Steel material and component

Abstract

Provided is a steel material having a sheared end face, the steel material characterized by: having a Vickers hardness of 450 Hv or more; the maximum value of tensile residual stress measured at the 1 / 4 position, the 2 / 4 position, and the 3 / 4 position of the sheet thickness at the sheared end face being 1300 MPa or less; and the maximum value of the tensile residual stress not being the tensile residual stress measured at the 2 / 4 position of the sheet thickness.

Description

Steel materials and parts

[0001] This invention relates to steel materials and components.

[0002] In recent years, the automotive industry has been demanding lighter vehicle bodies from the perspective of improving fuel efficiency or electric energy consumption. To achieve both vehicle weight reduction and collision safety, increasing the strength of the steel plates used is one effective method, and for this reason, the development of high-strength steel plates is progressing.

[0003] However, while high-strength steel plates can increase load-bearing capacity, hydrogen embrittlement cracking (also known as delayed fracture) can become a problem in corrosive environments. Hydrogen embrittlement cracking is a phenomenon in which a steel member subjected to high stress under usage conditions suddenly fractures due to hydrogen entering the steel from the environment.

[0004] Furthermore, automotive parts and the like are typically manufactured by cutting steel sheets to a predetermined size through shearing, and then pressing them into a predetermined shape. In shearing steel sheets, the steel sheet is generally placed between a punch and a die, and a shearing force is applied to the sheet to cut it. In this type of processing, tensile residual stress can occur on the sheared end face of the steel sheet, and if this tensile residual stress becomes large, there is a problem that the risk of hydrogen embrittlement cracking due to hydrogen entering the steel from the external environment increases.

[0005] In this regard, Patent Document 1 describes a steel material having a shear end face, wherein the shear end face comprises a burr, a fracture surface, and a burr, and the fracture surface includes a first portion and a second portion, the first portion being formed by a first crack that has progressed from the burr side to the burr side, and the second portion being formed by a second crack that has progressed from the burr side to the burr side, and the area ratio of the first portion to the fracture surface is 1.2 times or more the area ratio of the second portion to the fracture surface. Furthermore, Patent Document 1 teaches that by making the area ratio of the first portion to the fracture surface 1.2 times or more the area ratio of the second portion to the fracture surface, the tensile residual stress of the fracture surface is reduced, thereby improving the hydrogen embrittlement resistance at the shear end face.

[0006] International Publication No. 2023 / 148899

[0007] Generally, hydrogen embrittlement cracking is known to occur more easily as the strength of steel increases. On the other hand, the automotive industry and other sectors are also demanding further weight reduction of steel materials, and achieving such weight reduction requires making steel materials stronger than ever before. Therefore, even when steel materials with strength equivalent to or greater than conventional materials are sheared, it is necessary to suppress or reduce the occurrence of hydrogen embrittlement cracking at the sheared end face.

[0008] Therefore, the present invention aims to provide a steel material in which hydrogen embrittlement resistance at the shear end face is improved by a novel configuration.

[0009] To achieve the above objective, the inventors focused particularly on the relationship between the strength of the steel material and the tensile residual stress at the shear end face. As a result, the inventors discovered that even with high-strength steel material having a Vickers hardness of 450 Hv or more, the maximum value of the tensile residual stress at the shear end face can be controlled within a predetermined range, and that the maximum value of the tensile residual stress does not equal the tensile residual stress measured at the 2 / 4 position of the plate thickness at the shear end face. This allowed them to reduce the occurrence of hydrogen embrittlement cracking at the shear end face, thus completing the present invention.

[0010] The present invention, which has achieved the above objectives, is as follows: (1) A steel material having a shear end face, having a Vickers hardness of 450 Hv or more, wherein the maximum value of the tensile residual stress measured at the 1 / 4, 2 / 4, and 3 / 4 positions of the plate thickness on the shear end face is 1300 MPa or less, and the maximum value of the tensile residual stress is not the tensile residual stress measured at the 2 / 4 position of the plate thickness on the shear end face. (2) The steel material contains inclusions, wherein the number density of inclusions having a circular equivalent diameter of 0.01 to 3.00 μm in the region from the surface of the steel material to the 3 / 8 to 5 / 8 position of the plate thickness is 50.0 pieces / mm 2 The following conditions apply: the number density of inclusions with a circular equivalent diameter greater than 3.00 μm is 5.0 inclusions / mm². 2The steel material according to (1) above, characterized in that the average aspect ratio of inclusions having a circular equivalent diameter of 0.01 μm or more in the region from the surface of the steel material to the 3 / 8 to 5 / 8 position of the plate thickness is 6.00 or less. (3) The steel material according to (1) or (2) above, characterized in that the steel material has a chemical composition containing, by mass%, at least one of Mg and Ca: totaling 0.005 to 0.050%. (4) The steel material is composed of, in mass%, C: 0.15 to 0.60%, Si: 0.01 to 1.30%, Mn: 1.00 to 3.50%, P: 0.0200% or less, S: 0.0200% or less, Al: 0.001 to 1.00%, N: 0.0200% or less, O: 0.0200% or less, Co: 0 to 0.50%, Ni: 0 to 1.00%, Mo: 0 to 1.00%, Cr: 0 to 2.00%, Ti: 0 to 0.5000%, B: 0 to 0.0100%, Nb: 0 to 0.500%, V: 0 to 0.500%, Cu: 0 to 1.000%, W: 0 to 0.100%, (1) The steel material according to any one of (1) to (3) above, characterized in having a chemical composition consisting of Ta: 0 to 0.100%, Sn: 0 to 0.100%, Sb: 0 to 0.500%, As: 0 to 0.100%, Mg: 0 to 0.050%, Ca: 0 to 0.050%, Y: 0 to 0.050%, Zr: 0 to 0.500%, La: 0 to 0.050%, Ce: 0 to 0.050%, and the remainder being Fe and impurities. (5) The steel material according to any one of (1) to (4) above, characterized in having a metallic structure containing martensite of 85.0% or more by area percent. (6) A component characterized by containing the steel material according to any one of (1) to (5) above.

[0011] According to the present invention, it is possible to provide a steel material in which the hydrogen embrittlement resistance at the shear end face is improved.

[0012] This is a schematic enlarged view showing an example of a steel material having a shear end face. This is a schematic diagram to explain an example of the formation mechanism of a shear end face when a workpiece is sheared. This shows a state in which a crack is generated in the workpiece by further pressing the punch and die against the workpiece after burr formation. This shows a state in which a part of the workpiece is punched out by further pressing the punch and die against the workpiece after crack formation. This is a schematic diagram to explain the state of tensile residual stress on the shear end face when a crack is propagated from the punch side. This shows a cross-section when the cutting edges of the punch and die are chamfered. This is a schematic diagram showing a test piece used to evaluate hydrogen embrittlement resistance.

[0013] <Steel Material> The steel material according to the embodiment of the present invention has a shear end face, has a Vickers hardness of 450 Hv or more, and is characterized in that the maximum value of the tensile residual stress measured at the 1 / 4, 2 / 4, and 3 / 4 positions of the plate thickness on the shear end face is 1300 MPa or less, and the maximum value of the tensile residual stress is not the tensile residual stress measured at the 2 / 4 position of the plate thickness on the shear end face.

[0014] Generally, hydrogen embrittlement cracking is known to occur more easily as the strength of the steel increases. On the other hand, as mentioned earlier, in the shearing process of steel plates, cutting is performed by applying a shear force to the steel plate, which can cause tensile residual stress to occur on the sheared end face of the steel plate. When this tensile residual stress becomes large, there is a problem that the risk of hydrogen embrittlement cracking occurring due to hydrogen entering the steel from the external environment increases.

[0015] Figure 1 is a schematic enlarged view showing an example of a steel material having a sheared end face. Referring to Figure 1, the sheared end face 1 of the steel material 10 generally includes a burr 1a, a shear surface 1b, a fracture surface 1c, and a burr 1d. To explain in more detail using Figure 2, for example, when a punch 21 and a die 22 are placed on the first surface 10a side and the second surface 10b side, respectively, of a workpiece 5 corresponding to the steel material 10 before shearing, and the workpiece 5 is sheared by these, first, as shown in Figure 2A, the bottom surface 21a of the punch 21 is pressed against the first surface 10a of the workpiece 5, and a burr 1a is formed on the first surface 10a side during the process until the cutting edge (tip) of the punch 21 bites into the workpiece 5. After the burr 1a is formed, a shear surface 1b (not shown) is formed during the process as the cutting edge of the punch 21 bites into the workpiece 5. After the burr 1a and shear surface 1b are formed, as shown in Figure 2B, a first crack 1ex is generated from the punch 21 side toward the die 22 side, and similarly on the die 22 side, a second crack 1ey is generated from the die 22 side toward the punch 21 side as the cutting edge of the die 22 bites into the second surface 10b of the workpiece 5. Finally, as shown in Figure 2C, the first crack 1ex and the second crack 1ey propagate and merge to form a fracture surface 1c. Furthermore, by moving the punch 21 and die 22 further, the workpiece 5 is separated into scrap 15 and the target steel material 10. At this time, as shown in Figures 1 and 2C, a burr 1d is generally formed on the corner of the shear end surface 1 of the steel material 10 on the die 22 side.

[0016] In the shear end face 1 formed as described above, compressive residual stress and tensile residual stress may be generated due to damage and strain caused by shearing. If large tensile residual stress is present in the shear end face 1, the hydrogen embrittlement resistance of the shear end face 1 may decrease, leading to hydrogen embrittlement cracking.

[0017] Therefore, the inventors focused particularly on the relationship between the strength of the steel material and the tensile residual stress at the shear end face and conducted their research. As a result, the inventors found that even with high-strength steel material having a Vickers hardness of 450 Hv or more, by devising a shearing method, as will be explained in detail later in relation to the manufacturing method, the maximum value of the tensile residual stress at the shear end face can be controlled to a predetermined range, or more specifically, by controlling the maximum value of the tensile residual stress at the shear end face to 1300 MPa or less, the occurrence of hydrogen embrittlement cracking at the shear end face can be reduced.

[0018] While not intended to be bound by any particular theory, hydrogen embrittlement cracking generally tends to occur at points of stress concentration, and therefore is likely to occur at the point where the tensile residual stress is maximum within the shear end face. Consequently, in order to improve the hydrogen embrittlement resistance at the shear end face, it is considered important to sufficiently reduce the maximum value of the tensile residual stress at that shear end face. Therefore, in the high-strength steel material according to the embodiment of the present invention having a Vickers hardness of 450 Hv or more, it is important to control the maximum value of the tensile residual stress at the shear end face to 1300 MPa or less, thereby making it possible to suppress or reduce the occurrence of hydrogen embrittlement cracking at that shear end face.

[0019] However, subsequent studies by the inventors revealed that in high-strength steel materials having a Vickers hardness of 450 Hv or more, simply controlling the maximum tensile residual stress at the shear end face to 1300 MPa or less may not be sufficient to adequately suppress or reduce the occurrence of hydrogen embrittlement cracks at the shear end face. Therefore, the inventors conducted further studies focusing on the relationship between the location on the shear end face where the tensile residual stress is maximum and the occurrence of hydrogen embrittlement cracks. As a result, the inventors have found that by controlling the maximum tensile residual stress measured at the 1 / 4, 2 / 4, and 3 / 4 positions of the plate thickness on the shear end face so that it does not equal the tensile residual stress measured at the 2 / 4 position of the plate thickness on the shear end face (hereinafter, the region from the 2 / 4 position or the 3 / 8 to 5 / 8 position of the plate thickness may also be referred to as the center of the plate thickness), that is, by controlling it so that it equals the tensile residual stress measured at the 1 / 4 or 3 / 4 position of the plate thickness, stress concentration at specific locations on the shear end face can be reduced more significantly, thereby further improving the hydrogen embrittlement resistance at the shear end face.

[0020] While not intended to be bound by any particular theory, the following reasons can be considered for why controlling the maximum tensile residual stress so that it does not equal the tensile residual stress measured at the 2 / 4 position of the plate thickness on the shear end face further improves hydrogen embrittlement resistance. First, as steel materials become stronger, they may contain relatively large amounts of alloying elements such as Mn, Al, and Ti. These elements tend to cause central segregation during slab casting, and it is thought that they ultimately form a relatively large number of inclusions such as sulfides and oxides in the center of the plate thickness. Therefore, when such high-strength steel is sheared, it is thought that a relatively large number of inclusions will be exposed in the center of the plate thickness on the shear end face. Here, since inclusions are, so to speak, defects in the steel, they can become the starting point of stress concentration. Therefore, if there is a relatively high tensile residual stress in the center of the plate thickness, stress concentration in the center of the plate thickness will be promoted due to both the relatively high tensile residual stress and the inclusions, and the hydrogen embrittlement resistance at the shear end face will be greatly reduced. Therefore, from the viewpoint of improving hydrogen embrittlement resistance at the shear end face, it is preferable to sufficiently reduce tensile residual stress in the central part of the plate thickness where the number of inclusions exposed at the shear end face is relatively large.

[0021] From this perspective, in the steel material according to the embodiment of the present invention, it is important to control the maximum tensile residual stress measured at the 1 / 4, 2 / 4, and 3 / 4 positions of the plate thickness on the shear end face so that it does not equal the tensile residual stress measured at the 2 / 4 position of the plate thickness on the shear end face, that is, to sufficiently reduce the tensile residual stress in the center of the plate thickness. As a result, it is possible to suppress or reduce the occurrence of hydrogen embrittlement cracks caused by inclusions exposed in the center of the plate thickness on the shear end face, and to further improve the hydrogen embrittlement resistance characteristics at the shear end face. In the steel material according to the embodiment of the present invention, for the reasons mentioned above, by combining controlling the maximum tensile residual stress on the shear end face to 1300 MPa or less with controlling the maximum tensile residual stress so that it does not equal the tensile residual stress measured at the 2 / 4 position of the plate thickness on the shear end face, it is possible to more significantly improve the hydrogen embrittlement resistance characteristics at the shear end face compared to simply controlling the maximum tensile residual stress on the shear end face to 1300 MPa or less. The components of the steel material according to the embodiment of the present invention will be described in more detail below.

[0022] [Steel Material] The steel material according to the embodiment of the present invention encompasses any form having a sheared end face. For example, the steel material according to the embodiment of the present invention may be in the form of a blank material as sheared, or it may be in the form of a part formed from the blank material into a predetermined shape by cold pressing or hot pressing or the like.

[0023] [Vickers Hardness: 450 Hv or more] The steel material according to the embodiment of the present invention has a Vickers hardness of 450 Hv or more. The Vickers hardness is preferably 480 Hv or more, 500 Hv or more, 530 Hv or more, 550 Hv or more, 580 Hv or more, 600 Hv or more, 630 Hv or more, or 650 Hv or more. According to the embodiment of the present invention, even for a high-strength steel material having a Vickers hardness of 450 Hv or more, as described above, the maximum value of the tensile residual stress at the sheared end face is controlled to 1300 MPa or less, and by controlling so that the maximum value of the tensile residual stress does not become the tensile residual stress measured at the 2 / 4 position of the sheet thickness on the sheared end face, the occurrence of hydrogen embrittlement cracking at the sheared end face can be reduced. The upper limit of the Vickers hardness is not particularly limited, and for example, the Vickers hardness may be 800 Hv or less, 780 Hv or less, 750 Hv or less, 730 Hv or less, or 700 Hv or less.

[0024] [Measurement of Vickers Hardness] The Vickers hardness is determined as follows. First, a test piece is cut out from an arbitrary position excluding the end portion of the steel material so that a cross section perpendicular to the surface (thickness cross section) can be observed. The sheet thickness cross section of the test piece is wet-polished using #600 to #1500 silicon carbide paper, and then mirror-finished using a liquid obtained by dispersing diamond powder with a grain size of 1 to 6 μm in a diluent such as alcohol or pure water, and this thickness cross section is taken as the measurement surface. Next, using a micro Vickers hardness tester, the Vickers hardness is measured at a load of 1 kgf at intervals of 3 times or more the indentation. Specifically, a total of 20 points are randomly measured in the vicinity of the 1 / 4 position of the sheet thickness of the steel material, and the arithmetic average thereof is determined as the Vickers hardness of the steel material.

[0025] [Maximum tensile residual stress at the shear end face: 1300 MPa or less] In the steel material according to the embodiment of the present invention, the maximum value of the tensile residual stress at the shear end face is controlled to 1300 MPa or less. By controlling the maximum value of the tensile residual stress at the shear end face to 1300 MPa or less, stress concentration at specific locations on the shear end face can be reduced, thereby improving the hydrogen embrittlement resistance at the shear end face. From the viewpoint of improving hydrogen embrittlement resistance, a lower maximum value of the tensile residual stress at the shear end face is preferable, and may be, for example, 1200 MPa or less, 1100 MPa or less, 1000 MPa or less, 900 MPa or less, 800 MPa or less, or 700 MPa or less. The lower limit is not particularly limited, but for example, the maximum value of the tensile residual stress at the shear end face may be 300 MPa or more, 400 MPa or more, 500 MPa or more, or 600 MPa or more.

[0026] [Measurement of the maximum tensile residual stress at the shear end face] The maximum tensile residual stress at the shear end face is determined as follows. First, when taking a sample by cutting a part or the like, the cutting operation should be performed at a position sufficiently far from the shear end face, for example, at a position 50 mm or more away from the shear end face, so that the cutting operation does not affect the value of the tensile residual stress at the shear end face. Next, at the shear end face of the collected sample, X-rays with a spot diameter of φ500 μm are irradiated at three depth positions centered on the 1 / 4, 2 / 4, and 3 / 4 positions of the plate thickness, and the residual stress at each depth position is measured. Specifically, first, an X-ray stress measuring device (for example, Rigaku Corporation's "AutoMATE II") is used to irradiate the plate at a 1 / 4 position of thickness with X-rays in a 90-degree angular range from a direction parallel to the fracture surface to a direction perpendicular to the fracture surface, under the following measurement conditions, to obtain residual stress in each direction. Then, the maximum principal stress (corresponding to tensile residual stress) is calculated from the three obtained residual stresses. More specifically, the data obtained with the above X-ray stress measuring device is analyzed using Rigaku Corporation's 2θ-sin under the following analysis conditions: stress constant: -318 MPa / degree, Young's modulus: 223.3 GPa, and Poisson's ratio: 0.28. 2Using stress analysis software based on ψ diagrams, residual stresses are obtained in three different directions, and then the maximum principal stress (i.e., tensile residual stress) is calculated from the three obtained residual stresses. Next, similar measurements are performed at the 2 / 4 and 3 / 4 positions of the plate thickness, and the tensile residual stress at each depth position is calculated. Finally, the maximum value among the tensile residual stresses calculated at each depth position is determined as the maximum value of the tensile residual stress at the shear end face. As is clear from the above determination method, the "tensile residual stress" according to the embodiment of the present invention is calculated by measuring a predetermined position in the plate thickness direction at the shear end face with an X-ray stress measuring device. Therefore, the tensile residual stress is not limited to that caused solely by shearing, but also includes stresses added to the shear end face when forming the steel material including the shear end face into a specific part, and stresses added to the shear end face when assembling the steel material including the shear end face into a specific part. [Measurement conditions] X-ray tube: Cr tube Tube voltage: 40kV Tube current: 40mA 2θ: 146-167 degrees Diffraction plane: α-Fe{211} plane (2θ = 156.4 degrees)

[0027] [The maximum value of tensile residual stress is not the tensile residual stress measured at the 2 / 4 position of the sheet thickness on the sheared end face] In the steel material according to the embodiment of the present invention, the maximum value of tensile residual stress is controlled so as not to be the tensile residual stress measured at the 2 / 4 position of the sheet thickness on the sheared end face. As described earlier, with the increase in strength of steel materials, the steel materials may contain relatively large amounts of alloying elements such as Mn, Al and Ti, and these elements tend to cause center segregation during slab casting, and finally can form a relatively large amount of inclusions such as sulfides and oxides at the center portion of the sheet thickness. For this reason, when such a high-strength steel material is sheared, the number of inclusions exposed at the center portion of the sheet thickness on the sheared end face becomes relatively large, which can serve as starting points for stress concentration. Therefore, if a relatively high tensile residual stress exists at the center portion of the sheet thickness, stress concentration toward the center portion of the sheet thickness is promoted due to both the relatively high tensile residual stress and the inclusions, and the hydrogen embrittlement resistance at the sheared end face will be significantly degraded. Therefore, in the steel material according to the embodiment of the present invention, as described above, the maximum value of the tensile residual stress at the sheared end face is controlled to 1300 MPa or less, and by controlling so that the maximum value of the tensile residual stress does not become the tensile residual stress measured at the 2 / 4 position of the sheet thickness on the sheared end face, the stress concentration at a specific location on the sheared end face can be more significantly reduced, thereby making it possible to further improve the hydrogen embrittlement resistance at the sheared end face.

[0028] [Preferred Embodiment of Steel Material] In a preferred embodiment of the present invention, the steel material includes inclusions, and in the region from the surface of the steel material at the 3 / 8 position to the 5 / 8 position of the sheet thickness, the number density of inclusions having an equivalent circular diameter of 0.01 to 3.00 μm is 50.0 pieces / mm 2 or less, and the number density of inclusions having an equivalent circular diameter exceeding 3.00 μm is 5.0 pieces / mm 2 or less, and the average aspect ratio of inclusions having an equivalent circular diameter of 0.01 μm or more in the region from the surface of the steel material at the 3 / 8 position to the 5 / 8 position of the sheet thickness is 6.00 or less.

[0029] As is clear from the above explanation, in terms of the characteristics of the steel material itself, the inclusions contained within the steel material are considered to be an important factor in improving the hydrogen embrittlement resistance at the shear end face. Therefore, the inventors focused on the inclusions contained within the steel material in order to further improve the hydrogen embrittlement resistance at the shear end face of the steel material from the perspective of the characteristics of the steel material itself. To explain in more detail, it is known that corrosion of steel can generally start from inclusions. Hydrogen gas is generated along with such corrosion, and it is thought that some of the generated hydrogen gas penetrates into the steel. For this reason, from the perspective of hydrogen embrittlement, it is thought that if there are a relatively large number of inclusions on the surface that is in contact with the external environment, for example, the shear end face, it will promote hydrogen embrittlement. Also, as mentioned above, since inclusions are defects in the steel, they can also be the starting point of stress concentration. Therefore, for example, even when the same stress is applied, it is thought that the more inclusions there are, the more likely hydrogen embrittlement will occur. Therefore, if there are a relatively large number of inclusions at the shear end face, it is thought that hydrogen embrittlement is further accelerated not only by the intrusion of hydrogen gas generated by corrosion into the steel, but also by the increased susceptibility to hydrogen embrittlement due to such stress concentration.

[0030] Therefore, the inventors considered that reducing the number of inclusions exposed on the sheared end face when steel is sheared would be effective in improving hydrogen embrittlement resistance, and therefore focused their research on the number and morphology of inclusions present in the steel. As a result, the inventors controlled the number density of inclusions contained in the center of the steel plate thickness to within a predetermined range, and controlled the inclusions to have a more equiaxed grain shape. More specifically, they aimed to reduce the number density of inclusions having a circular equivalent diameter of 0.01 to 3.00 μm in the region from the surface of the steel plate to the 3 / 8 to 5 / 8 position of the plate thickness to 50.0 pieces / mm². 2 The following control is applied, and the number density of inclusions with a circular equivalent diameter greater than 3.00 μm is set to 5.0 inclusions / mm². 2By controlling the following parameters and controlling the average aspect ratio of inclusions with a circular diameter of 0.01 μm or more within the same region to 6.00 or less, it has been found that the number of inclusions exposed on the sheared end face when steel is sheared can be sufficiently reduced. As a result, the inventors have found that the hydrogen embrittlement resistance properties at the sheared end face of the steel can be significantly improved, even though the Vickers hardness is high and the steel has a strength of 450 MPa or more.

[0031] In a preferred embodiment of the present invention, the number density and morphology of inclusions are defined in the region from 3 / 8 to 5 / 8 of the plate thickness from the surface (hereinafter, this region may be referred to as the "center of the plate thickness"). The reason for focusing on the center of the plate thickness is explained below. High-strength steel materials of 450 Hv or more, which are the subject of the present invention, contain relatively large amounts of alloying elements such as Mn, Al, and Ti in order to increase their strength. These elements tend to cause central segregation during slab casting, and it is thought that a relatively large amount of inclusions such as sulfides and oxides are ultimately formed in the center of the plate thickness. Therefore, it is thought that many of the inclusions on the shear end face of the steel material will be exposed in the center of the plate thickness. For this reason, controlling the inclusions in the region from 3 / 8 to 5 / 8 of the plate thickness from the surface of the steel material is important.

[0032] Furthermore, it is thought that the larger the size of the inclusions, the greater the amount of hydrogen gas generated due to corrosion and the greater the degree of stress concentration on those inclusions. In addition, when the average aspect ratio of the inclusions is relatively large, that is, when the inclusions have a relatively flat shape, they have a shape that extends particularly in the rolling direction, so compared to when the inclusions have a more equiaxed shape, the frequency with which the inclusions are exposed on the shear end face increases when the steel is sheared, and as a result the area of ​​the inclusions exposed on the shear end face becomes larger. Therefore, in high-strength steel materials with a Vickers hardness of 450 Hv or more, and in which a relatively large amount of alloying elements such as Mn are added, in order to improve the hydrogen embrittlement resistance when sheared, it is extremely important to reduce the number density of inclusions with a specific grain size in the center of the plate thickness and to control the inclusions to have a more equiaxed grain shape.

[0033] Accordingly, as a result of further detailed investigation on the number density of inclusions at the sheet thickness central portion of the steel material, the present inventors found that, in order to improve the hydrogen embrittlement resistance, it is effective to divide the particle sizes of inclusions into two ranges and appropriately control the number density of inclusions in each particle size range. More specifically, the present inventors found that, in the region from 3 / 8 position to 5 / 8 position of the sheet thickness from the surface of the steel material, the number density of inclusions having a relatively small particle size, that is, inclusions having an equivalent circle diameter of 0.01 to 3.00 μm, is set to 50.0 pieces / mm 2 or less, and in the same region, the number density of inclusions having a relatively large particle size, that is, inclusions having an equivalent circle diameter of more than 3.00 μm, is set to 5.0 pieces / mm 2 or less is effective. While not intending to be bound by any particular theory, inclusions having an equivalent circle diameter of 0.01 to 3.00 μm are considered to mainly contribute to the generation of hydrogen gas due to corrosion and the penetration of the hydrogen gas into the steel. On the other hand, inclusions having an equivalent circle diameter of more than 3.00 μm, although also contributing to the generation of hydrogen gas and penetration into the steel, are considered to mainly act as starting points for stress concentration. Therefore, in a preferred embodiment of the steel material, it is not sufficient to only appropriately control the number density of one of the inclusions having an equivalent circle diameter of 0.01 to 3.00 μm and the inclusions having an equivalent circle diameter of more than 3.00 μm, and it is necessary to set the number densities of both of them to 50.0 pieces / mm 2 or less and 5.0 pieces / mm 2 or less, respectively, which is important for obtaining more excellent hydrogen embrittlement resistance. Regarding inclusions having an equivalent circle diameter of less than 0.01 μm, they do not necessarily adversely affect the hydrogen embrittlement resistance from the viewpoint of focusing on the shear end face. For this reason, in the steel material according to the embodiment of the present invention, the number density of fine inclusions having an equivalent circle diameter of less than 0.01 μm is not particularly controlled.

[0034] However, it was found that simply controlling the equivalent circular diameter of the inclusions appropriately is not sufficient to adequately reduce the number of inclusions exposed on the shear end face during shearing, especially when the inclusions have a flatter shape. Therefore, in a preferred embodiment of the steel material, in addition to controlling the number density of inclusions in the center of the plate thickness, it is extremely important to control the inclusions in the center of the plate thickness to a more equiaxed shape, and more specifically, to control the average aspect ratio of inclusions with an equivalent circular diameter of 0.01 μm or more in the region from the surface of the steel material to the 3 / 8 to 5 / 8 position of the plate thickness to 6.00 or less, thereby reliably achieving superior hydrogen embrittlement resistance. The fact that superior hydrogen embrittlement resistance can be achieved by controlling the tensile residual stress at the shear end face, dividing the particle size of inclusions in the center of the steel plate thickness into two ranges, appropriately controlling the number density of inclusions in each particle size range, and controlling the inclusions in the center of the plate thickness to a more equiaxed shape is not previously known and has been revealed for the first time by the present inventors. The following describes in more detail each component of the preferred embodiment of the steel material.

[0035] [Number density of inclusions with a circular equivalent diameter of 0.01 to 3.00 μm: 50.0 pieces / mm] 2 The following are the number densities of inclusions having a circular equivalent diameter greater than 3.00 μm: 5.0 pieces / mm 2 [The following] In the preferred embodiment, the steel material contains inclusions, and in the region from the surface of the steel material at a position 3 / 8 to 5 / 8 of the plate thickness, the number density of inclusions having a circular equivalent diameter of 0.01 to 3.00 μm is 50.0 pieces / mm². 2 The following conditions are met: The number density of inclusions with a circular equivalent diameter greater than 3.00 μm in the same region is 5.0 inclusions / mm². 2The following control is achieved: By controlling the number density of inclusions in the region from 3 / 8 to 5 / 8 of the plate thickness from the surface of the steel material to within the above range, it is possible to suppress the generation of hydrogen gas at the shear end face and its intrusion into the steel, as well as reduce stress concentration at the shear end face. As a result, it becomes possible to more significantly suppress the occurrence of hydrogen embrittlement cracks at the shear end face of the steel material. The inclusions include at least one of sulfides and oxides, and are not particularly limited, but may include, for example, MnS and Al2O3.

[0036] From the viewpoint of further improving hydrogen embrittlement resistance, it is preferable that the number density of inclusions having a circular equivalent diameter of 0.01 to 3.00 μm in the region from the surface of the steel material to the 3 / 8 to 5 / 8 position of the plate thickness be as low as possible, for example, 45.0 inclusions / mm². 2 Below, 40.0 pieces / mm 2 Below, 35.0 pieces / mm 2 Below, 30.0 pieces / mm 2 The following, or 25.0 pieces / mm 2 The following is also acceptable: The lower limit of the number density of inclusions having a circular equivalent diameter of 0.01 to 3.00 μm is not particularly limited, and is 0 inclusions / mm². 2 This may also be the case. For example, the number density of inclusions having a circular equivalent diameter of 0.01 to 3.00 μm is 3.0 inclusions / mm². 2 Above, 5.0 pieces / mm 2 Above, 8.0 pieces / mm 2 Above, 10.0 pieces / mm 2 Above, or 15.0 pieces / mm 2 The above is also acceptable. As mentioned earlier, inclusions having a circular diameter of less than 0.01 μm do not necessarily have a negative impact on hydrogen embrittlement resistance. Therefore, in the steel material according to the preferred embodiment, there is no need to particularly control the number density of such minute inclusions.

[0037] Similarly, from the viewpoint of further improving hydrogen embrittlement resistance, it is preferable that the number density of inclusions having a circular equivalent diameter greater than 3.00 μm in the region from the surface of the steel material to the 3 / 8 to 5 / 8 position of the plate thickness be as low as possible, for example, 4.5 inclusions / mm². 2 Below, 4.0 pieces / mm 2 Below, 3.5 pieces / mm2 The following, or 3.0 pieces / mm 2 The following is also acceptable: The lower limit of the number density of inclusions having a circular equivalent diameter greater than 3.00 μm is not particularly limited, and is 0 inclusions / mm². 2 This may also be the case. For example, the number density of inclusions having a circular equivalent diameter greater than 3.00 μm is 0.5 inclusions / mm². 2 Above, 1.0 pieces / mm 2 Above, 1.5 pieces / mm 2 More than or equal to 2.0 pieces / mm 2 That's fine too.

[0038] [Average aspect ratio of inclusions with a circular equivalent diameter of 0.01 μm or more: 6.00 or less] In the steel material according to the preferred embodiment, the average aspect ratio of inclusions with a circular equivalent diameter of 0.01 μm or more in the region from 3 / 8 to 5 / 8 of the plate thickness from the surface of the steel material is controlled to 6.00 or less. By controlling the average aspect ratio of inclusions with a circular equivalent diameter of 0.01 μm or more in this region to 6.00 or less, that is, by controlling the inclusions to a more equiaxed grain shape, the frequency of inclusions being exposed on the shear end surface when the steel material is sheared can be reduced compared to the case where the average aspect ratio of inclusions with a circular equivalent diameter of 0.01 μm or more is relatively large, that is, when the inclusions have a relatively flat shape, and thereby the area of ​​inclusions exposed on the shear end surface can be sufficiently reduced. As a result, it becomes possible to more significantly suppress the occurrence of hydrogen embrittlement cracks on the shear end surface.

[0039] From the viewpoint of further improving hydrogen embrittlement resistance, it is preferable that the average aspect ratio of inclusions having a circular diameter of 0.01 μm or more in the region from 3 / 8 to 5 / 8 of the plate thickness from the surface of the steel material be small, for example, 5.50 or less, 5.00 or less, 4.50 or less, 4.00 or less, 3.50 or less, 3.00 or less, or 2.50 or less. The lower limit is not particularly limited, but for example, the average aspect ratio of the inclusions may be 1.00 or more, 1.20 or more, or 1.50 or more.

[0040] [Method for Determining Number Density and Average Aspect Ratio of Inclusions] In this specification, the inclusions whose number density and average aspect ratio are to be determined are not limited to compounds that crystallize in the liquid phase, commonly referred to as "inclusions," but also include compounds that precipitate in the solid phase, so-called "precipitates." The number density and average aspect ratio of inclusions are determined as follows. First, a sample with a thickness cross section in a direction parallel to the rolling direction is taken. If the rolling direction is unknown, the thickness cross section is observed at 0°, 45°, 90°, and 135° to any direction, and the cross section with the highest aspect ratio of inclusions is identified as the thickness cross section parallel to the rolling direction. Next, the thickness cross section is polished using diamond paste (particle size 1 μm), and then an automatic inclusion analyzer is used to measure an area of ​​8 mm² within the range of 3 / 8 to 5 / 8 of the thickness from the surface. 2The field of view is set and the analysis is performed accordingly. As an automated inclusion analyzer, for example, MQA (Metal Quality Analyzer: a registered trademark of ASPEX) is used, and inclusions specified in JIS G 0555:2023 are measured using MQA analysis software (MQA-Metals Cleanliness Rating: MQA-MCR®). Next, among the inclusions that contain at least one of Mn, Fe, Ti, and Al, and at least one of S and O, as measured by elemental analysis of the automated inclusion analyzer, the number densities of inclusions with a circular equivalent diameter of 0.01 to 3.00 μm and inclusions with a circular equivalent diameter greater than 3.00 μm are calculated, and these are determined as the number density of inclusions with a circular equivalent diameter of 0.01 to 3.00 μm and the number density of inclusions with a circular equivalent diameter greater than 3.00 μm. If the MQA detection limit is greater than 0.01 μm, for example, if the MQA detection limit is approximately 0.10 μm, then inclusions with a circular diameter from the detection limit up to 3.00 μm are determined as "inclusions with a circular diameter of 0.01 to 3.00 μm". For inclusions containing at least one of Mn, Fe, Ti, and Al, and at least one of S and O, even if inclusions of 0.01 to 0.10 μm exist, their proportion is negligibly small compared to inclusions with a circular diameter of 0.10 to 3.00 μm. Therefore, even if the MQA detection limit is approximately 0.10 μm, it is possible to consider inclusions with a circular diameter from the detection limit up to 3.00 μm as "inclusions with a circular diameter of 0.01 to 3.00 μm". In addition, the area and aspect ratio (ratio of the major axis to the minor axis of each inclusion, major axis / minor axis) of all measured inclusions with a circular equivalent diameter of 0.01 μm or more are calculated, and based on the calculated area and aspect ratio, the average aspect ratio of inclusions with a circular equivalent diameter of 0.01 μm or more in the region from 3 / 8 to 5 / 8 of the plate thickness from the surface is determined by the following formula: Average aspect ratio = Σ (area of ​​inclusion × aspect ratio of inclusion) / Σ (area of ​​inclusion) In general steel manufacturing processes, inclusions are mainly formed in the steel material by the time casting is completed, and these are stretched or crushed by rolling, resulting in the inclusions present in the final steel material.Therefore, in the steel material according to the embodiment of the present invention, the major axis of the inclusion is the length in the direction parallel to the rolling direction, or, if the rolling direction is unknown, the length in the direction parallel to the rolling direction determined by the above method, and the minor axis is the length in the direction parallel to the plate thickness direction.

[0041] [At least one of Mg and Ca: 0.005 to 0.050% in total] In a more preferred embodiment of the present invention, the steel material has a chemical composition comprising, in mass%, at least one of Mg and Ca: 0.005 to 0.050% in total. Mg and Ca are effective elements for controlling the morphology of inclusions. Therefore, by making the total amount of at least one of Mg and Ca 0.005% by mass or more, the number of inclusions exposed on the shear end face when the steel material is sheared can be sufficiently reduced. As a result, it becomes possible to more significantly suppress the occurrence of hydrogen embrittlement cracks on the shear end face. From the viewpoint of further improving hydrogen embrittlement resistance, a higher total content of Mg and Ca is preferable, for example, 0.008% or more, 0.010% or more, 0.015% or more, 0.020% or more, or 0.025% or more by mass%. On the other hand, excessive amounts of these elements lead to saturation of the effect, and including more than necessary in the steel material increases manufacturing costs. Therefore, the total content of Mg and Ca is preferably 0.050% or less by mass, and may be, for example, 0.045% or less, 0.040% or less, 0.035% or less, or 0.030% or less.

[0042] [Preferred Chemical Composition of Steel] As described above, the present invention aims to provide a steel material with improved hydrogen embrittlement resistance at the shear end face. This objective is achieved by controlling the maximum tensile residual stress at the shear end face to 1300 MPa or less in a steel material having a Vickers hardness of 450 Hv or more, and by controlling the maximum tensile residual stress so that it does not equal the tensile residual stress measured at the 2 / 4 position of the plate thickness at the shear end face. Therefore, it is clear that the chemical composition of the steel material itself is not an essential technical feature for achieving the objective of the present invention. In addition to containing 0.0005 to 0.0500% by mass of at least one of Mg and Ca as described above, or by substituting therefor, the chemical composition of the steel material may contain in appropriate amounts any alloying elements commonly added in the art of the present invention. The following describes preferred chemical compositions for steel materials in detail. However, these descriptions are merely examples of preferred chemical compositions for obtaining steel materials having, for example, a Vickers hardness of 450 Hv or higher and a metallic structure such as martensite, which will be described later, in a desired area ratio. The present invention is not intended to be limited to steel materials having such specific chemical compositions. Furthermore, in the following descriptions, "%", which is the unit for the content of each element, means "mass percent" unless otherwise specified. In addition, in this specification, "~" indicating a numerical range means that the numbers written before and after it are included as the lower and upper limits, respectively, unless otherwise specified.

[0043] In a specific embodiment of the present invention, for example, the steel material has the following composition in mass%, C: 0.15-0.60%, Si: 0.01-1.30%, Mn: 1.00-3.50%, P: 0.0200% or less, S: 0.0200% or less, Al: 0.001-1.00%, N: 0.0200% or less, O: 0.0200% or less, Co: 0-0.50%, Ni: 0-1.00%, Mo: 0-1.00%, Cr: 0-2.00%, Ti: 0-0.5000%, B: 0-0.0100%, Nb: 0-0.500%, V: 0-0.500%, Cu: 0-1.000%, W: 0-0.100% Preferably, the chemical composition consists of Ta: 0-0.100%, Sn: 0-0.100%, Sb: 0-0.500%, As: 0-0.100%, Mg: 0-0.050%, Ca: 0-0.050%, Y: 0-0.050%, Zr: 0-0.500%, La: 0-0.050%, Ce: 0-0.050%, and the remainder being Fe and impurities. Each element will be described in more detail below.

[0044] [C: 0.15-0.60%] Carbon (C) is an effective element for increasing tensile strength at a low cost. It is also an effective element for stabilizing austenite. To obtain these effects fully, the C content should be 0.15% or more. The C content may be 0.18% or more, 0.20% or more, 0.22% or more, 0.25% or more, 0.28% or more, 0.30% or more, or 0.32% or more. On the other hand, excessive C content may reduce weldability. For this reason, the C content should be 0.60% or less. The C content may be 0.55% or less, 0.50% or less, 0.48% or less, 0.45% or less, 0.42% or less, 0.40% or less, or 0.38% or less.

[0045] [Si: 0.01-1.30%] Si is an element that acts as a deoxidizing agent. The presence of Si can suppress the formation of coarse oxides. To obtain this effect sufficiently, the Si content should be 0.01% or more. The Si content may be 0.05% or more, 0.10% or more, 0.30% or more, or 0.50% or more. On the other hand, if the Si content is excessive, the local ductility may decrease. For this reason, the Si content should be 1.30% or less. The Si content may be 1.20% or less, 1.00% or less, 0.80% or less, or 0.60% or less.

[0046] [Mn: 1.00–3.50%] Mn is an effective element for increasing the hardenability of steel and raising its strength. Mn is also an effective element for stabilizing austenite. To obtain these effects fully, the Mn content should be 1.00% or more. The Mn content may be 1.20% or more, 1.50% or more, 1.80% or more, 2.00% or more, 2.20% or more, 2.40% or more, or 2.50% or more. On the other hand, if Mn is present in excess, it may not only promote co-segregation with P and S, but also degrade corrosion resistance. For this reason, the Mn content should be 3.50% or less. The Mn content may be 3.20% or less, 3.00% or less, 2.80% or less, or 2.60% or less.

[0047] [P: 0.0200% or less] P is an element that causes embrittlement of welds and deterioration of plating properties. For this reason, the P content should be 0.0200% or less. The P content may also be 0.0180% or less, 0.0150% or less, 0.0120% or less, or 0.0100% or less. A lower P content is preferable, and it may even be 0%, but excessively reducing the P content requires a lot of time for refining, leading to a significant increase in costs. For this reason, the P content should be 0.0001% or more. The P content may also be 0.0005% or more, 0.0010% or more, or 0.0020% or more.

[0048] [S: 0.0200% or less] S is an element that generates nonmetallic inclusions such as MnS in steel. Excessive S content leads to significant formation of nonmetallic inclusions that become crack initiation points during cold working. For this reason, the S content should be 0.0200% or less. The S content may also be 0.0180% or less, 0.0150% or less, 0.0120% or less, or 0.0100% or less. A lower S content is preferable, and it may even be 0%, but excessively reducing the S content requires a lot of time in refining, leading to a significant increase in costs. For this reason, the S content should be 0.0001% or more. The S content may also be 0.0005% or more, 0.0010% or more, or 0.0020% or more.

[0049] [Al: 0.001-1.00%] Al is an element that acts as a deoxidizing agent for steel. To obtain this effect sufficiently, the Al content should be 0.001% or more. The Al content may be 0.005% or more, 0.01% or more, 0.02% or more, or 0.05% or more. On the other hand, if Al is present in excess, coarse Al oxide may be formed, which can become the starting point for cracking. For this reason, the Al content should be 1.00% or less. The Al content may be 0.95% or less, 0.90% or less, 0.80% or less, or 0.60% or less.

[0050] [N: 0.0200% or less] N is an element that causes blowholes during welding. Therefore, the N content should be 0.0200% or less. The N content may be 0.0180% or less, 0.0160% or less, 0.0120% or less, or 0.0100% or less. On the other hand, a lower N content is preferable and may be 0%, but excessively reducing the N content will lead to a significant increase in manufacturing costs. For this reason, the N content should be 0.0001% or more. The N content may be 0.0005% or more, 0.0010% or more, or 0.0020% or more.

[0051] [O: 0.0200% or less] O is an element that causes blowholes during welding. Therefore, the O content should be 0.0200% or less. The O content may also be 0.0180% or less, 0.0150% or less, 0.0120% or less, or 0.0100% or less. On the other hand, a lower O content is preferable and may be 0%, but excessively reducing the O content will lead to a significant increase in manufacturing costs. For this reason, the O content should be 0.0001% or more. The O content may also be 0.0005% or more, 0.0010% or more, or 0.0015% or more.

[0052] The basic chemical composition of the steel material according to the preferred embodiment is as described above. Furthermore, the steel material may optionally contain at least one of the following optional elements in place of a portion of the remaining Fe. For example, the steel material may contain at least one selected from the group consisting of Co: 0-0.50%, Ni: 0-1.00%, Mo: 0-1.00%, Cr: 0-2.000%, Ti: 0-0.500%, B: 0-0.0100%, Nb: 0-0.500%, V: 0-0.500%, Cu: 0-1.000%, W: 0-0.100%, and Ta: 0-0.100%. Alternatively, the steel material may contain at least one selected from the group consisting of Sn: 0-0.100%, Sb: 0-0.500%, and As: 0-0.100%. Furthermore, the steel material may contain at least one element selected from the group consisting of Mg: 0-0.0400%, Ca: 0-0.0400%, Y: 0-0.050%, Zr: 0-0.500%, La: 0-0.050%, and Ce: 0-0.050%. These optional elements will be described in detail below.

[0053] [Co: 0-0.50%] Co is an effective element for controlling the morphology of carbides and increasing the strength of steel. The Co content may be 0%, but to obtain these effects, it is preferable that the Co content be 0.001% or more. The Co content may be 0.01% or more, 0.02% or more, 0.05% or more, or 0.10% or more. On the other hand, if the Co content is excessive, coarse Co carbides may precipitate. For this reason, it is preferable that the Co content be 0.50% or less. The Co content may be 0.40% or less, 0.30% or less, or 0.20% or less.

[0054] [Ni: 0-1.00%] Ni is an element effective in increasing the strength of steel materials. Ni is also an element that is effective in improving wettability and promoting alloying reactions. The Ni content may be 0%, but in order to obtain these effects, it is preferable that the Ni content be 0.001% or more. The Ni content may be 0.01% or more, 0.02% or more, 0.05% or more, or 0.10% or more. On the other hand, if the Ni content is excessive, the weldability may decrease. For this reason, it is preferable that the Ni content be 1.00% or less. The Ni content may be 0.90% or less, 0.80% or less, 0.60% or less, or 0.30% or less.

[0055] [Mo: 0-1.00%] Mo is an element effective in increasing the strength of steel. Mo is also an element that has the effect of suppressing ferrite transformation that occurs during heat treatment in continuous annealing equipment or continuous hot-dip galvanizing equipment. The Mo content may be 0%, but in order to obtain these effects, the Mo content is preferably 0.001% or more. The Mo content may be 0.01% or more, 0.02% or more, 0.05% or more, or 0.08% or more. On the other hand, if the Mo content is excessive, the effect of suppressing ferrite transformation may saturate, or coarse intermetallic compounds and carbides may be formed. For this reason, the Mo content is preferably 1.00% or less. The Mo content may be 0.90% or less, 0.80% or less, 0.60% or less, or 0.30% or less.

[0056] [Cr: 0-2.00%] Cr is an element that suppresses pearlite transformation and is effective in increasing the strength of steel. The Cr content may be 0%, but to obtain such an effect, it is preferable that the Cr content be 0.001% or more. The Cr content may be 0.005% or more, 0.01% or more, 0.02% or more, or 0.05% or more. On the other hand, if the Cr content is excessive, coarse Cr carbides may be formed in the central segregation area. For this reason, it is preferable that the Cr content be 2.00% or less. The Cr content may be 1.80% or less, 1.50% or less, 1.00% or less, or 0.50% or less.

[0057] [Ti: 0-0.5000%] Ti is an element that contributes to increasing the strength of steel materials through precipitate strengthening, fine grain strengthening by suppressing the growth of ferrite crystal grains, and dislocation strengthening through the suppression of recrystallization. The Ti content may be 0%, but in order to obtain these effects, the Ti content is preferably 0.0010% or more. The Ti content may be 0.0050% or more, 0.0100% or more, or 0.0500% or more. On the other hand, if the Ti content is excessive, the precipitation of carbonitrides may increase. For this reason, the Ti content is preferably 0.5000% or less. The Ti content may be 0.4500% or less, 0.4000% or less, 0.3000% or less, or 0.1000% or less.

[0058] [B: 0-0.0100%] B is an element that suppresses the formation of ferrite and pearlite during the cooling process from the austenite temperature range and promotes the formation of low-temperature transformation structures such as martensite. B is also an element beneficial for increasing the strength of steel. The B content may be 0%, but to obtain these effects, it is preferable that the B content be 0.0001% or more. The B content may be 0.0003% or more, 0.0005% or more, or 0.0010% or more. On the other hand, if B is present in excess, coarse B oxides may be formed in the steel. For this reason, it is preferable that the B content be 0.0100% or less. The B content may be 0.0080% or less, 0.0060% or less, 0.0050% or less, or 0.0020% or less.

[0059] [Nb: 0-0.500%] Nb is an element effective in controlling the morphology of carbides and is also effective in improving toughness by refining the structure. The Nb content may be 0%, but to obtain these effects, it is preferable that the Nb content be 0.001% or more. The Nb content may be 0.002% or more, 0.010% or more, or 0.020% or more. On the other hand, if the Nb content is excessive, coarse Nb carbides may be formed. For this reason, it is preferable that the Nb content be 0.500% or less. The Nb content may be 0.450% or less, 0.400% or less, 0.300% or less, or 0.100% or less.

[0060] [V: 0-0.500%] V is an element that contributes to increasing the strength of steel materials through precipitation strengthening, fine grain strengthening by suppressing the growth of ferrite crystal grains, and dislocation strengthening through the suppression of recrystallization. The V content may be 0%, but to obtain these effects, it is preferable that the V content be 0.001% or more. The V content may be 0.002% or more, 0.010% or more, or 0.020% or more. On the other hand, if the V content is excessive, the precipitation of carbonitrides may increase. For this reason, it is preferable that the V content be 0.500% or less. The V content may be 0.450% or less, 0.400% or less, 0.300% or less, or 0.100% or less.

[0061] [Cu: 0-1.000%] Cu is an element effective in improving the strength of steel. The Cu content may be 0%, but to obtain such an effect, it is preferable that the Cu content be 0.001% or more. The Cu content may be 0.002% or more, 0.010% or more, or 0.030% or more. On the other hand, if the Cu content is excessive, the steel may become brittle during hot rolling, making hot rolling difficult. For this reason, it is preferable that the Cu content be 1.000% or less. The Cu content may be 0.800% or less, 0.600% or less, 0.500% or less, 0.450% or less, 0.400% or less, 0.300% or less, or 0.100% or less.

[0062] [W: 0-0.100%] W is an element effective in increasing the strength of steel. W also forms precipitates and crystals. Since precipitates and crystals containing W become hydrogen trapping sites, W is an element effective in further improving hydrogen embrittlement resistance. The W content may be 0%, but to obtain these effects, it is preferable that the W content be 0.001% or more. The W content may be 0.002% or more, 0.005% or more, or 0.010% or more. On the other hand, if W is included in excess, coarse W precipitates or crystals may be formed. For this reason, it is preferable that the W content be 0.100% or less. The W content may be 0.080% or less, 0.060% or less, 0.050% or less, or 0.030% or less.

[0063] [Ta: 0-0.100%] Ta is an element effective in controlling the morphology of carbides and increasing the strength of steel. The Ta content may be 0%, but to obtain these effects, it is preferable that the Ta content be 0.001% or more. The Ta content may be 0.002% or more, 0.005% or more, or 0.010% or more. On the other hand, if the Ta content is excessive, many fine Ta carbides may precipitate, and the ductility may decrease as the strength of the steel increases. For this reason, it is preferable that the Ta content be 0.100% or less. The Ta content may be 0.080% or less, 0.060% or less, 0.050% or less, or 0.020% or less.

[0064] [Sn: 0-0.100%] Sn is an element contained in steel when scrap is used as a raw material for steel. If the Sn content is high, it may cause embrittlement of ferrite. For this reason, the Sn content is preferably 0.100% or less. The Sn content may be 0.060% or less, 0.030% or less, or 0.020% or less. The lower the Sn content, the better, and it may even be 0%, but reducing the Sn content to less than 0.001% requires a lot of time for refining, leading to a significant increase in costs. For this reason, the Sn content may be 0.001% or more. The Sn content may be 0.002% or more, 0.005% or more, or 0.010% or more.

[0065] [Sb: 0-0.500%] Sb, like Sn, is an element that is present when scrap is used as a raw material for steel. Sb is also an element that causes a decrease in ductility. For this reason, the Sb content is preferably 0.500% or less. The Sb content may be 0.300% or less, 0.100% or less, 0.080% or less, 0.060% or less, 0.040% or less, 0.030% or less, or 0.020% or less. The lower the Sb content, the better, and it may even be 0%, but reducing the Sb content to less than 0.001% requires a lot of time for refining, leading to a significant increase in costs. For this reason, the Sb content may be 0.001% or more. The Sb content may be 0.002% or more, 0.005% or more, or 0.008% or more.

[0066] [As: 0-0.100%] As, like Sn and Sb, is an element that is present when scrap is used as a raw material for steel. As is also an element that causes a decrease in ductility. For this reason, the As content is preferably 0.100% or less. The As content may be 0.040% or less, 0.030% or less, or 0.020% or less. The lower the As content, the better, and it may even be 0%, but reducing the As content to less than 0.001% requires a lot of time for refining, leading to a significant increase in costs. For this reason, the As content may be 0.001% or more. The As content may be 0.002% or more, 0.003% or more, or 0.005% or more.

[0067] [Mg: 0-0.050%] Mg is an element whose presence in trace amounts can control the morphology of inclusions. The Mg content may be 0%, but to obtain such an effect, it is preferable that the Mg content be 0.0001% or more. The Mg content may be 0.0005% or more, 0.001% or more, 0.005% or more, 0.010% or more, 0.015% or more, or 0.020% or more. On the other hand, if Mg is included in excess, the effect will saturate, and including more Mg in the steel than necessary will lead to an increase in manufacturing costs. For this reason, it is preferable that the Mg content be 0.050% or less. The Mg content may be 0.045% or less, 0.040% or less, 0.035% or less, 0.030% or less, or 0.025% or less.

[0068] [Ca: 0-0.050%] Ca is a useful deoxidizing element and is also effective in controlling the morphology of inclusions. The Ca content may be 0%, but to obtain these effects, it is preferable that the Ca content be 0.0001% or more. The Ca content may be 0.0005% or more, 0.001% or more, 0.005% or more, 0.010% or more, 0.015% or more, or 0.020% or more. On the other hand, if Ca is included in excess, the effect will saturate, and including more Ca in the steel than necessary will lead to an increase in manufacturing costs. For this reason, it is preferable that the Ca content be 0.050% or less. The Ca content may be 0.045% or less, 0.040% or less, 0.035% or less, 0.030% or less, or 0.025% or less.

[0069] [Y: 0-0.050%] [Zr: 0-0.500%] [La: 0-0.050%] [Ce: 0-0.050%] Y, Zr, La, and Ce are elements whose presence in trace amounts can control the form of sulfides. The content of Y, Zr, La, and Ce may be 0%, but in order to obtain this effect, the content of Y, Zr, La, and Ce is preferably 0.0001% or more, and may be 0.0005% or more, 0.001% or more, or 0.002% or more. On the other hand, if these elements are present in excess, coarse oxides may be formed. For this reason, the content of Y, La, and Ce is preferably 0.050% or less, and may be 0.030% or less, 0.010% or less, 0.005% or less, or 0.003% or less. Similarly, the Zr content is preferably 0.500% or less, but may be 0.300% or less, 0.100% or less, 0.050% or less, 0.030% or less, 0.010% or less, 0.005% or less, or 0.003% or less.

[0070] In the steel material according to the preferred embodiment, the remainder other than the above-mentioned elements consists of Fe and impurities. Impurities are components that are mixed in during the industrial production of steel material due to various factors in the manufacturing process, including raw materials such as ore and scrap, and components that are included in a range that does not affect the effects of the present invention.

[0071] The chemical composition of the steel material according to the preferred embodiment can be measured by general analytical methods. For example, the chemical composition of the steel material can be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) on chips in accordance with JIS G 1201:2014. Specifically, a 35 mm square test piece can be taken from an arbitrary position at least 50 mm away from the shear end face (if a test piece cannot be taken from this position, a position avoiding the shear end face), for example, at the 1 / 4 position of the plate thickness, and measured using a Shimadzu ICPS-8100 or similar (measuring device) under conditions based on a pre-established calibration curve. C and S, which cannot be measured by ICP-AES, can be measured using the combustion-infrared absorption method, N can be measured using the inert gas fusion-thermal conductivity method, and O can be measured using the inert gas fusion-nondispersive infrared absorption method. If the steel material has a coating layer on its surface, the coating layer should be removed by mechanical grinding or the like before the chemical composition analysis can be performed.

[0072] [Preferred Metallurgical Structure of Steel] The metallurgical structure of the steel according to the preferred embodiment may be any metallurgical structure that satisfies the requirement of a Vickers hardness of 450 Hv or more. More specifically, as described above, the present invention aims to provide a steel material with improved hydrogen embrittlement resistance at the shear end face, and achieves this objective by controlling the maximum value of the tensile residual stress at the shear end face to 1300 MPa or less in a steel material having a Vickers hardness of 450 Hv or more, and by controlling so that the maximum value of the tensile residual stress does not become the tensile residual stress measured at the 2 / 4 position of the plate thickness at the shear end face. Therefore, it is clear that the metallurgical structure of the steel material itself is not an essential technical feature for achieving the objective of the present invention. The preferred metallurgical structures for steel materials having a Vickers hardness of 450 Hv or more will be described in detail below, but these descriptions are intended to be merely examples of preferred metallurgical structures for obtaining steel materials having a Vickers hardness of 450 Hv or more, and are not intended to limit the present invention to steel materials having such specific metallurgical structures. In the following description, the unit "%" for tissue fraction means "area %" unless otherwise specified. Furthermore, in the steel material according to the preferred embodiment, the metallographic structure is defined at a position 1 / 4 of the plate thickness in a flat area at least 50 mm away from the shear end face (or a position avoiding the shear end face if a sample cannot be taken from this position). If the surface of the steel material is provided with a coating layer such as plating, the position in the thickness direction shall be specified for the area excluding the coating layer.

[0073] [Martensite: 85.0% or more] In the steel material according to the preferred embodiment, the metal structure may contain 85.0% or more martensite by area percentage. Including 85.0% or more martensite makes it possible to more reliably achieve a Vickers hardness of 450 Hv or more. From the viewpoint of increasing strength, a higher area percentage of martensite is preferable, and may be, for example, 87.0% or more, 90.0% or more, 92.0% or more, or 95.0% or more. There is no particular upper limit, but for example, the area percentage of martensite may be 100.0% or less, 99.0% or less, or 97.0% or less. In the present invention, "martensite" includes not only as-quenched martensite (so-called fresh martensite) but also tempered martensite.

[0074] [Retained Austenite: 0-7.0%] Retained austenite (also called retained gamma) improves the elongation of steel through the TRIP effect, where it transforms into martensite by work-induced transformation during deformation of the steel. The area ratio of retained austenite may be 0%, but in order to obtain this effect, it is preferable that the metal structure contains retained austenite: 1.0-7.0% in area %. From the viewpoint of improving elongation, a higher area ratio of retained austenite is preferable, for example, it may be 2.0% or more, 3.0% or more, or 4.0% or more. On the other hand, if retained austenite is included in excess, it may not be possible to secure a sufficient area ratio of martensite, and the desired strength may not be achieved. Therefore, the area ratio of retained austenite should be 7.0% or less, for example, 6.0% or less or 5.0% or less.

[0075] [Residual structure: 10.0% or less] The residual structure other than martensite and retained austenite may be 0% in area percentage, but if residual structure is present, it shall be 10.0% or less in area percentage. If residual structure is included in excess, it may become impossible to control the area percentage of martensite within the desired range, and as a result, the desired strength may not be obtained. Therefore, the area percentage of residual structure shall be 10.0% or less, and may be 8.0% or less, 6.0% or less, 5.0% or less, 4.0% or less, 3.0% or less, or 2.0% or less. On the other hand, reducing the area percentage of residual structure to 0% requires a high degree of control in the steel manufacturing process, which may lead to a decrease in yield. Therefore, the area percentage of residual structure may be 0.5% or more or 1.0% or more. The residual structure is not particularly limited, but may include, for example, at least one of ferrite, bainite, and pearlite, or at least one of them.

[0076] [Identification and Calculation of Metal Microstructure] [Martensite] The identification and calculation of the metal microstructure is performed as follows. First, a sample with a plate thickness cross section perpendicular to the plate surface is taken, and this cross section is used as the observation surface. This cross section is polished, taking care not to leave any scratches from polishing. Then, the strain introduced by polishing the surface is removed by chemical polishing to obtain a cross section observation sample for EBSD (Electron-Back-Scatter-Diffraction) analysis in which no change in crystal orientation has occurred due to polishing. Here, scratches in crystal orientation due to polishing refer to regions observed in a straight line penetrating the metal microstructure in the IPF map (Inverse-Pole-Fig) in EBSD analysis. Since such scratches are not inherent to the microstructure, it is naturally necessary to exclude scratches from the observation field of view. It is preferable to find a field of view that does not contain scratches, or if scratches inevitably appear in the field of view, to perform polishing again.

[0077] The above-mentioned EBSD analysis sample is subjected to electron beam backscatter analysis. While the EBSD analysis conditions may be within the realm of common sense for those skilled in the art, a detailed example is described below. After inserting the sample into an EBSD-capable FE-SEM, the polished surface of the sample is tilted 60-70° relative to the electron beam incidence direction. The tilt direction of the sample must be such that the cross-section of the object to be measured faces the EBSD detector that will be inserted later. After tilting, the EBSD detector is inserted into the FE-SEM chamber and brought close to the sample. The position of the EBSD detector should be such that the electron beam detection sensitivity is high, but it is desirable to bring it as close as possible without colliding with the FE-SEM chamber, the sample, or the jig or table holding the sample. Subsequently, the electron beam detection sensitivity of the EBSD detector is adjusted. Since the adjustment of the detection sensitivity depends on the performance of the electron gun and EBSD detector of the FE-SEM used, it is desirable to perform the adjustment within the realm of common sense for those skilled in the art. The adjustment should ultimately result in conditions that allow for clear observation of the EBSD pattern. Subsequently, an electron beam is irradiated onto the observation field at Step Size intervals of 0.3 μm, and EBSD patterns are collected at each measurement point. Based on the EBSD patterns at each measurement point, indexing and crystal orientation calculation are performed. For indexing and crystal orientation calculation, it is desirable to use APEX software from AMETEK. The EBSD data obtained in this way is then analyzed using OIMAnalysis software (Orientation Imaging Microscope) version 7 or later, which is EBSD data analysis software from AMETEK. The obtained EBSD data is opened in OIMAnalysis, and only regions with a CI value (Confidence Index) of 0.1 or higher are extracted. The CI value is an indicator of the reliability of the indexing and crystal orientation analysis results. Areas with a CI value lower than 0.1 are highly likely to be regions where the orientations of contaminants on the sample surface or grain boundaries overlap during electron beam irradiation, and can be considered regions that do not have the original crystal orientation of the metal microstructure. Subsequently, the area ratio of regions with a GAM value (Grain Average Misorientation) of 0.5° or higher is calculated. A grain boundary refers to a boundary line where the orientation difference between measurement points is 15° or more.Here, the GAM value is the average orientation difference between measurement points in the region surrounded by grain boundaries. Microstructures formed at low temperatures, such as martensite, are characterized by the occurrence of orientation differences within grains due to transformation strain, making identification possible using the GAM value. Such investigations are carried out at five or more locations within a 100 x 100 μm field of view centered at a point 1 / 4 of the plate thickness from the steel surface, and the average area ratio is derived.

[0078] Retained austenite is formed at sub-nanometer levels between the martensite laths. Therefore, with the resolution of EBSD analysis, it is difficult to separate the two, and the average area ratio of the region where the GAM value obtained by the method described above is 0.5° or higher is the sum of the area ratio of martensite and retained austenite. For this reason, the area ratio of retained austenite, measured by the procedure described later, is subtracted from the area ratio obtained by the method described above, and the resulting value is determined as the area ratio of martensite at the 1 / 4 thickness position. As described above, the martensite in the metal structure according to the embodiment of the present invention is a structure that includes tempered martensite, and no particular distinction is made regarding the tempered state of the martensite.

[0079] [Retained Austenite] The area fraction of retained austenite is calculated by measurement using X-rays. First, the material from the surface of the sample up to 1 / 4 of the thickness in the thickness direction is removed by mechanical polishing and chemical polishing. Next, the integral intensity ratio of the diffraction peaks of the bcc phase (200) and (211) and the fcc phase (200), (220), and (311) obtained using MoKα rays as characteristic X-rays on the polished sample is used to calculate the structural fraction of retained austenite, and this is determined as the area fraction of retained austenite.

[0080] [Remaining Structure] The area percentage of the remaining structure is determined by subtracting the area percentage of martensite and retained austenite obtained above from 100%. The remaining structure may include, for example, at least one of ferrite, bainite, and pearlite, or at least one of them, but in this invention, the identification of these structural types and the determination of the area percentage of each structural type are not particularly necessary to achieve the objectives of this invention. If there is any need, it is not difficult to identify them using methods commonly applied by those skilled in the art.

[0081] [Plate Thickness] The steel material according to the embodiment of the present invention generally has a plate thickness of 1.0 to 6.0 mm. Although not particularly limited, the plate thickness may be 1.2 mm or more, 1.4 mm or more, or 1.6 mm or more, and / or 5.0 mm or less, 4.0 mm or less, 3.0 mm or less, or 2.5 mm or less.

[0082] [Coating Layer] The steel material according to the embodiment of the present invention may further have a coating layer, such as a plating layer, on at least one surface, preferably both surfaces, for the purpose of improving corrosion resistance, etc. The coating layer is not particularly limited, but includes at least one selected from the group consisting of zinc, aluminum, magnesium, and their alloys. More specifically, the coating layer may be a hot-dip plating layer or an electroplating layer. Examples of hot-dip plating layers include hot-dip galvanized layers, alloyed hot-dip galvanized layers, hot-dip aluminum plating layers, hot-dip Zn-Al alloy plating layers, hot-dip Zn-Al-Mg alloy plating layers, hot-dip Zn-Al-Mg-Si alloy plating layers, etc. Examples of electroplating layers include electroplated zinc plating layers, electroplated Zn-Ni alloy plating layers, etc. Preferably, the coating layer is a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, or an electroplated zinc layer. The amount of coating layer attached is not particularly limited and may be a general amount. As mentioned above, such a coating layer is excluded in determining the tensile strength, chemical composition, and metal structure.

[0083] As described above, the steel material according to the embodiment of the present invention is a high-strength steel material having a Vickers hardness of 450 Hv or more, yet it can achieve excellent hydrogen embrittlement resistance at the shear end face. Therefore, the steel material according to the embodiment of the present invention is particularly useful for parts in technical fields that require a high level of both high strength and hydrogen embrittlement resistance, or for use in such parts, and is especially useful for parts in the automotive field or for use in such parts. In a preferred embodiment, an automotive part (including not only passenger cars but also parts used in large vehicles such as trucks) containing the steel material according to the embodiment of the present invention is provided. Examples of automotive parts include skeletal parts such as front pillars, center pillars, side sills, and cross members, as well as bumpers, and other structural and reinforcing parts that require strength. These parts may be entirely composed of the steel material according to the embodiment of the present invention, or at least a part of them may contain the steel material according to the embodiment of the present invention, and therefore all or at least a part of these parts satisfy the characteristics of the steel material described above. In forming processes such as press forming, the characteristics of the steel material do not change significantly before and after forming in areas of steel that do not come into direct contact with the mold, or in areas where the degree of processing is relatively low even if they come into direct contact with the mold.

[0084] <Method for Manufacturing Steel Materials> Next, preferred methods for manufacturing steel materials according to the embodiments of the present invention will be described. The following description is intended to illustrate characteristic methods for manufacturing steel materials according to the embodiments of the present invention, and is not intended to limit the steel materials to those manufactured by the manufacturing methods described below.

[0085] A steel material according to an embodiment of the present invention, more specifically a steel material having a sheared end face, can be obtained by first manufacturing the steel material itself, and then shearing the manufactured steel material. Below, the shearing process of the steel material will be described in detail, and then the manufacturing of the steel material corresponding to the steel material according to the preferred embodiment described above will be described in detail.

[0086] [Shearing of Steel Materials] [Reduction of Maximum Tensile Residual Stress at Shear End Faces] The maximum value of tensile residual stress at shear end faces correlates with the crack propagation behavior in the plate thickness direction at the shear end face, and can be adjusted to a desired range by controlling the crack propagation behavior from the punch side and the die side. More specifically, as shown in Figure 3, a case will be described in which a part 11 of the workpiece 5 is punched out by a punch 21 and another part 12 of the workpiece 5 is punched out by a die 22. In this case, as shown in Figure 3, if the crack propagates preferentially from the punch 21 side, the tensile residual stress at the shear end face of the part 11 becomes large, while the tensile residual stress at the shear end face of the other part 12 becomes small. In Figure 3, "○" means that the tensile residual stress is small, and "×" means that the tensile residual stress is large. Therefore, by using part 11 as scrap 15 and adopting the other part 12 as a product (steel material 10), it is possible to reduce the maximum value of tensile residual stress at the shear end face of the steel material 10.

[0087] In addition, by changing the degree of stress concentration on the punch 21 side and the die 22 side, crack propagation can be adjusted, thereby reducing the maximum value of tensile residual stress to a desired range. More specifically, by chamfering only the cutting edge of the die 22 (i.e., chamfering only the cutting edge of the die 22 by C2 (mm) in Figure 4) or by making the degree of chamfering of the cutting edge of the die 22 greater than the degree of chamfering of the cutting edge of the punch 21 (i.e., making the chamfer C2 (mm) of the cutting edge of the die 22 greater than the chamfer C1 (mm) of the cutting edge of the punch 21 in Figure 4), stress concentration on the die 22 side becomes less likely, and crack propagation from the punch 21 side can be promoted. As a result, the maximum value of tensile residual stress at the shear end face of the steel material 10 can be sufficiently reduced, for example, to 1300 MPa or less. From the viewpoint of reducing the maximum value of tensile residual stress at the shear end face of the steel material 10, when a chamfer of C2 (mm) is applied only to the cutting edge of the die 22, C2 is preferably 0.5 mm or more, more preferably 0.9 mm or more, and most preferably 1.4 mm or more. Similarly, when the chamfer C2 (mm) of the cutting edge of the die 22 is larger than the chamfer C1 (mm) of the cutting edge of the punch 21, the C2-C1 is preferably 0.5 mm or more, more preferably 0.9 mm or more, and most preferably 1.4 mm or more.

[0088] [Controlling the maximum tensile residual stress to a position other than the 2 / 4 position of the plate thickness at the shear end face] The position where the tensile residual stress is maximum at the shear end face can also be adjusted by controlling the crack propagation behavior in the plate thickness direction at the shear end face. More specifically, by controlling the process so that crack propagation from the punch 21 side is dominant, that is, by chamfering only the cutting edge of the die 22 (i.e., by chamfering only the cutting edge of the die 22 in Figure 4 by C2 (mm)) or by making the degree of chamfering of the cutting edge of the die 22 greater than the degree of chamfering of the cutting edge of the punch 21 (i.e., by making the chamfer C2 (mm) of the cutting edge of the die 22 in Figure 4 greater than the chamfer C1 (mm) of the cutting edge of the punch 21), the stress near the contact point between the punch 21 and the workpiece 5 will be closer to compression. As a result, at the shear end face after cutting, the tensile residual stress is minimized at the 1 / 4 position of the plate thickness near the punch 21, while the tensile residual stress is maximized at the 3 / 4 position of the plate thickness. In other words, it is possible to control the maximum value of the tensile residual stress to a position other than the 2 / 4 position of the plate thickness on the shear end face. However, even with this control, due to the influence of some variation in measured values, the tensile residual stress at the 1 / 4 position of the plate thickness near the punch 21 may be higher than the tensile residual stress at the 3 / 4 position of the plate thickness, meaning that the tensile residual stress at the 1 / 4 position of the plate thickness near the punch 21 may be maximized. On the other hand, if the crack propagation from the punch 21 side and the crack propagation from the die 22 side are roughly equivalent, the tensile residual stress will be maximized at the 2 / 4 position of the plate thickness on the shear end face. Therefore, in order to control the maximum value of the tensile residual stress to a position other than the 2 / 4 position of the plate thickness on the shear end face, it is necessary to control the crack propagation so that it is dominant from the punch 21 side. To more reliably control the maximum value of tensile residual stress to a position other than the 2 / 4 position of the plate thickness at the shear end face, when a C2 (mm) chamfer is applied only to the cutting edge of the die 22, C2 is preferably 0.5 mm or more, more preferably 0.9 mm or more, and most preferably 1.4 mm or more.Similarly, when the chamfer C2 (mm) of the cutting edge of the die 22 is made larger than the chamfer C1 (mm) of the cutting edge of the punch 21, the C2-C1 is preferably 0.5 mm or more, more preferably 0.9 mm or more, and most preferably 1.4 mm or more.

[0089] The clearance between the punch 21 and the die 22 during shearing is not particularly limited and can be appropriately determined according to the material and shape of the workpiece 5. For example, if the workpiece 5 is plate-shaped, the clearance may be 5% or more of the plate thickness, or 25% or less of the plate thickness. Preferably, the clearance is 10-20% of the plate thickness. "Clearance" is defined in accordance with ISO 16630:2009.

[0090] [Manufacturing of the Steel Material Itself] A steel material according to a preferred embodiment can be manufactured by, for example, a casting step of casting molten steel with an adjusted chemical composition to form a slab, a hot rolling step of hot rolling the slab to obtain a hot-rolled steel sheet, a pickling step of pickling the obtained hot-rolled steel sheet, a cold rolling step of cold rolling the pickled hot-rolled steel sheet, an annealing step of annealing the obtained cold-rolled steel sheet, and a cooling step of cooling the annealed cold-rolled steel sheet. Each step will be described in detail below.

[0091] [Casting Process] The conditions for the casting process are not particularly limited. For example, after melting in a blast furnace or electric furnace, various secondary smelting processes may be carried out, and then a slab having the chemical composition described above in relation to steel materials may be cast by methods such as conventional continuous casting or ingot casting. From the viewpoint of productivity, it is preferable to cast the slab using the continuous casting method.

[0092] [Hot Rolling Process] [Slab Heating] First, the cast slab is heated to 1150-1300°C. If the heating temperature is below 1150°C, the deformation resistance during subsequent rolling may increase, potentially leading to a greater load on the equipment. On the other hand, if the heating temperature exceeds 1300°C, the growth of MnS and other elements may occur, making it impossible to control the number density of inclusions, particularly those with a circular diameter exceeding 3.00 μm, within the desired range in the center of the final steel plate thickness. Therefore, the heating temperature should be 1300°C or lower, preferably 1250°C or lower.

[0093] [Rough Rolling] In this method, for example, rough rolling may be performed on a heated slab before finish rolling to adjust the plate thickness. The conditions for rough rolling are not particularly limited, as long as the desired sheet bar dimensions are secured.

[0094] [Finish Rolling] The heated slab, or the slab that has been roughly rolled as needed, is then subjected to finish rolling. In finish rolling, the reduction ratio of at least two of the rolling passes at a rolling temperature of 950 to 1100°C is controlled to 30 to 60%, and the reduction ratio of all rolling passes at a rolling temperature below 950°C is controlled to less than 30%. By performing such finish rolling, the number density of inclusions having a circular diameter equivalent to 0.01 to 3.00 μm is set to 50.0 pieces / mm² in the center of the thickness of the final steel plate. 2 The following control becomes possible, and in addition, the average aspect ratio of inclusions having a circular diameter of 0.01 μm or more at the center of the plate thickness can be controlled to 6.00 or less.

[0095] More specifically, at relatively high rolling temperatures of 950-1100°C, the MnS and other elements constituting the inclusions become relatively harder than the base steel. Therefore, by applying sufficient reduction at a rolling temperature of 950-1100°C, or more specifically, by applying at least two rolling passes with a reduction ratio of 30-60%, stress can be concentrated in the inclusions, allowing them to be sufficiently crushed. As a result, the number density of inclusions with a circular diameter of 0.01-3.00 μm in the center of the final steel plate thickness is 50.0 inclusions / mm².2 The following control becomes possible, and similarly, the average aspect ratio of inclusions having a circular equivalent diameter of 0.01 μm or more at the center of the plate thickness can be controlled to 6.00 or less. If there are zero or one rolling pass with a reduction ratio of 30-60% at a rolling temperature of 950-1100°C, and / or if the reduction ratio of all rolling passes at that rolling temperature is less than 30%, the inclusions cannot be sufficiently crushed because the fracture limit stress of the inclusions is not reached. As a result, it becomes impossible to control the number density of inclusions having a circular equivalent diameter of 0.01-3.00 μm at the center of the final steel plate thickness to the desired level, and / or to control the aspect ratio of the inclusions at the center of the plate thickness to the desired level. Furthermore, if the reduction ratio at a rolling temperature of 950-1100°C exceeds 60%, the deformation resistance during rolling may increase, leading to a greater load on the equipment, and / or deterioration of the plate shape.

[0096] On the other hand, at relatively low rolling temperatures below 950°C, the MnS and other elements constituting the inclusions become relatively softer than the base steel. Therefore, if the reduction ratio exceeds 30% in even one rolling pass at a rolling temperature below 950°C, that is, if the maximum reduction ratio of the rolling passes at a rolling temperature below 950°C exceeds 30%, the inclusions will stretch excessively in the rolling direction, and it may become impossible to control the average aspect ratio of inclusions with a circular diameter of 0.01 μm or more at the center of the final steel plate thickness to 6.00 or less. Accordingly, in this manufacturing method, by applying relatively low reduction in all rolling passes at rolling temperatures below 950°C, or more specifically, by keeping the reduction ratio of all rolling passes below 30%, it is possible to maintain a more equiaxed grain shape without stretching the inclusions in the rolling direction. As a result, the average aspect ratio of inclusions with a circular diameter of 0.01 μm or more at the center of the final steel plate thickness can be controlled to 6.00 or less.

[0097] [Cooling and Winding] Next, the finish-rolled steel sheet is cooled to 600°C or less at an average cooling rate of 20°C / second or more and then wound up. If the average cooling rate is less than 20°C / second or the winding temperature is greater than 600°C, internal oxidation will proceed excessively on the surface of the steel sheet, which may cause defects in the appearance of the final product. For example, an average cooling rate of 25°C / second or more is preferable, and a winding temperature of 580°C or less is preferable. For example, an average cooling rate of 100°C / second or less is preferable, and a winding temperature of 400°C or more is preferable.

[0098] [Pickling Process] Next, the obtained hot-rolled steel sheet is pickled to remove the oxide scale formed on its surface. Pickling can be carried out under conditions suitable for removing the oxide scale, and may be done once or in multiple steps to ensure complete removal of the oxide scale.

[0099] [Cold Rolling Process] Pickled hot-rolled steel sheets are cold-rolled in the cold-rolling process with a reduction ratio of 35 to 80%. By setting the cold-rolling reduction ratio to 35% or more, the shape of the cold-rolled steel sheet can be kept flat, and a decrease in ductility in the final product can be suppressed. The cold-rolling reduction ratio is preferably 50% or more. On the other hand, by setting the cold-rolling reduction ratio to 80% or less, it is possible to prevent the rolling load from becoming excessive and making rolling difficult. The cold-rolling reduction ratio is preferably 70% or less. The number of rolling passes and the reduction ratio for each pass are not particularly limited and should be set appropriately so that the overall cold-rolling reduction ratio falls within the above range.

[0100] [Annealing Process] The obtained cold-rolled steel sheet is heated, for example, in a heating furnace and soaking furnace of a continuous annealing line, and then held at a maximum heating temperature of 830 to 900°C for 20 to 150 seconds. This promotes austenitization, and in the subsequent cooling process, the desired hard structure mainly composed of martensite can be obtained, thereby reliably achieving a Vickers hardness of 450 Hv or higher. If the maximum heating temperature is less than 830°C or the holding time at the maximum heating temperature is less than 20 seconds, austenitization tends to be insufficient, making it difficult to obtain the desired area ratio of martensite in the final steel material, and it may not be possible to achieve a Vickers hardness of 450 Hv or higher. On the other hand, if the maximum heating temperature exceeds 900°C or the holding time at the maximum heating temperature exceeds 150 seconds, the austenite coarses, reducing hardenability, and ferrite transformation or bainite transformation is more likely to occur. As a result, it may not be possible to achieve the desired strength in the final steel material.

[0101] [Coating Process] After the annealing process, a coating treatment may be applied to the surface of the cold-rolled steel sheet for the purpose of improving corrosion resistance, etc. The coating treatment may be a hot-dip plating, alloyed hot-dip plating, electroplating, etc. For example, the steel sheet may be hot-dip galvanized as a coating treatment, or an alloying treatment may be performed after the hot-dip galvanizing treatment. The coating layer includes, for example, at least one selected from the group consisting of zinc, aluminum, magnesium and their alloys. More specifically, the coating layer may be a hot-dip plating layer or an electroplating layer. Examples of hot-dip plating layers include a hot-dip galvanized (GI) layer, an alloyed hot-dip galvanized (GA) layer, a hot-dip aluminum plating layer, a hot-dip Zn-Al alloy plating layer, a hot-dip Zn-Al-Mg alloy plating layer, a hot-dip Zn-Al-Mg-Si alloy plating layer, etc. Examples of electroplating layers include an electroplated zinc layer, an electroplated Zn-Ni alloy plating layer, etc. Preferably, the coating layer is a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, or an electroplated zinc layer. The specific conditions for the coating treatment and alloying treatment are not particularly limited and may be any suitable conditions known to those skilled in the art.

[0102] [Cooling Process] Finally, the cold-rolled steel sheet after the annealing or coating process is cooled to below the Ms point at an average cooling rate of 1.0°C / second or higher in the next cooling process. This allows the resulting microstructure to consist of 85.0% or more martensite by area percentage, while optionally including 1.0% to 7.0% retained austenite by area percentage in the microstructure, thus ensuring a Vickers hardness of 450 Hv or higher. Here, the Ms point (°C) is calculated based on the following formula 1. Ms = 550 - 350 × [C] - 40 × [Mn] - 20 × [Cr] - 10 × [Mo] - 17 × [Ni] - 10 × [Cu] - 35 × [V] - 10 × [W] - 15 × [Co] ...Equation 1 In the equation, [C], [Mn], [Cr], [Mo], [Ni], [Cu], [V], [W], and [Co] are the mass %) content of each element in the steel material.

[0103] On the other hand, if the average cooling rate is less than 1.0°C / second, a relatively large amount of ferrite and bainite will form during the cooling process, making it impossible to achieve the desired Vickers hardness in the final steel material. Preferably, the average cooling rate is 10.0°C / second or more and 20.0°C / second or less.

[0104] [Skin Pass Rolling Process] For the purpose of correcting the shape of the steel material or adjusting the surface roughness, skin pass rolling may be applied to the steel sheet after, for example, the cold rolling process, the annealing process, or the coating process. The reduction ratio of skin pass rolling is preferably 1.0% or less.

[0105] The present invention will be described in more detail below with reference to examples, but the present invention is not limited in any way to these examples.

[0106] In the following examples, steel materials according to the embodiment of the present invention were manufactured under various conditions, and the Vickers hardness and hydrogen embrittlement resistance properties at the shear end face of the obtained steel materials were investigated.

[0107] First, molten steel was cast using a continuous casting method to form slabs with various chemical compositions as shown in Table 1. These slabs were then heated to the heating temperatures shown in Table 2 and hot-rolled. Hot rolling was carried out by rough rolling and finish rolling, with the finish rolling performed under the conditions shown in Table 2. Next, the finish-rolled steel sheets were cooled and coiled under the conditions shown in Table 2. Then, the obtained hot-rolled steel sheets were pickled and cold-rolled at the reduction ratio shown in Table 2 to obtain cold-rolled steel sheets with the thickness shown in Table 2. Next, the obtained cold-rolled steel sheets were heated in the heating furnace and soaking furnace of the continuous annealing line and held at the maximum heating temperature shown in Table 2 for the time shown in Table 2. Next, the annealed cold-rolled steel sheets were appropriately coated, more specifically by hot-dip galvanizing (GI) or alloyed hot-dip galvanizing (GA), and finally, the cold-rolled steel sheets or plated steel sheets were cooled in the cooling process under the conditions shown in Table 2.

[0108]

[0109]

[0110]

[0111] The properties of the obtained steel plates are shown in Table 3.

[0112]

[0113] [Measurement of Vickers Hardness] The Vickers hardness was determined as follows. First, a test piece was cut from any position except the edge of the steel plate so that a cross section perpendicular to the surface (thickness cross section) could be observed. The thickness cross section of the test piece was wet polished using #600 to #1500 silicon carbide sandpaper, and then polished to a mirror finish using a liquid in which diamond powder with a particle size of 1 to 6 μm was dispersed in a diluent such as alcohol or pure water. This thickness cross section was used as the measurement surface. Next, the Vickers hardness was measured using a micro Vickers hardness tester with a load of 1 kgf at intervals of more than three times the indentation. Specifically, a total of 20 points were randomly measured around the 1 / 4 position of the steel plate thickness, and the arithmetic mean of these measurements was determined as the Vickers hardness of the steel. The Vickers hardness of all the steel plates measured was 450 Hv or higher.

[0114] [Evaluation of Hydrogen Embrittlement Resistance] The hydrogen embrittlement resistance of the obtained steel plate was evaluated by the following method. Specifically, the steel plate was cut to a size of 50 mm square, and then a 10 mmφ punching shearing process was performed in the center of the cut steel plate. The clearance during punching was ±1% of the value shown in Table 4 relative to the steel plate thickness. Clearance is the ratio of the gap between the die and the punch to the thickness of the steel plate subjected to shearing, and is defined by the following formula: Clearance (%) = (Die diameter - Punch diameter) / (2 × Steel plate thickness) × 100 During the shearing process, circular dies and punches with appropriate chamfers on the cutting edges as shown in Table 4 were used. In Table 4, C1 (mm) means the chamfer on the cutting edge of the punch, and C2 (mm) means the chamfer on the cutting edge of the die. Then, the occurrence of cracks when hydrogen was introduced from the sheared end face was observed using a 50 mm square steel plate with a 10 mmφ hole as a sample. Two samples are prepared, each subjected to the same punching and shearing process. One of these samples is then subjected to a hydrogen intrusion treatment. The hydrogen intrusion treatment is carried out by immersing the sample in a pH 2.0 hydrochloric acid aqueous solution adjusted to 30°C for 48 hours, and the rust layer is removed from the sample after immersion using a nylon brush. Next, both the sample that underwent the hydrogen intrusion treatment and the sample that did not are subjected to a heat treatment at 500°C for 1 hour in air. This heat treatment releases residual stress on the shear end face and softens the structure, making it possible to suppress the formation of new cracks even if hydrogen penetrates from the sample surface during the subsequent observation surface processing. In other words, the cracks ultimately observed on the shear end face are limited to those that occurred after the predetermined shearing process and those caused by the hydrogen intrusion treatment in the hydrochloric acid aqueous solution described above. Crack observation is performed on the heat-treated sample in eight cross-sections, as shown in Figure 5, by cutting the sample at 0°, 45°, 90°, and 135° directions across the diameter of a 10 mmφ hole. Cutting is performed by wet cutting, and the resulting cross-section is finished to a mirror surface by wet polishing. Here, the directions crossing the diameter of the 10 mmφ hole are not particularly limited as long as they are at 45° angle intervals. For ease of understanding, please note that the hole diameter is drawn relatively large in Figure 5, and the scale of the 50 mm square size and the 10 mmφ hole diameter do not match.Furthermore, although Figure 5 shows a 50 mm square sample with each side parallel to the 0° and 90° directions, the sample is not necessarily limited to these directions. Eight cross-sections of the plate thickness were observed using an optical microscope for both the sample treated with hydrogen penetration and the sample that was not treated. The total depth of all cracks observed in the eight cross-sections was then calculated to evaluate the hydrogen embrittlement resistance of the sample. Specifically, the total crack depths of the two samples were compared, and the hydrogen embrittlement resistance was evaluated as follows. AAA: The total crack depth of samples treated with hydrogen immersion is 1.1 times or less the total crack depth of samples not treated with hydrogen immersion. AA: The total crack depth of samples treated with hydrogen immersion is more than 1.1 times but 1.3 times or less the total crack depth of samples not treated with hydrogen immersion. A: The total crack depth of samples treated with hydrogen immersion is more than 1.3 times but 1.5 times or less the total crack depth of samples not treated with hydrogen immersion. NG: The total crack depth of samples treated with hydrogen immersion is more than 1.5 times the total crack depth of samples not treated with hydrogen immersion.

[0115] Steel materials with improved hydrogen embrittlement resistance at the shear end face were evaluated as having AAA, AA, and A ratings for hydrogen embrittlement resistance. The results are shown in Table 4.

[0116]

[0117] Referring to Table 4, in examples H-1, I-1, H-2, I-2, H-3, I-3, H-4, and I-4, because chamfering C2 was not applied to the die cutting edge during shearing, the maximum values ​​of tensile residual stress measured at the 1 / 4, 2 / 4, and 3 / 4 positions of the plate thickness on the sheared end face exceeded 1300 MPa. In addition, in some cases, the maximum value of this tensile residual stress was the tensile residual stress measured at the 2 / 4 position of the plate thickness on the sheared end face (H-I and H-3), resulting in a decrease in hydrogen embrittlement resistance. Furthermore, in J-4 and K-4, although chamfering C2 (mm) was applied to the die cutting edge during shearing, the chamfering C1 (mm) on the punch cutting edge was larger, resulting in the maximum values ​​of tensile residual stress measured at the 1 / 4, 2 / 4, and 3 / 4 positions of the plate thickness on the sheared end face exceeding 1300 MPa, resulting in a decrease in hydrogen embrittlement resistance. Furthermore, in Example H-5, although the maximum tensile residual stress measured at the 1 / 4, 2 / 4, and 3 / 4 positions of the plate thickness on the shear end face was 1300 MPa or less, the hydrogen embrittlement resistance was reduced because the maximum tensile residual stress was measured at the 2 / 4 position of the plate thickness on the shear end face. On the other hand, in Example I-5, although the maximum tensile residual stress measured at the 1 / 4, 2 / 4, and 3 / 4 positions of the plate thickness on the shear end face was not the tensile residual stress measured at the 2 / 4 position of the plate thickness on the shear end face, the hydrogen embrittlement resistance was similarly reduced because the maximum tensile residual stress was greater than 1300 MPa.

[0118] In contrast, in examples C-1, D-1, E-1, F-1, G-1, J-1, K-1, C-2, D-2, E-2, F-2, G-2, J-2, K-2, C-3, D-3, E-3, F-3, G-3, J-3 and K-3, in which chamfering was applied only to the cutting edge of the die during shearing, and in examples A-1, B-1, A-2, B-2, A-3 and B-3, in which the chamfer C2 (mm) of the cutting edge of the die was larger than the chamfer C1 (mm) of the cutting edge of the punch, the maximum value of the tensile residual stress measured at the 1 / 4, 2 / 4, and 3 / 4 positions of the plate thickness on the sheared end face was 1300 MPa or less. In addition, the maximum value of the tensile residual stress was controlled so that it did not become the tensile residual stress measured at the 2 / 4 position of the plate thickness on the sheared end face, and as a result, the hydrogen embrittlement resistance was improved.

[0119] Among these, the number density of inclusions having a circular equivalent diameter of 0.01 to 3.00 μm in the region from 3 / 8 to 5 / 8 of the plate thickness from the surface of the steel material is 50.0 pieces / mm 2 The following control is applied, and the number density of inclusions with a circular equivalent diameter greater than 3.00 μm is set to 5.0 inclusions / mm². 2 Examples C-1, D-1, E-1, F-1, G-1, A-1, B-1, C-2, D-2, E-2, F-2, G-2, A-2, B-2, C-3, D-3, E-3, F-3, G-3, A-3 and B-3, which used steel materials controlled as described below, and in which the average aspect ratio of inclusions with a circular equivalent diameter of 0.01 μm or more within the same region was controlled to 6.00 or less, achieved a hydrogen embrittlement resistance evaluation of AA or AAA, demonstrating a more significant improvement in hydrogen embrittlement resistance.

[0120] 1 Shear end face 1a Burr 1b Shear surface 1c Fracture surface 1d Burr 5 Workpiece 10 Steel material 10a First surface 10b Second surface 11 Part 12 Other part 15 Scrap 21 Punch 21a Bottom surface of punch 22 Die

Claims

1. A steel material having a shear end face, having a Vickers hardness of 450 Hv or more, wherein the maximum value of the tensile residual stress measured at the 1 / 4, 2 / 4, and 3 / 4 positions of the plate thickness on the shear end face is 1300 MPa or less, and the maximum value of the tensile residual stress is not the tensile residual stress measured at the 2 / 4 position of the plate thickness on the shear end face.

2. The steel material contains inclusions, and in the region from 3 / 8 to 5 / 8 of the plate thickness from the surface of the steel material, the number density of inclusions having a circular equivalent diameter of 0.01 to 3.00 μm is 50.0 pieces / mm². 2 The following conditions apply: the number density of inclusions with a circular equivalent diameter greater than 3.00 μm is 5.0 inclusions / mm². 2 The steel material according to claim 1, characterized in that the average aspect ratio of inclusions having a circular equivalent diameter of 0.01 μm or more in the region from 3 / 8 to 5 / 8 of the plate thickness from the surface of the steel material is 6.00 or less.

3. The steel material according to claim 1 or 2, characterized in that the steel material has a chemical composition comprising, by mass%, at least one of Mg and Ca: totaling 0.005 to 0.050%.

4. The steel material has the following composition in mass%, C: 0.15-0.60%, Si: 0.01-1.30%, Mn: 1.00-3.50%, P: 0.0200% or less, S: 0.0200% or less, Al: 0.001-1.00%, N: 0.0200% or less, O: 0.0200% or less, Co: 0-0.50%, Ni: 0-1.00%, Mo: 0-1.00%, Cr: 0-2.00%, Ti: 0-0.5000%, B: 0-0.0100%, Nb: 0-0.500%, V: 0-0.500%, Cu: 0-1.000%, W: 0-0.100% A steel material according to any one of claims 1 to 3, characterized by having a chemical composition consisting of Ta: 0-0.100%, Sn: 0-0.100%, Sb: 0-0.500%, As: 0-0.100%, Mg: 0-0.050%, Ca: 0-0.050%, Y: 0-0.050%, Zr: 0-0.500%, La: 0-0.050%, Ce: 0-0.050%, and the remainder being Fe and impurities.

5. The steel material according to any one of claims 1 to 4, characterized in that the steel material has a metallic structure containing 85.0% or more martensite by area percentage.

6. A component characterized by comprising the steel material described in any one of claims 1 to 5.

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