Hot compression molded parts

Abstract

DEPCT6824 / 10 / 2567 This hot-stamping formed body part has the following components: Chemically defined, in the surface layer area where the benign surface area ratio is greater than 10 percent, the maximum value applies. The polarity density of the texture is 4.0 or less, and the surface boron reduction index is 4.0 or less. Equal to 0.05 or more;

Description

Hot stamped compact

[0001] The present invention relates to a hot-stamped product. This application claims priority based on Japanese Patent Application No. 2022-067023, filed on April 14, 2022, the contents of which are incorporated herein by reference.

[0002] In recent years, there has been a demand for lighter automobile bodies from the perspectives of environmental protection and resource conservation, and high-strength steel sheets are being used in automobile components. Automotive components are manufactured by press forming, but as the strength of steel sheets increases, not only do the forming loads increase but also formability decreases. Therefore, the formability of high-strength steel sheets into components with complex shapes becomes an issue.

[0003] To solve the above-mentioned problems, the application of hot stamping technology, in which steel sheets are heated to a high temperature in the austenite region where they soften and then press-formed, is being promoted. Hot stamping is attracting attention as a technology that achieves both formability into automotive parts and strength by performing quenching treatment in a die simultaneously with press working.

[0004] For example, Patent Document 1 discloses a high-strength electrogalvanized steel sheet with a high yield ratio and excellent bendability, in which the amount of diffusible hydrogen in the steel is 0.20 mass ppm or less.

[0005] International Publication No. 2020 / 079925

[0006] In order to further reduce the weight of automobile bodies compared to conventional models, it is effective to increase the strength of steel sheets. To increase the strength of steel sheets, it is effective to make the metallographic structure of the steel sheet a metallographic structure in which martensite with a high dislocation density is the main phase. However, a metallographic structure in which martensite with a high dislocation density is the main phase traps a large amount of hydrogen that penetrates from the outside, making automobile components more susceptible to hydrogen embrittlement cracking.

[0007] Hydrogen embrittlement cracking is a phenomenon in which a steel component subjected to high stress during use is fractured due to hydrogen that penetrates into the steel from the external environment. This phenomenon is also called delayed fracture due to the fracture mode. It is generally known that hydrogen embrittlement cracking of steel plate is more likely to occur as the tensile strength of the steel plate increases. This is thought to be because the higher the tensile strength of the steel plate, the greater the residual stress in the steel plate after part formation. The susceptibility to this hydrogen embrittlement cracking (delayed fracture) is called hydrogen embrittlement resistance.

[0008] In Patent Document 1, bendability is taken into consideration, but hydrogen embrittlement resistance is not taken into consideration.

[0009] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a hot-stamped steel sheet having high strength and excellent resistance to hydrogen embrittlement.

[0010] The gist of the present invention is as follows. (1) A hot stamped steel according to one aspect of the present invention has a chemical composition, in mass%, of C: more than 0.40% and 0.70% or less, Si: 0.010 to 3.000%, Mn: 0.10% or more and less than 0.60%, P: 0.100% or less, S: 0.0100% or less, N: 0.0200% or less, O: 0.0200% or less, Al: 0.0010 to 0.5000%, Nb: 0.0010 to 0.1000%, Ti: 0.010 to 0.200%, Cr: 0.010 to 0.800%, Mo: 0.0010 to 1.0000%, B: 0.0005 to 0.0200%, Co: 0 to 4.00%, The hot stamped steel sheet has a surface layer region extending from the surface of the hot stamped steel sheet to a depth of 1 / 25 of the sheet thickness, and the surface region region is composed of Ni: 0 to 3.00%, Cu: 0 to 3.00%, V: 0 to 3.00%, W: 0 to 3.00%, Ca: 0 to 1.0000%, Mg: 0 to 1.0000%, REM: 0 to 1.0000%, Sb: 0 to 1.000%, Sn: 0 to 1.000%, Zr: 0 to 1.000%, As: 0 to 0.100%, and the balance: Fe and impurities. The hot stamped steel sheet has a surface layer region extending from the surface of the hot stamped steel sheet to a depth of 1 / 25 of the sheet thickness, wherein the area fraction of bainite is greater than 10%, the maximum pole density of the texture is 4.0 or less, and the de-B index is 0.05 or more. (2) The hot stamped steel according to (1) above may have a chemical composition, in mass%, of one or more elements selected from the group consisting of Co: 0.01 to 4.00%, Ni: 0.01 to 3.00%, Cu: 0.01 to 3.00%, V: 0.01 to 3.00%, W: 0.01 to 3.00%, Ca: 0.0001 to 1.0000%, Mg: 0.0001 to 1.0000%, REM: 0.0001 to 1.0000%, Sb: 0.001 to 1.000%, Sn: 0.001 to 1.000%, Zr: 0.001 to 1.000%, and As: 0.001 to 0.100%.

[0011] According to the above aspects of the present invention, it is possible to provide a hot stamped steel sheet having high strength and excellent resistance to hydrogen embrittlement.

[0012] FIG. 10 is a diagram for explaining how to obtain a de-B index.

[0013] The present inventors have found that the hydrogen embrittlement resistance of a hot-stamped steel can be improved by generating a desired amount of bainite in the surface layer region, forming a texture with a desired crystal orientation, and achieving a desired debossing index.

[0014] The present inventors have found that, in order to obtain a hot stamped steel sheet having the above-described characteristics, it is particularly effective to carry out annealing under desired conditions during the production of a steel sheet to be subjected to hot stamping.

[0015] The hot stamped steel according to this embodiment will be described in detail below. First, the reasons for limiting the chemical composition of the hot stamped steel according to this embodiment will be described.

[0016] In the following, the numerical ranges indicated by "to" include the lower and upper limits. Numerical values ​​indicated as "less than" and "greater than" are not included in the numerical range. All percentages regarding chemical compositions are by mass.

[0017] The hot-stamped steel according to this embodiment has a chemical composition, in mass%, of more than 0.40% but not more than 0.70% C, 0.010 to 3.000% Si, 0.10% or more but less than 0.60% Mn, 0.100% or less P, 0.0100% or less S, 0.0200% or less N, 0.0200% or less O, 0.0010 to 0.5000% Al, 0.0010 to 0.1000% Nb, 0.010 to 0.200% Ti, 0.010 to 0.800% Cr, 0.0010 to 1.0000% Mo, 0.0005 to 0.0200% B, and the balance being Fe and impurities. Each element will be described below.

[0018] C: more than 0.40% and not more than 0.70% C is an element that improves the strength of a hot-stamped steel. If the C content is 0.40% or less, the desired strength cannot be obtained in the hot-stamped steel. Therefore, the C content is set to more than 0.40%. The C content is preferably 0.42% or more or 0.44% or more. On the other hand, if the C content is more than 0.70%, the amount of hydrogen trapped in martensite increases, making it impossible to obtain excellent hydrogen embrittlement resistance. Therefore, the C content is set to 0.70% or less. Preferably, the C content is 0.65% or less or 0.60% or less.

[0019] Si: 0.010 to 3.000% Si is an element that improves the strength of a hot stamped body through solid solution strengthening. If the Si content is less than 0.010%, the desired strength cannot be obtained. Therefore, the Si content is set to 0.010% or more. The Si content is preferably 0.050% or more, 0.100% or more, or 0.150% or more. On the other hand, if the Si content exceeds 3.000%, the amount of ferrite increases and the desired metal structure cannot be obtained. Therefore, the Si content is set to 3.000% or less. The Si content is preferably 2.000% or less, 1.000% or less, or 0.600% or less.

[0020] Mn: 0.10% or more, less than 0.60% Mn is an element that improves the hardenability of steel and increases the strength of hot-stamped steel. To obtain the desired strength, the Mn content is set to 0.10% or more. The Mn content is preferably 0.20% or more or 0.25% or more. On the other hand, if the Mn content is 0.60% or more, the desired texture cannot be obtained. Therefore, the Mn content is set to less than 0.60%. Preferably, the Mn content is 0.55% or less, 0.50% or less, or 0.45% or less.

[0021] P: 0.100% or less P is an impurity element that segregates at grain boundaries, thereby reducing grain boundary strength. This reduces the hydrogen embrittlement resistance of the hot-stamped steel. If the P content exceeds 0.100%, the above effect becomes significant. Therefore, the P content is set to 0.100% or less. The P content is preferably 0.050% or less or 0.010% or less. The lower limit of the P content is not particularly limited, but may be 0%. However, reducing the P content to less than 0.0001% significantly increases the dephosphorization cost, which is economically undesirable. Therefore, the P content may be set to 0.0001% or more, 0.001% or more, or 0.005% or more.

[0022] S: 0.0100% or less S is an impurity element that forms inclusions in steel. These inclusions trap large amounts of hydrogen, forming regions with locally high hydrogen concentrations, thereby degrading the hydrogen embrittlement resistance of the hot-stamped steel. If the S content exceeds 0.0100%, the above effect becomes significant. Therefore, the S content is set to 0.0100% or less. The S content is preferably 0.0080% or less, 0.0050% or less, or 0.0030% or less. The lower limit of the S content is not particularly limited, but may be 0%. However, reducing the S content to less than 0.0001% significantly increases the desulfurization cost, which is economically undesirable. Therefore, the S content may be set to 0.0001% or more, 0.0002% or more, 0.0003% or more, or 0.0010% or more.

[0023] N: 0.0200% or less N is an impurity element that forms nitrides in steel. These nitrides trap large amounts of hydrogen, forming regions with locally high hydrogen concentrations, which degrades the hydrogen embrittlement resistance of the hot-stamped steel. If the N content exceeds 0.0200%, the above effect becomes significant. Therefore, the N content is set to 0.0200% or less. The N content is preferably 0.0150% or less, 0.0100% or less, 0.0060% or less, or 0.0040% or less. The lower limit of the N content is not particularly limited, but may be 0%. However, reducing the N content to less than 0.0001% significantly increases the denitrification cost, which is economically undesirable. Therefore, the N content may be set to 0.0001% or more or 0.0010% or more.

[0024] O: 0.0200% or less When a large amount of O is contained in steel, it forms coarse oxides. These oxides trap a large amount of hydrogen and form regions with locally high hydrogen concentrations, thereby degrading the hydrogen embrittlement resistance of the hot-stamped steel. If the O content exceeds 0.0200%, the above effect becomes significant. Therefore, the O content is set to 0.0200% or less. The O content is preferably set to 0.0100% or less, 0.0070% or less, or 0.0040% or less. The O content may be 0%, but in order to disperse a large number of fine oxides during deoxidation of molten steel, the O content may be set to 0.0005% or more or 0.0010% or more.

[0025] Al: 0.0010 to 0.5000% Al is an element that deoxidizes molten steel to improve its soundness. If the Al content is less than 0.0010%, deoxidation is insufficient, resulting in the formation of coarse oxides. These oxides trap large amounts of hydrogen, forming regions with locally high hydrogen concentrations, thereby degrading the hydrogen embrittlement resistance of the hot-stamped steel. Therefore, the Al content is set to 0.0010% or more. The Al content is preferably 0.0050% or more, 0.0100% or more, or 0.0300% or more. On the other hand, if the Al content exceeds 0.5000%, coarse oxides are formed in the steel. These oxides trap large amounts of hydrogen, forming regions with locally high hydrogen concentrations, thereby degrading the hydrogen embrittlement resistance of the hot-stamped steel. Therefore, the Al content is set to 0.5000% or less. The Al content is preferably 0.4000% or less, 0.3000% or less, 0.2000% or less, or 0.1000% or less.

[0026] Nb: 0.0010 to 0.1000% Nb is an element that forms carbonitrides in steel and improves the strength of the hot-stamped body through precipitation strengthening. If the Nb content is less than 0.0010%, the desired strength cannot be obtained. Therefore, the Nb content is set to 0.0010% or more. The Nb content is preferably 0.0050% or more, 0.0090% or more, or 0.0150% or more. On the other hand, if the Nb content exceeds 0.1000%, a large amount of carbonitrides is formed in the steel, deteriorating the hydrogen embrittlement resistance of the hot-stamped body. Therefore, the Nb content is set to 0.1000% or less. The Nb content is preferably 0.0800% or less or 0.0600% or less.

[0027] Ti: 0.010 to 0.200% Ti is an element that forms carbonitrides in steel and improves the strength of the hot-stamped body through precipitation strengthening. If the Ti content is less than 0.010%, the desired strength cannot be obtained. Therefore, the Ti content is set to 0.010% or more. The Ti content is preferably 0.020% or more or 0.025% or more. On the other hand, if the Ti content exceeds 0.200%, a large amount of coarse carbonitrides is formed in the steel, which creates sites that locally increase the hydrogen concentration, thereby degrading the hydrogen embrittlement resistance of the hot-stamped body. Therefore, the Ti content is set to 0.200% or less. The Ti content is preferably 0.150% or less, 0.090% or less, 0.080% or less, 0.070% or less, 0.060% or less, or 0.050% or less.

[0028] Cr: 0.010 to 0.800% Cr is an element that dissolves in prior austenite grains during heating before hot stamping, thereby increasing the strength of the hot-stamped steel. If the Cr content is less than 0.010%, the desired strength cannot be obtained. Therefore, the Cr content is set to 0.010% or more. The Cr content is preferably set to 0.100% or more, 0.150% or more, or 0.200% or more. On the other hand, if the Cr content exceeds 0.800%, the desired texture cannot be obtained in the hot-stamped steel, and hydrogen embrittlement resistance deteriorates. Therefore, the Cr content is set to 0.800% or less. The Cr content is preferably set to 0.700% or less, 0.500% or less, or 0.400% or less.

[0029] Mo: 0.0010 to 1.0000% Mo is an element that dissolves in prior austenite grains during heating before hot stamping, thereby increasing the strength of the hot-stamped body. If the Mo content is less than 0.0010%, the desired strength cannot be obtained. Therefore, the Mo content is set to 0.0010% or more. The Mo content is preferably set to 0.0100% or more, 0.0500% or more, or 0.1000% or more. On the other hand, if the Mo content exceeds 1.0000%, the desired texture cannot be obtained in the hot-stamped body, and hydrogen embrittlement resistance deteriorates. Therefore, the Mo content is set to 1.0000% or less. The Mo content is preferably set to 0.8000% or less, 0.6000% or less, or 0.4000% or less.

[0030] B: 0.0005 to 0.0200% B is an element that improves the hardenability of steel. If the B content is less than 0.0005%, the desired strength cannot be obtained. Therefore, the B content is set to 0.0005% or more. The B content is preferably set to 0.0010% or more or 0.0015% or more. On the other hand, if the B content exceeds 0.0200%, coarse intermetallic compounds are formed in the hot-stamped steel. These intermetallic compounds become sites with locally elevated hydrogen concentrations, degrading the hydrogen embrittlement resistance of the hot-stamped steel. Therefore, the B content is set to 0.0200% or less. The B content is preferably set to 0.0150% or less, 0.0100% or less, 0.0080% or less, 0.0040% or less, or 0.0030% or less.

[0031] The balance of the chemical composition of the hot stamped steel may be Fe and impurities. Examples of impurities include elements that are inevitably mixed in from steel raw materials or scrap and / or during the steelmaking process and are permissible within a range that does not impair the properties of the hot stamped steel according to this embodiment.

[0032] The hot-stamped steel may contain the following elements as optional elements. When the following optional elements are not contained, the content is 0%.

[0033] Co: 0 to 4.00% Co is an element that improves the strength of the hot stamped steel by solid solution strengthening. To reliably obtain this effect, the Co content is preferably 0.01% or more. The Co content is more preferably 0.05% or more. However, even if a large amount of Co is added, the above effect saturates, so the Co content is set to 4.00% or less. If necessary, the upper limit of the Co content may be set to 1.00%, 0.50%, 0.10%, 0.05%, or 0.02%.

[0034] Ni: 0 to 3.00% Ni has the effect of increasing the strength of the hot-stamped body by dissolving in prior austenite grains during heating before hot stamping. To ensure this effect, the Ni content is preferably 0.01% or more. However, since the above effect saturates even when a large amount of Ni is added, the Ni content is preferably 3.00% or less. If necessary, the upper limit of the Ni content may be set to 1.50%, 1.00%, 0.50%, 0.10%, 0.05%, or 0.02%.

[0035] Cu: 0 to 3.00% Cu has the effect of increasing the strength of the hot-stamped body by dissolving in prior austenite grains during heating before hot stamping. To reliably obtain this effect, the Cu content is preferably 0.01% or more. The Cu content is more preferably 0.05% or more. However, since the above effect saturates even when a large amount of Cu is added, the Cu content is preferably 3.00% or less. If necessary, the upper limit of the Cu content may be set to 1.50%, 1.00%, 0.50%, 0.10%, 0.05%, or 0.02%.

[0036] V: 0 to 3.00% V forms carbonitrides in steel and has the effect of improving the strength of the hot-stamped body through precipitation strengthening. To reliably obtain this effect, the V content is preferably 0.01% or more. The V content is more preferably 0.05% or more. On the other hand, if the V content exceeds 3.00%, a large amount of coarse carbonitrides is formed in the steel. These carbonitrides become sites that locally increase the hydrogen concentration, degrading the hydrogen embrittlement resistance of the hot-stamped body. Therefore, the V content is set to 3.00% or less. If necessary, the upper limit of the V content may be set to 1.50%, 1.00%, 0.50%, 0.10%, 0.05%, or 0.02%.

[0037] W: 0 to 3.00% W has the effect of improving the strength of the hot-stamped steel. To reliably obtain this effect, the W content is preferably 0.01% or more. The W content is preferably 0.05% or more. However, even if a large amount of W is added, the above effect saturates, so the W content is set to 3.00% or less. If necessary, the upper limit of the W content may be set to 1.50%, 1.00%, 0.50%, 0.10%, 0.05%, or 0.02%.

[0038] Ca: 0 to 1.0000% Ca is an element that suppresses the generation of oxides that serve as fracture initiation sites, and contributes to improving the hydrogen embrittlement resistance of the hot-stamped steel. To reliably obtain this effect, the Ca content is preferably 0.0001% or more. However, since the above effect saturates even when a large amount of Ca is added, the Ca content is set to 1.0000% or less. If necessary, the upper limit of the Ca content may be set to 0.1000%, 0.0100%, 0.0050%, 0.0010%, 0.0005%, or 0.0002%.

[0039] Mg: 0 to 1.0000% Mg forms oxides and sulfides in molten steel, suppressing the formation of coarse MnS and dispersing many fine oxides to refine the metal structure. This contributes to improving the hydrogen embrittlement resistance of the hot-stamped steel. To reliably obtain these effects, the Mg content is preferably 0.0001% or more. However, since the above effects saturate even when a large amount is added, the Mg content is set to 1.0000% or less. If necessary, the upper limit of the Mg content may be set to 0.1000%, 0.0100%, 0.0050%, 0.0010%, 0.0005%, or 0.0002%.

[0040] REM: 0 to 1.0000% REM suppresses the formation of coarse oxides that become sites that cause localized increases in hydrogen concentration. This contributes to improving the hydrogen embrittlement resistance of the hot-stamped steel. To reliably obtain this effect, the REM content is preferably 0.0001% or more. However, since the above effect saturates even when a large amount is added, the REM content is set to 1.0000% or less. If necessary, the upper limit of the REM content may be set to 0.1000%, 0.0100%, 0.0050%, 0.0010%, 0.0005%, or 0.0002%. 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 to 1.000% Sb suppresses the formation of coarse oxides that become sites that accompany local increases in hydrogen concentration. This contributes to improving the hydrogen embrittlement resistance of the hot-stamped steel. To reliably obtain this effect, the Sb content is preferably 0.001% or more. However, even if a large amount is added, the above effect saturates, so the Sb content is set to 1.000% or less. If necessary, the upper limit of the Sb content may be set to 0.100%, 0.050%, 0.020%, 0.010%, 0.005%, or 0.002%.

[0042] Sn: 0 to 1.000% Sn suppresses the formation of coarse oxides that become sites that accompany local increases in hydrogen concentration. This contributes to improving the hydrogen embrittlement resistance of the hot-stamped steel. To reliably obtain this effect, the Sn content is preferably 0.001% or more. However, since the above effect saturates even when a large amount is added, the Sn content is set to 1.000% or less. If necessary, the upper limit of the Sn content may be set to 0.100%, 0.050%, 0.020%, 0.010%, 0.005%, or 0.002%.

[0043] Zr: 0 to 1.000% Zr suppresses the formation of coarse oxides that become sites that accompany local increases in hydrogen concentration. This contributes to improving the hydrogen embrittlement resistance of the hot-stamped steel. To ensure this effect, the Zr content is preferably 0.001% or more. However, even if a large amount is added, the above effect saturates, so the Zr content is set to 1.000% or less. If necessary, the upper limit of the Zr content may be set to 0.100%, 0.050%, 0.020%, 0.010%, 0.005%, or 0.002%.

[0044] As: 0 to 0.100% As refines prior austenite grains by lowering the austenite single-phase temperature. This contributes to improving the hydrogen embrittlement resistance of the hot-stamped steel. To reliably obtain this effect, the As content is preferably 0.001% or more. However, even if a large amount of As is added, the above effect saturates, so the As content is set to 0.100% or less. If necessary, the upper limit of the As content may be set to 0.100%, 0.050%, 0.020%, 0.010%, 0.005%, or 0.002%.

[0045] The chemical composition of the hot-stamped body described above may be measured by a common analytical method. For example, measurement may be performed using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). C and S may be measured using a combustion-infrared absorption method, N may be measured using an inert gas fusion-thermal conductivity method, and O may be measured using an inert gas fusion-non-dispersive infrared absorption method. If the surface of the hot-stamped body is provided with a plating layer, a paint film, or the like, the plating layer, paint film, or the like is removed by mechanical grinding before analyzing the chemical composition.

[0046] Next, the metallographic structure of the hot-stamped steel according to this embodiment will be described. In the hot-stamped steel according to this embodiment, in a surface layer region extending from the surface of the hot-stamped steel to a depth of 1 / 25 of the sheet thickness (thickness of the hot-stamped steel), the area fraction of bainite is more than 10%, the maximum value of the pole density of the texture is 4.0 or less, and the de-B index is 0.05 or more.

[0047] In this embodiment, the surface region refers to a region extending from the surface of the hot-stamped body to a depth of 1 / 25 of the sheet thickness (thickness of the hot-stamped body). When the hot-stamped body has a plating layer, a paint film, or the like on its surface, the term "surface" here refers to the interface between the plating layer and the base steel sheet, and for convenience, the plating layer, the paint film, or the like is excluded from the hot-stamped body. Specifically, when the hot-stamped body has a plating layer, a paint film, or the like on its surface, for convenience, the region where the iron concentration is less than 90% by mass in the GD-OES measurement, i.e., the plating layer, the paint film, or the like, is excluded from the hot-stamped body, as described below, and the measurement point where the iron concentration is 90% by mass (i.e., the interface between the base steel material and the plating layer, etc.) is regarded as the surface of the hot-stamped body. As described above, the plated layer, paint film, etc. are excluded from the hot-stamped body. However, when the thickness of the plated layer, paint film, etc. is so small as to be negligible relative to the thickness of the hot-stamped body (however, when only the plated layer is present, the thickness of the plated layer is often so small that it can be ignored in most cases), the thickness of the hot-stamped body may be measured as the thickness including the plated layer, paint film, etc.

[0048] (Surface layer region) Area fraction of bainite: more than 10% By generating bainite in the surface layer region, the dislocation density in the surface layer region can be reduced. As a result, hydrogen penetration from the external environment can be suppressed, and the hydrogen embrittlement resistance of the hot-stamped body can be improved. Furthermore, by generating bainite in the surface layer region, excessive softening of the surface layer can be suppressed, thereby further improving the hydrogen embrittlement resistance while maintaining the load-bearing capacity of the member. If the area fraction of bainite in the surface layer region is 10% or less, the hydrogen embrittlement resistance of the hot-stamped body will deteriorate. Therefore, the area fraction of bainite is set to more than 10%. Preferably, it is 20% or more, 40% or more, or 60% or more. There is no particular upper limit to the area fraction of bainite, but it may be 100%, 90%, or 80%.

[0049] The metallographic structure of the surface layer region may contain, in addition to bainite, 0 to 90% (0% or more and 90% or less) martensite, and a total of 0 to 65% (0% or more and 65% or less) ferrite and retained austenite. Note that martensite in this embodiment includes untempered martensite (fresh martensite) and tempered martensite. The area ratio of the metallographic structure is calculated for the surface layer region (the region from the surface to a depth of 1 / 25 of the plate thickness) by the following method.

[0050] A sample is cut out from an arbitrary position at least 50 mm away from the end face of the hot-stamped body (if a sample cannot be taken from this position, a position avoiding the end) so that a cross section of the sheet thickness parallel to the rolling direction can be observed. The size of the sample depends on the measuring device, but it should be large enough to allow observation of about 10 mm in the rolling direction.

[0051] The cross section of the sample is polished using #600 to #1500 silicon carbide paper, and then mirror-finished 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. The observation surface is then finished by electrolytic polishing. At any position in the longitudinal direction of the sample cross section, a region of 50 μm in length from the surface of the hot stamped compact to a depth of 1 / 25 of the plate thickness is measured at 0.1 μm measurement 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 may be used. For example, an EBSD analyzer consisting of a JEOL JSM-7001F and a TSL DVC5 type detector may be used. The vacuum level inside the EBSD analyzer is 9.6 × 10 －５ The pressure may be 10 Pa or less, the acceleration voltage may be 15 kV, and the irradiation current level may be 13.

[0052] The obtained crystal orientation information is used with the "Phase Map" function of the software "OIM Analysis (registered trademark)" attached to the EBSD analyzer to determine that the crystal structure is fcc as retained austenite. The area fraction of this retained austenite is obtained by calculating the area fraction of this retained austenite. Next, in the region with a bcc crystal structure, the "Grain Average Misorientation" function of the software "OIM Analysis (registered trademark)" attached to the EBSD analyzer is used to extract as bainite regions where the "Grain Average Misorientation" is greater than 0.50° and less than 0.75° under the condition that 5° grain boundaries are considered to be grain boundaries. The area fraction of bainite is obtained by calculating the area fraction of the extracted bainite. Next, the region where "Grain Average Misorientation" is 0.5° or less is extracted as ferrite. The area ratio of the extracted ferrite is calculated to obtain the area ratio of ferrite. The remaining region (the region where "Grain Average Misorientation" is greater than 0.75°) is extracted as martensite, and the area ratio of martensite is calculated to obtain the area ratio of martensite. In this embodiment, the rolling direction of the hot-stamped body is determined by the following method. First, a test piece is taken from an arbitrary position 50 mm or more away from the end of the hot-stamped body so that the thickness cross section can be observed. The thickness cross section of the taken test piece is mirror-polished and then observed using an optical microscope at magnifications of 100x, 200x, 500x, and 1000x. Depending on the size of the inclusion, an observation result at an appropriate magnification at which the size of the inclusion can be measured is selected. The observation range is 500 μm or more in width and across the entire thickness of the plate, and areas with dark brightness are determined to be inclusions. Observation may be performed from multiple fields of view. Next, using the plate thickness cross-section initially observed by the above method as a reference, a plane parallel to the plane rotated in 5° increments in the range of 0° to 180° around the plate thickness direction is observed in the same manner as above. The average value of the length of the major axes of the multiple inclusions in each cross-section is calculated for each cross-section. The cross-section with the largest average value of the length of the major axes of the inclusions obtained is identified.The direction parallel to the longitudinal axis of the inclusions in the cross section is determined to be the rolling direction.

[0053] (Surface layer region) Crystal orientation in the surface layer region: maximum value of the pole density of the texture is 4.0 or less By controlling the texture in the surface layer region, it is possible to suppress the penetration of hydrogen into the surface layer region from the external environment, thereby improving the hydrogen embrittlement resistance of the hot-stamped body. If the maximum value of the pole density of the texture in the surface layer region exceeds 4.0, the hydrogen embrittlement resistance of the hot-stamped body deteriorates. Therefore, the maximum value of the pole density of the texture in the surface layer region is set to 4.0 or less. Preferably, it is 3.5 or less, 3.0 or less, or 2.5 or less. There is no particular restriction on the lower limit of the pole density of the texture in the surface layer region, but it may be 1.0 or 1.2.

[0054] The texture in the surface region (the region from the surface to a depth of 1 / 25 of the sheet thickness) is obtained by the following method. A sample is cut out from an arbitrary position 50 mm or more away from the end face of the hot-stamped body (if a sample cannot be taken from this position, a position avoiding the end) so that a thickness cross section parallel to the rolling direction can be observed. The size of the sample depends on the measuring device, but it should be large enough to allow observation of about 10 mm in the rolling direction.

[0055] The cross section of the sample is polished using #600 to #1500 silicon carbide paper, and then mirror-finished using a liquid in which diamond powder with a grain size of 1 to 6 μm is dispersed in a diluent such as alcohol or pure water. Next, the sample is finished by electrolytic polishing. At any position in the longitudinal direction of the sample cross section, a region of 1000 μm in length from the surface to a depth of 1 / 25 of the plate thickness is measured using electron backscatter diffraction at measurement intervals of 5.0 μm to obtain crystal orientation information. For the measurement, an EBSD analyzer consisting of a thermal field emission scanning electron microscope and an EBSD detector may be used, such as an EBSD analyzer consisting of a JEOL JSM-7001F and a TSL DVC5 type detector. The vacuum level inside the EBSD analyzer is 9.6 × 10 －５ The pressure may be 10 Pa or less, the acceleration voltage may be 15 kV, and the irradiation current level may be 13.

[0056] The obtained crystal orientation information is used to calculate the intensity of crystal grains with a bcc crystal structure using the "Texture" function of the software "OIM Analysis (registered trademark)" attached to the EBSD analyzer, using a harmonic series expansion. In this case, the expansion order is set to 16, and the half-width when applying a Gaussian distribution is set to 5°. Next, the "Texture Plot" function is used for the output file after the intensity calculation to output the φ2 = 45° cross section in the crystal orientation distribution function (ODF). The maximum value of the pole density in the φ2 = 45° cross section is taken as the pole density of the texture in the surface region.

[0057] (Surface layer region) DeB index: 0.05 or more The deB index is an index that quantitatively represents the amount of reduction in the B concentration in the surface layer region. By reducing the B concentration in the surface layer region, the strength of the prior austenite before transformation decreases, improving the deformability of the prior austenite grains and making it easier for randomly oriented crystal grains to form in the surface layer region. If the deB index of the surface layer region is less than 0.05, it is not possible to obtain crystal grains with the desired texture in the surface layer region. Therefore, the deB index is set to 0.05 or more. Preferably, it is 0.20 or more, 0.30 or more, or 0.35 or more. There is no particular limitation on the upper limit of the deB index, but it may be 1.00, 0.80, or 0.60.

[0058] The deboronation index in the surface layer region is obtained by the following method. The element concentration distribution in the thickness direction of the hot-stamped body is measured using a glow discharge optical emission spectrometer (GD-OES: a Marcus-type high-frequency glow discharge optical emission spectrometer, GD-PROFILER-HR, manufactured by HORIBA, Ltd.). The measurement conditions are an analysis diameter of 4 mmφ, a sputtering rate of 4 μm / min, an argon pressure of 600 Pa, an RF output of 35 W, and a measurement interval of 0.02 μm or less. The measurement is performed for all elements contained in the hot-stamped body.

[0059] In addition, when the hot-stamped product has a plating layer or the like on its surface, the "surface" here refers to the interface between the plating layer or the like and the base steel sheet. In cases where the surface has a plating layer, paint film, or the like, the plating layer, paint, or the like is partially or completely removed by mechanical polishing or chemical polishing so that measurements can be made up to a depth of 200 μm from the surface of the base steel sheet (the interface between the base steel sheet and the plating layer, paint film, or the like) before GD-OES measurement. In GD-OES measurement, the measurement point where the iron concentration is 90 mass % is considered to be the surface of the hot-stamped product. In the following explanation, for convenience of explanation, the hot-stamped product may also be referred to as the base steel sheet.

[0060] Next, the B concentration is measured from the surface of the hot-stamped body to a depth of at least 100 μm from the surface. After measuring the B concentration at a depth of 100 μm from the surface, if the absolute value of the difference between the average B concentration in the region 80 to 100 μm and the maximum measured B concentration in the region 80 to 100 μm is 0.0006 mass% or less, and the absolute value of the difference between the average B concentration in the region 80 to 100 μm and the minimum measured B concentration in the region 80 to 100 μm is 0.0006 mass% or less, the measurement of the B concentration in the depth direction is terminated at a depth of 100 μm from the surface. If this requirement for termination of measurement is not met, the measurement of the B concentration in the depth direction is continued. Then, each time a new measurement value of the B concentration in the depth direction is obtained, the average value of the B concentration in the region from the deepest part to 20 μm from the deepest part toward the surface is calculated, and if the absolute value of the difference between the average value of the B concentration in the region from the deepest part to 20 μm from the deepest part toward the surface and the maximum measurement value of the B concentration in the region from the deepest part to 20 μm from the deepest part toward the surface is 0.0006 mass% or less, and the absolute value of the difference between the average value of the B concentration in the region from the deepest part to 20 μm from the deepest part toward the surface and the minimum measurement value of the B concentration in the region from the deepest part to 20 μm from the deepest part toward the surface is 0.0006 mass% or less, the measurement of the B concentration in the depth direction is terminated at that position. For example, when a measurement value of the B concentration is obtained at a depth of 150 μm from the surface, if the absolute value of the difference between the average B concentration in a region 130 to 150 μm deep from the surface and the maximum measured B concentration in the region 130 to 150 μm deep from the surface is 0.0006 mass% or less, and if the absolute value of the difference between the average B concentration in the region 130 to 150 μm deep from the surface and the minimum measured B concentration in the region 130 to 150 μm deep from the surface is 0.0006 mass% or less, the measurement of the B concentration in the depth direction is terminated at a depth of 150 μm from the surface. Even if the requirements for termination of measurement are not met and the measurement of the B concentration in the depth direction cannot be terminated, the measurement of the B concentration in the depth direction is terminated when the measurement of the B concentration at a depth of 200 μm from the surface is completed.Then, at the time when the measurement of the B concentration in the depth direction is completed, the average value of the B concentration in the region from the deepest part (the deepest position at which the B concentration used to calculate the B deconcentration index was obtained) to a position 20 μm toward the surface from the deepest part (hereinafter, the average B concentration in this region is referred to as the average B concentration in the deepest 20 μm) is used to calculate the B deconcentration index below. For convenience of measurement, for example, after measuring the B concentration from the surface to a depth of 200 μm, the shallowest depth position that satisfies the above-mentioned condition for ending the B concentration measurement in the depth direction is searched in the region from 100 to 200 μm deep from the surface, and if such a position is found, the B deconcentration index may be calculated without using the B concentration measurement results at positions deeper than that depth position. For example, the B concentration may be measured from the surface to a depth of 200 μm. In this case, if the shallowest depth position that satisfies the above-mentioned condition for ending the B concentration measurement in the depth direction is found in the region 100 μm or more from the surface, the measurement may be considered to have ended at that depth position, and the B deconcentration index is calculated.

[0061] In the region from the surface of the hot-stamped body to the deepest part and extending 20 μm from the deepest part toward the surface, the decrease in B concentration per unit depth (the value obtained by subtracting the B concentration at each measurement point from the average B concentration in the deepest 20 μm) is calculated, and the integral of the product of the decrease in B concentration per unit depth is determined to be the area of ​​the B-deficient region (the area of ​​region A in FIG. 1 ). However, if the value obtained by subtracting the B concentration at each measurement point from the average B concentration in the deepest 20 μm is negative, the integral is set to 0 (due to the de-B phenomenon near the surface, the B concentration at each measurement point is almost always smaller than the average B concentration in the deepest 20 μm, and this integral is positive). Next, the product of the average B concentration in the deepest 20 μm and the length of 200 μm is calculated as the reference area (the area of ​​rectangular region B in FIG. 1 ). The value obtained by dividing the B-deficient area (area of ​​region A) by the reference area (area of ​​region B) is determined to be the de-B index (area of ​​region A / area of ​​region B). Even when the requirement for measurement to be completed within 200 μm from the surface is met as described above, the length by which the average B concentration in the deepest 20 μm is multiplied is set to 200 μm to calculate the reference area (area of ​​region B).

[0062] The metallographic structure of a region other than the surface region of the hot-stamped body (for example, a region from a depth of 4 / 16 of the plate thickness from the surface to a depth of 5 / 16 of the plate thickness from the surface) is not particularly limited as long as the desired strength and hydrogen embrittlement resistance can be obtained. For example, the metallographic structure may be composed of a total of 90 to 100% (90% or more and 100% or less) of martensite and bainite, and 0 to 10% (0% or more and 10% or less) of ferrite and retained austenite, in area percentages. The metallographic structure of the region other than the surface region is measured by the following method. Note that the metallographic structure of the region other than the surface region is measured from a region from a depth of 1 / 8 of the plate thickness from the surface to a depth of 3 / 8 of the plate thickness from the surface. This is because the metallographic structure in this region represents a typical metallographic structure of a hot-stamped body.

[0063] A sample is cut out from an arbitrary position at least 50 mm away from the end face of the hot-stamped body (if a sample cannot be taken from this position, a position avoiding the end) so that a cross section of the sheet thickness parallel to the rolling direction can be observed. The size of the sample depends on the measuring device, but it should be large enough to allow observation of about 10 mm in the rolling direction.

[0064] The cross section of the sample is polished using #600 to #1500 silicon carbide paper, and then mirror-finished using a liquid in which diamond powder with a grain size of 1 to 6 μm is dispersed in a diluent such as alcohol or pure water. The observation surface is then finished by electrolytic polishing. At any position in the longitudinal direction of the sample cross section, a region of 50 μm in length, from a depth of 1 / 8 of the plate thickness from the surface to a depth of 3 / 8 of the plate thickness from the surface, is measured using electron backscatter diffraction at measurement intervals of 0.1 μm to obtain crystal orientation information. For the measurement, an EBSD analyzer consisting of a thermal field emission scanning electron microscope and an EBSD detector may be used, such as an EBSD analyzer consisting of a JEOL JSM-7001F and a TSL DVC5 detector. The vacuum level inside the EBSD analyzer is 9.6 × 10 －５ The pressure may be 10 Pa or less, the acceleration voltage may be 15 kV, and the irradiation current level may be 13.

[0065] The obtained crystal orientation information is used with the "Phase Map" function of the software "OIM Analysis (registered trademark)" attached to the EBSD analyzer to determine that the crystal structure is fcc as retained austenite. The area fraction of this retained austenite is calculated to obtain the area fraction of retained austenite. Next, regions with bcc crystal structure are determined to be bainite, martensite, and ferrite. For these regions, the "Grain Average Misorientation" function of the software "OIM Analysis (registered trademark)" attached to the EBSD analyzer is used to extract regions with a "Grain Average Misorientation" of 0.5° or less as ferrite under the condition that 5° grain boundaries are considered to be crystal grain boundaries. The area fraction of the extracted ferrite is calculated to obtain the area fraction of ferrite.

[0066] Next, the remaining region (the region where "Grain Average Misorientation" exceeds 0.5°) is taken as the total area ratio of martensite and bainite.

[0067] The hot-stamped product according to this embodiment may have a plating layer on its surface. By having a plating layer on its surface, corrosion resistance can be improved after hot stamping. Examples of the plating layer include an aluminum plating layer, an aluminum-zinc plating layer, an aluminum-silicon plating layer, a hot-dip galvanized layer, an electrogalvanized layer, a galvannealed hot-dip galvanized layer, a zinc-nickel plating layer, and an aluminum-magnesium-zinc based plating layer.

[0068] Next, a steel sheet for hot stamping for obtaining a hot-stamped steel according to this embodiment will be described. The steel sheet for hot stamping has the chemical composition described above. The metal structure of the steel sheet for hot stamping is not particularly limited as long as it can obtain the desired strength and hydrogen embrittlement resistance after hot stamping, but may, for example, be composed of, in area percentages, ferrite: 5 to 90%, bainite and martensite: 0 to 100%, pearlite: 10 to 95%, and retained austenite: 0 to 5%. In addition, iron carbides, alloy carbides, intermetallic compounds, and inclusions may be included.

[0069] Furthermore, the steel sheet for hot stamping may have a plating layer on its surface. By having a plating layer on its surface, corrosion resistance can be improved after hot stamping. Examples of the plating layer include an aluminum plating layer, an aluminum-zinc plating layer, an aluminum-silicon plating layer, a hot-dip galvanized layer, an electrogalvanized layer, a galvannealed hot-dip galvanized layer, a zinc-nickel plating layer, and an aluminum-magnesium-zinc based plating layer.

[0070]

[0033] Hereinafter, a method for manufacturing a steel sheet for hot stamping for obtaining a hot-stamped steel according to this embodiment will be described. In order to obtain the hot-stamped steel, it is particularly effective to control the annealing conditions in the method for manufacturing a steel sheet for hot stamping.

[0071] The conditions for the casting method of molten steel, heating before hot rolling, rough rolling, finish rolling, coiling, and cold rolling are not particularly limited, and general conditions may be used. Furthermore, in order to soften the hot-rolled steel sheet, the coil after coiling may be subjected to a softening heat treatment. The method of the softening heat treatment is not particularly limited, and general conditions may be used.

[0072] Annealing After cold rolling, it is preferable to perform annealing by heating in an oxidizing atmosphere for 15 seconds or more. Normally, annealing is preferably performed in a reducing atmosphere to suppress scale formation, but in this embodiment, annealing is performed in an oxidizing atmosphere to promote scale formation on the steel sheet surface. During heating for hot stamping, the scale formed on the steel sheet surface serves as an oxidation source, and C and B in the surface layer region are oxidized. The oxidized C and B are removed from the steel sheet surface, so the amounts of C and B in the surface layer region are reduced. As a result, the strength of prior austenite grains decreases, making them more susceptible to deformation, and randomly oriented crystal grains are more likely to be generated. This allows crystal grains with a desired texture to be generated in the surface layer region.

[0073] The heating temperature during annealing may be in the range of 730 to 900°C, and by retaining the steel within this heating temperature range for 15 seconds or more, it is possible to promote scale formation while suppressing scale spalling. The annealing time is preferably 100 seconds or more, more preferably 200 seconds or more, and even more preferably 300 seconds or more. On the other hand, annealing for more than 3600 seconds is not preferable because the prior austenite grain size becomes coarse, the grain boundary diffusion rate of B decreases, deboronation does not progress, and the deboronation index does not reach 0.05 or more. Therefore, the annealing time is preferably 3600 seconds or less. After annealing in an oxidizing atmosphere, the steel may be annealed again in an oxidizing or non-oxidizing atmosphere, as long as no treatment to remove oxide scale (e.g., pickling) is performed.

[0074] In the present embodiment, the oxidizing atmosphere may be any heating atmosphere in which oxide scale forms on the surface of the steel sheet, and may be any general condition. For example, a gas combustion atmosphere is preferably used in which the mixture ratio of air to fuel (air-fuel ratio) is controlled to 1.00 or more, more preferably 1.10 or more. Annealing in an oxidizing atmosphere is preferably performed to form an oxide scale of 15 μm or more on the surface of the steel sheet. It is preferable that the oxide scale on the surface of the steel sheet remains in subsequent processes. That is, it is preferable to perform the hot stamping described below with the oxide scale remaining. The oxide scale is removed by shot blasting after hot stamping. Furthermore, even when a plating layer is formed on the surface of the steel sheet for hot stamping, oxide scale remains at the interface between the base steel sheet and the plating layer. When a plating layer is formed, the oxide scale disappears after hot stamping due to an alloying reaction during heating before hot stamping.

[0075] The hot stamped steel sheet manufactured by the above-described method is hot stamped to obtain a hot stamped product according to this embodiment. The hot stamping conditions are not particularly limited, but it is preferable to, for example, heat the hot stamping steel sheet to a temperature range of 800°C to 1000°C and hold it at this temperature range for 60 to 1200 seconds.

[0076] If the heating temperature is less than 800°C, austenitization will be insufficient, and the hydrogen embrittlement resistance of the hot-stamped steel may be deteriorated or the desired strength may not be obtained. On the other hand, if the heating temperature exceeds 1000°C, the prior austenite will undergo excessive grain growth, and the hydrogen embrittlement resistance of the hot-stamped steel may be deteriorated or the desired strength may not be obtained. If the holding time is less than 60 seconds, austenitization will be insufficient, and the hydrogen embrittlement resistance of the hot-stamped steel may be deteriorated or the desired strength may not be obtained. If the holding time exceeds 1200 seconds, the prior austenite will undergo excessive grain growth, and the hydrogen embrittlement resistance of the hot-stamped steel may be deteriorated or the desired strength may not be obtained.

[0077] The heating atmosphere is not particularly limited and may be under ordinary conditions, such as air, a gas combustion atmosphere with a controlled air-to-fuel ratio, or a nitrogen atmosphere, and the dew point of these gases may be controlled. The workpiece is held in this temperature range and then hot stamped. After hot stamping, the workpiece is cooled to a temperature range of 250°C or less at an average cooling rate of 20°C / s or more.

[0078] Examples of the heating method before hot stamping include heating with an electric furnace or gas furnace, flame heating, electrical heating, high frequency heating, and induction heating.

[0079] The hot-stamped product according to this embodiment is obtained by the above method. To soften the product, a tempering treatment at 130 to 600°C may be performed after hot stamping, or a bake hardening treatment may be performed after painting. Furthermore, a portion of the hot-stamped product may be tempered by laser irradiation or the like to provide a partially softened region.

[0080] Next, examples of the present invention will be described, but the conditions in the examples are merely examples adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to these examples. Various conditions can be adopted in the present invention as long as they do not deviate from the gist of the present invention and the object of the present invention is achieved.

[0081] Slabs produced by casting molten steel having the chemical compositions shown in Tables 1A to 2C were held at a temperature of 1200°C or higher for 20 minutes or longer, and then rough rolling, finish rolling, cold rolling, and coiling were carried out under typical conditions. Annealing was then carried out under the conditions shown in Tables 3A to 3C. The annealing was carried out in an oxidizing atmosphere. In the annealing in the oxidizing atmosphere, the mixture ratio of air to fuel (air-fuel ratio) was controlled to 1.05 in a gas combustion atmosphere.

[0082] The obtained steel sheets for hot stamping were heated to the temperature ranges shown in Tables 3A to 3C in a furnace to which nitrogen gas was continuously supplied, held at the temperature ranges, and then hot stamped, followed by cooling to a temperature range of 250°C or less at an average cooling rate of 20°C / s or more. This resulted in the hot stamped products shown in Tables 4A to 4C. However, some of the examples were subjected to re-annealing, plating, or heat treatment for softening, as shown in the tables.

[0083] Note that underlines in the tables indicate values ​​outside the scope of the present invention, outside the preferred manufacturing conditions, or unfavorable characteristic values. Furthermore, the metallographic structure of the surface region of the hot-stamped steel according to the present invention example contained, in addition to bainite, 90% or more of martensite, and a total of 65% or less of ferrite and retained austenite, in terms of area percentage. Furthermore, the metallographic structure of the region other than the surface region of the hot-stamped steel according to the present invention example consisted, in terms of area percentage, of 90% or more of martensite and bainite, and 10% or less of ferrite and retained austenite.

[0084] The metallographic structure, deboronation index, and pole density of the texture of the hot-stamped body were measured by the methods described above. The mechanical properties of the hot-stamped body were evaluated by the following methods.

[0085] Tensile Strength The tensile (maximum) strength TS of the hot-stamped body was obtained by preparing a No. 5 test piece from an arbitrary position of the hot-stamped body and conducting a tensile test in accordance with JIS Z 2241:2011. The crosshead speed was 1 mm / min. A tensile strength TS of 2200 MPa or more was judged to have high strength and to pass, and a tensile strength of less than 2200 MPa was judged to not have high strength and to fail.

[0086] Hydrogen Embrittlement Resistance The hydrogen embrittlement resistance of the hot-stamped compact was evaluated by the following method. A test piece 68 mm long and 6 mm wide was taken from any position on the hot-stamped compact. The ends of the test piece were polished using #200 to #1500 silicon carbide paper, and then mirror-finished using a liquid in which diamond powder with a particle size of 1 to 6 μm was dispersed in a diluted solution such as alcohol or pure water. Furthermore, the corners of the test piece were chamfered using #200 to #1500 silicon carbide paper. A stress of 800 MPa or more was applied to the test piece, and it was immersed in 1 liter of hydrochloric acid adjusted to pH 4 at room temperature for 48 hours, and the presence or absence of cracks was determined.

[0087] A specimen that did not crack even at a load stress of 800 MPa or more was judged to have passed. A specimen that did not crack at 800 MPa was judged to have passed the test, a specimen that did not crack at 900 MPa was judged to have passed the test, a specimen that did not crack at 900 MPa was judged to have passed the test, a specimen that did not crack at 1000 MPa was judged to have passed the test, and a specimen that did not crack at 1100 MPa or more was judged to have passed the test, and a specimen that did not crack at 1100 MPa or more was judged to have passed the test, and a specimen that did not crack at 800 MPa was judged to have passed the test, and a specimen that did not crack at 1100 MPa or more was judged to have passed the test, and a specimen that did not crack at 800 MPa or more was judged to have passed the test, and a specimen that did not crack at 1 ...

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[0099]

[0100] Tables 4A to 4C show that the hot-stamped steels of the examples of the present invention have high strength and excellent hydrogen embrittlement resistance, while the hot-stamped steels of the comparative examples are inferior in one or more properties.

[0101] According to the above aspects of the present invention, it is possible to provide a hot stamped steel sheet having high strength and excellent resistance to hydrogen embrittlement.

Claims

DEPCT6824 / 10 / 25671. Hot-stamping formed body parts composed of (chemical composition in percentage by mass): C: greater than 0.40 percent and 0.70 percent or less; Si: 0.010 percent to 3.000 percent; Mn: 0.10 percent or more and less than 0.60 percent; P: 0.100 percent or less; S: 0.0100 percent or less; N: 0.0200 percent or less; O: 0.0200 percent or less; Al: 0.0010 percent to 0.5000 percent; N b: 0.0010% to 0.1000%; Ti: 0.010% to 0.200%; Cr: 0.010% to 0.800%; Mo: 0.0010% to 1.0000%; B: 0.0005% to 0.0200%; Co: 0% to 4.00%; Ni: 0% to 3.00%; Cu: 0% to 3.00%; V: 0% to 3.00%; W: 0% to 3.00%; Ca: 0% to 1.0000%; Mg: 0% to 1.0000%; REM: 0% to 1.0000%; Sb: 0% to 1.00%.1.000%; Sn: 0% to 1.000%; Zr: 0% to 1.000%; As: 0% to 0.100%; and the remainder: Fe and impurities, in the surface layer region from the surface of the hot-pressed body part to a depth of 1 / 25 of the sheet thickness from the surface, the bainite area ratio is greater than 10%, the maximum polar density of the texture is 4.0 or less, and the surface boron reduction index is 0.05 or more.

2. Hot-pressed body parts according to the above The first type of element, composed of one or more elements selected from a group of constituent elements (chemical elements in percentage by mass), is: Co: 0.01% to 4.00%; Ni: 0.01% to 3.00%; Cu: 0.01% to 3.00%; V: 0.01% to 3.00%; W: 0.01% to 3.00%; Ca: 0.0001% to 1.0000%; Mg: 0.0001% to 1.0000%; REM: 0.0001% to 1.0000%; Sb: 0.001% to 1.000%; Sn: 0.001% to 1.000%.0.000 percent; Zr: 0.001 percent to 1.000 percent; and As: 0.001 percent to 0.100 percent;

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