Steel sheet, member, method for manufacturing the same, method for manufacturing a hot-rolled steel sheet for cold-rolled steel sheet, and method for manufacturing a cold-rolled steel sheet

Abstract

The present invention provides: a steel sheet which has a tensile strength (TS) of not less than 440 MPa but less than 780 MPa, while exhibiting excellent collision characteristics; a member; a method for producing this steel sheet; and a method for producing this member.　The present invention provides a steel sheet which has a component composition having a carbon equivalent (CE) of not less than 0.18% but less than 0.46% and a specific steel structure, wherein: the average crystal grain size of ferrite is 25 µm or less; the value obtained by formula (coefficient of variation (CV) of ferrite grain size) × (carbon equivalent (CE)) is 0.25 or less; if the steel sheet is bent by 90° in the rolling direction (L) about the axis in the width direction (C) at a (curvature radius) / (sheet thickness) ratio of 4.2 and subsequently bent back to be flat again, the ratio (NFvoid / NF) of ferrite grains having a void at the interface to all ferrite grains is 5% or less in an L cross-section within a region that is 0-50 µm apart from the compressive / tensile deformation-side steel sheet surface; and the tensile strength is not less than 440 MPa but less than 780 MPa.

Description

Technical Field

[0001] The present invention relates to steel plates, members, methods for manufacturing them, a method for manufacturing a hot-rolled steel plate for a cold-rolled steel plate, and a method for manufacturing a cold-rolled steel plate, which are high-strength and have excellent collision characteristics. The steel plates of the present invention can be preferably used mainly for applications as steel plates for automobiles.

Background Art

[0002] From the perspective of global environmental conservation, while maintaining the strength of the automobile body, reducing its weight and improving the fuel efficiency of the automobile have always been important issues in the automobile industry in order to reduce CO 2 emissions. In order to reduce the weight while maintaining the strength of the automobile body, it is effective to reduce the thickness of the steel plate by increasing the strength of the steel plate used as a material for automobile parts. On the other hand, automobile parts made of steel plates are premised on ensuring the safety of people inside the vehicle during a collision. Therefore, high-strength steel plates used as materials for automobile parts are required to have excellent collision characteristics in addition to having a desired strength.

[0003] In recent years, the application of high-strength steel plates in automobile bodies has been expanding. From the perspective of collision characteristics, automobile parts are roughly classified into non-deformable members such as pillars and bumpers and energy-absorbing members such as side members, and the necessary collision characteristics are required to ensure the safety of passengers in the event of a collision while the automobile is running. In non-deformable members, in order to suppress large deformation during a collision, steel plates having a high tensile strength (hereinafter, also simply referred to as TS) are applied. On the other hand, in energy-absorbing members, it is required to stabilize the deformation and stably exhibit the collision energy absorption ability while the deformation during a collision is complicated. At this time, if member fracture occurs during deformation, the deformation cannot be stabilized and the desired collision characteristics cannot be obtained. Therefore, it is possible to contribute to the improvement of collision safety by suppressing member fracture during a collision and stably exhibiting a high absorbed energy. From the above, it is necessary to apply a high-strength steel plate having excellent collision characteristics and a TS of 440 MPa or more and less than 780 MPa to the energy-absorbing member.

[0004] In response to such requirements, for example, Patent Document 1 discloses a technology related to an ultra-high strength steel sheet with a TS of 1200 MPa or more, which is excellent in formability and impact resistance. Further, Patent Document 2 discloses a technology related to a high strength steel sheet with a maximum tensile strength of 780 MPa or more and applicable to an impact absorbing member during a collision.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0006] However, although Patent Document 1 examines the collision characteristics, the impact resistance is examined on the premise that the member does not break during a collision, and the collision characteristics from the viewpoint of member fracture resistance are not examined.

[0007] Further, in Patent Document 2, a crack determination of a dynamic axial crushing test by a drop weight is performed on a hat-shaped member, and the fracture resistance characteristics with a TS exceeding 780 MPa are evaluated. However, in the crack determination after crushing, the process from the occurrence of cracks during crushing to fracture, which is important for the collision characteristics, cannot be evaluated. The reason is that in the process of crushing, even if a slight crack that does not penetrate the plate thickness occurs at an early stage, it may reduce the absorbed energy. Also, when a crack occurs in the later stage of 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 only the crack determination after crushing is insufficient as an evaluation of the fracture resistance characteristics.

[0008] The present invention has been made in view of such circumstances, and an object thereof is to provide a steel sheet, a member, and a method for manufacturing the same, which are suitable for an energy absorption member of an automobile, have a tensile strength (TS) of 440 MPa or more and less than 780 MPa, and have excellent collision characteristics.

Means for Solving the Problems

[0009] As a result of intensive studies to solve the above problems, the present inventors have found the following.

[0010] The steel sheet has a component composition satisfying a carbon equivalent (CE) of 0.18 or more and less than 0.46%, and in terms of area ratio, ferrite: 55 to 90%, the total of tempered martensite and bainite: 5% or more, retained austenite: 2 to 10%, fresh martensite: 20% or less, and the total of ferrite, tempered martensite, bainite, retained austenite, and fresh martensite: 95% or more. The average crystal grain size of ferrite is 25 μm or less, and the coefficient of variation (CV) of the ferrite grain size × the carbon equivalent (CE) is 0.25 or less. When bent 90° in the rolling (L) direction around the width (C) direction with a curvature radius / plate thickness of 4.2 and then bent back flat again, in the L cross-section in the region of 0 to 50 μm from the surface of the steel sheet on the compression-tension deformation side, for all ferrite grains, the ratio (NF void / NF) of ferrite grains having voids at the interface is 5% or less, and the tensile strength is 440 MPa or more and less than 780 MPa. From these, it has been found that a steel sheet having high strength and excellent collision characteristics can be obtained.

[0011] The present invention has been made based on such findings, and the gist thereof is as follows. [1] A component composition satisfying a carbon equivalent (CE) of 0.18 or more and less than 0.46%, and In terms of area ratio, ferrite: 55 - 90%, the total of tempered martensite and bainite: 5% or more, retained austenite: 2 - 10%, fresh martensite: 20% or less, and the total of ferrite, tempered martensite, bainite, retained austenite, and fresh martensite: 95% or more, having a steel structure, The average crystal grain size of ferrite: 25 μm or less, The coefficient of variation (CV) of ferrite grain size × carbon equivalent (CE) is 0.25 or less, Radius of curvature / plate thickness: 4.2. After bending 90° in the rolling (L) direction with the width (C) direction as the axis and then bending back flat again, in the L cross-section within the range of 0 - 50 μm from the surface of the steel plate on the compression - tension deformation side, for all ferrite grains, the percentage (NF void / NF) of ferrite grains having voids at the interface is 5% or less, A steel plate with a tensile strength of 440 MPa or more and less than 780 MPa. [2] The component composition is in mass%, C: 0.02 - 0.12%, Si: 0.10 - 2.00%, Mn: 0.5 - 2.0%, P: 0.100% or less, S: 0.050% or less, Sol.Al: 0.005 - 0.100%, and N: 0.0100% or less, and the balance consists of Fe and inevitable impurities. The steel plate described in [1]. [3] 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 among REM: 0.005% or less. [4] The steel sheet according to any one of [1] to [3], having 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 sheet according to any one of [1] to [4] to at least one of forming and welding. [6] A steel slab having a carbon equivalent (CE) satisfying 0.18% or more and less than 0.46% and having the component composition described in [2] or [3] is heated in a temperature range of 1100 to 1300°C, hot-rolled at a finish rolling temperature of 800 to 950°C, with the cumulative reduction ratio of finish rolling being 60% or more, and in the cooling process from the finish rolling exit side to coiling, the residence time in the temperature range of 750 to 600°C is 10 s or less, and it is coiled at a coiling temperature of 600°C or less, and a cold rolling process in which the hot-rolled steel sheet obtained in the hot rolling process is pickled and cold-rolled with a cumulative reduction ratio of 20% or more; an annealing process in which the cold-rolled steel sheet obtained in the cold rolling process is heated to an annealing temperature of 740 to 850°C and held for 30 seconds or more; a quenching process in which, after the annealing process, the cooling stop temperature is cooled to (Ms - 250°C) to (Ms - 50°C); a tempering process in which, after the quenching process, the reheating temperature is heated to 300 to 500°C and held for 20 seconds or more; A method for manufacturing a steel sheet including the above. [7] A steel slab having a carbon equivalent (CE) satisfying 0.18% or more and less than 0.46% and having the component composition described in [2] or [3] is heated in a temperature range of 1100 to 1300°C, hot-rolled at a finish rolling exit side temperature of 800 to 950°C, with the cumulative reduction ratio of finish rolling being 60% or more, and in the cooling process from the finish rolling exit side to coiling, the residence time in the temperature range of 750 to 600°C is 10 s or less, and it is coiled at a coiling temperature of 600°C or less, A method for manufacturing a hot-rolled steel sheet for a cold-rolled steel sheet, including a hot rolling process for manufacturing a hot-rolled steel sheet having a structure with an area ratio of ferrite: 50% or less, and a total of fresh martensite and bainite: 50% or more. A method for manufacturing a cold-rolled steel sheet, including a cold rolling process of pickling the hot-rolled steel sheet obtained by the manufacturing method according to [8][7] and cold rolling it with a cumulative reduction ratio of 20% or more. [9] A method for manufacturing a steel sheet according to [6], including a plating process of applying electro-galvanizing, hot-dip galvanizing, or alloyed hot-dip galvanizing to the surface of the steel sheet after the annealing process and before the quenching process, or after the tempering process.

[10] The method for manufacturing a steel sheet according to [9], including a process of holding at a temperature range of 300 to 500 °C for 0 to 300 s before plating in the plating process after the annealing process and before the quenching process.

[11] A method for manufacturing a member having a process of performing at least one of forming and welding on a steel sheet manufactured by the method for manufacturing a steel sheet according to [6], [9], or

[10] .

Advantages of the Invention

[0012] According to the present invention, a steel sheet having a tensile strength (TS) of 440 MPa or more and less than 780 MPa and excellent collision characteristics can be obtained. A member obtained by performing forming, welding, etc. on the steel sheet of the present invention can be suitably used as an energy absorption member used in the automotive field.

Brief Description of the Drawings

[0013]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Mode for Carrying Out the Invention

[0014] The details of the present invention will be described below.

[0015] The steel plate of the present invention has a component composition satisfying a carbon equivalent (CE) of 0.18% or more and less than 0.46%, and in terms of area ratio, ferrite: 55 to 90%, the total of tempered martensite and bainite: 5% or more, retained austenite: 2 to 10%, fresh martensite: 20% or less, and the total of ferrite, tempered martensite, bainite, retained austenite, and fresh martensite: 95% or more. The average crystal grain size of ferrite is 25 μm or less, the coefficient of variation (CV) of the ferrite grain size × the carbon equivalent (CE) is 0.25 or less, and when bent 90° in the rolling (L) direction with the width (C) direction as the axis and then bent back flat again after a curvature radius / plate thickness of 4.2, in the L cross-section in the region of 0 to 50 μm from the steel plate surface on the compression-tension deformation side, for all ferrite grains, the ratio (NF void / NF) of the number of ferrite grains having voids at the interface is 5% or less, and the tensile strength is 440 MPa or more and less than 780 MPa.

[0016] Carbon equivalent (CE): 0.18% or more and less than 0.46% The carbon equivalent CE is a value obtained by converting the influence of elements other than C into the C amount as an index in the strength of steel. By setting the carbon equivalent CE to 0.18% or more and less than 0.46%, the area ratio of each metal structure such as ferrite described later can be controlled within the scope of the present invention, and the tensile strength (440 MPa or more and less than 780 MPa) and collision characteristics of the present invention can be obtained. Preferably, the carbon equivalent CE is 0.20% or more. Also, the carbon equivalent CE is preferably 0.43% or less.

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

[0018] Area ratio of ferrite: 55 - 90% When the area ratio of ferrite exceeds 90%, it becomes difficult to achieve both a tensile strength (TS) of 440 MPa or more and impact properties. When the area ratio of ferrite is less than 55%, the area ratio of the hard phase increases, and void generation at the interface of ferrite during deformation may be promoted. Therefore, the area ratio of ferrite is 55 - 90%. The area ratio of ferrite is preferably 60% or more. Also, the area ratio of ferrite is preferably 85% or less.

[0019] Total area ratio of tempered martensite and bainite: 5% or more Tempered martensite is effective in improving impact properties by suppressing member fracture during impact deformation, while also improving the energy absorption and strength during impact. Also, since it contributes to high strength, it is effective for improving impact properties and strength in a well - balanced manner. When the total area ratio of tempered martensite and bainite is less than 5%, such effects cannot be fully obtained. Therefore, the total area ratio is 5% or more, preferably 7% or more. Also, although the upper limit of the total area ratio is not limited, considering the balance with other microstructures and the tensile strength of the present invention (440 MPa or more and less than 780 MPa), the total area ratio is preferably 30% or less.

[0020] Area ratio of retained austenite: 2 - 10% Retained austenite is effective in delaying crack generation during impact and improving impact properties. Although the mechanism is not clear, it is considered as follows. Retained austenite work hardens during impact deformation, increasing the radius of curvature during bending deformation and dispersing the strain in the bent portion. By dispersing the strain, the stress concentration on the void generation part due to primary processing is alleviated, and as a result, the impact properties are improved. Such an effect cannot be obtained when the area ratio of retained austenite is less than 2%. Therefore, the area ratio of retained austenite is 2% or more, preferably 3% or more. On the other hand, when the area ratio of retained austenite exceeds 10%, the fracture resistance during impact may be reduced by fresh martensite generated by processing-induced transformation. Therefore, the area ratio of retained austenite is 10% or less, preferably 8% or less.

[0021] Area ratio of fresh martensite: 20% or less Fresh martensite is effective for increasing strength. However, voids are likely to occur at the grain boundaries with the soft phase, and when the area ratio of fresh martensite exceeds 20%, the generation of voids at the interface with ferrite is promoted, which may reduce the impact properties. Therefore, the area ratio of fresh martensite is 20% or less, preferably 15% or less. The lower limit of the area ratio of fresh martensite may be 0%.

[0022] Total area ratio of ferrite, tempered martensite, bainite, retained austenite and fresh martensite: 95% or more When the total area ratio of ferrite, tempered martensite, bainite, retained austenite, and fresh martensite is less than 95%, the area ratio of phases other than the above increases, making it difficult to achieve both high strength and impact properties. Examples of phases other than the above include pearlite and cementite. When these phases increase, they may serve as the starting points for void generation during impact deformation, reducing the impact properties. Also, when pearlite or cementite increases, the strength may decrease. If the above total area ratio is 95% or more, high strength and impact properties can be obtained regardless of the types and area ratios of the remaining phases. The total area ratio is preferably 97% or more. The total area ratio may be 100%. The total area ratio of pearlite and cementite, which form the remaining structure other than the above, is 5% or less. Preferably, the total area ratio of this remaining structure is 3% or less.

[0023] The area ratio of each structure refers to the ratio of the area of each phase in the observed area. The area ratio of each structure is measured as follows. After polishing the plate thickness cross-section of a steel plate cut perpendicular to the rolling direction, it is corroded with 3 vol% nital, and three fields are photographed at a magnification of 1500 times with a SEM (scanning electron microscope) at the 1 / 4 position of the plate thickness. 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 is taken as the area ratio of each structure in the present invention. In the image data, ferrite is black, bainite is black including island-shaped retained austenite, or gray including carbides with aligned orientations, tempered martensite is light gray including fine carbides with non-aligned orientations, and retained austenite can be distinguished as white. Here, fresh martensite also appears white, and it is difficult to distinguish fresh martensite and retained austenite in the SEM image. Therefore, the area ratio of fresh martensite is determined by subtracting the area ratio of retained austenite obtained 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 regarded as the area fraction of retained austenite. The volume fraction of retained austenite is determined by the ratio of the X-ray diffraction integrated intensities of the (200), (220), and (311) planes of fcc iron to the X-ray diffraction integrated intensities of the (200), (211), and (220) planes of bcc iron on the 1 / 4 plate thickness surface.

[0025] Average crystal grain size of ferrite: 25 μm or less In the steel plate of the present invention, high impact properties can be obtained by setting the average crystal grain size of ferrite to 25 μm or less. Although the mechanism is not clear, it is considered as follows. The fracture during impact, which causes deterioration of impact properties, starts from the generation and propagation of cracks. Cracks are likely to occur due to a decrease in work hardening ability and the generation and connection of voids in regions with high hardness differences. In addition, in the impact of an actual member, it deforms so as to be bent back in a direction perpendicular to the primary processing at the portion that has undergone primary processing during forming. At this time, when voids are generated in the region with high hardness difference in the primary processing part, stress concentrates around the voids, promoting the generation and propagation of cracks, and as a result, leading to fracture. The cause of void generation in the region with high hardness difference is that the amount of deformation of the soft phase becomes large relative to the hard phase. Therefore, by refining ferrite, the amount of deformation is reduced, the generation and propagation of voids in the primary processing part and the accompanying member fracture are suppressed, and high fracture resistance properties can be obtained. Therefore, the average crystal grain size of ferrite is 25 μm or less, preferably 20 μm or less. The lower limit of the average crystal grain size of ferrite is not particularly defined, but 3 μm or more is preferable.

[0026] The average crystal grain size of ferrite is measured by photographing 10 or more fields in a region of 40 μm × 50 μm at a magnification of 2000 times with a SEM (scanning electron microscope) at the 1 / 4 plate thickness position, and calculating and averaging the equivalent circle diameter from the area ratio of each ferrite grain using the above-mentioned Image-Pro from the obtained image data. In addition, the standard deviation of the ferrite grain size described later can be calculated from each ferrite grain size obtained using the above-mentioned Image-Pro.

[0027] Coefficient of variation (CV) of ferrite grain size × carbon equivalent (CE): 0.25 or less In the steel sheet of the present invention, high impact properties can be obtained by setting CV×CE to 0.25 or less. Although this mechanism is not clear, it is considered as follows. The generation and growth of voids in the primary working part, which becomes the starting point of fracture during impact, are promoted by local stress concentration. In order to suppress this, it is considered effective to reduce the variation in ferrite grain size in the steel structure and to soften the hard phase. Therefore, the former index (reducing the variation in ferrite grain size in the steel structure) is defined as CV, the latter index (softening of the hard phase) is defined as CE, and high fracture resistance can be obtained by setting CV×CE to 0.25 or less. Preferably, it is 0.22 or less.

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

[0029]

Equation

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

[0031] By controlling the reduction ratio of the finish rolling during hot rolling, the cooling process from the exit side of the finish rolling to coiling, and the coiling temperature, which will be described later, and making the hot-rolled structure mainly composed of fresh martensite and bainite, the desired average ferrite crystal grain size and CV×CE can be obtained. Regarding the hot-rolled structure, when the structure is mainly composed of fine fresh martensite and bainite, the number of nucleation sites when ferrite is generated 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] Radius of curvature / thickness of plate: When bent 90° in the rolling (L) direction about the width (C) direction with a radius of curvature / thickness of plate of 4.2 and then bent back flat again, in the L cross-section in the range of 0 to 50 μm from the surface of the steel sheet on the compression-tension deformation side, for all ferrite grains, the ratio (NF void / NF) of the number of ferrite grains having voids at the interface is 5% or less In the steel sheet of the present invention, high impact characteristics can be obtained by setting NF void / NF to 5% or less (NF is the number of all ferrite grains in the L cross-section in the range of 0 to 50 μm from the surface of the steel sheet on the compression-tension deformation side when bent 90° in the rolling (L) direction about the width (C) direction with a radius of curvature / thickness of plate of 4.2 and then bent back flat again. NF void is the number of ferrite grains having voids at the interface in the L cross-section in the range of 0 to 50 μm from the surface of the steel sheet on the compression-tension deformation side when bent 90° in the rolling (L) direction about the width (C) direction with a radius of curvature / thickness of plate of 4.2 and then bent back flat again.). This mechanism is not clear, but it is considered as follows. The fracture during impact that causes deterioration of impact characteristics starts from the generation and propagation of cracks. Cracks are likely to occur due to a decrease in work hardening ability and the generation and connection of voids in regions with high hardness differences. Also, in the impact of the actual member, the portion deformed during forming (primary processing) undergoes secondary deformation during impact, and the deformation history of the fracture initiation part is considered to be a portion that has undergone tensile deformation after undergoing compressive deformation by primary processing and secondary deformation. In the compression-tension deformation part, when voids are generated in the region with high hardness difference, stress concentrates around the voids, promoting the generation and propagation of cracks, and as a result, leading to fracture. Therefore, by reducing the region with high hardness difference by tempering martensite and bainite, and further suppressing macro stress concentration in the primary processing part during deformation by utilizing retained austenite as needed, and controlling the grain size of ferrite, micro stress concentration in coarse ferrite grains is suppressed, thereby suppressing void generation, propagation, and accompanying member fracture in the primary processing part, and obtaining high fracture resistance characteristics. Therefore, in order to obtain these effects, NF void / NF is set to 5% or less. Preferably, it is 3% or less. NFvoid / NF shall be 1% or more as the industrially obtainable lower limit.

[0033] In addition, the processing method is not limited as long as the primary bending processing conditions (curvature radius / thickness: 4.2, bending 90° in the rolling (L) direction with the width (C) direction as the axis) are satisfied. Examples of the primary bending processing method include bending by the V-block method and bending by draw forming. In the case of springback processing, press processing using a flat jig can be mentioned.

[0034] NF void The measurement method of / NF is as follows. The steel plate 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 after being bent back flat again, the plate thickness cross-section is polished, and the L cross-section within the range of 0 to 50 μm from the steel plate surface on the compression-tension side is observed. The L cross-section is photographed at a magnification of 2000 times with a SEM (scanning electron microscope) in three fields of view, and the number of all ferrite grains in the field of view and the number of ferrite grains having voids at the interfaces are counted from the obtained image data using Image-Pro manufactured by Media Cybernetics, and the ratio is obtained. The average value of the three fields of view is taken as NF void / NF. Note that the voids are darker black than ferrite and can be clearly distinguished from each structure.

[0035] In the present invention, performing a 90° bending process in the rolling (L) direction with the width (C) direction as the axis means that when the steel plate is viewed in the width (C) direction (refer to reference numeral D1 in FIG. 1) (in the view of the steel plate in the width direction (vertical cross-section view in the width direction)), bending by pressing is applied from one side of the steel plate surface in the directions perpendicular to the width direction and the rolling direction (refer to reference numerals D1 and D2 in FIG. 1) so that the distance between both ends becomes shorter, and pressing is performed until the angle formed by the flat portions not subjected to the bending process at both ends becomes 90°. In addition, the steel plate surface on the compression-tension side refers to the steel plate surface on the one side where the above pressing is performed (the steel plate surface in contact with the pressing portion such as a punch for applying the pressing). In addition, for the L cross-section after springback processing, it refers to a cross-section formed by cutting parallel to the direction of deformation due to bending and perpendicular to the steel plate surface, and is a cross-section perpendicular to the width direction.

[0036] Regarding the measurement position of ferrite grains after springback processing, it is a region including corners formed by bending and extending in the width (C) direction (refer to symbol D1 in Fig. 1). More specifically, in the region that becomes the lowermost part in the direction perpendicular to the width direction and the rolling direction (the pressing direction of the pressing part such as a punch) due to bending, the number of ferrite grains is measured within the range of 0 to 50 μm in the plate thickness direction.

[0037] The steel plate of the present invention may have an electro-galvanized layer, a hot-dip galvanized layer, or an alloyed hot-dip galvanized layer on its surface.

[0038] The tensile strength (TS) of the steel plate of the present invention is 440 MPa or more and less than 780 MPa. High strength as referred to in the present invention means that the tensile strength (TS) is 440 MPa or more. The upper limit of the tensile strength (TS) is less than 780 MPa from the viewpoint of harmony with collision characteristics and formability. The measurement method of the tensile strength (TS) is to take a JIS No. 5 tensile test piece (JIS Z2201) in a direction perpendicular to the rolling direction from the steel plate and conduct a tensile test in accordance with the provisions of JIS Z2241 (2011) with a strain rate of 10 -3 / s to obtain the tensile strength (TS).

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

[0040] The steel plate of the present invention is excellent in collision characteristics. Excellent collision characteristics as referred to in the present invention mean that the fracture resistance characteristics are good and the energy absorption is good. Good fracture resistance characteristics as referred to in the present invention mean the average value ΔS of the stroke at the point where the load decreases by 50% from the maximum load value when the following bending-orthogonal bending test is carried out 50It means that it is 35 mm or more. When it is said that the collision characteristics in the present invention are good, the axial crushing test described in the examples is carried out, and the average value F of the area in the range of stroke 0 to 100 mm in the graph of stroke - load at the time of crushing ave means that it is 32000 N or more.

[0041] The above bending - orthogonal bending test is carried out as follows. First, for the steel plate, after bending at 90° in the rolling (L) direction with the width (C) direction as the axis with a curvature radius / plate thickness of 4.2 and then performing a re - flattening bending process (primary bending process) to prepare a test piece. In the 90° bending process (primary bending process), as shown in Fig. 1, for the steel plate placed on the die A1 having a V - groove, the punch B1 is pushed in to obtain the test piece T1. Next, as shown in Fig. 2, for the test piece T1 placed on the support roll A2, the punch B2 is pushed in so that the bending direction is perpendicular to the rolling direction to perform orthogonal bending (secondary bending process). In Figs. 1 and 2, D1 indicates the width (C) direction and D2 indicates the rolling (L) direction.

[0042] The test piece T1 obtained by performing 90° bending (primary bending process) on the steel plate is shown in Fig. 3. Also, the test piece T2 obtained by performing orthogonal bending (secondary bending process) on the test piece T1 is shown in Fig. 4. The position indicated by the broken line in the test piece T2 in Fig. 4 corresponds to the position indicated by the broken line in the test piece T1 in Fig. 3 before orthogonal bending.

[0043] The conditions for orthogonal bending are as follows. [Orthogonal Bending Conditions] Test method: Roll support, punch pushing Roll diameter: φ30 mm Punch tip R: 0.4 mm Roll distance: (plate thickness × 2)+1.5 mm Stroke speed: 20 mm / min Test piece size: 60 mm × 60 mm Bending direction: Perpendicular to the rolling direction

[0044] In the stroke-load curve obtained when the above orthogonal bending is performed, determine the stroke at the point where the load has decreased by 50% from the maximum load. Let the average value of the strokes at the points where the load has decreased by 50% from the maximum load when the above bending-orthogonal bending test is performed three times be ΔS 50 be defined as such.

[0045] Also, the above axial compression test is performed as follows. First, in the axial compression test, considering the influence of the plate thickness, all tests are performed using steel plates with a thickness of 1.2 mm. Cut out the steel plate and use a die with a punch shoulder radius of 5.0 mm and a die shoulder radius of 5.0 mm to perform forming (bending) so that the depth becomes 40 mm, and produce the hat-shaped member 10 shown in FIGS. 5 and 6. Also, cut out the steel plate used as the material of the hat-shaped member separately into a size of 200 mm × 80 mm. Next, spot-weld the cut steel plate 20 and the hat-shaped member 10 to produce a test member 30 as shown in FIGS. 5 and 6. FIG. 5 is a front view of the test member 30 produced by spot-welding the hat-shaped member 10 and the steel plate 20. FIG. 6 is a perspective view of the test member 30. The position of the spot-welded portion 40 is set so that the distance between the end of the steel plate and the welded portion is 10 mm and the distance between the welded portions is 45 mm, as shown in FIG. 6. Next, as shown in FIG. 7, join the test member 30 to the base plate 50 by TIG welding to produce a sample for the axial compression test. Next, impact the produced sample for the axial compression test with an impactor 60 at a constant collision speed of 10 m / s to crush the sample for the axial compression test by 100 mm. As shown in FIG. 7, the crushing direction D3 is set to be parallel to the longitudinal direction of the test member 30. Obtain the area in the range of stroke 0 to 100 mm in the stroke-load graph at the time of crushing, and let the average value of the areas when three tests are performed be the absorbed energy (F ave ) be defined as such.

[0046] Next, the preferable range of the component composition of the steel plate will be described. Note that “%” representing the content of the component element means “mass %” unless otherwise specified.

[0047] C: 0.02 to 0.12% C facilitates the formation of phases other than ferrite and forms alloy compounds with elements such as Nb and Ti, and is an element necessary for improving strength. When the C content is less than 0.02%, even if the manufacturing conditions are optimized, the desired strength may not be ensured. Therefore, the C content is preferably 0.02% or more, more preferably 0.03% or more. On the other hand, when the C content exceeds 0.12%, the strength of martensite increases excessively, and the collision characteristics of the present invention may not be obtained even if the manufacturing conditions are optimized. Therefore, the C content is preferably 0.12% or less, more preferably 0.10% or less.

[0048] Si: 0.10 - 2.00% Si is an element that promotes the formation of ferrite and is also a solid-solution strengthening element. Therefore, it contributes to improving the balance between strength and ductility. To obtain this effect, the Si content is preferably 0.10% or more, more preferably 0.20% or more. On the other hand, when the Si content exceeds 2.00%, it may cause problems such as poor adhesion and surface quality deterioration during zinc plating. Therefore, the Si content is preferably 2.00% or less, more preferably 1.50% or less.

[0049] Mn: 0.5 - 2.0% Mn is an element that promotes the formation of martensite and is also a solid-solution strengthening element. It also contributes to stabilizing retained austenite. To obtain these effects, the Mn content is preferably 0.5% or more. The Mn content is more preferably 1.0% or more. On the other hand, when the Mn content exceeds 2.0%, the fraction of retained austenite may increase, and the collision characteristics may deteriorate. Therefore, the Mn content is preferably 2.0% or less, more preferably 1.8% or less.

[0050] P: 0.100% or less P is an element effective for strengthening steel. However, when the P content exceeds 0.100%, the alloying rate may be significantly delayed. Also, when P is contained in excess of 0.100%, embrittlement is caused by grain boundary segregation, and even if the steel structure of the present invention is satisfied, the fracture resistance characteristics during collision may deteriorate. 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 currently industrially feasible lower limit is about 0.002%, and it is preferably 0.002% or more.

[0051] S: 0.050% or less S forms inclusions such as MnS, which may cause cracks along the metal flow in the welded part, and even if the steel structure of the present invention is satisfied, the collision characteristics may deteriorate. Therefore, the amount of S should be as low as possible, but from the perspective of manufacturing cost, 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 currently industrially feasible lower limit is about 0.0002%, and it is preferably 0.0002% or more.

[0052] Sol.Al: 0.005 - 0.100% Al acts as a deoxidizer and is also 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, the strength may decrease. Therefore, the Sol.Al content is preferably 0.005% or more. On the other hand, when the Sol.Al content exceeds 0.100%, the slab quality 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 Since N forms coarse inclusions such as TiN, (Nb, Ti)(C, N), and AlN in steel, which deteriorate the impact properties, it is necessary to suppress its content. When the N content exceeds 0.0100%, the impact properties tend to deteriorate. Therefore, the N content is preferably 0.0100% or less. More preferably, the N content is 0.007% or less, and even more preferably 0.005% or less. The lower limit of the N content is not particularly limited, but the currently industrially feasible lower limit is about 0.0003%, and it is preferably 0.0003% or more.

[0054] The steel sheet of the present invention contains the above components and has a component composition including the balance of Fe (iron) and unavoidable impurities. In particular, the steel sheet according to an embodiment of the present invention preferably contains the above components and has a component composition in which the balance consists of Fe and unavoidable impurities.

[0055] The steel sheet of the present invention can appropriately contain the following components (optional elements) according to desired properties.

[0056] At least one selected from among 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 increase hardenability and are effective elements for strengthening steel. However, if the content exceeds 1.000% for Cr, 0.500% for Mo, and 0.500% for V, the above effects become saturated, and furthermore, the raw material cost increases. In addition, the second-phase fraction may become excessive, deteriorating the fracture resistance during impact. 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 a small content of Cr, Mo, and V, the lower limit of each content is not particularly limited. To more effectively obtain the hardenability effect, the content of Cr, Mo, and V is preferably 0.005% or more respectively.

[0057] Ti and Nb are effective elements for precipitation strengthening of steel. However, if the Ti content or the Nb content exceeds 0.500% respectively, the fracture resistance during impact may deteriorate. Therefore, when either Ti or Nb is contained, the Ti content and the Nb content are preferably 0.500% or less respectively. More preferably, the Ti content and the Nb content are 0.400% or less respectively. Since the effects of the present invention can be obtained even with a small content of Ti and Nb, the lower limit of each content is not particularly limited. To more effectively obtain the precipitation strengthening effect of steel, the Ti content and the Nb content are preferably 0.005% or more respectively.

[0058] Since B contributes to improving hardenability by suppressing the formation and growth of ferrite from austenite grain boundaries, it can be added as necessary. However, if the B content exceeds 0.0050%, the fracture resistance characteristics during collision may deteriorate. Therefore, when containing B, 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, the lower limit of the B content is not particularly limited. In order 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 for strengthening steel. However, if Ni and Cu each exceed 1.000%, the fracture resistance characteristics during collision may deteriorate. Therefore, when containing either Ni or Cu, the content of Ni and Cu is preferably 1.000% or less, respectively. More preferably, the Ni content and the Cu content are each 0.800% or less. Since the effects of the present invention can be obtained even with a small content of Ni and Cu, the lower limit of each content is not particularly limited. In order to more effectively obtain the effect of strengthening steel, the Ni content and the Cu content are preferably 0.005% or more, respectively.

[0060] Sb and Sn can be added as necessary from the viewpoint of suppressing nitridation, oxidation of the steel sheet surface, and decarburization of the region near the steel sheet surface. By suppressing such nitridation and oxidation, it is possible to prevent a decrease in the amount of martensite formed on the steel sheet surface, and there is an effect of improving the collision characteristics. However, if Sb and Sn each exceed 1.000%, the collision characteristics may decrease due to grain boundary embrittlement. Therefore, when containing either Sb or Sn, the Sb content and the Sn content are preferably 1.000% or less, respectively. More preferably, the Sb content and the Sn content are each 0.800% or less. Since the effects of the present invention can be obtained even with a small content of Sb and Sn, the lower limit of each content is not particularly limited. In order to more effectively obtain the effect of improving the collision characteristics, the Sb content and the Sn content are preferably 0.005% or more, respectively.

[0061] As is an element that segregates at grain boundaries and is an element contained as an impurity in raw material scraps. From the viewpoint of suppressing grain boundary embrittlement, it is preferably 1.000% or less. More preferably, the As content is 0.800% or less. The lower the As content, the more preferable it is, and the lower limit of the content is not particularly limited, but from the viewpoint of refining cost, it is preferably 0.005% or more.

[0062] Ca is an element effective for improving workability by controlling the form of sulfides. However, if the Ca content exceeds 0.0050%, it may have an adverse effect on the cleanliness of the steel and the properties may deteriorate. 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, the lower limit of the content is not particularly limited. 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%, the workability deteriorates. Therefore, when W is contained, the W content is set to 0.500% or less. When W is contained, the W content is preferably 0.005% or more, more preferably 0.050% or more. When W is contained, the W content 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%, the workability deteriorates. Therefore, when Ta is contained, the Ta content is set to 0.100% or less. When Ta is contained, the Ta content is preferably 0.001% or more, more preferably 0.010% or more. When Ta is contained, the Ta content is preferably 0.08% or less, more preferably 0.060% or less.

[0065] Mg is an element effective for improving workability by controlling the morphology of inclusions. On the other hand, if the Mg content exceeds 0.050%, it may have an adverse effect on the cleanliness of the steel. Therefore, when Mg is contained, the Mg content should be 0.050% or less. When Mg is contained, the Mg content is preferably 0.0005% or more, more preferably 0.001% or more. When Mg is contained, the Mg content is preferably 0.040% or less, more preferably 0.030% or less.

[0066] Zr is an element effective for improving workability by controlling the morphology of inclusions. On the other hand, if the Zr content exceeds 0.050%, it may have an adverse effect on the cleanliness of the steel. Therefore, when Zr is contained, the Zr content should be 0.050% or less. When Zr is contained, the Zr content is preferably 0.0005% or more, more preferably 0.001% or more. When Zr is contained, the Zr content is preferably 0.040% or less, more preferably 0.030% or less.

[0067] REM is an element effective for improving workability by controlling the morphology of sulfides. However, if the content of each of REM exceeds 0.005%, it may have an adverse effect on the cleanliness of the steel and there is a risk of deterioration of properties. Therefore, when any of REM is contained, it is preferable that the content of each of REM is 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 content of REM, the lower limit of the content of each is not particularly limited. In order to more effectively obtain the effect of improving workability, it is preferable that the content of each of REM is 0.001% or more. In addition, REM as used in the present invention refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanoids from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. The REM concentration in the present invention is the total content of one or more elements selected from the above-mentioned REM. The REM is not particularly limited, but is preferably scandium (Sc), yttrium (Y), lanthanum (La), cerium (Ce), neodymium (Nd), dysprosium (Dy), holmium (Ho), or erbium (Er).

[0068] In addition, when the above optional element is contained in an amount less than the aforementioned preferable lower limit value, the element shall be regarded as being contained as an inevitable impurity.

[0069] Hereinafter, an embodiment of the method for manufacturing the steel sheet of the present invention will be described in detail. Note that, unless otherwise specified, the temperature when heating or cooling the steel slab (steel material), steel sheet, etc. means the surface temperature of the steel slab (steel material), steel sheet, etc.

[0070] The method for manufacturing the steel sheet of the present invention includes, for example, heating a steel slab having the above composition in a temperature range of 1100 to 1300 °C, setting the finish rolling temperature (finish rolling outlet side temperature) to 800 to 950 °C, setting the reduction ratio of finish rolling to 60% or more, and in the cooling process from the finish rolling outlet side to coiling, setting the residence time in the temperature range of 750 to 600 °C to 10 s or less, and coiling at a coiling temperature of 600 °C or less, and has a hot rolling process. After coiling, the hot rolled steel sheet may have a structure in which, by area ratio, ferrite: 50% or less, and the total of fresh martensite and bainite: 50% or more. In addition, it has a cold rolling process in which the hot rolled steel sheet after the above hot rolling process is pickled and cold rolled with a cumulative reduction ratio of 20% or more. In addition, the cold rolled steel sheet after the above cold rolling process is heated to an annealing temperature of 740 to 850 °C and held for 30 seconds or more, a quenching process in which, after the annealing process, the cooling stop temperature is cooled to (Ms - 250 °C) to (Ms - 50 °C), and a tempering process in which, after the quenching process, the reheating temperature is heated to 300 to 500 °C and held for 20 seconds or more. In addition, the method for manufacturing the steel sheet of the present invention may have a plating process in which hot dip galvanizing or alloyed hot dip galvanizing is applied to the surface of the steel sheet before the quenching process or after the tempering process.

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

[0072] Finish rolling temperature: 800 - 950 °C When the finish rolling temperature (the temperature at the exit side of finish rolling) is less than 800 °C, ferrite transformation may occur during rolling, and the hot rolling 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, when the finish rolling temperature exceeds 950 °C, the crystal grains become coarser, and non-uniform ferrite grains may be generated after annealing. Therefore, the finish rolling temperature is 950 °C or lower, preferably 930 °C or lower.

[0073] Cumulative reduction ratio of finish rolling: 60% or more By setting the cumulative reduction ratio of finish rolling to 60% or more, the recrystallization rate during hot rolling increases, resulting in a fine hot rolling structure. Furthermore, by controlling the cooling process from the exit side of finish rolling to coiling and the coiling temperature, the formation of ferrite is suppressed, and a fine hot rolling structure mainly composed of fresh martensite and bainite is obtained. In the annealing process, it is considered that the nucleation sites of ferrite increase, and uniform and fine ferrite grains can be obtained. When the cumulative reduction ratio of finish rolling is less than 60%, these effects cannot be obtained. Therefore, the cumulative reduction ratio of finish rolling is 60% or more, preferably 70% or more. Although the upper limit is not particularly limited, considering the balance with the reduction ratio during cold rolling, the cumulative reduction ratio of finish rolling is preferably 99% or less, and more preferably 96% or less.

[0074] Residence time in the temperature range of 750 - 600 °C during the cooling process from the exit side of finish rolling to coiling: 10 s or less During the cooling process from the exit side of finish rolling to coiling, when the residence time in the temperature range of 750 - 600 °C exceeds 10 s, ferrite transformation may progress, and the hot rolling structure of the present invention may not be obtained. Therefore, the residence time in the temperature range of 750 - 600 °C is 10 s or less, preferably 8 s or less. The lower limit is not particularly limited, but considering the manufacturing cost, the residence time is preferably 1 s or more, more preferably 3 s or more.

[0075] Coiling temperature: 600°C or lower When the coiling temperature exceeds 600°C, ferrite transformation may progress after coiling, and the hot-rolled structure of the present invention may not be obtained. In addition, carbides in the hot-rolled steel sheet coarsen, and such coarsened carbides may not completely dissolve during soaking in annealing, so the required strength may not be obtained. Therefore, the coiling temperature is 600°C or lower, preferably 580°C or lower. The lower limit of the coiling temperature is not particularly limited, but from the viewpoint of making it difficult to generate shape defects in the steel sheet and preventing the steel sheet from becoming overly hard, the coiling temperature is preferably 400°C or higher.

[0076] Area ratio of ferrite in the hot-rolled steel sheet: 50% or less In the steel sheet of the present invention, controlling the structure of the hot-rolled steel sheet (hot-rolled plate) is important for obtaining ferrite with an average crystal grain size of 25 μm or less and CV×CE of 0.25 or less in the final structure. In the hot-rolling process, by controlling the rolling reduction during finish rolling, the cooling process from the exit side of finish rolling to coiling, and the coiling temperature, the formation of ferrite is suppressed, and for the hot-rolled steel sheet structure, by setting the area ratio of ferrite to 50% or less, a fine hot-rolled structure containing 50% or more of fresh martensite and bainite described below can be obtained. In the annealing process, it is considered that the nucleation sites of ferrite increase and uniform and fine ferrite grains can be obtained. Therefore, the area ratio of ferrite in the hot-rolled steel sheet is 50% or less, preferably 40% or less.

[0077] Total area ratio of fresh martensite and bainite in the hot-rolled steel sheet: 50% or more In the present invention, controlling the structure of the hot-rolled steel sheet to a structure mainly composed of fresh martensite and bainite is important for obtaining ferrite with an average crystal grain size of 25 μm or less and a CV×CE of 0.25 or less in the final structure for the same reasons as described above. Therefore, the total area ratio of fresh martensite and bainite in the hot-rolled steel sheet is 50% or more, preferably 60% or more.

[0078] Note that if the area ratio of ferrite in the hot-rolled steel sheet (hot-rolled plate) is 50% or less and the total area ratio of fresh martensite and bainite in the hot-rolled plate is 50% or more, the structure, strength, and collision characteristics of the present invention can be obtained regardless of the types and area ratios of phases other than the above. Examples of phases other than the 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 is less than 50%, non-uniform ferrite grains are formed during annealing, which may become the starting point of void generation during collision deformation and reduce the collision characteristics. These phases preferably have an area ratio of 15% or less.

[0079] After the hot-rolled steel sheet obtained by the hot-rolling process is subjected to preliminary treatments such as pickling and degreasing by a generally known method, cold rolling is performed as necessary. The conditions of the cold-rolling process when performing cold rolling will be described.

[0080] Cumulative reduction ratio of cold rolling: 20% or more If the cumulative reduction ratio of cold rolling is less than 20%, 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 ratio of cold rolling is 20% or more, preferably 30% or more.

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

[0082] Annealing temperature: 740 to 850 °C, holding time: 30 seconds or more When the annealing temperature is less than 740°C, excessive ferrite is generated and the steel structure of the present invention cannot be obtained. When the annealing temperature exceeds 850°C, austenite becomes excessive and ferrite may be insufficient. Therefore, the annealing temperature is 740 to 850°C. The annealing temperature is preferably 750°C or higher. Also, the annealing temperature is preferably 840°C or lower, more preferably 820°C or lower. Also, when the holding time is less than 30 seconds, the generation of austenite is insufficient, excessive ferrite is generated, and the steel structure of the present invention cannot be obtained. Therefore, the holding time is 30 seconds or longer, preferably 60 seconds or longer. The upper limit of the holding time is not particularly limited, 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, quenching is performed. The conditions of the quenching process will be described.

[0084] Cooling stop temperature: (Ms - 250°C) to (Ms - 50°C) When the cooling stop temperature exceeds (Ms - 50°C), the generation 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 lower, preferably (Ms - 100°C) or lower. On the other hand, when it is less than (Ms - 250°C), tempered martensite becomes excessive and the generation of retained austenite may be insufficient. Therefore, the cooling stop temperature is (Ms - 250°C) or higher, preferably (Ms - 200°C) or higher.

[0085] Ms can be obtained by the following formula (3). Ms (°C) = 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 not contained are set to 0. In addition, [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 finally obtained steel sheet after annealing and through the quenching process and tempering process of the present invention.

[0086] After the quenching process, tempering is performed. The conditions of the tempering process will be described.

[0087] Tempering temperature (reheating temperature): 300 - 500 °C, holding time: 20 seconds or more When it is less than 300 °C, the tempering of martensite is insufficient, and the hardness difference between ferrite and tempered martensite becomes large. As a result, during primary processing, the tempered martensite does not deform following the ferrite, and voids are likely to occur at the interface with ferrite, which is considered to reduce the impact properties. In addition, bainite transformation may be insufficient, and the steel structure and fracture resistance properties of the present invention may not be obtained. Therefore, the tempering temperature (reheating temperature) is 300 °C or more, preferably 350 °C or more. On the other hand, when the tempering temperature (reheating temperature) exceeds 500 °C, excessive ferrite is generated, and the steel structure of the present invention cannot be obtained. In addition, bainite transformation may be insufficient, and the steel structure and fracture resistance properties of the present invention may not be obtained. Therefore, the tempering temperature (reheating temperature) is 500 °C or less, preferably 450 °C or less. Also, when the holding time is less than 20 seconds, the tempering of martensite is insufficient, and the fracture resistance properties of the present invention cannot be obtained. In addition, bainite transformation may 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, preferably 30 seconds or more. The upper limit of the holding time is not particularly limited, but 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 of the plating process will be described.

[0089] In the method for manufacturing a steel sheet of the present invention, after the annealing step and before the quenching step, or after the tempering step, electro-galvanizing, hot-dip galvanizing, or alloyed hot-dip galvanizing may be applied to the surface of the steel sheet.

[0090] In the plating step after the annealing step and before the quenching step, it is preferable to include a step of holding at a temperature range of 300 to 500°C for 0 to 300 s before plating. When the temperature range is less than 300°C, martensitic transformation may occur, and C may be excessively concentrated in the untransformed austenite, decomposing during plating or plating alloying, and the retained austenite may decrease. On the other hand, when the temperature range exceeds 500°C, ferrite may be formed, and the steel structure of the present invention may not be obtained. Also, when the holding time exceeds 300 s, excessive bainitic transformation may progress, and the steel structure and fracture resistance characteristics of the present invention may not be obtained. Therefore, in the present invention, in the plating step after the annealing step and before the quenching step, it is preferable to include a step of holding at a temperature range of 300 to 500°C for 0 to 300 s before plating.

[0091] The electro-galvanizing treatment is preferably performed by immersing in a zinc solution at 50 to 60°C and applying an electric current. The hot-dip galvanizing treatment is preferably performed by immersing the steel sheet obtained as described above in a zinc plating bath at 440°C or higher and 500°C or lower. Thereafter, it is preferable to adjust the plating adhesion amount by gas wiping or the like. Note that an alloying step of performing an alloying treatment after the hot-dip galvanizing treatment step may be provided. When performing an alloying treatment on the zinc plating, it is preferable to hold at a temperature range of 450°C or higher and 580°C or lower for 1 s or more and 180 s or less for alloying.

[0092] After subjecting the steel sheet to hot dip galvanizing or alloyed hot dip galvanizing, temper rolling can be performed for the purpose of shape correction, adjustment of surface roughness, etc. However, since the bendability may deteriorate due to surface hardening when the temper rolling reduction rate exceeds 1.0%, the temper rolling reduction rate is preferably 1.0% or less. More preferably, it is 0.7% or less. Also, various coating treatments such as resin or oil and fat coating can be performed.

[0093] The conditions of other manufacturing methods are not particularly limited, but it is preferable to carry out under the following conditions.

[0094] The slab is preferably manufactured by the continuous casting method in order to prevent macrosegregation, and can also be manufactured by the ingot method or the thin slab casting method. To hot roll the slab, the slab may be once cooled to room temperature and then reheated for hot rolling. Also, the slab can be charged into a heating furnace without cooling to room temperature and hot rolled. Also, an energy-saving process in which hot rolling is performed immediately after slight heat retention can be applied. When heating the slab, it is preferable to heat it to 1100°C or higher in order to prevent an increase in rolling load and to dissolve carbides. Also, in order to prevent an increase in scale loss, the heating temperature of the slab is preferably 1300°C or lower.

[0095] When hot rolling the slab, from the viewpoint of preventing troubles during rolling when the heating temperature of the slab is lowered, the rough bar after rough rolling can also be heated. Also, a so-called continuous rolling process in which rough bars are joined together and finish rolling is continuously performed can be applied. Also, in order to reduce the rolling load and to uniformize the 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 of the passes of finish rolling.

[0096] The scale on the coiled steel sheet may be removed by pickling or the like. After pickling, cold rolling, annealing, and galvanizing are performed under the above conditions.

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

[0098] The member of the present invention is obtained by subjecting the steel sheet of the present invention to at least one of forming and welding. Further, the manufacturing method of the member of the present invention has a step of subjecting the steel sheet manufactured by the manufacturing method of the steel sheet 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 characteristics. Therefore, the member obtained using the steel sheet of the present invention also has high strength, excellent collision characteristics, and is less likely to cause member fracture during collision deformation. Therefore, the member of the present invention can be suitably used as an energy absorption member in automobile parts.

[0100] For the forming process, general processing methods such as press working can be used without limitation. Also, for welding, general welding such as spot welding and arc welding can be used without limitation.

Example

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

[0102] [Example 1] Steel with the component composition shown in Table 1 was melted in a vacuum melting furnace and slab rolled into steel slabs. These steel slabs were heated to 1100 - 1300 °C and hot rolled, cold rolled, annealed, quenched, and tempered under the conditions shown in Table 2 to manufacture steel sheets. When manufacturing the steel sheets under the conditions shown in Table 2, some of the steel sheets were subjected to plating treatment before the quenching process or after the tempering process. In the hot-dip galvanizing treatment, the steel sheet was immersed in a plating bath to form a hot-dip galvanized layer (GI) with a plating adhesion amount of 10 - 100 g / m 2 . Also, in the alloyed hot-dip galvanizing, after forming a hot-dip galvanized layer on the steel sheet, an alloying treatment was performed to form an alloyed hot-dip galvanized layer (GA). Note that the final thickness of each steel sheet was 1.2 mm.

[0103]

Table 1

[0104]

Table 2

[0105] After subjecting the obtained steel sheet to skin pass rolling with a reduction rate of 0.2%, the area ratios of ferrite (F), bainite (B), fresh martensite (FM), tempered martensite (TM), and retained austenite (RA) were determined according to the following method. Also, according to the above-described method, after bending 90° in the rolling (L) direction with the width (C) direction as the axis at a curvature radius / thickness: 4.2 and then bending it back flat again, when bending it back flat again, in the L cross-section in the 0 to 50 μm region from the steel sheet surface on the compression-tension deformation side, for all ferrite grains, the ratio (NF void / NF) of ferrite grains having voids at the interface was also measured.

[0106] NF void The method for measuring NF void / 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 / thickness: 4.2 and then bent back flat again. After that, the plate thickness cross-section is polished, and the L cross-section in the 0 to 50 μm region from the steel sheet surface on the compression-tension side is observed. Three fields of view of the L cross-section are photographed at a magnification of 2000 times with a scanning electron microscope (SEM), and the number of all ferrite grains in the field of view and the number of ferrite grains having voids at the interface are counted from the obtained image data using Image-Pro manufactured by Media Cybernetics, and the ratio is obtained. The average value of the three fields of view is taken as NF void / NF. Note that the voids can be clearly distinguished from each structure by being darker black than ferrite. Regarding the measurement position of the ferrite grains after bending back, the region including the corner formed by the bending and extending in the width (C) direction (see reference symbol D1 in FIG. 1) was used. More specifically, in the region that becomes the lowermost part in the direction perpendicular to the width direction and the rolling direction (the pressing direction of the pressing part such as a punch) due to the bending, the number of ferrite grains was measured in the 0 to 50 μm region in the plate thickness direction.

[0107] The area ratio of each structure was measured as follows. After polishing the plate thickness cross-section of the steel sheet cut perpendicular to the rolling direction, it was corroded with 3% by volume nital, and three fields were photographed at a magnification of 1500 times with a SEM (scanning electron microscope) at the 1 / 4 position of the plate thickness. The area ratio of each structure was determined from the obtained image data using Image-Pro manufactured by Media Cybernetics. The average value of the area ratios of the three fields was taken as the area ratio of each structure of the present invention. In the image data, ferrite was distinguished as black, bainite as black including island-like retained austenite or gray including carbides with aligned orientations, tempered martensite as light gray including fine non-aligned carbides, and retained austenite as white. Here, fresh martensite also exhibits white, and it is difficult to distinguish fresh martensite and retained austenite in the SEM image. Therefore, the area ratio of fresh martensite was determined by subtracting the area ratio of retained austenite obtained by the method described later from the total area ratio 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 these remaining structures were judged to be pearlite and / or cementite.

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

[0109] Also, the tensile properties and impact properties were determined according to the following test methods. 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 sheets in a direction perpendicular to the rolling direction, and the strain rate was 10-3 A tensile test was conducted in accordance with the provisions of JIS Z2241 (2011) with / s, and the tensile strength (TS) was determined. TS of 440 MPa or more and less than 780 MPa was considered qualified.

[0111] <Bending - Orthogonal Bending Test> For the obtained steel plate, after bending at 90° in the rolling (L) direction with the axis in the width (C) direction at a curvature radius / plate thickness of 4.2, it was bent back flat again (primary bending process) to prepare test pieces. In the 90° bending process (primary bending process), as shown in Fig. 1, for the steel plate placed on die A1 with a V - groove, punch B1 was pushed in to obtain test piece T1. Next, as shown in Fig. 2, for test piece T1 placed on support roll A2, punch B2 was pushed in to perform orthogonal bending (secondary bending process) with the bending direction being perpendicular to the rolling direction. In Figs. 1 and 2, D1 indicates the width (C) direction and D2 indicates the rolling (L) direction.

[0112] Test piece T1 after performing 90° bending (primary bending process) on the steel plate is shown in Fig. 3. Also, test piece T2 after performing orthogonal bending (secondary bending process) on test piece T1 is shown in Fig. 4. The position indicated by the dashed line in test piece T2 in Fig. 4 corresponds to the position indicated by the dashed line in test piece T1 in Fig. 3 before orthogonal bending.

[0113] The conditions for orthogonal bending are as follows. [Orthogonal Bending Conditions] Test method: Roll support, punch pushing Roll diameter: φ30 mm Punch tip R: 0.4 mm Roll distance: (plate thickness × 2)+1.5 mm Stroke speed: 20 mm / min Test piece size: 60 mm × 60 mm Bending direction: Perpendicular to the rolling direction

[0114] In the stroke-load curve obtained when the above orthogonal bending is applied, the stroke at the point where the load has decreased by 50% from the maximum load was determined. The average value of the strokes at the points where the load has decreased by 50% from the maximum load when the above bending-orthogonal bending test was carried out three times was defined as ΔS 50 . ΔS 50 was evaluated to have good fracture resistance characteristics when it was 35 mm or more. When determining ΔS 50 , the load for determining the stroke amount is important in the evaluation of fracture resistance characteristics. In the case of axial crushing deformation, fracture occurs due to the increase in cracks generated in the primary processed part of the member and penetration through the plate thickness. In the bending-orthogonal bending test, cracks occur in the test piece near the maximum load, and as the cracks grow, the cross-sectional area of the bent part decreases and the load decreases. That is, the ratio of the load decrease from the maximum load represents how much the cracks have grown with deformation. By setting the stroke ΔS 50 at the point where the load has decreased by 50% from the maximum load to 35 mm or more, fracture can be suppressed even considering the variations in actual crushing deformation. Therefore, ΔS 50 was evaluated to have good fracture resistance characteristics when it was 35 mm or more.

[0115] <Axial Crushing Test> In the axial crushing test, the influence of the plate thickness was considered, and all tests were carried out using steel plates with a thickness of 1.2 mm. Steel plates obtained in the above manufacturing process were cut out, and using a die with a punch shoulder radius of 5.0 mm and a die shoulder radius of 5.0 mm, they were formed (bent) to a depth of 40 mm to produce the hat-shaped member 10 shown in FIGS. 5 and 6. Also, the steel plate used as the material for the hat-shaped member was separately cut out to a size of 200 mm × 80 mm. Next, the cut steel plate 20 and the hat-shaped member 10 were spot welded to produce a test member 30 as shown in FIGS. 5 and 6. FIG. 5 is a front view of the test member 30 produced by spot welding the hat-shaped member 10 and the steel plate 20. FIG. 6 is a perspective view of the test member 30. As shown in FIG. 6, the position of the spot weld 40 was set such that the distance between the end of the steel plate and the weld was 10 mm and the distance between the welds was 45 mm. Next, as shown in FIG. 7, the test member 30 was joined to the base plate 50 by TIG welding to produce a sample for the axial crushing test. Next, an impactor 60 was collided with the produced sample for the axial crushing test at a constant collision speed 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 to be parallel to the longitudinal direction of the test member 30. The area in the range of stroke 0 to 100 mm in the stroke-load graph at the time of crushing was obtained, and the average value of the area when three tests were performed was taken as the absorbed energy (F ave ). When F ave was 32000 N or more, the absorbed energy was evaluated as good. Also, when both the fracture resistance characteristics and the absorbed energy were good, the collision characteristics were evaluated as good.

[0116] [Table 3]

[0117] The steel plates of the inventive examples had a TS of 440 MPa or more and less than 780 MPa and had excellent collision characteristics. On the other hand, the steel plates of the comparative examples had a TS of less than 440 MPa or exceeded 780 MPa, or had poor collision characteristics.

[0118] [Example 2] The steel sheet of No.1 (the present invention example) in Table 3 of Example 1 was formed by press working to manufacture the member of the present invention example. Further, the steel sheet of No.1 in Table 3 of Example 1 and the steel sheet of No.58 (the present invention example) in Table 3 of Example 1 were joined by spot welding to manufacture the member of the present invention example. The member of the present invention example manufactured using the steel sheet of the present invention is excellent in collision characteristics and has high strength. It was confirmed that it can be suitably used for automotive skeletal parts and the like in all of the members manufactured by forming the steel sheet of No.1 (the present invention example) in Table 3 of Example 1 and the members manufactured by spot welding the steel sheet of No.1 in Table 3 of Example 1 and the steel sheet of No.58 (the present invention example) in Table 3 of Example 1.

[0119] [Example 3] The galvanized steel sheet of No.1 (the present invention example) in Table 3 of Example 1 was formed by press working to manufacture the member of the present invention example. Further, the galvanized steel sheet of No.1 in Table 3 of Example 1 and the galvanized steel sheet of No.58 (the present invention example) in Table 3 of Example 1 were joined by spot welding to manufacture the member of the present invention example. The member of the present invention example manufactured using the steel sheet of the present invention is excellent in collision characteristics and has high strength. It was confirmed that it can be suitably used for automotive skeletal parts and the like in all of the members manufactured by forming the steel sheet of No.1 (the present invention example) in Table 3 of Example 1 and the members manufactured by spot welding the steel sheet of No.1 in Table 3 of Example 1 and the steel sheet of No.58 (the present invention example) in Table 3 of Example 1.

Explanation of symbols

[0120] 10 Hat-shaped member 20 Steel sheet 30 Test member 40 Spot weld part 50 Floor 60 Impactor A1 Die A2 Support roll B1 Punch B2 Punch D1 Width (C) direction D2 Rolling (L) direction D3 Crushing direction T1 test piece T2 test piece

Industrial Applicability

[0121] According to the present invention, a steel sheet having a TS of 440 MPa or more and less than 780 MPa and excellent impact characteristics can be obtained. If the member obtained from the steel sheet of the present invention is used as an automotive part, it can contribute to the weight reduction of the automobile and greatly contribute to the improvement of the performance of the automobile body.

Claims

1. By mass percentage, C: 0.02 to 0.12%, Si: 0.10 to 2.00%, Mn: 0.5 to 2.0%, P: 0.100% or less, S: 0.050% or less, Sol. Al: 0.005 to 0.100%, and N: 0.0100% or less, containing The carbon equivalent (CE) satisfies 0.18% or more and less than 0.46%, The balance consists of Fe and inevitable impurities, and By area ratio, ferrite: 55 to 90%, the total of tempered martensite and bainite: 5% or more, retained austenite: 2 to 10%, fresh martensite: 20% or less, the total of ferrite, tempered martensite, bainite, retained austenite and fresh martensite: 95% or more, having a steel structure, The average crystal grain size of ferrite: 25 μm or less, The coefficient of variation (CV) of ferrite grain size × carbon equivalent (CE) is 0.25 or less, Radius of curvature / plate thickness: When bent by 90° in the rolling (L) direction with the width (C) direction as the axis at 4.2 and then bent back flat again, in the L cross-section within the range of 0 to 50 μm from the surface of the steel plate on the compression-tension deformation side, for all ferrite grains, the ratio (NF ｖｏｉｄ / NF) of the number of ferrite grains having voids at the interface is 5% or less, A steel sheet with a tensile strength of 440 MPa or more and less than 780 MPa.

2. The above component composition further contains, by mass percentage, 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 claim 1, containing at least one selected from the group consisting of REM: 0.005% or less.

3. The steel sheet according to claim 1 or claim 2, having an electrogalvanized layer, a hot-dip galvanized layer, or an alloyed hot-dip galvanized layer on the surface of the steel sheet.

4. A member obtained by subjecting the steel sheet according to claim 1 or claim 2 to at least one of forming and welding.

5. A member obtained by subjecting the steel sheet according to claim 3 to at least one of forming and welding.

6. The carbon equivalent (CE) satisfies 0.18% or more and less than 0.46%, and a steel slab having the component composition according to claim 1 or claim 2 is heated in a temperature range of 1100 to 1300 °C, hot-rolled at a finish rolling temperature of 800 to 950 °C, the cumulative reduction ratio of finish rolling is 60% or more, and in the cooling process from the outlet side of finish rolling to coiling, the residence time in the temperature range of 750 to 600 °C is 10 s or less, and the coiling temperature is 600 °C or less for coiling, a hot rolling process; A cold rolling process in which the hot-rolled steel sheet obtained in the hot rolling process is pickled and cold-rolled with a cumulative reduction ratio of 20% or more; An annealing process in which the cold-rolled steel sheet obtained in the cold rolling process is heated to an annealing temperature of 740 to 850 °C and held for 30 seconds or more; A quenching process in which, after the annealing process, the cooling stop temperature is cooled to (Ms - 250 °C) to (Ms - 50 °C); A tempering process in which, after the quenching process, the reheating temperature is heated to 300 to 500 °C and held for 20 seconds or more; including having a steel structure with an area ratio of ferrite: 55 to 90%, the total of tempered martensite and bainite: 5% or more, retained austenite: 2 to 10%, fresh martensite: 20% or less, and the total of ferrite, tempered martensite, bainite, retained austenite and fresh martensite: 95% or more; the average crystal grain size of ferrite: 25 μm or less; the coefficient of variation (CV) of ferrite grain size × carbon equivalent (CE) is 0.25 or less; Radius of curvature / plate thickness: 4.

2. When bent 90° in the rolling (L) direction about the width (C) direction and then bent back flat again, in the L cross-section in the 0 to 50 μm region from the steel plate surface on the compression-tension deformation side, for all ferrite grains, the ratio of the number of ferrite grains having voids at the interface (NF ｖｏｉｄ / NF) is 5% or less, A method for manufacturing a steel sheet having a tensile strength of 440 MPa or more and less than 780 MPa.

7. having a steel structure with an area ratio of ferrite: 55 to 90%, the total of tempered martensite and bainite: 5% or more, retained austenite: 2 to 10%, fresh martensite: 20% or less, and the total of ferrite, tempered martensite, bainite, retained austenite and fresh martensite: 95% or more; the average crystal grain size of ferrite: 25 μm or less; the coefficient of variation (CV) of ferrite grain size × carbon equivalent (CE) is 0.25 or less; Radius of curvature / plate thickness: 4.

2. When bent 90° in the rolling (L) direction about the width (C) direction and then bent back flat again, in the L cross-section within the range of 0 to 50 μm from the surface of the steel plate on the compression-tension deformation side, for all ferrite grains, the ratio (NF ｖｏｉｄ / NF) of the number of ferrite grains having voids at the interface is 5% or less, A method for manufacturing a hot-rolled steel sheet used for a cold-rolled steel sheet having a tensile strength of 440 MPa or more and less than 780 MPa, The carbon equivalent (CE) satisfies 0.18% or more and less than 0.46%. A steel slab having the component composition described in claim 1 or claim 2 is heated in a temperature range of 1100 to 1300 °C, hot-rolled at a finish rolling temperature of 800 to 950 °C, the cumulative reduction ratio of finish rolling is set to 60% or more, and in the cooling process from the outlet side of finish rolling to coiling, the residence time in the temperature range of 750 to 600 °C is set to 10 s or less, and coiled at a coiling temperature of 600 °C or less. A method for manufacturing a hot-rolled steel sheet for cold-rolled steel sheet, including a hot-rolling process for manufacturing a hot-rolled steel sheet having a structure with an area ratio of ferrite: 50% or less and a total of fresh martensite and bainite: 50% or more in the area ratio of the hot-rolled steel sheet structure.

8. A method for manufacturing a cold-rolled steel sheet, including a cold-rolling process of pickling the hot-rolled steel sheet obtained by the method for manufacturing a hot-rolled steel sheet for cold-rolled steel sheet according to claim 7 and cold-rolling it at a cumulative reduction ratio of 20% or more.

9. The method for manufacturing a steel sheet according to claim 6, including a plating process of applying electro-galvanizing, hot-dip galvanizing, or alloyed hot-dip galvanizing to the surface of the steel sheet after the annealing process and before the quenching process, or after the tempering process.

10. The method for manufacturing a steel sheet according to claim 9, including a process of holding at a temperature range of 300 to 500 °C for 0 to 300 s before plating in the plating process after the annealing process and before the quenching process.

11. A method for manufacturing a member, including a process of performing at least one of forming and welding on the steel sheet manufactured by the method for manufacturing a steel sheet according to claim 6.

12. A method for manufacturing a member, including a process of performing at least one of forming and welding on the steel sheet manufactured by the method for manufacturing a steel sheet according to claim 9.

13. A method for manufacturing a member, including a process of performing at least one of forming and welding on the steel sheet manufactured by the method for manufacturing a steel sheet according to claim 10.

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