Hot stamp molded body and automobile component

Abstract

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 2.00-3.00, wherein: 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 of the prior austenite grains at the 1 / 4 thickness position is 5.0-15.0 μm; and, when the Vickers hardness is measured at the 1 / 4 thickness position of the first region and the second region, [1.05 ≤ HVMBγ2_1 / 4 / HVMBγ1_1 / 4 ≤ 1.25] is satisfied.

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 both excellent hydrogen embrittlement resistance and high notch tensile strength.

[0009] The present invention aims 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 process, and which has excellent hydrogen embrittlement resistance and high notch tensile strength, 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 body 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 are 5.0 to 15.0 μm, and when Vickers hardness is measured at the quarter-thickness positions in the first region and the second region, the following formula (i) is satisfied: 1.05≦HV ＭＢγ２＿１／４ / HV ＭＢγ１＿１／４≦1.25 (i) where the meanings of the symbols in the above formula are as follows: HV ＭＢγ１＿１／４ : The average value (HV 0.1) of the Vickers hardness measurement values ​​at the 1 / 4 thickness position of the first region, where the indentation made by pressing the indenter in the Vickers hardness measurement was determined to contain only martensite, bainite and / or retained austenite. HV ＭＢγ２＿１／４ Average value (HV0.1) of the Vickers hardness measurements at the 1 / 4 thickness position of the second region, where the indentation made by pressing the indenter in the Vickers hardness measurement was determined to contain only martensite, bainite and / or retained austenite.

[0012] (2) The hot-stamped steel according to (1) above, wherein G1 and G2 further satisfy the following formula (ii): |G2-G1|<3.0 (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 hydrogen embrittlement resistance and high notch tensile strength, 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-sized plate test piece according to the ASTM standard. FIG. 11 is a view illustrating the shape of a tensile test piece used to evaluate hydrogen embrittlement resistance and notch tensile strength. FIG. 12A is a view illustrating the procedure for cutting out a test piece from the burring portion and preparing a tensile 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] As a result of repeated experiments under various hot stamping conditions, it was found that increasing the cooling rate in the high strain region is effective in enabling hardening, which makes it possible to harden in the high strain region and suppress the decrease in strength.

[0020] On the other hand, in the low strain region, the cooling rate is relatively slowed to reduce the hardness compared to the high strain region, which makes it possible to improve the hydrogen embrittlement resistance and notch tensile strength of the hot stamped steel as a whole.

[0021] The present invention was made based on the above findings. Each of the features of the present invention will be described in detail below.

[0022] 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.

[0023] 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.

[0024] 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.

[0025] 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 quarter-thickness position of 2.00 to 3.00. As described above, in the region where high strain is introduced, diffusion transformation is promoted, which not only causes ferrite to form, particularly in the center of the plate thickness, resulting in softening, but also in reduced hydrogen embrittlement resistance and notch tensile strength. However, in the second region, the cooling rate is increased after hot stamping to enable quenching, so the area fraction of martensite is 95% or more not only at the quarter-thickness position but also at the half-thickness position.

[0026] 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.

[0027] In the hot stamped steel according to this embodiment, even when a large strain is applied, the steel has a second region in which the area ratio of martensite is 95% or more, which not only suppresses a decrease in strength but also makes it possible to improve hydrogen embrittlement resistance and notch tensile strength.

[0028] Furthermore, in order to provide a hot stamped steel sheet having a strength of 1.3 to 2.5 GPa with excellent hydrogen embrittlement resistance and high notch tensile strength, when the average grain size of the prior austenite grains at the 1 / 4 thickness position of the first region is G1 (μm) and the average grain size of the prior austenite grains at the 1 / 4 thickness position of the second region is G2 (μm), G1 and G2 are set to 5.0 to 15.0 μm. G1 is preferably 6.0 μm or more and 14.0 μm or less. That is, G1 is preferably 6.0 to 14.0 μm. Furthermore, G2 is preferably 6.0 μm or more and 14.0 μm or less. That is, G2 is preferably 6.0 to 14.0 μm.

[0029] There is a so-called trade-off relationship between hydrogen embrittlement resistance and notch tensile strength, and as the strength of the hot-stamped steel increases, the notch tensile strength improves, but the hydrogen embrittlement resistance tends to decrease. From the viewpoint of emphasizing the balance between hydrogen embrittlement resistance and notch tensile strength, the strength of the hot-stamped steel is preferably in the 1.5 GPa class. In order to provide a 1.5 GPa-class hot-stamped steel with excellent hydrogen embrittlement resistance and high notch tensile strength, 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] As mentioned above, although the cooling rate is different between the first and second regions, the heating temperature remains unchanged, so G1 and G2 are approximately the same. Therefore, G1 and G2 may satisfy the following formula (ii). Since G1 and G2 may be the same, the lower limit of |G2-G1| is 0. |G2-G1|<3.0 (ii)

[0031] 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").

[0032] 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.

[0033] 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.

[0034] 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

[0035] 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.

[0036] 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.

[0037] 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 －５ Pa 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°.

[0038] 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.

[0039] 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).

[0040] 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.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] 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.

[0050] 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).

[0051] 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.

[0052] 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.

[0053] In the example shown in FIG. 2B, the thickness t ２ is thick enough that t ２ / 4 position and t ２ The 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.

[0054] 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.

[0055] 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.

[0056] 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 ２ is, 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.

[0057] 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.

[0058] 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.

[0059] 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 ３ is, 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.

[0060] 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.

[0061] 7A is a schematic diagram illustrating an example of the second measurement region, and is a cross-sectional view of the 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 diagram illustrating an example of the second measurement region, and is a cross-sectional view of the portion ee in FIG. 6C. FIG. 8B is an enlarged view of the region indicated by the dotted line in FIG. 8A. In FIG. 7B and FIG. 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 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.

[0062] 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.

[0063] 2. Mechanical Properties In the hot stamped steel according to this embodiment, when the Vickers hardness is measured at the quarter thickness position of the first region and the second region, the following formula (i) is satisfied: 1.05≦HV ＭＢγ２＿１／４ / HV ＭＢγ１＿１／４ ≦1.25 (i) where the meanings of the symbols in the above formula are as follows: HV ＭＢγ１＿１／４ : The average value (HV 0.1) of the Vickers hardness measurement values ​​at the 1 / 4 thickness position of the first region, where the indentation made by pressing the indenter in the Vickers hardness measurement was determined to contain only martensite, bainite and / or retained austenite. HV ＭＢγ２＿１／４Average value (HV0.1) of the Vickers hardness measurements at the 1 / 4 thickness position of the second region, where the indentation made by pressing the indenter in the Vickers hardness measurement was determined to contain only martensite, bainite and / or retained austenite.

[0064] This is because the cooling rate in the second region is higher than that in the first region, making self-tempering relatively difficult and resulting in higher hardness. In other words, in the second region, the cooling rate is increased to enable quenching, thereby increasing hardness, whereas in the first region, the increase in hardness is relatively suppressed. By suppressing the increase in hardness in the first region, it is possible to improve the hydrogen embrittlement resistance and notch tensile strength of the hot-stamped body as a whole. The central value in the above formula (i) is preferably 1.20 or less, and more preferably 1.15 or less.

[0065] In addition, HV ＭＢγ１＿１／４ and HV ＭＢγ２＿１／４ is the above-mentioned HV ＭＢγ It is measured by the same method as HV. ＭＢγ１＿１／４ is the HV measured when determining the area fraction of martensite at the 1 / 4 thickness position of the first region. ＭＢγ and HV ＭＢγ２＿１／４ is the HV measured when determining the area fraction of martensite at the 1 / 4 thickness position of the second region. ＭＢγ Therefore, the measurement method will not be described here.

[0066] 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 emphasis is placed on the balance between the hydrogen embrittlement resistance and notch tensile strength of the hot-stamped steel, a strength of the 1.5 GPa range is preferable, and specifically, the tensile strength is preferably 1450 to 1540 MPa.

[0067] 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.

[0068] The hot-stamped steel according to this embodiment has excellent hydrogen embrittlement resistance and high notch tensile strength. However, as described above, there is a so-called trade-off between hydrogen embrittlement resistance and notch tensile strength. Therefore, in this embodiment, it can be determined that the steel has excellent hydrogen embrittlement resistance and high notch tensile strength when, for example, the sum of the fracture stress in a hydrogen environment, which is an index of hydrogen embrittlement resistance, and the notch tensile strength is 2600 MPa or more.

[0069] The fracture stress and notch tensile strength in a hydrogen environment can be evaluated by the following method. First, two tensile test specimens (width 9.0 mm, total length 120 mm, parallel portion width 2.0 mm, thickness 1.0 mm) having the shape shown in Fig. 11 are taken from each of the hot-stamped steel sheets. Preferably, the above tensile test specimens are taken from each of the first and second regions, and evaluation is performed for each region. In this case, the tensile test specimens are taken so that a position at a depth of 1 / 2 of the thickness from the surface of the base steel sheet overlaps with at least a part of the bottom of a U-notch formed at the center in the longitudinal direction of the tensile test specimen and on both sides in the width direction.

[0070] Note that, due to dimensional limitations, it may not be possible to obtain the above-mentioned tensile 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 2.0 mm and a length and thickness within the possible range is obtained from the burring section, etc., and a U-notch is then formed on both sides 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 welded to both sides of the longitudinal direction of the test specimen so that the test specimen is positioned in the center of the tensile 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 U-notch formed portion.

[0071] Furthermore, due to dimensional constraints, it may not be possible to obtain the 1.0 mm thick tensile test specimen shown in Figure 11 or the 1.0 mm thick test specimen shown in Figure 12D. In such cases, two 0.5 mm thick tensile test specimens are prepared as described above and then joined by screws or welding to obtain a 1.0 mm thick tensile test specimen. The joining by screws or welding is performed at the center position in the height direction of the grips formed on both ends of the tensile test specimen (on the dashed line in the figure).

[0072] One of the tensile test pieces was placed in a 3% NaCl aqueous solution as a cathode, and a current density of 1.0 mA / cm ２ A tensile test is carried out at a tension speed of 0.006 mm / min while generating hydrogen on the surface of the tensile test piece under the conditions of (a) and (b) to measure the breaking stress (maximum stress). The measured breaking stress is a value that serves as an index of hydrogen embrittlement resistance, and is defined as the tensile strength in a hydrogen environment.

[0073] The other tensile test piece is subjected to a tensile test in accordance with JIS Z 2241:2022 at 0.006 mm / min in air to measure the breaking stress (maximum stress), and the measured value is defined as the notch tensile strength.

[0074] 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 %."

[0075] 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%.

[0076] 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%.

[0077] 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%.

[0078] 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.

[0079] 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.

[0080] 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.

[0081] 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.

[0082] 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%.

[0083] 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%.

[0084] 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.

[0085] 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%.

[0086] 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%.

[0087] 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%.

[0088] 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%.

[0089] 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%.

[0090] 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%.

[0091] 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%.

[0092] 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%.

[0093] 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%.

[0094] 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%.

[0095] 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%.

[0096] 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%.

[0097] 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.

[0098] 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%.

[0099] 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%.

[0100] 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%.

[0101] 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%.

[0102] 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.

[0103] 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.

[0104] 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.

[0105] 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.

[0106] 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.

[0107] 5. Manufacturing Method An example of a method for manufacturing the hot-stamped steel according to this embodiment will be described.

[0108] 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.

[0109] 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.

[0110] 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.

[0111] 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.

[0112] 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.

[0113] 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.

[0114] Next, the blank is subjected to hot stamping. In this process, the blank is heated, followed by press forming and quenching. In quenching, the blank is cooled to a temperature below the Mf point while controlling the cooling rate so that the highly strained portion is higher than the portion not highly strained. When producing a hot stamped product having a burred portion, burring may be performed simultaneously with or consecutively to press forming, and then quenching may be performed immediately thereafter. This will be described in more detail below.

[0115] 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 120 seconds. After the blank is removed from the furnace, forming begins. At this time, the temperature at the start of forming is set to 700°C or higher. Note that from the time the blank is removed from the furnace until forming begins, it is air-cooled at an average cooling rate of 10 to 20°C / s.

[0116] The hot-stamped steel sheet is then quenched by cooling to a temperature below the Mf point without being released from the mold. In order to form the first and second regions in the hot-stamped steel sheet, it is important to control the cooling conditions after hot stamping.

[0117] For regions to which high strain is not imparted (hereinafter also referred to as "low strain regions"), the average cooling rate from the forming 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 and less than 80°C / s. On the other hand, for regions to which high strain is imparted (hereinafter also referred to as "high strain regions"), the average cooling rate from the forming start temperature to the Ms point is 200°C / s or more, and the average cooling rate from the Ms point to the Mf point is 100°C / s or more. There is no need to set an upper limit to the cooling rate for the high strain regions, but from the viewpoint of manufacturing equipment capacity, it is preferable that the average cooling rate from the forming start temperature to the Ms point be 2000°C / s or less, and the average cooling rate from the Ms point to the Mf point be 1800°C / s or less. After demolding in a temperature range below the Mf point, the material may be allowed to cool to room temperature, for example, at an average cooling rate of 5°C / s or less.

[0118] Methods for achieving a difference in cooling rate between the two sections include: (1) direct water cooling of only the high-strain section and die cooling of the low-strain section; (2) using different die materials in contact with the two sections, with the die material in contact with the high-strain section having a high thermal conductivity; and (3) using different pressures between the blank and die when forming the two sections, increasing the forming pressure in the high-strain section and increasing heat conduction to the die.

[0119] By performing hot stamping under the above conditions, it is possible to suppress an increase in hardness in the low strain region, thereby improving hydrogen embrittlement resistance and notch tensile strength, while suppressing the formation of ferrite in the high strain region, thereby forming a first region from the low strain region and a second region from the high strain region.

[0120] 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 120 seconds, the crystal grains may become coarse, and the hydrogen embrittlement resistance and notch tensile strength may decrease.

[0121] 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 ３ It is more preferable to set the temperature at the temperature point +50°C or lower.

[0122] Furthermore, if the forming start temperature is less than 700°C, a large amount of ferrite will be generated during air cooling from the time of heating until the start of forming, or during the subsequent cooling process, and the area ratio of martensite, particularly at the half-thickness position, may become less than 95%.

[0123] In the low strain region, 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 is generated during the cooling process, and the area ratio of martensite at the half-thickness position in particular may be less than 95%. On the other hand, if the average cooling rate is 150°C / s or more, the average cooling rate from the Ms point to the Mf point may also be likely to increase during production.

[0124] In the low strain region, if the average cooling rate from the Ms point to the Mf point is less than 10°C / s, the strength may be excessively reduced due to self-tempering. On the other hand, if the average cooling rate is 80°C / s or more, the effect of self-tempering may not be sufficiently obtained, and the hardness in the first region may increase, resulting in a risk of not being able to sufficiently improve the hydrogen embrittlement resistance and notch tensile strength.

[0125] In the highly strained portion, if the average cooling rate from the forming start temperature to the Ms point is less than 200°C / s, 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 become less than 95%. Furthermore, if the average cooling rate from the forming start temperature to the Ms point is 200°C / s or more, the average cooling rate from the Ms point to the Mf point will be 100°C / s or more in manufacturing.

[0126] As described above, after hot stamp forming, the mold is released at a temperature below the Mf point because, if the mold is released at a temperature equal to or higher than the Mf point, martensitic transformation occurs even after the mold is released, which may result in a decrease in the dimensional accuracy of the formed body.

[0127] 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.

[0128] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples.

[0129] Molten steels (steel types a to m) 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 e and g were further subjected to cold rolling to produce cold-rolled steel sheets.

[0130]

[0131]

[0132] The obtained hot-rolled or cold-rolled steel sheet was subjected to hot stamping, which involves a series of steps of press forming, burring, and quenching, under the conditions shown in Table 3, to produce a hot-stamped product having the shape shown in FIG. 1A and including a plate-like portion and a burred portion. The plate-like portion had a thickness of 2.6 mm, the tip end of the burred portion had a thickness shown in Table 5 (end thickness), and the diameter of the burred portion was 50 mm. In this case, the portion where burring was performed (the portion that will become the burred portion after hot stamping) was directly water-cooled, and the other portion (the portion that will become the plate-like portion after hot stamping) was die-cooled, thereby providing a difference in cooling rate between the two portions.

[0133]

[0134] 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 material is held at the heating temperature. "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.

[0135] 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.

[0136]

[0137]

[0138] 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 4 and 5. If the area fraction was less than 95%, the martensite area fraction ("M area fraction" in Tables 4 and 5) was determined to be less than 95% ("<95%).

[0139] 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.

[0140] 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

[0141] 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.

[0142] Furthermore, in the table, the average hardness at the 1 / 4 and 1 / 2 thickness positions of the plate-shaped portion, and the average hardness at the 1 / 4 and 1 / 2 thickness positions of the burred portion are respectively expressed as HV １＿１／４ , 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.

[0143] 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.

[0144] Next, the tensile strength, hydrogen embrittlement resistance, and notched tensile strength of each hot stamped steel were evaluated by the following methods.

[0145] <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. Then, a tensile test was performed using the half-size plate test pieces to measure the tensile strength TS (MPa). The tensile speed was a crosshead displacement rate of 3 mm / min, and the other conditions were in accordance with JIS Z 2241:2022.

[0146] <Hydrogen Embrittlement Resistance and Notched Tensile Strength> Test specimens were cut from each of the plate-like portion and the burred portion of the hot-stamped body to prepare two tensile test specimens each having the shape shown in FIG. 11 and FIG. 12D . FIGS. 12A to 12D are diagrams illustrating the procedure for cutting test specimens from the burred portion to prepare tensile test specimens. Due to dimensional constraints, it was not possible to obtain tensile test specimens having the shape shown in FIG. 11 from the burred portion. Therefore, as shown in FIGS. 12A and 12B , first, test specimens having a length of 8.0 mm, a thickness of 1.0 mm, and a height of 2.0 mm were cut from the tip of the burred portion 12. Then, as shown in FIG. 12C , U-notches were formed on both sides in the height direction at the center of the longitudinal direction of the test specimen. Then, as shown in FIG. 12D , dummy materials (shown by hatching) were welded to both sides in the longitudinal direction of the test specimen to obtain tensile test specimens having the same shape as that shown in FIG. 11 .

[0147] One of the tensile test pieces was placed in a 3% NaCl aqueous solution as a cathode, and a current density of 1.0 mA / cm ２ The tensile test was performed at a tension speed of 0.006 mm / min while generating hydrogen on the surface of the tensile test specimen under the above conditions, and the fracture stress (maximum stress) in a hydrogen environment was measured. Pt was used as the anode. The measured value was taken as the tensile strength under a hydrogen environment, TSH (MPa), and was used as an index of hydrogen embrittlement resistance.

[0148] The other tensile test piece was subjected to a tensile test in accordance with JIS Z 2241:2022 at 0.006 mm / min in air to measure the breaking stress (maximum stress), and the measured value was defined as the notched tensile strength TSN (MPa).

[0149] In this example, when the sum of the measured tensile strength in a hydrogen environment and the notched tensile strength, TSH+TSN (MPa), was 2600 MPa or more, it was determined that the steel had excellent hydrogen embrittlement resistance and high notched tensile strength.

[0150] The results are shown in Table 6.

[0151]

[0152] As shown in Tables 4 to 6, test numbers 1 to 4, 13 to 16, 20 to 22, and 24 to 34, which satisfied the provisions of the present invention, had high strength and were able to achieve both excellent hydrogen embrittlement resistance and high notch tensile strength.

[0153] In contrast, in Test Nos. 5 to 12, 17 to 19, and 23, in which the hot stamping conditions were inappropriate, the martensite area ratio was less than 95% in at least one of the plate portion and the burred portion, the average grain size exceeded the specified value, or the formula (i) was not satisfied. As a result, the effects of improving hydrogen embrittlement resistance and notch tensile strength could not be obtained.

[0154] Specifically, in test number 5, the heating temperature in the hot stamping process was excessive, and in test number 10, the heating time was excessive, so that the average crystal grain size became excessive in both the plate-like portion and the burred portion, and TSH+TSN decreased.

[0155] In test number 6, the average cooling rate from the forming start temperature to the Ms point in the burred portion was low, so the area ratio of martensite in the burred portion did not reach 95% or more. As a result, TSH + TSN in the burred portion decreased.

[0156] In Test No. 7, the area ratio of martensite was not 95% or more in the plate-like portion and the burred portion because the forming start temperature was low. As a result, TSH+TSN decreased in both the plate-like portion and the burred portion.

[0157] In test numbers 8, 17, and 23, the average cooling rate from the forming start temperature to the Ms point in the burred portion was slightly low, so that although the area fraction of martensite was 95% or more at the 1 / 4 thickness position in the burred portion, the area fraction of martensite was not 95% or more at the 1 / 2 thickness position in the burred portion. As a result, TSH + TSN in the burred portion decreased slightly.

[0158] In Test Nos. 9 and 18, the average cooling rate from the Ms point to the Mf point in the plate-shaped portion and the average cooling rate from the forming start temperature to the Ms point in the burred portion were low. Therefore, the area fraction of martensite in the plate-shaped portion and the burred portion did not reach 95% or more. As a result, TSH + TSN decreased in both the plate-shaped portion and the burred portion.

[0159] In Test No. 11, the heating temperature in the hot stamping process was low, so the area ratio of martensite was not 95% or more in the plate-like portion and the burred portion, resulting in a decrease in TSH+TSN in both the plate-like portion and the burred portion.

[0160] In test number 12, the average cooling rate from the forming start temperature to the Ms point in the plate-shaped part was excessive, so the self-tempering effect was not sufficient in the plate-shaped part, and the hardness increased. As a result, TSH + TSN in the plate-shaped part slightly decreased.

[0161] In Test No. 19, the average cooling rate from the forming start temperature to the Ms point in the plate-shaped part was low, so the area ratio of martensite in the plate-shaped part did not reach 95% or more. As a result, TSH + TSN in the plate-shaped part slightly decreased.

[0162] According to the present invention, it is possible to obtain 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 process, and an automotive part using the hot-stamped product.

Claims

1. A hot stamped body comprising: a first region having a metallographic structure in which, at the 1 / 4 thickness position, an 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 metallographic structure in which, at the 1 / 4 thickness position, an area fraction of martensite is 95% or more and the average aspect ratio of prior austenite grains is 2.00 to 3.00, wherein the area fractions of martensite at the 1 / 2 thickness positions of 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 1 / 4 thickness position of the first region and G2 (μm) is 5.0 to 15.0 μm, and when Vickers hardness is measured at the 1 / 4 thickness positions of the first region and the second region, the following formula (i) is satisfied: 1.05≦HV ＭＢγ２＿１／４ / HV ＭＢγ１＿１／４ ≦1.25 (i) where the meanings of the symbols in the above formula are as follows: HV ＭＢγ１＿１／４ : The average value (HV 0.1) of the Vickers hardness measurement values ​​at the 1 / 4 thickness position of the first region, where the indentation made by pressing the indenter in the Vickers hardness measurement was determined to contain only martensite, bainite and / or retained austenite. HV ＭＢγ２＿１／４ Average value (HV0.1) of the Vickers hardness measurements at the 1 / 4 thickness position of the second region, where the indentation made by pressing the indenter in the Vickers hardness measurement was determined to contain only martensite, bainite and / or retained austenite.

2. The hot-stamped steel according to claim 1, wherein G1 and G2 further satisfy the following formula (ii): |G2-G1|<3.0 (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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