Hot-stamped steel sheet and hot-stamped formed product

Abstract

A hot-stamped steel sheet having a prescribed chemical composition, when a Mo content of the steel sheet is measured by line analysis using an EPMA in a range of 0.05 mm in a sheet thickness direction centered on a depth position that is 1 / 4 of a sheet thickness of the steel sheet from a surface of the steel sheet, a maximum value of the Mo content, a minimum value of the Mo content, and an average value of the Mo content satisfy ([Mo] MAX − [Mo] MIN ) / [Mo] AVE <0.50, a standard deviation of Vickers hardness in a region of 0.3 mm in the sheet thickness direction and 0.6 mm in a direction orthogonal to the sheet thickness direction centered on the depth position that is 1 / 4 of the sheet thickness of the steel sheet from the surface of the steel sheet is 20 (Hv) or less.

Description

Technical Field

[0001] This invention relates to steel sheets for hot stamping and hot-stamped formed articles.

[0002] 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

[0003] In today's highly specialized industrial technology sector, materials used in various fields require specialized and high-performance properties. For example, regarding steel sheets for automobiles, high strength is required to improve fuel efficiency through vehicle lightweighting, driven by environmental concerns. By applying high-strength steel sheets to automobile bodies, it is possible to reduce the thickness of the steel sheets, thereby lightening the vehicle body, while simultaneously imparting the desired strength.

[0004] However, in the pressing process that forms the body components of a car, the thinner the steel sheet used, the more prone it is to cracking and wrinkling. Therefore, excellent press formability is also required for steel sheets used in automobiles.

[0005] Ensuring press formability is the opposite of achieving high strength in steel sheets, making it difficult to simultaneously satisfy both properties. Furthermore, when high-strength steel sheets are press-formed, the shape of the component changes significantly due to springback when removed from the mold, making it difficult to ensure dimensional accuracy. Thus, manufacturing high-strength vehicle body components through press forming is not easy.

[0006] To date, methods for manufacturing ultra-high strength vehicle body components, such as those disclosed in Patent Document 1, have proposed techniques for pressing heated steel sheets using low-temperature pressing dies. This technique, known as hot stamping or hot pressing, allows for the manufacture of complex-shaped components with high dimensional accuracy because it involves pressing steel sheets heated to a high temperature and then in a soft state. Furthermore, since the steel sheet is rapidly cooled through contact with the die, quenching can significantly increase strength simultaneously with the pressing process. For example, Patent Document 1 describes how hot stamping of steel sheets with a tensile strength of 500–600 MPa yields components with a tensile strength of 1400 MPa or higher.

[0007] Furthermore, as a technology for manufacturing high-strength hot-stamped components, Patent Document 2 discloses hot-stamped components with tensile strengths of 1770–1940 MPa and their manufacturing methods, while Patent Document 3 discloses hot-stamped components with tensile strengths of 1960–2130 MPa and their manufacturing methods. In the methods described in Patent Documents 2 and 3, hot-stamping is performed by heating the hot-stamping steel sheet to the two-phase region of ferrite and austenite, thereby creating a composite structure of ferrite and martensite with an average grain size of 7 μm or less, thus improving the ductility of the steel sheet constituting the component. However, according to the researchers of this invention, for hot-stamped components containing a composite structure of ferrite and martensite, cracking sometimes occurs in the early stages of deformation during a collision, originating from ferrite. In particular, if the tensile strength of the component exceeds 2300 MPa, it becomes difficult to ensure the collision safety of the vehicle body.

[0008] Patent Document 4 discloses a technique for manufacturing hot-stamped components with excellent toughness and a tensile strength of 1800 MPa or higher. In the method described in Patent Document 4, hot-stamping is performed by heating a steel sheet to the low-temperature region of austenite, followed by slow cooling within a temperature range below the Ms point. This forms a metallic structure containing tempered martensite with a pre-austenite grain size of 10 μm or less, thereby improving the toughness of the component. The technique disclosed in Patent Document 4 excels in obtaining hot-stamped components with a tensile strength of 1800 MPa that do not crack even in low-temperature impact tests. However, there is no description of components with a tensile strength of 2300 MPa or higher. Based on our research, it is known that even with hot-stamped components containing a single-phase tempered martensite structure as described in Patent Document 4, increasing the tensile strength to 2300 MPa or higher can cause localized hardness variations within the component, leading to cracking in the early stages of deformation during impact and potentially insufficient impact resistance.

[0009] Existing technical documents

[0010] Patent documents

[0011] Patent Document 1: Japanese Patent Application Publication No. 2002-102980

[0012] Patent Document 2: Japanese Patent Application Publication No. 2010-65294

[0013] Patent Document 3: Japanese Patent Application Publication No. 2010-65295

[0014] Patent Document 4: Japanese Patent Application Publication No. 2006-152427 Summary of the Invention

[0015] The problem that the invention aims to solve

[0016] As mentioned above, it has been difficult in the past to manufacture components with a tensile strength of 2300 MPa or more, especially hot-stamped components with a tensile strength of 2300 MPa or more that have excellent impact resistance (hot-stamped formed products) by hot stamping.

[0017] The purpose of this invention is to solve the above-mentioned problems and to provide a hot stamping steel sheet suitable as a raw material for hot stamping formed articles with excellent impact resistance and tensile strength of 2300 MPa or more, and a hot stamping formed article with excellent impact resistance and tensile strength of 2300 MPa or more.

[0018] Methods for solving problems

[0019] The present invention was made to solve the above-mentioned problems, and its main purpose is the following hot-stamping steel sheet.

[0020] [1] A hot-stamping steel sheet according to one embodiment of the present invention has the following chemical composition: C: more than 0.40% and less than 0.70% by mass%, Si: less than 2.00%, Mn: more than 0.01% and less than 0.50%, P: less than 0.200%, S: less than 0.0200%, sol.Al: 0.001 to 1.000%, N: less than 0.0200%, Mo: more than 0.01% 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%, Cu: 0 to 2.00%, Ni: 0 to 2.00%, Ca: 0~0.0100%, Mg: 0~0.0100%, REM: 0~0.1000%, Bi: 0~0.0500%, Remaining: Fe and impurities. When the Mo content of the steel plate is determined by line analysis of EPMA in a range of 0.05 mm in the thickness direction centered at a depth of 1 / 4 of the thickness of the steel plate from the surface of the steel plate, the maximum value, minimum value and average value of the Mo content satisfy the following formula (i). The standard deviation of the Vickers hardness in the region of 0.3 mm in the thickness direction and 0.6 mm in the direction orthogonal to the thickness direction centered at a depth of 1 / 4 of the thickness of the steel plate from the surface of the steel plate is 20 (Hv) or less.

[0021] ([Mo)) MAX -[Mo] MIN ) / [Mo] AVE <0.50 (i)

[0022] The meanings of the symbols in equation (i) above are as follows.

[0023] [Mo] MAX Maximum Mo content (mass%)

[0024] [Mo] MIN Minimum Mo content (mass%)

[0025] [Mo] AVE Average Mo content (mass%)

[0026] [2] According to the hot stamping steel sheet described in [1], the above chemical composition may also contain one or more elements selected from the following elements in mass %: 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%.

[0027] [3] The hot stamping steel sheet according to [1] or [2] may contain, in mass percent, one or more of the following elements: 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%.

[0028] [4] The hot stamping steel sheet according to any one of [1] to [3], wherein the above chemical composition may also contain one or more elements selected from the following elements in mass %: Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0100% and REM: 0.0001 to 0.1000%.

[0029] [5] The hot stamping steel sheet according to any one of [1] to [4], wherein the above chemical composition may also contain, in mass %: Bi: 0.0001 to 0.0500%.

[0030] [6] Another embodiment of the hot-stamped product of the present invention has a base steel plate having the following chemical composition: C: more than 0.40% and less than 0.70% by mass %, Si: less than 2.00%, Mn: more than 0.01% and less than 0.50%, P: less than 0.200%, S: less than 0.0200%, sol.Al: 0.001 to 1.000%, N: less than 0.0200%, M o: ≥0.01% and <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: 0~0.0100%. REM: 0~0.1000%, Bi: 0~0.0500%, Remaining: Fe and impurities. When the Mo content of the base steel plate is determined by linear analysis using EPMA in a range of 0.05 mm in the thickness direction centered at a depth of 1 / 4 of the thickness of the base steel plate from the surface of the base steel plate, the maximum value, minimum value and average value of the Mo content satisfy the following formula (ii). The metal structure of the base steel plate contains more than 90.0% martensite. The standard deviation of the Vickers hardness in the region of 0.3 mm in the thickness direction and 0.6 mm in the direction orthogonal to the thickness direction centered at a depth of 1 / 4 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 more than 2300 MPa.

[0031] ([Mo)) mMAX -[Mo] mMIN ) / [Mo] mAVE <0.50 (ii)

[0032] The meanings of the symbols in equation (ii) above are as follows.

[0033] [Mo] mMAX Maximum Mo content (mass%) in the base steel plate

[0034] [Mo] mMIN Minimum Mo content (mass%) in the base steel plate.

[0035] [Mo] mAVE Average Mo content (mass%) of the base steel plate

[0036] Invention Effects

[0037] According to the above-described solution of the present invention, it is possible to obtain a hot stamping steel sheet suitable as a raw material for hot stamping formed articles with excellent impact resistance and a tensile strength of 2300 MPa or more, and a hot stamping formed article with excellent impact resistance and a tensile strength of 2300 MPa or more. Attached Figure Description

[0038] Figure 1 This is a schematic diagram showing the location for measuring the hardness of hot-stamped steel sheets and hot-stamped products.

[0039] Figure 2 This is a schematic diagram illustrating an example of the shape of a hot-stamped product.

[0040] Figure 3 This is a schematic diagram showing the shape of a three-point bending test specimen.

[0041] Figure 4 This is a schematic diagram showing the configuration of the testing machine and the test specimen in a three-point bending test. Detailed Implementation

[0042] The inventors have conducted in-depth research on methods to suppress cracking during impact-induced deformation of hot-stamped products with a tensile strength of 2300 MPa or higher. In particular, they have conducted in-depth research on methods to suppress cracking during impact-induced deformation of hot-stamped products by controlling the chemical composition and microstructure of the hot-stamping steel sheet used in the hot-stamping process. As a result, the following insights have been obtained.

[0043] (A) For hot stamping products with a tensile strength of 2300MPa or above, local hardness changes are likely to occur. When the hot stamping product deforms, the stress is concentrated in the part with low hardness, and cracking occurs in the early stage of deformation.

[0044] (B) As a steel sheet for hot stamping, by using a steel sheet with small local variation in Mo concentration, cracking during deformation of hot stamped products is suppressed.

[0045] Although the reasons are unclear, it is presumed that: (a) in the low-concentration portion of Mo, the austenite coarsens during the hot stamping process, which tends to result in lower hardness in the hot-stamped product; (b) on the other hand, in the high-concentration portion of Mo, the austenite refines during the hot stamping process, which tends to result in higher hardness in the hot-stamped product.

[0046] (C) In hot stamping steel sheets, by reducing the local hardness variation, cracking during deformation of hot stamped products can be suppressed.

[0047] Although the reasons are unclear, it is presumed that: (a) in hot-stamped steel sheets, the variation in hardness increases if soft ferrite is present in certain areas; (b) in areas with a high ferrite content, the austenite coarsens during the hot-stamping process, and the hardness tends to be lower in the hot-stamped product; (c) on the other hand, in areas with a low ferrite content, the austenite refines during the heating process, and the hardness tends to be higher in the hot-stamped product.

[0048] (D) As a steel sheet for hot stamping, by using a steel sheet manufactured without annealing after the cold rolling process (also known as cold-rolled steel sheet or high-hardness cold-rolled sheet), cracking during deformation of the formed product can be suppressed.

[0049] Although the reasons are unclear, it is presumed that: (a) in cold-rolled steel sheets, due to the accumulation of processing strain during cold rolling, the austenite is refined during the hot stamping process, and the hardness of the hot-stamped product increases; (b) this effect is stronger in the regions with low Mo concentration and high ferrite content, and by using cold-rolled steel sheets, the local hardness variation in the hot-stamped product is reduced.

[0050] (E) In the process of manufacturing hot-stamped steel sheets, by annealing the hot-rolled steel sheets by heating them to above the Ac3 point and holding them for a long time (also known as the first hot-rolled sheet annealing), the local variation of Mo concentration in the hot-stamped steel sheets is reduced.

[0051] (F) In the process of manufacturing hot-stamping steel sheets, the local hardness variation of the hot-stamping steel sheets is reduced by annealing that follows the first hot-rolled annealing to a point above Ac3 and is held for a short time (also known as the second hot-rolled annealing).

[0052] Although the reasons are unclear, it is presumed that the cause is: (a) in the first hot-rolled plate annealing, the austenite tends to coarsen during annealing, and coarse ferrite is present locally after annealing; (b) in the second hot-rolled plate annealing, the austenite does not tend to coarsen during annealing, and ferrite is uniformly and finely dispersed after annealing.

[0053] Based on the insights gained from (A) to (F) above, the inventors have discovered that by using hot stamping steel sheets with small local variations in Mo concentration and consequently small local variations in hardness, it is possible to manufacture hot stamped products with small local variations in hardness, tensile strength of 2300 MPa or higher, and excellent impact resistance.

[0054] Hereinafter, the components of the hot stamping steel sheet of the present invention (the hot stamping steel sheet of this embodiment) will be described in detail.

[0055] Chemical composition of hot-stamped steel sheets

[0056] The hot-stamping steel sheet of this embodiment has the chemical composition shown below. The reasons for the limitation of each element are as follows. In the following description, "%" for content refers to "mass %". In addition, the values ​​at both ends of the numerical range indicated by "~" are included in the range. On the other hand, values ​​indicated by "less than" or "more than" are not included in the range.

[0057] C: More than 0.40% and less than 0.70%

[0058] Carbon (C) is an element that increases the tensile strength of hot-stamped steel sheets (the steel sheets used in hot-stamped products). When the C content is below 0.40%, the tensile strength of the hot-stamped steel sheet falls below 2300 MPa, resulting in insufficient strength in the hot-stamped product. Therefore, the C content is set to be greater than 0.40%. Preferred C contents are greater than 0.42%, greater than 0.43%, greater than 0.44%, or greater than 0.45%.

[0059] On the other hand, if the carbon content exceeds 0.70%, the strength of the hot-stamped product becomes too high, making it impossible to ensure impact resistance. Therefore, the carbon content is set to 0.70% or less. Preferred carbon contents are 0.65% or less, 0.60% or less, 0.55% or less, or 0.50% or less.

[0060] Si: Below 2.00%

[0061] Si is an element present in steel as an impurity that causes embrittlement. Its adverse effects become particularly pronounced if the Si content exceeds 2.00%. Therefore, the Si content is set below 2.00%. Preferred Si contents are below 1.50%, below 1.00%, below 0.75%, or below 0.50%.

[0062] There is no particular lower limit for the Si content, but excessively reducing the Si content will lead to an increase in steelmaking costs. Therefore, it is preferable to set the Si content to 0.001% or more. In addition, since Si has the effect of improving the hardenability of steel, it can also be actively included. From the viewpoint of improving hardenability, the Si content is preferably 0.10% or more, 0.20% or more, or 0.30% or more.

[0063] Mn: ≥0.01% and <0.50%

[0064] Mn is an element that degrades the impact resistance of hot-stamped articles. In particular, if the Mn content is 0.50% or higher, the impact resistance deteriorates significantly, and even if the manufacturing method for hot-stamped steel sheets described later is applied, it becomes impossible to ensure the impact resistance of the hot-stamped articles. Therefore, the Mn content is set to be less than 0.50%. The preferred Mn content is less than 0.45%, less than 0.40%, less than 0.35%, or less than 0.30%.

[0065] On the other hand, Mn is an element that combines with S, which is an impurity, to form MnS, thus suppressing the adverse effects caused by 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.

[0066] P: below 0.200%

[0067] Phosphorus (P) is an element present in steel as an impurity that causes embrittlement. If the P content exceeds 0.200%, its adverse effects become particularly significant, leading to a marked deterioration in weldability. Therefore, the P content is set to 0.200% or less. Preferred P contents are less than 0.100%, less than 0.050%, or less than 0.020%.

[0068] There is no specific lower limit for phosphorus (P) content, but excessively reducing P content will lead to an increase in steelmaking costs. Therefore, the P content can also be set at 0.001% or higher.

[0069] S: below 0.0200%

[0070] Sulfur (S) is an element present in steel as an impurity that causes embrittlement. If the S content exceeds 0.0200%, its adverse effects become particularly significant. Therefore, the S content is set to be below 0.0200%. Preferred S contents are below 0.0050%, below 0.0020%, or below 0.0010%.

[0071] There is no specific lower limit for sulfur content, but excessively reducing sulfur content will lead to an increase in steelmaking costs. Therefore, the sulfur content can also be set at 0.0001% or higher.

[0072] sol.Al: 0.001~1.000%

[0073] Al is an element that deoxidizes molten steel. If the sol.Al content (acid-soluble Al content) is less than 0.001%, deoxidation becomes 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.

[0074] On the other hand, if the sol.Al content is too high, the phase transformation point rises, making it difficult to heat the steel sheet to temperatures exceeding the Ac3 point during the hot stamping process. Therefore, the sol.Al content is set to 1.000% or less. Preferably, the sol.Al content is less than 0.500%, less than 0.100%, less than 0.060%, or less than 0.040%.

[0075] N: below 0.0200%

[0076] Nitrogen (N) is an element present in steel as an impurity and forms nitrides during continuous casting. These nitrides degrade the ductility of the hot-stamped steel sheet, therefore a low N content is preferred. If the N content exceeds 0.0200%, its adverse effects become particularly significant. Therefore, the N content is set to 0.0200% or less. Preferably, the N content is below 0.0100%, below 0.0080%, or below 0.0050%.

[0077] There is no specific lower limit for nitrogen (N) content, but excessively reducing the N content will lead to an increase in steelmaking costs. Therefore, the N content can also be set at 0.0010% or higher.

[0078] Mo: ≥0.01% and <0.50%

[0079] Mo is an element that improves the hardenability of steel and is effective in ensuring the strength of hot-stamped parts by forming a martensitic microstructure during the hot stamping process. To achieve this effect, the Mo content is set to 0.01% or more. Preferred Mo contents are 0.05% or more, 0.10% or more, or 0.15% or more.

[0080] On the other hand, if the Mo content is 0.50% or higher, even if the manufacturing method for hot-stamping steel sheets described later is applied, it is impossible to suppress local variations in Mo concentration within the hot-stamping steel sheet, thus failing to adequately ensure the impact resistance of the hot-stamped product. Therefore, the Mo content is set to be less than 0.50%. The Mo content is preferably less than 0.40%, less than 0.35%, or less than 0.30%.

[0081] B: 0.0002~0.0200%

[0082] Boron (B) is an element that improves the hardenability of steel and is effective in ensuring the strength of hot-stamped parts by forming a martensitic microstructure during the hot stamping process. To achieve this effect, the B content is set to 0.0002% or more. Preferred B contents are 0.0006% or more, 0.0010% or more, or 0.0015% or more.

[0083] On the other hand, when the boron content exceeds 0.0200%, carborides will form, impairing the hardenability improvement effect brought about by the presence of boron. Therefore, the boron content is set to 0.0200% or less. Preferred boron contents are less than 0.0050%, less than 0.0040%, or less than 0.0030%.

[0084] The hot-stamping steel sheet of this embodiment may also have a chemical composition including the above-described chemical composition, with the remainder being Fe and impurities. However, in order to improve properties, the hot-stamping steel sheet of this embodiment may further contain one or more elements selected from Ti, Nb, V, Zr, Cr, W, Cu, Ni, Ca, Mg, REM, and Bi within the range shown below. These elements (optional elements) are not necessarily required, therefore the lower limit is 0%.

[0085] Here, "impurities" refers to components that are mixed in from raw materials such as ore and scrap iron or through various factors during the industrial manufacturing of steel plates, and are permissible within a range that does not adversely affect the hot-stamping steel plate of this embodiment.

[0086] Ti: 0~0.200%

[0087] Nb: 0~0.200%

[0088] V: 0~0.200%

[0089] Zr: 0~0.200%

[0090] Ti, Nb, V, and Zr are elements that improve the impact resistance of hot-stamped parts by refining the metal structure. To achieve this effect, one or more of Ti, Nb, V, and Zr may be included as needed.

[0091] To achieve the above-mentioned effects, it is preferable to contain 0.001% or more of one or more selected from Ti, Nb, V and Zr, more preferably 0.005% or more of each, and even more preferably 0.010% or more of each.

[0092] On the other hand, when the contents of Ti, Nb, V, and Zr exceed 0.200%, the above effects saturate, and the manufacturing cost of the steel plate increases. Therefore, when Ti, Nb, V, and Zr are present, their contents are set to 0.200% or less.

[0093] Furthermore, when the contents of Ti, Nb, V, and Zr are high, the excessive precipitation of carbides from these elements impairs the ductility of the hot-stamped steel sheet. From the viewpoint of ensuring ductility, the preferred Ti content is less than 0.050% or less than 0.030%, the preferred Nb content is less than 0.050%, less than 0.030%, or less than 0.020%, the preferred V content is less than 0.100% or less than 0.050%, and the preferred Zr content is less than 0.100% or less than 0.050%.

[0094] Cr: 0–2.00%

[0095] W: 0~2.00%

[0096] Cu: 0–2.00%

[0097] Ni: 0~2.00%

[0098] Cr, W, Cu, and Ni are elements that improve the hardenability of steel. Therefore, it is also possible to include one or more of Cr, W, Cu, and Ni as needed.

[0099] To achieve the aforementioned effects, it is preferable to contain 0.001% or more of one or more selected from Cr, W, Cu, and Ni. More preferably, the Cr content is 0.05% or more, or 0.10% or more; more preferably, the W content is 0.05% or more, or 0.10% or more; more preferably, the Cu content is 0.10% or more; and more preferably, the Ni content is 0.10% or more.

[0100] On the other hand, if the contents of Cr, W, Cu, and Ni each exceed 2.00%, the impact resistance of the hot-stamped product deteriorates. Therefore, when these components are present, the contents of Cr, W, Cu, and Ni are each set to 2.00% or less. Preferably, the Cr content is less than 0.50%, less than 0.40%, or less than 0.30%; the preferred W content is less than 0.50%, less than 0.40%, or less than 0.30%; the preferred Cu content is less than 1.00% or less than 0.50%; and the preferred Ni content is less than 1.00% or less than 0.50%.

[0101] Ca: 0~0.0100%

[0102] Mg: 0~0.0100%

[0103] REM: 0~0.1000%

[0104] Ca, Mg, and REM are elements that improve the ductility of hot-stamped steel sheets by adjusting the shape of inclusions. Therefore, they may be included as needed. To obtain the above-mentioned effects, it is preferable to include 0.0001% or more of one or more of Ca, Mg, and REM.

[0105] On the other hand, when the content of Ca or Mg exceeds 0.0100%, or when the content of REM exceeds 0.1000%, not only does the above effect saturate, but excessive costs are also incurred. Therefore, when present, the content of Ca and Mg is set to 0.0100% or less, and the content of REM is set to 0.1000% or less.

[0106] In this embodiment, REM refers to a total of 17 elements including Sc, Y, and the lanthanides, and REM content refers to the total content of these elements. The lanthanides are industrially added in the form of mixed rare earth metals.

[0107] Bi: 0~0.0500%

[0108] Bi is an element that improves the impact resistance of hot-stamped articles by refining the solidification structure. Therefore, it may be included as needed. To achieve the above-mentioned effects, the Bi content is preferably 0.0001% or more. More preferably, the Bi content is 0.0003% or more, or 0.0005% or more.

[0109] On the other hand, when the Bi content exceeds 0.0500%, the above-mentioned effects saturate, resulting in excessive costs. Therefore, when Bi is present, the Bi content is set to 0.0500% or less. Preferably, the Bi content is 0.0100% or less, or 0.0050% or less.

[0110] As described above, the chemical composition of the hot stamping steel sheet of this embodiment may also contain essential elements and the remainder being Fe and impurities, or it may contain essential elements, further contain one or more optional elements, and the remainder being Fe and impurities.

[0111]

[0112] The local element concentration distribution of the hot-stamping steel sheet of this embodiment will be described. When the Mo content of the hot-stamping steel sheet of this embodiment is determined by line analysis within a range of 0.05 mm in the thickness direction centered at a depth of 1 / 4 of the thickness of the steel sheet at 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 equation (i).

[0113] ([Mo)) MAX -[Mo] MIN ) / [Mo] AVE <0.50 (i)

[0114] The meanings of the symbols in equation (i) above are as follows.

[0115] [Mo] MAX Maximum Mo content (mass%)

[0116] [Mo] MIN Minimum Mo content (mass%)

[0117] [Mo] AVE Average Mo content (mass%)

[0118] By ensuring that the Mo content of the hot-stamped steel sheet within the aforementioned range satisfies equation (i) above, the impact resistance of the hot-stamped product can be improved. The left-hand side value of equation (i) above is preferably less than 0.40 or less than 0.30.

[0119] The lower limit of the left-hand side value of equation (i) above is not limited. However, in order to significantly reduce the left-hand side value of equation (i), in the manufacturing method of hot-stamping steel sheet described later, it is necessary to excessively increase the soaking temperature of the first hot-rolled sheet annealing or excessively extend the soaking time. In this case, not only will the productivity of the hot-stamping steel sheet be impaired, but the local hardness variation of the hot-stamping steel sheet will also increase. Therefore, the left-hand side value of equation (i) above can also be 0.05 or more, 0.10 or more, or 0.15 or more.

[0120] In this embodiment, the distribution of local Mo content (concentration) is determined as follows.

[0121] First, test pieces were collected from hot-stamped steel plates, and longitudinal sections parallel to the rolling direction of the steel plates were polished with water-resistant abrasive paper. Then, after polishing with a diamond suspension, line analysis was performed using a field emission electron probe microanalysis (FE-EPMA) within a 0.05 mm range along the thickness direction, centered at a depth of 1 / 4 of the steel plate thickness in the thickness direction (1 / 4 depth position). EPMA measurements were performed at 0.2 μm intervals along the thickness direction, and the Mo content at each measurement location was determined by a 5-point moving average. Specifically, the average value of the Mo concentration measurements at 5 consecutive points was set as the Mo content at the third measurement location, and the Mo content at each measurement location within the aforementioned range was calculated. The left-hand side of equation (i) above was then calculated from the maximum, minimum, and average values ​​of the Mo content within the aforementioned range (the average value of the Mo content at all measurement locations). The line analysis is performed at any 10 points on the steel plate, and the average of the left-hand values ​​obtained at the 10 points is set as the left-hand value of the above equation (i) in the steel plate.

[0122] Hardness distribution of steel sheets for hot stamping

[0123] The 0.18mm steel plate used for hot stamping in this embodiment 2 The standard deviation of Vickers hardness in the region (centered at 1 / 4 depth of the steel plate, 0.3 mm in the plate thickness direction, and 0.6 mm in the direction orthogonal to the plate thickness direction) is less than 20 (Hv).

[0124] If the standard deviation of Vickers hardness in the aforementioned region exceeds 20 (Hv), cracking will occur in the early stages of deformation when the hot-stamped part deforms, significantly deteriorating its impact resistance. Therefore, the standard deviation of hardness in the aforementioned region is set to 20 (Hv) or less. Preferably, the standard deviation of hardness is set to 15 (Hv) or less, or 10 (Hv) or less.

[0125] Furthermore, in this embodiment, the steel sheet used for hot stamping is a cold-rolled steel sheet, and the average hardness becomes an indicator of the strain energy accumulated in the steel sheet. In order to improve the strain energy and the impact 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.

[0126] The standard deviation of hardness within the aforementioned region should ideally be small, but significantly reducing the standard deviation of hardness will lead to a decrease in the productivity of hot-stamped steel sheets. Therefore, the standard deviation of hardness can also exceed 5 (Hv) or exceed 10 (Hv). The average hardness within the aforementioned region should ideally be large, but significantly increasing the average hardness will not only lead to a decrease in the productivity of hot-stamped steel sheets, but also deteriorate the shearability of the hot-stamped steel sheets. Therefore, the average hardness can also be below 400 (Hv) or below 370 (Hv).

[0127] In this embodiment, the hardness of the hot-stamping steel sheet is determined as follows.

[0128] First, test pieces were collected from hot-stamped steel plates. The longitudinal section parallel to the rolling direction of the steel plate was ground with water-resistant abrasive paper. After further polishing with diamond suspension, the Vickers hardness was measured at 1 / 4 depth of the steel plate.

[0129] Specifically, such as Figure 1 As shown, Vickers hardness was measured at specified intervals within a range of 0.3 mm in the thickness direction and 0.6 mm in the direction orthogonal to the thickness direction, centered at 1 / 4 depth of the steel plate. The arithmetic mean and standard deviation were calculated from the measured values. The hardness was measured using a micro Vickers hardness tester, with the testing conditions set as a load of 0.49 N and a load holding time of 10 seconds. Higher loads would result in larger indentations, making it impossible to evaluate the localized hardness distribution, which is closely related to the impact resistance of hot-stamped parts. Therefore, the load was specified as 0.49 N.

[0130] <Strength of hot-stamped steel sheets>

[0131] For the hot-stamping steel sheet of this embodiment, in order to improve strain energy and impact resistance of the hot-stamped product, the tensile strength is preferably 900 MPa or more. More preferably, the tensile strength is 950 MPa or more, or 1000 MPa or more.

[0132] <Metal Structure of Hot-Stamped Steel Sheets>

[0133] The hot-stamping steel sheet of this embodiment is manufactured without annealing after the cold rolling process, thus having a metallic structure that extends along the rolling direction. By producing such a metallic structure, the strain energy of the hot-stamping steel sheet is increased, and the impact resistance of the hot-stamped product is improved. In contrast, for steel sheets that are annealed after cold rolling, the accumulated strain energy is insufficient, and the impact resistance of the hot-stamped product is reduced.

[0134] If the microstructure contains martensite (including tempered martensite), the steel sheet becomes significantly hardened, making it difficult to cut. Therefore, the microstructure of hot-stamping steel sheets is preferably dominated by ferrite, pearlite, and / or bainite extending along the rolling direction. The total volume fraction of ferrite extending along the rolling direction, pearlite extending along the rolling direction, and bainite extending along the rolling direction is preferably greater than 80.0%, greater than 90.0%, or greater than 95.0%.

[0135] In the microstructure, the remaining portion other than ferrite, pearlite, and bainite extending along the rolling direction may also be martensite and / or retained austenite, and may further contain precipitates such as cementite. The volume fraction of the remaining portion is preferably 20.0% or less. The volume fraction of martensite is preferably less than 10.0% or less than 5.0%.

[0136] The volume fraction of each component in the metal structure of hot-stamped steel sheet is calculated as follows.

[0137] First, test pieces were collected from hot-stamped steel plates. The longitudinal section parallel to the rolling direction of the steel plate was ground with water-resistant abrasive paper. After further polishing with diamond suspension, the microstructure was observed at a depth of 1 / 4 of the steel plate thickness from the surface of the steel plate.

[0138] Specifically, after etching the polished surface with nitric acid ethanol or electrolytic polishing, the microstructure was observed using an optical microscope and a scanning electron microscope (SEM). Image analysis based on brightness differences or differences in the morphology of iron carbides within the phases was performed on the obtained microstructure photographs to obtain the area ratios of ferrite, pearlite, bainite, and tempered martensite. Subsequently, the same observation locations were etched with Lepera etching, and the microstructure was observed again using an optical microscope and a scanning electron microscope (SEM). Image analysis of the obtained microstructure photographs was performed to calculate the total area ratio of retained austenite and martensite.

[0139] In addition, for the same observation position, after electrolytic grinding of the longitudinal section parallel to the rolling direction of the steel plate, the area ratio of retained austenite was determined based on the difference in crystal structure using a SEM equipped with an electron beam backscatter pattern resolution device (EBSP).

[0140] Based on these results, the area fractions of ferrite, pearlite, bainite, tempered martensite, martensite, and retained austenite were obtained. Then, the area fraction was set to be equal to the volume fraction, and the measured area fraction was set as the volume fraction of each microstructure.

[0141] In microstructure observation, tempered martensite can be distinguished from martensite by the presence of iron carbides within it. Furthermore, the fact that the iron carbides present within it elongate in multiple directions can be distinguished from bainite.

[0142] <Manufacturing Method of Steel Sheets for Hot Stamping>

[0143] A preferred manufacturing method for the hot-stamping steel sheet of this embodiment will be described.

[0144] The hot-stamping steel sheet of this embodiment can be manufactured by a manufacturing method including the following steps.

[0145] (I) A hot rolling process in which a slab having the above chemical composition is hot rolled and then coiled to produce a hot-rolled steel sheet.

[0146] (II) The first hot-rolled annealing process of producing hot-rolled annealed steel sheet by performing a first hot-rolled annealing on the above-mentioned hot-rolled steel sheet;

[0147] (III) The second hot-rolled annealing process for performing a second hot-rolled annealing on the above-mentioned hot-rolled annealed steel sheet;

[0148] (IV) A cold rolling process for producing cold-rolled steel sheets by cold rolling hot-rolled annealed steel sheets that have undergone the second hot-rolled annealing process described above.

[0149] The method for manufacturing the slab in the hot-stamping steel sheet manufacturing method of this embodiment is not particularly limited. In the preferred manufacturing method of the slab illustrated, steel having the above-described composition (chemical composition) is melted by known means and then made into steel ingots by continuous casting, or into steel ingots by any casting method and then into steel billets by a billet-making method. In the continuous casting process, in order to suppress the generation of surface defects caused by inclusions, it is preferable to generate external additional flow such as electromagnetic stirring in the molten steel within the mold. The steel ingots or billets can be temporarily cooled and then reheated for hot rolling, or the steel ingots in the high-temperature state after continuous casting or the steel billets in the high-temperature state after billet-making can be directly, or kept at a constant temperature, or subjected to auxiliary heating for hot rolling. In this embodiment, such steel ingots and billets are collectively referred to as "slabs" as raw materials for hot rolling.

[0150] [Hot rolling process]

[0151] To prevent the coarsening of austenite, the temperature of the slab supplied for hot rolling (slab heating temperature) is preferably set below 1250°C, and more preferably below 1200°C. On the other hand, if the slab heating temperature is too low, rolling becomes difficult, so the slab heating temperature can also be set above 1050°C.

[0152] Hot-rolled steel sheet is obtained by hot rolling a heated slab. In order to refine the metal structure of the hot-rolled steel sheet by causing austenite phase transformation after rolling, hot rolling is preferably carried out in a temperature range above Ar3.

[0153] In hot rolling processes that include roughing and finishing, the rough-rolled material can be heated between the roughing and finishing processes to complete the finishing at the aforementioned temperatures. In this case, it is preferable to heat the rough-rolled material in such a way that the rear end becomes hotter than the front end, thus suppressing the temperature variation along the entire length of the rough-rolled material at the start of finishing to below 140°C. This improves the uniformity of the product characteristics within the coil after the winding process.

[0154] Heating of the rough-rolled material can be carried out using any known method. For example, a solenoid induction heating device can be installed between the roughing mill and the finishing mill, and the heating rate can be controlled based on the temperature distribution along the length of the rough-rolled material on the upstream side of the induction heating device.

[0155] When coiling hot-rolled steel sheets, it is preferable to set the coiling temperature to 660°C or below in order to suppress local fluctuations in Mo concentration. More preferably, the coiling temperature is 640°C or below, or 620°C or below.

[0156] On the other hand, if the coiling temperature becomes too low, the steel sheet may harden significantly, causing cracks to form within the steel sheet during the manufacturing process. Therefore, the coiling temperature is preferably set to exceed 500°C or 550°C.

[0157] [The first annealing process for hot-rolled steel plates]

[0158] Hot-rolled and coiled steel sheets undergo a first hot-rolled annealing process to become hot-rolled annealed steel sheets. In this embodiment, the annealing performed on the hot-rolled steel sheet is referred to as hot-rolled annealing, and the steel sheet after hot-rolled annealing is referred to as hot-rolled annealed steel sheet. Before the first hot-rolled annealing, leveling processes such as surface finishing or descaling processes such as pickling may be performed.

[0159] In the first hot-rolled sheet annealing process, the soaking temperature is set to Ac3 point (°C) or higher, and the soaking time (holding time at the soaking temperature) is set to more than 1 hour. Furthermore, the average cooling rate from the soaking temperature to 500°C is set to more than 1°C / second. This is to suppress local variations in Mo concentration and improve the impact resistance of the hot-stamped product. A more preferred soaking temperature is (Ac3 point + 50°C) or higher, a more preferred soaking time is 2 hours or more or 6 hours or more, and a more preferred average cooling rate to 500°C is 2°C / second or higher. If the soaking temperature is too high or the soaking time is too long, the austenite becomes excessively coarse, and the local hardness variation in the hot-stamped steel sheet increases. Therefore, the soaking temperature is preferably set to (Ac3 point + 200°C) or lower or (Ac3 point + 100°C) or lower, and the soaking time is preferably set to 12 hours or lower or 10 hours or lower.

[0160] The Ac3 point is the temperature at which ferrite disappears in the metal structure when the steel plate is heated. In this embodiment, it is determined by the thermal expansion change when the steel plate is heated at 8°C / second.

[0161] [Second hot-rolled plate annealing process]

[0162] For steel sheets that have undergone a first hot-rolled annealing (hot-rolled annealed steel sheets), a second hot-rolled annealing is performed. The annealing performed on hot-rolled annealed steel sheets is also called hot-rolled annealing. Before the second hot-rolled annealing, leveling processes such as surface finishing or descaling processes such as pickling may be performed.

[0163] In the second hot-rolled sheet annealing process, the soaking temperature is set to above Ac3 and below (Ac3 + 50°C), and the soaking time is set to 1 second or more and less than 10 minutes. Furthermore, 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 localized hardness variations in the hot-stamped steel sheet and improve the impact resistance of the hot-stamped product. More preferably, the soaking temperature is above Ac3 and below (Ac3 + 25°C), more preferably, the soaking time is 10 seconds or more and less than 5 minutes, and more preferably, the average heating rate from 500°C to the soaking temperature is 2°C / second or more. If the average cooling rate from the soaking temperature to 500°C is too fast, the steel sheet becomes significantly hardened, making it difficult to cut; therefore, it is preferable to set the cooling rate to 15°C / second or less.

[0164] [Cold rolling process]

[0165] Hot-rolled steel sheets that have undergone a second annealing (hot-rolled annealed steel sheets) are cold-rolled to produce cold-rolled steel sheets using conventional methods. In the cold rolling process, the cold pressing ratio (cumulative reduction in cold rolling) is set to 10% or more. If the cold pressing ratio is less than 10%, the strain energy accumulated in the steel sheet is insufficient, and the localized hardness variation in the steel sheet increases, reducing the impact resistance of the hot-stamped products. Preferred cold pressing ratios are 20%, 30%, or 40% or more. While an upper limit for the cold pressing ratio is not strictly necessary, excessively increasing the cold pressing ratio will increase the load on the rolling equipment and lead to a decrease in productivity; therefore, the cold pressing ratio is preferably set to less than 70%, less than 60%, or less than 50%.

[0166] To reduce the weight of hot-stamped products, 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. Before cold rolling, the sheet may be leveled by surface finishing or descaled by pickling or other methods, according to known procedures.

[0167] It is preferable not to anneal the cold-rolled steel sheet. If the cold-rolled steel sheet is annealed, the strain energy accumulated during cold rolling is released. In addition, the local variation in hardness of the steel sheet may sometimes increase. When such steel sheet is used as hot stamping steel sheet, the impact resistance of the hot stamped product deteriorates. For cold-rolled steel sheets obtained in this way, degreasing or oiling can also be performed according to conventional methods.

[0168] By hot stamping the hot stamping steel sheet of this embodiment described above, a hot stamping formed article can be obtained. The hot stamping formed article manufactured using the hot stamping steel sheet of this embodiment (hereinafter, the hot stamping formed article of this embodiment) will be described.

[0169] The hot-stamped product of this embodiment includes a base steel sheet (a steel sheet constituting the hot-stamped product formed by hot stamping a steel sheet for hot stamping). It may also include only the base steel sheet.

[0170] <Chemical composition of the base steel sheet for hot stamping>

[0171] Since the chemical composition remains substantially unchanged through hot stamping, the chemical composition of the base steel sheet of the hot-stamped product (also referred to as the chemical composition of the hot-stamped product when the hot-stamped product consists only of the base steel sheet) is the same as that of the hot-stamped steel sheet described above. When the hot-stamped product has portions with a tensile strength of 2300 MPa or more and portions with a tensile strength lower than 2300 MPa, the aforementioned chemical composition is sufficient as long as the portion of the base steel sheet with a minimum tensile strength of 2300 MPa or more has such a composition.

[0172]

[0173] When the Mo content of the hot-stamped product of this embodiment is determined by line analysis within a range of 0.05 mm in the thickness direction centered at a depth position of 1 / 4 of the thickness of the base steel plate (the steel plate included in the hot-stamped product), the maximum value, minimum value and average value of the Mo content in the measurement results satisfy the following formula (ii).

[0174] ([Mo)) mMAX -[Mo] mMIN ) / [Mo] mAVE <0.50 (ii)

[0175] The meanings of the symbols in equation (ii) above are as follows.

[0176] [Mo] mMAX Maximum Mo content (mass%) in the base steel plate

[0177] [Mo] mMIN Minimum Mo content (mass%) in the base steel plate.

[0178] [Mo] mAVE Average Mo content (mass%) of the base steel plate

[0179] In the case where a hot-stamped part has a tensile strength of 2300 MPa or more and a tensile strength of less than 2300 MPa, the above formula (ii) can be satisfied as long as the part of the base steel plate has a tensile strength of at least 2300 MPa or more.

[0180] The smaller the local variation in Mo concentration in a hot-stamped part, the more effectively stress concentration can be mitigated in the softer areas during deformation, thus suppressing cracking. Therefore, the left-hand side value of equation (ii) above is preferably below 0.50. The left-hand side value of equation (ii) above is further preferably below 0.40 or below 0.30.

[0181] The lower limit of the left-hand side value of equation (ii) above is not limited, but significantly reducing the left-hand side value of equation (ii) above will lead to a decrease in the productivity of hot-stamped steel sheets. Therefore, the left-hand side value of equation (ii) above can also be 0.05 or more, 0.10 or more, or 0.15 or more.

[0182] The localized Mo concentration distribution in hot-stamped products can be determined by taking test pieces from the hot-stamped products, polishing and grinding the longitudinal section of the steel sheet, and then performing concentration analysis at 1 / 4 depth of the base steel sheet using the same method as for hot-stamping steel sheets. When the hot-stamped product has portions with tensile strengths of 2300 MPa or higher and portions with tensile strengths lower than 2300 MPa, concentration analysis is performed by taking test pieces from the portion of the base steel sheet where the tensile strength is at least 2300 MPa or higher.

[0183] <Metal structure of the base steel sheet for hot stamping>

[0184] The hot-stamped product manufactured using the hot-stamping steel sheet of this embodiment preferably has the following metal structure as the base steel sheet. When the hot-stamped product has portions having a tensile strength of 2300 MPa or more and portions having a tensile strength of less than 2300 MPa, the portion of the base steel sheet with a tensile strength of at least 2300 MPa or more preferably has the following metal structure.

[0185] Martensite: Over 90.0%

[0186] Martensite is an important microstructure for improving the tensile strength of hot-stamped steel sheets. If the martensite volume fraction is below 90.0%, the tensile strength of the hot-stamped product becomes below 2300 MPa, which is insufficient. Therefore, it is preferable to set the martensite volume fraction to be greater than 90.0%. More preferably, the martensite volume fraction is greater than 91.0%, greater than 93.0%, or greater than 95.0%.

[0187] There is no need to specifically define an upper limit for the volume fraction of martensite. However, in order to significantly increase the volume fraction of martensite, it is necessary to excessively increase the heating temperature of the steel sheet or excessively increase the cooling rate during the hot stamping process, which greatly reduces the productivity of hot stamped products. Therefore, the volume fraction of martensite is preferably set to 99.0% or less, or 98.0% or less.

[0188] The aforementioned martensite includes not only primary martensite that has not been tempered, but also tempered martensite that has been tempered and contains iron carbides inside.

[0189] The remaining portion of the metallic structure may also 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 limit of the volume fraction of ferrite, pearlite, bainite, retained austenite, and precipitates is 0%.

[0190] Ferrite, pearlite, and bainite improve the ductility of hot-stamped steel sheets; therefore, to achieve this effect, it is preferable to include one or more selected from ferrite, pearlite, and bainite. The volume fraction of ferrite is preferably set to 0.5% or more, or 1.0% or more, and the volume fractions of pearlite and bainite are preferably set to 1.0% or more, more preferably 2.0% or more, respectively.

[0191] On the other hand, if the content of ferrite, pearlite, and bainite is excessive, the impact resistance of the hot-stamped product will deteriorate. Therefore, the volume fraction of ferrite is preferably set to less than 3.0% or less than 2.0%, and the volume fractions of pearlite and bainite are preferably set to less than 10.0%, more preferably less than 5.0%, respectively.

[0192] Retained austenite improves the ductility of hot-stamped steel sheets. To achieve this effect, it is preferable to set the volume fraction of retained austenite to 0.5% or more, 1.0% or more, or 2.0% or more.

[0193] On the other hand, to prevent an excessive increase in the volume fraction of retained austenite, isothermal quenching at high temperatures is required after hot stamping, significantly reducing the productivity of hot-stamped products. Furthermore, excessive retained austenite can sometimes 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%.

[0194] The volume fraction of each microstructure in the metal structure of a hot-stamped product can be determined by taking test pieces from the hot-stamped product, polishing and grinding the longitudinal section of the steel sheet, and then observing the microstructure at 1 / 4 depth of the base steel sheet using the same method as for hot-stamping steel sheets. When the hot-stamped product has portions with tensile strength of 2300 MPa or higher and portions with tensile strength lower than 2300 MPa, test pieces are taken from the portion of the base steel sheet with a tensile strength of at least 2300 MPa or higher for microstructure observation.

[0195] <Strength of the base steel sheet for hot stamping>

[0196] In this embodiment, all or part of the hot-stamped formed article preferably has a tensile strength of 2300 MPa or more. Therefore, the tensile strength of all or part of the base steel sheet of the hot-stamped formed article is 2300 MPa or more. If the tensile strength of at least a portion is not 2300 MPa or more, the impact resistance of the hot-stamped formed article cannot be ensured. Therefore, the tensile strength of all or part of the hot-stamped formed article is set to 2300 MPa or more. Preferably, the tensile strength of all or part of the hot-stamped formed article is 2400 MPa or more, or 2500 MPa or more. On the other hand, excessively increasing the strength of the hot-stamped formed article will lead to a decrease in impact resistance, so it is preferable to set the tensile strength of the base steel sheet of the hot-stamped formed article to be less than 3000 MPa or less than 2800 MPa.

[0197] The hot-stamped formed article of this embodiment can have a tensile strength of 2300 MPa or higher throughout, or it can contain portions with a tensile strength of 2300 MPa or higher and portions with a tensile strength of less than 2300 MPa. By providing portions with different strengths, the deformation state of the hot-stamped formed article during impact can be controlled. Hot-stamped formed articles with portions of different strengths can be manufactured by joining two or more steel sheets with different chemical compositions and then hot-stamping them, by partially changing the heating temperature of the steel sheets or the cooling rate after hot-stamping during the hot-stamping process, or by partially reheating the hot-stamped formed article.

[0198] Hardness distribution of the base steel sheet for hot stamping

[0199] The hot-stamped formed article of this embodiment is 0.18mm. 2 The standard deviation of Vickers hardness in the region (0.3 mm in the thickness direction and 0.6 mm in the direction orthogonal to the thickness direction, centered at 1 / 4 depth of the base steel plate) is less than 20 (Hv).

[0200] If the standard deviation of Vickers hardness in the aforementioned region exceeds 20 (Hv), cracking will occur in the early stages of deformation when the hot-stamped part deforms, significantly deteriorating its impact resistance. Therefore, the standard deviation of hardness in the aforementioned region is set to 20 (Hv) or less. More preferably, the standard deviation of hardness is set to 15 (Hv) or less, or 10 (Hv) or less.

[0201] In the case where 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, the above-mentioned hardness distribution is sufficient as long as the portion of the base steel plate with a minimum tensile strength of 2300 MPa or more has the aforementioned hardness distribution.

[0202] A small standard deviation of hardness within the aforementioned region is preferable, but significantly reducing the standard deviation of hardness can lead to a decrease in the productivity of hot-stamped products. Therefore, the standard deviation of hardness can also exceed 5 (Hv) or exceed 10 (Hv).

[0203] The hardness distribution of the base steel sheet in a hot-stamped product can be determined by taking test pieces from the hot-stamped product, polishing and grinding the longitudinal section of the steel sheet, and then measuring the hardness at 1 / 4 depth of the base steel sheet using the same method as for hot-stamped steel sheets. When the hot-stamped product has portions with tensile strength of 2300 MPa or more and portions with tensile strength less than 2300 MPa, the hardness is measured by taking test pieces from the portion of the base steel sheet where at least the tensile strength is 2300 MPa or more.

[0204] <Manufacturing Method of Hot Stamped Parts>

[0205] A preferred manufacturing method for the hot-stamped formed article of this embodiment will be described.

[0206] The hot-stamped product of this embodiment is manufactured by a manufacturing method including a heating process of heating the hot-stamping steel sheet of this embodiment as described above, and a hot-stamping process of hot-stamping the heated hot-stamping steel sheet to obtain the hot-stamped product. In the hot-stamping process, forming and cooling are performed using a die.

[0207] In the heating process, the hot stamping steel sheet of this embodiment is heated before the hot stamping process. In the heating process of heating the hot stamping steel sheet, it is preferable to set the heating temperature to a temperature higher than the Ac3 point. If the heating temperature is lower than the Ac3 point, the volume fraction of martensite in the metal structure of the hot stamped product is insufficient, the strength of the product is reduced, and the impact resistance is deteriorated.

[0208] There is no particular upper limit to the heating temperature, but if the heating temperature is too high, excessive oxide scale will be generated in the hot stamping process, and the productivity of the formed products will decrease due to the accumulation of oxide scale in the mold. Therefore, the heating temperature is preferably below 1200°C or below 1150°C.

[0209] The heating rate of the steel sheet does not need to be specifically limited, but a higher heating rate allows for more effective utilization of the strain energy accumulated in the hot-stamped steel sheet, thus improving the impact resistance of the hot-stamped product. Therefore, it is preferable to set the average heating rate up to 700°C to be greater than 10°C / second, greater than 20°C / second, greater than 30°C / second, or greater than 50°C / second. On the other hand, if the heating rate is too high, the amount of coarse iron carbides formed in the metal structure of the hot-stamped product becomes excessive, reducing the ductility of the hot-stamped steel sheet. Therefore, it is preferable to set the average heating rate to be less than 150°C / second, less than 120°C / second, or less than 90°C / second.

[0210] In the hot stamping process of heated steel sheets, it is preferable to remove the heated steel sheet from the furnace and allow it to cool in the atmosphere before starting the hot stamping process at a temperature of 700°C or higher. If the starting temperature of the hot stamping is below 700°C, the volume fraction of martensite in the metal structure of the hot-stamped product will be insufficient, resulting in reduced strength and deteriorated impact resistance.

[0211] After hot stamping, the formed article is cooled while held in the mold and / or removed from the mold and cooled by any method. If the cooling rate is low, the volume fraction of martensite in the metal structure of the hot-stamped article is insufficient, resulting in reduced strength. Therefore, it is preferable to set the average cooling rate from the hot stamping start temperature to 400°C to 30°C / second or higher, 60°C / second or higher, or 90°C / second or higher. Similarly, if the cooling stop temperature is high, the volume fraction of martensite in the metal structure of the hot-stamped article is also insufficient, resulting in reduced strength. Therefore, it is preferable to set the cooling stop temperature using the above-mentioned cooling method to be below 90°C or below 50°C.

[0212] Reheating can also be applied to hot-stamped parts. Reheating reduces localized hardness variations in the hot-stamped parts, improving their impact resistance. To achieve this effect fully, it is preferable to set the reheating temperature to 90°C or higher. On the other hand, if the reheating temperature is too high, the steel sheet softens, resulting in insufficient strength in the formed part. Therefore, it is preferable to set the reheating temperature to below 200°C or below 150°C.

[0213] If the holding time at the heating temperature is too short, the above-mentioned effect cannot be fully obtained; on the other hand, if the holding time is too long, the strength of the molded article will be insufficient. Therefore, the lower limit of the holding time is preferably set to 5 minutes or more, or 10 minutes or more, and the upper limit of the holding time is preferably set to less than 30 minutes or less than 20 minutes.

[0214] The hot stamping steel sheet described in this embodiment can also be represented as follows.

[0215] (Note 1) A type of steel plate for hot stamping

[0216] It has the following chemical composition: in mass%

[0217] C: More than 0.40% but less than 0.70%

[0218] Si: below 2.00%

[0219] Mn: ≥0.01% and <0.50%

[0220] P: below 0.200%

[0221] S: Below 0.0200%

[0222] sol.Al: 0.001~1.000%

[0223] N: below 0.0200%

[0224] Mo: ≥0.01% and <0.50%

[0225] B: 0.0002~0.0200%

[0226] Remaining components: Fe and impurities.

[0227] When the Mo content of the steel plate is determined by line analysis using EPMA within a range of 0.05 mm in the thickness direction centered at a depth of 1 / 4 of the steel plate's thickness from the surface, the maximum, minimum, and average Mo content satisfy the following equation (i).

[0228] The standard deviation of Vickers hardness in a region centered at a depth of 1 / 4 of the thickness of the steel plate, with a depth of 0.3 mm in the thickness direction and 0.6 mm in a direction orthogonal to the thickness direction, is 20 (Hv) or less.

[0229] ([Mo)) MAX -[Mo] MIN ) / [Mo] AVE <0.50 (i)

[0230] The meanings of the symbols in equation (i) above are as follows.

[0231] [Mo] MAX Maximum Mo content (mass%)

[0232] [Mo] MIN Minimum Mo content (mass%)

[0233] [Mo] AVE Average Mo content (mass%)

[0234] (Note 2) A type of steel plate for hot stamping

[0235] It has the following chemical composition: containing, by mass%,

[0236] C: More than 0.40% but less than 0.70%

[0237] Si: below 2.00%

[0238] Mn: ≥0.01% and <0.50%

[0239] P: below 0.200%

[0240] S: Below 0.0200%

[0241] sol.Al: 0.001~1.000%

[0242] N: below 0.0200%

[0243] Mo: ≥0.01% and <0.50%

[0244] B: 0.0002~0.0200%,

[0245] Furthermore, it contains one or more of the following groups: A, B, C, and D.

[0246] Remaining components: Fe and impurities.

[0247] When the Mo content of the steel plate is determined by line analysis using EPMA within a range of 0.05 mm in the thickness direction centered at a depth of 1 / 4 of the steel plate's thickness from the surface, the maximum, minimum, and average Mo content satisfy the following equation (i).

[0248] The standard deviation of Vickers hardness in a region centered at a depth of 1 / 4 of the thickness of the steel plate, with a depth of 0.3 mm in the thickness direction and 0.6 mm in a direction orthogonal to the thickness direction, is 20 (Hv) or less.

[0249] [Group A] Selected from one or more of the following: Ti: 0.001–0.200%, Nb: 0.001–0.200%, V: 0.001–0.200%, and Zr: 0.001–0.200%.

[0250] [Group B] Selected from one or more of the following: Cr: 0.001–2.00%, W: 0.001–2.00%, Cu: 0.001–2.00%, and Ni: 0.001–2.00%.

[0251] [Group C] Selected from one or more of the following: Ca: 0.0001–0.0100%, Mg: 0.0001–0.0100%, and REM: 0.0001–0.1000%.

[0252] [Group D] Bi: 0.0001~0.0500%

[0253] (Note 3)

[0254] The hot-stamping steel sheet according to (Appendix 2) has a chemical composition containing the above-mentioned Group A in mass percent.

[0255] (Note 4)

[0256] The hot-stamping steel sheet according to (Appendix 2) has a chemical composition containing the above-mentioned group B in mass percent.

[0257] (Note 5)

[0258] According to (Appendix 2), the hot-stamping steel sheet has a chemical composition containing the above-mentioned group C in mass percent.

[0259] (Note 6)

[0260] According to (Appendix 2), the hot-stamping steel sheet has a chemical composition containing the above-mentioned group D in mass percent.

[0261] The present invention will be described in more detail below through embodiments, but the present invention is not limited to these embodiments.

[0262] Example

[0263] Molten steel was cast using a vacuum melting furnace to produce steels A to U with the chemical compositions shown in Table 1. The Ac3 point in Table 1 was determined by the thermal expansion change when cold-rolled steel sheets of steels A to U were heated at 8°C / second. Steels A to U were heated to 1200°C and held for 60 minutes, and then hot-rolled under the conditions shown in Table 2.

[0264]

[0265]

[0266] Specifically, in the temperature range above Ar3, steel A to U are rolled in 10 passes to produce hot-rolled steel sheets with a thickness of 2.2 to 3.2 mm. After hot rolling, the hot-rolled steel sheets are cooled to 640 to 660°C by water spray. The cooling end temperature is set as the coiling temperature. The hot-rolled steel sheets are placed in an electric heating furnace maintained at this coiling temperature and held for 60 minutes. Afterward, the hot-rolled steel sheets are furnace cooled to room temperature at an average cooling rate of 20°C / hour to simulate the slow cooling after coiling.

[0267] After pickling, the hot-rolled steel sheet undergoes a first hot-rolled annealing under the conditions shown in Table 2. Specifically, it is heated from room temperature to the homogenization temperature for 0.1 to 6 hours using an electric heating furnace at a heating rate of 100°C / hour. The steel sheet is then removed from the furnace and cooled to room temperature. The average cooling rate from the homogenization temperature to 500°C is 9–10°C / second. For some hot-rolled steel sheets, the first hot-rolled annealing is omitted.

[0268] After pickling, hot-rolled annealed steel sheets or hot-rolled steel sheets undergo a second hot-rolled annealing under the conditions shown in Table 2. Specifically, an electric heating furnace is used to heat the sheet from 500°C to the homogenization temperature at an average rate of 2–5°C / second, and homogenize it for 30 seconds to 1 hour. The sheet is then removed from the furnace and cooled to room temperature. The average cooling rate from the homogenization temperature to 500°C is 7–10°C / second. For some hot-rolled annealed steel sheets, the second hot-rolled annealing is omitted.

[0269] After pickling, the hot-rolled annealed steel sheet is cold-rolled under the conditions shown in Table 2 to produce a cold-rolled steel sheet with a thickness of 1.4 mm.

[0270] For some hot-rolled annealed steel plates, instead of cold rolling, they are made into 1.4mm thick grinding plates by mechanical grinding.

[0271] In addition, a portion of the cold-rolled steel sheet is heated from room temperature to 780°C for 120 seconds using a continuous annealing simulator at a heating rate of 10°C / second. It is then cooled to room temperature at an average cooling rate of 15°C / second to produce an annealed steel sheet.

[0272] Test pieces for EPMA measurement are collected from cold-rolled steel sheets, ground steel sheets, and annealed steel sheets (collectively referred to as hot-stamping steel sheets). After grinding a longitudinal section of the test piece parallel to the rolling direction of the steel sheet, the concentration distribution (maximum, minimum, and average values) of Mo is measured at a depth of 1 / 4 of the steel sheet thickness in the thickness direction (1 / 4 depth position) from the steel sheet surface using the method described above. The left-hand side of equation (i) is then calculated. Specifically, for EPMA measurement, a JXA-8530F irradiation device manufactured by Nippon Electronics Corporation is used, with the accelerating voltage set to 15.0 kV and the irradiation current set to 5.0 × 10⁻⁶ kV. -8 A. Linear analysis was performed along the thickness direction of the plate at measurement intervals of 0.20 μm. The maximum, minimum, and average values ​​of the Mo content were determined from the 5-point moving average of the obtained measurement data. These values ​​were then used to calculate the left-hand side of equation (i) above.

[0273] In addition, tensile test pieces of JIS13B were collected from the above-mentioned hot stamping steel sheet in a direction orthogonal to the rolling direction, and tensile tests were conducted at a tensile speed of 10 mm / min to determine the tensile strength.

[0274] In addition, a test piece for hardness testing was collected from the hot-stamping steel plate mentioned above. After grinding the longitudinal section of the test piece parallel to the rolling direction of the steel plate, the Vickers hardness was measured at 1 / 4 depth of the steel plate using the above method with a load of 0.49N according to JIS Z2244:2009. The average value and standard deviation of the Vickers hardness were then calculated.

[0275] Furthermore, test pieces for microstructure observation were collected from the aforementioned hot-stamping steel sheets. After grinding a longitudinal section of the test piece parallel to the rolling direction of the steel sheet, the microstructure at a depth of 1 / 4 of the steel sheet was observed using the method described above. Table 2 shows the results of investigating the Mo concentration distribution and mechanical properties of the hot-stamping steel sheets. In Table 2, underlined values ​​indicate values ​​outside the scope of this invention.

[0276] A hot-stamping blank plate with a width of 240mm and a length of 800mm is taken from the aforementioned hot-stamping steel plate, and manufactured through hot stamping. Figure 2The shape of the hat-shaped component shown is described. In the hot stamping process, a gas-fired furnace is used to heat the blank sheet (hot stamping steel sheet) to 950°C at an average heating rate of 11°C / s up to 700°C, and hold it at that temperature for 1 minute. Afterward, the blank sheet is removed from the furnace and cooled to 800°C, then clamped into a mold equipped with a cooling device for hat-forming, and then cooled to room temperature (25°C) within the mold. For test number 34 using steel U, the cooled hat-shaped component is reheated in an electric furnace at 140°C for 10 minutes.

[0277] Test pieces for microstructure observation are collected from the longitudinal wall of the obtained hat-shaped component (hot stamped product). After grinding the longitudinal section of the test piece, the microstructure at 1 / 4 depth of the steel plate is observed using the above method, and the volume fractions of martensite, retained austenite, and other materials (one or more of ferrite, pearlite, bainite, and precipitates) are determined.

[0278] In addition, a test piece for EPMA measurement was collected from the longitudinal wall of the cap-shaped component (hot stamped part). After grinding the longitudinal section of the test piece, the concentration distribution of Mo was measured at 1 / 4 depth of the steel plate using the method described above, and the left side value of the above equation (ii) was obtained.

[0279] In addition, tensile test specimens of JIS13B were collected from the longitudinal wall of the hat-shaped component along the length of the component, and tensile tests were conducted at a tensile speed of 10 mm / min to determine the tensile strength.

[0280] In addition, a test piece for hardness testing was collected from the longitudinal wall of the hat-shaped component. After grinding the longitudinal section of the test piece, Vickers hardness was measured at 1 / 4 depth of the steel plate using the above method with a load of 0.49 N according to JIS Z2244:2009, and the standard deviation of Vickers hardness was calculated.

[0281] In addition, such as Figure 3 As shown, a sealing plate with a thickness of 1.4 mm, a width of 130 mm, and a length of 800 mm was welded onto the cap-shaped component to manufacture a test specimen for the three-point bending test. The sealing plate was made of steel plate with a tensile strength of 1553 MPa.

[0282] The test subject as Figure 4As shown, an 800mm long test specimen is placed on two support rollers spaced 700mm apart, with the end plate facing down. A three-point bending test is performed at a test speed of 2m / s. The maximum load, the displacement from the point of contact between the test specimen and the impactor until cracking begins in the test specimen, and the absorbed energy until cracking begins are determined. If the maximum load is 23.0kN or more, the displacement at cracking is 35mm or more, and the absorbed energy is 0.80kJ or more, the impact resistance is considered good.

[0283] Table 3 shows the results of investigating the Mo concentration distribution of the cap-shaped component, observing the metal structure of the cap-shaped component, evaluating the mechanical properties of the cap-shaped component, and evaluating the impact resistance of the cap-shaped component.

[0284]

[0285] Test numbers 1, 6, 11, 16, 20, 22, 24, 26, 27, and 29-34, which meet the requirements of this invention, all describe hot-stamping steel sheets where the left-hand side of equation (i) representing the Mo concentration distribution is less than 0.50, and the standard deviation of Vickers hardness is less than 20. Furthermore, in the three-point bending test of the hot-stamped product, the highest load is 23.0 kN or more, the displacement due to cracking is 35 mm or more, and the absorbed energy is 0.80 kJ or more, demonstrating good impact resistance. Additionally, although not shown in the table, the microstructure of the hot-stamping steel sheet of this invention comprises a total of more than 80.0% by volume of ferrite, pearlite, and / or bainite extending along the rolling direction, with the remainder being one or more of martensite, retained austenite, and precipitates.

[0286] Compared to these, the comparative examples 2-5, 7-10, 12-15, 17-19, 21, 23, 25, and 28, whose chemical composition, Mo concentration distribution, or standard deviation of Vickers hardness of the hot-stamped steel sheet deviates from the scope of this invention, showed low maximum load, crack displacement, and energy absorption in the three-point bending test of the hot-stamped formed articles, indicating poor impact resistance.

[0287] Specifically, test number 13 using steel D had a low C content, resulting in a tensile strength of less than 2300 MPa for the hot-stamped product and a low maximum load in the three-point bending test.

[0288] Test number 14, using steel E, resulted in premature fracture in the tensile test of the hot-stamped product due to its excessively high carbon content, making it impossible to determine the tensile strength. Furthermore, the standard deviation of the Vickers hardness of the hot-stamped product exceeded 20 (Hv), and the maximum load, crack displacement, and energy absorption in the three-point bending test were low.

[0289] Test number 15, which used steel F, had a high Mn content, resulting in a Vickers hardness standard deviation exceeding 20 (Hv) for the hot-stamped product. The maximum load, crack displacement, and energy absorption in the three-point bending test were also low.

[0290] Test number 17 using steel H has a high Mo content, so the left side value of formula (i) in hot stamping steel sheet is 0.50 or higher, the left side value of formula (ii) in hot stamping formed product is 0.50 or higher, the standard deviation of Vickers hardness exceeds 20 (Hv), and the cracking displacement and energy absorption in the three-point bending test are low.

[0291] Test number 18 using steel I had insufficient Mo and B content, and test number 19 using steel J had insufficient Mo content. Therefore, the martensite volume fraction in the microstructure of the hot-stamped product was insufficient, resulting in a tensile strength below 2300 MPa. Furthermore, the standard deviation of the Vickers hardness of the hot-stamped product exceeded 20 (Hv), and the maximum load, crack displacement, and energy absorption in the three-point bending test were low.

[0292] Comparative examples 2-5, 7-10, 12, 21, 23, 25, and 28, whose chemical composition is within the scope of this invention but whose manufacturing conditions for hot-stamped products deviate from the above-mentioned range, have left-hand side values ​​of formula (i) of 0.50 or higher, or standard deviations of Vickers hardness exceeding 20 (Hv). In the three-point bending test of the hot-stamped products, the displacement of cracking and energy absorption are low, and the impact resistance is poor.

[0293] Specifically, test number 2 using steel A and test number 7 using steel B were used because the steel sheets for hot stamping were annealed after cold rolling during the manufacturing process (the steel sheets supplied for hot stamping were not in a cold-rolled state). Therefore, the standard deviation of Vickers hardness in the steel sheets for hot stamping exceeded 20 (Hv). Even for hot-stamped products, the standard deviation of Vickers hardness exceeded 20 (Hv), and the cracking displacement and energy absorption in the three-point bending test were low.

[0294] Test No. 3 using steel A and Test No. 8 using steel B were used because cold rolling was not performed in the manufacturing process of the hot stamping steel sheet (the steel sheet supplied for hot stamping is not in a cold-rolled state). Therefore, the standard deviation of Vickers hardness in the hot stamping steel sheet exceeded 20. Even for hot stamping formed products, the standard deviation of Vickers hardness exceeded 20 (Hv). The cracking in the three-point bending test resulted in low displacement and low energy absorption.

[0295] Test No. 4 using steel A and Test No. 9 using steel B were used because a second hot-rolled annealing was not performed in the manufacturing process of the hot-stamping steel sheet. Therefore, the standard deviation of Vickers hardness in the hot-stamping steel sheet exceeded 20 (Hv). Even for hot-stamped formed products, the standard deviation of Vickers hardness exceeded 20 (Hv). The cracking in the three-point bending test resulted in low displacement and low energy absorption.

[0296] Test No. 5 using steel A and Test No. 10 using steel B were used because the first hot-rolled annealing was not performed in the manufacturing process of the hot-stamped steel sheet. Therefore, the left side value of formula (i) in the hot-stamped steel sheet was 0.50 or more, and the left side value of formula (ii) in the hot-stamped formed product was 0.50 or more. The standard deviation of Vickers hardness exceeded 20 (Hv), and the displacement and energy absorption of cracking in the three-point bending test were low.

[0297] Test No. 12, which used steel C, and Test No. 21, which used steel K, showed that due to the high homogenization temperature and long homogenization time of the second hot-rolled annealing process in the hot-stamping steel sheet manufacturing process, the standard deviation of Vickers hardness in the hot-stamping steel sheet exceeded 20 (Hv). Even for hot-stamped formed products, the standard deviation of Vickers hardness exceeded 20 (Hv), and the cracking in the three-point bending test resulted in low displacement and low energy absorption.

[0298] Test number 23 using steel L has a short soaking time during the first hot-rolled annealing process in the hot-stamping steel sheet manufacturing process. Therefore, the left side value of formula (i) is 0.50 or more in the hot-stamping steel sheet, and the left side value of formula (ii) is 0.50 or more in the hot-stamped formed product. The standard deviation of Vickers hardness exceeds 20 (Hv), and the displacement and energy absorption during cracking in the three-point bending test are low.

[0299] Test number 25 of steel M was used. Due to the long heating time of the second hot-rolled plate annealing in the hot-stamping steel plate manufacturing process, the standard deviation of Vickers hardness in the hot-stamping steel plate exceeded 20 (Hv). Even for hot-stamped formed products, the standard deviation of Vickers hardness exceeded 20 (Hv), and the cracking displacement and energy absorption in the three-point bending test were low.

[0300] Test No. 28 using steel O has a high homogenization temperature during the second hot-rolled annealing process in the hot-stamping steel sheet manufacturing process. Therefore, the standard deviation of Vickers hardness in the hot-stamping steel sheet exceeds 20 (Hv). Even for hot-stamped formed products, the standard deviation of Vickers hardness exceeds 20 (Hv). The cracking in the three-point bending test results in low displacement and low energy absorption.

[0301] Industrial availability

[0302] According to the present invention, it is possible to obtain a hot stamping steel sheet suitable as a raw material for hot stamping formed articles with excellent impact resistance and a tensile strength of 2300 MPa or more.

[0303] By using this hot stamping steel sheet as raw material and performing hot stamping, it is possible to manufacture hot stamped products with a tensile strength of over 2300 MPa and excellent impact resistance.

Claims

1. A hot-stamping steel sheet having the following chemical composition: (in mass%) C: More than 0.40% but less than 0.70% Si: below 2.00% Mn: ≥0.01% and <0.50% P: below 0.200% S: Below 0.0200% sol.Al: 0.001~1.000% N: below 0.0200% Mo: ≥0.01% and <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: 0~0.0100% REM: 0~0.1000% Bi: 0~0.0500% Remaining components: Fe and impurities. When the Mo content of the steel plate is determined by line analysis using EPMA within a range of 0.05 mm in the thickness direction centered at a depth of 1 / 4 of the steel plate's thickness from the surface, the maximum, minimum, and average Mo content satisfy the following equation (i). The standard deviation of Vickers hardness in a region centered at a depth of 1 / 4 of the thickness of the steel plate, within a radius of 0.3 mm in the thickness direction and 0.6 mm in a direction orthogonal to the thickness direction, is less than 20 (Hv). ([Mo] MAX -[Mo] MIN ) / [Mo] AVE <0.50 (i) in, The meanings of the symbols in equation (i) are as follows. [Mo] MAX Maximum Mo content (mass%) [Mo] MIN Minimum Mo content (mass%) [Mo] AVE : Average Mo content (mass%).

2. The hot-stamping steel plate according to claim 1, wherein, The chemical composition, expressed in mass percent, contains one or more elements selected from the following: Ti: 0.001~0.200% Nb: 0.001~0.200% V: 0.001~0.200% and Zr：0.001～0.200％。 3. The hot-stamping steel sheet according to claim 1 or claim 2, wherein, The chemical composition, expressed in mass percent, contains one or more elements selected from the following: Cr：0.001～2.00％、 W：0.001～2.00％、 Cu: 0.001–2.00% and Ni: 0.001~2.00%.

4. The hot-stamping steel sheet according to any one of claims 1 to 3, wherein, The chemical composition, expressed in mass percent, contains one or more elements selected from the following: Ca: 0.0001~0.0100% Mg: 0.0001~0.0100% and REM: 0.0001~0.1000%.

5. The hot-stamping steel sheet according to any one of claims 1 to 4, wherein, The chemical composition, expressed in mass percent, contains: Bi: 0.0001~0.0500%.

6. A hot-stamped product having a base steel plate, The base steel plate has the following chemical composition: (in mass%) C: More than 0.40% but less than 0.70% Si: below 2.00% Mn: ≥0.01% and <0.50% P: below 0.200% S: Below 0.0200% sol.Al: 0.001~1.000% N: below 0.0200% Mo: ≥0.01% and <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: 0~0.0100% REM: 0~0.1000% Bi: 0~0.0500% Remaining components: Fe and impurities. When the Mo content of the base steel plate is determined by line analysis using EPMA within a thickness direction of 0.05 mm, centered at a depth of 1 / 4 of the thickness of the base steel plate, the maximum, minimum, and average Mo content satisfy the following equation (ii). The microstructure of the base steel plate contains more than 90.0% martensite. The standard deviation of Vickers hardness in a region centered at a depth of 1 / 4 the thickness of the base steel plate, within 0.3 mm in the thickness direction and within 0.6 mm in a direction orthogonal to the thickness direction, is less than 20 (Hv). The tensile strength of the base steel plate is above 2300 MPa. ([Mo] mMAX -[Mo] mMIN ) / [Mo] mAVE <0.50 (ii) in, The meanings of the symbols in equation (ii) are as follows. [Mo] mMAX Maximum Mo content (mass%) in the base steel plate [Mo] mMIN Minimum Mo content (mass%) in the base steel plate. [Mo] mAVE : Average Mo content (mass %) of the base steel plate.

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