Hot stamp molded body, steel sheet, and methods for manufacturing same

Abstract

This hot stamp molded body has a chemical composition of 0.40-1.00 mass% of C, 0.01-1.00 mass% of Si, 0.01-1.00 mass% (exclusive of 1.00 mass%) of Mn, at most 0.100 mass% of P, at most 0.01000 mass% of S, and 0.0010-1.0000 mass% of Al, wherein when the position at 1 / 4 of the thickness from the surface in the thickness direction is defined as a 1 / 4 depth position, the number density of ε-carbides having a circle-equivalent diameter of at least 5 nm at the 1 / 4 depth position is at least 20 carbides / μm2, and has a tensile strength of at least 2,100 MPa.

Description

Hot-stamped body and steel sheet, and manufacturing methods thereof

[0001] The present invention relates to a hot-stamped steel sheet and a method for manufacturing the same. This application claims priority based on Japanese Patent Application No. 2023-016207, filed on February 6, 2023, the contents of which are incorporated herein by reference.

[0002] In the field of automotive steel sheets, in response to recent tightening of environmental regulations and collision safety standards, the application of steel sheets having high tensile strength (high-strength steel sheets) is expanding in order to improve both fuel economy and collision safety. However, as the strength of steel sheets increases, the press formability of the steel sheets decreases, making it difficult to manufacture products with complex shapes.

[0003] Specifically, as the strength of steel sheets increases, their ductility decreases, resulting in the problem of fracture at highly processed locations when they are processed into complex shapes. Furthermore, as the strength of steel sheets increases, residual stress after processing causes springback and wall warping, resulting in poor dimensional accuracy. Therefore, it is not easy to press-form high-strength steel sheets, particularly those with a tensile strength of 780 MPa or more, into products with complex shapes. While roll forming, rather than press forming, makes it easier to process high-strength steel sheets, its application is limited to parts with a uniform cross section in the longitudinal direction.

[0004] Therefore, in recent years, hot stamping has been adopted as a technique for press-forming materials that are difficult to form, such as high-strength steel sheets, as disclosed in, for example, Patent Document 1. Hot stamping is a hot forming technique in which a material to be formed is heated and then formed.

[0005] In this technology, the material is heated before being formed. Therefore, the steel is soft during forming and has good formability. This allows even high-strength steel sheets to be formed into complex shapes with high precision. In addition, in hot stamping, the steel is quenched simultaneously with forming using a press die, so the steel after forming (hot-stamped product) has sufficient strength.

[0006] For example, Patent Document 1 discloses that hot stamping can impart a tensile strength of 1400 MPa or more to a steel member (hot-stamped body) obtained by forming a steel plate.

[0007] In recent years, countries around the world have been experiencing higher CO ２ Automobile companies have set reduction targets and are working to reduce fuel consumption while taking collision safety into consideration. Higher strength materials are required not only for gasoline-powered vehicles but also for electric vehicles, which are rapidly being developed, to protect not only passengers but also batteries from collisions and to offset the resulting weight increase. For example, hot-stamped steel sheets used in automobiles and the like require higher strength (greater than 1.5 GPa) than those described in the aforementioned Patent Document 1 and those generally used for hot-stamped steel sheets currently formed by hot stamping.

[0008] However, as the strength of the hot stamped steel sheet increases, the toughness tends to decrease, and there is a concern that sufficient impact absorption properties may not be obtained.

[0009] In response to these problems, Patent Document 2 discloses a hot-stamped steel sheet having a tensile strength of 2000 MPa or more. Patent Document 2 discloses that a hot-stamped steel sheet having excellent strength and toughness can be obtained by performing a double heat treatment to set the average grain size of prior austenite grains to 5.0 μm or less and the average Mn concentration at the grain boundaries of the prior austenite grains to 1.0 mass % or less.

[0010] However, as a result of investigations by the present inventors, it has been found that although the method of Patent Document 2 can achieve a certain degree of improvement in toughness, it cannot necessarily be said to fully meet the increasing demands of recent years.

[0011] Japanese Patent Publication No. 2002-102980 Japanese Patent No. 6966023

[0012] As described above, in recent years, there has been a demand for improved impact absorption in high-strength hot-stamped steel sheets (particularly those having a tensile strength of more than 1500 MPa or 2100 MPa or more), but conventional techniques have not necessarily been able to meet this demand. Therefore, in view of the above-mentioned problems, an object of the present invention is to provide a hot-stamped steel sheet having both high strength and excellent impact absorption, a steel sheet suitable as a material for the hot-stamped steel sheet, and methods for manufacturing the hot-stamped steel sheet and the hot-stamped steel sheet.

[0013] The present inventors have investigated methods for improving the impact absorption of high-strength hot-stamped steel sheets. As a result, they have found that the impact absorption can be improved by precipitating ε carbides in the hot-stamped steel sheet. Furthermore, they have found that, when obtaining a hot-stamped steel sheet containing such ε carbides, it is effective to reduce the number density of carbides of one or more of Nb, Ti, Fe, Mo, W, and Cr in the steel sheet (hot-stamping steel sheet) that serves as the raw material for the hot-stamped steel sheet.

[0014] The present invention has been made in light of the above findings. The gist of the present invention is as follows: [1] A hot-stamped steel according to one aspect of the present invention contains, in mass%, C: 0.40 to 1.00%, Si: 0.01 to 1.00%, Mn: 0.01% or more but less than 1.00%, P: 0.100% or less, S: 0.01000% or less, Al: 0.0010 to 1.0000%, N: 0.0150% or less, Nb: 0 to 0.100%, Ti: 0 to 0.100%, Cr: 0 to 0.50%, V: 0 to 0.50%, Mo: 0 to 0.50%, B: 0 to 0.0100%, Co: 0 to 1.00%, and Ni: 0 to 1.00%. 0.00%, Cu: 0 to 1.00%, W: 0 to 3.00%, O: 0 to 0.100%, Ca: 0 to 1.00%, Mg: 0 to 1.00%, REM: 0 to 0.0050%, Sb: 0 to 0.020%, Zr: 0 to 0.10%, Sn: 0 to 0.10%, As: 0 to 0.10%, and the balance: Fe and impurities, and when a position at 1 / 4 of the thickness from the surface in the thickness direction is defined as a 1 / 4 depth position, the number density of ε carbides having a circle equivalent diameter of 5 nm or more at the 1 / 4 depth position is 20 particles / μm ２or more, and the tensile strength is 2100 MPa or more. [2] In the hot-stamped steel according to [1], when a position 50 μm from the surface in the thickness direction is defined as the 50 μm depth position, the hardness at the 50 μm depth position may be lower than the hardness at the ¼ depth position. [3] In the hot-stamped steel according to [2], the hardness at the 50 μm depth position may be lower by HV100 or more in Vickers hardness than the hardness at the ¼ depth position. [4] In the hot-stamped steel according to any of [1] to [3], the chemical composition may include, in mass%, one or more elements selected from the group consisting of Nb: 0.010 to 0.100%, Ti: 0.010 to 0.100%, Cr: 0.03 to 0.50%, and V: 0.01 to 0.50%. [5] The hot-stamped steel according to any one of [1] to [4] may have a chemical composition, in mass%, of one or more elements selected from the group consisting of Mo: 0.05 to 0.50%, B: 0.0010 to 0.0100%, Co: 0.01 to 1.00%, Ni: 0.10 to 1.00%, Cu: 0.10 to 1.00%, and W: 0.10 to 3.00%. [6] The hot-stamped steel according to any one of [1] to [5] may have a chemical composition, in mass%, of one or more elements selected from the group consisting of O: 0.001 to 0.100%, Ca: 0.01 to 1.00%, Mg: 0.01 to 1.00%, REM: 0.0001 to 0.0050%, Sb: 0.001 to 0.020%, Zr: 0.01 to 0.10%, Sn: 0.01 to 0.10%, and As: 0.01 to 0.10%.[7] A steel sheet according to another embodiment of the present invention comprises, in mass%, C: 0.40 to 1.00%, Si: 0.01 to 1.00%, Mn: 0.01% or more and less than 1.00%, P: 0.100% or less, S: 0.01000% or less, Al: 0.0010 to 1.0000%, N: 0.0150% or less, Nb: 0 to 0.100%, Ti: 0-0.100%, Cr: 0-0.50%, V: 0-0.50%, Mo: 0-0.50%, B: 0-0.0100%, Co: 0-1.00%, Ni: 0- 1.00%, Cu: 0-1.00%, W: 0-3.00%, O: 0-0.100%, Ca: 0-1.00%, Mg: 0-1.00%, REM: 0-0.0 The steel sheet has a chemical composition consisting of 0.050%, Sb: 0 to 0.020%, Zr: 0 to 0.10%, Sn: 0 to 0.10%, As: 0 to 0.10%, and the balance: Fe and impurities. When a position at 1 / 4 of the sheet thickness from the surface in the sheet thickness direction is defined as a 1 / 4 depth position, the microstructure at the 1 / 4 depth position consists of, in area ratios, ferrite: more than 50% and 100% or less, pearlite: 0 to 40%, and bainite, martensite, and austenite: 0% or more and less than 10% in total, and the number density of carbides of one or more of Nb, Ti, Fe, Mo, W, and Cr having an equivalent circle diameter of 0.2 μm or more at the 1 / 4 depth position is 5.0 pieces / 10 μm. ２[8] In the steel plate according to [7], when a position 50 μm deep from the surface in the plate thickness direction is taken as a 50 μm depth position, the hardness at the 50 μm depth position may be lower than the hardness at the 1 / 4 depth position. [9] In the steel plate according to [7] or [8], the chemical composition may include, in mass%, one or more elements selected from the group consisting of Nb: 0.010 to 0.100%, Ti: 0.010 to 0.100%, Cr: 0.03 to 0.50%, and V: 0.01 to 0.50%.

[10] The steel sheet according to any one of [7] to [9], wherein the chemical composition may include, in mass%, one or more elements selected from the group consisting of Mo: 0.05 to 0.50%, B: 0.0010 to 0.0100%, Co: 0.01 to 1.00%, Ni: 0.10 to 1.00%, Cu: 0.10 to 1.00%, and W: 0.10 to 3.00%.

[11] A method for producing a hot-stamped product according to another aspect of the present invention is a method for producing a hot-stamped product according to [1], comprising a hot stamping step of heating the steel sheet according to [7] to a maximum heating temperature of not less than 950°C and not more than the higher of the Ac3 point and 800°C, holding the steel sheet at the maximum heating temperature for 60 to 720 seconds, and then cooling the steel sheet to 300°C or less at an average cooling rate of 10 to 500°C / second from the maximum heating temperature to 300°C; and a tempering step of tempering the steel sheet after the tempering step, wherein the tempering step comprises holding the steel sheet at 80 to 300°C for 6.0 seconds or more, cooling the steel sheet to less than 80°C at an average cooling rate of 20 to 500°C / second, reheating the steel sheet, and holding it at 80 to 300°C for 6.0 seconds or more, or holding the steel sheet at 80 to 300°C for 6.0 seconds or more, cooling the steel sheet to less than 80°C at an average cooling rate of 20 to 500°C / second, reheating the steel sheet, and holding it at 80 to 300°C for 6.0 seconds or more.

[12] A method for producing a steel sheet according to another aspect of the present invention is a method for producing a steel sheet according to [7], comprising, in mass%, C: 0.40 to 1.00%, Si: 0.01 to 1.00%, Mn: 0.01% or more and less than 1.00%, P: 0.100% or less, S: 0.01000% or less, Al: 0.0010 to 1.0000%, N: 0.0150% or less, Nb: 0 to 0.100%, Ti: 0 ~0.100%, Cr: 0-0.50%, V: 0-0.50%, Mo: 0-0.50%, B: 0-0.0100%, Co: 0-1.00%, Ni: 0-1.00%, Cu: 0-1.00 %, W: 0-3.00%, O: 0-0.100%, Ca: 0-1.00%, Mg: 0-1.00%, REM: 0-0.0050%, Sb: 0-0.020%, Zr: 0-0.10%, Sn the steel sheet after the hot rolling step is cooled to 750°C or less at an average cooling rate of 10 to 100°C / sec; the coiling step is started within 5.0 seconds from the completion of the hot rolling step, and the steel sheet after the hot rolling step is cooled to 750°C or less at an average cooling rate of 10 to 100°C / sec; the steel sheet after the cooling step is coiled at a coiling temperature of more than 500°C and not more than 750°C, and the average cooling rate from the coiling temperature to 500°C is more than 50°C / hr; and the cold rolling step is performed on the steel sheet after the coiling step at a thickness reduction rate of 10 to 60%.

[13] The method for producing a steel sheet according to

[12] may include, after the cold rolling step, a heat treatment step of heating the steel sheet to an annealing temperature of 700 to 920°C and holding it at the annealing temperature for 120 to 500 seconds in an atmosphere having an oxygen potential of -1.50 or more.

[14] The method for producing a steel sheet according to

[13] may include, after the heat treatment step, a skin pass step of subjecting the steel sheet to skin pass rolling at a reduction ratio of 0.05 to 2.0%.

[0015] According to the above aspects of the present invention, it is possible to provide a hot-stamped steel product having both high strength and excellent impact absorption, a steel sheet suitable as a material for the hot-stamped steel product, and methods for manufacturing the hot-stamped steel product and the steel sheet.

[0016] Hereinafter, a hot-stamped product according to one embodiment of the present invention (hot-stamped product according to this embodiment), a steel sheet according to one embodiment of the present invention (steel sheet according to this embodiment), and methods for manufacturing the same will be described.

[0017] <Hot-stamped body> The hot-stamped body according to this embodiment has a predetermined chemical composition described below, and at a quarter depth position, the number density of ε carbides having a circle equivalent diameter of 5 nm or more is 20 particles / μm ２ In this embodiment, the position at 1 / 4 of the thickness from the surface in the thickness direction is defined as the 1 / 4 depth position, and the position at 50 μm from the surface in the thickness direction is defined as the 50 μm depth position.

[0018] <Chemical Composition> The chemical composition of the hot-stamped steel according to this embodiment will be described. Unless otherwise specified, the % of the content of each element constituting the chemical composition is mass %. When the hot-stamped steel according to this embodiment is composed of a base steel and a coating formed on the surface thereof, the chemical composition of the hot-stamped steel according to this embodiment means the chemical composition of the base steel.

[0019] C: 0.40 to 1.00% C is an element that improves the hardenability of steel and improves the strength of the hot-stamped body obtained after the steel sheet is subjected to hot stamping. If the C content is less than 0.40%, it becomes difficult to ensure sufficient strength in the hot-stamped body. Therefore, the C content is set to 0.40% or more. The C content is preferably set to 0.45% or more. On the other hand, if the C content exceeds 1.00%, the strength of the hot-stamped body becomes too high, which may result in a decrease in bendability and ductility. Therefore, the C content is set to 1.00% or less. The C content is preferably set to 0.80% or less.

[0020] Si: 0.01 to 1.00% Si is an element effective in improving the hardenability of steel and ensuring stable strength of hot-stamped products. To achieve the above effects, the Si content is set to 0.01% or more. The Si content is preferably set to 0.10% or more. On the other hand, if the Si content in steel exceeds 1.00%, the heating temperature required for austenite transformation during heat treatment (quenching) becomes significantly higher. This may increase the cost of heat treatment or reduce the strength of the hot-stamped product due to residual ferrite during heating. Furthermore, if the Si content exceeds 1.00%, the behavior of scale formation during steel sheet production may change, impairing the appearance of the product surface. Therefore, the Si content is set to 1.00% or less. The Si content is preferably set to 0.90% or less.

[0021] Mn: 0.01% or more, less than 1.00% Mn contributes to improving the strength of hot-stamped steel through solid solution strengthening. Furthermore, Mn is a highly effective element for improving the hardenability of steel and ensuring the stable strength of hot-stamped steel. To achieve this effect, the Mn content is set to 0.01% or more. On the other hand, if the Mn content is 1.00% or more, coarse inclusions such as MnS are likely to be generated in the steel, which may result in reduced bendability and ductility. Therefore, the Mn content is set to less than 1.00%. The Mn content is preferably set to 0.90% or less.

[0022] P: 0.100% or less P is an element that segregates at grain boundaries and reduces the strength of the grain boundaries. If the P content exceeds 0.100%, the strength of the grain boundaries is significantly reduced, resulting in a decrease in the toughness of the hot-stamped steel. Therefore, the P content is set to 0.100% or less. The P content is preferably set to 0.050% or less, or 0.035% or less. There is no need to particularly set a lower limit for the P content, and the lower limit is 0%. However, if the P content is reduced to less than 0.0001%, the cost of dephosphorization increases significantly, which is economically undesirable. In practical operation, the P content may be set to 0.0001% or more.

[0023] S: 0.01000% or less S is an element that forms inclusions in steel. If the S content exceeds 0.01000%, a large amount of inclusions will be formed in the steel, reducing the toughness of the hot-stamped steel. Therefore, the S content is set to 0.01000% or less. The S content is preferably 0.00400% or less. There is no need to particularly limit the lower limit of the S content, and the lower limit is 0%. However, reducing the S content to less than 0.00015% significantly increases the desulfurization cost, which is economically undesirable. In practical operation, the S content may be set to 0.00015% or more, or 0.00020% or more.

[0024] Al: 0.0010 to 1.0000% Al is an element that deoxidizes molten steel to improve its soundness (suppressing the occurrence of defects such as blowholes in the steel). If the Al content is less than 0.0010%, deoxidation is insufficient. Therefore, the Al content is set to 0.0010% or more. The Al content is preferably 0.0100% or more, and more preferably 0.0200% or more. On the other hand, if the Al content exceeds 1.0000%, coarse oxides and nitrides are formed in the steel, reducing the toughness of the hot-stamped steel. Therefore, the Al content is set to 1.0000% or less. The Al content is preferably 0.5000% or less, and more preferably 0.3000% or less.

[0025] N: 0.0150% or less N is an element that forms nitrides in steel. Because these nitrides become the starting point of fracture, the N content is set to 0.0150% or less. The N content is preferably 0.0100% or less, more preferably 0.0050% or less. There is no need to specify a lower limit for the N content, and the lower limit is 0%. However, reducing the N content to less than 0.0001% significantly increases the cost of denitrification, which is economically undesirable. Therefore, the N content may be set to 0.0001% or more, 0.0004% or more, or 0.0010% or more.

[0026] The hot-stamped steel according to this embodiment may have a chemical composition containing the above elements (basic elements), with the balance being Fe and impurities. On the other hand, for the purpose of improving various properties, it may further contain one or more of the following elements (optional elements). Since the optional elements do not necessarily need to be contained, the lower limit of the content is 0%.

[0027] Nb: 0 to 0.100% Nb is an element that improves the strength of the hot-stamped body through solid solution strengthening and contributes to the refinement of prior austenite grains by forming carbonitrides. Therefore, Nb may be added as necessary. When Nb is added, the Nb content is preferably 0.010% or more to ensure the above effects. The Nb content is more preferably 0.035% or more. On the other hand, if the Nb content exceeds 0.100%, Nb-based carbonitrides are formed in excess, which may suppress the formation of ε-carbides that contribute to improving the toughness of the hot-stamped body. Therefore, the Nb content is preferably 0.100% or less. The Nb content is more preferably 0.080% or less.

[0028] Ti: 0 to 0.100% Ti forms fine carbides, carbonitrides, etc. with Nb in steel, and their grain refinement suppresses Cu hot embrittlement cracking during the hot rolling process and improves the hydrogen embrittlement resistance of the hot-stamped steel. Ti also preferentially bonds with N in the steel to form nitrides, suppressing the consumption of solute B due to the precipitation of BN and promoting the hardenability-improving effect of B, described below. Therefore, Ti may be contained. To achieve the above effects, the Ti content is preferably 0.010% or more. The Ti content is more preferably 0.020% or more. On the other hand, if the Ti content exceeds 0.100%, coarse TiN is formed, deteriorating the toughness of the hot-stamped steel. Therefore, the Ti content is set to 0.100% or less. The Ti content is preferably 0.080% or less, more preferably 0.060% or less.

[0029] Cr: 0 to 0.50% Cr is an element effective in improving the hardenability of steel and stably ensuring the strength of hot-stamped products. Therefore, Cr may be added. To obtain the above effects, the Cr content is preferably 0.03% or more, and more preferably 0.05% or more. On the other hand, if the Cr content exceeds 0.50%, the above effects saturate and costs increase. Therefore, the Cr content is set to 0.50% or less. The Cr content is preferably set to 0.30% or less.

[0030] V: 0 to 0.50% V is an element that improves the strength of the hot-stamped steel sheet by solid solution strengthening. Therefore, V may be added. To reliably obtain the above effect, the V content is preferably 0.01% or more. On the other hand, if the V content exceeds 0.50%, V-based carbonitrides are formed in excess, which may suppress the formation of ε-carbides that contribute to improving the toughness of the hot-stamped steel sheet. Therefore, the V content is set to 0.50% or less. The V content is preferably 0.40% or less.

[0031] Mo: 0 to 0.50% Mo is a highly effective element for improving the hardenability of steel and ensuring stable strength of hot-stamped bodies. In particular, when Mo is added in combination with B, a synergistic effect of improving hardenability is obtained. Therefore, Mo may be added. To obtain the above effect, the Mo content is preferably 0.05% or more, and more preferably 0.10% or more. On the other hand, if the Mo content exceeds 0.50%, not only will the above effect saturate, but the alloy cost will also increase. Therefore, the Mo content is set to 0.50% or less. The Mo content is preferably 0.40% or less.

[0032] B: 0 to 0.0100% B is an element that has the effect of improving the hardenability of steel even in small amounts. Furthermore, B segregates at grain boundaries, strengthening the grain boundaries. Therefore, B may be added. To obtain the above effects, the B content is preferably 0.0010% or more. On the other hand, if the B content exceeds 0.0100%, a large amount of coarse compounds precipitates, reducing the toughness of the hot-stamped body. Therefore, the B content is set to 0.0100% or less. The B content is preferably 0.0080% or less.

[0033] Co: 0 to 1.00% Co is an element that has the effect of raising the martensite start temperature (Ms point) and improving the toughness of the hot stamped body. Therefore, Co may be contained. To obtain the above effect, the Co content is preferably 0.01% or more. On the other hand, Co is an expensive element, and if the Co content exceeds 1.00%, the alloy cost increases. Therefore, the Co content is set to 1.00% or less. The Co content may also be set to 0.10% or less.

[0034] Ni: 0 to 1.00% Ni is an element effective for improving the hardenability of steel and stably ensuring the strength of hot-stamped steel sheets. Ni also has the effect of suppressing Cu hot embrittlement cracking during the production of steel sheets. Therefore, Ni may be added. To obtain the above effects, the Ni content is preferably 0.10% or more, and more preferably 0.20% or more. On the other hand, if the Ni content exceeds 1.00%, the above effects saturate and costs increase. Therefore, the Ni content is set to 1.00% or less. The Ni content is preferably 0.80% or less, and more preferably 0.50% or less.

[0035] Cu: 0 to 1.00% Cu is an element effective for improving the hardenability of steel and stably ensuring the strength of hot-stamped bodies. Cu is also an element that improves corrosion resistance in corrosive environments. Therefore, Cu may be contained. To obtain the above effects, the Cu content is preferably 0.10% or more. The Cu content is more preferably 0.20% or more. On the other hand, if the Cu content exceeds 1.00%, the above effects saturate and costs increase. Therefore, the Cu content is set to 1.00% or less. The Cu content is preferably set to 0.90% or less.

[0036] W: 0 to 3.00% W is an element effective for improving the hardenability of steel and stably ensuring the strength of hot-stamped steel. To obtain the above effects, the W content is preferably 0.10% or more. On the other hand, if the W content exceeds 3.00%, the above effects saturate and costs increase. Therefore, the Cu content is set to 3.00% or less.

[0037] O: 0 to 0.100% O is an element that, when present in large amounts in steel, forms coarse oxides that become the starting points for fracture, thereby degrading the toughness of the hot-stamped steel. Therefore, the O content is set to 0.100% or less. The O content is preferably set to 0.080% or less, 0.050% or less, or 0.030% or less. There is no particular need to specify a lower limit for the O content, and the lower limit is 0%, but the O content may be set to 0.001% or more or 0.005% or more in order to disperse a large number of fine oxides during deoxidation of molten steel.

[0038] Ca: 0 to 1.00% Ca is an element that deoxidizes molten steel and suppresses the generation of oxides that serve as starting points for fracture. Therefore, it may be added. To obtain the above effect, the Ca content is preferably 0.01% or more, and more preferably 0.05% or more. However, even if a large amount is added, the above effect saturates, so the Ca content is set to 1.00% or less. The Ca content is preferably 0.40% or less, 0.20% or less, or 0.15% or less.

[0039] Mg: 0 to 1.00% Mg is an element that has the effect of deoxidizing molten steel and improving the quality of the steel. Therefore, Mg may be added. To obtain the above effect, the Mg content is preferably 0.01% or more. The Mg content is more preferably 0.05% or more. On the other hand, if the Mg content exceeds 1.00%, oxides in the steel increase, adversely affecting the toughness of the hot-stamped steel. Therefore, the Mg content is set to 1.00% or less. The Mg content is preferably 0.40% or less, 0.20% or less, or 0.15% or less.

[0040] REM: 0 to 0.0050% REM is an element that deoxidizes molten steel and suppresses the generation of oxides that can serve as starting points for fracture. Therefore, it may be contained. To obtain the above effect, the REM content is preferably 0.0001% or more, and more preferably 0.0010% or more. On the other hand, even if a large amount is added, the above effect saturates, so the REM content is set to 0.0050% or less. The REM content is preferably 0.0040% or less, or 0.0020% or less. In this embodiment, REM refers to a total of 17 elements consisting of Sc, Y, and lanthanoids, and the REM content refers to the total content of these elements.

[0041] Sb: 0 to 0.020% Sb is an element that improves the deformability of the hot-stamped steel by deoxidizing molten steel and suppressing the generation of oxides that serve as fracture initiation sites. Therefore, Sb may be added. To obtain the above-mentioned effect, the Sb content is preferably 0.001% or more, and more preferably 0.005% or more. However, even if a large amount of Sb is added, the above-mentioned effect saturates, so the Sb content is set to 0.020% or less. The Sb content is preferably 0.015% or less.

[0042] Zr: 0 to 0.10% Zr is an element that contributes to inclusion control, particularly to finely dispersing inclusions, and improves the toughness of hot-stamped steel sheets. Therefore, it may be added. To obtain the above effects, the Zr content is preferably 0.01% or more, and more preferably 0.03% or more. On the other hand, if a large amount of Zr is added, deterioration of surface properties may become apparent. Therefore, the Zr content is set to 0.10% or less. The Zr content is preferably 0.08% or less.

[0043] Sn: 0 to 0.10% Sn is an element that improves hydrogen embrittlement resistance. Therefore, it may be contained. To obtain the above effects, the Sn content is preferably 0.01% or more. On the other hand, if the Sn content exceeds 0.10%, the effect saturates and the cost increases. Therefore, if Sn is contained, the Sn content is set to 0.10% or less.

[0044] As: 0 to 0.10% As is an element that reduces the austenite single-phase temperature, thereby refining prior austenite grains and contributing to improved hydrogen embrittlement resistance. Therefore, it may be contained. To obtain the above effect, the As content is preferably 0.01% or more. However, even if a large amount is contained, the above effect saturates, so the As content is set to 0.10% or less. The As content is preferably 0.06% or less.

[0045] Balance: Fe and Impurities In the chemical composition of the hot-stamped steel according to this embodiment, the elements other than those described above, i.e., the balance, are Fe and impurities. That is, the hot-stamped steel according to this embodiment may have a chemical composition containing the basic elements with the balance being Fe and impurities, or may have a chemical composition containing the basic elements and one or more optional elements with the balance being Fe and impurities. Here, "impurities" refer to components that are mixed in during the industrial production of steel sheet due to raw materials such as ore and scrap, or various factors in the manufacturing process, and are acceptable within a range that does not adversely affect the properties of the hot-stamped steel according to this embodiment. The industrial production method refers to a blast furnace steelmaking process or an electric furnace steelmaking process, and includes the level (impurity level) of components mixed in during production by either method.

[0046] The chemical composition of the hot-stamped body can be determined by the following method. It can be obtained by performing elemental analysis using a common method such as ICP-AES from a 1 / 4 depth position, which is a position that is 1 / 4 of the way through the thickness from the surface of the hot-stamped body in the thickness direction (a range of 1 / 8 to 3 / 8 of the thickness from the surface in the thickness direction is acceptable). C and S, which are difficult to measure with ICP-AES, can be measured using combustion-infrared absorption method, N can be measured using inert gas fusion-thermal conductivity method, and O can be measured using inert gas fusion-non-dispersive infrared absorption method.

[0047] [Microstructure] In this embodiment, the microstructures at the 1 / 4 depth position and the 50 μm depth position are specified. The 1 / 4 depth position is a position that shows a typical microstructure of a hot stamped body.

[0048] (At the 1 / 4 depth position, the number density of ε carbides with a circle equivalent diameter of 5 nm or more is 20 / μm ２ (The above is the case.) A high-strength hot-stamped steel sheet needs to contain a large amount of C and other alloy elements to obtain high tensile strength. However, as strength increases, the toughness of the hot-stamped steel sheet generally decreases, resulting in a decrease in impact absorption. In response to this, the inventors have conducted studies and found that impact absorption can be improved by allowing ε carbides to exist in the hot-stamped steel sheet and controlling their size and number density. Specifically, the number density of ε carbides with a circle equivalent diameter of 5 nm or more is 20 particles / μm ２ Therefore, in the hot stamped steel according to this embodiment, the number density of ε carbides having a circle equivalent diameter of 5 nm or more at the ¼ depth position is set to 20 particles / μm ２ The reason why ε carbides with an equivalent circle diameter of 5 nm or more are targeted is that ε carbides smaller than this do not sufficiently improve the impact absorption. On the other hand, although there is no upper limit to the equivalent circle diameter of the targeted ε carbides, if the size becomes too large, it becomes difficult to obtain a sufficient number density, which is not preferable. For example, ε carbides with a diameter of 5 to 50 nm are targeted. Furthermore, even if the equivalent circle diameter of ε carbides is 5 nm or more, it is possible to obtain a number density of 20 particles / μm. ２ On the other hand, if the number density is less than 200 pieces / μm, the effect is not sufficient. ２ If the number exceeds 200 / μm, the interface between the ε carbide and the base material may become the starting point of cracks. ２ In this embodiment, the ε carbide is a carbide having a structure of FexC (x: about 2 to 3).

[0049] The circle-equivalent diameter of ε carbides and the number density of ε carbides with circle-equivalent diameters of 5 nm or greater can be determined by observing a thin-film sample using a field-emission transmission electron microscope (JEM-2100F manufactured by JEOL Ltd.) equipped with an energy-dispersive X-ray spectroscopy facility. Specifically, a small piece approximately 10 mm square is cut from the hot-stamped compact, and then both sides are mechanically or chemically polished to prepare a thin-film TEM sample (approximately 60 μm thick, φ3 mm) at a depth of 1 / 4 of the original depth (a depth in the range of 1 / 8 to 3 / 8 of the thickness from the surface in the thickness direction is acceptable). This sample is prepared by polishing using coarse waterproof abrasive paper of approximately #120, then gradually finer waterproof abrasive paper, and finally waterproof abrasive paper of approximately #600, and punching the sample with a specimen punch. Then, double-sided jet electropolishing is performed, leaving a hole in the center, to prepare a TEM observation sample. A STRUERS TENUPOL-2 electropolishing device is used, and the electropolishing solution is a mixture of 5% perchloric acid and 95% glacial acetic acid solution, and a voltage of 70 V is used to create a thin-film TEM sample. Thin-film TEM observation is performed at an accelerating voltage of 200 kV. To reduce variation between locations, at least five fields of view, each approximately 100 to 300 nm square, are observed on the sample. The type of precipitate observed is identified using the diffraction pattern and EDX analysis results. To reduce variation between fields of view, the number density is counted for each field of view, and the average is used as the representative value for number density.

[0050] (Preferably, the hardness at the 50 μm depth position is smaller than the hardness at the ¼ depth position) In the hot stamped steel according to this embodiment, the hardness at the 50 μm depth position is preferably smaller (lower) than the hardness at the ¼ depth position. Reducing (softening) the hardness near the surface further improves impact absorption. Although an effect can be obtained if the hardness at the 50 μm depth position is smaller than the hardness at the ¼ depth position, to obtain a more pronounced effect, it is more preferable that the hardness at the 50 μm depth position is smaller than the hardness at the ¼ depth position by at least HV 100 in Vickers hardness. There is no upper limit to the difference in Vickers hardness, but it may be HV 300 or less in order to ensure the strength of the entire steel sheet. The hardness at the 50 μm depth position can be reduced (lowered), for example, by decarburization by annealing as described below. The reason for selecting the 50 μm depth position as the target is that, due to the measurement principle, there is a large variation in hardness near the outermost surface.

[0051] The hardness at a 1 / 4 depth position and at a 50 μm depth position is evaluated by Vickers hardness in accordance with JIS Z2244-1: 2020. For the measurement, the cross section of the polished test piece is measured at five points on each site with a load of 50 gf, and the average value of the three points excluding the maximum and minimum values ​​is taken as the measured value.

[0052] In the hot-stamped steel according to this embodiment, the constituent phases of the microstructure are not limited and may be controlled according to the target tensile strength, but it is preferable that the area fraction of martensite is 95% or more. Here, martensite includes so-called fresh martensite and tempered martensite. The area fraction of martensite can be measured in a manner similar to that of the steel sheet microstructure observation described below.

[0053] [Coating] The hot-stamped steel according to this embodiment may have a coating on part or all of its surface. The coating may be primarily composed of an Fe—Al alloy or an Fe—Zn alloy. The coating is also referred to as a film, alloyed plating layer, or intermetallic compound layer. A coating primarily composed of an Fe—Al alloy is a coating containing a total of 70 mass% or more of Fe and Al, and a coating primarily composed of an Fe—Zn alloy is a coating containing a total of 70 mass% or more of Fe and Zn. The coating primarily composed of an Fe—Al alloy may further contain, in addition to Fe and Al, Si, Mg, Ca, Sr, Ni, Cu, Mo, Mn, Cr, C, Nb, Ti, B, V, Sn, W, Sb, Zn, Co, In, Bi, Zr, Se, As, and REM, with the remainder being impurities. The coating mainly made of an Fe-Zn alloy may contain, in addition to Fe and Zn, Si, Mg, Ca, Sr, Ni, Cu, Mo, Mn, Cr, C, Nb, Ti, B, V, Sn, W, Sb, Al, Co, In, Bi, Zr, Se, As, and REM, with the remainder being impurities. The coating provides corrosion resistance, which has the effect of improving hydrogen embrittlement resistance when used in automobiles. The thickness of the coating is preferably 5 to 100 μm.

[0054] The chemical composition and thickness of the coating can be determined by observing the cross section under a scanning electron microscope. Specifically, a measurement sample is cut out from half the longitudinal portion (a position half the length from the longitudinal end) and one-quarter the width (a position one-quarter the width from the width end) of the hot-stamped body and observed. The observation range under the microscope is, for example, 400x magnification, and an area of ​​40,000 μm ２The thickness of the coating is within the above range. The cut sample is mechanically polished and then mirror-finished. Next, the thickness of the coating is measured in 10 random fields of view, and the average value is used as the coating thickness. When observed using a BSE image (or COMPO image), a clear difference in contrast is confirmed between the coating and the base steel (steel plate substrate). Therefore, the thickness of the coating can be determined by measuring the thickness from the outermost surface to the point where the contrast changes. Measurements are made at 20 equally spaced locations within the observation photograph, with the distance between measurement locations being 6.50 μm. Furthermore, five fields of view are observed as described above, and the average value is used to determine the coating thickness. Furthermore, the chemical composition of the coating can be determined by performing spot elemental analysis (beam diameter: 1 μm or less) on the same observation range as above using an electron probe microanalyzer (EPMA) to determine the Fe, Al, and Zn contents contained in the coating. A total of 10 points are analyzed in the coating in 10 random fields of view, and the average values ​​are used to determine the Fe, Al, and Zn contents in the coating. Even when elements other than Fe, Al, and Zn are contained, the same method is used to determine the content.

[0055] The surfaces that serve as the reference for the above-mentioned 1 / 4 depth position and 50 μm depth position are the surfaces of the hot-stamped body. However, when the hot-stamped body has a coating, that is, when the hot-stamped body has a base steel and a coating formed on the surface of the base steel, the surface means the surface of the base steel excluding the coating.

[0056] [Mechanical Properties] (Tensile Strength) The tensile strength of the hot-stamped product according to this embodiment is 2100 MPa or more, taking into consideration its contribution to improving fuel economy and collision safety when applied to automotive parts. There is no upper limit to the tensile strength, but since there is a concern that an increase in tensile strength may result in a decrease in impact absorption, the tensile strength may be less than 2900 MPa. The tensile strength can be determined by taking a No. 5 test piece as specified in JIS Z2241:2011 from a position on the hot-stamped product that is as flat as possible, and performing a tensile test on this test piece according to the test method as specified in JIS Z2241:2011.

[0057] (Impact Absorption) In the hot stamped steel according to this embodiment, excellent impact absorption can be obtained by controlling the chemical composition and the state of existence of ε carbide as described above. The target impact absorption is a limit bending angle (bending angle at maximum force) of 40° or more in the VDA (German Association of the Automotive Industry) bending test 238-100, converted to a sheet thickness of 2.0 mm. The sheet thickness conversion is performed based on "Bending Angle Correction Regarding Sheet Thickness", Materials Science and Engineering 418 (2018) 012076.

[0058] <Steel Sheet> Next, the steel sheet according to the present embodiment will be described. By hot stamping and tempering the steel sheet according to the present embodiment, the hot-stamped product according to the present embodiment described above can be obtained, and therefore the steel sheet according to the present embodiment is suitable as a raw material for the hot-stamped product according to the present embodiment (steel sheet for hot stamping).

[0059] [Chemical Composition] The chemical composition of the steel sheet according to this embodiment needs to be set so as to obtain preferred properties for the hot-stamped steel obtained by hot stamping and tempering. However, since the chemical composition does not substantially change by hot stamping and tempering, the chemical composition of the steel sheet according to this embodiment may be the same as that of the hot-stamped steel according to this embodiment.

[0060] [Microstructure] The microstructure of the steel sheet according to this embodiment will be described assuming that the position of 1 / 4 of the sheet thickness from the surface in the sheet thickness direction is the 1 / 4 depth position, the range from the surface to 50 μm in the sheet thickness direction is the surface layer portion, and the position 50 μm from the surface in the sheet thickness direction is the 50 μm depth position. The 1 / 4 depth position is a position that shows a typical microstructure of the steel sheet.

[0061] (At the 1 / 4 depth position, the area ratios are: ferrite: more than 50% but not more than 100%, pearlite: 0 to 40%, bainite, martensite (including fresh martensite and tempered martensite), and austenite: a total of 0% or more but less than 10%). In the steel sheet according to this embodiment, in consideration of workability, the microstructure is mainly ferrite (area ratio is more than 50%). The area ratio of ferrite may be 100%, but structures other than ferrite such as pearlite, bainite, martensite, and austenite (residual austenite) may also be included. Pearlite is a structure in which ferrite and cementite are arranged in a lamellar shape. In other words, a high area ratio of pearlite means that the amount of C present as cementite is large. If the amount of C present as cementite is large, this cementite (pearlite) does not sufficiently dissolve during heating in hot stamping, and sufficient ε carbide cannot be obtained even when hot stamping and tempering are performed. Therefore, the area ratio of pearlite is set to less than 40%. The area ratio of pearlite may be 0%, but may also be 5% or more. The presence of bainite, martensite, and austenite increases the strength of the steel sheet before hot stamping. As a result, the cut edge may become rough or cracks may occur in a trimming process or the like before hot stamping. Therefore, the total area ratio of bainite, martensite, and austenite is set to 0% or more and less than 10%.

[0062] The area fractions of ferrite, pearlite, bainite, martensite, and retained austenite in the microstructure of steel sheet can be determined using a field emission scanning electron microscope (FE-SEM) and X-ray diffraction measurement as follows: The L-section of the steel sheet (a cross section parallel to the rolling direction and thickness direction) is mirror-polished and then etched with nital. The same sample is observed using an FE-SEM at a magnification of 3000x in 10 fields of view, and the area fraction of each phase at the 1 / 4 depth position is calculated. In this case, the structure is identified based on the following structural characteristics in the FE-SEM image: Ferrite is a massive crystal grain that does not contain a substructure such as lath within it. Pearlite is a structure in which ferrite and cementite are alternately layered (the layered ferrite in pearlite is distinguished from the massive ferrite described above and is not included in the area fraction of massive ferrite). Bainite and tempered martensite are structures composed of lath-shaped crystal grains and carbides, but they have the following differences. First, bainite is observed as being divided into upper bainite and lower bainite. Upper bainite is a collection of lath-shaped crystal grains containing carbides between the laths. Lower bainite is a collection of lath-shaped crystal grains containing iron-based carbides with a major axis of 5 nm or more. Furthermore, the carbides belong to a single variant, i.e., a group of iron-based carbides elongated in the same direction. Here, a group of iron-based carbides elongated in the same direction means that the difference in the elongation direction of the iron-based carbides is within 5°. The area fraction of bainite is determined by the sum of the area fractions of upper bainite and lower bainite. Like lower bainite, tempered martensite is a collection of lath-shaped crystal grains containing iron-based carbides, but since carbides select two or more variants, the elongation direction of the iron-based carbides is two or more. In this way, the characteristics of ferrite, pearlite, tempered martensite, and bainite can be confirmed and identified using the FE-SEM.On the other hand, untempered fresh martensite and retained austenite are not sufficiently corroded by nital etching. Therefore, while they can be distinguished from other etched structures (tempered martensite, bainite, and ferrite) in FE-SEM observations, the difference between fresh martensite and retained austenite cannot be determined. Therefore, the area fraction of retained austenite is measured by X-ray diffraction. For X-ray diffraction, the surface of a test specimen cut into a 20 mm square is mechanically polished to 50 μm, and then the surface that is chemically polished is measured. The integrated intensities of the diffraction peaks of the BCC phase and FCC phase are measured by X-ray diffraction, and the ratio of the integrated intensity of the FCC phase to the sum of all integrated intensities is taken as the area fraction of retained austenite. Measurements are performed three times for each sample, and the average obtained is taken as the area fraction of retained austenite. The area fraction of fresh martensite is determined as the difference between the area fraction of the uncorroded region (fresh martensite or retained austenite) observed with an FE-SEM and the area fraction of retained austenite measured by X-ray diffraction. The area fraction can also be determined in a similar manner for a hot-stamped steel.

[0063] (At a 1 / 4 depth position, the number density of one or more carbides of Nb, Ti, Fe, Mo, W, and Cr having a circle equivalent diameter of 0.2 μm or more is 5.0 pieces / 10 μm ２ In the hot-stamped steel according to the present embodiment described above, the predetermined ε carbide is obtained by precipitating the ε carbide through hot stamping and tempering under predetermined conditions. However, if carbides are present in the steel sheet used as the base material and the carbides do not dissolve during hot stamping, the predetermined ε carbide cannot be obtained even after hot stamping and tempering. For example, coarse carbides formed in the steel sheet tend to remain undissolved during hot stamping. Therefore, the steel sheet according to the present embodiment reduces the number of coarse carbides. More specifically, at the 1 / 4 depth position, the number density of carbides of one or more of Nb, Ti, Fe, Mo, W, and Cr having a circle equivalent diameter of 0.2 μm or more is reduced to 5.0 particles / 10 μm. ２ Less than (0.50 pieces / μm ２Coarse carbides with a circle equivalent diameter of 0.2 μm or more are 5.0 pieces / 10 μm ２ If the number density of carbides is more than 10 μm, the number of carbides may not be present, and may be 0.0 / 10 μm. ２ However, in order not to significantly inhibit the formation of ε carbide after hot stamping, ２ (0.03 pieces / 10μm ２ ) or more. There is no upper limit to the circle-equivalent diameter of the target carbide, but it may be 2.0 μm or less so as not to excessively affect the variation in hardenability depending on the location during hot stamping. In other words, carbides with circle-equivalent diameters of 0.2 to 2.0 μm may be targeted.

[0064] The number density of carbides of one or more of Nb, Ti, Fe, Mo, W, and Cr having an equivalent circle diameter of 0.2 μm or more can be determined by the following method. The L-section of a steel plate is mirror-polished and then etched with nital. The sample is observed at a 1 / 4 depth position using a scanning electron microscope. Observed precipitates in a 50 μm square region at the same depth position are subjected to composition analysis using EDX. If the precipitate contains one or more of Nb, Ti, Fe, Mo, W, and Cr and C, it is determined to be the target carbide. To reduce variation in number density due to visual field, at least five or more visual fields of a 50 μm square region are observed, and the number densities of the carbides having an equivalent circle diameter of 0.2 μm or more are counted, and the average is used as the representative value of the number density.

[0065] (Preferably, the hardness at the 50 μm depth position is smaller than the hardness at the ¼ depth position) By making the hardness at the 50 μm depth position smaller than the hardness at the ¼ depth position at the stage of the steel sheet material, the hardness at the 50 μm depth position can be made smaller than the hardness at the ¼ depth position in the hot stamped steel. Therefore, in the steel sheet according to this embodiment, it is preferable that the hardness at the 50 μm depth position is smaller than the hardness at the ¼ depth position.

[0066] [Thickness] The thickness of the steel sheet according to this embodiment is not limited, but is preferably 1.0 to 3.5 mm, assuming use as a steel sheet for automobiles.

[0067] [Coating] The steel sheet according to this embodiment may have a coating on a portion of its surface. The coating may be an Al-based coating or a Zn-based coating. The coating is also referred to as a film or a plating layer. An Al-based coating is a coating containing 70% by mass or more of Al, and a Zn-based coating is a coating containing 70% by mass or more of Zn. The Al-based coating may contain Si, Mg, Ca, Sr, Ni, Cu, Mo, Mn, Cr, C, Nb, Ti, B, V, Sn, W, Sb, Zn, Co, In, Bi, Zr, Se, As, and REM in addition to Al, with the remainder being impurities. The Zn-based coating may further contain, in addition to Zn, Si, Mg, Ca, Sr, Ni, Cu, Mo, Mn, Cr, C, Nb, Ti, B, V, Sn, W, Sb, Al, Co, In, Bi, Zr, Se, As, and REM, with the remainder being impurities. The surfaces that serve as the reference for the above-mentioned 1 / 4 depth position, 50 μm depth position, and surface layer portion are the surfaces of the steel sheet, but when the steel sheet has a coating, that is, when the steel sheet has a base material and a coating formed on the surface of the base material, the surface means the surface of the base material excluding the coating.

[0068] <Manufacturing Method> The steel sheet according to the present embodiment and the hot-stamped steel according to the present embodiment can obtain the effects as long as they have the above-described characteristics, regardless of the manufacturing method, but can be preferably manufactured by the manufacturing method described below.

[0069] [Method for Manufacturing Steel Sheet] The steel sheet according to this embodiment can be obtained by a manufacturing method including the following steps. (I) a heating step of heating a slab having a predetermined chemical composition to 1150 to 1350°C; (II) a hot rolling step of hot rolling the slab after the heating step so that the finish rolling temperature is 800 to 950°C to obtain a steel sheet; (III) a cooling step which is started within 5 seconds from the completion of the hot rolling step and cools the steel sheet after the hot rolling step to 750°C or less at an average cooling rate of 10 to 100°C / second; (IV) a coiling step of coiling the steel sheet after the cooling step at a coiling temperature of more than 500°C and not more than 750°C, and the average cooling rate from the coiling temperature to 500°C is more than 50°C / hr; and (V) a cold rolling step of cold rolling the steel sheet after the coiling step at a sheet thickness reduction rate of 10 to 60%. The method for producing a steel sheet according to this embodiment may further include one or more of the following steps: (VI) a heat treatment step of heating the steel sheet after the cold rolling step to an annealing temperature of 700 to 920°C and holding the steel sheet at the annealing temperature for 120 to 500 seconds in an atmosphere having an oxygen potential of -1.50 or higher; (VII) a skin pass step of subjecting the steel sheet to skin pass rolling at a rolling reduction of 0.05 to 2.0% after the heat treatment step; and (VIII) a coating step of forming a coating on the surface of the steel sheet. Preferred conditions for each step will be described. Known conditions can be applied to conditions or steps not described.

[0070] (Heating Step) In the heating step, the slab is heated prior to hot rolling. The heating temperature is set to 1150 to 1350°C. If the heating temperature is less than 1150°C, the carbides formed during casting do not dissolve, and coarse carbides remain even after the hot rolling step. On the other hand, from the viewpoint of suppressing scale loss and saving energy, the slab heating temperature is set to 1350°C or less. The chemical composition of the slab to be subjected to the heating step may be the same as the chemical composition of the steel sheet to be obtained.

[0071] (Hot Rolling Process) In the hot rolling process, the slab after the heating process is hot rolled at a finish rolling temperature of 800 to 950°C to obtain a steel sheet. If the finish rolling temperature (surface temperature at the final pass exit) is less than 800°C, many unrecrystallized regions flattened in the rolling direction remain, which may cause anisotropy in the properties of the steel sheet. On the other hand, if the finish rolling temperature is more than 950°C, the crystal grains of the steel sheet become coarse.

[0072] (Cooling Process) After the completion of finish rolling, if the steel sheet is allowed to remain at a temperature above 750°C for a long period of time, coarse carbides are generated. Therefore, in the cooling process, the steel sheet after the hot rolling process is cooled to a cooling stop temperature of 750°C or less so that the average cooling rate is 10 to 100°C / s. This cooling is initiated within 5.0 seconds from the completion of the hot rolling process. If the average cooling rate to the cooling stop temperature of 750°C or less is less than 10°C / s, if the time from the completion of the hot rolling process to the start of the cooling process exceeds 5.0 seconds, or if the cooling stop temperature is above 750°C, a large amount of coarse carbides are generated. On the other hand, if the average cooling rate to the cooling stop temperature of 750°C or less exceeds 100°C / s, it is difficult to uniformly cool the steel sheet, and defects in the sheet shape may occur.

[0073] (Coiling Process) In the coiling process, the steel sheet after the cooling process is coiled at a temperature above 500°C and not higher than 750°C. After coiling, the average cooling rate from the coiling temperature to 500°C is set to be more than 50°C / hr. If the coiling temperature is 500°C or lower, hard phases such as bainite and martensite are formed, making cold rolling impossible or increasing the load during cold rolling. The coiling temperature is preferably 520°C or higher, more preferably 540°C or higher. On the other hand, if the coiling temperature is higher than 750°C, Cr and Mn concentrate in the ferrite grain boundaries and cementite present in pearlite, and may remain as undissolved carbides during subsequent annealing and hot stamping processes. On the other hand, coiling slows down the cooling rate. If the average cooling rate to 500°C is slow, an internal oxide layer develops, which increases the load on the pickling step and causes the formation of coarse carbides during cooling. Therefore, the average cooling rate from the coiling temperature to 500°C is set to exceed 50°C / hr.

[0074] (Cold Rolling Step) In the cold rolling step, the steel sheet after the coiling step is cold rolled at a thickness reduction rate (rolling reduction rate) of 10 to 60% to adjust the steel sheet to a predetermined thickness.

[0075] (Heat Treatment Step) In the heat treatment step, the steel sheet after the cold rolling step is heated to an annealing temperature of 700 to 920°C and held at the annealing temperature for 120 to 500 seconds in an atmosphere with an oxygen potential of -1.50 or higher. While the heat treatment step is not essential, performing heat treatment under the above conditions is preferable because it reduces the C content (decarburization) in the surface layer of the steel sheet, thereby softening the surface layer of the steel sheet. If the annealing temperature is less than 700°C, the oxygen potential is less than -1.50, or the holding time is less than 120 seconds, sufficient effects cannot be obtained. On the other hand, if the annealing temperature exceeds 920°C, the crystal grains become coarse. Furthermore, if the holding time exceeds 500 seconds, productivity deteriorates and material costs increase. While there is no upper limit for the oxygen potential, an excessively high oxygen potential oxidizes Fe and other alloy elements in the outermost layer, causing a scale pattern on the surface; therefore, it is preferable to set the oxygen potential to -0.50 or less. The oxygen potential is the partial molar Gibbs free energy of oxygen expressed by the oxygen partial pressure in the gas phase and the temperature.

[0076] <Coating Step> A coating may be formed on the surface as needed. The coating method is not particularly limited, and may be hot-dip plating, electroplating, vacuum deposition, cladding, thermal spraying, or the like. Hot-dip plating is the most widely used method industrially. Examples of coatings include Al-based coatings containing Al and Zn-based coatings containing Zn.

[0077] When an Al-based coating is formed by hot-dip plating, the coating bath often contains Fe as an impurity in addition to Al. Furthermore, as long as the coating bath contains 70% by mass or more of Al, the coating bath may contain Si, Mg, Ca, Sr, Ni, Cu, Mo, Mn, Cr, C, Nb, Ti, B, V, Sn, W, Sb, Zn, Co, In, Bi, Zr, Se, As, or misch metal in addition to the elements described above. When hot-dip plating is performed, the steel sheet after the heat treatment process may be cooled to room temperature and then heated again for plating. Alternatively, the steel sheet may be annealed and then cooled to a temperature close to the coating bath temperature (e.g., 650 to 750°C for Al-based coating, 420 to 500°C for Zn-based coating), and then hot-dip plated without first cooling to room temperature.

[0078] There are no particular limitations on the pre-treatment and post-treatment for coating, and pre-coating, solvent application, alloying treatment, temper rolling, etc. are possible. As an alloying treatment, for example, annealing at 450 to 800°C is possible. Furthermore, as a post-treatment, temper rolling is useful for shape adjustment, etc., and for example, a reduction of 0.1 to 0.5% is possible.

[0079] (Skin-pass process) The method for manufacturing a steel sheet according to this embodiment may further include a skin-pass process in which the steel sheet is subjected to skin-pass rolling after the heat treatment process or the coating process. By performing skin-pass rolling, the diffusion rate of elements in the material increases, making it easier for carbides to dissolve during hot stamping. In this case, the number density of ε carbides can be increased. To achieve this effect, the reduction ratio of the skin-pass rolling is preferably 0.05% or more, more preferably 0.1% or more. On the other hand, if the reduction ratio of the skin-pass rolling exceeds 2.0%, the load on the skin-pass process becomes large, which causes an increase in material costs. Therefore, when performing skin-pass rolling, the reduction ratio is preferably 2.0% or less.

[0080] [Method for manufacturing hot-stamped body] The method for manufacturing a hot-stamped body according to this embodiment can be achieved by a manufacturing method using the steel sheet according to this embodiment described above, which includes the following steps: (i) a hot stamping step in which the steel sheet according to this embodiment is heated to a maximum heating temperature of at least the higher of the Ac3 point and 800°C and not more than 950°C, held at this maximum heating temperature for 60 to 720 seconds, and then cooled to 300°C or less at an average cooling rate from the maximum heating temperature to 300°C of 10 to 500°C / second; and (ii) a tempering step in which the steel sheet after the hot stamping step is tempered. Each step will be described.

[0081] (Hot Stamping Process) In the hot stamping process, the steel sheet according to this embodiment is used as a raw material (steel sheet for hot stamping). This steel sheet is heated to a maximum heating temperature of at least the higher of the Ac3 point (°C) and 800°C, but not exceeding 950°C. The steel sheet is then held at the maximum heating temperature for 60 to 720 seconds, and then cooled to 300°C or below so that the average cooling rate from the maximum heating temperature to 300°C is 10 to 500°C / s. This process dissolves carbides present in the steel sheet and increases its strength. If the maximum heating temperature is below the Ac3 point or below 800°C, or if the holding time is less than 60 seconds, the carbides will not dissolve sufficiently, or austenite transformation will be insufficient, resulting in insufficient strength after the hot stamping process. On the other hand, if the maximum heating temperature is too high or the holding time at the maximum heating temperature is too long, the crystal grains will become coarse, resulting in insufficient toughness and bendability of the formed body after the hot stamping process. Therefore, the maximum heating temperature should be 950°C or less, and the holding time at the maximum heating temperature should be 720 seconds or less. Furthermore, when cooling from the maximum heating temperature to 300°C or less, if the average cooling rate to 300°C is less than 10°C / s, sufficient hardening will not occur, and the hot-stamped body will not obtain sufficient tensile strength. On the other hand, if the average cooling rate to 300°C exceeds 500°C / s, the cooling rate will vary greatly from part to part, causing distortion in the shape of the formed body. The Ac3 point can be determined from the change point in the thermal expansion coefficient when the heating rate is set to 5°C / s, using a plate Formaster test or the like.

[0082] (Tempering Process) In the tempering process, the steel sheet after the hot stamping process is tempered at a temperature of 80 to 300°C. When the cooling stop temperature in the hot stamping process is 80 to 300°C, the steel sheet may be held at that temperature. Alternatively, the steel sheet may be once cooled to a temperature below 80°C and then reheated to 80 to 300°C and held at that temperature. After the hot stamping process, the steel sheet may be held at 80 to 300°C, once cooled to a temperature below 80°C, and then reheated to 80 to 300°C and held at that temperature. When the cooling stop temperature in the hot stamping process is less than 80°C, the steel sheet may be once heated to 80 to 300°C and held at that temperature. The holding temperature at 80 to 300°C is set to 6 seconds or longer, regardless of whether reheating is performed, in order to sufficiently precipitate ε carbide. There is no upper limit to the holding time, but holding for longer than necessary reduces productivity, so the holding time may be set to 1,800 seconds or shorter.

[0083] Slabs having the chemical compositions shown in Tables 1-1 and 1-2 were prepared. These slabs were heated under the conditions shown in Table 2-1, hot-rolled, cooled, and coiled to produce hot-rolled steel sheets of 2.6 mm.

[0084] These hot-rolled steel sheets were cold-rolled at the reduction ratios shown in Table 2-2, and then, with some exceptions, were heat-treated under the conditions shown in Table 2-2. Furthermore, in some examples, a coating (hot-dip galvanized layer or hot-dip Al-plated layer) was formed by hot-dip galvanization. The hot-dip galvanized layer was converted into a galvannealed layer by alloying. In Table 2-2, coating type GA is the galvannealed layer, and Al is the hot-dip Al-plated layer. Furthermore, in some examples, skin-pass rolling was performed. In Table 2-2, "-" indicates that no skin-pass rolling was performed. Steel sheets Nos. 1 to 23 and Nos. 101 to 113 were thus obtained.

[0085] For the obtained steel sheets, the microstructure fraction at the 1 / 4 depth position and the number density of carbides of one or more of Nb, Ti, Fe, Mo, W, and Cr having an equivalent circle diameter of 0.2 μm or more at the 1 / 4 depth position were measured in the same manner as described above. Furthermore, Vickers hardness was measured at the 50 μm depth position and the 1 / 4 depth position in the same manner as described above. The results are shown in Table 2-3.

[0086]

[0087]

[0088]

[0089]

[0090] [Amendment under Rule 91 26.03.2025]

[0091] The obtained steel sheets were then hot stamped and tempered under the conditions shown in Table 3-1 to obtain hot stamped products. Product No. H1 in the table indicates that steel sheet No. 1 was used as the raw material (steel sheet for hot stamping), product No. H2 indicates that steel sheet No. 2 was used as the raw material (steel sheet for hot stamping), and the same applies to the others. In Table 3-1, "-" indicates that the treatment was not carried out.

[0092] For the obtained hot-stamped body, the area fraction of martensite in the microstructure at the 1 / 4 depth position and the number density of ε carbides with a circle equivalent diameter of 5 nm or more at the 1 / 4 depth position were measured in the same manner as described above. Furthermore, Vickers hardness was measured at the 50 μm depth position and the 1 / 4 depth position in the same manner as described above. The results are shown in Table 3-2.

[0093] Furthermore, the tensile strength (TS) and critical bending angle were determined as an index of the impact absorption of the hot stamped body obtained.

[0094] (Tensile Strength) The tensile strength was determined by taking a No. 5 test piece as described in JIS Z2241:2011 from the flattest possible position of the hot-stamped body and conducting a tensile test on this test piece in accordance with the test method described in JIS Z2241:2011. The measurement was performed twice for each steel plate, and the average was taken as the measured value. A tensile strength of 2000 MPa or more was determined to be a preferable strength.

[0095] (Critical bending angle) A test piece measuring 60 mm in width and 30 mm in length (the length was parallel to the rolling direction) was taken from the flattest possible position of the hot-stamped body. Using this test piece, the steel sheet was bent in accordance with VDA238-100 while applying a load with a punch between a pair of rolls, and the bending angle of the steel sheet at which the reaction force of the punch reached its maximum was defined as the critical bending angle of the steel sheet. Measurements were performed twice for each steel sheet, and the average was taken as the measured value. If the critical bending angle was 40° or more in terms of a sheet thickness of 2.0 mm, it was determined that the steel sheet had excellent impact absorption properties.

[0096]

[0097] [Amendment under Rule 91 26.03.2025]

[0098] As can be seen from Tables 1-1 to 3-2, the hot-stamped steel sheets (H1 to H23) that were examples of the invention had chemical compositions and ε-carbide number densities within the ranges of the present invention, resulting in high strength and excellent impact absorption. In contrast, the hot-stamped steel sheets (H101 to H103, H105 to H109) that were comparative examples were made from unsuitable steel sheets. The chemical compositions or ε-carbide number densities of the steel sheets were outside the ranges of the present invention, resulting in poor tensile strength or impact absorption (critical bending angle). (H101 fractured prematurely before reaching TS during tensile and bending tests.) Because cracks developed during blank processing prior to hot stamping, further testing was not performed on H104. For H110, the heating temperature and heating time during the heat treatment process were too high and too long, resulting in insufficient quenching and insufficient tensile strength for a hot-stamped steel sheet. For No. H111 and H112, the steel sheet used as the starting material was preferable, but the hot stamping conditions were unfavorable, and sufficient tensile strength was not obtained for the hot-stamped body. For No. H113, the steel sheet used as the starting material was preferable, but the tempering after hot stamping was insufficient, and the number density of ε carbide was outside the range of the present invention, resulting in poor impact absorption (critical bending angle).

[0099] According to the present invention, it is possible to provide a hot-stamped steel sheet having both high strength and excellent impact absorption, a steel sheet suitable as a raw material for the hot-stamped steel sheet, and a method for manufacturing the hot-stamped steel sheet. The hot-stamped steel sheet meets the recent demand for automotive steel sheets that combine high strength and impact absorption, and can improve the fuel efficiency and collision safety of automobiles.

Claims

1. By mass%, C: 0.40-1.00%, Si: 0.01-1.00%, Mn: 0.01% or more, less than 1.00%, P: 0.100% or less, S: 0.01000% or less, Al: 0.0010-1.0000%, N: 0.0150% or less, Nb: 0-0.100%, Ti: 0-0.100%, Cr: 0-0.50%, V: 0-0.50%, Mo: 0-0.50%, B: 0-0.0100%, Co: 0-1.00%, Ni: 0-1.00%, Cu: 0-1.00%, W: 0-3.00%, O: 0-0.100%, A steel sheet has a chemical composition consisting of Ca: 0-1.00%, Mg: 0-1.00%, REM: 0-0.0050%, Sb: 0-0.020%, Zr: 0-0.10%, Sn: 0-0.10%, As: 0-0.10%, and the balance: Fe and impurities, and when a position at 1 / 4 of the thickness from the surface in the thickness direction is defined as a 1 / 4 depth position, the number density of ε carbides having a circle equivalent diameter of 5 nm or more at the 1 / 4 depth position is 20 pieces / μm ２ or more, and having a tensile strength of 2100 MPa or more.

2. The hot stamped product according to claim 1, wherein, when a position 50 μm from the surface in the thickness direction is defined as a 50 μm depth position, the hardness at the 50 μm depth position is lower than the hardness at the 1 / 4 depth position.

3. The hot stamped product according to claim 2, wherein the hardness at the 50 μm depth position is lower by HV 100 or more in Vickers hardness than the hardness at the 1 / 4 depth position.

4. The hot stamped product according to any one of claims 1 to 3, wherein the chemical composition includes, in mass%, one or more selected from the group consisting of Nb: 0.010 to 0.100%, Ti: 0.010 to 0.100%, Cr: 0.03 to 0.50%, and V: 0.01 to 0.50%.

5. The hot stamped product according to any one of claims 1 to 3, wherein the chemical composition includes, in mass%, one or more elements selected from the group consisting of Mo: 0.05 to 0.50%, B: 0.0010 to 0.0100%, Co: 0.01 to 1.00%, Ni: 0.10 to 1.00%, Cu: 0.10 to 1.00%, and W: 0.10 to 3.00%.

6. The hot stamped product according to any one of claims 1 to 3, wherein the chemical composition includes, in mass%, one or more elements selected from the group consisting of O: 0.001 to 0.100%, Ca: 0.01 to 1.00%, Mg: 0.01 to 1.00%, REM: 0.0001 to 0.0050%, Sb: 0.001 to 0.020%, Zr: 0.01 to 0.10%, Sn: 0.01 to 0.10%, and As: 0.01 to 0.10%.

7. By mass: C: 0.40-1.00%, Si: 0.01-1.00%, Mn: 0.01% or more, less than 1.00%, P: 0.100% or less, S: 0.01000% or less, Al: 0.0010-1.0000%, N: 0.0150% or less, Nb: 0-0.100%, Ti: 0-0.100%, Cr: 0-0.50%, V: 0-0.50%, Mo: 0-0.50%, B: 0-0.0100%, Co: 0-1.00%, Ni: 0-1.00%, Cu: 0-1.00%, W: 0-3.00%, O: 0-0.100%, It has a chemical composition consisting of Ca: 0-1.00%, Mg: 0-1.00%, REM: 0-0.0050%, Sb: 0-0.020%, Zr: 0-0.10%, Sn: 0-0.10%, As: 0-0.10%, and the balance: Fe and impurities, and when a position at 1 / 4 of the sheet thickness in the sheet thickness direction from the surface is defined as a 1 / 4 depth position, the microstructure at the 1 / 4 depth position consists, in terms of area ratio, of ferrite: more than 50% and 100% or less, pearlite: 0-40%, and bainite, martensite, and austenite: a total of 0% or more and less than 10%, At the 1 / 4 depth position, the number density of one or more carbides of Nb, Ti, Fe, Mo, W, and Cr having a circle equivalent diameter of 0.2 μm or more is 5.0 pieces / 10 μm ２ Less than steel plate.

8. The steel plate according to claim 7, wherein, when a position 50 μm from the surface in the plate thickness direction is defined as a 50 μm depth position, the hardness at the 50 μm depth position is smaller than the hardness at the 1 / 4 depth position.

9. The steel sheet according to claim 7 or 8, wherein the chemical composition includes, in mass%, one or more elements selected from the group consisting of Nb: 0.010 to 0.100%, Ti: 0.010 to 0.100%, Cr: 0.03 to 0.50%, and V: 0.01 to 0.50%.

10. The steel sheet according to claim 7 or 8, wherein the chemical composition includes, in mass%, one or more elements selected from the group consisting of Mo: 0.05 to 0.50%, B: 0.0010 to 0.0100%, Co: 0.01 to 1.00%, Ni: 0.10 to 1.00%, Cu: 0.10 to 1.00%, and W: 0.10 to 3.00%.

11. A method for producing the hot stamped product according to claim 1, comprising: a hot stamping step of heating the steel sheet according to claim 7 to a maximum heating temperature of at least the higher of the Ac3 point and 800°C and not exceeding 950°C, holding the steel sheet at the maximum heating temperature for 60 to 720 seconds, and then cooling the steel sheet to not more than 300°C so that the average cooling rate from the maximum heating temperature to 300°C is 10 to 500°C / sec; and a tempering step of tempering the steel sheet after the hot stamping step, wherein in the tempering step, the steel sheet is held at 80 to 300°C for 6.0 seconds or more, the steel sheet is cooled to less than 80°C at an average cooling rate of 20 to 500°C / sec, and then reheated and held at 80 to 300°C for 6.0 seconds or more, or the steel sheet is held at 80 to 300°C for 6.0 seconds or more, cooled to less than 80°C at an average cooling rate of 20 to 500°C / s, reheated, and held at 80 to 300°C for 6.0 seconds or more.

12. A method for producing the steel plate according to claim 7, comprising the steps of: C: 0.40-1.00%, Si: 0.01-1.00%, Mn: 0.01% or more and less than 1.00%, P: 0.100% or less, S: 0.01000% or less, Al: 0.0010-1.0000%, N: 0.0150% or less, Nb: 0-0.100%, Ti: 0-0.100%, Cr: 0-0.50%, V: 0-0.50%, Mo: 0-0.50%, B: 0-0.0100%, Co: 0-1 A heating process of heating a slab having a chemical composition consisting of: 0.00%, Ni: 0-1.00%, Cu: 0-1.00%, W: 0-3.00%, O: 0-0.100%, Ca: 0-1.00%, Mg: 0-1.00%, REM: 0-0.0050%, Sb: 0-0.020%, Zr: 0-0.10%, Sn: 0-0.10%, As: 0-0.10%, and the balance: Fe and impurities, to 1150-1350 ° C.; The method for producing a steel sheet includes: a hot rolling step in which the slab after the heating step is hot rolled at a finish rolling temperature of 800 to 950°C to obtain a steel sheet; a cooling step which is started within 5.0 seconds from the completion of the hot rolling step and in which the steel sheet after the hot rolling step is cooled to 750°C or less at an average cooling rate of 10 to 100°C / sec; a coiling step in which the steel sheet after the cooling step is coiled at a coiling temperature of more than 500°C and not more than 750°C, and the average cooling rate from the coiling temperature to 500°C is more than 50°C / hr; and a cold rolling step in which the steel sheet after the coiling step is cold rolled at a sheet thickness reduction rate of 10 to 60%.

13. The method for producing a steel sheet according to claim 12, further comprising a heat treatment step of heating the steel sheet to an annealing temperature of 700 to 920°C after the cold rolling step, and holding the steel sheet at the annealing temperature for 120 to 500 seconds in an atmosphere having an oxygen potential of -1.50 or more.

14. The method for producing a steel plate according to claim 13, further comprising a skin pass process of subjecting the steel plate to skin pass rolling at a rolling reduction rate of 0.05 to 2.0% after the heat treatment process.

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