Structural member and method for manufacturing the same

Abstract

The manufacturing method for the structural member (10) comprises the steps of preparing a blank (30) and forming the blank (30) into a structural member (10). In the blank (30), the ends of the first metal plate (31) are joined together by welding with the ends of the second metal plate (32) overlapping each other. The structural member (10) includes a first member (11) and a second member (12). In the forming step, the first member (11) is restrained from a first direction (D1) by a mold (40), and the flanges (123, 123a, 123b) of the second member (12) are restrained from a second direction (D2), while the top plate (121) and vertical walls (122, 122a, 122b) of the second member (12) are formed by the mold (40) from the second direction (D2).

Description

Technical Field

[0001] The present disclosure relates to a structural member and a method for manufacturing the same.

Background Art

[0002] Structures such as automobile bodies are composed of a plurality of structural members. Each structural member is produced, for example, by processing a blank using a mold. The structure is formed by joining the formed structural members by welding.

[0003] In recent years, in the automotive field, the movement to join and integrate structural members for vehicle bodies at the blank stage has been accelerating. When forming a module composed of a plurality of structural members from a single blank, the number of parts of the vehicle body can be reduced compared to the case of joining the plurality of structural members after forming. As a result, the manufacturing process of the vehicle body is omitted, and it is possible to reduce the amount of greenhouse gas (life cycle GHG) emissions throughout the life cycle of the vehicle body.

[0004] However, conventionally, when forming a module composed of a plurality of structural members as one structural member, in this structural member, a portion having a negative angle with respect to the processing direction of the mold may occur. When there is a portion having a negative angle with respect to the processing direction, it becomes difficult to form the structural member.

[0005] Patent Document 1 discloses a technique for forming a negative angle portion (grooved bead portion) on the left and right vertical walls of a structural member having a hat-shaped cross section. In Patent Document 1, a mold including an upper mold, a lower mold, and a pair of slide molds attached to both side surfaces of the upper mold via cam mechanisms is used. The lower mold includes a central mold and a pair of split molds attached to both side surfaces of the central mold via cam mechanisms. In Patent Document 1, with the mold clamping operation of the mold, the pair of split molds are separated in the width direction of the mold. Also, with the mold clamping operation of the mold, the pair of slide molds approach each other in the width direction of the mold. Then, each vertical wall is sandwiched between the split mold and the slide mold, and a bead portion is formed on the vertical wall.

[0006] Patent Document 2 discloses a molding method in which multiple blanks are integrated and then press-formed. In Patent Document 2, parts of the multiple blanks are overlapped and a portion of the overlapping portion is temporarily fixed. After these blanks are formed into a press-formed product, the other portions of the overlapping portion are fixed in place. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2011-83807 [Patent Document 2] Japanese Patent Publication No. 2007-29966 [Overview of the project] [Problems that the invention aims to solve]

[0008] As mentioned above, when multiple components are joined at the blank stage and formed as a single structural component, a portion of this structural component may have a negative angle with respect to the die's machining direction. Furthermore, when multiple components are integrally formed as a single structural component, it may be necessary to form flanges parallel to the die's machining direction. For example, when multiple components, each having a hat shape in cross-section, are integrally formed as a single structural component, if the top plate of one adjacent component is bent relative to the top plate of the other, the top plate of one of the components may have a negative angle with respect to the die's machining direction. Also, if the top plate of one adjacent component is bent relative to the top plate of the other, it is conceivable that the flange of one of the components may be substantially parallel to the die's machining direction. Therefore, it is difficult to integrate such components at the blank stage and form them as a single structural component. When the top plates of one component and the top plate of the other component are bent at an angle of 150° or less, it becomes even more difficult to integrally form these components as a single structural component.

[0009] The object of this disclosure is to provide a method for manufacturing a structural member that allows the first member and the second member to be integrally molded as a single structural member, even when the top plate of the first member and the top plate of the second member are bent at an angle of 150° or less. [Means for solving the problem]

[0010] A method for manufacturing a structural member according to this disclosure comprises the steps of preparing a blank and forming the blank into a structural member using a mold. The blank includes a first metal plate and a second metal plate. In the blank, the ends of the first metal plate are joined together by welding with the ends of the second metal plate overlapping each other. The structural member includes a first member and a second member. The first member includes a first top plate, a pair of first vertical walls, and a pair of first flanges. The first vertical walls are connected to both side edges of the first top plate. The first flanges are connected to the first vertical walls on the opposite side of the first top plate. The first flanges protrude outward from the first vertical walls. The second member includes a second top plate, a pair of second vertical walls, and a pair of second flanges. The second top plate is bent relative to the first top plate at an angle of 150° or less. The second vertical walls are connected to both side edges of the second top plate. The second flanges are connected to the second vertical walls on the opposite side of the second top plate. The second flange protrudes outward from the second vertical wall. In the forming process, the first metal plate is formed into the first member and the second metal plate is formed into the second member. In the forming process, the first member is restrained from the first direction by the mold, and the second flange is restrained from the second direction perpendicular to the first direction, while the second top plate and the second vertical wall are formed from the second direction by the mold. [Effects of the Invention]

[0011] According to the manufacturing method of the structural member described herein, even when the top plate of the first member and the top plate of the second member are bent at an angle of 150° or less, the first member and the second member can be integrally molded as a single structural member. [Brief explanation of the drawing]

[0012] [Figure 1]FIG. 1 is a perspective view of a structural member according to an embodiment. [Figure 2] FIG. 2 is a cross-sectional view of the first member included in the structural member shown in FIG. 1. [Figure 3] FIG. 3 is a cross-sectional view of the second member included in the structural member shown in FIG. 1. [Figure 4] FIG. 4 is a longitudinal sectional view of the structural member shown in FIG. 1. [Figure 5A] FIG. 5A is a schematic diagram for explaining a method of manufacturing a structural member according to an embodiment. [Figure 5B] FIG. 5B is a schematic diagram for explaining a method of manufacturing a structural member according to an embodiment. [Figure 5C] FIG. 5C is a schematic diagram for explaining a method of manufacturing a structural member according to an embodiment. [Figure 5D] FIG. 5D is a schematic diagram for explaining a method of manufacturing a structural member according to an embodiment. [Figure 5E] FIG. 5E is a schematic diagram for explaining a method of manufacturing a structural member according to an embodiment. [Figure 5F] FIG. 5F is a schematic diagram for explaining a method of manufacturing a structural member according to an embodiment. [Figure 6] FIG. 6 is a partial longitudinal sectional view of a structural member manufactured by the manufacturing method according to an embodiment. [Figure 7] FIG. 7 is a partial longitudinal sectional view of a structural member manufactured by the manufacturing method according to an embodiment.<000.org / [Figure 8] FIG. 8 is a perspective view of a structural member according to a modification of an embodiment. [Figure 9] FIG. 9 is a perspective view of a structural member according to another modification of an embodiment. [Figure 10] FIG. 10 is a diagram showing the distribution of Vickers hardness of a spot weld cross section in the first embodiment.

MODE FOR CARRYING OUT THE INVENTION

[0013] A method for manufacturing a structural member according to an embodiment comprises the steps of preparing a blank and forming the blank into a structural member using a mold. The blank includes a first metal plate and a second metal plate. In the blank, the ends of the first metal plate are joined together by welding with the ends of the second metal plate overlapping each other. The structural member includes a first member and a second member. The first member includes a first top plate, a pair of first vertical walls, and a pair of first flanges. The first vertical walls are connected to both side edges of the first top plate. The first flanges are connected to the first vertical walls on the opposite side of the first top plate. The first flanges protrude outward from the first vertical walls. The second member includes a second top plate, a pair of second vertical walls, and a pair of second flanges. The second top plate is bent relative to the first top plate at an angle of 150° or less. The second vertical walls are connected to both side edges of the second top plate. The second flanges are connected to the second vertical walls on the opposite side of the second top plate. The second flange protrudes outward from the second vertical wall. In the molding process, the first metal plate is formed into the first member and the second metal plate is formed into the second member. In the molding process, the first member is restrained from the first direction by the mold, and the second flange is restrained from the second direction perpendicular to the first direction, while the second top plate and the second vertical wall are formed by the mold from the second direction (first configuration).

[0014] In the manufacturing method according to the first configuration, a structural member is formed from a blank including a first metal plate and a second metal plate whose ends are overlapped and joined by welding. More specifically, a first member of the structural member is formed from the first metal plate of the blank, and a second member of the structural member is formed from the second metal plate of the blank. In the process of forming the structural member from the blank, while the first member is constrained from the first direction by a mold, the top plate (second top plate) and the vertical wall (second vertical wall) of the second member are formed from the second direction by the mold in a state where the flange (second flange) of the second member is constrained from the second direction perpendicular to the first direction. By forming the second top plate from the second direction different from the first direction in this way, even if there is a portion with a negative angle with respect to the first direction due to the second top plate being bent with respect to the top plate (first top plate) of the first member, the second top plate can be formed. Further, since the second member is formed from the side with respect to the first direction, even if the second flange includes a portion substantially parallel to the first direction due to the second top plate being bent with respect to the first top plate, the second flange can be formed. Therefore, even when the first top plate and the second top plate are bent at an angle of 150° or less, the first member and the second member can be integrally formed as a single structural member. Thereby, compared with the case where the first member and the second member are individually formed as separate members, the number of parts in a structure such as a vehicle body can be reduced. As a result, the manufacturing process of the structure can be omitted, and the emission amount of life cycle GHG can be reduced.

[0015] When the second top plate is bent at an angle of 150° or less with respect to the first top plate, wrinkles are likely to occur at the bending portions of these top plates during molding. On the other hand, in the manufacturing method according to the first configuration, the second top plate is formed from the second direction in a state where the first member is constrained from the first direction and the second flange is constrained from the second direction perpendicular to the first direction. Thereby, during the molding of the structural member, the generation of wrinkles at the bending portions of the first top plate and the second top plate can be suppressed.

[0016] In the manufacturing method relating to the first configuration, the mold may include two movable molds. In this case, during the molding process, one of the two movable molds, which moves along the second direction, may be used to form the end of the second metal plate into a second flange and to restrain the second flange, while the other of the two movable molds, which moves along the second direction, may be used to form the other part of the second metal plate into a second top plate and a second vertical wall (second configuration).

[0017] In the manufacturing method relating to the second configuration, the mold may further include an upper mold and a lower mold. In this case, during the molding process, the first member may be clamped and restrained from a first direction by the upper mold and the lower mold, and the second flange may be clamped and restrained by the upper mold and one of the movable molds, while the second top plate and the second vertical wall may be molded by the other movable mold (third configuration).

[0018] The structural member according to this embodiment comprises a first member and a second member. The first member includes a first top plate, a pair of first vertical walls, and a pair of first flanges. The first vertical walls are connected to both side edges of the first top plate. The first flanges are connected to the first vertical walls on the opposite side of the first top plate. The first flanges protrude outward from the first vertical walls. The second member includes a second top plate, a pair of second vertical walls, and a pair of second flanges. The second top plate is bent relative to the first top plate at an angle of 150° or less. The second vertical walls are connected to both side edges of the second top plate. The second flanges are connected to the second vertical walls on the opposite side of the second top plate. The second flanges protrude outward from the second vertical walls. The longitudinal end of the first member is joined to the end of the second member by welding in a state where it overlaps with the longitudinal end of the second member. In the first member, the minimum Vickers hardness of the heat-affected zone of the welded area is 70% or more of the Vickers hardness of the unwelded area (fourth configuration).

[0019] In the fourth configuration, the first and second members are integrally formed as a single structural member by hot working, such as hot stamping. In this case, at least in the first member, the minimum Vickers hardness of the heat-affected zone of the weld is 70% or more of the Vickers hardness of the unwelded area. That is, because the first and second members are integrated at the blank stage and formed simultaneously as a single structural member, even if softening of the heat-affected zone (HAZ softening) occurs during welding before forming, the subsequent heat treatment reduces the HAZ softening, resulting in a smaller hardness difference between the unwelded area and the heat-affected zone. As a result, for example, fracture originating from the weld becomes less likely to occur during a collision of a vehicle body in which the structural member is used, and the structural member can be given good collision performance.

[0020] A structural member according to another embodiment comprises a first member and a second member. The first member includes a first top plate, a pair of first vertical walls, and a pair of first flanges. The first vertical walls are connected to both side edges of the first top plate. The first flanges are connected to the first vertical walls on the opposite side of the first top plate. The first flanges protrude outward from the first vertical walls. The second member includes a second top plate, a pair of second vertical walls, and a pair of second flanges. The second top plate is bent relative to the first top plate at an angle of 150° or less. The second vertical walls are connected to both side edges of the second top plate. The second flanges are connected to the second vertical walls on the opposite side of the second top plate. The second flanges protrude outward from the second vertical walls. The longitudinal end of the first member is joined to the end of the second member by welding in a state where it overlaps with the longitudinal end of the second member. In a cross-section along the longitudinal direction of the first member, the maximum Vickers hardness in the range from the edge of the weld in a direction perpendicular to the thickness direction of the first member up to 2.0 mm outward is defined as HVmax, and the Vickers hardness of the base material of the first member is defined as HVm. Then, HVmax - HVm is 7.0% or more of HVm (fifth configuration).

[0021] In the fifth configuration, the first and second members are formed integrally as a single structural member by cold working. In this case, the material is constrained at the weld joint connecting the first and second members, causing strain in the area near the weld during the forming of the structural member, resulting in work hardening in that area. As a result, in at least the first member, the Vickers hardness (maximum value) near the weld is 7.0% or more greater than the Vickers hardness of the base material. Thus, the work hardening of the area near the weld in the first member improves the yield strength of the structural member during shear deformation.

[0022] In the structural member relating to the fourth or fifth configuration, the total extended length of the first and second top plates, measured along the first and second top plates, may be 400 mm or more (sixth configuration).

[0023] Embodiments of this disclosure will be described below with reference to the drawings. In these drawings, the same or equivalent components are denoted by the same reference numerals, and the same description will not be repeated.

[0024] [Structural Member Composition] Figure 1 is a perspective view of the structural member 10 according to this embodiment. The structural member 10 comprises a first member 11 and a second member 12.

[0025] Referring to Figure 1, the first member 11 includes a top plate 111, a pair of vertical walls 112a, 112b, and a pair of flanges 113a, 113b.

[0026] The top plate 111 extends in the longitudinal direction of the first member 11. The vertical walls 112a and 112b are connected to both side edges of the top plate 111. One vertical wall 112a is provided on the side opposite to the other vertical wall 112b relative to the top plate 111. The flanges 113a and 113b are connected to the vertical walls 112a and 112b, respectively, on the opposite side of the top plate 111. The flanges 113a and 113b protrude outward from the vertical walls 112a and 112b.

[0027] The vertical walls 112a and 112b extend along the top plate 111 in the longitudinal direction of the first member 11. The flanges 113a and 113b each extend along the vertical walls 112a and 112b in the longitudinal direction of the first member 11. The height of the vertical walls 112a and 112b may be constant along the entire length of the first member 11 in the longitudinal direction, or it may vary along the longitudinal direction of the first member 11. For example, in a part of the longitudinal direction of the first member 11, the vertical walls 112a and 112b may substantially disappear, and the top plate 111 and flanges 113a and 113b may be flat.

[0028] Figure 2 is a cross-sectional view of the first member 11 shown in Figure 1, along line II-II. Figure 2 shows the cross-section (transverse plane) obtained when the first member 11 is cut by a plane perpendicular to its longitudinal direction.

[0029] Referring to Figure 2, the first member 11 has a hat-shaped cross-section in at least a portion of its longitudinal direction. That is, the first member 11 includes four ridges 114a, 114b, 115a, and 115b in a cross-sectional view in at least a portion of its longitudinal direction.

[0030] The ridge sections 114a and 114b are provided continuously with the top plate 111. The ridge section 114a connects one vertical wall 112a to the top plate 111. The ridge section 114a is the corner between the top plate 111 and the vertical wall 112a. The ridge section 114b is on the opposite side of the vertical wall 112a and connects the other vertical wall 112b to the top plate 111. The ridge section 114b is the corner between the top plate 111 and the vertical wall 112b. The ridge sections 114a and 114b may each have an arc shape in a cross-sectional view of the first member 11.

[0031] The ridge sections 115a and 115b are provided on the opposite side of the top plate 111, continuously attached to the vertical walls 112a and 112b, respectively. Ridge section 115a connects one vertical wall 112a to the flange 113a. Ridge section 115a is the corner between the vertical wall 112a and the flange 113a. Ridge section 115b connects the other vertical wall 112b to the flange 113b. Ridge section 115b is the corner between the vertical wall 112b and the flange 113b. Ridge sections 115a and 115b may each have an arc shape in a cross-sectional view of the first member 11.

[0032] The vertical walls 112a and 112b may be arranged symmetrically or asymmetrically in a cross-sectional view of the first member 11. For example, in a cross-sectional view of the first member 11, the heights of the vertical walls 112a and 112b may be equal or different. Also, the vertical walls 112a and 112b may be arranged parallel or non-parallel in a cross-sectional view of the first member 11. For example, the vertical walls 112a and 112b may be spaced further apart from each other as they move away from the top plate 111 in a cross-sectional view of the first member 11.

[0033] Similarly, the flanges 113a and 113b may be arranged symmetrically or asymmetrically in a cross-sectional view of the first member 11. For example, in a cross-sectional view of the first member 11, the lengths of the flanges 113a and 113b may be equal or different. Also, the flanges 113a and 113b may be parallel or non-parallel in a cross-sectional view of the first member 11.

[0034] Returning to Figure 1, the second member 12 is provided adjacent to the first member 11. The longitudinal end of the first member 11 is joined to the longitudinal end of the second member 12 by welding, with the first member 11 overlapping the longitudinal end of the second member 12. Therefore, a welded joint 20 is formed at the overlapping end of the first member 11 and the second member 12. The welded joint 20 is provided along a direction intersecting the longitudinal directions of the first member 11 and the second member 12, and crosses the structural member 10. The welded joint 20 may be formed intermittently or continuously. At the overlapping end of the first member 11 and the second member 12, the end of the first member 11 may be located outside the structural member 10 relative to the end of the second member, or the end of the first member 11 may be located inside the structural member 10 relative to the end of the second member.

[0035] Referring to Figure 1, the second member 12 includes a top plate 121, a pair of vertical walls 122a, 122b, and a pair of flanges 123a, 123b.

[0036] The top plate 121 extends in the longitudinal direction of the second member 12. The top plate 121 bends relative to the top plate 111 of the first member 11. The second member 12 may be arranged so as a whole bend relative to the first member 11. The vertical walls 122a and 122b are connected to both side edges of the top plate 121. One vertical wall 122a is provided on the opposite side of the top plate 121 from the other vertical wall 122b. The flanges 123a and 123b are connected to the vertical walls 122a and 122b, respectively, on the opposite side of the top plate 121. The flanges 123a and 123b project outward from the vertical walls 122a and 122b.

[0037] The vertical walls 122a and 122b extend along the top plate 121 in the longitudinal direction of the second member 12. The flanges 123a and 123b each extend along the vertical walls 122a and 122b in the longitudinal direction of the second member 12. The height of the vertical walls 122a and 122b may be constant along the entire length of the second member 12, or it may vary along the longitudinal direction of the second member 12. For example, in a part of the longitudinal direction of the second member 12, the vertical walls 122a and 122b may substantially disappear, and the top plate 121 and flanges 123a and 123b may be flat. The flanges 123a and 123b may be connected and integrated at both ends of the longitudinal direction of the second member 12, at the end opposite to the first member 11.

[0038] Figure 3 is a cross-sectional view of the second member 12 shown in Figure 1, taken along line III-III. Figure 3 shows the cross-section (transverse plane) of the second member 12 when it is cut by a plane perpendicular to its longitudinal direction.

[0039] Referring to Figure 3, the second member 12 has a hat-shaped cross-section in at least a portion of its longitudinal direction. That is, the second member 12 includes four ridges 124a, 124b, 125a, and 125b in a cross-sectional view in at least a portion of its longitudinal direction.

[0040] The ridge sections 124a and 124b are provided continuously with the top plate 121. The ridge section 124a connects one vertical wall 122a to the top plate 121. The ridge section 124a is the corner between the top plate 121 and the vertical wall 122a. The ridge section 124b is on the opposite side of the vertical wall 122a and connects the other vertical wall 122b to the top plate 121. The ridge section 124b is the corner between the top plate 121 and the vertical wall 122b. The ridge sections 124a and 124b may each have an arc shape in a cross-sectional view of the second member 12.

[0041] The ridge sections 125a and 125b are provided on the opposite side of the top plate 121, continuously attached to the vertical walls 122a and 122b, respectively. Ridge section 125a connects one vertical wall 122a to the flange 123a. Ridge section 125a is the corner between the vertical wall 122a and the flange 123a. Ridge section 125b connects the other vertical wall 122b to the flange 123b. Ridge section 125b is the corner between the vertical wall 122b and the flange 123b. Ridge sections 125a and 125b may each have an arc shape in a cross-sectional view of the second member 12.

[0042] The vertical walls 122a and 122b may be arranged symmetrically or asymmetrically in a cross-sectional view of the second member 12. For example, in a cross-sectional view of the second member 12, the heights of the vertical walls 122a and 122b may be equal or different. Also, the vertical walls 122a and 122b may be arranged parallel or non-parallel in a cross-sectional view of the second member 12. For example, the vertical walls 122a and 122b may be spaced further apart from each other as they move away from the top plate 121 in a cross-sectional view of the second member 12.

[0043] Similarly, the flanges 123a and 123b may be arranged symmetrically or asymmetrically in a cross-sectional view of the second member 12. For example, in a cross-sectional view of the second member 12, the lengths of the flanges 123a and 123b may be equal or different. Also, the flanges 123a and 123b may be parallel or non-parallel in a cross-sectional view of the second member 12.

[0044] Figure 4 is a cross-sectional view of the structural member 10 shown in Figure 1, taken along the line IV-IV. Figure 4 shows the cross-sections (longitudinal sections) of the structural member 10 when it is cut along the longitudinal directions of the first member 11 and the second member 12, and along the thickness direction of the top plates 111 and 121.

[0045] Referring to Figure 4, the top plate 111 of the first member 11 includes at least one flat portion 141. The top plate 121 of the second member 12 includes at least one flat portion 142. The flat portions 141 and 142 are substantially straight (flat) in a longitudinal cross-sectional view of the structural member 10.

[0046] In the top plates 111 and 121, curved portions 143 may be provided between adjacent flat portions 141 and 142. If the top plate 121 includes multiple flat portions 142, as in the example of this embodiment, curved portions 143 may also be provided between adjacent flat portions 142. The curved portion 143 has a concave arc shape inward toward the structural member 10 when viewed in a longitudinal cross-section of the structural member 10.

[0047] The curved portion 143 allows the flat portion 141 of the top plate 111 to be smoothly connected to the flat portion 142 of the top plate 121. The welded portion 20 may be positioned at the location of the curved portion 143 connecting adjacent flat portions 141 and 142. Alternatively, the welded portion 20 may be positioned close to the curved portion 143 on either of the adjacent flat portions 141 or 142.

[0048] The top plate 121 of the second member 12 is bent relative to the top plate 111 of the first member 11 so as to form an angle θ with the top plate 111. The angle θ is the angle formed by the flat portion 141 of the top plate 111 and the flat portion 142 of the top plate 121 in a longitudinal cross-sectional view of the structural member 10. More specifically, the angle θ is the angle between the extension lines (dash-dot lines) of the outer surfaces of the flat portions 141 and 142 in the longitudinal cross-section of the structural member 10, and the angle formed by these extension lines on the first member 11 side in the longitudinal direction of the structural member 10 and on the top plate 111 and 121 side in the height direction of the structural member 10. If at least one of the top plates 111 and 121 contains multiple flat portions, the angle θ is the smallest angle formed by the combination of flat portions 141 and 142 selected from the top plates 111 and 121, respectively. The angle θ is 150° or less, preferably 135° or less, and more preferably 120° or less. The angle θ may also be 110° or less, or 100° or less. The angle θ may also be 90° or more.

[0049] The structural member 10 is a relatively long member. In the structural member 10, the extended length L of the top plates 111 and 121 is, for example, 400 mm or more. The extended length L may be 600 mm or more, or 800 mm or more. The extended length L is, for example, 1500 mm or less. The extended length L is the total extended length of the top plates 111 and 121 in the longitudinal direction of the structural member 10, measured along the top plate 111 of the first member 11 and the top plate 121 of the second member 12. The extended length L is measured, for example, along the outer surface of the top plates 111 and 121 in the longitudinal section of the structural member 10 at the center of the width of the top plate 111 or 121.

[0050] Returning to Figure 1, the first member 11 and the second member 12 are formed from metal plates. The first member 11 and the second member 12 may also be formed from steel plates. The first member 11 and the second member 12 can each have a tensile strength of 440 MPa or more. The tensile strength of the first member 11 and the second member 12 may be 590 MPa or more, 780 MPa or more, or 980 MPa or more. The tensile strengths of the first member 11 and the second member 12 may be the same or different.

[0051] The tensile strength of the first member 11 is obtained in accordance with JIS Z 2241:2011 by taking a No. 5 test specimen of the full thickness from the flat part of the top plate 111 near the center in the longitudinal and width directions of the first member 11 and performing a tensile test. The tensile strength of the second member 12 is measured in accordance with JIS Z 2241:2011 by taking a No. 5 test specimen of the full thickness from the corner portion of the flange 123a or 123b at the end opposite to the first member 11 in the longitudinal direction of the second member 12 and performing a tensile test. The corner portion of the flange 123a is the end of the connection point between the portion of the flange 123a that extends in the longitudinal direction of the second member 12 along the vertical wall 122a and the portion of the flange 123a that extends in the width direction of the second member 12. The corner portion of flange 123b is the end of the connection point between the portion of flange 123b that extends longitudinally along the vertical wall 122b and the portion of flange 123b that extends widthwise along the second member 12. If only one test specimen for tensile strength testing can be taken from each of the first member 11 and the second member 12, the tensile strength test is performed on that single test specimen to determine the tensile strength. If two or more test specimens can be taken, up to three test specimens are taken and tensile strength tests are performed on them, and the average value of the measured tensile strengths is taken to determine the tensile strength.

[0052] If it is difficult to obtain test specimens for tensile testing from the first member 11 and the second member 12, the tensile strength of the first member 11 and the second member 12 can be determined based on their Vickers hardness. The Vickers hardness of the first member 11 can be determined in accordance with JIS Z 2244-1:2024 by preparing an optical microscope microscopy specimen of the entire thickness of the vertical cross-section of the flat part of the top plate 111 near the center in the longitudinal and width directions of the first member 11, measuring the Vickers hardness at five points in the 1 / 4t portion (1 / 4 plate thickness portion) of the specimen with a test load of 1.961 N (200 gf), and taking the average value of these measurements. The Vickers hardness of the second member 12 can be determined in accordance with JIS Z 2244-1:2024 by preparing an optical microscope microscopy specimen of the entire thickness of the vertical cross-section of the corner portion of flange 123a or 123b at the end opposite to the first member 11 in the longitudinal direction of the second member 12, measuring the Vickers hardness at five points in the 1 / 4t portion (1 / 4 plate thickness portion) of the specimen with a test load of 1.961 N (200 gf), and taking the average value of these measurements. If the Vickers hardness of the first member 11 and the second member 12 is Hv138 or higher, the tensile strength of the first member 11 and the second member 12 is 440 MPa or higher. If the Vickers hardness of the first member 11 and the second member 12 is Hv184 or higher, the tensile strength of the first member 11 and the second member 12 is 590 MPa or higher. If the Vickers hardness of the first member 11 and the second member 12 is Hv243 or higher, the tensile strength of the first member 11 and the second member 12 is 780 MPa or higher. If the Vickers hardness of the first member 11 and the second member 12 is Hv305 or higher, the tensile strength of the first member 11 and the second member 12 is 980 MPa or higher.

[0053] For example, a tensile strength of 440 MPa corresponds to Hv138. In this case, if a Vickers hardness test is performed with a test load of 1.961 N (200 gf) in accordance with JIS Z 2244-1:2024, the diagonal length d of the indentation will be 51.8 μm. If the plate thickness is 0.6 mm, the 1 / 4t portion is located 0.15 mm from the sample surface, but 2.5 times the diagonal length d is 0.1295 mm, which satisfies the provision in JIS Z 2244-1:2024 that the Vickers hardness test must be performed at a distance of at least 2.5 times the diagonal length d from the surface edge to avoid the influence of the surface edge. Also, for example, a tensile strength of 1180 MPa corresponds to Hv367. In this case, when a Vickers hardness test is performed with a test load of 1.961 N (200 gf), the diagonal length d of the indentation is 31.8 μm, and it is possible to measure the Vickers hardness in accordance with JIS Z 2244-1:2024.

[0054] The plate thickness of the first member 11 and the second member 12 may be 2.3 mm or less. The plate thickness of the first member 11 and the second member 12 may be, for example, 2.0 mm or less, preferably 1.5 mm or less, and more preferably 1.0 mm or less. The plate thickness of the first member 11 and the second member 12 may be 0.7 mm or more. The plate thicknesses of the first member 11 and the second member 12 may be the same or different. The plate thickness of the first member 11 and the second member 12 can be measured at the same location as the measurement location for tensile strength or Vickers hardness.

[0055] Such structural members 10 are used, for example, in the body of an automobile. One of the first member 11 and the second member 12 may be, for example, a front side member for the body. In this case, the other of the first member 11 and the second member 12 may be a joint member for connecting the front side member to another member, for example, an A-pillar post. Alternatively, one of the first member 11 and the second member 12 may be a cross member for the body. In this case, the other of the first member 11 and the second member 12 may be a joint member for connecting the cross member to another member, for example, a wheel well. However, the applications of the structural member 10 are not limited to these examples.

[0056] [Method for manufacturing structural members] Next, the manufacturing method for the structural member 10 will be described with reference to Figures 5A to 5F. The manufacturing method for the structural member 10 according to this embodiment comprises a preparation step and a molding step. The manufacturing method may further include a heating step.

[0057] (preparation process) Referring to Figure 5A, the preparation step involves preparing a blank 30. The blank 30 includes a first metal sheet 31 and a second metal sheet 32. The first metal sheet 31 and the second metal sheet 32 ​​may be steel sheets. The thicknesses of the first metal sheet 31 and the second metal sheet 32 ​​may be the same or different. The tensile strengths of the first metal sheet 31 and the second metal sheet 32 ​​may also be the same or different.

[0058] The blank 30 is a so-called tailor-welded blank. In the blank 30, the end of the first metal plate 31 overlaps the end of the second metal plate 32, and these ends are joined together by welding. As a result, a step is created on both surfaces of the blank 30 at the overlapping ends of the first metal plate 31 and the second metal plate 32. In addition, a welded joint 20 is formed at the overlapping ends of the first metal plate 31 and the second metal plate 32. The first metal plate 31 and the second metal plate 32 may be joined by intermittent welding or by continuous welding. Intermittent welding is a welding method in which the welded joint is formed intermittently, for example, spot welding. Continuous welding is a welding method in which a linear welded joint is formed, for example, laser welding.

[0059] (Heating process) In this embodiment, the structural member 10 (Figures 1 to 4) may be manufactured by cold working or by hot working (hot stamping). When the structural member 10 is manufactured by hot stamping, a heating step is performed before the forming step. In the heating step, the blank 30 is heated, for example, in a heating furnace. If the first metal plate 31 and the second metal plate 32 are steel plates, the first metal plate 31 and the second metal plate 32 are heated to an austenite transformation completion temperature (Ac3 It is preferable to heat it to a temperature of 1.5°C or higher.

[0060] (molding process) Referring to Figures 5B to 5F, in the molding process, the blank 30 is formed into a structural member 10 (Figures 1 to 4) using a mold 40. In the molding process, the first metal plate 31 is formed into the first member 11 (Figure 1), and the second metal plate 32 is formed into the second member 12 (Figure 1). In the molding process, the first member 11 is constrained from the first direction D1 by the mold 40, and the flanges 123a and 123b (Figure 1) of the second member 12 are constrained from the second direction D2, while the top plate 121 and vertical walls 122a and 122b (Figure 1) of the second member 12 are formed by the mold 40 from the second direction D2. Hereafter, unless otherwise necessary, the vertical walls 122a and 122b of the second member 12 will be collectively referred to as vertical wall 122, and the flanges 123a and 123b will be collectively referred to as flange 123. Similarly, when there is no particular need to distinguish between them, the vertical walls 112a and 112b of the first member 11 are collectively referred to as vertical wall 112, and the flanges 113a and 113b are collectively referred to as flange 113.

[0061] Figure 5B is a perspective view of the blank 30 and the mold 40. Figures 5C to 5F show cross-sections (longitudinal sections) of the blank 30 and the mold 40 when they are cut along the longitudinal and thickness directions of the blank 30 at the position where the top plates 111 and 121 (Figure 1) of the structural member 10 are formed.

[0062] First, the configuration of the mold 40 will be described with reference to Figures 5B and 5C. In this embodiment, the mold 40 includes an upper mold 41, a lower mold 42, and movable molds 43 and 44.

[0063] The upper die 41 and the lower die 42 are positioned opposite each other in a first direction D1. The upper die 41 and the lower die 42 are configured to be able to move closer to and further apart relative to each other in the first direction D1. The first direction D1 is the pressing direction of the upper die 41 and the lower die 42, and is, for example, the vertical direction.

[0064] The lower die 42 is the part of the mold 40 that is mainly used to process the first metal plate 31 and form it into the first member 11 (Figure 1). The lower die 42 can process the first metal plate 31 together with the upper die 41. The molding surface 411 of the upper die 41 has a shape that corresponds to at least the first member 11. The molding surface 421 of the lower die 42 has a shape that corresponds to at least the top plate 111 and vertical wall 112 (Figure 1) of the first member 11. Steps may be formed on the molding surfaces 411 and 421, corresponding to the step difference in the blank 30 at the overlapping end of the first metal plate 31 and the second metal plate 32. In this embodiment, the lower die 42 is supported from the opposite side of the upper die 41 by an elastic member 45. The elastic member 45 is a member that can expand and contract in a first direction D1, and includes, for example, a spring or a fluid pressure cylinder. The expansion and contraction of the elastic member 45 causes the lower die 42 to move in the first direction D1.

[0065] The movable molds 43 and 44 are positioned, for example, to the side of the lower mold 42. The movable molds 43 and 44 are the parts of the mold 40 that are mainly used to process the second metal plate 32 and form it into the second member 12 (Figure 1). The movable molds 43 and 44 can process the second metal plate 32 together with the upper mold 41. The forming surface 431 of the movable mold 43 has a shape that corresponds to at least a part of the flange 123 (Figure 1) of the second member 12. The forming surface 441 of the movable mold 44 has a shape that corresponds to at least a part of the top plate 121 and vertical wall 122 (Figure 1) of the second member 12. Since the movable mold 44 is used to form the top plate 121 and vertical wall 122, it is positioned inside the movable mold 43 which forms the flange 123. In a vertical cross-sectional view of the mold 40 (Figure 5C), the movable molds 43 and 44 may be arranged side by side in the first direction D1. The molding surfaces 412 and 413 of the upper mold 41 correspond to the molding surface 431 of the movable mold 43 and the molding surface 441 of the movable mold 44, respectively.

[0066] The movable molds 43 and 44 are each configured to be movable in a second direction D2. The second direction D2 is substantially perpendicular to the first direction D1 in a longitudinal cross-sectional view of the mold 40. The second direction D2 is, for example, the horizontal direction.

[0067] The movable molds 43 and 44 can be moved in a second direction D2, for example, by a cam mechanism. In this case, the mold 40 may include a cam driver 46. The cam driver 46 is able to move relative to and away from the movable molds 43 and 44 in a first direction D1. The movable molds 43 and 44 can function as sliding cams actuated by the cam driver 46.

[0068] The movable molds 43 and 44 each have sliding surfaces 432 and 442 opposite to the molding surfaces 431 and 441, respectively. The sliding surfaces 432 and 442 are inclined with respect to a first direction D1 in a longitudinal cross-sectional view of the mold 40, for example. The cam driver 46 has a sliding surface 461 corresponding to the sliding surfaces 432 and 442 of the movable molds 43 and 44.

[0069] As shown in Figure 5C, before the start of the molding process, the upper mold 41 and the lower mold 42 are separated in the first direction D1. Also, the cam driver 46 is separated from the movable molds 43 and 44 in the first direction D1. At the start of the molding process, the blank 30 is placed between the upper mold 41 and the lower mold 42 and movable molds 43 and 44.

[0070] Next, the upper die 41 is brought relatively closer to the lower die 42 in the first direction D1. Also, the cam driver 46 is brought relatively closer to the movable dies 43 and 44 in the first direction D1. For example, the upper die 41 is moved towards the lower die 42 along the first direction D1. Also, the cam driver 46 is moved towards the movable dies 43 and 44 along the first direction D1. In this embodiment, as shown in Figure 5D, the first metal plate 31 of the blank 30 is clamped between the upper die 41 and the lower die 42 from the first direction D1, and the forming of the first member 11 begins. The upper die 41 and the lower die 42 form, for example, the top plate 111 and the vertical wall 112 (Figure 1) before the flange 113.

[0071] As shown in Figure 5E, after the first metal plate 31 is sandwiched between the upper die 41 and the lower die 42, as the upper die 41 moves further, the lower die 42 is pushed in the first direction D1 by the upper die 41, and the elastic member 45 contracts. Also, as the cam driver 46 and the movable die 43 move closer together, the sliding surface 461 of the cam driver 46 comes into contact with the sliding surface 432 of the movable die 43 and slides on the sliding surface 432. As a result, the movable die 43 moves along the second direction D2. The movable die 43 is pushed towards the upper die 41 by the cam driver 46, forming the end of the second metal plate 32 into a flange 123. More specifically, the forming surface 431 of the movable die 43 and the forming surface 412 of the upper die 41 sandwich the end of the second metal plate 32, and the end of the second metal plate 32 is formed into a flange 123.

[0072] Referring to Figure 5F, in the molding process, the end of the second metal plate 32 is formed into a flange 123 by the movable die 43 and the flange 123 is restrained, and the other part of the second metal plate 32 is formed into a top plate 121 and a vertical wall 122 by the movable die 44. More specifically, the first member 11 is clamped and restrained from the first direction D1 by the upper die 41 and the lower die 42, and the flange 123 is clamped and restrained by the upper die 41 and the movable die 43, while the top plate 121 and the vertical wall 122 are formed by the movable die 44.

[0073] In this embodiment, as the cam driver 46 and the movable mold 44 approach each other, the sliding surface 461 of the cam driver 46 contacts the sliding surface 442 of the movable mold 44 and slides along the sliding surface 442. This causes the movable mold 44 to move along the second direction D2. The movable mold 44 is pushed out toward the upper mold 41 by the cam driver 46, forming the portion of the second metal plate 32 that is not constrained by the movable mold 43. More specifically, the forming surface 441 of the movable mold 44 and the forming surface 413 of the upper mold 41 sandwich the second metal plate 32, forming the top plate 121 and the vertical wall 122. This forms the structural member 10 from the blank 30.

[0074] If a heating process is performed before the molding process, the structural member 10 may be held within the mold 40. This allows the structural member 10 to be deheated (quenched) by the mold 40.

[0075] In this embodiment, the height of the portion of the lower mold 42 on the side of the movable mold 44 is greater than the height of the portion on the opposite side of the movable mold 44. Therefore, when the top plate 111 or 112 of the structural member 10 rises in the first direction D1 from the lower mold 42 to the movable mold 44, the rising portion of the top plate 111 or 112 can be supported by the lower mold 42. This prevents the top plate 111 or 112 from partially dropping due to gravity when the movable mold 44 is separated from the lower mold 42 in the second direction D2 during demolding.

[0076] Figure 6 schematically shows a portion of the longitudinal cross-section of a structural member 10 formed through a heating process. In other words, Figure 6 is a partial longitudinal cross-sectional view of a structural member 10 (hot-stamped member) formed by hot stamping. On the other hand, Figure 7 schematically shows a portion of the longitudinal cross-section of a structural member 10 formed without a heating process. In other words, Figure 7 is a partial longitudinal cross-sectional view of a structural member 10 (cold-worked member) formed by cold working. Figures 6 and 7 show the overlapping end of the first member 11 and the second member 12 and its vicinity.

[0077] Referring to Figure 6, a welded joint 20 exists at the overlapping end of the first member 11 and the second member 12. The welded joint 20 includes a weld metal portion 21 and a heat-affected zone 22. If a heating process is performed before the molding process in the manufacturing of the structural member 10, the heating process reduces HAZ softening in the welded joint 20. Specifically, if the structural member 10 is a hot-stamped member, the minimum Vickers hardness of the heat-affected zone 22 in the first member 11 is 70% or more of the Vickers hardness of the unwelded portion. In the first member 11, the minimum Vickers hardness of the heat-affected zone 22 is preferably 80% or more of the Vickers hardness of the unwelded portion, and more preferably 90% or more. In the first member 11, the minimum Vickers hardness of the heat-affected zone 22 is less than or equal to the Vickers hardness of the unwelded portion.

[0078] The Vickers hardness of the first member 11 can be measured by the Vickers hardness test specified in JIS Z 2244-1:2024. Specifically, first, the structural member 10 is cut by laser cutting or the like at a position passing through the weld center of the weld 20 to obtain a test piece including the first member 11 and the weld 20. Then, the test piece is embedded in resin so that the cross-section passing through the weld centers of the first member 11 and the weld 20 is positioned on the surface, and the cross-section is polished. Subsequently, the Vickers hardness is measured in accordance with JIS Z 2244-1:2024 at a position 1 / 4 of the plate thickness from the surface closer to the weld center, up to a position 12.0 mm outward from the weld center, with a test force of, for example, 0.49 N and a measurement interval (pitch) of 0.1 to 0.2 mm. The minimum value of the measured Vickers hardness is taken as the minimum Vickers hardness of the heat-affected zone 22 of the first member 11. Furthermore, the Vickers hardness is measured at a position at least 15.0 mm away from the welding center of the welded joint 20, and at a position where the plate thickness is 1 / 4 of the surface of the first member 11, in accordance with JIS Z 2244-1:2024, for example, with a test force of 0.49 N. This Vickers hardness is taken as the Vickers hardness of the non-welded part of the first member 11.

[0079] Whether or not a structural member 10 is a hot-stamped member can be determined by measuring the boron (B) content in the structural member 10 and determining the presence or absence of scale or an alloy layer (diffusion layer). Specifically, known component analysis is performed on the first member 11 to measure the B content in the base material of the first member 11. Furthermore, for the first member 11, cross-sectional observation using a scanning electron microscope (SEM) and component analysis by energy-dispersive X-ray spectroscopy (EDS) are performed to determine whether or not scale or an alloy layer (diffusion layer) exists within 20 μm of the surface layer. For example, if the first member 11 is made of aluminum-plated steel sheet, and the structural member 10 is a hot-stamped member, then an Fe-Al alloy layer or diffusion layer exists within 20 μm of the surface layer of the first member 11. Also, for example, if the first member 11 is made of zinc-plated steel sheet, and the structural member 10 is a hot-stamped member, then an Fe-Zn alloy layer or diffusion layer exists within 20 μm of the surface layer of the first member 11. If the first member 11 is made of a steel plate (bare material) without a plating layer, then if the structural member 10 is a hot-stamped member, scale will be present within 20 μm of the surface layer of the first member 11. If the first member 11 contains 0.001 to 0.005 mass% of B and scale or an alloy layer (diffusion layer) is present within 20 μm of the surface layer, then it can be determined that the structural member 10 is a hot-stamped member formed through a heating process. In this embodiment, the first member 11 is joined to the second member 12 by welding before the forming process, and the HAZ softening of the heat-affected zone of the welded part 20 is reduced in the heating process. Therefore, if the structural member 10 is a hot-stamped member, the minimum Vickers hardness of the heat-affected zone 22 of the first member 11 will be 70% or more of the Vickers hardness of the unwelded part.

[0080] If the structural member 10 is a hot-stamped member, the minimum Vickers hardness of the heat-affected zone 22 of the welded portion 20 in the second member 12 may be 70% or more of the Vickers hardness of the unwelded portion. In the second member 12, the minimum Vickers hardness of the heat-affected zone 22 is preferably 80% or more of the Vickers hardness of the unwelded portion, and more preferably 90% or more. In the second member 12, the minimum Vickers hardness of the heat-affected zone 22 is less than or equal to the Vickers hardness of the unwelded portion. The Vickers hardness of the second member 12 can be measured in the same way as in the first member 11.

[0081] Referring to Figure 7, when the structural member 10 is a cold-worked member, work hardening occurs in the vicinity of the weld 20 in the first member 11 due to the forming process, resulting in a higher hardness compared to the base material. More specifically, in the longitudinal section of the first member 11, when the maximum Vickers hardness in the range R1 near the weld 20 is HVmax, and the Vickers hardness of the base material of the first member 11 is HVm, then HVmax-HVm is 7.0% or more of HVm (ΔHV=HVmax-HVm≧0.070×HVm). Range R1 is the range from the edge of the weld 20 (heat-affected zone 22) in the longitudinal section of the first member 11 in a direction perpendicular to the plate thickness direction of the first member 11 to 2.0 mm outside of it.

[0082] Even if the structural member 10 is a cold-worked member, the Vickers hardness of the first member 11 can be measured by the Vickers hardness test specified in JIS Z 2244-1:2024. Specifically, first, the structural member 10 is cut by laser cutting or the like at a position passing through the weld center of the weld 20 to obtain a test piece including the first member 11 and the weld 20. Then, the test piece is embedded in resin so that the cross section passing through the weld centers of the first member 11 and the weld 20 is positioned on the surface, and the cross section is polished. Subsequently, within a range R1 up to 2.0 mm outward from the edge of the heat-affected zone 22 of the weld 20, the Vickers hardness is measured at five or more points at a position 1 / 4 of the plate thickness from the surface closer to the weld center, in accordance with JIS Z 2244-1:2024, for example, with a test force of 0.49 N. The maximum value of the measured Vickers hardness is then defined as HVmax. Furthermore, the Vickers hardness is measured at a position at least 15.0 mm away from the welding center of the weld 20, at a position 1 / 4 of the plate thickness from the surface of the first member 11, in accordance with JIS Z 2244-1:2024, for example, with a test force of 0.49 N. This Vickers hardness is taken as the Vickers hardness HVm of the base material of the first member 11. If the first member 11 and the second member 12 are joined by spot welding, HVmax is measured for, for example, the weld 20 of the ridge portion 114a or 114b (Figure 2), or the weld 20 closest to the ridge portion 114a or 114b among the welds 20 located on the top plate 111 or vertical wall 112 (Figures 1 and 2). HVmax may also be measured for the weld 20 closest to the ridge portion 114a or 114b among the welds 20 located on the vertical wall 112.

[0083] If the structural member 10 is a cold-worked member, then in the second member 12 as well, when the maximum Vickers hardness in the range R2 near the weld 20 is HVmax and the Vickers hardness of the base material of the second member 12 is HVm, HVmax-HVm may be 7.0% or more of HVm (ΔHV=HVmax-HVm≧0.070×HVm). Range R2 is the range from the edge of the weld 20 (heat-affected zone 22) in the longitudinal section of the second member 12 in a direction perpendicular to the plate thickness direction of the second member 12 to 2.0 mm outside thereto. The Vickers hardness HVmax and HVm in the second member 12 can be measured in the same way as in the first member 11.

[0084] [effect] In the manufacturing method according to this embodiment, a structural member 10 including a first member 11 and a second member 12 is formed from a blank 30 in which a first metal plate 31 and a second metal plate 32 are pre-overlapped and welded. In the forming process, the first member 11 is restrained from a first direction D1 by a mold 40, and the flange 123 of the second member 12 is restrained from a second direction D2 perpendicular to the first direction D1. With this restraint, the top plate 121 and vertical wall 122 of the second member 12 are formed by the mold 40 from the second direction D2. In this case, even if the top plate 121 of the second member 12 is bent relative to the top plate 111 of the first member 11, and there is a portion of the top plate 121 that intrudes into the first member 11 side in a longitudinal cross-sectional view of the structural member 10, that is, a portion that is at a negative angle with respect to the first direction D1, the top plate 121 can still be formed by the mold 40. Furthermore, since the second member 12 is formed from the side with respect to the first direction D1, even if the flange 123 of the second member 12 includes a portion substantially parallel to the first direction D1 due to the bending of the top plate 121 relative to the top plate 111, the flange 123 can be formed in the mold 40. Therefore, even if the top plate 121 is bent at an angle θ of 150° or less relative to the top plate 111, which is generally difficult to form, the first member 11 and the second member 12 can be integrated at the blank 30 stage and formed as a single structural member 10.

[0085] More specifically, in the manufacturing method according to this embodiment, the mold 40 includes two movable molds 43 and 44 that move independently in the second direction D2. In the molding process, the end of the second metal plate 32 can be formed into a flange 123 by the movable mold 43 that moves along the second direction D2. In this way, by forming the flange 123 from the side with respect to the first direction D1 using the movable mold 43, the flange 123 can be formed even if the flange 123 includes a portion that is parallel or nearly parallel to the first direction D1.

[0086] Furthermore, in the molding process, the remaining portion of the second metal plate 32 can be formed into the top plate 121 and vertical wall 122 by another movable die 44 that moves along the second direction D2. In this way, by forming the top plate 121 and vertical wall 122 from the side with respect to the first direction D1 by the movable die 44, the top plate 121 and the vertical wall 122 connected thereto can be formed even if the top plate 121 includes a portion that is at a negative angle with respect to the first direction D1.

[0087] In this embodiment, the first member 11 and the second member 12 can be integrally molded as a single structural member 10. This reduces the number of parts in a structure such as a vehicle body compared to the case where the first member 11 and the second member 12 are separate members. As a result, the manufacturing process of the structure can be omitted, and life cycle GHG emissions can be reduced.

[0088] In this embodiment, the first metal plate 31 and the second metal plate 32 are joined at their ends by overlap welding at the blank 30 stage before forming, and then formed into the first member 11 and the second member 12. When the structural member 10 is a hot-stamped member, the welded portion 20 of the blank 30 is heated in the heating process, so that in the first member 11 after the forming process, the minimum value of the Vickers hardness of the heat-affected zone of the welded portion 20 is 70% or more of the Vickers hardness of the unwelded portion. That is, by performing the heating process before the forming process, the HAZ softening of the heat-affected zone of the welded portion 20 that occurred during welding before the heating process is reduced. In the second member 12 as well, the minimum value of the Vickers hardness of the heat-affected zone of the welded portion 20 may also be 70% or more of the Vickers hardness of the unwelded portion. In this way, the softening of the heat-affected zone (HAZ) is reduced, and the hardness difference between the non-welded area and the heat-affected zone becomes smaller. As a result, when a vehicle body using the structural member 10 is hit, fracture originating from the welded area 20 becomes less likely. Therefore, according to the manufacturing method of this embodiment, the collision performance of the structural member 10 can be improved.

[0089] When the structural member 10 is a cold-worked member, the portion near the weld 20 is affected by the constraint of the weld 20 when the blank 30 is formed into the structural member 10. During the forming process, the material is constrained by the weld 20, causing the portion near the weld 20 to be pulled, resulting in strain and work hardening. As a result, the portion of the structural member 10 near the weld 20 becomes harder than the base material after the forming process. More specifically, in the first member 11, the maximum Vickers hardness HVmax in the area R1 near the weld 20 is 7.0% or more greater than the Vickers hardness HVm of the base material (ΔHV = HVmax - HVm ≥ 0.070 × HVm). Similarly, in the second member 12, the maximum Vickers hardness HVmax in the area R2 near the weld 20 may be 7.0% or more greater than the Vickers hardness HVm of the base material (ΔHV = HVmax - HVm ≥ 0.070 × HVm). By having a relatively hard area near the weld 20 in this way, the load-bearing capacity of the structural member 10 during shear deformation can be improved.

[0090] In the structural member 10 according to this embodiment, the top plate 111 of the first member 11 and the top plate 121 of the second member 12 are bent at an angle θ of 150° or less. Even in this case, if the total extended length L of the top plates 111 and 121 is relatively short, it may be possible to form the structural member 10 by general press forming while suppressing the occurrence of wrinkles in the bent parts of the top plates 111 and 121, for example by rotating the pressing direction. However, if the total extended length L of the top plates 111 and 121 is long, it becomes difficult to rotate the pressing direction, making it difficult to prevent wrinkles by rotating the pressing direction. On the other hand, in this embodiment, the first member 11 and the second member 12 are formed from two directions, the first direction D1 and the second direction D2. More specifically, the top plate 121 of the second member 12 is formed from the second direction D2 while the first member 11 is restrained from the first direction D1 and the flange 123 of the second member 12 is restrained from the second direction D2. This makes it possible to suppress the occurrence of wrinkles on the top plates 111 and 121 without rotating the press direction. Therefore, even structural members 10 with a total extended length L of top plates 111 and 121 of 400 mm or more can be easily formed.

[0091] In general press forming, that is, press forming in a single direction, when integrally forming the first member 11 and the second member 12, the smaller the angle θ between the top plate 111 of the first member 11 and the top plate 121 of the second member 12, the more difficult the forming becomes. For example, if the angle θ is less than 120°, wrinkles will occur in the bent parts of the top plates 111 and 121 in general press forming, making it difficult to properly form the first member 11 and the second member 12. However, in this embodiment, the first member 11 and the second member 12 are formed from two directions, the first direction D1 and the second direction D2. More specifically, the top plate 121 of the second member 12 is raised from the second direction D2 while the first member 11 is restrained from the first direction D1 and the flange 123 of the second member 12 is restrained from the second direction D2. In this case, the occurrence of wrinkles in the top plates 111 and 121 can be suppressed. In the manufacturing method according to this embodiment, even when the angle θ is 110° or less or 100° or less, wrinkles are less likely to occur on the top plates 111 and 121, and the first member 11 and the second member 12 can be well integrally molded.

[0092] When the first member 11 and the second member 12 are integrally formed by general press forming, if the first member 11 and the second member 12 are thick, wrinkles in the top plates 111 and 121 are easily flattened during forming, and the degree of wrinkles that occur in the top plates 111 and 121 is relatively small. However, if the first member 11 and the second member 12 are thin, wrinkles in the top plates 111 and 121 are not easily flattened, and in general press forming, noticeable wrinkles occur in the bent parts of the top plates 111 and 121. On the other hand, in this embodiment, the first member 11 and the second member 12 are formed from two directions, the first direction D1 and the second direction D2. More specifically, with the first member 11 restrained from the first direction D1 and the flange 123 of the second member 12 restrained from the second direction D2, the top plate 121 of the second member 12 is raised from the second direction D2. In this case, even if the first member 11 and / or the second member 12 are thin, the occurrence of wrinkles on the top plates 111 and 121 can be suppressed. When using the manufacturing method according to this embodiment, even if the thickness of one or both of the first member 11 and the second member 12 is, for example, 2.0 mm or less, 1.5 mm or less, or 1.0 mm or less, wrinkles are less likely to occur on the top plates 111 and 121, and the first member 11 and the second member 12 can be well integrally molded. Furthermore, even if the first member 11 and / or the second member 12 are thin and the angle θ of the top plates 111 and 121 is relatively small, the occurrence of wrinkles on the top plates 111 and 121 can be suppressed.

[0093] While embodiments relating to this disclosure have been described above, this disclosure is not limited to the embodiments described above, and various modifications are possible as long as they do not deviate from its spirit.

[0094] In the above embodiment, the structural member 10 includes a first member 11 and a second member 12. The structural member 10 may further include other members. For example, as shown in Figure 8, the structural member 10 may include a third member 13 in addition to the first member 11 and the second member 12.

[0095] As shown in Figure 8, the third member 13 may be arranged in parallel with the second member 12. The third member 13, like the second member 12, may have a hat-shaped cross-section in at least a portion of its longitudinal direction. The third member 13 may be joined to the second member 12 by welding, for example, at the stage of the blank 30 (Figure 5A) before molding. Alternatively, the third member 13 may be provided continuously with the second member 12, and there may be no joint such as a weld at the boundary between the second member 12 and the third member 13. Even with such a structural member 10, the manufacturing method according to the above embodiment can be applied. However, the third member 13 may be joined to the second member 12 by welding, for example, after the molding of the first member 11 and the second member 12. Although not shown, the third member 13 may be arranged in parallel with the first member 11. In this case as well, the third member 13 may be joined to the first member 11 before molding by welding, for example, or it may be joined to the first member 11 after the molding of the first member 11 and the second member 12. Alternatively, the third member 13 may be provided continuously with the first member 11, and there may be no joint such as a weld at the boundary between the first member 11 and the third member 13.

[0096] In the structural member 10 according to the above embodiment, a single first member 11 and a single second member 12 are provided, and the first member 11 and the second member 12 are arranged to form a substantially L-shape. However, as shown in Figure 9, the structural member 10 may also include, for example, two second members 12. In the example in Figure 9, the second members 12 are arranged on both sides in the longitudinal direction of the first member 11. That is, the first member 11 and the two second members 12 are arranged to form a substantially U-shape. The longitudinal end of one second member 12 is overlapped with one longitudinal end of the first member 11 and joined by welding. The longitudinal end of the other second member 12 may be overlapped with the other longitudinal end of the first member 11 and joined by welding. Even with such a structural member 10, the manufacturing method according to the above embodiment can be applied. It is preferable that each of the second members 12 is joined to the first member 11 at the stage of the blank 30 (Figure 5A) before molding. However, if the structural member 10 includes two second members 12, it is sufficient that at least one of the second members 12 is joined to the first member 11 by overlap welding before molding.

[0097] In the manufacturing method according to the above embodiment, the movable molds 43 and 44 move in the second direction D2 by a cam mechanism. More specifically, the movable molds 43 and 44 move in the second direction D2 by a common cam driver 46. However, it is also possible to provide a cam driver for each of the movable molds 43 and 44. For example, if the position of the sliding surface 432 of the movable mold 43 and the position of the sliding surface 442 of the movable mold 44 are different in directions perpendicular to the first direction D1 and the second direction D2 (directions perpendicular to the plane of the paper in Figures 5C to 5F), a cam driver can be provided for each of the movable molds 43 and 44. However, it is sufficient that the movable molds 43 and 44 are configured to be movable in the second direction D2. It is not necessarily required that a cam mechanism be used as a means to actuate the movable molds 43 and 44. For example, the movable molds 43 and 44 may be moved in the second direction D2 by an actuator such as a hydraulic cylinder. When the movable molds 43 and 44 are actuated by means other than a cam mechanism, the mold 40 is not provided with a cam driver 46.

[0098] In the above embodiment, the movable mold 44 is positioned inside the movable mold 43. However, the arrangement of the movable molds 43 and 44 is not limited to this. For example, the movable mold 43 may be housed within the movable mold 44 and configured to protrude from the movable mold 44 in the second direction D2.

[0099] In the above embodiment, the mold 40 includes an upper mold 41, a lower mold 42, and movable molds 43 and 44. However, the mold 40 does not necessarily have to adopt the configuration described in the above embodiment. The mold 40 is sufficient if it is configured to allow the top plate 121 and vertical wall 122 of the second member 12 to be molded from the second direction D2 while constraining the first member 11 from the first direction D1 and constraining the flange 123 of the second member 12 from the second direction D2.

[0100] In the above embodiment, the upper mold 41 is positioned above the lower mold 42. However, the positional relationship between the upper mold 41 and the lower mold 42 is not limited to this. For example, the lower mold 42 may be positioned above the upper mold 41, or the upper mold 41 and the lower mold 42 may not be positioned vertically.

[0101] In the above embodiment, a metal plate serving as a patch material may be provided on the blank 30 before the molding process. The patch material is overlapped on a part of the blank 30 and joined to the blank 30 by spot welding or laser welding. The blank 30 may be molded into a structural member 10 with the patch material joined to it. The patch material can be placed in the structural member 10 where reinforcement is required. [Examples]

[0102] The present disclosure will be further described below with reference to examples. However, the present disclosure is not limited to the following examples.

[0103] [First Embodiment] Press forming tests were conducted on structural members including the first and second members to investigate the Vickers hardness of the structural members. In the example, the first and second members were joined by welding before forming and integrally formed as a single structural member by hot stamping. In the comparative example, the first and second members were formed from separate blanks (steel plates), and the first and second members were joined by welding after forming.

[0104] In both the examples and comparative examples, the first member is joined to the second member by spot welding with their ends overlapping. For each of the examples and comparative examples, the Vickers hardness of the spot weld cross-section of the first member was investigated. For each spot weld cross-section of the examples and comparative examples, the Vickers hardness was measured at 0.2 mm intervals along a straight line at a position 1 / 4 of the way from the surface on the side closer to the weld center in the plate thickness direction, in accordance with JIS Z 2244-1:2024 (test force: 0.49 N). Figure 10 shows the distribution of Vickers hardness of the spot weld cross-section of the structural members for the examples and comparative examples. As shown in Figure 10, a heat-affected zone exists around the weld metal (nugget) of the weld. At a point approximately 1.0 mm away from the outer edge of the weld metal, a decrease in Vickers hardness (softening) due to heat effects occurs, but as you move further away from the outer edge of the weld metal, the Vickers hardness converges to the base metal hardness: 500 Hv.

[0105] Referring to Figure 10, the Vickers hardness was measured at 0.2 mm intervals from the center of the weld toward both outer sides of the weld metal, for example, at a distance of 7.0 mm or more. When the minimum measured Vickers hardness was taken as the minimum Vickers hardness of the heat-affected zone, the minimum Vickers hardness of the heat-affected zone in the comparative example was approximately 300 Hv. In the comparative example, the minimum Vickers hardness of the heat-affected zone was approximately 60% of the Vickers hardness of the non-welded area (base metal hardness): 500 Hv.

[0106] On the other hand, in the example, the minimum Vickers hardness of the heat-affected zone was 420 Hv, which was higher than the minimum Vickers hardness of the heat-affected zone in the comparative example. In the example, the minimum Vickers hardness of the heat-affected zone was 84% ​​of the Vickers hardness of the non-welded area (base material hardness): 500 Hv.

[0107] Thus, it was confirmed that when the first and second components are integrated from the blank stage and formed by hot stamping, the softening of the heat-affected zone is reduced.

[0108] [Second Example] We investigated the Vickers hardness of structural members by performing press forming (cold forming) analysis using commercially available analysis software (AutoForm R10, manufactured by AutoForm). In this analysis, two metal plates (steel plates) were overlapped and joined by spot welding to form a hat shape, and then the Vickers hardness near the weld was measured.

[0109] As described in the above embodiment, at a position 1 / 4 of the plate thickness from the surface closer to the center of the weld on one of the two surfaces of one metal plate, within a range of 2.0 mm outward from the edge of the heat-affected zone of the weld (range R1 or R2 in Figure 7), Vickers hardness was measured at five or more points in accordance with JIS Z 2244-1:2024 (test force: 0.49 N), and the maximum value HVmax of these Vickers hardness measurements was taken as the Vickers hardness near the weld. In this analysis, the Vickers hardness was investigated for each of the multiple welds (weld points). The results of this analysis are shown in Table 1.

[0110] [Table 1]

[0111] As shown in Table 1, in all welds formed before molding, the Vickers hardness increased after molding, with the Vickers hardness HVmax near the weld being 7.0% or more greater than the Vickers hardness HVm of the base material. On the other hand, when welds were formed after molding, there was almost no increase in Vickers hardness near the weld because no strain from molding occurred near the weld. When welds were formed after molding, the Vickers hardness HVmax near weld 1 in Table 1 was 279 HV, and the Vickers hardness HVmax near weld 2 was 282 HV, resulting in an increase of less than 2.0% in Vickers hardness HVmax near the weld relative to the base material's Vickers hardness HVm: 278 HV.

[0112] Thus, it was confirmed that when metal plates are welded together before forming and then cold-worked, the hardness of the area near the weld increases compared to the base material. More specifically, it was confirmed that the maximum Vickers hardness HVmax near the weld becomes 7.0% or more greater than the Vickers hardness HVm of the base material. [Explanation of Symbols]

[0113] 10: Structural members 11: First component 111: Top plate (first top plate) 112,112a,112b: Vertical wall (First vertical wall) 113, 113a, 113b: Flange (First flange) 12: Second component 121: Top plate (second top plate) 122, 122a, 122b: Vertical wall (second vertical wall) 123, 123a, 123b: Flange (second flange) 20: Welded section 22: Heat-affected zone 30: Blank 31: 1st metal plate 32:Second metal plate 40: Mold 41: Upper mold 42: Lower mold 43,44: Movable type

Claims

1. A method for manufacturing structural members, A step of preparing a blank comprising a first metal plate and a second metal plate, wherein the end of the first metal plate overlaps the end of the second metal plate and the ends of the first metal plate are joined together by welding, The process includes a step of forming the blank into a structural member including a first member and a second member using a mold that includes an upper mold and a lower mold, The first member includes a first top plate, a pair of first vertical walls connected to both side edges of the first top plate, and a pair of first flanges connected to the first vertical walls on the opposite side of the first top plate and projecting outward from the first vertical walls. The second member includes a second top plate that is bent relative to the first top plate at an angle of 150° or less with the first top plate, a pair of second vertical walls connected to both side edges of the second top plate, and a pair of second flanges connected to the second vertical wall on the opposite side of the second top plate and projecting outward from the second vertical wall. A manufacturing method in which, in the molding step, the first metal plate is formed into the first member and the second metal plate is formed into the second member, and the first member is restrained by the mold from a first direction which is the pressing direction of the upper and lower molds, and the second flange is restrained from a second direction which is perpendicular to the first direction, and the second top plate and the second vertical wall are formed by the mold from the second direction.

2. A manufacturing method according to claim 1, The mold further includes two movable molds, A manufacturing method comprising the molding process, wherein one of the two movable molds moves along the second direction to form the end of the second metal plate into the second flange and restrains the second flange, and the other of the two movable molds moves along the second direction to form the other portion of the second metal plate into the second top plate and the second vertical wall.

3. A manufacturing method according to claim 2, A manufacturing method comprising the molding process, in which the first member is clamped and restrained from a first direction by the upper mold and the lower mold, and the second flange is clamped and restrained by the upper mold and one of the movable molds, while the second top plate and the second vertical wall are molded by the other movable mold.

4. A structural member, A first member comprising a first top plate, a pair of first vertical walls connected to both side edges of the first top plate, and a pair of first flanges connected to the first vertical walls on the opposite side of the first top plate and projecting outward from the first vertical walls, A second member comprising: a second top plate bent relative to the first top plate at an angle of 150° or less with the first top plate; a pair of second vertical walls connected to both side edges of the second top plate; and a pair of second flanges connected to the second vertical wall on the opposite side of the second top plate and projecting outward from the second vertical wall; Equipped with, The longitudinal end of the first member is joined to the longitudinal end of the second member by welding, with the end of the second member overlapping the longitudinal end of the second member. A structural member wherein, in the first member, the minimum Vickers hardness of the heat-affected zone of the welded portion is 70% or more of the Vickers hardness of the unwelded portion.

5. A structural member, A first member comprising a first top plate, a pair of first vertical walls connected to both side edges of the first top plate, and a pair of first flanges connected to the first vertical walls on the opposite side of the first top plate and projecting outward from the first vertical walls, A second member comprising: a second top plate bent relative to the first top plate at an angle of 150° or less with the first top plate; a pair of second vertical walls connected to both side edges of the second top plate; and a pair of second flanges connected to the second vertical wall on the opposite side of the second top plate and projecting outward from the second vertical wall; Equipped with, The longitudinal end of the first member is joined to the longitudinal end of the second member by welding, with the end of the second member overlapping the longitudinal end of the second member. A structural member wherein, in a cross-section of the first member along its longitudinal direction, the maximum value of the Vickers hardness in the range from the edge of the weld in a direction perpendicular to the thickness direction of the first member up to 2.0 mm outward is HVmax, and the Vickers hardness of the base material of the first member is HVm, such that HVmax - HVm is 7.0% or more of HVm.

6. A structural member according to claim 4 or 5, A structural member wherein the total extended length of the first and second top plates, as measured along the first and second top plates, is 400 mm or more.

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