Steel sheet for hot stamping and hot-stamp molded body

Abstract

This steel sheet for hot stamping comprises a steel sheet, wherein, when decarburization indexes of the steel sheet at five positions are measured, DIAVE, which is the average of the decarburization indexes at the five positions, DIMAX, which is the maximum value of the decarburization indexes at the five positions, and DIMIN, which is the minimum value of the decarburization indexes at the five positions, satisfy expressions (i) and (ii). This steel sheet for stamping is used to obtain a hot-stamp molded body.　Expression (i): 0.10 ≤ DIAVE ≤ 0.35 Expression (ii): DIMAX - DIMIN ≤ 0.15

Description

Hot stamping steel sheets and hot stamped products

[0001] This disclosure relates to a steel sheet for hot stamping and a hot stamped steel. This application claims priority based on Japanese Patent Application No. 2024-045334, filed on March 21, 2024, the contents of which are incorporated herein by reference.

[0002] In recent years, there has been a demand for reducing the weight of automobile bodies from the perspectives of environmental protection and resource conservation, and high-strength steel sheets have been used in automobile components. The use of high-strength steel sheets allows for the reduction of the steel sheet thickness, thereby reducing the weight of the automobile body while providing the desired strength. Automotive components are manufactured by press-forming steel sheets. However, as the strength of steel sheets increases, not only does the forming load increase, but formability also decreases, making them more susceptible to cracks and wrinkles. Furthermore, when high-strength steel sheets are press-formed, the shape of the component changes significantly due to springback when the component is removed from the mold, making it difficult to ensure the dimensional accuracy of the component. Thus, it is not easy to manufacture high-strength automotive components by press-forming.

[0003] To solve the above-mentioned problems, a technique has been proposed in the past, as disclosed in, for example, Patent Document 1, in which a heated steel sheet is press-formed using a low-temperature press die. This technique, known as hot stamping or hot pressing, press-forms a steel sheet that has been heated to a high temperature and is in a soft state, making it possible to manufacture components with complex shapes with high dimensional accuracy. Furthermore, since the steel sheet is rapidly cooled by contact with the die, quenching makes it possible to significantly increase the strength of the steel sheet during press forming. Patent Document 1 discloses that hot stamping a steel sheet with a tensile strength of 500 to 600 MPa can produce a formed product with a tensile strength of 1400 MPa or more.

[0004] The strength of a hot-stamped steel sheet can be further increased by increasing the C content of the steel sheet. However, when the C content of the steel sheet is increased, the deformability decreases as the strength of the hot-stamped steel sheet increases, and when the hot-stamped steel sheet deforms during a collision, cracks tend to occur in the early stages of deformation. As such, it is not easy to produce a high-strength hot-stamped steel sheet with high deformability, and it is particularly difficult to achieve both strength and deformability when the tensile strength of the hot-stamped steel sheet is 1800 MPa or more.

[0005] As a technique for producing a hot-stamped product having excellent deformability, Patent Document 2 discloses a high-strength pressed part having a tensile strength of 1300 MPa or more and high impact absorption, and a method for producing the same.

[0006] Patent Document 3 discloses a hot stamped automotive component having a tensile strength of 1100 MPa or more and enhanced bendability from the viewpoint of impact energy absorption, and a method for manufacturing the same.

[0007] In the methods disclosed in Patent Documents 2 and 3, a steel sheet for hot stamping having a decarburized surface layer is hot stamped under predetermined conditions to form a soft layer on the surface of the hot-stamped member, thereby improving the crash resistance of the member.

[0008] Japanese Patent Publication No. 2002-102980 Japanese Patent Publication No. 2015-30890 International Publication No. 2018 / 179839

[0009] However, studies by the present inventors have revealed that it is not easy to form a uniform decarburized layer on a steel sheet for hot stamping, and that the amount of decarburization is likely to vary depending on the position within the steel sheet. Furthermore, it has been found that when a hot-stamped product is produced using a steel sheet with a non-uniform amount of decarburization, the C concentration in the surface layer region of the hot-stamped product varies, which may result in a partial decrease in the deformability of the hot-stamped product and an insufficient improvement in crash resistance.

[0010] The present disclosure has been made in view of the above circumstances, and aims to provide a steel sheet for hot stamping suitable as a raw material for a hot-stamped steel sheet having high strength and excellent deformability, and a hot-stamped steel sheet having high strength and excellent deformability. Here, "excellent deformability" means that the deformability is highly uniform and high regardless of the position within the hot-stamped steel sheet.

[0011] The gist of the present disclosure is as follows: [1] A steel sheet for hot stamping, comprising a steel sheet, wherein the chemical composition of the steel sheet is, in mass%, C: more than 0.310% and 0.700% or less, Si: less than 2.00%, Mn: 0.01 to 3.00%, P: 0.200% or less, S: 0.0200% or less, sol. Al: 0.001 to 1.000%, N: 0.0200% or less, O: 0.0200% or less, B: 0.0002 to 0.0200%, Cr: 0 to 2.00%, Mo: 0 to 2.00%, W: 0 to 2.00%, Cu: 0 to 2.00%, Ni: 0 to 2.00%, Ti: 0 to 0.200%, Nb: 0 to 0.200%, V: 0 to 0.200%, Zr: 0 to 0.200%, Ca: 0 to 0.1000%, Mg: 0 to 0.1000%, REM: 0 to 0.1000%, Sn: 0 to 0.200%, As: 0 to 0.100%, and The steel plate contains 0 to 0.0500% Bi, with the balance being Fe and impurities. When the decarburization index is measured at five positions, namely, a position 50 mm from the end face in the plate width direction, a position 1 / 4 in the plate width direction, a central position in the plate width direction, a position 3 / 4 in the plate width direction, and a position 50 mm from the end face opposite to the end face, the DI is an average value of the decarburization indexes at the five positions. ＡＶＥ and DI, which is the maximum value of the decarburization index at the five points. ＭＡＸ and DI, which is the minimum value of the decarburization index at the five points. ＭＩＮ and the steel sheet for hot stamping satisfies the following formulas (i) and (ii): 0.10≦DI ＡＶＥ ≦0.35...(i) DI ＭＡＸ -DI ＭＩＮ≦0.15 ... (ii) [2] The chemical composition of the steel plate is, in mass%, Cr: 0.01 to 2.00%, Mo: 0.01 to 2.00%, W: 0.01 to 2.00%, Cu: 0.01 to 2.00%, Ni: 0.01 to 2.00%, Ti: 0.001 to 0.200%, Nb: 0.001 to 0.200%, V: 0.001 to 0.200%, Zr: 0.001 to 0.200%, Ca: 0.0001 to 0.1000%, Mg: 0.0001 to 0.1000%, REM: 0.0001 to 0.1000%, Sn: 0.001 to 0.200%, [3] A hot stamped steel sheet according to the above [1], characterized in that it contains one or more elements selected from the group consisting of As: 0.001 to 0.100%, and Bi: 0.0001 to 0.0500%. [3] A hot stamped product comprising a steel sheet, wherein all or a part of the steel sheet has a chemical composition, in mass%, of C: more than 0.310% and 0.700% or less, Si: less than 2.00%, Mn: 0.01 to 3.00%, P: 0.200% or less, S: 0.0200% or less, sol. Al: 0.001 to 1.000%, N: 0.0200% or less, O: 0.0200% or less, B: 0.0002 to 0.0200%, Cr: 0 to 2.00%, Mo: 0 to 2.00%, W: 0 to 2.00%, Cu: 0 to 2.00%, Ni: 0 to 2.00%, Ti: 0 to 0.200%, Nb: 0 to 0.200%, V: 0 to 0.200%, Zr: 0 to 0.200%, Ca: 0 to 0.1000%, Mg: 0 to 0.1000%, REM: 0 to 0.1000%, Sn: 0 to 0.200%, As: 0 to 0.100%, and The steel sheet contains 0 to 0.0500% Bi, with the balance being Fe and impurities, and when CS, which is an average value of the C concentration in mass % in the surface layer region of the steel sheet, is measured at each of five locations that satisfy the following conditions A to D in a surface layer region that is a region from a depth of 20 μm to a depth of 40 μm from the surface of the steel sheet, the average value of the CS at the five locations is CS ＡＶＥ and CS, which is the maximum value of the CSs at the five locations. ＭＡＸ and CS, which is the minimum value of the CS at the five locations. ＭＩＮand a hot stamped steel sheet characterized in that the following formulas (iii) and (iv) are satisfied, and the tensile strength is 1800 MPa or more. ＡＶＥ ≦0.80×C １ ...(iii) CS ＭＡＸ -CS ＭＩＮ ≦0.100 mass% (iv) Condition A: Each of the five locations is 25 mm or more away from the end face. Condition B: The five locations are 100 mm or more away from each other. Condition C: Of the five locations, at least two locations are 25 to 75 mm away from the end face. Condition D: Of the locations that are 25 to 75 mm away from the end face, the distance between the two most distant locations is 400 mm or more. However, C in the above formula (iii) １ is the C content in mass % in the chemical composition of the steel plate. [4] The chemical composition of the steel plate is, in mass%, Cr: 0.01 to 2.00%, Mo: 0.01 to 2.00%, W: 0.01 to 2.00%, Cu: 0.01 to 2.00%, Ni: 0.01 to 2.00%, Ti: 0.001 to 0.200%, Nb: 0.001 to 0.200%, V: 0.001 to 0.200%, Zr: 0.001 to 0.200%, Ca: 0.0001 to 0.1000%, Mg: 0.0001 to 0.1000%, REM: 0.0001 to 0.1000%, Sn: 0.001 to 0.200%, The hot-stamped steel according to [3], further comprising one or more elements selected from the group consisting of As: 0.001 to 0.100%, and Bi: 0.0001 to 0.0500%.

[0012] According to the above aspects of the present disclosure, it is possible to provide a steel sheet for hot stamping suitable as a raw material for a hot-stamped steel sheet having high strength and excellent deformability, and a hot-stamped steel sheet having high strength and excellent deformability. The hot-stamped steel sheet according to the above aspects has excellent deformability and is free from cracks in the early stage of deformation during a collision, and is therefore suitable for use in automotive components such as pillars, bumpers, and door beams.

[0013] 10 is a diagram for explaining five locations in the hat member that satisfy conditions A to D. FIG. 11 is a diagram showing a hat member manufactured in an example.

[0014] The present inventors have investigated methods for improving the deformability of hot-stamped steel sheets having a tensile strength of 1800 MPa or more, and have obtained the following findings.

[0015] (A) When hot stamping is performed using a steel sheet for hot stamping having a decarburized layer, a soft layer with a low C concentration is formed in the surface layer region of the hot stamped body, improving the deformability of the hot stamped body.

[0016] (B) In order to decarburize the surface layer of a steel sheet for hot stamping, it is effective to anneal it in an atmosphere with a higher dew point than usual (hereinafter, this may be referred to as high dew point annealing). However, when high dew point annealing is performed, the amount of decarburization tends to vary depending on the position on the steel sheet. The variation in the amount of decarburization is often particularly large in the width direction of the steel sheet. The amount of decarburization is often high near the center of the steel sheet in the width direction and decreases toward the edge.

[0017] (C) When hot stamping is performed using a steel sheet for hot stamping with a large variation in the amount of decarburization, the C concentration in the surface region after hot stamping becomes low in areas with a high amount of decarburization, and high in areas with a low amount of decarburization. This causes the C concentration to vary depending on the position in the surface region of the hot-stamped steel. Furthermore, the bendability of the hot-stamped steel decreases in areas with a high C concentration in the surface region of the hot-stamped steel. Therefore, if the variation in the amount of decarburization of the steel sheet for hot stamping, which is the raw material, is large, the deformability of the hot-stamped steel may decrease partially.

[0018] (D) The variation in the amount of decarburization in a steel sheet for hot stamping can be effectively suppressed by employing a process that satisfies the following conditions in the production of the steel sheet for hot stamping: (a) A reheating treatment is performed to reheat the end face side in the sheet width direction of the hot-rolled and coiled steel sheet, and (b) When annealing the cold-rolled steel sheet, the dew point of the atmospheric gas in the furnace is set to the intermediate dew point.

[0019] Although the reason why the adoption of the above process can suppress fluctuations in the amount of decarburization in a steel sheet for hot stamping is unclear, the inventors speculate as follows. After hot rolling and coiling into a coil, the longer a portion of the coil stays in the high temperature range (near the center in the sheet width direction), the more likely decarburization is to occur during high dew-point annealing. The coiling temperature near the end faces of the steel sheet is lower than near the center in the sheet width direction, and the cooling rate after coiling is also faster. However, by actively reheating, the residence time in the high temperature range is extended, and the difference with the center in the sheet width is reduced. If the dew point during high dew-point annealing of a steel sheet after cold rolling is slightly lowered (intermediate dew-point annealing is performed), the decarburization rate does not change significantly in portions where the residence time in the high temperature range is long, but decarburization is promoted in portions where the residence time in the high temperature range is short.

[0020] From the findings of (A) to (D) above, the present inventors have found that, in the manufacturing process of a steel sheet for hot stamping, reheating the end face portion of the steel sheet that has been hot rolled and wound into a coil, and performing annealing after cold rolling in an atmosphere with an intermediate dew point, can suppress fluctuations in the amount of decarburization in the steel sheet for hot stamping; and further, by hot stamping using a steel sheet for hot stamping that has small fluctuations in the amount of decarburization as a starting material, fluctuations in the C concentration in the surface layer region of the hot stamped product can be suppressed, and a hot stamped product can be obtained that has a tensile strength of 1800 MPa or more, excellent uniformity in deformability, and high deformability regardless of the position within the hot stamped product.

[0021] Hereinafter, the steel sheet for hot stamping according to this embodiment will be described in detail. First, the reasons for limiting the chemical composition of the steel sheet for hot stamping according to this embodiment will be described.

[0022] The steel sheet constituting the steel sheet for hot stamping according to this embodiment has the following chemical composition. Note that the numerical ranges described below, separated by "to", include the lower and upper limits. Numerical values ​​indicated as "less than" and "greater than" do not include the numerical range. All percentages regarding the chemical composition represent mass%.

[0023] The steel sheet constituting the hot stamping steel sheet according to this embodiment has a chemical composition, in mass%, of C: more than 0.310% and 0.700% or less, Si: less than 2.00%, Mn: 0.01 to 3.00%, P: 0.200% or less, S: 0.0200% or less, sol. Al: 0.001 to 1.000%, N: 0.0200% or less, O: 0.0200% or less, B: 0.0002 to 0.0200%, and the balance: Fe and impurities. Each element will be described below.

[0024] C: More than 0.310% and 0.700% or less C is an element that improves the tensile strength of the steel sheet after hot stamping (the steel sheet that constitutes the hot-stamped body). If the C content is 0.310% or less, the tensile strength of the steel sheet after hot stamping will be less than 1800 MPa, resulting in insufficient strength of the hot-stamped body. Therefore, the C content is set to more than 0.310%. The C content is preferably 0.320% or more, 0.340% or more, 0.380% or more, 0.420% or more, or 0.450% or more. On the other hand, if the C content exceeds 0.700%, the strength of the hot-stamped body will be too high, making it impossible to obtain excellent deformability. Therefore, the C content is set to 0.700% or less. The C content is preferably 0.650% or less, 0.600% or less, 0.550% or less, or 0.500% or less.

[0025] Si: Less than 2.00% Si is an element that embrittles steel. If the Si content is 2.00% or more, the adverse effects become particularly significant. Therefore, the Si content is set to less than 2.00%. The Si content is preferably 1.50% or less, 1.00% or less, 0.75% or less, 0.50% or less, or 0.20% or less. The lower limit of the Si content is not particularly limited, but may be 0%. Since an excessive decrease in the Si content increases steelmaking costs, the Si content is preferably 0.001% or more. Furthermore, since Si has the effect of improving the hardenability of steel, it may be intentionally added. From the viewpoint of improving hardenability, the Si content is preferably 0.05% or more, 0.10% or more, 0.15% or more, or 0.20% or more.

[0026] Mn: 0.01 to 3.00% Mn is an element that combines with S to form MnS and suppresses the adverse effects of S. If the Mn content is less than 0.01%, the above effect cannot be achieved. Therefore, the Mn content is set to 0.01% or more. Furthermore, Mn is an element that improves the hardenability of steel and is an element that forms a metal structure mainly composed of martensite in the inner layer region of the steel sheet after hot stamping, thereby ensuring the strength of the hot-stamped body. From the viewpoint of ensuring strength, the Mn content is preferably 0.50% or more, 0.75% or more, 1.00% or more, or 1.25% or more. On the other hand, if the Mn content exceeds 3.00%, excellent deformability cannot be obtained in the hot-stamped body. Therefore, the Mn content is set to 3.00% or less. The Mn content is preferably 2.50% or less, 2.00% or less, or 1.50% or less.

[0027] P: 0.200% or less P is an element that embrittles steel. If the P content exceeds 0.200%, the adverse effects become particularly large, and weldability also deteriorates significantly. Therefore, the P content is set to 0.200% or less. The P content is preferably 0.100% or less, 0.050% or less, or 0.020% or less. The P content may be 0%. However, if the P content is reduced to less than 0.001%, the cost of dephosphorization increases significantly, which is not economically preferable, so the P content may be set to 0.001% or more.

[0028] S: 0.0200% or less S is an element that embrittles steel. If the S content exceeds 0.0200%, the adverse effects become particularly significant. Therefore, the S content is set to 0.0200% or less. The S content is preferably 0.0050% or less, 0.0020% or less, or 0.0010% or less. The S content may be 0%. However, if the S content is reduced to less than 0.0001%, the cost of desulfurization increases significantly, which is not economically preferable, so the S content may be set to 0.0001% or more.

[0029] Sol. Al: 0.001 to 1.000% Al is an element that has the effect of deoxidizing molten steel. If the sol. Al content (acid-soluble Al content) is less than 0.001%, deoxidation will be insufficient. Therefore, the sol. Al content is set to 0.001% or more. The sol. Al content is preferably 0.005% or more, 0.010% or more, or 0.020% or more. On the other hand, if the sol. Al content is too high, the transformation point will rise, and the Ac content will increase during the heating process of hot stamping. ３ It becomes difficult to heat the steel sheet to a temperature exceeding this point. Furthermore, the strength and deformability of the hot-stamped steel sheet decrease. Therefore, the sol. Al content is set to 1.000% or less. The sol. Al content is preferably 0.500% or less, 0.100% or less, 0.060% or less, or 0.040% or less.

[0030] N: 0.0200% or less N is an element that forms nitrides during continuous casting of steel. Since these nitrides reduce the deformability of the steel sheet after hot stamping, a low N content is preferable. If the N content exceeds 0.0200%, the adverse effects become particularly significant. Therefore, the N content is set to 0.0200% or less. The N content is preferably 0.0100% or less, 0.0080% or less, or 0.0050% or less. The N content may be 0%. However, excessive reduction of the N content significantly increases the denitrification cost, which is economically undesirable, and therefore the N content may be set to 0.0005% or more, or 0.0010% or more.

[0031] O: 0.0200% or less O is an element that forms oxide-based inclusions. If the O content exceeds 0.0200%, a large amount of coarse oxide-based inclusions is formed in the steel. This deteriorates the deformability of the hot-stamped steel. Therefore, the O content is set to 0.0200% or less. The O content is preferably 0.0150% or less, 0.0100% or less, 0.0060% or less, or 0.0040% or less. The O content may be 0%. However, excessive reduction of the O content significantly increases the steelmaking cost and is economically undesirable, so the O content may be set to 0.0005% or more or 0.0010% or more.

[0032] B: 0.0002 to 0.0200% B is an element that improves the hardenability of steel. It forms a metallographic structure mainly composed of martensite in the inner layer region of the steel sheet after hot stamping, and is an effective element for ensuring the strength of the hot-stamped body. If the B content is less than 0.0002%, the desired strength cannot be obtained in the hot-stamped body. In addition, the deformability of the hot-stamped body deteriorates. Therefore, the B content is set to 0.0002% or more. The B content is preferably 0.0005% or more, 0.0010% or more, 0.0015% or more, or 0.0020% or more. On the other hand, if the B content exceeds 0.0200%, boron dioxide is formed in the hot-stamped body, impairing the hardenability-improving effect of B. Therefore, the B content is set to 0.0200% or less. The B content is preferably 0.0080% or less, 0.0050% or less, 0.0040% or less, or 0.0030% or less.

[0033] The balance of the chemical composition of the steel sheet constituting the hot stamping steel sheet according to the present embodiment is composed of Fe and impurities. Examples of impurities include elements that are inevitably mixed in from steel raw materials or scrap and / or during the steelmaking process and are permissible within a range that does not impair the properties of the hot stamped steel according to the present embodiment.

[0034] The steel sheet constituting the steel sheet for hot stamping according to this embodiment may contain the following elements as optional elements in place of a portion of Fe. When the following optional elements are not contained, the content is 0%.

[0035] Cr: 0.01 to 2.00% Cr is an element that increases the hardenability of steel, thereby increasing the strength of the hot-stamped product. To reliably obtain this effect, the Cr content is preferably 0.01% or more. The Cr content is more preferably 0.05% or more, 0.10% or more, or 0.20% or more. On the other hand, if the Cr content exceeds 2.00%, the deformability of the hot-stamped product decreases. Therefore, the Cr content is set to 2.00% or less. The Cr content is preferably 1.50% or less, 1.00% or less, 0.50% or less, 0.40% or less, or 0.30% or less.

[0036] Mo: 0.01 to 2.00% Mo is an element that increases the hardenability of steel, thereby increasing the strength of the hot-stamped body. To reliably obtain this effect, the Mo content is preferably 0.01% or more. The Mo content is more preferably 0.05% or more, 0.10% or more, or 0.15% or more. On the other hand, if the Mo content exceeds 2.00%, the deformability of the hot-stamped body decreases. Therefore, the Mo content is set to 2.00% or less. The Mo content is preferably 1.00% or less, 0.50% or less, 0.40% or less, or 0.30% or less.

[0037] W: 0.01 to 2.00% W is an element that increases the hardenability of steel, thereby increasing the strength of the hot-stamped steel. To reliably obtain this effect, the W content is preferably 0.01% or more. The W content is more preferably 0.05% or more or 0.10% or more. On the other hand, if the W content exceeds 2.00%, the deformability of the hot-stamped steel decreases. Therefore, the W content is set to 2.00% or less. The W content is preferably 0.50% or less, 0.40% or less, or 0.30% or less.

[0038] Cu: 0.01 to 2.00% Cu is an element that increases the hardenability of steel, thereby increasing the strength of the hot-stamped product. To reliably obtain these effects, the Cu content is preferably 0.01% or more. The Cu content is more preferably 0.10% or more. On the other hand, if the Cu content exceeds 2.00%, the deformability of the hot-stamped product decreases. Therefore, the Cu content is set to 2.00% or less. The Cu content is preferably 1.00% or less, 0.75% or less, or 0.50% or less.

[0039] Ni: 0.01 to 2.00% Ni is an element that increases the hardenability of steel, thereby increasing the strength of the hot-stamped body. To reliably obtain this effect, the Ni content is preferably 0.01% or more. The Ni content is more preferably 0.10% or more. On the other hand, if the Ni content exceeds 2.00%, the deformability of the hot-stamped body decreases. Therefore, the Ni content is set to 2.00% or less. The Ni content is preferably 1.00% or less, 0.75% or less, or 0.50% or less.

[0040] Ti: 0.001 to 0.200% Ti is an element that forms carbonitrides in steel and increases the strength of hot-stamped steel through precipitation strengthening. Ti also refines the metal structure, improving the deformability of the hot-stamped steel. To ensure these effects, the Ti content is preferably 0.001% or more. The Ti content is more preferably 0.005% or more or 0.010% or more. On the other hand, if the Ti content exceeds 0.200%, a large amount of coarse carbonitrides is formed in the steel, reducing the deformability of the hot-stamped steel. Therefore, the Ti content is set to 0.200% or less. The Ti content is preferably 0.100% or less, 0.050% or less, or 0.030% or less.

[0041] Nb: 0.001 to 0.200% Nb is an element that forms carbonitrides in steel and increases the strength of hot-stamped steel through precipitation strengthening. Nb also refines the metal structure, improving the deformability of the hot-stamped steel. To ensure these effects, the Nb content is preferably 0.001% or more. The Nb content is more preferably 0.005% or more or 0.010% or more. On the other hand, if the Nb content exceeds 0.200%, a large amount of coarse carbonitrides is formed in the steel, reducing the deformability of the hot-stamped steel. Therefore, the Nb content is set to 0.200% or less. The Nb content is preferably 0.100% or less, 0.050% or less, less than 0.040%, or 0.030% or less.

[0042] V: 0.001 to 0.200% V is an element that forms carbonitrides in steel and increases the strength of hot-stamped steel through precipitation strengthening. V also refines the metal structure, improving the deformability of the hot-stamped steel. To ensure these effects, the V content is preferably 0.001% or more. The V content is more preferably 0.005% or more or 0.010% or more. On the other hand, if the V content exceeds 0.200%, a large amount of coarse carbonitrides is formed in the steel, reducing the deformability of the hot-stamped steel. Therefore, the V content is set to 0.200% or less. The V content is preferably 0.100% or less or 0.050% or less.

[0043] Zr: 0.001 to 0.200% Zr is an element that forms carbonitrides in steel and increases the strength of hot-stamped steel through precipitation strengthening. Zr also refines the metal structure, improving the deformability of the hot-stamped steel. To ensure these effects, the Zr content is preferably 0.001% or more. The Zr content is more preferably 0.005% or more or 0.010% or more. On the other hand, if the Zr content exceeds 0.200%, a large amount of coarse carbonitrides is formed in the steel, reducing the deformability of the hot-stamped steel. Therefore, the Zr content is set to 0.200% or less. The Zr content is preferably 0.100% or less, 0.050% or less, or 0.025% or less.

[0044] Ca: 0.0001 to 0.1000% Ca is an element that improves the deformability of the steel sheet after hot stamping by adjusting the shape of inclusions. To reliably obtain this effect, the Ca content is preferably 0.0001% or more. However, even if a large amount of Ca is added, the above effect saturates and excessive costs are incurred, so the Ca content is set to 0.1000% or less. The Ca content is preferably less than 0.0100%, less than 0.0050%, or less than 0.0020%.

[0045] Mg: 0.0001 to 0.1000% Mg is an element that improves the deformability of the steel sheet after hot stamping by adjusting the shape of inclusions. To reliably obtain these effects, the Mg content is preferably 0.0001% or more. However, even if a large amount of Mg is added, the above effects saturate and excessive costs are incurred, so the Mg content is set to 0.1000% or less. The Mg content is preferably less than 0.0100%, less than 0.0050%, or less than 0.0020%.

[0046] REM: 0.0001 to 0.1000% REM is an element that improves the deformability of a steel sheet after hot stamping by adjusting the shape of inclusions. To reliably obtain this effect, the REM content is preferably 0.0001% or more. However, even if a large amount of REM is added, the above effect saturates and excessive costs are incurred, so the REM content is set to 0.1000% or less. The REM content is preferably less than 0.0100%, less than 0.0050%, or less than 0.0020%. In this embodiment, REM refers to a total of 17 elements consisting of Sc, Y, and lanthanides, and the REM content refers to the total content of these elements.

[0047] Sn: 0.001 to 0.200% Sn is an element that has the effect of improving the corrosion resistance of a hot-stamped steel. To reliably obtain this effect, the Sn content is preferably 0.001% or more. The Sn content is more preferably 0.005% or more, 0.015% or more, or 0.030% or more. On the other hand, even if a large amount of Sn is added, the above effect saturates and excessive costs are incurred, so the Sn content is set to 0.200% or less. The Sn content is preferably 0.150% or less, 0.100% or less, or 0.050% or less.

[0048] As: 0.001 to 0.100% As is an element that has the effect of increasing the strength of a hot-stamped steel. To reliably obtain this effect, the As content is preferably 0.001% or more. However, even if a large amount of As is added, the above effect saturates and, furthermore, excessive costs are incurred, so the As content is set to 0.100% or less.

[0049] Bi: 0.0001 to 0.0500% Bi is an element that refines the solidification structure, thereby increasing the deformability of the hot-stamped steel. To ensure this effect, the Bi content is preferably 0.0001% or more. However, even if a large amount of Bi is added, the above effect saturates and excessive costs are incurred, so the Bi content is set to 0.0500% or less. The Bi content is preferably 0.0100% or less or 0.0050% or less.

[0050] The chemical composition of the steel sheet constituting the above-mentioned hot stamping steel sheet can be determined by collecting a test piece from the steel sheet constituting the hot stamping steel sheet and measuring the average element content throughout the sheet thickness using a common analytical method. For example, this can be measured using inductively coupled plasma optical emission spectrometry or inductively coupled plasma mass spectrometry. C and S can be measured using a combustion-infrared absorption method, and O and N can be measured using an inert gas fusion-infrared absorption method or an inert gas fusion-thermal conductivity method. If the steel sheet constituting the hot stamping steel sheet has a coating or plating layer on its surface, this is removed before measuring the chemical composition.

[0051] In the steel sheet for hot stamping according to this embodiment, when the decarburization index is measured at five positions, namely, a position 50 mm from the end face in the sheet width direction of the steel sheet constituting the steel sheet for hot stamping, a 1 / 4 position in the sheet width direction, a central position in the sheet width direction, a 3 / 4 position in the sheet width direction, and a position 50 mm from the end face opposite to the end face, the DI is an average value of the decarburization indexes at the five positions. ＡＶＥ and DI, which is the maximum value of the decarburization index at the five points. ＭＡＸ and DI, which is the minimum value of the decarburization index at the five points. ＭＩＮ and satisfies the following formulas (i) and (ii): 0.10≦DI ＡＶＥ ≦0.35…(i) DI ＭＡＸ -DI ＭＩＮ ≦0.15...(ii)

[0052] The 1 / 4 position in the width direction of the steel plate is a position that is 1 / 4 of the plate width away from the end face in the plate width direction, and can be expressed as the w / 4 position when the length of the steel plate in the plate width direction is w. Similarly, the 3 / 4 position in the width direction of the steel plate is a position that is 3 / 4 of the plate width away from the end face in the plate width direction, and can be expressed as the w x 3 / 4 position when the length of the steel plate in the plate width direction is w.

[0053] DI, which is the average value of the decarburization indexes at the above five locations ＡＶＥ If DI is less than 0.10, it is not possible to preferably control the C concentration in the surface layer region of the hot stamped steel. ＡＶＥ DI must be 0.10 or more. ＡＶＥ is preferably 0.12 or more, more preferably 0.15 or more or 0.20 or more. ＡＶＥ In order to excessively increase DI, a long annealing time is required in the manufacturing process of the steel sheet for hot stamping, which will be described later, and the productivity of the steel sheet is impaired. ＡＶＥ If DI exceeds 0.35, the strength of the hot stamped product decreases. ＡＶＥ DI must be 0.35 or less. ＡＶＥ is preferably 0.30 or less or 0.25 or less.

[0054] DI, which is the maximum value of the decarburization index of the above five points ＭＡＸ and DI, which is the minimum value of the decarburization index at the above five points. ＭＩＮ When the difference between DI and DI exceeds the right side of the above formula (ii), i.e., exceeds 0.15, it is not possible to preferably control the C concentration in the surface layer region of the hot stamped steel. ＭＡＸ -DI ＭＩＮ DI must be 0.15 or less. ＭＡＸ -DI ＭＩＮ is preferably 0.12 or less, more preferably 0.10 or less. ＭＡＸ -DI ＭＩＮ The lower limit of is not particularly limited, but may be 0.01 or more, or may be 0.02 or more.

[0055] The decarburization indexes at five positions, i.e., a position 50 mm from the end face in the sheet width direction of the steel sheet constituting the steel sheet for hot stamping, a 1 / 4 position in the sheet width direction, a central position in the sheet width direction, a 3 / 4 position in the sheet width direction, and a position 50 mm from the end face opposite to the end face, are measured by the following method.

[0056] Test pieces are taken from the steel sheet constituting the steel sheet for hot stamping, and the distributions of C concentration and Fe concentration in the sheet thickness direction at the above five locations of the steel sheet for hot stamping are measured by glow discharge optical emission spectrometry (GDS analysis). Here, the measurement range is from the surface of the steel sheet to a position at a depth of 120 μm or more, and the measurement interval is 0.10 μm or less.

[0057] In the case where a plating layer, a coating film, or the like is present on the surface of the steel sheet constituting the hot stamping steel sheet, the plating layer, the coating film, or the like is partly or entirely removed before the GDS measurement so that measurement can be performed up to a depth of 120 μm from the surface of the steel sheet. In this embodiment, the region where the Fe concentration is 95 mass% or more in the GDS measurement is determined to be the steel sheet, and the depth position of the measurement point where the Fe concentration first becomes 95 mass% or more from the start of the measurement is determined to be the surface of the steel sheet.

[0058] The average value of the C concentration in the region from the surface of the steel plate to a depth of 120 μm from the surface of the steel plate (C ＊ ) is calculated, and C ＊ and the C content in mass% in the chemical composition of the steel sheet for hot stamping (C ０ ) and the decarburization index (DI) is calculated using the following formula (v): DI = {(C ０ -C ＊ ) / C ０} × 0.6 ... (v)

[0059] The steel sheet constituting the steel sheet for hot stamping according to this embodiment is not particularly limited as long as it can obtain the desired strength and deformability after hot stamping, but may have the following metallographic structure. For example, the steel sheet may be composed of, in area percentages, ferrite: 5 to 90%, bainite and martensite: 0 to 50%, pearlite: 10 to 95%, and retained austenite: 0 to 10%. In addition, the steel sheet may contain precipitates and inclusions such as iron carbides, alloy carbides, and intermetallic compounds.

[0060] The metallographic structure of the steel sheet constituting the steel sheet for hot stamping is measured by the following method. The metallographic structure of the steel sheet is measured from the surface to a depth of 1 / 4 of the sheet thickness (a region from the surface to a depth of 1 / 8 of the sheet thickness to a depth of 3 / 8 of the sheet thickness).

[0061] A test piece is taken from a steel plate for hot stamping, and the cross section of the steel plate through the thickness is buffed. Then, the microstructure is observed in the above region. Specifically, the polished surface is subjected to nital etching or electrolytic polishing, and then a microstructure photograph is taken using an optical microscope and a scanning electron microscope (SEM). Image analysis is performed on the obtained microstructure photograph based on brightness differences or differences in the morphology of iron carbides present within the phase to obtain the area ratios of ferrite, pearlite, bainite, tempered martensite, precipitates, and inclusions. Subsequently, similar observation positions are subjected to Lepera corrosion, and then a microstructure photograph is taken using an optical microscope or a scanning electron microscope (SEM). Image analysis is performed on the obtained microstructure photograph to calculate the total area ratio of "retained austenite and fresh martensite."

[0062] Furthermore, after electrolytically polishing the plate thickness cross section at the same observation position, the area ratio of retained austenite is measured using an SEM equipped with an electron backscatter pattern analyzer (EBSP device). The area ratio of retained austenite is obtained by calculating the area ratio of regions with an fcc crystal structure from the crystal orientation information obtained by EBSP analysis. The area ratio of fresh martensite is obtained by subtracting the area ratio of retained austenite from the sum of the area ratios of the above-mentioned "retained austenite and fresh martensite." In the EBSP analysis, the measurement interval is 0.1 μm, and the degree of vacuum in the device is 9.6 × 10－５ The pressure is set to 25 Pa or less, the acceleration voltage is 25 kV, and the probe current level is 16. To display the EBSP map, version 7 or later of OIM Analysis (registered trademark) manufactured by EDAX / TSL Solution is used for the obtained crystal orientation information.

[0063] Based on these results, the area fractions of ferrite, pearlite, bainite, martensite (tempered martensite and fresh martensite), retained austenite, precipitates, and inclusions are obtained.

[0064] In structural observation, tempered martensite can be distinguished from fresh martensite by the presence of iron carbides inside. Tempered martensite can also be distinguished from bainite by the fact that the iron carbides inside are elongated in multiple directions rather than in a single direction. Elongation in a single direction means that the difference in the elongation direction is within 5°.

[0065]

[0033] The thickness of the steel sheet for hot stamping according to the present embodiment (the thickness of the steel sheet when the steel sheet for hot stamping consists of only a steel sheet) is not particularly limited, but from the viewpoint of reducing the weight of the vehicle body, it is preferably 2.5 mm or less, 2.0 mm or less, 1.8 mm or less, or 1.6 mm or less. On the other hand, from the viewpoint of ensuring the amount of impact absorption, the thickness is preferably 0.4 mm or more, 0.6 mm or more, 0.8 mm or more, or 1.0 mm or more.

[0066] The steel sheet for hot stamping according to this embodiment may have a plating layer on the surface of the steel sheet. The plating layer may be formed on both sides or one side of the steel sheet. By having a plating layer on the surface of the steel sheet, corrosion resistance can be improved after hot stamping. Examples of types of plating layer include hot-dip aluminum plating, hot-dip aluminum-zinc alloy plating, hot-dip aluminum-silicon alloy plating, hot-dip galvanizing, electrogalvanizing, alloyed hot-dip galvanizing, hot-dip zinc-aluminum alloy plating, and electrogalvanizing-nickel alloy plating.

[0067] Next, a hot-stamped steel according to this embodiment, which is obtained by hot stamping the above-described steel sheet for hot stamping as a raw material, will be described.

[0068] In the hot-stamped steel according to this embodiment, all or part of the steel sheet constituting the hot-stamped steel has the above-mentioned chemical composition. Since the chemical composition is the same as that of the steel sheet constituting the above-mentioned steel sheet for hot stamping, detailed description thereof will be omitted.

[0069] When the hot-stamped product has a portion having a tensile strength of 1800 MPa or more and a portion having a tensile strength of less than 1800 MPa, it is sufficient that at least the portion having a tensile strength of 1800 MPa or more has the above-described chemical composition. In order to analyze the chemical composition of the portion having a tensile strength of 1800 MPa or more in the analysis of the chemical composition, a tensile test described below is performed, and a test specimen for chemical composition analysis is taken from a tensile test specimen that has obtained a tensile strength of 1800 MPa or more, or from a portion adjacent to the portion from which the tensile test specimen was taken.

[0070] In the steel sheet constituting the hot stamped steel according to this embodiment, when CS, which is the average value of the C concentration in mass% in the surface layer region of the steel sheet, is measured at each of five locations that satisfy the following conditions A to D in a surface layer region that is a region from a depth of 20 μm to a depth of 40 μm from the surface of the steel sheet, the average value of the C concentration in mass% in the surface layer region of the steel sheet is CS, ＡＶＥ and CS, which is the maximum value of the CSs at the five locations. ＭＡＸ and CS, which is the minimum value of the CS at the five locations. ＭＩＮ and satisfies the following formulas (iii) and (iv): ＡＶＥ ≦0.80×C １ ...(iii) CS ＭＡＸ -CS ＭＩＮ≦0.100 mass% (iv) Condition A: Each of the five locations is 25 mm or more away from the end face. Condition B: The five locations are 100 mm or more away from each other. Condition C: Of the five locations, at least two locations are 25 to 75 mm away from the end face. Condition D: Of the locations that are 25 to 75 mm away from the end face, the distance between the two most distant locations is 400 mm or more. However, C in the above formula (iii) １ is the C content in mass % in the chemical composition of the steel sheet constituting the hot stamped body.

[0071] CS is the average value of the CS of the above five locations. ＡＶＥ is greater than the right side of the above formula (iii), i.e., 0.80 × C １ If the C content exceeds 50%, the C concentration in the surface layer region of the steel sheet constituting the hot stamped steel sheet is too high, making it impossible to obtain the desired deformability in the hot stamped steel sheet. ＡＶＥ is 0.80 x C １ % by mass or less. ＡＶＥ is preferably 0.75 x C １ Mass% or less, 0.70×C １ Mass% or less or 0.65 x C １ % by mass or less. ＡＶＥ Although there is no particular restriction on the lower limit of CS, if it is too low, a long annealing time is required in the manufacturing process of the steel sheet for hot stamping, which will be described later, and the productivity of the steel sheet is impaired. In addition, the heating time in the heating process for heating the steel sheet for hot stamping, which will be described later, must be shortened, which makes it difficult to produce a hot stamped steel. ＡＶＥ The lower limit of is preferably 0.020% by mass or more, 0.050% by mass or more, or 0.100% by mass or more.

[0072] CS is the maximum value of the CS of the above five locations ＭＡＸ and the minimum value of the CS of the above five locations, CS ＭＩＮWhen the difference between the C concentration and the C content exceeds the right-hand side of the formula (iv), i.e., exceeds 0.100 mass%, this indicates that the C concentration fluctuates significantly in the surface layer region of the steel sheet constituting the hot-stamped steel. This makes it impossible to obtain the desired deformability in the hot-stamped steel. ＭＡＸ -CS ＭＩＮ is 0.100 mass% or less. ＭＡＸ -CS ＭＩＮ is preferably 0.080% by mass or less, more preferably 0.050% by mass or less. ＭＡＸ -CS ＭＩＮ The lower limit of the content is not particularly limited, but may be 0.010% by mass or more, or 0.020% by mass or more.

[0073] Conditions A to D will now be described in detail with reference to the drawings. An example of a hot-stamped product is the product shown in FIG. 2. FIG. 2 shows a hat member produced in an example that will be described later. Here, five locations that satisfy conditions A to D when the hot-stamped product is a hat member such as that shown in FIG. 2 will be described.

[0074] FIG. 1 is a diagram illustrating five locations in a hat member that satisfy conditions A to D. The values ​​in the diagram are in mm. Points 1 to 5 in the diagram are examples of five locations that satisfy conditions A to D. In FIG. 1, points 1 to 5 are each 25 mm or more away from the end face. That is, condition A is satisfied. Here, the end face refers to the surface where the cross section of the steel sheet constituting the hot-stamped body is exposed (including cases where the cross section is painted). The distance from the measurement point to the end face refers to the shortest distance along the surface of the member from the measurement point to the end face. Points 1 to 5 are 100 mm or more away from each other. That is, condition B is satisfied. Here, the distance between measurement points refers to the shortest distance along the surface of the member between the measurement points. Points 1, 2, 4, and 5 are within the range of 25 to 75 mm from the end face. That is, condition C is satisfied. Of points 1, 2, 4, and 5, points 1 and 5 are the furthest apart, at 900 mm. That is, condition D is satisfied. Five locations are selected as described above, and the average value of the carbon concentration (CS) in the surface layer region is measured at these locations.

[0075] The CS at five locations that satisfy conditions A to D is measured by the following method. Test pieces are taken from the hot-stamped steel, and the distributions of C and Fe concentrations in the thickness direction at the five locations on the hot-stamped steel are measured by glow discharge optical emission spectrometry (GDS analysis). The measurement range is from the surface of the steel sheet to a depth of 40 μm or more, and the measurement interval is 0.10 μm or less.

[0076] In addition, when the hot-stamped steel sheet has a plating layer, a coating film, or the like on its surface, the steel sheet is subjected to GDS measurement after removing part or all of the plating layer, coating film, or the like so that measurement can be performed up to a depth of 40 μm from the surface of the steel sheet. In this embodiment, the region where the Fe concentration is 95 mass% or more in the GDS measurement is determined to be the steel sheet, and the depth position of the measurement point where the Fe concentration first becomes 95 mass% or more from the start of measurement is determined to be the surface of the steel sheet.

[0077] From the results of the GDS measurement, the average value of the carbon concentration (CS) in the surface layer region, which is the region from 20 μm deep from the surface of the steel sheet to 40 μm deep from the surface, is calculated. Using the CS obtained at five locations, the average value of the CS at the five locations, CS ＡＶＥ and CS, which is the maximum value of the CS at the five locations. ＭＡＸ and CS, which is the minimum value of the CS at the five locations. ＭＩＮ When the hot stamped steel sheet has a portion having a tensile strength of 1800 MPa or more and a portion having a tensile strength of less than 1800 MPa, the above-mentioned CS is obtained at least in the portion having a tensile strength of 1800 MPa or more. ＡＶＥ , C.S. ＭＡＸ and C.S. ＭＩＮ The test piece to be used for the GDS measurement may be obtained by subjecting the sample to a tensile test described later to a process adjacent to a portion from which a tensile test piece having a tensile strength of 1800 MPa or more was obtained.

[0078] The metal structure of the steel sheet constituting the hot stamped steel is not particularly limited as long as the desired strength and deformability can be obtained, but it is preferable that the steel sheet has the metal structure shown below.

[0079] Since martensite is an effective structure for increasing the tensile strength of a steel sheet after hot stamping, it is preferable that the area ratio of martensite is greater than 90.0% in the region from 200 μm deep from the surface of the steel sheet constituting the hot-stamped steel sheet to the center of the sheet thickness (hereinafter sometimes referred to as the inner layer region). If the area ratio of martensite in the inner layer region is 90.0% or less, the tensile strength of the hot-stamped steel sheet may be less than 1800 MPa, resulting in insufficient strength. Therefore, it is preferable that the area ratio of martensite in the inner layer region is greater than 90.0%. The area ratio of martensite in the inner layer region is more preferably greater than 91.0%, greater than 93.0%, or greater than 95.0%.

[0080] Although there is no particular need to set an upper limit for the area ratio of martensite in the inner layer region, in order to significantly increase the area ratio of martensite, it is necessary to excessively increase the heating temperature of the steel sheet for hot stamping or the cooling rate in the hot stamping step described below, which would significantly impair the productivity of the hot-stamped steel. Therefore, it is preferable that the area ratio of martensite in the inner layer region is 99.0% or less or 98.0% or less.

[0081] In this embodiment, the martensite includes not only fresh martensite that has not been tempered, but also tempered martensite that has been tempered and contains iron carbides therein.

[0082] The remainder of the metal structure of the inner layer region may contain ferrite, pearlite, bainite, or retained austenite, and may further contain precipitates such as cementite present alone, and inclusions such as oxides. Since it is not necessary to contain ferrite, pearlite, bainite, retained austenite, precipitates, and inclusions, the lower limits of the area ratios of ferrite, pearlite, bainite, retained austenite, precipitates, and inclusions are all 0%.

[0083] Retained austenite has the effect of improving the deformability of a steel sheet after hot stamping. To achieve this effect, the area fraction of retained austenite in the inner layer region is preferably 0.5% or more, 1.0% or more, or 2.0% or more. On the other hand, excessively increasing the area fraction of retained austenite requires austempering treatment at high temperature after hot stamping, which significantly reduces the productivity of the hot-stamped steel sheet. Furthermore, excessive retained austenite may deteriorate the deformability of the hot-stamped steel sheet. Therefore, the area fraction of retained austenite in the inner layer region is preferably less than 9.0%, less than 7.0%, less than 5.0%, or less than 4.0%.

[0084] When the hot-stamped steel has a portion having a tensile strength of 1800 MPa or more and a portion having a tensile strength of less than 1800 MPa, it is sufficient that at least the portion having a tensile strength of 1800 MPa or more has the above-described metal structure.

[0085] The area ratio of the metallographic structure of the steel sheet constituting the hot-stamped steel is measured by the same method as that for the steel sheet for hot stamping described above. Test specimens for microstructure observation may be taken from a portion adjacent to a portion from which a tensile test specimen having a tensile strength of 1800 MPa or more was taken, in a tensile test described later.

[0086] The thickness of the hot-stamped product according to this embodiment (the thickness of the steel plate when the hot-stamped product is made of only a steel plate) is not particularly limited, but from the viewpoint of reducing the vehicle body weight, it is preferably 2.5 mm or less, 2.0 mm or less, 1.8 mm or less, or 1.6 mm or less. On the other hand, from the viewpoint of ensuring the amount of impact absorption, the thickness is preferably 0.4 mm or more, 0.6 mm or more, 0.8 mm or more, or 1.0 mm or more.

[0087] Tensile Strength All or a portion of the hot-stamped steel according to this embodiment has a tensile strength of 1800 MPa or more. To achieve this, it is necessary that all or a portion of the steel plate constituting the hot-stamped steel according to this embodiment has a tensile strength of 1800 MPa or more. If the tensile strength of at least a portion of the hot-stamped steel is not 1800 MPa or more, it will be impossible to ensure the deformation load required for deformation of the hot-stamped steel. Therefore, the tensile strength of all or a portion of the hot-stamped steel is set to 1800 MPa or more. Preferably, the tensile strength of all or a portion of the hot-stamped steel is 1900 MPa or more, 2000 MPa or more, 2100 MPa or more, 2300 MPa or more, or 2500 MPa or more. On the other hand, excessively increasing the strength of the hot-stamped steel causes a decrease in deformability, so the tensile strength of the hot-stamped steel is preferably less than 3000 MPa or less than 2800 MPa.

[0088] The hot-stamped steel according to this embodiment may have a tensile strength of 1800 MPa or more throughout (the entire hot-stamped steel), or it may have a mixture of portions with a tensile strength of 1800 MPa or more and portions with a tensile strength of less than 1800 MPa. By providing portions with different strengths, it becomes possible to control the deformation state of the hot-stamped steel during collision. A hot-stamped steel having portions with different strengths can be produced by a method of joining two or more types of steel sheet with different chemical compositions and then hot stamping, a method of partially changing the heating temperature of the steel sheet or the cooling rate after hot stamping in the hot stamping step, and a method of partially reheating the hot-stamped steel.

[0089] When a hot-stamped steel sheet has a portion having a tensile strength of 1800 MPa or more and a portion having a tensile strength of less than 1800 MPa, it is difficult to ensure deformability in the portion having a tensile strength of 1800 MPa or more, and cracks often occur during collision. However, in the hot-stamped steel sheet according to this embodiment, the deformability of the hot-stamped steel sheet can be improved in the portion having a tensile strength of 1800 MPa or more by controlling the chemical composition of the steel sheet constituting the hot-stamped steel sheet and the C concentration distribution in the surface layer region, as described above. Note that, in this embodiment, "a portion" refers to the minimum amount necessary to ensure the properties of the hot-stamped steel sheet. For example, 5% or more, 10% or more, 15% or more, 20% or more, 25% or more, or 30% or more of the steel sheet in the hot-stamped steel sheet by volume may have the above-described chemical composition, C concentration distribution, and tensile strength.

[0090] The tensile strength of a hot-stamped steel sheet can be measured by taking a small, rectangular piece from the hot-stamped steel sheet, processing it into a tensile test piece without surface grinding, and conducting a tensile test. Specifically, it is preferable to take a No. 13B plate test piece in accordance with JIS Z 2241:2022 and conduct a tensile test at a tensile speed of 10 mm / min. If it is not possible to take a No. 13B test piece, a tensile test piece having a parallel portion of any width can be taken, and a tensile test can be conducted at a tensile speed of 10 mm / min, and the tensile strength can be determined from the maximum test force and the original cross-sectional area of ​​the parallel portion. If the hot-stamped steel sheet contains a mixture of high-strength and low-strength portions, a tensile test piece can be taken from the high-strength portion. If the tensile strength cannot be measured by the above-mentioned method, the Vickers hardness can be measured and converted to tensile strength. The Vickers hardness can be measured by the following method. A test piece is cut out from the hot-stamped steel sheet so that a cross section perpendicular to the surface (thickness cross section) can be observed. After buffing the cross section of the steel plate, a Vickers hardness test is performed in accordance with JIS Z 2244-1:2024. The Vickers hardness test is performed using a micro Vickers hardness tester at a depth of 1 / 4 of the plate thickness (the region from the surface to a depth of 1 / 8 of the plate thickness to a depth of 3 / 8 of the plate thickness) with a load of 9.8 N and a load holding time of 10 seconds. Measurement points are set at any five points at a depth of 1 / 4 of the plate thickness. The interval between each measurement point is set to at least three times the size of the indentation. The Vickers hardness is measured at these five measurement points and the average value is calculated to obtain the Vickers hardness. The obtained Vickers hardness is multiplied by 3.1 to convert it to tensile strength.

[0091] The hot-stamped product according to this embodiment may have a plating layer on its surface. Having a plating layer on its surface can improve corrosion resistance after hot stamping. The plating layer may be formed on both sides or one side of the hot-stamped product. Examples of the plating layer include an aluminum-based plating layer formed by hot stamping a steel sheet for hot stamping having a hot-dip aluminum-plated layer, a hot-dip aluminum-zinc alloy-based plating layer, a hot-dip aluminum-silicon alloy-plated layer, or the like; and a zinc-based plating layer formed by hot stamping a steel sheet for hot stamping having a hot-dip galvanized layer, an electrogalvanized layer, a galvannealed layer, a hot-dip zinc-aluminum alloy-based plating layer, an electrogalvanized-nickel alloy-based plating layer, or the like.

[0092] Next, a method for manufacturing a steel sheet for hot stamping according to this embodiment will be described.

[0093] The steel sheet for hot stamping is produced by a production method including a hot rolling process in which a slab having the above-mentioned chemical composition is hot rolled and coiled to obtain a hot rolled steel sheet, and then a reheating process, a cold rolling process in which the hot rolled steel sheet is cold rolled to obtain a cold rolled steel sheet, and an annealing process in which the cold rolled steel sheet is annealed to obtain an annealed steel sheet.

[0094] The method for producing a slab used in the method for producing a steel sheet for hot stamping is not particularly limited. Steel having the above-described chemical composition is melted by a known means and then formed into a steel ingot by a continuous casting method, or formed into a steel billet by a method such as blooming after being formed into a steel ingot by any casting method. In the continuous casting process, it is preferable to generate an external additional flow, such as electromagnetic stirring, in the molten steel in the mold to suppress the occurrence of surface defects due to inclusions. The steel ingot or billet may be cooled and then reheated for hot rolling. Alternatively, the steel ingot in a high temperature state after continuous casting or the billet in a high temperature state after blooming may be subjected to hot rolling directly, with warming, or with supplementary heating. Such steel ingots and billets are collectively referred to as "slabs" as raw materials for hot rolling.

[0095] In order to prevent coarsening of austenite, the heating temperature of the slab to be subjected to hot rolling is preferably less than 1250° C., more preferably less than 1200° C. If the slab heating temperature is too low, rolling becomes difficult, so the slab heating temperature may be 1050° C. or higher.

[0096] The heated slab is hot-rolled to obtain a hot-rolled steel sheet. Hot-rolling is carried out by using Ar to transform austenite after the rolling is completed, thereby refining the metal structure of the hot-rolled steel sheet. ３ It is preferable to complete the reaction in a temperature range above this point.

[0097] When the hot-rolled steel sheet is coiled after hot rolling, the coiling temperature is preferably 450°C or higher and lower than 600°C. If the coiling temperature is lower than 450°C or higher than 600°C, even if the coiled hot-rolled steel sheet is subjected to a reheating treatment described below, the temperature of the end face of the steel sheet cannot be sufficiently increased, and the decarburization index of the steel sheet for hot stamping may not be preferably controlled. As a result, the desired deformability may not be obtained in the hot-stamped steel sheet. The coiling temperature is the average temperature in the width direction of the hot-rolled steel sheet to be coiled, and this average temperature can be measured with a radiation thermometer.

[0098] A hot-rolled steel sheet (hot-rolled coil) that has been hot-rolled and wound into a coil is subjected to a recuperation treatment by covering it with a heat-retaining cover. This causes the temperature of the end surface of the hot-rolled steel sheet to rise due to transformation heat and heat transfer from the center of the coil. The amount of temperature rise at the end surface of the hot-rolled steel sheet due to the recuperation treatment is preferably 50°C or more. By increasing the temperature rise at the end surface of the hot-rolled steel sheet by 50°C or more, the difference between the amount of decarburization at the end surface in the sheet width direction and the amount of decarburization near the center in the sheet width direction can be reduced in the steel sheet for hot stamping. The recuperation treatment time (the time from covering the end surface to removing the cover) is preferably 4.0 to 6.0 hours. If the recuperation time is less than 4.0 hours, the amount of temperature rise may be insufficient. On the other hand, if the recuperation time exceeds 6.0 hours, thermally stable iron carbides may be formed, which may deteriorate the deformability of the hot-stamped product. Furthermore, during the recuperation treatment, the residence time of the end surface of the hot-rolled steel sheet in a temperature range of 600°C or higher is preferably less than 5.0 hours. In the recuperation treatment, if the residence time in the temperature range of 600°C or higher is too long, the amount of decarburization becomes excessive, and DI ＡＶＥ may become high.

[0099] The temperature rise at the end face of the hot-rolled steel sheet due to the reheating treatment is the difference in the average temperature at the end face of the hot-rolled steel sheet before and after the cover is removed. The average temperature at the end face of the hot-rolled steel sheet can be obtained by measuring the temperature of the entire side face of the hot-rolled coil with a thermoviewer.

[0100] After removing the cover, the coil is unwound, pickled as necessary, and then cold-rolled according to a conventional method to obtain a cold-rolled steel sheet. In the cold-rolling process, the cumulative reduction in cold rolling is preferably 40% or more. If the cumulative reduction is less than 40%, the metal structure of the steel sheet for hot stamping may become coarse. If the metal structure of the steel sheet for hot stamping is coarse, the metal structure of the hot-stamped body will become coarse after hot stamping, causing a decrease in the deformability of the hot-stamped body. On the other hand, an excessive increase in the cumulative reduction increases the load on the rolling equipment and causes a decrease in productivity, so the cumulative reduction is preferably less than 70%. After cold rolling, treatment such as degreasing may be performed according to a conventional method.

[0101] After cold rolling, the steel sheet is annealed to obtain an annealed steel sheet. In the annealing step, the dew point of the atmospheric gas in the furnace is preferably set to −30.0° C. or higher and lower than −10.0° C. The atmosphere is preferably, for example, a nitrogen-hydrogen atmosphere containing 1% by volume or higher and lower than 4% by volume of hydrogen.

[0102] In this embodiment, the difference between the amount of decarburization at the end face side in the sheet width direction of the annealed steel sheet (steel sheet for hot stamping) and the amount of decarburization near the center in the sheet width direction can be reduced by annealing the cold-rolled steel sheet that has been reheated and cold-rolled in an atmosphere with a medium dew point of not less than −30.0° C. and less than −10.0° C. The annealing temperature can be determined by measuring the temperature at the center in the sheet width direction of the cold-rolled steel sheet to be annealed with a radiation thermometer.

[0103] The soaking temperature during annealing is preferably set to more than 700°C. If the soaking temperature is 700°C or lower, it may not be possible to favorably control the decarburization index in the steel sheet for hot stamping. As a result, it may not be possible to obtain the desired deformability in the hot-stamped steel sheet. On the other hand, if the heating rate is too slow, the soaking temperature is too high, or the soaking time is too long, the metal structure of the annealed steel sheet may become coarse due to grain growth, and the deformability of the hot-stamped steel sheet may decrease. Therefore, it is preferable to set the average heating rate up to the soaking temperature to 1°C / sec or higher, the soaking temperature to 800°C or lower, and the soaking time (holding time at the soaking temperature) to less than 600 seconds. In addition, it is preferable to set the heating rate up to 700°C or higher, (Ac ３ It is preferable that the residence time in the temperature range below -30°C is more than 420 seconds and less than 600 seconds.

[0104] 700℃ or more, (Ac ３ If the residence time in the temperature range below the temperature point (-30°C) is 420 seconds or less, the decarburization index of the steel sheet for hot stamping may not be controlled in a favorable manner. As a result, the desired deformability may not be obtained in the hot-stamped steel sheet. On the other hand, if the residence time in the above temperature range is 600 seconds or more, excessive decarburization may occur in the steel sheet for hot stamping, and the strength of the hot-stamped steel sheet after hot stamping may be insufficient.

[0105] The annealed steel sheet produced by the above-mentioned method may be plated in a conventional manner to obtain a plated steel sheet. The annealed steel sheet or plated steel sheet thus obtained may be temper rolled in a conventional manner.

[0106] In addition, Ac ３ The point can be determined from the change in thermal expansion when a cold-rolled steel sheet is heated at a heating rate of 8°C / second.

[0107] The hot-stamped steel according to this embodiment can be obtained by a production method including a heating step of heating a steel sheet for hot stamping (annealed steel sheet or plated steel sheet) produced by the above-described method, and a hot stamping step of hot stamping the heated steel sheet for hot stamping. In order to stably obtain the hot-stamped steel according to this embodiment, it is preferable to perform hot stamping by the following method.

[0108] In the heating step, the above-mentioned steel sheet for hot stamping is heated prior to the hot stamping step. In the heating step, in order to form a metal structure mainly composed of martensite in the inner layer region of the hot stamped body and to obtain a desired strength, the heating temperature is set to be higher than 900°C and Ac ３ It is preferable that the heating temperature is more than 60 seconds. ３ The point is the Ac of the cold-rolled steel sheet obtained by the above method. ３ It should be the same value as the point.

[0109] On the other hand, if the heating temperature is too high or the holding time at the heating temperature is too long, the metal structure of the hot-stamped steel may become coarse, which may reduce the deformability of the hot-stamped steel and the strength of the hot-stamped steel. Therefore, the heating temperature is preferably less than 1050°C and the holding time is preferably less than 600 seconds.

[0110] In the hot stamping process, it is preferable that the heated steel sheet for hot stamping is removed from the heating furnace and allowed to cool in the atmosphere, and then hot stamping is started in a temperature range above 750°C. If the starting temperature for hot stamping is 750°C or lower, ferrite may be excessively formed in the metal structure of the hot stamped body, which may result in a decrease in the strength of the hot stamped body. After forming by hot stamping, the hot stamped body is cooled while held in the die, and / or the hot stamped body is removed from the die and cooled by any method.

[0111] If the cooling rate is low, the area fraction of martensite in the metallographic structure of the hot-stamped body may be insufficient, resulting in a decrease in the strength of the hot-stamped body. Therefore, the average cooling rate from the hot stamping start temperature to 400°C is preferably 30°C / sec or more, 60°C / sec or more, or 90°C / sec or more. Furthermore, if the cooling stop temperature is high, the area fraction of martensite in the metallographic structure of the hot-stamped body may be insufficient, resulting in a decrease in the strength of the hot-stamped body. Therefore, the cooling stop temperature for the above cooling is preferably less than 90°C.

[0112] The hot-stamped product according to this embodiment is obtained by the above method. It is preferable to perform a reheating treatment after hot stamping. The reheating treatment improves the deformability of the hot-stamped product. The reheating treatment is preferably performed at a heating temperature of 130°C or higher and with a holding time at the heating temperature of 10 minutes or longer. On the other hand, if the heating temperature is too high or the holding time at the heating temperature is too long, the strength of the hot-stamped product may decrease. Therefore, the heating temperature is preferably less than 170°C, and the holding time is preferably less than 20 minutes. The hot-stamped product may also be subjected to a blasting treatment, or a painting and baking treatment.

[0113] Next, examples of the present disclosure will be described, but the conditions in the examples are examples adopted to confirm the feasibility and effects of the present disclosure, and the present disclosure is not limited to these examples. Various conditions may be adopted in the present disclosure as long as they do not deviate from the gist of the present disclosure and the object of the present disclosure is achieved.

[0114] Steels having the chemical compositions shown in Tables 1A and 1B were melted and cast to obtain steel ingots. The resulting steel ingots were heated to 1200°C and hot-rolled in a temperature range of 920°C or higher. They were then cooled and wound into coils at a coiling temperature of 410 to 630°C. The resulting hot-rolled coils were covered with heat-retaining covers and held for 3.4 to 6.0 hours for recuperation treatment. Recuperation treatment was omitted for some hot-rolled coils. After removing the covers, the hot-rolled coils were allowed to cool, yielding hot-rolled steel sheets with a thickness of 2.6 mm. The hot-rolled steel sheets were pickled and then cold-rolled to obtain cold-rolled steel sheets with a thickness of 1.4 mm. The cumulative reduction during cold rolling was 46%.

[0115] In addition, blank cells in Tables 1A and 1B indicate that the content of the element in question was below the detection limit. ３ The points were determined from the change in thermal expansion when the cold-rolled steel sheets of each steel were heated at 8°C / second.

[0116] The obtained cold-rolled steel sheets were continuously annealed under the annealing conditions shown in Tables 2A and 2B. The atmosphere during soaking in the annealing furnace was a nitrogen-hydrogen atmosphere containing 2% by volume of hydrogen, and the dew points were as shown in Tables 2A and 2B. After soaking, the sheets were cooled to room temperature to obtain annealed steel sheets (steel sheets for hot stamping) having a sheet width of 1000 mm. In No. 4, the residence time of the end surface of the hot-rolled steel sheet in a temperature range of 600°C or higher during the recuperation treatment was less than 3.0 hours. The "residence time" in Tables 2A and 2B refers to the time period from 700°C or higher (Ac ３ This indicates the residence time in a temperature range below -30°C.

[0117] Note that, as a result of observing the metallographic structure of the steel sheet for hot stamping according to the present invention example by the above-mentioned method, it was found that the metallographic structure at a depth of ¼ of the sheet thickness from the surface of the steel sheet consisted of, in terms of area ratio, ferrite: 5 to 90%, bainite and martensite: 0 to 50%, pearlite: 10 to 95%, and retained austenite: 0 to 10%.

[0118] The obtained steel sheet for hot stamping was divided in the longitudinal direction (rolling direction), and test pieces for GDS measurement were taken from five positions in the width direction in the center of the longitudinal direction, specifically, from five positions: 50 mm from the end face in the sheet width direction of the steel sheet, 1 / 4 position in the sheet width direction, the center position in the sheet width direction, 3 / 4 position in the sheet width direction, and 50 mm from the end face opposite to the end face. The surface of the test piece was used as the measurement starting surface, and GDS measurement was performed from the measurement starting surface to a depth of 130 μm in the sheet thickness direction using the above-mentioned method, and the decarburization index at the five positions was determined. The measurement points from the start of measurement to the depth of 130 μm were 1,800 points. From the measurement results at the five positions, the average value of the decarburization index at the five positions, DI, was calculated. ＡＶＥ and DI, which is the maximum value of the decarburization index at five points. ＭＡＸ and DI, which is the minimum value of the five decarburization indices. ＭＩＮ The results are shown in Tables 3A and 3B.

[0119] Next, a hot stamping blank measuring 960 mm in the width direction and 240 mm in the length direction was taken from the longitudinal center of the hot stamping steel sheet, and hot stamping was performed to obtain a hat member (hot-stamped product) having the shape shown in FIG. 2 . In the hot stamping process, the hot stamping blank was heated under the conditions shown in Tables 3A and 3B. The hot stamping blank was then removed from the heating furnace and allowed to cool. It was then sandwiched between dies equipped with a cooling device and subjected to hat forming (hot stamping) with a forming start temperature of 770°C or higher. Subsequently, the average cooling rate from the forming start temperature to 400°C was 50°C / sec or higher, and the die was cooled to a cooling stop temperature of 80°C or lower. Furthermore, oxide scale (iron oxide) formed on the surface of the hat member was removed by shot blasting. The hat member was then placed in an electric heating furnace and subjected to a reheating treatment in which it was heated to 160°C and held there for 10 minutes.

[0120] A test specimen was taken from the longitudinal center of the top plate of the obtained hat member, and the chemical composition was measured by the above-mentioned method, and the metallographic structure was observed. The C content measured for the hat member is shown in Tables 3A and 3B. The contents of elements other than C were the same as those shown in Tables 1A and 1B. In the metallographic structure of the hot-stamped steel according to the present invention, in the inner layer region of the steel plate at the longitudinal center of the top plate of the hat member, the area ratio of martensite was 92.0% or more, and the total area ratio of structures other than martensite was 8.0% or less.

[0121] In addition, a No. 13B plate-shaped test piece was taken from the longitudinal center of the top plate of the hat member along the longitudinal direction of the hat member in accordance with JIS Z 2241:2022, and a tensile test was carried out at a tensile speed of 10 mm / min to determine the tensile strength.

[0122] If the obtained tensile strength was 1800 MPa or more, it was judged to have high strength and pass. On the other hand, if the obtained tensile strength was less than 1800 MPa, it was judged to have insufficient strength and fail. Note that the hat member having the shape shown in Figure 2 exhibits the lowest tensile strength at the longitudinal center of the top plate.

[0123] As shown in FIG. 1, test pieces for GDS measurement were taken from three locations on the top plate portion of the hat member and two locations on the vertical wall portion. The surface of the test piece was used as the measurement starting surface, and GDS measurement was performed from the measurement starting surface to a depth of 50 μm in the plate thickness direction by the above-mentioned method. The CS value, which is the average value of the CS values ​​at the five locations in the surface layer region, which is the region from a depth of 20 μm to a depth of 40 μm from the surface of the steel plate, was calculated. ＡＶＥ and CS, which is the maximum value of the CS at the five locations. ＭＡＸ and CS, which is the minimum value of the CS at the five locations. ＭＩＮ The measurement points were 720, ranging from the surface of the steel plate to a depth of 50 μm.

[0124] In addition, 30 mm square VDA bending test specimens were taken adjacent to the GDS measurement specimens from three locations on the top plate portion and two locations on the vertical wall portion of the hat member, and bending tests were performed in accordance with the German Association of the Automotive Industry standard VDA 238-100. The test specimens were bent so that the ridge direction of the bend was perpendicular to the rolling direction of the steel plate, and the bending angle (VDA bending angle) was determined when the bending load decreased by 30 N from the highest point. If a crack occurred before the bending load reached the highest point (early fracture occurred), the bending angle at the time the crack occurred was determined and used as the VDA bending angle.

[0125] When the tensile strength of the steel sheet constituting the hot-stamped steel sheet was less than 2300 MPa, if the average VDA bend angle at five locations was 60.0° or more and the minimum VDA bend angle at five locations was 55.0° or more, the hot-stamped steel sheet was judged to have high uniformity of deformability and excellent deformability and to pass. Furthermore, when the tensile strength was 2300 MPa or more, if the average VDA bend angle at five locations was 40.0° or more and the minimum VDA bend angle at five locations was 35.0° or more, the hot-stamped steel sheet was judged to have high uniformity of deformability and excellent deformability and to pass. If these conditions were not met, the hot-stamped steel sheet was judged to have poor deformability and to fail.

[0126]

[0127]

[0128]

[0129]

[0130]

[0131]

[0132] As can be seen from Tables 3A and 3B, the hot-stamped steels according to the examples of the present invention had high strength and excellent deformability, while the hot-stamped steels according to the comparative examples were inferior in one or more of the above properties.

[0133] According to the above aspects of the present disclosure, it is possible to provide a steel sheet for hot stamping suitable as a raw material for a hot-stamped steel having high strength and excellent deformability, and a hot-stamped steel having high strength and excellent deformability.

Claims

1. A hot stamping steel sheet comprising a steel plate, the chemical composition of which is, in mass%, C: more than 0.310% and 0.700% or less, Si: less than 2.00%, Mn: 0.01 to 3.00%, P: 0.200% or less, S: 0.0200% or less, sol. Al: 0.001 to 1.000%, N: 0.0200% or less, O: 0.0200% or less, B: 0.0002 to 0.0200%, Cr: 0 to 2.00%, Mo: 0 to 2.00%, W: 0 to 2.00%, Cu: 0 to 2.00%, Ni: 0 to 2.00%, Ti: 0 to 0.200%, Nb: 0 to 0.200%, V: 0 to 0.200%, Zr: 0 to 0.200%, Ca: 0 to 0.1000%, Mg: 0 to 0.1000%, REM: 0 to 0.1000%, Sn: 0 to 0.200%, As: 0 to 0.100%, and The steel plate contains 0 to 0.0500% Bi, with the balance being Fe and impurities. When the decarburization index is measured at five positions, namely, a position 50 mm from the end face in the plate width direction, a position 1 / 4 in the plate width direction, a central position in the plate width direction, a position 3 / 4 in the plate width direction, and a position 50 mm from the end face opposite to the end face, the DI is an average value of the decarburization indexes at the five positions. ＡＶＥ and DI, which is the maximum value of the decarburization index at the five points. ＭＡＸ and DI, which is the minimum value of the decarburization index at the five points. ＭＩＮ and the steel sheet for hot stamping satisfies the following formulas (i) and (ii): 0.10≦DI ＡＶＥ ≦0.35...(i) DI ＭＡＸ -DI ＭＩＮ ≦0.15...(ii) 2. The chemical composition of the steel plate is, in mass%, Cr: 0.01 to 2.00%, Mo: 0.01 to 2.00%, W: 0.01 to 2.00%, Cu: 0.01 to 2.00%, Ni: 0.01 to 2.00%, Ti: 0.001 to 0.200%, Nb: 0.001 to 0.200%, V: 0.001 to 0.200%, Zr: 0.001 to 0.200%, Ca: 0.0001 to 0.1000%, Mg: 0.0001 to 0.1000%, REM: 0.0001 to 0.1000%, Sn: 0.001 to 0.200%, The steel sheet for hot stamping according to claim 1, further comprising at least one element selected from the group consisting of As: 0.001 to 0.100%, and Bi: 0.0001 to 0.0500%.

3. A hot stamped product comprising a steel plate, wherein all or part of the steel plate has a chemical composition, in mass %, of C: more than 0.310% and 0.700% or less, Si: less than 2.00%, Mn: 0.01 to 3.00%, P: 0.200% or less, S: 0.0200% or less, sol. Al: 0.001 to 1.000%, N: 0.0200% or less, O: 0.0200% or less, B: 0.0002 to 0.0200%, Cr: 0 to 2.00%, Mo: 0 to 2.00%, W: 0 to 2.00%, Cu: 0 to 2.00%, Ni: 0 to 2.00%, Ti: 0 to 0.200%, Nb: 0 to 0.200%, V: 0 to 0.200%, Zr: 0 to 0.200%, Ca: 0 to 0.1000%, Mg: 0 to 0.1000%, REM: 0 to 0.1000%, Sn: 0 to 0.200%, As: 0 to 0.100%, and The steel sheet contains 0 to 0.0500% Bi, with the balance being Fe and impurities, and when CS, which is an average value of the C concentration in mass % in the surface layer region of the steel sheet, is measured at each of five locations that satisfy the following conditions A to D in a surface layer region that is a region from a depth of 20 μm to a depth of 40 μm from the surface of the steel sheet, the average value of the CS at the five locations is CS ＡＶＥ and CS, which is the maximum value of the CSs at the five locations. ＭＡＸ and CS, which is the minimum value of the CS at the five locations. ＭＩＮ and a hot stamped steel sheet characterized in that the following formulas (iii) and (iv) are satisfied, and the tensile strength is 1800 MPa or more. ＡＶＥ ≦0.80×C １ ...(iii) CS ＭＡＸ -CS ＭＩＮ ≦0.100 mass% (iv) Condition A: Each of the five locations is 25 mm or more away from the end face. Condition B: The five locations are 100 mm or more away from each other. Condition C: Of the five locations, at least two locations are 25 to 75 mm away from the end face. Condition D: Of the locations that are 25 to 75 mm away from the end face, the distance between the two most distant locations is 400 mm or more. However, C in the above formula (iii) １ is the C content in mass % in the chemical composition of the steel plate.

4. The chemical composition of the steel plate is, in mass%, Cr: 0.01 to 2.00%, Mo: 0.01 to 2.00%, W: 0.01 to 2.00%, Cu: 0.01 to 2.00%, Ni: 0.01 to 2.00%, Ti: 0.001 to 0.200%, Nb: 0.001 to 0.200%, V: 0.001 to 0.200%, Zr: 0.0001 to 0.200%, Ca: 0.0001 to 0.1000%, Mg: 0.0001 to 0.1000%, REM: 0.0001 to 0.1000%, Sn: 0.001 to 0.200%, The hot-stamped steel according to claim 3, further comprising at least one element selected from the group consisting of As: 0.001 to 0.100%, and Bi: 0.0001 to 0.0500%.

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