Hot stamp molded body and automobile component

Abstract

Provided is a hot stamp molded body comprising: a first region having a metal structure in which, at a 1 / 4 thickness position, the area fraction of martensite is 95% or more and the average aspect ratio of prior austenite grains is 1.10 or less; and a second region having a metal structure in which, at a 1 / 4 thickness position, the area fraction of martensite is 95% or more and the average aspect ratio of prior austenite grains is 2.00 to 3.00. The area fraction of martensite at a 1 / 2 thickness position of the first region and the second region is 95% or more. The average crystal grain size G1 (μm) of the prior austenite grains at the 1 / 4 thickness position of the first region and the average crystal grain diameter G2 (μm) of the prior austenite grains at the 1 / 4 thickness position of the second region satisfy [|G2-G1|< 3.0].

Description

Hot stamped compacts and automotive parts

[0001] The present invention relates to a hot-stamped product and an automobile part.

[0002] To achieve both weight reduction and crashworthiness in automobiles, there is a demand for higher strength materials used in automobiles. For this reason, hot-stamped steel with a strength of up to 1.8 GPa has been put to practical use in car bodies in recent years. Furthermore, if chassis parts also require high strength similar to that of car bodies, it will be necessary to consider using hot-stamped steel in the future.

[0003] In the automotive parts described above, burring may be performed in order to assemble the parts together, etc. For example, Patent Document 1 discloses a method for manufacturing a hot-pressed product having a burred portion.

[0004] The manufacturing method disclosed in Patent Document 1 includes a heating step of heating a plate material and a forming step of forming the heated plate material. In the forming step, the plate material is quenched and burred in addition to being formed.

[0005] Japanese Patent Application Laid-Open No. 2019-58916

[0006] However, as a result of investigations by the present inventors, it was found that ferrite may be generated in regions where high strain is applied, such as burred parts, during hot stamping, resulting in local softening. This is thought to be due to the promotion of diffusional transformation by the high strain. Furthermore, various investigations using a hot working simulator revealed that the greater the processing strain, the lower the hardenability, and that it is necessary to increase the critical cooling rate to obtain a sound full martensite structure during the subsequent cooling process.

[0007] Because chassis parts use relatively thick steel, the cooling rate due to die cooling is slower than that of the thinner steel used in the body. From this perspective, it is thought that a decrease in hardenability is more likely to become apparent in areas where high processing strain has been introduced. Furthermore, even in body parts that already use hot stamping steel sheets, the structure is becoming more complex in order to control the deformation form of each part, and high strain may be introduced in some areas.

[0008] Furthermore, from the viewpoint of further improving safety, automobile parts and the like are also required to have excellent toughness even in areas where high processing strain has been introduced.

[0009] An object of the present invention is to solve the above problems and to provide a hot-stamped product in which a decrease in strength after hot stamping in a high strain region is suppressed even when high strain is locally introduced in the hot stamping step, and which has excellent toughness in a high strain region, and an automotive part using the hot-stamped product.

[0010] The present invention has been made to solve the above-mentioned problems, and provides the following hot-stamped product and automotive part.

[0011] (1) A hot stamped steel sheet comprising: a first region having a metallographic structure in which, at a quarter-thickness position, an area fraction of martensite is 95% or more and an average aspect ratio of prior austenite grains is 1.10 or less; and a second region having a metallographic structure in which, at the quarter-thickness position, an area fraction of martensite is 95% or more and an average aspect ratio of prior austenite grains is 2.00 to 3.00, wherein the area fractions of martensite at half-thickness positions in the first region and the second region are 95% or more, and where G1 (μm) is the average grain size of the prior austenite grains at the quarter-thickness position in the first region and G2 (μm) is the average grain size of the prior austenite grains at the quarter-thickness position in the second region, G1 and G2 satisfy the following formula (i): |G2 - G1| < 3.0 ... (i)

[0012] (2) The average hardness at the 1 / 2 thickness position of the first region is HV １＿１／２(HV 0.1), the average hardness at the 1 / 2 thickness position of the second region is HV ２＿１／２ (HV0.1), HV １＿１／２ and HV ２＿１／２ The hot stamped steel sheet according to the above (1), wherein HV satisfies the following formula (ii): 0.95≦HV ２＿１／２ / HV １＿１／２ ≦1.05...(ii)

[0013] (3) Chemical composition, in mass%, is: C: 0.10 to 0.60%, Si: 0.01 to 2.00%, Mn: 0.10 to 3.00%, P: 0.050% or less, S: 0.0200% or less, N: 0.0200% or less, O: 0.100% or less, Al: 0.001 to 0.100%, Cr: 0.01 to 1.00%, Nb: 0 to 0.200%, Ti: 0 to 0.200%, Mo: 0 to 1.00%, B: 0 to 0.0100%, Co: 0 to 4.00%, Ni: 0 to 2.00%, Cu: 0 to 1.00%, V: 0 to 1.00%, W: 0 to 1.00%, The hot stamped steel according to (1) or (2), comprising: Ca: 0 to 0.100%, Mg: 0 to 0.100%, REM: 0 to 0.100%, Sb: 0 to 0.100%, Zr: 0 to 0.100%, Sn: 0 to 1.00%, As: 0 to 0.100%, and the balance: Fe and impurities.

[0014] (4) An automobile part using the hot stamped product according to any one of (1) to (3) above.

[0015] According to the present invention, even when high strain is locally introduced in the hot stamping process, a decrease in strength in the high strain region after hot stamping is suppressed, and a hot-stamped product having excellent toughness in the high strain region and an automotive part using the hot-stamped product can be obtained.

[0016] FIG. 1A is a schematic perspective view showing a molded product having a circular burring portion. FIG. 1B is a plan view showing a molded product having a circular burring portion. FIG. 1C is a side view showing a molded product having a circular burring portion. FIG. 2A is a schematic view for explaining an example of a second measurement region, and is a cross-sectional view of the a-a portion in FIG. 1C. FIG. 2B is an enlarged view of the area indicated by the dotted line in FIG. 2A. FIG. 3A is a plan view showing a molded product having an elliptical burring portion. FIG. 3B is a cross-sectional view showing a molded product having an elliptical burring portion. FIG. 4A is a schematic perspective view showing a molded product having an arc-shaped flange portion. FIG. 4B is a plan view showing a molded product having an arc-shaped flange portion. FIG. 4C is a side view showing a molded product having an arc-shaped flange portion. FIG. 5A is a schematic view for explaining an example of a second measurement region, and is a cross-sectional view of the b-b portion in FIG. 4C. FIG. 5B is an enlarged view of the area indicated by the dotted line in FIG. 5A. FIG. 6A is a schematic perspective view showing a molded product having a flange deformation portion. FIG. 6B is a plan view showing a molded product having a flange deformation portion. FIG. 6C is a cross-sectional view of portion c-c in FIG. 6B. FIG. 7A is a schematic view illustrating an example of the second measurement region, and is a cross-sectional view of portion dd in FIG. 6C. FIG. 7B is an enlarged view of the region indicated by the dotted line in FIG. 7A. FIG. 8A is a schematic view illustrating an example of the second measurement region, and is a cross-sectional view of portion e-e in FIG. 6C. FIG. 8B is an enlarged view of the region indicated by the dotted line in FIG. 8A. FIG. 9A is a schematic view illustrating an example of the first measurement region, and is a cross-sectional view of portion f-f in FIG. 1B. FIG. 9B is an enlarged view of the region indicated by the dotted line in FIG. 9A. FIG. 10 is a schematic view showing the shape of a sub-size plate test piece according to the ASTM standard. FIG. 11 is a view illustrating the shape of a Charpy test piece used to evaluate toughness. FIG. 12A is a view illustrating the procedure for cutting a test piece from a burred portion and preparing a Charpy test piece. Fig. 12B is a diagram for explaining the procedure for cutting out a test piece from the burring portion to prepare a tensile test piece. Fig. 12C is a diagram for explaining the procedure for cutting out a test piece from the burring portion to prepare a tensile test piece. Fig. 12D is a diagram for explaining the procedure for cutting out a test piece from the burring portion to prepare a tensile test piece.

[0017] The present inventors have investigated methods for suppressing the deterioration of hardenability in a high strain region, and as a result have come to the following findings.

[0018] An example of a high strain region is a burring portion. In the burring portion, the die may be designed to lightly squeeze the steel sheet (in the following description, the steel sheet before hot stamping is also referred to as the "blank"), so there is a high possibility that a direct water-cooled die cannot be applied. Furthermore, there is a possibility that the die and the blank may not be in sufficient contact with each other over the entire burring portion. Due to the promotion of diffusion transformation by introducing high strain and the difficulty of ensuring a cooling rate, it is thought that softening due to the formation of ferrite is likely to occur.

[0019] Therefore, after repeated experiments under various hot stamping conditions, we found that increasing the starting temperature in the high strain region is effective in enabling quenching. When hot stamping is performed, the blank is first heated in a furnace. However, the blank cools while being removed from the furnace and transported to the hot stamping equipment, which can lower the starting temperature. To prevent a decrease in the starting temperature, after the blank is removed from the furnace, it is necessary to keep the blank warm or reheat it during transport from the furnace to the hot stamping equipment, at least in the region where high strain is expected to be applied. This makes it possible to quench in the high strain region and suppress a decrease in strength and toughness.

[0020] The present invention was made based on the above findings. Each of the features of the present invention will be described in detail below.

[0021] 1. Metallographic Structure of Hot-Stamped Steel The hot-stamped steel according to this embodiment has a first region having a metallographic structure in which, at a depth position of ¼ of the thickness from the surface of the base steel sheet (referred to herein as the "¼-thickness position"), an area fraction of martensite is 95% or more and an average aspect ratio of prior austenite grains is 1.10 or less, and a second region having a metallographic structure in which, at the ¼-thickness position, an area fraction of martensite is 95% or more and an average aspect ratio of prior austenite grains is 2.00 to 3.00. The area fractions of martensite in the first region and the second region at a depth position of ½ of the thickness from the surface of the base steel sheet (referred to herein as the "½-thickness position") are 95% or more.

[0022] The first region is a region where high strain was not introduced during hot stamping. Therefore, the average aspect ratio of the prior austenite grains at the 1 / 4 thickness position in the first region is 1.10 or less. The substantial lower limit of the average aspect ratio is 1.00. In addition, the metal structure is mainly composed of martensite due to the quenching associated with the hot stamping. Specifically, the area ratio of martensite at the 1 / 4 thickness position and the 1 / 2 thickness position is 95% or more. In the present invention, martensite includes not only as-quenched fresh martensite, but also tempered martensite and self-tempered martensite.

[0023] In the metal structure of this embodiment, the remainder other than martensite is bainite, retained austenite, ferrite, and pearlite. However, in the present invention, as will be described later, the area ratio of martensite is measured based on Vickers hardness, and therefore martensite whose hardness has been excessively reduced by tempering or self-tempering is excluded from martensite and included in the remainder.

[0024] On the other hand, the second region is a region where high strain is introduced during hot stamping, resulting in an average aspect ratio of prior austenite grains at the 1 / 4 thickness position of 2.00 to 3.00. As described above, in the region where high strain is introduced, diffusion transformation is promoted, which may result in the formation of ferrite, particularly in the center of the plate thickness, resulting in not only softening but also a decrease in toughness. However, in the second region, the starting temperature during hot stamping is increased to enable quenching, so the area fraction of martensite is 95% or more not only at the 1 / 4 thickness position but also at the 1 / 2 thickness position.

[0025] In addition, in forming that does not cause fracture, it is generally difficult to make the average aspect ratio of the prior austenite grains exceed 3.00. Therefore, the average aspect ratio of the prior austenite grains at the 1 / 4 thickness position in the second region is substantially 3.00 or less. In other words, a region in which the average aspect ratio of the prior austenite grains at the 1 / 4 thickness position exceeds 3.00 is considered to be a region that has not been formed appropriately, and is therefore excluded from the second region.

[0026] In the hot stamped steel according to this embodiment, even when a large strain is applied, the second region has a martensite area ratio of 95% or more, which makes it possible to suppress a decrease in strength and to improve toughness.

[0027] Furthermore, there is no difference in the heating temperature before hot stamping between the first region and the second region. Therefore, when the average grain size of the prior austenite grains at the 1 / 4 position in the thickness direction of the first region is G1 (μm) and the average grain size of the prior austenite grains at the 1 / 4 position in the thickness direction of the second region is G2 (μm), G1 and G2 are approximately the same. Specifically, G1 and G2 satisfy the following formula (i). Since G1 and G2 may be the same, the lower limit of |G2-G1| is 0. |G2-G1|<3.0 (i)

[0028] In addition, in a hot stamped steel sheet having a strength of 1.3 to 2.5 GPa, from the viewpoint of providing excellent toughness, G1 is preferably 5.0 μm or more and preferably 15.0 μm or less. G1 is more preferably 6.0 μm or more and more preferably 14.0 μm or less. That is, G1 is preferably 5.0 to 15.0 μm and more preferably 6.0 to 14.0 μm. For the same reason, G2 is preferably 5.0 μm or more and preferably 15.0 μm or less. G2 is more preferably 6.0 μm or more and more preferably 14.0 μm or less. That is, G2 is preferably 5.0 to 15.0 μm and more preferably 6.0 to 14.0 μm.

[0029] There is a so-called trade-off relationship between tensile strength and toughness, and the higher the strength of the hot-stamped steel sheet, the lower its toughness tends to be. From the viewpoint of emphasizing the balance between tensile strength and toughness, the strength of the hot-stamped steel sheet is preferably in the 1.5 GPa class. Furthermore, in a 1.5 GPa-class hot-stamped steel sheet, from the viewpoint of providing excellent toughness, it is preferable that G1 be in the range of 8.0 to 14.0 μm and G2 be in the range of 8.0 to 14.0 μm.

[0030] In the present invention, the metallographic structure of a hot-stamped steel is measured by the following method. First, measurement regions are determined. In this case, a region of the hot-stamped steel that has been subjected to a low degree of processing and has not been subjected to high strain can be selected as the measurement region of the first region (hereinafter also referred to as the "first measurement region"), while a region where high strain has been introduced, such as a burred portion, can be selected as the measurement region of the second region (hereinafter also referred to as the "second measurement region").

[0031] Next, for each of the above measurement regions (first measurement region and second measurement region), the martensite area fraction is measured at the 1 / 4 thickness position and the 1 / 2 thickness position, and the average grain size and average aspect ratio of the prior austenite grains at the 1 / 4 thickness position are measured. In the present invention, the martensite area fraction is measured by combining observation with a field emission scanning electron microscope (FE-SEM) and Vickers hardness measurement, and the average grain size and average aspect ratio of the prior austenite grains are measured by analysis using electron backscatter diffraction (EBSD). Each measurement procedure will be described in more detail.

[0032] After the measurement area is etched with nital, a 300 μm long and 2000 μm wide region centered at the 1 / 4 and 1 / 2 thickness positions of the measurement area is photographed at 1000x magnification using an FE-SEM. That is, an FE-SEM equipped with a secondary electron detector is used. Image analysis is then performed on the obtained microstructure photograph to measure the area fraction of structures determined to be ferrite or pearlite, and the remaining structures are determined to be martensite, bainite, or retained austenite. If the total area fraction determined to be martensite, bainite, or retained austenite is 95% or greater, a mesh is drawn at 70 μm intervals in the 300 μm long and 2000 μm wide region, and Vickers hardness is measured at all intersections of the mesh (4 × 28 = 112 points). The test force for the Vickers hardness measurement is 0.9807 N (0.1 kgf). Images are then photographed again at 1000x magnification using an FE-SEM. Then, by comparing the microstructure photographs taken before and after the Vickers hardness measurement, it is determined whether the indentation made by pressing the indenter in the Vickers hardness measurement (hereinafter simply referred to as "indentation") contains a structure other than martensite, bainite, and retained austenite, i.e., ferrite or pearlite.

[0033] Then, among the obtained Vickers hardness measurements, the average value of the measurements that are identified as containing only martensite, bainite, and / or retained austenite within the range of the depressions remaining on the surface is calculated, and if the following formula is satisfied, it is determined that the area ratio of martensite in the metal structure is 95% or more. Note that "HV0.1" below refers to the "hardness symbol" when a Vickers hardness test is conducted with a test force of 0.9807 N (0.1 kgf) (see JIS Z 2244-1:2020). HV ＭＢγ ≧957.1×[C]+221.0 where the meanings of the symbols in the above formula are as follows: HV ＭＢγ [C]: C content (mass%) of hot stamped body

[0034] If it is not possible to secure an area of ​​300 μm in length and 2000 μm in width within the measurement area, for example, four areas of 300 μm in length and 500 μm in width may be secured, and the above-mentioned FE-SEM observation and Vickers hardness measurement may be performed in each area.

[0035] Next, the measurement area is polished using #600 to #1500 silicon carbide paper, and then polished to a mirror finish using a liquid in which diamond powder with a particle size of 1 to 6 μm is dispersed in a diluted solution such as alcohol or pure water. Subsequently, the sample is polished for 8 minutes at room temperature using colloidal silica with a particle size of 0.25 μm that does not contain an alkaline solution to remove the strain introduced into the surface layer of the sample.

[0036] Then, EBSD measurements are performed at 0.1 μm intervals on a region of 300 μm length and 500 μm width centered at a quarter-thickness position of the measurement region to obtain crystal orientation information. For the measurements, an EBSD analyzer comprising an FE-SEM and an EBSD detector may be used, for example, an EBSD analyzer comprising a JEOL JSM-7001F and an AMETEK Hikari detector. In this case, for example, the degree of vacuum in the EBSD analyzer is 9.6×10 －５Pa or less, the acceleration voltage is 20 kV, the working distance (WD) is 15 mm, the irradiation current level is 18, and the tilt angle of the test piece is 70°.

[0037] The EBSD pattern is collected using the "AMETEK OIM Data Collection" software function provided with the EBSD analyzer. The analysis is performed by selecting the crystal structure as Iron (Alpha) or Iron (Gamma). Using the obtained crystal orientation information, the crystal orientation of the prior austenite grains is calculated from the crystal orientation relationship between general prior austenite grains and crystal grains with a bcc structure after transformation.

[0038] The crystal orientation of the prior austenite grains may be calculated by the following method: First, the prior austenite grains are identified by the method described in "Study on High-Precision Method for Reconstructing Austenite Structure of Steel" (Hata Kengo, Wakita Masayuki, Fujiwara Tomoya, Kono Kaori, Nippon Steel & Sumitomo Metal Technical Report No. 404 (2016), pp. 24-30).

[0039] In addition, when adopting the method described in the above-mentioned document, in the present invention, the orientation relationship between ferrite and austenite is a K-S relationship. In the present invention, structures determined to have a bcc structure by EBSD include pearlite, fresh martensite, tempered martensite, and bainite in addition to ferrite, and structures determined to have an fcc structure include retained austenite. Therefore, a structure determined to have a bcc structure by EBSD is considered to be ferrite, and a structure determined to have an fcc structure is considered to be austenite. Furthermore, the allowable angle described in the above-mentioned document is set to 3 degrees, and the allowable error is set to 5 degrees.

[0040] For austenite grains that could not be reconstructed even after the above procedure, the data is saved as ferrite grains (bcc structure). In other words, they are not converted to austenite (fcc structure). For each identified prior austenite grain, excluding prior austenite grains that are not entirely included in the field of view, such as those at the edges of the field of view, the grain size is determined using the Area Fraction of the "Grain Size (diameter)" chart in "AMETEK OIM Analysis (registered trademark)," and the aspect ratio is calculated by calculating the reciprocal of the "Grain Shape Aspect Ratio." Here, the reciprocal of the "Grain Shape Aspect Ratio" is used because the "Grain Shape Aspect Ratio" is the minor axis / major axis. In this case, the boundary where the misorientation is 15 degrees or more is defined as the austenite grain boundary.

[0041] In the present invention, the crystal grain size means the circle-equivalent diameter, and the aspect ratio means the ratio of the length in the major axis direction to the length in the direction perpendicular thereto.

[0042] The shape of the hot-stamped product according to this embodiment is not particularly limited, but has a portion including a first region to which no large strain has been applied and a portion including a second region to which large strain has been applied. The thickness of the portion to which no large strain has been applied is preferably 1.0 to 4.0 mm, and the thickness of the portion to which large strain has been applied is preferably 0.6 to 3.0 mm, and more preferably 1.0 to 3.0 mm.

[0043] Furthermore, suitable measurement regions will be described using a typical hot-stamped product as an example. Examples of hot-stamped products according to this embodiment include a product having a circular burring portion as shown in Fig. 1A, a product having an elliptical burring portion as shown in Fig. 3A, a product having an arc-shaped flange portion as shown in Fig. 4A, and a product having a deformed flange portion as shown in Fig. 6A. Suitable measurement regions for each product will be described in detail below.

[0044] Fig. 1A is a schematic perspective view showing a molded body having a circular burred portion, Fig. 1B is a plan view, and Fig. 1C is a side view. As shown in Fig. 1A, the molded body 100 has a plate-like portion 10 and a cylindrical burred portion 12 rising from the plate-like portion 10. The thickness t １ is, for example, 1.0 to 4.0 mm, and the thickness t ２ is, for example, 0.6 to 3.0 mm. The height L of the burring portion 12 (the distance in the thickness direction of the plate-shaped portion 10 between the plate-shaped portion 10 and the tip of the burring portion 12) is, for example, 3 to 30 mm. When viewed in the thickness direction of the plate-shaped portion 10, the diameter (inner diameter) φ of the tip of the burring portion 12 is, for example, 15 to 100 mm.

[0045] In the present invention, the molded body may be plated. In this specification, the "thickness" refers to the thickness of the base material, which is the total thickness minus the plating thickness. The plating thickness is measured using a high-frequency glow discharge optical emission surface analyzer (GDS). The specific measurement method is described below.

[0046] Three measurement positions are arbitrarily selected from each of the first and second regions of the hot-stamped body, each having a coating layer. At each measurement position, the concentrations of the elements Fe, Mn, Zn, Si, Al, O, Cr, Ni, Mg, Cu, and Sn are measured while sputtering is performed from the surface of the coating layer.

[0047] The content of each element is analyzed in the depth direction, and the depth at which the Fe concentration first reaches 90% by mass or more is determined. The average depth values ​​at each measurement point are then calculated, and this average is used as the plating thickness for each of the first and second regions. If the Fe concentration does not reach 90% by mass or more up to the depth that can be analyzed in a single GDS measurement, i.e., if the plating layer is thicker than the measurable depth, a portion of the plating layer equivalent to 80 to 90% of the previously measured depth is removed by polishing at an arbitrary position other than the previously measured position within the same region. The depth of the plating layer removed by polishing is determined from the change in plate thickness before and after polishing, and a new GDS analysis is performed from the polished surface. The plating thickness is measured by combining the results of the first and second measurements.

[0048] As a GDS measurement device, for example, a Marcus-type high-frequency glow discharge optical emission spectrometer GD-Profiler 2 (manufactured by HORIBA) is used. At this time, measurement is performed under the following conditions: discharge condition: 35 W, Ar pressure during measurement: 600 Pa, discharge range: diameter 4 mmφ, electrode distance: 0.15 to 0.25 mm, and measurement pitch in the plate thickness direction: 0.01 to 0.05 μm.

[0049] 1A to 1C, the burring portion 12 is formed to rise perpendicularly to the plate-shaped portion 10, but the burring portion 12 may rise at an angle relative to the plate-shaped portion 10. For example, the burring portion 12 may be formed so that the diameter becomes smaller toward the tip end (the further away from the plate-shaped portion 10 in the thickness direction of the plate-shaped portion 10).

[0050] In the molded body 100, high strain is introduced into the burring portion 12, and the strain increases toward the tip and toward the outer edge. Therefore, a cross section can be cut out at a position distance d from the tip of the burring portion 12 toward the plate-like portion 10, and a second measurement region can be selected from the cross section. The distance d is, for example, 0.5 to 1.0 mm.

[0051] 2A is a schematic diagram for explaining an example of the second measurement area, and is a cross-sectional view of the aa portion of FIG. 1C, and FIG. 2B is an enlarged view of the area indicated by the dotted line in FIG. 2A. In FIG. 2B, hatching is not used to avoid cluttering the drawing. As shown in FIG. 2B, ２ / 4 position and t ２ The observation area of ​​the FE-SEM is a region of 300 μm in length and 2000 μm in width indicated by a broken line, with the center at each of the positions of 1 / 2 and t ２ The region of 300 μm in length and 500 μm in width, indicated by the dashed dotted line and centered at the position of / 4, can be used as the EBSD analysis region.

[0052] In the example shown in FIG. 2B, the thickness t ２ is thick enough that t ２ / 4 position and t ２The regions of 300 μm in length and 2000 μm in width, each centered at the position of / 2, do not overlap. ２ If is thin, t ２ / 4 position and t ２ In such a case, the observation areas are selected from positions offset from each other in the circumferential direction of the cylindrical burring portion 12.

[0053] 3A is a plan view showing a molded body having an elliptical burring portion, and FIG. 3B is a cross-sectional view. In the molded body 100 shown in FIG. 1A, the burring portion 12 is circular when viewed from the thickness direction of the plate-like portion 10, but as in the molded body 200 shown in FIG. 3A, the burring portion 22 may be elliptical when viewed from the thickness direction of the plate-like portion 20. When viewed from the thickness direction of the plate-like portion 20, the minor axis (inner diameter) φ of the tip of the burring portion 22 is １ is, for example, 10 to 80 mm, and the major diameter (inner diameter) φ ２ is, for example, 15 to 100 mm.

[0054] When the burring portion 22 has an elliptical shape, a higher strain is imparted to the regions on both sides of the burring portion 22 with a larger degree of curvature in the major axis direction. Therefore, the second measurement region can be selected from the region on one side of the major axis direction, as indicated by the dotted line in FIG. 3B. The positions of the FE-SEM observation region and the EBSD analysis region are the same as those described above, and therefore will not be described here. Note that FIG. 3B is a cross-sectional view taken at a position a distance d away from the tip of the burring portion 22 toward the plate-like portion 20, where the distance d is, for example, 0.5 to 1.0 mm.

[0055] Fig. 4A is a schematic perspective view showing a molded body having an arc-shaped flange portion, Fig. 4B is a plan view, and Fig. 4C is a side view. As shown in Fig. 4A, molded body 300 has plate-shaped portion 30 and arc-shaped flange portion 32 rising from plate-shaped portion 30. The thickness t １ is, for example, 1.0 to 4.0 mm, and the thickness t ２is, for example, 0.6 to 3.0 mm. The height L of the flange portion 32 (the distance in the thickness direction of the plate-shaped portion 30 between the plate-shaped portion 30 and the tip of the flange portion 32) is, for example, 3 to 30 mm. When viewed in the thickness direction of the plate-shaped portion 30, the flange portion 32 has an arc shape, and the radius of curvature R of the surface on the inner diameter side thereof is, for example, 50 to 500 mm.

[0056] In the molded body 300, high strain is introduced into the flange portion 32, and the strain increases toward the outside of the arc (the side farther from the center of curvature). Therefore, a cross section can be cut out at a position distance d from the tip of the flange portion 32 toward the plate-like portion 30, and a second measurement region can be selected from the cross section. The distance d is, for example, 0.5 to 1.0 mm.

[0057] 5A is a schematic diagram illustrating an example of the second measurement area, and is a cross-sectional view of the portion bb in FIG. 4C, and FIG. 5B is an enlarged view of the area indicated by the dotted line in FIG. 5A. In FIG. 5B, hatching is not used to avoid cluttering the drawing. As shown in FIG. 5B, the area t ２ / 4 position and t ２ The observation area of ​​the FE-SEM is a region of 2000 μm in length and 300 μm in width, as shown by the broken line, with the center at each of the positions of 1 / 2. ２ The region of 500 μm in length and 300 μm in width, indicated by the dashed dotted line and centered at the position of / 4, can be used as the EBSD analysis region.

[0058] Fig. 6A is a schematic perspective view showing a molded body having a flange deformation portion, Fig. 6B is a plan view, and Fig. 6C is a cross-sectional view of part c-c in Fig. 6B. As shown in Fig. 6A, molded body 400 has a plate-like portion 40, a cylindrical portion 42 rising from plate-like portion 40, and an annular inner flange portion 44 protruding inward from cylindrical portion 42 and having a hole in the center. The thickness t of plate-like portion 40 is １ is, for example, 1.0 to 4.0 mm, and the thickness t ２ and the thickness t of the inner flange portion 44 ３is, for example, 0.6 to 3.0 mm. The height L of the cylindrical portion 42 (the distance between the plate-shaped portion 40 and the inner flange portion 44 in the thickness direction of the plate-shaped portion 40) is, for example, 3 to 30 mm. When viewed in the thickness direction of the plate-shaped portion 40, the diameter (inner diameter) φ of the hole in the inner flange portion 44 is, for example, 10 to 100 mm.

[0059] In the molded body 400, a large strain is introduced into the inner flange portion 44, and the strain increases toward the inside (the side closer to the hole). ３ / 4 away and t ３ Cross sections can be cut out at positions 1 / 2 apart, and a second measurement area can be selected from the cross sections.

[0060] Fig. 7A is a schematic diagram illustrating an example of the second measurement region, and is a cross-sectional view of the dd portion of Fig. 6C, and Fig. 7B is an enlarged view of the region indicated by the dotted line in Fig. 7A. Fig. 8A is a schematic diagram illustrating an example of the second measurement region, and is a cross-sectional view of the ee portion of Fig. 6C, and Fig. 8B is an enlarged view of the region indicated by the dotted line in Fig. 8A. In Figs. 7B and 8B, hatching is not used to avoid cluttering the drawings. As shown in Fig. 7B, the distance t from the outer surface of the inner flange portion 44 to the plate-like portion 40 side is ３ In the cross section at a distance of 1 / 4 mm, a region of 2000 μm in length and 300 μm in width, indicated by a dashed line and centered at a position distant from the hole in the inner flange portion 44 by a distance d, can be used as the FE-SEM observation region, and a region of 500 μm in length and 300 μm in width, indicated by a dashed line, can be used as the EBSD analysis region. In addition, as shown in FIG. 8B , a region of 500 μm in length and 300 μm in width, indicated by a dashed line, can be used as the EBSD analysis region. ３ In the cross section at a position 0.5 mm away from the hole, the observation area of ​​the FE-SEM can be an area 2000 μm long and 300 μm wide, as indicated by the dashed line, centered at a position distance d from the hole in the inner flange portion 44. The distance d is, for example, 0.5 to 1.0 mm.

[0061] As described above, each molded body has a plate-like portion, but the plate-like portion is a region where high strain has not been introduced. Therefore, it is preferable to select the first measurement region from the plate-like portion. The first measurement region will be described in more detail using Figure 1B and Figures 9A and 9B. Figure 9A is a schematic diagram illustrating an example of the first measurement region, and is a cross-sectional view of the f-f portion in Figure 1B, and Figure 9B is an enlarged view of the region indicated by the dotted line in Figure 9A. In Figure 9B, hatching is not used to avoid cluttering the drawing. As shown in Figure 1B, a cross-section parallel to the thickness direction of the plate-like portion 10 and at a distance D from the boundary where the plate-like portion 10 and the burring portion 12 are connected can be cut out, and the first measurement region can be selected from that cross-section. The distance D is, for example, 7 to 8 mm. Then, as shown in Figure 9B, a cross-section t from the surface of the plate-like portion 10 is cut out. １ / 4 position and t １ The observation area of ​​the FE-SEM was a region of 300 μm in length and 2000 μm in width, as indicated by the broken line, with the center at each of the positions of t / 2. １ The region of 300 μm in length and 500 μm in width, indicated by the dashed dotted line and centered at the position of / 4, can be used as the EBSD analysis region.

[0062] 2. Mechanical Properties In the hot stamped steel according to this embodiment, the metallographic structure is mainly martensite in the thickness center of the second region as well as in the first region, thereby suppressing local softening. Therefore, it is possible to suppress a significant decrease in the hardness in the thickness center of the second region relative to the hardness in the thickness center of the first region. In other words, the hardness in the thickness center of the first region and the hardness in the thickness center of the second region are equivalent. From this perspective, in the hot stamped steel according to this embodiment, the average hardness at the half-thickness position of the first region is set to HV １＿１／２ (HV 0.1), the average hardness at the 1 / 2 thickness position of the second region is HV ２＿１／２ (HV0.1), HV １＿１／２ and HV ２＿１／２It is preferable that the value of HV satisfies the following formula (ii): The value in the formula (ii) is preferably 0.97 or more and 1.03 or less. 0.95≦HV ２＿１／２ / HV １＿１／２ ≦1.05...(ii)

[0063] The average hardness HV at the 1 / 2 thickness position of the first region １ and the average hardness HV at the 1 / 2 thickness position of the second region ２ The average value of the Vickers hardness measurements at the half-thickness position at all intersections of the mesh with 70 μm intervals described above is used as the hardness.

[0064] Furthermore, the strength of the hot-stamped steel according to this embodiment is not particularly limited. However, the tensile strength measured by a tensile test using a test piece collected from the first region is preferably in the 1.3 to 2.5 GPa range, i.e., 1250 to 2540 MPa range. This is because, when the hot-stamped steel is used as an automobile component, having this strength is effective in reducing the automobile's weight and ensuring collision safety. Furthermore, as described above, when the balance between the tensile strength and toughness of the hot-stamped steel is important, a strength of the 1.5 GPa range is preferable, and specifically, the tensile strength is preferably 1450 to 1540 MPa.

[0065] In the present invention, tensile strength is measured using a sub-size plate-shaped test piece (width 10 mm, total length 100 mm, parallel portion width 6.25 mm, parallel portion length 32 mm, gauge length 25 mm, thickness: original thickness of the first region) specified in ASTM A370-22, the shape of which is shown in Figure 10. The tensile speed is a crosshead displacement rate of 3 mm / min. Other conditions are in accordance with JIS Z 2241:2022. As described above, when the molded body is plated, tensile strength is measured using the plated test piece, but the "thickness: original thickness of the first region" refers to the thickness of the base material portion, calculated by subtracting the plating thickness from the total thickness. In other words, the original cross-sectional area of ​​the test piece used to calculate tensile strength is the original cross-sectional area of ​​the base material portion excluding the plating layer. The plating thickness is measured using the same method as described above.

[0066] The hot-stamped steel according to this embodiment has excellent toughness even in the second region where high processing strain has been introduced. The toughness in the second region can be evaluated by the following method. First, three Charpy test specimens (width 10 mm, length 55 mm, thickness 1.0 mm) having the shape shown in FIG. 11 are taken from the second region of the hot-stamped steel. In this case, the Charpy test specimens are taken so that the half-thickness position overlaps at least a part of the bottom of a notch formed in the center in the longitudinal direction of the Charpy test specimen and on one side in the width direction.

[0067] Note that, due to dimensional limitations, it may not be possible to obtain the above-mentioned Charpy test specimens from the burring section, etc. In such cases, as shown in an example in Figures 12A to 12D, a test specimen with a width of 10 mm and a length and thickness within the possible range is obtained from the burring section, etc., and a notch is then formed on one side of the longitudinal center of the obtained test specimen in the width direction. However, the length of the obtained test specimen must be 6.0 mm or more. Then, other dummy materials are joined by welding to both sides of the longitudinal direction of the test specimen so that the test specimen is positioned at the center of the Charpy test specimen shown in Figure 12D. At this time, the test specimen is laser-welded while being clamped between copper jigs to prevent the heat from welding from affecting the notched portion.

[0068] After the Charpy test specimens shown in FIG. 11 or FIG. 12D are obtained, three of them are stacked and fastened with screws, and then a Charpy impact test is carried out at room temperature in accordance with JIS Z 2242:2023 to determine the absorbed energy (J). The absorbed energy is then divided by the sum of the original cross-sectional areas of the notched portions of the three stacked test specimens to obtain the Charpy impact value (J / cm ２ ) is calculated (see JIS G 0202:2013).

[0069] 11 and 12D, a Charpy test specimen having a height of 10 mm, a length of 55 mm, and a thickness of 1.0 mm is prepared, but the thickness may be, for example, 0.75 mm depending on the thickness of the burred portion, etc. When the thickness of the test specimen is to be less than 1.0 mm, three or more test specimens are taken and adjusted so that the total thickness is within the range of 2.8 to 3.5 mm.

[0070] 3. Chemical Composition The chemical composition of the hot-stamped steel according to this embodiment is not particularly limited as long as it has the strength described above. The hot-stamped steel according to this embodiment preferably has, for example, the chemical composition shown below. The reasons for limiting each element are as follows. In the following description, "%" for the content means "mass %."

[0071] C: 0.10 to 0.60% C is an element that improves the strength of the hot-stamped steel. Therefore, the C content is preferably 0.10% or more. The C content is more preferably more than 0.10%, 0.15% or more, 0.20% or more, or 0.25% or more. From the viewpoint of weldability, the C content is preferably 0.60% or less. More preferably, the C content is 0.50% or less, 0.40% or less, 0.38% or less, 0.37% or less, 0.35% or less, or 0.30% or less. The C content is preferably 0.10 to 0.50%, 0.10 to 0.40%, more than 0.10% to 0.38%, 0.15 to 0.37%, 0.20 to 0.35%, or 0.25 to 0.30%.

[0072] Si: 0.01 to 2.00% Si is an element that improves the strength of hot-stamped steel sheets through solid solution strengthening. Therefore, the Si content is preferably 0.01% or more. The Si content is more preferably 0.05% or more, 0.10% or more, 0.20% or more, 0.30% or more, or 0.40% or more. On the other hand, if the Si content exceeds 2.00%, the amount of ferrite increases, and the desired metal structure may not be obtained. Therefore, the Si content is preferably 2.00% or less. The Si content is more preferably 1.80% or less, 1.50% or less, 1.20% or less, 1.00% or less, or 0.80% or less. The Si content is preferably 0.05 to 1.80%, 0.10 to 1.50%, 0.20 to 1.20%, 0.30 to 1.00%, or 0.40 to 0.80%.

[0073] Mn: 0.10 to 3.00% Mn is an element that improves the hardenability of steel and contributes to improving strength. Therefore, the Mn content is preferably 0.10% or more. The Mn content is more preferably 0.20% or more, 0.50% or more, 1.00% or more, 1.30% or more, or 1.50% or more. On the other hand, if the Mn content exceeds 3.00%, Mn segregation may become significant. Therefore, the Mn content is preferably 3.00% or less. The Mn content is more preferably 2.80% or less, 2.50% or less, 2.30% or less, or 2.00% or less. The Mn content is preferably 0.20 to 2.80%, 0.50 to 2.50%, 1.00 to 2.30%, 1.30 to 2.00%, or 1.50 to 2.00%.

[0074] P: 0.050% or less P is an impurity element and may cause a decrease in weldability. Therefore, the P content is preferably 0.050% or less. The P content is more preferably 0.030% or less, 0.020% or less, or 0.010% or less. The lower limit of the P content does not need to be particularly limited, and may be 0%. However, excessive reduction of the P content significantly increases the dephosphorization cost, which is economically undesirable. Therefore, the P content may be more than 0% or may be 0.0001% or more.

[0075] S: 0.0200% or less S is an impurity element and may cause a decrease in weldability. Therefore, the S content is preferably 0.0200% or less. The S content is more preferably 0.0180% or less, 0.0150% or less, 0.0100% or less, 0.0060% or less, or 0.0040% or less. The lower limit of the S content does not need to be particularly limited, and may be 0%. However, excessive reduction of the S content significantly increases the desulfurization cost, which is economically undesirable. Therefore, the S content may be more than 0% or may be 0.0001% or more.

[0076] N: 0.0200% or less N is an impurity element and may cause a decrease in weldability. Therefore, the N content is preferably 0.0200% or less. The N content is more preferably 0.0180% or less, 0.0150% or less, 0.0100% or less, 0.0060% or less, or 0.0040% or less. The lower limit of the N content does not need to be particularly limited and may be 0%. However, excessive reduction of the N content significantly increases the denitrification cost, which is economically undesirable. Therefore, the N content may be more than 0% or may be 0.0001% or more.

[0077] O: 0.100% or less If a large amount of O is contained in steel, it may cause a decrease in weldability. Therefore, the O content is preferably 0.100% or less. The O content is more preferably 0.0700% or less, 0.0500% or less, 0.0300% or less, 0.0100% or less, or 0.0050% or less. The lower limit of the O content does not need to be particularly limited, and may be 0%. However, from the viewpoint of reducing refining costs, the O content may be more than 0%, or may be 0.0001% or more, or 0.0005% or more.

[0078] Al: 0.001 to 0.100% Al is an element that has the effect of deoxidizing molten steel and improving the quality of the steel. Therefore, the Al content is preferably 0.001% or more. The Al content is more preferably 0.005% or more, 0.010% or more, 0.015% or more, 0.020% or more, or 0.025% or more. On the other hand, if the Al content exceeds 0.100%, the effect saturates, so the Al content is preferably 0.100% or less. The Al content is more preferably 0.080% or less, 0.060% or less, 0.040% or less, 0.020% or less, or 0.010% or less. The Al content is preferably 0.005 to 0.080%, 0.010 to 0.070%, 0.015 to 0.060%, 0.020 to 0.050%, or 0.025 to 0.040%. The Al content is also preferably 0.001 to 0.020% or 0.001 to 0.010%.

[0079] Cr: 0.01 to 1.00% Cr is an element that improves the hardenability of steel. Therefore, the Cr content is preferably 0.01% or more. The Cr content is more preferably 0.03% or more, 0.05% or more, 0.10% or more, or 0.15% or more. On the other hand, if the Cr content exceeds 1.00%, the above effect saturates, so the Cr content is preferably 1.00% or less. The Cr content is more preferably 0.80% or less, 0.60% or less, 0.50% or less, or 0.40% or less. The Cr content is preferably 0.03 to 0.80%, 0.05 to 0.60%, 0.10 to 0.50%, or 0.15 to 0.40%.

[0080] The basic chemical composition of the hot-stamped steel according to the embodiment of the present invention is as described above. Furthermore, the hot-stamped steel may contain at least one of the following optional elements in place of a portion of the remaining Fe, as necessary. The optional elements will be described in detail below.

[0081] Nb: 0 to 0.200% Nb is an element that forms carbonitrides in steel and improves the strength of hot-stamped steel through precipitation strengthening. The Nb content may be 0.0005% or more, but to ensure this effect, the Nb content is preferably 0.001% or more or 0.002% or more. However, since the above effect saturates even when a large amount is added, the Nb content is preferably 0.200% or less. The Nb content may be 0.180% or less, 0.150% or less, 0.100% or less, 0.050% or less, or 0.010% or less. The Nb content is preferably 0.0005 to 0.180%, 0.001 to 0.150%, 0.002 to 0.100%, 0.003 to 0.050%, or 0.004 to 0.010%.

[0082] Ti: 0 to 0.200% Ti is an element that forms carbonitrides in steel and improves the strength of hot-stamped steel through precipitation strengthening. The Ti content may be 0.001% or more, but to ensure this effect, the Ti content is preferably 0.010% or more or 0.020% or more. However, since the above effect saturates even when a large amount of Ti is added, the Ti content is preferably 0.200% or less. The Ti content may be 0.180% or less, 0.150% or less, 0.100% or less, 0.070% or less, or 0.040% or less. The Ti content is preferably 0.001 to 0.180%, 0.005 to 0.150%, 0.010 to 0.100%, 0.015 to 0.070%, or 0.020 to 0.040%.

[0083] Mo: 0 to 1.00% Mo is an element that improves the hardenability of steel. The Mo content may be 0.001% or more, but to ensure this effect, the Mo content is preferably 0.003% or more or 0.005% or more. However, since the above effect saturates even when a large amount is added, the Mo content is preferably 1.00% or less. The Mo content may be 0.80% or less, 0.60% or less, 0.50% or less, 0.30% or less, or 0.10% or less. The Mo content is preferably 0.001 to 0.80%, 0.002 to 0.60%, 0.003 to 0.50%, 0.004 to 0.30%, or 0.005 to 0.10%.

[0084] B: 0 to 0.0100% B is an element that improves the hardenability of steel. The B content may be 0.0001% or more, but to ensure this effect, the B content is preferably 0.0005% or more or 0.0010% or more. However, since the above effect saturates even when a large amount of B is added, the B content is preferably 0.0100% or less. The B content may be 0.0080% or less, 0.0060% or less, 0.0050% or less, 0.0030% or less, or 0.0020% or less. The B content is preferably 0.0001 to 0.0080%, 0.0003 to 0.0060%, 0.0005 to 0.0050%, 0.0007 to 0.0030%, or 0.0010 to 0.0020%.

[0085] Co: 0 to 4.00% Co is an element that improves the strength of hot-stamped steel through solid solution strengthening. The Co content may be 0.001% or more, but to ensure this effect, the Co content is preferably 0.01% or more or 0.05% or more. However, since the above effect saturates even when a large amount of Co is added, the Co content is preferably 4.00% or less. The Co content may be 3.00% or less, 2.00% or less, 1.00% or less, 0.50% or less, or 0.10% or less. The Co content is preferably 0.001 to 3.00%, 0.005 to 2.00%, 0.01 to 1.00%, 0.03 to 0.50%, or 0.05 to 0.10%.

[0086] Ni: 0 to 2.00% Ni dissolves in austenite grains during heating in the hot stamping process, thereby enhancing the strength of the hot stamped steel. The Ni content may be 0.001% or more, but 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 2.00% or less. The Ni content may be 1.80% or less, 1.60% or less, 1.40% or less, 1.20% or less, 1.00% or less, 0.50% or less, or 0.10% or less. The Ni content is preferably 0.001 to 1.80%, 0.005 to 1.60%, 0.01 to 1.40%, 0.02 to 1.20%, 0.03 to 1.00%, 0.04 to 0.50%, or 0.05 to 0.10%.

[0087] Cu: 0 to 1.00% Cu dissolves in austenite grains during heating in the hot stamping process, thereby enhancing the strength of the hot stamped steel. The Cu content may be 0.001% or more, but to ensure this effect, the Cu content is preferably 0.01% or more or 0.05% or more. However, since the above effect saturates even when a large amount is added, the Cu content is preferably 1.00% or less. The Cu content may be 0.80% or less, 0.60% or less, 0.50% or less, 0.30% or less, or 0.10% or less. The Cu content is preferably 0.001 to 0.80%, 0.005 to 0.60%, 0.01 to 0.50%, 0.03 to 0.30%, or 0.05 to 0.10%.

[0088] V: 0 to 1.00% V forms carbonitrides in steel and has the effect of improving the strength of hot-stamped steel through precipitation strengthening. The V content may be 0.001% or more, but to ensure this effect, the V content is preferably 0.01% or more or 0.05% or more. However, since the above effect saturates even when a large amount is added, the V content is preferably 1.00% or less. The V content may be 0.80% or less, 0.60% or less, 0.50% or less, 0.30% or less, or 0.10% or less. The V content is preferably 0.001 to 0.80%, 0.005 to 0.60%, 0.01 to 0.50%, 0.03 to 0.30%, or 0.05 to 0.10%.

[0089] W: 0 to 1.00% W is an element that improves the hardenability of steel. The W content may be 0.001% or more, but to ensure this effect, the W content is preferably 0.005% or more or 0.01% or more. However, since the above effect saturates even when a large amount is added, the W content is preferably 1.00% or less. The W content may be 0.80% or less, 0.60% or less, 0.50% or less, 0.30% or less, or 0.10% or less. The W content is preferably 0.001 to 0.80%, 0.005 to 0.60%, 0.01 to 0.50%, 0.03 to 0.30%, or 0.05 to 0.10%.

[0090] Ca: 0 to 0.100% Ca is an element that can control the morphology of sulfides. The Ca content may be 0.0001% or more, but to ensure this effect, the Ca content is preferably 0.0005% or more or 0.0010% or more. On the other hand, since the above effect saturates even when a large amount of Ca is added, the Ca content is preferably 0.100% or less. The Ca content may be 0.0800% or less, 0.0600% or less, 0.0400% or less, 0.0200% or less, 0.0100% or less, 0.0080% or less, 0.0060% or less, 0.0040% or less, 0.0030% or less, or 0.0020% or less. The Ca content is preferably 0.0001 to 0.0800%, 0.0001 to 0.0600%, 0.0001 to 0.0400%, 0.0001 to 0.0200%, 0.0001 to 0.0100%, 0.0001 to 0.0080%, 0.0003 to 0.0060%, 0.0005 to 0.0040%, 0.0007 to 0.0030%, or 0.0010 to 0.0020%.

[0091] Mg: 0 to 0.100% Mg is an element that can control the morphology of sulfides. The Mg content may be 0.0001% or more, but to ensure this effect, the Mg content is preferably 0.0005% or more or 0.0010% or more. However, even if a large amount is added, the above effect saturates, so the Mg content is preferably 0.100% or less. The Mg content may be 0.0800% or less, 0.0600% or less, 0.0400% or less, 0.0200% or less, 0.0100% or less, 0.0080% or less, 0.0060% or less, 0.0040% or less, 0.0030% or less, or 0.0020% or less. The Mg content is preferably 0.0001 to 0.0800%, 0.0001 to 0.0600%, 0.0001 to 0.0400%, 0.0001 to 0.0200%, 0.0001 to 0.0100%, 0.0001 to 0.0080%, 0.0003 to 0.0060%, 0.0005 to 0.0040%, 0.0007 to 0.0030%, or 0.0010 to 0.0020%.

[0092] REM: 0 to 0.100% REM is an element that can control the morphology of sulfides. The REM content may be 0.0001% or more, but to ensure this effect, the REM content is preferably 0.0005% or more or 0.0010% or more. However, even if a large amount is added, the above effect saturates, so the REM content is preferably 0.100% or less. The REM content may be 0.0800% or less, 0.0600% or less, 0.0400% or less, 0.0200% or less, 0.0100% or less, 0.0080% or less, 0.0060% or less, 0.0040% or less, 0.0030% or less, or 0.0020% or less. The REM content is preferably 0.0001 to 0.0800%, 0.0001 to 0.0600%, 0.0001 to 0.0400%, 0.0001 to 0.0200%, 0.0001 to 0.0100%, 0.0001 to 0.0080%, 0.0003 to 0.0060%, 0.0005 to 0.0040%, 0.0007 to 0.0030%, or 0.0010 to 0.0020%.

[0093] In this embodiment, "REM" refers to a total of 17 elements, including Sc, Y, and lanthanides, and "REM content" refers to the content of one type of REM when there is one type of REM, and to the total content of two or more types of REM when there are two or more types of REM. REM is also generally supplied as misch metal, which is an alloy of multiple types of REM. Therefore, one or more individual elements may be added, or they may be added in the form of misch metal, for example.

[0094] Sb: 0 to 0.100% Sb is an element that suppresses oxidation of the surface of the base steel sheet. To reliably obtain this effect, the Sb content is preferably 0.001% or more. However, since the above effect saturates even when a large amount of Sb is added, the Sb content is preferably 0.100% or less. The Sb content may be 0.080% or less, 0.060% or less, 0.050% or less, 0.030% or less, or 0.010% or less. The Sb content is preferably 0.0001 to 0.080%, 0.0005 to 0.060%, 0.001 to 0.050%, 0.003 to 0.030%, or 0.005 to 0.010%.

[0095] Zr: 0 to 0.100% Zr is an element that suppresses oxidation of the surface of the base steel sheet. To reliably obtain this effect, the Zr content is preferably 0.001% or more. However, since the above effect saturates even when a large amount is added, the Zr content is preferably 0.100% or less. The Zr content may be 0.080% or less, 0.060% or less, 0.050% or less, 0.030% or less, or 0.010% or less. The Zr content is preferably 0.0001 to 0.080%, 0.0005 to 0.060%, 0.001 to 0.050%, 0.003 to 0.030%, or 0.005 to 0.010%.

[0096] Sn: 0 to 1.00% Sn is an element that suppresses oxidation of the surface of the base steel sheet. 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 of Sn is added, the Sn content is preferably 1.00% or less. The Sn content may be 0.800% or less, 0.500% or less, 0.200% or less, 0.100% or less, 0.050% or less, or 0.010% or less. The Sn content is preferably 0.0001 to 0.800%, 0.0005 to 0.500%, 0.001 to 0.200%, 0.002 to 0.100%, 0.003 to 0.050%, or 0.005 to 0.010%.

[0097] As: 0 to 0.100% As has the effect of increasing the strength of the hot-stamped steel. To reliably obtain this effect, the As content is preferably 0.001% or more. However, since the above effect saturates even when a large amount of As is added, the As content is preferably 0.100% or less. The As content may be 0.080% or less, 0.050% or less, 0.020% or less, 0.010% or less, or 0.005% or less. The As content is preferably 0.001 to 0.080%, 0.002 to 0.050%, 0.003 to 0.020%, 0.004 to 0.015%, or 0.005 to 0.010%.

[0098] The chemical composition of the hot-stamped steel according to this embodiment comprises the remainder, other than the above elements, of Fe and impurities, which are components that are mixed in due to various factors in the manufacturing process, including raw materials such as ore and scrap, during industrial production of the hot-stamped steel.

[0099] As described above, when emphasis is placed on the balance between the hydrogen embrittlement resistance and notch tensile strength of the hot stamped steel sheet, it is preferable that the hot stamped steel sheet have a strength of 1.5 GPa class. To achieve this strength, the chemical composition of the hot stamped steel sheet should be, in mass %, C: 0.18 to 0.26%, Si: 0.05 to 0.30%, Mn: 0.80 to 1.60%, P: 0.050% or less, S: 0.0200% or less, N: 0.0200% or less, O: 0.100% or less, Al: 0.005 to 0.080%, Cr: 0.10 to 0.50%, Nb: 0 to 0.020%, Ti: 0.010 to 0.050%, Mo: 0.010 to 0.050%, and Preferably, the composition is Fe: 0.050%, B: 0.0010 to 0.0050%, Co: 0 to 4.00%, Ni: 0 to 2.00%, Cu: 0 to 1.00%, V: 0 to 1.00%, W: 0 to 1.00%, Ca: 0 to 0.100%, Mg: 0 to 0.100%, REM: 0 to 0.100%, Sb: 0 to 0.100%, Zr: 0 to 0.100%, Sn: 0 to 1.00%, As: 0 to 0.100%, and the balance: Fe and impurities.

[0100] The chemical composition of the hot stamped body is, in mass%, C: 0.18 to 0.26%, Si: 0.05 to 0.30%, Mn: 0.80 to 1.60%, P: 0.050% or less, S: 0.0200% or less, N: 0.0200% or less, O: 0.100% or less, Al: 0.005 to 0.080%, Cr: 0.10 to 0.50%, Nb: 0 to 0.020%, Ti: 0.010 to 0.050%, Mo: 0 to 0.050%, B : 0.0010 to 0.0050%, Co: 0 to 4.00%, Ni: 0 to 2.00%, Cu: 0 to 1.00%, V: 0 to 1.00%, W: 0 to 1.00%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, REM: 0 to 0.0100%, Sb: 0 to 0.100%, Zr: 0 to 0.100%, Sn: 0 to 1.00%, As: 0 to 0.100%, and the balance: Fe and impurities are more preferable.

[0101] The chemical composition of the hot-stamped product 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 hot-stamped product has a plating layer on its surface, the plating layer may be removed by mechanical grinding before analyzing the chemical composition.

[0102] The hot stamped body according to this embodiment may be used for any purpose, including, but not limited to, automobile parts, such as lower arms, wheels, subframes, center pillars, front side members, front pillars, rear side members, side sills, and bumpers.

[0103] 5. Manufacturing Method An example of a method for manufacturing the hot-stamped steel according to this embodiment will be described.

[0104] First, a steel plate (blank) is manufactured as the raw material for the hot stamped body. Molten steel having the above-mentioned chemical composition is produced, and this molten steel is used to produce a slab. The slab to be subjected to hot rolling can be a continuous cast slab or one produced by a thin slab caster, etc. The above-mentioned steel plate manufacturing method is compatible with processes such as continuous casting-direct rolling (CC-DR), in which hot rolling is performed immediately after casting.

[0105] The slab heating temperature is preferably 1100°C or higher. A slab heating temperature in a temperature range below 1100°C leads to a decrease in the finish rolling temperature, and the strength during finish rolling tends to be high. As a result, rolling may become difficult or the shape of the steel sheet after rolling may be defective, so the slab heating temperature is preferably 1100°C or higher.

[0106] The finish rolling completion temperature is preferably 800°C or higher. If the finish rolling completion temperature is below 800°C, the rolling load will increase, which may make rolling difficult or may result in defective shape of the steel sheet after rolling, and therefore the lower limit of the finish rolling completion temperature is preferably 800°C. There is no need to particularly set an upper limit for the finish rolling completion temperature, but if the finish rolling completion temperature is set too high, the slab heating temperature must be set too high to ensure that temperature, and therefore the upper limit of the finish rolling completion temperature is preferably 1100°C. After the completion of finish rolling, the average cooling rate to the coiling temperature described below is preferably 10 to 100°C / s.

[0107] The coiling temperature is preferably 700°C or lower. If the coiling temperature exceeds 700°C, the thickness of the oxide formed on the steel sheet surface may be excessively increased, which may reduce pickling properties. If cold rolling is then performed, the lower limit of the coiling temperature is preferably 400°C. If the coiling temperature is less than 400°C, the strength of the hot-rolled steel sheet increases extremely, which is likely to induce sheet breakage and shape defects during cold rolling. Therefore, the lower limit of the coiling temperature is preferably 400°C. However, if the coiled hot-rolled steel sheet is softened by heating it in a box annealing furnace or continuous annealing equipment, it may be coiled at a low temperature of less than 400°C. Note that rough-rolled sheets may be joined together during hot rolling and continuously finished. The rough-rolled sheet may also be coiled once.

[0108] The hot-rolled steel sheet may be subjected to pickling, and the hot-rolled steel sheet after the pickling treatment may be cold-rolled to produce a cold-rolled steel sheet. When cold-rolling is performed, the reduction ratio may be, for example, 30 to 80%. The pickling treatment may be performed by immersing the steel sheet in an aqueous solution containing an inhibitor and having an acid concentration of 3 to 20 mass % at a temperature of 80°C or higher but lower than 100°C for 30 seconds or longer. Furthermore, the hot-rolled steel sheet or cold-rolled steel sheet may be annealed to produce a hot-rolled annealed steel sheet or cold-rolled annealed steel sheet. The annealing is performed by passing the steel sheet through a continuous annealing line, and the annealing temperature may be, for example, in the range of 550 to 750°C.

[0109] The obtained steel sheets may be subjected to a plating treatment before hot stamping. Examples of applicable plating treatments include, but are not limited to, hot-dip galvanizing, alloyed hot-dip galvanizing, electrogalvanizing, Zn—Ni plating (electroalloy galvanizing), Sn plating, Al—Si plating, alloyed electrogalvanizing, hot-dip zinc-aluminum alloy plating, hot-dip zinc-aluminum-magnesium alloy plating, hot-dip zinc-aluminum-magnesium-Si alloy plating, and zinc-vapor-deposited Al plating. The plating treatment can be carried out by passing the sheet through a continuous line.

[0110] Next, the blank is subjected to hot stamping. In this process, the blank is heated, and then press-formed and quenched. When producing a hot-stamped product having a burred portion, burring may be performed simultaneously with or consecutively to press-forming, and then quenched immediately. This process will be described in more detail below.

[0111] First, the blank was heated at an average heating rate of 2 to 200°C / s using Ac ３ Ac above points ３ After heating to a temperature below +65°C, the blank is held at that temperature for 10 to 180 seconds. Forming begins after the blank is removed from the furnace. At this time, in order to ensure hardenability and suppress the formation of ferrite, the forming start temperature is set to 750°C or higher for at least the portions where high strain is to be applied (hereinafter also referred to as "high strain portions"). On the other hand, for portions where high strain is not to be applied (hereinafter also referred to as "low strain portions"), the forming start temperature is set to 700°C or higher when the thickness is 2.0 mm or more, and 600°C or higher when the thickness is less than 2.0 mm.

[0112] Here, while the blank is removed from the furnace and transported to the hot stamping equipment, it is air-cooled at an average cooling rate of 10 to 30°C / s. As a result, it is usually difficult to stably maintain the forming start temperature at 750°C or higher in the highly strained portion. Therefore, at least the highly strained portion must be kept warm or reheated during transport from the furnace to the hot stamping equipment. Examples of methods for keeping warm or reheating include using heating equipment or heat-retaining equipment installed on the transport machine during transport from the furnace to the hot stamping equipment, or transporting dummy material for the cover on top of each other.

[0113] The molded product is then quenched by cooling to a temperature below the Mf point without demolding. At this time, the average cooling rate from the molding start temperature to the Ms point is 50°C / s or more and less than 150°C / s, and the average cooling rate from the Ms point to the Mf point is 10°C / s or more. After demolding in a temperature range below the Mf point, the molded product may be allowed to cool, for example, to room temperature.

[0114] By performing hot stamping under the above conditions, it is possible to suppress the generation of ferrite not only in the low strain portion but also in the high strain portion, thereby making it possible to suppress the decrease in strength and toughness. As a result, the first region is formed from the low strain portion and the second region is formed from the high strain portion.

[0115] Heating temperature is Ac ３ If the heating temperature is below the Ac point, structures other than austenite, such as ferrite, pearlite, bainite, and cementite, may remain after heating, and the area ratio of martensite may become less than 95% during the subsequent cooling process. ３ If the temperature exceeds the temperature point +65°C or the heating time exceeds 180 seconds, the crystal grains may become coarse and the toughness may decrease.

[0116] In addition, when the strength of the hot stamped body is set to 1.5 GPa class, G1 is set to 8.0 to 14.0 μm, and G2 is set to 8.0 to 14.0 μm, the heating temperature is set to Ac ３ Point +10℃ or more Ac ３ It is preferable that the temperature is set to 60°C or lower than the temperature at which the ３ Point +15℃ or more Ac ３It is more preferable to set the temperature at the temperature point +50°C or lower.

[0117] Furthermore, when the forming start temperature in the high strain region is less than 750°C, a large amount of ferrite is generated during the cooling process, and the area ratio of martensite at the half thickness position in particular may be less than 95%. Similarly, when the forming start temperature in the low strain region is less than 700°C for a thickness of 2.0 mm or more, or less than 600°C for a thickness of less than 2.0 mm, a large amount of ferrite is generated during the cooling process, and the area ratio of martensite may be less than 95%.

[0118] If the average cooling rate from the forming start temperature to the Ms point is less than 50°C / s, a large amount of ferrite will be generated during the cooling process, and the area ratio of martensite may be less than 95%. If the average cooling rate from the Ms point to the Mf point is less than 10°C / s, self-tempering may cause a decrease in strength. If the mold is released at a temperature equal to or higher than the Mf point, martensitic transformation may occur even after release, which may result in a decrease in the dimensional accuracy of the molded body.

[0119] Here, in the present invention, Ac ３ The Ac point, Ms point and Mf point are calculated based on the following formulas (I) to (III), respectively. ３ (°C) = 850 + 10 x (C + N) x Mn + 350 x Nb + 250 x Ti + 40 x B + 10 x Cr + 100 x Mo ... (I) Ms (°C) = 550 - 361 x C - 39 x Mn - 35 x V - 20 x Cr - 17 x Ni - 10 x Cu - 5 x (Mo + W) + 15 x Co + 30 x Al ... (II) Mf (°C) = 410.5 - 407.3 x C - 7.3 x Si - 37.8 x Mn - 20.5 x Cu - 19.5 x Ni - 19.8 x Cr - 4.5 x Mo ... (III) In the above formulas, the element symbols represent the content (mass%) of each element.

[0120] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples.

[0121] Molten steels (steel types A to H) having the chemical compositions shown in Table 1 were cast by a continuous casting method to produce slabs. These slabs were heated to a temperature of 1100°C or higher, and then subjected to rough rolling, finish rolling, cooling, and coiling under the conditions shown in Table 2 to produce hot-rolled steel sheets. Thereafter, the hot-rolled steel sheets of steel types A, C, F, and G were further subjected to cold rolling to produce cold-rolled steel sheets.

[0122]

[0123]

[0124] The obtained hot-rolled or cold-rolled steel sheet was subjected to hot stamping, which involves a series of press forming, burring, and quenching steps, under the conditions shown in Table 3. This produced a hot-stamped product having the shape shown in FIG. 1A and a plate-shaped portion and a burred portion. The thicknesses (end thicknesses) of the tip ends of the plate-shaped portion and the burred portion were as shown in Tables 4 and 5, and the diameter of the burred portion was 50 mm. In this case, for examples marked "Yes" in the "Heat Insulation" column of Table 3, dummy material heated to the same temperature as the steel sheet was placed on the portion where burring was planned (the portion that would become the burred portion after hot stamping) and / or the portion where burring was not planned (the portion that would become the plate-shaped portion after hot stamping), thereby suppressing temperature drop during transportation. The degree of temperature drop was also controlled by adjusting the thickness of the dummy material.

[0125]

[0126] In the table, "heating rate" and "heating temperature" refer to the average heating rate and heating temperature during heating before hot stamping. ３ The difference between the heating temperature and the ３ "Holding time" means the time during which the blank is held at the heating temperature, and "transport time" means the time from when the blank is removed from the furnace to when forming begins. "St-Ms cooling rate" means the average cooling rate from the forming start temperature St to the Ms point, and "Ms-Mf cooling rate" means the average cooling rate from the Ms point to the Mf point.

[0127] Next, the cross sections shown in Figures 1B and 1C, 2A and 2B, and 9A and 9B were used as measurement areas for the plate-like portion and the burred portion, respectively, and metallographic observation and Vickers hardness measurement were carried out by the following methods. The measurement results are shown in Tables 4 and 5.

[0128]

[0129]

[0130] First, the measurement area was etched with nital, and after etching, it was photographed at 1000x magnification using an FE-SEM. The FE-SEM used was equipped with a secondary electron detector. Image analysis was then performed on the obtained structural photographs to measure the area fraction of structures determined to be ferrite or pearlite, and the remaining structures were determined to be martensite, bainite, or retained austenite. The total area fraction determined to be martensite, bainite, or retained austenite is shown as "M + B + γ area fraction (%)" in Tables 5 and 6. If the area fraction was less than 95%, the martensite area fraction ("M area fraction" in Tables 5 and 6) was determined to be less than 95% ("<95%).

[0131] On the other hand, when the area ratio was 95% or more, a mesh was drawn at 70 μm intervals in an area of ​​300 μm length and 2000 μm width, and Vickers hardness was measured at all intersections of the mesh. The test force for the Vickers hardness measurement was 0.9807 N (0.1 kgf). Images were then taken again at 1000x magnification using an FE-SEM. Then, by comparing the structural photographs taken before and after the Vickers hardness measurement, it was determined whether the area of ​​the depressions remaining on the surface contained a structure other than martensite, bainite, and retained austenite, i.e., ferrite or pearlite.

[0132] Then, of the Vickers hardness measurements obtained, the average value of the measurements that identified the indentations as containing only martensite, bainite, and / or retained austenite was calculated, and if the following formula was satisfied, it was determined that the area fraction of martensite in the metal structure was 95% or more (≧95%). On the other hand, if the following formula was not satisfied, it was considered that the area fraction of bainite was relatively high or that the martensite had been excessively tempered, so it was determined that the martensite area fraction was less than 95% (<95%), and no further metal structure observation was performed. HV ＭＢγ ≧957.1×[C]+221.0 where the meanings of the symbols in the above formula are as follows: HV ＭＢγ [C]: C content (mass%) of hot stamped body

[0133] In the table, the HV at the 1 / 4 and 1 / 2 thickness positions of the plate-shaped part ＭＢγ and HV respectively. ＭＢγ１＿１／４ and HV ＭＢγ１＿１／２ and HV at the 1 / 4 and 1 / 2 thickness positions of the burred portion. ＭＢγ and HV respectively. ＭＢγ２＿１／４ and HV ＭＢγ２＿１／２ Also, HV ＭＢγ１＿１／４ and HV ＭＢγ２＿１／４ The ratios are also shown in Tables 4 and 5.

[0134] Furthermore, in the table, the average hardness at the 1 / 2 thickness position of the plate-shaped portion, and the average hardness at the 1 / 4 thickness position and the 1 / 2 thickness position of the burred portion are expressed in HV. １＿１／２ , HV ２＿１／４ and HV ２＿１／２ The average hardness is the average value of the measured Vickers hardness at all intersections of the mesh with 70 μm intervals.

[0135] Next, the observation area was mirror-polished and electrolytically polished under the conditions described above, and then measured using EBSD to identify prior austenite grains using the reconstruction method described above. The grain size and aspect ratio of each identified prior austenite grain were determined, and the measured values ​​for all prior austenite grains in the observation area were averaged to determine the average grain size and average aspect ratio.

[0136] Next, the tensile strength and toughness of each hot stamped steel sheet were evaluated by the following methods.

[0137] <Tensile strength> Subsize plate test pieces (width 10 mm, total length 100 mm, parallel portion width 6.25 mm, parallel portion length 32 mm, gauge length 25 mm, thickness: original thickness of plate portion) specified in ASTM A370-22, the shape of which is shown in Fig. 10, were prepared from the plate portion of each hot-stamped body. Tensile strength was measured by conducting a tensile test using the half-size plate test pieces. In this test, the pulling speed was a crosshead displacement speed of 3 mm / min, and the other conditions were in accordance with JIS Z 2241:2022.

[0138] <Toughness> Test specimens were cut out from the burred portion of the hot-stamped body, and three test specimens each having the shape shown in FIG. 12D were prepared. FIGS. 12A to 12D are diagrams illustrating the procedure for cutting test specimens from the burred portion and preparing Charpy test specimens. As shown in FIGS. 12A and 12B, first, test specimens having a height of 10 mm, a width of 8.0 mm, and a thickness of 1.0 mm were cut out from the tip of the burred portion 12. Then, as shown in FIG. 12C, a notch was formed on one side in the height direction at the center in the longitudinal direction of the test specimen. Thereafter, as shown in FIG. 12D, dummy materials indicated by hatching were joined by welding to both sides in the longitudinal direction of the test specimen to prepare Charpy test specimens.

[0139] Three of the Charpy test pieces were stacked and fastened with screws, and then a Charpy impact test was carried out at room temperature in accordance with JIS Z 2242:2023 to determine the absorbed energy (J). The absorbed energy was then divided by the sum of the original cross-sectional areas of the notched portions of the three stacked test pieces to determine the Charpy impact value (J / cm２ In this example, the Charpy impact value was calculated as 33.0 J / cm ２ When the toughness was equal to or greater than this, it was determined that the specimen had excellent toughness.

[0140] The results are shown in Table 6.

[0141]

[0142] As shown in Table 6, test numbers 1, 2, 4, 6, 7, 9, 11, 15, 19 and 20, which satisfied the requirements of the present invention, had high strength and excellent toughness in the burred portion (high strain region).

[0143] In contrast, in Test Nos. 3, 5, 8, 10, 12 to 14, 16 to 18, and 21 to 23, in which the hot stamping conditions were inappropriate, the martensite area ratio was less than 95% at least in the burred portion, resulting in a decrease in toughness in the burred portion.

[0144] Specifically, in test numbers 3, 5, 8, 10, 12, 16 to 18, and 21 to 23, the area ratio of martensite in the burred portion was not 95% or more due to the low forming start temperature. In particular, in test numbers 3, 16 to 18, and 21 to 23, although the area ratio of martensite was 95% or more at the 1 / 4 thickness position in the burred portion, the area ratio of martensite was not 95% or more at the 1 / 2 thickness position.

[0145] In test number 13, the average cooling rate from the forming start temperature to the Ms point in the plate-shaped portion and the burred portion was low, so the area ratio of martensite in the plate-shaped portion and the burred portion did not reach 95% or more. In test number 14, the heating temperature in the hot stamp forming process was low, so the area ratio of martensite in the plate-shaped portion and the burred portion did not reach 95% or more.

[0146] According to the present invention, even when high strain is locally introduced in the hot stamping process, a decrease in strength in the high strain region after hot stamping is suppressed, and a hot-stamped product having excellent toughness in the high strain region and an automotive part using the hot-stamped product can be obtained.

Claims

1. A hot stamped steel sheet comprising: a first region having a metallographic structure in which, at a quarter-thickness position, an area fraction of martensite is 95% or more and an average aspect ratio of prior austenite grains is 1.10 or less; and a second region having a metallographic structure in which, at the quarter-thickness position, an area fraction of martensite is 95% or more and an average aspect ratio of prior austenite grains is 2.00 to 3.00, wherein the area fractions of martensite at half-thickness positions in the first region and the second region are 95% or more, and where G1 (μm) is the average grain size of the prior austenite grains at the quarter-thickness position in the first region and G2 (μm) is the average grain size of the prior austenite grains at the quarter-thickness position in the second region, G1 and G2 satisfy the following formula (i): |G2 - G1| < 3.0 ... (i) 2. The average hardness at the 1 / 2 thickness position of the first region is HV １＿１／２ (HV 0.1), the average hardness at the 1 / 2 thickness position of the second region is HV ２＿１／２ (HV0.1), HV １＿１／２ and HV ２＿１／２ The hot stamped steel according to claim 1, wherein HV satisfies the following formula (ii): 0.95≦HV ２＿１／２ / HV １＿１／２ ≦1.05...(ii) 3. Chemical composition, in mass%, is: C: 0.10 to 0.60%, Si: 0.01 to 2.00%, Mn: 0.10 to 3.00%, P: 0.050% or less, S: 0.0200% or less, N: 0.0200% or less, O: 0.100% or less, Al: 0.001 to 0.100%, Cr: 0.01 to 1.00%, Nb: 0 to 0.200%, Ti: 0 to 0.200%, Mo: 0 to 1.00%, B: 0 to 0.0100%, Co: 0 to 4.00%, Ni: 0 to 2.00%, Cu: 0 to 1.00%, V: 0 to 1.00%, W: 0 to 1.00%, 3. The hot-stamped steel according to claim 1 or 2, comprising: Ca: 0 to 0.100%, Mg: 0 to 0.100%, REM: 0 to 0.100%, Sb: 0 to 0.100%, Zr: 0 to 0.100%, Sn: 0 to 1.00%, As: 0 to 0.100%, and the balance: Fe and impurities.

4. An automobile part using the hot stamped product according to any one of claims 1 to 3.

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