Steel sheet, component, and method for producing steel sheet

Abstract

Provided is a steel sheet that is characterized by having a prescribed chemical composition and that is characterized in that: the steel structure at a position 1 / 4 of the thickness from the surface includes 8-30% of retained austenite in area%; when the Mn concentration in a depth range of 10-30 µm from the surface is measured through EPMA and when a region having an Mn concentration that is 1.5-fold or greater than the average Mn concentration in said depth range is defined as an Mn-concentrated region, the number density in the Mn-concentrated region having an area of 0.3 µm2 or more is 10 pieces / 1000 µm2 or more; when GDS measurement is performed, the ratio X / Y of the average C concentration X in said depth range and the bulk C concentration Y is 0.20-0.50; and the steel sheet has a Vickers hardness of 300 Hv or more. Also provided is a method for producing said steel sheet.

Description

Steel plates, parts, and methods for manufacturing steel plates

[0001] This invention relates to steel plates, parts, and methods for manufacturing steel plates.

[0002] In recent years, there has been a growing demand for improved fuel efficiency in automobiles due to regulations on greenhouse gas emissions as a measure against global warming. As a result, the application of high-strength steel sheets is expanding to reduce vehicle weight and ensure collision safety. However, increasing the strength of steel sheets generally reduces properties such as elongation, making it difficult to achieve both strength and formability in high-strength steel sheets.

[0003] In this regard, for example, Patent Document 1 describes a steel having a predetermined chemical composition, in a range of 1 / 8 to 3 / 8 thickness centered at a position 1 / 4 thickness from the surface, containing, by volume fraction, soft ferrite: 0% to 30%, retained austenite: 3% to 40%, fresh martensite: 0% to 30%, total of pearlite and cementite: 0% to 10%, with the remainder being hard ferrite, and in the aforementioned range of 1 / 8 to 3 / 8 thickness, the number ratio of retained austenite with an aspect ratio of 2.0 or more to the total retained austenite is 50% or more, and the hardness is 80% or less of the hardness in the aforementioned range of 1 / 8 to 3 / 8 thickness. The invention describes a steel sheet characterized in that, when the region is defined as a soft layer, a soft layer with a thickness of 1 to 100 μm exists in the thickness direction from the surface, the volume fraction of crystal grains with an aspect ratio of less than 3.0 among the ferrite contained in the soft layer is 50% or more, the volume fraction of retained austenite in the soft layer is less than 50% of the volume fraction of retained austenite in the range of 1 / 8 thickness to 3 / 8 thickness, and when the emission intensity of the wavelength indicating Si is analyzed by high-frequency glow discharge analysis from the surface in the thickness direction, a peak of emission intensity of the wavelength indicating Si appears in the range of more than 0.2 μm and 5.0 μm or less from the surface. Furthermore, Patent Document 1 teaches that according to the above configuration, a high-strength steel sheet can be provided that has excellent ductility and hole-expanding properties, excellent chemical conversion treatment properties and plating adhesion, and good bendability after processing.

[0004] Patent Document 2 describes a steel having a predetermined chemical composition, in a range of 1 / 8 to 3 / 8 thickness centered at a position 1 / 4 thickness from the surface, containing, by volume fraction, soft ferrite: 0% to 30%, retained austenite: 3% to 40%, fresh martensite: 0% to 30%, total of pearlite and cementite: 0% to 10%, with the remainder being hard ferrite, and in the aforementioned range of 1 / 8 to 3 / 8 thickness, the proportion of retained austenite with an aspect ratio of 2.0 or more to the total retained austenite is 50% or more, and a region having a hardness of 80% or less of the hardness of the aforementioned range of 1 / 8 to 3 / 8 thickness is defined as a soft layer. The invention describes a steel sheet characterized in that, when defined, a soft layer with a thickness of 1 to 100 μm exists in the thickness direction from the surface, the volume fraction of crystal grains with an aspect ratio of 3.0 or more among the ferrite contained in the soft layer is 50% or more, the volume fraction of retained austenite in the soft layer is 80% or less of the volume fraction of retained austenite in the range of 1 / 8 thickness to 3 / 8 thickness, and when the emission intensity of the wavelength indicating Si is analyzed by high-frequency glow discharge analysis from the surface in the thickness direction, a peak of emission intensity of the wavelength indicating Si appears in the range of more than 0.2 μm and 10.0 μm or less from the surface. Furthermore, Patent Document 2 teaches that the above configuration can provide a high-strength steel sheet with excellent ductility and hole-expanding properties, excellent chemical conversion treatment properties and plating adhesion, and good fatigue properties and hydrogen embrittlement resistance of the bent part.

[0005] International Publication No. 2019 / 187090, International Publication No. 2019 / 187060

[0006] In automotive parts, improved impact resistance is desired from the perspective of ensuring occupant safety. For this reason, steel sheets used as materials for automotive parts, for example, require ductility such as elongation from the standpoint of processability, while at the same time, they are required to exhibit high impact resistance after being processed into parts by press forming or other methods so that they are less likely to break when subjected to impacts such as automobile collisions. In order to obtain excellent impact resistance, it is necessary to increase the ability to absorb impact energy, and considering the deformation modes during a collision, it is particularly important to improve bendability among the deformability, and more specifically, to improve the bendability after processing as described in Patent Document 1.

[0007] Therefore, the present invention aims to provide a steel sheet and a method for manufacturing the same, which, through a novel configuration, possess high strength, improved elongation, and excellent bendability after processing.

[0008] To achieve the above objective, the inventors focused particularly on the microstructure of the steel sheet. Specifically, the inventors first found that by optimizing the chemical composition and microstructure of the steel sheet, and in particular controlling the microstructure to have a Vickers hardness of 300 Hv or higher, and by constructing the microstructure with a predetermined amount of retained austenite, it is possible to significantly improve the elongation of the steel sheet while achieving high strength. In addition, the inventors found that by softening the surface layer of the steel sheet through decarburization and controlling the number density of a predetermined Mn-enriched region in the surface layer to a specific range, it is possible to significantly improve the bendability even after processing such as press forming, thus completing the present invention.

[0009] The present invention, which has achieved the above objectives, is as follows. (1) In mass percent, C: 0.12-0.40%, Si: 0.60-2.50%, Mn: 1.00-4.00%, P: 0.050% or less, S: 0.0100% or less, Al: 0.001-1.500%, N: 0.0100% or less, O: 0.0100% or less, Cr: 0-1.00%, Mo: 0-1.00%, Cu: 0-1.00%, Ni: 0-1.00%, Co: 0-1.00%, W: 0-1.00%, B: 0-0.0050%, Ta: 0-1.000%, Sn: 0-1.000%, Sb: 0-0.500%, The chemical composition consists of Ti: 0-0.100%, Nb: 0-0.200%, V: 0-1.00%, As: 0-0.100%, Zn: 0-1.000%, Ca: 0-0.0100%, Mg: 0-0.0100%, Zr: 0-0.0100%, Hf: 0-0.0100%, Sr: 0-0.0100%, Bi: 0-0.0100%, REM: 0-0.0100%, and the remainder being Fe and impurities. The microstructure at a thickness of 1 / 4 of the surface contains, by area percentage, 8-30% retained austenite. When the Mn concentration in the depth range of 10 to 30 μm from the surface is measured by EPMA, and the region having a Mn concentration of 1.5 times or more the average Mn concentration in the said depth range is defined as the Mn-enriched region, then 0.3 μm 2 The number density of Mn-enriched regions with the above area is 10 particles / 1000 μm 2 The steel plate is characterized in that, in GDS measurement, the ratio X / Y of the average C concentration X in the depth range to the bulk C concentration Y is 0.20 to 0.50, and it has a Vickers hardness of 300 Hv or more. (2) 0.3 μm 2 The number density of Mn-enriched regions with the above area is 20 particles / 1000 μm 2The steel sheet according to (1) above, characterized by the above. (3) The steel sheet according to (1) or (2) above, characterized by having a tensile strength of 980 MPa or more. (4) The steel sheet according to any one of (1) to (3) above, characterized by having a hot-dip zinc coating layer or an alloyed hot-dip zinc coating layer on at least one surface. (5) An automotive part, characterized by including the steel sheet according to any one of (1) to (4) above. (6) (A) A first heat treatment step including heating a steel sheet having the chemical composition according to (1) above to perform soaking heat treatment and then cooling, satisfying the following conditions (A1) to (A3), pH₂O: partial pressure of water vapor pH₂: partial pressure of hydrogen (A2) The soaking heat treatment includes retaining the steel sheet in the temperature range of Ac3 - 10°C to 950°C for 50 to 200 seconds, and the atmosphere in the temperature range of Ac3 - 10°C to 950°C satisfies the following formula 2, and (A3) The cooling includes cooling the steel sheet to room temperature, and the average cooling rate in the temperature range of 450 to 700°C is 50°C / second or more. (B) A skin pass rolling step of performing rolling with a equivalent plastic strain of 0.005 to 0.050 on the steel sheet after the first heat treatment step, and (C) A second heat treatment step including heating and then cooling the steel sheet after the skin pass rolling step, satisfying the following conditions (C1) and (C2) (C1) The heating includes heating the steel sheet from room temperature to a maximum heating temperature of Ac1 + 30°C to Ac3 - 30°C, the soaking heat treatment includes retaining the steel sheet in the temperature range of Ac1 + 30°C to Ac3 - 30°C for 50 to 200 seconds, the atmosphere at 1°C or higher of Ac1 in the heating and soaking heat treatment satisfies the above formula 1, and the heating satisfies the following formulas 3 to 6, and T(t): Temperature (K) at time t t: Elapsed time in one-second increments (seconds) when the time when the steel plate temperature exceeds Ac1 is set to 0 tf: Elapsed time (seconds) when the steel plate temperature first exceeds Ac1 + 30°C (C2) A method for manufacturing a steel plate according to any one of (1) to (5) above, characterized in that the holding time in the temperature range of 300 to 500°C is 50 to 500 seconds.

[0010] According to the present invention, it is possible to provide a steel sheet that has high strength, improved elongation, and excellent bendability after processing, as well as a method for manufacturing the same.

[0011] <Steel Plate> The steel plate according to the embodiment of the present invention has the following composition in mass%, C: 0.12-0.40%, Si: 0.60-2.50%, Mn: 1.00-4.00%, P: 0.050% or less, S: 0.0100% or less, Al: 0.001-1.500%, N: 0.0100% or less, O: 0.0100% or less, Cr: 0-1.00%, Mo: 0-1.00%, Cu: 0-1.00%, Ni: 0-1.00%, Co: 0-1.00%, W: 0-1.00%, B: 0-0.0050%, Ta: 0-1.000%, Sn: 0-1.000%, Sb: 0-0.500% The chemical composition consists of Ti: 0-0.100%, Nb: 0-0.200%, V: 0-1.00%, As: 0-0.100%, Zn: 0-1.000%, Ca: 0-0.0100%, Mg: 0-0.0100%, Zr: 0-0.0100%, Hf: 0-0.0100%, Sr: 0-0.0100%, Bi: 0-0.0100%, REM: 0-0.0100%, and the remainder being Fe and impurities. The microstructure at a thickness of 1 / 4 of the surface contains, by area percentage, 8-30% retained austenite. When the Mn concentration in the depth range of 10 to 30 μm from the surface is measured by EPMA, and the region having a Mn concentration of 1.5 times or more the average Mn concentration in the said depth range is defined as the Mn-enriched region, then 0.3 μm 2 The number density of Mn-enriched regions with the above area is 10 particles / 1000 μm 2As described above, in the GDS measurement, the ratio X / Y of the average C concentration X in the depth range to the bulk C concentration Y is 0.20 to 0.50, and it is characterized by having a Vickers hardness of 300 Hv or more.

[0012] As described above, when the strength of the steel sheet is increased, properties such as elongation generally decrease. Therefore, first, the present inventor optimized the chemical composition and steel structure of the steel sheet, particularly controlled the steel structure to have a Vickers hardness of 300 Hv or more, and configured the steel structure to contain a predetermined amount of retained austenite. More specifically, by configuring the steel structure of the steel sheet to contain 8 to 30% of retained austenite in area%, while achieving high strength of the steel sheet, for example, high strength with a tensile strength of 980 MPa or more, the elongation of the steel sheet can be significantly improved.

[0013] On the other hand, steel sheets used as materials for automobile parts are required to exhibit high impact resistance characteristics after being processed into parts by press forming or the like in order to be difficult to break even when subjected to impacts such as automobile collisions. In this regard, as described above, in order to obtain excellent impact resistance characteristics, it is necessary to enhance the impact energy absorption ability. Considering the deformation mode during a collision, among the deformation abilities, particularly bendability, more specifically, enhancing the bendability after processing, that is, after receiving strain, is important.

[0014] Therefore, the present inventor focused on the surface layer of the steel sheet in particular to improve the bendability after processing. As a result, the present inventor found that by appropriately decarburizing the surface layer of the steel sheet, more specifically, controlling the ratio X / Y of the average C concentration X in the depth range of 10 to 30 μm from the surface of the steel sheet to the bulk C concentration Y within the range of 0.20 to 0.50 in the GDS measurement, the surface layer of the steel sheet can be softened and the bendability, particularly the bendability after processing, can be improved. Hereinafter, in this specification, unless it is clear from the context that the "surface layer of the steel sheet" includes the range from the surface of the steel sheet to a depth of 10 μm, it means the depth range of 10 to 30 μm from the surface of the steel sheet.

[0015] However, subsequent investigations by the inventors revealed that simply decarburizing the surface layer of the steel sheet may not be sufficient to adequately improve its bendability after processing. Therefore, the inventors further investigated the steel structure of the decarburized surface layer. As a result, the inventors measured the Mn concentration in the decarburized surface layer, more specifically in the depth range of 10 to 30 μm from the surface of the steel sheet, using EPMA. They defined the region having an Mn concentration of 1.5 times or more the average Mn concentration in that depth range as the Mn-enriched region, and found that 0.3 μm 2 The number density of Mn-enriched regions with the above area is 10 particles / 1000 μm 2 By controlling the process within the above range, we found that, in combination with the softening of the steel sheet surface layer due to decarburization as described earlier, the bendability after processing can be significantly improved.

[0016] Although not intended to be bound by any particular theory, in the steel sheet according to the embodiment of the present invention, it is considered that the steel structure in the surface layer portion of the steel sheet acts as follows, and the bending property after processing is improved. More specifically, in the steel sheet according to the embodiment of the present invention, as described above, since the steel structure of the steel sheet is configured to contain retained austenite: 8 to 30% in area%, it is natural that relatively a large amount of retained austenite exists in the surface layer portion of the steel sheet. Generally, when retained austenite is contained in the steel structure, it is known that the work hardening rate increases due to the TRIP (Transformation-Induced Plasticity) effect, and thus properties such as elongation are improved. Here, since Mn is an element that stabilizes austenite, it is considered that by making relatively many regions where Mn is concentrated exist in the surface layer portion of the steel sheet, the retained austenite existing in the surface layer portion can be sufficiently stabilized. In this case, even when the steel sheet is processed by press forming or the like, at least a part of the retained austenite, preferably a relatively large part of the retained austenite, is considered to remain in the steel structure after processing without transforming into a hard phase such as martensite. Therefore, in the steel sheet according to the embodiment of the present invention, by making relatively many regions where Mn is concentrated exist in the surface layer portion of the steel sheet, that is, when the Mn concentration in the depth range of 10 to 30 μm from the surface of the steel sheet is measured by EPMA, and the region having a Mn concentration of 1.5 times or more the average Mn concentration in the depth range is defined as the Mn-concentrated region, 0.3 μm 2 or more in area, the number density of the Mn-concentrated regions is 10 per 1000 μm 2 or more within the above range, it is considered that the work hardening rate in the high strain region can be increased. As a result, it is considered that the bending property of the steel sheet can be improved even after processing such as press forming. In particular, in the steel sheet according to the embodiment of the present invention, in addition to the effect of improving the work hardening rate in the high strain region caused by the presence of a predetermined amount of Mn-concentrated regions in the surface layer portion of the steel sheet containing a relatively large amount of retained austenite, in combination with the softening of the surface layer portion of the steel sheet due to decarburization, it is considered that the bending property after processing can be significantly improved.

[0017] Therefore, the steel sheet according to the embodiment of the present invention, despite having high strength due to the optimization of the chemical composition and steel structure of the steel sheet, also has improved elongation and can sufficiently enhance bendability after processing. This makes it particularly useful in the automotive field, where a high level of both high strength and formability is required, as well as high impact resistance.

[0018] The steel sheets according to embodiments of the present invention will be described in more detail below. In the following description, "%", which is the unit for the content of each element, means "mass%" unless otherwise specified. In this specification, "~", which indicates a numerical range, is used to mean that the numbers written before and after it are included as the lower limit and upper limit, respectively, unless otherwise specified.

[0019] [C: 0.12-0.40%] Carbon (C) is an essential element for ensuring the strength and hardness of steel plates. To obtain these effects fully, the C content should be 0.12% or more. The C content may be 0.14% or more, 0.16% or more, 0.18% or more, 0.20% or more, 0.22% or more, 0.24% or more, or 0.26% or more. On the other hand, if the C content is excessive, the toughness may decrease due to an excessive increase in strength, which may lead to premature fracture or a decrease in uniform elongation. For this reason, the C content should be 0.40% or less. The C content may be 0.38% or less, 0.35% or less, 0.32% or less, 0.30% or less, or 0.28% or less.

[0020] [Si: 0.60-2.50%] Silicon (Si) is an element that contributes to improved strength and formability by suppressing the formation of iron carbides and increasing the stability of retained austenite. To fully obtain these effects, the Si content should be 0.60% or more. The Si content may be 0.80% or more, 1.00% or more, 1.20% or more, or 1.40% or more. On the other hand, excessive Si content can reduce the toughness of the steel, potentially leading to premature fracture. Therefore, the Si content should be 2.50% or less. The Si content may be 2.40% or less, 2.20% or less, 2.00% or less, 1.80% or less, or 1.60% or less.

[0021] [Mn: 1.00–4.00%] Manganese (Mn) is an element that enhances hardenability and contributes to improved strength and hardness. To fully obtain these effects, the Mn content should be 1.00% or more. The Mn content may be 1.20% or more, 1.40% or more, 1.60% or more, 1.80% or more, 2.00% or more, or 2.20% or more. On the other hand, excessive Mn content can lead to excessive formation of the hard phase, which can reduce toughness, causing premature fracture or reduced uniform elongation. Therefore, the Mn content should be 4.00% or less. The Mn content may be 3.80% or less, 3.60% or less, 3.40% or less, 3.20% or less, 3.00% or less, 2.80% or less, 2.60% or less, or 2.40% or less.

[0022] [P: 0.050% or less] P (phosphorus) is a solid solution strengthening element and is effective in increasing the strength of steel plates, but excessive addition may degrade weldability and toughness. Therefore, the P content should be 0.050% or less. Preferably, the P content is 0.045% or less, 0.035% or less, or 0.020% or less. The P content may be 0%, but reducing the P content to an extreme degree will increase the cost of removing P. For this reason, from an economic standpoint, the P content may be 0.0001% or more, 0.0005% or more, or 0.001% or more.

[0023] [S: 0.0100% or less] S (sulfur) is an element contained as an impurity and can form MnS in steel, degrading toughness and hole-expanding properties. Therefore, the S content should be 0.0100% or less. Preferably, the S content is 0.0050% or less, 0.0040% or less, 0.0030% or less, or 0.0020% or less. The S content may be 0%, but extremely low S content increases desulfurization costs. For this reason, from an economic standpoint, the S content may be 0.0001% or more, 0.0003% or more, or 0.0005% or more.

[0024] [Al: 0.001 to 1.500%] Al (aluminum) is an element that acts as a deoxidizing agent. To obtain this effect sufficiently, the Al content should be 0.001% or more. The Al content may be 0.010% or more, 0.020% or more, 0.030% or more, 0.040% or more, or 0.050% or more. On the other hand, if Al is included in excess, the effect will saturate, and including more Al in the steel plate than necessary will lead to an increase in manufacturing costs. Therefore, the Al content should be 1.500% or less. The Al content may be 1.400% or less, 1.200% or less, 1.000% or less, 0.800% or less, 0.600% or less, 0.300% or less, or 0.100% or less.

[0025] [N: 0.0100% or less] Nitrogen (N) is an element contained as an impurity, and if the N content is high, coarse nitrides may form in the steel, reducing its bendability and hole-expanding properties. Therefore, the N content should be 0.0100% or less. Preferably, the N content is 0.0080% or less, 0.0060% or less, or 0.0050% or less. The N content may be 0%, but reducing the N content to an extreme degree will increase the cost of removing N. For this reason, from an economic standpoint, the N content may be 0.0001% or more, 0.0003% or more, or 0.0005% or more.

[0026] [O: 0.0100% or less] O (oxygen) is an element contained as an impurity, and if the O content is high, coarse oxides may form in the steel, reducing its bendability and hole-expanding properties. Therefore, the O content should be 0.0100% or less. Preferably, the O content is 0.0080% or less, 0.0060% or less, or 0.0050% or less. The O content may be 0%, but extremely low O content increases manufacturing costs. For this reason, from the viewpoint of manufacturing costs, the O content may be 0.0001% or more, 0.0003% or more, or 0.0005% or more.

[0027] The basic chemical composition of the steel sheet according to the embodiment of the present invention is as described above. Furthermore, the steel sheet may optionally contain at least one of the following elements in place of a portion of the remaining Fe.

[0028] [Cr: 0-1.00%, Mo: 0-1.00%, Cu: 0-1.00%, Ni: 0-1.00%, Co: 0-1.00%, W: 0-1.00%, B: 0-0.0050%, Ta: 0-1.000%, Sn: 0-1.000%, Sb: 0-0.500%, Ti: 0-0.100%, Nb: 0-0.200%, and V: 0-1.00%] Cr (chromium), Mo (molybdenum), Cu (copper), Ni (nickel), Co (cobalt), W (tungsten), B (boron), Ta (tantalum), Sn (tin), Sb (antimony), Ti (titanium), Nb (niobium), and V (vanadium) are all elements that are effective in increasing the strength of steel sheets. The content of these elements may be 0%, but in order to obtain such an effect, at least one of these elements may be included in the steel sheet as needed. However, excessive content of these elements may lead to saturation of the effect and an increase in manufacturing costs. Therefore, the content of Cr, Mo, Cu, Ni, Co, W, and V may be 1.00% or less, 0.60% or less, 0.50% or less, 0.30% or less, or 0.20% or less, respectively. Similarly, the content of Ta and Sn may be 1.000% or less, 0.600% or less, 0.500% or less, 0.300% or less, or 0.200% or less. Similarly, the content of B may be 0.0050% or less, 0.0030% or less, or 0.0020% or less. Similarly, the content of Sb may be 0.500% or less, 0.300% or less, 0.100% or less, or 0.050% or less. Similarly, the Ti content may be 0.100% or less, 0.080% or less, 0.060% or less, or 0.040% or less. Similarly, the Nb content may be 0.200% or less, 0.100% or less, 0.060% or less, or 0.040% or less. For the lower limits of these elements, for example, the content of Cr, Mo, Cu, Ni, Co, W, Sn, Sb, and V may be 0.001% or more, 0.005% or more, or 0.01% or more. Similarly, the B content may be 0.0001% or more, 0.0003% or more, or 0.0005% or more. Similarly, the content of Ta, Ti, and Nb may be 0.001% or more, 0.005% or more, or 0.010% or more.

[0029] [As: 0-0.100%] Arsenic (As) is an effective element for improving corrosion resistance. The As content may be 0%, but to obtain such an effect, the As content is preferably 0.001% or more, and may be 0.005% or more, or 0.010% or more. On the other hand, if the As content is excessive, the effect will saturate, and including more As than necessary in the steel sheet will lead to an increase in manufacturing costs. Therefore, the As content should be 0.100% or less, and may be 0.050% or less, 0.030% or less, or 0.020% or less.

[0030] [Zn: 0-1.000%, Ca: 0-0.0100%, Mg: 0-0.0100%, Zr: 0-0.0100%, Hf: 0-0.0100%, Sr: 0-0.0100%, Bi: 0-0.0100%, and REM: 0-0.0100%] Zinc (Zn) is an effective element for controlling the shape of inclusions in steel, calcium (Ca), magnesium (Mg), zirconium (Zr), hafnium (Hf), strontium (Sr), and rare earth metals (REM) are elements that contribute to the fine dispersion of inclusions in steel, and bismuth (Bi) is an element that reduces the microsegregation of substitutional alloy elements such as Mn and Si in steel. The content of these elements may be 0%, but since each contributes to improving the workability of the steel sheet, at least one of these elements may be included in the steel sheet as needed. However, excessive amounts of these elements may lead to saturation of the effect and an increase in manufacturing costs. Therefore, the Zn content should be 1.000% or less, and may also be 0.500% or less, 0.200% or less, 0.100% or less, 0.050% or less, or 0.020% or less. Similarly, the Ca, Mg, Zr, Hf, Sr, Bi, and REM content should be 0.0100% or less, and may also be 0.0080% or less, 0.0060% or less, or 0.0030% or less. Regarding the lower limits for these elements, for example, the Zn content may be 0.001% or more, or 0.005% or more. Similarly, the Ca, Mg, Zr, Hf, Sr, Bi, and REM content may be 0.0001% or more, or 0.0005% or more, respectively. In this specification, REM refers to the collective term for 17 elements, including scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanides from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. The REM content is the total content of these elements.

[0031] In the steel sheet according to the embodiment of the present invention, the remainder other than the above-mentioned elements consists of Fe and impurities. Impurities are components that are mixed in during the industrial manufacture of steel sheets due to various factors in the manufacturing process, including raw materials such as ore and scrap, and components that are included in a range that does not affect the effects of the present invention.

[0032] The chemical composition of the steel sheet according to the embodiment of the present invention can be measured by general analytical methods. For example, the chemical composition of the steel sheet can be measured using inductively coupled plasma atomic emission spectroscopy (ICP-AES). C and S can be measured using the combustion-infrared absorption method, N can be measured using the inert gas fusion-thermal conductivity method, and O can be measured using the inert gas fusion-nondispersive infrared absorption method.

[0033] [Steel structure] [Retained austenite: 8-30%] In the steel sheet according to the embodiment of the present invention, the steel structure at a position 1 / 4 of the thickness from the surface of the steel sheet contains retained austenite: 8-30% by area %. Retained austenite improves the uniform elongation of the steel sheet through the TRIP effect, in which it transforms into a hard phase such as martensite by work-induced transformation during deformation of the steel sheet. In order to fully obtain such an effect, the area ratio of retained austenite is set to 8% or more. From the viewpoint of further improving uniform elongation, a higher area ratio of retained austenite is preferable, and may be, for example, 9% or more, 10% or more, 12% or more, 14% or more, 16% or more, 18% or more, or 20% or more. However, if the area ratio of retained austenite becomes too high, the area ratio of hard phases such as martensite decreases, and therefore the desired strength may not be achieved. Therefore, the area ratio of retained austenite is set to 30% or less. The area ratio of retained austenite may be 28% or less, 26% or less, 24% or less, or 22% or less.

[0034] As described above, the present invention aims to provide a steel sheet that is high in strength, yet possesses improved elongation and excellent bendability after processing. This is achieved primarily by optimizing the chemical composition of the steel sheet, controlling the steel structure of the steel sheet so that its Vickers hardness is 300 Hv or higher, and configuring it to contain 8-30% retained austenite by area percentage. Therefore, the remaining structural elements other than retained austenite in the steel sheet's structure are not particularly limited as long as the Vickers hardness is 300 Hv or higher, and are clearly not essential technical features for achieving the objectives of the present invention. Accordingly, in a steel sheet according to an embodiment of the present invention, the remaining structural elements other than retained austenite can be appropriately selected within the range in which the Vickers hardness of the steel sheet is 300 Hv or higher.

[0035] [Identification of Steel Microstructure and Calculation of Area Ratio] The area ratio of retained austenite is measured by X-ray diffraction. Specifically, the steel plate is polished mechanically and chemically from the surface to a depth of 1 / 4 in the thickness direction. Then, using MoKα1 rays as characteristic X-rays on the polished sample, the integral intensity ratio of the diffraction peaks of the bcc phase (200) and (211) and the fcc phase (200), (220), and (311) is used to calculate the microstructure fraction of retained austenite, which is then determined as the area ratio of retained austenite.

[0036] [0.3 μm in the surface layer] 2 Number density of Mn-enriched regions with the above area: 10 particles / 1000 μm 2 [End] In the steel plate according to the embodiment of the present invention, the Mn concentration in a depth range of 10 to 30 μm from the surface of the steel plate is measured by EPMA, and when the region having a Mn concentration of 1.5 times or more the average Mn concentration in said depth range is defined as the Mn enrichment region, then 0.3 μm 2 The number density of Mn-enriched regions with the above area is 10 particles / 1000 μm 2 It is controlled to the above extent. 0.3 μm in the surface layer of the steel plate 2By controlling the number density of the Mn-enriched regions having the above area to within this range, as explained earlier, it becomes possible to sufficiently stabilize the retained austenite present in the surface layer. As a result, even when the steel sheet is processed by press forming or the like, at least a portion of the retained austenite present in the surface layer, preferably a relatively large portion of the retained austenite, remains in the steel structure after processing without transforming into a hard phase such as martensite. Therefore, the work hardening rate in the high-strain region can be increased, and the combination of this effect of improving the work hardening rate in the high-strain region due to the presence of such Mn-enriched regions in the surface layer of the steel sheet, and the softening of the surface layer of the steel sheet by decarburization, which will be explained later, makes it possible to significantly improve the bendability after processing.

[0037] The flexibility of a steel sheet is largely determined by the microstructure of the steel sheet closer to the surface. For this reason, in the embodiment of the present invention, the microstructure is defined up to a depth of 30 μm from the surface of the steel sheet. However, it is relatively difficult to accurately measure the number density of Mn-enriched regions in the microstructure up to a depth of 10 μm from the surface with high reproducibility using EPMA. Therefore, in the embodiment of the present invention, the Mn concentration in the depth range of 10 to 30 μm from the surface of the steel sheet is measured by EPMA, and the Mn-enriched region obtained therefrom is 0.3 μm 2 The number density of Mn-enriched regions with the above area is 10 particles / 1000 μm, as described above. 2 The above controls the concentration. For the same reason, the carbon concentration in the surface layer, which will be explained in more detail later, is measured by EPMA in a depth range of 10 to 30 μm from the surface of the steel plate.

[0038] From the viewpoint of further improving the bendability after processing, the 0.3 μm layer of the surface of the steel plate 2 A higher number density of Mn-enriched regions having the above area is preferable, for example, 15 particles / 1000 μm 2 Above, 20 pieces / 1000μm 2 Above, 25 pieces / 1000μm 2 Above, 30 pieces / 1000μm 2 More than or equal to 35 particles / 1000 μm 2The above is also acceptable. The upper limit is not particularly limited, but for example, 0.3 μm in the surface layer of the steel plate. 2 The number density of Mn-enriched regions with the above area is 200 particles / 1000 μm 2 Below, 100 pieces / 1000μm 2 The following, or 80 particles / 1000 μm 2 The following is also acceptable.

[0039] [0.3 μm in the surface layer] 2 Method for determining the number density of Mn-enriched regions having the above area: 0.3 μm in the depth range of 10 to 30 μm from the surface of the steel plate. 2 The number density of Mn-enriched regions having the above area is determined using an FE-EPMA (electron emission electron beam microanalyzer) as follows. First, a sample is taken from the thickness cross section perpendicular to the surface of the steel plate, and the observation surface is mechanically polished to a mirror finish. A total of 100,000 measurements are taken at 0.10 μm intervals in a rectangular region containing a depth range of 20 μm in the thickness direction of the sample and a width of 50 μm in the width direction. The Mn concentration at each measurement point is measured, and the arithmetic mean of the Mn concentrations at nine measurement points—the Mn concentrations at the original measurement point plus the Mn concentrations at the eight adjacent measurement points in the thickness direction, width direction, and intermediate directions (diagonal directions)—is determined as the Mn concentration at the original measurement point. Next, the area of ​​the region (Mn-enriched region) where the Mn concentration at each measurement point is 1.5 times or more the average Mn concentration of the entire rectangular region (the arithmetic mean of the Mn concentrations of the first 100,000 points measured) is measured by binarization using image processing software. Finally, 0.3 μm 2 The number of Mn-enriched regions with the above area was counted, and the obtained value was set to 0.3 μm. 2 Number density of Mn-enriched regions with the above area (number of particles / 1000 μm 2 This value is determined as follows: Here, the area of ​​one measurement point pixel is 0.01 μm ２ The area is defined as (0.1 μm × 0.1 μm), and the region where 30 or more measurement points with a Mn concentration of 1.5 times or more the average Mn concentration are connected is defined as 0.3 μm. ２A Mn-enriched region having the above area is defined. Linking is defined as a connection between two points if, for a given measurement point where the Mn concentration is 1.5 times or more the average Mn concentration, any one of the eight pixels in the surrounding area (up, down, left, right, or diagonally) contains a pixel with a Mn concentration of 1.5 times or more. For measurement, for example, JEOL's JXA-8530F is used, with an acceleration voltage of 15 kV. The characteristic X-ray spectroscopy method is wavelength-dispersive. Since the element to be analyzed is Mn, LiF is used as the spectroscopic crystal. The output Mn concentration is a value converted from the detection intensity of characteristic X-rays to mass percent using the program included with the JXA-8530F, assuming calibration with standard materials.

[0040] [Ratio X / Y of average C concentration X in the surface layer to bulk C concentration Y: 0.20 to 0.50] In the steel sheet according to the embodiment of the present invention, the ratio X / Y of the average C concentration X in the depth range of 10 to 30 μm from the surface of the steel sheet to the bulk C concentration Y is controlled to 0.20 to 0.50 in high-frequency glow discharge emission spectrometry (GDS) measurement. By controlling the ratio X / Y of the average C concentration X in the surface layer of the steel sheet to the bulk C concentration Y to 0.20 to 0.50, the surface layer of the steel sheet can be softened to improve its bendability, particularly its bendability after processing. Combined with the effect of improving the work hardening rate in the high-strain region due to the presence of the Mn-enriched region in the surface layer of the steel sheet described above, it becomes possible to significantly improve the bendability after processing.

[0041] If the ratio X / Y is less than 0.20, the strength of the steel sheet may decrease due to excessive decarburization. Therefore, the ratio X / Y should be 0.20 or higher, preferably 0.22 or higher, 0.24 or higher, 0.26 or higher, 0.28 or higher, 0.30 or higher, 0.32 or higher, or 0.34 or higher. On the other hand, if the ratio X / Y is greater than 0.50, the softening of the surface layer of the steel sheet may not be sufficient, and the desired bendability after processing may not be achieved. Therefore, the ratio X / Y should be 0.50 or lower, preferably 0.48 or lower, 0.46 or lower, 0.44 or lower, 0.42 or lower, 0.40 or lower, 0.38 or lower, or 0.36 or lower.

[0042] [Method for Determining the Ratio X / Y of Average C Concentration X at the Surface and Bulk C Concentration Y] The ratio X / Y of the average C concentration X and bulk C concentration Y in the depth range of 10 to 30 μm from the surface of the steel plate is measured using radiofrequency glow discharge emission spectrometry (GDS). Specifically, the surface of the steel plate to be measured is placed in an Ar atmosphere, and a voltage is applied to generate a glow plasma, and the elemental concentration profile in the depth direction is measured by sputtering the surface of the steel plate. Then, the elements contained in the steel plate are identified from the emission spectral wavelengths of the elements emitted when atoms are excited in the glow plasma, and the emission intensity of the identified elements at each measurement time is estimated. The relationship between measurement time and measurement depth can be calculated from the sputtering time and the depth of the sputtered mark after measurement is complete. That is, the measurement depth at each measurement time can be estimated as sputtered mark depth × (each measurement time / total measurement time). The sputtering time is set so that the sputtered mark depth is at least 30 μm or more. In the case of plated steel sheets, the depth of the sputter marks is set to exceed the plating thickness + 30 μm. The depth of the sputter marks may be measured from the surface using a microscope or by observing the cross-section. Let X be the arithmetic mean of the C concentration in the depth range of 10 to 30 μm from the surface of the steel sheet. In the case of Zn-based plated steel sheets such as hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets, the depth at which the Zn concentration falls below 0.5 mass% is set to 0 μm depth, and the arithmetic mean of the C concentration in the range of 10 to 30 μm is determined as the average C concentration X in the depth range of 10 to 30 μm from the surface of the steel sheet. The bulk C concentration Y is also measured by GDS in the same way. Using a steel sheet with one side reduced in thickness by 1 / 4 of the sheet thickness by mechanical grinding as a sample, GDS measurement is performed on the ground surface to a depth range of at least 30 μm. To confirm that measurements were taken to a depth range of 30 μm or more, the sputtering depth of the sample after measurement is measured using the method described above, and the time is converted to depth. Since the vicinity of the outermost surface is affected by contamination, the arithmetic mean of the C concentration in the depth range of 10 to 30 μm is determined as the bulk C concentration Y. A commercially available analytical instrument can be used for high-frequency GDS analysis. In this embodiment, for example, a high-frequency glow discharge emission spectrometer GD-Profiler 2 (registered trademark) manufactured by Horiba, Ltd. is used.The detection pitch will be 0.1 seconds. Other measurement conditions will be Ar gas pressure: 600 Pa, anode diameter: 4 mmφ, RF output: 35 W.

[0043] [Vickers hardness: 300 Hv or higher] The steel sheet according to the embodiment of the present invention has a Vickers hardness of 300 Hv or higher. Preferably, the Vickers hardness is 310 Hv or higher, 330 Hv or higher, 350 Hv or higher, 360 Hv or higher, or 380 Hv or higher. According to the embodiment of the present invention, by having a steel structure with a Vickers hardness of 300 Hv or higher, and further configuring the steel structure to contain retained austenite: 8 to 30% by area%, it is possible to achieve both elongation and tensile strength of the steel sheet. Furthermore, by combining the effect of improving the work hardening rate in the high strain region due to the presence of a Mn-enriched region in the surface layer of the steel sheet and the softening of the surface layer of the steel sheet due to decarburization, it is possible to significantly improve the bendability after processing. The upper limit of the Vickers hardness is not particularly limited, but for example, the Vickers hardness may be 600 HV or less, 580 HV or less, 550 HV or less, or 520 HV or less.

[0044] [Measurement of Vickers Hardness] Vickers hardness is determined as follows. First, a test piece is cut from any position except the edge of the steel plate so that a cross section perpendicular to the surface (thickness cross section) can be observed. The thickness cross section of the test piece is polished using #600 to #1500 silicon carbide sandpaper, and then polished to a mirror finish using a liquid in which diamond powder with a particle size of 1 to 6 μm is dispersed in a diluent such as alcohol or pure water, and this thickness cross section is used as the measurement surface. Next, the Vickers hardness is measured using a micro Vickers hardness tester with a load of 500 gf at intervals of at least three times the indentation. Specifically, a total of 20 points are randomly measured around the 1 / 4 position of the steel plate thickness, and the arithmetic mean of these measurements is determined as the Vickers hardness of the steel plate.

[0045] [Plate Thickness] The steel plate according to the embodiment of the present invention is not particularly limited, but generally has a plate thickness of 0.6 to 8.0 mm. For example, the plate thickness may be 1.0 mm or more, 1.2 mm or more, 1.4 mm or more, 1.6 mm or more, or 2.0 mm or more, and / or 7.0 mm or less, 6.0 mm or less, 5.0 mm or less, 4.0 mm or less, 3.0 mm or less, or 2.5 mm or less.

[0046] [Plating Layer] The steel sheet according to the embodiment of the present invention may be a plated steel sheet having a plating layer on at least one surface, preferably both surfaces. The plating layer is not particularly limited, but may be, for example, a hot-dip galvanized layer (GI), an alloyed hot-dip galvanized layer (GA), or an electroplated galvanized layer (EG). These zinc plating layers may have any composition known to those skilled in the art, and may contain additive elements such as Al and Mg in addition to Zn. Furthermore, the amount of the plating layer is not particularly limited and may be a general amount.

[0047] As described above, the steel sheet according to the embodiment of the present invention optimizes the chemical composition and structure of the steel sheet, particularly controlling the structure to have a Vickers hardness of 300 Hv or higher, and configuring the structure to contain 8-30% retained austenite by area percentage. This achieves high strength, more specifically 980 MPa or higher, while significantly improving the elongation of the steel sheet. Furthermore, by appropriately controlling the Mn and C concentrations in the surface layer of the steel sheet, the bendability after processing can also be significantly improved. Therefore, the steel sheet according to the embodiment of the present invention is particularly useful for use in parts in technical fields that require a high level of both high strength and formability, and also require high impact resistance, and is especially useful for use in automotive parts. In a preferred embodiment, an automotive part (including parts used not only in passenger cars but also in large vehicles such as trucks) containing the steel sheet according to the embodiment of the present invention is provided. Examples of automotive parts include structural parts such as front pillars, center pillars, side sills, and cross members, as well as bumpers, and other structural and reinforcing parts that require strength. These parts only need to include a steel sheet according to an embodiment of the present invention in at least a portion of them, and therefore at least a portion of these parts satisfy the characteristics of the steel sheet described above. In parts of the steel sheet that do not come into direct contact with the mold during forming such as press forming, or that come into direct contact with the mold but undergo relatively little processing, the characteristics of the steel sheet do not change particularly before and after forming.

[0048] [Mechanical Properties] [Tensile Strength (TS) and Uniform Elongation (uEL)] According to the steel sheet having the above chemical composition and steel structure, high tensile strength, specifically a tensile strength (TS) of 980 MPa or more, can be achieved. The tensile strength is preferably 1080 MPa or more, 1180 MPa or more, or 1200 MPa or more. There is no particular upper limit, but for example, the tensile strength may be 1800 MPa or less, 1600 MPa or less, or 1400 MPa or less. Furthermore, according to the steel sheet according to the embodiment of the present invention, despite having such very high tensile strength, improved elongation, specifically a uniform elongation (uEL) of 12.0% or more, can be achieved. For example, the uniform elongation may be 13.0% or more, 14.0% or more, 15.0% or more, or 16.0% or more. For example, when the tensile strength is 980 MPa or more, a uniform elongation of 15.0% or more can be achieved. Similarly, for example, when the tensile strength is 1180 MPa or more, a uniform elongation of 12.0% or more can be achieved. There is no particular upper limit to the uniform elongation, but for example, the uniform elongation may be 25.0% or less, 22.0% or less, or 20.0% or less. Tensile strength and uniform elongation are determined by taking a JIS No. 5 test specimen from a direction (C direction) where the longitudinal direction of the test specimen is preferably parallel to the direction perpendicular to the rolling direction of the steel sheet, and performing a tensile test in accordance with JIS Z 2241:2022. If the rolling direction of the steel sheet cannot be determined, a JIS No. 5 test specimen may be taken from any direction within the surface of the steel sheet. If it is difficult to take a JIS No. 5 test specimen, a JIS No. 13B test specimen may be used, or a small test specimen with a similar shape to a JIS No. 13B test specimen may be used.

[0049] <Method for Manufacturing Steel Sheets> Next, preferred methods for manufacturing steel sheets according to embodiments of the present invention will be described. The following description is intended to illustrate characteristic methods for manufacturing steel sheets according to embodiments of the present invention, and is not intended to limit the steel sheets to those manufactured by the manufacturing methods described below.

[0050] A method for manufacturing a steel sheet according to an embodiment of the present invention includes (A) a first heat treatment step that includes raising the temperature of a steel sheet having the chemical composition described above, soaking it, and then cooling it, and satisfying the following conditions (A1) to (A3): (A1) the raising of temperature includes heating the steel sheet from room temperature to a maximum heating temperature of Ac3-10°C to 950°C, the atmosphere in the temperature range of Ac1 to Ac3-30°C satisfies the following formula 1, and the residence time in the temperature range of Ac1 to Ac3-30°C is 50 to 200 seconds. pH2O: partial pressure of water vapor pH2: partial pressure of hydrogen (A2) The soaking treatment includes leaving the steel plate in the temperature range of Ac3-10°C to 950°C for 50 to 200 seconds, and the atmosphere in the temperature range of Ac3-10°C to 950°C satisfies the following formula 2, and (A3) The cooling includes cooling the steel sheet to room temperature, and the average cooling rate in the temperature range of 450 to 700°C is 50°C / second or more. (B) A skin pass rolling step in which the steel sheet after the first heat treatment step is rolled to an equivalent plastic strain of 0.005 to 0.050. (C) A second heat treatment step that includes raising the temperature of the steel sheet after the skin pass rolling step, soaking it in heat, and then cooling it, and satisfies the following conditions (C1) and (C2): (C1) The raising of temperature includes heating the steel sheet from room temperature to a maximum heating temperature of Ac1 + 30°C to Ac3 - 30°C, the soaking in heat includes leaving the steel sheet in the temperature range of Ac1 + 30°C to Ac3 - 30°C for 50 to 200 seconds, the atmosphere at Ac1°C or higher in the raising of temperature and soaking in heat satisfies formula 1 above, and the raising of temperature satisfies the following formulas 3 to 6, T(t): Temperature (K) at time t t: Elapsed time in one second, with the time when the steel plate temperature exceeded Ac1 being set as 0 tf: Elapsed time in seconds when the steel plate temperature first exceeded Ac1 + 30°C (C2) is characterized by including a holding time of 50 to 500 seconds in the temperature range of 300 to 500°C. Each process will be explained in detail below.

[0051] [Manufacturing of Hot-Rolled or Cold-Rolled Steel Sheets] The steel sheet to be used in the first heat treatment process may be a hot-rolled steel sheet or a cold-rolled steel sheet. Therefore, a hot-rolled or cold-rolled steel sheet is manufactured prior to the first heat treatment process. The method for manufacturing a hot-rolled or cold-rolled steel sheet is not particularly limited, and any suitable method known to those skilled in the art can be applied. For example, a hot-rolled steel sheet can be obtained by first casting molten steel using a continuous casting method or the like to form a slab, then directly or after cooling the formed slab and reheating it, and then hot-rolling it. When reheating is performed, it is preferable to heat the slab to, for example, 1200°C or higher and hold it for 20 minutes or more. Hot rolling is usually carried out by rough rolling and finish rolling. The temperature and reduction ratio of each rolling can be appropriately determined according to the desired steel structure and sheet thickness. For example, the finishing rolling temperature may be 800 to 1000°C, the total reduction ratio in rough rolling may be 75 to 90%, and the total reduction ratio in finishing rolling may be 60 to 95%. The rolling method may be a reverse type in which one stand repeatedly moves back and forth, or a tandem type consisting of multiple stands arranged in series, and the reduction ratio at each stand may be 10 to 50%.

[0052] Hot-rolled steel sheets can be wound at a predetermined temperature. The winding temperature can be appropriately determined according to the desired steel structure, etc., and may be, for example, 400 to 700°C. After winding, the hot-rolled steel sheets may be pickled, and then optionally cold-rolled. The reduction ratio for cold rolling can be appropriately determined according to the desired steel structure and sheet thickness, and may be, for example, 30 to 80%. After the cold-rolling process, the sheets may be cooled to room temperature by air cooling, for example.

[0053] [(A) First Heat Treatment Process] First, a steel sheet having the chemical composition described above, more specifically a hot-rolled steel sheet or a cold-rolled steel sheet, is heated and soaked in a first heat treatment process, and then cooled. The first heat treatment process must satisfy the following conditions (A1) to (A3). (A1) The heating includes heating the steel sheet from room temperature to a maximum heating temperature of Ac3-10°C to 950°C, the atmosphere in the temperature range of Ac1 to Ac3-30°C satisfies the following formula 1, and the residence time in the temperature range of Ac1 to Ac3-30°C is 50 to 200 seconds. pH2O: partial pressure of water vapor pH2: partial pressure of hydrogen (A2) The soaking treatment includes leaving the steel plate in the temperature range of Ac3-10°C to 950°C for 50 to 200 seconds, and the atmosphere in the temperature range of Ac3-10°C to 950°C satisfies the following formula 2, and (A3) The cooling includes cooling the steel plate to room temperature, and the average cooling rate in the temperature range of 450 to 700°C is 50°C / second or more.

[0054] [(A1) Heating from room temperature to the maximum heating temperature of Ac3-10°C to 950°C, oxygen potential in Ac1 to Ac3-30°C: greater than -1.50 and less than -0.20, and residence time in Ac1 to Ac3-30°C: 50 to 200 seconds] In the first heat treatment step, the steel plate is first heated, specifically by heating the steel plate from room temperature to the maximum heating temperature of Ac3-10°C to 950°C, so that the steel structure of the steel plate is sufficiently austenitized. During heating, the atmosphere in the temperature range of Ac1 to Ac3-30°C satisfies the following equation 1, and the residence time in this temperature range is controlled to be 50 to 200 seconds. pH2O: Partial pressure of water vapor; pH2: Partial pressure of hydrogen

[0055] If the maximum heating temperature is lower than Ac3-10°C, it may not be possible to sufficiently austenitize the steel structure in the surface layer. In such cases, the distribution of Mn to austenite in the surface layer is not promoted in the second heat treatment step, which will be explained in detail later, and the surface layer of the final steel sheet will have a thickness of 0.3 μm. 2If the area exceeds the specified size, it becomes impossible to achieve the desired number density of Mn-enriched regions. The middle side of equation 1, i.e., log(pH2O / pH2), is also called the oxygen potential, and the larger this value, the more the decarburization of the surface layer of the steel sheet can be advanced. In the first heat treatment step, by controlling the oxygen potential in the temperature range of Ac1 to Ac3-30°C to satisfy equation 1, and controlling the residence time in that temperature range to within the range of 50 to 200 seconds, it becomes possible to appropriately decarburize the surface layer of the steel sheet. As a result, in the GDS measurement of the finally obtained steel sheet, it becomes possible to control the ratio X / Y of the average C concentration X in the depth range of 10 to 30 μm from the surface of the steel sheet to the bulk C concentration Y within the range of 0.20 to 0.50. If the oxygen potential in the temperature range of Ac1 to Ac3-30°C is -1.50 or less and / or the residence time in that temperature range is less than 50 seconds, decarburization cannot proceed sufficiently, and the ratio X / Y value in the final steel sheet may exceed 0.50. In such cases, the softening of the surface layer of the steel sheet is insufficient, making it impossible to achieve the desired bendability after processing. For this reason, the oxygen potential in the temperature range of Ac1 to Ac3-30°C should be greater than -1.50, preferably -1.45 or higher. Similarly, the residence time in that temperature range should be 50 seconds or more, preferably 80 seconds or more.

[0056] On the other hand, if the oxygen potential in the temperature range of Ac1 to Ac3 -30°C is -0.20 or higher and / or the residence time in that temperature range exceeds 200 seconds, decarburization may proceed too much, and the ratio X / Y value in the final steel sheet may be less than 0.20. In such cases, the strength of the steel sheet may decrease due to excessive softening of the surface layer of the steel sheet. For this reason, the oxygen potential in the temperature range of Ac1 to Ac3 -30°C should be less than -0.20, preferably -0.15 or lower. Similarly, the residence time in that temperature range should be 200 seconds or less, preferably 150 seconds or less. In this manufacturing method, Ac1 and Ac3 (°C) are calculated based on the following formulas 8 and 9, respectively. In the following formulas 7 and 8, the mass % of the element should be substituted for the element symbol. For elements that are not present, 0 mass % should be substituted. Ac1 (°C) = 727-32.7×C+14.9×Si+2.0×Mn-17.0×Cu-14.2×Ni+17.8×Cr+25.6×Mo...Formula 7 Ac3(°C)=912-230.5×C+31.6×Si-20.4×Mn-39.8×Cu-18.1×Ni-14.8×Cr+16.8×Mo+100×Al...Formula 8

[0057] [(A2) Oxygen potential in the Ac3-10°C to 950°C range: greater than -5.00 and less than -1.50, and residence time in the Ac3-10°C to 950°C range: 50 to 200 seconds] Next, the steel plate is left in the Ac3-10°C to 950°C temperature range for 50 to 200 seconds, and the atmosphere in this temperature range is controlled to satisfy the following equation 2. pH2O: Partial pressure of water vapor; pH2: Partial pressure of hydrogen

[0058] By controlling the residence time in the temperature range of 50 to 200 seconds while satisfying the oxygen potential in the middle of the above equation 2, i.e., the temperature range of Ac3-10°C to 950°C, it is possible to completely austenitize the steel structure in the surface layer of the steel sheet while suppressing further decarburization, that is, while maintaining the low carbon concentration state in the surface layer of the steel sheet created under the conditions of (A1). In order to make the oxygen potential in the temperature range of Ac3-10°C to 950°C below -5.00, it becomes necessary to excessively reduce the water vapor partial pressure in that temperature range, which reduces productivity. Therefore, the oxygen potential in the temperature range of Ac3-10°C to 950°C should be greater than -5.00, preferably -4.00 or higher. Also, if the residence time in the temperature range of Ac3-10°C to 950°C is less than 50 seconds, it may not be possible to completely austenitize the steel structure in the surface layer. In such cases, the distribution of Mn to austenite in the surface layer is not promoted in the second heat treatment process, which will be explained in detail later, and a 0.3 μm layer is formed in the surface layer of the final steel sheet. 2 If the desired number density of Mn-enriched regions with the above area is not achieved, it becomes impossible to obtain the desired number density. For this reason, the residence time in the temperature range of Ac3-10°C to 950°C is set to 50 seconds or more, preferably 80 seconds or more.

[0059] On the other hand, if the oxygen potential in the temperature range of Ac3 -10°C to 950°C is -1.50 or higher and / or the residence time in that temperature range exceeds 200 seconds, the progress of decarburization cannot be sufficiently suppressed, and excessive ferrite may be formed in the surface layer during the subsequent cooling treatment. In such cases, the distribution of Mn to austenite in the surface layer is not promoted in the second heat treatment step, which will be explained in detail later, and a 0.3 μm layer is formed in the surface layer of the final steel sheet. 2 If the area exceeds the specified size, it becomes impossible to achieve the desired number density of Mn-enriched regions. For this reason, the oxygen potential in the temperature range of Ac3-10°C to 950°C should be less than -1.50, preferably -1.45 or less. Similarly, the residence time in this temperature range should be 200 seconds or less, preferably 150 seconds or less.

[0060] [(A3) Average cooling rate in the 450-700°C temperature range: 50°C / second or more] Next, the steel sheet is cooled to room temperature, and the average cooling rate in the 450-700°C temperature range is controlled to 50°C / second or more. This suppresses the formation of ferrite in the low-carbon concentration surface layer, and the surface layer can be composed of low-carbon concentration bainite and / or martensite. Here, the low-carbon concentration bainite and / or martensite in the surface layer is reverse-transformed into austenite in the subsequent second heat treatment step. In this second heat treatment step, by promoting the distribution of Mn to this reverse-transformed austenite, the surface layer of the final steel sheet has a thickness of 0.3 μm. 2 The desired number density of Mn-enriched regions having the above area is 10 particles / 1000 μm. 2 The above can be achieved. Therefore, in this manufacturing method, it is very important to satisfy conditions (A1) to (A3) in the first heat treatment step so that the surface layer is composed of bainite and / or martensite with a low carbon concentration.

[0061] If the average cooling rate in the temperature range of 450-700°C is less than 50°C / second, it may not be possible to sufficiently suppress the formation of ferrite in the low-carbon concentration surface layer. In such cases, the surface layer of the final steel sheet may have a ferrite layer of 0.3 μm. 2 If the area exceeds the specified limit, it becomes impossible to achieve the desired number density of Mn-enriched regions. Therefore, the average cooling rate in the 450-700°C temperature range should be 50°C / second or higher, preferably 55°C / second or higher. The upper limit is not particularly limited, but for example, the average cooling rate in the 450-700°C temperature range may be 150°C / second or lower, or 100°C / second or lower.

[0062] [(B) Skin Pass Rolling Process] After the first heat treatment process, the steel sheet is subjected to skin pass rolling in the next skin pass rolling process so that the equivalent plastic strain is 0.005 to 0.050. By subjecting the steel sheet to skin pass rolling under these conditions and introducing appropriate strain to the surface layer of the steel sheet, a driving force for Mn distribution to austenite can be provided in the next second heat treatment process. As a result, the distribution of Mn to austenite can be promoted in the second heat treatment process, and the surface layer of the final steel sheet has a thickness of 0.3 μm. 2 The desired number density of Mn-enriched regions having the above area is 10 particles / 1000 μm. 2 It becomes possible to achieve the above.

[0063] If the equivalent plastic strain is less than 0.005, sufficient strain cannot be introduced into the surface layer of the steel sheet, making it impossible to provide the driving force for Mn distribution to the austenite in the subsequent second heat treatment process. As a result, the distribution of Mn to the austenite cannot be promoted in the second heat treatment process, and the surface layer of the final steel sheet has a thickness of 0.3 μm. 2 If the Mn-enriched region has an area greater than the above, it becomes impossible to achieve the desired number density. Therefore, the equivalent plastic strain should be 0.005 or higher, preferably 0.008 or higher. On the other hand, if the equivalent plastic strain exceeds 0.050, it also becomes difficult to achieve the desired number density of the Mn-enriched region in the surface layer. Although the details of the mechanism are not clear, it is thought that if excessive strain is introduced into the surface layer, the growth rate of austenite increases during the heating and soaking process of the second heat treatment step. As a result, austenite grows faster than the diffusion of Mn from ferrite to austenite, making it difficult for Mn distribution to occur. Therefore, the equivalent plastic strain should be 0.050 or lower, preferably 0.040 or lower. In this manufacturing method, equivalent plastic strain is calculated using the following formula, where the plate thickness, plate width, and plate length before skin pass rolling are t0 (mm), w0 (mm), and l0 (mm), respectively, and the plate thickness, plate width, and plate length after skin pass rolling are t1 (mm), w1 (mm), and l1 (mm), respectively.

[0064] [(C) Second Heat Treatment Process] The steel sheet after the skin pass rolling process is heated and soaked in the next second heat treatment process, and then cooled, and the second heat treatment process must satisfy the following conditions (C1) and (C2). (C1) The heating includes heating the steel sheet from room temperature to a maximum heating temperature of Ac1 + 30°C to Ac3 - 30°C, the soaking treatment includes keeping the steel sheet in the temperature range of Ac1 + 30°C to Ac3 - 30°C for 50 to 200 seconds, the atmosphere at Ac1°C or higher in the heating and soaking treatment satisfies the following formula 1, and the heating satisfies the following formulas 3 to 6, and pH2O: Partial pressure of water vapor; pH2: Partial pressure of hydrogen T(t): Temperature (K) at time t t: Elapsed time in one second, with the time when the steel plate temperature exceeded Ac1 being set as 0 tf: Elapsed time in seconds when the steel plate temperature first exceeded Ac1 + 30°C (C2) Residence time in the temperature range of 300 to 500°C is 50 to 500 seconds

[0065] [(C1) Heating from room temperature to the maximum heating temperature of Ac1 + 30°C to Ac3 - 30°C, residence time in the Ac1 + 30°C to Ac3 - 30°C range: 50 to 200 seconds, oxygen potential above Ac1°C: greater than -1.50 and less than -0.20, and heating that satisfies equations 3 to 6] In the second heat treatment step, first the steel plate is heated, specifically the steel plate is heated from room temperature to the maximum heating temperature of Ac1 + 30°C to Ac3 - 30°C, i.e., to the two-phase region of ferrite and austenite, and the residence time in the temperature range of Ac1 + 30°C to Ac3 - 30°C is controlled to be 50 to 200 seconds. This makes it possible to reverse the transformation of bainite and / or a portion of martensite in the surface layer formed in the first heat treatment step into austenite, and to distribute Mn to the austenite. As a result, in combination with the control of equations 3-6 during heating, which will be explained later, the surface layer of the final steel sheet has a thickness of 0.3 μm. 2 It becomes possible to achieve the desired number density of Mn-enriched regions having the above area.

[0066] If the maximum heating temperature is lower than Ac1 + 30°C, or if the residence time in the temperature range of Ac1 + 30°C to Ac3 - 30°C is less than 50 seconds, austenitization will not proceed sufficiently within the steel sheet, resulting in the retained austenite not achieving the desired area ratio or the desired Vickers hardness. On the other hand, if the maximum heating temperature is higher than Ac3 - 30°C, austenitization in the surface layer may proceed excessively, resulting in insufficient distribution of Mn to the austenite. In such cases, the surface layer of the final steel sheet may have a density of 0.3 μm. 2 If the area exceeds the specified size, it becomes impossible to achieve the desired number density in the Mn-enriched region. If the residence time in the temperature range of Ac1 + 30°C to Ac3 - 30°C exceeds 200 seconds, decarburization proceeds excessively, and the ratio X / Y of the average C concentration X in the depth range of 10 to 30 μm from the surface of the steel plate to the bulk C concentration Y, as described later, may not satisfy the desired range. Therefore, the residence time in the temperature range of Ac1 + 30°C to Ac3 - 30°C should be 50 to 200 seconds, preferably 90 to 180 seconds.

[0067] Furthermore, in the second heat treatment process, by controlling the oxygen potential in the temperature range of Ac1°C or higher during the heating and soaking process described above to satisfy the following equation 1, the low carbon concentration state in the surface layer of the steel sheet created in the first heat treatment process can be maintained. Finally, in the GDS measurement of the resulting steel sheet, the ratio X / Y of the average C concentration X in the depth range of 10 to 30 μm from the surface of the steel sheet to the bulk C concentration Y can be controlled to be within the range of 0.20 to 0.50. pH2O: Partial pressure of water vapor; pH2: Partial pressure of hydrogen

[0068] If the oxygen potential in the temperature range above Ac1°C is -1.50 or lower, carbon contained in the bulk of the steel sheet will diffuse to the surface layer, and due to this recarburization of the surface layer, the ratio X / Y value in the final steel sheet may exceed 0.50. For this reason, the oxygen potential in the temperature range above Ac1°C should be greater than -1.50, preferably -1.45 or higher. On the other hand, if the oxygen potential in the temperature range above Ac1°C is -0.20 or higher, decarburization will proceed too much, and the ratio X / Y value in the final steel sheet may fall below 0.20. In such cases, the strength of the steel sheet may decrease due to excessive softening of the surface layer. For this reason, the oxygen potential in the temperature range above Ac1°C should be less than -0.20, preferably -0.15 or lower.

[0069] Furthermore, in the second heat treatment process, when raising the temperature from room temperature to the maximum heating temperature of Ac1 + 30°C to Ac3 - 30°C, by controlling the temperature increase to satisfy the following equations 3 to 6, and in combination with the control of the residence time from Ac1 + 30°C to Ac3 - 30°C described earlier, it becomes possible to sufficiently promote the distribution of Mn to austenite. As a result, the surface layer of the final steel sheet has a thickness of 0.3 μm. 2 It becomes possible to achieve the desired number density of Mn-enriched regions having the above area. T(t): Temperature (K) at time t t: Elapsed time in one-second increments (seconds), with the time when the steel plate temperature exceeded Ac1 being set as 0 tf: Elapsed time (seconds) when the steel plate temperature first exceeded Ac1 + 30°C

[0070] Equation 3 above represents the degree of Mn enrichment in austenite. The greater the value of the left side of Equation 3 is than 0.010, the more sufficiently Mn is enriched in the austenite. To explain in more detail, f in Equation 3 above... γ represents the fraction of austenite at time t, and is expressed by equation 4 above. In equation 4 above, C represents the carbon content (mass%) of the steel plate. On the other hand, 1-f γΔMn represents the fraction of the non-austenite portion and corresponds to the fraction of ferrite. ΔMn represents the difference between the Mn concentration in austenite and the Mn concentration in ferrite, and is expressed in equation 6 above. In equation 6, Mn and Si represent the Mn and Si content (mass%) of the steel sheet, respectively. Equation 5 represents the diffusion coefficient of Mn at temperature T(t) at time t. Therefore, equation 10 below represents the degree of distribution from ferrite to austenite at time t.

[0071] Here, the distribution of Mn from ferrite to austenite proceeds particularly on the lower temperature side of the two-phase region. For this reason, in the second heat treatment step, when the steel sheet is heated from room temperature to the maximum heating temperature of Ac1 + 30°C to Ac3 - 30°C, the time when the steel sheet temperature exceeds Ac1 is set to t=0 (zero), and the degree of Mn enrichment in austenite is evaluated by accumulating the above equation 10 every second until the time tf when the steel sheet temperature first exceeds the lower limit of that temperature range, i.e., Ac1 + 30°C. As a result of repeated experiments, the inventors have found that when the value of the left side of the above equation 3 is controlled to be greater than 0.010, Mn can be sufficiently enriched in the austenite, and as a result, the surface layer of the final steel sheet has a thickness of 0.3 μm. 2 The desired number density of Mn-enriched regions having the above area is 10 particles / 1000 μm. 2 We found that the above can be achieved. 0.3 μm in the surface layer of the steel plate. 2 From the viewpoint of further increasing the number density of Mn-enriched regions having the above area, it is preferable that the value of the left side of equation 3 above be larger, for example, 0.015 or more.

[0072] [(C2) Holding time at 300-500°C: 50-500 seconds] Finally, the steel sheet is cooled after heating and soaking treatment, and the holding time in the 300-500°C temperature range during cooling is controlled to be within the range of 50-500 seconds. By holding the steel sheet in such a relatively low temperature range for a sufficient amount of time, the austenite in the steel can be stabilized, and it is possible to achieve the desired retained austenite area ratio in the final resulting steel structure. If the holding temperature is less than 300°C or the residence time in the 300-500°C temperature range is less than 50 seconds, the austenite in the steel cannot be sufficiently stabilized, and the retained austenite area ratio in the final resulting steel structure may be less than 8%. Therefore, the holding time in the 300-500°C temperature range should be 50 seconds or more, preferably 60 seconds or more. On the other hand, if the holding temperature exceeds 500°C or the holding time in the 300-500°C temperature range exceeds 500 seconds, a large portion of the austenite may decompose due to excessive heat treatment, resulting in a retained austenite area ratio of less than 8% in the final steel structure. Therefore, the residence time in the 300-500°C temperature range should be 500 seconds or less, preferably 450 seconds or less.

[0073] [Plating Treatment] When manufacturing plated steel sheets, for example, the steel sheet after the second heat treatment process may be immersed in the plating bath (approximately 460°C in the case of a Zn bath), or it may be immersed during the second heat treatment process, either after the soaking treatment or while it is cooling to room temperature. In that case, for example, it may be immersed before or after holding at 300-500°C, or it may be immersed in the middle of holding at 300-500°C. When manufacturing alloyed hot-dip galvanized steel (GA), the steel may be reheated and alloyed after immersion in the plating bath. The alloying treatment temperature may be in the range of 460-600°C. Alternatively, it may be hot-dip galvanized steel without alloying treatment (GI), or electroplated zinc (EG). These zinc platings may be carried out according to any suitable method known to those skilled in the art. Similarly, these zinc platings may have any composition known to those skilled in the art, and may contain additive elements such as Al and Mg in addition to Zn. Furthermore, the amount of these zinc platings deposited is not particularly limited and may be a general amount.

[0074] According to the steel sheet manufactured by the above manufacturing method, while controlling the Vickers hardness of the steel sheet to 300 Hv or higher, and configuring the steel structure of the steel sheet to contain 8-30% retained austenite by area percentage, it is possible to significantly improve the elongation of the steel sheet while achieving high strength, for example, a tensile strength of 980 MPa or higher. Furthermore, by a specific combination of controlled first and second heat treatment processes and skin pass rolling processes, the ratio X / Y of the average C concentration X in the depth range of 10-30 μm from the surface of the steel sheet to the bulk C concentration Y is controlled to 0.20-0.50, and when the region having a Mn concentration of 1.5 times or more the average Mn concentration in the said depth range is defined as the Mn-enriched region, then 0.3 μm 2 The number density of Mn-enriched regions with the above area is 10 particles / 1000 μm 2 The process can be controlled within the above range. As a result, the combination of softening of the steel sheet surface layer due to decarburization based on the control of the ratio X / Y, and the effect of improving the work hardening rate in the high-strain region due to the presence of a Mn-enriched region in the steel sheet surface layer, makes it possible to significantly improve the bendability after processing. Therefore, steel sheets manufactured by the above manufacturing method are particularly useful in the automotive sector, where a high level of both high strength and formability is required, as well as high impact resistance.

[0075] The present invention will be described in more detail below with reference to examples, but the present invention is not limited in any way to these examples.

[0076] In the following embodiments, steel sheets according to the present invention were manufactured under various conditions, and the tensile strength (TS), uniform elongation (uEL), and bendability after processing (after pre-straining) of the obtained steel sheets were investigated.

[0077] First, molten steel was cast using a continuous casting method to form slabs with various chemical compositions as shown in Table 1. Next, these slabs were heated to over 1200°C and held for 20 minutes or more, then hot-rolled, and the resulting hot-rolled steel sheets were wound at 500°C and cooled. Hot rolling was performed in rough rolling and finish rolling stages, with the final temperature of the finish rolling being 950°C. Next, the obtained hot-rolled steel sheets were pickled, and cold-rolled at the reduction ratios shown in Table 2, except for Examples 29 and 38. The obtained cold-rolled steel sheets and the hot-rolled steel sheets of Examples 29 and 38 were heat-treated in the first heat treatment step under the conditions shown in Table 2 and cooled to room temperature. Next, the obtained steel sheets were subjected to skin-pass rolling at the equivalent plastic strain shown in Table 2, then heat-treated in the second heat treatment step under the conditions shown in Table 2, and further alloyed hot-dip galvanizing was performed as appropriate at the alloying temperature shown in Table 2 to obtain plated steel sheets or cold-rolled steel sheets with a thickness of 1.0 to 2.0 mm. In Examples 29 and 38, plated steel sheets with a thickness of 2.3 mm were obtained by applying alloyed hot-dip galvanizing to hot-rolled steel sheets.

[0078]

[0079]

[0080]

[0081] The properties of the obtained steel plates were measured and evaluated by the following method.

[0082] [Tensile Strength (TS) and Uniform Elongation (uEL)] Tensile strength (TS) and uniform elongation (uEL) were determined by taking a JIS No. 5 test specimen from the direction in which the longitudinal direction of the test specimen is parallel to the direction perpendicular to the rolling direction of the steel plate (direction C), and performing a tensile test in accordance with JIS Z 2241:2022.

[0083] [Bendability after processing] The bendability after processing was evaluated by performing a bending test in accordance with VDA (German Association of the Automotive Industry) 238-100:2017-04 on a test specimen pre-strained by 5%. More specifically, a tensile test specimen with a parallel section width of 30 mm, a parallel section length of 60 mm, and a shoulder radius of curvature of 25 mm was taken from the direction (C direction) where the longitudinal direction of the test specimen is parallel to the direction perpendicular to the rolling direction of the steel plate. After pre-straining to a strain of 5% was applied using a tensile testing machine, a bending test specimen with a length of 60 mm was taken from the parallel section. The bending test was performed so that the longitudinal direction of the test specimen and the bending ridge direction were perpendicular, that is, so that the bending ridge direction was parallel to the rolling direction. The bending angle (°) was evaluated by converting the displacement at the maximum load obtained in the bending test into an angle according to the VDA standard.

[0084] The following cases (i) and (ii) were evaluated as steel sheets that, despite having high strength, possessed improved elongation and excellent bendability after processing. The results are shown in Table 3. (i) When TS is 980 MPa or more and less than 1180 MPa, uEL is 15.0% or more, and the bending angle after 5% pre-strain is 70° or more. (ii) When TS is 1180 MPa or more, uEL is 12.0% or more, and the bending angle after 5% pre-strain is 60° or more.

[0085]

[0086] Referring to Tables 1-3, Comparative Examples 20, 22, and 24 had low C, Si, and Mn content, respectively, which resulted in a decrease in the area fraction of retained austenite or Vickers hardness, and a decrease in TS. Comparative Examples 21, 23, and 25 had high C, Si, and Mn content, respectively, which led to an excessive increase in strength and a decrease in the toughness of the steel. As a result, the steel plate fractured before reaching the maximum stress during the tensile test (premature fracture).

[0087] In Comparative Example 41, it is believed that decarburization could not be sufficiently advanced because the first heat treatment step was not performed. As a result, the ratio X / Y value in the final steel sheet exceeded 0.50, and the bendability after processing decreased.

[0088] In Comparative Example 42, the maximum heating temperature in the first heat treatment step was low, which prevented sufficient austenitization of the steel structure in the surface layer of the steel sheet. As a result, the distribution of Mn to austenite in the surface layer was not promoted in the second heat treatment step. Consequently, the surface layer of the final steel sheet had a thickness of 0.3 μm. 2 The number density of Mn-enriched regions with the above area is 10 particles / 1000 μm 2 The result was less than [a certain value], and the bendability after processing decreased.

[0089] In Comparative Example 43, it is believed that the decarburization could not be sufficiently advanced because the residence time in the temperature range of Ac1 to Ac3-30°C during the first heat treatment process was short. As a result, the ratio X / Y value in the final steel sheet exceeded 0.50, and the bendability after processing decreased.

[0090] In Comparative Example 44, the residence time in the temperature range of Ac3-10°C to 950°C during the first heat treatment step was short, which prevented sufficient austenitization of the steel structure in the surface layer of the steel sheet. As a result, the distribution of Mn to austenite in the surface layer was not promoted during the second heat treatment step. Consequently, the surface layer of the final steel sheet showed a thickness of 0.3 μm. 2 The number density of Mn-enriched regions with the above area is 10 particles / 1000 μm 2 The result was less than [a certain value], and the bendability after processing decreased.

[0091] In Comparative Example 45, it is believed that decarburization could not be sufficiently carried out because the oxygen potential (i.e., the middle side of Equation 1) in the temperature range of Ac1 to Ac3-30°C during the first heat treatment process was -1.50 or less. As a result, the ratio X / Y value in the final steel sheet exceeded 0.50, and the bendability after processing decreased.

[0092] In Comparative Example 46, the oxygen potential (i.e., the middle side of Equation 2) in the temperature range of Ac3 -10°C to 950°C during the first heat treatment step was -1.50 or higher, which prevented sufficient suppression of decarburization. As a result, excessive ferrite was generated in the surface layer during the subsequent cooling treatment, and it is thought that the distribution of Mn to austenite in the surface layer was not promoted during the second heat treatment step. Consequently, the surface layer of the finally obtained steel sheet had a thickness of 0.3 μm. 2 The number density of Mn-enriched regions with the above area is 10 particles / 1000 μm 2 The result was less than [a certain value], and the bendability after processing decreased.

[0093] In Comparative Example 47, the average cooling rate in the temperature range of 450-700°C during the first heat treatment process was slow, which is thought to have prevented sufficient suppression of ferrite formation in the low-carbon concentration surface layer. As a result, the surface layer of the final steel sheet had a ferrite layer of 0.3 μm. 2 The number density of Mn-enriched regions with the above area is 10 particles / 1000 μm 2 The result was less than [a certain value], and the bendability after processing decreased.

[0094] In Comparative Example 48, the equivalent plastic strain in the skin pass rolling process was low, which prevented sufficient strain from being introduced into the surface layer of the steel sheet. Therefore, it is believed that the driving force for Mn distribution to austenite in the subsequent second heat treatment process could not be provided. As a result, the distribution of Mn to austenite could not be promoted in the subsequent second heat treatment process, and ultimately, the surface layer of the resulting steel sheet had a thickness of 0.3 μm. 2 The number density of Mn-enriched regions with the above area is 10 particles / 1000 μm 2 The result was less than [a certain value], and the bendability after processing decreased.

[0095] In Comparative Example 49, the equivalent plastic strain was high during the skin pass rolling process, which is thought to have caused the austenite to grow faster than the diffusion of Mn into the austenite during the subsequent second heat treatment process, thus preventing the distribution of Mn into the austenite from being promoted. As a result, the surface layer of the final steel sheet had a thickness of 0.3 μm. 2The number density of Mn-enriched regions with the above area is 10 particles / 1000 μm 2 The result was less than [a certain value], and the bendability after processing decreased.

[0096] In Comparative Example 50, it is believed that austenitization did not proceed sufficiently within the steel sheet because the maximum heating temperature in the second heat treatment process was low. As a result, the desired retained austenite area ratio and Vickers hardness could not be obtained in the final steel structure, and the TS and uEL decreased.

[0097] In Comparative Example 51, the maximum heating temperature in the second heat treatment step was too high, which prevented the surface layer from being properly divided into two phase regions of ferrite and austenite, resulting in insufficient distribution of Mn to the austenite. As a result, the surface layer of the final steel sheet had a thickness of 0.3 μm. 2 The number density of Mn-enriched regions with the above area is 10 particles / 1000 μm 2 The result was less than [a certain value], and the bendability after processing decreased.

[0098] In Comparative Example 52, it is believed that austenitization did not proceed sufficiently within the steel sheet because the residence time in the temperature range of Ac1 + 30°C to Ac3 - 30°C during the second heat treatment process was short. As a result, the retained austenite area ratio in the final steel structure was less than 8%, and the uEL decreased.

[0099] In Comparative Example 53, the oxygen potential (i.e., the middle side of Equation 1) in the temperature range of Ac1°C or higher during the second heat treatment process was -1.50 or less, which is thought to have caused carbon contained in the bulk of the steel sheet to diffuse to the surface layer. As a result of this recarburization to the surface layer, the ratio X / Y in the final steel sheet exceeded 0.50, and the bendability after processing decreased.

[0100] In Comparative Example 54, the heating temperature increase from room temperature to the maximum heating temperature of Ac1 + 30°C to Ac3 - 30°C in the second heat treatment step did not satisfy Equation 3, and therefore it is thought that the distribution of Mn to austenite could not be sufficiently promoted. As a result, the surface layer of the finally obtained steel sheet was 0.3 μm thick. 2The number density of Mn-enriched regions with the above area is 10 particles / 1000 μm 2 The result was less than [a certain value], and the bendability after processing decreased.

[0101] In Comparative Example 55, it is believed that the austenite in the steel could not be sufficiently stabilized because the heating step in the second heat treatment process and the holding temperature after the soaking treatment were too low. As a result, the retained austenite area ratio of the final steel microstructure was less than 8%, and the uEL decreased.

[0102] In Comparative Example 56, the high temperature during the heating step in the second heat treatment process and the high holding temperature during the low-temperature holding after the soaking treatment resulted in excessive heat treatment, causing a large amount of austenite to decompose. Consequently, the area ratio of retained austenite in the final resulting steel structure was less than 8%, and the uEL decreased.

[0103] In Comparative Example 57, it is believed that the austenite in the steel could not be sufficiently stabilized because the heating time in the second heat treatment step and the holding time in the low-temperature holding after the soaking treatment were too short. As a result, the retained austenite area ratio of the final steel microstructure was less than 8%, and the uEL decreased.

[0104] In Comparative Example 58, the heating process in the second heat treatment step and the holding time during the low-temperature holding after soaking were too long. As a result of excessive heat treatment, a large amount of austenite decomposed, and the area ratio of retained austenite in the final steel structure was less than 8%, leading to a decrease in uEL.

[0105] In contrast, in all the examples (inventive examples) of steel sheets, by having a predetermined chemical composition and appropriately controlling each condition in the manufacturing method, it was possible to control the structure to have a Vickers hardness of 300 Hv or more, and to configure the structure to contain retained austenite: 8-30% by area%, thereby achieving a TS of 980 MPa or more and a uEL of 15.0% or more, or a TS of 1180 MPa or more and a uEL of 12.0% or more. Furthermore, in all the examples (inventive examples) of steel sheets, the ratio X / Y of the average C concentration X of the surface layer of the steel sheet to the bulk C concentration Y was controlled to be within the range of 0.20 to 0.50 in GDS measurement, and the surface layer of the steel sheet was 0.3 μm 2 The number density of Mn-enriched regions with the above area is 10 particles / 1000 μm 2 The above control was achieved, and these combinations significantly improved the bendability after processing.

Claims

1. In mass percent, C: 0.12-0.40%, Si: 0.60-2.50%, Mn: 1.00-4.00%, P: 0.050% or less, S: 0.0100% or less, Al: 0.001-1.500%, N: 0.0100% or less, O: 0.0100% or less, Cr: 0-1.00%, Mo: 0-1.00%, Cu: 0-1.00%, Ni: 0-1.00%, Co: 0-1.00%, W: 0-1.00%, B: 0-0.0050%, Ta: 0-1.000%, Sn: 0-1.000%, Sb: 0-0.500% The chemical composition consists of Ti: 0-0.100%, Nb: 0-0.200%, V: 0-1.00%, As: 0-0.100%, Zn: 0-1.000%, Ca: 0-0.0100%, Mg: 0-0.0100%, Zr: 0-0.0100%, Hf: 0-0.0100%, Sr: 0-0.0100%, Bi: 0-0.0100%, REM: 0-0.0100%, and the remainder being Fe and impurities. The microstructure at a thickness of 1 / 4 of the surface contains, by area percentage, 8-30% retained austenite. When the Mn concentration in the depth range of 10 to 30 μm from the surface is measured by EPMA, and the region having a Mn concentration of 1.5 times or more the average Mn concentration in the said depth range is defined as the Mn-enriched region, then 0.3 μm 2 The number density of Mn-enriched regions with the above area is 10 particles / 1000 μm 2 The steel plate is characterized in that, in GDS measurement, the ratio X / Y of the average C concentration X in the depth range to the bulk C concentration Y is 0.20 to 0.50, and it has a Vickers hardness of 300 Hv or more.

2. 0.3 μm 2 The number density of Mn-enriched regions with the above area is 20 particles / 1000 μm 2 The steel plate according to claim 1, characterized in that it is as described above.

3. The steel plate according to claim 1 or 2, characterized by having a tensile strength of 980 MPa or more.

4. The steel sheet according to claim 1 or 2, characterized in that it has a hot-dip galvanized layer or an alloyed hot-dip galvanized layer on at least one surface.

5. An automobile part characterized by comprising the steel plate described in claim 1 or 2.

6. (A) A first heat treatment step comprising heating a steel sheet having the chemical composition described in claim 1 to a soaking temperature, and then cooling, satisfying the following conditions (A1) to (A3): (A1) The heating includes heating the steel sheet from room temperature to a maximum heating temperature of Ac3-10°C to 950°C, the atmosphere in the temperature range of Ac1 to Ac3-30°C satisfies the following formula 1, and the residence time in the temperature range of Ac1 to Ac3-30°C is 50 to 200 seconds. pH2O: partial pressure of water vapor pH2: partial pressure of hydrogen (A2) The soaking treatment includes leaving the steel plate in the temperature range of Ac3-10°C to 950°C for 50 to 200 seconds, and the atmosphere in the temperature range of Ac3-10°C to 950°C satisfies the following formula 2, and (A3) The cooling includes cooling the steel sheet to room temperature, and the average cooling rate in the temperature range of 450 to 700°C is 50°C / second or more. (B) A skin pass rolling step in which the steel sheet after the first heat treatment step is rolled to an equivalent plastic strain of 0.005 to 0.

050. (C) A second heat treatment step that includes raising the temperature of the steel sheet after the skin pass rolling step, soaking it in heat, and then cooling it, and satisfies the following conditions (C1) and (C2): (C1) The raising of temperature includes heating the steel sheet from room temperature to a maximum heating temperature of Ac1 + 30°C to Ac3 - 30°C, the soaking in heat includes leaving the steel sheet in the temperature range of Ac1 + 30°C to Ac3 - 30°C for 50 to 200 seconds, the atmosphere at Ac1°C or higher in the raising of temperature and soaking in heat satisfies formula 1 above, and the raising of temperature satisfies the following formulas 3 to 6, A method for manufacturing a steel sheet according to claim 1 or 2, characterized in that (C2) the holding time in the temperature range of 300 to 500°C is 50 to 500 seconds. T(t): Temperature (K) at time t t: Elapsed time in one-second increments (seconds) when the time when the steel sheet temperature exceeds Ac1 is set to 0 tf: Elapsed time (seconds) when the steel sheet temperature first exceeds Ac1 + 30°C (C2)

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