Hot stamped molded parts

Abstract

This hot-stamped object has a predetermined chemical composition, and when a region ranging, in the thickness direction from the surface, from a position corresponding to 1 / 8 the thickness to a position corresponding to 3 / 8 the thickness is defined as a 1 / 4 depth position, the microstructure at the 1 / 4 depth position comprises, in areal proportion, 80.0% or more of martensite and 0.0-5.0%, excluding 5.0%, of retained austenite. In the microstructure at the 1 / 4 depth position, the number density of iron-based carbide grains present in the martensite and have a circle-equivalent diameter exceeding 0.5 μm is less than 0.050 grains / μm2 and the average distance between any of the iron-based carbide grains and another iron-based carbide grain closest thereto is 3.0 μm or greater. In the microstructure at the 1 / 4 depth position, the prior austenite grain size is 20.0 μm or less.

Description

[Technical Field]

[0001] This invention relates to a hot-stamped molded body. This application claims priority based on Japanese Patent Application No. 2023-065610, filed in Japan on April 13, 2023, and the contents of that application are incorporated herein by reference. [Background technology]

[0002] In the field of automotive steel sheets, the application of high-strength steel sheets with high tensile strength is expanding in order to improve both fuel efficiency and collision safety, against the backdrop of increasingly stringent environmental regulations and collision safety standards. However, as strength increases, the press formability of the steel sheets decreases, making it difficult to manufacture products with complex shapes.

[0003] Specifically, as the strength of the steel sheet increases, its ductility decreases, leading to the problem of fracture at highly processed areas when processed into complex shapes. Furthermore, the increased strength of the steel sheet also causes residual stress after processing, resulting in springback and wall warping, which reduces dimensional accuracy. Therefore, press forming high-strength steel sheets, especially those with a tensile strength of 780 MPa or more, into products with complex shapes is not easy. While roll forming makes it easier to process high-strength steel sheets, its application is limited to parts with a uniform cross-section along the longitudinal direction.

[0004] Therefore, in recent years, as disclosed in Patent Documents 1 to 3, for example, hot stamping has been adopted as a technique for press-forming materials that are difficult to form, such as high-strength steel sheets. Hot stamping is a hot forming technique in which the material to be formed is heated before forming.

[0005] In this technology, the material is heated before shaping. Therefore, the steel is soft during shaping and has good formability. This allows even high-strength steel plates to be precisely shaped into complex forms. In addition, with hot stamping, quenching is performed simultaneously with shaping using a press die, so the shaped steel material (steel component, hot-stamped molded body) has sufficient strength.

[0006] For example, Patent Document 1 discloses that it is possible to impart a tensile strength of 1400 MPa or more to a steel member obtained by forming a steel plate by hot stamping.

[0007] In recent years, countries around the world have set higher CO2 reduction targets, and automobile companies are working to reduce fuel consumption while prioritizing collision safety. Not only in gasoline-powered vehicles, but also in electric vehicles, which are rapidly under development, there is a need for materials that are even stronger to protect not only passengers but also batteries from collisions, and to offset the increased weight. For example, in steel components used in automobiles, there is a need for steel materials that are stronger and have superior collision resistance, exceeding the strength of those described in the aforementioned Patent Document 1 or those currently used in hot-stamped steel components (hot-stamped molded bodies).

[0008] Regarding high-strength steel materials with a tensile strength of 1.5 GPa or higher, for example, Patent Document 2 discloses a hot-press-formed press-formed product that has excellent toughness and a tensile strength of 1.8 GPa or higher. Patent Document 3 discloses a steel material having an extremely high tensile strength of 2.0 GPa or higher, and further possessing good toughness and ductility. Patent Document 4 discloses a steel material having a high tensile strength of 1.8 GPa or higher, and further possessing good toughness. Patent Document 5 discloses a steel material having an extremely high tensile strength of 2.0 GPa or higher, and further possessing good toughness.

[0009] However, as a result of our investigations, we have found that, in Patent Documents 2 to 5, sufficient impact resistance (resistance to bending cracks and resistance to crack propagation during impact deformation) may not be obtained to meet the increasing demands of recent years.

Prior Art Documents

Patent Documents

[0010]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Summary of the Invention

Problems to be Solved by the Invention

[0011] As described above, hot stamping formed bodies having a tensile strength of 1.5 GPa (1500 MPa) or more have been disclosed. However, in any case, there is room for improvement in the impact resistance characteristics. Therefore, an object of the present invention is to provide a hot stamping formed body having high strength and excellent impact resistance characteristics.

Means for Solving the Problems

[0012] The inventors of the present invention examined the improvement of impact resistance characteristics on the premise of a hot stamping formed body with an increased C content in order to obtain high strength. As a result, the following findings were obtained. (i) When obtaining a hot stamping formed body with high strength (for example, a tensile strength of 1.5 GPa or more), as disclosed in Patent Documents 2 and 3, a large amount of carbon (C) is contained to ensure high strength. In this case, undissolved carbides tend to remain in the hot stamping formed body. If a large number of coarse carbides (iron-based carbides) with a circle-equivalent diameter exceeding 0.5 μm are present in the microstructure of the hot stamping formed body, they can become the starting points of cracks during bending or act as the starting points of crack propagation. (ii) The collision resistance characteristics (bending property and crack propagation resistance property) improve as the amount of coarse carbides decreases. Also, even when coarse carbides are present, by ensuring an average distance between carbides of a certain amount or more, the crack propagation resistance property improves. (iii) In the microstructure of the hot stamping formed body, when a large amount of ferrite or pearlite is present, the collision resistance characteristics (bending property and crack propagation resistance property) decrease, and when a large amount of retained austenite is present, the crack propagation resistance property decreases. Also, when the prior austenite grain size becomes coarse, the collision resistance characteristics (bending property and crack propagation resistance property) decrease.

[0013] The present invention has been made in view of the above findings. The gist of the present invention is as follows. [1] A hot-stamped molded article according to one aspect of the present invention has the following composition in mass%, C: 0.20-0.70%, Si: 0.010-2.000%, Mn: 0-2.00%, P: 0.100% or less, S: 0.0100% or less, N: 0.0100% or less, O: 0.0200% or less, Al: 0.0010-0.5000%, Mo: 0.0010-1.00% 00%, B:0.0005~0.0100%, Ti:0.010~0.100%, Nb:0~0.100%, Cr:0~1.00%, Co:0~3.00%, Ni:0 ~3.00%, Cu:0~1.00%, V:0~1.000%, W:0~1.00%, Ca:0~1.0000%, Mg:0~1.0000%, REM:0~1.00 The chemical composition consists of 00%, Sb: 0~1.000%, Zr: 0~1.000%, As: 0~1.000%, one or more elements selected from Ta, Re, Os, Ir, Tc, Pb, Se, Bi, and Sn: totaling 0~1.000%, and the remainder being Fe and impurities. When the 1 / 4 depth position is defined as the range from 1 / 8 of the thickness to 3 / 8 of the thickness in the thickness direction from the surface, the microstructure at the 1 / 4 depth position contains, by area percentage, martensite: 80.0% or more, and retained austenite: 0.0% or more and less than 5.0%, and in the microstructure at the 1 / 4 depth position, the number density of iron-based carbides with an equivalent circle diameter greater than 0.5 μm present in the martensite is 0.050 particles / μm 2 The average distance between the iron-based carbide and the nearest other iron-based carbide is 3.0 μm or more, and the prior austenite grain size in the microstructure at the 1 / 4 depth position is 20.0 μm or less. The hot-stamped molded articles described in [2][1] may have a decarburization index Dc of 0.085 or higher. The hot-stamped molded article described in [3] [1] or [2] may have a surface layer that extends from the surface to 50 μm, and the microstructure of the surface layer may contain, by area percentage, more than 5.0% ferrite. The hot-stamped molded article described in any of [4][1] to [3] has a chemical composition in mass% of: C: greater than 0.40% and 0.70% or less, Si: 0.010 to 2.000%, Mn: 0 to 1.00%, P: 0.100% or less, S: 0.0100% or less, N: 0.0100% or less, O: 0.0200% or less, Al: 0.0010 to 0.5000%, Mo: 0.0010 to 1.0000%, B: 0.0005 to 0.0100%, Ti: 0.010 to 0.100%, Nb: 0 to 0.100%. The composition may consist of Cr: 0-1.00%, Co: 0-3.00%, Ni: 0-3.00%, Cu: 0-1.00%, V: 0-1.000%, W: 0-1.00%, Ca: 0-1.0000%, Mg: 0-1.0000%, REM: 0-1.0000%, Sb: 0-1.000%, Zr: 0-1.000%, As: 0-1.000%, one or more elements selected from Ta, Re, Os, Ir, Tc, Pb, Se, Bi, and Sn: totaling 0-1.000%, and the remainder: Fe and impurities. The hot-stamped molded article described in any of [5][1] to [4] may have a coating on its surface. The hot-stamped molded article described in [6][5] has the coating, Even if it contains a total of 70% by mass or more of Fe and Al good. The hot-stamped molded article described in [7][5] has the coating, Even if it contains a total of 70% by mass or more of Fe and Zn good. [Effects of the Invention]

[0014] According to the above aspects of the present invention, it is possible to provide a hot-stamped molded article that has high strength and excellent impact resistance. [Brief explanation of the drawing]

[0015] [Figure 1] This figure shows an example of an impact force-displacement curve obtained from an instrumented impact test. [Modes for carrying out the invention]

[0016] <Hot-stamped molded parts> A hot-stamped molded article according to one embodiment of the present invention (a hot-stamped molded article according to this embodiment) will be described. In the following explanation, the range from 1 / 8 of the thickness to 3 / 8 of the thickness in the thickness direction from the surface will be defined as the 1 / 4 depth position, and the range from the surface up to 50 μm will be described as the surface layer. However, the surface that serves as the reference for the surface layer and the 1 / 4 depth position is the surface of the hot-stamped molded body. If the hot-stamped molded body has a coating (having a base material and a coating), it means the surface of the base material excluding the coating.

[0017] The hot-stamped molded article according to this embodiment has a predetermined chemical composition, and when the range from 1 / 8 of the thickness to 3 / 8 of the thickness in the thickness direction from the surface is defined as the 1 / 4 depth position, the microstructure at the 1 / 4 depth position contains, by area ratio, martensite: 80.0% or more and retained austenite: 0.0% or more and less than 5.0%, and in the microstructure at the 1 / 4 depth position, the number density of iron-based carbides with an equivalent circle diameter of more than 0.5 μm present in the martensite is 0.050 particles / μm 2 The average distance between the iron-based carbide and the nearest other iron-based carbide is 3.0 μm or more, and the prior austenite grain size in the microstructure at the 1 / 4 depth position is 20.0 μm or less. The hot-stamped molded article according to this embodiment may have a coating on its surface. In this case, the hot-stamped molded article has a base material made of steel and a coating formed on the surface of the base material. In this case, the chemical composition and microstructure described above refer to the chemical composition and microstructure of the base material. The following explains each of these points.

[0018] [Chemical composition] The chemical composition of the hot-stamped molded article according to this embodiment is as follows (in mass%): C: 0.20-0.70%, Si: 0.010-2.000%, Mn: 0-2.00%, P: 0.100% or less, S: 0.0100% or less, N: 0.0100% or less, O: 0.0200% or less, Al: 0.0010-0.5000%, Mo: 0.0010-1.0000%, B: 0.0005-0.0100%, Ti: 0.010-0.100%, Nb: 0-0.100%, Cr: 0-1 The composition consists of 0.00%, Co: 0-3.00%, Ni: 0-3.00%, Cu: 0-1.00%, V: 0-1.000%, W: 0-1.00%, Ca: 0-1.0000%, Mg: 0-1.0000%, REM: 0-1.0000%, Sb: 0-1.000%, Zr: 0-1.000%, As: 0-1.000%, one or more elements selected from Ta, Re, Os, Ir, Tc, Pb, Se, Bi, and Sn: totaling 0-1.000%, and the remainder: Fe and impurities. The reasons for limiting the content of each element are as follows: In the following, the range indicated by "A~B" represents a range with A as the lower limit and B as the upper limit (A or greater, B or less). However, values ​​indicated as "greater than" or "less than" are not included in the lower or upper limit. For example, "greater than A~B" indicates a range that is greater than A and less than or equal to B.

[0019] C: 0.20~0.70% Carbon (C) is an element that enhances the hardenability of steel and increases the strength of hot-stamped molded articles obtained after quenching steel sheets, such as hot stamping. If the C content is less than 0.20%, it becomes difficult to ensure sufficient strength in the hot-stamped molded article. Therefore, the C content should be 0.20% or more. Preferably, the C content should be 0.30% or more, and to obtain even higher tensile strength, a C content of 0.33% or more or more than 0.40% is more preferable. Even more preferably, the C content should be more than 0.44%. On the other hand, if the carbon content exceeds 0.70%, the number density and average distance of coarse iron-based carbides fall outside the specified range, resulting in a decrease in impact resistance (bendability and crack propagation resistance). Therefore, the carbon content should be 0.70% or less. Preferably, the carbon content should be 0.60% or less, and more preferably 0.55% or less. In other words, the C content is 0.20 to 0.70%, preferably 0.30 to 0.70%, more preferably greater than 0.40 to 0.70%, even more preferably greater than 0.44 to 0.70%, and may also be 0.30 to 0.60%, 0.33 to 0.60%, greater than 0.40 to 0.60%, greater than 0.44 to 0.60%, or greater than 0.44 to 0.55%.

[0020] Si: 0.010~2.000% Si is an effective element for ensuring the impact resistance (bendability and crack propagation resistance) of hot-stamped molded products by improving the hardenability of steel, suppressing the increase of coarse iron-based carbides, and ensuring the number density and average distance of coarse iron-based carbides. To obtain the above effects, the Si content should be 0.010% or more. Preferably, the Si content should be 0.100% or more, and more preferably 0.200% or more. On the other hand, if the Si content in the steel exceeds 2,000%, the amount of retained austenite increases, and the crack propagation resistance decreases. Therefore, the Si content should be 2,000% or less. Preferably, the Si content is 1,500% or less, and more preferably 1,000% or less. In other words, the Si content is 0.010 to 2.000%, preferably 0.100 to 1.500%, and more preferably 0.200 to 1.000%.

[0021] Mn: 0~2.00% Mn is not required to be included (it can be 0%), but it is a very effective element for improving the hardenability of steel and ensuring strength after quenching. Furthermore, Mn is an element that lowers the Ac3 (transformation point) and promotes lower quenching temperatures. For this reason, it may be included. To obtain the above effects, the Mn content is preferably 0.05% or more, more preferably 0.15% or more, and even more preferably 0.17% or more or 0.20% or more. On the other hand, if the Mn content exceeds 2.00%, the amount of coarse iron-based carbides increases, making it impossible to satisfy the number density and average distance of coarse iron-based carbides, and thus reducing the impact resistance properties (bendability and crack propagation resistance) after quenching. For this reason, the Mn content should be 2.00% or less. Preferably, the Mn content is 1.50% or less, more preferably 1.00% or less, and even more preferably 0.80% or less, or 0.60% or less. In other words, the Mn content is 0 to 2.00%, for example, preferably 0.05 to 1.50% or 0 to 1.00%, more preferably 0.15 to 1.00%, and even more preferably 0.17 to 0.80%, or 0.20 to 0.60%.

[0022] P:0.100% or less P is an impurity element, and its segregation at grain boundaries reduces grain boundary strength and decreases crack propagation resistance. Therefore, the P content should be 0.100% or less. Preferably, the P content is 0.050% or less, 0.030% or less, or 0.020% or less. There is no need to specify a lower limit for the phosphorus (P) content; the lower limit is 0%. However, reducing the P content to less than 0.0001% significantly increases the cost of P removal, making it economically undesirable. Therefore, the P content may be set at 0.0001% or higher, 0.001% or higher, 0.003% or higher, or 0.005% or higher. In other words, the P content is 0 to 0.100%, preferably 0.0001 to 0.100%.

[0023] S: 0.0100% or less S is an impurity element that forms inclusions in steel. These inclusions serve as fracture initiation points and crack propagation paths, so a high S content reduces impact resistance (bendability and crack propagation resistance). Therefore, the S content should be 0.0100% or less. Preferably, the S content is 0.0080% or less, 0.0050% or less, 0.0030% or less, 0.0020% or less, or 0.0010% or less. There is no need to specify a lower limit for the sulfur content; the lower limit is 0%. However, reducing the sulfur content to less than 0.0001% significantly increases the cost of sulfur removal, making it economically undesirable. Therefore, the sulfur content may be set at 0.0001% or higher, 0.0002% or higher, or 0.0003% or higher. In other words, the S content is 0 to 0.0100%, preferably 0.0001 to 0.0100%.

[0024] N: 0.0100% or less N is an impurity element that forms nitrides in steel. Since these nitrides serve as fracture initiation points and crack propagation paths, a high N content reduces impact resistance (bendability and crack propagation resistance). Therefore, the N content should be 0.0100% or less. Preferably, the N content is 0.0080% or less, or 0.0050% or less. There is no need to specify a lower limit for the N content; the lower limit is 0%. However, reducing the N content to less than 0.0001% significantly increases the cost of de-nitrogenization, making it economically undesirable. Therefore, the N content may be set at 0.0001% or higher, 0.0004% or higher, or 0.0010% or higher. In other words, the N content is 0 to 0.0100%, preferably 0.0001 to 0.0100%.

[0025] O: 0.0200% or less O is an impurity element that forms coarse oxides of Al, Ti, Mg, etc., which act as the starting point for fracture. When the O content increases, the amount of coarse oxides of Al, Ti, Mg, etc. increases, and the impact resistance (bendability and crack propagation resistance) of the hot-stamped molded product decreases. Therefore, the O content should be 0.0200% or less. Preferably, the O content is 0.0100% or less, more preferably 0.0050% or less, even more preferably 0.0040% or less, 0.0030% or less, and 0.0020% or less. There is no specific requirement to specify a lower limit for the oxygen content; the lower limit is 0%. However, reducing the oxygen content to less than 0.0001% significantly increases deoxidation costs, making it economically undesirable. Therefore, the oxygen content may be 0.0001% or higher, 0.0005% or higher, or 0.0010% or higher. In other words, the O content is 0 to 0.0200%, preferably 0.0001 to 0.0200%.

[0026] Al: 0.0010~0.5000% Al is an element that deoxidizes molten steel and improves its integrity (suppressing the occurrence of defects such as blowholes in the steel). If the Al content is less than 0.0010%, deoxidation is not sufficient, leading to an increase in blowholes and coarse oxides, and a decrease in impact resistance (bendability and crack propagation resistance). Therefore, the Al content should be 0.0010% or more. Preferably, the Al content is 0.0050% or more, 0.0100% or more, 0.0150% or more, 0.0200% or more, or 0.0250% or more. On the other hand, if the Al content exceeds 0.5000%, the amount of coarse oxides in the steel increases, and the impact resistance (bendability and crack propagation resistance) of the hot-stamped molded product decreases. Therefore, the Al content should be 0.5000% or less. Preferably, the Al content is 0.4000% or less, 0.3000% or less, 0.2000% or less, 0.1500% or less, 0.1000% or less, or 0.0750% or less. In other words, the Al content is 0.0010 to 0.5000%, for example, preferably 0.0050 to 0.4000%, more preferably 0.0100 to 0.3500% or 0.0150 to 0.3000%, even more preferably 0.0200 to 0.2000%, 0.0250 to 0.1500%, 0.0200 to 0.1000%, or 0.0200 to 0.0750%. In this embodiment, the Al content refers to the total Al content.

[0027] Mo: 0.0010~1.0000% Mo is an element that enhances the strength of hot-stamped molded products by solid-solubilizing into the prior austenite grains during heating before hot stamping. To ensure this effect is obtained, the Mo content should be 0.0010% or more. Preferably, the Mo content is 0.0100% or more. On the other hand, if the Mo content exceeds 1.0000%, the amount of Mo-based inclusions increases. Since these inclusions serve as crack initiation points and crack propagation paths, a high Mo content reduces the impact resistance (flexibility and crack propagation resistance) of the hot-stamped molded product. Therefore, the Mo content should be 1.0000% or less. Preferably, the Mo content is 0.8000% or less or 0.6000% or less. In other words, the Mo content is 0.0010 to 1.0000%, preferably 0.0100 to 0.8000%, or 0.0100 to 0.6000%.

[0028] B: 0.0005~0.0100% B is an element that improves the hardenability of steel. If the B content is less than 0.0005%, the hardenability decreases and the amount of martensite decreases, making it impossible to obtain the desired strength, and also reducing impact resistance (bendability and crack propagation resistance). Therefore, the B content should be 0.0005% or more. Preferably, the B content is 0.0010% or more, and more preferably 0.0015% or more. On the other hand, if the B content exceeds 0.0100%, the amount of coarse nitrides such as BN increases, and the impact resistance (bendability and crack propagation resistance) of the hot-stamped molded product decreases. Therefore, the B content should be 0.0100% or less. Preferably, the B content is 0.0080% or less, 0.0060% or less, 0.0040% or less, or 0.0030% or less. In other words, the B content is 0.0005 to 0.0100%, preferably 0.0010 to 0.0080%, more preferably 0.0015 to 0.0060%, 0.0015 to 0.0040%, or 0.0015 to 0.0030%.

[0029] Ti: 0.010~0.100% Ti is an element that fixes nitrogen as TiN, improves hardenability through solid solution B, and enhances the strength of hot-stamped molded articles through precipitation strengthening by the formation of fine carbides and carbonitrides. If the Ti content is less than 0.010%, these effects cannot be obtained. Therefore, the Ti content should be 0.010% or more. Preferably, the Ti content is 0.020% or more, or 0.030% or more. On the other hand, if the Ti content exceeds 0.100%, the amount of coarse nitrides and carbonitrides such as TiN and (Ti,Nb)(C,N) in the steel increases, reducing the impact resistance (bendability and crack propagation resistance) of the hot-stamped molded product. Therefore, the Ti content should be 0.100% or less. Preferably, the Ti content is 0.080% or less, 0.060% or less, 0.050% or less, or 0.040% or less. That is, the Ti content is 0.010 to 0.100%, preferably 0.020 to 0.080%, more preferably 0.030 to 0.060%, 0.030 to 0.050%, 0.010 to 0.040%, or 0.020 to 0.040%.

[0030] The chemical composition of the hot-stamped molded article according to this embodiment may contain the above-mentioned elements (basic elements), with the remainder being Fe and impurities. Alternatively, one or more of the following elements (arbitrary elements) may be further included in place of a portion of the Fe.

[0031] Nb: 0~0.100% Nb is an element that forms carbides and carbonitrides in steel, thereby increasing the strength of hot-stamped molded articles through precipitation strengthening. Therefore, it may be included. To obtain the above effect, the Nb content is preferably 0.001% or more. More preferably, the Nb content is 0.005% or more, 0.009% or more, or 0.015% or more. On the other hand, if the Nb content exceeds 0.100%, the amount of carbides and carbonitrides such as NbC and (Nb,Ti)(C,N) in the steel increases, reducing the toughness of the hot-stamped molded product. Therefore, the Nb content should be 0.100% or less. Preferably, the Nb content is 0.080% or less, 0.060% or less, or 0.050% or less. In other words, the Nb content is 0 to 0.100%, preferably 0.001 to 0.100%, more preferably 0.005 to 0.080%, and even more preferably 0.009 to 0.060%, or 0.015 to 0.050%.

[0032] Cr: 0~1.00% Cr is an element that enhances the strength of hot-stamped molded articles by solid-solubilizing into the prior austenite grains during heating before hot stamping. Therefore, it may be included. To obtain the above effect, the Cr content is preferably 0.01% or more. More preferably, the Cr content is 0.10% or more, or 0.20% or more. On the other hand, if the Cr content exceeds 1.00%, the hydrogen embrittlement resistance (hydrogen embrittlement resistance) of the hot-stamped molded product decreases. Therefore, the Cr content should be 1.00% or less. Preferably, the Cr content is 0.70% or less, 0.65% or less, 0.60% or less, or 0.55% or less. In other words, the Cr content is 0 to 1.00%, for example, preferably 0.01 to 1.00%, more preferably 0.10 to 0.70%, and even more preferably 0.20 to 0.65%, 0.20 to 0.60%, or 0.20 to 0.55%.

[0033] Co: 0~3.00% Co is an element that increases the strength of hot-stamped molded articles through solid solution strengthening. Therefore, it may be included. To obtain the above effect, the Co content is preferably 0.01% or more. More preferably 0.10% or more, and even more preferably 0.20% or more. On the other hand, even if a large amount is included, the above effect will saturate, so the Co content should be 3.00% or less. Preferably, it should be 2.50% or less. More preferably, it should be 2.20% or less, 2.00% or less, 1.00% or less, 0.70% or less, 0.65% or less, 0.60% or less, or 0.55% or less. That is, the Co content is 0-3.00%, for example, preferably 0.01-2.50%, more preferably 0.01-2.20%, 0.01-2.00%, 0.10-2.20%, 0.10-2.00%, or 0.20-2.00%, and even more preferably 0.20-1.00%, 0.20-0.70%, 0.20-0.65%, 0.20-0.60%, or 0.20-0.55%.

[0034] Ni: 0~3.00% Ni is an element that dissolves in austenite, improving the hardenability and tensile strength of steel. Therefore, it may be included. To obtain the above effects, it is preferable to have a Ni content of 0.001% or more. More preferably, the Ni content is 0.01% or more, or 0.10% or more. On the other hand, if the Ni content exceeds 3.00%, the above effect saturates and causes an increase in alloy costs. Therefore, the Ni content should be 3.00% or less. Preferably, the Ni content is 2.50% or less or 2.00% or less, more preferably 1.00% or less, 0.70% or less, 0.65% or less, 0.60% or less, or 0.55% or less. That is, the Ni content is 0 to 3.00%, for example, preferably 0.001 to 2.50% or 0.01 to 2.00%, more preferably 0.10 to 1.00%, and even more preferably 0.10 to 0.70%, 0.10 to 0.65%, 0.10 to 0.60%, or 0.10 to 0.55%.

[0035] Cu: 0~1.00% Cu is an element that enhances the strength of hot-stamped molded articles by solid-solubilizing into the prior austenite grains during heating before hot stamping. Therefore, it may be included. To obtain the above effect, it is preferable to have a Cu content of 0.01% or more. More preferably, it is 0.10% or more. On the other hand, the above effect will saturate even if a large amount is included, so the Cu content should be 1.00% or less. Preferably, the Cu content is 0.80% or less, 0.60% or less, or 0.30% or less. In other words, the Cu content is 0 to 1.00%, for example, preferably 0.01 to 0.80% or 0.01 to 0.60%, more preferably 0.10 to 0.80%, even more preferably 0.10 to 0.60%, or 0.10 to 0.30%.

[0036] V: 0~1.000% V is an element that has the effect of forming carbonitrides in steel and increasing the strength of hot-stamped molded products through precipitation strengthening. Therefore, it may be included. To obtain the above effect, it is preferable to have a V content of 0.010% or more. On the other hand, if the V content exceeds 1.000%, a large amount of carbonitrides are formed in the steel, reducing the hydrogen embrittlement resistance of the hot-stamped molded product. Therefore, the V content should be 1.000% or less. Preferably, the V content is 0.800% or less, 0.600% or less, or 0.300% or less. In other words, the V content is 0 to 1.000%, preferably 0.010 to 0.800%, more preferably 0.010 to 0.600%, and even more preferably 0.010 to 0.300%.

[0037] W: 0~1.00% W is an element that has the effect of increasing the strength of hot-stamped molded articles. Therefore, it may be included. To obtain the above effect, it is preferable that the W content be 0.01% or more, and more preferably 0.10% or more. On the other hand, the above effect will saturate even if a large amount is included, so the W content should be 1.00% or less. Preferably, the W content is 0.80% or less, 0.60% or less, or 0.30% or less. In other words, the W content is 0 to 1.00%, for example, preferably 0.01 to 0.80%, more preferably 0.01 to 0.60%, even more preferably 0.01 to 0.30%, or 0.10 to 0.30%.

[0038] Ca: 0~1.0000% Ca is an element that suppresses the formation of coarse oxides that serve as the starting point for fracture. Therefore, it may be included. To obtain the above effect, it is preferable to have a Ca content of 0.0001% or more, and more preferable to have a Ca content of 0.0010% or more. On the other hand, even if a large amount is included, the above effect will saturate, so the Ca content should be 1.0000% or less. Preferably, the Ca content is 0.4000% or less, 0.1000% or less, 0.0700% or less, 0.0200% or less, or 0.0100% or less. That is, the Ca content is 0 to 1.0000%, for example, preferably 0.0001 to 0.4000%, 0.0001 to 0.1000%, or 0.0001 to 0.0700%, more preferably 0.0010 to 0.1000%, and even more preferably 0.0010 to 0.0700%, 0.0010 to 0.0200%, or 0.0010 to 0.0100%.

[0039] Mg: 0~1.0000% Mg is an element that suppresses the formation of coarse MnS by forming fine oxides and sulfides in molten steel. Furthermore, Mg has the effect of refining the metal structure by dispersing a large number of fine oxides. Therefore, it may be included. To obtain the above effects, it is preferable to have an Mg content of 0.0001% or more. On the other hand, if the Mg content exceeds 1.0000%, the amount of oxides in the steel increases, which adversely affects the toughness of the hot-stamped molded product. Therefore, the Mg content should be 1.0000% or less. Preferably, the Mg content is 0.4000% or less, 0.1000% or less, 0.0700% or less, 0.0200% or less, or 0.0100% or less. That is, the Mg content is 0 to 1.0000%, for example, preferably 0.0001 to 0.4000%, more preferably 0.0001 to 0.1000%, even more preferably 0.0001 to 0.0700%, 0.0001 to 0.0200%, 0.0001 to 0.0100%, 0.0010 to 0.1000%, 0.0010 to 0.0700%, 0.0010 to 0.0200%, or 0.0010 to 0.0100%.

[0040] REM: 0~1.0000% REM is an element that suppresses the formation of coarse oxides that serve as the starting point for fracture. Therefore, it may be included. To obtain the above effect, it is preferable to have a REM content of 0.0001% or more, and more preferable to have a REM content of 0.0010% or more. On the other hand, even if a large amount is included, the above effect will saturate, so the REM content should be 1.0000% or less. Preferably, the REM content is 0.4000% or less, 0.1000% or less, 0.0700% or less, 0.0200% or less, or 0.0100% or less. That is, the REM content is 0 to 1.0000%, for example, preferably 0.0001 to 1.0000%, 0.0001 to 0.4000%, or 0.0001 to 0.1000%, more preferably 0.0010 to 0.1000%, and even more preferably 0.0010 to 0.0700%, 0.0010 to 0.0200%, or 0.0010 to 0.0100%. In this embodiment, REM refers to a total of 17 elements consisting of Sc, Y, and lanthanides, and the REM content refers to the total content of these elements.

[0041] Sb: 0~1.000% Sb is an element that improves the deformability of hot-stamped molded articles by suppressing the formation of oxides that serve as the starting point for fracture. Therefore, it may be included. To obtain the above effect, the Sb content is preferably 0.001% or more, more preferably 0.002% or more, and even more preferably 0.010% or more. On the other hand, even if a large amount is included, the above effect will saturate, so the Sb content should be 1.000% or less. Preferably, the Sb content is 0.400% or less, 0.100% or less, 0.050% or less, or 0.020% or less. That is, the Sb content is 0 to 1.000%, for example, preferably 0.001 to 1.000%, 0.001 to 0.400%, 0.001 to 0.100%, 0.001 to 0.050%, or 0.001 to 0.020%, more preferably 0.002 to 0.400%, 0.002 to 0.100%, or 0.010 to 0.050%, and even more preferably 0.001 to 0.020%, 0.002 to 0.020%, or 0.010 to 0.020%.

[0042] Zr: 0~1.000% Zr is an element that contributes to inclusion control, particularly the fine dispersion of inclusions, and enhances the toughness of hot-stamped molded articles. Therefore, it may be included. To obtain the above effects, it is preferable to have a Zr content of 0.001% or more, and more preferably 0.010% or more. On the other hand, if a large amount of Zr is included, deterioration of the surface properties may become apparent. Therefore, the Zr content should be 1.000% or less. Preferably, the Zr content is 0.400% or less, 0.200% or less, or 0.100% or less. That is, the Zr content is 0 to 1.000%, for example, preferably 0.001 to 1.000%, 0.001 to 0.400%, 0.001 to 0.200%, or 0.001 to 0.100%, more preferably 0.010 to 0.200%, and even more preferably 0.010 to 0.100%.

[0043] As: 0~1.000% As is an element that contributes to improving hydrogen embrittlement resistance by lowering the austenite single-phase formation temperature, thereby refining the prior austenite grains. For this reason, it may be included. To obtain the above effect, it is preferable to have an As content of 0.001% or more, and more preferable to have an As content of 0.005% or more. On the other hand, the above effect will saturate even if a large amount is included, so the As content should be 1.000% or less. Preferably, the As content is 0.400% or less, 0.200% or less, or 0.100% or less. That is, the As content is 0 to 1.000%, for example, preferably 0.001 to 1.000%, 0.001 to 0.400%, or 0.005 to 0.400%, more preferably 0.001 to 0.200%, or 0.005 to 0.400%, and even more preferably 0.001 to 0.100%, or 0.005 to 0.100%.

[0044] One or more elements selected from Ta, Re, Os, Ir, Tc, Pb, Se, Bi, and Sn: Total of 0-1.000% Ta, Re, Os, Ir, Tc, Pb, Se, Bi, and Sn are elements that improve hydrogen embrittlement resistance. Therefore, they may be included. To obtain the above effect, it is preferable that the total content be 0.010% or more. On the other hand, if the total content of these elements exceeds 1.000%, the effect saturates and the cost increases. Therefore, if they are included, the total content should be 1.000% or less. Preferably, it should be 0.800% or less, more preferably 0.500% or less, 0.400% or less, 0.200% or less, or 0.100% or less. That is, the total content of one or more elements selected from Ta, Re, Os, Ir, Tc, Pb, Se, Bi, and Sn is 0 to 1.000%, for example, preferably 0.010 to 0.800%, more preferably 0.010 to 0.500%, and even more preferably 0.010 to 0.400%, 0.010 to 0.200%, or 0.010 to 0.100%.

[0045] As described above, the chemical composition of the hot-stamped molded article according to this embodiment is that it contains basic elements with the remainder being Fe and impurities, or that it contains basic elements and one or more arbitrary elements with the remainder being Fe and impurities. Examples of impurities include elements that are introduced from steel raw materials or scrap and / or during the steelmaking process, and are acceptable as long as they do not impair the properties of the hot-stamped molded articles according to this embodiment.

[0046] The chemical composition of the hot-stamped molded body described above can be measured using general analytical methods after removing the decarburized layer, as described later, by mechanical grinding or the like. For example, it can be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Elements that are difficult to measure with ICP-AES can be measured by other methods. For example, C and S can be measured using combustion-infrared absorption spectroscopy, N can be measured using inert gas fusion-thermal conductivity spectroscopy, and O can be measured using inert gas fusion-nondispersive infrared absorption spectroscopy. Tc can be measured using ICP-MS (Inductively Coupled Plasma-Mass Spectrometry). If the hot-stamped molded body has a coating on its surface, the coating and decarburized layer should be removed by mechanical grinding before analyzing the chemical composition of the base material.

[0047] [Microorganisms] <The microstructure at a 1 / 4 depth contains, by area percentage, martensite: 80.0% or more, and retained austenite: 0.0% to less than 5.0%.> If the area ratio of martensite falls below a predetermined amount and a large amount of ferrite, pearlite, or bainite is present, the impact resistance (bendability and crack propagation resistance) decreases. Tensile strength also decreases. Therefore, the area ratio of martensite is set to 80.0% or more (including 100.0%). In this embodiment, martensite includes so-called fresh martensite and tempered martensite (including self-tempered martensite). To further improve the impact resistance (bendability and crack propagation resistance), the area ratio of martensite is preferably 85.0% or more, more preferably 90.0% or more, and even more preferably 95.0% or more. In other words, the martensite area is 80.0% or more (~100.0%), for example, preferably 85.0~100.0%, more preferably 90.0~100.0%, and even more preferably 95.0~100.0%. Furthermore, retained austenite transforms into hard martensite through processing-induced transformation during impact deformation. Therefore, if the area ratio of retained austenite is high, the crack propagation resistance decreases. In the hot-stamped molded article according to this embodiment, the area ratio of retained austenite is set to less than 5.0% (including 0.0%). To further improve the crack propagation resistance, the area ratio of retained austenite is preferably less than 4.0%, and more preferably less than 2.0%. That is, the area ratio of retained austenite is 0.0% or more and less than 5.0%, preferably 0.0% or more and less than 4.0%, and more preferably 0.0% or more and less than 2.0%.

[0048] The microstructure consists of martensite and retained austenite, with the remainder being ferrite, pearlite, and bainite. These area ratios should total 20.0% or less (including 0.0%) in order to ensure 80.0% or more martensite. Preferably, it is less than 20.0%, more preferably 15.0% or less, and even more preferably 10.0% or less or 5.0% or less.

[0049] The area ratio of each tissue (each phase) in the microstructure can be determined by the following method. Cut a sample from any position at least 50 mm away from the edge of the hot-stamped molded body (or a position avoiding the edge if it is not possible to take a sample from this position) so that the metal structure at 1 / 4 depth (the range from 1 / 8 of the thickness from the surface to 3 / 8 of the thickness from the surface) can be observed in a cross section parallel to the rolling direction and the thickness direction (the thickness direction of the raw steel sheet). Depending on the measuring device, the sample size should be large enough to allow observation of approximately 10 mm in the rolling direction.

[0050] Tissue identification will be performed using the above sample in the following manner. The cross-section (observation surface) parallel to the rolling direction and thickness direction is polished with silicon carbide sandpaper from #600 to #1500, and then finished to a mirror surface using a diluted solution such as alcohol or a pure dispersion of diamond powder with a particle size of 1 to 6 μm. Next, the sample is polished for 8 minutes at room temperature with colloidal silica with a particle size of 0.25 μm that does not contain alkaline solutions to remove strain introduced into the surface layer of the sample. At any position in the longitudinal direction of the sample cross-section, the crystal orientation information is obtained by electron backscatter diffraction measurement at measurement intervals of 0.1 μm in the range from 1 / 8 of the thickness from the surface to 3 / 8 of the thickness from the surface, with a length of 200 μm in the rolling direction and the center at a position 1 / 4 of the thickness from the surface. For the measurement, an EBSD analyzer consisting of a thermal field emission scanning electron microscope (JEOL JSM-7001F) and an EBSD detector (AMETEK Hikari detector) is used. At this time, the vacuum level inside the EBSD analyzer is 9.6 × 10⁻⁶. -5The parameters shall be Pa or less, the acceleration voltage 20kV, the working distance (WD) 15mm, and the irradiation current level 18. When acquiring EBSD patterns, the camera settings in the "OIM Data Collection" function of the software attached to the EBSD analyzer shall be set to EXPOSURE TIME 3.65 and Gain 0.39. In addition, when detecting bands in the EBSD pattern, the Hough transform shall be set to Max Peak Count 9 and Min Peak Count 5. For analysis, Iron(Alpha), a database for bcc crystal structures, and Iron(Gamma), a database for fcc crystal structures, shall be selected. If the crystal structure of the tissue to be analyzed is a bct structure, it shall be analyzed as a bcc structure. Using the "Phase Map" function included in the "OIM Analysis®" software attached to the EBSD analyzer, the obtained crystal orientation information is used to identify regions with an fcc crystal structure as retained austenite. The area fraction of this retained austenite is then calculated to obtain the area fraction of retained austenite. Next, regions with a bcc crystal structure are identified as bainite, ferrite, pearlite, and martensite (fresh martensite and tempered martensite). For these regions, using the "Grain Orientation Spread MAP" function included in the aforementioned "OIM Analysis®," under the condition that boundaries with a crystal orientation difference of 15° or more are considered grain boundaries (15° grain boundaries), regions with a "Grain Orientation Spread" of 1° or less are extracted as ferrite. The area fraction of the extracted ferrite is then calculated to obtain the area fraction of ferrite. Next, the "Highlight" function included in the aforementioned "OIM Analysis (registered trademark)" is used to separate the ferrite region (the region where the "Grain Orientation Spread" is 1° or less) from the remaining region (the region where the "Grain Orientation Spread" is greater than 1°). Subsequently, for the ferrite region (the region where the "Grain Orientation Spread" is 1° or less), the "Grain Average IQ MAP" function included in the aforementioned "OIM Analysis (registered trademark)" is used to determine the maximum value Iα of the "Grain Average IQ" of the ferrite region, under the condition that 15° grain boundaries are considered crystal grain boundaries. Next, for the remaining region (the region where the "Grain Orientation Spread" is greater than 1°), the "Grain Average IQ MAP" function installed in the aforementioned "OIM Analysis®" is used to extract regions greater than Iα / 2 as "bainite and pearlite" and regions less than or equal to Iα / 2 as "martensite (fresh martensite and tempered martensite)" under the condition that 15° grain boundaries are considered crystal grain boundaries. The area percentages of the extracted "bainite and pearlite" and "martensite (fresh martensite and tempered martensite)" are calculated to obtain the area percentages of "bainite and pearlite" and "martensite (fresh martensite and tempered martensite)," respectively. The area ratio for each organization can be obtained from the "Phase MAP," "Grain Orientation Spread MAP," and "Grain Average IQ MAP" functions included in the aforementioned "OIM Analysis (registered trademark)." If the ferrite area ratio is 0%, a separate sample is taken from the hot-stamped molded body being measured, heated to a temperature of Ac3-70°C, held in this temperature range for 10 minutes, and then heat-treated by quenching from this temperature range to room temperature at an average cooling rate of 100°C / s or more to generate ferrite. The Iα of the ferrite in that sample is determined using the procedure described above. Using the determined Iα of the ferrite, the area ratios of bainite, pearlite, and martensite in the hot-stamped molded body being measured are determined using the procedure described above. To observe the same region as the EBSD measurement area using a scanning electron microscope (SEM), a Vickers indentation is imprinted near the observation site. Subsequently, surface contaminants are polished off the area including the Vickers indentation, and then nital etching is performed. Next, the same field of view as the EBSD observation surface is observed using a thermal field emission scanning electron microscope (FE-SEM: JEOL JSM-7001F) with an acceleration voltage of 15kV and a secondary electron image at a magnification of 3000x. Through observation, the location of the tissue corresponding to the location identified as martensite in the EBSD tissue identification described above is identified.

[0051] If the rolling direction of the hot-stamped molded product is not clear, first, before cutting out a sample so that a cross-section parallel to the rolling direction and thickness direction can be observed, the rolling direction should be determined by the following method. A test piece is taken from any position at least 50 mm away from the edge of the hot-stamped molded body, so that a cross-section parallel to the thickness direction can be observed. After finishing the above cross-section of the taken sample with mirror polishing, it is observed using an optical microscope at magnifications of 100x, 200x, 500x, and 1000x, and the observation result at an appropriate magnification that allows for measurement of the inclusion dimensions is selected according to the dimensions of the inclusion. The observation range is at least 500 μm in width and the entire thickness of the plate, and areas with dark brightness are determined to be inclusions. When observing, multiple fields of view may be used so that at least two or more inclusions can be observed. Next, using the cross-section initially observed using the above method as a reference, the planes parallel to the planes rotated in 5° increments within the range of 0 to 180° around the thickness direction are observed using the above method. For each inclusion in each obtained cross-section, its maximum length is determined as the length of the inclusion, and the length of the inclusion in the direction perpendicular to the direction of its maximum length is determined as the thickness of the inclusion. The average aspect ratio (length / thickness) of multiple inclusions is calculated for each cross-section, and the cross-section with the highest average aspect ratio of the inclusions is identified. The direction parallel to the longitudinal direction of the inclusions in that cross-section is determined to be the rolling direction.

[0052] In the microstructure at a depth of <1 / 4, the number density of iron-based carbides with an equivalent circle diameter greater than 0.5 μm present in the martensite is 0.050 particles / μm 2 Less than > If there are many coarse carbides (iron-based carbides) with an equivalent circular diameter of more than 0.5 μm present in the martensite, these can act as crack initiation points during bending or as crack propagation initiation points. Therefore, in the hot-stamped molded body according to this embodiment, the number density of such coarse iron-based carbides is reduced. Specifically, the number density of iron-based carbides with an equivalent circular diameter of more than 0.5 μm present in the martensite is set to 0.050 particles / μm 2 If the above is true, the impact resistance (bendability and crack propagation resistance) will be significantly reduced, so the number density of iron-based carbides with an equivalent circle diameter greater than 0.5 μm present in the martensite should be 0.050 particles / μm 2 The number density should be less than 0.030 particles / μm². From the perspective of further improving impact resistance (bendability and crack propagation resistance), the number density should be 0.030 particles / μm².2 Less than is preferable, 0.020 pieces / μm 2 Less than is more preferable, 0.010 pieces / μm 2 Less than is even more preferable. The collision resistance properties (flexibility and crack propagation resistance properties) improve as the amount of coarse iron-based carbides decreases. Although the lower limit of the number density is not limited, it is not easy to make the number density 0 pieces / μm 2 so the lower limit may be 0.0001 pieces / μm 2 The number density is more preferably 0.001 pieces / μm 2 or more, even more preferably 0.002 pieces / μm 2 or more, 0.003 pieces / μm 2 or more, 0.005 pieces / μm 2 or more. That is, the number density of iron-based carbides having a circle equivalent diameter exceeding 0.5 μm present in martensite is, for example, 0.0001 pieces / μm 2 or more and less than 0.050 pieces / μm 2 It may also be 0.001 pieces / μm 2 or more and less than 0.050 pieces / μm 2 It may also be 0.001 pieces / μm 2 or more and less than 0.030 pieces / μm 2 It may also be 0.002 pieces / μm 2 or more and less than 0.030 pieces / μm 2 less than, 0.003 pieces / μm 2 or more and less than 0.020 pieces / μm 2 less than, 0.005 pieces / μm 2 or more and less than 0.010 pieces / μm 2 It may also be less than.

[0053] <In the microstructure at the 1 / 4 depth position, the average distance between an iron-based carbide having a circle equivalent diameter exceeding 0.5 μm present in martensite and another iron-based carbide having a circle equivalent diameter exceeding 0.5 μm present in the nearest other martensite is 3.0 μm or more> Even when the above-mentioned coarse iron-based carbides are present, the crack propagation resistance properties are improved by ensuring a certain amount or more of the average carbide distance between the coarse iron-based carbides. Therefore, in the hot-stamped molded article according to this embodiment, the average distance between an iron-based carbide with an equivalent circle diameter greater than 0.5 μm present in the martensite and an iron-based carbide with an equivalent circle diameter greater than 0.5 μm present in the nearest other martensite is set to 3.0 μm or more. In other words, in the hot-stamped molded article according to this embodiment, crack propagation resistance is improved by reducing the presence of coarse iron-based carbides in close proximity. The average distance is preferably 5.0 μm or more, more preferably 8.0 μm or more. The upper limit of the average distance is not limited, but the average distance may be 30.0 μm or less. The average distance is preferably 20.0 μm or less or 15.0 μm or less. In other words, the average distance is 3.0 μm or more, but for example it may be 3.0 to 30.0 μm, 5.0 to 30.0 μm, 5.0 to 20.0 μm, or 5.0 to 15.0 μm, or 8.0 to 30.0 μm.

[0054] The identification of iron-based carbides, as well as the equivalent circle diameter of iron-based carbides, the number density of iron-based carbides with an equivalent circle diameter greater than 0.5 μm, and the measurement of the average distance between an iron-based carbide with an equivalent circle diameter greater than 0.5 μm and the nearest other iron-based carbide with an equivalent circle diameter greater than 0.5 μm can be performed by the following method. For the identification of iron-based carbides, the metallographic structure is identified using the FE-SEM described above, and the same sample with an indentation is observed using a scanning transmission electron microscope (STEM: JEOL JEM-2100). In the field of view where the metallographic structure was observed, precipitates are extracted using the extraction replica method in a range of 200 μm in length in the rolling direction and from 1 / 8 of the thickness from the surface to 3 / 8 of the thickness, with the center at 1 / 4 of the thickness from the surface. For precipitate extraction, carbon is deposited onto the observation surface where the metallographic structure was observed with the FE-SEM as described above, and the sample is immersed in a stripping solution to dissolve only the base material. The floating replica film is then washed and collected on a grid. For the areas identified as martensite in the metal structure (excluding the grid area), extracted precipitates are observed at 5000x magnification using a scanning transmission electron microscope (STEM), and point analysis is performed using energy-dispersive X-ray spectroscopy (EDX: JEOL JED-2300T) attached to the microscope. Point analysis is performed at the centroid of the observed precipitate (the centroid determined from the observed image (planar shape)). Quantitative analysis of the precipitates by EDX is performed for all alloying elements mentioned above, excluding C, N, B, O, P, and S, in addition to Fe, and precipitates with 70% or more iron (Fe) are determined to be iron-based carbides. For observation of precipitates by STEM, the acceleration voltage is 200kV, and for point analysis of precipitates by EDX, the irradiation current is 2.56nA, and measurements are taken for 60 seconds at each point. Observation is performed at 200μm. 2 Perform the above / 5 or more field tests on each field of view. The equivalent circular diameter of the precipitate determined to be an iron-based carbide is obtained. In this process, the longest length of the iron-based carbide is defined as the major axis and the shortest length as the minor axis. From the major and minor axes of the iron-based carbide, (major axis × minor axis) 0.5 The value calculated as such is defined as the equivalent diameter of a circle. The maximum length here is defined as the maximum distance between two parallel lines that are tangent to the outer circumference of an iron-based carbide when the iron-based carbide is sandwiched between the two parallel lines. The minimum length is defined as the minimum distance between the two parallel lines that are tangent to the outer circumference of an iron-based carbide when the iron-based carbide is sandwiched between the two parallel lines as described above. Among these, carbides with an equivalent circle diameter exceeding 0.5 μm are extracted, and the number density in each field of view is calculated by dividing the number of such carbides by the area in each field of view. The average of these number densities is defined as the number density of iron-based carbides with an equivalent circle diameter exceeding 0.5 μm in the hot-stamped molded body according to this embodiment. Furthermore, the distance between iron-based carbides with an equivalent circle diameter exceeding 0.5 μm is measured, and the distance between the closest carbides is determined in each field of view. Specifically, iron-based carbides exceeding 0.5 μm are extracted, and their centroid coordinates are calculated. Next, one iron-based carbide is arbitrarily selected, and a particle search is performed radially around the centroid coordinate of that iron-based carbide. In this case, the step angle for the radial search is set to 1 degree or less. For all carbides detected from the step angle, the distance at which the distance between the outer edges of the carbides is shortest is determined and used as the measured value. This process is performed for all detected carbides to determine the distance between the closest carbides. The average value of the distance between the closest carbides measured in each field of view using the method described above is used as the average distance between the closest other iron-based carbides.

[0055] <In the microstructure at a 1 / 4 depth, the prior austenite grain size is 20.0 μm or less.> When the prior austenite grains become coarser, the impact resistance properties (bendability and crack propagation resistance) decrease. Therefore, in the hot-stamped molded article according to this embodiment, the prior austenite grain size is set to 20.0 μm or less. From the viewpoint of further improving impact resistance properties (bendability and crack propagation resistance), the prior austenite grain size is preferably 15.0 μm or less, and more preferably 13.0 μm or less, 12.0 μm or less, 11.0 μm or less, and 10.0 μm or less. On the other hand, there is no particular limit to the lower limit of the prior austenite grain size, but in order to improve the hardenability of the hot-stamped molded article in the hot-stamping process and obtain a predetermined martensite fraction, the prior austenite grain size is preferably 2.0 μm or more, and more preferably 3.0 μm or more. In other words, the prior austenite grain size is 20.0 μm or less, but may be, for example, 2.0 to 20.0 μm, 2.0 to 15.0 μm, 3.0 to 15.0 μm, 3.0 to 13.0 μm, 3.0 to 12.0 μm, 3.0 to 11.0 μm, or 3.0 to 10.0 μm.

[0056] The prior austenite grain size (prior γ grain size) can be determined by the following method. Cut a sample from any position at least 50 mm away from the edge of the hot-stamped molded body (if it is not possible to take a sample from this position, take a sample from a position that avoids the edge) so that a cross-section parallel to the rolling direction and thickness direction can be observed. Depending on the measuring device, the sample size should be such that approximately 10 mm can be observed in the rolling direction. The cross-section of the sample described above, which will serve as the observation surface, is polished using silicon carbide sandpaper ranging from #600 to #1500. Then, a mirror finish is achieved using a liquid containing diamond powder with a particle size of 1-6 μm dispersed in a diluent such as alcohol or pure water. Next, the observation surface is finished by electropolishing. At any position along the longitudinal direction of the sample cross-section, a range of 200 μm in length, centered at a point 1 / 4 of the thickness from the surface, from 1 / 8 of the thickness to 3 / 8 of the thickness from the surface, is measured at 0.1 μm intervals using electron backscatter diffraction to obtain crystal orientation information. For the measurement, an EBSD analyzer consisting of a thermal field emission scanning electron microscope and an EBSD detector is suitable; specifically, an EBSD analyzer consisting of a JEOL JSM-7001F and an AMETEK Hikari detector is appropriate. In this case, the vacuum level inside the EBSD analyzer should be 9.6 × 10⁻⁶. -5The parameters shall be Pa or less, the acceleration voltage 20kV, the working distance (WD) 15mm, and the irradiation current level 18. When acquiring EBSD patterns, the camera settings in the "OIM Data Collection" function of the software attached to the EBSD analyzer shall be set to EXPOSURE TIME 3.65 and Gain 0.39. In addition, when detecting bands in the EBSD pattern, the Hough transform shall be set to Max Peak Count 9 and Min Peak Count 5. For analysis, Iron(Alpha), a database for bcc crystal structures, and Iron(Gamma), a database for fcc crystal structures, shall be selected. If the crystal structure of the tissue to be analyzed is a bct structure, it shall be analyzed as a bcc structure. Using the obtained crystal orientation information, the crystal orientation of the prior austenite grains is calculated from the crystal orientation relationship between the prior austenite grains and the transformed grains with a BCC structure, and this is used to calculate the average grain size of the prior austenite grains. The crystal orientation of the prior austenite grains is calculated using the following method. First, a crystal orientation map of the prior austenite grains is created using the method described in Nippon Steel & Sumitomo Metal Technical Report No. 404 (2016), pp. 24-30. The orientation relationship between ferrite (bcc structure) and austenite (fcc structure) is limited to a tolerance of 3 degrees from the KS relationship, and the tolerance for the orientation difference between austenites that are judged to be common austenite when reconstructing the austenite structure is limited to 5 degrees. The analysis is performed under these conditions to reconstruct the austenite structure before the phase transformation. The grain boundaries of the reconstructed austenite are determined to be those where the orientation difference between adjacent crystal grains is 15 degrees or more. In addition, prior austenite grains that are not entirely included in the field of view, such as those at the edges of the field of view, are excluded, and the average grain size of the prior austenite grains is obtained by analyzing the "Area Fraction" in the "Grain Size (diameter)" chart within the "OIM Analysis (registered trademark)" mentioned above.

[0057] Preferably, the decarburization index Dc is 0.085 or higher. In addition to the microstructure control described above, the presence of a decarburized layer within a certain range from the surface of the hot-stamped molded product enables further improvement in impact resistance (flexibility and crack propagation resistance). Conventionally, the parameters evaluated for decarburized layers include the thickness of the decarburized layer and the hardness distribution within the decarburized layer. However, according to the inventors' research, while these parameters correlate with impact resistance (flexibility and crack propagation resistance), controlling them alone does not necessarily improve impact resistance. For example, the inventors' research revealed that even when the thickness (decarburization depth) of the decarburized layer is the same, if the hardness distribution in the surface region is steep (the degree of hardness change is large), the improvement in flexibility is smaller compared to when the distribution is gentle, and the correlation with the thickness (decarburization depth) of the decarburized layer becomes weaker. Therefore, in the hot-stamped molded article according to this embodiment, a decarburization index is used as a new indicator, and by controlling it, the impact resistance characteristics are improved compared to conventional methods. This index takes into account not only the thickness of the decarburized layer (decarburization depth) but also the hardness information of the surface region, so it has a high correlation with impact resistance performance. Specifically, the decarburization index Dc is set to 0.085 or higher. This decarburization index quantifies the amount of carbon reduction up to 200 μm from the surface of the hot-stamped molded article. From the viewpoint of ensuring flexibility, the decarburization index Dc is preferably 0.100 or higher, and more preferably 0.120 or higher, 0.130 or higher, 0.140 or higher, or 0.150 or higher. The upper limit for the decarburization index Dc is 1.000, based on the calculation method. From the viewpoint of ensuring tensile strength, the decarburization index Dc is preferably 0.800 or less. More preferably, the decarburization index Dc is 0.500 or less, and even more preferably 0.200 or less. That is, Dc is 0.085 or greater, and may be 0.085 or greater and 0.800 or less, 0.085 or greater and 0.800 or less, 0.100 or greater and 0.800 or less, 0.085 or greater and 0.500 or less, 0.085 or greater and 0.200 or less, 0.100 or greater and 0.500 or less, 0.120 or greater and 0.500 or less, 0.130 or greater and 0.500 or less, 0.140 or greater and 0.200 or less, 0.150 or greater and 0.500 or less, or 0.150 or greater and 0.200 or less.

[0058] The decarburization index Dc can be determined by the following method. The elemental concentration distribution in the thickness direction of a hot-stamped molded body will be measured using a Glow Discharge Optical Emission Spectrometry (GD-OES) analyzer. The measurement range will be from the surface of the hot-stamped molded body to a position 200 μm from the surface (200 μm depth), and the measurement interval will be 0.02 μm or less. Measurements will be performed for all elements contained in the hot-stamped molded body.

[0059] When a hot-stamped molded body has a coating on its surface, the surface referred to here is the interface between the coating and the base material. In cases where the surface has a coating, the coating is partially or completely removed by mechanical polishing or chemical polishing so that measurements can be taken up to a depth of 200 μm from the surface of the base material (the interface between the base material and the coating) before being subjected to GD-OES measurement. In GD-OES measurement, the region where the Fe concentration (Fe content) is 90% by mass or more is considered the base material, and the measurement point where the Fe concentration first reaches 90% by mass or more from the surface is considered the surface of the base material. Next, the average value of the measured C concentration (C content) from 180 μm (180 μm depth) to 200 μm depth from the surface of the hot-stamped molded body (more than 1000 points) is calculated, and this average value is considered to be the C concentration in the part unaffected by decarburization. However, if measurements can be taken to a depth where it can be determined that the carbon content has reached the same level as the average carbon content of the base material (the portion unaffected by decarburization), the measurement range may be extended to the surface beyond the 200 μm depth (however, measurements must be taken from 50 μm or more from the surface). In that case, if the absolute difference between the average carbon concentration in the region from the deepest point to 20 μm on the surface and the maximum carbon concentration measurement in the region 20 μm on the surface is 0.05 mass% or less, and the absolute difference between the average carbon concentration in the region 20 μm on the surface and the minimum carbon concentration measurement in the region 20 μm on the surface is 0.05 mass% or less, then the average carbon concentration in the region 20 μm on the surface may be considered as the carbon concentration in the portion unaffected by decarburization. If the deepest point is 120 μm, then "the measured C concentration in the region from the deepest point to 20 μm from the surface" means the C concentration contained within the area from 100 μm to 120 μm.

[0060] In the region from the surface of the hot-stamped molded body to the C concentration at a position where decarburization does not occur, the decrease in C concentration per unit depth (the value obtained by subtracting the C concentration at each measurement point from the C concentration at the position where decarburization does not occur) is calculated, and the integral of the product of the unit depth and the decrease in C concentration is taken as the area of ​​the C-deficient region (Area A). Here, unit depth refers to the measurement interval of GD-OES. Next, the product of the C concentration at the position where decarburization does not occur and 200 (μm) is taken as the reference area (Area B), and the value obtained by dividing the C-deficient area (Area A) by the reference area (Area B) (Area A / Area B) is taken as the decarburization index Dc.

[0061] Furthermore, in this embodiment, the decarburization depth is defined as the distance from the surface to the first location where, based on the GDS analysis, the carbon content is determined to be equivalent to the average carbon content of the base material (the carbon concentration at a location determined to be unaffected by decarburization) (however, the analysis is performed on a range of 50 μm or more from the surface). In the hot-stamped molded article according to this embodiment, from the viewpoint of productivity, the decarburization depth is preferably 180 μm or less. More preferably, it is 150 μm or less. Alternatively, it may be less than 1 / 8 the thickness of the flat portion of the molded article. The decarburization depth is preferably 10 μm or more.

[0062] Preferably, the microstructure of the surface layer contains more than 5.0% ferrite by area. By incorporating a certain amount of softer tissue into the surface layer of the molded body than the interior, and by appropriately controlling the structure of that surface layer, it is possible to further improve impact resistance (bendability and crack propagation resistance). For this reason, it is preferable that the area ratio of ferrite in the microstructure of the surface layer exceeds 5.0%. The area ratio of ferrite is preferably 10.0% or more, and more preferably 20.0% or more. In the microstructure of the surface layer, the remainder other than ferrite consists of more than 5.0% martensite (fresh martensite, tempered martensite) and / or bainite, and less than 5.0% retained austenite and pearlite.

[0063] The microstructure of the surface layer should be observed in the same way as at the 1 / 4 depth position, and the area ratio of each layer should be measured. However, the measurement location should not be at the 1 / 4 depth position, but rather within a range of 50 μm from the surface in the thickness direction.

[0064] [covering] The hot-stamped molded body according to this embodiment may have a coating on part or all of its surface. The coating may be primarily composed of an Fe-Al alloy (Fe-Al coating) or primarily composed of an Fe-Zn alloy (Fe-Zn coating). The coating is also called a film, alloyed plating layer, or intermetallic compound layer. A coating mainly composed of Fe-Al alloys is a coating containing a total of 70% by mass or more of Fe and Al, and a coating mainly composed of Fe-Zn alloys is a coating containing a total of 70% by mass or more of Fe and Zn. In addition to Fe and Al, a coating mainly composed of Fe-Al alloys may also contain Si, Mg, Ca, Sr, Ni, Cu, Mo, Mn, Cr, C, Nb, Ti, B, V, Sn, W, Sb, Zn, Co, In, Bi, Zr, Se, As, and REM, with the remainder being impurities. In addition to Fe and Zn, a coating mainly composed of Fe-Zn alloys may also contain Si, Mg, Ca, Sr, Ni, Cu, Mo, Mn, Cr, C, Nb, Ti, B, V, Sn, W, Sb, Al, Co, In, Bi, Zr, Se, As, and REM, with the remainder being impurities. The coating provides corrosion resistance, resulting in improved hydrogen embrittlement resistance in automotive applications. The thickness of the coating is preferably 10 to 100 μm.

[0065] Such coatings can be formed, for example, if the coating is mainly Fe-Al alloy, by performing heat treatment such as hot stamping on a steel sheet that has an Al-based coating (Al-type coating). Similarly, if the coating is mainly Fe-Zn alloy, it can be formed by performing heat treatment such as hot stamping on a steel sheet that has a Zn-based coating (Zn-type coating). An Al-based coating is a coating containing 70% by mass or more of Al, and a Zn-based coating is a coating containing 70% by mass or more of Zn. An Al-based coating may also contain Si, Mg, Ca, Sr, Ni, Cu, Mo, Mn, Cr, C, Nb, Ti, B, V, Sn, W, Sb, Zn, Co, In, Bi, Zr, Se, As, and REM, with the remainder being impurities. A Zn-based coating may also contain Si, Mg, Ca, Sr, Ni, Cu, Mo, Mn, Cr, C, Nb, Ti, B, V, Sn, W, Sb, Al, Co, In, Bi, Zr, Se, As, and REM, with the remainder being impurities.

[0066] The chemical composition and thickness of the coating can be determined by line analysis of the cross-section using a field emission electron beam microanalyzer (FE-EPMA) (qualitative and quantitative analysis as shown below). Specifically, a sample is cut from any position at least 50 mm away from the edge of the hot-stamped molded body (or a position avoiding the edge if a sample cannot be taken from this position) so that a cross-section parallel to the rolling direction and thickness direction can be observed. The size of the sample should be such that approximately 10 mm can be observed in the rolling direction, although this depends on the measuring device. Next, the cross-section (observation surface) parallel to the rolling direction and thickness direction is polished with silicon carbide sandpaper from #600 to #1500, and then finished to a mirror surface using a liquid in which diamond powder with a particle size of 1 to 6 μm is dispersed in a diluent such as alcohol or pure water. Line analysis is performed on the sample using FE-EPMA. The observation range of the FE-EPMA (JXA-8530F, manufactured by JEOL) is 500x magnification, and the content of each element is quantitatively analyzed by line analysis. FE-EPMA uses an acceleration voltage of 15kV and a beam diameter of 100nm to perform line analysis (qualitative analysis) across the entire wavelength range using a high-speed scanning method to detect the elements present. Next, with the same acceleration voltage and beam diameter, an irradiation time of 1000ms per point, and a measurement pitch of 60nm, line analysis (quantitative analysis) is performed using the ZAF method for Fe, Al, Zn, Si, and other elements detected in the qualitative analysis described above, over a length of 200μm in the depth direction from the surface of the coating to the plate thickness, and the quantitative values ​​of each element are determined. The region where the Fe concentration is 90 mass% or more is defined as the base material, and the region where the Fe concentration is less than 90 mass% is defined as the coating. The depth distance from the surface of the coating to the position where the Fe concentration first reaches 90 mass% or more is measured and defined as the thickness of the coating. The coating thickness measurement described above is performed five times per field of view, with a distance of 10 μm or more between measurement points. This is done for any 10 fields of view. The average value of the coating thickness obtained from the above 50 measurements is taken as the coating thickness. The chemical composition of the coating is determined during the process of measuring the coating thickness as described above. Specifically, in the quantitative analysis for measuring the coating thickness described above, the average value of each element is calculated for the elements Fe, Al, Zn, and Si analyzed, as well as other elements detected in the qualitative analysis described above, in the range from the "surface of the coating" to the "position where the Fe concentration first falls below 90% by mass," and the chemical composition of the coating is determined. This measurement is performed five times per field of view, with a distance of 10 μm or more between measurement points. This is done for any 10 fields of view. The average value of the chemical composition of the coating per measurement obtained is calculated for a total of 50 measurements and is used as the chemical composition of the coating.

[0067] [Characteristics] (Tensile strength) In the hot-stamped molded article according to this embodiment, considering its contribution to the weight reduction of the automobile body, a tensile strength of 1500 MPa or higher is preferred. Preferably, the tensile strength is 1800 MPa or higher, more preferably 2000 MPa or higher, and even more preferably 2200 MPa or higher. The tensile strength may be 3000 MPa or less in order to ensure resistance to hydrogen embrittlement.

[0068] Tensile strength is obtained by taking a sub-sized plate-shaped test specimen (parallel section length: 32 mm, parallel section width: 6.25 mm) in accordance with ASTM A370:2022 standard from a flat portion of the hot-stamped molded product (for example, the top plate portion or other flat portions of a hat-shaped member) with the material thickness intact (without removing the coating if applicable), with the tensile direction parallel to the rolling direction, and performing a tensile test in accordance with JIS Z 2241:2022 at 20°C with a gauge length of 25 mm and a crosshead displacement rate of 1.0 mm / min. The tensile strength is calculated by dividing the maximum test force by the cross-sectional area obtained by multiplying the material thickness (thickness excluding the measured coating thickness if applicable) and the parallel section width of 6.25 mm.

[0069] (Collision resistance characteristics) As described above, the hot-stamped molded article according to this embodiment exhibits excellent impact resistance (bendability and crack propagation resistance) due to the control of its chemical composition and microstructure. As described later, the bending test is performed by taking a sample with a width of 30 mm and a length of 60 mm in the direction perpendicular to the rolling direction from a flat portion of the hot-stamped molded body (for example, the top portion or other flat portion in the case of a hat-shaped member), with the material thickness of the hot-stamped molded body intact (without removing the coating if there is one), and conducting the test in accordance with the VDA standard VDA238-100:2017, ensuring that the direction of the bending ridge is perpendicular to the rolling direction. The product of the maximum bending angle obtained by the bending test and the tensile strength is preferably 80,000 (MPa·°) or more. More preferably 90,000 (MPa·°) or more. Even more preferably 100,000 (MPa·°) or more. Crack propagation resistance is determined by testing in accordance with JIS Z 2242:2018 and JIS B 7755:2011. A test piece measuring 10 mm in width and 55 mm in length is taken from a flat portion of the hot-stamped molded body (for example, the top plate portion or other flat portions of a hat-shaped member), with the material thickness of the hot-stamped molded body intact (without removing the coating if applicable). The test piece conforms to JIS Z 2242:2018 except for the thickness, including the notch shape described later. The rolling direction is oriented along the length of the test piece. A 2 mm deep V-notch (notch angle: 45°, notch bottom radius: 0.25 mm, notch bottom width: 8 mm, notch position (center): 27.5 mm from the end in the length direction of the test piece) is made in this test piece. Three of these pieces are then stacked and fixed with screws, and an instrumented impact test is performed. However, if the plate thickness is 2.00 mm or less, the test will be conducted with three plates stacked together, but if the plate thickness is greater than 2.00 mm, the test will be conducted with a single plate without stacking. The instrumented impact test is performed at 20°C, and the time from the start to the end of the test and the impact force are measured. Next, the displacement is calculated from the product of the test speed and the measured time of the instrumented impact test. Since the fracture surface length of the Charpy specimen is 8 mm, the average value of the impact force measured in the region where the displacement is 8 mm or more is used as the background. After subtracting the background from the impact force of all measurement points, an impact force-displacement curve is created. Figure 1 shows an example of an impact force-displacement curve obtained from an instrumented impact test. Since the impact force obtained from the instrumented Charpy test contains noise due to natural vibrations, smoothing is performed by applying a 30-point moving average. For the obtained impact force-displacement curve, the area under the curve from a displacement of 0 mm to 8 mm was calculated, and this value was taken as the total impact energy. Next, the impact force at which a sharp decline in the impact force-displacement curve begins (the time of crack initiation in Figure 1) was found, and the corresponding displacement (displacement at crack initiation) was determined. The area under the curve from this displacement of 0 mm to the displacement at crack initiation was calculated and taken as the crack initiation energy. The value obtained by subtracting the crack initiation energy from the total impact energy is taken as the crack propagation energy. The ratio of crack propagation energy to total impact energy (crack propagation energy / (crack initiation energy + crack propagation energy)) is used as an indicator of crack propagation resistance. A sharp decline in the impact force-displacement curve refers to a case where the amount of impact force reduction per unit displacement is 50% or more of the maximum impact force at which the measurement was taken. In terms of crack propagation resistance, it is preferable that the ratio of crack propagation energy to (crack initiation energy + crack propagation energy) is 0.10 or higher. More preferably, it is 0.20 or higher, and even more preferably 0.30 or higher.

[0070] (thickness) In hot-stamped molded articles, the thickness of the flat portion (the thickness of the steel plate used as the material (excluding the coating)) is preferably 0.8 to 3.0 mm from the viewpoint of hardenability. The thickness may also be 1.2 to 3.0 mm, 1.4 to 3.0 mm, or 1.5 to 3.0 mm.

[0071] <Manufacturing method> The hot-stamped molded article according to this embodiment can be manufactured by a manufacturing method that includes the following steps. (I) A preheat treatment step in which a steel plate having a predetermined chemical composition is heated to a temperature range of over 950°C and up to 1200°C such that the average heating rate is 2°C / s or more, held in the said temperature range for 1 second to 1200 seconds, and then cooled after holding so that the average cooling rate from the said temperature range to a cooling stop temperature of 100°C or less is less than 15°C / s, and the average cooling rate between 700°C and 500°C is less than 10°C, (II) A hot stamping process in which the steel sheet after the preheat treatment process is heated to a temperature range of Ac3 to 1100°C at an average heating rate of 2°C / s or more and less than 50°C / s, held in this temperature range for 10 seconds or more and 600 seconds or less, and after holding, forming is started in a temperature range of 650°C or higher, and hot stamping (simultaneous cooling in the mold) is performed so that the average cooling rate to 250°C is 10°C / s or more. Furthermore, when forming a coating on the surface of a hot-stamped molded body, a coating step may be included between the preheat treatment step and the hot stamping step. The following describes the preferred conditions for each process. For conditions not described, publicly known conditions may be applied. Here, Ac3 can be calculated using the content of each element with the following formula. Ac3(°C) = 910 - 203 × C 0.5 +66×Si-25×Mn+700×P-11×Cr+109×Al+400×Ti-15.2×Ni+104×V+31.5×Mo···(1) In formula (1) above, the element symbols indicate the mass percentage content of each element, and 0 is substituted if the element is not present.

[0072] <Preheat treatment process> In the preheat treatment process, a steel plate having a predetermined chemical composition (including cases where the surface is coated) is heated to a preheat treatment temperature in the temperature range of over 950°C and up to 1200°C, such that the average heating rate is 2°C / s or more. This temperature range is then maintained for 1 second to 1200 seconds. After maintenance, the plate is cooled so that the average cooling rate from this temperature range (specifically, the temperature at the end of maintenance) to a cooling stop temperature of 100°C or less is less than 15°C / s, and the average cooling rate between 700°C and 500°C is less than 10°C / s. In this process, the iron-based carbides, which have been melted, are re-precipitationd during cooling, and the iron-based carbides are grown by Ostwald growth, thereby increasing the distance between the iron-based carbides. Although some iron-based carbides generated in the preheat treatment process may remain undissolved in the subsequent hot stamping process, the average distance between carbides is maintained above a predetermined level, thus improving crack propagation resistance. Preheat treatment is performed before the hot stamping process, but it differs from annealing performed on cold-rolled steel sheets in the following respects. Conventional annealing of cold-rolled steel sheets is based on the idea of ​​homogenizing the metal structure of the base material and heating at a low temperature within a range that does not coarseize the prior austenite grain size, thereby suppressing the coarsening of iron-based carbides and ensuring fine dispersion, thereby securing ductility, flange properties, and elongation. In contrast, the preheat treatment in this embodiment differs in that it is based on the idea of ​​remelting the iron-based carbides formed in the cold-rolling annealing stage by heating at a high temperature, and then controlling the distance between carbides by coarsening them through Ostwald growth during the cooling process, thereby controlling the crack propagation resistance after hot stamping.

[0073] If the average heating rate to the preheat treatment temperature is less than 2°C / s, the prior austenite grains become coarser, and the impact resistance properties (flexibility and crack propagation resistance) decrease. Although there is no upper limit specified for the average heating rate, productivity decreases from the standpoint of fuel consumption, so the average heating rate is preferably 50°C / s or less, and more preferably 10°C / s or less. Furthermore, if the preheat treatment temperature (holding temperature range) is below 950°C, the iron-based carbides in the steel sheet will not dissolve, and after the hot stamping process, the number density of circles with an equivalent diameter of more than 0.5 μm and the average distance between iron-based carbides will not be within the desired range. In this case, the impact resistance properties (bendability and crack propagation resistance properties) will decrease. On the other hand, if the preheat treatment temperature exceeds 1200°C, the prior austenite grains become coarser. In this case, the impact resistance properties (bendability and crack propagation resistance) decrease. If the holding time in the above temperature range is less than 1 second, the iron-based carbides in the steel sheet remain undissolved, and after the hot stamping process, the number density and average distance of iron-based carbides with an equivalent circle diameter greater than 0.5 μm will not be within the desired range. On the other hand, if the holding time exceeds 1200 seconds, the prior austenite grains become coarser. In this case, the impact resistance properties (bendability and crack propagation resistance) decrease.

[0074] If the average cooling rate from the holding temperature to the cooling stop temperature below 100°C is 15°C / s or higher, the Ostwald growth of the iron-based carbides generated during cooling is insufficient, and the average distance of iron-based carbides with an equivalent circle diameter of more than 0.5 μm after the hot stamping process does not fall within the desired range. There is no specific lower limit for the average cooling rate from the holding temperature to the cooling stop temperature below 100°C, but from the viewpoint of productivity, an average cooling rate of 2°C / s or higher, and more preferably 5°C / s or higher, is preferred. Ostwald growth is particularly affected by the temperature range of 700-500°C. Therefore, even if the average cooling rate from the holding temperature to the cooling stop temperature of 100°C or less is less than 15°C / s, if the average cooling rate between 700-500°C is 10°C / s or higher, the Ostwald growth of the iron-based carbides generated during cooling will not be sufficient, and the average distance of iron-based carbides with an equivalent circle diameter of more than 0.5 μm after the hot stamping process will not be within the desired range. Therefore, it is necessary to satisfy both cooling rates. There is no specific lower limit for the average cooling rate between 700-500°C, but from the viewpoint of productivity, an average cooling rate of 1°C / s or higher is preferable, and 4°C / s or higher is more preferable. Furthermore, if hot stamping is performed without cooling to below 100°C, untransformed austenite with concentrated carbon remains. During the heating process of hot stamping, this untransformed austenite decomposes into iron-based carbides, potentially resulting in an excessive number density of iron-based carbides with an equivalent circle diameter exceeding 0.5 μm. Therefore, cooling should be performed to below 100°C under the conditions described above.

[0075] Since the chemical composition does not change during the preheat treatment process and the subsequent hot stamping process, the chemical composition of the steel sheet subjected to the preheat treatment process should be the same as the chemical composition of the hot stamped molded product to be ultimately obtained. Furthermore, the steel sheet can be hot-rolled, cold-rolled, or any other type, and is not limited to these. The manufacturing method is also not limited, but it may be manufactured under conditions such as those described later. The microstructure of the steel sheet subjected to the preheat treatment process is not limited, but from the standpoint of workability, a structure consisting of ferrite and pearlite is preferred. The remaining structure may include bainite and retained austenite.

[0076] <Coating process> When forming a coating on the surface, the coating process may be included after the preheat treatment process and before the hot stamping process. In the coating process, a coating is formed on the surface of the steel sheet to form a coated steel sheet. The coating method is not particularly limited, and various methods are possible, including hot-dip galvanizing, electroplating, vacuum deposition, cladding, and thermal spraying. Hot-dip galvanizing is the most widely used method industrially. Examples of coatings include aluminum-based coatings containing aluminum and zinc-based coatings containing zinc. If an Al-based coating is applied, the coating becomes an Fe-Al-based coating in the subsequent hot stamping process. Similarly, if a Zn-based coating is applied, the coating becomes an Fe-Zn-based coating in the subsequent hot stamping process.

[0077] When forming an Al-based coating by hot-dip plating, the plating bath often contains Fe as an impurity in addition to Al. Furthermore, as long as the Al content is 70% by mass or more, the plating bath may also contain Si, Mg, Ca, Sr, Ni, Cu, Mo, Mn, Cr, C, Nb, Ti, B, V, Sn, W, Sb, Zn, Co, In, Bi, Zr, Se, As, and mischmetal in addition to the elements mentioned above. When performing hot-dip plating, the steel sheet may be heated after the preheat treatment process and hot-dip plating may be performed at a temperature close to the plating bath temperature (for example, 650-750°C for Al-based plating, and 400-500°C for Zn-based plating).

[0078] There are no particular limitations on the pre-treatment or post-treatment of the coating; pre-coating, solvent application, alloying treatment, etc., are possible. For example, for alloying treatment, heat treatment can be performed at 450-600°C for Zn-based plating and 650-750°C for Al-based plating.

[0079] <Hot stamping process> In the hot stamping process, the steel sheet, after the preheating or coating process, is heated to a hot stamping heating temperature in the range of Ac3 (°C) to 1100°C at an average heating rate of 2°C / s or more and less than 50°C / s. This temperature range is then held for 10 seconds or more and less than 600 seconds. After holding, forming is started at a temperature of 650°C or higher, and hot stamping (simultaneous cooling in the mold) is performed so that the average cooling rate down to 250°C is 10°C / s or more. This process primarily involves controlling the prior austenite grain size and the area ratio of each phase within the microstructure.

[0080] If the average heating rate to the hot stamping heating temperature is less than 2°C / s, the prior austenite grains become coarser. On the other hand, if the average heating rate is 50°C / s or higher, the amount of undissolved iron-based carbides with an equivalent circle diameter greater than 0.5 μm increases, making it impossible to maintain the number density and average distance of iron-based carbides with an equivalent circle diameter greater than 0.5 μm within the specified range. Furthermore, if the hot stamping heating temperature (and the subsequent holding temperature range) is below Ac3 (°C), the ferrite area ratio increases in the microstructure at 1 / 4 depth of the hot stamped molded body, and the predetermined martensite area ratio cannot be obtained. In addition, the amount of undissolved iron-based carbides increases, and the number density of iron-based carbides with an equivalent circle diameter greater than 0.5 μm and the average distance between iron-based carbides with an equivalent circle diameter greater than 0.5 μm do not fall within the desired range. If the heating temperature exceeds 1100°C, the prior austenite grains become coarser, and the impact resistance properties (bendability and crack propagation resistance properties) decrease. Furthermore, from the viewpoint of suppressing the decrease in impact resistance properties (bendability and crack propagation resistance properties), the heating temperature is preferably 1000°C or lower. More preferably 950°C or lower. Furthermore, if the holding time is less than 10 seconds, an excessive amount of undissolved iron-based carbides with an equivalent diameter of more than 0.5 μm will be produced, making it impossible to maintain the number density and average distance of iron-based carbides with an equivalent diameter of more than 0.5 μm within the specified range. On the other hand, if the holding time exceeds 600 seconds, the prior austenite grains become coarser.

[0081] If the starting temperature for hot stamping is below 650°C, the amount of ferrite increases, and the hot stamped molded article cannot meet the predetermined martensite area ratio. If the average cooling rate up to 250°C is less than 10°C / s, at least one of ferrite, pearlite, and bainite increases during cooling, and the hot-stamped molded article cannot meet the predetermined martensite area ratio.

[0082] [Preferred manufacturing method for steel sheets to be subjected to preheat treatment process] The steel sheet to be subjected to the preheat treatment process is obtained by manufacturing conditions that include, for example, the following "heating process," "hot rolling process," and "winding process," and further include a "cold rolling process" and / or "annealing process" as needed. Furthermore, when obtaining a hot-stamped molded body with a coating, a "coating formation process" may be performed to form a coating on the steel sheet to be subjected to the preheat treatment process.

[0083] "Heating process" In the heating process, steel materials such as slabs having a predetermined chemical composition are heated before being subjected to hot rolling. Preferably, the heating temperature is 1100°C or higher, and the holding time in this temperature range is 20 minutes or more. After this holding time, hot rolling is performed.

[0084] From the viewpoint of rolling load in the hot rolling process, it is preferable that the heating temperature be 1100°C or higher and the holding time be 20 minutes or longer. More preferably, the heating temperature be 1200°C or higher and the holding time be 25 minutes or longer. The heating temperature is preferably 1350°C or lower and the holding time is preferably 120 minutes or less.

[0085] "Hot rolling process" The hot rolling process typically includes rough rolling, finish rolling, and winding. Of these, finish rolling is preferably performed in a temperature range of 800°C or higher, considering the sheet shape. More preferably, it is 830°C or higher. The finish rolling temperature is preferably 1050°C or lower.

[0086] After finish rolling, the steel sheet is wound into a coil. This yields a hot-rolled steel sheet. The winding temperature is preferably 750°C or lower to promote scale removal in the subsequent pickling process. Furthermore, from the standpoint of the rolling load in the cold rolling process, the winding temperature is preferably 600°C or higher.

[0087] [Cold rolling process] The steel sheet used in the preheat treatment process may be a hot-rolled steel sheet after the hot-rolling process, or it may be a cold-rolled steel sheet obtained by cold-rolling a hot-rolled steel sheet. When producing cold-rolled steel sheets, cold rolling should be carried out using a normal cumulative reduction ratio, for example, a cumulative reduction ratio of 30-90%. When cold rolling is performed, a reheating treatment (hot roll annealing) may be carried out on the hot-rolled steel sheet before cold rolling to soften it.

[0088] [Annealing process] Cold-rolled steel sheets may be annealed after cold rolling in order to homogenize the steel structure. Annealing conditions can be exemplified by heating to a temperature range of 750-900°C (annealing temperature), holding at this temperature range for 10-600 seconds, and then cooling to 500°C or below at an average cooling rate of 5°C / s or higher. From the viewpoint of promoting recrystallization of the steel structure and homogenizing the steel structure, an annealing temperature of 750°C or higher and a holding time of 10 seconds or more in this temperature range are preferable. From the viewpoint of productivity, an annealing temperature of 900°C or lower and a holding time of 600 seconds or less in this temperature range are preferable. On the other hand, when the decarburization index Dc of a hot-stamped molded product is to be 0.085 or higher, it is preferable to decarburize the surface region (a certain range from the surface) of the steel sheet by controlling the annealing atmosphere in the annealing process. The annealing atmosphere is preferably an H2O-containing atmosphere containing 2 to 20 volume% hydrogen and the remainder being impurities such as nitrogen and oxygen, with a dew point of -10°C to 20°C. To increase the decarburization index by increasing the oxygen potential in the atmosphere and promoting decarburization, the hydrogen concentration is often 2 volume% or higher and the dew point is -10°C or higher, preferably 3 volume% or higher and the dew point is 0°C or higher, and more preferably 4 volume% or higher and the dew point is 5°C or higher. On the other hand, in order to suppress condensation on the equipment and not hinder productivity, the hydrogen concentration is often 20 volume% or less and the dew point is 20°C or lower, preferably 15 volume% or less and the dew point is 15°C or lower, and more preferably 10 volume% or less and the dew point is 10°C or lower. Furthermore, to promote the decarburization reaction and increase the decarburization index, the annealing temperature is preferably 780°C or higher, more preferably 790°C or higher. Also, from the viewpoint of productivity, the annealing temperature is preferably 890°C or lower, and more preferably 880°C or lower. To promote the decarburization reaction and increase the decarburization index, the holding time in this temperature range is preferably 20 seconds or more, and more preferably 60 seconds or more. From the viewpoint of productivity, the holding time in this temperature range is preferably 590 seconds or less. Furthermore, if the ferrite area ratio of the surface layer of the hot-stamped molded article exceeds 5.0%, in addition to the above annealing conditions, it is preferable to control the annealing temperature to 820°C or higher and the holding time in this temperature range to 90 seconds or more. [Examples]

[0089] Slabs (steel grades 1 to 61) having the chemical compositions shown in Tables 1-1 to 1-4 were prepared. (The "Total Ta, etc." column in the tables indicates the total content of one or more elements selected from Ta, Re, Os, Ir, Tc, Pb, Se, Bi, and Sn.) This slab was subjected to heating, hot rolling, cold rolling, and annealing processes under the conditions shown in Tables 2-1 to 2-4, yielding steel plates No. 1 to 113. A "-" in the table indicates that the process was not performed. Furthermore, the average cooling rate in the annealing process in Tables 2-1 to 2-4 represents the average cooling rate from holding to a cooling stop temperature of 500°C or lower.

[0090] The obtained steel sheets were subjected to a preheat treatment process and a hot stamping process under the conditions shown in Tables 2-5 to 2-12 to obtain hot stamped molded bodies. In the hot stamping process, a hat shape was formed using a mold. In addition, some examples were subjected to a coating process before the hot stamping process. In the coating process, a plating layer was formed on the surface by molten zinc plating or molten aluminum plating.

[0091] [Table 1-1]

[0092] [Table 1-2]

[0093] [Table 1-3]

[0094] [Table 1-4]

[0095] [Table 2-1]

[0096] Table 2-2

[0097] Table 2-3

[0098] Table 2-4

[0099] Table 2-5

[0100] Table 2-6

[0101] Table 2-7

[0102] Table 2-8

[0103] Table 2-9

[0104] Table 2-10

[0105] Table 2-11

[0106] [Table 2-12]

[0107] Regarding the obtained hot-stamped molded articles, the microstructure at 1 / 4 depth and the surface layer was observed in the manner described above, and the area percentage of each structure was determined. Although not shown in the table, apart from the ferrite in the surface layer, the total consisted of more than 5.0% martensite and / or bainite, and less than 5.0% retained austenite and / or pearlite. Furthermore, in the microstructure at a 1 / 4 depth position, the number density of iron-based carbides with an equivalent circle diameter greater than 0.5 μm present in the martensite and the average distance between them and the nearest other iron-based carbides with an equivalent circle diameter greater than 0.5 μm were determined. In addition, the decarburization index and decarburization depth were determined. The results are shown in Tables 3-1 to 3-4. In the cases where hot-dip Zn plating or hot-dip Al plating was performed, a coating mainly composed of Fe-Zn alloy or Fe-Al alloy was formed with a thickness of 30 μm.

[0108] [Table 3-1]

[0109] [Table 3-2]

[0110] [Table 3-3]

[0111] [Table 3-4]

[0112] A sample was taken from the top portion of the hat-shaped hot-stamped molded body, and its tensile strength was measured in the following manner. Furthermore, as impact resistance characteristics, bending properties and crack propagation resistance were evaluated. The results are shown in Tables 3-5 to 3-8.

[0113] [Tensile strength] From the top plate portion of the hot-stamped molded body, a sub-sized plate-shaped test specimen (parallel section length: 32 mm, parallel section width: 6.25 mm) conforming to ASTM A370:2022 was taken with the material thickness intact (without removing the coating if present), with the tensile direction parallel to the rolling direction. Tensile strength was obtained by performing a tensile test in accordance with JIS Z 2241:2022 at 20°C with a gauge length of 25.0 mm and a crosshead displacement rate of 1.0 mm / min. The tensile strength was calculated by dividing the maximum test force by the cross-sectional area obtained by multiplying the material thickness (thickness excluding the measured coating thickness if present) and the parallel section width of 6.25 mm. We determined that a material possessed high strength if its tensile strength was 1500 MPa or higher.

[0114] [Bendability] A sample measuring 30 mm in width (perpendicular to rolling direction) and 60 mm in length (perpendicular to rolling direction) was taken from the top plate portion of the hot-stamped molded product, maintaining the original material thickness (without removing any coatings). A bending test was then performed on this sample, ensuring that the direction of the bending ridge was perpendicular to the rolling direction. The bending test was conducted in accordance with the VDA standard VDA238-100:2017, and the maximum bending angle was determined. A material was judged to have excellent bendability if the product of its tensile strength and maximum bending angle was 80,000 (MPa·°) or higher. A material was judged to have even better bendability if the product of its tensile strength and maximum bending angle was 90,000 (MPa·°) or higher, and even better bendability if it was 100,000 (MPa·°) or higher.

[0115] [Crack propagation resistance properties] Crack propagation resistance was determined by testing in accordance with JIS Z 2242:2018 and JIS B 7755:2011. Specifically, a test specimen measuring 10 mm in width and 55 mm in length was taken from the top plate portion of the hot-stamped molded body, maintaining the material thickness of the hot-stamped molded body (without removing the coating if present), with the rolling direction oriented along the length of the specimen. A 2 mm deep V-notch (notch angle: 45°, notch bottom radius: 0.25 mm, notch bottom width: 8 mm, notch position (center): 27.5 mm from the end along the length of the specimen) was made in this specimen, and then three specimens were stacked and fixed with screws, and an instrumented impact test was performed. Here, if the plate thickness was 2.00 mm or less, the test was performed with three stacked specimens, but if the plate thickness was greater than 2.00 mm, the test was performed with a single specimen without stacking. The instrumented impact test was conducted at 20°C, and the time from the start to the end of the test and the impact force were measured. Displacement was calculated from the product of the test speed and the measured time. Since the fracture surface length of the Charpy specimen was 8 mm, the average of the impact forces measured in the region where the displacement was 8 mm or more was used as the background. After subtracting the background from the impact forces of all measurement points, an impact force-displacement curve was created. Since the impact force obtained in the instrumented Charpy test contains noise due to natural vibrations, smoothing was performed by applying a 30-point moving average. Figure 1 shows an example (schematic diagram) of an impact force-displacement curve. For the obtained impact force-displacement curve, the area under the curve from a displacement of 0 mm to 8 mm was calculated, and the obtained value was taken as the total impact energy. Next, using the procedure described above, the impact force at which a sharp decline in the impact force-displacement curve begins was searched, and the corresponding displacement (displacement at crack initiation) was determined. The area under the curve from a displacement of 0 mm to the displacement at crack initiation was calculated and taken as the crack initiation energy. The value obtained by subtracting the crack initiation energy from the total impact energy was taken as the crack propagation energy. The ratio of crack propagation energy to total impact energy was used as an indicator of crack propagation resistance. If this ratio of crack propagation energy to total impact energy (crack propagation energy / total impact energy) is 0.10 or higher, it was judged to have excellent crack propagation resistance. If the ratio of crack propagation energy to total impact energy is 0.20 or higher, it was judged to have even better crack propagation resistance, and if it is 0.30 or higher, it was judged to have even better crack propagation resistance.

[0116] [Table 3-5]

[0117] [Table 3-6]

[0118] [Table 3-7]

[0119] [Table 3-8]

[0120] As can be seen from the results in Tables 1-1 to 3-8, in hot-stamped molded articles with a tensile strength of 1500 MPa or more, excellent impact resistance was obtained in examples (inventive examples) where the chemical composition, the area ratio of the microstructure at the 1 / 4 depth position, the number density of iron-based carbides with an equivalent circle diameter of more than 0.5 μm present in the martensite, the average distance, and the prior austenite grain size were within the range of the present invention.

[0121] In the comparative examples, the tensile strength was either less than 1500 MPa, or even if the tensile strength was 1500 MPa or more, one or more of the following factors were outside the scope of the present invention: chemical composition, area ratio of the microstructure at 1 / 4 depth, number density of iron-based carbides with an equivalent circle diameter of more than 0.5 μm present in the martensite, average distance, and prior austenite grain size. As a result, sufficient impact resistance could not be obtained. [Industrial applicability]

[0122] According to the present invention, it is possible to provide a hot-stamped molded article with high strength and excellent impact resistance. Therefore, it has high potential for industrial application.

Claims

1. In mass percent, C: 0.20-0.70%, Si: 0.010-2.000%, Mn: 0 to 2.00%, P: 0.100% or less, S: 0.0100% or less, N: 0.0100% or less, O: 0.0200% or less, Al: 0.0010-0.5000%, Mo: 0.0010-1.0000%, B: 0.0005-0.0100%, Ti: 0.010 to 0.100%, Nb: 0 to 0.100%, Cr: 0-1.00%, Co: 0-3.00%, Ni: 0-3.00%, Cu: 0 to 1.00%, V: 0-1.000%, W: 0-1.00%, Ca: 0-1.0000%, Mg: 0 to 1.0000%, REM: 0-1.0000%, Sb: 0 to 1.000%, Zr: 0 to 1.000%, As: 0 to 1.000%, One or more selected from Ta, Re, Os, Ir, Tc, Pb, Se, Bi, and Sn: total of 0 to 1,000%, and Remainder: Fe and impurities It has a chemical composition consisting of, When the depth range from 1 / 8 of the thickness to 3 / 8 of the thickness in the thickness direction from the surface is defined as the 1 / 4 depth position, The microstructure at the aforementioned 1 / 4 depth position is, in terms of area ratio, Martensite: 80.0% or more, Residual austenite: 0.0% or more and less than 5.0% Includes, In the microstructure at the 1 / 4 depth position, the number density of iron-based carbides with an equivalent circle diameter greater than 0.5 μm present in the martensite is 0.0001 or more, or 0.050 particles / μm. ２ The average distance between the iron-based carbide and the nearest other iron-based carbide is 3.0 μm or more. In the microstructure at the 1 / 4 depth position, the prior austenite grain size is 20.0 μm or less. A hot-stamped molded body characterized by the following features.

2. The decarburization index Dc is 0.085 or higher. A hot-stamped molded body according to claim 1, characterized in that...

3. When the area from the surface up to 50 μm is defined as the surface layer, the microstructure of the surface layer is, by area ratio, Ferrite: over 5.0%, including, A hot-stamped molded article according to claim 1 or 2, characterized in that...

4. The aforementioned chemical composition is, in mass%, C: more than 0.40%, less than 0.70%, Si: 0.010-2.000%, Mn: 0 to 1.00%, P: 0.100% or less, S: 0.0100% or less, N: 0.0100% or less, O: 0.0200% or less, Al: 0.0010-0.5000%, Mo: 0.0010-1.0000%, B: 0.0005-0.0100%, Ti: 0.010 to 0.100%, Nb: 0 to 0.100%, Cr: 0-1.00%, Co: 0-3.00%, Ni: 0-3.00%, Cu: 0 to 1.00%, V: 0-1.000%, W: 0-1.00%, Ca: 0-1.0000%, Mg: 0 to 1.0000%, REM: 0-1.0000%, Sb: 0 to 1.000%, Zr: 0 to 1.000%, As: 0 to 1.000%, One or more selected from Ta, Re, Os, Ir, Tc, Pb, Se, Bi, and Sn: total of 0 to 1,000%, and Remainder: Fe and impurities Consists of, A hot-stamped molded article according to claim 1 or 2, characterized in that...

5. The surface has a coating, The hot-stamped molded article according to feature 1 or 2.

6. The coating contains a total of 70% by mass or more of Fe and Al. The hot-stamped molded article according to feature 5.

7. The coating contains a total of 70% by mass or more of Fe and Zn. The hot-stamped molded article according to feature 5.

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