Hot-stamp molded body

Abstract

A hot-stamping formed body includes a first region having a microstructure in which an area fraction of martensite is 95% or more and an average aspect ratio of prior-austenite grains is 1.10 or less, and a second region having a microstructure in which an area fraction of martensite is 95% or more and an average aspect ratio of prior-austenite grains is 2.00 or more. An average grain diameter G1 (µm) of prior-austenite grains in the first region and an average grain diameter G2 (µm) of prior-austenite grains in the second region satisfy [G1 ≤ 15.0], [G2 ≤ 25.0], and [G2-G1 ≥ 3.0]. A tensile strength measured by a tensile test using a test specimen taken from the first region is 1250 to 2540 MPa.

Description

TECHNICAL FIELD

[0001] The present invention relates to a hot-stamping formed body.BACKGROUND ART

[0002] In order to both reduce the weight and ensure the collision safety of the automobiles, there is a demand to increase the strength of materials used in automobiles. For this reason, in recent years, hot-stamp steels of up to 1.8 GPa grade have been put to practical use in automobile bodies. Furthermore, with regard to parts of chassis also, if there is a demand to increase the strength in a similar manner to the automobile body, in the future it will be necessary to consider using hot-stamp steel for parts of chassis also.

[0003] Automotive parts such as those described above are sometimes subjected to a burring processing to assemble the parts together or the like. For example, Patent Document 1 discloses a method for producing a hot pressed product having a burring portion.

[0004] Patent Document 1 discloses a production method that includes a heating step of heating a sheet material, and a forming step of subjecting the heated sheet material to forming. In the forming step, in addition to subjecting the sheet material to forming, quenching and a burring processing are also performed.LIST OF PRIOR ART DOCUMENTSPATENT DOCUMENT

[0005] Patent Document 1: JP2019-58916ASUMMARY OF INVENTIONTECHNICAL PROBLEM

[0006] However, as a result of studies conducted by the present inventors, it has been found that during forming in hot stamping, in some cases ferrite may form and cause partial softening in a region where high strain is imparted, such as in a burring portion. This is thought to be because diffusion transformation is promoted by the high strain. Furthermore, based on the results of various studies conducted using a hot processing simulator, it has been revealed that the greater the strain by processing is, the more hardenability decreases and the more it is necessary to increase a critical cooling rate for obtaining a sound full martensitic structure in the subsequent cooling process.

[0007] Because relatively thick steel material is used for chassis parts, the rate of cooling by die quenching is lower as comparison to that of thin steel material that is used for the automobile body. From this viewpoint also, it is considered that a decrease in hardenability will tend to become manifest in a region where high strain by processing has been introduced. Further, even in the case of automotive body parts for which steel sheets for hot stamping are already being used, the structures are becoming more complicated in order to control the deformation mode of each part, and high strain is partially introduced in some cases.

[0008] In addition, from the perspective of further improving safety, automotive parts and the like are required to have both excellent resistance to hydrogen embrittlement and high notched tensile strength.

[0009] An objective of the present invention is to solve the problem described above and provide a hot-stamping formed body in which a decrease in strength after hot-stamping forming in a high strain region is suppressed even in a case where high strain is partially introduced in a hot stamping process, and which has excellent resistance to hydrogen embrittlement and high notched tensile strength.SOLUTION TO PROBLEM

[0010] The gist of the present invention, which has been made to solve the problem described above, is a hot-stamping formed body described hereunder.

[0011] (1) A hot-stamping formed body, including: a first region having a microstructure in which an area fraction of martensite is 95% or more and an average aspect ratio of prior-austenite grains is 1.10 or less, and a second region having a microstructure in which an area fraction of martensite is 95% or more and an average aspect ratio of prior-austenite grains is 2.00 or more; wherein: when an average grain diameter of prior-austenite grains in the first region is represented by G1 (µm) and an average grain diameter of prior-austenite grains in the second region is represented by G2 (µm), G1 is 15.0 µm or less and G2 is 25.0 µm or less, and G1 and G2 satisfy Formula (i) below, and a tensile strength measured by a tensile test using a test specimen taken from the first region is 1250 to 2540 MPa: G 2 − G 1 ≥ 3.0 (2) The hot-stamping formed body according to (1) above, wherein: the average grain diameter G1 of prior-austenite grains in the first region is 5.0 to 15.0 µm, and the average grain diameter G2 of prior-austenite grains in the second region is 10.0 to 25.0 µm. (3) The hot-stamping formed body according to (1) or (2) above, having a chemical composition containing, by mass%, C: 0.10 to 0.60%, Si: 0.01 to 2.00%, Mn: 0.10 to 3.00%, P: 0.050% or less, S: 0.0200% or less, N: 0.0200% or less, O: 0.100% or less, Al: 0.001 to 0.100%, Cr: 0.01 to 1.00%, Nb: 0 to 0.200%, Ti: 0 to 0.200%, Mo: 0 to 1.00%, B: 0 to 0.0100%, Co: 0 to 4.00%, Ni: 0 to 2.00%, Cu: 0 to 1.00%, V: 0 to 1.00%, W: 0 to 1.00%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, REM: 0 to 0.0100%, Sb: 0 to 0.100%, Zr: 0 to 0.100%, Sn: 0 to 1.00%, As: 0 to 0.100%, and the balance: Fe and impurities. (4) A hot-stamping formed body, including: a first region having a microstructure in which an area fraction of martensite is 95% or more and an average aspect ratio of prior-austenite grains is 1.10 or less, and a second region having a microstructure in which an area fraction of martensite is 95% or more and an average aspect ratio of prior-austenite grains is 2.00 or more; wherein: when an average grain diameter of prior-austenite grains in the first region is represented by G1 (µm) and an average grain diameter of prior-austenite grains in the second region is represented by G2 (µm), G1 is 15.0 µm or less and G2 is 25.0 µm or less, and G1 and G2 satisfy Formula (i) below, and a tensile strength measured by a tensile test using a test specimen taken from the first region is 1250 to 2040 MPa: G 2 − G 1 ≥ 3.0 (5) The hot-stamping formed body according to (4) above, wherein: an average grain diameter G1 of prior-austenite grains in the first region is 5.0 to 15.0 µm, and an average grain diameter G2 of prior-austenite grains in the second region is 10.0 to 25.0 µm. (6) The hot-stamping formed body according to (4) or (5) above, having a chemical composition containing, by mass%, C: 0.10 to 0.40%, Si: 0.01 to 2.00%, Mn: 0.10 to 3.00%, P: 0.050% or less, S: 0.0200% or less, N: 0.0200% or less, O: 0.100% or less, Al: 0.010 to 0.100%, Cr: 0.01 to 1.00%, Nb: 0 to 0.200%, Ti: 0 to 0.200%, Mo: 0 to 1.00%, B: 0 to 0.0100%, Co: 0 to 4.00%, Ni: 0 to 2.00%, Cu: 0 to 1.00%, V: 0 to 1.00%, W: 0 to 1.00%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, REM: 0 to 0.0100%, Sb: 0 to 0.100%, Zr: 0 to 0.100%, Sn: 0 to 1.00%, As: 0 to 0.100%, and the balance: Fe and impurities. ADVANTAGEOUS EFFECTS OF INVENTION

[0012] According to the present invention, a hot-stamping formed body can be obtained in which, even in a case where high strain is partially introduced in a hot stamping process, a decrease in strength after hot-stamping forming in a high strain region is suppressed, and which has excellent resistance to hydrogen embrittlement and high notched tensile strength.BRIEF DESCRIPTION OF DRAWINGS

[0013] [Figure 1] Figure 1 is a schematic diagram illustrating a formed body having a circular burring portion. [Figure 2] Figure 2 is a schematic diagram for describing one example of a second measurement region. [Figure 3] Figure 3 is a schematic diagram illustrating a formed body that has a burring portion with an oval shape. [Figure 4] Figure 4 is a schematic diagram illustrating a formed body that has a flange portion with an arc shape. [Figure 5] Figure 5 is a schematic diagram for describing one example of a second measurement region. [Figure 6] Figure 6 is a schematic diagram illustrating a formed body that has a flange deformation portion. [Figure 7] Figure 7 is a schematic diagram for describing one example of a second measurement region. [Figure 8] Figure 8 is a schematic diagram for describing one example of a first measurement region. [Figure 9] Figure 9 is a schematic diagram illustrating the shape of a sub-sized sheet-shaped test specimen according to ASTM standards. [Figure 10] Figure 10 is a view for describing the shape of a tensile test specimen used for evaluating resistance to hydrogen embrittlement and notched tensile strength. [Figure 11] Figure 11 is a view for describing a procedure for cutting out a test specimen from a burring portion and preparing a tensile test specimen. DESCRIPTION OF EMBODIMENTS

[0014] The present inventors investigated methods for suppressing a decrease in hardenability in a high strain region and, as a result, have obtained the following findings.

[0015] A burring portion may be mentioned as one example of a high strain region. In some cases a die set is designed to lightly squeeze a steel sheet (in the following description, a steel sheet before being subjected to hot-stamping forming is also referred to as a "blank") at a burring portion, and therefore the possibility that a direct water cooling die set cannot be applied is high. In addition, there is also a possibility that the die set and the blank will enter a state in which they cannot come into close contact with each other sufficiently over the entire burring portion. It is considered that softening caused by formation of ferrite tends to easily occur due to the promotion of diffusion transformation that is caused by the introduction of high strain and due to the difficulty of securing the cooling rate.

[0016] Therefore, as a result of repeated experiments under various hot stamping conditions, the present inventors have discovered that increasing the heating temperature before hot-stamping forming and causing the grains to coarsen in the microstructure before processing is effective for securing hardenability.

[0017] However, if the heating temperature for the entire blank is increased, in low strain regions also, the microstructure will coarsen and the specific surface area of the grain boundaries will become smaller, which may result in a decrease in resistance to hydrogen embrittlement and notched tensile strength.

[0018] Therefore, it is possible to increase hardenability in a high strain region and suppress a decrease in the strength of a hot-stamping formed body as a whole by subjecting a region at which it is planned to impart high strain in a steel sheet before hot-stamping forming to heating at a higher temperature in comparison to the other regions.

[0019] Note that, in a high strain region, the aspect ratio of grains increases due to the influence of processing. For grains with the same grain diameter, the specific surface area of the grain boundary will be larger in the grain that has the larger aspect ratio. Therefore, even if grains are coarsened in high strain regions, the degree to which the resistance to hydrogen embrittlement and the notched tensile strength are reduced will be relatively low.

[0020] The present invention has been made based on the above findings. The respective requirements of the present invention are described in detail hereunder.1. Microstructure of Hot-stamping Formed Body

[0021] A hot-stamping formed body according to the present embodiment has a first region having a microstructure in which an area fraction of martensite is 95% or more and an average aspect ratio of prior-austenite grains is 1.10 or less, and a second region having a microstructure in which an area fraction of martensite is 95% or more and an average aspect ratio of prior-austenite grains is 2.00 or more.

[0022] The first region is a region where high strain was not introduced during hot-stamping forming. Therefore, the average aspect ratio of prior-austenite grains in the first region is 1.10 or less. Note that, the practical lower limit of the average aspect ratio is 1.00. In addition, the microstructure is mainly composed of martensite due to quenching that accompanies the hot-stamping forming, and specifically the area fraction of martensite is 95% or more. Note that, in the present invention, the term "martensite" is taken to also include tempered martensite and self-tempered martensite in addition to fresh martensite that is as-quenched martensite.

[0023] In the microstructure of the present embodiment, the balance other than martensite is bainite, retained austenite, ferrite, and pearlite. However, in the present invention, as described later, because the area fraction of martensite is measured based on Vickers hardness, martensite whose hardness has been excessively decreased by tempering or self-tempering is excluded from martensite and is included in the balance.

[0024] On the other hand, the second region is a region where high strain was introduced during hot-stamping forming, and consequently the average aspect ratio of prior-austenite grains is 2.00 or more. As described above, in a region where high strain has been introduced, because diffusion transformation is promoted, not only does ferrite form and the steel material softens, but in some cases the resistance to hydrogen embrittlement and the notched tensile strength may also decrease. However, in the second region, because the hardenability is increased by making the heating temperature before hot-stamping forming a high temperature, the area fraction of martensite is 95% or more.

[0025] Note that, in forming performed in a manner so that a fracture does not occur, it is generally difficult to make the average aspect ratio of prior-austenite grains more than 3.00. Therefore, the average aspect ratio of prior-austenite grains in the second region is practically 3.00 or less. In other words, a region where the average aspect ratio of prior-austenite grains is more than 3.00 is considered to be a region where appropriate forming was not performed, and is therefore excluded from the second region.

[0026] In the hot-stamping formed body according to the present embodiment, by having the second region in which the area fraction of martensite is 95% or more even when high strain has been imparted thereto, in addition to suppressing a decrease in strength, it is also possible to enhance resistance to hydrogen embrittlement and the notched tensile strength.

[0027] As described above, in the second region, hardenability is increased by making the heating temperature before hot-stamping forming a high temperature. On the other hand, the heating temperature for the first region is a lower temperature than the heating temperature for the second region to ensure that the grains do not coarsen. Because of this difference, the grain diameter in the second region is larger than the grain diameter in the first region. Specifically, when the average grain diameter of prior-austenite grains in the first region is represented by G1 (µm), and the average grain diameter of prior-austenite grains in the second region is represented by G2 (µm), G1 and G2 satisfy the following Formula (i): G 2 − G 1 ≥ 3.0

[0028] By G1 being 3.0 µm or more smaller in comparison to G2, that is, by the microstructure in the first region being a relatively fine-grained structure, excellent resistance to hydrogen embrittlement and high notched tensile strength are obtained.

[0029] Preferably, the value of G2-G1 is 4.0 µm or more, and more preferably is 5.0 µm or more. On the other hand, although it is not necessary to set an upper limit for the value of G2-G1, since it is industrially difficult to change the heating temperatures for both regions to an extreme degree, G2-G1 is 17.0 µm or less.

[0030] Further, if the grains in the first region and the second region are excessively coarse, respectively, there is a risk that the resistance to hydrogen embrittlement and notched tensile strength in each region will decrease. Therefore, G1 is to be 15.0 µm or less, and G2 is to be 25.0 µm or less. Note that, as described above, the difference between the upper limit values of the average grain diameter in the two regions is due to the difference between the aspect ratios of the grains.

[0031] Note that, although a lower limit value for each of G1 and G2 is not particularly limited, from the viewpoint of providing excellent resistance to hydrogen embrittlement and high notched tensile strength in a hot-stamping formed body of 1.3 to 2.5 GPa grade, for example, preferably G1 is within the range of 5.0 to 15.0 µm, and preferably G2 is within the range of 10.0 to 25.0 µm. More preferably, G1 is 8.0 µm or more, and more preferably G2 is 15.0 µm or more.

[0032] Further, a so-called "trade-off relationship" exists between the resistance to hydrogen embrittlement and the notched tensile strength, and consequently, as the strength of a hot-stamping formed body increases, although the notched tensile strength increases, the resistance to hydrogen embrittlement tend to decrease. From the viewpoint of placing importance on the balance between resistance to hydrogen embrittlement and notched tensile strength, preferably the strength of the hot-stamping formed body is strength of 1.5 GPa grade. Furthermore, in a hot-stamping formed body of 1.5 GPa grade, from the viewpoint of providing excellent resistance to hydrogen embrittlement and high notched tensile strength, preferably G1 is within the range of 8.0 to 14.0 µm and G2 is within the range of 17.0 to 25.0 µm.

[0033] In the present invention, measurement of the microstructure of the hot-stamping formed body is performed by the following method. First, measurement regions are determined. At such time, in the hot-stamping formed body, a region where the degree of processing is low and high strain has not been introduced can be selected as a measurement region of the first region (hereunder, also referred to as "first measurement region"), while on the other hand a region where high strain has been introduced, such as a burring portion, can be selected as a measurement region of the second region (hereunder, also referred to as "second measurement region").

[0034] Next, measurement of the martensite area fraction as well as measurement of the average grain diameter and average aspect ratio of prior-austenite grains is performed in each of the aforementioned measurement regions (the first measurement region and the second measurement region, respectively). In the present invention, measurement of the martensite area fraction is performed by combining observation with a field emission-scanning electron microscope (FE-SEM) and Vickers hardness measurement, and measurement of the average grain diameter and average aspect ratio of prior-austenite grains is performed by analysis using electron back scatter diffraction (EBSD). Each measurement procedure is described in further detail hereunder.

[0035] The aforementioned measurement regions are etched with nital, and thereafter, in each measurement region, a secondary electron image of a region of 300 µm in length by 2000 µm in width centered at a position at a depth of 1 / 4 of the thickness from the surface of the base material (hereunder, referred to as a "1 / 4 thickness position") is obtained at a magnification of 1000× using an FE-SEM. In other words, an FE-SEM equipped with a secondary electron detector is used. The obtained micro-structure image is then subjected to image analysis to measure the area fraction of micro-structure that is determined to be ferrite or pearlite, and the remaining micro-structure is determined to be martensite, bainite, or retained austenite. If the total of the area fractions that are determined to be martensite, bainite, or retained austenite is 95% or more, a mesh is drawn with 70 µm intervals in the region of 300 µm in length by 2000 µm in width, and measurement of the Vickers hardness is performed at all of the intersection points (4 x 28 = 112 points) of the mesh. The test force in the Vickers hardness measurement is set to 0.98 N (0.1 kgf). Thereafter, an image is obtained once again at a magnification of 1000× using an FE-SEM. Then, by visually comparing the micro-structure images that were obtained before and after the Vickers hardness measurement respectively, it is determined whether or not a micro-structure other than martensite, bainite, and retained austenite, that is, ferrite or pearlite, is contained in an indentation made by pressing the indenter during the Vickers hardness measurement (hereunder, such an indentation is also referred to simply as "indentation").

[0036] Then, among the obtained Vickers hardness measurement values, the average value of those measurement values identified as containing only martensite, bainite, or retained austenite within the area of the indentations remaining in the surface is calculated, and if the following formula is satisfied, it is determined that the area fraction of martensite in the microstructure is 95% or more. Note that, "HV0.1" described hereunder means the "hardness symbol" in a case where a Vickers hardness test is conducted with a test force set to 0.98 N (0.1 kgf) (see JIS Z 2244-1: 2020). HV MBγ ≥ 957.1 × C + 221.0 where, the meaning of each symbol in the above formula is as follows: HV MBγ : Average value of Vickers hardness measurement values with respect to indentations identified as containing only martensite, bainite, or retained austenite (HV0.1); [C]: Content of C in hot-stamping formed body (mass%).

[0037] Note that, in a case where a region of 300 µm in length by 2000 µm in width cannot be secured within the measurement region, for example, four regions of 300 µm in length by 500 µm in width may be secured, and the aforementioned observation using an FE-SEM and Vickers hardness measurement may be performed in each region.

[0038] Next, each of the aforementioned measurement regions is polished using silicon carbide paper with a grit size of 600 to 1500, and thereafter finished to a mirror finish using a liquid in which diamond powder with a particle size of 1 to 6 µm is dispersed in a diluent such as alcohol or in pure water. Next, polishing is performed for 8 minutes at room temperature using colloidal silica with a particle size of 0.25 µm that does not contain an alkaline solution, to thereby remove strain introduced into the outer layer of the sample.

[0039] Then, measurement by EBSD is performed at intervals of 0.1 µm with respect to a region of 300 µm in length by 500 µm in width that is centered at the 1 / 4 thickness position of the measurement region to obtain crystal orientation information. The measurement may be performed using an EBSD analysis system consisting of an FE-SEM and an EBSD detector, for example, an EBSD analysis system consisting of JSM-7001F manufactured by JEOL and Hikari detector manufactured by AMETEK may be used. At such time, for example, the degree of vacuum in the EBSD analysis system is set to 9.6 × 10 -5< Pa or less, the acceleration voltage is set to 20 kV, the working distance (WD) is set to 15 mm, the irradiation current level is set to 18, and the inclination angle of the test specimen is set to 70°.

[0040] When collecting EBSD patterns, the functions of the "AMETEK OIM Data Collection" software that comes with the EBSD analysis system are to be used. For the analysis, the crystal structure is to be selected as Iron (Alpha) and Iron (Gamma). The obtained crystal orientation information is used to calculate the crystal orientation of the prior-austenite grains from the crystal orientation relationship between typical prior-austenite grains and grains that have a bcc structure after transformation.

[0041] The crystal orientation of the prior-austenite grains may be calculated by the following method. First, identification of the prior-austenite grains is performed by the method described in "Development of a Reconstruction Method for Prior Austenite Microstructure Using EBSD Data of Ferrite Microstructure" (Kengo Hata, Masayuki Wakita, Kazuki Fujiwara, Kaori Kawano; Nippon Steel & Sumitomo Metal Corporation Technical Report No. 404 (2016), pp 24-30).

[0042] Note that, when adopting the method described in the above document, in the present invention the orientation relationship between ferrite and austenite is to be the K-S relationship. In the present invention, pearlite, fresh martensite, tempered martensite, and bainite may be mentioned as examples of micro-structures which, in addition to ferrite, are determined to have the bcc structure by EBSD, while retained austenite may be mentioned as an example of micro-structure which is determined to have the fcc structure. Therefore, micro-structure which is determined as having the bcc structure by EBSD is regarded as ferrite, and micro-structure which is determined as having the fcc structure is regarded as austenite. Further, the allowable angle described in the above document is to be set to 3 degrees, and the tolerance error is to be set to 5 degrees.

[0043] For austenite grains that could not be reconstructed by performing the aforementioned procedure, the data is saved as ferrite grains (bcc structure). In other words, they are not converted to austenite (fcc structure). For each identified prior-austenite grain excluding those prior-austenite grains which are not entirely included in the field of view, such as prior-austenite grains at the edge of the field of view, the grain diameter is determined using the Area Fraction of the "Grain Size (diameter)" chart in "OIM Analysis (registered trademark) manufactured by AMETEK", and the aspect ratio is determined by calculating the inverse of the "Grain Shape Aspect Ratio". Here, the reason the inverse of the "Grain Shape Aspect Ratio" is used is because the "Grain Shape Aspect Ratio" is the minor axis / major axis. Further, at such time, a boundary where the orientation difference is 15 degrees or more is regarded as a crystal grain boundary of austenite.

[0044] Note that, in the present invention the grain diameter means the equivalent circular diameter, and the aspect ratio means the ratio between the length in the longitudinal direction and the length in the direction perpendicular thereto.

[0045] Although the shape of the hot-stamping formed body according to the present embodiment is not particularly limited, the shape has a region where high strain has not been introduced that includes the first region, and a region where high strain has been introduced that includes the second region. The thickness of the region where high strain has not been introduced is preferably 1.0 to 4.0 mm, and the thickness of the region where high strain has been introduced is preferably 0.6 to 3.0 mm, and more preferably is 1.0 to 3.0 mm.

[0046] Suitable measurement regions will now be described taking typical hot-stamping formed bodies as examples. For example, a formed body having a circular burring portion that is illustrated in Figure 1, a formed body having a burring portion with an oval shape that is illustrated in Figure 3, a formed body having a flange portion with an arc shape that is illustrated in Figure 4, and a formed body having a flange deformation portion that is illustrated in Figure 6 may be mentioned as examples of the hot-stamping formed body according to the present embodiment. Suitable measurement regions in each of these formed bodies are described in detail below.

[0047] Figure 1 is a schematic diagram illustrating a formed body having a circular burring portion, in which (a) is a perspective view, (b) is a plan view, and (c) is a side view. As illustrated in Figure 1, a formed body 100 has a sheet-shaped portion 10, and a cylindrical burring portion 12 rising from the sheet-shaped portion 10. A thickness t 1 of the sheet-shaped portion 10 is, for example, 1.0 to 4.0 mm, and a thickness t 2 at the tip of the burring portion 12 is, for example, 0.6 to 3.0 mm. A height L (distance in the thickness direction of the sheet-shaped portion 10 between the sheet-shaped portion 10 and the tip of the burring portion 12) of the burring portion 12 is, for example, 3 to 30 mm. Further, when viewed from the thickness direction of the sheet-shaped portion 10, a diameter (inner diameter) ϕ of the tip of the burring portion 12 is, for example, 15 to 100 mm.

[0048] In the present invention, in some cases the formed body may be plated. In the specification of the present application, the aforementioned term "thickness" refers to the thickness of the base material portion obtained by subtracting the plating thickness from the total thickness. Further, the plating thickness is measured using a radio frequency glow discharge optical emission spectrometer (GDS). The specific measurement method is described hereunder.

[0049] In the hot-stamping formed body, three arbitrary measurement positions are determined from each of the first region and the second region which have a plating layer. At each measurement point, the respective concentrations of the elements Fe, Mn, Zn, Si, Al, O, Cr, Ni, Mg, Cu, and Sn are measured while sputtering from the surface of the plating layer.

[0050] The content of each element is analyzed in the depth direction, and the depth until the Fe concentration first reaches 90% or more is determined. The average value of the depths at the respective measurement points is then calculated, and for each of the first region and second region, the relevant average value that is calculated is taken as the plating thickness. Note that, if the Fe concentration does not become 90% or more before reaching the depth that can be analyzed in one GDS measurement, in other words, in a case where the plating layer is thicker than the depth that can be measured, a portion of the plating layer that is equivalent to 80 to 90% of the previously measured depth is removed by polishing at an arbitrary position other than the previously measured position within the same region, and after ascertaining the depth of the portion of the plating layer removed by polishing based on the change in the sheet thickness between before and after polishing, a new GDS analysis is performed from the surface after polishing, and the measurement results obtained in the first and second and subsequent measurements are then combined to thereby measure the plating thickness.

[0051] For example, a Marcus-type high-frequency glow discharge optical emission spectrometer with the model name "GD-Profiler 2" (manufactured by HORIBA) is used as the GDS measurement device. At such time, for example, measurement is performed under discharge conditions of 35 W, an Ar pressure during measurement of 600 Pa, a discharge range of 4 mm ϕ in diameter, a distance between electrodes of 0.15 to 0.25 mm, and a measurement pitch in the sheet thickness direction of 0.01 to 0.05 µm.

[0052] Note that, although in Figure 1 the burring portion 12 is formed so as to rise perpendicularly with respect to the sheet-shaped portion 10, the burring portion 12 may rise at an angle with respect to the sheet-shaped portion 10. For example, the burring portion 12 may be formed so that the diameter thereof becomes progressively smaller toward the tip side (as the distance from the sheet-shaped portion 10 increases in the thickness direction of the sheet-shaped portion 10).

[0053] In the formed body 100, high strain is introduced into the burring portion 12, and the strain becomes progressively higher toward the tip side and toward the outer edge side. Therefore, a cross section at a position that is separated from the tip of the burring portion 12 by a distance d toward the sheet-shaped portion 10 side can be cut out, and a second measurement region can be selected from the cross section. The distance d is, for example, 0.5 to 1.0 mm.

[0054] Figure 2 is a schematic diagram for describing an example of the second measurement region, in which (a) is a cross-sectional diagram of an a-a portion in Figure 1(c), and (b) is an enlarged view of a region indicated by a dotted line in (a). In Figure 2(b), to avoid complicating the drawing, hatching is not applied. As illustrated in Figure 2(b), a region of 300 µm in length by 2000 µm in width that is indicated by a broken line and is centered at a position that is a distance of t 2 / 4 from the outer edge of the burring portion 12 can be adopted as an observation region for an FE-SEM, and a region of 300 µm in length by 500 µm in width indicated by a dashed line can be adopted as an EBSD analysis region.

[0055] Figure 3 is a schematic diagram illustrating a formed body having a burring portion that is an oval shape, in which (a) is a plan view, and (b) is a cross-sectional diagram. Although in the formed body 100 illustrated in Figure 1, the burring portion 12 is a circular shape as viewed from the thickness direction of the sheet-shaped portion 10, as shown in a formed body 200 illustrated in Figure 3, a burring portion 22 may be an oval shape as viewed from the thickness direction of a sheet-shaped portion 20. When viewed from the thickness direction of the sheet-shaped portion 20, a minor axis (inner diameter) ϕ 1 of the tip of the burring portion 22 is, for example, 10 to 80 mm, and a major axis (inner diameter) ϕ 2 is, for example, 15 to 100 mm.

[0056] In a case where the burring portion 22 is an oval shape, since higher strain is imparted to the region with a larger degree of curvature on the major axis side, the second measurement region can be selected from the region indicated by the dotted line in Figure 3(b). The positions of the FE-SEM observation region and the EBSD analysis region are the same as those described above, and hence a description thereof is omitted here. Note that, Figure 3(b) is a cross-sectional diagram at a position separated from the tip of the burring portion 22 by a distance d toward the sheet-shaped portion 20 side, with the distance d being, for example, 0.5 to 1.0 mm.

[0057] Figure 4 is a schematic diagram illustrating a formed body having an arc-shaped flange portion, in which (a) is a perspective view, (b) is a plan view, and (c) is a side view. As illustrated in Figure 4, a formed body 300 has a sheet-shaped portion 30, and an arc-shaped flange portion 32 rising from the sheet-shaped portion 30. A thickness t 1 of the sheet-shaped portion 30 is, for example, 1.0 to 4.0 mm, and a thickness t 2 at the tip of the flange portion 32 is, for example, 0.6 to 3.0 mm. A height L (distance in the thickness direction of the sheet-shaped portion 30 between the sheet-shaped portion 30 and the tip of the flange portion 32) of the flange portion 32 is, for example, 3 to 30 mm. Further, when viewed from the thickness direction of the sheet-shaped portion 30, the flange portion 32 has an arc shape, and a radius of curvature R thereof is, for example, 50 to 500 mm.

[0058] In the formed body 300, high strain is introduced into the flange portion 32, and the strain becomes progressively higher toward the outer side of the arc (the side farther from the center of curvature). Therefore, a cross section at a position separated from the tip of the flange portion 32 by a distance d toward the sheet-shaped portion 30 side can be cut out, and a second measurement region can be selected from the cross section. The distance d is, for example, 0.5 to 1.0 mm.

[0059] Figure 5 is a schematic diagram for describing an example of the second measurement region, in which (a) is a cross-sectional diagram of a b-b portion in Figure 4(c), and (b) is an enlarged view of a region indicated by a dotted line in (a). In Figure 5(b), to avoid complicating the drawing, hatching is not applied. As illustrated in Figure 5(b), a region of 2000 µm in length by 300 µm in width that is indicated by a broken line and is centered at a position that is a distance of t 2 / 4 away from the outer side of the arc of the flange portion 32 can be adopted as an observation region for an FE-SEM, and a region of 500 µm in length by 300 µm in width indicated by a dashed line can be adopted as an EBSD analysis region.

[0060] Figure 6 is a schematic diagram illustrating a formed body having a flange deformation portion, in which (a) is a perspective view, (b) is a plan view, and (c) is a cross-sectional diagram along a c-c portion in (b). As illustrated in Figure 6, a formed body 400 has a sheet-shaped portion 40, a cylindrical part 42 that rises from the sheet-shaped portion 40, and an annular inner flange portion 44 protruding inward from the cylindrical part 42 and having a hole in the center thereof. A thickness t 1 of the sheet-shaped portion 40 is, for example, 1.0 to 4.0 mm, and a thickness t 2 of the cylindrical part 42 and a thickness t 3 of the inner flange portion 44 are, for example, 0.6 to 3.0 mm. A height L (distance in the thickness direction of the sheet-shaped portion 40 between the sheet-shaped portion 40 and the inner flange portion 44) of the cylindrical part 42 is, for example, 3 to 30 mm. Further, when viewed from the thickness direction of the sheet-shaped portion 40, a diameter (inner diameter) ϕ of the hole of the inner flange portion 44 is, for example, 10 to 100 mm.

[0061] In the formed body 400, high strain is introduced into the inner flange portion 44, and the strain becomes progressively higher toward the inner side (the side closer to the hole). Therefore, a cross section at a position separated from the outer surface of the inner flange portion 44 by a distance of t 3 / 4 toward the sheet-shaped portion 40 side can be cut out, and a second measurement region can be selected from the cross section.

[0062] Figure 7 is a schematic diagram for describing an example of the second measurement region, in which (a) is a cross-sectional diagram of a d-d portion in Figure 6(c), and (b) is an enlarged view of a region indicated by a dotted line in (a). In Figure 6(b), to avoid complicating the drawing, hatching is not applied. As illustrated in Figure 6(b), a region of 2000 µm in length by 300 µm in width that is indicated by a broken line and is centered at a position separated by a distance d from the hole of the inner flange portion 44 can be adopted as an observation region for an FE-SEM, and a region of 500 µm in length by 300 µm in width indicated by a dashed line can be adopted as an EBSD analysis region. The distance d is, for example, 0.5 to 1.0 mm.

[0063] As mentioned above, although each of the formed bodies has a sheet-shaped portion, the sheet-shaped portion is a region into which high strain is not introduced. Therefore, as the first measurement region, it is preferable to select a region from the sheet-shaped portion. The first measurement region will now be described more specifically using Figures 1 and 8. Figure 8 is a schematic diagram for describing an example of the first measurement region, in which (a) is a cross-sectional diagram of an e-e portion in Figure 1(b), and (b) is an enlarged view of a region indicated by a dotted line in (a). In Figure 8(b), to avoid complicating the drawing, hatching is not applied. As illustrated in Figure 1, a cross section which is parallel to the thickness direction of the sheet-shaped portion 10 and which is separated by a distance D from the boundary where the sheet-shaped portion 10 and the burring portion 12 are connected can be cut out, and the first measurement region can be selected from the cross section. The distance D is, for example, 7 to 8 mm. Further, as illustrated in Figure 8(b), a region of 300 µm in length by 2000 µm in width that is indicated by a broken line and is centered at a position that is a distance of t 1 / 4 from the surface of the sheet-shaped portion 10 can be adopted as an observation region for an FE-SEM, and a region of 300 µm in length by 500 µm in width indicated by a dashed line can be adopted as an EBSD analysis region.2. Mechanical Properties

[0064] In the hot-stamping formed body according to the present embodiment, a tensile strength measured by a tensile test using a test specimen taken from the first region is 1250 to 2540 MPa. By having the aforementioned strength, in a case where the hot-stamping formed body is used as a part for an automobile, it is possible to contribute to reducing the weight of the automobile and ensuring collision safety. The aforementioned tensile strength is preferably 1250 to 2040 MPa. Further, as described above, when placing importance on the balance between the resistance to hydrogen embrittlement and the notched tensile strength of the hot-stamping formed body, preferably the hot-stamping formed body has a strength of 1.5 GPa grade, and specifically, preferably the aforementioned tensile strength is 1450 to 1540 MPa.

[0065] In the present invention, measurement of the tensile strength is carried out using a sub-sized sheet-shaped test specimen defined in ASTM A370-22 (a test specimen having a width of 10 mm, an overall length of 100 mm, a parallel portion width of 6.25 mm, a parallel portion length of 32 mm, a gage length of 25 mm, and a thickness that is the original thickness of the first region) that has a shape illustrated in Figure 9, with the speed of tensile test used in the measurement test being set to a crosshead separation rate of 3 mm / min. With regard to the other conditions, the measurement test is to be performed in accordance with JIS Z 2241: 2022. Note that, as described above, in a case where the formed body is plated, although measurement of the tensile strength is to be performed using a test specimen in a state in which the test specimen has been plated, the phrase "thickness that is the original thickness of the first region" refers to the thickness of the base material portion that is obtained by subtracting the plating thickness from the total thickness. In other words, the original cross-sectional area of the test specimen used to calculate the tensile strength is the original cross-sectional area of the base material portion that excludes the plating layer. Note that, the plating thickness is to be measured by the same method as described above.

[0066] Further, as described above, by also making the microstructure in the second region a micro-structure that is mainly composed of martensite, in addition to the first region, local softening is suppressed. Therefore, it is possible to reduce the difference between the hardness in the first region and the hardness in the second region. From this viewpoint, in the hot-stamping formed body according to the present embodiment, when the average hardness in the first region is defined as HV 1 (HV0.1), and the average hardness in the second region is defined as HV 2 (HV0.1), preferably HV 1 and HV 2 satisfy the following Formula (ii). The right-hand side value of the following Formula (ii) is more preferably 30.0, and further preferably is 20.0. HV 1 − HV 2 ≤ 40.0

[0067] Note that, the average values of the Vickers hardness measurement values at all of the intersection points of the mesh with 70 µm intervals described above in the first region and the second region, respectively, are to be used as the average hardness HV 1 in the first region and the average hardness HV 2 in the second region, respectively.

[0068] In addition, preferably HV 1 and HV 2 satisfy the following Formula (iii) and Formula (iv), respectively: 997.6 × C + 230.4 ≤ HV 1 ≤ 1133.9 × C + 281.8 997.6 × C + 230.4 ≤ HV 2 ≤ 1133.9 × C + 281.8 where, [C] in the above formulas means the content of C (mass%) in the hot-stamping formed body.

[0069] The hot-stamping formed body according to the present embodiment has excellent resistance to hydrogen embrittlement and high notched tensile strength. However, as described above, a so-called "trade-off relationship" exists between the resistance to hydrogen embrittlement and the notched tensile strength. Therefore, in the present embodiment, for example, in a case where the sum of fracture stress under a hydrogen environment that serves as an index of resistance to hydrogen embrittlement and the notched tensile strength is 2400 MPa or more, it can be determined that the hot-stamping formed body has excellent resistance to hydrogen embrittlement and high notched tensile strength.

[0070] The fracture stress under a hydrogen environment and the notched tensile strength can be evaluated by the following methods, respectively. First, two tensile test specimens, each of which has the shape illustrated in Figure 10 are taken from the hot-stamping formed body. It is preferable to take the aforementioned tensile test specimens from the first region and the second region, respectively, and to perform evaluation for each region. At such time, each tensile test specimen is to be taken so that at least one part of the bottom of a two-sided U-notch formed in the tensile test specimen overlaps with a position at a depth of 1 / 2 of the thickness from the surface of the base material.

[0071] Note that, in some cases it may not be possible to take the aforementioned tensile test specimens from the burring portion or the like due to dimensional constraints. In such a case, as illustrated in one example in Figure 11, a test specimen having a size of the possible range is taken from the burring portion or the like, and then a two-sided U-notch is formed in the center of the taken test specimen. However, the length of the taken test specimen is to be 6.0 mm or more. Next, another dummy material is joined by welding to both sides of the test specimen so that the test specimen is arranged at the center of the tensile test specimen illustrated in Figure 10. At such time, laser welding is performed while clamping the test specimen with a copper jig so as to prevent heat generated by the welding from affecting the portion where the two-sided U-notch is formed.

[0072] Furthermore, in some cases it may not be possible to take the tensile test specimen having a thickness of 1.0 mm illustrated in Figure 10 or the test specimen having a thickness of 1.0 mm illustrated in Figure 11 due to dimensional constraints. In such a case, two tensile test specimens, each of which has a thickness of 0.5 mm are prepared by the procedure described above, and these two tensile test specimens are then joined together by fastening with screws or by welding to obtain a tensile test specimen having a thickness of 1.0 mm. Note that, the joining by fastening with screws or by welding is to be performed at the center position in the height direction (on the dashed line in the drawing) of grip sections formed at both ends of the tensile test specimen.

[0073] Next, one of the tensile test specimens is used as a cathode in a 3% NaCl aqueous solution, and a tensile test is performed at a speed of tensile test of 0.006 mm / min while generating hydrogen on the surface of the tensile test specimen under a condition of a current density of 1.0 mA / cm 2< , and the fracture stress (maximum stress) is measured. The measured fracture stress is a value that serves as an index of the resistance to hydrogen embrittlement, and is taken as the tensile strength in a hydrogen environment.

[0074] Further, the other tensile test specimen is used to perform a tensile test in accordance with JIS Z 2241: 2022 in air at a speed of tensile test of 0.006 mm / min, the fracture stress (maximum stress) is measured, and the obtained measurement value is taken as the notched tensile strength.3. Chemical Composition

[0075] The chemical composition of the hot-stamping formed body according to the present embodiment is not particularly limited as long as the hot-stamping formed body has the strength described above. Preferably the hot-stamping formed body according to the present embodiment has, for example, the chemical composition described below. The reasons for limiting each element are as follows. Note that, the symbol "%" in relation to content in the following description means "mass percent".C: 0.10 to 0.60%

[0076] C is an element that increases the strength of the hot-stamping formed body. Therefore, the content of C is preferably 0.10% or more. The content of C is more preferably more than 0.10%, 0.15% or more, 0.20% or more, or 0.25% or more. From the viewpoint of weldability, preferably the content of C is 0.60% or less. More preferably, the content of C 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. Preferably, the content of C is 0.10 to 0.50%, 0.10 to 0.40%, more than 0.10% and 0.38% or less, 0.15 to 0.37%, 0.20 to 0.35%, or 0.25 to 0.30%.Si: 0.01 to 2.00%

[0077] Si is an element that increases the strength of the hot-stamping formed body by solid-solution strengthening. Therefore, the content of Si is preferably 0.01% or more. The content of Si is more preferably 0.05% or more, 0.10% or more, 0.20% or more, 0.30% or more, or 0.40% or more. On the other hand, if the content of Si is more than 2.00%, in some cases the amount of ferrite will increase and it may not be possible to obtain the desired microstructure. Therefore, the content of Si is preferably 2.00% or less. The content of Si is more preferably 1.80% or less, 1.50% or less, 1.20% or less, 1.00% or less, or 0.80% or less. The content of Si is preferably 0.05 to 1.80%, 0.10 to 1.50%, 0.20 to 1.20%, 0.30 to 1.00%, or 0.40 to 0.80%.Mn: 0.10 to 3.00%

[0078] Mn is an element that increases hardenability of the steel and contributes to improving strength. Therefore, the content of Mn is preferably 0.10% or more. The content of Mn is more preferably 0.20% or more, 0.50% or more, 1.00% or more, 1.30% or more, or 1.50% or more. On the other hand, if the content of Mn is more than 3.00%, in some cases Mn segregation will become noticeable. Therefore, preferably the content of Mn is 3.00% or less. The content of Mn is more preferably 2.80% or less, 2.50% or less, 2.30% or less, or 2.00% or less. Preferably, the content of Mn is 0.20 to 2.80%, 0.50 to 2.50%, 1.00 to 2.30%, 1.30 to 2.00%, or 1.50 to 2.00%.P: 0.050% or less

[0079] P is an impurity element, and may cause a decrease in weldability. Therefore, the content of P is preferably 0.050% or less. The content of P is more preferably 0.030% or less, 0.020% or less, or 0.010% or less. Although the lower limit of the content of P is not particularly limited, reducing the content of P to less than 0.0001% will significantly increase the dephosphorization cost, and therefore is not economically preferable. Therefore, the content of P may be made 0.0001% or more.S: 0.0200% or less

[0080] S is an impurity element, and may cause a decrease in weldability. Therefore, the content of S is preferably 0.0200% or less. The content of S is more preferably 0.0180% or less, 0.0150% or less, 0.0100% or less, 0.0060% or less, or 0.0040% or less. Although the lower limit of the content of S is not particularly limited, reducing the content of S to less than 0.0001% will significantly increase the desulfurization cost, and therefore is not economically preferable. Therefore, the content of S may be made 0.0001% or more.N: 0.0200% or less

[0081] N is an impurity element, and may cause a decrease in weldability. Therefore, the content of N is preferably 0.0200% or less. The content of N is more preferably 0.0180% or less, 0.0150% or less, 0.0100% or less, 0.0060% or less, or 0.0040% or less. Although the lower limit of the content of N is not particularly limited, reducing the content of N to less than 0.0001% will significantly increase the denitrification cost, and therefore is not economically preferable. Therefore, the content of N may be made 0.0001% or more.O: 0.100% or less

[0082] If a large amount of O is contained in the steel, O may cause a decrease in weldability. Therefore, the content of O is preferably 0.100% or less. The content of O is more preferably 0.0700% or less, 0.0500% or less, 0.0300% or less, 0.0100% or less, or 0.0050% or less. From the viewpoint of reducing the refining cost, the content of O may be 0.0001% or more, or 0.0005% or more.Al: 0.001 to 0.100%

[0083] Al is an element that produces an effect of deoxidizing the molten steel and making the steel sounder. Therefore, the content of Al is preferably 0.001% or more. The content of Al is more preferably 0.005% or more, 0.010% or more, 0.015% or more, 0.020% or more, or 0.025% or more. On the other hand, since the effect will be saturated if the content of Al is more than 0.100%, the content of Al is preferably 0.100% or less. The content of Al is more preferably 0.080% or less, 0.060% or less, 0.040% or less, 0.020% or less, or 0.010% or less. The content of Al is preferably 0.010 to 0.100%. Further, the content of Al 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%.Cr: 0.01 to 1.00%

[0084] Cr is an element that enhances hardenability of the steel. Therefore, the content of Cr is preferably 0.01% or more. The content of Cr is more preferably 0.03% or more, 0.05% or more, 0.10% or more, or 0.15% or more. On the other hand, since the aforementioned effect will be saturated if the content of Cr is more than 1.00%, the content of Cr is preferably 1.00% or less. The content of Cr is more preferably 0.80% or less, 0.60% or less, 0.50% or less, or 0.40% or less. The content of Cr is preferably 0.03 to 0.80%, 0.05 to 0.60%, 0.10 to 0.50%, or 0.15 to 0.40%.

[0085] The basic chemical composition of the hot-stamping formed body according to an embodiment of the present invention is as described above. In addition, as necessary, the hot-stamping formed body may contain at least one type of element among the following optional selective elements in lieu of a part of the Fe of the balance. Hereunder, the optional selective elements are described in detail.Nb: 0 to 0.200%

[0086] Nb is an element that forms carbo-nitrides in the steel and increases the strength of the hot-stamping formed body by precipitation strengthening. Although the content of Nb may be 0.0005% or more, in order to reliably obtain the aforementioned effect, preferably the content of Nb is set to 0.001% or more, or 0.002% or more. On the other hand, since the aforementioned effect will be saturated even if a large amount of Nb is contained, preferably the content of Nb is set to 0.200% or less. The content of Nb may be 0.180% or less, 0.150% or less, 0.100% or less, 0.050% or less, or 0.010% or less. Preferably, the content of Nb is 0.0005 to 0.180%, 0.001 to 0.150%, 0.002 to 0.100%, 0.003 to 0.050%, or 0.004 to 0.010%.Ti: 0 to 0.200%

[0087] Ti is an element that forms carbo-nitrides in the steel and increases the strength of the hot-stamping formed body by precipitation strengthening. Although the content of Ti may be 0.001% or more, in order to reliably obtain the aforementioned effect, preferably the content of Ti is set to 0.010% or more, or 0.020% or more. On the other hand, since the aforementioned effect will be saturated even if a large amount of Ti is contained, preferably the content of Ti is set to 0.200% or less. The content of Ti may be 0.180% or less, 0.150% or less, 0.100% or less, 0.070% or less, or 0.040% or less. Preferably, the content of Ti is 0.001 to 0.180%, 0.005 to 0.150%, 0.010 to 0.100%, 0.015 to 0.070%, or 0.020 to 0.040%.Mo: 0 to 1.00%

[0088] Mo is an element that enhances hardenability of the steel. Although the content of Mo may be 0.001% or more, in order to reliably obtain the aforementioned effect, preferably the content of Mo is set to 0.003% or more, or 0.005% or more. On the other hand, since the aforementioned effect will be saturated even if a large amount of Mo is contained, preferably the content of Mo is set to 1.00% or less. The content of Mo may be 0.80% or less, 0.60% or less, 0.50% or less, 0.30% or less, or 0.10% or less. Preferably, the content of Mo is 0.001 to 0.80%, 0.002 to 0.60%, 0.003 to 0.50%, 0.004 to 0.30%, or 0.005 to 0.10%.B: 0 to 0.0100%

[0089] B is an element that enhances hardenability of the steel. Although the content of B may be 0.0001% or more, in order to reliably obtain the aforementioned effect, preferably the content of B is set to 0.0005% or more, or 0.0010% or more. On the other hand, since the aforementioned effect will be saturated even if a large amount of B is contained, preferably the content of B is set to 0.0100% or less. The content of B may be 0.0080% or less, 0.0060% or less, 0.0050% or less, 0.0030% or less, or 0.0020% or less. Preferably, the content of B is 0.0001 to 0.0080%, 0.0003 to 0.0060%, 0.0005 to 0.0050%, 0.0007 to 0.0030%, or 0.0010 to 0.0020%.Co: 0 to 4.00%

[0090] Co is an element that increases the strength of the hot-stamping formed body by solid-solution strengthening. Although the content of Co may be 0.001% or more, in order to reliably obtain the aforementioned effect, preferably the content of Co is set to 0.01% or more, or 0.05% or more. On the other hand, since the aforementioned effect will be saturated even if a large amount of Co is contained, preferably the content of Co is set to 4.00% or less. The content of Co may be 3.00% or less, 2.00% or less, 1.00% or less, 0.50% or less, or 0.10% or less. Preferably, the content of Co is 0.001 to 3.00%, 0.005 to 2.00%, 0.01 to 1.00%, 0.03 to 0.50%, or 0.05 to 0.10%.Ni: 0 to 2.00%

[0091] Ni produces an effect of increasing the strength of the hot-stamping formed body by dissolving in austenite grains during heating in the hot-stamping forming process. Although the content of Ni may be 0.001% or more, in order to reliably obtain the aforementioned effect, preferably the content of Ni is set to 0.01% or more. On the other hand, since the aforementioned effect will be saturated even if a large amount of Ni is contained, preferably the content of Ni is set to 2.00% or less. The content of Ni may be 1.80% or less, 1.60% or less, 1.40% or less, 1.20% or less, 1.00% or less, 0.50% or less, or 0.10% or less. Preferably, the content of Ni is 0.001 to 1.80%, 0.005 to 1.60%, 0.01 to 1.40%, 0.02 to 1.20%, 0.03 to 1.00%, 0.04 to 0.50%, or 0.05 to 0.10%.Cu: 0 to 1.00%

[0092] Cu produces an effect of increasing the strength of the hot-stamping formed body by dissolving in austenite grains during heating in the hot-stamping forming process. Although the content of Cu may be 0.001% or more, in order to reliably obtain the aforementioned effect, preferably the content of Cu is set to 0.01% or more, or 0.05% or more. On the other hand, since the aforementioned effect will be saturated even if a large amount of Cu is contained, preferably the content of Cu is set to 1.00% or less. The content of Cu may be 0.80% or less, 0.60% or less, 0.50% or less, 0.30% or less, or 0.10% or less. Preferably, the content of Cu is 0.001 to 0.80%, 0.005 to 0.60%, 0.01 to 0.50%, 0.03 to 0.30%, or 0.05 to 0.10%.V: 0 to 1.00%

[0093] V forms carbo-nitrides in the steel and produces an effect that increases the strength of the hot-stamping formed body by precipitation strengthening. Although the content of V may be 0.001% or more, in order to reliably obtain the aforementioned effect, preferably the content of V is set to 0.01% or more, or 0.05% or more. On the other hand, since the aforementioned effect will be saturated even if a large amount of V is contained, preferably the content of V is set to 1.00% or less. The content of V may be 0.80% or less, 0.60% or less, 0.50% or less, 0.30% or less, or 0.10% or less. Preferably, the content of V is 0.001 to 0.80%, 0.005 to 0.60%, 0.01 to 0.50%, 0.03 to 0.30%, or 0.05 to 0.10%.W: 0 to 1.00%

[0094] W is an element that enhances hardenability of the steel. Although the content of W may be 0.001% or more, in order to reliably obtain the aforementioned effect, preferably the content of W is set to 0.005% or more, or 0.01% or more. On the other hand, since the aforementioned effect will be saturated even if a large amount of W is contained, preferably the content of W is set to 1.00% or less. The content of W may be 0.80% or less, 0.60% or less, 0.50% or less, 0.30% or less, or 0.10% or less. Preferably, the content of W is 0.001 to 0.80%, 0.005 to 0.60%, 0.01 to 0.50%, 0.03 to 0.30%, or 0.05 to 0.10%.Ca: 0 to 0.0100%

[0095] Ca is an element that makes it possible to control the morphology of sulfides. Although the content of Ca may be 0.0001% or more, in order to reliably obtain the aforementioned effect, preferably the content of Ca is set to 0.0005% or more, or 0.0010% or more. On the other hand, since the aforementioned effect will be saturated even if a large amount of Ca is contained, preferably the content of Ca is set to 0.0100% or less. The content of Ca may be 0.0080% or less, 0.0060% or less, 0.0040% or less, 0.0030% or less, or 0.0020% or less. Preferably, the content of Ca is 0.0001 to 0.0080%, 0.0003 to 0.0060%, 0.0005 to 0.0040%, 0.0007 to 0.0030%, or 0.0010 to 0.0020%.Mg: 0 to 0.0100%

[0096] Mg is an element that makes it possible to control the morphology of sulfides. Although the content of Mg may be 0.0001% or more, in order to reliably obtain the aforementioned effect, preferably the content of Mg is set to 0.0005% or more, or 0.0010% or more. On the other hand, since the aforementioned effect will be saturated even if a large amount of Mg is contained, preferably the content of Mg is set to 0.0100% or less. The content of Mg may be 0.0080% or less, 0.0060% or less, 0.0040% or less, 0.0030% or less, or 0.0020% or less. Preferably, the content of Mg is 0.0001 to 0.0080%, 0.0003 to 0.0060%, 0.0005 to 0.0040%, 0.0007 to 0.0030%, or 0.0010 to 0.0020%.REM: 0 to 0.0100%

[0097] REM is an element that makes it possible to control the morphology of sulfides. Although the content of REM may be 0.0001% or more, in order to reliably obtain the aforementioned effect, preferably the content of REM is set to 0.0005% or more, or 0.0010% or more. On the other hand, since the aforementioned effect will be saturated even if a large amount of REM is contained, preferably the content of REM is set to 0.0100% or less. The content of REM may be 0.0080% or less, 0.0060% or less, 0.0040% or less, 0.0030% or less, or 0.0020% or less. Preferably, the content of REM is 0.0001 to 0.0080%, 0.0003 to 0.0060%, 0.0005 to 0.0040%, 0.0007 to 0.0030%, or 0.0010 to 0.0020%.

[0098] In the present embodiment, the term "REM" refers to a total of 17 elements which are Sc, Y, and the lanthanoids, and in a case where one type of REM is contained, the term "content of REM" refers to the content of the relevant one type of REM, and in a case where two or more types of REM elements are contained, the term "content of REM" refers to the total content of the two or more types of REM elements. Further, REM is generally supplied as a misch metal that is an alloy of a plurality of types of REM element. Therefore, REM may be contained by adding one or more types of individual elements, or for example, may be added in the form of a misch metal.Sb: 0 to 0.100%

[0099] Sb is an element that suppresses oxidation of the surface of the base material. In order to reliably obtain the aforementioned effect, preferably the content of Sb is set to 0.001% or more. On the other hand, since the aforementioned effect will be saturated even if a large amount of Sb is contained, preferably the content of Sb is set to 0.100% or less. The content of Sb may be 0.080% or less, 0.060% or less, 0.050% or less, 0.030% or less, or 0.010% or less. Preferably, the content of Sb is 0.0001 to 0.080%, 0.0005 to 0.060%, 0.001 to 0.050%, 0.003 to 0.030%, or 0.005 to 0.010%.Zr: 0 to 0.100%

[0100] Zr is an element that suppresses oxidation of the surface of the base material. In order to reliably obtain the aforementioned effect, preferably the content of Zr is set to 0.001% or more. On the other hand, since the aforementioned effect will be saturated even if a large amount of Zr is contained, preferably the content of Zr is set to 0.100% or less. The content of Zr may be 0.080% or less, 0.060% or less, 0.050% or less, 0.030% or less, or 0.010% or less. Preferably, the content of Zr is 0.0001 to 0.080%, 0.0005 to 0.060%, 0.001 to 0.050%, 0.003 to 0.030%, or 0.005 to 0.010%.Sn: 0 to 1.00%

[0101] Sn is an element that suppresses oxidation of the surface of the base material. In order to reliably obtain the aforementioned effect, preferably the content of Sn is set to 0.001% or more. On the other hand, since the aforementioned effect will be saturated even if a large amount of Sn is contained, preferably the content of Sn is set to 1.00% or less. The content of Sn may be 0.800% or less, 0.500% or less, 0.200% or less, 0.100% or less, 0.050% or less, or 0.010% or less. Preferably, the content of Sn is 0.0001 to 0.800%, 0.0005 to 0.500%, 0.001 to 0.200%, 0.002 to 0.100%, 0.003 to 0.050%, or 0.005 to 0.010%.As: 0 to 0.100%

[0102] As produces an effect of increasing the strength of the hot-stamping formed body. In order to reliably obtain the aforementioned effect, preferably the content of As is set to 0.001% or more. On the other hand, since the aforementioned effect will be saturated even if a large amount of As is contained, preferably the content of As is set to 0.100% or less. The content of As may be 0.080% or less, 0.050% or less, 0.020% or less, 0.010% or less, or 0.005% or less. Preferably, the content of As is 0.001 to 0.080%, 0.002 to 0.050%, 0.003 to 0.020%, 0.004 to 0.010%, or 0.005 to 0.005%.

[0103] In the chemical composition of the hot-stamping formed body according to the present embodiment, the balance other than the elements described above is Fe and impurities. The term "impurities" refers to components which, when industrially producing the hot-stamping formed body, are mixed in due to various factors during the production processes, such as from raw material such as ore or scrap.

[0104] As described above, when placing importance on the balance between the resistance to hydrogen embrittlement and the notched tensile strength of the hot-stamping formed body, it is preferable for the hot-stamping formed body to have a strength of 1.5 GPa grade. When it is desired to obtain the aforementioned strength of 1.5 GPa grade, preferably the chemical composition of the hot-stamping formed body contains, by mass%, C: 0.18 to 0.26%, Si: 0.05 to 0.30%, Mn: 0.80 to 1.60%, P: 0.050% or less, S: 0.0200% or less, N: 0.0200% or less, O: 0.100% or less, Al: 0.005 to 0.080%, Cr: 0.10 to 0.50%, Nb: 0 to 0.020%, Ti: 0.010 to 0.050%, Mo: 0 to 0.050%, B: 0.0010 to 0.0050%, Co: 0 to 4.00%, Ni: 0 to 2.00%, Cu: 0 to 1.00%, V: 0 to 1.00%, W: 0 to 1.00%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, REM: 0 to 0.0100%, Sb: 0 to 0.100%, Zr: 0 to 0.100%, Sn: 0 to 1.00%, As: 0 to 0.100%, and the balance: Fe and impurities.

[0105] The chemical composition of the hot-stamping formed body described above may be measured by a general analytical method. For example, the chemical composition may be measured using inductively coupled plasma-atomic emission spectrometry (ICP-AES). Note that, C and S may be measured using a infrared absorption method after combustion, N may be measured using an thermal conductimetric method after fusion in a current of inert gas, and O may be measured using an inert gas fusion-non-dispersive infrared absorption method. In a case where a plating layer is provided on the surface of the hot-stamping formed body, it suffices to perform analysis of the chemical composition after removing the plating layer by mechanical grinding.4. Production Method

[0106] One example of a method for producing the hot-stamping formed body according to the present embodiment will now be described.

[0107] First, a steel sheet (blank) that will serve as a starting material of the hot-stamping formed body is produced. Molten steel having the chemical composition described above is produced, and the molten steel is used to produce a slab. As a slab to be provided for hot rolling, a continuously cast slab, or a slab produced by a thin slab caster or the like can be used. The above method for producing a steel sheet is compatible with a process such as continuous casting-direct rolling (CC-DR) in which hot rolling is performed immediately after casting.

[0108] Preferably, the slab heating temperature is set to 1100°C or more. A slab heating temperature in a temperature region of less than 1100°C will result in a decrease in the rolling finishing temperature, and therefore the strength during finish rolling will also tend to be high. Since there is a possibility that, as a result, rolling will become difficult or a defective shape of the steel sheet after rolling will be caused or the like, it is preferable to set the slab heating temperature to 1100°C or more.

[0109] The finish rolling completion temperature is preferably 800°C or more. If the finish rolling completion temperature is lower than 800°C, the rolling load will be high and there is a possibility that rolling will become difficult or a defective shape of the steel sheet after rolling will be caused or the like, and for this reason the lower limit of the finish rolling completion temperature is preferably set to 800°C. Although it is not particularly necessary to set an upper limit of the finish rolling completion temperature, if the finish rolling completion temperature is set to an excessively high temperature, it will be necessary to make the slab heating temperature excessively high in order to ensure the finish rolling completion temperature, and for this reason the upper limit of the finish rolling completion temperature is preferably 1100°C. After finish rolling is completed, an average cooling rate until a coiling temperature described later is preferably 10 to 100°C / s.

[0110] It is preferable to make the coiling temperature 700°C or less. If the coiling temperature is more than 700°C, the thickness of oxides formed on the steel sheet surface may be excessively increased, which will cause the pickling property to decrease. In the case of performing cold rolling thereafter, it is preferable to set the lower limit of the coiling temperature at 400°C. If the coiling temperature is less than 400°C, the strength of the hot-rolled steel sheet will increase extremely, which will cause the steel sheet to easily fracture or become a defective shape during cold rolling, and for this reason the lower limit of the coiling temperature is preferably set at 400°C. However, if it is intended to subject the coiled hot-rolled steel sheet to softening by heating the coiled hot-rolled steel sheet in a box type annealing furnace or a continuous annealing line, the steel sheet may be coiled at a low temperature of less than 400°C. Note that, when performing hot rolling, rough-rolled sheets may be joined to one another and finishing rolling may be continuously performed. Further, the rough-rolled sheet may be coiled temporarily.

[0111] A pickling treatment may be performed on the aforementioned hot-rolled steel sheet, and the hot-rolled steel sheet after the pickling treatment may be subjected to cold rolling to obtain a cold-rolled steel sheet. In the case of performing cold rolling, the rolling reduction can be set to, for example, 30 to 80%. Further, a pickling treatment can be performed by immersing the steel sheet for 30 seconds or more in an aqueous solution at a temperature of 80°C or more to less than 100°C in which a concentration of acid is 3 to 20% by mass and an inhibitor is included. In addition, the aforementioned hot-rolled steel sheet or cold-rolled steel sheet may be subjected to annealing to obtain a hot-rolled annealed steel sheet or a cold-rolled annealed steel sheet. The annealing is performed by passing the hot-rolled steel sheet or cold-rolled steel sheet through a continuous annealing line, and the annealing temperature can be set, for example, within the range of 550 to 750°C.

[0112] Each kind of obtained steel sheet may be subjected to a plating treatment before performing hot-stamping forming. The plating to be applied is not particularly limited, and examples thereof include hot-dip galvanizing, hot-dip galvannealing, electro-galvanizing, Zn-Ni plating (zinc alloy electroplating), Sn plating, Al-Si plating, electro-galvannealing, hot-dip zinc-aluminum alloy plating, hot-dip zinc-aluminum-magnesium alloy plating, hot-dip zinc-aluminum-magnesium-Si alloy plating, and zinc-Al alloy deposition. Further, the plating treatment can be performed by passing the steel sheet through a continuous line.

[0113] Next, the blank is subjected to hot-stamping forming. In the present process, the blank is heated, and thereafter press forming and quenching are performed. In the quenching, the blank is held in a die set and cooled to a temperature equal to or lower than the Mf point by heat dissipation to the die set. When producing a hot-stamping formed body having a burring portion, the burring processing may be performed simultaneously with or successively to press forming, and thereafter quenching may be immediately performed.

[0114] In order to form the aforementioned first region and second region in the hot-stamping formed body, it is important to control the heating conditions before hot-stamping forming.

[0115] In the case of a region where it is not planned to impart high strain (hereinafter, also referred to as "low strain region"), the region is heated at an average heating rate of 2 to 200°C / s until reaching a temperature in the range of the Ac 3 point or more to the Ac 3 point + 65°C or less, and then held in that temperature region for 10 to 120 seconds. On the other hand, in the case of a region where it is planned to impart high strain (hereinafter, also referred to as "high strain region"), the region is heated at an average heating rate which is within the range of 2 to 200°C / s and is also equal to or higher than the average heating rate in the low strain region until reaching a temperature which is within the range of the Ac 3 point + 40°C or more to the Ac 3 point + 110°C or less and is also 40°C or more higher than the heating temperature in the low strain region, and is then held in that temperature range for 10 to 120 seconds.

[0116] Methods for creating a difference in the heating rate and in the heating temperature between the two regions that may be mentioned include: (1) a method in which a surface treatment such as a black coating is applied only to the high strain region to change the emissivity of the surface; (2) a method in which, during heating in a furnace, a temperature distribution is created in the furnace; (3) a method in which, during heating in a furnace, a shield is inserted between the low strain region and the heat source in the furnace; and (4) a method in which, during conduction heating, the sheet width of the high strain region is made less than the sheet width of the low strain region to thereby change the electrical resistance.

[0117] After the aforementioned blank is taken out from the furnace, forming is started. At such time, for both regions, the temperature at the start of forming is to be 700°C or more. Thereafter, quenching is performed by cooling to a temperature less than the Mf point without die opening. At such time, the average cooling rate from the temperature at the beginning of press forming to the Ms point is to be 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 to be 10°C / s or more. After die opening in a temperature region less than the Mf point, it suffices to allow cooling to room temperature at an average cooling rate of, for example, 5°C / s or less.

[0118] By performing hot-stamping forming according to the conditions described above, on the one hand, in the low strain region it is possible to suppress coarsening of grains in the microstructure and also suppress the occurrence of a reduction in strength as well as a decrease in the resistance to hydrogen embrittlement and the notched tensile strength, while on the other hand, in the high strain region, it is possible to suppress the formation of ferrite. Thus, the first region is formed from the low strain region, and the second region is formed from the high strain region.

[0119] If the heating temperature in the low strain region falls below the Ac 3 point, micro-structures other than austenite, such as ferrite, pearlite, bainite, or cementite may remain after heating, and the area fraction of martensite may become less than 95% during the subsequent cooling process. On the other hand, if the heating temperature in the low strain region is more than the Ac 3 point + 65°C or the heating time is more than 120 seconds, the grains may coarsen and the resistance to hydrogen embrittlement and notched tensile strength may decrease.

[0120] Further, if the heating temperature in the high strain region falls below the Ac 3 point + 40°C, coarsening of grains may be insufficient, sufficient hardenability may not be obtained, ferrite may be formed, and the area fraction of martensite after cooling may become less than 95%. On the other hand, if the heating temperature in the high strain region is more than the Ac 3 point + 110°C or the heating time is more than 120 seconds, the grains may coarsen excessively and the resistance to hydrogen embrittlement and notched tensile strength may decrease.

[0121] Note that, in the case of providing the hot-stamping formed body with a strength of 1.5 GPa grade and making G1 fall within the range of 8.0 to 14.0 µm and G2 fall within the range of 17.0 to 25.0 µm, it is preferable to set the heating temperature in the low strain region within the range of the Ac 3 point + 10°C or more to the Ac 3 point + 60°C or less, and to set the heating temperature in the high strain region within the range of the Ac 3 point + 70°C or more to the Ac 3 point + 110°C or less. The heating temperature in the low strain region is more preferably set within the range of the Ac 3 point + 15°C or more to the Ac 3 point + 50°C or less, and the heating temperature in the high strain region is more preferably set within the range of the Ac 3 point + 75°C or more to the Ac 3 point + 100°C or less.

[0122] In addition, if the difference between the heating temperature in the high strain region and the heating temperature in the low strain region is less than 40°C, the difference between the average grain diameter G1 in the first region and the average grain diameter G2 in the second region will be small, and it will not be possible to satisfy the aforementioned Formula (i). As a result, it will not be possible to achieve both refinement of the micro-structure in the low strain region and suppression of ferrite formation in the high strain region.

[0123] Further, if the temperature at the beginning of press forming is less than 700°C, a large amount of ferrite may be formed during air cooling in the period from after heating until the start of forming, or during the subsequent cooling process, and consequently the area fraction of martensite may become less than 95%.

[0124] If the average cooling rate from the temperature at the beginning of press forming to the Ms point is less than 50°C / s, a large amount of ferrite may be formed during the cooling process and consequently the area fraction of martensite may become less than 95%. If the average cooling rate from the Ms point to the Mf point is less than 10°C / s, the strength may decrease due to self-tempering. If the die opens at a temperature equal to or higher than the Mf point, martensitic transformation may occur even after die opening, which may result in a decrease in the dimensional accuracy of the formed body.

[0125] Here, in the present invention, the Ac 3 point, the Ms point, and the Mf point are to be calculated based on the following Formulas (I) to (III), respectively: Ac 3 ° C = 850 + 10 × C + N × Mn + 350 × Nb + 250 × Ti + 40 × B + 10 × Cr + 100 × Mo where, each symbol of an element in the above formulas represents the content (mass%) of the corresponding element.

[0126] Hereunder, the present invention is described more specifically by way of examples, although the present invention is not limited to these examples.EXAMPLE

[0127] Molten steels (steel types a to r) having the chemical compositions shown in Table 1 were cast by a continuous casting process to produce slabs. After heating these slabs to a temperature of 1100°C or more, rough rolling, finish rolling, cooling, and coiling were performed under the conditions shown in Table 2 to produce hot-rolled steel sheets. Thereafter, the hot-rolled steel sheets of steel types f and k were further subjected to cold rolling to produce cold-rolled steel sheets.[Table 1]

[0128] Table 1SteelChemical composition (mass%, balance: Fe and impurities)Ac 3 point (°C)Ms point (°C)Mf point (°C)CSiMnPSNOAlCrNbTiMoBOthera0.210.221.230.0140.00170.00320.0050.0330.190.0030.0210.0070.0015862423273b0.200.051.150.0090.00120.00290.0040.0270.180.0020.0190.0050.0014860430282c0.250.291.320.0080.00090.00240.0050.0340.180.0030.0240.0350.0019866406253e0.150.112.780.0130.00260.00580.0050.0290.190.0040.0310.0035Co: 0.06866385240f0.320.231.710.0080.00360.00550.0050.0260.160.0030.0024V: 0.29858355211g0.380.271.320.0150.00300.00340.0020.0330.190.025863359200k0.220.871.340.0100.00490.0050.0030.0260.190.0030.0021Ni:0.07, REM:0.0010856414259l0.210.241.330.0150.00410.00280.0050.0310.650.0250.0019Mg:0.0030, Sn:0.001866410260n0.310.241.710.0150.00350.00390.0020.0430.180.0190.0050.0021W:0.31, Sb:0.022863368214o0.331.781.310.0120.00240.00400.0050.0360.180.0030.0180.0019Ca: 0.0060859377210p0.350.231.180.0150.00370.00230.0030.0450.170.0020.0210.0018Zr:0.005, As:0.005862376218q0.340.260.210.0130.00260.00470.0030.0450.280.0080.0180.650.0019Cu: 0.05926411253r0.510.330.300.0120.00160.00330.0040.0290.110.0040.0110.0320.0013860353187 [Table 2]

[0129] Table 2SteelHot rolling processCold rolling processHeating temperature (°C)Finish rolling completion temperature (°C)Average cooling rate (°C / s)Coiling temperature (°C)Rolling reduction (%)a1230870256900b1230910306200c1210930306300e1200920306000f12309702558033g1160970405600k11908903066035l1190980306200n1180880356200o1110890206800p1120900255300q1190990306000r1200980306100

[0130] Each of the obtained hot-rolled steel sheets or cold-rolled steel sheets was subjected to hot-stamping forming in which press forming, a burring processing, and quenching were performed as a series of processes under the conditions shown in Tables 3 and 4 to thereby produce a hot-stamping formed body with the shape shown in Figure 1 having a sheet-shaped portion and a burring portion. Note that, the thickness of the sheet-shaped portion was 2.6 mm, the thickness (edge thickness) of the tip of the burring portion was the thickness shown in Tables 5 and 6, and the diameter of the burring portion was 50 mm. At such time, a black coating was applied to a region where it was planned to perform a burring processing (the region that would be the burring portion after hot-stamping forming) to thereby change the emissivity of the surface of that region and thus provide a difference in the heating rate and the heating temperature, respectively, between the aforementioned region and a region where it was not planned to perform a burring processing (the region that would be the sheet-shaped portion after hot-stamping forming).

[0131] Note that, in the tables, the terms "Heating rate" and "Heating temperature" refer to the average heating rate and heating temperature during heating before hot-stamping forming. The term "Difference from Ac 3 point" refers to a value obtained by subtracting the Ac 3 point from the aforementioned heating temperature. The term "Holding time" refers to the time for which the steel sheet is held at the aforementioned heating temperature. The term "Cooling rate from St to Ms" refers to the average cooling rate from a temperature at the beginning of press forming St to the Ms point, and the term "Cooling rate from Ms to Mf" refers to the average cooling rate from the Ms point to the Mf point. The term "Difference of heating temperature" refers to a value obtained by subtracting the heating temperature in the region where it was not planned to perform a burring processing from the heating temperature in the region where it was planned to perform a burring processing.

[0132] Next, the cross sections illustrated in Figures 1, 2, and 8 were adopted as the measurement regions for the sheet-shaped portion and the burring portion, respectively, and observation of the microstructure and measurement of the Vickers hardness were carried out by the method described below. The measurement results are shown in Tables 5 and 6.[Table 5]

[0133] Table 5Test No.SteelMicrostructure and Vickers hardnessRemarksSheet-shaped portionBurring portionDifference of grain diameter G2-G1 (µm)M+B+γ area fraction (%)HV MSγ (HV)957.1 > [C]+ 221.0M area fraction (%)G1 (µm)Average aspect ratioHV 1 (HV0.1)M+B+γ area fraction (%)HV MBγ (HV)957.1 <[C]+ 221.0M area fraction (%)G2 (µm)Average aspect ratioHV 2 (HN0.1)Edge thickness (mm)1a99458.3422.0≥95%9.41.03455.798464.1422.0≥95%17.22.35458.81.47.8Inventive example2a96461.7422.0≥95%9.11.10451.2100470.1422.0≥95%20.82.28470.11.511.7Inventive example3a99471.7422.0≥95%9.81.01469.098468.7422.0≥95%24.72.13463.31.614.9Inventive example4a99459.4422.0≥95%11.41.10456.899473.8422.0≥95%17.32.29471.11.55.9Inventive example5a96455.7422.0≥95%11.81.08445.5100464.9422.0≥95%23.4222464.91.511.6Inventive example6a100470.8422.0≥95%13.21.03470.8100476.3422.0≥95%17.72.38476.31.44.5Inventive example7a96469.8422.0≥95%13.71.02459.099457.1422.0≥95%24.42.24454.51.510.7Inventive example8a95458.8422.0≥95%8.31.02447.9100469.8422.0≥95%23.32.34469.81.415.0Inventive example9a97460.4422.0≥95%19.81.10452.696461.5422.0≥95%19.82.27451.01.50.0Comparative example10a99461.1422.0≥95%13.81.02458.596463.8422.0≥95%28.22.04453.21.714.4Comparative example11a98457.8422.0≥95%18.21.08452.692-422.0<95%--434.41.4-Comparative example12a99465.0422.0≥95%24.81.05462.4100468.7422.0≥95%27.32.12468.71.62.5Comparative example13a92-422.0<95%--426.789-422.0<95%--417.71.4-Comparative example14a90-422.0<95%--419.393-422.0<95%--427.51.4-Comparative example15a100386.8422.0<95%--386.8100380.4422.0<95%--380.41.4-Comparative example16a100470.8422.0≥95%13.11.03470.892-422.0<95%--429.11.4-Comparative example17a86-422.0<95%--391.4100476.0422.0≥95%17.92.38476.01.4-Comparative example18a100469.2422.0≥95%18.21.04469.2100479.3422.0≥95%17.82.41479.31.4-0.4Comparative example19a100456.7422.0≥95%41.71.02456.7100462.4422.0≥95%45.32.36462.41.43.6Comparative example [Table 6]

[0134] Table 6Test No.SteelMicrostructure and Vickers hardnessRemarksSheet-shaped portionBurring portionDifference of grain diameter G2-G1 (µm)M+B+y area fraction (%)HV MBγ (HV)957.1 ×[C]+ 221.0M area fraction (%)G1 (µm)Average aspect ratioHV 1 (HV0.1)M+B+y area fraction (%)HV MBγ (HV)957.1 ×[C]+ 221.0M area fraction (%)G2 (µm)Average aspect ratioHV 2 (HV0.1)Edge thickness (mm)20b97447.9412.4≥95%9.51.04440.598448.7412.4≥95%18.12.03443.71.78.6Inventive example21b96460.6412.4≥95%9.91.10450.2100464.2412.4≥95%24.62.11464.21.614.7Inventive example22b96460.5412.4≥95%12.21.07450.197459.5412.4≥95%18.32.01451.71.76.1Inventive example23b96455.4412.4≥95%13.81.09445.297458.0412.4≥95%17.92.15450.31.64.1Inventive example24b96464.8412.4≥95%13.71.08454.299451.4412.4≥95%24.22.23448.91.510.5Inventive example25b96453.5412.4≥95%20.11.02443.498460.9412.4≥95%20.22.46455.71.40.1Comparative example26c97476.0460.3≥95%8.91.03467.797480.0460.3≥95%20.22.52471.61.411.3Inventive example27c97471.3460.3≥95%10.91.02463.299474.1460.3≥95%17.12.10471.41.76.2Inventive example28c97479.7460.3≥95%12.61.04471.397471.0460.3≥95%23.12.51462.91.410.5Inventive example29c100475.6460.3≥95%24.91.02475.699471.6460.3≥95%27.82.28468.91.52.9Comparative example30e97396.9364.6≥95%8.41.05391.096397.7364.6≥95%19.22.02389.81.710.8Inventive example31e96399.4364.6≥95%12.51.06391.496400.1364.6≥95%16.72.19392.11.64.2Inventive example32f97559.9527.3≥95%11.71.05549.1100564.7527.3≥95%17.72.03564.71.76.0Inventive example33g98620.0584.7≥95%14.61.01611.696621.4584.7≥95%19.22.03604.51.74.6Inventive example34k98471.4431.6≥95%11.91.05466.096472.1431.6≥95%17.92.19461.21.66.0Inventive example351100469.8422.0≥95%13.81.04469.8100467.2422.0≥95%19.32.15467.21.65.5Inventive example36n96561.6517.7≥95%14.21.10547.199562.5517.7≥95%18.92.52558.91.44.7Inventive example37o98558.5536.8≥95%12.41.03551.398558.2536.8≥95%18.52.27551.01.56.1Inventive example38p99591.5556.0≥95%13.71.07587.6100589.5556.0≥95%17.32.08589.51.73.6Inventive example39q96593.1546.4≥95%14.21.05577.497587.8546.4≥95%22.52.48576.21.48.3Inventive example40r100744.9709.1≥95%11.51.04744.9100749.8709.1≥95%15.62.33749.81.44.1Inventive example

[0135] First, each of the aforementioned measurement regions was etched using Nital, and after etching, an FE-SEM was used to obtain an image of the microstructure at a magnification of 1000×. The FE-SEM that was used was equipped with a secondary electron detector. The obtained micro-structure image was subjected to image analysis to measure the area fractions of micro-structures determined to be ferrite or pearlite, and the other micro-structures were determined to be martensite, bainite, or retained austenite. The total of the area fractions determined to be martensite, bainite, or retained austenite is shown as "M+B+γ area fraction (%)" in Tables 5 and 6. Further, if the area fraction in question was less than 95%, it was determined that the martensite area fraction ("M area fraction" in Tables 5 and 6) was less than 95% ("<95%").

[0136] On the other hand, if the area fraction was 95% or more, a mesh was drawn with 70 µm intervals in a region of 300 µm in length by 2000 µm in width, and measurement of the Vickers hardness was performed at all of the intersection points of the mesh. The test force in the Vickers hardness measurement was set to 0.98 N (0.1 kgf). Thereafter, an image was obtained once again at a magnification of 1000× using the FE-SEM. Then, by visually comparing the micro-structure images obtained before and after the Vickers hardness measurement respectively, it was determined whether or not a micro-structure other than martensite, bainite, and retained austenite, that is, ferrite or pearlite, was contained within the area of indentations that remained in the surface.

[0137] Then, among the obtained Vickers hardness measurement values, the average value of those measurement values for which it was identified that only martensite, bainite, or retained austenite were contained in the indentations was calculated, and if the following formula was satisfied it was determined that the area fraction of martensite in the microstructure was 95% or more ("≥ 95%"). On the other hand, if the following formula was not satisfied, it was considered that the area fraction of bainite was relatively high or that martensite was excessively tempered, and therefore it was determined that the martensite area fraction was less than 95% ("<95%"), and a subsequent observation of the microstructure was not performed. HV MBγ ≥ 957.1 × C + 221.0 where, the meaning of each symbol in the above formula is as follows: HV MBγ : Average value of Vickers hardness measurement values with respect to indentations identified as containing only martensite, bainite, or retained austenite (HV0.1); [C]: Content of C in hot-stamping formed body (mass%).

[0138] Further, the average values of the Vickers hardness measurement values at all of the intersection points of the mesh with 70 µm intervals in the observation region of the sheet-shaped portion and the burring portion are shown as HV 1 and HV 2 , respectively, in Tables 5 and 6.

[0139] Next, after subjecting the aforementioned observation regions to mirror polishing and electropolishing under the conditions described above, measurement was performed using EBSD to identify the prior-austenite grains by the reconstruction method described above. The grain diameter and aspect ratio of each identified prior-austenite grain were determined, and the average grain diameter and average aspect ratio were determined by averaging the measurement values obtained for all the prior-austenite grains in the relevant observation region.

[0140] Next, the tensile strength, resistance to hydrogen embrittlement, and notched tensile strength of each hot-stamping formed body were evaluated by the following methods.<Tensile strength>

[0141] A sub-sized sheet-shaped test specimen defined in ASTM A370-22 (a test specimen having a width of 10 mm, an overall length of 100 mm, a parallel portion width of 6.25 mm, a parallel portion length of 32 mm, a gage length of 25 mm, and a thickness that was the original thickness of the sheet-shaped portion) that had the shape illustrated in Figure 9 was prepared from the sheet-shaped portion of each hot-stamping formed body. A tensile test was then conducted using the sub-sized sheet-shaped test specimen, and the tensile strength TS (MPa) was measured. At such time, the speed of tensile test was set to a crosshead separation rate of 3 mm / min, and the other conditions for the tensile test were in accordance with JIS Z 2241: 2022.<Resistance to hydrogen embrittlement and notched tensile strength>

[0142] Test specimens were cut from each of the sheet-shaped portion and the burring portion of the hot-stamping formed body, and two tensile test specimens having the shape illustrated in Figure 10 were prepared. Figure 11 is a view for describing the procedure used to cut out the test specimen from the burring portion and prepare a tensile test specimen. Due to dimensional constraints, it was not possible to take a tensile test specimen having the shape illustrated in Figure 10 from the burring portion. Therefore, as illustrated in Figure 11(a) and (b), first, a test specimen having a length of 8.0 mm, a thickness of 1.0 mm, and a height of 2.0 mm was cut out from the tip of the burring portion 12. Then, as illustrated in Figure 11(c), a two-sided U-notch was formed in the center of the test specimen, and thereafter, as illustrated in Figure 11(d), dummy material indicated by hatching was joined by welding to both sides of the test specimen to make a tensile test specimen having the same shape as the shape illustrated in Figure 10.

[0143] Next, one of the tensile test specimens was used as a cathode in a 3% NaCl aqueous solution, and a tensile test was conducted at a speed of tensile test of 0.006 mm / min while generating hydrogen on the surface of the tensile test specimen under a condition of a current density of 1.0 mA / cm 2< , and the fracture stress (maximum stress) under a hydrogen environment was measured. Note that, Pt was used as an anode. The obtained measurement value was defined as the tensile strength TSH (MPa) under a hydrogen environment, and adopted as an index of resistance to hydrogen embrittlement.

[0144] Further, the other tensile test specimen was used to conduct a tensile test in accordance with JIS Z 2241: 2022 in air at a speed of tensile test of 0.006 mm / min, and the fracture stress (maximum stress) was measured. The obtained measurement value was defined as the notched tensile strength TSN (MPa).

[0145] In the present examples, if the sum TSH + TSN (MPa) of the measured tensile strength under a hydrogen environment and the measured notched tensile strength was 2400 MPa or more, it was determined that the hot-stamping formed body had excellent resistance to hydrogen embrittlement and high notched tensile strength.

[0146] The results are shown in Table 7.[Table 7]

[0147] Table 7Test No.SteelResult of evaluationRemarksSheet-shaped portionBurring portionHV 1 -HV 2 TS (MPa)TSH (MPa)TSN (MPa)TSH+ TSN (MPa)TSH (MPa)TSN (MPa)TSH+ TSN (MPa)1a14819731714268794817042652-3.1Inventive example2a14679661709267593317112644-18.9Inventive example3a152497217072679916171026265.7Inventive example4a14859601709266995317102663-14.3Inventive example5a14589641709267391717112628-19.4Inventive example6a14789531712266595117052656-5.5Inventive example7a149295017112661919170926284.5Inventive example8a14709441678262292417232647-21.9Inventive example9a147160415632167938170626441.6Comparative example10a149095017042654627157121985.3Comparative example11a1471607157021777151597231218.2Comparative example12a15036301562219260715682175-6.3Comparative example13a141671115872298730158023109.0Comparative example14a14167251595232074415832327-8.2Comparative example15a12351098144425421122143225546.4Comparative example16a1482944170926537381587232541.7Comparative example17a12727221482220494617092655-84.6Comparative example18a14596231571219493817122650-10.1Comparative example19a14775461549209555315622115-5.7Comparative example20b14519661703266994717062653-3.2Inventive example21b14639671714268191917152634-14.0Inventive example22b14669581704266295217132665-1.6Inventive example23b14599481711265994517082653-5.1Inventive example24b147694717002647919170226215.3Inventive example25b14546031567217094317052648-123Comparative example26c15209741704267893817012639-3.9Inventive example27c15059601706266694917022651-8.2Inventive example28c153294617062652923171326368.4Inventive example29c153361115752186609156121706.7Comparative example30e12711142146226041132145425861.2Inventive example31e1272113514582593114414492593-0.7Inventive example32f17856482010265864020102650-15.6Inventive example33g198840322072610395220025957.1Inventive example34k151495017002650946169426404.8Inventive example35l152995216912643948170226502.6Inventive example36n17786451999264464320102653-11.8Inventive example37o179264720102657643200926520.3Inventive example38p19393942206260040122022603-1.9Inventive example39q190540621972603399220226011.2Inventive example40r24883232298262132922812610-4.9Inventive example

[0148] As shown in Table 5 to 7, in each of Test Nos. 1 to 8, 20 to 24, 26 to 28, and 30 to 40 which fulfilled the requirements defined in the present invention, the hot-stamping formed body had high strength, and a decrease in strength was also suppressed in the burring portion, and in addition it was possible to achieve both excellent resistance to hydrogen embrittlement and high notched tensile strength.

[0149] In contrast, in Test Nos. 9 to 19, 25, and 29 in which the hot stamping conditions were not suitable, in at least either one of the sheet-shaped portion and the burring portion the martensite area fraction was less than 95% or the average grain diameter was more than the specified value. Consequently, softening occurred due to formation of ferrite, or the advantageous effect of improving the resistance to hydrogen embrittlement and notched tensile strength could not be obtained.

[0150] Specifically, in Test No. 9, the heating temperature in the region where it was not planned to perform a burring processing was high and there was little difference between the heating temperature in that region and the heating temperature in the region where it was planned to perform a burring processing, and therefore the average grain diameter G1 in the sheet-shaped portion was excessive and the difference (G2-G1) between the average grain diameter G1 in the sheet-shaped portion and the average grain diameter G2 in the burring portion did not meet the requirement defined in the present invention. As a result, the value of TSH + TSN decreased in the sheet-shaped portion.

[0151] In Test No. 10, the heating temperature in the region where it was planned to perform a burring processing was high, and therefore the average grain diameter G2 in the burring portion was excessive. As a result, the value of TSH + TSN decreased in the burring portion.

[0152] In Test No. 11, since the heating temperature in the region where it was not planned to perform a burring processing was high and the heating temperature in the region where it was planned to perform a burring processing was low, the average grain diameter G1 in the sheet-shaped portion was excessive, and the area fraction of martensite in the burring portion did not become 95% or more. As a result, the value of TSH + TSN decreased in both the sheet-shaped portion and the burring portion.

[0153] In Test No. 12, in the region where it was not planned to perform a burring processing and the region where it was planned to perform a burring processing, the heating temperature was high and the heating time was long, and furthermore, the difference in the heating temperature between the two regions was small. Therefore, the average grain diameter G1 in the sheet-shaped portion and the average grain diameter G2 in the burring portion were each excessive, and the difference between the two average grain diameters (G2-G1) also did not meet the requirement defined in the present invention. As a result, the value of TSH + TSN decreased in both the sheet-shaped portion and the burring portion.

[0154] In Test No. 13, the forming start temperature was low in the region where it was not planned to perform a burring processing and in the region where it was planned to perform a burring processing, and consequently the area fraction of martensite did not become 95% or more in the sheet-shaped portion or in the burring portion. As a result, the value of TSH + TSN decreased in both the sheet-shaped portion and the burring portion.

[0155] In Test No. 14, the average cooling rate from the forming start temperature to the Ms point was low in the region where it was not planned to perform a burring processing and in the region where it was planned to perform a burring processing, and consequently the area fraction of martensite did not become 95% or more in the sheet-shaped portion or in the burring portion. As a result, the value of TSH + TSN decreased in both the sheet-shaped portion and the burring portion.

[0156] In Test No. 15, the average cooling rate from the Ms point to the Mf point was low in the region where it was not planned to perform a burring processing and in the region where it was planned to perform a burring processing, and consequently the area fraction of martensite did not become 95% or more in the sheet-shaped portion or in the burring portion. As a result, the required strength could not be obtained in the sheet-shaped portion.

[0157] In Test No. 16, the heating temperature in the region where it was planned to perform a burring processing was low, and the difference between the heating temperature in the region where it was planned to perform a burring processing and the heating temperature in the region where it was not planned to perform a burring processing was small, and consequently the area fraction of martensite did not become 95% or more in the burring portion. As a result, the value of TSH + TSN decreased in the burring portion.

[0158] In Test No. 17, the heating temperature in the region where it was not planned to perform a burring processing was low, and consequently the area fraction of martensite did not become 95% or more in the sheet-shaped portion. As a result, in the sheet-shaped portion, the required strength could not be obtained and, furthermore, the value of TSH + TSN decreased.

[0159] In Test No. 18, the heating temperature in the region where it was not planned to perform a burring processing was high and there was little difference between the heating temperature in that region and the heating temperature in the region where it was planned to perform a burring processing, and therefore the average grain diameter G1 in the sheet-shaped portion was excessive and the difference (G2-G1) between the average grain diameter G1 in the sheet-shaped portion and the average grain diameter G2 in the burring portion did not meet the requirement defined in the present invention. As a result, the value of TSH + TSN decreased in the sheet-shaped portion.

[0160] In Test No. 19, in the region where it was not planned to perform a burring processing and the region where it was planned to perform a burring processing, the heating temperature was high and the heating time was long, and furthermore, the difference in the heating temperature between the two regions was small. Therefore, the average grain diameter G1 in the sheet-shaped portion and the average grain diameter G2 in the burring portion were each markedly excessive. As a result, the value of TSH + TSN decreased in both the sheet-shaped portion and the burring portion.

[0161] In Test No. 25, the heating temperature in the region where it was not planned to perform a burring processing was high and there was little difference between the heating temperature in that region and the heating temperature in the region where it was planned to perform a burring processing, and therefore the average grain diameter G1 in the sheet-shaped portion was excessive and the difference (G2-G1) between the average grain diameter G1 in the sheet-shaped portion and the average grain diameter G2 in the burring portion did not meet the requirement defined in the present invention. As a result, the value of TSH + TSN decreased in the sheet-shaped portion.

[0162] In Test No. 29, the heating temperature was high in the region where it was not planned to perform a burring processing and in the region where it was planned to perform a burring processing, and furthermore the difference in the heating temperature between the two regions was small. Therefore, the average grain diameter G1 in the sheet-shaped portion and the average grain diameter G2 in the burring portion were each excessive, and the difference between the two average grain diameters (G2-G1) also did not meet the requirement defined in the present invention. As a result, the value of TSH + TSN decreased in both the sheet-shaped portion and the burring portion.INDUSTRIAL APPLICABILITY

[0163] According to the present invention, a hot-stamping formed body can be obtained in which, even in a case where high strain is partially introduced in a hot stamping process, a decrease in strength after hot-stamping forming in a high strain region is suppressed.

Claims

1. A hot-stamping formed body, comprising: a first region having a microstructure in which an area fraction of martensite is 95% or more and an average aspect ratio of prior-austenite grains is 1.10 or less, and a second region having a microstructure in which an area fraction of martensite is 95% or more and an average aspect ratio of prior-austenite grains is 2.00 or more; wherein: when an average grain diameter of prior-austenite grains in the first region is represented by G1 (µm) and an average grain diameter of prior-austenite grains in the second region is represented by G2 (µm), G1 is 15.0 µm or less and G2 is 25.0 µm or less, and G1 and G2 satisfy Formula (i) below, and a tensile strength measured by a tensile test using a test specimen taken from the first region is 1250 to 2540 MPa: G 2 − G 1 ≥ 3.

02. The hot-stamping formed body according to claim 1, wherein: an average grain diameter G1 of prior-austenite grains in the first region is 5.0 to 15.0 µm, and an average grain diameter G2 of prior-austenite grains in the second region is 10.0 to 25.0 µm.

3. The hot-stamping formed body according to claim 1 or claim 2, having a chemical composition comprising, by mass%, C: 0.10 to 0.60%, Si: 0.01 to 2.00%, Mn: 0.10 to 3.00%, P: 0.050% or less, S: 0.0200% or less, N: 0.0200% or less, O: 0.100% or less, Al: 0.001 to 0.100%, Cr: 0.01 to 1.00%, Nb: 0 to 0.200%, Ti: 0 to 0.200%, Mo: 0 to 1.00%, B: 0 to 0.0100%, Co: 0 to 4.00%, Ni: 0 to 2.00%, Cu: 0 to 1.00%, V: 0 to 1.00%, W: 0 to 1.00%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, REM: 0 to 0.0100%, Sb: 0 to 0.100%, Zr: 0 to 0.100%, Sn: 0 to 1.00%, As: 0 to 0.100%, and the balance: Fe and impurities.

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