Structural member and method for manufacturing the same

Abstract

The present invention provides a structural member that can reduce tensile residual stress in the welded joint in a curved region. [Solution] The structural member (100) comprises an inner member (10), an outer member (20), a welded portion (32), and a support member (50). The inner member (10) includes a top plate (11) and a vertical wall (13). The vertical wall (13) includes a curved region (15). The curved region (15) curves concavely inward of the structural member (100) when viewed from the top plate (11) side. The outer member (20) includes a top plate (21) and a vertical wall (23). The welded portion (32) extends along the end of the vertical wall (23). The support member (50) supports the vertical wall (13) from within the inner member (10) in the curved region (15). In the curved region (15), a hole (133) is formed in the vertical wall (13) that opens to its inner surface. In the curved region (15), a portion of the support member (50) is inserted into the hole (133) and joined to the weld metal (321).

Description

[[Technical Field]]

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

[0002] For example, a plurality of structural members are used in structures such as automobiles. Some structural members have a curved region that curves concavely inward in a plan view.

[0003] For example, Patent Document 1 discloses a structural member having a concave curved portion and a non-concave curved portion in a plan view. The structural member of Patent Document 1 includes a first member and a second member. The first member and the second member each include a main surface portion (top plate) and two wall forming portions (vertical walls). The wall forming portion of the first member is disposed inside the wall forming portion of the second member and is joined to the wall forming portion of the second member by welding. In Patent Document 1, the height of the wall forming portion of the first member in the concave curved portion is smaller than the height of the wall forming portion of the first member in the non-concave curved portion. [[Prior Art Documents]] [[Patent Documents]]

[0004] [[Patent Document 1]] Japanese Unexamined Patent Publication No. 2023-122747 [[Summary of the Invention]] [[Problem to be Solved by the Invention]]

[0005] When a structural member includes a curved region that curves concavely inward in a plan view, as described in Patent Document 1, the tensile residual stress of the welded portion tends to increase in the curved region. For a structural member including a curved region, measures are particularly desired to reduce the tensile residual stress of the welded portion in the curved region and improve the fatigue strength of the welded portion.

[0006] An object of the present disclosure is to provide a structural member capable of reducing the tensile residual stress of a welded portion in a curved region. [Means for solving the problem]

[0007] The structural member according to this disclosure comprises an inner member, an outer member, a welded portion, and a support member. The inner member includes a first top plate, a first vertical wall, and a second vertical wall. The first vertical wall is continuous with the first top plate. The first vertical wall includes a curved region. The curved region curves concavely inward into the structural member when viewed from the first top plate side. The second vertical wall is continuous with the first top plate on the opposite side of the first vertical wall. The outer member includes a second top plate, a third vertical wall, and a fourth vertical wall. The second top plate faces the first top plate. The third vertical wall is continuous with the second top plate. The third vertical wall is superimposed on the first vertical wall from the outside of the inner member. The third vertical wall is joined to the first vertical wall by welding. The fourth vertical wall is continuous with the second top plate on the opposite side of the third vertical wall. The fourth vertical wall is joined to the second vertical wall. The weld extends along the end of the third vertical wall. The support member supports the first vertical wall from within the inner member in the curved region. In the curved region, a hole is formed in the first vertical wall that opens into the inner surface of the first vertical wall. In the curved region, a portion of the support member is inserted into the hole and connects with the weld metal of the weld. The other portion of the support member is fixed to at least one of the first top plate and the second vertical wall. [Effects of the Invention]

[0008] According to the structural member described herein, tensile residual stress in the welded joint can be reduced in the curved region. [Brief explanation of the drawing]

[0009] [Figure 1] Figure 1 is a perspective view of a structural member according to an embodiment. [Figure 2] Figure 2 is another perspective view of the structural member according to the embodiment. [Figure 3] Figure 3 is a cross-sectional view of the structural member shown in Figure 2 at the location of the curved region. [Figure 4A] Figure 4A is a schematic diagram illustrating the manufacturing method of a structural member according to the embodiment. [Figure 4B]Figure 4B is a schematic diagram illustrating the manufacturing method of a structural member according to the embodiment. [Figure 4C] Figure 4C is a schematic diagram illustrating the manufacturing method of a structural member according to the embodiment. [Figure 5A] Figure 5A is a schematic diagram illustrating the behavior of structural members during welding heating. [Figure 5B] Figure 5B is a schematic diagram illustrating the behavior of structural members during cooling. [Figure 6] Figure 6 is a plan view of a structural member according to a modified example of the above embodiment. [Figure 7] Figure 7 is a cross-sectional view of the structural member at the location of the curved region according to a modified example of the above embodiment. [Figure 8] Figure 8 is a schematic diagram illustrating a method for manufacturing a structural member according to a modified example of the above embodiment. [Figure 9] Figure 9 is a cross-sectional view of the structural member at the location of the curved region according to a modified example of the above embodiment. [Modes for carrying out the invention]

[0010] The structural member according to this embodiment comprises an inner member, an outer member, a welded portion, and a support member. The inner member includes a first top plate, a first vertical wall, and a second vertical wall. The first vertical wall is continuous with the first top plate. The first vertical wall includes a curved region. The curved region curves concavely inward into the structural member when viewed from the first top plate side. The second vertical wall is continuous with the first top plate on the opposite side of the first vertical wall. The outer member includes a second top plate, a third vertical wall, and a fourth vertical wall. The second top plate faces the first top plate. The third vertical wall is continuous with the second top plate. The third vertical wall is superimposed on the first vertical wall from the outside of the inner member. The third vertical wall is joined to the first vertical wall by welding. The fourth vertical wall is continuous with the second top plate on the opposite side of the third vertical wall. The fourth vertical wall is joined to the second vertical wall. The weld extends along the end of the third vertical wall. The support member supports the first vertical wall from within the inner member in the curved region. In the curved region, a hole is formed in the first vertical wall that opens into the inner surface of the first vertical wall. In the curved region, a portion of the support member is inserted into the hole and connects with the weld metal of the weld. The other portion of the support member is fixed to at least one of the first top plate and the second vertical wall (first configuration).

[0011] In a structural member, if the vertical wall (first vertical wall) of the inner member includes a curved region that curves inward in a concave shape when viewed from above, when the vertical wall (third vertical wall) of the outer member is overlapped with the first vertical wall from the outside and welded together, in the curved region, the portion of the first vertical wall near the weld line expands due to welding heat and displaces toward the internal space of the inner member. On the other hand, the portion of the first vertical wall away from the weld line experiences less thermal expansion compared to the portion near the weld line and is constrained by the top plate (first top plate). Therefore, when overlapping and welding the first vertical wall of the inner member and the third vertical wall of the outer member, in the curved region, the first vertical wall tilts so that its free end is located toward the internal space of the inner member relative to the first top plate. After welding the first vertical wall of the inner member and the third vertical wall of the outer member, these vertical walls are naturally cooled to, for example, room temperature. As they cool, the portion of the first vertical wall near the weld contracts, and the first vertical wall attempts to return from its tilted state to its original state. As a result, the weld is pulled in the height direction of the first vertical wall, generating relatively high tensile residual stress in the weld.

[0012] The present inventor has found that the tensile residual stress in a welded portion has a high correlation with the amount of out-of-plane deformation of the first vertical wall of the inner member. Directly, the larger the amount of out-of-plane deformation of the first vertical wall during cooling (the amount of recovery from that during welding heating), the higher tensile residual stress is generated in the welded portion. Based on this finding, in the structural member according to the first configuration, the first vertical wall is supported from the inside of the inner member by the support member in the curved region. More specifically, in the curved region, a part of the support member is inserted into the hole of the first vertical wall, and the part of the support member is bonded to the weld metal of the welded portion. This means that when the first vertical wall of the inner member and the third vertical wall of the outer member are overlap-welded, a part of the support member is directly heated by welding heat. Compressive plastic strain is introduced into the support member during this heating, and the length of the support member is shortened. Therefore, in the cooling process after welding, the first vertical wall is pulled from the inside of the inner member by the support member, and the movement (recovery) of the first vertical wall to the outside of the inner member is suppressed. Accordingly, in the curved region, the amount of out-of-plane deformation of the first vertical wall caused by cooling is reduced, and the tensile residual stress in the welded portion can be reduced.

[0013] In the structural member according to the first configuration, the support member may be formed in a rod shape (second configuration).

[0014] In the structural member according to the first or second configuration, the other part of the support member may be fixed to the second vertical wall (third configuration).

[0015] A method for manufacturing a structural member according to an embodiment includes step i) and step ii). In step i), an inner member, an outer member, and a support member are prepared. The inner member includes a first top plate, a first vertical wall, and a second vertical wall. The first vertical wall is continuous with the first top plate. The second vertical wall is continuous with the first top plate on a side opposite to the first vertical wall. The outer member includes a second top plate, a third vertical wall, and a fourth vertical wall. The third vertical wall is continuous with the second top plate. The fourth vertical wall is continuous with the second top plate on a side opposite to the third vertical wall. The first vertical wall includes a curved region. The curved region has a shape recessed inward of the inner member when viewed from the first top plate side. The first vertical wall has a hole. The hole is formed in the curved region and opens to at least an inner surface of the first vertical wall. In step ii), with a part of the support member inserted into the hole inside the inner member and another part of the support member fixed to at least one of the first top plate and the second vertical wall, the third vertical wall is overlapped with the first vertical wall from outside the inner member, and a part of the support member is heated from the outer side of the first vertical wall while welding the first vertical wall and the third vertical wall along an end of the third vertical wall, thereby forming a welded portion in which a part of the support member is bonded to weld metal (fourth configuration).

[0016] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. In these drawings, the same or corresponding components are denoted by the same reference numerals, and repeated description thereof will not be provided.

[0017] [Structural Member] FIG. 1 and FIG. 2 are perspective views of a structural member 100 according to the present embodiment. The structural member 100 is used, for example, in vehicles such as automobiles. The structural member 100 may be a chassis component of an automobile. Among chassis components, the structural member 100 may be, for example, a suspension component such as a suspension member, a suspension arm, or a suspension frame, or may be a chassis frame or the like. In the present embodiment, an example in which the structural member 100 is a lower arm, which is a type of suspension arm, will be described.

[0018] Referring to Figures 1 and 2, the structural member 100 comprises an inner member 10, an outer member 20, and welded portions 31, 32, and 33. In this embodiment, the structural member 100 includes mounting portions 41, 42, and 43.

[0019] The structural member 100 is attached to the vehicle body or wheels at mounting portions 41, 42, and 43. Mounting portion 41 is positioned outward in the vehicle width direction (left-right direction) relative to mounting portions 42 and 43 when the structural member 100 is installed in the vehicle. Mounting portion 41 is connected to the wheels, for example, via a ball joint and a steering knuckle (not shown). Mounting portions 42 and 43 are positioned in the vehicle length direction (front-rear direction) when the structural member 100 is installed in the vehicle. Mounting portion 42 is positioned in front of mounting portion 43. Mounting portions 42 and 43 are each connected to the vehicle body via bushings (not shown).

[0020] The inner member 10 includes a top plate 11 and vertical walls 12, 13, and 14. Vertical walls 12 and 13 are continuous with the top plate 11. Vertical wall 12 extends along the top plate 11 from mounting portion 41 to mounting portion 42. Vertical wall 13 extends along the top plate 11 from mounting portion 42 to mounting portion 43. Vertical wall 14 is continuous with the top plate 11 on the opposite side of vertical walls 12 and 13. Vertical wall 14 extends along the top plate 11 from mounting portion 41 to mounting portion 43.

[0021] The outer member 20 is positioned below the inner member 10 when the structural member 100 is installed in the vehicle. Alternatively, the outer member 20 is positioned above the inner member 10 when the structural member 100 is installed in the vehicle.

[0022] The outer member 20 includes a top plate 21 and vertical walls 22, 23, and 24. The top plate 21 is positioned opposite the top plate 11 of the inner member 10, either below or above the top plate 11. The vertical walls 22 and 23 are continuous with the top plate 21. Vertical wall 22 extends along the top plate 21 from mounting portion 41 to mounting portion 42. Vertical wall 23 extends along the top plate 21 from mounting portion 42 to mounting portion 43. Vertical wall 24 is continuous with the top plate 21 on the opposite side of vertical walls 22 and 23. Vertical wall 24 extends along the top plate 21 from mounting portion 41 to mounting portion 43.

[0023] The vertical walls 22, 23, and 24 of the outer member 20 correspond to the vertical walls 12, 13, and 14 of the inner member 10, respectively. The vertical walls 22, 23, and 24 of the outer member 20 are joined to the vertical walls 12, 13, and 14 of the inner member 10, respectively.

[0024] The vertical wall 22 of the outer member 20 is joined to the vertical wall 12 of the inner member 10. The vertical wall 22 is, for example, overlapped with the vertical wall 12 from the outside of the inner member 10 and joined to the vertical wall 12 by welding. In this case, a welded portion 31 is formed at the end of the vertical wall 22. The welded portion 31 can extend along the end of the vertical wall 22. The welded portion 31 extends, for example, from mounting portion 41 to mounting portion 42. The welded portion 31 is a lap fillet weld that is continuously formed by, for example, arc welding or laser-arc hybrid welding.

[0025] The vertical wall 23 of the outer member 20 is overlapped with the vertical wall 13 from the outside of the inner member 10 and joined to the vertical wall 13 by welding. Therefore, a welded portion 32 is formed at the end of the vertical wall 23. The welded portion 32 extends along the end of the vertical wall 23. The welded portion 32 extends, for example, from mounting portion 42 to mounting portion 43. The welded portion 32 is a lap fillet weld that is continuously formed by, for example, arc welding or laser-arc hybrid welding.

[0026] The vertical wall 24 of the outer member 20 is joined to the vertical wall 14 of the inner member 10. The vertical wall 24 is, for example, overlapped with the vertical wall 14 from the outside of the inner member 10 and joined to the vertical wall 14 by welding. In this case, a welded portion 33 is formed at the end of the vertical wall 24. The welded portion 33 can extend along the end of the vertical wall 24. The welded portion 33 extends, for example, from mounting portion 41 to mounting portion 43. The welded portion 33 is a lap fillet weld that is continuously formed by, for example, arc welding or laser-arc hybrid welding.

[0027] In this embodiment, the vertical wall 13 of the inner member 10 includes a curved region 15. The curved region 15 is the area of ​​the vertical wall 13 that curves inward in a concave shape when viewed from the top plate 11 side of the structural member 100 (inner member 10). The vertical wall 23 of the outer member 20 corresponds to the vertical wall 13 of the inner member 10 and curves along the vertical wall 13 at the location of the curved region 15.

[0028] The curved region 15 can be identified in the same manner as the concave curved portion in Patent Document 1. That is, the shape of the structural member 100 when viewed from the top plate 11 side of the inner member 10 along the height direction of the vertical wall 13 is defined as the plan view shape. First, in the plan view shape of the structural member 100, one evaluation point corresponding to the toe of the welded portion 32 is identified. Next, at the toe of the welded portion 32, two points 4 mm apart on both sides in the extending direction of the welded portion 32 are identified from that evaluation point, and a circle of curvature is identified from these three points. The length of the line segment connecting the evaluation point on the toe of the welded portion 32 and the center of curvature of the identified circle of curvature is defined as the radius of curvature R. If the line segment overlaps with the structural member 100, the radius of curvature R is considered positive, and if the line segment does not overlap with the structural member 100, the radius of curvature R is considered negative. When the radius of curvature R is negative, the evaluation point is located in the portion of the vertical wall 13 that curves inward in a concave shape in plan view. On the other hand, when the radius of curvature R is positive, the evaluation points are located on the portion of the vertical wall 13 that curves outward in a plan view.

[0029] In the plan view of the structural member 100, the region where evaluation points satisfying a radius of curvature R [mm] of -400 ≤ R < 0 are clustered can be defined as the curved region 15. That is, by measuring the radius of curvature R of an evaluation point using the procedure described above, if the radius of curvature R [mm] satisfies -400 ≤ R < 0, it is determined that the evaluation point is included in the curved region 15. On the other hand, at the location of an evaluation point where R < -400, the curvature of the vertical wall 13 in the plan view of the structural member 100 is small, and the vertical wall 13 has a shape close to a straight line, so it is determined that the evaluation point is not included in the curved region 15. The region where evaluation points satisfying a radius of curvature R [mm] of R < -400 or R > 0 are clustered among the vertical wall 13 is a region that is substantially straight when viewed from the top plate 11 side of the inner member 10, or a region that curves convexly outward from the structural member 100.

[0030] In this embodiment, other vertical walls 14 of the inner member 10 may also include curved regions 16. The curved region 16 is a region of the vertical wall 14 that curves inward in a concave shape when viewed from the top plate 11 side of the structural member 100 (inner member 10). The vertical wall 24 of the outer member 20 corresponds to the vertical wall 14 of the inner member 10 and curves along the vertical wall 14 at the location of the curved region 16. The curved region 16 can be identified in the same way as the curved region 15 of the vertical wall 13.

[0031] The inner member 10 and the outer member 20 are formed from, for example, metal plates. The inner member 10 and the outer member 20 may be formed from steel plates or from non-ferrous metal plates. The tensile strength of the inner member 10 and the outer member 20 is, for example, 780 MPa or more, preferably 980 MPa or more. The tensile strength of the inner member 10 and the outer member 20 is, for example, 1470 MPa or less, but may exceed 1470 MPa. The tensile strength of the inner member 10 may be the same as or different from the tensile strength of the outer member 20.

[0032] The thickness of the inner member 10 and the outer member 20 is preferably 4.0 mm or less. The thickness of the inner member 10 and the outer member 20 is, for example, 1.6 mm or more, but may be less than 1.6 mm. The thickness of the inner member 10 may be the same as or different from the thickness of the outer member 20.

[0033] Figure 3 is a cross-sectional view of the structural member 100 at the location of the curved region 15 (section III-III in Figure 2). The cross-sectional view of the structural member 100 refers to the cross-section obtained when the structural member 100 is cut by a plane that runs along the thickness direction of the top plate 11 of the inner member 10. Figure 3 shows the cross-section of the structural member 100 cut by a plane that passes through the vertical wall 13 of the inner member 10 and the vertical wall 23 of the outer member 20, as well as the other vertical wall 14 of the inner member 10 and the other vertical wall 24 of the outer member 20. The configurations of the inner member 10 and the outer member 20 will be described in more detail below with reference to Figure 3.

[0034] Referring to Figure 3, the inner member 10 has a roughly inverted U-shaped cross-section. The inner member 10 opens on the opposite side of the top plate 11 when viewed alone. In the inner member 10, the vertical wall 13 on the curved region 15 side includes a vertical wall body 131 and a ridge portion 132. The vertical wall body 131 is connected to the top plate 11 via the ridge portion 132. That is, the ridge portion 132 is the corner portion between the top plate 11 and the vertical wall body 131. The ridge portion 132 may have an outwardly convex curved shape in a cross-sectional view of the structural member 100. On the other hand, the vertical wall body 131 may have a substantially straight shape in a cross-sectional view of the structural member 100.

[0035] The other vertical walls 14 of the inner member 10 are positioned on the opposite side of the vertical wall 13 from the top plate 11. Each vertical wall 14 includes a vertical wall body 141 and a ridge portion 142. The vertical wall body 141 is connected to the top plate 11 via the ridge portion 142. That is, the ridge portion 142 is the corner portion between the top plate 11 and the vertical wall body 141. The ridge portion 142 may have a curved shape that is convex outward in a cross-sectional view of the structural member 100. For example, in the cross-section of the structural member 100, the direction in which a straight line extends from the R-end of one ridge portion 132 of the inner member 10 on the top plate 11 side to the R-end of the other ridge portion 142 on the top plate 11 side can be defined as the width direction of the inner member 10, and the direction perpendicular to this width direction can be defined as the height direction of the vertical walls 13 and 14.

[0036] The vertical wall body 141 may be substantially straight in a cross-sectional view of the structural member 100. The vertical wall body 141 faces the vertical wall body 131 in a cross-sectional view of the structural member 100. The vertical wall body 141 may be arranged parallel to the vertical wall body 131 in a cross-sectional view of the structural member 100, or it may be arranged non-parallel to the vertical wall body 131.

[0037] The outer member 20 has a generally U-shaped cross-section. The outer member 20 opens on the opposite side of the top plate 21 when viewed alone. However, the outer member 20 is joined to the inner member 10, thereby forming a closed cross-section together with the inner member 10. The top plate 21 of the outer member 20 faces the top plate 11 of the inner member 10. In the outer member 20, the vertical wall 23 on the curved region 15 side includes a vertical wall body 231 and a ridge portion 232. The vertical wall body 231 is connected to the top plate 21 via the ridge portion 232. That is, the ridge portion 232 is the corner portion between the top plate 11 and the vertical wall body 231. The ridge portion 232 may have an outwardly convex curved shape in the cross-sectional view of the structural member 100. On the other hand, the vertical wall body 231 may have a substantially straight shape in the cross-sectional view of the structural member 100.

[0038] The vertical wall body 231 is superimposed on the outer surface of the vertical wall body 131 of the inner member 10 and joined to the vertical wall body 131 by a weld 32. The weld 32 includes weld metal 321. The weld metal 321 is located outside the space formed by the inner member 10 and the outer member 20.

[0039] The other vertical walls 24 of the outer member 20 are positioned on the opposite side of the vertical wall 23 from the top plate 21. The vertical wall 24 includes a vertical wall body 241 and a ridge portion 242. The vertical wall body 241 is connected to the top plate 21 via the ridge portion 242. That is, the ridge portion 242 is the corner portion between the top plate 21 and the vertical wall body 241. The ridge portion 242 may have an outwardly convex curved shape in a cross-sectional view of the structural member 100. On the other hand, the vertical wall body 241 may have a substantially straight shape in a cross-sectional view of the structural member 100. The vertical wall body 241 faces the vertical wall body 231 in a cross-sectional view of the structural member 100. The vertical wall body 241 may be positioned parallel to or non-parallel to the vertical wall body 231 in a cross-sectional view of the structural member 100.

[0040] In the example shown in Figure 3, the vertical wall body 241 is superimposed on the outer surface of the vertical wall body 141 of the inner member 10 and joined to the vertical wall body 141 by a weld 33. The weld 33 includes weld metal 331. The weld metal 331 is located outside the space formed by the inner member 10 and the outer member 20.

[0041] As shown in Figure 3, the structural member 100 further comprises a support member 50. The support member 50 is provided on the structural member 100 to support the vertical wall 13 from within the inner member 10 in the curved region 15. The support member 50 is positioned within the inner member 10. More specifically, the support member 50 is positioned within the inner member 10 at a distance from the top plate 21 of the outer member 20. In the example in Figure 3, the support member 50 is positioned on the top plate 11 side with respect to the open end faces of the vertical walls 13 and 14 within the inner member 10.

[0042] The support member 50 is made of metal. The support member 50 may be made of steel, or it may be made of a non-ferrous metal such as aluminum alloy or copper. The support member 50 may be made of the same type of metal material as the inner member 10 and the outer member 20, or it may be made of a different type of metal material than the inner member 10 and the outer member 20.

[0043] In the curved region 15 of the inner member 10, a hole 133 is formed in the vertical wall 13. The hole 133 opens at least to the inner surface of the vertical wall 13. Part of the support member 50 is inserted into this hole 133. Part of the support member 50 is joined to the weld metal 321. The other part of the support member 50 is fixed to at least one of the top plate 11 and the other vertical wall 14 of the inner member 10.

[0044] In this embodiment, the support member 50 is formed in the shape of a rod. The support member 50 may be a round rod having a circular cross-section, or a square rod having a square cross-section. The support member 50 may have a cross-section of another polygon, or a cross-section such as L-shaped or U-shaped. When the support member 50 is in the shape of a rod, one end of the support member 50 in the longitudinal direction is placed in the hole 133 of the vertical wall 13 and integrated with the weld metal 321. One end of the support member 50 in the longitudinal direction is covered by the weld metal 321 when viewed from the outer surface side of the vertical wall 13. That is, the support member 50 is not exposed from the weld metal 321 on the outer surface side of the vertical wall 13.

[0045] In this embodiment, a nut 60 is attached to the support member 50. The nut 60 may be in contact with the vertical wall 13 from the inside of the inner member 10.

[0046] The other end of the support member 50 in the longitudinal direction is fixed to the top plate 11 or the vertical wall 14. In this embodiment, the other end of the support member 50 in the longitudinal direction is fixed to the vertical wall 14. The method of fixing the other end of the support member 50 in the longitudinal direction to the vertical wall 14 is not particularly limited. The other end of the support member 50 in the longitudinal direction is fixed so as not to separate from the vertical wall 14 by welding such as arc welding or laser welding, brazing, mechanical joining using a screw mechanism, or adhesive. Alternatively, the other end of the support member 50 in the longitudinal direction may be fixed so as not to separate from the vertical wall 14 by friction joining, for example.

[0047] In this embodiment, the other end of the support member 50 in the longitudinal direction is not connected to the weld metal 331 of the welded portion 33. That is, the other end of the support member 50 in the longitudinal direction is separated from the weld metal 331 by the vertical wall 14.

[0048] [Method for manufacturing structural members] Next, the manufacturing method for the structural member 100 will be described with reference to Figures 4A to 4C. The manufacturing method for the structural member 100 according to this embodiment comprises a preparation step and a welding step.

[0049] (preparation process) As shown in Figure 4A, the preparation step involves preparing the inner member 10, the outer member 20, and the support member 50. Figure 4A, and Figures 4B and 4C described later, show the cross-sections of each member at the location of the curved region 15, that is, the cross-sections corresponding to the cross-section of the structural member 100 shown in Figure 3.

[0050] Referring to Figure 4A, the inner member 10 and the outer member 20 can be formed, for example, by press working of a metal sheet. In the inner member 10, a hole 133 is formed in the curved region 15 of the vertical wall 13. The hole 133 opens to at least the inner surface of the vertical wall 13. In this embodiment, the hole 133 also opens to the outer surface of the vertical wall 13. That is, the hole 133 is a through hole that penetrates the vertical wall 13.

[0051] A nut 60 is attached to the support member 50. In this case, a threaded portion corresponding to the nut 60 may be formed on the outer surface of the rod-shaped support member 50 so that the support member 50 and the nut 60 interlock.

[0052] As shown in Figure 4B, the support member 50 is positioned inside the inner member 10. After positioning the support member 50 inside the inner member 10, the nut 60 is rotated to bring it closer to the vertical wall 13 and press it against the inner surface of the vertical wall 13. This holds one end of the support member 50 in the longitudinal direction to the vertical wall 13. The end face of the support member 50 on the vertical wall 13 side may be substantially coplanar with the outer surface of the vertical wall 13 in a cross-sectional view of the inner member 10, as shown in Figure 4B, or it may be positioned inside or outside the inner member 10 relative to the outer surface of the vertical wall 13. The other end of the support member 50 in the longitudinal direction is fixed to the vertical wall 14, for example, by the method described above.

[0053] (Welding process) Referring to Figure 4C, in the welding process, the vertical wall 13 of the inner member 10 and the vertical wall 23 of the outer member 20 are joined by welding. The vertical walls 13 and 23 are joined, for example, by arc welding. The vertical walls 13 and 23 may also be joined by other welding methods such as laser-arc hybrid welding.

[0054] In the welding process, a portion of the support member 50 is inserted into the hole 133 within the inner member 10, and the other portion of the support member 50 is fixed to at least one of the top plate 11 and the vertical wall 14. Then, the vertical wall 23 of the outer member 20 is overlapped with the vertical wall 13 from the outside of the inner member 10. While welding the vertical walls 13, 23 along the end of the vertical wall 23, a portion of the support member 50 is heated from the outside of the vertical wall 13, thereby forming a welded portion 32 in which a portion of the support member 50 is joined to the weld metal 321.

[0055] During the welding process of the vertical walls 13 and 23, a portion of the support member 50 is located within the hole 133 of the vertical wall 13, and a portion of the support member 50 is directly heated by the welding heat. In this embodiment, one end of the support member 50 in the longitudinal direction is directly heated by the welding heat.

[0056] When one end of the support member 50 in the longitudinal direction is heated by the welding heat, the support member 50 attempts to expand in the longitudinal direction. Also, melting occurs at one end of the support member 50 due to the welding heat, and the material of the support member 50 mixes with the material of the vertical walls 13 and 23 and the welding material (welding wire), which have also been melted by the welding heat. As a result, one end of the support member 50 is bonded to the weld metal 321 of the welded joint 32. The weld metal 321 is also bonded to the vertical walls 13 and 23.

[0057] Although not shown in the diagram, the vertical walls 12 and 14 of the inner member 10 are also joined to the vertical walls 22 and 24 of the outer member 20, respectively. The vertical walls 12 and 14 are typically joined to the vertical walls 22 and 24 by welding. For example, the vertical walls 12 and 14 are joined to the vertical walls 22 and 24 by arc welding. The vertical walls 12 and 14 may also be joined to the vertical walls 22 and 24 by another welding method such as laser-arc hybrid welding. The joining of the vertical walls 12 and 14 of the inner member 10 to the vertical walls 22 and 24 of the outer member 20 may be performed after welding the vertical walls 13 and 23, or before welding the vertical walls 13 and 23.

[0058] In this embodiment, the vertical wall 14 of the inner member 10 does not have a hole like the vertical wall 13 on the curved region 15 side, and the other end of the support member 50 in the longitudinal direction is joined to the inner surface of the vertical wall 14. Normally, the welding material does not penetrate to the inner surface of the vertical wall 14, so the other end of the support member 50 is not joined to the weld metal 331 (Figure 3) of the welded portion 33.

[0059] This is how the structural member 100 is manufactured. However, the method for manufacturing the structural member 100 according to this embodiment may include one or more other steps in addition to the preparation and welding steps described above, as needed.

[0060] [effect] When the vertical wall 13 of the inner member 10 is overlapped with the vertical wall 23 of the outer member 20 and welded by arc welding or the like, the end (free end) and the surrounding portion of the vertical wall 13 are prone to thermal expansion in the tangential direction of the weld line in the concave curved region 15. When such thermal expansion occurs in the curved region 15, the portion of the vertical wall 13 adjacent to the weld 32 is displaced inward in the width direction of the inner member 10 from the pre-welding state shown by the dashed line in Figure 5A. As a result, the vertical wall 13 tilts inward in the width direction of the inner member 10, causing out-of-plane deformation of the vertical wall 13. The vertical wall 13 of the inner member 10 is welded to the vertical wall 23 of the outer member 20 in this out-of-plane deformed state.

[0061] After welding, the inner member 10 and the outer member 20 are cooled. At this time, as shown in Figure 5B, the portion of the vertical wall 13 adjacent to the weld 32 shrinks due to cooling and is displaced outward in the width direction of the inner member 10. That is, the vertical wall 13 tries to return to its original state, and out-of-plane deformation of the vertical wall 13 occurs. As a result, as shown by the thin arrow in Figure 5B, the weld 32 is pulled in the height direction of the vertical wall 13, and tensile residual stress is generated in the weld 32.

[0062] In this embodiment, when the vertical wall 13 of the inner member 10 is welded to the vertical wall 23 of the outer member 20, the vertical wall 13 is supported from within the inner member 10 by the support member 50 in the curved region 15. More specifically, the vertical walls 13 and 23 are welded with a portion of the support member 50 inserted into the hole 133 of the vertical wall 13 and the other portion of the support member 50 fixed to the vertical wall 14. As a result, a portion of the support member 50 is directly heated from the outside of the vertical wall 13 by the welding heat. During the welding process, as a portion of the support member 50 is heated, the support member 50 attempts to expand due to thermal expansion, while the vertical wall 13 tilts inward into the inner member 10, compressing the support member 50. Therefore, compressive plastic strain is introduced into the support member 50. When the support member 50 is rod-shaped as in this embodiment, the support member 50 is compressed in the longitudinal direction by the vertical wall 13, and compressive plastic strain in the longitudinal direction is introduced into the support member 50. As a result, the longitudinal length of the support member 50 becomes shorter than before heating. Therefore, when the inner member 10 and the outer member 20 are cooled after welding, in the curved region 15, the vertical wall 13 is pulled from inside the inner member 10 by the support member 50 which is connected to the weld metal 321 of the weld 32, and out-of-plane deformation (return) of the vertical wall 13 in the width direction is suppressed. Thus, in the curved region 15, the amount of out-of-plane deformation of the vertical wall 13 due to cooling is reduced, and the tensile residual stress of the weld 32 can be reduced.

[0063] In this embodiment, when a portion of the support member 50 is directly heated from the outside of the vertical wall 13 by welding heat, a portion of the support member 50 in the structural member 100 is bonded to the weld metal 321 of the welded portion 32. That is, the welding material melts not only into the vertical wall 13 of the inner member 10 and the vertical wall 23 of the outer member 20, but also into the support member 50, and the support member 50 becomes integrated with the weld metal 321.

[0064] In this embodiment, the support member 50 is formed in the shape of a rod. In this case, the proportion of the volume of the support member 50 to the volume of the structural member 100 is very small. Therefore, the tensile residual stress of the welded joint 32 can be reduced without significantly increasing the weight of the structural member 100.

[0065] 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.

[0066] In the above embodiment, a single support member 50 is provided in the concave curved region 15 of the vertical wall 13 of the inner member 10. However, as shown in Figure 6, multiple support members 50 may be provided in the curved region 15. In the example of Figure 6, similar to the above embodiment, one support member 50 is attached to the curved region 15 of the vertical wall 13 of the inner member 10 and to the vertical wall 14 located on the opposite side of the vertical wall 13 from the top plate 11. Another support member 50 is attached to the curved region 15 of the vertical wall 13 of the inner member 10 and to the vertical wall 12 located on the opposite side of the vertical wall 13 from the top plate 11.

[0067] In the above embodiment, a hole 133 is provided in the vertical wall 13 of the inner member 10, while no holes are provided in the other vertical walls 14. However, if the vertical wall 14 includes a curved region 16, as shown in Figure 7, a hole 143 similar to that of the vertical wall 13 may also be provided in the curved region 16 of the vertical wall 14. In this case, when welding the vertical wall 14 of the inner member 10 to the vertical wall 24 of the outer member 20, a part of the support member 50 is inserted into the hole 143 to hold or fix it, the vertical wall 24 is superimposed on the vertical wall 14 from the outside of the inner member 10, the vertical walls 14 and 24 are welded along the end of the vertical wall 24, and a part of the support member 50 is heated from the outside of the vertical wall 14. As a result, even in the curved region 16 which is different from the curved region 15, the vertical wall 14 can be pulled from inside the inner member 10 by the support member 50 during cooling after welding, and out-of-plane deformation of the vertical wall 14 is suppressed. Therefore, in both the curved regions 15 and 16, the amount of out-of-plane deformation of the vertical walls 13 and 14 due to cooling is reduced, and the tensile residual stress of the welded joints 32 and 33 can be reduced.

[0068] When a portion of the support member 50 is directly heated by welding heat from the outside of the vertical wall 14, in the structural member 100, a portion of the support member 50 is bonded with the weld metal 331 of the welded portion 33. That is, the welding material melts not only into the vertical wall 14 of the inner member 10 and the vertical wall 24 of the outer member 20, but also into the support member 50, and the support member 50 becomes integrated with the weld metal 331.

[0069] In the above embodiment, before welding the vertical wall 13 of the inner member 10 and the vertical wall 23 of the outer member 20, the hole 133 is a through-hole penetrating the vertical wall 13. However, the hole 133 does not necessarily have to be a through-hole. As shown in Figure 8, the hole 133 may be a recess that opens to the inner surface of the vertical wall 13 but not to the outer surface of the vertical wall 13. Even in this case, after welding the vertical walls 13 and 23, a part of the support member 50 placed in the hole 133 is joined to the weld metal 321 of the welded portion 32, similar to the above embodiment (Figure 3).

[0070] Although not shown in the illustration, if a hole 143 is provided in the curved region 16 of another vertical wall 14 of the inner member 10, the hole 143 may be a recess that opens to the inner surface of the vertical wall 14 but does not open to the outer surface of the vertical wall 14.

[0071] In the above embodiment, before welding the vertical wall 13 of the inner member 10 and the vertical wall 23 of the outer member 20, the support member 50 is held to the vertical wall 13 by a nut 60 with a portion of it positioned in the hole 133. However, the method of holding the support member 50 with the vertical wall 13 is not limited to this. The support member 50 only needs to be held to the vertical wall 13 so that it does not come out of the hole 133. For example, a knot or stepped portion may be formed on the support member 50, and this knot or stepped portion may restrict the movement of the support member 50 to the outside of the vertical wall 13. Alternatively, the support member 50 may be fixed to the vertical wall 13 with a portion of it inserted into the hole 133 by welding such as arc welding or laser welding, or by brazing.

[0072] Similarly, even if a hole 143 is provided in the curved region 16 of another vertical wall 14 of the inner member 10, the method of fixing the support member 50 to the vertical wall 14 is not particularly limited. The support member 50 may be held to the vertical wall 14 using a nut 60, a joint, or a stepped portion, or it may be fixed to the vertical wall 14 by welding such as arc welding or laser welding, or by brazing. However, if the method of holding the support member 50 to the vertical wall 14 using a nut 60, a joint, or a stepped portion is adopted, the vertical walls 14 and 24 are welded to fix the support member 50 to the vertical wall 14, and then the vertical walls 13 and 23 are welded.

[0073] In the above embodiment, the support member 50 is formed in the shape of a rod. However, the shape of the support member 50 is not limited to this. The support member 50 may be formed in the shape of a plate, for example. The plate-shaped support member 50 may be arranged so that its front and back surfaces face the top plate 11 of the inner member 10 and the top plate 21 of the outer member 20. Alternatively, as shown in Figure 9, the plate-shaped support member 50 may be arranged so that its end faces face the top plate 11 of the inner member 10 and the top plate 21 of the outer member 20. In this case, it is preferable that the hole 133 in the vertical wall 13 of the inner member 10 opens not only on the inner surface of the vertical wall 13 but also on the end surface of the vertical wall 13 before welding the vertical wall 13 of the inner member 10 and the vertical wall 23 of the outer member 20. For example, the hole 133 may be a notch formed on the end surface of the vertical wall 13. This allows a part of the support member 50 to be inserted into the hole 133 from the opposite side of the top plate 11.

[0074] Similarly, if a hole 143 is provided in the curved region 16 of another vertical wall 14 of the inner member 10, the hole 143 may be a notch or the like that opens not only on the inner surface of the vertical wall 14 but also on the end face of the vertical wall 14.

[0075] In the above embodiment, in the curved region 15, a portion of the support member 50 is inserted into the hole 133 of the vertical wall 13 of the inner member 10 and joined to the weld metal 321 of the welded portion 32, while the other portion of the support member 50 is fixed to the other vertical wall 14 of the inner member 10. However, the other portion of the support member 50 may be fixed to the top plate 11 of the inner member 10.

[0076] If the support member 50 is rod-shaped, for example, by bending the support member 50, one end of the support member 50 in the longitudinal direction can be fixed to the vertical wall 13, and the other end of the support member 50 in the longitudinal direction can be fixed to the top plate 11. If the support member 50 is plate-shaped, a part of the support member 50 can be fixed to the vertical wall 13, while the other part of the support member 50 can be fixed to either the top plate 11 or the vertical wall 14, or both. However, from the viewpoint of ease of manufacturing the structural member 100, it is preferable that the area to which the support member 50 is fixed to the inner member 10 is small. Therefore, it is preferable that the other part of the support member 50 is fixed to only one of the top plate 11 or the vertical wall 14. For example, if the other part of the support member 50 is fixed to the vertical wall 14, it is preferable that, in a cross-sectional view of the structural member 100 at the position of the curved region 15, the support member 50 is positioned inside the inner member 10 with a gap between it and the top plate 11. [Examples]

[0077] The present disclosure will be further described below with reference to examples. However, the present disclosure is not limited to the following examples.

[0078] To confirm the effects of this disclosure, arc welding analysis of the vertical walls 13 and 23 of the inner member 10 and outer member 20 having the same shape as the above embodiment was performed using general-purpose finite element analysis software (ABAQUS, manufactured by Dassault Systèmes). In the comparative example, arc welding analysis was performed between the vertical wall 13 of the inner member 10 and the vertical wall 23 of the outer member 20 without providing the support member 50. In the embodiment, the support member 50 was installed in the curved region 15 of the vertical wall 13 at a position where high tensile residual stress occurred in the welded portion 32 in the comparative example, and the same arc welding analysis as in the comparative example was performed. However, in the embodiment, the end of the support member 50 was directly heated by the arc welding heat generated when welding the vertical walls 13 and 23.

[0079] In the example, the support member 50 was cylindrical (round bar) with a diameter of 4.0 mm, and the material properties of the support member 50 were assumed to be 980 MPa class steel. In both the example and the comparative example, the plate thickness of the inner member 10 and the outer member 20 was 2.6 mm, and the material properties of the inner member 10 and the outer member 20 were assumed to be 980 MPa class steel, respectively.

[0080] In the example, the installation point (center) of the support member 50 was used as the evaluation point, and the residual stress of the welded joint 32 and the out-of-plane deformation of the vertical wall 13 of the inner member 10 at this evaluation point were evaluated as a reduction rate compared to the comparative example. The reduction rates for residual stress and out-of-plane deformation are expressed as percentages of the reduction in residual stress and out-of-plane deformation from the comparative example, with the tensile residual stress and out-of-plane deformation of the comparative example set to 100%, respectively. The residual stress here is the residual stress at a position approximately 2 mm from the toe of the welded joint 32 toward the top plate 11 of the inner member 10, and is the residual stress component in a direction perpendicular to the extending direction of the welded joint 32 and along the outer surface of the vertical wall 13 of the inner member 10. The out-of-plane deformation (during heating) is the out-of-plane deformation (tilt) of the vertical wall 13 at the evaluation point during the welding heating process from before welding to welding heating, and the out-of-plane deformation (during cooling) is the out-of-plane deformation (return) of the vertical wall 13 at the evaluation point during the cooling process from welding heating to cooling. The out-of-plane deformation was defined as the amount of movement (maximum movement) in the width direction of the toe of the welded joint 32.

[0081] The evaluation results for the examples and comparative examples are shown in Table 1.

[0082] [Table 1]

[0083] As shown in Table 1, in the example, the out-of-plane deformation of the vertical wall 13 measured at the installation point (evaluation point) of the support member 50 during heating was reduced by 16% compared to the comparative example. Furthermore, in the example, the out-of-plane deformation of the vertical wall 13 during cooling was reduced by 31% compared to the comparative example. As a result, in the example, the residual stress (tensile residual stress) of the welded joint 32 was reduced by 94% compared to the comparative example.

[0084] This analysis confirmed that by installing a support member 50 inside the inner member 10 and directly heating the support member 50 with welding heat to introduce compressive plastic strain, the amount of out-of-plane deformation of the vertical wall 13, particularly during cooling after welding, can be reduced in the curved region 15. Furthermore, it was confirmed that the tensile residual stress in the welded joint 32 was significantly reduced as a result of the reduced out-of-plane deformation during cooling. [Explanation of Symbols]

[0085] 100: Structural member 10: Inner component 11: Top plate (first top plate) 13: Vertical wall (First vertical wall) 133: Hole 12,14: Vertical wall (Second vertical wall) 143: Hole 15,16: Curved region 20:Outer member 21: Top plate (second top plate) 23: Vertical wall (Third vertical wall) 22,24: Vertical wall (4th vertical wall) 31, 32, 33: Welded section 321,331: Weld metal 50: Support member

Claims

1. A structural member, An inner member comprising a first top plate, a first vertical wall continuous with the first top plate and including a curved region that curves concavely inward towards the structural member when viewed from the first top plate side, and a second vertical wall continuous with the first top plate on the opposite side of the first vertical wall, An outer member including a second top plate facing the first top plate, a third vertical wall continuous with the second top plate, overlapping the first vertical wall from the outside of the inner member and joined to the first vertical wall by welding, and a fourth vertical wall on the opposite side of the third vertical wall that is continuous with the second top plate and joined to the second vertical wall, A welded portion extending along the end of the third vertical wall, A support member that supports the first vertical wall from within the inner member in the curved region, Equipped with, In the curved region, the first vertical wall has a hole that opens to the inner surface of the first vertical wall, and a part of the support member is inserted into the hole and bonded to the weld metal of the welded portion. The other portion of the support member is a structural member fixed to at least one of the first top plate and the second vertical wall.

2. A structural member according to claim 1, The support member is a structural member formed in the shape of a rod.

3. A structural member according to claim 1, The other portion of the support member is a structural member that is fixed to the second vertical wall.

4. A method for manufacturing structural members, A step of preparing a first top plate, an inner member including a first vertical wall continuous with the first top plate, a second vertical wall continuous with the first top plate on the opposite side of the first vertical wall, an outer member including a second top plate, a third vertical wall continuous with the second top plate, a fourth vertical wall continuous with the second top plate on the opposite side of the third vertical wall, and a support member, wherein the first vertical wall includes a curved region having a concave shape on the inside of the inner member when viewed from the first top plate side, and has a hole formed in the curved region that opens to at least the inner surface of the first vertical wall. The process involves inserting a portion of the support member into the hole within the inner member, fixing the other portion of the support member to at least one of the first top plate and the second vertical wall, overlapping the third vertical wall with the first vertical wall from the outside of the inner member, and welding the first vertical wall and the third vertical wall along the end of the third vertical wall while heating the portion of the support member from the outside of the first vertical wall to form a welded portion in which the portion of the support member is joined with the weld metal, A manufacturing method that includes the following features.

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