Steel plate and method for manufacturing the same

Abstract

Provided are a steel plate and a method for manufacturing the same, the steel plate being characterized by having a prescribed chemical composition, and in that: the metal structure at the 1 / 4 thickness position, which is the range of 1 / 8 to 3 / 8 of the plate thickness of the steel plate in the plate thickness direction of the steel plate from the surface of the steel plate centered at the position at 1 / 4 of the plate thickness from the surface, contains, as area ratio, 0 to 60% ferrite and 0 to 3% retained austenite, with the remainder containing martensite and one or more materials selected from bainite, pearlite, and cementite; the ferrite fraction in the range up to 2 μm from the surface of the steel plate in the plate thickness direction of the steel plate is 95% or above; the in-plane average particle diameter of the ferrite in the in-plane direction is 2.0 μm or below; the ferrite fraction in the range of 5 to 20 μm from the surface of the steel plate in the plate thickness direction of the steel plate is smaller than 90%; and the tensile strength is 950 MPa or higher.

Description

[Technical Field]

[0001] This invention relates to a steel plate and a method for manufacturing the same. This invention claims priority based on Japanese Patent Application No. 2023-030697, filed in Japan on March 1, 2023, the contents of which are incorporated herein by reference. [Background technology]

[0002] High-strength steel sheets are used in automobiles to reduce weight, improve fuel efficiency, lower carbon dioxide emissions, and ensure passenger safety in the event of a collision. Furthermore, high-strength steel sheets used in automobile parts require not only strength but also properties necessary for part formation, such as bendability.

[0003] When a steel sheet is formed into a predetermined component, and also when it is subjected to bending deformation during a collision, the flexibility of a steel sheet generally decreases as its strength increases. Therefore, the inventors considered it important to improve both the flexibility for formability and the flexibility for collision performance while maintaining high strength.

[0004] For example, methods for improving the bendability of high-strength steel sheets include softening the surface layer of the steel sheet (Patent Document 1) and defining the metal structure (Patent Document 2). In these methods, the deformability is improved by making the surface layer of the steel sheet a ferrite-dominant phase or a decarburized ferrite phase, but a hard phase that can become the initiation point of cracks remains, so there is room to improve both of the bending characteristics mentioned above. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Patent No. 7103509 [Patent Document 2] Patent No. 6536294 [Patent Document 3] Patent No. 6154794 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] There are two methods for evaluating the bendability of thin steel sheets: one involves observing whether cracks occur on the surface after bending (a typical example being the 90° V bending test), and the other involves determining cracks based on load changes during bending (a typical example being the VDA (German Association of the Automotive Industry) bending test). For example, Patent Document 3 proposes a method for evaluating crackability during impact using the VDA bending test. In evaluation using the VDA bending test, fracture is determined by the peak or decrease in load, but the inventor's research revealed that fracture occurs before the load peak. Therefore, when the presence or absence of cracks is important, such as for formability, good bendability is required using the 90° V bending test, while when load transmission characteristics are important, such as for impact performance, good bendability is required using the VDA bending test.

[0007] In view of the above, the object of the present invention is to provide a steel sheet that is highly strong and has excellent formability as a bendability and impact characteristics as a bendability, and a method for manufacturing the same. [Means for solving the problem]

[0008] The gist of this invention is as follows:

[0009] (1) A steel sheet according to one aspect of the present invention has the following composition by mass%, C: 0.070~0.15%, Si: 0.10~2.00%, Mn: 1.00~4.00%, sol.Al: 0.001~1.500%, P: 0.0010~0.0300%, S: 0.0200% or less, N: 0.0100% or less, O: 0.0100% or less, Ti: 0~0.200%, B: 0~0.0100%, Cr: 0~1%. 000%, Mo: 0~1.000%, Ni: 0~1.000%, Cu: 0~1.000%, Sn: 0~0.500%, Nb: 0~0.200%, V: 0~0.500%, W: 0~0.50 0%, Ca:0~0.0100%, Mg:0~0.0100%, Bi:0~0.0100%, Sb:0~0.1000%, Zr:0~0.0100%, REM:0~0.1000%, and the remainder The steel plate has a chemical composition consisting of Fe and impurities, and its microstructure at a 1 / 4 thickness position centered at a position 1 / 4 of the thickness of the steel plate in the thickness direction from the surface of the steel plate, in the range of 1 / 8 to 3 / 8 of the thickness from the surface, contains, by area percentage, 0 to 60% ferrite and 0 to 3% retained austenite, with the remainder being one or more selected from martensite, bainite, pearlite, and cementite, and the ferrite fraction in the range from the surface of the steel plate to 2 μm in the thickness direction of the steel plate is 95% or more, and the in-plane average grain size of the ferrite in the in-plane direction is 2.0 μm or less, and the ferrite fraction in the range of 5 to 20 μm from the surface of the steel plate in the thickness direction of the steel plate is less than 90%, and the tensile strength is 950 MPa or more. (2) The steel plate described in (1) above may have a tensile strength of less than 1300 MPa. (3) In the steel plate described in (1) or (2) above, the transition region based on the C concentration in the thickness direction of the steel plate may be 150 μm or less. (4) In the steel sheet described in any one of the above items (1) to (3), the fresh martensite fraction may be 10% or less in the range of 5 to 20 μm from the surface of the steel sheet in the thickness direction of the steel sheet. (5) In the steel plate described in any one of the above items (1) to (4), the ferrite fraction may be 50% or more in the range of 5 to 20 μm from the surface of the steel plate in the thickness direction of the steel plate. (6) A method for manufacturing a steel sheet according to one aspect of the present invention, with the following composition in mass%, C: 0.070~0.15%, Si: 0.10~2.00%, Mn: 1.00~4.00%, sol.Al: 0.001~1.500%, P: 0.0010~0.0300%, S: 0.0200% or less, N: 0.0100% or less, O: 0.0100% or less, Ti: 0~0.200%, B: 0~0.0100%, Cr: 0~1.000% %, Mo: 0~1.000%, Ni: 0~1.000%, Cu: 0~1.000%, Sn: 0~0.500%, Nb: 0~0.200%, V: 0~0.500%, W: 0~0.500%, Ca: 0~0.0100%, Mg: 0~0.0100%, Bi: 0~0.0100%, Sb: 0~0.1000%, Zr: 0~0.0100%, REM: 0~0.1000%, and remainder: Fe and impurities. A cold rolling step in which a steel sheet having a chemical composition consisting of is cold-rolled; a grinding step in which the surface of the steel sheet after the cold rolling step is ground; an annealing step in which the steel sheet whose surface has been ground in the grinding step is annealed; and a quenching step in which quenching is performed after the annealing step. The invention is characterized by having the following: in the grinding step, the surface of the steel sheet that has undergone the cold rolling step is ground to a thickness of 0.1 μm or more; in the annealing step, the dew point is set to -15°C to 20°C; in the annealing step, the annealing temperature is set to 750°C or higher; and in the quenching step, quenching is performed to 300°C or lower at an average cooling rate of 0.4°C / second or higher. (7) In the steel plate manufacturing method described in (6) above, the dew point may be set to -15°C to 20°C during the heating process at least up to the annealing temperature in the annealing step. (8) The method for manufacturing steel plates described in (6) or (7) above may further include a tempering step in which the steel is tempered at 150°C or higher after the quenching step. [Effects of the Invention]

[0010] According to the present invention, it is possible to provide a steel sheet having high strength, excellent bendability as formability, and excellent bendability as collision characteristics, and a method for manufacturing the same.

Embodiments for Carrying out the Invention

[0011] The steel sheet and the manufacturing method thereof according to an embodiment of the present invention will be described below. In the description of the following embodiments, the range indicated with "~" is generally included in the range with the values at both ends as the lower limit value and the upper limit value. However, numerical values indicated with "more than" or "less than" are not included in the range. Hereinafter, each configuration of the steel sheet according to the present embodiment will be described.

[0012] [Chemical Composition of Steel Sheet] The steel sheet according to the present embodiment contains the following elements. In the present embodiment, the % of the content of each element means mass %.

[0013] C: 0.070 to 0.15% C (carbon) is an essential element for strengthening the steel sheet. When the C content is less than 0.070%, sufficient tensile strength cannot be obtained. Therefore, the C content is set to 0.070% or more. From the viewpoint of ensuring the ferrite phase and improving elongation, the C content is preferably 0.08% or more. On the other hand, when the C content exceeds 0.15%, the ductility of the material decreases and the bendability deteriorates. Therefore, the C content is set to 0.15% or less. From the viewpoint of weldability, the C content is preferably 0.14% or less.

[0014] Si: 0.10 to 2.00% Si (silicon) is a solid solution strengthening element and is an element effective for strengthening the steel sheet. To obtain this effect, the Si content is set to 0.10% or more. From the viewpoint of ensuring a desired ferrite fraction in a wide annealing temperature range, the Si content is preferably 0.30% or more. On the one hand, if Si is contained excessively, the chemical conversion treatment property of the steel sheet and the wettability with hot-dip galvanizing will be significantly deteriorated. Therefore, the Si content is set to 2.00% or less. Also, from the viewpoint of causing embrittlement of steel parts and reducing cold formability, the Si content is preferably 1.8% or less.

[0015] Mn: 1.00 - 4.00% Mn (manganese) is a strong austenite stabilizing element and is an element effective for improving the hardenability of the steel sheet. To obtain this effect, the Mn content is set to 1.00% or more. From the viewpoint of ensuring strength, the Mn content is preferably 1.50% or more. On the one hand, if Mn is contained excessively, the weldability and low-temperature toughness will be deteriorated. Therefore, the Mn content is set to 4.00% or less. From the viewpoint of promoting co-segregation with P or S and causing significant deterioration of workability, the Mn content is preferably 3.20% or less.

[0016] sol.Al: 0.001 - 1.500% Al (aluminum) is an element having a deoxidizing effect on steel. To obtain this effect, the sol.Al content is set to 0.001% or more. The sol.Al content is preferably 0.005% or more. On the one hand, even if Al is contained excessively, the effect is saturated, which not only causes cost increase, but also raises the transformation temperature of steel and increases the load during hot rolling. Therefore, the sol.Al content is set to 1.500% or less. The sol.Al content is preferably 1.000% or less.

[0017] P: 0.0010 - 0.0300% P (phosphorus) is a solid solution strengthening element and is an element effective for increasing the strength of the steel sheet. To obtain this effect, the P content is set to 0.0010% or more. The P content is preferably 0.0050% or more. On the one hand, if the P content exceeds 0.0300%, the steel sheet will be embrittled due to segregation of P at the grain boundaries. Also, the weldability and toughness will be deteriorated. Therefore, the P content is set to 0.0300% or less. The P content is preferably 0.0200% or less.

[0018] S: 0.0200% or less S (sulfur) is an element that causes hot brittleness and also inhibits weldability and corrosion resistance. If the sulfur content exceeds 0.0200%, hot workability, weldability, and corrosion resistance will be significantly reduced, so the sulfur content should be kept below 0.0200%. Preferably, the sulfur content is below 0.0100%. A low sulfur (S) content is preferable, and it may even be 0%, but reducing the S content to less than 0.0001% significantly increases manufacturing costs. Therefore, the S content may be 0.0001% or higher. The S content may also be 0.0010% or higher.

[0019] N: 0.0100% or less Nitrogen (N) is an element that forms coarse nitrides in steel, degrading its bending resistance and hole-expanding properties. When the N content exceeds 0.0100%, the above degradation becomes significant, so the N content should be kept below 0.0100%. Preferably, the N content is below 0.0050%. A low N content is preferable, and it may even be 0%, but since extremely low N content would increase the cost of removing N, from an economic standpoint, the N content may be set at 0.0005% or more.

[0020] O: 0.0100% or less Oxygen (O) is an element that forms coarse oxides in steel, degrading its bending resistance and hole-expanding properties. When the O content exceeds 0.0100%, the above degradation becomes significant, so the O content should be kept below 0.0100%. Preferably, the O content is below 0.0070%. A low oxygen content is preferable, and it may be 0%, but from the perspective of manufacturing costs, the oxygen content may be 0.0001% or more. The oxygen content may also be 0.0010% or more.

[0021] The steel sheet according to this embodiment may contain the above elements, with the remainder being Fe and impurities. However, for the purpose of improving various properties, it may further contain one or more elements (arbitrary elements) selected from the following: Ti, B, Cr, Mo, Ni, Cu, Sn, Nb, V, W, Ca, Mg, Bi, Sb, Zr, and REM. Since the arbitrary elements are not required to be included, the lower limit is 0%.

[0022] Ti: 0~0.200% Titanium (Ti) is an element that suppresses the formation of BN, which is a factor that reduces hardenability, by fixing N as TiN in steel. Furthermore, Ti is an element that refines the austenite grain size during heating, thereby improving toughness. To obtain this effect, a Ti content of 0.005% or more is preferable. A Ti content of 0.010% or more is more preferable. On the other hand, excessive Ti content reduces the ductility of the steel sheet. Therefore, when Ti is included, the Ti content should be 0.200% or less. Preferably, the Ti content should be 0.050% or less.

[0023] B: 0~0.0100% Boron (B) is an element that, during welding, segregates at austenite grain boundaries, strengthening them and contributing to improved resistance to molten metal brittle cracking. To achieve this effect, it is preferable to have a B content of 0.0005% or more. It is more preferable to have a B content of 0.0008% or more. On the other hand, if the B content exceeds 0.0100%, carbides and nitrides are formed, the above effects become saturated, and the hot workability decreases. Therefore, the B content should be 0.0100% or less. Preferably, the B content is 0.0050% or less.

[0024] Cr: 0~1.000% Mo: 0~1.000% Ni: 0~1.000% Cu: 0~1.000% Sn: 0~0.500% Cr (chromium), Mo (molybdenum), Ni (nickel), Cu (copper), and Sn (tin) are all elements that are effective in increasing the strength of steel sheets. To obtain the above effects, it is preferable to include one or more elements selected from Cr, Mo, Ni, Cu, and Sn in an amount of 0.001% or more, more preferably 0.010% or more, and even more preferably 0.050% or more. On the other hand, excessive amounts of these elements lead to saturation of the effect and increased costs. Therefore, when including them, the content of Cr, Mo, Ni, and Cu should all be 1.000% or less, and the Sn content should be 0.500% or less. Preferably, the content of Cr, Mo, Ni, and Cu should all be 0.600% or less, and the Sn content should be 0.300% or less.

[0025] Nb: 0~0.200% V: 0~0.500% W: 0~0.500% Nb (niobium), V (vanadium), and W (tungsten) are carbide-forming elements and are effective in increasing the strength of steel sheets. To obtain the above effects, it is preferable to include one or more elements selected from Nb, V, and W in an amount of 0.001% or more, more preferably 0.005% or more, and even more preferably 0.010% or more. On the other hand, if these elements are included in excess, the effect saturates and the cost increases. Therefore, when including these elements, the Nb content should be 0.200% or less, and the V and W content should both be 0.500% or less. Preferably, the Nb content should be 0.100% or less, and the V and W content should both be 0.300% or less.

[0026] Ca: 0~0.0100% Mg: 0~0.0100% Bi: 0~0.0100% Sb: 0~0.1000% Zr: 0~0.0100% REM: 0~0.1000% Ca (calcium), Mg (magnesium), Sb (antimony), Zr (zirconium), and REM (rare earth elements) are elements that contribute to the fine dispersion of inclusions in steel, while Bi (bismuth) is an element that reduces the microsegregation of substitutional alloy elements such as Mn and Si in steel. Each of these elements contributes to improving the bending resistance of steel sheets. Therefore, they may be included as needed. To obtain the above effects, it is preferable to include at least 0.0001% of one or more elements selected from Ca, Mg, Bi, Sb, Zr, and REM, and more preferably at least 0.0010%. On the other hand, excessive amounts of these elements degrade ductility. Therefore, the content of Ca, Mg, Bi, Sb, and Zr should all be 0.0100% or less. In addition, the content of REM should be 0.1000% or less. Preferably, the content of Ca, Mg, Bi, Sb, and Zr should all be 0.0080% or less, and more preferably 0.0060% or less. The REM content is preferably 0.0800% or less, more preferably 0.0600% or less, and even more preferably 0.0200% or less.

[0027] Here, REM refers to the 17 elements totaling Sc, Y, and lanthanides, and REM content means the total content of these elements. Industrially, lanthanides are added in the form of mischmetals.

[0028] The chemical composition of the steel sheet according to this embodiment can be determined by the following method. The chemical composition of the steel sheet mentioned above can be measured using a general chemical composition method. For example, ICP-AES (Inductively Coupled Plasma-Atomic The measurement should be performed using Emission Spectrometry. Sol.Al can be measured by ICP-AES using the filtrate obtained after heating and decomposing the sample with acid. Furthermore, C and S can be measured using the combustion-infrared absorption method, N using the inert gas fusion-thermal conductivity method, and O using the inert gas fusion-nondispersive infrared absorption method. If the steel sheet has a plating layer on its surface, the plating layer should be removed by mechanical grinding before the chemical composition analysis can be performed.

[0029] As described above, the steel sheet according to this embodiment has a chemical composition containing C, Si, Mn, sol.Al, P, S, O, and N, with the remainder being Fe and impurities, or it contains C, Si, Mn, sol.Al, P, S, O, and N, and further contains one or more elements selected from Ti, B, Cr, Mo, Ni, Cu, Sn, Nb, V, W, Ca, Mg, Bi, Sb, Zr, and REM, with the remainder being Fe and impurities.

[0030] [Metal structure of steel plates] In this embodiment, the steel sheet has a microstructure in the range from 1 / 8 to 3 / 8 of the sheet thickness, centered at a point 1 / 4 of the sheet thickness from the surface, extending in the thickness direction from the surface of the steel sheet. This microstructure contains, by area ratio, 0 to 60% ferrite, 0 to 3% retained austenite, and the remainder consists of one or more elements selected from martensite, bainite, pearlite, and cementite. In this embodiment, this "range from 1 / 8 to 3 / 8 of the sheet thickness from the surface of the steel sheet" is referred to as the "1 / 4 thickness position." In this embodiment, the thickness direction of the steel sheet is the direction perpendicular to the surface of the steel sheet. Area ratio means the proportion of each element to the total microstructure in the above range. The remainder of the above microstructure may consist of one or more elements selected from martensite, bainite, pearlite, and cementite.

[0031] From the viewpoint of ensuring strength, the metal structure at the 1 / 4 thickness position more preferably contains 0 to 30% ferrite. From the viewpoint of ensuring strength, the microstructure at the 1 / 4 thickness position preferably has a total of 40-100% tempered martensite and fresh martensite. From the viewpoint of ensuring flexibility, the microstructure at the 1 / 4 thickness position is more preferably such that the sum of tempered martensite and bainite is 40-100%. From the viewpoint of ensuring strength, the microstructure at the 1 / 4 thickness position preferably has a total of 0-5% pearlite and cementite, more preferably 0-3% pearlite and cementite.

[0032] The area percentages of ferrite, retained austenite, martensite (including tempered martensite and fresh martensite), and bainite contained in the metal structure at the 1 / 4 thickness position described above can be measured using the method shown below. A sample is taken from a cross-section of the steel sheet parallel to the rolling direction and thickness direction, which is used as the observation surface. The observation surface is then polished and etched with Nital. In this embodiment, the rolling direction is the longitudinal direction of the steel sheet when it is rolled and stretched, and is parallel to the plane of the steel sheet. Since highly ductile non-ferrous inclusions such as MnS contained in the steel sheet are also stretched along with the steel sheet during rolling, the rolling direction coincides with the direction in which highly ductile non-ferrous inclusions such as MnS elongate. For this reason, the cutting direction in which the aspect ratio of highly ductile non-ferrous inclusions such as MnS is largest can be determined by observing a cross-section cut with a plane perpendicular to the surface of the steel sheet. For the sake of explanation, a method for determining the rolling direction when the rolling direction of the steel sheet is unknown, such as when the steel sheet has been processed into a part, will be described later.

[0033] Next, when observing the microstructure at the 1 / 4 thickness position, the range from 1 / 8 thickness to 3 / 8 thickness (from the 1 / 8 thickness position to the 3 / 8 thickness position) centered on the 1 / 4 thickness position from the surface of the steel plate, is observed at a magnification of 5000x, with a field of view of 250 μm. 2 In summary, a total of five fields of view will be observed using a field emission scanning electron microscope (FE-SEM). The area fractions of ferrite, retained austenite, martensite, pearlite, cementite, and bainite will then be measured.

[0034] Here, regarding the identification of each phase, a region that has a substructure within the grain and, when observed with a scanning electron microscope, has multiple long-side directions for the carbides is identified as tempered martensite. Similarly, when the brightness within the grain changes finely relative to the grain size during the same observation, it is determined that a substructure is present. Furthermore, areas where cementite precipitates in a lamellar pattern are identified as pearlite or cementite. Areas with low brightness and no visible underlying structure are identified as ferrite. Areas with high brightness and no underlying structure revealed by etching are identified as fresh martensite or retained austenite. The remainder is identified as bainite. The area ratio of each tissue was calculated using the point counting method. In the point counting method, measurement points were spaced 2 μm apart, and measurements were taken at 300 or more points per field of view. The arithmetic mean of the calculated values ​​for each field of view was taken to obtain the area ratio of each tissue.

[0035] The area ratio of fresh martensite can be determined by subtracting the area ratio of retained austenite, as determined by the EBSD method described later, from the area ratio of fresh martensite or retained austenite. The sum of this and the area ratio of tempered martensite, calculated by the point counting method, is considered the area ratio of martensite. If the area ratio of fresh martensite or retained austenite, calculated by the point counting method, is smaller than the area ratio of retained austenite, as determined by the EBSD method described later, the area ratio of fresh martensite is set to zero.

[0036] In the steel sheet according to this embodiment, the area ratio of retained austenite at the 1 / 4 thickness position is evaluated by performing high-resolution crystal structure analysis using the EBSD method (electron beam backscatter diffraction). Specifically, a sample is taken from a cross section of the steel sheet parallel to the rolling direction and thickness direction, and the observation surface is polished to a mirror finish. Furthermore, electrolytic polishing or mechanical polishing using colloidal silica is performed to remove the processed surface layer.

[0037] Next, at a position where the steel plate is 1 / 4 thickness, the magnification was 5000x, and the size of one field of view was 250 μm. 2 Based on the above, crystal structure analysis will be performed on the five fields of view using the EBSD method. The step distance between evaluation points will be set to 0.01 to 0.20 μm. Data obtained by the EBSD method will be analyzed using TSL's "OIM Analysys 6.0". Based on the observation results at each location, regions identified as FCC iron will be identified as retained austenite, and the area percentage of retained austenite at each 1 / 4 thickness position will be calculated. If the area percentage of fresh martensite or retained austenite calculated by the point counting method is smaller than the area percentage of retained austenite obtained by the EBSD method, the area percentage of fresh martensite or retained austenite calculated by the point counting method will be used as the area percentage of retained austenite.

[0038] [Ferrite fraction of the outermost layer] In this embodiment, the steel sheet has a ferrite fraction of 95% or more in the range from the surface of the steel sheet to 2 μm in the thickness direction of the steel sheet. Furthermore, the steel sheet in this embodiment has an in-plane average grain size of ferrite of 2.0 μm or less. As will be described later, in the steel sheet manufacturing method according to this embodiment, the surface of the steel sheet that has undergone the cold rolling process is ground and annealed, and the grain size is refined during annealing due to the strain introduced by grinding. As a result, ferrite with an in-plane average grain size of 2.0 μm or less is generated in the surface layer of the steel sheet with a ferrite fraction of 95% or more, and soft ferrite is formed in the outermost layer of the steel sheet. As a result, the occurrence of microcracks on the surface of the steel sheet, which is particularly observed in 90°V bending tests, can be suppressed. Furthermore, the propagation of such microcracks can also be suppressed.

[0039] The ferrite fraction can be measured using the method described below. The ferrite fraction refers to the proportion of ferrite structure to the total metallic structure in the range from the surface of the steel sheet to 2 μm in the thickness direction of the steel sheet.

[0040] The ferrite fraction in the range from the surface of the steel plate to 2 μm in the thickness direction of the steel plate can be measured using the method described below. A sample is taken from a cross-section of the steel plate parallel to the rolling direction and thickness direction, which is used as the observation surface. The observation surface is then polished and etched with Nital. Next, at a magnification of 5000x, one field of view is 250 μm. 2 In summary, a total of five fields of view will be observed using a field emission scanning electron microscope. The area fraction of ferrite will then be measured for each field.

[0041] Here, areas with low brightness and no visible substructure are identified as ferrite tissue. The area ratio of the ferrite tissue is calculated using the same point counting method as described above.

[0042] The range from the surface of the steel plate to 2 μm in the thickness direction of the steel plate refers to the range within 2 μm along the thickness direction of the steel plate, from the surface of the steel plate toward the inside of the steel plate.

[0043] The in-plane average grain size of ferrite can be measured using the method described below. Prepare two samples: one with a cross-section parallel to the rolling direction and thickness direction of the steel plate as the observation surface, and another with a cross-section parallel to the width direction and thickness direction of the steel plate as the observation surface. Polish the observation surface of each sample and etch it with nital. First, photograph the surface of the sample with a cross-section parallel to the rolling direction and thickness direction at a magnification of 5000x, and then move the imaging range in the rolling direction to obtain a 250 μm layer. 2 The above linked images constitute one field of view, and a total of five linked field images are acquired using a field emission scanning electron microscope. For samples where the cross-section parallel to the plate width and plate thickness directions is used as the observation surface, the surface layer is imaged at a magnification of 5000x, and a 250 μm layer is obtained by moving the imaging range in the plate width direction. 2 The above linked images constitute one field of view, and a total of five linked field images are acquired using a field emission scanning electron microscope. For the total of 10 linked field images obtained from each sample, the particle size of ferrite grains at depths of 0.5, 1.0, 1.5, and 2.0 μm from the surface of the steel plate in the thickness direction is measured. Specifically, at each depth position, a straight line parallel to the surface of the steel plate is assumed, and for the ferrite grains at which this line intersects, the particle size of the ferrite grain at that depth position is calculated as (length of the line) / (number of ferrite grains intersecting the line - 1). The length of the line, i.e., the observation distance, is set to 50 μm or more. By taking the arithmetic mean of all these ferrite grain sizes, i.e., the particle sizes obtained from a total of 40 lines at four depth positions in each of the 10 fields of view, the in-plane average particle size of ferrite is obtained. Furthermore, if the rolling direction of the steel sheet is unknown, such as after it has been processed into a part, the above measurement can be performed by preparing two samples: one with an observation surface on a cross section parallel to the reference direction and the thickness direction, and another with an observation surface on a cross section perpendicular to the reference direction and parallel to the thickness direction within the sheet surface. By measuring the ferrite particle size using the method described above, the average particle size (in-plane average particle size) of ferrite in the measurement range at the point where it intersects with a straight line parallel to the steel sheet surface can be evaluated. The fine grain size of the ferrite particles parallel to the steel plate surface allows the bending strain generated on the steel surface during bending deformation to disperse into the individual fine ferrite particles. This suppresses the localization of strain within the particles that occurs with coarse ferrite particles, making it less likely for cracks to form during bending deformation and less likely for cracks to propagate in the thickness direction of the plate. Therefore, it is not the small cross-sectional area of ​​the ferrite particles that matters, but rather that the ferrite particle size along a straight line parallel to the steel plate surface is within a specified range (2.0 μm or less). The steel plate in this embodiment has such a configuration, which makes it possible to suppress the occurrence and propagation of microcracks on the surface of the steel plate, particularly as seen in 90°V bending tests.

[0044] [Tensile strength of steel plate] The steel sheet according to this embodiment has a tensile strength of 950 MPa or more. Having a tensile strength of 950 MPa or more makes it suitable for use as a steel sheet for automobiles. When considering the contribution to weight reduction of automobiles, a tensile strength of 980 MPa or higher, or more preferably 1050 MPa or higher, and even more preferably 1100 MPa or higher, is desirable.

[0045] In this embodiment, the steel plate is more preferably having a tensile strength of less than 1300 MPa. Having a tensile strength of less than 1300 MPa has the advantage of making it easier to ensure elongation. On the other hand, if the tensile strength exceeds 1400 MPa, the weldability deteriorates, so the tensile strength should be kept below 1400 MPa.

[0046] In this embodiment, it is more preferable that the transition region based on carbon concentration is 150 μm or less in the thickness direction of the steel sheet. In this embodiment, the transition region is the region where the carbon concentration is 20% to 90% of the carbon concentration of the steady-state region, which will be described later. If the transition region based on carbon concentration is 150 μm or less, the change in carbon concentration from the low-carbon region to the high-carbon region from the surface to the interior becomes steeper compared to the case where the transition region based on carbon concentration is greater than 150 μm for the same amount of decarburization. Therefore, a low-carbon region of sufficient thickness can be secured in the surface layer, and the occurrence and propagation of microcracks in the surface layer can be suppressed. If the transition region is 100 μm or less, it is more preferable because the bendability can be further improved.

[0047] C concentration refers to the carbon concentration in steel sheets. C concentration can be measured using a GDS (Glow Discharge Emission Spectrometer) as shown below.

[0048] Specifically, after degreasing and cleaning the sample surface, the carbon (C) concentration is continuously measured from the sample surface. After measurement, the amount of material loss is measured with a micrometer, and by assuming that the material loss occurred at a constant rate, the carbon (C) concentration at each depth can be obtained. The measurement time is set so that the measurement depth of the steady-state carbon concentration is 50 μm or more. After noise reduction processing using moving average processing with a unit interval of 1.0 μm in the depth direction, the steady-state region is defined as a range where the arithmetic mean falls within ±5% of the steady-state carbon concentration, which is the average carbon concentration of the base material before decarburization.

[0049] The steady state measured by this measurement method is defined as having a C concentration of 100%. In this embodiment, the transition region is defined as the region in the thickness direction of the steel plate where the C concentration is between 20% and 90%.

[0050] In this embodiment, it is more preferable that the fresh martensite fraction of the steel sheet is 10% or less in the range of 5 to 20 μm from the surface of the steel sheet in the thickness direction of the steel sheet. The microstructure in this range is important for suppressing the propagation of microcracks. By having a fresh martensite fraction of 10% or less in this range, the effect of suppressing the propagation of microcracks is improved. The fresh martensite fraction in this range is more preferably 5% or less. The fresh martensite fraction refers to the proportion of the fresh martensite structure to the total metallic structure in the range of 5 to 20 μm from the surface of the steel sheet in the thickness direction of the steel sheet.

[0051] The fresh martensite fraction is determined by calculating the proportion of fresh martensite in the metal structure within the above range, based on the area ratio obtained by the method for measuring the area ratio of the metal structure described above.

[0052] The range of 5 to 20 μm from the surface of the steel plate in the thickness direction means the range of 5 μm or more and 20 μm or less from the surface of the steel plate toward the inside of the steel plate, along the thickness direction from the surface of the steel plate.

[0053] In this embodiment, it is more preferable that the ferrite fraction of the steel sheet is 50% or more in the range of 5 to 20 μm from the surface of the steel sheet in the thickness direction of the steel sheet. By having a ferrite fraction of 50% or more in this range, the hard phase that serves as the initiation point for fracture is largely eliminated, and the ferrite phase with good ductility suppresses crack propagation. The ferrite fraction in this range is more preferably 70% or more. The ferrite fraction refers to the proportion of the ferrite structure to the total metallic structure in the range of 5 to 20 μm from the surface of the steel sheet in the thickness direction of the steel sheet.

[0054] In the steel sheet of this embodiment, the formation and propagation of microcracks are suppressed by the appropriate arrangement of each microstructure having the characteristics described above in the thickness direction of the sheet.

[0055] A 90° V-bending test can be used as a method to observe the occurrence of microcracks on the surface. The 90° V-bending test is a method that allows observation of microcracks on the surface of steel sheets, i.e., initial microcracks, and can be used to evaluate the bending performance corresponding to the formability of the steel sheet. Suppression of initial microcracks means that the properties in the 90° V-bending test are improved, which means that the formability of the steel sheet is improved.

[0056] Furthermore, the VDA bending test can be used as a method to evaluate the propagation of microcracks. The VDA bending test is a method that determines cracks based on changes in the load applied to the object being measured, and it can evaluate the bending performance corresponding to the impact performance. Suppressing the propagation of microcracks means that the characteristics in the VDA bending test are improved, which means that the impact performance of the steel sheet for automobiles is improved.

[0057] (Plating layer) The steel sheet according to this embodiment may have a plating layer, such as zinc plating, on the surface of the steel sheet as the base material. The zinc plating layer is, for example, a hot-dip galvanized layer. In this embodiment, the zinc plating layer means a plating layer containing 80% by mass or more of Zn. The presence of a hot-dip galvanized layer on the surface improves corrosion resistance. There are no specific restrictions on the amount of zinc plating layer to be applied. However, from the viewpoint of continuous weldability, the application amount should be 150 g / m². 2 Preferably, it should be 100g / m 2 The following is more preferable. On the other hand, in terms of improving corrosion resistance, the amount of adhesion should be 20 g / m 2 It is preferable that the above conditions are met.

[0058] The chemical composition of the zinc plating layer is not limited, but preferably it contains, for example, Al: 0.1-2.0%, Fe: 5.0% or less, with the remainder being Zn and impurities.

[0059] The amount and chemical composition of the zinc plating layer are determined by the following method: The plating layer is melted using hydrochloric acid containing an inhibitor, and the amount of plating is determined by comparing the weight before and after melting. The chemical composition of the plating layer is then measured by quantitative analysis of the resulting solution using ICP.

[0060] Furthermore, if the steel sheet has a plating layer on its surface, the position in the thickness direction in this embodiment is defined as the depth relative to the surface of the base steel sheet (the interface between the Fe phase and the plating layer).

[0061] [Method of manufacturing steel plates] Next, the method for manufacturing the steel plate according to the above embodiment will be described.

[0062] The steel sheet manufacturing method of this embodiment includes a cold rolling step of cold rolling a steel sheet having a predetermined chemical composition, a grinding step of grinding the surface of the steel sheet that has undergone the cold rolling step, and an annealing step of annealing the steel sheet whose surface has been ground in the grinding step.

[0063] <Cold rolling process> In the cold rolling process, steel sheets having the following chemical composition are cold-rolled. In mass%, C: 0.070~0.15%, Si: 0.10~2.00%, Mn: 1.00~4.00%, sol.Al: 0.001~1.500%, P: 0.0010~0.0300%, S: 0.0200% or less, N: 0.0100% or less, O: 0.0100% or less, Ti: 0~0.200%, B: 0~0.0100%, Cr: 0~1.000%, Mo: 0~1.000%, Ni: 0~1,000%, Cu: 0~1.000%, Sn: 0~0.500%, Nb: 0~0.200%, V: 0~0.500%, W: 0 to 0.500%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Bi: 0 to 0.0100%, Sb: 0 to 0.1000%, Zr: 0 to 0.0100%, REM: 0 to 0.1000%, and the balance: Fe and impurities

[0064] The conditions of cold rolling are not particularly limited, and a cold-rolled steel sheet can be manufactured by subjecting the above hot-rolled steel sheet to cold rolling under normal conditions. There are no restrictions on the manufacturing conditions of the steel sheet to be subjected to the cold rolling process. For example, after casting molten steel having the above-described chemical composition into a steel slab under normal conditions and then subjecting it to hot rolling under normal conditions, a hot-rolled steel sheet can be manufactured, and cold rolling can be performed on this hot-rolled steel sheet.

[0065] <Grinding process> In the grinding process, the surface of the steel sheet that has undergone the above cold rolling process is ground by an average of 0.1 μm or more in the plate thickness direction. By grinding the surface of the steel sheet by 0.1 μm or more, strong strain is introduced into the interior of the steel sheet. Therefore, recrystallization is promoted in the surface layer of the steel sheet structure during heating in the subsequent annealing process, and fine ferrite crystal grains can be obtained on the outermost surface of the steel sheet. From the perspective of the amount of strain introduced into the interior of the steel sheet, it is more preferable to grind the surface of the steel sheet by 0.15 μm or more.

[0066] In the grinding process, it is important to introduce large strain into the surface layer of the steel sheet.

[0067] The amount (μm) of grinding the steel sheet is calculated based on the change in the weight of the steel sheet before and after grinding. The amount of weight reduction of the steel sheet before and after grinding is divided by the area of the ground steel sheet to obtain the weight reduction (g) per 1 m 2 per. Then, the value of an offline test performed under the same conditions in advance (1 m 2The grinding amount (μm) is calculated based on a value that represents the relationship between the weight reduction per unit and the grinding amount (μm).

[0068] One example of a grinding method is to use a grinding brush and grind at a predetermined rotational speed, reduction amount, and grinding speed. While not particularly limited, for example, a Hotani D-100 grinding brush could be used, with a rotational speed of 1000-1500 rpm, a reduction amount of 2.0 mm, and a grinding speed of approximately 100 mpm. Alternatively, the amount of material removed may be adjusted by performing the grinding process multiple times, typically 2 to 10 times. Furthermore, it is preferable to grind the entire surface of the steel plate.

[0069] While it is possible to grind only one side of the steel plate or both sides, grinding both sides is preferable considering its versatility as an automotive steel plate.

[0070] <Annealing process> The annealing process includes a heating process of heating a steel sheet having a predetermined chemical composition (the same chemical composition as the steel sheet according to this embodiment to be obtained) to a predetermined annealing temperature (maximum heating temperature), a holding process of holding the heated steel sheet at the annealing temperature for a certain period of time, and a cooling process of cooling from the annealing temperature to a predetermined temperature. From a productivity standpoint, it is preferable to perform annealing by passing the steel plates through a continuous annealing line.

[0071] (Heating process) During the heating process, the steel plate is heated to its annealing temperature. During the heating process to reach the annealing temperature in the annealing process, the heating rate is not particularly limited, but it is important to control the atmosphere as described below.

[0072] During the heating process, the dew point is set to -15°C to 20°C. By setting the dew point to -15°C to 20°C, recrystallization is promoted by the strain introduced during the grinding process, allowing fine crystal grains to be obtained on the outermost layer of the steel sheet. This promotes the decarburization reaction, making it possible to achieve a ferrite fraction of 95% or more on the outermost layer of the steel sheet.

[0073] If the dew point inside the furnace is below -15°C, a sufficient ferrite fraction cannot be obtained; therefore, the dew point should be set to -15°C or higher. From the viewpoint of obtaining a high ferrite fraction, the dew point should more preferably be -10°C or higher. On the other hand, if the dew point exceeds 20°C, decarburization proceeds excessively, significantly reducing the hard phase on the surface of the base steel sheet, and forming coarse oxides in the refined layer, resulting in decreased plating adhesion and powdering properties. For this reason, the dew point should be kept below 20°C.

[0074] (retention process) After heating to the annealing temperature under the above conditions, the steel plate is held at the predetermined maximum heating temperature for at least 5 seconds. If the holding time is less than 5 seconds, it will not be possible to secure a sufficient amount of austenite, which will later become the hard phase. There is no particular upper limit on the holding time. However, since excessively long holding times hinder the manufacturability of the steel sheet, it is preferable from a cost perspective that the holding time be less than 500 seconds. Furthermore, from the viewpoint of ensuring sufficient austenite at low cost, a holding time of approximately 10 to 120 seconds is more preferable.

[0075] The annealing temperature should be 750°C or higher to ensure sufficient austenite, which will later become the hard phase.

[0076] If the annealing temperature exceeds 1000°C, the grain size during annealing becomes coarser, making it difficult to obtain sufficient fine ferrite grains that contribute to improving the flexibility of the surface layer. Therefore, the annealing temperature should be kept below 1000°C. Preferably, the annealing temperature is 900°C or lower.

[0077] During the holding process, following the heating process, the dew point may be set to -15°C to 20°C.

[0078] In addition, the above annealing process may be carried out without setting the dew point to -15°C to 20°C, and a separate process may be provided before or after the annealing process in which the steel sheet is heated to a dew point of -15°C to 20°C.

[0079] <Heatening and tempering processes> In the steel sheet manufacturing method of this embodiment, a quenching process and a tempering process may be carried out after the holding process in the annealing process. This can reduce the fresh martensite fraction and further suppress the propagation of microcracks. In particular, from the viewpoint of reducing the fresh martensite fraction in the range of 5 to 20 μm from the surface of the steel sheet in the thickness direction of the steel sheet, it is preferable to carry out a tempering process in addition to the quenching process.

[0080] In the quenching process, the steel sheet, which was heated and held in the annealing process, is cooled and quenched so that the temperature of the steel sheet is 300°C or lower. By quenching the steel sheet to a temperature of 300°C or lower, the amount of fresh martensite, which is the starting point for fracture, can be reduced. From the viewpoint of further reducing the amount of fresh martensite, which is the starting point for fracture, it is more preferable to quench the steel sheet to 250°C or lower. The average cooling rate during quenching should be 0.4°C / second or higher to obtain a hard layer. While there is no specific upper limit to the average cooling rate during quenching, cooling at 500°C / second or higher is difficult from a cost perspective. By performing quenching at an average cooling rate of 0.4°C / second or higher, the ferrite fraction in the surface layer (5-20 μm) can be kept below 90%. Excessive softening of the surface layer not only leads to a decrease in bending strength but also makes it difficult to ensure the strength of the steel sheet.

[0081] In the tempering process, the steel sheet, which has been cooled and held in the quenching process, is tempered to a temperature of 150°C or higher. The tempering conditions include a holding time of 2 seconds or more. There is no particular upper limit, but it is preferable to keep the tempering holding time to 500 seconds or less as the effect will saturate. To obtain sufficiently soft tempered martensite, it is preferable to temper the steel sheet to a temperature of 200°C or higher. Furthermore, to ensure strength, it is even more preferable to temper the steel sheet to a temperature of 500°C or higher.

[0082] In the above explanation, the quenching process is performed immediately after the annealing process. However, after the holding process of the annealing process, the steel plate may be cooled to a predetermined temperature, and then reheated to perform the quenching process.

[0083] (Plating process) In the steel sheet manufacturing method according to this embodiment, a plating process may be performed between the annealing process and the quenching process, between the quenching process and the tempering process, or after the tempering process. The plating process may also be performed as part of the quenching process.

[0084] Furthermore, a plating process may be carried out after the tempering process. If a plating process is carried out after the tempering process, the aforementioned plating layer may be formed by electroplating. [Examples]

[0085] Slabs having the chemical compositions shown in Tables 1A and 1B were hot-rolled under the hot-rolling conditions shown in Tables 2A and 2B, and then coiled to obtain hot-rolled steel sheets of various thicknesses shown in Tables 2A and 2B. These hot-rolled steel sheets were then cold-rolled under the cold-rolling conditions shown in Tables 2A and 2B to obtain cold-rolled steel sheets of various thicknesses shown in Tables 2A and 2B (cold-rolling process). Note that underlined values ​​in the following table indicate that they are outside the scope of the present invention.

[0086] [Table 1A]

[0087] [Table 1B]

[0088] [Table 2A]

[0089] [Table 2B]

[0090] These cold-rolled steel sheets were subjected to surface grinding (grinding process). The amount of grinding performed on the steel sheet surface is shown in Tables 3A and 3B. Then, the material was heated to the annealing temperature at the dew points shown in Tables 3A and 3B and held there (annealing process). The conditions controlled during annealing are shown in Tables 3A and 3B.

[0091] After holding, quenching was performed at the average cooling rate and cooling stop temperature shown in Tables 3A and 3B, and tempering was performed at the heat treatment temperature and heat treatment time shown in Tables 3A and 3B.

[0092] In addition, in some experimental cases, hot-dip galvanizing was performed under the conditions shown in Tables 3A and 3B (plating type (GA: alloyed hot-dip galvanizing, GI: hot-dip galvanizing), steel sheet temperature before plating, alloying temperature), and a plating layer was formed on cold-rolled steel sheets after annealing.

[0093] [Table 3A]

[0094] [Table 3B]

[0095] For the obtained steel sheets, the ferrite fraction and the in-plane average grain size of ferrite in the range from the surface of the steel sheet to 2 μm were measured using the following method.

[0096] (Ferrite fraction (Vα)) A sample was taken from a cross-section of the steel plate parallel to the rolling direction and thickness direction, using this as the observation surface. The observation surface was polished and then etched with Nital at a magnification of 5000x, with one field of view measuring 250 μm. 2 In total, five fields of view were observed using a field emission scanning electron microscope. The area fraction of ferrite was then measured for each field. Here, areas with low brightness and no underlying tissue were identified as ferrite tissue. The area ratio of ferrite tissue was calculated using a point counting method, with measurements taken at 300 or more measurement points per field of view. The ferrite fraction (Vα) was obtained by arithmetic mean of the calculated values ​​for each field of view.

[0097] (In-plane average grain size of ferrite) Two types of steel plates were prepared: one with an observation surface parallel to the rolling direction and thickness direction, and another with an observation surface parallel to the width direction and thickness direction. The observation surfaces of each sample were polished and then etched with Nital. The surface layer of each sample was etched at a magnification of 5000x, with a field of view of 250 μm. 2 In total, five fields of view were observed using a field emission scanning electron microscope. For each sample, the grain size in the rolling direction and the width direction of ferrite grains located at positions of 0.5, 1.0, 1.5, and 2.0 μm from the surface of the steel plate in the thickness direction was measured. The in-plane average grain size of ferrite was obtained by arithmetic mean calculation of all these ferrite grain sizes.

[0098] (Structure at 1 / 4 thickness) The tissue at the 1 / 4 thickness position was observed using the following method. A sample was taken from a cross-section of the steel plate parallel to the rolling direction and thickness direction, and the observation surface was polished and then etched with Nital. Next, in the range from 1 / 8 thickness to 3 / 8 thickness (from 1 / 8 thickness from the surface to 3 / 8 thickness from the surface), centered at a magnification of 5000x, one field of view was 250 μm. 2 In total, five fields of view were observed using a field emission scanning electron microscope. The area fractions of ferrite, retained austenite, martensite, bainite, pearlite, and cementite were then measured. The area percentages of ferrite were denoted as (Vα), retained austenite as (Vγ), bainite as (VB), fresh martensite as (VfM), and tempered martensite as (VtM). Pearlite and cementite were listed as other structures, and their total values ​​are given.

[0099] The identification of each phase was carried out as follows: Regions with a substructure within the grain and multiple long-side orientations of carbides when observed with a scanning electron microscope were identified as tempered martensite. Regions where cementite precipitated in a lamellar pattern were identified as pearlite or cementite. Regions with low brightness and no visible substructure were identified as ferrite. Regions with high brightness and no substructure revealed by etching were identified as fresh martensite or retained austenite. The remainder was identified as bainite. The area ratio of each tissue was calculated using the point counting method. In the point counting method, measurements were taken at 300 or more measurement points per field of view. The area ratio of each tissue was obtained by taking the arithmetic mean of the calculated values ​​for each field of view.

[0100] The area ratio of fresh martensite was determined by subtracting the area ratio of retained austenite, obtained by the EBSD method described above, from the area ratio of fresh martensite or retained austenite. The sum of this and the area ratio of tempered martensite, calculated by the point counting method, was considered the area ratio of martensite.

[0101] The area ratio of retained austenite at the 1 / 4 thickness position was evaluated by performing high-resolution crystal structure analysis using EBSD. Samples were taken from cross-sections parallel to the rolling direction and thickness direction of the steel sheet, and the observation surfaces were polished to a mirror finish. Electrolytic polishing or mechanical polishing using colloidal silica was then performed to remove the surface processing layer. Next, at a position where the steel plate is 1 / 4 thickness, the magnification was 5000x, and the size of one field of view was 250 μm. 2 Based on the above, crystal structure analysis was performed on five fields of view using the EBSD method. The step distance between evaluation points was set to 0.01 to 0.20 μm. Data obtained by the EBSD method were analyzed using TSL's "OIM Analysys 6.0". Based on the observation results at each location, regions identified as FCC iron were identified as retained austenite, and the area percentage of each retained austenite at the 1 / 4 thickness position was calculated.

[0102] Furthermore, the ferrite fraction (Vα) and fresh martensite fraction (VfM) in the range of 5 to 20 μm from the surface of the steel plate were measured using the same method as described above.

[0103] The results above are shown in Tables 4A and 4B.

[0104] [Table 4A]

[0105] [Table 4B]

[0106] (Transition region based on C concentration) The carbon (C) concentration of steel plates was measured using a glow discharge emission spectrometer (GD-Profiler2, manufactured by HORIBA). After degreasing and cleaning the sample surface, continuous C concentration measurements were performed from the sample surface. After measurement, the amount of thinning was measured with a micrometer, and the C concentration at each depth was obtained by assuming that the thinning occurred at a constant rate. The measurement time was set so that a steady-state region with a C concentration of 50 μm or more was obtained. The steady-state region was defined as a region with a variation range of ±5% after noise reduction. Based on the results obtained from the above measurements, the region in the thickness direction of the steel plate where the carbon concentration is between 20% and 90% was defined as the transition region.

[0107] Furthermore, the tensile strength of the steel plate was measured using the following method. (Yield stress (YS)) A JIS No. 5 test specimen was taken from the steel plate perpendicular to the rolling direction, and the yield stress was measured in accordance with JIS Z 2241:2011. (Tensile Strength (TS)) A JIS No. 5 tensile test piece was taken from the direction perpendicular to the rolling direction and the thickness direction of the steel plate (width direction), and a tensile test was conducted in accordance with JIS Z 2241:2011 to measure the tensile strength (TS). (Elongation (EL)) A JIS No. 5 test piece was taken from the direction perpendicular to the rolling direction of the steel plate, and the elongation of the steel plate was measured in accordance with JIS Z 2241:2011.

[0108] For the obtained steel plates, the following two bending tests were carried out to evaluate the performance in each bending test.

[0109] <VDA Bending Test> According to VDA238-100, a bending test was carried out, and the bending performance was evaluated by scoring according to the bending angle of the VDA standard bending as follows. The test piece was taken in the direction where the bending ridge line is parallel to the rolling direction. 0 points: Bending angle < 120 - TS × 0.05 1 point: Bending angle ≥ 120 - TS × 0.05 2 points: Bending angle ≥ 120 - TS × 0.04

[0110] <90° V-Bending Test> A 90° V-bending test was carried out in accordance with JIS Z 2248. The test piece shape was a strip shape of 30 mm × 150 mm. As follows, the bending performance was evaluated by scoring according to the limit r / t obtained by 90° V-bending. Here, t in the limit r / t is the plate thickness, and r is the minimum value of the bending radius at which no crack occurred. 0 points: Limit r / t > 2.0 × (TS / 1000) 1 point: Limit r / t ≤ 2.0 × (TS / 1000) 2 points: Limit r / t ≤ 1.5 × (TS / 1000)

[0111] Those with a total score of 3 or more for the score of the VDA bending test and the score of the 90° V-bending test were judged to have good bending property (OK), and the others were judged to be defective (NG).

[0112] The results above are shown in Tables 5A and 5B.

[0113] [Table 5A]

[0114] [Table 5B]

[0115] As can be seen from the results above, in experiments No. 44 to 47, 49, and 50, where the chemical composition fell outside the scope of the present invention, the bending score was poor. In experiments No. 48 and 51, cracks occurred during hot rolling, making it impossible to conduct further tests.

[0116] Furthermore, even when the chemical composition was within the range of the present invention, in experiments No. 2, 3, 20, and 21, where the amount of grinding during the grinding process was small, the ferrite fraction and the in-plane average grain size of ferrite in the range from the surface of the steel sheet up to 2 μm fell outside the range of the present invention, resulting in a poor bending score.

[0117] Furthermore, even when the chemical composition was within the range of the present invention, in experiments No. 5 and 18, where the dew point during the heating process of the annealing step fell outside the range of the present invention, the ferrite fraction or the in-plane average grain size of ferrite in the range from the surface of the steel sheet up to 2 μm fell outside the range of the present invention, resulting in a poor bending score. Furthermore, even when the chemical composition was within the scope of the present invention, the desired tensile strength could not be obtained in Experiment No. 6, where the annealing temperature in the annealing process was outside the scope of the present invention, and in Experiments No. 11 and 12, where the conditions in the quenching process were outside the scope of the present invention. Furthermore, even though the chemical composition was within the range of the present invention, in Experiment No. 52, where the cooling stop temperature during quenching was outside the range of the present invention, the bending score was poor.

[0118] In experimental examples other than those mentioned above, the chemical composition and manufacturing method conditions were within the scope of the present invention, and good results were obtained in the bending score. [Industrial applicability]

[0119] The steel sheet disclosed herein has high strength and excellent formability (bendability as a formability) and impact characteristics (bendability as a impact property), making it highly applicable to industrial use.

Claims

1. In mass percent, C: 0.070-0.15%, Si: 0.10-2.00%, Mn: 1.00-4.00%, Sol. Al: 0.001–1.500%, P: 0.0010-0.0300%, S: 0.0200% or less, N: 0.0100% or less, O: 0.0100% or less, Ti: 0-0.200%, B: 0 to 0.0100%, Cr: 0-1.000%, Mo: 0-1.000%, Ni: 0-1.000%, Cu: 0 to 1.000%, Sn: 0-0.500%, Nb: 0 to 0.200%, V: 0 to 0.500%, W: 0-0.500%, Ca: 0-0.0100%, Mg: 0 to 0.0100%, Bi: 0 to 0.0100%, Sb: 0 to 0.1000%, Zr: 0 to 0.0100%, REM: 0 to 0.1000%, and Remainder: Fe and impurities, It has a chemical composition consisting of, The metal structure at a 1 / 4 thickness position, centered at a point 1 / 4 of the thickness of the steel plate in the thickness direction from the surface of the steel plate, and ranging from 1 / 8 to 3 / 8 of the thickness from the surface, contains, by area percentage, 0 to 60% ferrite and 0 to 3% retained austenite. The remainder consists of one or more elements selected from martensite, bainite, pearlite, and cementite. The ferrite fraction in the range from the surface of the steel plate to 2 μm in the thickness direction of the steel plate is 95% or more, and the in-plane average particle size of the ferrite in the in-plane direction is 2.0 μm or less. In the range of 5 to 20 μm from the surface of the steel plate in the thickness direction of the steel plate, the ferrite fraction is less than 90%. The tensile strength is 950 MPa or higher. A steel plate characterized by the following features.

2. The tensile strength is less than 1300 MPa. The steel plate according to feature 1.

3. The steel plate according to claim 1 or 2, characterized in that the transition region based on the C concentration in the thickness direction of the steel plate is 150 μm or less.

4. In the range of 5 to 20 μm from the surface of the steel plate in the thickness direction of the steel plate, the fresh martensite fraction is 10% or less. The steel plate according to feature 1 or 2.

5. In the range of 5 to 20 μm from the surface of the steel plate in the thickness direction of the steel plate, the ferrite fraction is 50% or more. The steel plate according to feature 1 or 2.

6. A method for manufacturing a steel sheet according to Claim 1, In mass percent, C: 0.070-0.15%, Si: 0.10-2.00%, Mn: 1.00-4.00%, Sol. Al: 0.001–1.500%, P: 0.0010-0.0300%, S: 0.0200% or less, N: 0.0100% or less, O: 0.0100% or less, Ti: 0-0.200%, B: 0 to 0.0100%, Cr: 0-1.000%, Mo: 0-1.000%, Ni: 0-1.000%, Cu: 0 to 1.000%, Sn: 0-0.500%, Nb: 0 to 0.200%, V: 0 to 0.500%, W: 0-0.500%, Ca: 0-0.0100%, Mg: 0 to 0.0100%, Bi: 0 to 0.0100%, Sb: 0 to 0.1000%, Zr: 0 to 0.0100%, REM: 0 to 0.1000%, and Remainder: Fe and impurities, A cold rolling process in which a steel sheet having a chemical composition consisting of the above is cold-rolled, A grinding step in which the surface of the steel sheet that has undergone the cold rolling step is ground, An annealing step is performed to anneal the steel plate whose surface has been ground in the grinding step, A quenching step is performed after the annealing step, It has, In the grinding process, the surface of the steel sheet that has undergone the cold rolling process is ground to a thickness of 0.1 μm or more. In the aforementioned annealing process, the dew point is set to -15°C to 20°C. In the aforementioned annealing process, the annealing temperature is set to 750°C or higher. In the aforementioned quenching process, the quenching is performed at an average cooling rate of 0.4°C / second or higher, down to 300°C or lower. A method for manufacturing steel plates, characterized by the following:

7. In the aforementioned annealing process, The method for manufacturing a steel sheet according to claim 6, characterized in that the dew point is set to -15°C to 20°C during the heating process up to at least the annealing temperature.

8. The method for manufacturing a steel sheet according to claim 6 or 7, further comprising a tempering step of tempering at 150°C or higher after the quenching step.

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