Method for manufacturing structural member

Abstract

The present invention makes it possible for a structural member having a closed cross section to be manufactured through press molding by a simpler method. According to the present invention, press molding is performed to obtain a target component shape (1) comprising a top plate part (1A) and left and right vertical wall parts (1B), wherein the left and right vertical wall parts (1B) each have a recessed part (1Bc) that protrudes toward the other vertical wall part (1B), and the inner surface (2) of each of the recessed parts (1Bc) has a cross-section that can be in surface contact with the inner surface of the other vertical wall part (1B). The present invention comprises: a first molding step (5A) for molding a metal plate (10) into a first intermediate component (11) having recessed parts (1Bc); a second molding step (5B) for producing a second intermediate component (12) by applying bending deformation to the first intermediate component (11) in a direction bringing the recessed parts (1Bc) toward each other, the bending deformation being applied to a region (11A) that is to become the top plate part (1A); and a third molding step (5C) for further applying bending deformation to the second intermediate component (12) in a state in which the left and right vertical wall parts (1B) are being pressed toward each other, the bending deformation being applied to the region (11A) that is to become the top plate part (1A).

Description

Manufacturing method of structural member

[0001] The present invention relates to a manufacturing technology for structural members. The present invention relates to a technology suitable for a manufacturing method for manufacturing structural members by press molding, which are mainly used for automobile parts, home appliance parts, etc. The present invention also relates to a technology suitable for manufacturing structural members having a closed cross-sectional shape.

[0002] When high strength is required for structural components made of steel, such as in automobiles, high-strength steel plates are commonly used as the material. To increase the strength of structural components, it is also common to construct them with a structure having a closed cross-section. This type of structure is found in many parts of automobile bodies, such as side sills and front side members.

[0003] In press forming of metal sheets, the sheet metal is sandwiched between a pair of dies. Therefore, it is generally impossible to create parts with concave and convex cross-sectional shapes on the vertical plane perpendicular to the die stroke direction, or parts with closed cross-sections, in a single process. Therefore, for example, the aforementioned side sills and front side members are composed of two hat-shaped parts, or a combination of a hat-shaped part and a flat part. In other words, it is common to form multiple parts individually by press forming, and then combine these formed parts by welding or other methods to form a closed-section skeleton.

[0004] However, manufacturing and combining two parts requires the preparation of separate dies for each type of part, the separate press forming processes for the two types of parts, and the subsequent assembly of the two parts. Therefore, if a part with a closed cross section could be manufactured from a single metal plate, the above-mentioned work could be significantly reduced. To achieve this, various efforts have been made.

[0005] For example, roll forming is sometimes used to manufacture parts with closed cross sections. Roll forming excels at forming long parts with a uniform cross-sectional shape, as described above. Furthermore, roll forming facilitates multi-stage forming, enabling the production of complex cross sections. Furthermore, joining can be performed in the forming line. Therefore, roll forming is relatively easy for forming parts that can form a closed cross section during use. Furthermore, several methods for producing closed cross-section parts by press forming have been proposed using various innovations. For example, Patent Document 1 discloses a method for forming a cylindrical closed cross-section part with different diameters along its length. Furthermore, Patent Documents 2 and 3 disclose press forming methods for closed cross-section parts in which both ends of the cross section are bent and fastened to prevent the shape from opening after forming.

[0006] JP 2014-004626 A JP 2013-244511 A JP 2013-244512 A

[0007] As mentioned above, roll forming excels at producing parts with a constant cross section. However, there are various restrictions and difficulties when forming products where the cross section shape varies from part to part. In addition, it is necessary to have production equipment specialized for roll forming. However, roll forming is not as widespread as press machines. Therefore, there is great significance in manufacturing structural components by press forming using widely used press machines.

[0008] On the other hand, Patent Document 1 describes that the cross section opens after press forming. In particular, high-tensile steel sheets are used for automobile frames. When these high-tensile steel sheets are press-formed to manufacture parts, a significant springback occurs, resulting in a large opening after demolding. Furthermore, when joining the formed parts, a dedicated reworking jig is required to reclose the open cross section, which increases costs.

[0009] To address this issue, Patent Documents 2 and 3 disclose methods of fastening by bending both ends of the cross section. This is an effective method when both ends are to be joined. However, this method cannot be applied to joining other parts. In particular, press molding using a core as shown in Patent Document 3 poses the following problem. That is, in order to remove the core from the molded product after molding, a mechanism for moving the core or the molded product in a direction different from the operating direction of the press is required, as well as the operation time required for removal. This results in increased costs in terms of equipment cost and production time.

[0010] The present invention has been made in light of the above-mentioned points, and aims to provide a manufacturing method that enables structural members having closed cross sections to be manufactured by press molding in a simpler manner.

[0011] In order to solve the problem, one aspect of the present invention is a manufacturing method of a structural member by press-forming a metal plate into a target part shape having a cross section that includes a top plate portion, and left and right vertical wall portions that are connected to both widthwise ends of the top plate portion via ridge portions and that face each other in the widthwise direction of the top plate portion, and in which a recess is formed in one or both of the left and right vertical wall portions at a midpoint in the height direction of the vertical wall portion, protruding toward the other opposing vertical wall portion, and an inner surface of the recess can be in surface contact with an inner surface of the other vertical wall portion, a second forming step of producing a second intermediate part by applying bending deformation to the region that will become the top plate portion, or to the region that will become the top plate portion and the region that will become the ridge portion, in a direction in which the inner surface of the recess approaches the region that will become the other vertical wall portion; and a third forming step of pressing the regions that will become the left and right vertical wall portions of the second intermediate part in a direction in which the regions that will become the left and right vertical wall portions approach each other, and further applying bending deformation to the region that will become the top plate portion.

[0012] According to an aspect of the present invention, it is possible to manufacture a press-formed product having a closed cross section from a single metal plate by press working. When the target part shape has a flange portion, the manufactured structural member becomes a closed cross section part in which the joint is not at the cross section end. Furthermore, according to an aspect of the present invention, a general-purpose press machine can be used, and a mold structure having a general structure for automotive parts can also be used. Therefore, according to an aspect of the present invention, production is possible without reducing the production speed using the conventional press process for automotive parts. Therefore, according to an aspect of the present invention, parts having a closed cross section that have traditionally been manufactured using roll forming or the combination of multiple parts can be produced inexpensively.

[0013] FIG. 1 is a cross-sectional view of a target part shape according to an embodiment of the present invention. FIG. 2 is a perspective view of a target part shape according to an embodiment of the present invention. FIG. 3 is a diagram illustrating another example of the cross-sectional shape of a target part shape. FIG. 4 is a diagram illustrating a manufacturing process for a structural member according to this embodiment. FIG. 5 is a schematic diagram illustrating the configuration of a mold used in a first molding process. FIG. 6 is a schematic diagram illustrating the state at the bottom dead center of molding in the first molding process. FIG. 7 is a schematic diagram illustrating a second molding process. (a) shows the shape of a first intermediate part, and (b) is an example of the mold configuration used in the second molding process. FIG. 8 is an enlarged schematic diagram of the molding surface of the lower die used in the second molding process. FIG. 9 is a schematic diagram illustrating a third molding process. FIG. 10 is a schematic diagram illustrating another example of molding surface 41A. FIG. 11 is a schematic diagram illustrating another example of the third molding process. FIG. 12 is a conceptual diagram illustrating molding of the region that will become the top plate portion in the third molding process. FIG. 13 is a schematic diagram illustrating another example of molding surface 41A.

[0014] Next, an embodiment of the present invention will be described with reference to the drawings. This embodiment is a method for manufacturing a target structural member by press-forming a metal sheet into a target part shape. The present invention does not particularly limit the material of the metal sheet to be processed. An example of the metal sheet is a steel sheet with a tensile strength of approximately 270 MPa to 2000 MPa. The present invention is a technology suitable for processing a high-strength steel sheet with a tensile strength of 980 MPa or more.

[0015] (Target Part Shape 1) As shown in FIG. 1, the target part shape 1 in this embodiment has a cross-sectional shape including a top plate portion 1A and left and right vertical wall portions 1B. The left and right vertical wall portions 1B are connected to both widthwise ends of the top plate portion 1A via ridge portions 1D. The left and right vertical wall portions 1B are arranged opposite each other in the widthwise direction of the top plate portion 1A. The ridge portions 1D have an outwardly convex arc-shaped cross-sectional shape and are joints that join (connect) the top plate portion 1A and the vertical wall portions 1B. This ridge portion 1D may be considered to be part of the top plate portion 1A. FIG. 1 shows an example in which the top surface of the top plate portion 1A is flat.

[0016] Furthermore, in the cross section of the target part shape 1, at least one of the left and right vertical wall portions 1B has a recess 1Bc formed at a midpoint in the height direction of the vertical wall portion. The recess 1Bc protrudes toward the other opposing vertical wall portion 1B. The inner surface of the recess 1Bc is capable of surface contact with the inner surface of the other vertical wall portion 1B. FIG. 1 shows an example in which the recess 1Bc is provided on both the left and right vertical wall portions 1B. This results in an example in which the target part shape 1 is bilaterally symmetrical. Note that the dimensions shown in FIG. 1 and elsewhere are included as dimensions for the example and do not limit the present invention in any way.

[0017] Furthermore, as shown in FIG. 2 , the target part shape 1 of this embodiment has a part shape in which the cross section extends along the longitudinal direction. The longitudinal direction is a direction intersecting the cross section. The recesses 1Bc are also formed to extend along the longitudinal direction. The recesses 1Bc may be formed intermittently along the longitudinal direction. However, it is preferable that the shape of each recess 1Bc is such that the longitudinal dimension is longer than the height dimension of the vertical wall portion 1B. The target part shape 1 of this embodiment is not limited to the cross-sectional shape shown in FIG. 1 . The cross section of the target part shape 1 may have a cross-sectional shape in which at least one of the left and right vertical wall portions 1B has a recess 1Bc formed in at least one of the left and right vertical wall portions 1B that can come into surface contact with the inner surface of the other vertical wall portion 1B.

[0018] Other examples of the cross-sectional shape of the target part shape 1 are shown in FIG. 3 . The cross-sectional shape of the target part shape 1 is not limited to the example shown in FIG. 3 . FIG. 3( a) shows an example in which the shape of the top plate portion 1A is changed from the cross-sectional shape of FIG. 1 to an arc-shaped cross section that is convex upward. Note that, in this specification, the term "arc" or "arc-shaped" is not limited to a partial circle. The term "arc" or "arc-shaped" as used herein includes curved shapes that are rounded toward one side, such as an ellipse or a partial parabola. "Rounded" refers to the absence of any sharp curvature along the curve. FIGS. 3( b) and 3( c) show examples in which the inner surface of the recess 1Bc is offset from the center position in the width direction. FIG. 3( b) shows an example in which the left and right recesses 1Bc have different shapes. FIG. 3( c) shows an example in which the recess 1Bc is provided only on one vertical wall portion 1B, and the inner surface 2 of the recess 1Bc is shaped to be in surface contact with the inner surface 3 of the other vertical wall portion 1B.

[0019] Figures 3(d) and (e) illustrate cases where the left and right recesses 1Bc have different vertical ranges and positions. The left and right recesses 1Bc may be arranged so that they do not completely overlap in the vertical direction. In this case, the inner surface 2 of each recess 1Bc is in surface contact with the inner surface 3 of the corresponding vertical wall portion 1B other than the recess 1Bc. Figure 3(f) shows an example where the width above the recess 1Bc is different from the width below the recess 1Bc. Figure 3(g) shows an example where there is no flange portion 1C.

[0020] In this embodiment, the cross-sectional shape of the vertical wall portion 1B other than the recessed portion 1Bc is a straight line. However, the vertical wall portion 1B other than the recessed portion 1Bc may have a curved portion in whole or in part. However, this is limited to the case where the left and right vertical wall portions 1B have a shape that allows surface contact at the recessed portion 1Bc.

[0021] The following description will be given taking the target part shape 1 of the structural member to be manufactured as shown in FIG. 1 as an example. The target part shape 1 shown in FIG. 1 has a flat top plate 1A, recesses 1Bc formed in both left and right vertical wall portions 1B, and a shape that allows the inner surfaces of the left and right recesses 1Bc to come into surface contact with each other. Furthermore, in the target part shape 1 shown in FIG. 1, outward flange portions 1C are connected to the lower ends of the left and right vertical wall portions 1B, respectively. In this specification, the outward flange portions 1C are also simply referred to as flange portions 1C. Before assembly of the manufactured part, the structural member having the target part shape 1 may have a gap between the inner surfaces 2 of the left and right recesses 1Bc. However, considering the effort required for assembly, a smaller gap is preferable.

[0022] (Manufacturing Method) As shown in Figure 4, the manufacturing method for a structural member of this embodiment includes a first forming step 5A, a second forming step 5B, a third forming step 5C, and a joining process step 5D. In this embodiment, these steps 5A to 5C are used to press a metal plate 10 into a structural member (formed part 13) having a target part shape 1. The joining process step 5D may not be included. The metal plate 10 to be pressed is also referred to as a blank 10.

[0023] <First forming step 5A> The first forming step 5A is a step of press-forming the metal plate 10 into a first intermediate part 11. The shape of the first intermediate part 11 is a shape in which the recess 1Bc and the flange portion 1C are formed in the metal plate 10 (see FIGS. 6 and 7(a)). The first forming step 5A is performed using a pair of dies 20 and 21 that face each other with the metal plate 10 sandwiched therebetween, as shown in FIG.

[0024] A concave shape 20Ba that forms the recess 1Bc of the vertical wall portion 1B is formed on the molding surface of one of the molds 20. A convex shape 21Ca corresponding to the concave shape 20Ba is formed on the molding surface of the other mold 21. In this example, one of the molds 20 is the lower mold 20, and the other mold 21 is the upper mold 21. Furthermore, the first molding step 5A of this embodiment has left and right bending blades 20C. These bending blades 20C bend and deform the metal plate 10 at the ridge line position connecting the vertical wall portion 1B and the outward flange portion 1C, thereby forming the left and right outward flange portions 1C. The outward flange portions 1C are formed so as to rise in the direction opposite to the recessed direction of the recess 1Bc. In this example, the bending blades 20C are provided on the lower mold 20.

[0025] The mold of this example will be described in more detail. The lower mold 20 of this example has a lower mold pad 20B and left and right bending blades 20C. The lower mold pad 20B and the left and right bending blades 20C are provided on a main body 20A. A lower mold gas cushion 20D is interposed between the lower mold pad 20B and the main body 20A. The lower mold gas cushion 20D is a component that allows the lower mold pad 20B to move in the pressing direction and also biases the lower mold pad 20B toward the metal plate 10.

[0026] A recessed shape 20Ba is formed on the upper surface of the lower die pad 20B at a position where the recess 1Bc will be formed. A bending blade 20C is disposed on each side of the lower die pad 20B. The surface 20Ca of the bending blade 20C facing the lower die pad 20B is the surface that raises each end of the metal sheet 10 to form the left and right flange portions 1C. The lower die pad 20B is supported by a lower die gas cushion 20D so that it can move downward below the top of the bending blade 20C.

[0027] The upper mold 21 is arranged so that its molding surface faces the upper surface of the metal plate 10. The upper mold 21 has a main body 21A, left and right upper dies 21C, and an upper pad 21B. The lower surfaces (molding surfaces) of the left and right upper dies 21C face the upper surfaces (molding surfaces) of the lower pads 20B, and are each formed with a convex shape 21Ca corresponding to the concave shape 20Ba of the lower pad 20B. The upper pad 21B is arranged between the left and right upper dies 21C. An upper gas cushion 21D is interposed between the upper pad 21B and the main body 21A. The upper gas cushion 21D is a component that allows the upper pad 21B to move in the pressing direction and biases the upper pad 21B toward the metal plate 10.

[0028] Furthermore, the widthwise outer edge of each upper die 21C is positioned outward from the outer edge of the lower pad 20B when viewed from above. The lower corners 21Cb of the outer edge of each upper die 21C are rounded to form a radius. This forms a shoulder that forms a ridge at the connection between the vertical wall portion 1B and the flange portion 1C. The bending blade 20C also has a step 20Cb at its lower part. The upper surface of this step 20Cb faces the shoulder 21Cb from above and below. The upper surface of this step 20Cb also has a radius that corresponds to the radius of the shoulder 21Cb of the upper die 21C. Here, the upper surface of the lower pad 20B and the lower surfaces of the upper die 21C and upper pad 21B form a molding surface for molding the metal sheet 10.

[0029] In this embodiment, the vertical wall portion 1B, both the portion facing the top plate portion 1A relative to the recess 1Bc and the portion facing the flange portion 1C, are flat (see FIG. 1). Therefore, the molding surface is flat except for the recessed shape 20Ba corresponding to the recess 1Bc. The portion of the vertical wall portion 1B facing the top plate portion 1A above the recess 1Bc is also referred to as the upper vertical wall portion 1Ba. The portion of the vertical wall portion 1B facing the flange portion below the recess 1Bc is also referred to as the lower vertical wall portion 1Bb. If the upper vertical wall portion 1Ba or the lower vertical wall portion 1Bb is not entirely or partially flat, a shape corresponding to the shape of that portion can be formed on the upper surface (molding surface) of the lower pad 20B and the upper surfaces (molding surfaces) of the upper die 21C and upper pad 21B. That is, in this embodiment, in the first molding step 5A, the surface shape of the region that will become the vertical wall portion is molded, as necessary, to the surface shape of the vertical wall portion 1B in the target part shape. Therefore, in the following description, the region that becomes the vertical wall portion will also be referred to as the vertical wall portion 1B.

[0030] Next, the operation of the mold will be described. As shown in Figure 5, when no load is applied, the metal plate 10 is placed on the lower die pad 20B, which is located above the bending blade 20C. In this state, the upper die 21 is lowered relatively toward the metal plate 10, i.e., toward the lower die 20. Then, the upper die pad 21B is brought into contact with the metal plate 10, and the metal plate 10 is held (sandwiched) between the lower die pad 20B and the upper die pad 21B. This prevents the metal plate 10 from shifting position during molding.

[0031] Next, by lowering the upper die 21 toward the lower die 20, the upper die gas cushion 21D contracts, and the lower die pad 20B and the upper die 21C form the recess 1Bc in the metal plate 10. Furthermore, as the upper die 21C descends, the lower die gas cushion 20D contracts. Then, both ends of the metal plate 10 are bent by the outer end of the upper die 21C and the lower bending blade 20C, forming the left and right flange portions 1C.

[0032] FIG. 6 is a schematic diagram illustrating the state at the bottom dead center of press forming using an upper mold 21 and a lower mold 20. In this first forming step 5A, the metal plate 10 is formed into the shape of a first intermediate part 11. As shown in FIGS. 6 and 7(a), this first intermediate part 11 has left and right recesses 1Bc and left and right flanges 1C. In addition, in the target part shape 1, the upper vertical wall portion 1Ba and the lower vertical wall portion 1Bb may not be flat along the cross-sectional direction. For example, these portions may be curved rather than linear in the cross-sectional direction. In this case, the first forming step 5A forms the regions that will become the upper vertical wall portion 1Ba and the lower vertical wall portion 1Bb into the curved shape. The flanges 1C may be formed in a separate process.

[0033] <Second Molding Step 5B> The second molding step 5B is a step of press-molding the first intermediate part 11 into a second intermediate part 12. Specifically, the second molding step 5B applies bending deformation to the region 11A that will become the top plate portion 1A of the first intermediate part 11 in a direction in which the inner surface 2 of the recess 1Bc approaches the region that will become the other vertical wall portion 1B. Alternatively, the second molding step 5B applies bending deformation to the region 11A that will become the top plate portion 1A and the region that will become the ridge portion 1D of the first intermediate part 11 in a direction in which the inner surface 2 of the recess 1Bc approaches the region that will become the other vertical wall portion 1B. Then, the second intermediate part 12 is produced by this press molding. In this embodiment, the region 11A that will become the top plate portion 1A of the first intermediate part 11 is applied bending deformation in the direction in which the recesses 1Bc of the left and right vertical wall portions 1B approach each other (see FIG. 7B).

[0034] At this time, it is preferable that the second intermediate component 12 be formed by applying bending deformation in the second forming step 5B, in the region 11A that will become the top plate portion 1A and the region that will become the ridge portion 1D, into a curved portion (convex portion) with a cross-sectional shape that is convex in the outward bending direction. In this embodiment, the cross-sectional shape that is convex in the outward bending direction is exemplified as follows: a first convex cross-sectional shape and a second convex cross-sectional shape. Note that the cross-sectional shape that is convex in the outward bending direction is not limited to the first convex cross-sectional shape and the second convex cross-sectional shape.

[0035] The cross-sectional shape of the first convexity is a curved cross-sectional shape that is convex in the outward bending direction, as shown in Figure 9(a). The curved cross-sectional shape may be a partial arc shape or a rounded shape, such as an ellipse or a parabola. It is preferable that the cross-sectional shape of the region 11A that will become the top plate portion 1A has a curved cross-sectional shape with a smaller radius of curvature than the cross-sectional shape of the top plate portion 1A in the target part shape 1.

[0036] The cross-sectional shape of the second convex portion is a curved cross-sectional shape that is convex outward in the bending direction, as shown in FIG. 11( a). The cross-sectional shape of the second convex portion includes a top plate flat portion 12Aa formed in the region 11A that will become the top plate portion 1A, and left and right inclined portions 12Ab that join the top plate flat portion 12Aa and the vertical wall portion 1B. The left and right inclined portions 12Ab are arranged in a V-shape, sandwiching the top plate flat portion 12Aa. That is, the cross-sectional shape of the convex portion is a trapezoid without a base, as shown in the enlarged view of FIG. 12( a). The top plate flat portion 12Aa and the inclined portion 12Ab are joined via a first ridge portion 12Ac. The inclined portion 12Ab and the vertical wall portion 1B are joined via a second ridge portion 12Ad. The first ridge portion 12Ac and the second ridge portion 12Ad are bent portions and have an outwardly convex arc-shaped cross-sectional shape.

[0037] The inclined portions 12Ab are connected to both widthwise ends of the top plate flat portion 12Aa. The inclined portions 12Ab extend obliquely from the top plate flat portion 12Aa toward the vertical wall portion. The lower ends of the inclined portions 12Ab in the extending direction are joined to the vertical wall portion 1B via the second ridge portion 12Ad. A virtual straight line passing through the first ridge portion 12Ac and the second ridge portion 12Ad that define the left and right inclined portions 12Ab is inclined downward and outward in the widthwise direction of the top plate. In this embodiment, the cross-sectional shape of the inclined portions 12Ab is linear. However, the cross-sectional shape of the inclined portions 12Ab is not limited to a linear shape. The cross-sectional shape of the inclined portions 12Ab may be, for example, a curved cross-sectional shape that bulges outward midway in the extending direction. The cross-sectional curved shape of the inclined portion 12Ab may be a shape of a part of a circular arc, or may be a rounded shape such as an ellipse or a parabola.

[0038] The second forming step 5B is performed using a pair of dies 30, 31 that face each other with the metal plate 10 sandwiched between them, as shown in Fig. 7. The pair of dies includes a lower die 30 and an upper die 31, as shown in Fig. 7(b). A recessed concave surface 30A is formed on the upper surface (forming surface) of the lower die 30 at a position facing the region 11A that will become the top plate portion 1A of the first intermediate part 11. The concave surface 30A has a shape that follows the curved portion that constitutes the convex portion.

[0039] When the curved portion constituting the convex portion has the cross-sectional shape of the second convex portion, the molding surface 30A of the lower die 30 has a curved cross-section that follows the cross-sectional shape of the second convex portion. Also, when the curved portion constituting the convex portion has the cross-sectional shape of the second convex portion, the molding surface 30A of the lower die 30 has a shape as shown in Fig. 8, which is an enlarged schematic view. That is, as shown in Fig. 8, the concave surface 30A has a shape that follows the cross-sectional shape of the convex portion that is the top plate side of the second intermediate part.

[0040] In the case of the cross-sectional shape of the second convex portion, the cross-sectional shape of the concave surface 30A constituting the molding surface includes a top plate flat portion molding surface 30Aa, left and right inclined portion molding surfaces 30Ab, and left and right vertical wall portion upper molding surfaces 30Ac, as shown in the enlarged view of FIG. 8. The angle of the inclined portion molding surface 30Ab relative to the top plate flat portion molding surface 30Aa is less than 180 degrees. The angle of the vertical wall portion upper molding surface 30Ac relative to the inclined portion molding surface 30Ab is also less than 180 degrees. The top plate flat portion molding surface 30Aa and the inclined portion molding surface 30Ab are connected by a first ridge portion molding surface having an arc-shaped cross section. The inclined portion molding surface 30Ab and the vertical wall portion upper molding surface 30Ac are connected by a second ridge portion molding surface having an arc-shaped cross section. As shown in FIG. 8, the molding surface 30A of the lower die 30 has a shape synonymous with the shape of the top plate side of the second intermediate part before demolding (see FIG. 7(b)). That is, the molding surface 30Aa has the same shape as the top plate flat portion 12Aa.

[0041] Furthermore, the cross-sectional line length of the top plate flat portion 12Aa of the second intermediate part before demolding is defined as w1 [mm]. The cross-sectional line length of the flat portion of the top plate portion 1A in the target part shape is defined as Tf [mm]. In this case, it is preferable to design the part so that the following formula (1) is satisfied. The flat portion of the top plate portion 1A in the target part shape corresponds, for example, to the portion indicated by the symbol 12Aa in FIG. 12(d). 1 / 3 Tf ≦ w1 ≦ Tf (1). Furthermore, the top plate flat portion 12Aa is positioned within the region that will become the top plate portion 1A, but it is preferable to position it at an equal widthwise position, i.e., in the center position, of the region that will become the top plate portion 1A. However, if the product has asymmetry in the width direction, the top plate flat portion 12Aa may be positioned offset to one side in the width direction within the region that will become the top plate portion 1A.

[0042] Furthermore, the angle of the inclined portion 12Ab relative to the flat portion 12Aa of the top plate is set to, for example, 120 degrees or more and 165 degrees or less. The angle of the upper portion of the vertical wall portion relative to the inclined portion 12Ab may be less than 180 degrees. That is, it is preferable that there is an angle between the inclined portion 12Ab and the upper portion of the vertical wall portion. Furthermore, it is preferable that the angle of the upper portion of the vertical wall portion relative to the inclined portion 12Ab is less than 180 degrees on the inner side. The sum of the cross-sectional line lengths of the flat portion 12Aa of the top plate, the left and right first ridge portions 12Ac, the left and right inclined portions 12Ab, and the left and right second ridge portions 12Ad is preferably equal to, for example, the sum of the cross-sectional line lengths of the top plate portion 1A and the ridge portion 1D. Or, a value slightly larger than that sum is preferable. This slightly larger value corresponds to the line length of the upper end of the vertical wall portion 1B in the product. For example, at least a part of the cross-sectional line of the second ridge portion 12Ad may constitute the upper end portion of the vertical wall portion 1B.

[0043] The upper die 31 also has a punch 31A that protrudes toward the concave surface 30A of the lower die 30. A tip 31Aa of the punch 31A is configured to be able to press the region 11A that will become the top plate portion 1A toward the concave surface 30A of the lower die 30. The cross-sectional shape of the tip 31Aa of the punch 31A is modeled after the cross-sectional shape of the top plate side of the second intermediate part 12.

[0044] Next, the operation of the mold in the second molding step 5B will be described. The first intermediate part 11 is placed on the lower die 30 with the protrusion of the recess 1Bc of the first intermediate part 11 facing upward (see FIG. 7(b)). That is, the first intermediate part 11 after the first molding step 5A is inverted and set on the upper surface of the lower die 30. However, the inversion is not essential. If the die 30 is used as the upper die and the die 31 as the lower die, and a jig that can stably position the first intermediate part 11 is used, the inversion is not necessarily required. The first intermediate part 11 has a shape as shown in FIG. 7(a).

[0045] The upper die 31 is then lowered toward the lower die 30, and the first intermediate part 11 is sandwiched between the lower die 30 and the punch 31A of the upper die 31. The region 11A that will become the top plate portion 1A is then bent into a shape with a greater curvature than the desired shape of the top plate portion 1A, thereby producing a second part shape. The second intermediate part 12 is bent in the region 11A that will become the top plate portion 1A and in the region that will become the ridge portion 1D. Therefore, compared to the first intermediate part 11, the second intermediate part 12 is bent in a V-shape, with the regions that will become the left and right vertical wall portions 1B closer together.

[0046] <Third Forming Step 5C> The third forming step 5C is a step of forming the top plate portion 1A of the second intermediate part 12 and bending the ridge portion 1D, which is the connection portion between the top plate portion 1A and the vertical wall portion 1B. In the third forming step 5C, this processing forms the top plate portion 1A into a target shape and brings the left and right vertical wall portions 1B closer to each other to form them into the target part shape.

[0047] Specifically, in the third forming step 5C, as shown in FIG. 9 , the second intermediate part 12 is pressed in a direction that brings the regions that will become the left and right vertical wall portions 1B closer together, and a process of further bending the region 11A that will become the top plate portion 1A is performed. The bending may be performed while the left and right cams are pressing the left and right vertical wall portions 1B closer together. In this case, the bending in the third forming step 5C preferably deforms the convex portion of the region 12A that will become the top plate portion 1A in a direction that increases the radius of curvature of the cross section. Before being formed in the third forming step 5C, the second intermediate part 12 has a recess 1Bc and a flange portion 1C, and has a generally mountain-like shape as a whole, as shown in FIG. 9( a).

[0048] As shown in FIG. 9 , the third forming step 5C is performed using a pair of dies facing each other across the metal plate 10 and left and right cams 42. The pair of dies includes a lower die 40 and an upper die 41. The upper surface of the lower die 40 is flat, allowing contact with the flange portion 1C, which is the lower portion of the second intermediate part 12. A positioning protrusion 40A protrudes from the upper surface. The protrusion 40A has a width that matches the width between the lower vertical wall portions 1Bb in the target part shape 1. Furthermore, the side surface facing the lower vertical wall portion 1Bb has a shape that follows the shape of the lower vertical wall portion 1Bb in the target part shape 1. In this example, since the lower vertical wall portion 1Bb is flat, the side surface of the protrusion 40A facing the lower vertical wall portion 1Bb is also flat. Furthermore, the height of the protrusion 40A is equal to the height of the lower vertical wall portion 1Bb. The height of the protrusion 40A may be lower than the height of the lower vertical wall portion 1Bb.

[0049] The upper die 41 has a molding surface 41A on its lower surface that press-forms the cross section of the top plate portion 1A. The molding surface 41A is shaped to match the target top plate portion 1A. The left and right cams 42 are configured to be able to move toward and away from each other, sandwiching the second intermediate part 12. The opposing surfaces of the left and right cams 42 are shaped to match the shape of the target vertical wall portion 1B (see FIG. 9(c)). The upper surface of the cam 42 is positioned at approximately the same height as or slightly lower than the upper end of the vertical wall portion 1B of the part placed on the lower die 40. A slightly lower position refers to, for example, a position five times the thickness of the molded product below the upper end of the vertical wall portion 1B. In other words, the top plate portion 1A and the ridge portion connecting the top plate portion 1A and the vertical wall portion 1B are positioned above the upper surface of the cam 42. In this embodiment, the lower end surface of the cam 42 is set to have a gap with the upper surface of the lower die 40 so that the flange portion 1C does not abut against the wall surface (pressing surface) of the cam 42.

[0050] Next, the operation of the mold shown in Fig. 9 in the third molding step 5C will be described. In the third molding step 5C, first, as shown in Fig. 9(a), the second intermediate part 12 is placed on the lower die 40. In this state, as shown in Fig. 9(a), the ridge portion of the second intermediate part 12 connecting the vertical wall portion 1B and the flange portion 1C abuts against the upper surface of the lower die 40.

[0051] From this state, as shown in FIG. 9( b), the left and right cams 42 are moved closer to each other, deforming the second intermediate component 12 so that the regions that will become the left and right vertical wall portions 1B are closer together. Next, as shown in FIG. 9( c), the left and right cams 42 are moved closer together until the inner surfaces 2 of the recesses 1Bc of the left and right vertical wall portions 1B come into contact with each other. The gap distance between the inner surfaces 2 of the recesses 1Bc of the left and right vertical wall portions 1B is preferably 0 mm or greater and less than half the width of the top plate portion 1A. The smaller this gap distance, the easier it is to align the inner surfaces 2 of the recesses 1Bc of the left and right vertical wall portions 1B. At this time, the protrusion 40A also positions the component. Furthermore, by clamping the lower vertical wall portion 1Bb between the protrusion 40A and the lower side of the cam 42, the shape of the lower vertical wall portion 1Bb can be further formed into the target component shape 1.

[0052] In this way, by bending and deforming the second intermediate part 12 with the left and right cams 42, the region 12A that will become the top plate portion 1A is deformed in a direction that reduces the radius of curvature. Then, the left and right cams 42 are brought closer to each other, restraining the left and right vertical wall portions 1B in a state where they are close to each other, or the next process is executed in synchronization with the completion of the approach of the left and right cams 42. That is, as shown in FIG. 9( c), the upper die 41 is lowered, and the upper die 41 bends and deforms the region 12A that will become the top plate portion 1A in a direction that increases the radius of curvature. The direction in which the radius of curvature increases is the direction in which the region becomes flat. At this time, it is preferable that both sides of the lower surface of the upper die 41 abut against the upper surfaces of the left and right cams 42, thereby regulating the stroke amount of the upper die 41.

[0053] Here, the region 12A that will become the top plate portion 1A is formed from a convex shape with a small radius of curvature toward a flatter shape. This causes springback in the direction of closing the cross section at the ridge connecting the top plate portion 1A and the vertical wall portion 1B. In other words, springback occurs in the direction of bringing the left and right vertical wall portions 1B closer together. This springback also acts to increase the radius of curvature of the cross section of the top plate portion 1A, i.e., to flatten the shape of the top plate portion. Therefore, if the top plate portion 1A is formed using an upper die 41 with a completely flat forming surface 41A, a convex shape remains on the top plate portion 1A after demolding. Therefore, if a flat top plate portion 1A is required after springback, it is desirable to set the forming surface 41A of the upper die 41 to a mold shape (forming surface shape) with a convex shape, such as that shown in FIG. 10, that compensates for the deformation due to springback.

[0054] Here, the line length of each side after the third forming step 5C may be set slightly longer or shorter than the line length of each side up to the second forming step 5B. In this case, tensile or compressive deformation occurs within the metal sheet 10, making it possible to reduce the amount of springback at the bending ridge line.

[0055] <Alternative Example of Third Forming Step 5C> Next, an alternative example of the third forming step 5C will be described, which is suitable when the curved portion constituting the convex portion has the cross-sectional shape of the second convex portion. Even when the curved portion constituting the convex portion has the cross-sectional shape of the second convex portion, the above-described third forming step 5C, which is performed using a mold such as that shown in FIG. 9 , may be applied. Similar to the above-described third forming step 5C, this alternative example of the third forming step 5C is a process of forming the top plate portion 1A of the second intermediate part 12 and bending the ridge portion 1D, which is the connection portion between the top plate portion 1A and the vertical wall portion 1B. In this alternative example of the third forming step 5C, when forming the top plate portion 1A into the target shape, the left and right vertical wall portions 1B are brought closer together to form the target part shape.

[0056] Another example of the third forming process 5C, similar to the third forming process 5C described above, involves pressing the second intermediate part 12 in a direction that brings the regions that will become the left and right vertical wall portions 1B closer together, and then further bending the region 11A that will become the top plate portion 1A. This other example of the third forming process 5C is performed using the same process and mechanism as the third forming process 5C described above, except for the specific method of bending the region 12A that will become the top plate portion 1A. In this other example of the third forming process 5C, the convex portion on the side of the region 12A that will become the top plate portion 1A has a trapezoidal cross-sectional shape, as shown in FIG. 12(a), with a flat top plate portion 12Aa and inclined portions 12Ab on both the left and right sides. The top plate flat portion 12Aa is the highest point (top surface). Another example of the third forming process 5C is performed using a pair of molds 40, 41 that face each other across the second intermediate part 12, and left and right cams 42, as shown in FIG. 11. The pair of dies consists of a lower die 40 and an upper die 41 .

[0057] The upper surface of the lower die 40 is a flat surface that can come into contact with the flange portion 1C, which is the lower part of the second intermediate part 12. A positioning protrusion 40A protrudes from the upper surface. The protrusion 40A has a width that matches the width between the lower vertical wall portions 1Bb in the target part shape 1. The side surface facing the lower vertical wall portion 1Bb has a shape that follows the shape of the lower vertical wall portion 1Bb in the target part shape 1. In this example, the lower vertical wall portion 1Bb is flat. Therefore, the side surface of the protrusion 40A facing the lower vertical wall portion 1Bb is a flat surface. The height of the protrusion 40A is equal to the height of the lower vertical wall portion 1Bb. The height of the protrusion 40A may be shorter than the height of the lower vertical wall portion 1Bb.

[0058] As shown in FIG. 12(a), the upper die 41 has a molding surface 41A that presses the top plate flat portion 12Aa against its lower surface. The molding surface 41A has a surface shape that follows the target top plate portion 1A. Recessed relief portions 41B are formed at each widthwise end of the molding surface 41A. The width T of the molding surface 41A is equal to or slightly smaller than the width of the top plate flat portion 12Aa. The molding surface 41A faces the top plate flat portion 12Aa from above and below, and is capable of applying a downward pressing force to the top plate flat portion 12Aa. It is preferable that the surface of the top plate flat portion 12Aa be perpendicular to the pressing direction.

[0059] Furthermore, by pressing the flat portion 12Aa of the top plate with the forming surface 41A, the left and right inclined portions 12Ab are deformed so as to be convex outward (see FIG. 12(c)). The relief portions 41B are formed to prevent interference with this deformation. However, FIG. 12(c) shows the deformation in an exaggerated manner, and it is not necessary to deform the first ridge portion 12Ac and the second ridge portion 12Ad to the extent that they are reverse-bent.

[0060] The left and right cams 42 are configured to be able to move toward and away from each other, sandwiching the second intermediate part 12. The opposing surfaces of the left and right cams 42 are shaped to conform to the shape of the target vertical wall portion 1B (see FIG. 11(c)). The upper surfaces of the cams 42 are positioned at approximately the same height as or slightly lower than the upper end of the region that will become the vertical wall portion 1B in the part placed on the lower die 40. A slightly lower position is, for example, a position that is five times the plate thickness of the molded product lower than the upper end of the vertical wall portion 1B.

[0061] As a result, the second intermediate component 12 is set so that the top plate 1A and the portion that becomes the ridge 1D connecting the top plate 1A and the vertical wall 1B are positioned above the upper surface of the cam 42. In this embodiment, the lower end surface of the cam 42 is set so as to have a gap between it and the upper surface of the lower die 40. This prevents the flange 1C from coming into contact with the wall surface (pressing surface) of the cam 42 (see FIG. 11(a)).

[0062] Next, the operation of the mold in another example of the third molding step 5C will be described. In the third molding step 5C, first, as shown in Fig. 11(a), the second intermediate part 12 is placed on the lower die 40. In this state, as shown in Fig. 11(a), the ridge portion of the second intermediate part 12 connecting the vertical wall portion 1B and the flange portion 1C abuts against the upper surface of the lower die 40.

[0063] From this state, as shown in FIG. 11( b), the left and right cams 42 are moved closer to each other to deform the second intermediate part 12 so that the regions that will become the left and right vertical wall portions 1B are brought closer together. Next, as shown in FIG. 11( c), the left and right cams 42 are moved closer together until the inner surfaces 2 of the recesses 1Bc of the left and right vertical wall portions 1B come into contact with each other. At this time, the protrusions 40A position the part. Furthermore, by sandwiching the lower vertical wall portion 1Bb between the protrusions 40A and the lower sides of the cams 42, the shape of the lower vertical wall portion 1Bb can be further shaped into the target part shape 1. In this way, by bending and deforming the second intermediate part 12 with the left and right cams 42, the radius of curvature of the region 12A that will become the top plate portion 1A is deformed in a direction that reduces the radius of curvature, as shown in FIG. 12( a) → FIG. 12( b).

[0064] Then, the left and right cams 42 are moved closer to each other to restrain the left and right vertical wall portions 1B, or pressing is performed in synchronization with the completion of the approach of the left and right cams 42, as shown in FIG. 11( c). That is, the upper die 41 is lowered, and the molding surface 41A of the upper die 41 presses the top plate flat portion 12Aa downward. Pressing downward means pressing the top plate flat portion 12Aa in a direction approaching the vertical wall portion. By pressing the top surface of the top plate flat portion 12Aa, the surface of the top plate flat portion 12Aa comes into surface contact with the molding surface 41A of the upper die 41, so that even if the top plate flat portion 12Aa is tilted during initial setting, it is automatically adjusted to a horizontal plane.

[0065] Furthermore, as the flat portion 12Aa of the top plate is pressed downward, the left and right inclined portions 12Ab deform so as to bulge outward, as shown in Fig. 12(b) → Fig. 12(c). That is, the radius of curvature of the cross-sectional shape of the inclined portions 12Ab decreases when viewed from the inner surface side. As the inclined portions 12Ab deform, the cross-sections of the first ridge portion 12Ac and the second ridge portion 12Ad, which are bent portions continuing from the ends of the inclined portions 12Ab, also deform in a direction in which the radius of curvature increases.

[0066] Here, it is desirable to set the overall width W of the upper die 41 in the left and right directions larger than the target width of the target part shape at that height position. The stroke amount of the upper die 41 is regulated by adjusting the distance between both sides of the lower surface of the upper die 41 and the upper surfaces of the left and right cams 42. In FIG. 12(c), the bulging left and right inclined portions 12Ab are in contact with the relief portions 41B. However, it is not necessary for the bulging left and right inclined portions 12Ab to be in contact with the relief portions 41B. The outward bulging amount of the left and right inclined portions 12Ab may be adjusted by adjusting the depth D of the relief portions.

[0067] When the top plate flat portion 12Aa is pressed downward to the bottom dead center and then released from the mold, springback occurs in the region 12A that will become the top plate portion, resulting in a shape as shown in the schematic diagram of FIG. 12(d). This demolding primarily generates springback in the left and right inclined portions 12Ab, the first ridge portion 12Ac, and the second ridge portion 12Ad. In the left and right inclined portions 12Ab, springback occurs in the direction in which the radius of curvature increases. In other words, in the left and right inclined portions 12Ab, springback acts in the direction in which the inclined portions 12Ab become flat. In other words, during demolding, springback occurs in the direction in which the left and right vertical wall portions 1B open apart. Meanwhile, in the first ridge portion 12Ac and the second ridge portion 12Ad, springback acts in the direction in which the radius of curvature decreases from a state in which the radius of curvature has increased. In other words, during demolding, springback occurs in the direction in which the left and right vertical wall portions 1B approach each other.

[0068] That is, through the molding process using the left and right cams 42 and the upper die 41, all or part of the top plate flat portion 12Aa is bent slightly convexly from its pre-molding flat shape, and then bent in a direction that straightens out the convex bend. During this process, all or part of the first ridge portion 12Ac and the second ridge portion 12Ad are deformed to straighten or bend the convex bend formed in the second molding process. All or part of the inclined portion 12Ab is bent convexly outward. As a result of these molding processes, at the bottom dead center of molding, the top plate flat portion 12Aa, the first ridge portion 12Ac, and the second ridge portion 12Ad are subjected to bending moments that primarily close the side walls, while the inclined portion 12Ab is subjected to bending moments that primarily open the vertical walls. Even after springback due to mold release, these bending moments cancel each other out, or the closing direction prevails. This results in the production of a molded product with a closed cross section. Furthermore, the order in which the left and right cams 42 and the upper die 41 contact the molded product, or the order in which they reach the bottom dead center position, may be either the left and right cams 42 first, the left and right cams 42 and the upper die 41 simultaneously, or the upper die 41 first.

[0069] As described above, in another example of the third forming step 5C, the region 12A including the top plate portion 1A and the ridge portion 1D is formed from the convex shapes 12Aa and 12Ad with a small radius of curvature toward a flatter shape. This causes springback in the direction of closing the cross section of the ridge portion 1D and a portion of the vertical wall portion 1B connecting the top plate portion 1A and the vertical wall portion 1B. In other words, springback is generated in the direction of moving the left and right vertical wall portions 1B closer together. As a result, the surfaces to be joined of the left and right vertical wall portions 1B can be achieved with a more tightly closed shape.

[0070] The forming surface of the upper mold may not have a relief portion, as shown in Fig. 13. Here, the line length of each side after the alternative example of the third forming step 5C may be set to be slightly longer or shorter than the line length of each side up to the second forming step 5B. In this case, tensile or compressive deformation occurs within the metal sheet 10, respectively, making it possible to reduce the amount of springback at the bending ridge line.

[0071] <Joining Process 5D> In the joining process 5D, the left and right vertical wall portions 1B are joined at the recesses 1Bc of the part that has attained the target part shape after the third forming process 5C. The joining process is performed, for example, by spot welding at 50 mm intervals along the longitudinal direction. However, there are no particular restrictions on the joining method. In addition to welding, known joining processes such as adhesive bonding and riveting may be used. The joining process can more reliably form a structural member with a closed cross-sectional shape. However, the joining process is not necessarily performed. When assembling the manufactured structural member, if the structural member is assembled so that the recesses 1Bc are in surface contact with each other, a structure with a closed cross-sectional shape will be obtained.

[0072] (Operation and Others) According to this embodiment, it is possible to produce a press-formed product having a closed cross section from a single metal plate 10 through a multi-step press forming process using a combination of a press machine and a die having a general structure. That is, in this embodiment, a general-purpose press machine can be used, and the die structure is also a general structure for automotive parts, so that production is possible without reducing the production speed in the conventional press process for automotive parts. Therefore, it is possible to inexpensively produce parts having a closed cross section that would previously have required roll forming or the combination of multiple parts.

[0073] In this embodiment, in the third forming step 5C, bending deformation is applied to the region 11A that will become the top plate 1A while the left and right vertical wall portions 1B are brought close to each other. This makes it possible to eliminate or minimize the gap between the recesses 1Bc of the left and right vertical wall portions 1B. As a result, the metal plate 10 is made of a high-strength material, and a structural member having a closed cross-sectional shape can be easily produced by press forming.

[0074] In this case, if the curved portion constituting the convex portion has the cross-sectional shape of the second convex portion, the following effect is achieved. That is, according to this embodiment, the surfaces of the left and right vertical wall portions 1B to be joined together can be made more closed by another example of the third molding step 5C. Furthermore, according to this embodiment, when molding the top plate portion 1A side in another example of the third molding step 5C, a flat top plate flat portion 12Aa is formed in advance in the area that will become the top plate portion in a previous step. As a result, a flat portion with high flatness and better dimensional accuracy can be provided on the top plate portion 1A.

[0075] As described above, according to this embodiment, even if the metal plate 10 is a high-strength steel plate, it is possible to provide a flat portion with good dimensional accuracy on the top plate portion 1A while further reducing the gap between the left and right vertical wall portions 1B. In other words, this embodiment is a technology suitable for high-strength materials with large springback.

[0076] Here, closed-section parts, such as those targeted by the present invention, are often used in conjunction with other parts. In such cases, it is desirable for the joint with the other part to be flat. Therefore, high flatness and dimensional accuracy may be required for the top plate portion 1A to be attached to other parts. Simply bending the top plate portion 1A side in the second forming process 5B would result in a cross-sectional curvature greater than the target part shape in the region that will become the top plate portion 1A. In this case, the curvature must be reduced to form the top plate portion 1A in the subsequent third forming process 5C, necessitating mold adjustments. Particularly when a flat surface is desired for the top plate portion 1A, the following problem arises. That is, if the top plate portion 1A is given a curvature in an intermediate process and then molded back to a flat top plate portion 1A in the third forming process 5C, adjusting the dimensional accuracy of the flat surface may be difficult. In contrast, in this embodiment, when an alternative example of the third forming process 5C is adopted, a flat top plate flat portion 12Aa is formed in the region that will become the top plate portion 1A in the second forming process 5B. Therefore, the flat portion 12Aa of the top plate can be molded as it is in the third molding step 5C without changing its shape as part of the top plate 1A. Therefore, when another example of the third molding step 5C is adopted, according to this embodiment, it is easier to obtain a molded product having a top plate shape with a flat portion having high dimensional accuracy.

[0077] Furthermore, the structural member manufactured in this embodiment has a closed cross section, and an attachment portion is formed by a flange portion 1C that is continuous with the lower vertical wall portion 1Bb extending downward. If the flange portion 1C is not present, the manufactured structural member can be used in a state having a closed cross section by, for example, assembling the lower end portion or longitudinal end portion of the lower vertical wall portion 1Bb.

[0078] (Other) The present disclosure may have the following configurations: (1) Disclosure 1 is a manufacturing method for a structural member in which a metal plate is press-formed into a target part shape including a top plate portion and left and right vertical wall portions connected to both widthwise ends of the top plate portion via ridge portions and opposing each other in the width direction of the top plate portion, and in which a recess is formed in one or both of the left and right vertical wall portions at a midpoint in the height direction of the vertical wall portion, protruding toward the other opposing vertical wall portion, and the inner surface of the recess is in surface contact with the inner surface of the other vertical wall portion, the method comprising: a first forming step of press-forming the metal plate into a first intermediate part having the recess; and a second forming step of bending the first intermediate part in a direction in which the inner surface of the recess approaches the region that will become the top plate portion, or in a region that will become the top plate portion and a region that will become the top plate portion and the ridge portion, to produce a second intermediate part. and a third forming step of further applying a bending deformation to the region that will become the top plate portion while pressing the regions that will become the left and right vertical wall portions of the second intermediate part in a direction in which the regions that will become the left and right vertical wall portions approach each other. (2) Disclosure 2 relates to a method for manufacturing a structural member, wherein in the second forming step, the application of the bending deformation forms a convex portion having a cross-sectional curved shape that is convex in an outward bending direction in the region that will become the top plate portion, and in the third forming step, the application of the bending deformation deforms the convex portion in a direction that increases the radius of curvature of the cross section. (3) Disclosure 3 describes a method for manufacturing a structural member, wherein a cross-sectional shape of the second intermediate part on the side of the region that will become the top plate portion has a top plate flat portion that is formed in the region that will become the top plate portion and has a flat surface shape, and left and right inclined portions that are connected to both widthwise ends of the top plate flat portion and extend obliquely toward the vertical wall portion to connect the top plate flat portion and the region that will become the vertical wall portion, and wherein the bending deformation in the third forming step is performed by pressing the top plate flat portion in a direction that brings the top plate flat portion closer to the vertical wall portion. (4) Disclosure 4 describes a method for manufacturing a structural member, wherein in the third forming step, by pressing the top plate flat portion, the left and right inclined portions are deformed in directions that reduce the radius of curvature of the cross section.(5) Disclosure 5 describes a method for manufacturing a structural member, wherein the first shaping step is performed by press-forming the metal plate with a pair of dies opposing each other in the thickness direction of the metal plate, wherein a concave shape that forms the recess is formed on the shaping surface of one of the dies, and a convex shape corresponding to the concave shape is formed on the shaping surface of the other die. (6) Disclosure 6 describes a method for manufacturing a structural member, wherein the target part shape includes an outward flange portion connected to an end of a vertical wall portion, and the first shaping step involves bending deformation at a position of a ridge line connecting the vertical wall portion and the outward flange portion so that the outward flange portion rises in a direction opposite to the protruding direction of the recess. (7) Disclosure 7 describes a method for manufacturing a structural member, wherein the second shaping step is performed by bending the first intermediate part into the shape of the second intermediate part using a die having a concave portion on a shaping surface facing the region that will become the top plate portion of the first intermediate part, and a punch that presses the region that will become the top plate portion toward the concave portion of the die. (8) Disclosure 8 describes a method for manufacturing a structural member, in which the third molding step is performed using left and right cams that press the regions that will become the left and right vertical wall portions against the second intermediate part in directions in which the regions approach the left and right vertical wall portions, and a mold that presses the region that will become the top plate portion into the shape of the top plate portion of a target part shape while being pressed by the left and right cams. (9) Disclosure 9 describes a method for manufacturing a structural member, in which surfaces of the left and right cams that contact the second intermediate part have a shape that follows the shape of the vertical wall portions. (10) Disclosure 10 describes a method for manufacturing a structural member, in which the regions that will become the left and right vertical wall portions are brought closer together by the pressing in the third molding step by setting a distance between an inner surface of a recess in the second intermediate part and a surface of the region that will become the other vertical wall portion facing the recess to be 0 mm or more and ½ the width of the top plate portion. (11) Disclosure 11 describes a method for manufacturing a structural member, the method including a step of forming an outward flange portion, wherein the target part shape includes an outward flange portion connected to an end of a vertical wall portion. (12) Disclosure 12 describes a method for manufacturing a structural member, the method including, as a step after the third molding step, a joining process of bringing a concave surface of the recess into surface contact with an inner surface of the other vertical wall portion, and joining the inner surface of the recess and the inner surface of the other vertical wall portion to form a closed cross section.

[0079] Next, examples based on this embodiment will be described. "First Example" First, the first example will be described. (Target Part Shape 1) In this example, the target part shape 1 of the target structural member (press-molded product) was the shape shown in FIGS. 1 and 2. That is, the target part shape 1 of this example has a top plate portion 1A, left and right vertical wall portions 1B, and left and right flange portions 1C. Furthermore, the target part shape 1 of this example has recesses 1Bc formed in the left and right vertical wall portions 1B, and the inner surfaces 2 of the left and right recesses 1Bc are in surface contact with each other. By allowing the inner surfaces 2 of the left and right recesses 1Bc to be in surface contact with each other, the structural member has a closed cross section formed above the recesses 1Bc, and the lower flange portion 1C has a structure that provides an attachment portion for another component. Note that when the structural member is in an upright position, the left and right flange portions 1C extend horizontally to form foot portions. Furthermore, the connections between the various portions are configured to be continuous, forming arc-shaped arcs.

[0080] The metal plate 10 to be press-formed was a cold-rolled steel plate with a thickness of 1.4 mm and a yield strength of 1200 to 1300 MPa. The dimensions of the structural member to be manufactured are as shown in Figures 1 and 2. That is, the target structural member had a height of 100 mm, a width of the top plate portion 1A of 15 mm, a longitudinal length of 600 mm, and a vertical length of 15.1 mm at the bottom surface (vertical wall joint) of the recess 1Bc. The depth (step) of the recess 1Bc was 6.1 mm. Furthermore, the connection portions of each side in the cross section were configured to be continuous, forming an arc-shaped curve. Specifically, each side was connected by a curve with an inner radius of curvature of 4.2 mm.

[0081] (Manufacturing) In this example, similar to the embodiment, press molding was carried out in the order of the first molding step 5A, the second molding step 5B, and the third molding step 5C to manufacture a structural member.

[0082] <First forming step 5A> In the first forming step 5A of this example, the metal plate 10 was press-formed by the method described in the embodiment using the mold shown in Fig. 5 to produce the first intermediate part 11. That is, left and right recesses 1Bc and left and right flange portions 1C were formed in the metal plate 10 (see Fig. 7(a)).

[0083] <Second forming step 5B> In the second forming step 5B of this example, the first intermediate part 11 was press-formed by the method described in the embodiment using the mold shown in Fig. 7(b) to produce a second intermediate part 12. However, the forming surface shape of the mold used in the second forming step 5B was a forming surface with an arc-shaped cross section. In this example, the tip portion 31Aa of the punch 31A provided in the upper die 41 was shaped to have a width of 20.8 mm and a leading edge R of 8.5 mm, and the punch was bent using the mold so that the angle between the left and right vertical wall portions 1B was approximately 30 degrees.

[0084] <Third Forming Step 5C> In the third forming step 5C of this example, the second intermediate part 12 was press-formed using the mold shown in FIG. 9 using the method described in the embodiment to produce a part having the target part shape 1. The protrusion 40A on the lower die 40 was set to a 12.2 mm wide, arc-shaped tip with a 4.2 mm radius to match the target part shape 1. The recessed shape 20Ba on the lower surface of the upper die 41 was set to an arc-shaped recess 1Bc with a 15 mm wide and a 5 mm radius of curvature. In this example, the minimum approach distance between the left and right cams 42 was changed, and the third forming step 5C was performed under three conditions, the first to third examples below. However, the conditions for the first forming step 5A and the second forming step 5B in the first to third examples were the same.

[0085] [Conditions for the First Example] In the first example, the minimum approach distance between the left and right cams 42 was set to 4.2 mm (three times the plate thickness). In other words, the left and right cams 42 restrained the second intermediate part 12 so that the gap (distance) between the left and right recesses 1Bc was 1.4 mm (equivalent to the plate thickness). Then, in this state, the region 11A that would become the top plate portion 1A was bent using the upper die 41. Note that the minimum approach distance between the left and right cams 42 is the distance between the surfaces that contact the recesses 1Bc.

[0086] [Conditions for the Second Example] In the second example, the minimum approach distance between the left and right cams 42 was set to 2.8 mm (twice the plate thickness). In other words, the left and right cams 42 restrained the second intermediate part 12 so that the gap (distance) between the left and right recesses 1Bc was 0.0 mm (surface contact state). However, bending of the region 11A that would become the top plate portion 1A by the upper die 41 was not performed.

[0087] [Conditions for the Third Example] In the third example, the minimum approach distance between the left and right cams 42 was set to 2.8 mm (twice the plate thickness). In other words, the left and right cams 42 restrained the second intermediate part 12 so that the gap (distance) between the left and right recesses 1Bc was 0.0 mm (surface contact state). Then, in this state, the region 11A that would become the top plate portion 1A was bent using the upper die 41.

[0088] (Evaluation) Structural members were manufactured under the conditions of the first to third examples above, and the gap distance between the inner surfaces 2 of the recesses 1Bc provided in the left and right vertical wall portions 1B after release from the mold in the third molding step 5C was determined. The value at the center position in the height direction of the inner surfaces 2 of the recesses 1Bc was used as the gap distance. The results of the evaluation are shown below. First example: gap distance = 1.5 mm Second example: gap distance = 5.3 mm Third example: gap distance = 0 mm (no gap)

[0089] As can be seen from the first and third examples, in the third molding step 5C, the region 11A that will become the top plate portion 1A is bent using the upper die 41, and the following was found: That is, it was found that the gap distance after demolding can be set to a value equivalent to the separation distance in a restrained state by the left and right cams 42. In other words, it was found that the manufacturing method based on the present invention makes it possible to easily control the gap distance between the recesses 1Bc after demolding, and further, to set the separation distance small.

[0090] On the other hand, the second example revealed the following: Even when the left and right cams 42 were used to restrict the separation distance so as to be small, if the region 11A that will become the top plate portion 1A in the upper die 41 was not bent in the third forming step 5C, the separation distance after demolding became wider than the width of the top plate portion 1A. In other words, the distance between the left and right vertical wall portions 1B widens in a V-shape, and the higher the strength of the material of the metal plate 10, the more likely it is that the joining process between the left and right recesses 1Bc will become difficult.

[0091] Next, for the structural members manufactured under the conditions of the first to third examples, the left and right recesses 1Bc were spot-welded together at 50 mm intervals along the longitudinal direction. We then investigated whether the final target product shape could be achieved. For the structural members manufactured in the first and second examples, it was confirmed that the final target product shape could be achieved by spot-welding the lower vertical wall portion 1Bb while pressing it from the left and right. However, the structural member manufactured in the second example required a stronger pressing force from the left and right. Furthermore, for the structural member manufactured in the third example, spot welding was possible without the need to press the lower vertical wall portion 1Bb, and it was confirmed that the desired target part shape 1 could be easily achieved.

[0092] Second Example Next, a second example will be described. In the second example, an evaluation is performed for the case where the curved portion constituting the convex portion has the cross-sectional shape of the second convex portion (see FIG. 12(a)). (Target Part Shape 1) In this example, the target part shape 1 of the target structural member (press-formed product) was the shape shown in FIGS. 1 and 2. That is, the target part shape 1 of this example has a top plate portion 1A, left and right vertical wall portions 1B connected to the top plate portion 1A via a ridge portion 1D, and left and right flange portions 1C. Furthermore, the target part shape 1 of this example has a shape in which recesses 1Bc are formed in the left and right vertical wall portions 1B, and the inner surfaces 2 of the left and right recesses 1Bc can be in surface contact with each other. By making the inner surfaces 2 of the left and right recesses 1Bc in surface contact with each other, the structural member has a closed cross section formed above the recesses 1Bc, and the lower flange portion 1C has a portion for attachment to another component. The top plate 1A is assumed to be a flat surface, as it will be used as a joint with other components. When the structural member is in an upright position, the left and right flanges 1C extend horizontally to form legs. The connections between each part are configured to be continuous, forming an arc-shaped arc.

[0093] The metal plate 10 to be press-formed was a cold-rolled steel plate with a thickness of 1.4 mm and a tensile strength of 1470 MPa. It was confirmed that similar evaluations were obtained when a metal plate with a tensile strength of 1570 MPa was used. The dimensions of the structural member to be manufactured are as shown in Figures 1 and 2. That is, the target structural member has a height of 100 mm, a width of the top plate portion 1A of 15 mm, a longitudinal length of 600 mm, and a vertical length of 15.1 mm at the bottom surface (vertical wall joint) of the recess 1Bc. The depth (step) of the recess 1Bc is 6.1 mm. Furthermore, the connection portions of each side in the cross section were configured to be continuous, forming an arc-shaped curve. Specifically, each side was connected by a curve with an inner radius of curvature of 4.2 mm.

[0094] (Manufacturing) In this example, a structural member was manufactured by press-forming in the order of the first molding step 5A, the second molding step 5B, and the third molding step 5C, as in the embodiment. The target structural member was manufactured using two methods: an inventive example based on the present invention and a comparative reference example. In the inventive example, as described below, a molding surface based on this embodiment was used as the molding surface shape of the mold used in the second molding step 5B. In the reference example, as described below, a molding surface with an arc-shaped cross section was used as the molding surface shape of the mold used in the second molding step 5B.

[0095] (Example of the Invention) <First Forming Step 5A> In the first forming step 5A of this example, the metal plate 10 was press-formed by the method described in the embodiment using the mold shown in Fig. 5 to produce the first intermediate part 11. That is, left and right recesses 1Bc and left and right flange portions 1C were formed in the metal plate 10 (see Fig. 7(a)).

[0096] <Second forming step 5B> In the second forming step 5B of this example, a mold having a lower die 30 with a forming surface 30A shaped as shown in Fig. 11 was used to press-form the first intermediate part 11 as shown in Fig. 7(b) by the method described in the embodiment, to produce a second intermediate part 12. In this example, the shape of the tip 31Aa of the punch 31A provided in the upper die 41 was the same as the surface shape of the lower die 30.

[0097] In the example of the invention, the cross-sectional shape of the molding surface 30A of the lower die 30, which forms the shape of the area that will become the top plate portion 1A, is set as follows. Here, the cross-sectional shape of the molding surface 30A is synonymous with the shape of the area that will become the top plate portion 1A in the second intermediate part 12. For this reason, the cross-sectional shape will be described according to the names of the various parts of the second intermediate part 12.

[0098] [Settings] The settings are as follows (see FIG. 8): Angle between the left and right vertical wall portions 1B and the top plate flat portion 12Aa: 30 degrees Length w1 of the top plate flat portion 12Aa: 7.8 mm Radius of curvature r1 of the first ridge portion 12Ac: 4.1 mm Distance d1 between the left and right second ridge portions 12Ad: 20.8 mm Radius of curvature r2 of the second joint portion: 4.1 mm

[0099] <Third Forming Step 5C> In the third forming step 5C of this example, the second intermediate part 12 was press-formed using the mold shown in Figures 11 and 12 by the method described in the embodiment. Then, a part having target part shape 1 was produced. Here, the protrusion 40A provided on the lower die 40 was set to a circular arc shape with a width of 12.2 mm and a tip rounded with a radius of 4.2 mm to match target part shape 1. The forming surface of the upper die 41 was shaped as shown in Figure 12(a).

[0100] The dimensions are as follows: Overall width W of the molding surface of the upper die 41 from left to right: 16 mm Width T of the convex molding surface 41A that contacts the top plate flat portion 12Aa: 5.2 mm Depth D of the relief portion: 1.0 mm Radius of curvature R of the bottom of the arc-shaped portion located on the left and right outer sides of the relief portion: 4.0 mm

[0101] (Reference Example) In the Reference Example, as in the invention example, a structural member was manufactured by press-molding in the order of the first molding step 5A, the second molding step 5B, and the third molding step 5C. The first molding step 5A was performed under the same molding conditions as the invention example. Next, the second molding step 5B was performed under the same molding conditions as the invention example, except for the shape of the molding surface of the mold.

[0102] The molding surface shape of the mold in the Reference Example was as follows. The tip 31Aa of the punch 31A provided on the upper die 41 in the Reference Example had a width of 20.8 mm and a tip radius of 8.5 mm. Regarding the shape of the bottom portion of the concave molding surface 30A of the lower die 30, the shape of the molding surfaces 30Aa and 30Ab, which are trapezoidal in the invention example (see FIG. 8), was adjusted to match the shape of the tip 31Aa of the punch 31A, resulting in an arc-shaped cross section with a radius of curvature of 8.5 mm. Then, using a mold having the above molding surface shape, the mold was bent so that the angle between the left and right vertical wall portions 1B was approximately 30 degrees. The top of the region that would become the top plate of the second intermediate part produced in the Reference Example had an arc-shaped cross section.

[0103] Next, another example of the third molding step 5C was performed under the same molding conditions as the invention example, except for the different molding surface shape of the upper die 41. The molding surface shape of the upper die 41 in the reference example did not have a convex portion in the center (the molding surface 41A portion in FIG. 12(a)). Otherwise, the molding surface was a concave shape similar to the molding surface shape of the invention example (see FIG. 12(a)). The concave molding surface had an overall width W of 15 mm and a circular arc shape with a base curvature radius R of 5 mm.

[0104] (Evaluation) Structural members were manufactured under the conditions of the above-mentioned invention example and reference example, and the gap distance between the inner surfaces 2 of the recesses 1Bc provided in the left and right vertical wall portions 1B after release from the mold in another example of the third molding step 5C was measured. The value at the center position in the height direction of the inner surfaces 2 of the recesses 1Bc was used as the gap distance. The evaluation results are shown below. Invention example: gap distance = 0 mm (no gap) Reference example: gap distance = 2.0 mm

[0105] This evaluation revealed that the manufacturing method according to the present invention allows for easy control of the gap distance between the recesses 1Bc after demolding. Furthermore, it was found that the gap distance can be set small. Furthermore, in the inventive example, the top shape of the second intermediate part is formed into a trapezoidal shape. This revealed the following, compared to the reference example, where the top shape of the second intermediate part is simply bent in the second molding step to form an arc-shaped cross section. That is, in the inventive example, in which the top shape of the second intermediate part is formed into a trapezoidal shape, it was found that the gap between the left and right vertical wall portions after the third molding step can be controlled to be small.

[0106] Here, whether the second intermediate part of the invention example or the reference example was used, when the region 12A that will become the top plate portion 1A was not bent in the alternative example of the third forming step 5C, the separation distance after demolding was wider than the width of the top plate portion 1A. That is, even when the left and right vertical wall portions were constrained by the left and right cams 42 to reduce the separation distance, when the region 12A that will become the top plate portion 1A was not bent in the upper die 41 in the alternative example of the third forming step 5C, the separation distance after demolding was wider than the width of the top plate portion 1A. That is, the distance between the left and right vertical wall portions 1B widens in a V-shape, and the higher the strength of the material of the metal plate 10, the more likely it is that the joining process between the left and right recesses 1Bc will be difficult.

[0107] Next, we investigated whether the structural members manufactured under each of the conditions of the inventive example and the reference example could be spot-welded at 50 mm intervals along the longitudinal direction to form the final target product shape. It was confirmed that the structural members manufactured in both the inventive example and the reference example could be formed into the final target product shape by spot-welding the lower vertical wall portion 1Bb while pressing it from the left and right. However, the structural member manufactured in the reference example required a stronger pressing force from the left and right. Furthermore, it was confirmed that the structural member manufactured in the inventive example could be spot-welded without pressing the lower vertical wall portion 1Bb, and could easily form the desired target part shape 1.

[0108] The entire contents of Japanese Patent Application Nos. 2023-220210 (filed December 27, 2023) and 2024-035651 (filed March 8, 2024), from which this application claims priority, are incorporated herein by reference. While the present invention has been described with reference to a limited number of embodiments, the scope of the invention is not limited thereto, and modifications of each embodiment based on the above disclosure would be obvious to one skilled in the art.

[0109] 1 Target part shape 1A Top plate portion 1B Vertical wall portion 1Ba Upper vertical wall portion 1Bb Lower vertical wall portion 1Bc Recessed portion 1C Outward flange portion 1D Ridge portion 2 Inner surface of recess 5A First forming process 5B Second forming process 5C Third forming process 5D Joining process 10 Metal plate (blank) 11 First intermediate part 11A Region to become top plate portion 12 Second intermediate part 12A Region to become top plate portion 12Aa Top plate flat portion 12Ab Inclined portion 12Ac First ridge portion 12Ad Second ridge portion 20 Upper mold 20 Lower mold 20B Lower mold pad 20Ba Concave shape 20C Bending blade 21 Upper mold 21B Upper mold pad 21C Upper mold die 30 Lower mold die 30A Concave surface 31 Upper die 31A Punch 31Aa Tip 40 Lower die 40A Protrusion 41 Upper die 41A Forming surface 42 Cam

Claims

1. A method for manufacturing a structural member by press-forming a metal plate into a target component shape having a cross-section in which a top plate portion, left and right vertical wall portions connected to both end portions in the width direction of the top plate portion via ridge line portions and facing each other in the width direction of the top plate portion are provided, and a concave portion protruding toward the other vertical wall portion facing it is formed at a position in the middle in the height direction of one or both of the left and right vertical wall portions, and the inner surface of the concave portion is capable of surface contact with the inner surface of the other vertical wall portion, the method comprising: a first forming step of press-forming the metal plate into a first intermediate component having the concave portion; a second forming step of applying a bending deformation to the first intermediate component in a direction in which the inner surface of the concave portion approaches a region that will become the other vertical wall portion, with respect to a region that will become the top plate portion, or with respect to a region that will become the top plate portion and a region that will become the ridge line portion, to produce a second intermediate component; and a third forming step of further applying a bending deformation to a region that will become the top plate portion while pressing regions that will become the left and right vertical wall portions in a direction in which the regions that will become the left and right vertical wall portions approach each other.

2. In the second forming step, a convex portion having a cross-sectional curve shape convex in the bending outward direction is formed in the region that will become the top plate portion by applying the bending deformation. In the third forming step, the convex portion is deformed in a direction in which the radius of curvature of the cross-section increases by applying the bending deformation. The method for manufacturing a structural member according to claim 1.

3. The cross-sectional shape on the side of the region that will become the top plate portion in the second intermediate component has a top plate flat portion formed flat within the region that will become the top plate portion, and left and right inclined portions respectively connected to both end portions in the width direction of the top plate flat portion and extending obliquely toward the vertical wall portion side to connect the top plate flat portion and the region that will become the vertical wall portion. The application of the bending deformation in the third forming step is performed by pressing the top plate flat portion in a direction in which the top plate flat portion approaches the vertical wall portion side. The method for manufacturing a structural member according to claim 1.

4. In the third forming step, by pressing the top plate flat portion, the left and right inclined portions are respectively deformed in a direction in which the radius of curvature of the cross-section decreases. The method for manufacturing a structural member according to claim 3.

5. The first forming step is performed by press-forming the metal plate with a pair of molds facing each other in the thickness direction of the metal plate. A concave shape for forming the concave portion is formed on the forming surface of one mold, and a convex shape corresponding to the concave shape is formed on the forming surface of the other mold. The manufacturing method of the structural member according to any one of claims 1 to 4.

6. The target part shape includes an outward flange portion connected to the end of the vertical wall portion. In the first forming step, at the position of the ridge line portion connecting the vertical wall portion and the outward flange portion, the outward flange portion is bent and deformed so as to rise in a direction opposite to the protruding direction of the concave portion. The manufacturing method of the structural member according to any one of claims 1 to 5.

7. The second forming step is a bending forming using a mold having a concave portion on the forming surface facing the region that becomes the top plate portion in the first intermediate part, and a punch for pushing the region that becomes the top plate portion into the concave portion of the mold, and the first intermediate part is formed into the shape of the second intermediate part. The manufacturing method of the structural member according to any one of claims 1 to 6.

8. The third forming step is performed on the second intermediate part using left and right cams that press the regions that become the left and right vertical wall portions in a direction in which the regions that become the left and right vertical wall portions approach each other, and a mold that presses the region that becomes the top plate portion into the shape of the top plate portion in the target part shape in the pressing state by the left and right cams. The manufacturing method of the structural member according to any one of claims 1 to 7.

9. The surfaces of the left and right cams that contact the second intermediate part are shaped to follow the shape of the vertical wall portion. The manufacturing method of the structural member according to claim 8.

10. The approach of the regions that become the left and right vertical wall portions due to the pressing in the third forming step is set so that the separation distance between the inner surface of the concave portion in the second intermediate part and the surface of the region that becomes the other vertical wall portion facing the concave portion is 0 [mm] or more and 1 / 2 or less of the width of the top plate portion. The manufacturing method of the structural member according to any one of claims 1 to 9.

11. The target part shape includes an outward flange portion connected to the end of the vertical wall portion, and includes a step of forming the outward flange portion. The manufacturing method of the structural member according to any one of claims 1 to 10.

12. As a step after the third forming step, a joining process is provided in which the concave surface of the concave portion is brought into surface contact with the inner surface of the other vertical wall portion to join the inner surface of the concave portion and the inner surface of the other vertical wall portion to form a closed cross section. The method for manufacturing a structural member according to any one of claims 1 to 11.

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