the group consisting of REM: 0.005% or less.

3. The steel sheet according to claim 1 or 2, which has an electrogalvanized layer, a hot-dip galvanized layer, or a galvannealed layer on a surface thereof.

4. A member obtained by subjecting the steel plate according to any one of claims 1 to 3 to at least one of forming and welding.

5. a hot rolling process in which a steel slab having a carbon equivalent (CE) of 0.46 or more and a component composition according to claim 1 or 2 is heated to a temperature range of 1100 to 1300°C, hot rolled at a finish rolling temperature of 800 to 950°C, with a cumulative reduction in finish rolling of 60% or more, with a residence time in a temperature range of 750 to 600°C of 10 seconds or less in a cooling process from the finish rolling exit side to coiling, and coiled at a coiling temperature of 600°C or less; a cold rolling step of pickling the hot-rolled steel sheet obtained in the hot rolling step and cold-rolling it at a cumulative reduction rate of 20% or more; An annealing step in which the cold-rolled steel sheet obtained in the cold rolling step is heated to an annealing temperature of 750 to 880 ° C. and held for 30 seconds or more; After the annealing step, a quenching step of cooling to a cooling stop temperature: (Ms-250°C) to (Ms-50°C); After the quenching process, a tempering process is performed in which the steel sheet is heated to a reheating temperature of 300 to 500 ° C. and held for 20 seconds or more. Including, In mass%, C: 0.07-0.20%, Si: 0.10-2.00%, Mn: 1.5-4.0%, P: 0.100% or less, S: 0.050% or less, Sol. Al: 0.005 to 0.100%, and N: 0.0100% or less; Carbon equivalent (CE) is 0.46% or more, Further optionally, Cr: 1.000% or less, Mo: 0.500% or less, V: 0.500% or less, Ti: 0.500% or less, Nb: 0.500% or less, B: 0.0050% or less, Ni: 1.000% or less, Cu: 1.000% or less, Sb: 1.000% or less, Sn: 1.000% or less, As: 1.000% or less, Ca: 0.0050% or less, W: 0.500% or less, Ta: 0.100% or less, Mg: 0.050% or less, Zr: 0.050% or less, and REM: 0.005% or less, The balance is composed of Fe and unavoidable impurities; and a steel structure having, in area ratios, ferrite: 10 to 50%, a total of tempered martensite and bainite: 30% or more, retained austenite: 3 to 20%, fresh martensite: 15% or less, and a total of ferrite, tempered martensite, bainite, retained austenite and fresh martensite: 90% or more, Average grain size of ferrite: 25 μm or less, The coefficient of variation (CV) of ferrite grain size × carbon equivalent (CE) is 0.28 or less, When the steel sheet is bent 90° in the rolling (L) direction with the width (C) direction as the axis at a curvature radius / thickness of 4.2 and then bent back flat again, the ratio of the number of ferrite grains having voids at the interface to the total number of ferrite grains (NF) in the L cross section within a region of 0 to 50 μm from the steel sheet surface on the compression-tensile deformation side is ｖｏｉｄ / NF) is 15% or less, A method for manufacturing a steel sheet having a tensile strength of 780 MPa or more.

6. In mass %, C: 0.07-0.20%, Si: 0.10-2.00%, Mn: 1.5-4.0%, P: 0.100% or less, S: 0.050% or less, Sol. Al: 0.005 to 0.100%, and N: 0.0100% or less; Carbon equivalent (CE) is 0.46% or more, Further optionally, Cr: 1.000% or less, Mo: 0.500% or less, V: 0.500% or less, Ti: 0.500% or less, Nb: 0.500% or less, B: 0.0050% or less, Ni: 1.000% or less, Cu: 1.000% or less, Sb: 1.000% or less, Sn: 1.000% or less, As: 1.000% or less, Ca: 0.0050% or less, W: 0.500% or less, Ta: 0.100% or less, Mg: 0.050% or less, Zr: 0.050% or less, and REM: 0.005% or less, The balance is composed of Fe and unavoidable impurities; and a steel structure having, in area ratios, ferrite: 10 to 50%, a total of tempered martensite and bainite: 30% or more, retained austenite: 3 to 20%, fresh martensite: 15% or less, and a total of ferrite, tempered martensite, bainite, retained austenite and fresh martensite: 90% or more, Average grain size of ferrite: 25 μm or less, The coefficient of variation (CV) of ferrite grain size × carbon equivalent (CE) is 0.28 or less, When the steel sheet is bent 90° in the rolling (L) direction with the width (C) direction as the axis at a curvature radius / thickness of 4.2 and then bent back flat again, the ratio of the number of ferrite grains having voids at the interface to the total number of ferrite grains (NF) in the L cross section within a region of 0 to 50 μm from the steel sheet surface on the compression-tensile deformation side is ｖｏｉｄ / NF) is 15% or less, A method for manufacturing a hot-rolled steel sheet used for a cold-rolled steel sheet having a tensile strength of 780 MPa or more, A steel slab having a carbon equivalent (CE) of 0.46 or more and a component composition according to claim 1 or 2 is heated to a temperature range of 1100 to 1300°C, hot rolled at a finish rolling temperature of 800 to 950°C, the cumulative reduction rate of the finish rolling is 60% or more, and in the cooling process from the finish rolling exit side to coiling, the residence time in a temperature range of 750 to 600°C is 10 seconds or less, and the coiling temperature is 600°C or less, A method for manufacturing a hot-rolled steel sheet for use in a cold-rolled steel sheet, comprising a hot rolling step for manufacturing a hot-rolled steel sheet having a structure in which, in terms of area ratios of the hot-rolled steel sheet structure, ferrite is 20% or less and the total of fresh martensite and bainite is 80% or more.

7. A method for producing a cold-rolled steel sheet, comprising a cold-rolling step of pickling the hot-rolled steel sheet obtained by the method for producing a hot-rolled steel sheet for a cold-rolled steel sheet according to claim 6 and cold-rolling the hot-rolled steel sheet at a cumulative reduction of 20% or more.

8. The method for producing a steel sheet according to claim 5, further comprising a plating step of applying electrogalvanizing, hot-dip galvanizing, or alloyed hot-dip galvanizing to a surface of the steel sheet after the annealing step and before the quenching step, or after the tempering step.

9. The method for producing a steel sheet according to claim 8, further comprising the step of holding the steel sheet in a temperature range of 300 to 500 ° C. for 0 to 300 seconds before plating, in a plating step after the annealing step and before the quenching step.

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 5, 8 or 9.

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