Hot stamping steel sheets and hot stamped products

Abstract

The present invention provides a steel sheet for hot stamping that has a prescribed chemical composition, in which: when the Mo content of the steel sheet is measured, by line analysis using EPMA, in a range of 0.05 mm in the thickness direction centered on a depth position at one-quarter of the thickness of the steel sheet from the surface of the steel sheet, the maximum value of the Mo content, the minimum value of the Mo content, and the average value of the Mo content are such that ([Mo]MAX–[Mo]MIN) / [Mo]AVE<0.50; and the standard deviation of the Vickers hardness in a region of 0.6 mm in a direction orthogonal to the thickness direction and 0.3 mm in the thickness direction centered on a depth position at one-quarter of the thickness of the steel sheet from the surface of the steel sheet is 20 (Hv) or less.

Description

[Technical Field]

[0001] The present invention relates to a steel sheet for hot stamping and a hot stamped product. This application claims priority based on Japanese Patent Application No. 2021-081622, filed on May 13, 2021, the contents of which are incorporated herein by reference. [Background technology]

[0002] In today's highly specialized industrial technology fields, materials used in each technology field are required to have specialized and advanced performance. For example, in the case of automotive steel sheets, high strength is required to improve fuel efficiency by reducing the weight of the vehicle body, in consideration of the global environment. When high-strength steel sheets are used in automobile bodies, the desired strength can be imparted to the vehicle body while reducing the thickness of the steel sheet and reducing the vehicle body weight.

[0003] However, in press forming, a process for forming automobile body components, the thinner the steel sheet used, the more likely it is that cracks and wrinkles will occur, so automotive steel sheets also need to have excellent press formability.

[0004] Since ensuring press formability and increasing the strength of steel sheets are contradictory elements, it is difficult to simultaneously satisfy both of these characteristics. 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 die, making it difficult to ensure the dimensional accuracy of the component. Thus, it is not easy to manufacture high-strength car body components by press forming.

[0005] To date, a technique for press-forming a heated steel sheet using a low-temperature press die has been proposed as a method for manufacturing ultra-high-strength vehicle body components, as disclosed in Patent Document 1, for example. This technique is called hot stamping or hot pressing, and since a steel sheet heated to a high temperature and in a soft state is press-formed, it is possible to manufacture components with complex shapes with high dimensional accuracy. Furthermore, since the steel sheet is rapidly cooled by contact with the die, it is possible to significantly increase the strength by quenching at the same time as press-forming. For example, Patent Document 1 describes that a component with a tensile strength of 1400 MPa or more can be obtained by hot stamping a steel sheet with a tensile strength of 500 to 600 MPa.

[0006] As a technique for producing a hot-stamped member with even higher strength, Patent Document 2 discloses a hot-stamped member having a tensile strength of 1770 to 1940 MPa and a manufacturing method thereof, and Patent Document 3 discloses a hot-stamped member having a tensile strength of 1960 to 2130 MPa and a manufacturing method thereof. In the methods described in Patent Documents 2 and 3, a hot-stamping steel sheet is heated to a two-phase region of ferrite and austenite and then hot-stamped, resulting in a metal structure of the hot-stamped member consisting of a composite structure of ferrite and martensite with an average grain size of 7 μm or less, thereby improving the ductility of the steel sheet constituting the member. However, according to the inventors' investigations, when a hot-stamped member consisting of a composite structure of ferrite and martensite deforms during a collision, cracks may occur initiating in the ferrite at the initial stage of deformation. It has been found that, particularly when the tensile strength of the member exceeds 2300 MPa, it becomes difficult to ensure the collision safety of the vehicle body.

[0007] Patent Document 4 discloses a technique for producing a hot-stamped member with excellent toughness and a tensile strength of 1800 MPa or more. In the method described in Patent Document 4, a hot-stamping steel sheet is heated to a low-temperature austenite range, hot-stamped, and then cooled relatively slowly in a temperature range below the Ms point to form a metallographic structure consisting of tempered martensite with a prior austenite grain size of 10 μm or less, thereby improving the toughness of the member. The technique disclosed in Patent Document 4 is advantageous in that it can produce a hot-stamped member with a tensile strength of 1800 MPa that is free from cracking even in low-temperature impact tests. However, there is no mention of a member with a tensile strength of 2300 MPa or more. According to the inventors' investigations, even in a hot-stamped member with a tempered martensite single-phase structure as described in Patent Document 4, increasing the tensile strength to 2300 MPa or more can result in localized hardness fluctuations within the member, which can lead to cracking during the initial deformation phase during a collision and result in insufficient crashworthiness. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2002-102980 [Patent Document 2] Japanese Patent Publication No. 2010-65294 [Patent Document 3] Japanese Patent Application Publication No. 2010-65295 [Patent Document 4] Japanese Patent Publication No. 2006-152427 Summary of the Invention [Problem to be solved by the invention]

[0009] As described above, it has been difficult with conventional techniques to manufacture a component having a tensile strength of 2300 MPa or more by hot stamping, particularly a hot-stamped component (hot-stamped product) having a tensile strength of 2300 MPa or more and excellent impact resistance.

[0010] The present invention aims to solve the above problems and to provide a steel sheet for hot stamping suitable as a raw material for hot-stamped products having excellent crash resistance and a tensile strength of 2300 MPa or more, and a hot-stamped product having excellent crash resistance and a tensile strength of 2300 MPa or more. [Means for solving the problem]

[0011] The present invention has been made to solve the above-mentioned problems, and the gist of the present invention is the following steel sheet for hot stamping.

[0012] [1] A steel sheet for hot stamping according to one embodiment of the present invention comprises, in mass%, C: more than 0.40% and 0.70% or less, Si: less than 2.00%, Mn: 0.01% or more and less than 0.50%, P: 0.200% or less, S: 0.0200% or less, sol.Al: 0.001 to 1.000%, N: 0.0200% or less, Mo: 0.0 1% or more, less than 0.50%, B: 0.0002~0.0200%, Ti: 0~0.200%, Nb: 0~0.200%, V: 0~0.200%, Zr: 0~0.200%, Cr: 0~2.00%, W: 0~2.00%, Cu: 0~2.00%, Ni: 0~2.00%, Ca: 0~0.0100%, Mg: 0~0 and a chemical composition of 0.0100%, REM: 0-0.1000%, Bi: 0-0.0500%, and the balance: Fe and impurities, and when the Mo content of the steel plate is measured by linear analysis using an EPMA in a range of 0.05 mm in the thickness direction centered at a depth position of 1 / 4 of the plate thickness of the steel plate from the surface of the steel plate, the maximum Mo content, the minimum Mo content, and the average Mo content satisfy the following formula (i): and the standard deviation of Vickers hardness in a range of 0.3 mm in the thickness direction centered at a depth position of 1 / 4 of the plate thickness of the steel plate from the surface of the steel plate and 0.6 mm in the direction perpendicular to the plate thickness direction is 20 (Hv) or less. The average value of the Vickers hardness is 280 (Hv) or more. . ([Mo] MAX -[Mo] MIN ) / [Mo] AVE <0.50 (i) However, the meanings of the symbols in the above formula (i) are as follows: [Mo] MAX : Maximum Mo content (mass%) [Mo] MIN : Minimum Mo content (mass%) [Mo] AVE : Average value of Mo content (mass%) [2] The steel sheet for hot stamping described in [1] may have a chemical composition containing, in mass%, one or more elements selected from Ti: 0.001 to 0.200%, Nb: 0.001 to 0.200%, V: 0.001 to 0.200%, and Zr: 0.001 to 0.200%. [3] The steel sheet for hot stamping according to [1] or [2] may have a chemical composition containing, in mass%, one or more elements selected from Cr: 0.001 to 2.00%, W: 0.001 to 2.00%, Cu: 0.001 to 2.00%, and Ni: 0.001 to 2.00%. [4] In the steel sheet for hot stamping according to any one of [1] to [3], the chemical composition may contain, in mass%, one or more elements selected from Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0100%, and REM: 0.0001 to 0.1000%. [5] In the steel sheet for hot stamping according to any one of [1] to [4], the chemical composition may contain, in mass %, Bi: 0.0001 to 0.0500%. [6] A hot stamped product according to another embodiment of the present invention has a base steel sheet, and the base steel sheet contains, in mass%, C: more than 0.40% and 0.70% or less, Si: less than 2.00%, Mn: 0.01% or more and less than 0.50%, P: 0.200% or less, S: 0.0200% or less, sol.Al: 0.001 to 1.000%, N: 0.0200% or less, Mo: 0.01% or more and less than 0.50%, B: 0.0002 to 0.0200%, Ti: 0 to 0.200%, Nb: 0 to 0.200%, V: 0 to 0.200%, Zr: 0 to 0.200%, Cr: 0 to 2.00%, W: 0 to 2.00%, C A base steel plate has a chemical composition of u: 0 to 2.00%, Ni: 0 to 2.00%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, REM: 0 to 0.1000%, Bi: 0 to 0.0500%, and the balance: Fe and impurities, and when the Mo content of the base steel plate is measured by linear analysis using an EPMA within a range of 0.05 mm in the plate thickness direction centered at a depth position of 1 / 4 of the plate thickness of the base steel plate from the surface of the base steel plate, the maximum value, the minimum value, and the average value of the Mo content satisfy the following formula (ii), and the metallographic structure of the base steel plate has a martensite structure of 90.0 volume % or more, the standard deviation of Vickers hardness in a region from the surface of the base steel plate to 0.3 mm in the thickness direction and 0.6 mm in the direction perpendicular to the thickness direction, centered at a depth position of 1 / 4 of the thickness of the base steel plate, is 20 (Hv) or less, and the tensile strength of the base steel plate is 2300 MPa or more. ([Mo] mMAX -[Mo] mMIN ) / [Mo] mAVE <0.50 (ii) However, the meanings of the symbols in the above formula (ii) are as follows: [Mo] mMAX : Maximum Mo content of base steel sheet (mass%) [Mo] mMIN : Minimum Mo content of base steel plate (mass%) [Mo] mAVE : Average value of Mo content in base steel sheet (mass%) [Effects of the Invention]

[0013] According to the above-described aspect of the present invention, it is possible to obtain a steel sheet for hot stamping that is suitable as a material for a hot stamped product having excellent impact resistance and a tensile strength of 2300 MPa or more, and a hot stamped product having excellent impact resistance and a tensile strength of 2300 MPa or more. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 2 is a schematic diagram showing hardness measurement positions of a steel sheet for hot stamping and a hot stamped product. [Figure 2] 1 is a schematic diagram showing an example of the shape of a hot stamped product. FIG. [Figure 3] FIG. 1 is a schematic diagram showing the shape of a three-point bending test specimen. [Figure 4] FIG. 1 is a schematic diagram showing the arrangement of a testing machine and a test specimen in a three-point bending test. DETAILED DESCRIPTION OF THE INVENTION

[0015] The present inventors have conducted extensive research into methods for suppressing the occurrence of cracks during deformation due to collision in hot-stamped products having a tensile strength of 2300 MPa or more. In particular, they have conducted extensive research into methods for suppressing the occurrence of cracks during deformation due to collision in hot-stamped products by controlling the chemical composition and structure of a steel sheet for hot stamping used in the hot-stamped product. As a result, they have obtained the following findings.

[0016] (A) Hot stamped products with a tensile strength of 2300 MPa or more are prone to localized variations in hardness, and when the hot stamped product deforms, stress concentrates in the areas with low hardness, causing cracks to occur in the early stages of deformation.

[0017] (B) By using a steel sheet for hot stamping in which the local variation in Mo concentration is small, the occurrence of cracks during deformation of a hot stamped product is suppressed. The reason for this is not clear, but it is thought to be due to the following: (a) In the portion with a low Mo concentration, austenite coarsens during the process of heating the steel sheet in the hot stamping step, and the hardness of the hot stamped product tends to be low; and (b) On the other hand, in the portion with a high Mo concentration, austenite refines during the process of heating the steel sheet, and the hardness of the hot stamped product tends to be high.

[0018] (C) In a steel sheet for hot stamping, by minimizing local variations in hardness, the occurrence of cracks during deformation of a hot stamped product can be suppressed. The reason for this is not clear, but it is thought to be due to the following: (a) In steel sheets for hot stamping, the localized presence of soft ferrite increases fluctuations in hardness; (b) In areas with a high ferrite fraction, austenite coarsens during the heating process of the steel sheet in the hot stamping process, and the hot-stamped product tends to have low hardness; and (c) On the other hand, in areas with a low ferrite fraction, austenite refines during the heating process of the steel sheet, and the hot-stamped product tends to have high hardness.

[0019] (D) By using a steel sheet for hot stamping that has been produced without annealing after undergoing a cold rolling process (also called an as-cold-rolled steel sheet or full hard steel sheet), the occurrence of cracks when the formed product is deformed is suppressed. The reason for this is not clear, but it is thought to be due to the following: (a) In as-cold-rolled steel sheet, processing strain during cold rolling accumulates, and therefore, in the hot stamping process, the austenite becomes finer as the steel sheet is heated, increasing the hardness of the hot-stamped product; and (b) this effect is stronger in areas with low Mo concentration and high ferrite fraction, so that the use of as-cold-rolled steel sheet reduces local hardness variations in the hot-stamped product.

[0020] (E) In the process of producing a steel sheet for hot stamping, the hot-rolled steel sheet is annealed by heating it to above the Ac3 point and holding it there for a long time (also called the first hot-rolled sheet annealing), thereby reducing the local variation in Mo concentration in the steel sheet for hot stamping.

[0021] (F) In the process of producing a steel sheet for hot stamping, the first hot-rolled sheet annealing is followed by an annealing step (also referred to as a second hot-rolled sheet annealing) in which the steel sheet is heated to above the Ac3 point and held there for a short period of time, thereby reducing the variation in local hardness of the steel sheet for hot stamping. The reason for this is not clear, but it is thought to be due to the following: (a) in the first annealing of the hot-rolled sheet, austenite tends to coarsen during annealing, resulting in the localization of coarse ferrite after annealing; and (b) in the second annealing of the hot-rolled sheet, austenite does not coarsen during annealing, resulting in the uniform, fine dispersion of ferrite after annealing.

[0022] From the findings of (A) to (F) above, the present inventors have found that by hot stamping using a steel sheet for hot stamping in which the local variation in Mo concentration is small and further the local variation in hardness is small, it is possible to produce a hot stamped product which has excellent impact resistance and a tensile strength of 2300 MPa or more and which has small local variation in hardness. Hereinafter, each requirement of the steel sheet for hot stamping according to the embodiment of the present invention (steel sheet for hot stamping according to the present embodiment) will be described in detail.

[0023] <Chemical composition of steel sheets for hot stamping> The steel sheet for hot stamping according to this embodiment has the chemical composition shown below. The reasons for limiting each element are as follows. In the following description, "%" for the content means "mass %." Furthermore, numerical ranges indicated with "to" include both ends of the range. On the other hand, numerical values indicated with "less than" and "greater than" do not include the value in the range.

[0024] C: More than 0.40%, less than 0.70% C is an element that has the effect of increasing the tensile strength of the steel sheet after hot stamping (the steel sheet included in the hot-stamped product). If the C content is 0.40% or less, the tensile strength of the steel sheet after hot stamping will be less than 2300 MPa, and the strength of the hot-stamped product will be insufficient. Therefore, the C content is set to more than 0.40%. The preferred C content is more than 0.42%, more than 0.43%, more than 0.44%, or more than 0.45%. On the other hand, if the C content exceeds 0.70%, the strength of the hot stamped product becomes too high and it becomes impossible to ensure impact resistance. Therefore, the C content is set to 0.70% or less. The C content is preferably 0.65% or less, 0.60% or less, 0.55% or less, or 0.50% or less.

[0025] Si: Less than 2.00% Silicon is contained in steel as an impurity and is an element that embrittles steel. If the Si content is 2.00% or more, the adverse effects become particularly severe. Therefore, the Si content is set to less than 2.00%. The preferred Si content is less than 1.50%, less than 1.00%, less than 0.75%, or less than 0.50%. Although there is no particular lower limit for the Si content, excessively reducing the Si content increases steelmaking costs. Therefore, it is preferable that the Si content be 0.001% or more. Furthermore, because 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 0.10% or more, 0.20% or more, or 0.30% or more.

[0026] Mn: 0.01% or more, less than 0.50% Mn is an element that deteriorates the impact resistance of hot-stamped products. In particular, if the Mn content is 0.50% or more, the impact resistance is significantly deteriorated, and even if the manufacturing method of a steel sheet for hot stamping described later is applied, the impact resistance of the hot-stamped product cannot be ensured. Therefore, the Mn content is set to less than 0.50%. The Mn content is preferably less than 0.45%, less than 0.40%, less than 0.35%, or less than 0.30%. On the other hand, Mn is an element that combines with the impurity S to form MnS, thereby suppressing the harmful effects of S. To achieve this effect, the Mn content is set to 0.01% or more. The Mn content is preferably 0.05% or more, or 0.10% or more. Furthermore, Mn is an element that improves the hardenability of steel. From the viewpoint of improving hardenability, the Mn content is preferably 0.15% or more, 0.20% or more, or 0.25% or more.

[0027] P:0.200% or less P is contained in steel as an impurity and is an element that embrittles steel. If the P content exceeds 0.200%, the adverse effects become particularly severe, and weldability also deteriorates significantly. Therefore, the P content is set to 0.200% or less. The preferred P content is less than 0.100%, less than 0.050%, or less than 0.020%. Although there is no particular lower limit for the P content, an excessively low P content increases the steelmaking cost, so the P content may be set to 0.001% or more.

[0028] S: 0.0200% or less S is contained in steel as an impurity and is an element that embrittles steel. When 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 less than 0.0050%, less than 0.0020%, or less than 0.0010%. Although there is no particular lower limit for the S content, an excessively low S content leads to an increase in steelmaking costs, so the S content may be set to 0.0001% or more.

[0029] sol.Al:0.001~1.000% 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 rises, making it difficult to heat the steel sheet to a temperature exceeding the Ac3 point during the manufacturing process of the steel sheet for hot stamping. Therefore, the sol.Al content is set to 1.000% or less. The sol.Al content is preferably less than 0.500%, less than 0.100%, less than 0.060%, or less than 0.040%.

[0030] N: 0.0200% or less N is an element contained in steel as an impurity and forms nitrides during continuous casting of steel. These nitrides deteriorate the ductility of steel sheets after hot stamping, so 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 less than 0.0100%, less than 0.0080%, or less than 0.0050%. Although there is no particular lower limit for the N content, an excessively low N content leads to an increase in steelmaking costs, so the N content may be set to 0.0010% or more.

[0031] Mo: 0.01% or more, less than 0.50% Mo is an element that improves the hardenability of steel and is effective in forming a metal structure mainly composed of martensite during the hot stamping process, thereby ensuring the strength of hot stamped products. To achieve this effect, the Mo content is set to 0.01% or more. The preferred Mo content is 0.05% or more, 0.10% or more, or 0.15% or more. On the other hand, if the Mo content is 0.50% or more, even if the manufacturing method of a steel sheet for hot stamping described later is applied, it is not possible to suppress local fluctuations in the Mo concentration in the steel sheet for hot stamping, and it is not possible to ensure sufficient impact resistance of the hot stamped product. Therefore, the Mo content is set to less than 0.50%. The Mo content is preferably less than 0.40%, less than 0.35%, or less than 0.30%.

[0032] B: 0.0002 to 0.0200% B is an element that improves the hardenability of steel, forms a metal structure mainly composed of martensite during the hot stamping process, and is an element that is effective in ensuring the strength of hot stamped products. To achieve this effect, the B content is set to 0.0002% or more. The preferred B content is 0.0006% or more, 0.0010% or more, or 0.0015% or more. On the other hand, if the B content exceeds 0.0200%, boron dioxide is formed, which reduces the effect of improving hardenability due to the inclusion of B. Therefore, the B content is set to 0.0200% or less. The B content is preferably less than 0.0050%, less than 0.0040%, or less than 0.0030%.

[0033] The steel sheet for hot stamping according to this embodiment may have a chemical composition containing the above chemical components with the balance being Fe and impurities, but in order to improve properties, etc., the steel sheet for hot stamping according to this embodiment may further contain one or more elements selected from Ti, Nb, V, Zr, Cr, W, Cu, Ni, Ca, Mg, REM, and Bi in the ranges shown below. These elements (optional elements) do not necessarily need to be contained, so the lower limit is 0%. Here, the term "impurities" refers to components that are mixed in from raw materials such as ores and scraps or due to various factors in the manufacturing process when industrially manufacturing a steel sheet, and are acceptable within a range that does not adversely affect the steel sheet for hot stamping according to the present embodiment.

[0034] Ti: 0 to 0.200% Nb: 0 to 0.200% V: 0 to 0.200% Zr: 0 to 0.200% Ti, Nb, V, and Zr are elements that have the effect of improving the impact resistance of hot stamped products by refining the metal structure. To achieve this effect, one or more elements selected from Ti, Nb, V, and Zr may be added as needed. To obtain the above-mentioned effects, it is preferable to contain one or more selected from Ti, Nb, V and Zr in an amount of 0.001% or more each, more preferably 0.005% or more each, and even more preferably 0.010% or more each. On the other hand, if the content of Ti, Nb, V, and Zr exceeds 0.200%, the above effects saturate and the manufacturing cost of the steel sheet increases. Therefore, if these elements are contained, the content of Ti, Nb, V, and Zr is set to 0.200% or less. Furthermore, if the contents of Ti, Nb, V, and Zr are high, large amounts of carbides of these elements will precipitate, impairing the ductility of the steel sheet after hot stamping. From the viewpoint of ensuring ductility, the Ti content is preferably less than 0.050% or less than 0.030%, the Nb content is preferably less than 0.050%, less than 0.030%, or less than 0.020%, the V content is preferably less than 0.100% or less than 0.050%, and the Zr content is preferably less than 0.100% or less than 0.050%.

[0035] Cr: 0 to 2.00% W: 0~2.00% Cu: 0 to 2.00% Ni: 0 to 2.00% Cr, W, Cu and Ni are elements that have the effect of improving the hardenability of steel, and therefore, one or more elements selected from Cr, W, Cu and Ni may be contained as needed. To obtain the above effects, it is preferable to contain at least 0.001% of one or more selected from Cr, W, Cu, and Ni. The Cr content is more preferably 0.05% or more, or 0.10% or more, the W content is more preferably 0.05% or more, or 0.10% or more, the Cu content is more preferably 0.10% or more, and the Ni content is more preferably 0.10% or more. On the other hand, if the Cr, W, Cu, and Ni contents exceed 2.00% each, the impact resistance of the hot-stamped product deteriorates. Therefore, when these elements are contained, the Cr, W, Cu, and Ni contents are each set to 2.00% or less. The Cr content is preferably less than 0.50%, less than 0.40%, or less than 0.30%, the W content is preferably less than 0.50%, less than 0.40%, or less than 0.30%, the Cu content is preferably less than 1.00% or less than 0.50%, and the Ni content is preferably less than 1.00% or less than 0.50%.

[0036] Ca: 0 to 0.0100% Mg: 0 to 0.0100% REM: 0 to 0.1000% Ca, Mg, and REM are elements that improve the ductility of the steel sheet after hot stamping by adjusting the shape of inclusions. Therefore, they may be added as needed. To achieve the above effects, it is preferable to add at least 0.0001% of one or more elements selected from Ca, Mg, and REM. On the other hand, if the Ca or Mg content exceeds 0.0100%, or if the REM content exceeds 0.1000%, not only will the above effects saturate but excessive costs will also be incurred. Therefore, if these elements are added, the Ca and Mg contents should each be 0.0100% or less, and the REM content should be 0.1000% or less.

[0037] In this embodiment, REM refers to a total of 17 elements, including Sc, Y, and lanthanoids, and the REM content refers to the total content of these elements. Lanthanoids are industrially added in the form of misch metal.

[0038] Bi: 0 to 0.0500% Bi is an element that improves the impact resistance of hot-stamped products by refining the solidification structure. Therefore, it may be added as needed. To achieve the above effect, the Bi content is preferably 0.0001% or more. The Bi content is more preferably 0.0003% or more, or 0.0005% or more. On the other hand, if the Bi content exceeds 0.0500%, the above effects saturate and excessive costs are incurred. Therefore, if Bi is added, the Bi content is set to 0.0500% or less. The Bi content is preferably 0.0100% or less, or 0.0050% or less.

[0039] As described above, the chemical composition of the steel sheet for hot stamping according to the present embodiment may contain essential elements with the balance being Fe and impurities, or may contain essential elements and one or more optional elements with the balance being Fe and impurities.

[0040] The local element concentration distribution of the steel sheet for hot stamping according to this embodiment will be described. When the Mo content of the steel sheet for hot stamping according to this embodiment is measured by linear analysis in a range of 0.05 mm in the sheet thickness direction centered at a depth position of ¼ of the sheet thickness from the surface of the steel sheet, the maximum value, minimum value, and average value of the Mo content in the measurement results satisfy the following formula (i): ([Mo] MAX -[Mo] MIN ) / [Mo] AVE <0.50 (i) However, the meanings of the symbols in the above formula (i) are as follows: [Mo] MAX : Maximum Mo content (mass%) [Mo] MIN : Minimum Mo content (mass%) [Mo] AVE : Average value of Mo content (mass%) When the Mo content of the steel sheet for hot stamping falls within the above range and satisfies the above formula (i), the impact resistance of the hot stamped product can be improved. The value of the left side of the above formula (i) is preferably less than 0.40 or less than 0.30. Although there is no lower limit for the value of the left side of the above equation (i), in order to significantly reduce the value of the left side of the above equation (i), it is necessary to excessively increase the soaking temperature or prolong the soaking time in the first annealing of the hot-rolled sheet in the manufacturing method of the steel sheet for hot stamping, which will be described later. In this case, not only is the productivity of the steel sheet for hot stamping impaired, but local variations in hardness of the steel sheet for hot stamping also increase. Therefore, the value of the left side of the above equation (i) may be 0.05 or more, 0.10 or more, or 0.15 or more.

[0041] In this embodiment, the distribution of the local Mo content (concentration) is determined as follows. First, a test piece was taken from the hot stamping steel sheet, and the longitudinal section parallel to the rolling direction of the steel sheet was polished with waterproof abrasive paper. After buffing with a diamond suspension, a line analysis was performed using a field emission electron probe microanalyzer (FE-EPMA) within a 0.05 mm range in the thickness direction, centered at a depth of 1 / 4 of the steel sheet's thickness (1 / 4 depth position) from the surface of the steel sheet. EPMA measurements were performed at 0.2 μm intervals in the thickness direction, and the Mo content at each measurement position was calculated from a five-point moving average. Specifically, the average of the Mo concentration measurements at five consecutive points was used as the Mo content at the third measurement position, and the Mo content at each measurement position within the range was calculated. The maximum, minimum, and average Mo content (average of the Mo content at all measurement positions) within the range thus obtained were used to calculate the left-hand side of equation (i) above. However, this line analysis is carried out at any 10 locations on the steel sheet, and the average value of the left-hand side values obtained at the 10 locations is taken as the left-hand side value of the above equation (i) for that steel sheet.

[0042] <Hardness distribution of steel sheets for hot stamping> The hot stamping steel sheet according to this embodiment has a thickness of 0.18 mm 2The standard deviation of Vickers hardness within this region (a region of 0.3 mm in the thickness direction and 0.6 mm in the direction perpendicular to the thickness direction, centered at the 1 / 4 depth position of the steel plate) is 20 (Hv) or less (20 or less in Hv). If the standard deviation of Vickers hardness within the above range exceeds 20 (Hv), cracks will occur in the early stage of deformation when the hot stamped product is deformed, resulting in a significant deterioration in impact resistance. Therefore, the standard deviation of hardness within the above range is set to 20 (Hv) or less. The standard deviation of hardness is preferably set to 15 (Hv) or less, or 10 (Hv) or less. Furthermore, the steel sheet for hot stamping according to this embodiment is an as-cold-rolled steel sheet, and the average hardness is an index of the strain energy accumulated in the steel sheet. In order to increase the strain energy and improve the crash resistance of the hot-stamped product, it is preferable to set the average hardness to 280 (Hv) or more, 295 (Hv) or more, or 310 (Hv) or more. A smaller standard deviation of hardness within the above range is preferable, but a large decrease in the standard deviation of hardness leads to a decrease in the productivity of the steel sheet for hot stamping. Therefore, the standard deviation of hardness may be greater than 5 (Hv) or greater than 10 (Hv). A larger average hardness within the above range is preferable, but a large increase in the average hardness not only leads to a decrease in the productivity of the steel sheet for hot stamping but also deteriorates the cuttability of the steel sheet for hot stamping. Therefore, the average hardness may be 400 (Hv) or less, or 370 (Hv) or less.

[0043] In this embodiment, the hardness of the steel sheet for hot stamping is determined as follows. First, a test piece is taken from a steel plate for hot stamping, and the longitudinal cross section parallel to the rolling direction of the steel plate is polished with waterproof abrasive paper, and then buffed with a diamond suspension, after which the Vickers hardness is measured at a position 1 / 4 of the way into the steel plate. Specifically, as shown in Figure 1, Vickers hardness is measured at 45 points at specified intervals within a range of 0.3 mm in the thickness direction and 0.6 mm perpendicular to the thickness direction, centered at the 1 / 4 depth position of the steel plate, and the arithmetic mean and standard deviation are calculated from the measurements. A micro Vickers hardness tester is used to measure the hardness, with a load of 0.49 N and a load holding time of 10 seconds. A high load increases the size of the indentation, making it impossible to evaluate the local hardness distribution, which is closely related to the impact resistance of hot-stamped products. For this reason, the load is set to 0.49 N.

[0044] <Strength of steel sheets for hot stamping> In order to increase strain energy and improve the crash resistance of the hot stamped product, the steel sheet for hot stamping according to this embodiment preferably has a tensile strength of 900 MPa or more, more preferably 950 MPa or more, or 1000 MPa or more.

[0045] <Metal structure of steel sheets for hot stamping> The steel sheet for hot stamping according to this embodiment is manufactured without annealing after the cold rolling process, and therefore has a metal structure that is elongated in the rolling direction. By providing such a metal structure, the strain energy of the steel sheet for hot stamping is increased, and the crash resistance of the hot-stamped product is improved. In a steel sheet that is annealed after cold rolling, the accumulated strain energy is insufficient, and the crash resistance of the hot-stamped product is reduced. If the metal structure contains martensite (including tempered martensite), the steel sheet becomes significantly hardened and difficult to cut, so the metal structure of a steel sheet for hot stamping is preferably composed mainly of ferrite, pearlite, and / or bainite extended in the rolling direction. The total volume fraction of ferrite extended in the rolling direction, pearlite extended in the rolling direction, and bainite extended in the rolling direction is preferably more than 80.0%, more than 90.0%, or more than 95.0%. In the metal structure, the remainder other than the ferrite, pearlite, and bainite elongated in the rolling direction may be martensite and / or retained austenite, and may further contain precipitates such as cementite. The volume fraction of the remainder is preferably 20.0% or less. The volume fraction of martensite is preferably less than 10.0% or less than 5.0%.

[0046] The volume fraction of each structure in the metal structure of the steel sheet for hot stamping is determined as follows. First, a test piece is taken from a steel plate for hot stamping, and the longitudinal cross section parallel to the rolling direction of the steel plate is polished with waterproof abrasive paper, and then further buffed with a diamond suspension. After that, the structure is observed at a depth of 1 / 4 of the steel plate thickness from the surface of the steel plate. Specifically, the polished surface is subjected to nital etching or electrolytic polishing, followed by structural observation using an optical microscope and a scanning electron microscope (SEM). The resulting structural photographs are analyzed based on brightness differences or differences in the morphology of iron carbides present within the phases to determine the area fractions of ferrite, pearlite, bainite, and tempered martensite. Similar observation locations are then subjected to Lepera etching, followed by structural observation using an optical microscope and a scanning electron microscope (SEM). Image analysis of the resulting structural photographs is then performed to calculate the total area fraction of retained austenite and martensite. In addition, at the same observation position, a longitudinal cross section parallel to the rolling direction of the steel sheet is electrolytically polished, and then the area fraction of retained austenite is measured based on differences in crystal structure using an SEM equipped with an electron backscatter pattern analyzer (EBSP). Based on these results, the area fractions of ferrite, pearlite, bainite, tempered martensite, martensite, and retained austenite are obtained. The area fractions are then assumed to be equal to the volume fractions, and the measured area fractions are used as the volume fractions of each structure. In structural observation, tempered martensite can be distinguished from martensite in that iron carbides are present inside, and can also be distinguished from bainite in that the iron carbides present inside are elongated in multiple directions.

[0047] <Method of manufacturing steel sheets for hot stamping> A preferred method for producing a steel sheet for hot stamping according to this embodiment will be described. The steel sheet for hot stamping according to this embodiment can be manufactured by a manufacturing method including the following steps. (I) A hot rolling process in which a slab having the above-mentioned chemical composition is hot-rolled and then coiled to form a hot-rolled steel sheet. (II) First hot-rolled sheet annealing step: the hot-rolled steel sheet is subjected to first hot-rolled sheet annealing to obtain a hot-rolled annealed steel sheet. (III) A second hot-rolled sheet annealing step in which the hot-rolled and annealed steel sheet is subjected to a second hot-rolled sheet annealing. (IV) A cold rolling process in which the hot-rolled annealed steel sheet that has been subjected to the second hot-rolled sheet annealing is cold-rolled to form a cold-rolled steel sheet.

[0048] The method for producing the slab used in the method for producing a steel sheet for hot stamping according to this embodiment is not particularly limited. In a preferred method for producing a slab, a steel having the above-described chemical composition is melted by a known means and then formed into a steel ingot by continuous casting, or a steel ingot is formed by any casting method and then rolled into a billet. 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 before being subjected to hot rolling. Alternatively, the steel ingot at a high temperature after continuous casting or the billet at a high temperature after blooming may be subjected to hot rolling directly, with warming, or with supplementary heating. In this embodiment, such steel ingots and billets are collectively referred to as "slabs" as raw materials for hot rolling.

[0049] [Hot rolling process] The temperature of the slab used in hot rolling (slab heating temperature) is preferably less than 1250°C, more preferably 1200°C or less, in order to prevent coarsening of austenite. On the other hand, if the slab heating temperature is too low, rolling becomes difficult, so the slab heating temperature may be 1050°C or more. The heated slab is hot-rolled to obtain a hot-rolled steel sheet. The hot-rolling is preferably completed in a temperature range of Ar3 point or higher in order to refine the metal structure of the hot-rolled steel sheet by transforming austenite after the rolling is completed.

[0050] When the hot rolling consists of rough rolling and finish rolling, the rough-rolled material may be heated between rough rolling and finish rolling in order to complete the finish rolling at the above temperature. In this case, it is desirable to heat the rough-rolled material so that the rear end is hotter than the front end, thereby suppressing the temperature fluctuation over the entire length of the rough-rolled material at the start of finish rolling to 140°C or less. This improves the uniformity of the product properties in the coil after the coiling process.

[0051] The rough-rolled material may be heated by any known means. For example, a solenoid-type induction heating device may be provided between the roughing mill and the finishing mill, and the heating temperature may be controlled based on the temperature distribution in the longitudinal direction of the rough-rolled material upstream of the induction heating device.

[0052] When the hot-rolled steel sheet is coiled after hot rolling, the coiling temperature is preferably 660°C or less in order to suppress local fluctuations in the Mo concentration. The coiling temperature is more preferably 640°C or less, or 620°C or less. On the other hand, if the coiling temperature is too low, the steel sheet becomes significantly hardened, which may cause cracks in the steel sheet during the steel sheet manufacturing process. Therefore, the coiling temperature is preferably set to more than 500°C or more than 550°C.

[0053] [First hot-rolled sheet annealing process] The hot-rolled and coiled steel sheet is subjected to a first hot-rolled sheet annealing to become a hot-rolled annealed steel sheet. In this embodiment, the annealing performed on the hot-rolled steel sheet is referred to as hot-rolled sheet annealing, and the steel sheet after hot-rolled sheet annealing is referred to as a hot-rolled annealed steel sheet. Before the first hot-rolled sheet annealing, flattening by skin-pass rolling or the like, or descaling by pickling or the like may be performed. In the first hot-rolled sheet annealing process, the soaking temperature is set to the Ac3 point (°C) or higher, and the soaking time (holding time at the soaking temperature) is set to more than 1 hour. The average cooling rate from the soaking temperature to 500°C is set to more than 1°C / second. This is to suppress local fluctuations in the Mo concentration and improve the crash resistance of the hot-stamped product. The soaking temperature is more preferably (Ac3 point + 50°C) or higher, the soaking time is more preferably 2 hours or more or 6 hours or more, and the average cooling rate to 500°C is more preferably 2°C / second or higher. If the soaking temperature is too high or the soaking time is too long, the austenite will become excessively coarse, resulting in large local fluctuations in the hardness of the hot-stamping steel sheet. Therefore, the soaking temperature is preferably set to no more than (Ac3 point + 200°C) or no more than (Ac3 point + 100°C), and the soaking time is preferably set to no more than 12 hours or no more than 10 hours. The Ac3 point is the temperature at which ferrite disappears from the metal structure when the steel sheet is heated, and in this embodiment, it is determined from the change in thermal expansion when the steel sheet is heated at 8°C / second.

[0054] [Second hot-rolled sheet annealing process] A second hot-rolled annealing is performed on a steel sheet that has undergone the first hot-rolled annealing (hot-rolled annealed steel sheet). The annealing performed on hot-rolled annealed steel sheet is also called hot-rolled annealing. Before the second hot-rolled annealing, flattening by skin-pass rolling or descaling by pickling may be performed. In the second hot-rolled sheet annealing process, the soaking temperature is set to the Ac3 point or higher (Ac3 point + 50°C) or lower, and the soaking time is set to 1 second or higher and less than 10 minutes. The average heating rate from 500°C to the soaking temperature is set to more than 1°C / second, and the average cooling rate from the soaking temperature to 500°C is set to more than 1°C / second. This is to suppress local hardness variations in the hot stamping steel sheet and improve the crash resistance of the hot-stamped product. The soaking temperature is more preferably set to the Ac3 point or higher (Ac3 point + 25°C) or lower, the soaking time is more preferably set to 10 seconds or higher and less than 5 minutes, and the average heating rate from 500°C to the soaking temperature is more preferably set to 2°C / second or higher. If the average cooling rate from the soaking temperature to 500°C is too fast, the steel sheet will significantly harden, making it difficult to cut. Therefore, the cooling rate is preferably set to 15°C / second or lower.

[0055] [Cold rolling process] The steel sheet that has undergone the second hot-rolled sheet annealing (hot-rolled annealed steel sheet) is cold-rolled according to conventional methods to produce a cold-rolled steel sheet. In the cold-rolling process, the cold-rolling ratio (cumulative reduction ratio in cold rolling) is set to 10% or more. If the cold-rolling ratio is less than 10%, the strain energy accumulated in the steel sheet will be insufficient, and local hardness variations in the steel sheet will increase, resulting in reduced crash resistance of the hot-stamped product. A preferred cold-rolling ratio is 20% or more, 30% or more, or 40% or more. There is no particular need to set an upper limit for the cold-rolling ratio, but an excessively high cold-rolling ratio will increase the load on the rolling equipment and reduce productivity, so a cold-rolling ratio of less than 70%, less than 60%, or less than 50% is preferred. In order to reduce the weight of the hot stamped product, the thickness of the cold rolled steel sheet is preferably 2.0 mm or less, more preferably 1.8 mm or less, and even more preferably 1.6 mm or less. Prior to cold rolling, flattening by skin pass rolling or the like, or descaling by pickling or the like may be carried out according to a known method. It is preferable not to anneal the cold-rolled steel sheet. Annealing the cold-rolled steel sheet releases the strain energy accumulated during cold rolling. Furthermore, the local hardness variation of the steel sheet may increase. When such a steel sheet is used as a steel sheet for hot stamping, the impact resistance of the hot-stamped product deteriorates. The cold-rolled steel sheet thus obtained may be subjected to treatments such as degreasing and oiling according to conventional methods.

[0056] A hot-stamped product can be obtained by hot stamping the steel sheet for hot stamping according to the present embodiment described above. A hot-stamped product manufactured using the steel sheet for hot stamping according to the present embodiment (hereinafter, the hot-stamped product according to the present embodiment) will be described. The hot-stamped product according to this embodiment has a base steel sheet (a steel sheet constituting the hot-stamped product obtained by hot stamping a steel sheet for hot stamping). It may be made of only the base steel sheet.

[0057] <Chemical composition of base steel sheet for hot stamped products> Since the chemical composition does not substantially change due to hot stamping, the chemical composition of the base steel sheet of the hot-stamped product (when the hot-stamped product consists only of the base steel sheet, this can also be said to be the chemical composition of the hot-stamped product) is the same as that of the steel sheet for hot stamping described above. When the hot-stamped product has a portion having a tensile strength of 2300 MPa or more and a portion having a tensile strength of less than 2300 MPa, it is sufficient that at least the portion of the base steel sheet having a tensile strength of 2300 MPa or more has the above-mentioned chemical composition.

[0058] In the hot stamped product according to this embodiment, when the Mo content is measured by linear analysis in a range of 0.05 mm in the thickness direction centered at a depth position of ¼ of the thickness of the base steel plate from the surface of the base steel plate (the steel plate included in the hot stamped product), the maximum Mo content, the minimum Mo content and the average Mo content in the measurement results satisfy the following formula (ii): ([Mo] mMAX -[Mo] mMIN ) / [Mo] mAVE <0.50 (ii) However, the meanings of the symbols in the above formula (ii) are as follows: [Mo] mMAX : Maximum Mo content of base steel sheet (mass%) [Mo] mMIN : Minimum Mo content of base steel plate (mass%) [Mo] mAVE : Average value of Mo content in base steel sheet (mass%) When a hot stamped product has a portion having a tensile strength of 2300 MPa or more and a portion having a tensile strength of less than 2300 MPa, it is sufficient that the above formula (ii) is satisfied at least in the portion of the base steel plate where the tensile strength is 2300 MPa or more. The smaller the local variation in Mo concentration in the hot-stamped product, the more the stress concentration in the soft parts during deformation of the hot-stamped product is alleviated, and the less likely cracks are to occur. Therefore, the value of the left side of the above formula (ii) is preferably less than 0.50. The value of the left side of the above formula (ii) is more preferably less than 0.40 or less than 0.30. Although there is no lower limit for the value of the left side of the formula (ii), a large decrease in the value of the left side of the formula (ii) leads to a decrease in the productivity of the steel sheet for hot stamping, and therefore the value of the left side of the formula (ii) may be 0.05 or more, 0.10 or more, or 0.15 or more.

[0059] The local Mo concentration distribution in a hot-stamped product can be determined by taking a test piece from the hot-stamped product, buffing the longitudinal cross section of the steel sheet, and then performing a concentration analysis at a position 1 / 4 of the depth of the base steel sheet using the same method as for a steel sheet for hot stamping. When the hot-stamped product has a portion having a tensile strength of 2300 MPa or more and a portion having a tensile strength of less than 2300 MPa, a test piece is taken from at least a portion of the base steel sheet where the tensile strength is 2300 MPa or more, and the concentration analysis is performed.

[0060] <Metal structure of base steel sheet for hot stamped products> In a hot-stamped product manufactured using the steel sheet for hot stamping according to this embodiment, the base steel sheet preferably has the following metallographic structure: When the hot-stamped product has a portion having a tensile strength of 2300 MPa or more and a portion having a tensile strength of less than 2300 MPa, it is preferable that at least the portion of the base steel sheet having a tensile strength of 2300 MPa or more has the following metallographic structure.

[0061] Martensite: Over 90.0% Martensite is an important structure for increasing the tensile strength of steel sheets after hot stamping. If the volume fraction of martensite is 90.0% or less, the tensile strength of the hot stamped product will be less than 2300 MPa, resulting in insufficient strength. Therefore, it is preferable that the volume fraction of martensite is greater than 90.0%. More preferably, the volume fraction of martensite is greater than 91.0%, greater than 93.0%, or greater than 95.0%. Although there is no particular need to set an upper limit for the volume fraction of martensite, in order to significantly increase the volume fraction of martensite, it would be necessary to excessively increase the heating temperature of the steel sheet or the cooling rate in the hot stamping process, which would significantly impair the productivity of hot stamped products. Therefore, it is preferable that the volume fraction of martensite is 99.0% or less, or 98.0% or less. The martensite includes not only fresh martensite that has not been tempered, but also tempered martensite that has been tempered and contains iron carbides. The remainder of the metal structure may contain ferrite, pearlite, bainite, or retained austenite, and may further contain precipitates such as cementite. Since it is not necessary to contain ferrite, pearlite, bainite, retained austenite, and precipitates, the lower limits of the volume fractions of ferrite, pearlite, bainite, retained austenite, and precipitates are all 0%.

[0062] Since ferrite, pearlite, and bainite have the effect of improving the ductility of the steel sheet after hot stamping, in order to obtain this effect, it is preferable to contain one or more selected from ferrite, pearlite, and bainite. The volume fraction of ferrite is preferably 0.5% or more, or 1.0% or more, and the volume fractions of pearlite and bainite are each preferably 1.0% or more, and more preferably 2.0% or more. On the other hand, excessive content of ferrite, pearlite, and bainite deteriorates the impact resistance of the hot stamped product, so the volume fraction of ferrite is preferably less than 3.0% or less than 2.0%, and the volume fractions of pearlite and bainite are each preferably less than 10.0%, and more preferably less than 5.0%.

[0063] Retained austenite has the effect of improving the ductility of the steel sheet after hot stamping. To obtain this effect, the volume fraction of retained austenite is preferably 0.5% or more, 1.0% or more, or 2.0% or more. On the other hand, excessively increasing the volume fraction of retained austenite requires austempering at high temperatures after hot stamping, which significantly reduces the productivity of hot-stamped products. Furthermore, excessive retained austenite content can degrade the impact resistance of hot-stamped products. Therefore, it is preferable to set the volume fraction of retained austenite to less than 9.0%, less than 7.0%, less than 5.0%, or less than 4.0%.

[0064] The volume fraction of each structure in the metal structure of a hot-stamped product can be determined by taking a test piece from the hot-stamped product, buffing the longitudinal section of the steel sheet, and then observing the structure at a position 1 / 4 of the depth of the base steel sheet in the same manner as for the steel sheet for hot stamping. When the hot-stamped product has a portion having a tensile strength of 2300 MPa or more and a portion having a tensile strength of less than 2300 MPa, a test piece is taken from at least a portion of the base steel sheet having a tensile strength of 2300 MPa or more, and the structure is observed.

[0065] <Strength of base steel sheet for hot stamped products> Preferably, all or a portion of the hot-stamped product according to this embodiment has a tensile strength of 2300 MPa or more. To achieve this, the tensile strength of all or a portion of the base steel sheet of the hot-stamped product is 2300 MPa or more. If the tensile strength of at least a portion is not 2300 MPa or more, the crash resistance of the hot-stamped product cannot be ensured. Therefore, the tensile strength of all or a portion of the hot-stamped product is set to 2300 MPa or more. Preferably, the tensile strength of all or a portion of the hot-stamped product is 2400 MPa or more, or 2500 MPa or more. On the other hand, since excessively increasing the strength of the hot-stamped product leads to a decrease in crash resistance, it is preferable that the tensile strength of the base steel sheet of the hot-stamped product is less than 3000 MPa or less than 2800 MPa.

[0066] The hot-stamped product according to this embodiment may have a tensile strength of 2300 MPa or more throughout (the entire product), or may have a mixture of portions with a tensile strength of 2300 MPa or more and portions with a tensile strength of less than 2300 MPa. By providing portions with different strengths, it becomes possible to control the deformation state of the hot-stamped product during a collision. A hot-stamped product having portions with different strengths can be produced by a method of joining two or more types of steel sheets with different chemical compositions and then hot stamping them, a method of partially changing the heating temperature of the steel sheet or the cooling rate after hot stamping during the hot stamping process, or a method of partially reheating the hot-stamped product.

[0067] <Hardness distribution of base steel sheet for hot stamped products> The hot stamped product according to this embodiment has a thickness of 0.18 mm 2 The standard deviation of Vickers hardness within this region (a region of 0.3 mm in the thickness direction and 0.6 mm in the direction perpendicular to the thickness direction, centered at the 1 / 4 depth position of the base steel plate) is 20 (Hv) or less. If the standard deviation of Vickers hardness within the above range exceeds 20 (Hv), cracks will occur in the early stage of deformation when the hot stamped product is deformed, resulting in a significant deterioration in impact resistance. Therefore, the standard deviation of hardness within the above range is set to 20 (Hv) or less. The standard deviation of hardness is more preferably set to 15 (Hv) or less, or 10 (Hv) or less. When a hot stamped product has a portion having a tensile strength of 2300 MPa or more and a portion having a tensile strength of less than 2300 MPa, it is sufficient that the base steel plate has the above-mentioned hardness distribution at least in the portion having a tensile strength of 2300 MPa or more. Although it is preferable that the standard deviation of hardness within the above range is small, a large decrease in the standard deviation of hardness leads to a decrease in productivity of hot stamped products. Therefore, the standard deviation of hardness may be more than 5 (Hv) or more than 10 (Hv). The hardness distribution of the base steel sheet in a hot-stamped product can be determined by taking a test piece from the hot-stamped product, buffing the longitudinal section of the steel sheet, and then measuring the hardness at a position 1 / 4 of the depth of the base steel sheet using the same method as for a steel sheet for hot stamping. When the hot-stamped product has a portion having a tensile strength of 2300 MPa or more and a portion having a tensile strength of less than 2300 MPa, a test piece is taken from at least the portion of the base steel sheet where the tensile strength is 2300 MPa or more, and the hardness is measured.

[0068] <Method of manufacturing hot stamped products> A preferred method for producing a hot-stamped product according to this embodiment will be described.

[0069] The hot-stamped product according to the present embodiment is manufactured by a manufacturing method including a heating step of heating the steel sheet for hot stamping according to the present embodiment described above, and a hot stamping step of hot stamping the heated steel sheet for hot stamping to obtain a hot-stamped product. In the hot stamping step, forming by a die and cooling are performed.

[0070] In the heating step, the steel sheet for hot stamping according to this embodiment is heated prior to the hot stamping step. In the heating step for heating the steel sheet for hot stamping, the heating temperature is preferably set to a temperature above the Ac3 point. If the heating temperature is below the Ac3 point, the volume fraction of martensite in the metal structure of the hot stamped product will be insufficient, resulting in a decrease in the strength of the product and a deterioration in impact resistance.

[0071] Although there is no particular upper limit to the heating temperature, if the heating temperature is too high, excessive scale will form on the hot stamped product, and the accumulation of scale inside the die will reduce the productivity of the product. Therefore, the heating temperature is preferably 1200°C or less, or 1150°C or less. The heating rate of the steel sheet does not need to be particularly limited, but the higher the heating rate, the more effectively the strain energy accumulated in the steel sheet for hot stamping can be utilized, improving the crash resistance of the hot-stamped product. Therefore, it is preferable to set the average heating rate up to 700°C to more than 10°C / s, more than 20°C / s, more than 30°C / s, or more than 50°C / s. On the other hand, if the heating rate is too high, excessive coarse iron carbides will be generated in the metal structure of the hot-stamped product, reducing the ductility of the steel sheet after hot stamping. Therefore, it is preferable to set the average heating rate to less than 150°C / s, less than 120°C / s, or less than 90°C / s.

[0072] In the step of hot stamping the heated steel sheet for hot stamping, it is preferable to remove the heated steel sheet from the heating furnace, allow it to cool in the air, and then start hot stamping at a temperature of 700° C. or higher. If the hot stamping start temperature is less than 700° C., the volume fraction of martensite in the metal structure of the hot stamped product will be insufficient, resulting in a decrease in the strength of the product and a deterioration in its crash resistance. After forming by hot stamping, the formed product is cooled while held in the die, and / or removed from the die and cooled by any method. If the cooling rate is low, the volume fraction of martensite in the metal structure of the hot stamped product will be insufficient, resulting in a decrease in the strength of the formed product. Therefore, it is preferable to set the average cooling rate from the hot stamping start temperature to 400°C to 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 volume fraction of martensite in the metal structure of the hot stamped product will be insufficient, resulting in a decrease in the strength of the formed product. Therefore, it is preferable to set the cooling stop temperature for the above cooling to less than 90°C or less than 50°C.

[0073] The hot stamped product may be subjected to a reheating treatment. The reheating treatment reduces local variations in hardness of the hot stamped product, improving the impact resistance of the hot stamped product. To fully achieve this effect, the reheating temperature is preferably 90°C or higher. On the other hand, if the reheating temperature is too high, the steel sheet becomes soft and the strength of the product is insufficient, so the reheating temperature is preferably less than 200°C or less than 150°C. If the holding time at the heating temperature is too short, the above effects cannot be fully obtained, while if the holding time is too long, the strength of the molded product will be insufficient. Therefore, the lower limit of the holding time is preferably 5 minutes or more, or 10 minutes or more, and the upper limit is preferably less than 30 minutes, or less than 20 minutes.

[0074] The steel sheet for hot stamping according to the present embodiment described above can also be expressed as follows. (Appendix 1) In mass%, C: more than 0.40%, less than 0.70%, Si: less than 2.00% Mn: 0.01% or more, less than 0.50% P: 0.200% or less, S: 0.0200% or less, sol.Al: 0.001~1.000%, N: 0.0200% or less, Mo: 0.01% or more, less than 0.50% B: 0.0002~0.0200%, Remainder: Fe and impurities and having a chemical composition When the Mo content of the steel plate is measured by line analysis using an EPMA in a range of 0.05 mm in the plate thickness direction centered at a depth position of ¼ of the plate thickness from the surface of the steel plate, the maximum value of the Mo content, the minimum value of the Mo content, and the average value of the Mo content satisfy the following formula (i): The standard deviation of Vickers hardness in a region 0.3 mm in the thickness direction and 0.6 mm in the direction perpendicular to the thickness direction, centered at a depth position of 1 / 4 of the thickness of the steel plate from the surface of the steel plate, is 20 (Hv) or less, Steel plate for hot stamping. ([Mo] MAX -[Mo] MIN ) / [Mo] AVE <0.50 (i) However, the meanings of the symbols in the above formula (i) are as follows: [Mo] MAX : Maximum Mo content (mass%) [Mo] MIN : Minimum Mo content (mass%) [Mo] AVE : Average value of Mo content (mass%)

[0075] (Appendix 2) In mass%, C: more than 0.40%, less than 0.70%, Si: less than 2.00% Mn: 0.01% or more, less than 0.50% P: 0.200% or less, S: 0.0200% or less, sol.Al: 0.001~1.000%, N: 0.0200% or less, Mo: 0.01% or more and less than 0.50%; and B: 0.0002 to 0.0200%, Furthermore, it contains one or more selected from the group consisting of the following groups A, B, C, and D: Remainder: Fe and impurities and having a chemical composition When the Mo content of the steel plate is measured by line analysis using an EPMA in a range of 0.05 mm in the plate thickness direction centered at a depth position of ¼ of the plate thickness from the surface of the steel plate, the maximum value of the Mo content, the minimum value of the Mo content, and the average value of the Mo content satisfy the following formula (i): The standard deviation of Vickers hardness in a region 0.3 mm in the thickness direction and 0.6 mm in the direction perpendicular to the thickness direction, centered at a depth position of 1 / 4 of the thickness of the steel plate from the surface of the steel plate, is 20 (Hv) or less, Steel plate for hot stamping. [Group A] One or more selected from Ti: 0.001 to 0.200%, Nb: 0.001 to 0.200%, V: 0.001 to 0.200%, and Zr: 0.001 to 0.200% [Group B] One or more selected from Cr: 0.001 to 2.00%, W: 0.001 to 2.00%, Cu: 0.001 to 2.00%, and Ni: 0.001 to 2.00% [Group C] One or more selected from Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0100%, and REM: 0.0001 to 0.1000% [Group D] Bi: 0.0001-0.0500%

[0076] (Appendix 3) The steel sheet for hot stamping according to (Appendix 2), which has a chemical composition containing, by mass %, the Group A.

[0077] (Appendix 4) The steel sheet for hot stamping according to (Appendix 2), which has a chemical composition containing, in mass %, the B group.

[0078] (Appendix 5) The steel sheet for hot stamping according to (Appendix 2), which has a chemical composition containing, by mass %, the C group.

[0079] (Appendix 6) The steel sheet for hot stamping according to (Appendix 2), having a chemical composition containing, by mass %, the D group.

[0080] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples. [Example]

[0081] Molten steel was cast using a vacuum melting furnace to produce Steels A to U having the chemical compositions shown in Table 1. The Ac3 points in Table 1 were determined from the change in thermal expansion when cold-rolled steel sheets of Steels A to U were heated at 8°C / sec. Steels A to U were heated to 1200°C and held there for 60 minutes, after which they were hot-rolled under the hot-rolling conditions shown in Table 2.

[0082] [Table 1]

[0083] [Table 2]

[0084] Specifically, steels A to U were subjected to 10 passes of rolling in a temperature range above the Ar3 point to produce hot-rolled steel sheets with thicknesses of 2.2 to 3.2 mm. After hot rolling, the hot-rolled steel sheets were cooled to 640 to 660°C using a water spray, and the cooling end temperature was set as the coiling temperature. The hot-rolled steel sheets were charged into an electric heating furnace maintained at this coiling temperature and held there for 60 minutes. Thereafter, the hot-rolled steel sheets were furnace-cooled to room temperature at an average cooling rate of 20°C / hour to simulate slow cooling after coiling. After pickling, the hot-rolled steel sheets were subjected to a first hot-rolled sheet annealing under the conditions shown in Table 2. Specifically, the sheets were heated from room temperature to the soaking temperature at a heating rate of 100°C / hour using an electric heating furnace, and soaked for 0.1 to 6 hours. The steel sheets were then removed from the heating furnace and allowed to cool to room temperature. The average cooling rate from the soaking temperature to 500°C was 9 to 10°C / second. The first hot-rolled sheet annealing was omitted for some hot-rolled steel sheets. After the hot-rolled annealed steel sheets or hot-rolled steel sheets were pickled, they were subjected to a second hot-rolled sheet annealing under the conditions shown in Table 2. Specifically, they were heated to the soaking temperature using an electric heating furnace at an average heating rate of 2 to 5°C / s from 500°C to the soaking temperature, and then soaked for 30 seconds to 1 hour. The steel sheets were then removed from the heating furnace and allowed to cool to room temperature. The average cooling rate from the soaking temperature to 500°C was 7 to 10°C / s. For some hot-rolled annealed steel sheets, the second hot-rolled sheet annealing was omitted.

[0085] The hot-rolled and annealed steel sheets were pickled and then cold-rolled under the conditions shown in Table 2 to obtain cold-rolled steel sheets having a thickness of 1.4 mm. Some of the hot-rolled and annealed steel sheets were not cold-rolled but were ground mechanically to a thickness of 1.4 mm.

[0086] Furthermore, using a continuous annealing simulator, a portion of the cold-rolled steel sheet was heated from room temperature to 780°C at a heating rate of 10°C / s, soaked for 120 seconds, and then cooled to room temperature at an average cooling rate of 15°C / s to obtain an annealed steel sheet.

[0087] Test pieces for EPMA measurement were taken from the cold-rolled steel sheets, ground steel sheets, and annealed steel sheets (these steel sheets are collectively referred to as hot stamping steel sheets) obtained in this manner, and the longitudinal cross sections of these test pieces parallel to the rolling direction of the steel sheets were polished. After that, the Mo concentration distribution (maximum, minimum, and average values) was measured by the above-mentioned method at a depth position of 1 / 4 of the steel sheet thickness from the surface of the steel sheet in the thickness direction of the steel sheet (1 / 4 depth position), and the value of the left side of the above equation (i) was obtained. Specifically, the EPMA measurement was performed using a JXA-8530F manufactured by JEOL Ltd., with an acceleration voltage of 15.0 kV and a probe current of 5.0 × 10 -8 A was used for the analysis, and linear analysis was performed in the thickness direction at a measurement interval of 0.20 μm. The maximum, minimum, and average values of the Mo content were determined from the five-point moving average of the obtained measurement data. These values were used to calculate the left side of the above equation (i).

[0088] Further, JIS No. 13B tensile test pieces were taken from the above steel sheets for hot stamping along a direction perpendicular to the rolling direction, and tensile tests were carried out at a pulling rate of 10 mm / min to determine the tensile strength. Furthermore, test pieces for hardness measurement were taken from the above-mentioned steel sheet for hot stamping, and the longitudinal cross section of this test piece parallel to the rolling direction of the steel sheet was polished. Thereafter, Vickers hardness measurement was carried out at a position of 1 / 4 depth of the steel sheet using the above-mentioned method under a load of 0.49 N in accordance with JIS Z 2244:2009, and the average value and standard deviation of the Vickers hardness were determined. Furthermore, test specimens for microstructure observation were taken from the above-mentioned steel sheets for hot stamping, and the longitudinal cross sections of these test specimens parallel to the rolling direction of the steel sheets were polished. Then, the metal structure at a 1 / 4 depth position of the steel sheets was observed by the above-mentioned method. Table 2 shows the results of investigating the Mo concentration distribution of the steel sheets for hot stamping and the results of investigating the mechanical properties of the steel sheets for hot stamping. In Table 2, underlined values indicate values outside the range of the present invention.

[0089] A hot stamping blank measuring 240 mm wide and 800 mm long was cut from the above-mentioned hot stamping steel sheet, and a hat component with the shape shown in Figure 2 was manufactured by hot stamping. In the hot stamping process, the blank (hot stamping steel sheet) was heated to 950 °C at an average heating rate of 11 °C / s up to 700 °C using a gas heating furnace and held at that temperature for 1 minute. The blank was then removed from the heating furnace and allowed to cool to 800 °C. It was then sandwiched between a die equipped with a cooling device to form a hat shape, and subsequently cooled to room temperature (25 °C) in the die. For test number 34 using steel U, the cooled hat component was reheated in an electric heating furnace, where it was held at 140 °C for 10 minutes.

[0090] A test piece for microstructure observation was taken from the vertical wall of the obtained hat member (hot stamped product), and the vertical cross section of this test piece was polished. After that, the metal structure at a 1 / 4 depth position of the steel plate was observed using the method described above, and the volume fractions of martensite, retained austenite, and others (one or more of ferrite, pearlite, bainite, and precipitates) were determined. In addition, a test piece for EPMA measurement was taken from the vertical wall of the hat member (hot stamped product), and the vertical cross section of this test piece was polished. After that, the Mo concentration distribution was measured at a 1 / 4 depth position of the steel plate using the above-mentioned method, and the value of the left side of the above equation (ii) was calculated.

[0091] Also, a JIS No. 13B tensile test piece was taken from the vertical wall of the hat member along the longitudinal direction of the member, and a tensile test was carried out at a tensile speed of 10 mm / min to determine the tensile strength. Furthermore, test pieces for hardness measurement were taken from the vertical wall portion of the hat member, and after polishing the vertical cross section of this test piece, Vickers hardness measurements were carried out at a depth of 1 / 4 of the steel plate using the above-mentioned method under a load of 0.49 N in accordance with JIS Z 2244:2009, and the standard deviation of the Vickers hardness was calculated.

[0092] Additionally, a specimen for the three-point bending test was prepared by welding a closing plate with a thickness of 1.4 mm, width of 130 mm, and length of 800 mm to the hat member, as shown in Fig. 3. A steel plate with a tensile strength of 1553 MPa was used for the closing plate. This test specimen, 800 mm long, was placed on two support rolls arranged with a roll spacing of 700 mm, with the closing plate facing downwards, as shown in Figure 4, and a three-point bending test was performed at a test speed of 2 m / s to determine the maximum load, the displacement from when the test specimen came into contact with the impactor until cracks began to appear in the test specimen, and the absorbed energy until cracks began to appear. A maximum load of 23.0 kN or more, a crack occurrence displacement of 35 mm or more, and an absorbed energy of 0.80 kJ or more was considered to have good crash resistance.

[0093] Table 3 shows the results of investigating the Mo concentration distribution in the hat member, observing the metal structure of the hat member, evaluating the mechanical properties of the hat member, and evaluating the impact resistance of the hat member.

[0094] [Table 3]

[0095] In all of the hot stamping steel sheets of Test Nos. 1, 6, 11, 16, 20, 22, 24, 26, 27, and 29 to 34, which satisfied the requirements of the present invention, the value of the left side of the above formula (i), which shows the Mo concentration distribution, was less than 0.50, and the standard deviation of the Vickers hardness was 20 or less. Furthermore, in a three-point bending test of the hot stamped products, the maximum load was 23.0 kN or more, the crack initiation displacement was 35 mm or more, and the absorbed energy was 0.80 kJ or more, indicating good crash resistance. Furthermore, although not shown in the table, the metal structure of the hot stamping steel sheets according to the present invention contained a total of more than 80.0 volume % of ferrite, pearlite, and / or bainite, which were elongated in the rolling direction, with the remainder being one or more of martensite, retained austenite, and precipitates.

[0096] In contrast, the comparative examples, test numbers 2 to 5, 7 to 10, 12 to 15, 17 to 19, 21, 23, 25, and 28, in which the chemical composition, Mo concentration distribution, or standard deviation of Vickers hardness of the hot stamping steel sheet was outside the range of the present invention, had low maximum load, crack initiation displacement, and absorbed energy in the three-point bending test of the hot stamped product, and therefore had poor impact resistance.

[0097] Specifically, in test number 13 using steel D, the C content of the steel was too low, so the tensile strength of the hot stamped product was less than 2300 MPa, and the maximum load in the three-point bending test of the hot stamped product was low.

[0098] Test No. 14, which used Steel E, had an excessively high C content, so the hot-stamped product fractured prematurely in a tensile test, making it impossible to determine its tensile strength. Furthermore, the standard deviation of the Vickers hardness of the hot-stamped product was over 20 (Hv), and the maximum load, crack initiation displacement, and absorbed energy in a three-point bending test were low.

[0099] In test number 15, which used steel F, the Mn content of the steel was too high, so the standard deviation of the Vickers hardness of the hot stamped product exceeded 20 (Hv), and the maximum load, crack initiation displacement, and absorbed energy in the three-point bending test were low. In test number 17, which used steel H, the Mo content of the steel was too high. Therefore, the value of the left side of equation (i) in the hot stamping steel sheet was 0.50 or more, the value of the left side of equation (ii) in the hot stamped product was 0.50 or more, the standard deviation of Vickers hardness exceeded 20 (Hv), and the crack initiation displacement and absorbed energy in the three-point bending test were low.

[0100] Test No. 18, which used Steel I, had too low a Mo and B content, and Test No. 19, which used Steel J, had too low a Mo content, resulting in an insufficient martensite volume fraction in the metal structure of the hot-stamped part, and the tensile strength of the hot-stamped part was less than 2300 MPa. Furthermore, the standard deviation of the Vickers hardness of the hot-stamped part exceeded 20 (Hv), and the maximum load, crack initiation displacement, and absorbed energy in a three-point bending test were low.

[0101] Although the chemical composition was within the range of the present invention, the comparative examples, test numbers 2 to 5, 7 to 10, 12, 21, 23, 25, and 28, in which the manufacturing conditions of the hot stamped products were outside the above-mentioned range, had hot stamping steel sheets in which the value of the left side of equation (i) was 0.50 or more or the standard deviation of Vickers hardness exceeded 20 (Hv), and the crack initiation displacement and absorbed energy in a three-point bending test of the hot stamped products were low, resulting in poor impact resistance. Specifically, in test number 2 using Steel A and test number 7 using Steel B, annealing was performed after cold rolling in the manufacturing process of the steel sheet for hot stamping (the steel sheet to be subjected to hot stamping was not in the cold-rolled state), so the standard deviation of Vickers hardness in the steel sheet for hot stamping exceeded 20 (Hv), and the standard deviation of Vickers hardness in the hot-stamped product also exceeded 20 (Hv), and the crack initiation displacement and absorbed energy in the three-point bending test were low. In test number 3 using Steel A and test number 8 using Steel B, cold rolling was not performed in the manufacturing process of the steel sheet for hot stamping (the steel sheet to be subjected to hot stamping was not in the cold-rolled state), so the standard deviation of Vickers hardness in the steel sheet for hot stamping exceeded 20, and the standard deviation of Vickers hardness in the hot-stamped product also exceeded 20 (Hv), and the crack initiation displacement and absorbed energy in the three-point bending test were low. In test number 4 using Steel A and test number 9 using Steel B, the second hot-rolled sheet annealing was not performed in the manufacturing process of the steel sheet for hot stamping, so the standard deviation of Vickers hardness in the steel sheet for hot stamping exceeded 20 (Hv). The hot-stamped product also had a standard deviation of Vickers hardness exceeding 20 (Hv), and the crack initiation displacement and absorbed energy in the three-point bending test were low. In test number 5 using steel A and test number 10 using steel B, the first hot-rolled sheet annealing was not performed in the manufacturing process of the steel sheet for hot stamping. Therefore, the value of the left side of equation (i) was 0.50 or more in the steel sheet for hot stamping, and the value of the left side of equation (ii) was 0.50 or more in the hot-stamped product, the standard deviation of Vickers hardness exceeded 20 (Hv), and the crack initiation displacement and absorbed energy in the three-point bending test were low. In test number 12 using steel C and test number 21 using steel K, the soaking temperature and soaking time for the second hot-rolled sheet annealing in the manufacturing process for the steel sheet for hot stamping were high and long, respectively, and as a result, the standard deviation of Vickers hardness in the steel sheet for hot stamping exceeded 20 (Hv). The hot-stamped product also had a standard deviation of Vickers hardness exceeding 20 (Hv), and the crack initiation displacement and absorbed energy in the three-point bending test were low. In test number 23 using steel L, the soaking time for the first annealing of the hot-stamped steel sheet was short in the manufacturing process for the hot-stamped steel sheet. As a result, the value of the left side of equation (i) was 0.50 or more in the hot-stamped steel sheet, and the value of the left side of equation (ii) was 0.50 or more in the hot-stamped product, the standard deviation of Vickers hardness exceeded 20 (Hv), and the crack initiation displacement and absorbed energy in the three-point bending test were low. In test number 25 using steel M, the soaking time for the second hot-rolled sheet annealing in the manufacturing process for the steel sheet for hot stamping was long, so the standard deviation of Vickers hardness in the steel sheet for hot stamping exceeded 20 (Hv), and the standard deviation of Vickers hardness in the hot-stamped product also exceeded 20 (Hv), and the crack initiation displacement and absorbed energy in the three-point bending test were low. In test No. 28 using Steel O, the soaking temperature for the second hot-rolled sheet annealing in the manufacturing process for the steel sheet for hot stamping was high, so the standard deviation of Vickers hardness in the steel sheet for hot stamping exceeded 20 (Hv), and the standard deviation of Vickers hardness in the hot-stamped product also exceeded 20 (Hv), and the crack initiation displacement and absorbed energy in the three-point bending test were low. [Industrial Applicability]

[0102] According to the present invention, it is possible to obtain a steel sheet for hot stamping which is excellent in impact resistance and has a tensile strength of 2300 MPa or more and is suitable as a material for hot stamped products. By hot stamping this steel sheet for hot stamping as a raw material, it is possible to manufacture hot stamped products with a tensile strength of 2300 MPa or more and excellent impact resistance.

Claims

1. In mass%, C: more than 0.40%, less than 0.70%, Si: less than 2.00% Mn: 0.01% or more and less than 0.50% P: 0.200% or less, S: 0.0200% or less, sol. Al: 0.001 to 1.000%, N: 0.0200% or less, Mo: 0.01% or more and less than 0.50% B: 0.0002 to 0.0200%, Ti: 0-0.200%, Nb: 0 to 0.200%, V: 0-0.200%, Zr: 0-0.200%, Cr: 0-2.00%, W: 0-2.00%, Cu: 0-2.00%, Ni: 0-2.00%, Ca: 0-0.0100%, Mg: 0 to 0.0100%, REM: 0-0.1000%, Bi: 0-0.0500%, The balance is Fe and impurities. and having a chemical composition When the Mo content of the steel plate is measured by linear analysis using EPMA in a range of 0.05 mm in the plate thickness direction centered at a depth position of ¼ of the plate thickness from the surface of the steel plate, the maximum value of the Mo content, the minimum value of the Mo content, and the average value of the Mo content satisfy the following formula (i): The standard deviation of Vickers hardness in a region of 0.3 mm in the thickness direction and 0.6 mm in a direction perpendicular to the thickness direction, centered at a depth position of 1 / 4 of the thickness of the steel plate from the surface of the steel plate, is 20 (Hv) or less, The average value of the Vickers hardness is 280 (Hv) or more. Steel plate for hot stamping. ([Mo] ＭＡＸ -[Mo] ＭＩＮ ) / [Mo] ＡＶＥ <0.50 ・・・(i) The meanings of the symbols in the formula (i) are as follows: [Mo] ＭＡＸ : Maximum Mo content (mass%) [Mo] ＭＩＮ : Minimum Mo content (mass%) [Mo] ＡＶＥ : Average value of Mo content (mass%)

2. The chemical composition is, in mass %, Ti: 0.001 to 0.200%, Nb: 0.001-0.200%, V: 0.001 to 0.200%, and Zr: 0.001 to 0.200%, Contains one or more selected from The steel sheet for hot stamping according to claim 1.

3. The chemical composition is, in mass %, Cr: 0.001-2.00%, W: 0.001-2.00%, Cu: 0.001 to 2.00%, and Ni: 0.001 to 2.00%, Contains one or more selected from The steel sheet for hot stamping according to claim 1 or 2.

4. The chemical composition is, in mass %, Ca: 0.0001-0.0100%, Mg: 0.0001 to 0.0100%, and REM: 0.0001-0.1000%, Contains one or more selected from The steel sheet for hot stamping according to any one of claims 1 to 3.

5. The chemical composition is, in mass %, Bi: 0.0001 to 0.0500%, containing The steel sheet for hot stamping according to any one of claims 1 to 4.

6. It has a base steel plate, The base steel plate is, in mass%, C: more than 0.40%, less than 0.70%, Si: less than 2.00% Mn: 0.01% or more and less than 0.50% P: 0.200% or less, S: 0.0200% or less, sol. Al: 0.001 to 1.000%, N: 0.0200% or less, Mo: 0.01% or more and less than 0.50% B: 0.0002 to 0.0200%, Ti: 0-0.200%, Nb: 0 to 0.200%, V: 0-0.200%, Zr: 0-0.200%, Cr: 0-2.00%, W: 0-2.00%, Cu: 0-2.00%, Ni: 0-2.00%, Ca: 0-0.0100%, Mg: 0 to 0.0100%, REM: 0-0.1000%, Bi: 0-0.0500%, The balance is Fe and impurities. and having a chemical composition When the Mo content of the base steel plate is measured by linear analysis using EPMA in a range of 0.05 mm in the plate thickness direction centered at a depth position of ¼ of the plate thickness of the base steel plate from the surface of the base steel plate, the maximum value of the Mo content, the minimum value of the Mo content, and the average value of the Mo content satisfy the following formula (ii): The metal structure of the base steel plate contains 90.0% by volume or more of martensite, a standard deviation of Vickers hardness in a region of 0.3 mm in the thickness direction and 0.6 mm in a direction perpendicular to the thickness direction, centered at a depth position of ¼ of the thickness of the base steel plate from the surface of the base steel plate, is 20 (Hv) or less; The tensile strength of the base steel plate is 2300 MPa or more. Hot stamped product. ([Mo] ｍＭＡＸ -[Mo] ｍＭＩＮ ) / [Mo] ｍＡＶＥ <0.50 ・・・(ii) However, the meanings of the symbols in the above formula (ii) are as follows: [Mo] ｍＭＡＸ : Maximum Mo content of base steel plate (mass%) [Mo] ｍＭＩＮ : Minimum Mo content of base steel plate (mass%) [Mo] ｍＡＶＥ : Average value of Mo content in base steel plate (mass%)

Images