Hot press-formed body and automobile component

Abstract

A hot press-formed body having: a first region having a metallurgical structure in which the area ratio of martensite at the 1 / 4-thickness position is 95% or greater and the average aspect ratio of prior austenite grains is 1.10 or less; and a second region having a metallurgical structure in which the area ratio of martensite at the 1 / 4-thickness position is 95% or greater and the average aspect ratio of prior austenite grains is 2.00 to 3.00, the area ratio of martensite at the 1 / 2-thickness position of the first region and the second region is 95% or greater, the average grain diameter of the prior austenite grains at the 1 / 4-thickness position is 5.0 to 15.0 μm, and in a case where a Vickers hardness is measured at the 1 / 4-thickness position of the first region and the second region, [1.05 ≤ HV MBγ2_1 / 4 / HV MBγ1_1 / 4 ≤ 1.25] is satisfied.

Description

Technical Field

[0001] This invention relates to hot-stamped formed bodies and automotive parts. Background Technology

[0002] To balance lightweighting and crash safety in automobiles, high-strength materials are required. Therefore, hot-stamped steel with a strength reaching 1.8 GPa has been put into practical use in car bodies in recent years. Furthermore, given the similar high-strength requirements for chassis components, the future development of hot-stamped steel applications also needs to be explored.

[0003] In automotive parts like those described above, flanging is sometimes performed for assembly of the parts. For example, Patent Document 1 discloses a method for manufacturing a hot-pressed product with flanged portions.

[0004] The manufacturing method disclosed in Patent Document 1 includes a heating step for heating the sheet material and a forming step for shaping the heated sheet material. In the forming step, in addition to shaping the sheet material, quenching and flanging are also performed.

[0005] Existing technical documents

[0006] Patent documents

[0007] Patent Document 1: Japanese Patent Application Publication No. 2019-58916 Summary of the Invention

[0008] The problem the invention aims to solve

[0009] However, the inventors conducted research and found that during hot stamping, ferrite is sometimes generated and locally softened in areas where high strain is applied, such as the flange. This is believed to be due to the diffusion phase transformation promoted by high strain. Furthermore, according to various studies using hot working simulators, the greater the processing strain, the lower the hardenability, and the critical cooling rate required to obtain a sound full martensitic structure needs to be increased during the subsequent cooling process.

[0010] Because the chassis components use thicker steel, the cooling rate based on mold cooling is lower compared to the thinner steel used in the body. From this perspective, it can also be argued that the hardenability of areas with high processing strain tends to decrease significantly. Furthermore, in body components that already utilize hot-stamped steel sheets, high strain is sometimes introduced locally to complicate the structure in order to control the deformation patterns of each component.

[0011] Furthermore, from the perspective of further improving safety, automotive components are also required to have both excellent resistance to hydrogen embrittlement and high notched tensile strength.

[0012] The purpose of this invention is to solve the above-mentioned problems and provide a hot-stamped formed body and an automotive part using the same, wherein the hot-stamped formed body can suppress the strength reduction after hot stamping in the high-strain region even when high strain is locally introduced during the hot stamping process, and has excellent resistance to hydrogen embrittlement and high notched tensile strength.

[0013] Solution for solving the problem

[0014] The present invention was made to solve the above-mentioned problems, and its main purpose is to provide the following hot stamping formed articles and automotive parts.

[0015] (1) A hot-stamped formed body, comprising:

[0016] The first region has a metallographic structure with a martensite area of ​​over 95% at the 1 / 4 thickness position and an average aspect ratio of the original austenite grains of less than 1.10; and

[0017] The second region has a metallographic structure with a martensite area of ​​over 95% at the 1 / 4 thickness position and an average aspect ratio of 2.00 to 3.00 for the original austenite grains.

[0018] The martensite area ratio at the half-thickness position of the first and second regions is over 95%.

[0019] When the average grain diameter of the original austenite grains at the 1 / 4 thickness position of the first region is set as G1 (μm), and the average grain diameter of the original austenite grains at the 1 / 4 thickness position of the second region is set as G2 (μm), G1 and G2 are 5.0~15.0 μm.

[0020] When the Vickers hardness is measured at a position where the thickness of the first region and the second region is 1 / 4, the following equation (i) is satisfied.

[0021] 1.05≤HV MBγ2_1 / 4 / HV MBγ1_1 / 4 ≤1.25…(i)

[0022] The meanings of the symbols in the above formula are as follows.

[0023] HV MBγ1_1 / 4 The indentation formed by pressing the indenter at the 1 / 4 position of the thickness of the first region is determined as the average value (HV0.1) of the Vickers hardness measurements containing only martensite, bainite, and / or retained austenite.

[0024] HV MBγ2_1 / 4The indentation formed by pressing the indenter at the 1 / 4 position of the thickness of the second region was determined as the average value (HV0.1) of the Vickers hardness measurements containing only martensite, bainite, and / or retained austenite.

[0025] (2) The hot stamping formed body according to (1) above, wherein G1 and G2 further satisfy the following formula (ii).

[0026] |G2-G1|<3.0…(ii)

[0027] (3) The hot-stamped formed body according to (1) or (2) above has the following chemical composition in mass %:

[0028] C: 0.10~0.60%

[0029] Si: 0.01~2.00%

[0030] Mn: 0.10~3.00%

[0031] P: below 0.050%

[0032] S: Below 0.0200%

[0033] N: below 0.0200%

[0034] O: Below 0.100%

[0035] Al: 0.001~0.100%

[0036] Cr: 0.01~1.00%

[0037] Nb: 0~0.200%

[0038] Ti: 0~0.200%

[0039] Mo: 0~1.00%

[0040] B: 0~0.0100%

[0041] Co: 0~4.00%,

[0042] Ni: 0~2.00%

[0043] Cu: 0~1.00%

[0044] V: 0~1.00%

[0045] W: 0~1.00%

[0046] Ca: 0~0.100%

[0047] Mg: 0~0.100%

[0048] REM: 0~0.100%

[0049] Sb: 0~0.100%

[0050] Zr: 0~0.100%

[0051] Sn: 0~1.00%

[0052] As: 0~0.100%

[0053] Balance: Fe and impurities.

[0054] (4) An automotive part that uses the hot stamping formed body described in any one of (1) to (3) above.

[0055] The effects of the invention

[0056] According to the present invention, it is possible to obtain hot-stamped formed articles with excellent resistance to hydrogen embrittlement and high notched tensile strength, even when high strain is locally introduced in the hot stamping process, and automotive parts using the same. Attached Figure Description

[0057] Figure 1A It is a schematic three-dimensional drawing showing a molded body with a circular flange.

[0058] Figure 1B This is a top view showing a molded body with a circular flange.

[0059] Figure 1C This is a side view of a molded body with a circular flange.

[0060] Figure 2A This is a schematic diagram illustrating an example of the second measurement region. Figure 1C A cross-sectional view of part aa.

[0061] Figure 2B yes Figure 2A An enlarged view of the area indicated by the dashed line.

[0062] Figure 3A This is a top view of a molded body with an elliptical flange.

[0063] Figure 3B It is a cross-sectional view of a molded body with an elliptical flange.

[0064] Figure 4A It is a schematic three-dimensional drawing representing a molded body with a flange having an arc shape.

[0065] Figure 4B This is a top view of a molded body with a flange having an arc shape.

[0066] Figure 4C This is a side view of a molded body with a flange having an arc shape.

[0067] Figure 5A This is a schematic diagram illustrating an example of the second measurement region. Figure 4C Cross-sectional view of the bb section.

[0068] Figure 5B yes Figure 5A An enlarged view of the area indicated by the dashed line.

[0069] Figure 6A It is a schematic three-dimensional view showing a molded body with a flanged deformable part.

[0070] Figure 6B This is a top view showing a molded body with a flanged deformed portion.

[0071] Figure 6C yes Figure 6B A cross-sectional view of the cc portion.

[0072] Figure 7A This is a schematic diagram illustrating an example of the second measurement region. Figure 6C A cross-sectional view of the dd portion.

[0073] Figure 7B yes Figure 7A An enlarged view of the area indicated by the dashed line.

[0074] Figure 8A This is a schematic diagram illustrating an example of the second measurement region. Figure 6C A cross-sectional view of the ee portion.

[0075] Figure 8B yes Figure 8A An enlarged view of the area indicated by the dashed line.

[0076] Figure 9A This is a schematic diagram illustrating an example of the first measurement region. Figure 1B A cross-sectional view of the ff portion.

[0077] Figure 9B yes Figure 9A An enlarged view of the area indicated by the dashed line.

[0078] Figure 10 This is a schematic diagram illustrating the shape of a small-sized plate-shaped test piece according to ASTM standards.

[0079] Figure 11It is a diagram showing the shape of a tensile test specimen used to illustrate resistance to hydrogen embrittlement and the evaluation of notched tensile strength.

[0080] Figure 12A This diagram illustrates the steps for cutting a test piece from the flange to create a tensile test piece.

[0081] Figure 12B This diagram illustrates the steps for cutting a test piece from the flange to create a tensile test piece.

[0082] Figure 12C This diagram illustrates the steps for cutting a test piece from the flange to create a tensile test piece.

[0083] Figure 12D This diagram illustrates the steps for cutting a test piece from the flange to create a tensile test piece. Detailed Implementation

[0084] The inventors have studied methods for suppressing the decrease in hardenability in high strain regions, and the following insights have been obtained.

[0085] As an example of a high-strain region, the flanged section can be cited. Sometimes, the die is designed to lightly compress the steel sheet (hereinafter referred to as the "blank" before hot stamping) at the flanged section, making direct water-cooled dies highly unsuitable. Furthermore, the die and blank may not achieve a sufficiently tight seal throughout the entire flanged section. Due to the promotion of diffusion phase transformation caused by high strain and the difficulty in ensuring a sufficient cooling rate, softening due to ferrite formation is considered likely.

[0086] Therefore, repeated experiments under various hot stamping conditions revealed that increasing the cooling rate in the high-strain region is effective for quenching. This allows for quenching in the high-strain region, suppressing strength reduction.

[0087] On the other hand, in the low-strain region, the cooling rate is relatively low, resulting in a decrease in hardness compared to the high-strain region. This, in turn, improves the overall hydrogen embrittlement resistance and notched tensile strength of the hot-stamped part.

[0088] This invention is based on the above-mentioned insights. The elements of this invention will now be described in detail.

[0089] 1. Metallographic structure of hot-stamped bodies

[0090] The hot-stamped formed body of this embodiment has: a first region having a martensite area ratio of 95% or more and an average aspect ratio of the original austenite grains of 1.10 or less at a depth of 1 / 4 of the thickness from the surface of the base steel plate (referred to as the "1 / 4 thickness position" in this application specification); and a second region having a martensite area ratio of 95% or more and an average aspect ratio of the original austenite grains of 2.00 to 3.00 at the 1 / 4 thickness position. Furthermore, the martensite area ratio at a depth of 1 / 2 of the thickness from the surface of the base steel plate in both the first and second regions is 95% or more.

[0091] The first region is the area where high strain was not introduced during hot stamping. Therefore, the average aspect ratio of the original austenite grains at the 1 / 4 thickness position in the first region is 1.10 or less. It should be noted that the practical lower limit of the average aspect ratio is 1.00. Furthermore, the microstructure, which becomes martensitic through quenching accompanying hot stamping, specifically has an area fraction of over 95% for martensite at the 1 / 4 and 1 / 2 thickness positions. It should be noted that in this invention, martensite includes not only primary martensite in the quenched state but also tempered martensite and self-tempered martensite.

[0092] In the metallographic structure of this embodiment, the balance other than martensite consists of bainite, retained austenite, ferrite, and pearlite. However, in this invention, as described later, since the area ratio of martensite is determined based on Vickers hardness, martensite whose hardness has been excessively reduced due to tempering or self-tempering is excluded from the martensite and included in the balance.

[0093] 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 2.00 to 3.00 for the original austenite grains at the 1 / 4 thickness position. As mentioned above, in the region where high strain is introduced, diffusion phase transformation is promoted, thus, particularly in the center of the plate thickness, not only is ferrite formed and softening occurs, but hydrogen embrittlement resistance and notched tensile strength are sometimes reduced. However, in the second region, quenching is possible by increasing the cooling rate after hot stamping, so the martensite area fraction becomes 95% or more not only at the 1 / 4 thickness position but also at the 1 / 2 thickness position.

[0094] It should be noted that in forming processes that do not result in fracture, it is generally difficult to make the average aspect ratio of the original austenite grains exceed 3.00. Therefore, the average aspect ratio of the original austenite grains at the 1 / 4 thickness position of the second region is substantially below 3.00. In other words, the region where the average aspect ratio of the original austenite grains at the 1 / 4 thickness position exceeds 3.00 is considered a region where proper forming was not performed and is therefore excluded from the second region.

[0095] In the hot stamped body of this embodiment, by having a second region with a martensite area ratio of more than 95% even under high strain, it is possible to suppress the reduction of strength and improve the hydrogen embrittlement resistance and notched tensile strength.

[0096] Furthermore, in hot-stamped formed articles with strength in the range of 1.3 to 2.5 GPa, from the perspective of possessing excellent resistance to hydrogen embrittlement and high notched tensile strength, when the average grain diameter of the original austenite grains at the 1 / 4 position of the thickness of the first region is set as G1 (μm), and the average grain diameter of the original austenite grains at the 1 / 4 position of the thickness of the second region is set as G2 (μm), G1 and G2 are preferably 5.0 to 15.0 μm. G1 is preferably 6.0 μm or more, and more preferably 14.0 μm or less. That is, G1 is preferably 6.0 to 14.0 μm. Similarly, G2 is preferably 6.0 μm or more, and more preferably 14.0 μm or less. That is, G2 is preferably 6.0 to 14.0 μm.

[0097] There is a trade-off between hydrogen embrittlement resistance and notched tensile strength; higher strength in a hot-stamped form generally results in higher notched tensile strength, while hydrogen embrittlement resistance tends to decrease. From the perspective of balancing hydrogen embrittlement resistance and notched tensile strength, the strength of the hot-stamped form is preferably in the 1.5 GPa range. Furthermore, within the 1.5 GPa range of hot-stamped forms, considering both excellent hydrogen embrittlement resistance and high notched tensile strength, G1 is preferably in the range of 8.0–14.0 μm, and G2 is preferably in the range of 8.0–14.0 μm.

[0098] It should be noted that, as described above, a difference in cooling rate is set between the first and second regions, but since the heating temperature does not change, G1 and G2 are of the same degree. Therefore, G1 and G2 can also satisfy the following equation (ii). Since G1 and G2 can be the same, the lower limit of |G2-G1| is 0.

[0099] |G2-G1|<3.0…(ii)

[0100] In this invention, the metallographic structure of the hot-stamped formed body is measured by the following method. First, the measurement area is determined. At this time, in the hot-stamped formed body, the parts with low processing degree and no high strain can be selected as the first measurement area (hereinafter also referred to as "first measurement area"), while the parts with high strain, such as the flange, can be selected as the second measurement area (hereinafter also referred to as "second measurement area").

[0101] Next, for the aforementioned measurement areas (each of the first and second measurement areas), the martensite area ratio at the 1 / 4 and 1 / 2 thickness positions, as well as the average grain diameter and average aspect ratio of the original austenite grains at the 1 / 4 thickness position, are measured. In this invention, the martensite area ratio is measured by combining observation based on field emission scanning electron microscopy (FE-SEM) and Vickers hardness measurement, while the average grain diameter and average aspect ratio of the original austenite grains are measured using analysis via electron backscatter diffraction (EBSD). Each measurement step will be described in more detail below.

[0102] After etching the aforementioned measurement area with nitric acid and ethanol, FE-SEM images were taken at 1000x magnification using a secondary electron microscope (FE-SEM) to capture 300 μm vertically and 2000 μm horizontally, centered at the 1 / 4 and 1 / 2 thickness positions of the measurement area, respectively. That is, an FE-SEM equipped with a secondary electron detector was used. The obtained microstructure images were then analyzed to determine the area fraction of the microstructure identified as ferrite or pearlite; otherwise, it was determined to be martensite, bainite, or retained austenite. If the total area fraction of the microstructure identified as martensite, bainite, or retained austenite was 95% or more, a grid with 70 μm intervals was drawn in the 300 μm vertical and 2000 μm horizontal area, and Vickers hardness was measured at all grid intersections (4 × 28 = 112 points). The test force for the Vickers hardness measurement was set to 0.9807 N (0.1 kgf). Then, FE-SEM images were taken again at 1000x magnification. Then, by comparing the microstructure photographs taken before and after the Vickers hardness test, it is determined whether the indentation (hereinafter also referred to as "indentation") formed by pressing in the indenter during the Vickers hardness test contains microstructure other than martensite, bainite and retained austenite, that is, whether it contains ferrite or pearlite.

[0103] Then, the average value of the obtained Vickers hardness measurements that are determined to contain only martensite, bainite, and / or retained austenite in the area of ​​the depression remaining on the surface is calculated. If the following formula is satisfied, it is judged that the area ratio of martensite in the metallographic structure is greater than 95%. It should be noted that "HV0.1" below refers to the "hardness symbol" when the Vickers hardness test is performed with the test force set to 0.9807 N (0.1 kgf) (refer to JIS Z 2244-1:2020).

[0104] HV MBγ ≥957.1×[C]+221.0

[0105] The meanings of the symbols in the above formula are as follows.

[0106] HV MBγ The average value of Vickers hardness measurements (HV0.1) for depressions identified as containing only martensite, bainite, and / or retained austenite.

[0107] [C]: C content (mass%) of the hot-stamped part

[0108] It should be noted that if it is not possible to ensure a vertical area of ​​300μm and a horizontal area of ​​2000μm within the measurement area, for example, if it is possible to ensure four vertical areas of 300μm and a horizontal area of ​​500μm, the above-mentioned observation based on FE-SEM and Vickers hardness measurement can be performed in each area.

[0109] Next, for the aforementioned measurement area, after grinding with #600 to #1500 silicon carbide paper, a mirror finish was achieved by dispersing diamond powder with a particle size of 1-6 μm in a diluent such as alcohol or pure water. Then, at room temperature, the surface was ground for 8 minutes with colloidal silica with a particle size of 0.25 μm and without an alkaline solution to remove the strain introduced into the sample surface.

[0110] Then, for a region 300 μm vertically and 500 μm horizontally centered at 1 / 4 of the thickness of the measurement area, EBSD measurements were performed at 0.1 μm intervals to obtain crystal orientation information. An EBSD analysis apparatus consisting of an FE-SEM and an EBSD detector can be used for the measurement; for example, an EBSD analysis apparatus consisting of a JEOL JSM-7001F detector and an AMETEK Hikari detector can be used. At this time, for example, the vacuum level inside the EBSD analysis apparatus is set to 9.6 × 10⁻⁶. -5 Below Pa, the accelerating voltage is 20kV, the working distance (WD) is 15mm, the irradiation current level is 18, and the tilt angle of the test piece is 70°.

[0111] When acquiring EBSD patterns, the "AMETEK OIM DataCollection" function of the software accompanying the EBSD analyzer was used. For analysis, Iron (Alpha) and Iron (Gamma) were selected as the crystal structures. Using the obtained crystal orientation information, the crystal orientation of the original austenite grains was calculated based on the general crystal orientation relationship between the original austenite grains and the transformed grains with a bcc structure.

[0112] The crystal orientation of the original austenite grains can be calculated using the following method. First, the original austenite grains are determined using the method described in "A Study on the High-Precision Reconstruction Method for Austenite Structure in Steel" (Kengo Hata, Masayuki Wakita, Tomoya Fujiwara, Kaori Kono, Nippon Steel & Sumitomo Metal Technical Report No. 404 (2016), pp. 24-30).

[0113] It should be noted that, when using the method described in the aforementioned documents, in this invention, the orientation relationship between ferrite and austenite is set as a KS relationship. In this invention, in addition to ferrite, pearlite, primary martensite, tempered martensite, and bainite can be cited as microstructures with a bcc structure as determined by EBSD, and retained austenite can be cited as a microstructure with an fcc structure. Therefore, by EBSD, microstructures determined to have a bcc structure are considered ferrite, and microstructures determined to have an fcc structure are considered austenite. Furthermore, the allowable angle described in the aforementioned documents is set to 3 degrees, and the allowable error is set to 5 degrees.

[0114] Even after performing the above steps, austenite grains that cannot be reconstructed are stored as ferrite grains (bcc structure). That is, they are not converted to austenite (fcc structure). For each identified original austenite grain, excluding the grains at the ends of the field of view, the grain diameter is determined using the Area Fraction of the "Grain Size (diameter)" diagram in "AMETEK OIM Analysis (registered trademark)". The reciprocal of the "Grain Shape Aspect Ratio" is then calculated, and the aspect ratio is determined accordingly. Here, it is set as the reciprocal of the "Grain Shape Aspect Ratio" because the "Grain Shape Aspect Ratio" is the ratio of the minor axis to the major axis. In addition, at this time, boundaries with an orientation difference of 15 degrees or more are considered as austenite grain boundaries.

[0115] It should be noted that in this invention, the grain diameter refers to the equivalent circle diameter, and the aspect ratio refers to the ratio of the length along the major axis to the length in the direction orthogonal to it.

[0116] The shape of the hot-stamped formed article of this embodiment is not particularly limited, and it has a portion including a first region that is not subjected to high strain and a portion including a second region that is subjected to high strain. The thickness of the portion not subjected to high strain is preferably 1.0 to 4.0 mm, and the thickness of the portion subjected to high strain is preferably 0.6 to 3.0 mm, more preferably 1.0 to 3.0 mm.

[0117] Furthermore, taking a representative hot-stamped formed body as an example, the preferred measurement area will be explained. Examples of hot-stamped formed bodies in this embodiment include... Figure 1A The molded body shown has a circular flange. Figure 3A The molded body shown has an elliptical flange. Figure 4A The molded body shown has a flange portion with an arc shape, and Figure 6AThe molded body shown has a flange deformation portion. The preferred measurement areas in each molded body will be described in detail below.

[0118] Figure 1A This is a schematic three-dimensional drawing showing a molded body with a circular flange. Figure 1B It is a top view. Figure 1C It is a side view. For example... Figure 1A As shown, the molded body 100 has a plate-shaped portion 10 and a cylindrical flange portion 12 erected from the plate-shaped portion 10. The thickness t1 of the plate-shaped portion 10 is, for example, 1.0 to 4.0 mm, and the thickness t2 at the front end of the flange portion 12 is, for example, 0.6 to 3.0 mm. The height L of the flange portion 12 (the distance between the plate-shaped portion 10 and the front end of the flange portion 12 in the thickness direction of the plate-shaped portion 10) is, for example, 3 to 30 mm. In addition, when viewed from the thickness direction of the plate-shaped portion 10, the diameter (inner diameter) φ of the front end of the flange portion 12 is, for example, 15 to 100 mm.

[0119] In this invention, plating is sometimes applied to the molded body. In this specification, "thickness" refers to the thickness of the base material portion obtained by subtracting the plating thickness from the total thickness. Furthermore, the plating thickness is measured using a high-frequency glow discharge surface analyzer (GDS). The specific measurement method is described below.

[0120] Three arbitrary measurement points are determined from the first and second regions of the hot-stamped body, which are coated. At each measurement point, the concentrations of each element, Fe, Mn, Zn, Si, Al, O, Cr, Ni, Mg, Cu, and Sn, are measured while sputtering is performed from the surface of the coating.

[0121] The content of each element is analyzed along the depth direction to determine the depth at which the Fe concentration first reaches 90% by mass or more. Then, the average depth of each measurement point is calculated, and this average value is used as the coating thickness for the first and second regions, respectively. It should be noted that if the depth that can be analyzed in a single GDS measurement is insufficient to achieve an Fe concentration of 90% by mass or more, i.e., the coating is thicker than the measurable depth, then at any location within the same region different from the previously measured location, 80-90% of the coating equivalent to the previous measurement depth is removed by grinding. The depth of the coating removed by grinding is determined based on the change in plate thickness before and after grinding. On this basis, GDS analysis is performed again on the ground surface, and the results of the first, second, and subsequent measurements are combined to determine the coating thickness.

[0122] For example, a Marcus-type high-frequency glow discharge luminescence analyzer GD-Profiler2 (manufactured by HORIBA) is used as the measuring device for GDS. The measurement is performed under the following conditions: discharge conditions of 35W, Ar pressure of 600Pa, discharge range of 4mm φ, electrode distance of 0.15~0.25mm, and measurement spacing in the plate thickness direction of 0.01~0.05μm.

[0123] It should be noted that, in Figure 1A In ~C, the flange portion 12 is formed to stand vertically relative to the plate-shaped portion 10, but the flange portion 12 may also stand in an inclined manner relative to the plate-shaped portion 10. For example, the flange portion 12 may also be formed to have a smaller diameter the closer it is to the front end side (the further away from the plate-shaped portion 10 in the thickness direction of the plate-shaped portion 10).

[0124] In the molded body 100, high strain is introduced into the flange portion 12, with the strain being higher towards the front end and the outer edge. Therefore, a cross-section can be cut at a distance d from the front end of the flange portion 12 toward the plate-shaped portion 10, and a second measurement area can be selected from this cross-section. The distance d is, for example, 0.5 to 1.0 mm.

[0125] Figure 2A This is a schematic diagram illustrating an example of the second measurement region. Figure 1C A cross-sectional view of part aa. Figure 2B yes Figure 2A An enlarged view of the area indicated by the dashed lines. Figure 2B To avoid complicating the accompanying diagrams, shading has not been used. (For example...) Figure 2B As shown, the regions centered at positions t2 / 4 and t2 / 2 from the outer edge of the flange 12, with a vertical dimension of 300 μm and a horizontal dimension of 2000 μm, can be used as the observation area for FE-SEM. The regions centered at positions t2 / 4 and t2 / 2 from the outer edge of the flange 12, with a vertical dimension of 300 μm and a horizontal dimension of 500 μm, can be used as the resolution area for EBSD.

[0126] It should be noted that, in Figure 2B In the example shown, the thickness t2 at the front end of the flange 12 is sufficiently thick, so the 300 μm longitudinal and 2000 μm transverse regions centered at positions t2 / 4 and t2 / 2, respectively, do not overlap. However, when the thickness t2 at the front end of the flange 12 is thin, the 300 μm longitudinal and 2000 μm transverse regions centered at positions t2 / 4 and t2 / 2, respectively, sometimes overlap. In such cases, observation areas are selected from positions that are offset from each other circumferentially on the cylindrical flange 12.

[0127] Figure 3AThis is a top view showing a molded body with an elliptical flange. Figure 3B It is a cross-sectional view. Figure 1A In the molded body 100 shown, when viewed from the thickness direction of the plate-shaped portion 10, the flange portion 12 is circular, but it can also be as shown in the figure. Figure 3A As shown in the molded body 200, when viewed from the thickness direction of the plate-shaped portion 20, the flange portion 22 is elliptical in shape. When viewed from the thickness direction of the plate-shaped portion 20, the minor diameter (inner diameter) φ1 of the front end of the flange portion 22 is, for example, 10~80mm, and the major diameter (inner diameter) φ2 is, for example, 15~100mm.

[0128] When the flange 22 is elliptical, the areas with greater curvature on both sides along the major axis are subjected to higher strain, thus allowing for [the desired effect]. Figure 3B The region on one side of the major axis, indicated by the dashed line, is selected as the second measurement area. The locations of the observation area for FE-SEM and the analysis area for EBSD are the same as described above, therefore, descriptions are omitted. It should be noted that... Figure 3B This is a cross-sectional view taken from the front end of the flanged part 22 toward the plate-shaped part 20 at a distance d, for example, 0.5~1.0 mm.

[0129] Figure 4A This is a schematic three-dimensional drawing showing a molded body with a flange portion having a circular arc shape. Figure 4B It is a top view. Figure 4C It is a side view. For example... Figure 4A As shown, the molded body 300 has a plate-shaped portion 30 and an arc-shaped flange portion 32 rising from the plate-shaped portion 30. The thickness t1 of the plate-shaped portion 30 is, for example, 1.0 to 4.0 mm, and the thickness t2 at the front end of the flange portion 32 is, for example, 0.6 to 3.0 mm. The height L of the flange portion 32 (the distance between the plate-shaped portion 30 and the front end of the flange portion 32 in the thickness direction of the plate-shaped portion 30) is, for example, 3 to 30 mm. Furthermore, when viewed from the thickness direction of the plate-shaped portion 30, the flange portion 32 is arc-shaped, and the radius of curvature R of its inner diameter side surface is, for example, 50 to 500 mm.

[0130] In the molded body 300, high strain is introduced into the flange portion 32, with the strain increasing towards the outer edge of the arc (the side furthest from the center of curvature). Therefore, a cross-section can be cut at a distance d from the front end of the flange portion 32 toward the plate-like portion 30, and a second measurement area can be selected from this cross-section. The distance d is, for example, 0.5 to 1.0 mm.

[0131] Figure 5A This is a schematic diagram illustrating an example of the second measurement region. Figure 4C A cross-sectional view of the bb section. Figure 5B yes Figure 5AAn enlarged view of the area indicated by the dashed lines. Figure 5B To avoid complicating the accompanying diagrams, shading has not been used. (For example...) Figure 5B As shown, the region with a vertical dimension of 2000 μm and a horizontal dimension of 300 μm, centered at the positions t2 / 4 and t2 / 2 of the outer side of the arc from the flange 32, can be used as the observation area for FE-SEM. The region with a vertical dimension of 500 μm and a horizontal dimension of 300 μm, centered at the position t2 / 4 of the outer side of the arc from the flange 32, can be used as the resolution area for EBSD.

[0132] Figure 6A This is a schematic three-dimensional view showing a molded body with a flanged deformable portion. Figure 6B It is a top view. Figure 6C yes Figure 6B A cross-sectional view of the cc portion. (See diagram below.) Figure 6A As shown, the molded body 400 has a plate-shaped portion 40, a cylindrical portion 42 erected from the plate-shaped portion 40, and an annular inner flange portion 44 protruding inward from the cylindrical portion 42 and having a hole at its center. The thickness t1 of the plate-shaped portion 40 is, for example, 1.0 to 4.0 mm, and the thickness t2 of the cylindrical portion 42 and the thickness t3 of the inner flange portion 44 are, 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. Furthermore, when viewed from 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.

[0133] In the molded body 400, high strain is introduced into the inner flange portion 44, with the strain increasing towards the inner side (the side closer to the hole). Therefore, cross sections can be cut from the outer surface of the inner flange portion 44 towards the plate-shaped portion 40 at positions away from t3 / 4 and t3 / 2, respectively, and a second measurement area can be selected from these cross sections.

[0134] Figure 7A This is a schematic diagram illustrating an example of the second measurement region. Figure 6C A cross-sectional view of the dd portion. Figure 7B yes Figure 7A An enlarged view of the area indicated by the dashed lines. Additionally, Figure 8A This is a schematic diagram illustrating an example of the second measurement region. Figure 6C A cross-sectional view of the ee portion. Figure 8B yes Figure 8A An enlarged view of the area indicated by the dashed lines. Figure 7B and Figure 8B To avoid complicating the accompanying diagrams, shading has not been used. (For example...) Figure 7BAs shown, in the cross-section located at a position t3 / 4 away from the outer surface of the inner flange 44 towards the plate-shaped portion 40, the region of 2000 μm longitudinally and 300 μm transversely, centered at a distance d from the hole in the inner flange 44, can be designated as the observation area for FE-SEM by a dashed line, and the region of 500 μm longitudinally and 300 μm transversely, centered by a dotted line, can be designated as the resolution area for EBSD. Furthermore, as... Figure 8B As shown, in a cross-section located at a position t3 / 2 away from the outer surface of the inner flange 44 towards the plate-shaped portion 40, a region of 2000 μm longitudinally and 300 μm transversely, centered at a distance d from the hole in the inner flange 44, can be used as the observation area for FE-SEM. The distance d is, for example, 0.5~1.0 mm.

[0135] As described above, every molded body has a plate-like portion, but this plate-like portion is the area where high strain is not introduced. Therefore, it is preferable to select the plate-like portion as the first measurement area. Figure 1B and Figure 9A , Figure 9B The first measurement area will be described in more detail. Figure 9A This is a schematic diagram illustrating an example of the first measurement region. Figure 1B A cross-sectional view of the ff portion. Figure 9B yes Figure 9A An enlarged view of the area indicated by the dashed lines. Figure 9B To avoid complicating the accompanying diagrams, shading has not been used. (For example...) Figure 1B As shown, a cross-section parallel to the thickness direction of the plate-shaped portion 10, and a distance D away from the boundary connecting the plate-shaped portion 10 and the flange portion 12, can be cut out, and a first measurement area can be selected from this cross-section. The distance D is, for example, 7~8 mm. Furthermore, as... Figure 9B As shown, the regions centered at positions t1 / 4 and t1 / 2 from the surface of the plate-shaped portion 10, with a vertical dimension of 300 μm and a horizontal dimension of 2000 μm, indicated by dashed lines, can be used as the observation area for FE-SEM. The regions centered at positions t1 / 4 from the surface of the plate-shaped portion 10, with a vertical dimension of 300 μm and a horizontal dimension of 500 μm, indicated by dotted lines, can be used as the resolution area for EBSD.

[0136] 2. Mechanical properties

[0137] In the hot stamping formed body of this embodiment, when the Vickers hardness is measured at a position 1 / 4 of the thickness of the first region and the second region, the following formula (i) is satisfied.

[0138] 1.05≤HV MBγ2_1 / 4 / HV MBγ1_1 / 4 ≤1.25…(i)

[0139] The meanings of the symbols in the above formula are as follows.

[0140] HV MBγ1_1 / 4 The indentation formed by pressing the indenter at the 1 / 4 position of the thickness of the first region is determined as the average value (HV0.1) of the Vickers hardness measurements containing only martensite, bainite, and / or retained austenite.

[0141] HV MBγ2_1 / 4 The indentation formed by pressing the indenter at the 1 / 4 position of the thickness of the second region was determined as the average value (HV0.1) of the Vickers hardness measurements containing only martensite, bainite, and / or retained austenite.

[0142] This is because the cooling rate is increased in the second region compared to the first region, making self-tempering relatively difficult and resulting in higher hardness. In other words, the cooling rate is increased in the second region to enable quenching, thereby increasing the hardness, while the increase in hardness is relatively suppressed in the first region. By suppressing the increase in hardness in the first region, the overall hydrogen embrittlement resistance and notched tensile strength of the hot-stamped part can be improved. The intermediate value of equation (i) above is preferably 1.20 or less, more preferably 1.15 or less.

[0143] It should be noted that HV MBγ1_1 / 4 and HV MBγ2_1 / 4 By comparing with the above HV MBγ The same method was used to determine it. That is, HV. MBγ1_1 / 4 The HV was measured when determining the area ratio of martensite at the 1 / 4 position of the thickness of the first region. MBγ HV MBγ2_1 / 4 The HV was measured when determining the area ratio of martensite at the 1 / 4 position of the thickness of the second region. MBγ Therefore, the description of the determination method is omitted.

[0144] Furthermore, in this embodiment of the hot-stamped formed body, the strength is not particularly limited. However, the tensile strength measured by tensile testing of a test piece collected from the first region is preferably in the range of 1.3 to 2.5 GPa, i.e., 1250 to 2540 MPa. This is because, with such strength, when the hot-stamped formed body is used as a component of an automobile, it is effective for reducing the weight of the automobile and ensuring crash safety. In addition, as mentioned above, when balancing the hydrogen embrittlement resistance and notched tensile strength of the hot-stamped formed body, a strength in the range of 1.5 GPa is preferred; specifically, the tensile strength is preferably 1450 to 1540 MPa.

[0145] In this invention, tensile strength is determined using... Figure 10The tensile strength was measured using a small-sized plate-shaped test piece (10 mm width, 100 mm total length, 6.25 mm parallel width, 32 mm parallel length, 25 mm gauge length, thickness: original thickness of the first region) as specified in ASTM A370-22, with a tensile speed of 3 mm / min as measured by crosshead displacement. Other conditions were performed according to JIS Z 2241:2022. It should be noted that, as described above, when the molded body is coated, the tensile strength is determined using a test piece in the coated state, but "thickness: original thickness of the first region" is set to the thickness of the base material portion obtained by subtracting the coating thickness from the total thickness. That is, the original cross-sectional area of ​​the test piece used for calculating the tensile strength is the original cross-sectional area of ​​the base material portion after removing the coating. It should be noted that the coating thickness is determined using the same method as described above.

[0146] The hot-stamped part of this embodiment exhibits excellent resistance to hydrogen embrittlement and high notched tensile strength. However, as mentioned above, there is a trade-off between resistance to hydrogen embrittlement and notched tensile strength. Therefore, in this embodiment, for example, when the sum of the fracture stress and the notched tensile strength in a hydrogen environment, which is an indicator of resistance to hydrogen embrittlement, is 2600 MPa or more, it can be determined that it has excellent resistance to hydrogen embrittlement and high notched tensile strength.

[0147] Fracture stress and notched tensile strength under hydrogen conditions can be evaluated using the following methods. First, two samples are collected from each of the hot-stamped parts. Figure 11 Tensile test specimens of the shape shown (width 9.0 mm, total length 120 mm, parallel portion width 2.0 mm, thickness 1.0 mm) are used. Preferably, these tensile test specimens are collected from the first region and the second region respectively, and each region is evaluated. The tensile test specimens are collected such that at least a portion of the bottom of the U-shaped notch formed on both sides in the width direction at a depth of 1 / 2 the thickness from the surface of the base steel plate overlaps with the center of the tensile test specimen in the length direction.

[0148] It should be noted that, due to size limitations, it is sometimes impossible to collect the aforementioned tensile test specimens from the flange or other areas. In such cases, if... Figure 12A As shown in example ~D, a test piece with a width of 2.0 mm and a possible range in length and thickness is collected from the flange, etc. Based on this, a U-shaped notch is formed on both sides of the width direction at the center of the collected test piece in the length direction. The length of the collected test piece is set to 6.0 mm or more. Then, the test piece is placed... Figure 12DThe tensile test specimen shown is joined to the sides along its length by welding other alternative materials to the center. Laser welding is performed while the specimen is held in a copper clamp to prevent the heat effect from affecting the area with the U-shaped notch.

[0149] Additionally, due to size limitations, data collection is sometimes impossible. Figure 11 The tensile test piece with a thickness of 1.0 mm shown is or Figure 12D The test piece shown is 1.0 mm thick. In this case, following the above procedure, two tensile test pieces with a thickness of 0.5 mm are made, and then joined together by screws or welding to form a tensile test piece with a thickness of 1.0 mm. It should be noted that the joint based on screw fixing or welding is made at the center position in the height direction of the clamping parts formed at both ends of the tensile test piece (on the dashed line in the figure).

[0150] Then, a tensile test piece was placed in a 3% NaCl aqueous solution as the cathode, and a current density of 1.0 mA / cm² was applied. 2 Hydrogen is generated on the surface of the tensile test specimen under certain conditions, and a tensile test is performed at a tensile speed of 0.006 mm / min to determine the fracture stress (maximum stress). The measured fracture stress is a value that serves as an indicator of hydrogen embrittlement resistance and is set as the tensile strength under hydrogen conditions.

[0151] In addition, for another tensile test specimen, a tensile test according to JIS Z 2241:2022 was performed at 0.006 mm / min under atmospheric conditions to determine the fracture stress (maximum stress), and the obtained measured value was taken as the notched tensile strength.

[0152] 3. Chemical composition

[0153] Regarding the chemical composition of the hot-stamped molded article of this embodiment, there are no particular limitations as long as it possesses the aforementioned strength. The hot-stamped molded article of this embodiment preferably has, for example, the chemical composition shown below. The reasons for limiting each element are explained below. It should be noted that in the following description, "%" for content refers to "mass %".

[0154] C: 0.10~0.60%

[0155] Carbon (C) is an element that improves the strength of hot-stamped formed articles. Therefore, the C content is preferably 0.10% or more. More preferably, the C content is more than 0.10%, 0.15% or more, 0.20% or more, or 0.25% or more. From the perspective 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 preferred C content is 0.10 to 0.50%, 0.10 to 0.40%, more than 0.10% but less than 0.38%, 0.15 to 0.37%, 0.20 to 0.35%, or 0.25 to 0.30%.

[0156] Si: 0.01~2.00%

[0157] Si is an element that enhances the strength of hot-stamped parts through solid solution strengthening. Therefore, the Si content is preferably 0.01% or more. More preferably, the Si content is 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, when the Si content exceeds 2.00%, the ferrite content increases, and sometimes the desired metallographic structure cannot be obtained. Therefore, the Si content is preferably 2.00% or less. More preferably, the Si content is 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 0.05~1.80%, 0.10~1.50%, 0.20~1.20%, 0.30~1.00%, or 0.40~0.80%.

[0158] Mn: 0.10~3.00%

[0159] Mn is an element that improves the hardenability of steel, thereby contributing to increased strength. Therefore, the Mn content is preferably 0.10% or more. More preferably, the Mn content is 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, when the Mn content exceeds 3.00%, Mn segregation sometimes becomes significant. Therefore, the Mn content is preferably 3.00% or less. More preferably, the Mn content is 2.80% or less, 2.50% or less, 2.30% or less, or 2.00% or less. The preferred Mn content is 0.20~2.80%, 0.50~2.50%, 1.00~2.30%, 1.30~2.00%, or 1.50~2.00%.

[0160] P: below 0.050%

[0161] Phosphorus (P) is an impurity element that can potentially reduce weldability. Therefore, the P content is preferably 0.050% or less. More preferably, it is 0.030%, 0.020%, or 0.010% or less. The lower limit of the P content is not particularly limited and can be 0%. However, excessively reducing the P content significantly increases the cost of P removal, which is not economically preferable. Therefore, the P content can be set to be greater than 0% or greater than 0.0001%.

[0162] S: Below 0.0200%

[0163] Sulfur (S) is an impurity element that can potentially reduce weldability. Therefore, the S content is preferably 0.0200% or less. More preferably, the S content is 0.0180%, 0.0150%, 0.0100%, 0.0060%, or 0.0040% or less. The lower limit of the S content is not particularly limited and can be 0%. However, excessively reducing the S content significantly increases the cost of desulfurization, which is not economically preferable. Therefore, the S content can be set to be greater than 0% or greater than 0.0001%.

[0164] N: below 0.0200%

[0165] Nitrogen (N) is an impurity element that can potentially reduce weldability. Therefore, the N content is preferably 0.0200% or less. More preferably, the N content is 0.0180%, 0.0150%, 0.0100%, 0.0060%, or 0.0040% or less. The lower limit of the N content is not particularly limited and can be 0%. However, excessively reducing the N content significantly increases the cost of N removal, which is not economically preferable. Therefore, the N content can be set to be greater than 0% or greater than 0.0001%.

[0166] O: Below 0.100%

[0167] When oxygen (O) is present in large quantities in steel, it may cause a decrease in weldability. Therefore, the O content is preferably 0.100% or less. More preferably, the O content is 0.0700%, 0.0500%, 0.0300%, 0.0100%, or 0.0050% or less. The lower limit of the O content is not particularly limited and can be 0%. However, from the perspective of reducing refining costs, the O content can be set to be greater than 0%, or greater than 0.0001%, or greater than 0.0005%.

[0168] Al: 0.001~0.100%

[0169] Al is an element that deoxidizes molten steel, thereby improving its quality. Therefore, the Al content is preferably 0.001% or more. More preferably, the Al content is 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; therefore, the Al content is preferably 0.100% or less. More preferably, the Al content is 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%. Furthermore, the Al content is preferably 0.001 to 0.020% or 0.001 to 0.010%.

[0170] Cr: 0.01~1.00%

[0171] Cr is an element that improves the hardenability of steel. Therefore, the Cr content is preferably 0.01% or more. More preferably, the Cr content is 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; therefore, the Cr content is preferably 1.00% or less. More preferably, the Cr content is 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%.

[0172] The basic chemical composition of the hot-stamped molded body according to embodiments of the present invention is as described above. Furthermore, the hot-stamped molded body may, as needed, contain at least one of the following selected elements to replace a portion of the balance Fe. The selected elements will be described in detail below.

[0173] Nb: 0~0.200%

[0174] Nitrogen (Nb) is an element that forms carbonitrides in steel, thereby increasing the strength of hot-stamped parts through precipitation strengthening. The Nb content can be 0.0005% or more, but to reliably achieve this effect, the Nb content is preferably 0.001% or more or 0.002% or more. On the other hand, even with a large content, the above effect is saturated, therefore the Nb content is preferably 0.200% or less. The Nb content can be 0.180% or less, 0.150% or less, 0.100% or less, 0.050% or less, or 0.010% or less. The preferred Nb content is 0.0005~0.180%, 0.001~0.150%, 0.002~0.100%, 0.003~0.050%, or 0.004~0.010%.

[0175] Ti: 0~0.200%

[0176] Ti is an element that forms carbonitrides in steel, thereby increasing the strength of hot-stamped parts through precipitation strengthening. The Ti content can be 0.001% or more, but to reliably achieve this effect, the Ti content is preferably 0.010% or more or 0.020% or more. On the other hand, even with a large content, the above effect is saturated, so the Ti content is preferably 0.200% or less. The Ti content can be 0.180% or less, 0.150% or less, 0.100% or less, 0.070% or less, or 0.040% or less. The preferred Ti content is 0.001~0.180%, 0.005~0.150%, 0.010~0.100%, 0.015~0.070%, or 0.020~0.040%.

[0177] Mo: 0~1.00%

[0178] Mo is an element that improves the hardenability of steel. The Mo content can be 0.001% or more, but to reliably achieve this effect, the Mo content is preferably 0.003% or more or 0.005% or more. On the other hand, even with a large content, the above effect is saturated, therefore the Mo content is preferably 1.00% or less. The Mo content can 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 0.001~0.80%, 0.002~0.60%, 0.003~0.50%, 0.004~0.30%, or 0.005~0.10%.

[0179] B: 0~0.0100%

[0180] Bo (B) is an element that improves the hardenability of steel. The B content can be 0.0001% or more, but to reliably achieve this effect, the B content is preferably 0.0005% or more or 0.0010% or more. On the other hand, even with a large content, the above effect is saturated, therefore the B content is preferably 0.0100% or less. The B content can be 0.0080% or less, 0.0060% or less, 0.0050% or less, 0.0030% or less, or 0.0020% or less. The preferred B content is 0.0001~0.0080%, 0.0003~0.0060%, 0.0005~0.0050%, 0.0007~0.0030%, or 0.0010~0.0020%.

[0181] Co: 0~4.00%

[0182] Co is an element that improves the strength of hot-stamped parts through solid solution strengthening. The Co content can be 0.001% or more, but to reliably achieve this effect, the Co content is preferably 0.01% or more or 0.05% or more. On the other hand, even with a large content, the above effect is saturated, therefore the Co content is preferably 4.00% or less. The Co content can 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 0.001~3.00%, 0.005~2.00%, 0.01~1.00%, 0.03~0.50%, or 0.05~0.10%.

[0183] Ni: 0~2.00%

[0184] Ni has the effect of increasing the strength of the hot-stamped body by dissolving in the austenite grains during heating in the hot stamping process. The Ni content can be 0.001% or more, but it is preferable to set the Ni content to 0.01% or more to reliably achieve this effect. On the other hand, even with a large content, the above effect is saturated, so the Ni content is preferably 2.00% or less. The Ni content can 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 0.001~1.80%, 0.005~1.60%, 0.01~1.40%, 0.02~1.20%, 0.03~1.00%, 0.04~0.50%, or 0.05~0.10%.

[0185] Cu: 0~1.00%

[0186] Cu has the effect of increasing the strength of the hot-stamped body by dissolving in the austenite grains during heating in the hot stamping process. The Cu content can be 0.001% or more, but to reliably obtain this effect, the Cu content is preferably 0.01% or more or 0.05% or more. On the other hand, even with a large content, the above effect is saturated, so the Cu content is preferably 1.00% or less. The Cu content can 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 0.001~0.80%, 0.005~0.60%, 0.01~0.50%, 0.03~0.30%, or 0.05~0.10%.

[0187] V: 0~1.00%

[0188] V has the effect of forming carbonitrides in steel, thereby improving the strength of hot-stamped parts through precipitation strengthening. The V content can be 0.001% or more, but to reliably obtain this effect, the V content is preferably 0.01% or more or 0.05% or more. On the other hand, even with a large content, the above effect is saturated, so the V content is preferably 1.00% or less. The V content can 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 0.001~0.80%, 0.005~0.60%, 0.01~0.50%, 0.03~0.30%, or 0.05~0.10%.

[0189] W: 0~1.00%

[0190] W is an element that improves the hardenability of steel. The W content can be 0.001% or more, but to reliably achieve this effect, the W content is preferably 0.005% or more or 0.01% or more. On the other hand, even with a large content, the above effect is saturated, therefore the W content is preferably 1.00% or less. The W content can be 0.80% or less, 0.60% or less, 0.50% or less, 0.30% or less, or 0.10% or less. The preferred W content is 0.001~0.80%, 0.005~0.60%, 0.01~0.50%, 0.03~0.30%, or 0.05~0.10%.

[0191] Ca: 0~0.100%

[0192] Ca is an element that controls the form of sulfides. The Ca content can be 0.0001% or more, but to reliably achieve this effect, the Ca content is preferably 0.0005% or more or 0.0010% or more. On the other hand, even with a large content, the above effect is saturated, so the Ca content is preferably 0.100% or less. The Ca content can 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 preferred Ca content is 0.0001~0.0800%, 0.0001~0.0600%, 0.0001~0.0400%, 0.0001~0.0200%, 0.0001~0.0100%, 0.0001~0.0080%, 0.0003~0.0060%, 0.0005~0.0040%, 0.0007~0.0030%, or 0.0010~0.0020%.

[0193] Mg: 0~0.100%

[0194] Mg is an element that controls the form of sulfides. The Mg content can be 0.0001% or more, but to reliably achieve this effect, the Mg content is preferably set to 0.0005% or more or 0.0010% or more. On the other hand, even with a large content, the above effect is saturated, therefore the Mg content is preferably set to 0.100% or less. The Mg content can 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 preferred Mg content is 0.0001~0.0800%, 0.0001~0.0600%, 0.0001~0.0400%, 0.0001~0.0200%, 0.0001~0.0100%, 0.0001~0.0080%, 0.0003~0.0060%, 0.0005~0.0040%, 0.0007~0.0030%, or 0.0010~0.0020%.

[0195] REM: 0~0.100%

[0196] REM is an element that controls the form of sulfides. The REM content can be 0.0001% or more, but to reliably achieve this effect, the REM content is preferably set to 0.0005% or more or 0.0010% or more. On the other hand, even with a large content, the above effect is saturated, therefore the REM content is preferably set to 0.100% or less. The REM content can 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 preferred REM content is 0.0001~0.0800%, 0.0001~0.0600%, 0.0001~0.0400%, 0.0001~0.0200%, 0.0001~0.0100%, 0.0001~0.0080%, 0.0003~0.0060%, 0.0005~0.0040%, 0.0007~0.0030%, or 0.0010~0.0020%.

[0197] In this embodiment, "REM" refers to a total of 17 elements, including Sc, Y, and the lanthanides. "REM content" refers to the content of one REM element and the total content of two or more REM elements. Furthermore, REM is often supplied as a mixed rare earth element in alloys of multiple REM elements. Therefore, it can be included by adding a single element or by adding two or more elements, for example, it can be added in the form of a mixed rare earth element.

[0198] Sb: 0~0.100%

[0199] Sb is an element that inhibits oxidation of the surface of the base steel sheet. To reliably achieve this effect, the Sb content is preferably set to 0.001% or more. On the other hand, even with a large content, the above effect becomes saturated; therefore, the Sb content is preferably set to 0.100% or less. The Sb content can be 0.080% or less, 0.060% or less, 0.050% or less, 0.030% or less, or 0.010% or less. The preferred Sb content is 0.0001~0.080%, 0.0005~0.060%, 0.001~0.050%, 0.003~0.030%, or 0.005~0.010%.

[0200] Zr: 0~0.100%

[0201] Zr is an element that inhibits oxidation on the surface of the base steel sheet. To reliably achieve this effect, the Zr content is preferably set to 0.001% or more. On the other hand, even with a large content, the above effect becomes saturated; therefore, the Zr content is preferably set to 0.100% or less. The Zr content can 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 0.0001~0.080%, 0.0005~0.060%, 0.001~0.050%, 0.003~0.030%, or 0.005~0.010%.

[0202] Sn: 0~1.00%

[0203] Sn is an element that inhibits oxidation on the surface of the base steel sheet. To reliably achieve this effect, the Sn content is preferably set to 0.001% or more. On the other hand, even with a large content, the above effect becomes saturated, therefore the Sn content is preferably 1.00% or less. The Sn content can 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 0.0001~0.800%, 0.0005~0.500%, 0.001~0.200%, 0.002~0.100%, 0.003~0.050%, or 0.005~0.010%.

[0204] As: 0~0.100%

[0205] As (As) has the effect of improving the strength of hot-stamped molded articles. To reliably achieve this effect, the As content is preferably set to 0.001% or more. On the other hand, even with a large content, the above effect is saturated; therefore, the As content is preferably set to 0.100% or less. The As content can be 0.080% or less, 0.050% or less, 0.020% or less, 0.010% or less, or 0.005% or less. The preferred As content is 0.001~0.080%, 0.002~0.050%, 0.003~0.020%, 0.004~0.015%, or 0.005~0.010%.

[0206] In the chemical composition of the hot-stamped molded body of this embodiment, the balance other than the aforementioned elements is Fe and impurities. Impurities refer to components that are mixed in during the industrial manufacturing of hot-stamped molded bodies, primarily from raw materials such as ores and waste, due to various reasons in the manufacturing process.

[0207] As described above, considering the balance between the hydrogen embrittlement resistance and notched tensile strength of the hot-stamped form, a strength of 1.5 GPa is preferred. To achieve this strength, the chemical composition of the hot-stamped form is preferably, by mass%, as follows: C: 0.18–0.26%, Si: 0.05–0.30%, Mn: 0.80–1.60%, P: ≤0.050%, S: ≤0.0200%, N: ≤0.0200%, O: ≤0.100%, Al: 0.005–0.080%, Cr: 0.10–0.50%, Nb: 0–0.020%, Ti: 0.010–0.050%. Mo: 0~0.050%, B: 0.0010~0.0050%, Co: 0~4.00%, Ni: 0~2.00%, Cu: 0~1.00%, V: 0~1.00%, W: 0~1.00%, Ca: 0~0.100%, Mg: 0~0.100%, REM: 0~0.100%, Sb: 0~0.100%, Zr: 0~0.100%, Sn: 0~1.00%, As: 0~0.100%, Balance: Fe and impurities.

[0208] The preferred chemical composition of the hot-stamped formed body, by mass%, is: C: 0.18~0.26%, Si: 0.05~0.30%, Mn: 0.80~1.60%, P: less than 0.050%, S: less than 0.0200%, N: less than 0.0200%, O: less than 0.100%, Al: 0.005~0.080%, Cr: 0.10~0.50%, Nb: 0~0.020%, Ti: 0.010~0.050%, Mo: 0~0 0.050%, B: 0.0010~0.0050%, Co: 0~4.00%, Ni: 0~2.00%, Cu: 0~1.00%, V: 0~1.00%, W: 0~1.00%, Ca: 0~0.0100%, Mg: 0~0.0100%, REM: 0~0.0100%, Sb: 0~0.100%, Zr: 0~0.100%, Sn: 0~1.00%, As: 0~0.100%, Balance: Fe and impurities.

[0209] The chemical composition of the hot-stamped formed body described above can be determined using general analytical methods. For example, it can be determined using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). It should be noted that C and S are determined using the combustion-infrared absorption method, N using the inert gas melting-thermal conductivity method, and O using the inert gas melting-non-dispersive infrared absorption method. If the surface of the hot-stamped formed body has a coating, the chemical composition can be analyzed after removing the coating by mechanical grinding.

[0210] 4. Uses

[0211] The application of the hot-stamped formed body of this embodiment is not particularly limited, and it can be suitable for use as an automotive part. Examples of automotive parts include lower control arms, wheels, subframes, center pillars, front longitudinal beams, rear longitudinal beams, lower side beams, and bumpers.

[0212] 5. Manufacturing method

[0213] An example of a method for manufacturing a hot-stamped molded body according to this embodiment will be described.

[0214] First, a steel sheet (billet) is manufactured as the raw material for hot stamping. Molten steel with the above-mentioned chemical composition is produced, and a slab is manufactured using this molten steel. As a slab for hot rolling, a continuously cast slab or a slab manufactured by a thin slab continuous casting machine can be used. Among the above-mentioned steel sheet manufacturing methods, a process such as continuous casting-direct rolling (CC-DR) that involves hot rolling immediately after casting is suitable.

[0215] The slab heating temperature is preferably set to 1100°C or higher. Slab heating temperatures below 1100°C will lead to a decrease in the finishing rolling temperature, and therefore the strength during finishing rolling will tend to be higher. As a result, rolling may become difficult, or the rolled steel sheet may have an undesirable shape. Therefore, the slab heating temperature is preferably set to 1100°C or higher.

[0216] The finishing rolling temperature is preferably set to 800°C or higher. If the finishing rolling temperature is below 800°C, the rolling load becomes high, which may make rolling difficult or result in poor shape of the rolled steel sheet. Therefore, the lower limit of the finishing rolling temperature is preferably set to 800°C. The upper limit of the finishing rolling temperature does not need to be specifically specified, but if the finishing rolling temperature is too high, the slab heating temperature must be too high to ensure its temperature. Therefore, the upper limit of the finishing rolling temperature is preferably 1100°C. After finishing rolling, the average cooling rate up to the coiling temperature (described later) is preferably set to 10~100°C / second.

[0217] The coiling temperature is preferably set below 700°C. When the coiling temperature exceeds 700°C, the thickness of the oxides formed on the steel sheet surface may increase excessively, reducing pickling properties. Subsequently, in the case of cold rolling, the lower limit of the coiling temperature is preferably set to 400°C. When the coiling temperature is below 400°C, the strength of the hot-rolled steel sheet increases drastically, which can easily induce sheet fracture and shape defects during cold rolling; therefore, the lower limit of the coiling temperature is preferably set to 400°C. However, if the coiled hot-rolled steel sheet is softened by heating it using a box-type annealing furnace or continuous annealing equipment, coiling can also be performed at a low temperature below 400°C. It should be noted that rough-rolled sheets can also be joined together during hot rolling for continuous finishing. Alternatively, the rough-rolled sheets can be temporarily coiled.

[0218] The hot-rolled steel sheet can also be pickled, and then cold-rolled to produce a cold-rolled steel sheet. In the case of cold rolling, the reduction rate can be set to, for example, 30-80%. Alternatively, the pickling process can be performed by immersing the sheet in an aqueous solution at a temperature of 80°C or higher but lower than 100°C, with the addition of an inhibitor and an acid concentration of 3-20% by mass, for at least 30 seconds. Furthermore, the hot-rolled or cold-rolled steel sheet can be annealed to produce a hot-rolled annealed sheet or a cold-rolled annealed sheet. Annealing is performed by passing the sheet through a continuous annealing line, and the annealing temperature can be set to, for example, in the range of 550-750°C.

[0219] For the various steel sheets obtained, plating treatment can be performed before hot stamping. There are no particular limitations on applicable plating methods, but examples include hot-dip galvanizing, alloyed hot-dip galvanizing, electro-galvanizing, Zn-Ni plating (electro-galvanized alloy), Sn plating, Al-Si plating, alloyed electro-galvanizing, hot-dip galvanized aluminum alloy, hot-dip galvanized aluminum-magnesium alloy, hot-dip galvanized aluminum-magnesium-Si alloy, and vapor-deposited Al. Furthermore, plating treatment can be performed by passing the sheets through a continuous production line.

[0220] Next, the blank is hot-stamped. In this process, the blank is heated, then stamped and quenched. During quenching, the cooling rate is controlled to be higher for the areas with high strain compared to areas without high strain, and the blank is cooled to a temperature below the Mf point. When manufacturing a hot-stamped body with a flange, the flange may be performed simultaneously or continuously with the stamping, followed immediately by quenching. This will be explained in more detail below.

[0221] First, the billet is heated at an average heating rate of 2~200℃ / second to above point Ac3 and below point Ac3 +65℃, and then held in this temperature range for 10~120 seconds. After the billet is removed from the furnace, molding begins. At this time, the initial molding temperature is set to above 700℃. It should be noted that during the period from removing the billet from the furnace to the start of molding, air cooling is performed at an average cooling rate of 10~20℃ / second.

[0222] Then, without demolding, it is cooled to a temperature below the Mf point, thereby undergoing quenching. In order to form the aforementioned first and second regions in the hot-stamped body, the control of the cooling conditions after hot stamping becomes important.

[0223] For areas without high strain (hereinafter referred to as "low strain areas"), the average cooling rate from the molding start temperature to the Ms point is set to 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 set to 10°C / s or more and less than 80°C / s. On the other hand, for areas with high strain (hereinafter referred to as "high strain areas"), the average cooling rate from the molding start temperature to the Ms point is set to 200°C / s or more, and the average cooling rate from the Ms point to the Mf point is set to 100°C / s or more. It is not necessary to set an upper limit on the cooling rate of high strain areas, but from the perspective of manufacturing equipment capabilities, it is preferable to set the average cooling rate from the molding start temperature to the Ms point to 2000°C / s or less, and the average cooling rate from the Ms point to the Mf point to 1800°C / s or less. After demolding in the temperature range below the Mf point, cooling to room temperature at an average cooling rate of 5°C / s or less is sufficient.

[0224] As a method of setting a difference in cooling rate between two parts, the following can be cited: (1) directly water cooling only the high strain part and mold cooling the low strain part; (2) making the mold material that is in contact with the two parts different, and making the mold material that is in contact with the high strain part have high thermal conductivity; (3) making the pressure between the blank and the mold different when molding the two parts, increasing the molding pressure of the high strain part, and increasing the heat conduction to the mold, etc.

[0225] By performing hot stamping under the above conditions, the increase in hardness is suppressed in low-strain regions, thereby improving hydrogen embrittlement resistance and notched tensile strength. On the other hand, ferrite formation is suppressed in high-strain regions. Thus, a first region is formed from the low-strain regions, and a second region is formed from the high-strain regions.

[0226] When the heating temperature is below Ac3, ferrite, pearlite, bainite, cementite, and other microstructures other than austenite remain after heating, and the martensite area ratio may be less than 95% during the subsequent cooling process. On the other hand, when the heating temperature is above Ac3 + 65°C or the heating time exceeds 120 seconds, the grains coarsen, and the hydrogen embrittlement resistance and notched tensile strength may decrease.

[0227] It should be noted that when the strength of the hot-stamped part is set to 1.5 GPa, G1 is set to 8.0~14.0 μm, and G2 is set to the range of 8.0~14.0 μm, it is preferable to set the heating temperature to Ac3 point +10°C or higher and Ac3 point +60°C or lower, and more preferably to Ac3 point +15°C or higher and Ac3 point +50°C or lower.

[0228] In addition, when the molding start temperature is less than 700°C, a large amount of ferrite is generated during the air cooling process from heating to the start of molding or during the subsequent cooling process, especially the area ratio of martensite at the 1 / 2 thickness position may be less than 95%.

[0229] In low-strain regions, when the average cooling rate from the forming start temperature to point Ms is less than 50°C / second, a large amount of ferrite is generated during cooling, especially at the 1 / 2 thickness position where the martensite area ratio may be less than 95%. On the other hand, when the average cooling rate is 150°C / second or higher, the average cooling rate from point Ms to point Mf may easily increase during manufacturing.

[0230] In low-strain regions, when the average cooling rate from Ms to Mf is less than 10°C / s, the strength reduction caused by self-tempering may become excessive. On the other hand, when the average cooling rate is 80°C / s or higher, the effect of self-tempering cannot be fully obtained, and the hardness of the first region increases. As a result, the improvement in hydrogen embrittlement resistance and notched tensile strength may not be fully achieved.

[0231] In high-strain regions, when 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 cooling, especially at the 1 / 2 thickness position where the martensite area ratio may be less than 95%. Conversely, when the average cooling rate from the forming start temperature to the Ms point is set to 200°C / s or more, in manufacturing, the average cooling rate from the Ms point to the Mf point is 100°C / s or more.

[0232] Furthermore, as mentioned above, demolding is performed in a temperature range below the Mf point after hot stamping. This is because if demolding occurs at temperatures above the Mf point, a martensitic phase transformation will also occur after demolding, which may reduce the dimensional accuracy of the molded body.

[0233] In this invention, points Ac3, Ms, and Mf are calculated based on equations (I) to (III) below.

[0234] Ac3(℃)=850+10×(C+N)×Mn+350×Nb+250×Ti+40×B+10×Cr+100×Mo…(I)

[0235] Ms(℃)=550-361×C-39×Mn-35×V-20×Cr-17×Ni-10×Cu-5×(Mo+W)+15×Co+30×Al…(II)

[0236] Mf(℃)=410.5-407.3×C-7.3×Si-37.8×Mn-20.5×Cu-19.5×Ni-19.8×Cr-4.5×Mo…(III)

[0237] In the above formula, the element symbols represent the content (mass%) of each element.

[0238] The present invention will be described in more detail below through embodiments, but the present invention is not limited to these embodiments.

[0239] Example

[0240] Molten steel (steel grades a~m) with the chemical compositions shown in Table 1 is produced by continuous casting to form slabs. These slabs are then heated to temperatures above 1100°C and subjected to rough rolling, finish rolling, cooling, and coiling under the conditions shown in Table 2 to produce hot-rolled steel sheets. The hot-rolled steel sheets of grades e and g are then further cold-rolled to produce cold-rolled steel sheets.

[0241] [Table 1]

[0242]

[0243] [Table 2]

[0244]

[0245] For the obtained hot-rolled or cold-rolled steel sheets, under the conditions shown in Table 3, hot stamping, flanging, and quenching are performed as a series of processes to produce steel sheets in the form of... Figure 1AThe hot-stamped formed body has the shape shown and includes a plate-shaped portion and a flanged portion. The plate-shaped portion has a thickness of 2.6 mm, the front end thickness (end thickness) of the flanged portion is as shown in Table 5, and the diameter of the flanged portion is 50 mm. In this case, the portion that has undergone flanged processing (the portion that becomes the flanged portion after hot stamping) is directly water-cooled, while other portions (the portions that become the plate-shaped portion after hot stamping) are cooled using a mold, thereby creating a difference in cooling rate between the two portions.

[0246] [Table 3]

[0247]

[0248] It should be noted that the "Heating Rate" and "Heating Temperature" in the table refer to the average heating rate and heating temperature during heating before hot stamping. "Difference from point Ac3" is the value obtained by subtracting point Ac3 from the aforementioned heating temperature. "Holding Time" refers to the time held at the aforementioned heating temperature. "Cooling Rate between St and Ms" refers to the average cooling rate from the molding start temperature St to point Ms, and "Cooling Rate between Ms and Mf" refers to the average cooling rate from point Ms to point Mf.

[0249] Next, Figure 1B , Figure 1C , Figure 2A , Figure 2B and Figure 9A , Figure 9B The cross sections shown are used as measurement areas in the plate-like portion and the flanged portion, respectively. The metallographic structure was observed and the Vickers hardness was measured using the following methods. The measurement results are shown in Tables 4 and 5.

[0250] [Table 4]

[0251]

[0252] [Table 5]

[0253]

[0254] First, the measured area was etched with nitric acid and ethanol, and then photographed at 1000x magnification using a FE-SEM. The FE-SEM used was equipped with a secondary electron detector. Then, the obtained tissue images were analyzed to determine the area ratio of the tissue identified as ferrite or pearlite; otherwise, it was determined as martensite, bainite, or retained austenite. The total area ratio identified as martensite, bainite, or retained austenite is expressed as “M+B+γ area ratio (%)” in Tables 4 and 5. Then, if this area ratio was less than 95%, the area ratio of martensite (“M area ratio” in Tables 4 and 5) was determined to be less than 95% (“<95%”).

[0255] On the other hand, with an area ratio of over 95%, a grid with 70 μm intervals was drawn in an area of ​​300 μm vertically and 2000 μm horizontally, and Vickers hardness was measured at all grid intersections. The test force in the Vickers hardness measurement was set to 0.9807 N (0.1 kgf). Then, FE-SEM was used again to take images at 1000x magnification. Then, by comparing the microstructure images taken before and after the Vickers hardness measurement, it was determined whether the area of ​​the residual depression on the surface contained microstructure other than martensite, bainite, and retained austenite, i.e., whether it contained ferrite or pearlite.

[0256] Then, the average value of the obtained Vickers hardness measurements is calculated to determine the indentation as containing only martensite, bainite, and / or retained austenite. If the following formula is satisfied, the area ratio of martensite in the metallographic structure is judged to be 95% or more ("≥95%"). On the other hand, if the following formula is not satisfied, it is considered that the area ratio of bainite is relatively high or the martensite has been over-tempered, and therefore the area ratio of martensite is judged to be less than 95% ("<95%"). Subsequent metallographic structure observation is not performed.

[0257] HV MBγ ≥957.1×[C]+221.0

[0258] The meanings of the symbols in the above formula are as follows.

[0259] HV MBγ The average value of Vickers hardness measurements (HV0.1) for depressions identified as containing only martensite, bainite, and / or retained austenite.

[0260] [C]: C content (mass%) of the hot-stamped part

[0261] In the table, the HV values ​​at the 1 / 4 and 1 / 2 thickness positions of the plate-shaped portion are... MBγ Represented as HV MBγ1_1 / 4 and HV MBγ1_1 / 2 HV at the 1 / 4 and 1 / 2 thickness positions of the flanged portion MBγ Represented as HV MBγ2_1 / 4 and HV MBγ2_1 / 2 Additionally, HV MBγ1_1 / 4 With HV MBγ2_1 / 4 The ratios are shown in Tables 4 and 5.

[0262] 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 flanged portion are respectively expressed as HV. 1_1 / 4 HV 1_1 / 2 HV 2_1 / 4and HV 2_1 / 2 The average hardness refers to the average value of the Vickers hardness measurements at all intersections of the 70μm grid.

[0263] Next, for the aforementioned observation area, after mirror polishing and electrolytic polishing under the above conditions, measurements were performed using EBSD to determine the original austenite grains through the aforementioned reconstruction method. For each determined original austenite grain, the grain diameter and aspect ratio were calculated. The measured values ​​of all original austenite grains in the observation area were averaged to determine the average grain diameter and average aspect ratio.

[0264] Next, for each hot-stamped part, the tensile strength, hydrogen embrittlement resistance and notched tensile strength were evaluated by the following methods.

[0265] Tensile Strength

[0266] Made from the plate-shaped parts of each hot-stamped body Figure 10 A small-sized plate test piece of the shape specified in ASTM A370-22 (width 10 mm, overall length 100 mm, parallel section width 6.25 mm, parallel section length 32 mm, gauge length 25 mm, thickness: original thickness of the plate section) is used. Then, a tensile test is performed using this half-size plate test piece to determine the tensile strength TS (MPa). The tensile speed is 3 mm / min as measured by the crosshead displacement speed, and other conditions are in accordance with JIS Z 2241:2022.

[0267] <Hydrogen embrittlement resistance and notched tensile strength>

[0268] Two test pieces were cut from the plate-shaped part and the flanged part of the hot-stamped body, respectively. Figure 11 and Figure 12D Tensile test specimens of the shape shown. Figure 12A ~D is a diagram illustrating the steps for cutting the test piece from the flange to prepare the tensile test piece. Due to size limitations, it is not possible to collect samples from the flange. Figure 11 Tensile test specimens of the shape shown. Therefore, as... Figure 12A As shown in Figure B, firstly, a test piece with a length of 8.0 mm, a thickness of 1.0 mm, and a height of 2.0 mm is cut from the front end of the flange 12. Then, as... Figure 12C As shown, a U-shaped notch is formed on both sides of the height direction at the center of the length direction of the test piece, and then, as... Figure 12D As shown, the alternative material shown in the shaded area is welded together on both sides along the length of the test piece to form a material that is similar to... Figure 11 Tensile test specimens of the same shape as those shown.

[0269] Then, a tensile test piece was placed in a 3% NaCl aqueous solution as the cathode, and a current density of 1.0 mA / cm² was applied. 2 Under certain conditions, hydrogen was generated on the surface of the tensile test specimen, and a tensile test was conducted at a tensile speed of 0.006 mm / min to determine the fracture stress (maximum stress) in the hydrogen environment. It should be noted that Pt was used as the anode. The measured value was then used as the tensile strength TSH (MPa) in the hydrogen environment, serving as an indicator of hydrogen embrittlement resistance.

[0270] In addition, for another tensile test specimen, a tensile test according to JIS Z 2241:2022 was performed at atmospheric pressure at a rate of 0.006 mm / min to determine the fracture stress (maximum stress). The obtained measured value was then used as the notched tensile strength TSN (MPa).

[0271] In this embodiment, when the sum of the tensile strength and notched tensile strength in the measured hydrogen environment (TSH+TSN) is 2600 MPa or higher, it is judged to have excellent hydrogen embrittlement resistance and high notched tensile strength.

[0272] These results are shown in Table 6.

[0273] [Table 6]

[0274]

[0275] As shown in Tables 4-6, among the test numbers 1-4, 13-16, 20-22 and 24-34 that meet the requirements of this invention, the test results exhibit high strength and can balance excellent resistance to hydrogen embrittlement with high notched tensile strength.

[0276] In contrast, in tests 5-12, 17-19, and 23 where the hot stamping conditions were unsuitable, the martensite area ratio of at least one of the plate-shaped portion and the flanged portion was less than 95%, or the average grain diameter exceeded the specified value, or the above formula (i) was not satisfied. Therefore, the improvement in hydrogen embrittlement resistance and notched tensile strength could not be obtained.

[0277] Specifically, in test number 5, the heating temperature in the hot stamping process was too high, and in test number 10, the heating time was too long. As a result, the average grain diameter was too large in both the plate-shaped part and the flanged part, and the TSH+TSN was reduced.

[0278] In experiment number 6, due to the low average cooling rate from the forming start temperature to Ms point in the flanged section, the martensite area ratio in the flanged section did not reach more than 95%. As a result, TSH+TSN decreased in the flanged section.

[0279] In experiment number 7, due to the low initial forming temperature, the martensite area ratio in both the plate-like and flanged portions did not reach 95%. As a result, the TSH+TSN ratio decreased in both the plate-like and flanged portions.

[0280] In tests 8, 17, and 23, due to the slightly lower average cooling rate from the forming start temperature to point Ms in the flanged section, although the martensite area ratio at the 1 / 4 thickness position was above 95%, the martensite area ratio at the 1 / 2 thickness position did not reach above 95%. As a result, the TSH+TSN ratio was slightly reduced in the flanged section.

[0281] In tests 9 and 18, the average cooling rate from point Ms to point Mf in the plate-like section and the average cooling rate from the forming start temperature to point Ms in the flanged section were low. Therefore, the martensite area ratio did not reach 95% or higher in both the plate-like and flanged sections. Consequently, the TSH+TSN ratio decreased in both the plate-like and flanged sections.

[0282] In experiment number 11, the heating temperature in the hot stamping process was low, so the martensite area ratio in both the plate section and the flanged section did not reach more than 95%. As a result, the TSH+TSN ratio decreased in both the plate section and the flanged section.

[0283] In experiment number 12, because the average cooling rate from the molding start temperature to the Ms point in the plate-shaped section was too high, the self-tempering effect could not be fully achieved in the plate-shaped section, resulting in an increase in hardness. As a result, the TSH+TSN ratio decreased slightly in the plate-shaped section.

[0284] In experiment number 19, the average cooling rate from the forming start temperature to Ms point in the plate-shaped section was low, therefore the martensite area ratio in the plate-shaped section did not reach more than 95%. As a result, the TSH+TSN ratio was slightly reduced in the plate-shaped section.

[0285] Industrial availability

[0286] According to the present invention, it is possible to obtain hot-stamped parts and automotive parts using the same, even when high strain is locally introduced in the hot stamping process, which can suppress the decrease in strength after hot stamping in the high strain region.

Claims

1. A hot-stamped formed body, comprising: The first region has a metallographic structure with a martensite area of ​​over 95% at the 1 / 4 thickness position and an average aspect ratio of the original austenite grains of less than 1.10; and The second region has a metallographic structure with a martensite area of ​​over 95% at the 1 / 4 thickness position and an average aspect ratio of 2.00 to 3.00 for the original austenite grains. The martensite area ratio at the half-thickness position of the first and second regions is over 95%. When the average grain diameter of the original austenite grains at the 1 / 4 thickness position of the first region is set as G1 (μm), and the average grain diameter of the original austenite grains at the 1 / 4 thickness position of the second region is set as G2 (μm), G1 and G2 are 5.0~15.0 μm. When the Vickers hardness is measured at a position one-quarter of the thickness of the first and second regions, the following equation (i) is satisfied. 1.05≤HV MBγ2_1 / 4 / HV MBγ1_1 / 4 ≤1.25…(in) in, The meanings of the symbols in the above formula are as follows: HV MBγ1_1 / 4 The indentation formed by pressing the indenter at the 1 / 4 position of the thickness of the first region is determined as the average value of the Vickers hardness measurements containing only martensite, bainite and / or retained austenite (HV0.1). HV MBγ2_1 / 4 The indentation formed by pressing the indenter at the 1 / 4 position of the thickness of the second region is determined as the average value of the Vickers hardness measurements containing only martensite, bainite and / or retained austenite (HV0.1).

2. The hot-stamped formed body according to claim 1, wherein, G1 and G2 further satisfy the following equation (ii), |G2-G1|<3.0…(ii).

3. The hot-stamped formed article according to claim 1 or 2, wherein its chemical composition, in mass percent, is as follows: C：0.10~0.60%、 Si: 0.01~2.00% Mn: 0.10~3.00% P: below 0.050% S: Below 0.0200% N: below 0.0200% O: Below 0.100% Al：0.001~0.100%、 Cr：0.01~1.00%、 Nb: 0~0.200% Ti: 0~0.200% Mo: 0~1.00% B：0~0.0100%、 Co: 0~4.00%, Ni: 0~2.00% Cu: 0~1.00% V：0~1.00%、 W：0~1.00%、 Ca: 0~0.100% Mg: 0~0.100% REM: 0~0.100% Sb: 0~0.100% Zr：0~0.100%、 Sn: 0~1.00% As: 0~0.100% Balance: Fe and impurities.

4. An automotive part that uses the hot-stamped formed body according to any one of claims 1 to 3.

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