Structural member and method for producing same

Abstract

This method for producing a structural member (10) comprises a step for preparing a blank (30) and a step for molding the blank (30) into the structural member (10). In the blank (30), an end of a first metal plate (31) is joined to an end of a second metal plate (32) by welding with the ends overlapping each other. The structural member (10) includes a first member (11) and a second member (12). In the molding step, a top plate (121) and vertical walls (122, 122a, 122b) of the second member (12) are molded from a second direction (D2) by a mold (40) with the first member (11) constrained from a first direction (D1) by the mold (40) and flanges (123, 123a, 123b) of the second member (12) constrained from the second direction (D2).

Description

Structural member and manufacturing method thereof

[0001] FIELD OF THE DISCLOSURE The present disclosure relates to structural members and methods of manufacturing the same.

[0002] A structure such as an automobile body is composed of multiple structural members. Each structural member is fabricated, for example, by processing a blank using a mold. The structure is then formed by joining the formed structural members together by welding.

[0003] In recent years, the automotive industry has seen an accelerating trend toward joining and integrating structural components for vehicle bodies at the blank stage. When a module consisting of multiple structural components is molded from a single blank, the number of vehicle body parts can be reduced compared to when multiple structural components are joined after molding. As a result, processes during the manufacturing of the vehicle body are omitted, enabling a reduction in greenhouse gas (GHG) emissions throughout the vehicle's life cycle.

[0004] However, when a module that was previously composed of multiple structural members is molded into a single structural member, the structural member may have a portion that forms a negative angle with respect to the processing direction of the mold. If a portion that forms a negative angle with respect to the processing direction exists, molding the structural member becomes difficult.

[0005] Patent Document 1 discloses a technique for forming negative angle portions (groove-shaped bead portions) on the left and right vertical walls of a structural member having a hat-shaped cross section. Patent Document 1 uses a mold including an upper mold, a lower mold, and a pair of slide molds attached to both sides of the upper mold via a cam mechanism. The lower mold includes a center mold and a pair of split molds attached to both sides of the center mold via a cam mechanism. In Patent Document 1, as the mold is clamped, the pair of split molds move apart in the width direction of the mold. Also, as the mold is clamped, the pair of slide molds move closer to each other in the width direction of the mold. Then, each vertical wall is sandwiched between the split molds and the slide mold, and a bead portion is formed on the vertical wall.

[0006] Patent Document 2 discloses a forming method in which a plurality of blanks are integrated and then press-formed. In Patent Document 2, the blanks are stacked so that they partially overlap, and a portion of the overlapping portion is temporarily fastened. After these blanks are formed into a press-formed product, the remaining portion of the overlapping portion is fixed.

[0007] JP 2011-83807 A JP 2007-29966 A

[0008] As described above, when multiple components are joined at the blank stage and molded into a single structural component, portions of the structural component may form a negative angle with respect to the mold processing direction. Furthermore, when molding multiple components integrally into a single structural component, it may be necessary to mold flanges parallel to the mold processing direction. For example, when molding multiple components each having a hat-like cross section integrally into a single structural component, if one top plate of adjacent components is positioned so that it bends relative to the other top plate, the top plate of one of the components may form a negative angle with respect to the mold processing direction. Furthermore, if one top plate of adjacent components is positioned so that it bends relative to the other top plate, it is possible that the flange of one of the components will be substantially parallel to the mold processing direction. Therefore, it is difficult to integrate such components at the blank stage and mold them into a single structural component. If the top plate 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 mold these components integrally into a single structural component.

[0009] An object of the present disclosure is to provide a method for manufacturing a structural member that can integrally mold a first member and a second member into 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.

[0010] A method for manufacturing a structural member according to the present disclosure includes 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, an end of the first metal plate is overlapped with an end of the second metal plate, and the ends are joined by welding. 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 so as to form an angle of 150° or less with the first top plate. 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 a first member and the second metal plate is formed into a second member. In the forming process, the first member is constrained by a mold from a first direction and the second flange is constrained from a second direction perpendicular to the first direction, and in this state, the second top plate and the second vertical wall are formed by the mold from the second direction.

[0011] According to the manufacturing method of a structural member disclosed herein, even if 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.

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

[0013] A method for manufacturing a structural member according to an embodiment includes 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, an end of the first metal plate is overlapped with an end of the second metal plate, and the ends are joined by welding. 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 so as to form an angle of 150° or less with the first top plate. 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 constrained by a mold from a first direction and the second flange is constrained from a second direction perpendicular to the first direction, and the second top plate and the second vertical wall are formed by the mold from the second direction (first configuration).

[0014] In a manufacturing method according to a first aspect, a structural member is formed from a blank including a first metal plate and a second metal plate whose ends are overlapped and welded together. More specifically, a first component of the structural member is formed from the first metal plate of the blank, and a second component 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, the first component is constrained from a first direction by a mold, and a flange (second flange) of the second component is constrained from a second direction perpendicular to the first direction. Forming the second component from a second direction different from the first direction in this manner allows the second component to be formed even if the second component has a portion that forms a negative angle with respect to the first direction due to the second component being bent relative to the top component (first top component). Furthermore, because the second member is molded laterally in the first direction, the second flange can be molded even if the second top plate is bent relative to the first top plate, resulting in a portion of the second flange that is substantially parallel to the first direction. Therefore, even if the first and second top plates are bent at an angle of 150° or less, the first and second members can be integrally molded as a single structural member. This reduces the number of parts in a structure, such as a vehicle body, compared to when the first and second members are molded separately as separate members. As a result, manufacturing processes for the structure can be omitted, thereby reducing lifecycle GHG emissions.

[0015] When the second top plate is bent at an angle of 150° or less relative to the first top plate, wrinkles are likely to occur at the bent portions of these top plates during molding. In contrast, in the manufacturing method according to the first configuration, the first member is constrained from a first direction, and the second flange is constrained from a second direction perpendicular to the first direction, and the second top plate is molded from the second direction. This makes it possible to suppress the occurrence of wrinkles at the bent portions of the first and second top plates during molding of the structural member.

[0016] In the manufacturing method according to the first configuration, the mold may include two movable dies. In this case, in the molding step, one of the two movable dies, which moves along the second direction, may mold an end of the second metal plate into the second flange and constrain the second flange, and the other of the two movable dies, which moves along the second direction, may mold the other portion of the second metal plate into the second top plate and the second vertical wall (second configuration).

[0017] In the manufacturing method according to the second configuration, the mold may further include an upper mold and a lower mold. In this case, in the molding step, 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 are molded by the other movable mold (third configuration).

[0018] The structural member according to the embodiment 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 so as to form an angle of 150° or less with the first top plate. 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. A longitudinal end of the first member is joined to an end of the second member by welding while overlapping the longitudinal end of the second member. 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 non-welded portion (fourth configuration).

[0019] In a 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 non-welded portion. That is, because the first and second members are integrated at the blank stage and simultaneously formed as a single structural member, even if softening of the heat-affected zone (HAZ) occurs during welding prior to forming, the HAZ softening is reduced by subsequent heat treatment, thereby reducing the difference in hardness between the non-welded portion and the heat-affected zone. As a result, fracture originating from the weld is less likely to occur during a collision of a vehicle body in which the structural member is used, and the structural member can have good crashworthiness.

[0020] A structural member according to another embodiment 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 so as to form an angle of 150° or less with the first top plate. 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. A longitudinal end of the first member is joined to an end of the second member by welding while overlapping the longitudinal end of the second member. In a cross section along the longitudinal direction of the first member, when the maximum value of the Vickers hardness in the range from the edge of the weld in a direction perpendicular to the plate thickness direction of the first member 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, 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 joining the first and second members, causing tension and strain in the vicinity of the weld during the formation of the structural member, resulting in work hardening in that area. As a result, at least in the first member, the Vickers hardness (maximum value) of the portion near the weld is 7.0% or more higher than the Vickers hardness of the base material. In this way, work hardening in the portion near the weld in the first member improves the yield strength of the structural member during shear deformation.

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

[0023] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. In these drawings, the same or equivalent components are designated by the same reference numerals, and the same description will not be repeated.

[0024] 1 is a perspective view of a structural member 10 according to this embodiment. The structural member 10 includes a first member 11 and a second member 12.

[0025] Referring to FIG. 1, the first member 11 includes a top plate 111, a pair of vertical walls 112a and 112b, and a pair of flanges 113a and 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 top plate 111 on the opposite side from the other vertical wall 112b. 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, 112b extend in the longitudinal direction of the first member 11 along the top plate 111. The flanges 113a, 113b extend in the longitudinal direction of the first member 11 along the vertical walls 112a, 112b, respectively. The heights of the vertical walls 112a, 112b may be constant over the entire longitudinal length of the first member 11, or may vary along the longitudinal direction of the first member 11. For example, the vertical walls 112a, 112b may essentially disappear in a portion of the longitudinal direction of the first member 11, and the top plate 111 and the flanges 113a, 113b may be flat.

[0028] Fig. 2 is a cross-sectional view taken along line II-II of the first member 11 shown in Fig. 1. Fig. 2 shows a cross section (transverse section) of the first member 11 cut along a plane perpendicular to the longitudinal direction thereof.

[0029] 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 the cross section in at least a portion of its longitudinal direction.

[0030] The ridge portions 114a and 114b are provided continuously with the top plate 111. The ridge portion 114a connects one of the vertical walls 112a to the top plate 111. The ridge portion 114a is a corner portion between the top plate 111 and the vertical wall 112a. The ridge portion 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 portion 114b is a corner portion between the top plate 111 and the vertical wall 112b. The ridge portions 114a and 114b may each have an arc shape when viewed in cross section of the first member 11.

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

[0032] The vertical walls 112a, 112b may be arranged symmetrically or asymmetrically in a cross-sectional view of the first member 11. For example, the heights of the vertical walls 112a, 112b may be equal to or different from each other in a cross-sectional view of the first member 11. Furthermore, the vertical walls 112a, 112b may be arranged parallel to or non-parallel to each other in a cross-sectional view of the first member 11. For example, the vertical walls 112a, 112b may be spaced 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, 113b may be provided symmetrically or asymmetrically in a cross-sectional view of the first member 11. For example, the lengths of the flanges 113a, 113b may be equal to or different from each other in a cross-sectional view of the first member 11. Furthermore, the flanges 113a, 113b may be parallel or non-parallel in a cross-sectional view of the first member 11.

[0034] Returning to FIG. 1 , the second member 12 is disposed adjacent to the first member 11. The longitudinal end of the first member 11 is overlapped with the longitudinal end of the second member 12 and joined to the end of the second member 12 by welding. Therefore, a weld 20 is formed at the overlapping end of the first member 11 and the second member 12. The weld 20 is provided across the structural member 10 in a direction intersecting the longitudinal directions of the first member 11 and the second member 12. The weld 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 FIG. 1, the second member 12 includes a top plate 121, a pair of vertical walls 122a and 122b, and a pair of flanges 123a and 123b.

[0036] The top plate 121 extends in the longitudinal direction of the second member 12. The top plate 121 is bent relative to the top plate 111 of the first member 11. The second member 12 may be arranged so that the entire second member 12 is bent 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 protrude outward from the vertical walls 122a and 122b.

[0037] The vertical walls 122a, 122b extend in the longitudinal direction of the second member 12 along the top plate 121. The flanges 123a, 123b extend in the longitudinal direction of the second member 12 along the vertical walls 122a, 122b, respectively. The heights of the vertical walls 122a, 122b may be constant over the entire longitudinal length of the second member 12, or may vary along the longitudinal direction of the second member 12. For example, the vertical walls 122a, 122b may essentially disappear in a portion of the longitudinal direction of the second member 12, and the top plate 121 and the flanges 123a, 123b may be flat. The flanges 123a, 123b may be connected and integrated at the ends of the second member 12 opposite the first member 11, among both longitudinal end portions.

[0038] Fig. 3 is a cross-sectional view of the second member 12 taken along line III-III of Fig. 1. Fig. 3 shows a cross section (transverse cross section) of the second member 12 cut along a plane perpendicular to the longitudinal direction thereof.

[0039] 3, the second member 12 has a hat-shaped cross section along at least a portion of its length. That is, the second member 12 includes four ridges 124a, 124b, 125a, and 125b in cross section along at least a portion of its length.

[0040] The ridge portions 124a and 124b are provided continuously with the top plate 121. The ridge portion 124a connects one of the vertical walls 122a to the top plate 121. The ridge portion 124a is a corner portion between the top plate 121 and the vertical wall 122a. The ridge portion 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 portion 124b is a corner portion between the top plate 121 and the vertical wall 122b. The ridge portions 124a and 124b may each have an arc shape when viewed in cross section of the second member 12.

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

[0042] The vertical walls 122a, 122b may be arranged symmetrically or asymmetrically in a cross-sectional view of the second member 12. For example, the heights of the vertical walls 122a, 122b may be equal to or different from each other in a cross-sectional view of the second member 12. Furthermore, the vertical walls 122a, 122b may be arranged parallel to or non-parallel to each other in a cross-sectional view of the second member 12. For example, the vertical walls 122a, 122b may be spaced 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, 123b may be provided symmetrically or asymmetrically in a cross-sectional view of the second member 12. For example, the lengths of the flanges 123a, 123b may be equal to or different from each other in a cross-sectional view of the second member 12. Furthermore, the flanges 123a, 123b may be parallel or non-parallel in a cross-sectional view of the second member 12.

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

[0045] 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 have a substantially straight (flat) shape when viewed in vertical cross section of the structural member 10.

[0046] In the top plates 111 and 121, a curved portion 143 may be provided between adjacent flat portions 141 and 142. When the top plate 121 includes a plurality of flat portions 142 as in the example of this embodiment, a curved portion 143 may also be provided between adjacent flat portions 142. The curved portion 143 has an arched shape that is concave toward the inside of the structural member 10 in a vertical cross-sectional view of the structural member 10.

[0047] The flat portion 141 of the top plate 111 can be smoothly connected to the flat portion 142 of the top plate 121 via the curved portion 143. The welded portion 20 may be disposed at the position of the curved portion 143 that connects the adjacent flat portions 141 and 142. Alternatively, the welded portion 20 may be disposed at a position close to the curved portion 143 on either one of the adjacent flat portions 141, 142.

[0048] The top plate 121 of the second member 12 is bent relative to the top plate 111 of the first member 11 at an angle θ. The angle θ is the angle between the flat portion 141 of the top plate 111 and the flat portion 142 of the top plate 121 in a vertical cross-sectional view of the structural member 10. More specifically, the angle θ is the angle between extension lines (two-dot chain lines) of the outer surfaces of the flat portions 141 and 142 in the vertical cross-section of the structural member 10, and is the angle between 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. When at least one of the top plates 111 and 121 includes multiple flat portions, the smallest angle formed by the combination of the flat portions 141 and 142 selected from the top plates 111 and 121, respectively, is defined as the angle θ. The angle θ is 150° or less, preferably 135° or less, and more preferably 120° or less. The angle θ may be 110° or less, or 100° or less. The angle θ may be 90° or more.

[0049] The structural member 10 is a relatively long member. In the structural member 10, the extension length L of the top plates 111, 121 is, for example, 400 mm or more. The extension length L may be 600 mm or more, or 800 mm or more. The extension length L is, for example, 1500 mm or less. The extension length L is the total extension length of the top plates 111, 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 extension length L is measured, for example, along the outer surfaces of the top plates 111, 121 in a vertical cross section of the structural member 10 at the center of the width of the top plate 111 or 121.

[0050] Returning to FIG. 1 again, the first member 11 and the second member 12 are formed of metal plates. The first member 11 and the second member 12 may be formed of steel plates. The first member 11 and the second member 12 may 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 strength 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 measured in accordance with JIS Z 2241:2011 by taking a No. 5 test piece of the full thickness from a flat portion of the top plate 111 near the center in the longitudinal and width directions of the first member 11 and conducting 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 piece of the full thickness from a corner portion of the flange 123a or 123b at one of both longitudinal ends of the second member 12 opposite the first member 11 and conducting a tensile test. The corner portion of the flange 123a refers to the end of the connection between the portion of the flange 123a extending in the longitudinal direction of the second member 12 along the vertical wall 122a and the portion of the flange 123a extending in the width direction of the second member 12. The corner portion of flange 123b refers to the end of the connection between the portion of flange 123b extending in the longitudinal direction of second member 12 along vertical wall 122b and the portion of flange 123b extending in the width direction of second member 12. When only one test piece for the tensile strength test can be taken from each of first member 11 and second member 12, the tensile strength is determined by conducting the tensile strength test on that one test piece, and when two or more test pieces can be taken, the tensile strength test is conducted on up to three test pieces, and the tensile strength is determined by averaging the measured tensile strengths.

[0052] When 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 Vickers hardness. The Vickers hardness of the first member 11 can be determined in accordance with JIS Z 2244-1:2024 by preparing a full-thickness optical microscope structure observation sample of a vertical cross section of a flat portion 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 at a ¼t portion (¼ plate thickness portion) of the sample 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 conforms to JIS Z 2244-1:2024. A full-thickness optical microscope microstructure observation sample is prepared for a vertical cross section of a corner portion of the flange 123a or 123b at one of the longitudinal ends of the second member 12 opposite the first member 11. The Vickers hardness is measured at five points in the 1 / 4t portion (1 / 4 plate thickness portion) of the sample with a test load of 1.961 N (200 gf), and the average value is calculated. When the Vickers hardness of the first member 11 and the second member 12 is Hv 138 or higher, the tensile strength of the first member 11 and the second member 12 is 440 MPa or higher. When the Vickers hardness of the first member 11 and the second member 12 is Hv 184 or higher, the tensile strength of the first member 11 and the second member 12 is 590 MPa or higher. When the Vickers hardness of the first member 11 and the second member 12 is Hv 243 or more, the tensile strength of the first member 11 and the second member 12 is 780 MPa or more. When the Vickers hardness of the first member 11 and the second member 12 is Hv 305 or more, the tensile strength of the first member 11 and the second member 12 is 980 MPa or more.

[0053] For example, a tensile strength of 440 MPa corresponds to Hv 138. In this case, if a Vickers hardness test is performed in accordance with JIS Z 2244-1:2024 at a test load of 1.961 N (200 gf), the diagonal length d of the indentation is 51.8 μm. If the plate thickness is 0.6 mm, the 1 / 4 t 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 JIS Z 2244-1:2024 requirement that the Vickers hardness test 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 Hv 367. In this case, when a Vickers hardness test is carried out with a test load of 1.961 N (200 gf), the diagonal length d of the indentation is 31.8 μm, and the Vickers hardness can be measured in accordance with JIS Z 2244-1:2024.

[0054] The thickness of the first member 11 and the second member 12 may be 2.3 mm or less. The thickness of the first member 11 and the second member 12 is, for example, 2.0 mm or less, preferably 1.5 mm or less, and more preferably 1.0 mm or less. The thickness of the first member 11 and the second member 12 may be 0.7 mm or more. The thickness of the first member 11 and the second member 12 may be the same or different. The thickness of the first member 11 and the second member 12 can be measured at the same locations as those used to measure tensile strength or Vickers hardness.

[0055] Such a structural member 10 is used, for example, in the body of an automobile or the like. One of the first member 11 and the second member 12 may be, for example, a front side member for the vehicle body. In this case, the other of the first member 11 and the second member 12 may be a coupling member for connecting the front side member to another member, such as an A-pillar post. Alternatively, one of the first member 11 and the second member 12 may be a cross member for the vehicle body. In this case, the other of the first member 11 and the second member 12 may be a coupling member for connecting the cross member to another member, such as a wheel house. However, the uses of the structural member 10 are not limited to these examples.

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

[0057] 5A , in the preparation step, a blank 30 is prepared. The blank 30 includes a first metal plate 31 and a second metal plate 32. The first metal plate 31 and the second metal plate 32 may be steel plates. The plate thicknesses of the first metal plate 31 and the second metal plate 32 may be the same or different. Furthermore, the tensile strengths of the first metal plate 31 and the second metal plate 32 may be the same or different.

[0058] The blank 30 is a so-called tailor-welded blank. In the blank 30, an end of a first metal plate 31 is overlapped with an end of a second metal plate 32, and these ends are joined by welding. Therefore, a step is formed on both surfaces of the blank 30 at the overlapping end of the first metal plate 31 and the second metal plate 32. Furthermore, a weld 20 is formed at the overlapping end 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 continuous welding. Intermittent welding is a welding method in which a weld is formed intermittently, such as spot welding. Continuous welding is a welding method in which a linear weld is formed, such as laser welding.

[0059] (Heating Step) In this embodiment, the structural member 10 (FIGS. 1 to 4) may be manufactured by cold working or hot working (hot stamping). When the structural member 10 is manufactured by hot stamping, a heating step is carried out before the forming step. In the heating step, the blank 30 is heated in, for example, a heating furnace. When 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 (A ｃ３ It is preferable that the temperature is higher than the above temperature.

[0060] 5B to 5F , in the forming process, the blank 30 is formed into the structural member 10 ( FIGS. 1 to 4 ) using a mold 40. In the forming process, the first metal plate 31 is formed into the first member 11 ( FIG. 1 ), and the second metal plate 32 is formed into the second member 12 ( FIG. 1 ). In the forming process, the first member 11 is constrained in the first direction D1 by the mold 40, and the flanges 123a, 123b ( FIG. 1 ) of the second member 12 are constrained in the second direction D2 by the mold 40. In the following description, unless otherwise required, the vertical walls 122a, 122b of the second member 12 will be collectively referred to as the vertical walls 122, and the flanges 123a, 123b will be collectively referred to as the flanges 123. Similarly, unless there is a particular need to distinguish between them, the vertical walls 112a and 112b of the first member 11 will be collectively referred to as the vertical wall 112, and the flanges 113a and 113b will be collectively referred to as the flange 113.

[0061] Fig. 5B is a perspective view of the blank 30 and the mold 40. Figs. 5C to 5F show cross sections (longitudinal cross sections) of the blank 30 and the mold 40 cut along the length and thickness directions of the blank 30 at positions of the blank 30 that will be formed into the top plates 111, 121 (Fig. 1) of the structural member 10.

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

[0063] The upper mold 41 and the lower mold 42 are arranged to face each other in a first direction D1. The upper mold 41 and the lower mold 42 are configured to be able to move relatively close to and away from each other in the first direction D1. The first direction D1 is the pressing direction of the upper mold 41 and the lower mold 42, and is, for example, the vertical direction.

[0064] The lower die 42 is a portion of the mold 40 that primarily processes the first metal plate 31 to form the first member 11 ( FIG. 1 ). The lower die 42 can process the first metal plate 31 together with the upper die 41. The forming surface 411 of the upper die 41 has a shape corresponding to at least the first member 11. The forming surface 421 of the lower die 42 has a shape corresponding to at least the top plate 111 and vertical wall 112 ( FIG. 1 ) of the first member 11. The forming surfaces 411, 421 may each have a step corresponding to the step of the blank 30 at the overlapping end positions 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 the 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 dies 43 and 44 are disposed, for example, to the sides of the lower die 42. The movable dies 43 and 44 are portions of the mold 40 that are primarily used to process the second metal plate 32 and form it into the second member 12 ( FIG. 1 ). The movable dies 43 and 44 can process the second metal plate 32 together with the upper die 41. The molding surface 431 of the movable die 43 has a shape corresponding to at least a portion of the flange 123 ( FIG. 1 ) of the second member 12. The molding surface 441 of the movable die 44 has a shape corresponding to at least a portion of the top plate 121 and vertical wall 122 ( FIG. 1 ) of the second member 12. Since the movable die 44 is used to form the top plate 121 and vertical wall 122, it is disposed inside the movable die 43 that forms the flange 123. In a vertical cross-sectional view of the mold 40 ( FIG. 5C ), the movable dies 43 and 44 may be disposed side by side in the first direction D1. The molding surfaces 412 and 413 of the upper mold 41 correspond to the molding surfaces 431 and 441 of the movable mold 43 and 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 a direction that is substantially perpendicular to the first direction D1 in a vertical cross-sectional view of the mold 40. The second direction D2 is, for example, a horizontal direction.

[0067] The movable dies 43, 44 can be moved in the second direction D2 by, for example, a cam mechanism. In this case, the mold 40 can include a cam driver 46. The cam driver 46 can move relatively close to and away from the movable dies 43, 44 in the first direction D1. The movable dies 43, 44 can function as slide cams operated by the cam driver 46.

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

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

[0070] Next, the upper die 41 is moved relatively close to the lower die 42 in the first direction D1. The cam driver 46 is also moved relatively close to the movable dies 43 and 44 in the first direction D1. For example, the upper die 41 is moved toward the lower die 42 along the first direction D1. The cam driver 46 is also moved toward the movable dies 43 and 44 along the first direction D1. In this embodiment, as shown in FIG. 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. For example, the top plate 111 and the vertical wall 112 ( FIG. 1 ) are formed by the upper die 41 and the lower die 42 prior to the flange 113.

[0071] As shown in FIG. 5E , after the first metal plate 31 is sandwiched between the upper mold 41 and the lower mold 42, as the upper mold 41 continues to move, the lower mold 42 is pushed in the first direction D1 by the upper mold 41, causing the elastic member 45 to contract. Furthermore, as the cam driver 46 and the movable mold 43 approach each other, the sliding surface 461 of the cam driver 46 contacts and slides on the sliding surface 432 of the movable mold 43. This causes the movable mold 43 to move along the second direction D2. The movable mold 43 is pushed toward the upper mold 41 by the cam driver 46, forming the end of the second metal plate 32 into the flange 123. More specifically, the forming surface 431 of the movable mold 43 and the forming surface 412 of the upper mold 41 sandwich the end of the second metal plate 32, forming the end of the second metal plate 32 into the flange 123.

[0072] 5F , in the forming process, the movable die 43 forms the end of the second metal plate 32 into the flange 123 and restrains the flange 123, and the movable die 44 forms the other portion of the second metal plate 32 into the top plate 121 and the vertical wall 122. 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 top plate 121 and the vertical wall 122 are formed by the movable die 44 in a state in which the flange 123 is clamped and restrained by the upper die 41 and the movable die 43.

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

[0074] When a heating step is performed before the molding step, the structural member 10 may be held in the mold 40. In this way, the structural member 10 is heat-removed (quenched) by the mold 40.

[0075] In this embodiment, the height of the portion of the lower mold 42 facing the movable mold 44 is greater than the height of the portion opposite 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 makes it possible to prevent the top plate 111 or 112 from partially sagging due to the influence of gravity when the movable mold 44 is separated from the lower mold 42 in the second direction D2 during demolding.

[0076] Fig. 6 is a diagram schematically showing a portion of a longitudinal cross section of a structural member 10 formed through a heating process. That is, Fig. 6 is a partial longitudinal cross section of a structural member 10 (hot-stamped member) formed by hot stamping. On the other hand, Fig. 7 is a diagram schematically showing a portion of a longitudinal cross section of a structural member 10 formed without a heating process. That is, Fig. 7 is a partial longitudinal cross section of a structural member 10 (cold-worked member) formed by cold working. Figs. 6 and 7 show the overlapping end portion of the first member 11 and the second member 12 and the vicinity thereof.

[0077] Referring to FIG. 6 , a weld 20 is present at the overlapping end of the first member 11 and the second member 12. The weld 20 includes a weld metal portion 21 and a heat-affected zone 22. When a heating process is performed before the forming process in the manufacture of the structural member 10, HAZ softening is reduced in the weld 20 due to heating in the heating process. Specifically, when 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 non-welded portion. In the first member 11, the minimum Vickers hardness of the heat-affected zone 22 is preferably 80% or more, and more preferably 90% or more, of the Vickers hardness of the non-welded portion. In the first member 11, the minimum Vickers hardness of the heat-affected zone 22 is equal to or less than the Vickers hardness of the non-welded portion.

[0078] The Vickers hardness of the first member 11 can be measured by a Vickers hardness test specified in JIS Z 2244-1:2024. Specifically, first, a test specimen including the first member 11 and the welded portion 20 is obtained by cutting the structural member 10 at a position passing through the weld center of the welded portion 20 using laser cutting or the like. The test specimen is then embedded in resin so that a cross section passing through the weld center of the first member 11 and the welded portion 20 is located on the surface, and the cross section is polished. Next, Vickers hardness is measured at a position 1 / 4 of the plate thickness from the surface closest to the weld center on both surfaces of the first member 11 to a position 12.0 mm outward from the weld center, in accordance with JIS Z 2244-1:2024, using a test force of 0.49 N and a measurement interval (pitch) of 0.1 to 0.2 mm. The minimum value of the measured Vickers hardness is defined 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 15.0 mm or more away from the weld center of the welded portion 20 and at a position ¼ of the plate thickness from the surface of the first member 11 in accordance with JIS Z 2244-1:2024, using a test force of, for example, 0.49 N. This Vickers hardness is defined as the Vickers hardness of the non-welded portion of the first member 11.

[0079] Whether the structural member 10 is a hot-stamped member can be determined by measuring the boron (B) content in the structural member 10 and determining whether or not scale or an alloy layer (diffusion layer) is present. Specifically, a known compositional analysis is performed on the first member 11 to measure the B content in the base material of the first member 11. Furthermore, the first member 11 is subjected to cross-sectional observation using, for example, a scanning electron microscope (SEM) and compositional analysis using energy dispersive X-ray analysis (EDS) to determine whether or not scale or an alloy layer (diffusion layer) is present within the surface layer of 20 μm. For example, if the first member 11 is formed of an aluminum-based plated steel sheet, and the structural member 10 is a hot-stamped member, an Fe—Al alloy layer or diffusion layer will be present within the surface layer of 20 μm of the first member 11. Furthermore, if the first member 11 is formed of a zinc-based plated steel sheet, and the structural member 10 is a hot-stamped member, an Fe—Zn alloy layer or diffusion layer will be present within the surface layer of 20 μm of the first member 11. When the first member 11 is formed of a steel plate (bare material) without a plating layer, 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% B and scale or an alloy layer (diffusion layer) is present within 20 μm of the surface layer, the structural member 10 can be determined to be 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 HAZ softening of the heat-affected zone of the weld 20 is reduced during the heating process. Therefore, when the structural member 10 is a hot-stamped member, the minimum Vickers hardness of the heat-affected zone 22 of the first member 11 is 70% or more of the Vickers hardness of the non-welded portion.

[0080] When 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 also be 70% or more of the Vickers hardness of the non-welded portion. In the second member 12, the minimum Vickers hardness of the heat-affected zone 22 is preferably 80% or more, and more preferably 90% or more, of the Vickers hardness of the non-welded portion. In the second member 12, the minimum Vickers hardness of the heat-affected zone 22 is equal to or less than the Vickers hardness of the non-welded portion. The Vickers hardness of the second member 12 can be measured in the same manner as for the first member 11.

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

[0082] Even when 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 at a position passing through the weld center of the weld 20 by laser cutting or the like to obtain a test specimen including the first member 11 and the weld 20. The test specimen is then embedded in resin so that a cross section passing through the weld center of the first member 11 and the weld 20 is located on the surface, and the cross section is polished. Next, Vickers hardness is measured at five or more points within a range R1 extending 2.0 mm outward from the edge of the heat-affected zone 22 of the weld 20, at a position ¼ of the plate thickness from the surface closest to the weld center, in accordance with JIS Z 2244-1:2024, using a test force of 0.49 N, for example. The maximum value of the measured Vickers hardness is designated HVmax. Furthermore, the Vickers hardness is measured in accordance with JIS Z 2244-1:2024 at a position 15.0 mm or more away from the weld center of the weld 20 and at a position ¼ of the plate thickness from the surface of the first member 11, using a test force of, for example, 0.49 N. This Vickers hardness is defined as the Vickers hardness HVm of the base material of the first member 11. When the first member 11 and the second member 12 are joined by spot welding, HVmax is measured, for example, for the weld 20 on the ridge line 114a or 114b ( FIG. 2 ), or for the weld 20 closest to the ridge line 114a or 114b among the welds 20 arranged on the top plate 111 or the vertical wall 112 ( FIGS. 1 and 2 ). HVmax may also be measured for the weld 20 closest to the ridge line 114a or 114b among the welds 20 arranged on the vertical wall 112.

[0083] When the structural member 10 is a cold-worked member, the maximum Vickers hardness in the range R2 near the weld 20 in the second member 12 may also be HVmax, and the Vickers hardness of the base metal of the second member 12 may be HVm, where HVmax - HVm is 7.0% or more of HVm (ΔHV = HVmax - HVm ≥ 0.070 × HVm). The range R2 is the range in the longitudinal cross section of the second member 12, extending from the edge of the weld 20 (heat-affected zone 22) in a direction perpendicular to the thickness direction of the second member 12 to a position 2.0 mm outward. The Vickers hardnesses HVmax and HVm of the second member 12 can be measured in the same manner as for 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 overlap-welded in advance. In the forming process, the first member 11 is constrained from a first direction D1 by a mold 40, and the flange 123 of the second member 12 is constrained from a second direction D2 perpendicular to the first direction D1. 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 a portion of the top plate 121 extends into the first member 11 in a vertical cross-sectional view of the structural member 10, i.e., a portion that forms a negative angle with respect to the first direction D1, the top plate 121 can be formed by the mold 40. Furthermore, because the second member 12 is formed laterally in the first direction D1, even if the flange 123 of the second member 12 includes a portion that is substantially parallel to the first direction D1 because the top plate 121 is bent with respect to the top plate 111, the flange 123 can be formed using the mold 40. Therefore, even if the top plate 121 is bent with respect to the top plate 111 at an angle θ of 150° or less, which is generally difficult to form, the first member 11 and the second member 12 can be integrated at the blank 30 stage and molded integrally as a single structural member 10.

[0085] More specifically, in the manufacturing method according to this embodiment, the mold 40 includes two movable dies 43, 44 that move independently in the second direction D2. In the molding process, the movable die 43 that moves along the second direction D2 can mold the end of the second metal plate 32 into the flange 123. By molding the flange 123 from the side with respect to the first direction D1 using the movable die 43 in this manner, the flange 123 can be molded 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, another movable die 44 that moves along the second direction D2 can mold other portions of the second metal plate 32 into the top plate 121 and the vertical wall 122. In this way, by molding the top plate 121 and the vertical wall 122 from the side with respect to the first direction D1 by the movable die 44, it is possible to mold the top plate 121 and the vertical wall 122 connected thereto even when the top plate 121 includes a portion that forms 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 allows the number of parts in a structure such as a vehicle body to be reduced compared to when the first member 11 and the second member 12 are separate members. As a result, the manufacturing process for the structure can be omitted, and lifecycle 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 lap welding at the blank 30 stage before forming, and are 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 Vickers hardness of the heat-affected zone of the welded portion 20 is 70% or more of the Vickers hardness of the non-welded portion. In other words, 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, the minimum Vickers hardness of the heat-affected zone of the welded portion 20 may also be 70% or more of the Vickers hardness of the non-welded portion. In this way, HAZ softening in the heat-affected zone is reduced and the difference in hardness between the non-welded zone and the heat-affected zone is reduced, making it less likely that fracture will occur originating from the welded zone 20 during a collision of a vehicle body in which the structural member 10 is used. Therefore, the manufacturing method according to this embodiment can improve the collision performance of the structural member 10.

[0089] If the structural member 10 is a cold-worked member, when the blank 30 is formed into the structural member 10, the portion near the weld 20 is affected by the constraint of the weld 20. During the forming process, the material is constrained by the weld 20, which pulls the portion near the weld 20, causing strain in that portion and resulting in work hardening. As a result, in the structural member 10 after the forming process, the portion near the weld 20 becomes harder than the base material. More specifically, in the first member 11, the maximum Vickers hardness HVmax in the range 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 range 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). The presence of a relatively hard portion near the weld 20 in this manner can improve the yield strength of the structural member 10 during shear deformation.

[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 extension length L of the top plates 111 and 121 is relatively short, it may be possible to form the structural member 10 using conventional press molding while suppressing the occurrence of wrinkles at the bent portions of the top plates 111 and 121, for example, by rotating the press direction. However, if the total extension length L of the top plates 111 and 121 becomes long, it becomes difficult to rotate the press direction, making it difficult to prevent wrinkles caused by rotating the press 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 constrained from the first direction D1 and the flange 123 of the second member 12 is constrained from the second direction D2. This makes it possible to prevent wrinkles from occurring in the top plates 111, 121 without rotating the press direction. Therefore, even if the structural member 10 has a total extension length L of the top plates 111, 121 of 400 mm or more, for example, it can be easily formed.

[0091] When integrally molding the first member 11 and the second member 12 using typical press molding, i.e., press molding with a unidirectional processing direction, 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 molding process becomes. For example, if the angle θ is less than 120°, wrinkles will occur at the bent portions of the top plates 111 and 121 during typical press molding, making it difficult to properly mold the first member 11 and the second member 12. However, in this embodiment, the first member 11 and the second member 12 are molded from two directions, the first direction D1 and the second direction D2. More specifically, while the first member 11 is constrained from the first direction D1 and the flange 123 of the second member 12 is constrained from the second direction D2, the top plate 121 of the second member 12 is raised 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 in the top plates 111, 121, and the first member 11 and the second member 12 can be molded integrally well.

[0092] When the first member 11 and the second member 12 are integrally molded by typical press molding, if the first member 11 and the second member 12 are thick, wrinkles in the top plates 111 and 121 are easily flattened during molding, 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 difficult to flatten, and typical press molding results in significant wrinkles at bent portions of the top plates 111 and 121. On the other hand, in this embodiment, the first member 11 and the second member 12 are molded from two directions, a first direction D1 and a 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, it is possible to suppress the occurrence of wrinkles in the top plates 111, 121. When the manufacturing method according to this embodiment is used, even if the plate 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 unlikely to occur in the top plates 111, 121, and the first member 11 and the second member 12 can be favorably molded as a single unit. Furthermore, even if the first member 11 and / or the second member 12 are thin and the angle θ of the top plates 111, 121 is relatively small, it is possible to suppress the occurrence of wrinkles in the top plates 111, 121.

[0093] Although the embodiments of the present disclosure have been described above, the present disclosure is not limited to the above-described embodiments, and various modifications are possible without departing from the spirit of the present disclosure.

[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 FIG. 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 FIG. 8 , the third member 13 may be arranged parallel to the second member 12. Like the second member 12, the third member 13 may have a hat-shaped cross section along at least a portion of its longitudinal direction. The third member 13 may be joined to the second member 12 by, for example, welding at the stage of the blank 30 ( FIG. 5A ) before forming. Alternatively, the third member 13 may be provided continuously with the second member 12, and no joint such as a weld may exist at the boundary between the second member 12 and the third member 13. The manufacturing method according to the above embodiment can also be applied to such a structural member 10. However, the third member 13 may be joined to the second member 12 by, for example, welding after the first member 11 and the second member 12 have been formed. Although not shown, the third member 13 may be arranged parallel to the first member 11. In this case, the third member 13 may also be joined to the first member 11 by, for example, welding before forming, or after the first member 11 and the second member 12 have been formed. Alternatively, the third member 13 may be provided continuously with the first member 11 , and no joint such as a weld may be present at the boundary between the first member 11 and the third member 13 .

[0096] The structural member 10 according to the above embodiment includes a single first member 11 and a single second member 12, and the first member 11 and the second member 12 are arranged to form a substantially L-shape. However, as shown in FIG. 9 , the structural member 10 may also include, for example, two second members 12. In the example shown in FIG. 9 , the second members 12 are arranged on both longitudinal sides 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 may be 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. The manufacturing method according to the above embodiment can also be applied to such a structural member 10. Preferably, each second member 12 is joined to the first member 11 at the stage of the blank 30 ( FIG. 5A ) before being formed. However, when the structural member 10 includes two second members 12, it is sufficient that at least one second member 12 is joined to the first member 11 by lap welding before forming.

[0097] In the manufacturing method according to the above embodiment, the movable dies 43 and 44 are moved in the second direction D2 by a cam mechanism. More specifically, the movable dies 43 and 44 are moved in the second direction D2 by a common cam driver 46. However, a cam driver can be provided for each of the movable dies 43 and 44. For example, if the position of the sliding surface 432 of the movable die 43 and the position of the sliding surface 442 of the movable die 44 are different in a direction perpendicular to the first direction D1 and the second direction D2 (a direction perpendicular to the paper surface of Figures 5C to 5F), a cam driver can be provided for each of the movable dies 43 and 44. However, it is sufficient that the movable dies 43 and 44 are configured to be movable in the second direction D2. A cam mechanism does not necessarily have to be used as a means for operating the movable dies 43 and 44. For example, the movable dies 43 and 44 may be moved in the second direction D2 by an actuator such as a hydraulic cylinder. If the movable dies 43 and 44 are operated by a means other than a cam mechanism, the mold 40 does not have to be provided with a cam driver 46.

[0098] In the above embodiment, the movable die 44 is disposed inside the movable die 43. However, the arrangement of the movable dies 43, 44 is not limited to this. For example, the movable die 43 may be configured to be housed inside the movable die 44 and to protrude from the movable die 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 have the configuration described in the above embodiment. The mold 40 only needs to be configured so that the top plate 121 and the vertical wall 122 of the second member 12 can be molded from the second direction D2 while constraining the first member 11 from the first direction D1 and the flange 123 of the second member 12 from the second direction D2.

[0100] In the above embodiment, the upper mold 41 is disposed 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 disposed above the upper mold 41, or the upper mold 41 and the lower mold 42 may not be disposed one above the other.

[0101] In the above embodiment, the blank 30 before the forming process may be provided with a metal plate as a patch material. The patch material is overlapped on a portion of the blank 30 and joined to the blank 30 by spot welding, laser welding, or the like. The blank 30 may be formed into the structural member 10 with the patch material joined thereto. The patch material can be placed in a location in the structural member 10 that requires reinforcement.

[0102] The present disclosure will be described in more detail below with reference to examples, although the present disclosure is not limited to the following examples.

[0103] [First Example] A press forming test was conducted on a structural member including a first member and a second member to investigate the Vickers hardness of the structural member. In the first example, the first member and the second member were joined by welding before forming, and then integrally formed into a single structural member by hot stamping. In the comparative example, the first member and the second member were formed from separate blanks (steel plates), and the first member and the second member were joined by welding after forming.

[0104] In both the examples and comparative examples, the first member was spot-welded to the second member with their ends overlapping. The Vickers hardness of the spot-welded cross section of the first member was investigated for each example and comparative example. For each spot-welded cross section of the examples and comparative examples, the Vickers hardness was measured at 0.2 mm intervals along a straight line located 1 / 4 of the way through the plate thickness from the surface closest to the weld center, in accordance with JIS Z 2244-1:2024 (test force: 0.49 N). Figure 10 shows the distribution of Vickers hardness at the spot-welded 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 portion (nugget portion) 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. However, at further distances from the outer edge of the weld metal, the Vickers hardness converges to the base metal hardness of 500 Hv.

[0105] 10, when the Vickers hardness was measured at 0.2 mm intervals from the weld center toward both outer sides of the weld metal zone, for example, at a distance of 7.0 mm or more, and 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, which was approximately 60% of the Vickers hardness (base metal hardness) of the non-welded zone: 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 (base material hardness) of the non-welded portion: 500 Hv.

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

[0108] [Example 2] A press forming (cold) analysis was performed using commercially available analysis software (AutoForm R10, manufactured by AutoForm, Inc.) to investigate the Vickers hardness of a structural member. In this analysis, two metal plates (steel plates) were overlapped and spot-welded to form a hat shape, and then the Vickers hardness was measured near the weld.

[0109] As explained in the above embodiment, the Vickers hardness was measured at five or more points (test force: 0.49 N) in accordance with JIS Z 2244-1:2024 at a position 1 / 4 of the plate thickness from the surface closest to the weld center on both 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), and the maximum value of these Vickers hardness values, HVmax, was taken as the Vickers hardness near the weld. In this analysis, the Vickers hardness was investigated for each of multiple welds (weld points). Table 1 shows the results of this analysis.

[0110]

[0111] As shown in Table 1, in all welds formed before forming, the Vickers hardness increased after forming, and the Vickers hardness HVmax near the weld was 7.0% or more higher than the Vickers hardness HVm of the base material. On the other hand, when the welds are formed after forming, there is no strain due to forming near the weld, so there is almost no increase in Vickers hardness near the weld. When the welds are formed after forming, 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 in the Vickers hardness HVmax near the weld relative to the Vickers hardness HVm of the base material of 278 HV, of less than 2.0%.

[0112] In this way, when metal plates are welded together before forming and then cold-worked, it has been confirmed that the hardness of the portion near the weld is higher than that of the base material. More specifically, it has been confirmed that the maximum Vickers hardness HVmax near the weld is 7.0% or more higher than the Vickers hardness HVm of the base material.

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

Claims

1. A method for manufacturing a structural member, comprising the steps of: preparing a blank including a first metal plate and a second metal plate, the blank having an end portion of the first metal plate overlapping an end portion of the second metal plate and the ends joined by welding; and using a mold to form the blank into a structural member including a first member and a second member, wherein 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 wall on the opposite side of the first top plate and protruding outward from the first vertical wall; and the second member includes a second top plate bent relative to the first top plate so as to form 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 protruding outward from the second vertical wall. In the molding step, the first metal plate is molded into the first member and the second metal plate is molded into the second member, and the first member is constrained from a first direction by the mold and the second flange is constrained from a second direction perpendicular to the first direction, and the second top plate and the second vertical wall are molded from the second direction by the mold.

2. A manufacturing method as described in claim 1, wherein the mold includes two movable dies, and in the molding step, one of the two movable dies, which moves along the second direction, molds the end of the second metal plate into the second flange and restrains the second flange, and the other of the two movable dies, which moves along the second direction, molds the other portion of the second metal plate into the second top plate and the second vertical wall.

3. A manufacturing method as claimed in claim 2, wherein the mold further includes an upper mold and a lower mold, and in the molding step, the first member is clamped and restrained from the 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, and the second top plate and the second vertical wall are molded by the other movable mold.

4. A structural member comprising: a first member including 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 wall on the opposite side of the first top plate and protruding outward from the first vertical wall; and a second member including a second top plate bent relative to the first top plate so as to form 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 protruding outward from the second vertical wall, wherein the longitudinal ends of the first member are joined to the longitudinal ends of the second member by welding while overlapping the longitudinal ends of the second member, and wherein the minimum Vickers hardness of the heat-affected zone of the weld in the first member is 70% or more of the Vickers hardness of the non-welded portion.

5. A structural member comprising: a first member including 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 wall on the opposite side of the first top plate and protruding outward from the first vertical wall; and a second member including a second top plate bent relative to the first top plate so as to form 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 protruding outward from the second vertical wall, wherein an end of the first member in the longitudinal direction is joined to an end of the second member by welding in a state where it is overlapped with an end of the second member in the longitudinal direction, A structural member in which, in a cross section along the longitudinal direction of the first member, the maximum value of Vickers hardness in a range from an edge of the weld in a direction perpendicular to the plate thickness direction of the first member 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, and HVmax - HVm is 7.0% or more of HVm.

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

Images