Structural member for automobile body and method for manufacturing same

Abstract

A structural member, which is for an automobile body and composed of a steel plate, is characterized by comprising: a reference part of the steel plate; and a plate thickness reduction part adjacent to the reference part and formed in a region including at least a portion of an end face of the steel plate, wherein, when the length of the end face in a direction parallel to the plate thickness direction is defined as a plate thickness td, the plate thickness td is 0.50-0.95 times the plate thickness t of the reference part, the plate thickness reduction part is formed from the end face to a position separated from the end face by a predetermined distance L in a direction perpendicular to the end face, and the predetermined distance L is 0.30-3.0 times the plate thickness t.

Description

Structural components for automobile bodies and methods for manufacturing the same

[0001] This disclosure relates to structural components for automobile bodies and methods for manufacturing the same. This application claims priority based on Japanese Patent Application No. 2024-157239, filed in Japan on September 11, 2024, the contents of which are incorporated herein by reference.

[0002] The end faces of steel materials, which are made of steel plates and used as structural components for automobile bodies, are formed by cutting the steel material. During this process, large plastic strain is applied to the end face, and at the same time, large residual stress is generated. If hydrogen enters the end face where residual stress is present, the risk of hydrogen embrittlement increases. Situations where steel materials come into contact with a hydrogen source include, for example, corrosion of steel plates during use in automobiles, and chemical treatment or electrodeposition coating during automobile manufacturing. In recent years, there has been a demand for the use of high-strength steel materials in automobile parts, but steel materials tend to become more susceptible to hydrogen embrittlement as their strength increases. Therefore, the end faces of structural components for automobile bodies, with their high residual stress and high steel strength, can be considered high-risk areas for hydrogen embrittlement.

[0003] Japanese Patent Publication No. 2022-31258 Japanese Patent No. 6784346

[0004] Patent documents 1 and 2 disclose techniques for reducing residual stress.

[0005] Patent Document 1 describes a die having a first surface that contacts one surface of a steel plate and a second surface that is continuous with the first surface via a first ridge portion that serves as a cutting edge; a punch having a third surface that contacts the other surface of the steel plate and a fourth surface that is continuous with the third surface via a second ridge portion that serves as a cutting edge and forms a shear angle α between it and the first ridge portion; a holder that sandwiches the steel plate between the die and the first surface; and a moving device that moves the die or the punch relative to each other along the thickness direction of the steel plate between a position where the fourth surface of the punch faces the holder and a position where the fourth surface faces the second surface of the die, wherein the first ridge portion includes at least one of a first inclined surface and a first curved surface inclined with respect to the first surface, and the second ridge portion includes at least one of a second inclined surface and a second curved surface inclined with respect to the third surface. A shearing apparatus is described in which, when viewed from a direction perpendicular to both the direction of relative movement and the direction in which the fourth surface and the holder face each other, the first inclined surface is used to calculate the second inclined angle difference ΔCB by subtracting the smaller angle between the second inclined surface and the third surface from the smaller angle between the first inclined surface and the first surface, and the radius of curvature difference ΔR is used to calculate the second inclined angle difference ΔCB by subtracting the radius of curvature of the second curved surface from the radius of curvature of the first curved surface [mm], using the formula ΔCB [degrees] = 80 × ΔR, and the sum of the first inclined angle difference ΔCA and the second inclined angle difference ΔCB is taken as the total inclined angle difference ΔCT, the total inclined angle difference ΔCT satisfies (0.15 × α² + 0.05 × α + 1) < ΔCT ≤ 40.

[0006] Furthermore, Patent Document 2 describes a method for manufacturing a pressed part by press-forming a metal sheet having a sheared end face, which includes a first press-forming step in which it is estimated that tensile residual stress will be generated in a part of the sheared end face of the metal sheet in a direction along the shear edge after demolding, and a tensile residual stress relief step as a post-step of the first press-forming step in which a region including at least the portion of the sheared end face in which the tensile residual stress is estimated to be generated is stretched out in the thickness direction of the sheet, and the stretched shape formed by the stretching in the tensile residual stress relief step is set such that the stretch height decreases as it moves away from the sheared end face.

[0007] However, even with these techniques, residual stress could not be sufficiently reduced.

[0008] This disclosure has been made in view of the above-mentioned problems, and the object of this disclosure is to provide a structural member for an automobile body and a method for manufacturing the same that can reduce residual stress present on the end face of a steel material.

[0009] The gist of this disclosure is as follows: (1) A structural member for an automobile body made of a steel plate, comprising: a reference portion of the steel plate; and a plate thickness reduction portion formed in a region adjacent to the reference portion and including at least a part of the end face of the steel plate, wherein the length of the end face in a direction parallel to the plate thickness direction is the plate thickness t ｄ In that case, the plate thickness t ｄ (2) Thickness t of the reference portion is 0.50 to 0.95 times the thickness t of the reference portion, the thickness reduction portion is formed from the end face to a position separated by a predetermined length L in a direction perpendicular to the end face, and the predetermined length L is 0.30 to 3.0 times the thickness t of the reference portion, a structural member for an automobile body. ｄ (1) The structural member for an automobile body, characterized in that the thickness of the plate is 0.50 to 0.90 times the plate thickness t. (3) Plate thickness t of the plate thickness reduction portion ｄ(1) or (2) A structural member for an automobile body, characterized in that it has a plate thickness inclined surface that gradually decreases toward the end face. (4) A structural member for an automobile body, characterized in that when the Vickers hardness of the steel plate is V (Hv), the residual stress of the end face is less than 2.7 × V (MPa). (5) A structural member for an automobile body, characterized in that the Vickers hardness of the steel plate is 340 Hv or more. (6) A structural member for an automobile body, characterized in that a plating layer is formed on the plate thickness reduction portion. (7) A method for manufacturing a structural member for an automobile body, characterized in that it includes a plate thickness reduction step of forming the plate thickness reduction portion of the steel plate. (8) The method for manufacturing a structural member for an automobile body according to (7), characterized in that the plate thickness reduction step is a step of pressing, rolling, or cutting the end of the steel plate.

[0010] According to this disclosure, it is possible to provide a structural member for an automobile body and a method for manufacturing the same that can reduce residual stress present on the end face of a steel material.

[0011] This is a cross-sectional view showing an example of the structure of the end face of a steel material. This is a cross-sectional view showing an example of the structure of the plate thickness reduction portion according to this embodiment test specimen used in the embodiment. This is a schematic side view showing a method of processing the end (forming a processed part) by press working. This is a schematic side view showing a method of processing the end (forming a processed part) by press working. This is a schematic side view showing a method of processing the end (forming a processed part) by cutting. This is a schematic side view showing a method of processing the end (forming a processed part) by cutting. This is a schematic side view showing a method of processing the end (forming a processed part) by rolling. This is a schematic side view illustrating a method of processing the ends (forming a processed part) by rolling.

[0012] Preferred embodiments of the present invention will be described in detail below with reference to the attached drawings. In this specification and drawings, components having substantially the same functional configuration are denoted by the same reference numerals, and redundant descriptions will be omitted.

[0013] <1. An Example of End Face Structure> Figure 1 is a cross-sectional view showing an example of the structure of an end face 901 of a steel material 110, which is a common structural member for automobile bodies. In Figure 1, the direction parallel to the plate thickness direction is defined as upward towards the upper surface 110c and downward towards the lower surface 110b. The steel material 110 is a member obtained by processing a steel plate. The end face 901 is composed of, for example, a burr 901a, a shear surface 901b, and a fracture surface 901c from the upper end. The burr 901a is a curved surface. The shear surface 901b is a flat surface. The fracture surface 901c is often inclined with respect to the shear surface 901b in a direction perpendicular to the plate thickness and perpendicular to the end face. The end face 901 is formed, for example, by shearing the steel material 110. In this case, a large plastic strain is applied to the end face 901 of the steel material 110, so a large residual stress is generated in the end face 901. If hydrogen enters the end face 901, where residual stress exists, the end face 901 undergoes hydrogen embrittlement. When the steel material 110 is used as an automobile part, high strength is required, making the steel material 110 susceptible to hydrogen embrittlement. Therefore, in this embodiment, hydrogen embrittlement of the steel material 110 is suppressed by reducing the residual stress on the end face 901. This improves the delayed fracture resistance. In the following description, "steel material," including "steel material 110" and "steel material 100," will be described as "steel material made of steel plate."

[0014] <2. Steel Material According to This Embodiment> Next, a specific example of the steel material 100 according to this embodiment will be described based on Figures 2 to 10. In Figures 2 to 10, among the directions parallel to the plate thickness direction, the direction from the center in the plate thickness direction toward the upper surface 100c is defined as the upward direction, and the direction from the center in the plate thickness direction toward the lower surface 100b is defined as the downward direction. Figure 2 is a cross-sectional view showing a first example of the steel material 100 according to this embodiment. The hardness of the steel material 100 is not particularly limited, but for example, the Vickers hardness at the reference position 100a1 may be 340 Hv or more. That is, according to this disclosure, it is possible to reduce residual stress present on the end face while maintaining high strength, and the effect of suppressing hydrogen embrittlement of high-strength steel material can be obtained. The same applies to other examples described later. Examples of automobile parts in which the steel material 100 is used include A-pillars, B-pillars, side sills, floor cloths, roof cloths, front side members, rear side members, floor side members, dash cloths, door impact beams, bumper reinforcements, crash boxes, and lower arms. At the end 10 of the steel material 100, a plate thickness reduction portion 10a and a plate thickness reduction surface 10a1 are formed in a region including at least a part of the end face 1. The plate thickness reduction surface 10a1 is formed on the upper surface 100c (the surface on the side of the sag 901a in Figure 1) of the steel material 100. The plate thickness reduction surface 10a1 reduces the plate thickness t indicated by the plate thickness reduction portion 10a to the plate thickness t ｄ This is the part that has been processed to reduce its thickness. In the example in Figure 2, the thickness-reduced surface 10a1 forms a step relative to the upper surface 100c. Furthermore, the thickness-reduced surface 10a1 is formed in a position that includes the portion that was the sag 901a of the end face 901 shown in Figure 1, and the sag 901a has disappeared due to the thickness-reducing process. Note that the portion of the steel plate adjacent to the thickness-reduced portion 10a, where the thickness has not been reduced, is defined as the reference portion 100a, and the thickness of the reference portion 100a is defined as the thickness t of the steel material.

[0015] plate thickness t of plate thickness reduction portion 10a ｄ The distance from the lower end of the plate thickness reduction surface 10a1 to the lower surface 100b of the steel material 100 (the surface on the side where the plate thickness reduction surface 10a1 is not formed), and the length in the direction parallel to the plate thickness direction of the end face 1, is 0.50 to 0.95 times the plate thickness t. Therefore, the thickness t of the plate thickness reduction surface 10a1 is ｅ(The plate thickness t, the plate thickness t of the plate thickness reduction part ｄ ｄ and the difference between them) is 0.05 to 0.50 times the plate thickness t of the steel material made of a steel plate. The plate thickness t may be the plate thickness of the steel plate generally measured, and may be the plate thickness of a part other than the plate thickness reduction part 10a of the steel material, that is, the reference part 100a. Also, the plate thickness t may be the plate thickness at the reference position 100a1. The reference position 100a1 is defined as a position parallel to either the upper surface 100c or the lower surface 100b (the surface located on the side opposite to the upper surface 100c in the steel material 100) of the steel material 100 from the end face 1 of the steel material 100 in the reference part 100a and separated by 4 to 6 times the plate thickness t toward the center of the steel material 100. For example, when the distance from an arbitrary position other than the plate thickness reduction part 10a to the end face 1 is 4 to 6 times the plate thickness t of the steel material 100, that position may be set as the reference position 100a1. The center of the steel material 100 is, for example, the center in the width direction of the steel material 100. The reference position 100a1 is preferably on a flat surface. When it has a nub or a special shape and is not flat, a flat part at a position within the range of 4 to 6 times the plate thickness t from the end face 1 is used as the reference position 100a1. In the following description, there may be cases where the plate thickness t of the steel material is described as the plate thickness at the reference position 100a1.

[0016] Thus, in the steel material 100 according to this embodiment, the plate thickness t of the plate thickness reduction part 10a ｄ ｄ is smaller than the plate thickness t of the steel material and is 0.50 to 0.95 times the plate thickness t. Preferably, the plate thickness t of the plate thickness reduction part 10a ｄ ｄ is from 0.50 to 0.90 times the plate thickness t. As shown in the examples described later, in this case, the residual stress existing on the end face 1 can be reduced. For example, when the Vickers hardness at the reference position 100a1 is V (Hv), the residual stress on the end face 1 can be made less than 2.7 × V (MPa).

[0017] The length L of the plate thickness reduction portion 10a (a predetermined length in a direction parallel to either the upper surface 100c or the lower surface 100b of the steel material 100 from the end surface 1 and perpendicular to the end surface) is 0.30 to 3.0 times the plate thickness t. When L is 0.30 times or more the plate thickness t, the residual stress on the end surface can be sufficiently reduced. When it is 3.0 times or less, the forming process at the end portion becomes easier. The length L of the plate thickness reduction portion 10a is more preferably 1.0 to 3.0 times the plate thickness t, and even more preferably 2.0 to 3.0 times the plate thickness t.

[0018] Incidentally, the method of forming the plate thickness reduction portion 10a, that is, the method of reducing the plate thickness of the end portion 10 of the steel material 100 is not particularly limited. Examples of the method of forming the plate thickness reduction portion 10a include pressing, rolling, or cutting the end portion 10 of the steel material 100.

[0019] A plating layer may be formed on the upper surface 100c and the lower surface 100b of the steel material 100 (hereinafter, these may be collectively referred to as the "surface of the steel material 100"). In this case, as a method of reducing the plate thickness of the end portion 10 of the steel material 100, by adopting the method of pressing or rolling the end portion 10, the plating layer can be left on the plate thickness reduction surface 10a1. That is, in this case, a plating layer is formed on the surface of the steel material 100 including the plate thickness reduction surface 10a1.

[0020] FIG. 3 is a cross-sectional view showing a second example of the steel material 100 according to the present embodiment. A plate thickness reduction portion 10a and a plate thickness reduction surface 10a1 are formed at the end portion 10 of the steel material 100. The plate thickness reduction surface 10a1 is formed on the upper surface 100c side of the steel material 100. The plate thickness reduction surface 10a1 is a portion where the processing of reducing the plate thickness t to the plate thickness t shown by the plate thickness reduction portion 10a has been performed. In the example of FIG. 3, the plate thickness reduction surface 10a1 is a plate thickness inclined surface. That is, the plate thickness reduction surface 10a1 is inclined downward toward the end surface 1 (gradually toward the upper surface 100c of the steel material 100). The plate thickness reduction surface 10a1 is formed at a position including the portion that was the sag 901a of the end surface shown in FIG. 1, and the sag 901a has disappeared due to the plate thickness reduction processing. ｄ The plate thickness t of the plate thickness reduction portion 10a

[0021] ｄ(This is the distance from the lower end of the plate thickness reduction surface 10a1 to the lower surface 100b of the steel material 100 (the surface on the side where the plate thickness reduction surface 10a1 is not formed), and is the length in the direction parallel to the plate thickness direction of the end face 1), and is 0.50 to 0.95 times the plate thickness t. Therefore, the thickness t of the plate thickness reduction surface 10a1 ｅ (The plate thickness t, the plate thickness t of the plate thickness reduction part ｄ and the difference therebetween) is 0.05 to 0.50 times the plate thickness t at a position different from the plate thickness reduction part 10a (for example, the reference position 100a1). The definition of the reference position 100a1 is the same as in the first example.)

[0022] The inclination angle θ of the plate thickness reduction surface 10a first with respect to the upper surface 100c is not particularly limited, and may be, for example, more than 0 to 20 degrees. In this case, the residual stress of the end face 1 can be further reduced.)

[0023] Thus, in the steel material 100 according to the present embodiment, the plate thickness t of the plate thickness reduction part 10a ｄ is smaller than the plate thickness t and is 0.50 to 0.95 times the plate thickness t. Preferably, the plate thickness t of the plate thickness reduction part 10a ｄ is 0.50 to 0.90 times the plate thickness t. As shown in the examples described later, in this case, the residual stress existing on the end face 1 can be reduced. For example, when the Vickers hardness of the reference position 100a1 is V (Hv), the residual stress of the end face 1 can be made less than 2.7 × V (MPa).

[0024] The length L of the plate thickness reduction part 10a is 0.30 to 3.0 times the plate thickness t. When the length L is 0.30 times or more the plate thickness t, the residual stress of the end face can be sufficiently reduced. When the length L is 3.0 times or less the plate thickness t, the forming process of the end part becomes easy. The length L of the plate thickness reduction part 10a is more preferably 1.0 to 3.0 times the plate thickness t, and even more preferably 2.0 to 3.0 times the plate thickness t.)

[0025] The method for forming the plate thickness reduction portion 10a, that is, the method for reducing the plate thickness of the end portion 10 of the steel material 100, is not particularly limited. Examples of methods for forming the plate thickness reduction portion 10a include pressing, rolling, or cutting the end portion 10 of the steel material 100. It is preferable to form the plate thickness reduction portion 10a by pressing because it can reduce residual stress.

[0026] A plating layer may be formed on the surface of the steel material 100. In this case, by using a method of pressing or rolling the end 10 of the steel material 100 to reduce the plate thickness of the end 10, the plating layer can be left on the surface of the plate thickness reduction surface 10a1. That is, in this case, a plating layer is formed on the surface of the steel material 100 including the plate thickness reduction surface 10a1.

[0027] Figure 4 is a cross-sectional view showing a third example of the steel material 100 according to this embodiment. A plate thickness reduction portion 10a and a plate thickness reduction surface 10a1 are formed at the end 10 of the steel material 100. The plate thickness reduction surface 10a1 is formed on the lower surface 100b (the surface on the fracture surface 1c side) of the steel material 100. The plate thickness reduction surface 10a1 reduces the plate thickness t indicated by the plate thickness reduction portion 10a to the plate thickness t ｄ This is the part that has been processed to reduce its thickness. In the example in Figure 4, the plate thickness reduction surface 10a1 forms a step with respect to the lower surface 100b. Part of the fracture surface 1c is lost due to the plate thickness reduction surface 10a1. In other words, the third example is an example in which the formation position of the plate thickness reduction surface 10a1 is changed compared to the first example.

[0028] plate thickness t of plate thickness reduction portion 10a ｄ The distance from the upper end of the plate thickness reduction surface 10a1 to the upper surface 100c of the steel material 100 (the surface on the side where the plate thickness reduction surface 10a1 is not formed), and the length in the direction parallel to the plate thickness direction of the end face 1, is 0.50 to 0.95 times the plate thickness t. Therefore, the thickness t of the plate thickness reduction surface 10a1 is ｅ (plate thickness t and plate thickness t of the plate thickness reduction portion) ｄ The difference between (t) and (t) is 0.05 to 0.50 times the plate thickness t at a position different from the plate thickness reduction portion 10a (for example, the reference position 100a1). The definition of the reference position 100a1 is the same as in the first example.

[0029] Thus, in the steel material 100 according to this embodiment, the plate thickness t of the plate thickness reduction portion 10a ｄ The thickness of the reduced portion 10a is smaller than the plate thickness t, and is 0.50 to 0.95 times the plate thickness t. Preferably, the plate thickness t of the reduced portion 10a is ｄ This is 0.50 to 0.90 times the plate thickness t. As shown in the embodiments described later, in this case, the residual stress present on the end face 1 can be reduced. For example, when the Vickers hardness at the reference position 100a1 is V (Hv), the residual stress on the end face 1 can be reduced to less than 2.7 × V (MPa).

[0030] The length L of the plate thickness reduction portion 10a is 0.30 to 3.0 times the plate thickness t. A length L of 0.30 times or more of the plate thickness t allows for sufficient reduction of residual stress at the end face. A length of 3.0 times or less facilitates the formation of the end face. A length L of the plate thickness reduction portion 10a is more preferably 1.0 to 3.0 times the plate thickness t, and even more preferably 2.0 to 3.0 times the plate thickness t.

[0031] The method for forming the plate thickness reduction portion 10a, that is, the method for reducing the plate thickness of the end portion 10 of the steel material 100, is not particularly limited. Examples of methods for forming the plate thickness reduction portion 10a include pressing, rolling, or cutting the end portion 10 of the steel material 100.

[0032] A plating layer may be formed on the surface of the steel material 100. In this case, by using a method of pressing or rolling the end 10 of the steel material 100 to reduce the plate thickness of the end 10, the plating layer can be left on the surface of the plate thickness reduction surface 10a1. That is, in this case, a plating layer is formed on the surface of the steel material 100 including the plate thickness reduction surface 10a1.

[0033] Figure 5 is a cross-sectional view showing a fourth example of the steel material 100 according to this embodiment. A plate thickness reduction portion 10a and a plate thickness reduction surface 10a1 are formed at the end 10 of the steel material 100. The plate thickness reduction surface 10a1 is formed on the lower surface 100b (the surface on the fracture surface 1c side) of the steel material 100. The plate thickness reduction surface 10a1 reduces the plate thickness t indicated by the plate thickness reduction portion 10a to the plate thickness t ｄThis is the part that has been processed to reduce its thickness. In the example in Figure 5, the plate thickness reduction surface 10a1 is a plate thickness inclined surface. That is, the plate thickness reduction surface 10a1 is inclined upward toward the end face 1 (gradually toward the lower surface 100b of the steel material 100). A portion of the fracture surface 1c is lost due to the plate thickness reduction surface 10a1.

[0034] plate thickness t of plate thickness reduction portion 10a ｄ The distance from the upper end of the plate thickness reduction surface 10a1 to the upper surface 100c of the steel material 100 (the surface on the side where the plate thickness reduction surface 10a1 is not formed), and the length in the direction parallel to the plate thickness direction of the end face 1, is 0.50 to 0.95 times the plate thickness t. Therefore, the thickness t of the plate thickness reduction surface 10a1 is ｅ (plate thickness t and plate thickness t of the plate thickness reduction portion) ｄ The difference between (t) and (t) is 0.05 to 0.50 times the plate thickness t at a position different from the plate thickness reduction portion 10a (for example, the reference position 100a1). The definition of the reference position 100a1 is the same as in the first example.

[0035] The inclination angle θ of the plate thickness reduction surface 10a1 with respect to the lower surface 100b is not particularly limited, but may be, for example, greater than 0 to 20 degrees. In this case, the residual stress on the end face 1 can be further reduced.

[0036] Thus, in the steel material 100 according to this embodiment, the plate thickness t of the plate thickness reduction portion 10a ｄ The thickness of the reduced portion 10a is smaller than the plate thickness t, and is 0.50 to 0.95 times the plate thickness t. Preferably, the plate thickness t of the reduced portion 10a is ｄ This is 0.50 to 0.90 times the plate thickness t. As shown in the embodiments described later, in this case, the residual stress present on the end face 1 can be reduced. For example, when the Vickers hardness at the reference position 100a1 is V (Hv), the residual stress on the end face 1 can be reduced to less than 2.7 × V (MPa).

[0037] The length L of the plate thickness reduction portion 10a is 0.30 to 3.0 times the plate thickness t. A length L of 0.30 times or more of the plate thickness t allows for sufficient reduction of residual stress at the end face. A length of 3.0 times or less facilitates the formation of the end face. A length L of the plate thickness reduction portion 10a is more preferably 1.0 to 3.0 times the plate thickness t, and even more preferably 2.0 to 3.0 times the plate thickness t.

[0038] The method for forming the plate thickness reduction portion 10a, that is, the method for reducing the plate thickness of the end portion 10 of the steel material 100, is not particularly limited. Examples of methods for forming the plate thickness reduction portion 10a include pressing, rolling, or cutting the end portion 10 of the steel material 100.

[0039] A plating layer may be formed on the surface of the steel material 100. In this case, by using a method of pressing or rolling the end 10 of the steel material 100 to reduce the plate thickness of the end 10, the plating layer can be left on the surface of the plate thickness reduction surface 10a1. That is, in this case, a plating layer is formed on the surface of the steel material 100 including the plate thickness reduction surface 10a1.

[0040] Figure 6 is a cross-sectional view showing a fifth example of the steel material 100 according to this embodiment. A plate thickness reduction portion 10a and a plate thickness reduction surface 10a1 are formed at the end 10 of the steel material 100. The plate thickness reduction surface 10a1 is formed on the upper surface 100c side of the steel material 100. The plate thickness reduction surface 10a1 reduces the plate thickness t indicated by the plate thickness reduction portion 10a to the plate thickness t ｄ This is the part that has been processed to reduce its thickness. In the example in Figure 6, it is a combination of a step and a plate thickness inclined surface. That is, a slight step is formed on the inside of the plate thickness reduction surface 10a1 (the inside side of the steel material 100) relative to the upper surface 100c, and it slopes downward from the lower end of the step towards the end face 1 (gradually toward the upper surface 100c side of the steel material 100). The plate thickness reduction surface 10a1 is formed in a position that includes the part that was the sag 901a of the end face 901 shown in Figure 1, and the sag 901a disappears due to the plate thickness reduction surface 10a1.

[0041] plate thickness t of plate thickness reduction portion 10a ｄ The distance from the lower end of the plate thickness reduction surface 10a1 to the lower surface 100b of the steel material 100 (the surface on the side where the plate thickness reduction surface 10a1 is not formed), and the length in the direction parallel to the plate thickness direction of the end face 1, is 0.50 to 0.95 times the plate thickness t. Therefore, the thickness t of the plate thickness reduction surface 10a1 is ｅ (plate thickness t and plate thickness t of the plate thickness reduction portion) ｄThe difference between (t) and (t) is 0.05 to 0.50 times the plate thickness t at a position different from the plate thickness reduction portion 10a (for example, the reference position 100a1). The definition of the reference position 100a1 is the same as in the first example.

[0042] The inclination angle θ of the plate thickness reduction surface 10a1 with respect to the upper surface 100c is not particularly limited, but may be, for example, greater than 0 to 20 degrees. In this case, the residual stress on the end face 1 can be further reduced.

[0043] Thus, in the steel material 100 according to this embodiment, the plate thickness t of the plate thickness reduction portion 10a ｄ The thickness of the reduced portion 10a is smaller than the plate thickness t, and is 0.50 to 0.95 times the plate thickness t. Preferably, the plate thickness t of the reduced portion 10a is ｄ This is 0.50 to 0.90 times the plate thickness t. As shown in the embodiments described later, in this case, the residual stress present on the end face 1 can be reduced. For example, when the Vickers hardness at the reference position 100a1 is V (Hv), the residual stress on the end face 1 can be reduced to less than 2.7 × V (MPa).

[0044] The length L of the plate thickness reduction portion 10a is 0.30 to 3.0 times the plate thickness t. A length L of 0.30 times or more of the plate thickness t allows for sufficient reduction of residual stress at the end face. A length of 3.0 times or less facilitates the formation of the end face. A length L of the plate thickness reduction portion 10a is more preferably 1.0 to 3.0 times the plate thickness t, and even more preferably 2.0 to 3.0 times the plate thickness t.

[0045] The method for forming the plate thickness reduction portion 10a, that is, the method for reducing the plate thickness of the end portion 10 of the steel material 100, is not particularly limited. Examples of methods for forming the plate thickness reduction portion 10a include pressing, rolling, or cutting the end portion 10 of the steel material 100.

[0046] A plating layer may be formed on the surface of the steel material 100. In this case, by using a method of pressing or rolling the end 10 of the steel material 100 to reduce the plate thickness of the end 10, the plating layer can be left on the surface of the plate thickness reduction surface 10a1. That is, in this case, a plating layer is formed on the surface of the steel material 100 including the plate thickness reduction surface 10a1.

[0047] Figure 7 is a cross-sectional view showing a sixth example of the steel material 100 according to this embodiment. A plate thickness reduction portion 10a and a plate thickness reduction surface 10a1 are formed at the end 10 of the steel material 100. The plate thickness reduction surface 10a1 is formed on the lower surface 100b (the surface on the fracture surface 1c side) of the steel material 100. The plate thickness reduction surface 10a1 reduces the plate thickness t indicated by the plate thickness reduction portion 10a to the plate thickness t ｄ This is the part that has been processed to reduce its thickness. In the example in Figure 7, it is a combination of a step and a plate thickness inclined surface. That is, a slight step is formed on the inside of the plate thickness reduction surface 10a1 (the inside side of the steel material 100) relative to the top surface 100c, and it slopes upward from the top end of the step towards the end face 1 (gradually towards the top surface 100c side of the steel material 100). A portion of the fracture surface 1c disappears due to the plate thickness reduction process.

[0048] plate thickness t of plate thickness reduction portion 10a ｄ The distance from the upper end of the plate thickness reduction surface 10a1 to the upper surface 100c of the steel material 100 (the surface on the side where the plate thickness reduction surface 10a1 is not formed), and the length in the direction parallel to the plate thickness direction of the end face 1, is 0.50 to 0.95 times the plate thickness t. Therefore, the thickness t of the plate thickness reduction surface 10a1 is ｅ (plate thickness t and plate thickness t of the plate thickness reduction portion) ｄ The difference between (t) and (t) is 0.05 to 0.50 times the plate thickness t at a position different from the plate thickness reduction portion 10a (for example, the reference position 100a1). The definition of the reference position 100a1 is the same as in the first example.

[0049] The inclination angle θ of the surface of the plate thickness reduction surface 10a1 with respect to the lower surface 100b is not particularly limited, but may be, for example, greater than 0 to 20 degrees. In this case, the residual stress on the end face 1 can be further reduced.

[0050] Thus, in the steel material 100 according to this embodiment, the plate thickness t of the plate thickness reduction portion 10a ｄ The thickness of the reduced portion 10a is smaller than the plate thickness t, and is 0.50 to 0.95 times the plate thickness t. Preferably, the plate thickness t of the reduced portion 10a is ｄThis is 0.50 to 0.90 times the plate thickness t. As shown in the embodiments described later, in this case, the residual stress present on the end face 1 can be reduced. For example, when the Vickers hardness at the reference position 100a1 is V (Hv), the residual stress on the end face 1 can be reduced to less than 2.7 × V (MPa).

[0051] The length L of the plate thickness reduction portion 10a is 0.30 to 3.0 times the plate thickness t. When L is 0.30 times or more the plate thickness t, the residual stress on the end face can be sufficiently reduced. When it is 3.0 times or less the plate thickness t, the end face forming process is facilitated. It is more preferable that the length L of the plate thickness reduction portion 10a is 1.0 to 3.0 times the plate thickness t, and even more preferable that it is 2.0 to 3.0 times the plate thickness t.

[0052] The method for forming the plate thickness reduction portion 10a, that is, the method for reducing the plate thickness of the end portion 10 of the steel material 100, is not particularly limited. Examples of methods for forming the plate thickness reduction portion 10a include pressing, rolling, or cutting the end portion 10 of the steel material 100.

[0053] A plating layer may be formed on the surface of the steel material 100. In this case, by using a method of pressing or rolling the end 10 of the steel material 100 to reduce the plate thickness of the end 10, the plating layer can be left on the surface of the plate thickness reduction surface 10a1. That is, in this case, a plating layer is formed on the surface of the steel material 100 including the plate thickness reduction surface 10a1.

[0054] Figures 8 to 10 are cross-sectional views showing seventh to ninth examples of the steel material 100 according to this embodiment. In the above-described example, the plate thickness reduction surface 10a1 is formed on either the lower surface 100b or the upper surface 100c, but in the seventh to ninth examples, the plate thickness reduction surface is formed on both the upper surface 100c and the lower surface 100b. In the example shown in Figure 10, the plate thickness reduction surface 10a1 is formed on the upper surface 100c. １ The step is a plate thickness reduction surface 10a1 on the lower surface 100b. ２Although a plate thickness inclined surface is shown, a plate thickness inclined surface may be formed on the upper surface 100c and a step on the lower surface 100b. That is, Figures 8 to 10 are examples that combine the example in which the step shown in the first and third examples is formed on the plate thickness reduction portion 10a and the example in which the plate thickness inclined surface shown in the second and fourth examples is formed on the plate thickness reduction portion 10a, and the upper surface 100c has a thickness t ｅ１ plate thickness reduction surface 10a1 １ A layer is formed, and the lower surface 100b has a thickness t ｅ２ plate thickness reduction surface 10a1 ２ This forms a plate thickness reduction surface 10a1, which is formed by combining the aforementioned combinations with the fifth and sixth examples. １ and the plate thickness reduction surface 10a1 ２ It may form.

[0055] The above describes examples of the plate thickness reduction portion 10a and the plate thickness reduction surface 10a1, but the present invention is not limited to the above examples. It is sufficient that the plate thickness reduction portion 10a is formed on the end 10 of the steel material 100 by some means. For example, it is preferable that the plate thickness reduction portion 10a is formed on all end 10 of the steel material 100, because residual stress often exists at the end 10 (more specifically, the end face 1).

[0056] <3. Method for Manufacturing Automobile Structural Members> Next, a method for manufacturing the steel material 100 will be described. First, the steel material 110 is prepared. The steel material 110 is not particularly limited, but is a member obtained by processing a steel plate. The steel material 110 has an end face 901, for example, that is generated when blanking a coil. There is a large residual stress on the end face 901. Next, after press forming with a die, a plate thickness reduction process is performed to reduce the plate thickness of the end of the steel material 110. The specific method of press forming is not particularly limited, and examples include shearing, bending, or drawing. Also, the specific method of reducing the plate thickness is not particularly limited, and examples include pressing, rolling, or cutting the end of the steel material 110. The plate thickness reduction process forms a plate thickness reduction portion 10a and a plate thickness reduction surface 10a1 at the end. If the steel material 110 has a plating layer, pressing or rolling the end is preferred. This allows the plating layer to remain on the surface of the plate thickness reduction surface 10a1. Furthermore, since the plate thickness reduction process is performed by press working, residual stress can be further reduced. The steel material 100 is manufactured through the above process. Note that the press forming and plate thickness reduction processes may be performed in any order. For example, the plate thickness reduction process may be performed on the end face 901 of the steel material 110 before press forming.

[0057] As described above, since the steel material 100 according to this embodiment has a plate thickness reduction portion 10a formed at the end portion 10, the residual stress on the end face 1 can be reduced. This suppresses hydrogen embrittlement. In other words, the delayed fracture resistance characteristics of the steel material 100 can be improved.

[0058] Next, an example of this embodiment will be described. The steel materials A and B used in this embodiment are alloyed hot-dip galvanized steel sheets, and their strengths are as shown in Table 1. The yield stress YS, tensile strength TS, and elongation EL are average values ​​measured in the rolling direction and perpendicular to the rolling direction using a JIS No. 5 tensile test specimen in accordance with JIS Z2241:2023.

[0059]

[0060] Furthermore, the plate thickness t of steel materials A and B is 1.6 mm. The reference position 100a1 was set parallel to either the upper surface 100c or the lower surface 100b of the steel material from the end face 1, and at a distance of 1.6 mm × 5 times toward the center of the steel material. The plate thickness at this reference position 100a1 was measured by the following method, and it was confirmed that the plate thickness was 1.6 mm. That is, the cross section including the reference position 100a1 was exposed by mirror polishing, and the cross section was observed with a microscope. As a result, it was confirmed that the plate thickness at the reference position 100a1 was 1.6 mm.

[0061] Next, as shown in Figures 11(a) to (c), a hat-shaped test specimen 200 was manufactured using steel materials A and B. Specifically, a hat-shaped member 210 was manufactured by sandwiching steel materials A and B in a mold. As shown in Figure 11(b), the length of the hat-shaped member 210 was 400 mm, and as shown in Figure 11(c), the hat-shaped member 210 had a top plate portion 210a, a vertical wall portion 210b, and a flange portion 210c. The width of the top plate portion 210a was 80 mm, the height of the vertical wall portion 210b was 60 mm, the inclination angle of the vertical wall portion 210b from the top plate portion 210a was 95 degrees, and the total width of the hat-shaped member 210 was 160 mm.

[0062] Next, as shown in Figure 11(a), the end of the hat-shaped member 210 in the longitudinal center was punched out in a semicircular shape with a diameter of 10 mm to form the test section 300. The end 10 formed in the test section 300 was processed using the method shown in Figures 12 to 15 to form the plate thickness reduction section 10a and plate thickness reduction surface 10a1 shown in Figure 2 or 3 on the end 10. Next, the flange section 210c of the hat-shaped member 210 and the back plate 220 were joined by spot welding. A total of 16 spot welds 240 were formed at a position 20 mm from the flange end and at positions 40 mm apart.

[0063] Figure 12 shows a method for forming the plate thickness reduction portion 10a and plate thickness reduction surface 10a1 shown in Figure 2 on the end portion 10 by press working. In this method, the flange portion 210c is sandwiched between the plate holding member 1000 and the lower die 1100, and the end portion 10 is pressed with an indenter 1200. Here, the bottom surface 1210 of the indenter 1200 is flat.

[0064] Figure 13 shows a method for forming the plate thickness reduction portion 10a and plate thickness reduction surface 10a1 shown in Figure 3 on the end portion 10 by press working. In this method, the flange portion 210c is sandwiched between the plate holding member 1000 and the lower die 1100, and the end portion 10 is pressed with an indenter 1200. Here, the bottom surface 1210 of the indenter 1200 is inclined.

[0065] Figure 14 shows a method for forming the plate thickness reduction portion 10a and plate thickness reduction surface 10a1 shown in Figure 2 on the end portion 10 by cutting. In this method, the flange portion 210c is sandwiched between support members 2000, and the end portion 10 is cut with a cutting tool 2100. Here, the cutting tool 2100 is pressed perpendicularly against the end portion 10.

[0066] Figure 15 shows a method for forming the plate thickness reduction portion 10a and plate thickness reduction surface 10a1 shown in Figure 3 on the end portion 10 by cutting. In this method, the flange portion 210c is sandwiched between the support member 2000, and the end portion 10 is cut with a cutting tool 2100. Here, the cutting tool 2100 is pressed against the end portion 10 at an oblique angle.

[0067] Figures 16, 17, and 18 show a method for forming a plate thickness reduction portion 10a and a plate thickness reduction surface 10a1 at the end portion 10 by rolling. In these methods, a plate thickness reduction surface is formed on both the lower surface 100b and the upper surface 100c of the end portion 10.

[0068] In the method shown in Figure 16, the end portion 10 is clamped between the roll 3000 and rolled. The rolling surface 3010 of the roll 3000 is flat. As a result, the plate thickness reduction surface 10a1 shown in Figure 2 is formed on both the lower surface 100b and the upper surface 100c.

[0069] In the method shown in Figure 17, the end portion 10 is clamped between the roll 3000 and rolled. The rolling surface 3010 of the roll 3000 is inclined. As a result, the plate thickness reduction surface 10a1 shown in Figure 3 is formed on both the lower surface 100b and the upper surface 100c.

[0070] In the method shown in Figure 18, the end portion 10 is sandwiched between rolls 3000a and 3000b and rolled. The rolling surface 3010a of roll 3000a is flat, but the rolling surface 3010b of roll 3000b is inclined. As a result, as shown in Figure 10, the plate thickness reduction surface 10a1 that was formed on the lower surface 100b in Figure 2 is formed on the upper surface 100c, and the plate thickness reduction surface 10a1 that was formed on the upper surface 100c in Figure 3 is formed on the lower surface 100b. Note that rolls 3000a and 3000b shown in Figure 18 may be mounted in reverse. In that case, the plate thickness reduction surface 10a1 shown in Figure 2 will be formed on the lower surface 100b, and the plate thickness reduction surface 10a1 shown in Figure 3 will be formed on the upper surface 100c.

[0071] Next, the residual stress at the center of each end face 1 in the thickness direction was measured using XRD. The residual stress was measured in (1) the thickness direction and (2) a direction perpendicular to the thickness direction and parallel to the end face, and the larger value was taken as the residual stress of that end face 1. The measurements were performed using an X-ray diffractometer (Rigaku Corporation, model RINT-TTR III), with an X-ray output of 50 kV and 300 mA, a copper target, a TTR goniometer (horizontal goniometer), a Kβ filter with a slit width of 0.05 mm, a longitudinal limiting slit width of 2 mm, a light-receiving slit width of 8 mm, and two light-receiving slits open. The measurement conditions were a scan speed of 5 deg. / min, a step width of 0.01 deg, and a scan axis of 2θ (5-90°). Next, the end portion 10 was immersed in hydrochloric acid at pH 2 for 48 hours. Then, the entire end surface 1, which had undergone plate thickness reduction processing, was observed with a microscope at a 50x field of view to check for the presence or absence of cracks larger than 100 μm. The results are shown in Tables 2 to 4.

[0072]

[0073] Here, the parameters in Tables 2 to 4 are explained. The material is either steel A or B. The flat surface hardness V is the Vickers hardness V (Hv) at the reference position 100a1. Specifically, the sample was embedded in resin, mirror polished, and the average of five measurements taken at a position 1 / 4 of the plate thickness was taken as the flat surface hardness V. The test load at this time was 4.903 N, and other conditions were carried out in accordance with JIS Z 2244-1:2024. The end shape creation method is shown as the processing method applied to the end 10. The end shape is shown as the shape of the plate thickness reduction portion 10a. The plate thickness reduction ratio is t ｄ This is the value of / t. If the requirements of this embodiment are met, the plate thickness reduction ratio will be 0.50 to 0.95. Plate thickness t of the plate thickness reduction portion 10a ｄ This was measured as the length in the thickness direction of end face 1. Specifically, it was defined as the distance between the end face 1 on the fracture surface 1c side, the intersection point of a straight line drawn in the thickness direction from end face 1 on the fracture surface 1c side with the opposite surface, and the end on the fracture surface 1c side. The length L of the thickness reduction portion is as described above. If the requirements of this embodiment are met, the length L of the thickness reduction portion will be 0.30t or more and 3.0t or less. The residual stress is the residual stress measured by the method described above. The value is shown as a ratio to the Vickers hardness V (Hv) at the reference position 100a1. For hydrogen embrittlement, the evaluation results are shown based on the presence or absence of cracks. Examples / Comparative Examples indicate whether each example belongs to an example or a comparative example.

[0074] Table 2 shows an example where no processing was applied to the end portion 10. In the example shown in Table 2, no processing was applied to the end portion 10, resulting in high residual stress on the end face 1 and the observation of cracks due to hydrogen embrittlement. Therefore, all the examples shown in Table 2 are comparative examples.

[0075]

[0076] Table 3 shows examples in which the plate thickness reduction portion 10a and plate thickness reduction surface 10a1 shown in Figure 2 are formed by press working. Examples No. 3 to 5 and No. 9 that satisfy the requirements of this embodiment are examples. In these examples, residual stress was low and no cracks due to hydrogen embrittlement were observed.

[0077] No. 6 is a comparative example because the plate thickness reduction ratio does not meet the requirements of this embodiment. In No. 6, residual stress increased, and cracks due to hydrogen embrittlement were also observed. Nos. 7 and 8 are comparative examples because the length L of the plate thickness reduction portion 10a does not meet the requirements of this embodiment. In all of these examples, residual stress increased, and cracks due to hydrogen embrittlement were also observed.

[0078]

[0079] Table 4 shows examples of forming a plate thickness reduction section 10a including the inclined plate thickness reduction surface 10a1 shown in Figure 3 by pressing, rolling, or cutting. In these examples, the plate thickness reduction rate and the length L of the plate thickness reduction section 10a are varied.

[0080] All of these examples satisfy the requirements of this embodiment and are therefore considered embodiments. In these examples, residual stress was low, and no cracks due to hydrogen embrittlement were observed. Furthermore, when the edges were formed by cutting and rolling, the residual stress was higher than when pressed. For these reasons, it is preferable to form the edges of the plate thickness reduction section by pressing.

[0081] Although preferred embodiments of the present invention have been described in detail above with reference to the attached drawings, the present invention is not limited to these examples. It is clear to any person with ordinary skill in the art to which the present invention belongs that various modifications or alterations can be conceived within the scope of the technical idea described in the claims, and these are also understood to fall within the technical scope of the present invention.

[0082] 1 End face 10 End part 10a Plate thickness reduction part 10a1 Plate thickness reduction surface 100 Steel material 100a Reference part 100a1 Reference position 100b Bottom surface 100c Top surface t Plate thickness

Claims

1. A structural member for an automobile body made of steel plate, comprising: a reference portion of the steel plate; and a plate thickness reduction portion formed in a region adjacent to the reference portion and including at least a part of the end face of the steel plate, wherein the length of the end face in a direction parallel to the plate thickness direction is the plate thickness t ｄ In that case, the plate thickness t ｄ A structural member for an automobile body, characterized in that the thickness of the reference portion is 0.50 to 0.95 times the thickness t of the reference portion, the thickness reduction portion is formed from the end face to a position separated by a predetermined length L in a direction perpendicular to the end face, and the predetermined length L is 0.30 to 3.0 times the thickness t.

2. Thickness t of the portion with reduced thickness ｄ The structural member for an automobile body according to claim 1, characterized in that the thickness of the plate is 0.50 to 0.90 times the plate thickness t.

3. The plate thickness decreasing portion has a plate thickness t ｄ A structural member for an automobile body according to claim 1 or 2, characterized in that it has a plate thickness inclined surface that gradually decreases toward the end face.

4. The structural member for an automobile body according to claim 1 or 2, characterized in that, when the Vickers hardness of the steel plate is V (Hv), the residual stress on the end face is less than 2.7 × V (MPa).

5. The structural member for an automobile body according to claim 1 or 2, characterized in that the Vickers hardness of the steel plate is 340 Hv or higher.

6. The structural member for an automobile body according to claim 1 or 2, characterized in that a plating layer is formed on the portion where the plate thickness is reduced.

7. A method for manufacturing a structural member for an automobile body according to claim 1, characterized in that it includes a plate thickness reduction step for forming the plate thickness reduction portion of the steel plate.

8. The method for manufacturing a structural member for an automobile body according to claim 7, characterized in that the plate thickness reduction step is a step of pressing, rolling, or cutting the end of the steel plate.

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