Hot stamped products

Abstract

This hot stamp molded article comprises a steel plate. All or part of the steel plate has a predetermined chemical composition, the metal structure in a 1 / 4 depth position of the plate thickness of the steel plate from the surface thereof includes martensite at a rate exceeding 90.0% by volume, the average value of the Vickers hardness in a region of 0.3 mm in the plate thickness direction and 0.6 mm in a direction orthogonal to the plate thickness direction is 670 or more, the standard deviation in the Vickers hardness in the region is 20 or less, and the tensile strength is 2300 MPa or more.

Description

[Technical field]

[0001] The present invention relates to a hot stamped product. This application claims priority based on Japanese Patent Application Nos. 2020-022634 and 2020-022635, filed on February 13, 2020, the contents of which are incorporated herein by reference. [Background technology]

[0002] In today's world, where industrial technology fields have become highly specialized, materials used in each technical field are required to have special and advanced performance. For example, steel sheets for automobiles are required to have high strength in order to improve fuel efficiency by reducing the weight of the car 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 car body while reducing the weight of the car body by reducing the thickness of the steel sheet.

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

[0004] Since ensuring press formability and increasing the strength of steel sheets are mutually exclusive, it is difficult to simultaneously satisfy both characteristics. In addition, when high-strength steel sheets are press-formed, the shape of the part changes significantly due to springback when the part is removed from the die, making it difficult to ensure the dimensional accuracy of the part. Thus, it is not easy to manufacture high-strength car body parts by press forming.

[0005] So far, as a method for manufacturing ultra-high strength car body members, for example, as disclosed in Patent Document 1, a technique of press-forming a heated steel sheet using a low-temperature press die has been proposed. 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 members of complex shapes with high dimensional accuracy. In addition, since the steel sheet is quenched by contact with the die, it is possible to significantly increase the strength by quenching at the same time as the press forming. For example, Patent Document 1 describes that a member 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 manufacturing 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 steel sheet for hot stamping is heated to a two-phase region of ferrite and austenite and then hot stamped, and the metal structure of the hot stamped member is made into a composite structure of ferrite and martensite having an average grain size of 7 μm or less, thereby increasing the ductility of the steel sheet constituting the member.

[0007] Patent Document 4 discloses a technique for manufacturing a hot stamped member having excellent toughness and a tensile strength of 1800 MPa or more. In the method described in Patent Document 4, a steel plate for hot stamping is heated to a low temperature range of austenite, then hot stamped, and cooled relatively slowly in a temperature range below the Ms point to form a metal 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 excellent in that it can obtain a 1800 MPa-class hot stamped member that does not crack even in a low-temperature impact test. [Prior art documents] [Patent documents]

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

[0009] However, according to the investigations of the present inventors, in the case of a hot stamped member having a composite structure of ferrite and martensite as described in Patent Documents 2 and 3, when the member deforms during a collision, cracks may occur originating from the ferrite at the initial stage of deformation. In particular, when the tensile strength of the member exceeds 2,300 MPa, it has been found that it is difficult to ensure the collision safety of the vehicle body. Moreover, Patent Document 4 does not disclose any member having a tensile strength of 2300 MPa or more. According to the study by the present inventors, even in the case of a hot stamped member having a tempered martensite single phase structure as described in Patent Document 4, if the tensile strength is increased to 2300 MPa or more, especially when the forming temperature during hot stamping of the steel sheet is low, localized hardness variations occur inside the member, and it is found that the recent high demand for crash resistance cannot be fully met. Moreover, it was found that such localized hardness variations are large especially when the material steel sheet for hot stamping is a plated steel sheet.

[0010] As described above, it has been difficult with conventional techniques to manufacture a member having a tensile strength of 2300 MPa or more by hot stamping, in particular a hot stamped member (molded product) having excellent impact resistance and a tensile strength of 2300 MPa or more.

[0011] The present invention has an object to solve the above problems and to provide a hot stamped product having excellent impact resistance and a portion with a tensile strength of 2300 MPa or more. [Means for solving the problem]

[0012] The present invention has been made to solve the above-mentioned problems, and the gist of the present invention is the following hot stamp formed product.

[0013] (1) A hot stamped product according to one aspect of the present invention is a hot stamped product having a steel plate, the hot stamped product being made of all or a part of the steel plate. of , The tensile strength is 2300 MPa or more, In the portion having a tensile strength of 2300 MPa or more, By mass%, C: more than 0.40%; 0.60 %, 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 % or more, less than 0.500% , N: 0.0200% or less, Mo: 0.01% or more, 0.35 %, B: 0.0002-0.0200%, Ti: 0-0.200%, Nb: 0-0.200%, V: 0-0.200%, Zr: 0-0.200%, Cr: 0 % or more, less than 0.50% , W:0 % or more, less than 0.50% and a chemical composition of Cu: 0-2.00%, Ni: 0-2.00%, Ca: 0-0.0100%, Mg: 0-0.0100%, REM: 0-0.1000%, Bi: 0-0.0500%, and the balance: Fe and impurities. At a depth position of 1 / 4 of the sheet thickness from the surface of the steel sheet, the metal structure contains, by volume %, more than 90.0% martensite, and the average Vickers hardness in an area 0.3 mm in the sheet thickness direction and 0.6 mm in the direction perpendicular to the sheet thickness direction is 670 or more, and the standard deviation of the Vickers hardness in the area is 20 or less. R . (2) In the hot stamped product described in (1) above, In the portion having a tensile strength of 2300 MPa or more, The yield ratio may be 0.65 or greater. (3) A hot stamped product according to another aspect of the present invention includes a steel sheet and a plating layer formed on a surface of the steel sheet, and the hot stamped product is a hot stamped product having a plating layer formed on a surface of the steel sheet. of , The tensile strength is 2300 MPa or more, and in the portion where the tensile strength is 2300 MPa or more, By mass%, C: more than 0.40%; 0.60 %, 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 % or more, less than 0.500% , N: 0.0200% or less, Mo: 0.01% or more, 0.35 %, B: 0.0002-0.0200%, Ti: 0-0.200%, Nb: 0-0.200%, V: 0-0.200%, Zr: 0-0.200%, Cr: 0 % or more, less than 0.50% , W:0 % or more, less than 0.50% a metal structure at a depth position of 1 / 4 of the sheet thickness of the steel sheet from a boundary between the steel sheet and the plating layer contains, by volume %, more than 90.0% martensite, an average Vickers hardness in an area of ​​0.3 mm in the sheet thickness direction and 0.6 mm in a direction perpendicular to the sheet thickness direction is 670 or more, and a standard deviation of the Vickers hardness in the area is 20 or less, 、 The yield ratio is 0.65 or more. (4) In the hot stamp formed product according to any one of (1) to (3) above, the chemical composition may contain, in mass%, 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%. (5) The hot stamp formed product according to any one of (1) to (4) above, wherein the chemical composition is, in mass%, Cr: 0.001 % or more, less than 0.50% , W:0.001 % or more, less than 0.50% , Cu: 0.001 to 2.00%, and Ni: 0.001 to 2.00%. (6) In the hot stamp formed product according to any one of (1) to (5) above, the chemical composition may contain, by mass%, one or more selected from Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0100%, and REM: 0.0001 to 0.1000%. (7) In the hot stamped product according to any one of (1) to (6) above, the chemical composition may contain, in mass %, Bi: 0.0001 to 0.0500%. Effect of the Invention

[0014] According to the above-described aspect of the present invention, it is possible to obtain a hot stamped product having excellent impact resistance and a portion with a tensile strength of 2300 MPa or more. [Brief description of the drawings]

[0015] [Figure 1] FIG. 2 is a schematic diagram showing hardness measurement positions of a hot stamped product. [Diagram 2] FIG. 2 is a schematic diagram showing an example of the shape of a hot stamped product. [Diagram 3] FIG. 2 is a schematic diagram showing the shape of a three-point bending test specimen. [Figure 4] FIG. 2 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 PREFERRED EMBODIMENTS

[0016] 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, and have obtained the following findings.

[0017] (A) Hot stamped products with a tensile strength of 2,300 MPa or more have large local variations in hardness. Although the reason is not clear, it is presumed to be due to the following: (a) in the material before hot stamping (hot stamping steel sheet) such that the tensile strength after hot stamping becomes 2300 MPa or more, local concentration unevenness of Mn and Mo is strong; (b) in parts where the concentration of Mn and Mo is low, the hot stamping steel sheet exhibits a metal structure with a high ferrite fraction, and in this part, austenite coarsens during the process of heating the hot stamping steel sheet, and in the formed product after hot stamping, the hardness tends to be low; (c) on the other hand, in parts where the concentration of Mn and Mo is high, the hot stamping steel sheet exhibits a metal structure with a high pearlite fraction, and in this part, austenite is refined during the process of heating the hot stamping steel sheet, and in the formed product after hot stamping, the hardness tends to be high.

[0018] (B) The greater the local hardness variation of the hot stamping formed product, the more likely cracks are to occur at the initial stage of deformation when the formed product deforms. This is considered to be because stress concentrates in the parts with low hardness.

[0019] (C) In a hot stamping formed product having a plating layer on the surface, the local hardness variation is more likely to be greater than in the case without a plating layer. Although the reason is not clear, it is presumed to be due to the following: (a) the hardness variation becomes smaller as the strain energy accumulated in the hot stamping steel sheet is higher; (b) in the plated steel sheet manufactured through an annealing process, the strain energy accumulated during cold rolling is released during annealing.

[0020] (D) By using a steel sheet (also called as cold-rolled-as-is steel sheet or full hard) manufactured without annealing after going through the process of cold rolling as the hot stamping steel sheet, the occurrence of cracks when the formed product deforms is suppressed. The reason for this is not clear, but it is assumed to be due to the following: (a) In as-cold-rolled steel sheet, processing strain during cold rolling accumulates, so that austenite becomes finer during the heating process of the steel sheet for hot stamping, and the hardness of the hot stamped product increases; and (b) this effect is stronger in areas with low concentrations of Mn and Mo, so that the use of as-cold-rolled steel sheet reduces local hardness variations in the hot stamped product.

[0021] (E) In the step of hot stamping, by increasing the temperature at which hot stamping begins (forming start temperature), the occurrence of cracks during deformation of the formed product can be suppressed. The reason for this is not clear, but it is presumed to be due to the following: (a) in steel plate for hot stamping, the higher the concentration of Mn or Mo in an area, the more easily strain accumulates in austenite during hot stamping, resulting in higher hardness in a hot stamped product; and (b) when hot stamping is performed at a high temperature, the accumulation of strain in austenite is suppressed, resulting in lower hardness in a hot stamped product. However, this effect is greater in areas with high concentrations of Mn or Mo than in areas with low concentrations of Mn or Mo, so that hot stamping at a high temperature reduces localized hardness variations in a hot stamped product.

[0022] (F) If the hot stamped product is reheated at a low temperature after hot stamping, the occurrence of cracks during deformation of the product can be suppressed. Although the reason for this is not clear, it is assumed that this is due to the following: (a) reheating reduces the amount of carbon present in solid solution in martensite, thereby decreasing the hardness of the hot stamped product; and (b) this effect is stronger in areas with high concentrations of Mn and Mo, and reheating reduces localized hardness variations in the hot stamped product.

[0023] From the findings of (A) to (F) above, the inventors have found that by using an as-cold-rolled steel sheet as a base steel sheet, heating the as-cold-rolled steel sheet, and then starting hot stamping at a high temperature, it is possible to manufacture a hot-stamped product having a tensile strength of 2300 MPa or more, small local hardness variation, and excellent impact resistance. Alternatively, they have found that even when a plated steel sheet is used as the base steel sheet, by heating the plated steel sheet, starting hot stamping at a high temperature, and then performing a reheat treatment at a low temperature after hot stamping, it is possible to produce a hot stamped product having a plated layer on the surface which has a tensile strength of 2300 MPa or more, small local hardness variation, and excellent impact resistance. Hereinafter, each requirement of a hot-stamped product according to one embodiment of the present invention (a hot-stamped product according to the present embodiment) and a method for producing the same will be described in detail.

[0024] <Chemical composition of steel sheets in hot stamped products> All or a part of the steel plate included in the hot stamped product according to this embodiment has the chemical composition shown below (when the hot stamped product is made of a steel plate, it can be said that all or a part of the hot stamped product has the chemical composition shown below). The reasons for limiting each element are as follows. In the following explanation, "%" for the content means "mass %". Furthermore, numerical ranges indicated with "to" include both ends of the numerical range. On the other hand, numerical values ​​indicated with "less than" and "greater than" do not include the value in the range. 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 (when the steel plate of the hot stamped product of this embodiment 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 having a tensile strength of 2300 MPa or more has the following chemical composition. When the hot stamped product includes a steel sheet and a plating layer formed on the surface of the steel sheet, the chemical composition described below means the chemical composition of the steel sheet excluding the plating layer.

[0025] 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 (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 is less than 2300 MPa, and the strength of the formed product is insufficient. Therefore, the C content is made to be 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 the crash resistance. Therefore, the C content is set to 0.70% or less. The preferred C content is 0.65% or less, 0.60% or less, 0.55% or less, or 0.50% or less.

[0026] Si: Less than 2.00% Silicon is contained in steel as an impurity and is an element that embrittles steel. If the silicon content is 2.00% or more, the adverse effects become particularly large. Therefore, the silicon content is set to less than 2.00%. The preferred silicon content is less than 1.50%, less than 1.00%, less than 0.75%, or less than 0.50%. From the viewpoint of ensuring plating properties, the silicon content is preferably 0.40% or less, 0.30% or less, or 0.20% or less. Although there is no particular lower limit for the Si content, excessively lowering the Si content leads to an increase in steelmaking costs. Therefore, it is preferable that the Si content is 0.001% or more. In addition, since Si has the effect of improving the hardenability of steel, it may be intentionally added. From the viewpoint of improving hardenability, it is preferable that the Si content is 0.10% or more, 0.20% or more, or 0.30% or more.

[0027] Mn: 0.01% or more, less than 0.50% Mn is an element that deteriorates the impact resistance of a hot stamped product. If the Mn content is 0.50% or more, the impact resistance is significantly deteriorated, and even if the manufacturing method of a hot stamped product described later is applied, the impact resistance of the 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 S, an impurity, to form MnS and suppress the harmful effects of S. To obtain 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.

[0028] 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 large, and further, weldability is significantly deteriorated. Therefore, the P content is set to 0.200% or less. The P content is preferably less than 0.100%, less than 0.050%, or less than 0.020%. From the viewpoint of ensuring galvanizability, the P content is preferably less than 0.020%, less than 0.015%, or less than 0.010%. Although there is no particular lower limit for the P content, an excessive decrease in the P content leads to an increase in steelmaking costs, so the P content may be set to 0.001% or more.

[0029] S: 0.0200% or less S is contained in steel as an impurity and is an element that embrittles steel. If the S content exceeds 0.0200%, the adverse effects become particularly large. Therefore, the S content is set to 0.0200% or less. The preferred S content is 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.

[0030] 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 and the Ac 3 It becomes difficult to heat the steel sheet to a temperature exceeding the above point. 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%.

[0031] N: 0.0200% or less N is an element contained in steel as an impurity and forms nitrides during continuous casting of steel. Since these nitrides deteriorate the ductility of the steel sheet after hot stamping, it is preferable that the N content is low. If the N content exceeds 0.0200%, the adverse effects become particularly large. 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.

[0032] 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 to ensure the strength of hot stamped products. To obtain 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, Mo is an element that deteriorates the impact resistance of a hot stamped product. If the Mo content is 0.50% or more, the impact resistance is significantly deteriorated, and even if the manufacturing method of a hot stamped product described later is applied, the impact resistance of the product cannot be ensured. 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%.

[0033] B: 0.0002~0.0200% B is an element that improves the hardenability of steel, forms a metal structure mainly composed of martensite, and is an effective element for ensuring the strength of hot stamped products. To obtain 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 carbohydrates are formed, which impairs the effect of improving hardenability due to the inclusion of B. Therefore, the B content is set to 0.0200% or less. The preferred B content is less than 0.0050%, less than 0.0040%, or less than 0.0030%.

[0034] The hot stamped product 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 characteristics, etc., the hot stamped product 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. 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 due to various factors in raw materials such as ores and scraps and in the manufacturing process when industrially manufacturing a steel sheet, and that are permissible within a range that does not adversely affect the hot stamped product according to the present embodiment.

[0035] Ti: 0 to 0.200% Nb: 0 to 0.200% V: 0~0.200% 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. In order to obtain this effect, one or more elements selected from Ti, Nb, V and Zr may be contained as necessary. In order 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, when the contents of Ti, Nb, V and Zr exceed 0.200%, respectively, the above effects are saturated and the manufacturing cost of the steel sheet increases. Therefore, when these elements are contained, the contents of Ti, Nb, V and Zr are each set to 0.200% or less. In addition, when the contents of Ti, Nb, V and Zr are high, carbides of these elements are precipitated in large amounts, 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%.

[0036] Cr: 0~2.00% W:0~2.00% Cu: 0-2.00% 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 necessary. To obtain the above effects, it is preferable to contain at least 0.001% of each of at least one selected from Cr, W, Cu, and Ni. More preferably, the Cr content is at least 0.05% or at least 0.10%, the W content is at least 0.05% or at least 0.10%, the Cu content is at least 0.10%, and the Ni content is at least 0.10%. On the other hand, if the content of Cr, W, Cu and Ni exceeds 2.00%, the impact resistance of the hot stamped product is deteriorated. Therefore, when they are contained, the content of Cr, W, Cu and Ni is set to 2.00% or less. The preferred 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%.

[0037] Ca: 0 to 0.0100% Magnesium: 0 to 0.0100% REM: 0~0.1000% Ca, Mg and REM are elements that have the effect of improving the ductility of the steel sheet after hot stamping by adjusting the shape of inclusions. Therefore, they may be contained as necessary. When it is desired to obtain the above effect, it is preferable to contain at least one selected from Ca, Mg and REM in an amount of at least 0.0001% each. 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 occur. Therefore, if these elements are included, the Ca and Mg contents should each be 0.0100% or less, and the REM content should be 0.1000% or less.

[0038] 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.

[0039] Bi: 0 to 0.0500% Bi is an element that has the effect of improving the impact resistance of a hot stamped product by refining the solidification structure. Therefore, it may be contained as necessary. To obtain the above effect, the Bi content is preferably 0.0001% or more. More preferably, the Bi content is 0.0003% or more, or 0.0005% or more. On the other hand, if the Bi content exceeds 0.0500%, the above effects are saturated and excessive costs are incurred. Therefore, if Bi is contained, the Bi content is set to 0.0500% or less. More preferably, the Bi content is 0.0100% or less, or 0.0050% or less.

[0040] As described above, the chemical composition of the hot stamped product according to the present embodiment may contain the essential elements with the balance being Fe and impurities, or may contain the essential elements and further contain one or more optional elements with the balance being Fe and impurities.

[0041] <Metal structure of steel sheets in hot stamped products> The metal structure (microstructure) of the steel plate included in the hot stamped product according to this embodiment will be described. All or a part of the steel plate included in the hot stamped product according to this embodiment has a metal structure containing the amount of martensite shown below (when the hot stamped product is made of a steel plate, it can be said that all or a part of the hot stamped product has a metal structure containing the amount of martensite shown below). In the following description of the metal structure, "%" means "volume %". 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 having a tensile strength of 2300 MPa or more has the following metal structure. When the hot stamped product includes a steel sheet and a plating layer formed on the surface of the steel sheet, the metal structure described below refers to the metal structure of the steel sheet. In the hot stamped product according to this embodiment, the metal structure is specified at a depth position of 1 / 4 of the sheet thickness from the surface of the steel sheet (the interface between the steel sheet and the plating layer if a plating layer is present).

[0042] Martensite: Over 90.0% by volume Martensite is an important structure for increasing the tensile strength of a steel sheet after hot stamping. If the volume fraction of martensite is 90.0% or less, the tensile strength of the hot stamped product (the tensile strength of the steel sheet in the hot stamped product) is less than 2300 MPa, resulting in insufficient strength. Therefore, the volume fraction of martensite is set to be more than 90.0%. The preferred volume fraction of martensite is more than 91.0%, more than 93.0%, or more 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 is necessary to excessively increase the heating temperature of the steel sheet or the cooling rate in the hot stamping process, which significantly impairs 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 fresh martensite that has not been tempered, and tempered martensite that has been tempered and contains iron carbides therein. The remainder of the metal structure may contain ferrite, pearlite, bainite, or residual austenite, and may further contain precipitates such as cementite. Since it is not necessary to contain ferrite, pearlite, bainite, residual austenite, and precipitates, the lower limits of the volume fractions of ferrite, pearlite, bainite, residual austenite, and precipitates are all 0%.

[0043] 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, if ferrite, pearlite and bainite are contained in excess, the impact resistance of the hot stamped product deteriorates, 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%, more preferably less than 5.0%.

[0044] Retained austenite has the effect of improving the ductility of a 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, in order to excessively increase the volume fraction of the retained austenite, it is necessary to perform austempering treatment at high temperature after hot stamping, which significantly reduces the productivity of hot stamped products. In addition, if the retained austenite is contained in excess, the impact resistance of the hot stamped product may deteriorate. Therefore, it is preferable to set the volume fraction of the retained austenite to less than 9.0%, less than 7.0%, less than 5.0%, or less than 4.0%.

[0045] In this embodiment, the volume ratio of each texture is calculated as follows. First, a test piece is taken from the hot stamped product, the longitudinal section of the steel sheet is buffed, and then the structure is observed at a depth position of 1 / 4 of the sheet thickness of the steel sheet from the surface of the steel sheet (the boundary between the base steel sheet and the plated layer if a plated layer is present) in the sheet thickness direction of the steel sheet. 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, the test piece is taken from the portion having a tensile strength of 2300 MPa or more and observed. Specifically, the polished surface is etched with nital or electrolytically polished, and then the structure is observed using an optical microscope and a scanning electron microscope (SEM), and the obtained structure photograph is subjected to image analysis based on the brightness difference or the difference in the morphology of the iron carbides present in the phase to obtain the area ratios of ferrite, pearlite, bainite, and tempered martensite.Then, the same observation positions are etched with Lepera, and then the structure is observed using an optical microscope and a scanning electron microscope (SEM), and the obtained structure photograph is subjected to image analysis to calculate the total area ratio of retained austenite and martensite. In addition, for the same observation position, the longitudinal section is electrolytically polished, and then the area ratio of retained austenite is measured using a 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 considered to be equal to the volume fractions, and the measured area fractions are taken 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 from bainite in that the iron carbides present inside are elongated in multiple directions.

[0046] <Strength of hot stamped products> The hot stamped product according to the present embodiment has a tensile strength of 2300 MPa or more in all or part of the steel plate of the hot stamped product according to the present embodiment. For this purpose, the tensile strength of all or part of the steel plate of the hot stamped product according to the present embodiment is 2300 MPa or more. If the tensile strength of at least part of the steel plate is not 2300 MPa or more, the impact absorption amount of the hot stamped product cannot be ensured. Therefore, the tensile strength of all or part of the hot stamped product is set to 2300 MPa or more. Preferably, the tensile strength of all or part of the hot stamped product is 2400 MPa or more, or 2500 MPa or more. On the other hand, excessively increasing the strength of the hot stamped product leads to a decrease in impact resistance, so the tensile strength of the hot stamped product is preferably less than 3000 MPa, or less than 2800 MPa.

[0047] It is preferable that all or a part of the hot stamped product according to this embodiment has a tensile strength of 2300 MPa or more and a yield ratio of 0.65 or more. By setting the yield ratio to 0.65 or more, it is possible to further improve the impact resistance. More preferably, the yield ratio is 0.68 or more, or 0.70 or more in all or a part of the hot stamped product. On the other hand, the upper limit of the yield ratio is not particularly limited, but in order to greatly increase the yield ratio, it is necessary to excessively increase the reheating temperature in the reheating step described later, which leads to a decrease in the strength of the formed product. Therefore, it is preferable that the yield ratio is less than 0.90, less than 0.85, or less than 0.80.

[0048] The hot stamped product according to the present embodiment may have a tensile strength of 2300 MPa or more throughout (the entire product), but may have a mixture of a portion with a tensile strength of 2300 MPa or more and a portion 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 collision. A hot stamped product having portions with different strengths can be manufactured 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 in the hot stamping process, or a method of partially reheating the hot stamped product.

[0049] The tensile strength and yield ratio are determined by taking a JIS No. 13B tensile test piece along the longitudinal direction of the member and conducting a tensile test at a tension rate of 10 mm / min. The yield ratio is calculated by dividing the yield stress of the steel plate by the tensile strength. The yield stress is taken as 0.2% proof stress when the steel plate undergoes continuous yielding, and as the stress at the upper yield point when the steel plate undergoes discontinuous yielding. Since the plating layer has a small effect on the tensile strength and yield ratio, a plating layer may be present on the surface of the test piece.

[0050] <Hardness distribution of hot stamped products> In the hot stamped product according to this embodiment, the thickness of the part having a tensile strength of 2300 MPa or more is 0.18 mm. 2 That is, within a region extending from the surface of the steel sheet (the boundary between the base steel sheet and the plating layer in the case where a plating layer is present) to a depth position of 1 / 4 of the thickness of the steel sheet in the thickness direction of the steel sheet, and extending 0.3 mm in the thickness direction and 0.6 mm in the direction perpendicular to the thickness direction, the average Vickers hardness is 670 (Hv) or more, and the standard deviation of the Vickers hardness within the above region is 20 (Hv) or less. A Vickers hardness average of 670 (Hv) or more corresponds to the hardness measurement region being in a region with a tensile strength of 2300 MPa or more, and if the Vickers hardness average value is less than 670 (Hv), the strength of the molded product will be insufficient. Therefore, the Vickers hardness average value in the above region is set to 670 (Hv) or more. The Vickers hardness average value is preferably 695 (Hv) or more, or 720 (Hv) or more. Furthermore, if the standard deviation of the Vickers hardness in the above range exceeds 20 (Hv), cracks will occur in the early stage of deformation when the molded product is deformed, and impact resistance will be significantly deteriorated. Therefore, the standard deviation of the hardness in the above range is set to 20 (Hv) or less. The standard deviation of the hardness is preferably set to 15 (Hv) or less, 12 (Hv) or less, or 10 (Hv) or less.

[0051] In this embodiment, the Vickers hardness of the hot stamped product is determined as follows. First, a test piece is taken from the hot stamped product, the longitudinal section of the steel sheet is polished with waterproof abrasive paper, and then buffed with a diamond suspension, and the Vickers hardness is measured at a depth position (1 / 4 depth position) of 1 / 4 of the thickness of the steel sheet from the surface of the steel sheet (the interface between the steel sheet and the plating layer if a plating layer is present) in the thickness direction of the steel sheet. If the hot stamped product has a portion with a tensile strength of 2300 MPa or more and a portion with a tensile strength of less than 2300 MPa, the test piece is taken from the portion with a tensile strength of 2300 MPa or more and the measurement is performed. Specifically, as shown in Figure 1, Vickers hardness is measured at 45 points at specified intervals in a range of 0.3 mm in the plate thickness direction and 0.6 mm in the direction perpendicular to the plate thickness direction, centered at the 1 / 4 depth position, in accordance with JIS Z 2244: 2009, and the arithmetic mean value and standard deviation are calculated from the measured values. A micro Vickers hardness tester is used to measure the hardness, and the measurement conditions are a load of 0.49 N and a load holding time of 10 seconds. If the load is high, the dimensions of the indentation will become large, making it impossible to evaluate the local hardness distribution, which is closely related to crash resistance. For this reason, the load is set to 0.49 N. Regarding the relationship between the hardness distribution and crash resistance of the hot stamped product, for example, WO 2018 / 151325 states that small hardness variation in a cross section perpendicular to the longitudinal direction of the product is important for ensuring crash resistance. However, WO 2018 / 151325 determines the macroscopic hardness variation in the entire cross-sectional region of the product by measuring the Vickers hardness at the center in the plate thickness direction at 1 mm intervals with a load of 1 kgf, and it can be said that the hardness distribution is different from that of the hot stamped product according to the present embodiment.

[0052] [Plating layer] The hot stamped product according to the present embodiment may have a plating layer on the surface of the steel sheet. By providing the plating layer on the surface, it is possible to prevent the generation of scale during hot stamping and further improve the corrosion resistance of the hot stamped product. The type of plating is not particularly limited as long as it meets the above-mentioned purpose. The plating layer of the hot stamped product can be formed by hot stamping using a plated steel sheet, as described later. Examples of the type of plating layer include a zinc-based plating layer and an aluminum-based plating layer formed by hot stamping using a zinc-based plated steel sheet and an aluminum-based plated steel sheet. The plating layer may be formed on one side or both sides.

[0053] Next, a steel sheet for hot stamping suitable for producing the above-mentioned hot stamped product (hereinafter, a steel sheet for hot stamping according to the present embodiment) will be described.

[0054] <Chemical composition of steel sheets for hot stamping> Since the chemical composition does not substantially change due to hot stamping, the chemical composition of the steel sheet for hot stamping is the same as that of the above-mentioned hot stamped product.

[0055] <Metal structure of steel sheets for hot stamping> The steel sheet for hot stamping according to this embodiment is a steel sheet (also called as-cold-rolled steel sheet or full hard) having a metal structure extended in the rolling direction with high strain energy, which is manufactured without annealing after the cold rolling process, or a plated steel sheet. The metal structure is formed in this way in order to reduce local hardness variations in the hot stamped product and improve the impact resistance of the product. It is preferable to use an as-cold-rolled steel sheet, which has a large accumulated strain energy, because it can reduce local hardness variations with fewer manufacturing steps. On the other hand, it is preferable to use a plated steel sheet from the viewpoint of preventing the generation of scale in the manufacturing process and further improving the corrosion resistance of the hot stamped product. In either the case of an as-cold-rolled steel sheet or a plated steel sheet, if the metal structure contains martensite, the steel sheet becomes extremely hard and difficult to cut. Therefore, in the case of an as-cold-rolled steel sheet, the metal structure of a steel sheet for hot stamping is preferably mainly composed of ferrite, pearlite, and / or bainite extended in the rolling direction. It is more preferable that the total volume ratio of ferrite extended in the rolling direction, pearlite extended in the rolling direction, and bainite extended in the rolling direction is more than 90.0%, or more than 95.0%. In the case of a plated steel sheet, it is preferable that the metal structure is mainly composed of ferrite, pearlite, and / or bainite. The volume fraction in the metal structure of a steel sheet for hot stamping can be determined by taking a test piece from the steel sheet for hot stamping, buffing the longitudinal section parallel to the rolling direction of the steel sheet, and then observing the structure in the same manner as for hot stamped products at a depth position of 1 / 4 of the sheet thickness from the surface of the steel sheet (the interface between the steel sheet and the plating layer in the case of a plated steel sheet) in the sheet thickness direction of the steel sheet.

[0056] The type of plated steel sheet is not particularly limited, and examples include hot-dip galvanized steel sheet, alloyed hot-dip galvanized steel sheet, hot-dip aluminum plated steel sheet, hot-dip Zn-Al alloy plated steel sheet, hot-dip Zn-Al-Mg alloy plated steel sheet, hot-dip Zn-Al-Mg-Si alloy plated steel sheet, etc. The plating layer may be provided on one side or both sides of the steel sheet.

[0057] <Strength of the hot stamping steel sheet> In order to reduce local hardness variations in the hot stamping molded product and enhance the impact resistance of the hot stamping molded product, for the steel sheet as cold rolled, it is preferable that the tensile strength exceeds 900 MPa. A more preferable tensile strength exceeds 950 MPa or exceeds 1000 MPa.

[0058] <Manufacturing method> The manufacturing method of the hot stamping molded product according to this embodiment and the preferable manufacturing method of the hot stamping steel sheet according to this embodiment will be described.

[0059] [Manufacturing method of hot stamping molded product] The hot stamping molded product according to this embodiment can be manufactured by a manufacturing method including the following steps (I) and (II), or a manufacturing method including steps (i), (ii), and (iii). (I) Heating step of heating the hot stamping steel sheet as cold rolled having the above chemical composition (II) Hot stamping step of performing hot stamping on the heated hot stamping steel sheet to obtain a hot stamping molded product (i) Heating step of heating the hot stamping steel sheet having the above chemical composition and having a plating layer on the surface (ii) Hot stamping step of performing hot stamping on the heated hot stamping steel sheet to obtain a hot stamping molded product (iii) Reheating step of reheating the molded product after the hot stamping step In the hot stamping steps (II) and (ii), molding and cooling are performed by a mold. Preferred conditions will be described for each step.

[0060] [Heating step] (I), (i) In the heating step, prior to the hot stamping step, a steel sheet for hot stamping, which has a predetermined chemical composition and is in the form of a cold-rolled steel sheet or a plated steel sheet, such as the steel sheet for hot stamping according to the present embodiment, is heated. In the heating step for heating the steel sheet for hot stamping, the heating temperature is set to more than 1050° C. and 3 It is preferable that the heating temperature is more than 1050°C. When the heating temperature is more than 1050°C, the hot stamping start temperature can be more than 1050°C in the hot stamping step described later, and it is easy to ensure the impact resistance of the hot stamped product. 3 By exceeding this point, the volume fraction of martensite is ensured in the metal structure of the hot stamped product, the strength of the product is improved, and impact resistance is easily ensured. 3 The point is the temperature at which ferrite disappears from the metal structure when the steel sheet is heated, and can be determined from the change in thermal expansion of the steel sheet during the heating process. The heating temperature is above 1100°C and below Ac 3 It is preferable that the concentration is more than 1000 ppm.

[0061] The upper limit of the heating temperature is not particularly limited, but if the heating temperature is too high, when the steel sheet for hot stamping is a cold-rolled steel sheet, excessive scale is generated on the hot stamped product, and the productivity of the product decreases due to the accumulation of scale in the die. When the steel sheet for hot stamping is a plated steel sheet, the amount of plating is reduced, and the corrosion resistance of the hot stamped product is deteriorated. 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 smaller the local hardness variation of the hot stamped product, and the better the impact resistance. 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, by suppressing the heating rate, it is possible to suppress the generation of coarse iron carbides in the metal structure of the hot stamped product, and to improve 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.

[0062] [Hot stamping process] (II), (ii) In the process of hot stamping the heated steel sheet for hot stamping, the heated steel sheet is removed from the heating furnace and allowed to cool in the air, and then hot stamping is started. The hot stamping start temperature is preferably higher than 1050°C. By having a hot stamping start temperature of more than 1050°C, excessive accumulation of strain in austenite during hot stamping is suppressed, local hardness fluctuation of the formed product is reduced, and impact resistance is improved. The hot stamping start temperature is preferably higher than 1100°C. Although there is no particular upper limit to the hot stamping start temperature, in order to increase the start temperature, it is necessary to increase the heating temperature of the steel sheet in the above-mentioned heating step. In this case, excessive scale is formed on the hot stamped product, which reduces the productivity of the product or deteriorates the corrosion resistance of the hot stamped product. Therefore, the start temperature is preferably 1200°C or less, or 1150°C or less. After forming by hot stamping, the formed product is cooled while being held in the die, and / or the formed product is removed from the die and cooled by any method. By increasing the cooling rate, the volume fraction of martensite is secured in the metal structure of the hot stamped product, and the strength of the formed product is improved, so 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. In addition, by lowering the cooling stop temperature, the volume fraction of martensite is secured in the metal structure of the hot stamped product, and the strength of the formed product is improved. In addition, after the reheating process described later, the generation of ferrite, pearlite, or bainite is suppressed, and the impact resistance is improved. Therefore, it is preferable to set the cooling stop temperature by the above cooling to less than 90 ° C. or less than 50 ° C.

[0063] [Reheating process] (iii) When a plated steel sheet is used as a steel sheet for hot stamping, reheating is performed on the steel sheet after hot stamping (hot stamped product). When the reheating temperature is 90°C or higher, the local hardness variation of the formed product is reduced, and the impact resistance is improved. On the other hand, when the reheating temperature is less than 150°C, the softening of the steel sheet is suppressed, and the strength of the formed product is ensured. In addition, the precipitation of coarse iron carbides is suppressed, and the impact resistance is improved. Therefore, the reheating temperature is preferably 90°C or higher and less than 150°C. The reheating temperature is more preferably 100°C or higher, 110°C or higher, or 120°C or higher. In addition, the reheating temperature is more preferably less than 140°C or less than 130°C. By increasing the holding time at the reheating temperature, the effect of suppressing the local hardness variation can be sufficiently obtained. Therefore, the holding time is preferably 5 minutes or more, or 10 minutes or more. On the other hand, if the holding time is short, the strength of the molded product can be ensured. Therefore, the holding time is preferably less than 20 minutes, or less than 15 minutes. Moreover, by performing reheating under the above conditions, the yield ratio can be increased. When a cold-rolled steel sheet is used as the hot stamping steel sheet, the reheating step does not need to be performed. As described above, the variation in hardness is small when the strain energy accumulated in the hot stamping steel sheet is high. This is because the processing strain during cold rolling is accumulated in the cold-rolled steel sheet, so that the target standard deviation of the Vickers hardness can be achieved without reheating. However, even when a cold-rolled steel sheet is used as the hot stamping steel sheet, the yield ratio can be increased by reheating. Therefore, reheating may be performed on a hot stamped product that does not have a plating layer on its surface. In order to fully obtain the effect of increasing the yield ratio, it is preferable to perform reheating under the same conditions as when a plated steel sheet is used as the hot stamping steel sheet described above.

[0064] [Method of manufacturing steel sheets for hot stamping] The steel sheet for hot stamping according to the present embodiment, which is used for producing a hot stamped product, is preferably produced by the following production method.

[0065] The manufacturing method of the slab used in the manufacturing method of the hot stamping steel sheet according to the present embodiment is not particularly limited. In a preferred manufacturing method of the slab exemplified, the steel having the above-mentioned component composition (chemical composition) is melted by a known means, and then made into a steel ingot by a continuous casting method, or made into a steel billet by a method of making a steel ingot by any casting method and then blooming. In the continuous casting process, in order to suppress the occurrence of surface defects caused by inclusions, it is preferable to cause an external additional flow such as electromagnetic stirring in the molten steel in the mold. The steel ingot or billet may be cooled once and then reheated for hot rolling, or the steel ingot in a high temperature state after continuous casting or the steel billet in a high temperature state after blooming may be used for hot rolling as it is, or after keeping warm, or after supplementary heating. In the present embodiment, such steel ingots and billets are collectively referred to as "slabs" as materials for hot rolling.

[0066] During hot rolling, the slab is heated. The temperature of the slab used for hot rolling (slab heating temperature) is preferably less than 1250°C, more preferably less than 1200°C, in order to prevent coarsening of austenite. If the slab heating temperature is low, rolling becomes difficult, so the slab heating temperature may be 1050°C or higher. The heated slab is then hot-rolled to obtain a hot-rolled steel sheet. Hot rolling is carried out using an Ar alloy to transform austenite after rolling to refine the metal structure of the hot-rolled steel sheet. 3 It is preferable to complete the reaction at a temperature above the Ar 3 The point is the temperature at which transformation from austenite to ferrite begins in the metal structure when the steel sheet is cooled, and can be determined from the change in thermal expansion of the steel sheet during cooling.

[0067] 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 of the rough rolled material is hotter than the front end, thereby suppressing the temperature fluctuation over the entire length of the rough rolled material at the start of the finish rolling to 140°C or less. This improves the uniformity of the product characteristics in the coil after the coiling process.

[0068] The rough rolled material may be heated by a known means. For example, a solenoid type induction heating device may be provided between the rough rolling mill and the finish rolling 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.

[0069] When the hot-rolled steel sheet is coiled after hot rolling, the coiling temperature is preferably set to more than 600° C. If the coiling temperature is 600° C. or less, the hot-rolled steel sheet becomes excessively hard, making it difficult to perform cold rolling, and the impact resistance of the hot stamped product may deteriorate. More preferably, the coiling temperature is more than 620° C. or more than 650° C. On the other hand, if the coiling temperature is too high, the amount of coarse iron carbides generated in the metal structure of the hot stamped product becomes excessive, and the ductility of the steel sheet after hot stamping decreases. Therefore, the coiling temperature is preferably 750° C. or less, or 700° C. or less. The hot-rolled steel sheet may be annealed before the cold rolling process.

[0070] When the steel sheet for hot stamping is to be a cold-rolled steel sheet, the hot-rolled and coiled steel sheet is cold-rolled according to a conventional method to obtain a cold-rolled steel sheet. In cold rolling, the cold rolling ratio (the cumulative rolling reduction in cold rolling) is preferably 10% or more. If the cold rolling ratio is less than 10%, the local hardness of the hot stamped product varies greatly, and the impact resistance of the product decreases. More preferably, the cold rolling ratio is 20% or more, 30% or more, or 40% or more. There is no need to particularly limit the upper limit of the cold rolling ratio, but an excessive increase in the cold rolling ratio increases the load on the rolling equipment and leads to a decrease in productivity, so the cold rolling ratio is preferably less than 70%, less than 60%, or less than 50%. 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 descaling by pickling or the like may be performed according to a known method. The cold rolled steel sheet thus obtained may be subjected to treatment such as degreasing according to a conventional method. When the steel sheet for hot stamping is used as it is cold-rolled, the cold-rolled steel sheet is not annealed. By not annealing, the strain energy accumulated during cold rolling can be used to reduce local variations in hardness of the hot stamped product, improving the impact resistance of the product.

[0071] On the other hand, when the steel sheet for hot stamping is to be a plated steel sheet, cold rolling may not be performed or may be performed under the above conditions. By performing cold rolling, the metal structure is refined and the impact resistance of the hot stamped product is improved. When the hot stamping steel sheet is used as a plated steel sheet, the hot rolled steel sheet or cold rolled steel sheet manufactured by the above-mentioned method is plated according to a conventional method. When plating a cold rolled steel sheet, in order to refine the metal structure of the plated steel sheet by recrystallization, it is preferable to set the lower limit of the soaking temperature in the annealing process of continuous hot dip plating to 600°C, 650°C, or 700°C. On the other hand, if the heating rate is too slow, the soaking temperature is too high, or the soaking time is too long, the metal structure of the plated steel sheet becomes coarse due to grain growth, and the impact resistance of the hot stamped product decreases. In addition, iron carbides may become spherical and coarse, and the ductility of the steel sheet after hot stamping may decrease. Therefore, it is preferable to set the average heating rate to the soaking temperature to 1°C / s or more, the soaking temperature to 800°C or less, or 760°C or less, and the soaking time (holding time at the soaking temperature) to less than 300 seconds, or less than 120 seconds. After subjecting the cold-rolled steel sheet to continuous annealing to obtain an annealed steel sheet, plating may be performed on the annealed steel sheet. However, if the heating rate in continuous annealing is too slow, the metallographic structure of the annealed steel sheet coarsens due to grain growth, and the impact resistance of the hot-stamped molded product decreases. In addition, the iron carbide spheroidizes and coarsens, and the ductility of the steel sheet after hot stamping decreases. Therefore, it is preferable that the average heating rate up to the soaking temperature in continuous annealing is 1 °C / second or more. The plated steel sheet thus obtained may be subjected to temper rolling according to a conventional method.

[0072] Hereinafter, the present invention will be described more specifically by way of examples, but the present invention is not limited to these examples.

Examples

[0073] (Example 1) Using a vacuum melting furnace, molten steel was cast to produce steels A to V having the chemical compositions shown in Table 1. Ac in Table 1 3 The point was determined from the thermal expansion change when cold-rolled steel sheets having the chemical compositions of steels A to V were heated at 8 °C / second. After heating steels A to V to 1200 °C and holding for 60 minutes, hot rolling was performed under the hot rolling conditions shown in Table 2.

[0074]

Table 1

[0075]

Table 2

[0076] Specifically, in the temperature range above the Ar 3 point, steels A to V were rolled 10 passes to obtain hot-rolled steel sheets with a thickness of 2.2 to 3.2 mm. After hot rolling, the hot-rolled steel sheet was cooled to 640 to 660 °C with water spray, the cooling end temperature was taken as the coiling temperature, and the hot-rolled steel sheet was charged into an electric heating furnace maintained at this coiling temperature and held for 60 minutes. Then, the hot-rolled steel sheet was furnace-cooled to room temperature at an average cooling rate of 20 °C / hour to simulate slow cooling after coiling.

[0077] A part of the hot-rolled steel sheet was pickled to prepare a base material for cold rolling, and cold-rolled under the cold rolling conditions shown in Table 2 to obtain a cold-rolled steel sheet having a thickness of 1.4 mm. In addition, a part of the hot-rolled steel sheet was mechanically ground to obtain a hot-rolled ground sheet having a thickness of 1.4 mm.

[0078] In addition, a part of the cold-rolled steel sheet was heated to 780°C at an average heating rate of 5°C / s using a continuous annealing simulator, soaked for 120 seconds, and then cooled to room temperature at an average cooling rate of 5°C / s to obtain an annealed steel sheet.

[0079] Test pieces for microstructural observation were taken from the cold-rolled steel sheets, hot-rolled ground steel sheets, and annealed steel sheets thus obtained (these steel sheets are collectively referred to as hot stamping steel sheets). The longitudinal cross sections of these test pieces parallel to the rolling direction of the steel sheets were polished, and then the microstructural observation was performed by the above-mentioned method at a depth position of 1 / 4 of the sheet thickness of the steel sheets from the surface of the steel sheets, and the total volume fraction of ferrite extended in the rolling direction, pearlite extended in the rolling direction, and bainite extended in the rolling direction was determined.

[0080] In addition, JIS 13B tensile test pieces were taken from the above hot stamping steel sheets along the direction perpendicular to the rolling direction, and tensile tests were performed at a tension speed of 10 mm / min to determine the tensile strength. Table 2 shows the results of observing the metal structure of the hot stamping steel sheets and investigating the mechanical properties of the hot stamping steel sheets.

[0081] From the above-mentioned hot stamping steel plate, a hot stamping blank having a width of 240 mm and a length of 800 mm was taken, and a hat member having the shape shown in FIG. 2 was manufactured by hot stamping. In the hot stamping process, the blank (hot stamping steel plate) was heated to the heating temperature shown in Table 3-1 at an average heating rate of 22°C / sec up to 700°C using a gas heating furnace, and was held at that temperature for 1 minute. Thereafter, the blank was removed from the heating furnace and allowed to cool, and was hat-formed by being sandwiched between a die equipped with a cooling device at the start temperature shown in Table 3-1, and then cooled in the die to the cooling stop temperature shown in Table 3-1. In addition, a part of the hat member was reheated under the conditions shown in Table 3-1 using an electric heating furnace. The mark "-" in the hot stamping conditions in Table 3-1 indicates that the reheating process was not performed.

[0082] A test piece for microstructure observation was taken from the vertical wall of the obtained hat component (hot stamp formed product), and the vertical cross section of this test piece was polished. After that, the metal structure was observed at a depth position of 1 / 4 of the plate thickness from the surface of the steel plate by the method described above.

[0083] In addition, JIS No. 13B tensile test pieces were taken from the vertical wall of the hat member along the longitudinal direction of the member, and tensile tests were carried out at a tensile speed of 10 mm / min to determine the tensile strength, yield stress, and yield ratio. In addition, test pieces for hardness measurement were taken from the vertical wall portion of the hat member, and the vertical cross section of this test piece was polished. Then, Vickers hardness measurement was performed in accordance with JIS Z 2244:2009 at a depth position of 1 / 4 of the plate thickness from the surface of the steel plate using the above-mentioned method and a load of 0.49 N, and the average value and standard deviation of the Vickers hardness were obtained.

[0084] In addition, a test specimen for the three-point bending test was manufactured 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 distance of 700 mm, with the closing plate facing down, 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. If the maximum load was 23.0 kN or more, the crack occurrence displacement was 35 mm or more, and the absorbed energy was 0.80 kJ or more, it was determined that the crash resistance was good.

[0085] Tables 3-1 and 3-2 show the results of observing the metal structure of the hat member, the results of evaluating the mechanical properties of the hat member, and the results of evaluating the impact resistance of the hat member. In Tables 3-1 and 3-2, underlined values ​​are outside the scope of the present invention.

[0086] [Table 3-1]

[0087] [Table 3-2]

[0088] In all of the test numbers 1, 6, 7, 12, 13, 23, 25, 26, 28, 30 to 32, and 34 to 40, which satisfy the provisions of the present invention, the tensile strength of the hot stamped products was 2300 MPa or more, the average Vickers hardness was 670 or more, and the standard deviation of the Vickers hardness was 20 or less. In addition, in a three-point bending test of the formed products, the maximum load was 23.0 kN or more, the crack occurrence displacement was 35 mm or more, and the absorbed energy was 0.80 kJ or more, indicating good crash resistance.

[0089] In addition, for test numbers 6, 12, 25, 31, 34, and 40, which were subjected to reheating treatment in the manufacturing process of the hot stamped product, the tensile strength of the hot stamped product was 2300 MPa or more, the average Vickers hardness was 670 or more, and the standard deviation of the Vickers hardness was 10 or less. In addition, the yield ratio was 0.65 or more, the maximum load in a three-point bending test of the formed product was 23.0 kN or more, the crack occurrence displacement was 45 mm or more, and the absorbed energy was 0.95 kJ or more, and the impact resistance was particularly good.

[0090] In contrast, in the comparative examples, test numbers 15 to 22, which used steel plates whose chemical compositions were outside the range of the present invention, the tensile strength of the hot stamped products was less than 2300 MPa, the average Vickers hardness was less than 670, the maximum load in the three-point bending test of the formed products was low or the standard deviation of the Vickers hardness exceeded 20, and the maximum load, crack initiation displacement, and absorbed energy in the three-point bending test of the formed products were low, and the impact resistance was poor.

[0091] Specifically, in test number 15 using steel D, the C content of the steel was too low, so the hot stamped product had a tensile strength of less than 2300 MPa, an average Vickers hardness of less than 670, and the maximum load of the product was low.

[0092] Test No. 16, which used steel E, had a high average Vickers hardness because the C content of the steel was too high, and early fracture occurred in the tensile test, making it impossible to determine the tensile strength, yield stress, and yield ratio. The standard deviation of the Vickers hardness was over 20, and the maximum load, crack initiation displacement, and absorbed energy of the molded product were low.

[0093] In test numbers 17 and 18, which used steels F and G, the Mn content of the steels was too high, and in test number 19, which used steel H, the Mo content of the steel was too high. In both cases, the standard deviation of Vickers hardness exceeded 20, and the maximum load, crack initiation displacement, and absorbed energy of the molded products were low.

[0094] In test No. 20 using steel I, the Mo and B contents of the steel were too low, in test No. 21 using steel J, the Mo content of the steel was too low, and in test No. 22 using steel K, the sol.Al content of the steel was too high. As a result, the martensite volume fraction in the metal structure of the hot stamped product was insufficient, the tensile strength was less than 2300 MPa, the average Vickers hardness was less than 670, the standard deviation of Vickers hardness was more than 20, and the maximum load, crack initiation displacement, and absorbed energy of the formed product were low.

[0095] In the comparative examples, test numbers 2 to 5, 8 to 11, 14, 24, 27, 29, and 33, in which the chemical composition was within the range of the present invention but the manufacturing conditions of the hot stamped products were outside the above-mentioned range, the standard deviation of the Vickers hardness of the hot stamped products exceeded 20, and the maximum load, crack initiation displacement, and absorbed energy of the formed products were low, or the crack initiation displacement and absorbed energy were low, and the impact resistance was poor. Specifically, in test No. 2 using Steel A, test No. 8 using Steel B, and test No. 27 using Steel M, 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 the Vickers hardness of the formed product was more than 20, and the maximum load, crack initiation displacement, and absorbed energy were low. In test number 5 using Steel A, test number 11 using Steel B, and test number 29 using Steel N, cold rolling was not performed in the manufacturing process of the steel plate for hot stamping (the steel plate to be subjected to hot stamping was not as-cold-rolled), so the standard deviation of the Vickers hardness of the formed product was more than 20, and the maximum load, crack initiation displacement, and absorbed energy were low. In test numbers 3 and 4 using Steel A, test number 9 and 10 using Steel B, test number 24 using Steel L, and test number 33 using Steel P, the standard deviation of the Vickers hardness of the formed products exceeded 20, and the crack initiation displacement and absorbed energy were low because the forming start temperature in the hot stamping process was too low. In test number 14 using steel C, an annealed steel sheet was used as the steel sheet for hot stamping, and the starting temperature for forming in the hot stamping process was too low, so the standard deviation of the Vickers hardness of the formed product exceeded 20, and the maximum load, the displacement at which cracks occurred, and the absorbed energy were low.

[0096] Example 2 Molten steel was cast using a vacuum melting furnace to produce steels a to w having the chemical compositions shown in Table 4. 3 The points were obtained from the change in thermal expansion when plated steel sheets having the chemical compositions of Steels A to W were heated at 8°C / sec. Steels A to W were heated to 1200°C and held for 60 minutes, and then hot-rolled under the hot-rolling conditions shown in Table 5.

[0097] Specifically, Ar 3 The steels a to w were rolled 10 times in a temperature range above this point to produce hot-rolled steel sheets with a thickness of 2.2 to 3.2 mm. After hot rolling, the hot-rolled steel sheets were cooled to 640 to 660°C with a water spray, and the end-of-cooling 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.

[0098] A part of the hot-rolled steel sheet was pickled to prepare a base material for cold rolling, and cold-rolled to prepare a cold-rolled steel sheet having a thickness of 1.4 mm under the cold-rolling conditions shown in Table 5. In addition, a part of the hot-rolled steel sheet (an example that was not subjected to cold rolling) was mechanically ground to prepare a hot-rolled ground sheet having a thickness of 1.4 mm.

[0099] The obtained steel sheets (cold-rolled steel sheets and hot-rolled ground steel sheets) were heated to the annealing soaking temperature shown in Table 5 at an average heating rate of 5°C / s using a hot-dip galvanizing simulator and soaked for 120 seconds. The steel sheets were then cooled and immersed in a hot-dip galvanizing bath or hot-dip aluminum plating bath to be hot-dip galvanized or hot-dip aluminum plated. Some of the base steel sheets were subjected to alloying treatment by heating to 520°C after hot-dip galvanizing.

[0100] [Table 4]

[0101] [Table 5]

[0102] From the thus obtained hot-dip galvanized steel sheet, alloyed hot-dip galvanized steel sheet, and hot-dip aluminum-plated steel sheet (these steel sheets are collectively referred to as hot-stamping steel sheets), a hot-stamping blank having a width of 240 mm and a length of 800 mm was taken, and a hat member having the shape shown in FIG. 2 was manufactured by hot stamping. In the hot stamping process, the blank was heated to the heating temperature shown in Table 6-1 at an average heating rate of 11°C / sec or more up to 700°C using a gas heating furnace, and was held at that temperature for 1 minute. Thereafter, the blank was taken out of the heating furnace and allowed to cool, and was sandwiched between a die equipped with a cooling device at the start temperature shown in Table 6-1 to form a hat shape, and was then cooled in the die to the cooling stop temperature shown in Table 6-1. In addition, a part of the hat member was reheated under the conditions shown in Table 6-1 using an electric heating furnace. The mark "-" in the hot stamping conditions in Table 6-1 indicates that the reheating process was not performed.

[0103] A test piece for microstructure observation was taken from the vertical wall of the obtained hat member (hot stamp formed product). The vertical cross section of this test piece was polished, and then the metal structure was observed at a depth position of 1 / 4 of the sheet thickness of the substrate steel sheet from the interface between the substrate steel sheet and the plating layer by the method described above.

[0104] In addition, JIS No. 13B tensile test pieces were taken from the vertical wall of the hat member along the longitudinal direction of the member, and tensile tests were carried out at a tensile speed of 10 mm / min to determine the tensile strength, yield stress, and yield ratio. In addition, test pieces for hardness measurement were taken from the vertical wall portion of the hat member, and the vertical cross section of this test piece was polished. After that, Vickers hardness was measured at a depth position of 1 / 4 of the sheet thickness of the steel sheet from the interface between the steel sheet and the plating layer using the method described above with a load of 0.49 N, and the average value and standard deviation of the Vickers hardness were obtained. In addition, a test specimen for the three-point bending test was manufactured 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.

[0105] As shown in Figure 4, a test specimen with a length of 800 mm was placed on two support rolls arranged with a roll distance of 700 mm, with the closing plate facing downwards, 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. If the maximum load was 23.0 kN or more, the displacement at which cracks occurred was 35 mm or more, and the absorbed energy was 0.80 kJ or more, the crash resistance was determined to be good.

[0106] Tables 6-1 and 6-2 show the results of observing the metal structure of the hat member, the results of evaluating the mechanical properties of the hat member, and the results of evaluating the impact resistance of the hat member. In Tables 6-1 and 6-2, underlined values ​​are outside the scope of the present invention.

[0107] [Table 6-1]

[0108] [Table 6-2]

[0109] In all of the test numbers 101 to 103, 107 to 109, 113, 115, 116, 118, 128, 129, 131, 135, and 137 to 143, which satisfy the provisions of the present invention, the tensile strength of the hot stamped product was 2300 MPa or more, the average Vickers hardness was 670 or more, and the standard deviation of the Vickers hardness was 20 or less. In addition, the yield ratio was 0.65 or more, the maximum load in the three-point bending test of the formed product was 23.0 kN or more, the crack occurrence displacement was 35 mm or more, and the absorbed energy was 0.80 kJ or more, indicating good crash resistance.

[0110] In contrast, test numbers 120 to 127, which were comparative examples using steel plates whose chemical compositions were outside the range of the present invention, had low maximum load, crack initiation displacement, and / or absorbed energy, and thus had poor impact resistance.

[0111] Specifically, in test number 120 using steel f, the C content of the steel was too low, so the tensile strength of the hot stamped product was less than 2300 MPa, the average Vickers hardness was less than 670, and the maximum load of the formed product was low.

[0112] Test No. 121, which used steel g, had a high average Vickers hardness because the C content of the steel was too high, and early fracture occurred in the tensile test, making it impossible to determine the tensile strength, yield stress, and yield ratio. The standard deviation of the Vickers hardness was over 20, and the maximum load, crack initiation displacement, and absorbed energy of the molded product were low.

[0113] In test numbers 122 and 123, which used steels h and i, the Mn content of the steel was too high, and in test number 124, which used steel j, the Mo content of the steel was too high. In both cases, the standard deviation of Vickers hardness exceeded 20, and the crack initiation displacement and absorbed energy were low.

[0114] In test No. 125 using steel k, the Mo and B contents of the steel were too low, in test No. 126 using steel l, the Mo content of the steel was too low, and in test No. 127 using steel m, the sol.Al content of the steel was too high. As a result, the martensite volume fraction in the metal structure of the hot stamped product was insufficient, the tensile strength was less than 2300 MPa, the average Vickers hardness was less than 670, the standard deviation of Vickers hardness was more than 20, and the maximum load, crack initiation displacement, and absorbed energy of the formed product were low.

[0115] The comparative examples, test numbers 104 to 106, 110 to 112, 114, 117, 119, 130, 132 to 134, and 136, in which the chemical compositions were within the range of the present invention but the manufacturing conditions of the hot stamped products were outside the above-mentioned range, had low maximum load, low cracking displacement, and low absorbed energy in the three-point bending test of the formed products, and therefore had poor impact resistance. Specifically, in test numbers 104 and 105 using steel a, test numbers 110 and 111 using steel b, test number 114 using steel c, test number 119 using steel e, and test number 130 using steel n, the standard deviation of the Vickers hardness of the formed products exceeded 20, and the crack initiation displacement and absorbed energy were low because the forming start temperature in the hot stamping process was too low. In test number 106 using steel a and test number 136 using steel p, the reheating temperature in the reheating process was too high, so the tensile strength was less than 2300 MPa, the average Vickers hardness was less than 670, and the maximum load was low. In test number 112 using steel b, test number 117 using steel d, and test number 132 using steel o, the reheating temperature in the reheating process was too low, or reheating treatment was not performed, so the standard deviation of Vickers hardness was more than 20, the yield ratio was less than 0.65, and the maximum load, crack initiation displacement, and absorbed energy were low. In test number 133 using steel o, the cooling stop temperature in the hot stamping process was high and the reheating temperature in the reheating process was too high, resulting in an insufficient martensite volume fraction, a tensile strength of less than 2300 MPa, an average Vickers hardness of less than 670, a standard deviation of Vickers hardness of more than 20, and low maximum load, crack initiation displacement, and absorbed energy of the formed product. In test number 134 using steel o, the forming start temperature in the hot stamping process was low and reheating treatment was not performed, so the standard deviation of the Vickers hardness was more than 20, the yield ratio was less than 0.65, and the maximum load, the displacement at which cracks occurred, and the absorbed energy were low. [Industrial Applicability]

[0116] According to the present invention, it is possible to obtain a hot stamped product having excellent impact resistance and a portion with a tensile strength of 2300 MPa or more.

Claims

1. A hot stamped product comprising a steel plate, The tensile strength of all or a part of the steel plate is 2300 MPa or more, In the portion having a tensile strength of 2300 MPa or more, In mass percent, C: more than 0.40%, less than 0.60%, 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% or more and less than 0.500%; N: 0.0200% or less, Mo: 0.01% or more, less than 0.35% 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% or more and less than 0.50% W: 0% or more and less than 0.50%; 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: Fe and impurities, and having a chemical composition At a depth position of 1 / 4 of the plate thickness from the surface of the steel plate, The metal structure contains, by volume percent, more than 90.0% martensite; The average Vickers hardness in a region of 0.3 mm in the sheet thickness direction and 0.6 mm in the direction perpendicular to the sheet thickness direction is 670 or more, and the standard deviation of the Vickers hardness in the region is 20 or less. Hot stamped product.

2. In the portion having a tensile strength of 2300 MPa or more, The yield ratio is 0.65 or more. The hot stamped product according to claim 1.

3. A steel sheet and a plating layer formed on a surface of the steel sheet, The tensile strength of all or a part of the steel plate is 2300 MPa or more, In the portion having a tensile strength of 2300 MPa or more, In mass percent, C: more than 0.40%, less than 0.60%, 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% or more and less than 0.500%; N: 0.0200% or less, Mo: 0.01% or more, less than 0.35% 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% or more and less than 0.50% W: 0% or more and less than 0.50%; 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: Fe and impurities, and having a chemical composition At a depth position of 1 / 4 of the sheet thickness of the steel sheet from the boundary between the steel sheet and the plating layer, The metal structure contains, by volume percent, more than 90.0% martensite; The average Vickers hardness in a region of 0.3 mm in the sheet thickness direction and 0.6 mm in the direction perpendicular to the sheet thickness direction is 670 or more, and the standard deviation of the Vickers hardness in the region is 20 or less, The yield ratio is 0.65 or more. Hot stamped product.

4. The chemical composition, 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 hot stamped product according to any one of claims 1 to 3.

5. The chemical composition, in mass%, Cr: 0.001% or more and less than 0.50% W: 0.001% or more and less than 0.50% Cu: 0.001 to 2.00%, and Ni: 0.001 to 2.00%, Contains one or more selected from The hot stamped product according to any one of claims 1 to 4.

6. The chemical composition, 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 hot stamped product according to any one of claims 1 to 5.

7. The chemical composition, in mass%, Bi: 0.0001 to 0.0500%, Contains The hot stamped product according to any one of claims 1 to 6.

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