Automotive part and method for producing same

Abstract

The purpose of the present invention is to provide, through a novel configuration, an automotive part having improved crack resistance at an end surface thereof, and a method for producing the automotive part. Provided is an automotive part having an end surface, the automotive part being characterized in that the Vickers hardness of the end surface is lower than the Vickers hardness inward from the end surface, and the difference between the Vickers hardness of the end surface and the Vickers hardness inward from the end surface is 50 HV 0.025 or greater, or is 10% or more of the hardness of the end surface.

Description

Automotive parts and their manufacturing methods

[0001] The present invention relates to automotive parts and methods for manufacturing the same.

[0002] In recent years, the automotive industry has been demanding lighter vehicle bodies from the perspective of improving fuel efficiency. 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] Furthermore, hot stamping (hot pressing) is a known technique for press-forming materials that are difficult to form, such as high-strength steel plates. Hot stamping is a hot forming technique in which the material to be formed is heated before forming.

[0004] In this regard, Patent Document 1 discloses a method for manufacturing a molded article, comprising: a first step of obtaining a blank from a steel sheet; a second step of covering the surface of the blank obtained in the first step, including the end face; a third step of heating the blank whose surface, including the end face, has been covered in the second step; and a fourth step of hot stamping the blank heated in the third step. Patent Document 1 teaches that the above manufacturing method can provide a method for manufacturing a molded article that can suppress the generation of scale.

[0005] Patent Document 2 discloses a method for producing an un-pressed pre-alloyed steel coil, plate or blank, comprising the following sequential steps: - providing an un-pressed pre-coated steel coil, plate or blank consisting of a heat-treatable steel substrate covered with a pre-coat of aluminum, an aluminum-based alloy or an aluminum alloy, wherein aluminum-based alloy refers to an alloy in which aluminum is the main element by weight percent, aluminum alloy refers to an alloy in which aluminum accounts for more than 50% by weight, said pre-coat is obtained directly from hot-dip aluminum plating without additional heat treatment, and the thickness of said pre-coat on each surface of the steel coil, plate or blank is between 10 and 35 micrometers; then - heating said un-pressed steel coil, plate or blank in a furnace for a duration t between t １ and t １ｍｉｎ to t １ｍａｘ , at a temperature θ between 750°C and 1000°C in an atmosphere containing at least 5% oxygen, wherein t １ is the duration, t １ｍｉｎ = 23500 / (θ １ - 729.5) and t １ｍａｘ = 4.946×10 ４１ ×θ １ －１３．０８ , t １ represents the total duration in the furnace, θ １ is expressed in °C, t １ｍｉｎ and t １ｍａｘ are expressed in seconds; then - cooling said un-pressed steel coil, plate or blank to a temperature θi at a cooling rate Vr1; then - holding said un-pressed steel coil, plate or blank at a temperature θ ２ between 100°C and 500°C for a duration t ２ between 3 minutes and 45 minutes, so as to obtain a diffusible hydrogen content of less than 0.35 ppm.

[0006] Patent Document 3 discloses a method for manufacturing high-strength steel, comprising: a first step of applying a decarburization inhibitor to areas of the steel that will not be punched during hot stamping; a second step of hot stamping the steel; and a third step of punching out the areas where the decarburization inhibitor has not been applied. Patent Document 3 teaches that according to the above manufacturing method, steel can be manufactured with high hardness, high strength, and a predetermined shape, and that punching (outer shape punching) after hot stamping can also be performed efficiently, leading to an improvement in manufacturing yield.

[0007] Japanese Patent Publication No. 2008-119702, Japanese Patent Publication No. 2021-517204, Japanese Patent Publication No. 2008-284599

[0008] As described above, efforts are being made to increase the strength of steel sheets used in automotive parts in order to achieve both weight reduction and collision safety for the vehicle body. However, in such automotive parts, cracks can occur starting from the edges, for example, during collisions or additional molding. Therefore, there is a high need for such automotive parts to suppress cracking from the edges, that is, to have crack resistance at the edges.

[0009] Therefore, the present invention aims to provide an automotive part with improved crack resistance at the end face through a novel configuration, and a method for manufacturing the same.

[0010] To achieve the above objective, the inventors focused particularly on the Vickers hardness of the end faces of automotive parts. As a result, the inventors discovered that crack resistance at the end faces can be improved by controlling the Vickers hardness of the end face to be lower than the Vickers hardness of the area inside the end face, and by controlling the difference between the Vickers hardness of the end face and the area inside the end face to be 50HV0.025 or more, or to be 10% or more of the Vickers hardness of the end face, thus completing the present invention.

[0011] The present invention, having achieved the above objectives, is as follows: (1) An automobile part having an end face, wherein the Vickers hardness of the end face is lower than the Vickers hardness of the area inside the end face, and the difference between the Vickers hardness of the end face and the Vickers hardness of the area inside the end face is 50HV0.025 or more, or 10% or more of the Vickers hardness of the end face. (2) The automobile part according to (1) above, wherein the automobile part is a hot-stamped molded body. (3) The automobile part according to (1) or (2) above, wherein at least one surface of the automobile part, other than the end face, further has a plating layer. (4) The carbon content inside the end face ([C] ｉｎｓｉｄｅ ) C content of the above end face ([C] ｅｎｄ ｓｕｒｆａｃｅ ) ratio ([C] ｅｎｄ ｓｕｒｆａｃｅ / [C] ｉｎｓｉｄｅ (1) The automotive part according to any one of (1) to (3) above, characterized in that the ratio is 0.80 or less. (5) The automotive part according to any one of (1) to (4) above, characterized in that the end face contains 50% or more ferrite by area %. (6) The automotive part according to any one of (1) to (5) above, characterized in that it has a tensile strength of 1.5 GPa or more. (7) The automotive part according to (6) above, characterized in that it has a tensile strength of 2.0 GPa or more. (8) A method for manufacturing the automotive part according to any one of (2) to (7) above, comprising the following steps: a shearing step of shearing a steel sheet to form the end face, and then a forming step of press-forming the sheared steel sheet by holding it at 800 to 1000°C for 0.5 to 5.0 minutes in an atmosphere with a dew point of 0 to 40°C.

[0012] According to the present invention, it is possible to provide an automobile part with improved crack resistance at the end face and a method for manufacturing the same.

[0013] Figures 1(A) and 1(B) are schematic diagrams illustrating automotive parts according to embodiments of the present invention. Figure 2 is a schematic cross-sectional diagram illustrating a shearing process in a manufacturing method for automotive parts according to a preferred embodiment of the present invention.​​

[0014] <Automobile Component> An automobile component according to an embodiment of the present invention is an automobile component having an end face, wherein the Vickers hardness of the end face is lower than the Vickers hardness at a position inward of the end face, and a difference between the Vickers hardness of the end face and the Vickers hardness at the position inward of the end face is 50 HV0.025 or more, or 10% or more of the Vickers hardness of the end face.

[0015] To achieve both weight reduction of a vehicle body and crash safety, increasing the strength of steel sheets used for automobile components has been promoted. However, in such automobile components, cracks may initiate from the end face, for example, during collision or additional forming. Therefore, for such automobile components, there is high demand for the ability to suppress cracking starting from the end face, that is, high crack resistance at the end face. Here, the crack resistance at the end face can be evaluated by a hole expansion ratio obtained from a hole expansion test using an initial hole, where a test piece having the same composition as that of the automobile component, an end face obtained by the same manufacturing method and treatment as the end face of the test piece, and the end face formed by a hole with a diameter of 10 mm is used. When the automobile component has an end face formed by a hole with a diameter of 10 mm, the evaluation can also be performed by the hole expansion ratio obtained from a hole expansion test using said hole as the initial hole. More specifically, the initial hole is expanded by a conical punch having a vertex angle of 60° until a crack penetrating the plate thickness occurs, the hole expansion ratio is calculated from the hole diameter at the time of crack occurrence, and evaluation can be performed based on the calculated hole expansion ratio.

[0016] Therefore, the inventors focused their research on the Vickers hardness of the end faces of automotive parts. As a result, the inventors found that crack resistance at the end faces can be improved by controlling the Vickers hardness of the end faces to be low, and by controlling the difference between the Vickers hardness of the end faces and the Vickers hardness of the area inside the end faces to be 50HV0.025 or higher, or by controlling it to be 10% or more of the Vickers hardness of the end faces. In conventional automotive parts with end faces, the Vickers hardness of the end faces is equivalent to the Vickers hardness of the area inside the end faces, so it is thought that cracks may occur starting from the end faces. By controlling the Vickers hardness of the end faces to be low, and by controlling the difference between the Vickers hardness of the end faces and the Vickers hardness of the area inside the end faces to be above a predetermined value, the end faces become softer than the interior, local deformation is permitted, and as a result, the hole expansion ratio is improved, and crack resistance at the end faces is improved. The following describes in more detail each component of the automotive part according to the embodiment of the present invention.

[0017] [Automotive Parts] Automotive parts according to the embodiments of the present invention include any form having an end face. Furthermore, the form of the end face is not particularly limited. Figures 1A and 1B show schematic diagrams of some automotive parts according to the embodiments of the present invention, but the invention is not limited to these cases. An automotive part according to one embodiment shown in Figure 1A is an automotive part 10 that has been punched out. The automotive part 10 has a surface 10b and an end face 10a formed by the punching process. For the automotive part 10, the Vickers hardness of the end face is lower than the Vickers hardness inside the end face, and the difference between the Vickers hardness of the end face 10a and the Vickers hardness inside the end face 10a is 50HV0.025 or more, or 10% or more of the Vickers hardness of the end face 10a. Another automotive part according to another embodiment shown in Figure 1B is an automotive part 10 that has been punched out in a curved shape, has a surface 10b and an end face 10a formed by punching out in a curved shape. The difference in Vickers hardness between the end face and the area inside the end face of the automobile part shown in Figure 1B is the same as the difference in Vickers hardness of the automobile part shown in Figure 1A.

[0018] The automotive parts according to the embodiments of the present invention may be in the form of a blank material with its end faces formed as is, or in the form of a part formed into a predetermined shape by cold pressing or hot stamping (hot pressing), and among these, the form of a part formed into a predetermined shape by hot stamping (hot pressing) is preferred. That is, the automotive parts according to the preferred embodiments of the present invention may be hot-stamped molded bodies. In hot stamping (hot pressing) molding, the material is heated before molding, so the steel material is soft and has good formability during molding. Therefore, even high-strength steel material can be molded into complex shapes with high precision, and since quenching is performed simultaneously with molding by a press die, it is known that the steel material has sufficient strength after molding. Therefore, in hot-stamped molded bodies, the problem of crack resistance at the end faces becomes more pronounced.

[0019] [Difference between Vickers hardness at end face and Vickers hardness inside the end face: 50 HV0.025 or more] [Difference between Vickers hardness at end face and Vickers hardness inside the end face: 10% or more of the Vickers hardness at the end face] In the automobile part according to the embodiment of the present invention, the difference between the Vickers hardness at the end face and the Vickers hardness at a position inward of the end face is 50 HV0.025 or more, or 10% or more of the Vickers hardness at the end face. In the automobile part according to the embodiment of the present invention, specifically, for example, the Vickers hardness at the end face is smaller than the Vickers hardness at the position inward of the end face by 50 HV0.025 or more, or smaller by 10% or more of the Vickers hardness at the end face. The difference in Vickers hardness described above can be controlled, for example, by softening a region including the end face, hardening a region located inward of the end face, or the like. By controlling the difference in Vickers hardness to be 50 HV0.025 or more, or controlling the difference to be 10% or more of the Vickers hardness at the end face, the crack resistance at the end face can be improved. The difference in Vickers hardness described above is preferably 55 HV0.025 or more, 60 HV0.025 or more, 65 HV0.025 or more, 70 HV0.025 or more, 75 HV0.025 or more, or 80 HV0.025 or more. The upper limit of the difference in Vickers hardness described above is not particularly limited, and may be, for example, 250 HV0.025 or less or 200 HV0.025 or less. Further, the difference in Vickers hardness described above is preferably 11% or more, 12% or more, 13% or more, 14% or more, or 15% or more with respect to the Vickers hardness at the end face. The upper limit of the difference in Vickers hardness described above is not particularly limited, and may be, for example, 40% or less or 30% or less with respect to the Vickers hardness at the end face.

[0020] [Measurement of Vickers Hardness] The Vickers hardness of the end face is determined as follows. First, a test piece is cut from a position including the end face of the automobile part so that a cross section (thickness cross section) perpendicular to the end face and perpendicular to the surface of the automobile part can be observed. The thickness cross section of the test piece is polished using #600 to #1500 silicon carbide sandpaper, and then finished to a mirror surface using a liquid in which diamond powder with a particle size of 1 to 6 μm is dispersed in a diluent such as alcohol or pure water. Etching is performed to prepare the sample for observation with an optical microscope, and this thickness cross section is used as the measurement surface. Next, in accordance with JIS 2244-1:2024, the Vickers hardness is measured using a micro-Vickers hardness tester with a load of 25 gf, i.e., 0.2452 N, at intervals of at least three times the diagonal length of the indentation. More specifically, the Vickers hardness is measured at a position 1 / 4 of the thickness from the surface of the automobile part, and at distances of 0.05 mm and 0.10 mm from the end face. Similarly, the Vickers hardness is measured at positions 3 / 4 of the way from the surface of the automotive part, and at distances of 0.05 mm and 0.10 mm from the end face. The arithmetic mean of the Vickers hardness at these four points is determined as the "Vickers hardness of the end face".

[0021] Furthermore, the "Vickers hardness inside the end face" is determined in the same way as the "Vickers hardness of the end face." More specifically, however, the Vickers hardness is measured at a position 1 / 4 of the way from the surface of the automobile part, and at distances of 0.50 mm and 0.60 mm from the end face. Similarly, the Vickers hardness is measured at a position 3 / 4 of the way from the surface of the automobile part, and at distances of 0.50 mm and 0.60 mm from the end face. The arithmetic mean of the Vickers hardness of these four points is determined as the "Vickers hardness inside the end face." Using the "Vickers hardness of the end face" and the "Vickers hardness inside the end face" determined in this way, the difference between the Vickers hardness of the end face and the Vickers hardness inside the end face is determined by subtracting the "Vickers hardness of the end face" from the "Vickers hardness inside the end face."

[0022] [Preferred Embodiment of Automotive Parts] [[C] ｅｎｄ ｓｕｒｆａｃｅ ​ / [C] ｉｎｓｉｄｅ : 0.70 or less] In a preferred embodiment of the present invention, the C content (C) inside the end face of the automobile part is 0.70 or less. ｉｎｓｉｄｅ C content of the end face relative to ) ([C] ｅｎｄ ｓｕｒｆａｃｅ ) ratio ([C] ｅｎｄ ｓｕｒｆａｃｅ / [C] ｉｎｓｉｄｅ ) is characterized by being 0.70 or less. In automotive parts, for example, by controlling the carbon content at the end face so that the carbon content at the end face is smaller than the carbon content inside the end face, i.e., [C] ｅｎｄ ｓｕｒｆａｃｅ / [C] ｉｎｓｉｄｅ By controlling it to be lower, the Vickers hardness and / or strength of the end face is reduced, thereby improving crack resistance at the end face. Therefore, [C] ｅｎｄ ｓｕｒｆａｃｅ / [C] ｉｎｓｉｄｅ This may be 0.60 or less, 0.55 or less, 0.50 or less, 0.45 or less, or 0.40 or less. [C] ｅｎｄ ｓｕｒｆａｃｅ / [C] ｉｎｓｉｄｅ The lower limit is not particularly limited, but may be, for example, 0.01 or higher, 0.05 or higher, or 0.10 or higher.

[0023] [C content ratio ([C] ｅｎｄ ｓｕｒｆａｃｅ / [C] ｉｎｓｉｄｅ ) Calculation method] C content ratio ([C] ｅｎｄ ｓｕｒｆａｃｅ / [C] ｉｎｓｉｄｅ The carbon content of the end face ([C]) is determined by an electron probe microanalyzer (EPMA) as follows: ｅｎｄ ｓｕｒｆａｃｅ ), and the C content inside the end face ([C] ｉｎｓｉｄｅ ) is obtained as follows, and [C] ｅｎｄ ｓｕｒｆａｃｅ to [C] ｉｎｓｉｄｅ ​​​​​​​​​It is determined by dividing by [a certain factor]. First, a sample is taken from a position including the end face of the automobile part, with the end face and the thickness cross section perpendicular to the surface of the automobile part as the observation surface. For this sample, measurements are taken at a total of 250,000 points at 0.40 μm intervals in a rectangular area including 200 μm in the range of 0 to 200 μm perpendicular to the end face, and 200 μm centered at a position 1 / 4 of the thickness from the surface in the thickness direction of the automobile part. Although a rectangular area centered at a position 1 / 4 of the thickness from the surface in the thickness direction of the automobile part is preferred, if a sample cannot be taken, a rectangular area centered at a position 3 / 4 of the thickness from the surface in the thickness direction of the automobile part may be used. The C concentration at a depth of 0.01 mm (10 μm) from the end face is used as the C content of the end face ([C] ｅｎｄ ｓｕｒｆａｃｅ ) is determined as follows. Similarly, the C concentration at a depth of 0.18 mm (180 μm) from the end face is determined as the C content inside the end face ([C] ｉｎｓｉｄｅ ) will be decided as such.

[0024] [Metal structure of the end face: Ferrite: 50% or more] In a preferred embodiment of the present invention, the automotive part is characterized in that the end face contains 50% or more ferrite by area percentage. By controlling the metal structure of the end face to be predominantly ferrite, the strength of the end face is reduced, thereby significantly improving the crack resistance of the end face. For this reason, a higher area percentage of ferrite at the end face is preferable, and may be, for example, 55% or more, 60% or more, 65% or more, 70% or more, 75% or more, or 80% or more. There is no particular upper limit, but for example, the area percentage of ferrite at the end face may be 100% or less, 99% or less, 97% or less, 95% or less, or 90% or less.

[0025] ​[Identification of ferrite on the end face and calculation of area ratio] The identification of ferrite on the end face and calculation of the area ratio are performed using an optical microscope and a scanning electron microscope (SEM) after etching with Nital solution. First, a sample is taken from a cross section perpendicular to the end face of the automobile part and perpendicular to the surface of the automobile part, which is used as the observation surface. Next, the observation surface is etched with Nital solution. For the etched end face of the automobile part, a 30 μm region including the end face in a direction perpendicular to the end face, and a region extending across the entire direction perpendicular to the surface of the automobile part are observed, and the area ratio of ferrite is obtained by performing image analysis. A massive crystal grain that does not contain understructures such as lath inside the structure is considered to be ferrite.

[0026] [Preferred Chemical Composition of Automotive Parts] As described above, the present invention aims to provide automotive parts with improved crack resistance at the end faces and a method for manufacturing the same. This is achieved by controlling the Vickers hardness of the end face to be lower than the Vickers hardness of the area inside the end face, while controlling the difference between the Vickers hardness of the end face and the Vickers hardness of the area inside the end face to be 50HV0.025 or more, or to be 10% or more of the Vickers hardness of the end face. Therefore, it is clear that the chemical composition of the automotive parts itself is not an essential technical feature for achieving the objectives of the present invention. The chemical composition of automotive parts can contain appropriate amounts of any alloying elements that are commonly added in the art of the present invention. The preferred chemical composition of automotive parts will be described in detail below, but these descriptions are intended to be merely examples of preferred chemical compositions for obtaining automotive parts having, for example, a tensile strength of 1.5 GPa or more and a metallic structure such as martensite, which will be described later, in a desired area ratio, and are not intended to limit the present invention to automotive parts having such specific chemical compositions. Furthermore, in the following explanation, "%", 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 before and after it are included as the lower and upper limits, respectively, unless otherwise specified.

[0027] In a specific embodiment of the present invention, for example, the automotive part has the following composition in mass%, C: 0.20-0.70%, Si: 0.01-1.30%, Mn: 0.05-3.00%, P: 0.100% or less, S: 0.0100% or less, N: 0.0200% or less, O: 0.0200% or less, Al: 0.001-1.000%, Cr: 0-1.00%, Nb: 0-0.200%, Ti: 0-0.200%, Mo: 0-1.00%, B: 0-0.1000%, Co: 0-4.00%, Ni: 0-3.00%, Cu: 0-3.00%, V: 0-3.00%, W: 0-1.00%. It is preferable to have a chemical composition consisting of Ca: 0-1,000%, Mg: 0-1,000%, REM: 0-1,000%, Sb: 0-1,000%, Zr: 0-1,000%, Sn: 0-1,000%, As: 0-0.100%, and the remainder being Fe and impurities. Each element will be described in more detail below.

[0028] [C: 0.20-0.70%] Carbon (C) is an element that improves the strength of automobile parts. Automobile parts sometimes require high strength, for example, 1.5 GPa or more. To ensure sufficient strength, the carbon content is preferably 0.30% or more. The carbon content may also be 0.34% or more, 0.42% or more, or 0.45% or more. In particular, by setting the carbon content to 0.30% or more, it is possible to obtain automobile parts with even higher strength, specifically automobile parts with a tensile strength of 2.0 GPa or more. Similarly, by setting the carbon content to 0.40% or more, it is possible to obtain automobile parts with a tensile strength of 2.3 GPa or more. Similarly, by setting the carbon content to 0.45% or more, it is possible to obtain automobile parts with a tensile strength of 2.5 GPa or more. On the other hand, if carbon is included in excess, the processability may decrease, so it is preferable that the carbon content is 0.70% or less. The C content may be 0.64% or less, 0.60% or less, 0.56% or less, or 0.52% or less.

[0029] [Si: 0.01-1.30%] Si is an element that improves the strength of automobile parts through solid solution strengthening. To ensure sufficient strength, the Si content is preferably 0.01% or more. The Si content may be 0.05% or more, 0.10% or more, 0.20% or more, 0.25% or more, 0.30% or more, or 0.40% or more. On the other hand, if the Si content is excessive, the processability may decrease, so the Si content is preferably 1.30% or less. The Si content may be 1.20% or less, 1.00% or less, 0.80% or less, 0.60% or less, or 0.50% or less.

[0030] [Mn: 0.05-3.00%] Mn is an element that enhances the hardenability of steel and contributes to improving its strength. To obtain this effect, the Mn content is preferably 0.05% or more. The Mn content may be 0.10% or more, 0.50% or more, 1.00% or more, 1.30% or more, or 1.50% or more. On the other hand, if Mn is included in excess, Mn segregation may occur, which may lead to uneven strength of the automobile parts, so the Mn content is preferably 3.00% or less. The Mn content may be 2.80% or less, 2.50% or less, 2.30% or less, or 2.00% or less.

[0031] [P: 0.100% or less] P is an impurity element, and excessive addition may degrade weldability and toughness. Therefore, it is preferable that the P content be 0.100% or less. The P content may also be 0.070% or less, 0.050% or less, or 0.010% or less. There is no particular lower limit to the P content, but reducing it to less than 0.0001% significantly increases the cost of P removal, which is economically undesirable. Therefore, the P content may be 0.0001% or more.

[0032] [S: 0.0100% or less] S is an impurity element and may form MnS in steel, degrading its toughness. Therefore, it is preferable that the S content be 0.0100% or less. The S content may also be 0.0080% or less, 0.0050% or less, 0.0030% or less, or 0.0020% or less. There is no particular lower limit to the S content, but reducing it to less than 0.0001% significantly increases the cost of S removal, which is economically undesirable. Therefore, the S content may be 0.0001% or more.

[0033] [N: 0.0200% or less] N is an impurity element, and a high N content can lead to the formation of coarse nitrides in the steel, reducing its flexibility. Therefore, it is preferable that the N content be 0.0200% or less. The N content may also be 0.0180% or less, 0.0150% or less, 0.0100% or less, 0.0060% or less, or 0.0040% or less. There is no particular lower limit to the N content, but reducing it to less than 0.0001% significantly increases the cost of removing N, which is economically undesirable. Therefore, the N content may be 0.0001% or more.

[0034] [O: 0.0200% or less] O is an impurity element, and a high O content can lead to the formation of coarse oxides in the steel, reducing its bendability. Therefore, it is preferable that the O content be 0.0200% or less. The O content may also be 0.0150% or less, 0.0100% or less, 0.0070% or less, or 0.0040% or less. From the viewpoint of reducing refining costs, the O content may be 0.0001% or more.

[0035] [Al: 0.001 to 1.000%] Al is an element that deoxidizes molten steel and makes the steel sound. To obtain this effect, the Al content is preferably 0.001% or more. The Al content may be 0.003% or more, 0.005% or more, 0.010% or more, or 0.030% or more. On the other hand, if there is an excess of Al, the workability may decrease, so the Al content is preferably 1.000% or less. The Al content may be 0.800% or less, 0.600% or less, 0.400% or less, 0.200% or less, or 0.100% or less.

[0036] The basic chemical composition of an automotive part according to a specific embodiment of the present invention is as described above. Furthermore, the automotive part may optionally contain at least one of the following optional elements in place of a portion of the remaining Fe. For example, the automotive part may contain Cr: 0 to 1.00%. The automotive part may also contain at least one element selected from the group consisting of Nb: 0 to 0.200%, Ti: 0 to 0.200%, Mo: 0 to 1.00%, B: 0 to 0.1000%, Co: 0 to 4.00%, Ni: 0 to 3.00%, Cu: 0 to 3.00%, V: 0 to 3.00%, and W: 0 to 1.00%. The automotive part may also contain at least one element selected from the group consisting of Ca: 0 to 1.000%, Mg: 0 to 1.000%, and REM: 0 to 1.000%. Furthermore, the automotive parts may contain at least one element selected from the group consisting of Sb: 0-1,000%, Zr: 0-1,000%, and Sn: 0-1,000%. The automotive parts may also contain As: 0-0.100%. These optional elements will be described in detail below.

[0037] [Cr: 0-1.00%] Cr is an element that increases the strength of automotive parts by solid-solubilizing into the prior austenite grains during heating before hot stamping. The Cr content may be 0.001% or more, but to obtain this effect, the Cr content is preferably 0.05% or more, 0.10% or more, 0.15% or more, or 0.20% or more. On the other hand, the above effect will saturate even if a large amount is included, so the Cr content is preferably 1.00% or less. The Cr content may be 0.80% or less, 0.60% or less, 0.50% or less, or 0.40% or less.

[0038] [Nb: 0-0.200%] Nb is an element that forms carbonitrides in steel, improving the strength of automobile parts through precipitation strengthening. The Nb content may be 0.001% or more, but to reliably obtain this effect, the Nb content is preferably 0.010% or more or 0.020% or more. On the other hand, even if a large amount is included, the above effect will saturate, so the Nb content is preferably 0.200% or less. The Nb content may be 0.180% or less, 0.150% or less, 0.100% or less, 0.080% or less, or 0.060% or less.

[0039] [Ti: 0-0.200%] Ti is an element that forms carbonitrides in steel, improving the strength of automobile parts through precipitation strengthening. The Ti content may be 0.001% or more, but to reliably obtain this effect, the Ti content is preferably 0.010% or more or 0.020% or more. On the other hand, even if a large amount is included, the above effect will saturate, so the Ti content is preferably 0.200% or less. The Ti content may be 0.180% or less, 0.150% or less, 0.100% or less, 0.080% or less, or 0.060% or less.

[0040] [Mo: 0-1.00%] Mo is an element that improves the hardenability of steel. The Mo content may be 0.001% or more, but to reliably obtain this effect, the Mo content is preferably 0.005% or more or 0.01% or more. On the other hand, even if a large amount is included, the above effect will saturate, so the Mo content is preferably 1.00% or less. The Mo content may be 0.80% or less, 0.60% or less, 0.50% or less, 0.30% or less, or 0.10% or less.

[0041] [B: 0 to 0.1000%] B is an element that improves the hardenability of steel. The B content may be 0.0001% or more, but to reliably obtain this effect, the B content is preferably 0.0005% or more or 0.0010% or more. On the other hand, even if a large amount is included, the above effect will saturate, so the B content is preferably 0.1000% or less. The B content may be 0.0500% or less, 0.0100% or less, 0.0050% or less, 0.0030% or less, or 0.0015% or less.

[0042] [Co: 0-4.00%] Co is an element that improves the strength of automobile parts through solid solution strengthening. The Co content may be 0.001% or more, but to reliably obtain this effect, the Co content is preferably 0.01% or more or 0.05% or more. On the other hand, the above effect will saturate if a large amount is included, so the Co content is preferably 4.00% or less. The Co content may be 3.00% or less, 2.00% or less, 1.00% or less, 0.80% or less, or 0.60% or less.

[0043] [Ni: 0-3.00%] Ni has the effect of increasing the strength of automotive parts by solid-solubilizing into austenite grains during heating in the hot stamping molding process. The Ni content may be 0.001% or more, but to reliably obtain this effect, the Ni content is preferably 0.01% or more. On the other hand, the above effect will saturate if a large amount is included, so the Ni content is preferably 3.00% or less. The Ni content may be 2.80% or less, 2.50% or less, 2.00% or less, 1.50% or less, 1.00% or less, or 0.80% or less.

[0044] [Cu: 0-3.00%] Cu has the effect of increasing the strength of automotive parts by solid-solubilizing into austenite grains during heating in the hot stamping molding process. The Cu content may be 0.001% or more, but to reliably obtain this effect, the Cu content is preferably 0.01% or more or 0.05% or more. On the other hand, the above effect will saturate even if a large amount is included, so the Cu content is preferably 3.00% or less. The Cu content may be 2.00% or less, 1.00% or less, 0.50% or less, 0.30% or less, or 0.10% or less.

[0045] [V: 0-3.00%] V has the effect of improving the strength of automobile parts by forming carbonitrides in the steel and strengthening through precipitation. The V content may be 0.001% or more, but to reliably obtain this effect, the V content is preferably 0.01% or more or 0.05% or more. On the other hand, even if a large amount is included, the above effect will saturate, so the V content is preferably 3.00% or less. The V content may be 2.00% or less, 1.00% or less, 0.50% or less, 0.30% or less, or 0.10% or less.

[0046] [W: 0-1.00%] W is an element that improves the hardenability of steel. The W content may be 0.001% or more, but to reliably obtain this effect, the W content is preferably 0.005% or more or 0.01% or more. On the other hand, even if a large amount is included, the above effect will saturate, so the W content is preferably 1.00% or less. The W content may be 0.80% or less, 0.60% or less, 0.50% or less, 0.30% or less, or 0.10% or less.

[0047] [Ca: 0-1.000%] Ca is an element that can control the morphology of inclusions. The Ca content may be 0.0001% or more, but to reliably obtain this effect, the Ca content is preferably 0.0005% or more or 0.001% or more. On the other hand, the above effect will saturate if a large amount is included, so the Ca content is preferably 1.000% or less. The Ca content may be 0.500% or less, 0.100% or less, 0.050% or less, 0.010% or less, 0.005% or less, or 0.002% or less.

[0048] [Mg: 0-1.000%] Mg is an element that can control the morphology of inclusions. The Mg content may be 0.0001% or more, but to reliably obtain this effect, the Mg content is preferably 0.0005% or more or 0.001% or more. On the other hand, the above effect will saturate if a large amount is included, so the Mg content is preferably 1.000% or less. The Mg content may be 0.500% or less, 0.100% or less, 0.050% or less, 0.010% or less, 0.005% or less, or 0.002% or less.

[0049] [REM: 0-1.000%] REM is an element that can control the morphology of inclusions. The REM content may be 0.0001% or more, but to reliably obtain this effect, the REM content is preferably 0.0005% or more or 0.001% or more. On the other hand, even if a large amount is included, the above effect will saturate, so the REM content is preferably 1.000% or less. The REM content may be 0.500% or less, 0.100% or less, 0.050% or less, 0.010% or less, 0.005% or less, or 0.002% or less. In this embodiment, REM is a collective term for 17 elements including scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanides from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71, and the REM content is the total content of these elements.

[0050] [Sb: 0-1.000%] Sb is an element that suppresses the formation of oxides. To reliably obtain this effect, the Sb content is preferably 0.001% or more. On the other hand, even if a large amount is included, the above effect will saturate, so the Sb content is preferably 1.000% or less. The Sb content may be 0.800% or less, 0.500% or less, 0.200% or less, 0.100% or less, or 0.050% or less.

[0051] [Zr: 0-1.000%] Zr is an element that suppresses the formation of oxides. To reliably obtain this effect, the Zr content is preferably 0.001% or more. On the other hand, even if a large amount is included, the above effect will saturate, so the Zr content is preferably 1.000% or less. The Zr content may be 0.800% or less, 0.500% or less, 0.200% or less, 0.100% or less, or 0.050% or less.

[0052] [Sn: 0-1.000%] Sn is an element that suppresses the formation of oxides. To reliably obtain this effect, the Sn content is preferably 0.001% or more. On the other hand, even if a large amount is included, the above effect will saturate, so the Sn content is preferably 1.000% or less. The Sn content may be 0.800% or less, 0.500% or less, 0.200% or less, 0.100% or less, or 0.050% or less.

[0053] [As: 0-0.100%] As contributes to the refinement of prior austenite grains by lowering the austenite single-phase formation temperature. To reliably obtain this effect, the As content is preferably 0.001% or more. On the other hand, the above effect becomes saturated even if a large amount is included, so the As content is preferably 0.100% or less. The As content may also be 0.080% or less, 0.050% or less, 0.020% or less, 0.010% or less, or 0.005% or less.

[0054] In the automotive part according to a preferred embodiment, the remainder other than the above-mentioned elements consists of Fe and impurities. Impurities include components that are mixed in due to various factors in the manufacturing process, such as raw materials like ore and scrap, when automotive parts are manufactured industrially, as well as components that are included in a range that does not affect the effects of the present invention.

[0055] [0.4 × [Si] + [Mn] + 0.45 × [Ni] + 0.8 × [Cr] + 2 × [Mo] ≤ 1.40] The chemical composition of an automotive part according to a preferred embodiment of the present invention satisfies the following formula (1): 0.4 × [Si] + [Mn] + 0.45 × [Ni] + 0.8 × [Cr] + 2 × [Mo] ≤ 1.40 ...Formula (1) where [Si], [Mn], [Ni], [Cr], and [Mo] are the mass %) content of Si, Mn, Ni, Cr, and Mo. Formula (1) above is a parameter related to hardenability, and is the sum of values ​​obtained by multiplying the elements related to hardenability by coefficients corresponding to the degree of influence of each element. The larger the value of Formula (1) above, the better the hardenability. By controlling the chemical composition of the automotive part to satisfy the above formula (1), the hardenability, especially at the end face, can be appropriately controlled, making it possible to sufficiently form ferrite at the end face even after hot stamping. From the viewpoint of further ferrite formation at the end face, a smaller value for the left side of the above formula (1) is preferable. The value for the left side of the above formula (1) may be 1.35 or less, 1.30 or less, 1.25 or less, or 1.20 or less. On the other hand, the lower limit of the left side of the above formula (1) is not particularly limited and may be, for example, 1.00 or more, 1.05 or more, or 1.10 or more.

[0056] The chemical composition of the automotive part according to the preferred embodiment can be measured by general analytical methods. For example, the chemical composition of the automotive part can be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) on chips in accordance with JIS G 1201:2014. Specifically, a test piece can be taken from an arbitrary position 0.50 mm away from the end face (if a test piece cannot be taken from this position, a position avoiding the end face can be used), and measured using a Shimadzu ICPS-8100 or similar (measuring device) under conditions based on a pre-prepared 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 surface of an automobile part is coated with a plating or other coating, the coating should be removed by mechanical grinding or similar means before the chemical composition analysis can be performed.

[0057] [Preferred Metal Structure of Automotive Parts] As described above, the present invention aims to provide automotive parts with improved crack resistance at the end faces and a method for manufacturing the same. This is achieved by controlling the Vickers hardness of the end face to be lower than the Vickers hardness of the area inside the end face, while controlling the difference between the Vickers hardness of the end face and the Vickers hardness of the area inside the end face to be 50HV0.025 or more, or to be 10% or more of the Vickers hardness of the end face. Therefore, it is clear that the metal structure of the automotive part itself is not an essential technical feature for achieving the objectives of the present invention. The following describes in detail preferred metal structures for automotive parts having a tensile strength of, for example, 1.5 GPa or more. However, these descriptions are intended merely as examples of preferred metal structures for obtaining a tensile strength of, for example, 1.5 GPa or more, and are not intended to limit the present invention to automotive parts having such specific metal structures. In the following description, "%", which is the unit of microstructure fraction, means "area %" unless otherwise specified. Furthermore, in the automotive part according to the preferred embodiment, the metal structure at a position 1 / 4 of the plate thickness on a flat portion at least 0.20 mm away from the end face (or, if a sample cannot be taken from this position, a position avoiding the end face) is defined. If the automotive part surface 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.

[0058] [Martensite: 80% or more] In the automotive parts according to the preferred embodiment, the metal structure may contain 80% or more martensite by area percentage. From the viewpoint of increasing strength, a higher area percentage of martensite is preferable, for example, 82% or more, 84% or more, 86% or more, 88% or more, 90% or more, 92% or more, or 94% or more. There is no particular upper limit, but for example, the area percentage of martensite may be 100% or less, 99% or less, or 97% or less. In the present invention, "martensite" includes not only as-quenched martensite (so-called fresh martensite) but also tempered martensite.

[0059] [Residual structure: 20% or less] Residual structures other than martensite may be 0% in area percentage, but if residual structures are present, their area percentage shall be 20% or less. If residual structures are present 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 structures shall be 20% or less, and may be 10% or less, 8% or less, 6% or less, 5% or less, 4% or less, or 3% or less. On the other hand, reducing the area percentage of residual structures to 0% requires a high degree of control in the manufacturing process of automotive parts, which may lead to a decrease in yield. Therefore, the area percentage of residual structures may be 0.5% or more, 1% or more, or 2% or more. The residual structures are not particularly limited, but may include, for example, at least one of ferrite, bainite, pearlite, and retained austenite, or at least one of them.

[0060] [Identification and Calculation of Metallographic Structure] [Martensite] For the metallic structure, the identification of martensite and the calculation of its area ratio are performed using secondary electron images captured with a thermal field emission scanning electron microscope (FE-SEM) and X-ray diffraction. First, a sample is taken from a steel plate so that the thickness cross section perpendicular to the plate surface becomes the observation surface. Although it is preferable that the thickness cross section be parallel to the rolling direction, it is not necessary for the thickness cross section to be parallel to the rolling direction if the rolling direction of the steel plate cannot be determined. Next, the cross section of the sample is polished using silicon carbide paper from #600 to #1500, then finished to a mirror surface using a liquid in which diamond powder with a particle size of 1 to 6 μm is dispersed in a diluent such as alcohol or pure water, and then nital etching is performed. Next, the region from the surface of the steel plate to 1 / 4 of the plate thickness on the observation surface is used as the observation field, and secondary electron images are observed using a thermal field emission scanning electron microscope (JEOL JSM-7001F). From the obtained secondary electron images, the total area ratio of martensite and retained austenite, and the total area ratio of ferrite and bainite are measured. First, regions with high brightness and where the underlying structure is not revealed by etching are identified as fresh martensite and retained austenite. Next, regions with a underlying structure and where multiple cementite deposits with different elongation directions are precipitated are identified as tempered martensite. Since martensite is not sufficiently etched by nital etching, it can be distinguished from other structures that are etched. However, since retained austenite is also not sufficiently etched like martensite, the area ratio of martensite is determined by subtracting the area ratio of retained austenite obtained by the X-ray diffraction method described below from the total area ratio of tempered martensite, fresh martensite, and retained austenite.

[0061] The area fraction of retained austenite is calculated by X-ray diffraction. First, the region from the surface of the steel plate to the 1 / 4 position of the plate thickness is removed by mechanical and chemical polishing. Next, the surface of the polished sample is treated with MoKα rays as characteristic X-rays to obtain diffraction peaks at (200) and (211) for the bcc phase, and at (200), (220), and (311) for the fcc phase. The structural fraction of retained austenite is calculated from the integral intensity ratio of these diffraction peaks, and this is taken as the area fraction of retained austenite.

[0062] [Residual Structure] The area percentage of the residual structure is determined by subtracting the area percentage of martensite obtained above from 100%. The residual structure may include or be at least one of, for example, ferrite, bainite, pearlite, and retained austenite, 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 by methods commonly applied by those skilled in the art.

[0063] [Sheet Thickness] The flat portion of the automobile part according to the embodiment of the present invention generally has a thickness of 0.2 to 2.3 mm. Although not particularly limited, the sheet thickness may be 0.3 mm or more or 0.4 mm or more. Similarly, the sheet thickness may be 1.8 mm or less, 1.5 mm or less, 1.2 mm or less, 1.0 mm or less, or 0.8 mm or less. For example, by setting the sheet thickness to 0.2 mm or more, the effect of improving dimensional accuracy and shape accuracy can be obtained. On the other hand, by setting the sheet thickness to 0.8 mm or less, the effect of reducing the weight of the component becomes significant. The thickness of the automobile part is measured with a micrometer.

[0064] [Coating Layer] In an automobile part according to a preferred embodiment of the present invention, a coating layer, such as a plating layer, may be further provided on at least one surface other than the end face 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 coating layers are excluded in determining tensile strength, chemical composition, and metal structure.

[0065] As described above, the automotive parts according to the embodiment of the present invention can achieve excellent crack resistance at the end faces. Therefore, they are useful for use in automotive parts where crack resistance at the end faces is required. Examples of automotive parts include structural parts such as front pillars, center pillars, side sills, and cross members, as well as bumper reinforcements and other structural and reinforcing parts that require strength. These parts only need to include the automotive parts according to the embodiment of the present invention in at least a portion of them, and therefore at least a portion of these automotive parts will satisfy the characteristics of automotive parts described above. In parts of automotive parts that do not directly contact the mold during molding such as press forming, or where the degree of processing is relatively low even if they directly contact the mold, the characteristics of the automotive parts do not change particularly before and after molding.

[0066] [Mechanical Properties] [Crack Resistance at End Face: Hole Expansion Ratio (λ)] According to the embodiment of the present invention, crack resistance at the end face can be improved, more specifically, using a test piece having the same composition as the automobile part, and an end face obtained by the same manufacturing method and processing as the end face of the test piece, and having an end face formed by a hole with a diameter of 10 mm, in a hole expansion test with the said hole as the initial hole, the difference between the Vickers hardness of the end face and the Vickers hardness inside the end face is less than 50HV0.025, and a higher hole expansion ratio can be achieved than that of an automobile part having a Vickers hardness of less than 10% of the Vickers hardness of the end face. The hole expansion ratio is determined by performing a hole expansion test in accordance with JIS Z 2256:2020. More specifically, first, an automobile part having an end face formed by a punched hole (hole diameter d0 = 10 mm) is prepared. Next, if a burr is present, the burr should face the die side, and the initial hole should be widened using a conical punch with a 60° apex angle until a crack penetrates the plate thickness. The hole diameter d1 mm at the time of crack occurrence is measured, and the hole expansion ratio λ (%) for each test piece is calculated using the following formula. This hole expansion test is performed three times, and the average value is determined as the hole expansion ratio λ. λ = 100 × {(d1 - d0) / d0}

[0067] [Tensile Strength (TS)] The tensile strength (TS) of the automobile part according to the embodiment of the present invention may be, for example, 1.5 GPa or more, 1.6 GPa or more, 1.7 GPa or more, 1.8 GPa or more, 1.9 GPa or more, or 2.0 GPa or more, preferably 2.1 GPa, 2.2 GPa or more, 2.3 GPa or more, or 2.4 GPa or more, and more preferably 2.5 GPa or more. The upper limit of the tensile strength is not particularly limited, but may be 4.0 GPa or less, 3.5 GPa or less, or 3.0 GPa or less. The tensile strength is measured by taking a JIS No. 5 test piece from the automobile part and performing a tensile test in accordance with JIS Z 2241:2022. If it is difficult to take a JIS No. 5 test piece from the sample to be measured, the tensile test can be performed on a small test piece and the tensile strength can be determined by converting it to the value for a JIS No. 5 test piece. For example, ASTM E8 Sub-size (6 mm wide) can be used as a micro-test specimen. The specimen should be taken from a part that is not substantially affected by work hardening, thermal effects of welds, or heat and strain during cutting. If it is difficult to obtain a tensile test specimen, the Vickers hardness should be used for evaluation. The Vickers hardness corresponding to the tensile strength may be, for example, 450 HV 0.025 or higher, 480 HV 0.025 or higher, 500 HV 0.025 or higher, 520 HV 0.025 or higher, 550 HV 0.025 or higher, or 570 HV 0.025 or higher, preferably 600 HV 0.025 or higher, 620 HV 0.025 or higher, 650 HV 0.025 or higher, or 670 HV 0.025 or higher, and more preferably 700 HV 0.025 or higher. There is no particular upper limit to the Vickers hardness, but it may be 1060HV 0.025 or less, 940HV 0.025 or less, or 820HV 0.025 or less. Specifically, for example, a Vickers hardness of 450HV 0.025 or higher corresponds to a tensile strength of 1.5 GPa or higher, a Vickers hardness of 570HV 0.025 or higher corresponds to a tensile strength of 2.0 GPa or higher, a Vickers hardness of 650HV 0.025 or higher corresponds to a tensile strength of 2.3 GPa or higher, and a Vickers hardness of 700HV 0.025 or higher corresponds to a tensile strength of 2.5 GPa or higher. The Vickers hardness is measured by the measurement method described in "Vickers hardness inside the end face" above.

[0068] <Method for Manufacturing Automotive Parts> Next, preferred methods for manufacturing automotive parts according to embodiments of the present invention will be described. The following description is intended to illustrate characteristic methods for manufacturing automotive parts according to embodiments of the present invention, and is not intended to limit the automotive parts to those manufactured by the manufacturing methods described below.

[0069] Automotive parts according to embodiments of the present invention, more specifically automotive parts having end faces, can be obtained, for example, by first manufacturing a steel sheet, then forming an end face on the manufactured steel sheet by shearing, and finally shaping the steel sheet having an end face into a predetermined shape by cold pressing or hot stamping (hot pressing). The shearing is performed under conditions where the clearance ratio (CL ratio) between the punch and the die is 5 to 25% and the punching speed is 10 mm / second or more.

[0070] Furthermore, a preferred embodiment of the present invention provides a method for manufacturing automotive parts, characterized by including a shearing step of shearing a steel sheet to form an end face, and a forming step of press-forming the sheared steel sheet by holding it at 800 to 1000°C for 0.5 to 5.0 minutes in an atmosphere with a dew point of 0 to 40°C.

[0071] In a preferred embodiment of the method for manufacturing automotive parts, an end face is formed on the steel sheet in a shearing process, and then in a forming process, an automotive part having an end face is manufactured by hot stamping (hot pressing) in a high dew point atmosphere, more specifically by press forming by holding at 800 to 1000°C for 0.5 to 5.0 minutes in an atmosphere with a dew point of 0 to 40°C. Large processing strain is introduced into the end face formed by the shearing process, and when the steel sheet having the end face is heat-treated in a high dew point atmosphere during hot stamping, decarburization from the end face is promoted, the Vickers hardness of the end face decreases, and it is thought that an automotive part having crack resistance at the end face can be manufactured. Below, the shearing process for forming the end face and the forming process will be described in detail first, and then the manufacturing of the steel sheet itself will be described in detail.

[0072] [Shearing Process] In a preferred embodiment, the shearing process involves shearing the steel plate to form an end face. The shearing can be carried out by appropriate means known to those skilled in the art, such as punching with a press. The shape of the punched-out section is not particularly limited and may be a hole, a curve, or a straight line. The shearing is carried out under conditions where the clearance ratio (CL ratio) between the punch and the die is 5 to 25% and the punching speed is 10 mm / second or more.

[0073] Figure 2 is a schematic cross-sectional view illustrating the shearing process, but is not limited to this case. In the shearing process, first, a steel plate is placed between punching tools such as dies 20 and 21. Then, as shown in Figure 2, the steel plate is sheared by punching with the dies 20 and 21, and cut into automobile parts 10 and scrap 11. End faces 10a are formed on such automobile parts 10 by the shearing process.

[0074] [Forming Process] In a preferred embodiment, the forming process involves pressing the sheared steel sheet by holding it at 800 to 1000°C for 0.5 to 5.0 minutes in an atmosphere with a dew point of 0 to 40°C. In the method for manufacturing automotive parts according to a preferred embodiment of the present invention, hot stamping in a high dew point atmosphere, more specifically, by holding it at 800 to 1000°C for 0.5 to 5.0 minutes in an atmosphere with a dew point of 0 to 40°C and pressing it, makes it possible to promote decarburization from the end faces of the automotive parts. If the dew point is below 0°C, the heating temperature is below 800°C, and / or the holding time is less than 0.5 minutes, decarburization at the end faces of the automotive parts will be insufficient. As a result, in the final automotive parts, it may not be possible to make the difference between the Vickers hardness of the end face and the Vickers hardness inside the end face 50HV0.025 or more, or 10% or more of the Vickers hardness of the end face. To obtain such effects, the dew point is preferably 0°C or higher, and more preferably 6°C or higher from the viewpoint of further promoting decarburization of the end face and sufficiently forming ferrite on the end face. On the other hand, if the dew point is above 40°C, the heating temperature is above 1000°C, and / or the holding time is above 5.0 minutes, an external oxide layer may be formed on the end face of the automobile part, and the strength of the final automobile part may decrease due to excessive decarburization. For this reason, the dew point is preferably 40°C or lower, and more preferably 30°C or lower, 20°C or lower, or 10°C or lower.

[0075] The heating temperature in an atmosphere with a dew point of 0 to 40°C should be such that the steel plate is heated to a temperature in the austenite region, for example, 800 to 1000°C, and preferably 850 to 950°C. The heating method for hot stamping is not limited, but examples include furnace heating, electric heating, and induction heating. The holding time after heating can be appropriately set from 0.5 to 5.0 minutes. More preferably, it is 1.0 to 4.0 minutes, and even more preferably 1.0 to 2.0 minutes. After the heat treatment, cooling (quenching) can be performed at an average cooling rate of, for example, 150°C / second or more. The average cooling rate may be 170°C / second or more, or 200°C / second or more. The upper limit of the average cooling rate is not particularly limited, but may be, for example, 1000°C / second or less, or 500°C / second or less.

[0076] [Manufacturing of the Steel Sheet Itself] A steel sheet according to a preferred embodiment can be manufactured by, for example, a casting process in which molten steel adjusted to the chemical composition described above is cast to form a slab; a hot rolling process in which the slab is hot-rolled to obtain a hot-rolled steel sheet; a pickling process in which the obtained hot-rolled steel sheet is pickled; a cold rolling process in which the pickled hot-rolled steel sheet is cold-rolled; and an annealing process in which the obtained cold-rolled steel sheet is annealed. Each process will be described in detail below.

[0077] [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 plates may be cast by a conventional continuous casting or ingot casting method.

[0078] [Hot Rolling Process] [Slab Heating] First, the cast slab is heated. From the viewpoint of productivity, the slab to be used is preferably cast by the continuous casting method, but it may also be manufactured by the ingot casting method or the thin slab casting method. The slab to be used contains a relatively large amount of alloying elements in order to obtain a high-strength steel plate. For this reason, it is necessary to heat the slab before subjecting it to hot rolling to solid dissolve the alloying elements in the slab. If the heating temperature is low, the alloying elements will not solid dissolve sufficiently in the slab, leaving coarse alloy carbides, which may cause brittle cracking during hot rolling. For this reason, the heating temperature is preferably 1100°C or higher, and more preferably 1200°C or higher. The upper limit of the heating temperature is not particularly limited, but from the viewpoint of the capacity of the heating equipment and productivity, it is preferably 1300°C or lower.

[0079] [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.

[0080] [Finish Rolling] The roughly rolled slab is then subjected to finish rolling. The conditions for finish rolling, such as the temperature and reduction ratio, are not particularly limited and can be appropriately determined according to the desired metal structure and sheet thickness. For example, the final temperature of finish rolling may be 850 to 1050°C, and the reduction ratio of each pass in finish rolling may be 10 to 50%.

[0081] [Cooling and Winding] Next, the finish-rolled steel sheet is cooled to 500°C or less at an average cooling rate of preferably 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 500°C, segregation of P may occur during the hot rolling process, causing the hot-rolled steel sheet to become brittle. For example, the average cooling rate is preferably 25°C / second or more, and the winding temperature is preferably 480°C or less. For example, the average cooling rate is preferably 100°C / second or less, and the winding temperature is preferably 300°C or more. There is no particular upper limit to the winding temperature, but for example, the winding temperature may be 450°C or more.

[0082] [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.

[0083] [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.

[0084] [Annealing Process] The annealing process is an operation that includes heat treatment to adjust the metal structure and properties of cold-rolled steel sheets. The maximum heating temperature in the annealing process is not particularly limited, but may be, for example, 900°C or lower. On the other hand, from the viewpoint of productivity, the maximum heating temperature is preferably 500°C or higher.

[0085] [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.

[0086] [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.

[0087] 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.

[0088] In the following embodiments, automotive parts according to the present invention were manufactured under various conditions, and the characteristics of the obtained automotive parts were investigated.

[0089] The details of the steel plates used in this embodiment are as follows: ・Steel plate 1: Alloyed hot-dip galvanized steel plate, C content: 0.46 mass%, left side of equation (1): 1.34, plate thickness 1.8 mm, tensile strength 2.5 GPa ・Steel plate 2: Alloyed hot-dip galvanized steel plate, C content: 0.33 mass%, left side of equation (1): 1.52, plate thickness 1.4 mm, tensile strength 2.0 GPa Here, "left side of equation (1)" is the left side of the following equation (1) relating to the parameters of hardenability, as explained above. 0.4 × [Si] + [Mn] + 0.45 × [Ni] + 0.8 × [Cr] + 2 × [Mo] ≤ 1.40 …Equation (1) However, [Si], [Mn], [Ni], [Cr] and [Mo] are the mass %) of Si, Mn, Ni, Cr and Mo.

[0090] [Example 1: Manufacturing of Test Piece A1] In Example 1, a test piece A1 as an automobile part was manufactured by shearing, then heating, and shaping. First, a shearing process was performed on the steel plate 1 to punch out a hole with a diameter of 10 mm, thereby forming an end face on the steel plate 1. Next, the sheared steel plate 1 was heated and shaped to obtain a test piece A1 as an automobile part having an end face formed by a hole with a diameter of 10 mm. More specifically, the heating was performed under the conditions of a dew point of 6°C, a temperature of 890°C, and a holding time of 2 minutes, and the average cooling rate of the quenching during shaping was 150°C / second or more.

[0091] [Comparative Example 1: Manufacturing of Test Piece a1] In Comparative Example 1, unlike Example 1, test piece a1 was manufactured by heating, cooling, and then shearing. First, the steel plate 1 was heated under conditions of dew point: 6°C, temperature: 890°C, and holding time: 2 minutes, and then cooled at an average cooling rate of 150°C / second or more. Next, the cooled steel plate 1 was subjected to shearing to punch a hole with a diameter of 10 mm, and test piece a1 having an end face formed by a hole with a diameter of 10 mm was obtained.

[0092] [Example 2: Manufacturing of Test Piece A2] Test piece A2 was obtained in the same manner as in Example 1, except that steel plate 2 was used instead of steel plate 1.

[0093] [Comparative Example 2: Manufacturing of Test Specimen a2] Test specimen a2 was obtained in the same manner as in Comparative Example 1, except that steel plate 2 was used instead of steel plate 1.

[0094] The properties of the obtained automotive parts were evaluated using the following method.

[0095] [Crack Resistance at End Face; Hole Expansion Ratio (λ)] The hole expansion ratio (λ) was determined by performing a hole expansion test in accordance with JIS Z 2256:2020. In each example and comparative example, a test piece was manufactured as an automotive part having an end face formed by a hole with a diameter of 10 mm. This hole was used as the initial hole (hole diameter d0 = 10 mm), and if a burr was present, the burr was positioned on the die side. The initial hole was expanded using a conical punch with a vertex angle of 60° until a crack penetrating the plate thickness occurred. The hole diameter d1 mm at the time of crack occurrence was measured, and the hole expansion ratio λ (%) for each test piece was calculated using the following formula. This hole expansion test was performed three times, and the average value was determined as the hole expansion ratio λ. λ = 100 × {(d1 - d0) / d0}

[0096] For test specimens of automotive parts having end faces, a specimen was evaluated as having superior crack resistance at the end face if the difference between the Vickers hardness of the end face and the Vickers hardness inside the end face was less than 50HV0.025, and the hole expansion ratio was higher than that of a specimen where the difference was less than 10% of the Vickers hardness of the end face. The evaluation results are shown in Tables 1 and 2. The "C content ratio" in Tables 1 and 2 is [C] ｅｎｄ ｓｕｒｆａｃｅ / [C] ｉｎｓｉｄｅ It represents.

[0097]

[0098]

[0099] Referring to Tables 1 and 2, it is believed that in the case of test pieces a1 and a2 in Comparative Examples 1 and 2, because they were heated, molded, and then sheared, decarburization from the end face did not proceed sufficiently, and the difference between the Vickers hardness of the end face and the Vickers hardness inside the end face was less than 50HV0.025, and less than 10% of the Vickers hardness of the end face. As a result, λ was low.

[0100] ​In contrast, test specimens A1 and A2 according to Examples 1 and 2 were controlled so that the Vickers hardness of the end face was lower than the Vickers hardness of the area inside the end face, while controlling the difference between the Vickers hardness of the end face and the area inside the end face to be 50HV 0.025 or more, or 10% or more of the Vickers hardness of the end face. As a result, they had a higher λ and superior crack resistance at the end face compared to test specimens where the difference between the Vickers hardness of the end face and the area inside the end face was less than 50HV 0.025 and less than 10% of the Vickers hardness of the end face. In particular, in Example 1, an optimized chemical composition, more specifically, a steel sheet having a chemical composition containing 0.45% by mass or more of C and satisfying 0.4 × [Si] + [Mn] + 0.45 × [Ni] + 0.8 × [Cr] + 2 × [Mo] ≤ 1.40, was sheared, then heated and held in an atmosphere with a dew point of 6°C, and press-formed to manufacture an automobile part. As a result, even with a high strength of 2.5 GPa or more, 50% or more ferrite was formed at the end face, and the crack resistance at the end face was excellent.

[0101] (Further investigation) [Example 3: Manufacturing of test piece A3] In Example 3, test piece A3 as an automobile part was manufactured by shearing, then heating, and shaping. First, a shearing process was performed on the steel plate 1 to punch out a straight line, forming an end face on the steel plate 1. Next, the sheared steel plate 1 was heated and shaped to obtain test piece A3 as an automobile part having an end face. More specifically, heating was performed under the conditions of dew point: 6°C, temperature: 890°C, and holding time: 2 minutes, and the average cooling rate of quenching during shaping was 150°C / second or more.

[0102] [Comparative Example 3: Manufacturing of Test Piece a3] In Comparative Example 3, unlike Example 3, test piece a3 was manufactured by heating, cooling, and then shearing. First, the steel plate 1 was heated under conditions of a dew point of 6°C, a temperature of 890°C, and a holding time of 2 minutes, and then cooled at an average cooling rate of 150°C / second or more. Next, the cooled steel plate 1 was subjected to a shearing process to punch out a straight line, and test piece a3 with an end face was obtained.

[0103] Table 3 shows the difference in Vickers hardness and the ferrite area ratio at the end face of test specimens A3 and a3.

[0104]

[0105] [Example 4: Manufacturing of Test Piece A4] Test piece A2 was obtained in the same manner as in Example 1, except that steel plate 2 was used instead of steel plate 1.

[0106] [Comparative Example 4: Manufacturing of Test Piece a4] Test piece a2 was obtained in the same manner as in Comparative Example 1, except that steel plate 2 was used instead of steel plate 1.

[0107] Table 4 shows the difference in Vickers hardness between test specimens A4 and a4.

[0108]

[0109] Referring to Tables 3 and 4, it is believed that test pieces a3 and a4 of Comparative Examples 3 and 4, because they were heated, molded, and then blanked, did not decarburize sufficiently from the end face, resulting in a difference of less than 50HV0.025 between the Vickers hardness of the end face and the Vickers hardness of the area inside the end face, and less than 10% of the Vickers hardness of the end face. In contrast, test pieces A3 and A4 of Examples 3 and 4, because they were blanked, heated, and molded, had a Vickers hardness of the end face lower than the Vickers hardness of the area inside the end face, resulting in a difference of 50HV0.025 or more between the Vickers hardness of the end face and the area inside the end face, and more than 10% of the Vickers hardness of the end face. Therefore, it is believed that test pieces A3 and A4, which are intended as automotive parts, have excellent crack resistance at the end face, similar to hole punching, even though they are processed in a straight line.

[0110] In all the test specimens used as automotive parts in the embodiments, the chemical composition was Si: 0.01-1.30%, Mn: 0.05-3.00%, P: 0.100% or less, S: 0.0100% or less, N: 0.0200% or less, O: 0.0200% or less, Al: 0.001-1.000%, Cr: 0-1.00%, Nb: 0-0.200%, Ti: 0-0.200%, Mo: 0-1.00%, B: 0-0.1000%. The composition was as follows: Co: 0-4.00%, Ni: 0-3.00%, Cu: 0-3.00%, V: 0-3.00%, W: 0-1.00%, Ca: 0-1.000%, Mg: 0-1.000%, REM: 0-1.000%, Sb: 0-1.000%, Zr: 0-1.000%, Sn: 0-1.000%, As: 0-0.100%, with the remainder being Fe and impurities. The metallic structure was martensite: 80% or more. The remaining structure other than martensite consisted of at least one of ferrite, bainite, pearlite, and retained austenite.

[0111] 10 Automotive parts 10a End face 10b Surface 11 Scrap 20, 21 Mold

Claims

1. An automobile part having an end face, characterized in that the Vickers hardness of the end face is lower than the Vickers hardness of the area inside the end face, and the difference between the Vickers hardness of the end face and the Vickers hardness of the area inside the end face is 50HV0.025 or more, or 10% or more of the Vickers hardness of the end face.

2. The automobile part according to claim 1, characterized in that the automobile part is a hot-stamped molded body.

3. The automobile part according to claim 1 or 2, characterized in that at least one surface of the automobile part, other than the end face, further has a plating layer.

4. C content inside the end face ([C] ｉｎｓｉｄｅ The carbon content of the end face relative to the given carbon content ([C] ｅｎｄ ｓｕｒｆａｃｅ ) ratio ([C] ｅｎｄ ｓｕｒｆａｃｅ / [C] ｉｎｓｉｄｅ The automobile part according to any one of claims 1 to 3, characterized in that the ratio is 0.80 or less.​​ 5. The automotive part according to any one of claims 1 to 4, characterized in that the end face contains 50% or more ferrite by area percentage.

6. An automobile part according to any one of claims 1 to 5, characterized by having a tensile strength of 1.5 GPa or more.

7. The automobile part according to claim 6, characterized in that it has a tensile strength of 2.0 GPa or more.

8. A method for manufacturing an automobile part according to any one of claims 2 to 7, comprising the following steps: a shearing step of shearing a steel sheet to form the end face; and a forming step of press-forming the sheared steel sheet by holding it at 800 to 1000°C for 0.5 to 5.0 minutes in an atmosphere with a dew point of 0 to 40°C.

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