Structural member and method for manufacturing same

Abstract

This method for manufacturing structural members (10, 10A, 10B) is provided with: a heating step for heating a blank (M) comprising a metal plate; and a molding step for molding the heated blank (M) into structural members (10, 10A, 10B) using a mold (20). The die (20) comprises a lower die (21), a pressure plate (23) and an upper die (22). The top surface (211) of the lower mold (21) comprises a top surface main body (211A) and an extending part (211B). And the pressure plate (23) is opposite to the top surface main body (211A). In the molding step, the upper die (22) and the lower die (21) are brought close to each other and the blank (M) is pressed in a state where the blank (M) is sandwiched between the top surface main body (211A) and the pressure plate (23) and the blank (M) is not sandwiched between the protruding portion (211B), the upper die (22), and the pressure plate (23).

Description

Technical Field

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

[0002] An automobile is composed of a plurality of structural members. For example, the structural members include columns, longitudinal beams, side sills, cross beams, floors, roof panels, etc. The structural members are manufactured, for example, by performing stamping on a metal plate. Patent Documents 1 to 3 disclose manufacturing methods for forming a metal plate into a structural member using a die including an upper die and a lower die.

[0003] In the manufacturing method of Patent Document 1, first, a metal plate is placed on a lower die, and the metal plate is pressed by a blank holder. Next, while moving the upper die closer to the lower die and causing one end edge in the length direction of the metal plate to move in the plane, the metal plate is formed into a structural member by clamping the metal plate with the upper die and the lower die. In Patent Document 1, as a structural member manufactured by this manufacturing method, a center pillar (B pillar) is exemplified.

[0004] Patent Document 2 discloses a manufacturing method suitable for structural members such as longitudinal beams, side sills, and cross beams. In Patent Document 2, the structural member to be manufactured has a substantially hat-shaped cross section. That is, the structural member includes a top plate, two longitudinal walls, and two flanges. An outward flange that stands up from the top plate and each longitudinal wall is provided at an end in the length direction of the structural member. In Patent Document 2, the outward flange and other parts are formed by a common lower die. In Patent Document 2, at least the region of the metal plate that becomes the outward flange and the region in the vicinity thereof start to be formed into the structural member in a state separated from the top surface of the lower die.

[0005] Patent Document 3 discloses a manufacturing method for a structural member having a letter T shape (letter T-shaped part) such as a cross beam. The letter T-shaped part includes: a top plate having a letter T shape; a longitudinal wall continuous with the top plate; and a flange continuous with the lower end of the longitudinal wall. The top plate includes a longitudinal side portion and a transverse side portion connected to the longitudinal side portion.

[0006] The manufacturing method of Patent Document 3 includes: a first forming step of forming a metal plate into an intermediate shape part; a trimming step of trimming the intermediate shape part to obtain a trimmed part; and a second forming step of forming the trimmed part into a letter T-shaped part using a die including an upper die and a lower die. In the first forming step, a convex-shaped portion is formed in a portion of the longitudinal side portion of the top plate adjacent to the transverse side portion. In addition, in the first forming step, a bent R portion is formed by lifting the connecting portion between the portion of the longitudinal wall continuous with the transverse side portion of the top plate and the flange. The convex-shaped portion and the bent R portion are flattened by the upper die in the second forming step.

[0007] Prior art documents

[0008] Patent documents

[0009] Patent Document 1: Japanese Patent Publication No. 6436166

[0010] Patent Document 2: Japanese Patent Publication No. 5958644

[0011] Patent Document 3: Japanese Unexamined Patent Application Publication No. 2019-013952 Summary of the Invention

[0012] Problems to be Solved by the Invention

[0013] As described in, for example, Patent Document 1 and Patent Document 3, among the structural members for automobiles, there is a structural member having a T-shaped letter in a plan view. The top plate of the structural member includes a top plate main body extending in the length direction of the structural member and a protruding portion protruding from the top plate main body in the width direction of the structural member. The longitudinal wall of the structural member is provided in a continuous manner with the top plate main body and the protruding portion. A flange protruding from the longitudinal wall in the width direction of the structural member is also provided in the structural member. The flange is connected to the longitudinal wall on the opposite side of the top plate.

[0014] In the case of manufacturing such a structural member by stamping, cracks sometimes occur in the longitudinal wall. More specifically, during stamping, cracks are likely to occur at the edge of the portion of the longitudinal wall that is continuous with the protruding portion of the top plate and extends along the height direction of the structural member. In addition, cracks sometimes occur at the connecting portion between the portion of the longitudinal wall that extends along the height direction of the structural member and the flange. Cracks during stamping are particularly likely to occur when forming a structural member from a steel plate having a relatively high tensile strength.

[0015] The subject of the present disclosure is to suppress the generation of cracks, particularly to suppress the generation of cracks at the edge of the portion of the longitudinal wall that extends along the height direction of the structural member and at the connecting portion between the longitudinal wall and the flange, in the manufacture of structural members for automobiles.

[0016] Means for Solving the Problems

[0017] The manufacturing method of the present disclosure is a manufacturing method of a structural member for an automobile. The manufacturing method includes a heating step of heating a blank made of a metal plate, and a forming step of forming the heated blank into a structural member using a mold. The mold includes a lower die, a blank holder, and an upper die. The lower die includes a top surface, a shoulder, a side surface, and a flange surface. The top surface includes a top surface main body and a protruding portion. The protruding portion protrudes outward from the side edge of the top surface main body. The shoulder is continuous with the side edge of the top surface main body and the protruding portion. The side surface is connected to the top surface main body and the protruding portion via the shoulder. The flange surface is connected to the side surface on the opposite side of the top surface. The blank holder faces the top surface main body. The upper die is disposed on the side of the blank holder. In the forming step, with the blank held between the top surface main body and the blank holder and not held by the protruding portion, the upper die, and the blank holder, the upper die is brought closer to the lower die so as to press the blank using the upper die, the shoulder, the side surface, and the flange surface.

[0018] Effects of the Invention

[0019] According to the present disclosure, generation of cracks can be suppressed in the manufacture of a structural member for an automobile. According to the present disclosure, generation of cracks can be particularly suppressed at the edge of a portion extending in the height direction of the structural member in a longitudinal wall and at a connection portion between the longitudinal wall and a flange in the manufacture of a structural member for an automobile. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 is a perspective view of the structural member according to the first embodiment.

[0021] Figure 2 is Figure 1 a transverse cross-sectional view of the structural member shown.

[0022] Figure 3A is a schematic view for explaining the manufacturing method of the above-described structural member.

[0023] Figure 3B is a schematic view for explaining the manufacturing method of the above-described structural member.

[0024] Figure 3C is a schematic view for explaining the manufacturing method of the above-described structural member.

[0025] Figure 3D is a schematic view for explaining the manufacturing method of the above-described structural member.

[0026] Figure 3E is a schematic view for explaining the manufacturing method of the above-described structural member.

[0027] Figure 3F is a schematic view for explaining the manufacturing method of the above-described structural member.

[0028] Figure 3GIt is a schematic diagram for explaining the manufacturing method of the above-mentioned structural member.

[0029] Figure 3H It is a schematic diagram for explaining the manufacturing method of the above-mentioned structural member.

[0030] Figure 4 It is Figure 1 A partial enlarged view of the structural member shown.

[0031] Figure 5 It is Figure 1 A partial enlarged view of the structural member shown.

[0032] Figure 6 It is a perspective view of the structural member of the second embodiment.

[0033] Figure 7 It is a perspective view of the structural member of a modification of the second embodiment.

[0034] Figure 8 It is a perspective view of the structural member of the third embodiment.

[0035] Figure 9 It is Figure 8 A top view of the blank used in the stamping forming of the structural member shown.

[0036] Figure 10 It is a perspective view of the die used in the manufacturing method of the fourth embodiment.

[0037] Figure 11 It is Figure 10 A transverse cross-sectional view of the die shown.

[0038] Figure 12 It is a perspective view of the die related to the modification of each of the above embodiments.

[0039] Figure 13 It is a chart obtained by analysis, showing the relationship between the distance from the end of the longitudinal wall included in the structural member to the end on the top plate side and the sheet thickness reduction rate.

[0040] Figure 14 It is a schematic diagram for explaining the analysis conditions. Detailed implementation manners

[0041] The manufacturing method of the embodiment is a manufacturing method of a structural member for an automobile. This manufacturing method includes a heating process of heating a blank made of a metal plate, and a forming process of forming the heated blank into a structural member using a mold. The mold includes a lower die, a blank holder, and an upper die. The lower die includes a top surface, a shoulder, a side surface, and a flange surface. The top surface includes a top surface main body and a protruding portion. The protruding portion protrudes outward from the side edge of the top surface main body. The shoulder is continuous with the side edge of the top surface main body and the protruding portion. The side surface is connected to the top surface main body and the protruding portion via the shoulder. The flange surface is connected to the side surface on the opposite side of the top surface. The blank holder faces the top surface main body. The upper die is disposed on the side of the blank holder. In the forming process, with the blank held by the top surface main body and the blank holder and not held by the protruding portion, the upper die, and the blank holder, the upper die and the lower die are relatively moved closer to press the blank with the upper die, the shoulder, the side surface, and the flange surface (the first configuration).

[0042] In the manufacturing method of the first configuration, a structural member is formed from a blank using a mold including an upper die, a lower die, and a blank holder. A top surface main body and a protruding portion protruding outward from the top surface main body are provided on the top surface of the lower die. In the forming process, with the blank held by the top surface main body of the lower die and the blank holder and not held by the protruding portion of the top surface of the lower die and the blank holder, the heated blank is pressed using the upper die and the lower die. In this case, the portion of the blank corresponding to the protruding portion of the top surface of the lower die, that is, the portion of the protruding portion of the top plate formed into the structural member is not constrained by the mold. Therefore, the portion of the blank formed into the protruding portion of the top plate is difficult to be cooled by the mold, and the material can flow in this portion during the forming of the structural member. The material flows from the portion that becomes the protruding portion of the top plate toward the portion that becomes the longitudinal wall (the portion formed along the side surface of the lower die). Thus, the generation of cracks in the structural member can be suppressed. In particular, the generation of cracks can be suppressed at the edge of the portion of the longitudinal wall that is continuous with the protruding portion of the top plate and extends in the height direction of the structural member, and at the connection portion between the longitudinal wall and the flange.

[0043] For example, when cracks are generated at the edge of the longitudinal wall during the forming process, it is necessary to perform a trimming process after the forming process and laser cut the outer peripheral portion of the formed product to remove the portion where the cracks are generated. In contrast, in the manufacturing method of the first configuration, by not constraining the protruding portion of the top plate with the mold, the generation of cracks at the edge of the longitudinal wall during the forming process can be suppressed. Therefore, there is no need for a trimming process after the forming process. When there is no need for a trimming process, compared with the case where a trimming process is performed, the amount of the blank input into the forming process can be reduced. Thus, the yield rate in the manufacturing of the structural member can be increased. In addition, by reducing the input amount of the blank and not performing a trimming process, the transportation amount, electric energy, etc. in the manufacturing of the structural member are reduced, and therefore, the emission amount of greenhouse gases can also be reduced.

[0044] Alternatively, the lower die may include a first lower die and a second lower die. The second lower die is disposed adjacent to the first lower die. The second lower die includes a protruding portion on the top surface. By creating a height difference in advance between the first lower die and the second lower die, when the upper die and the lower die approach each other, the first lower die can perform stamping on the blank between the first lower die and the upper die prior to the second lower die (second configuration).

[0045] In the second configuration, the lower die includes a first die and a second die separated from the first die. Since a height difference is created in advance between the first die and the second die, when forming the structural member, the first die contacts the blank prior to the second die including the protruding portion on the top surface. In this case, it is more difficult to restrain the portion of the blank corresponding to the protruding portion on the top surface of the lower die, that is, the portion of the protruding portion of the top plate formed into the structural member and its vicinity, and the flow of the material from the top plate side to the longitudinal wall side can be more promoted. Therefore, the generation of cracks at the edge of the portion extending in the height direction in the longitudinal wall and at the connecting portion between the longitudinal wall and the flange can be more suppressed.

[0046] Alternatively, the first lower die may be configured to be able to move up and down by means of a buffer mechanism. In this case, in the forming process, the upper die can be lowered toward the first lower die and the second lower die (third configuration).

[0047] The structural member of the embodiment is a structural member for an automobile. The structural member includes a top plate, a ridge line portion, a longitudinal wall, and a flange. The top plate includes a top plate main body and a protruding portion. The protruding portion protrudes outward from the side edge of the top plate main body. The ridge line portion is continuous with the side edge and the protruding portion of the top plate main body. The longitudinal wall is connected to the top plate main body and the protruding portion by means of the ridge line portion. The flange is connected to the longitudinal wall on the opposite side of the top plate. The flange protrudes outward from the longitudinal wall to the outside of the structural member. When the plate thickness reduction rate based on the plate thickness of the top plate main body is set to T [%], the length of the region having a plate thickness reduction rate T satisfying the following formula at the edge of the portion of the longitudinal wall continuous with the protruding portion is 2.0 times or more the plate thickness of the top plate main body (fourth configuration).

[0048] 0.9×Tmax≤T≤Tmax

[0049] Wherein, Tmax [%] is the maximum value of the plate thickness reduction rate at the edge of the portion of the longitudinal wall continuous with the protruding portion.

[0050] In the structural member of the fourth configuration, the length of the region satisfying the above formula at the edge of the portion of the longitudinal wall continuous with the protruding portion of the top plate is 2.0 times or more the plate thickness of the top plate main body. This means that the plate thickness distribution at the edge of the portion of the longitudinal wall continuous with the protruding portion of the top plate, in other words, the portion of the longitudinal wall extending in the height direction of the structural member, is made relatively uniform. In this case, when using the structural member, it is difficult to cause stress concentration in the plate thickness reduction portion located at the edge of the longitudinal wall, and cracks can be suppressed from occurring at the edge of the longitudinal wall. That is, the structural member can have excellent durability.

[0051] In the structural member of the fourth configuration, it is difficult for cracks to occur at the edge of the longitudinal wall. Therefore, when joining the structural member to other members by, for example, spot welding, it is possible to form a spot weld near the edge of the longitudinal wall. Thereby, the structural member can be firmly joined to other members, and the load transfer ability between the structural member and other members can be improved.

[0052] Alternatively, the arithmetic mean roughness Ra at the edge of the portion of the longitudinal wall continuous with the protruding portion may be 3.00 μm or less (fifth configuration).

[0053] Alternatively, the structural member may have a Vickers hardness of 325 Hv or more (sixth configuration).

[0054] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. In each figure, the same or corresponding components are denoted by the same reference numerals, and the same will not be described repeatedly.

[0055] <First Embodiment>

[0056] [Configuration of Structural Member]

[0057] Figure 1 is a perspective view of the structural member 10 of the first embodiment. Figure 2 is a cross-sectional view (cross-sectional view taken along line II-II) of the structural member 10. The structural member 10 is used as a structural member for an automobile. The structural member 10 is, for example, a cross member disposed on the floor or the back surface of the floor, a lateral extension provided at the end of the cross member, and the like.

[0058] Refer to Figure 1 and the structural member 10 includes a top plate 11, two ridge line portions 12, two longitudinal walls 13, and two flanges 14.

[0059] The top plate 11 includes a top plate main body 111 and two protruding portions 112. In the present embodiment, the top plate main body 111 has a longitudinally elongated shape when the structural member 10 is viewed from above. Hereinafter, the direction in which the top plate main body 111 extends is referred to as the longitudinal direction of the structural member 10, the direction that is substantially perpendicular to the longitudinal direction when the structural member 10 is viewed from above is referred to as the width direction of the structural member 10. In addition, the direction that is substantially perpendicular to the longitudinal direction and the width direction is referred to as the height direction of the structural member 10.

[0060] The top plate main body 111 includes two side edges 111a. The side edges 111a extend in the longitudinal direction of the structural member 10, respectively. The protruding portions 112 are provided adjacent to the side edges 111a, respectively.

[0061] The protruding portions 112 protrude outward from the side edges 111a of the top plate main body 111 in the width direction of the structural member 10. The protruding portions 112 are provided at one end portion of the top plate main body 111 in the longitudinal direction. The top plate 11 substantially has a letter T shape when the structural member 10 is viewed from above by including the top plate main body 111 and two protruding portions 112. The protruding portions 112 are, for example, located on substantially the same or approximately the same plane as the top plate main body 111.

[0062] The ridge line portions 12 are continuous with the side edges 111a of the top plate main body 111, respectively. Each ridge line portion 12 is also continuous with the protruding portion 112 of the top plate 11. Each ridge line portion 12 extends from the top plate main body 111 to the protruding portion 112. Each ridge line portion 12 mainly has a corner portion 121 between the portion corresponding to the top plate main body 111 and the portion corresponding to the protruding portion 112. When the structural member 10 is viewed from above, the corner portion 121 has a radius of curvature of, for example, 5.0 mm or more and 100.0 mm or less. The radius of curvature of the corner portion 121 when the structural member 10 is viewed from above is preferably 5.0 mm or more and 50.0 mm or less, and more preferably 5.0 mm or more and 25.0 mm or less. Refer to Figure 2 , each ridge line portion 12 substantially has an arc shape when the cross-section of the structural member 10 is observed.

[0063] Refer to Figure 1 and Figure 2 , the longitudinal walls 13 each include a first portion 131 and a second portion 132. The first portion 131 is connected to the side edge 111a of the top plate main body 111 by means of the ridge line portion 12. The first portion 131 extends along the top plate main body 111 in the longitudinal direction of the structural member 10. The first portion 131 of one longitudinal wall 13 faces the first portion 131 of the other longitudinal wall 13 in the width direction of the structural member 10. The second portion 132 is connected to the protruding portion 112 of the top plate 11 by means of the ridge line portion 12. The second portion 132 extends from the protruding portion 112 of the top plate 11 to the flange 14 in the height direction of the structural member 10.

[0064] Reference Figure 1 and Figure 2 The flanges 14 are connected to the longitudinal walls 13 on opposite sides of the top plate 11, respectively. Each flange 14 is connected to the longitudinal wall 13 by a ridge line portion 15. Each ridge line portion 15 extends along the first portion 131 and the second portion 132 of the longitudinal wall 13. Each ridge line portion 15 substantially has an arc shape when observed in the cross-section of the structural member 10, for example.

[0065] Each flange 14 protrudes outward from the longitudinal wall 13 of the structural member 10. More specifically, each flange 14 protrudes outward from the first portion 131 of the longitudinal wall 13 in the width direction of the structural member 10. Each flange 14 extends in the length direction of the structural member 10 along the first portion 131 of the longitudinal wall 13 and is connected to the second portion 132 of the longitudinal wall 13.

[0066] [Manufacturing method of structural member]

[0067] Hereinafter, while referring to Figures 3A to 3H , the manufacturing method of the structural member 10 will be described. Figures 3A to 3H is a schematic view for explaining the manufacturing method of the structural member 10. The manufacturing method of the structural member 10 includes: a step of preparing a blank M; a step of heating the blank M; and a step of forming the heated blank M into the structural member 10. The structural member 10 is manufactured by hot forming (hot stamping).

[0068] (Preparation step)

[0069] Referring to Figure 3A , in the preparation step, a blank M made of a metal plate is prepared. The blank M may also be made of a steel plate. The blank M is, for example, a blank having the shape of the unfolded structural member 10 ( Figure 1 and Figure 2 ). Such a blank can be formed by performing blanking on a metal strip (coil) using a die having the desired shape. Alternatively, the blank can also be formed by performing drilling on the coil using a laser.

[0070] (Heating step)

[0071] The heating step is a step of heating the prepared blank M. In the heating step, the blank M is heated to a temperature suitable for hot stamping. The blank M is heated by a known heating furnace, for example.

[0072] (Forming step)

[0073] The heated blank M is conveyed to the die 20 shown in Figures 3B to 3D . In the forming step, the heated blank M is formed into the structural member 10 ( Figure 1 and Figure 2 ) using the die 20. First, referring to Figures 3B to 3D , the configuration of the die 20 will be described.Figure 3B is a perspective view of the mold 20. Figure 3C is a transverse cross-sectional view of the mold 20 (section view taken along line IIIC-IIIC). Figure 3D is a longitudinal cross-sectional view of the mold 20 (section view taken along line IIID-IIID).

[0074] Refer to Figure 3B , the mold 20 includes a lower die 21, two upper dies 22, and a blank holder 23. The lower die 21 is a punch, and the upper die 22 is a die corresponding to the lower die 21. At the start of the forming process, the lower die 21 is arranged opposite to the upper die 22 and the blank holder 23. The lower die 21 is arranged, for example, below the upper die 22 and the blank holder 23. The lower die 21, the upper die 22, and the blank holder 23 are mounted, for example, on a known stamping machine (not shown).

[0075] Refer to Figure 3B and Figure 3C , the lower die 21 includes a top surface 211, two shoulders 212, two side surfaces 213, and two flange surfaces 214.

[0076] The top surface 211 is an upward-facing surface opposite to the blank holder 23. The top surface 211 is the surface for forming the top plate 11 ( Figure 1 and Figure 2 ) of the structural member 10. Therefore, the top surface 211 has a shape corresponding to the top plate 11. The top surface 211 corresponds to the top plate 11 and substantially has the shape of the letter T when looking down at the lower die 21.

[0077] The top surface 211 includes a top surface main body 211A and two protruding portions 211B. The top surface main body 211A corresponds to the top plate main body 111 ( Figure 1 and Figure 2 ) of the structural member 10 and has a longitudinally elongated shape when looking down at the lower die 21. Hereinafter, the direction in which the top surface main body 211A extends is referred to as the length direction of the mold 20, and the direction substantially perpendicular to the length direction when looking down at the mold 20 is referred to as the width direction of the mold 20. In addition, the direction substantially perpendicular to the length direction and the width direction is referred to as the height direction of the mold 20. The length direction, width direction, and height direction of the mold 20 are respectively consistent with the length direction, width direction, and height direction of the structural member 10.

[0078] The top surface main body 211A includes two side edges 211a. These side edges 211a are side edges extending in the length direction of the top surface main body 211A. The protruding portions 211B are respectively provided adjacent to the side edges 211a.

[0079] The protruding portions 211B respectively correspond to the protruding portions 112 ( Figure 1 and Figure 2) correspondingly projects outward from the side edge 211a of the top surface main body 211A. The projecting portion 211B is provided at one end portion in the length direction of the top surface main body 211A. The projecting portion 211B is, for example, located on substantially the same or approximately the same plane as the top surface main body 211A.

[0080] The shoulders 212 are continuous with the side edge 211a of the top surface main body 211A respectively. Each shoulder 212 is also continuous with the projecting portion 211B of the top surface 211. Each shoulder 212 corresponds to the ridge line portion 12 of the structural member 10 ( Figure 1 and Figure 2 ) and extends from the top surface main body 211A to the projecting portion 211B.

[0081] Refer to Figure 3C and Figure 3D , the side surface 213 corresponds to the first part 131 and the second part 132 of the longitudinal wall 13 of the structural member 10 ( Figure 1 and Figure 2 ) and includes a first part 213A and a second part 213B. The first part 213A is connected to the side edge 211a of the top surface main body 211A by means of the shoulder 212 and extends in the length direction of the die 20. The second part 213B is connected to the projecting portion 211B of the top surface 211 by means of the shoulder 212. The second part 213B extends from the projecting portion 211B of the top surface 211 to the flange surface 214 in the height direction of the lower die 21.

[0082] The flange surfaces 214 are connected to the side surfaces 213 on the opposite sides of the top surface 211 respectively. Each flange surface 214 projects outward from the side surface 213 to the outside of the lower die 21 corresponding to the flange 14 of the structural member 10. More specifically, each flange surface 214 projects outward from the first part 213A of the side surface 213 in the width direction of the die 20. Each flange surface 214 extends in the length direction of the die 20 along the first part 213A of the side surface 213 and is connected to the second part 213B of the side surface 213.

[0083] Next, refer to Figure 3C and Figure 3D , two upper dies 22 are, for example, mounted on sliders that can be lifted and lowered in a stamping machine (not shown). Each upper die 22 includes a forming surface 221. The forming surface 221 has a shape corresponding to the shoulder 212, the side surface 213, and the flange surface 214 of the lower die 21. A blank holder 23 is disposed between the two upper dies 22. That is, the upper dies 22 are disposed on both sides of the blank holder 23. These upper dies 22 can be formed separately or integrally.

[0084] The blank holder 23 is connected to the slider of a punching press (not shown) by means of a retractable elastic member 24, for example. The blank holder 23 faces the top surface main body 211A of the lower die 21. The blank holder 23 does not face the protruding portion 211B of the top surface 211 of the lower die 21. That is, the blank holder 23 is arranged at a position inside the continuous portion of the shoulder 212 of the lower die 21 and the protruding portion 211B in the width direction of the die 20. The blank holder 23 substantially has the shape of the letter I when viewed from above the die 20, for example.

[0085] In the forming process, the blank M is formed into the structural member 10 using the die 20 configured as described above ( Figure 1 and Figure 2 ). In the forming process, while the blank M is clamped between the top surface main body 211A of the lower die 21 and the blank holder 23 and is not substantially clamped by the protruding portion 211B provided on the top surface 211 of the lower die 21, the upper die 22, and the blank holder 23, the upper die 22 and the lower die 21 are brought close to each other, and the blank M is stamped using the upper die 22, the shoulder 212, the side surface 213, and the flange surface 214 of the lower die 21. Hereinafter, the forming process will be described in more detail.

[0086] As Figure 3C and Figure 3D shown, at the start of the forming process, the upper die 22 and the blank holder 23 mounted on the slider of a punching press (not shown) are at the top dead center. In this state, the blank M heated in the heating process is placed on the top surface 211 of the lower die 21. Thereafter, the upper die 22 and the blank holder 23 are lowered together with the slider of the punching press toward the lower die 21, and the upper die 22 and the blank holder 23 are brought close to the lower die 21.

[0087] When the upper die 22 and the blank holder 23 are brought close to the lower die 21, as Figure 3E shown, first, the blank M on the lower die 21 is pressed by the blank holder 23. More specifically, the portion of the blank M located on the top surface main body 211A of the lower die 21 is clamped between the lower die 21 and the blank holder 23. On the other hand, the portion of the blank M located on the protruding portion 211B of the top surface 211 of the lower die 21 is not clamped by the lower die 21 and the blank holder 23.

[0088] Figure 3F is a schematic view for more detailedly explaining the portion of the blank M ( Figure 3E ) constrained by the blank holder 23. As Figure 3F shown, in the lower die 21, each shoulder 212 mainly has a corner portion 212A between the portion corresponding to the top surface main body 211A and the portion corresponding to the protruding portion 211B. The lower die 21 includes a region A adjacent to the corner portion 212A when viewed along the stamping direction. As shown in Figure 3FIn the figure, the area A is shown by hatching. The area A extends from the corner 212A towards the inside of the top surface 211 until it reaches a region that is 10 times the thickness of the blank M( Figure 3E ). The blank holder 23 is arranged so as to overlap at least a part of the area A when viewed in the stamping direction (height direction). That is, at least a part of the range in the blank M that corresponds to the area A adjacent to the corner 212A in the stamping direction is pressed by the blank holder 23. On the other hand, the part of the blank M that is located outside the area A adjacent to the corner 212A in the width direction of the lower die 21 is not pressed by the blank holder 23. That is, the blank holder 23 does not restrain the part of the blank M that is located outside the end on the protruding part 211B side of the corner 212A in the width direction during the forming process.

[0089] Return to Figure 3E , while keeping the following state unchanged, further lower the slider of the stamping machine (not shown): clamp the blank M using the top surface main body 211A of the lower die 21 and the blank holder 23, and do not clamp the blank M using the protruding part 211B of the top surface 211 of the lower die 21 and the blank holder 23. As a result, the elastic member 24 connecting the blank holder 23 and the slider contracts, and the upper die 22 relatively descends with respect to the blank holder 23. As a result, the upper die 22 approaches the lower die 21 further, and the stamping and forming of the blank M by the upper die 22 and the lower die 21 starts. As Figure 3G shown, the part of the blank M that is located on the protruding part 211B of the top surface 211 of the lower die 21 becomes a state where it is slightly separated from the protruding part 211B during the forming process.

[0090] As Figure 3H shown, when the upper die 22 reaches the bottom dead center, the upper die 22 and the lower die 21 completely stamp the blank M to form the structural member 10. The structural member 10 is heat removed (quenched) due to contact with the die 20, making it high-strength.

[0091] Referring to Figure 4 , for the structural member 10 after the forming process, the edge 132a of the second part 132 of each longitudinal wall 13 includes a deformed region R1. The deformed region R1 is within the range from the rounded end (on the longitudinal wall 13 side) of the edge 12a of the ridge line portion 12 connecting the end edge 112a of the protruding part 112 of the top plate 11 and the end edge 132a of the second part 132 of the longitudinal wall 13 to the rounded end (on the longitudinal wall 13 side) of the edge 15a of the ridge line portion 15 connecting the end edge 132a of the second part 132 of the longitudinal wall 13 and the end edge 14a of the flange 14.

[0092] The entire deformed region R1 has a plate thickness reduction rate T [%] that satisfies the following formula (1).

[0093] 0.9×Tmax ≤ T ≤ Tmax (1)

[0094] The Tmax [%] in the above formula (1) is the maximum value of the plate thickness reduction rate T at the edge 132a of the second part 132 of each longitudinal wall 13. The plate thickness reduction rate T is based on the plate thickness of the top plate main body 111. When the plate thickness of the top plate main body 111 is set to t0 and the plate thickness at an arbitrarily selected position at the edge 132a of each longitudinal wall 13 is set to t1, the plate thickness reduction rate T [%] at this position can be obtained by (t0 - t1) / t0 × 100. The plate thickness t0 of the top plate main body 111 is the plate thickness of the part in the top plate main body 111 where strain caused by forming does not substantially occur. That is, the plate thickness t0 is substantially equal to the plate thickness of the blank M before forming. The plate thickness t0 is measured at the central part of the top plate main body 111 and at the part with a flat shape. For example, the plate thickness t0 is the plate thickness of the top plate main body 111 measured at a position more than 5 mm away from the ridge line part 12 and the end part in the longitudinal direction of the top plate main body 111. When there are steps, raised parts, and through holes in the top plate main body 111, not only the ridge line part 12 and the end part in the longitudinal direction of the top plate main body 111, but also the plate thickness of the top plate main body 111 measured at a position more than 5 mm away from the steps, raised parts, and through holes is set as the plate thickness t0.

[0095] In the edge 132a of the second part 132 of each longitudinal wall 13, the length of the deformation region R1 is 2.0 times or more the plate thickness t0 of the top plate main body 111. The length of the deformation region R1 is preferably 2.5 times or more the plate thickness t0 of the top plate main body 111. The length of the deformation region R1 can also be 6.0 times or less the plate thickness t0 of the top plate main body 111.

[0096] The structural member 10 can have a Vickers hardness of 325 Hv or more. The Vickers hardness HV of the structural member 10 can be evaluated based on the Vickers hardness of the top plate 11. For example, using a commercially available measuring instrument (fully automatic Vickers hardness tester HV-100, Mitutoyo Corporation, Japan), the Vickers hardness [Hv] of any 5 parts is measured in the top plate 11 by performing a Vickers hardness test according to JIS Z 2244:2009. The average value of the Vickers hardness of these 5 parts can be set as the Vickers hardness HV of the structural member 10. For example, the test force is set to 294.2 N (the value of HV30), and the holding time of the test force is set to 15 s to measure the Vickers hardness of the top plate 11.

[0097] [Effect]

[0098] In the present embodiment, the structural member 10 is formed from a blank M using a die 20 including a lower die 21, an upper die 22, and a blank holder 23. In the forming process, while the portion of the blank M located on the top surface main body 211A of the lower die 21 is pressed by the blank holder 23 and the portion of the blank M located on the protruding portion 211B of the lower die 21 is not pressed by the blank holder 23, the heated blank M is stamped using the upper die 22 and the lower die 21. In this case, the portion of the blank M located on the protruding portion 211B of the lower die 21, that is, the portion that becomes the protruding portion 112 of the top plate 11 of the structural member 10, is not constrained by the die 20. The portion of the blank M that becomes the protruding portion 112 of the top plate 11 is in a state of being separated from the die 20 during the forming process. Therefore, during the forming process, it is difficult to cool the protruding portion 112 of the top plate 11, and the flow of the material can be generated in the protruding portion 112. The material flows from the protruding portion 112 of the top plate 11 toward the second portion 132 of the longitudinal wall 13 in the height direction of the structural member 10. Thereby, the generation of cracks in the structural member 10 can be suppressed. In particular, it is possible to suppress the generation of cracks at the edge 132a of the second portion 132 of the longitudinal wall 13 that is continuous with the protruding portion 112 of the top plate 11 and at the connecting portion between the second portion 132 of the longitudinal wall 13 and the flange 14.

[0099] For example, when cracks are generated at the edge of the longitudinal wall during the forming process, as a countermeasure, a blank material with a surplus is used, and the shape outside the product is relaxed to manufacture a formed product. In this case, it is necessary to perform a trimming process after the forming process and laser cut the outer peripheral portion of the formed product to obtain the product shape. In contrast, in the present embodiment, by not constraining the protruding portion 112 of the top plate 11 with the die 20, it is possible to suppress the generation of cracks at the edge of the longitudinal wall 13 during the forming process. Therefore, there is no need for a trimming process after the forming process. When there is no need for a trimming process, compared with the case where a trimming process is performed, the amount of the blank M input into the forming process can be reduced. Thus, the yield rate in the manufacture of the structural member 10 can be increased. In addition, by reducing the input amount of the blank M, without performing a trimming process, the transportation amount, electric energy, etc. in the manufacture of the structural member 10 are reduced. Therefore, the emission amount of greenhouse gases can also be reduced.

[0100] As described above, when manufacturing the structural member 10 by the manufacturing method of the present embodiment, there is no need to laser cut the outer peripheral portion of the structural member 10 after the forming process. Therefore, the edge of the completed structural member 10 becomes smoother than the edge of the structural member completed through a trimming process with laser cutting.

[0101] In the case where laser cutting of a structural member is performed after a forming process (hot stamping), the arithmetic mean roughness Ra of the edge portion (laser cut surface) generated due to laser cutting is measured. As a result, the arithmetic mean roughness Ra of the laser cut surface is at least 4.20 μm even at its minimum. Also in other automotive components, the arithmetic mean roughness Ra of the edge portion (laser cut surface) generated due to laser cutting is measured. As a result, the arithmetic mean roughness Ra of the laser cut surface is greater than 3.00 μm.

[0102] In contrast, in the present embodiment, for the structural member 10 after the forming process, laser cutting is not performed on the edge 132a of the second portion 132 of the longitudinal wall 13 and the edge 12a of the ridge line portion 12 connecting the longitudinal wall 13 and the top plate 11. Therefore, the arithmetic mean roughness Ra at the edges 132a and 12a is intentionally smaller than the arithmetic mean roughness Ra of the surface on which laser cutting is performed after the forming process. More specifically, in the structural member 10, the arithmetic mean roughness Ra at the edge 132a of the second portion 132 of the longitudinal wall 13 becomes 3.00 μm or less. Similarly, the arithmetic mean roughness Ra at the edge 12a of the ridge line portion 12 becomes 3.00 μm or less.

[0103] Refer to Figure 5 , it is possible to measure the arithmetic mean roughness Ra at the edge 132a of the second portion 132 of the longitudinal wall 13 and the edge 12a of the ridge line portion 12 using a commercially available surface roughness measuring instrument. More specifically, using a surface roughness measuring instrument (laser microscope, manufactured by Keyence Corporation), in accordance with JIS B 0601:2013, in the region ( Figure 5 the thick line region) formed by the edge 132a of the longitudinal wall 13 and the edge 12a of the ridge line portion 12, a roughness curve is created at a position 0.1 mm from the plate surface. The evaluation length of the roughness curve can be set to 12.5 mm and the reference length can be set to 2.5 mm to obtain the arithmetic mean roughness Ra. Similarly, using the above-mentioned surface roughness measuring instrument, in accordance with JIS B 0601:2013, in the region formed by the edge 132a of the longitudinal wall 13 and the edge 12a of the ridge line portion 12, a roughness curve is created at positions 0.1 mm from the plate thickness center and the plate back surface. The evaluation length of the roughness curve can be set to 12.5 mm and the reference length can be set to 2.5 mm to obtain the arithmetic mean roughness Ra. The obtained arithmetic mean roughness Ra is all 3.00 μm or less.

[0104] In the structural member 10 of the present embodiment, the second part 132 of the longitudinal wall 13 has a deformation region R1 at the edge 132a that satisfies the above formula (1). The length of the deformation region R1 is 2.0 times or more the plate thickness t0 of the top plate main body 111. This means that the plate thickness distribution at the edge 132a of the second part 132 is made uniform in each longitudinal wall 13. In this case, when the structural member 10 is used, it is difficult to cause stress concentration at the plate thickness reduction portion at the edge 132a of the second part 132 of each longitudinal wall 13, and the generation of cracks at the edge 132a can be suppressed. That is, the structural member 10 can be made to have excellent durability.

[0105] In the structural member 10 of the present embodiment, at the edge 132a of the second part 132 of the longitudinal wall 13, the situation of local plate thickness reduction is reduced, and it is difficult to generate cracks at the edge 132a. Therefore, when the structural member 10 and other members are joined by, for example, spot welding, a spot weld can be formed near the edge 132a of the longitudinal wall 13. Thereby, the structural member 10 and other members can be firmly joined, and the load transfer ability between the structural member 10 and other members can be improved.

[0106] According to the manufacturing method of the present embodiment, when the blank M is formed into the structural member 10, the generation of cracks in the second part 132 of the longitudinal wall 13 and its vicinity can be suppressed. Therefore, as the material of the structural member 10, a high-strength material can be used. For example, the structural member 10 can be formed of a steel plate having a tensile strength of 1000 MPa or more or 2000 MPa or more after hot stamping. By forming the structural member 10 of a high-strength material, the strength of the structural member 10 can be ensured, and the structural member 10 can be made thin-walled and lightweight.

[0107] Generally, a structural member formed by hot stamping has the following characteristics: it is harder at the part where no plate thickness reduction occurs during forming, and the hardness is insufficient at the part where plate thickness reduction has occurred. Therefore, in the case of a general structural member, it is conceivable that deformation concentrates at the plate thickness reduction portion, and there is a possibility of generating cracks according to the degree of deformation concentration. However, in the structural member 10 of the present embodiment, the plate thickness distribution is made uniform as described above. That is, in the case of the structural member 10, the plate thickness reduction is dispersed, and the reduction of local hardness is suppressed. Therefore, the deformation concentration when the structural member 10 is used can be reduced, and the durability of the structural member 10 can be improved.

[0108] When observing the structural member 10 of the present embodiment from the side of the top plate 11, the radius of curvature of the corner portion 121 of the ridge line portion 12 is, for example, 100.0 mm or less, preferably 50.0 mm or less, and more preferably 25.0 mm or less. When the radius of curvature of the corner portion 121 is small, the crossing angle between the first portion 131 and the second portion 132 of the longitudinal wall 13 continuous with the ridge line portion 12 can be set to, for example, approximately a right angle. Thereby, the structural member 10 can be made into a member with excellent space efficiency. That is, the degree of freedom in spatial coordination with other components can be increased, and the dimensional constraints in the design of the structural member 10 or other components can be reduced. In addition, even when the area where the structural member 10 is arranged is narrow, the load transfer capacity of the structural member 10 can be ensured. Specifically, when a load in the length direction is input to the structural member 10 arranged in a narrow area, the second portion 132 of the longitudinal wall 13 can receive the load in the form of a surface, and the load is easily transferred from the second portion 132 to other portions. Therefore, the structural member 10 can exhibit good transfer ability for the load in the length direction.

[0109] <Second Embodiment>

[0110] Figure 6 It is a perspective view of the structural member 10A of the second embodiment. The basic configuration of the structural member 10A of the present embodiment is the same as the structure of the structural member 10 of the first embodiment. However, the structural member 10A is different from the structural member 10 of the first embodiment in terms of the shape of the top plate 11.

[0111] As Figure 6 shown, in the structural member 10A, a part of the top plate main body 111 bulges upward compared with other parts. The height of the longitudinal wall 13 changes locally according to the shape of the top plate main body 111.

[0112] The structural member 10A of the present embodiment can also be manufactured by the manufacturing method described in the first embodiment. When manufacturing the structural member 10A, a preforming process of forming an intermediate formed product from a metal plate (blank) may be performed before the forming process. In the preforming process, an intermediate formed product is formed from a metal plate by, for example, deep drawing. The intermediate formed product may also be, for example, a member formed with a raised portion of the top plate main body 111. The intermediate formed product may also be a member gently having the shape of the protruding portion 112 of the top plate 11 and the second portion 132 of the longitudinal wall 13. Typically, the preforming process is performed cold. In this case, the intermediate formed product obtained in the preforming process is heated, and the heated intermediate formed product is provided as a blank to the forming process.

[0113] As Figure 7As shown, the structural member 10A may also have a notch 16 at the connecting portion between, for example, the second part 132 of the longitudinal wall 13 and the flange 14. The notch 16 may be provided only on one side in the width direction in the structural member 10A, or may be provided on both sides in the width direction. The notch 16 can be pre-formed for the blank before it is formed into the structural member 10A. The structural member 10 of the first embodiment ( Figure 1 and Figure 2 ) can also have a notch 16, but the illustration thereof is omitted here.

[0114] However, in order to make it difficult for stress concentration to occur when using the structural members 10 and 10A, it is preferable that the structural members 10 and 10A do not have a notch. That is, the protruding portion 112 of the top plate 11, the longitudinal wall 13, and the edge of the flange 14 are preferably smoothly continuous. In this case, in addition to being able to suppress stress concentration, the load transfer capacity of the structural members 10 and 10A during a vehicle collision can be improved. In addition, it is possible to prevent water from entering the structural members 10 and 10A through the notch, and rusting of the structural members 10 and 10A can be suppressed.

[0115] <Third Embodiment>

[0116] Figure 8 is a perspective view of the structural member 10B of the third embodiment. The structural member 10B of this embodiment has a shape different from that of the structural members 10 and 10A of the above embodiments. The structural members 10 and 10A of the above embodiments can be used, for example, as cross members arranged on the floor or the back of the floor, lateral extensions provided at the ends of the cross members, and the like. On the other hand, the structural member 10B of this embodiment can be used as a B-pillar, for example.

[0117] Referring to Figure 8 , the structural member 10B includes a top plate 11 including a top plate main body 111 and a protruding portion 112, similarly to the structural members 10 and 10A of the above embodiments. In addition, the structural member 10B includes a longitudinal wall 13 including a first part 131 and a second part 132, similarly to the structural members 10 and 10A. In the structural member 10B, the first part 131 and the second part 132 of the longitudinal wall 13 are connected to the top plate 11 and the flange 14 by ridge lines 12 and 15. The flange 14 is provided on the opposite side of the top plate 11 with respect to the first part 131 and the second part 132 of the longitudinal wall 13.

[0118] The structural member 10B of this embodiment can also be manufactured by the manufacturing method described in the first embodiment. When manufacturing the structural member 10B, as Figure 9As shown, a blank that can prepare the shape of the deployable structural member 10B is used as the blank M. As described in the above embodiment, the blank can be formed by subjecting a metal strip (coil) to blanking by a die, or can be formed by performing drilling by a laser.

[0119] The structural member 10B is manufactured by the manufacturing method described in the first embodiment. Therefore, similar to the structural members 10 and 10A of the above embodiment, cracks are hardly generated at the edges of the longitudinal wall 13 and the like during the forming process. Therefore, for the structural member 10B, there is no need for a trimming process after the forming process. That is, there is no need to perform laser cutting on the outer peripheral portion of the structural member 10B after the forming process. Thus, in the structural member 10B, similar to the structural members 10 and 10A of the above embodiment, the arithmetic mean roughness Ra at the edge of the second portion 132 of the longitudinal wall 13 and the edge of the ridge line portion 12 becomes 3.00 μm or less.

[0120] In the structural member 10B, the edge of the second portion 132 of each longitudinal wall 13 can have the same deformation region R1 as the structural members 10 and 10A of the above embodiment. The structural member 10B can have a Vickers hardness of 325 Hv or more in the same manner as the structural members 10 and 10A of the above embodiment.

[0121] In the structural member 10B of the present embodiment, the ridge line portion 12 includes a corner portion 121 in the same manner as the structural members 10 and 10A of the above embodiment. When looking down at the structural member 10B, the radius of curvature of the corner portion 121 is, for example, 20.0 mm or more and 300.0 mm or less. The radius of curvature of the corner portion 121 when looking down at the structural member 10B is preferably 15.0 mm or more and 200.0 mm or less, and more preferably 15.0 mm or more and 100.0 mm or less. When the radius of curvature of the corner portion 121 is small, the same effect as that of the structural members 10 and 10A of other embodiments can be achieved. That is, the crossing angle between the first portion 131 and the second portion 132 of the longitudinal wall 13 continuous with the ridge line portion 12 can be set to a right angle or an angle close to a right angle, and the structural member 10B can be arranged in a narrow area. Thus, the situation where the size of the structural member 10B is restricted in the spatial arrangement relationship with other components is reduced. In addition, when a load in the length direction is input to the structural member 10B, the second portion 132 of the longitudinal wall 13 can bear the load in a planar form. Therefore, the structural member 10B can exhibit good load transfer ability for the load in the length direction.

[0122] <Fourth Embodiment>

[0123] Figure 10 Is a perspective view of the die 20A used in the manufacturing method of the present embodiment.Figure 11 It is a transverse sectional view (section XI-XI) of the die 20A. The die 20A is different from the die 20 described in the first embodiment only in the constitution of the lower die 21. The structural members 10, 10A, and 10B of the above embodiment can also be formed using the die 20A.

[0124] Refer to Figure 10 , the lower die 21 includes a first lower die 215 and a second lower die 216. The first lower die 215 is separated from the second lower die 216 and can operate independently. The second lower die 216 includes an extending portion 211B of the top surface 211. The second lower die 216 also includes a second portion 213B of the side surface 213. The second lower die 216 is arranged adjacent to the first lower die 215.

[0125] Refer to Figure 11 , the first lower die 215 is configured to be able to move up and down by means of a buffer mechanism 25. The buffer mechanism 25 can also be a mechanism commonly used in a known stamping machine (not shown), for example, including die cushions, buffer pins, etc.

[0126] The operation of the die 20A is the same as that of the die 20 in the above embodiment. However, in the die 20A, a height difference is generated in advance between the first lower die 215 and the second lower die 216. More specifically, the first lower die 215 is supported by the buffer mechanism 25 and is positioned slightly above the second lower die 216 before the start of the forming process. Therefore, when the upper die 22 descends toward the first lower die 215 and the second lower die 216, the first lower die 215 presses the blank M between the first lower die 215 and the upper die 22 prior to the second lower die 216. At the moment when the blank M is pressed by the upper die 22 and the first lower die 215, the second lower die 216 does not press the blank M.

[0127] If the upper die 22 in the state of clamping the blank M together with the first lower die 215 approaches the second lower die 216 further, the first lower die 215 supported by the buffer mechanism 25 is pressed down by the upper die 22. As a result, the height difference between the first lower die 215 and the second lower die 216 gradually becomes smaller. When the upper die 22 reaches the bottom dead center, the height difference between the first lower die 215 and the second lower die 216 disappears, and the blank M is pressed between the second lower die 216 and the upper die 22 in addition to the first lower die 215.

[0128] In the present embodiment, a height difference is pre-generated between the first lower die 215 and the second lower die 216. Therefore, when forming the structural members 10, 10A, 10B of the above embodiment, the first lower die 215 contacts the blank M prior to the second lower die 216 including the protruding portion 211B of the top surface 211. In this case, it is more difficult to restrain the portion of the blank M corresponding to the protruding portion 211B of the top surface 211 of the lower die 21, that is, the portion of the protruding portion 112 of the top plate 11 of the structural members 10, 10A, 10B and its vicinity, and the material flow from the protruding portion 112 of the top plate 11 to the second portion 132 of the longitudinal wall 13 can be more promoted. Therefore, the generation of cracks at the edge of the second portion 132 extending in the height direction in the longitudinal wall 13 and at the connecting portion between the longitudinal wall 13 and the flange 14 can be further suppressed.

[0129] As described above, the embodiments of the present disclosure have been described, but the present disclosure is not limited to the above embodiments, and various modifications can be made without departing from the gist thereof.

[0130] For example, in the above first to third embodiments, the protruding portion 112 of the top plate 11 is provided only at one end portion in the length direction of the structural members 10, 10A, 10B. However, the protruding portion 112 may also be provided at both end portions in the length direction of the structural members 10, 10A, 10B. In this case, the hot stamping of the structural members 10, 10A, 10B is also performed without pressing each protruding portion 112 by the blank holder 23.

[0131] When the protruding portion 112 of the top plate 11 is provided at both end portions in the length direction of the structural members 10, 10A, 10B, the structural members 10, 10A, 10B can also be divided into two after the forming process. Thereby, two structural members 10, structural members 10A, or structural members 10B having the protruding portion 112 only at one end portion in the length direction can be manufactured in one forming process.

[0132] In the above embodiment, with the die 20 or the die 20A mounted on the press, the upper die 22 is disposed above the lower die 21. Further, when forming the blank M into the structural members 10, 10A, 10B, the upper die 22 and the lower die 21 are brought closer by moving the upper die 22 toward the lower die 21. However, conversely to the above embodiment, the upper die 22 may be disposed below the lower die 21. Further, the upper die 22 may be relatively brought closer to the lower die 21 by moving the lower die 21 toward the upper die 22.

[0133] In the above embodiment, the length (width) of the blank holder 23 in the width direction of the die 20 or the die 20A is substantially constant. However, the width of the blank holder 23 does not have to be constant throughout its entirety. For example Figure 12As shown, the width of the blank holder 23 may also be widened on the side of the protruding portion 211B of the lower die 21. However, in this case, the blank holder 23 is also configured not to press the blank at a position on the outer side in the width direction of the region A adjacent to the corner portion 212A in the lower die 21.

[0134] Embodiment

[0135] Hereinafter, the present disclosure will be described in further detail using embodiments. However, the present disclosure is not limited to the following embodiments.

[0136] [First Embodiment]

[0137] To confirm the effects of the present disclosure, for the stamping forming of the structural member 10 of the first embodiment ( Figure 1 and Figure 2 ), CAE analysis was performed using commercially available software (stamping forming simulation system JSTAMP ver2.18, manufactured by JSOL Corporation and LS-DYNAver971rev7.12, manufactured by ANSYS Inc.). The analysis conditions and results are shown in Table 1.

[0138]

Table 1

[0139] Table 1-1

[0140]

[0141] Table 1-2

[0142]

[0143] In Table 1, the example of the embodiment with the buffer set to "none" is the case of forming the structural member 10 ( Figures 3B to 3G ) using the die 20 of the first embodiment ( Figure 1 and Figure 2 ). On the other hand, the example of the embodiment with the buffer set to "yes" is the case of forming the structural member 10 ( Figure 10 and Figure 11 ) using the die 20A of the fourth embodiment ( Figure 1 and Figure 2) Example. The cushion stroke (Cushion St.) is the distance of the downward movement of the first lower die 215 by the cushion mechanism 25, and is equal to the magnitude of the height difference between the first lower die 215 and the second lower die 216. The comparative example is an example of forming the structural member 10 using a die forming structure in which only the shape of the blank holder is different from that of the die 20. The blank holder 23 of the die 20 has an I-shaped letter in a plan view and is configured not to press the protruding portion 112 of the top plate 11 of the structural member 10. In contrast, the blank holder used in the comparative example has a T-shaped letter in a plan view and is configured to press substantially the entire surface of the top plate 11. That is, in the comparative example, the structural member 10 is formed in a state where the blank holder presses not only the top plate main body 111 but also the protruding portion 112.

[0144] As shown in Table 1, in Comparative Examples 1 to 4 in which a blank holder having a T-shaped letter in a plan view is used and the structural member 10 is formed while the blank holder presses the protruding portion 112 of the top plate 11, cracks occurred in the second portion 132 extending in the height direction of the structural member 10 in the longitudinal wall 13. On the other hand, in Examples 1 to 11 in which a blank holder 23 having an I-shaped letter in a plan view is used and the structural member 10 is formed without the blank holder 23 pressing the protruding portion 112 of the top plate 11, no cracks occurred in the structural member 10.

[0145] In Examples 1 to 11, compared with Comparative Examples 1 to 4, the maximum plate thickness reduction rate of the edge 132a of the second portion 132 of the longitudinal wall 13 is intentionally small. The maximum plate thickness reduction rate in Examples 1 to 11 is less than 20%. In addition, in Examples 1 to 11, the length of the deformation region R1 that satisfies the above formula (1) is ensured to be 2.0 times or more of the plate thickness t of the blank. From this result, it can be considered that if a blank holder 23 having an I-shaped letter in a plan view is used and the structural member 10 is formed without the blank holder 23 pressing the protruding portion 112 of the top plate 11, it is possible to suppress the reduction of the plate thickness at the edge 132a of the second portion 132 of the longitudinal wall 13 and avoid cracks, and it is possible to make the plate thickness distribution of the edge 132a uniform.

[0146] Figure 13 is a graph showing the relationship between the distance from the rounded end on the edge 12a side of the ridge line portion 12 and the plate thickness reduction rate T at the edge 132a of the second portion 132 of the longitudinal wall 13. As Figure 13 shown, in Examples 1 to 4, compared with Comparative Example 1, the change in the plate thickness reduction rate T is gentle. In addition, from Figure 13 it can be seen that the maximum plate thickness reduction rate in Examples 1 to 4 is significantly smaller than the maximum plate thickness reduction rate in Comparative Example 1. That is, it can be seen that in Examples 1 to 4, the reduction of the plate thickness at the edge 132a of the second portion 132 of the longitudinal wall 13 is suppressed, and the plate thickness distribution of the edge 132a is made uniform.

[0147] In this analysis, the Vickers hardness HV of the structural member 10 was measured for each of the examples and comparative examples using the method described in the first embodiment. The Vickers hardness HV is set to be the average value of the Vickers hardness [Hv] obtained by measuring five locations: a position 5 mm inward from the edge of the extension 112 side of the top plate 11, a center point in the width direction, a point 5 mm from both ends in the width direction, and a midpoint between them. As shown in Table 1, the structural member 10 is formed by hot stamping, so in any of the examples and comparative examples, the Vickers hardness of the structural member 10 is 325Hv or more.

[0148] In addition, in this analysis, the Vickers hardness of the portion where the plate thickness reduction rate is the largest (maximum plate thickness reduction portion) and the portion where the plate thickness reduction does not occur (normal portion) are measured for Example 1 and Comparative Example 1, respectively. With respect to Example 1, the Vickers hardness of the portion with the maximum plate thickness reduction is 360Hv, and the Vickers hardness of the normal portion is 410Hv. On the other hand, with respect to Comparative Example 1, the Vickers hardness of the portion with the maximum plate thickness reduction is 306Hv, and the Vickers hardness of the normal portion is 414Hv. Based on this result, it can be considered that: when the structural member 10 is formed using the manufacturing method disclosed in the present invention, the dispersion of the plate thickness reduction is utilized to suppress the reduction in local hardness. Therefore, the following effect can be estimated: when the structural member 10 is deformed due to a collision, the concentration of deformation can be suppressed. Therefore, it is assumed that the structural member 10 formed using the manufacturing method disclosed in the present invention has excellent collision resistance.

[0149] [Second embodiment]

[0150] For the above-described Example 5, the yield was calculated by dividing the weight (kg) of the structural member 10 by the weight (kg) of the metal strip (coil) before cutting out the blank that is a blank for stamping. As a reference example, for the case where the structural member 10 is formed by drawing using a die including a punch, a die, and a blank holder, the same CAE analysis as above was performed, and the yield was calculated. In addition, for Example 5 and the reference example, a LCA (Life Cycle Assessment) tool for components (which is a tool capable of analyzing the emissions of life cycle greenhouse gases advocated by the automotive subcommittee of the World Steel Association, WAS, and is software for analysis in EXCEL format downloaded from the WAS homepage (https: / / www.worldautosteel.org / life-cycle-thinking / case-studies / comparing-material-usage-in-production-vehicle-efficient-designs / )) was used to calculate the emissions of greenhouse gases (CO2-eq) during the manufacture of the structural member 10. The calculation results are shown in Table 2.

[0151]

Table 2

[0152] Table 2-1

[0153]

[0154] Table 2-2

[0155]

[0156] In the reference example where the structural member 10 is formed by drawing, although no cracks occurred in the structural member 10, compared with Example 5, the yield was significantly reduced. The reason is that the blank needs the surplus material held by the blank holder and the die in the drawing process, and the coil for cutting out the blank also becomes larger. In the reference example, after drawing, the surplus material of the formed product is laser cut to adjust it to the shape of the structural member 10. Therefore, the weights of the blank and the coil are relatively large compared to the weight of the structural member 10, and the yield is as low as 58%.

[0157] On the other hand, in Example 5 where a die 20 including a blank holder 23 having an I-shaped cross section in a plan view is used for performing a constrained bending forming, a high yield rate can be ensured. In Example 5, there is no need for a scrap of the blank held by the blank holder and the die, and no crack is generated in the structural member 10 either. Therefore, there is no need for laser cutting after the constrained bending forming. Accordingly, the blank cut out from the coil can be set to the developed shape of the structural member 10, and the size of the coil can be reduced as compared with the reference example. In Example 5, the weights of the blank and the coil are small relative to the weight of the structural member 10, and the yield rate is as high as 82%.

[0158] In Example 5, as compared with the reference example, the weight of the coil is small and there is no need for laser cutting either. Therefore, in Example 5, the amount of greenhouse gas emissions during the manufacture of the structural member 10 can also be reduced as compared with the reference example.

[0159] For Example 5 and Reference Example 1, the arithmetic mean roughness Ra of the edge 132a of the longitudinal wall 13 was calculated by the method described in the above First Embodiment. In this embodiment, the arithmetic mean roughness Ra was calculated within a range of ±6.25 mm in the height direction from the center of the edge 132a of the longitudinal wall 13. As shown in Table 2, for Example 5 where laser cutting is not performed after forming, the arithmetic mean roughness Ra is at least 1.91 μm and at most 2.10 μm. On the other hand, for the reference example where laser cutting is performed after forming, the arithmetic mean roughness Ra is at least 4.27 μm. Accordingly, it was confirmed that the arithmetic mean roughness Ra of the edge 132a of the longitudinal wall 13 becomes 3.00 μm or less when laser cutting is not performed on the edge 132a of the longitudinal wall 13 after hot stamping.

[0160] [Third Embodiment]

[0161] For the stamping forming of the structural member 10B ( Figure 8 ) of the Third Embodiment, the same software as that of the First Embodiment was used, and the same CAE analysis as that of the First Embodiment was performed. The analysis conditions and results are shown in Table 3.

[0162] [Table 3]

[0163] Table 3-1

[0164]

[0165] Table 3-2

[0166]

[0167] As shown in Table 3, in Comparative Example 5 where a blank holder in the shape of the letter "T" in a top view is used and the entire surface of the top plate 11 (the top plate main body 111 and the protruding portion 112) is pressed by the blank holder while forming the structural member 10B, cracks occurred in the second portion 132 of the vertical wall 13 that extends along the height direction of the structural member 10. On the other hand, in Example 12 where a blank holder 23 in the shape of the letter "I" in a top view is used and the protruding portion 112 of the top plate 11 is not pressed by the blank holder 23 while forming the structural member 10B, no cracks occurred in the structural member 10B.

[0168] In addition, in Example 12, compared with Comparative Example 5, the maximum plate thickness reduction rate of the edge of the second portion 132 of the vertical wall 13 is intentionally smaller, and the length of the deformation region R1 that satisfies the above formula (1) is ensured to be 2.0 times or more of the plate thickness t of the blank. That is, in Example 12 of forming the structural member 10B, similar to the above Examples 1 to 11 of forming the structural member 10, it is possible to suppress the reduction in the plate thickness at the edge of the second portion 132 of the vertical wall 13, and it is possible to make the plate thickness distribution at this edge uniform.

[0169] For Example 12 and Comparative Example 5 respectively, the Vickers hardness HV of the structural member 10B was measured by the same method as that of the first embodiment. In addition, for Example 12 and Comparative Example 5 respectively, the arithmetic mean roughness Ra of the edge 132a of the vertical wall 13 was calculated by the same method as that of the second embodiment.

[0170] As shown in Table 3, since the structural member 10B is formed by hot stamping, in either Example 12 or Comparative Example 5, the Vickers hardness of the structural member 10B becomes 325 Hv or more. The minimum arithmetic mean roughness Ra of Example 12 is 2.87 μm, and even the maximum does not exceed 3.00 μm. The minimum arithmetic mean roughness Ra of Comparative Example 5 is 6.41 μm.

[0171] [Fourth Embodiment]

[0172] Regarding the stamping forming of the structural member 10 of the first embodiment ( Figure 1 and Figure 2 ), while changing the range of pressing the blank by the blank holder 23, the same CAE analysis as that of the first embodiment was carried out. The analysis conditions and results are shown in Figure 14 and Table 4.

[0173]

Table 4

[0174] Table 4

[0175]

[0176] Refer to Figure 14, in Embodiments 13 and 14, between the forming steps, at least a part of region A adjacent to the corner 212A of the shoulder 212 of the lower die 21, the blank holder 23 presses the blank. In Embodiments 13 and 14, at a position outside the width direction of the lower die 21 relative to region A, the blank is not pressed by the blank holder 23. On the other hand, in Comparative Example 6, in the longitudinal direction of the lower die 21, the blank holder 23 is disposed in front of region A, and between the forming steps, the blank holder 23 does not press the blank located in region A. In Comparative Example 7, in the width direction of the lower die 21, the blank holder 23 is disposed at a position outside region A, and between the forming steps, in addition to region A, the blank holder 23 also presses the blank outside the width direction of region A.

[0177] As shown in Table 4, in Embodiments 13 and 14, the structural member 10 can be formed without generating cracks and wrinkles. In Embodiments 13 and 14, no forming defects such as necking are generated. In contrast, in Comparative Example 7, cracks are generated during the forming process of the structural member 10. In Comparative Example 6, although no cracks or necking are generated, wrinkles are generated in the structural member 10.

[0178] Therefore, by setting it as follows, it is confirmed that the formability of the structural member 10 is improved: the blank is pressed by the blank holder 23 in at least a part of region A adjacent to the corner 212A of the lower die 21, and on the other hand, in the width direction of the lower die 21, the blank is not pressed by the blank holder 23 at a position outside region A.

[0179] Description of Reference Numerals

[0180] 10, 10A, 10B, structural member; 11, top plate; 111, top plate main body; 111a, side edge; 112, protruding portion; 112a, end edge; 12, ridge line portion; 13, longitudinal wall; 132a, end edge; 14, flange; 20, 20A, die; 21, lower die; 211, top surface; 211A, top surface main body; 211a, side edge; 211B, protruding portion; 212, shoulder; 213, side surface; 214, flange surface; 215, first lower die; 216, second lower die; 22, upper die; 23, blank holder; 25, buffer mechanism.

Claims

1. A manufacturing method, which is a manufacturing method of a structural member for an automobile, and includes a heating process of heating a blank made of a metal plate, and a forming process of forming the heated blank into the structural member using a mold. The mold includes: The lower die, which includes a top surface, a shoulder, a side surface, and a flange surface, the top surface includes a top surface main body and an extending portion extending outward from a side edge of the top surface main body, the shoulder is continuous with the side edge and the extending portion of the top surface main body, the side surface is connected to the top surface main body and the extending portion by means of the shoulder, and the flange surface is connected to the side surface on the opposite side of the top surface; A blank holder, which is opposed to the top surface main body; and An upper die, which is disposed beside the blank holder, In the forming process, while clamping the blank by the top surface main body and the blank holder and not clamping the blank by the extending portion, the upper die, and the blank holder, the upper die is brought closer to the lower die relatively, and the blank is stamped by the upper die, the shoulder, the side surface, and the flange surface.

2. The manufacturing method according to claim 1, wherein, The lower die includes: a first lower die; and a second lower die, which is disposed adjacent to the first lower die and includes the extending portion, By previously generating a height difference between the first lower die and the second lower die, when the upper die is brought closer to the lower die relatively, the first lower die stamps the blank between the first lower die and the upper die prior to the second lower die.

3. The manufacturing method according to claim 2, wherein, The first lower die is configured to be able to be lifted and lowered by a buffer mechanism, In the forming process, the upper die is lowered toward the first lower die and the second lower die.

4. A structural member, which is a structural member for an automobile, and includes: A top plate, which includes a top plate main body and a protruding portion protruding outward from a side edge of the top plate main body; A ridge line portion, which is continuous with the side edge and the protruding portion of the top plate main body; A vertical wall, which is connected to the top plate main body and the extending portion by means of the ridge line portion; And A flange, which is connected to the vertical wall on the opposite side of the top plate and extends outward from the vertical wall to the outside of the structural member, When the plate thickness reduction rate based on the plate thickness of the top plate main body is set to T [%], at the edge of the portion of the vertical wall continuous with the extending portion, the length of the region having a plate thickness reduction rate T satisfying the following formula is 2.0 times or more of the plate thickness, 0.9×Tmax≤T≤Tmax, wherein, Tmax [%] is the maximum value of the plate thickness reduction rate at the edge of the portion of the vertical wall continuous with the extending portion.

5. The structural member according to claim 4, wherein, The arithmetic mean roughness Ra at the edge of the portion of the vertical wall continuous with the extending portion is 3.00 μm or less.

6. The structural member according to claim 4 or 5, wherein, This structural member has a Vickers hardness of 325 Hv or more.

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