EP4644014A1

EP4644014A1 - Structural member and method for manufacturing same

Info

Publication number: EP4644014A1
Application number: EP23912116.3A
Authority: EP
Inventors: Kenta IKEGAMI; Masahiro Kubo
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2022-12-26
Filing date: 2023-12-26
Publication date: 2025-11-05
Also published as: CN120418017A; JPWO2024143337A1; JP7525817B1; WO2024143337A1

Abstract

A method for producing a structural member (10) includes a preparing step of a starting material (M), and a forming step of subjecting the starting material (M) to cold press working by using a press tooling (20). The press tooling (20) includes a pad (23), an upper die (22), and a lower die (21). The lower die (21) includes first and second lower dies (211, 212). The forming step includes first and second steps. In the first step, in a state where a side surface (212c) of the second lower die (212) is retreated with respect to a side surface (211c) of the first lower die (211) as viewed from a pressing direction, the starting material (M) is held by the pad (23) and the first lower die (211). In the second step, the starting material (M) is sandwiched between the upper die (22) and the first lower die (211), and the second lower die (212) is moved so that the side surface (212c) is aligned with the side surface (211c), so as to sandwich the starting material (M) between the upper die (22) and the second lower die (212).

Description

TECHNICAL FIELD

The present disclosure relates to a structural member and a method for producing the same.

BACKGROUND ART

For example, a structure such as a vehicle body of an automobile and the like is constituted by a large number of structural members. As structural members for an automobile, for example, a pillar, a side member, a side sill (rocker), a cross member, a floor panel, a roof panel, and the like can be listed. Such structural members are typically produced by performing press working on a metal sheet.
For example, Patent Literature 1 discloses a method for producing a structural member having an L-shape in plan view. The structural member includes a top plate, a ridgeline portion, and a vertical wall connected to the top plate via the ridgeline portion. The ridgeline portion includes a corner portion curved along a longitudinal direction of the ridgeline portion. In the method for producing in Patent Literature 1, a metal sheet is pressed by using a press tooling that includes an upper die, a lower die, and a pad, so as to form the structural member.
For example, Patent Literature 2 discloses a method for producing a structural member having a hat shape in transverse cross-sectional view. A bead portion is formed in a vertical wall of the structural member. In the method for producing in Patent Literature 2, an intermediate formed body formed from a metal sheet is prepared, and the intermediate formed body is formed into the structural member by using a press tooling that includes an upper die, a lower die, and a slide die attached to the upper die via a cam mechanism. The lower die includes a central die and a split die attached to the central die via the cam mechanism. In Patent Literature 2, a portion corresponding to the vertical wall of the intermediate formed body is pressed by the slide die and the split die with a clamping operation of the press tooling, so that the bead portion of the vertical wall is formed.
For example, Patent Literature 3 discloses a method for producing a structural member having a T-shape in plan view. The method for producing in Patent Literature 3 includes a first forming step of forming a metal sheet into an intermediate shape member by draw forming, a trimming step of trimming the intermediate shape member, and a second forming step of forming the intermediate shape member after the trimming into a structural member by form shaping. In the second forming step, the structural member having a target shape is press-formed by a press tooling that includes an upper die and a lower die.

CITATION LIST

PATENT LITERATURE

Patent Literature 1: Japanese Patent Application Publication No. 2022-072562
Patent Literature 2: Japanese Patent Application Publication No. 2011-083807
Patent Literature 3: Japanese Patent No. 6690605

SUMMARY OF INVENTION

TECHNICAL PROBLEM

Incidentally, some structural members have a structure in which a member body and a flange are arranged with a corner portion sandwiched between them. Each of the member body and the flange includes a top plate, a ridgeline portion, and a vertical wall. The ridgeline portion and the vertical wall of the flange are connected to the ridgeline portion and the vertical wall of the member body via the corner portion.
When producing such a structural member by press working, cracks may be generated in the flange (continuous flange) continuously provided to the member body via the corner portion. In particular, when the curvature radius of the corner portion is small, cracks in the continuous flange are easily generated in press working. Cracks of the continuous flange during press working are more easily generated when the structural member is formed from a high-strength metal sheet.
An object of the present disclosure is to provide a method for producing a structural member capable of suppressing the generation of cracks in a continuous flange.

SOLUTION TO PROBLEM

A method for producing a structural member according to the present disclosure includes a preparing step of preparing a starting material made of a metal sheet, and a forming step of performing forming by subjecting the starting material to cold press working by using a press tooling. The press tooling includes a pad, an upper die, and a lower die. The lower die includes a first lower die, a second lower die, and a corner portion. The first lower die includes a first crest, a first shoulder portion, and a first side surface. The first crest intersects a pressing direction. The first shoulder portion extends along an end edge of the first crest. The first side surface is connected to the first crest via the first shoulder portion. The second lower die incudes a second crest, a second shoulder portion, and a second side surface. The second crest intersects the pressing direction. The second shoulder portion extends along an end edge of the second crest. The second side surface is connected to the second crest via the second shoulder portion. The second lower die is a separate body from the first lower die. The corner portion is provided between the first shoulder portion and the first side surface, and the second shoulder portion and the second side surface. The corner portion has a shape recessed inwardly of the lower die as viewed from the pressing direction. The forming step includes a first step and a second step. In the first step, in a state where the first lower die and the second lower die are arranged so that the second side surface is at a position retreated with respect to the first side surface as viewed from the pressing direction, the pad and the upper die are relatively brought closer to the lower die in the pressing direction, and the starting material is sandwiched between the pad and the first lower die to be held. In the second step, in the state where the starting material is held by the pad and the first lower die, the upper die is further relatively brought closer to the lower die in the pressing direction to sandwich the starting material between the upper die and the first lower die, and the second lower die is moved so that the second side surface is aligned with the first side surface as viewed from the pressing direction, so as to sandwich the starting material between the upper die and the second lower die.

ADVANTAGEOUS EFFECTS OF INVENTION

With a method for producing a structural member according to the present disclosure, the generation of cracks in a continuous flange can be suppressed.

BRIEF DESCRIPTION OF DRAWINGS

[FIG. 1] FIG. 1 is a perspective view of a structural member according to a first embodiment.
[FIG. 2] FIG. 2 is a plan view of the structural member illustrated in FIG. 1.
[FIG. 3] FIG. 3 is a side view of the structural member illustrated in FIG. 1.
[FIG. 4A] FIG. 4A is a schematic diagram for describing a method for producing the structural member according to the first embodiment.
[FIG. 4B] FIG. 4B is a schematic diagram for describing the method for producing the structural member according to the first embodiment.
[FIG. 4C] FIG. 4C is a schematic diagram for describing the method for producing the structural member according to the first embodiment.
[FIG. 4D] FIG. 4D is a schematic diagram for describing the method for producing the structural member according to the first embodiment.
[FIG. 4E] FIG. 4E is a schematic diagram for describing the method for producing the structural member according to the first embodiment.
[FIG. 4F] FIG. 4F is a schematic diagram for describing the method for producing the structural member according to the first embodiment.
[FIG. 4G] FIG. 4G is a schematic diagram for describing the method for producing the structural member according to the first embodiment.
[FIG. 5] FIG. 5 is a partial enlarged view of the structural member illustrated in FIG. 1.
[FIG. 6] FIG. 6 is a perspective view of a structural member according to a second embodiment.
[FIG. 7] FIG. 7 is a side view of the structural member illustrated in FIG. 6.
[FIG. 8] FIG. 8 is a schematic diagram of a structural member for describing conditions of analysis.
[FIG. 9] FIG. 9 is a schematic diagram of a structural member for describing conditions of analysis.
[FIG. 10] FIG. 10 is a schematic diagram of a structural member for describing conditions of analysis.
[FIG. 11] FIG. 11 is a graph illustrating the relationship between the distance from an edge of a continuous flange and the Vickers hardness for a structural member having a tensile strength of 1180 MPa.

DESCRIPTION OF EMBODIMENTS

A method for producing a structural member according to an embodiment includes a preparing step of preparing a starting material made of a metal sheet, and a forming step of performing forming by subjecting a starting material to cold press working by using a press tooling. The press tooling includes a pad, an upper die, and a lower die. The lower die includes a first lower die, a second lower die, and a corner portion. The first lower die includes a first crest, a first shoulder portion, and a first side surface. The first crest intersects a pressing direction. The first shoulder portion extends along an end edge of the first crest. The first side surface is connected to the first crest via the first shoulder portion. The second lower die includes a second crest, a second shoulder portion, and a second side surface. The second crest intersects the pressing direction. The second shoulder portion extends along an end edge of the second crest. The second side surface is connected to the second crest via the second shoulder portion. The second lower die is a separate body from the first lower die. The corner portion is provided between the first shoulder portion and the first side surface, and the second shoulder portion and the second side surface. The corner portion has a shape recessed inwardly of the lower die as viewed from the pressing direction. The forming step includes a first step and a second step. In the first step, in a state where the first lower die and the second lower die are arranged so that the second side surface is at a position retreated with respect to the first side surface as viewed from the pressing direction, the pad and the upper die are relatively brought closer to the lower die in the pressing direction, and a starting material is sandwiched between the pad and the first lower die to be held. In the second step, in the state where the starting material is held by the pad and the first lower die, the upper die is further relatively brought closer to the lower die in the pressing direction to sandwich the starting material between the upper die and the first lower die, and the second lower die is moved so that the second side surface is aligned with the first side surface as viewed from the pressing direction, so as to sandwich the starting material between the upper die and the second lower die (a first configuration).
In the method for producing according to the first configuration, a structural member is formed from a starting material by cold press working by using the press tooling that includes the pad, the upper die, and the lower die. The lower die includes the second lower die arranged with the corner portion sandwiched between the first lower die and the second lower die. At the start of the forming step, the second lower die is arranged at a position retreated from the first lower die as viewed from the pressing direction. Therefore, up to the middle of the forming step, a portion of the starting material that is eventually formed by the second lower die, in other words, a flange (continuous flange) continuously provided to a member body via a corner portion of the structural member, is not restrained by the second lower die. Since the upper die is relatively brought closer to the lower die including the second lower die in the pressing direction to apply tension to the starting material in the state where the second lower die does not restrain the continuous flange, the inflow of a material from the crest side to the side surface side of the second lower die is facilitated at the position of the continuous flange. In this way, distortion generated in the continuous flange can be dispersed. Accordingly, for example, even when the curvature radius of the corner portion is small, or when the strength of the starting material is high, the generation of cracks in the continuous flange can be suppressed.
For example, in the forming step, when the generation of cracks is expected in the continuous flange, particularly an end edge of the continuous flange, in order to prevent the generation of cracks, the forming step may be performed in a state where a surplus portion is provided in advance to a metal sheet (blank), and a trimming step may be performed after the forming step to remove the unnecessary surplus portion. On the other hand, in the method for producing according to the first configuration, cracks in the continuous flange can be suppressed by not restraining the continuous flange in an initial stage of the forming step. Therefore, no surplus portion is required, and the trimming step of the surplus portion after the forming step can be omitted. In a case where the trimming step of the surplus portion is omitted, as compared with the case where the trimming step is performed, the amount of the starting material to be input in the forming step can be reduced. Thus, the yield in the production of the structural member can be improved. In addition, since the input amount of the starting material is reduced, and the trimming step is not performed, the transport amount, the electricity amount, and the like in the production of the structural member are reduced, and therefore, the emission amount of greenhouse gas can also be reduced.
In the method for producing according to the first configuration, a moved distance of the second lower die in the second step is preferably 9.0 times or less of a thickness of the starting material (a second configuration).
In the first step of the forming, the second lower die is arranged at the position retreated with respect to the first lower die, and is moved to be aligned with the position of the first lower die in the second step of the forming. In the second configuration, the moved distance of the second lower die is 9.0 times or less of the thickness of the starting material. In this case, in the forming step, the generation of wrinkles in the structural member can be suppressed.
In the method for producing according to the first or second configuration, the second lower die may further include at least a part of the corner portion. In this case, the second shoulder portion and the second side surface are continuous with the corner portion (a third configuration).
In the third configuration, at least a part of the corner portion of the lower die is included in the second lower die that is arranged at the position retreated with respect to the first lower die at the start of the forming step. In this case, in addition to the continuous flange, at least a part of the corner portion adjacent to the continuous flange is not restrained up to the middle of the forming step. As a result, as compared with a case where, for example, the entire corner portion is included in the first lower die, and the entire corner portion is restrained by the first lower die from an early stage of the forming step, the bending of the continuous flange in a middle stage of the forming step becomes gentle, and the distortion of the continuous flange is reduced. Since the second lower die is moved in a later stage of the forming step, the continuous flange and the corner portion are formed by being bent up by the second lower die with the flow of the material. Accordingly, it becomes further difficult for cracks to be generated in the continuous flange.
In the method for producing according to any one of the first to third configurations, the pad can further hold a portion of the starting material adjacent to the corner portion on a bending outer side of the corner portion (a fourth configuration).
In the fourth configuration, in the forming step, the pad holds the starting material with the first lower die, and further holds the starting material in the vicinity of the corner portion between the first lower die and the second lower die. In this case, it is possible to prevent wrinkles from being generated in the starting material at the position of the corner portion.
In the method for producing according to any one of the first to fourth configurations, the second crest may be inclined with respect to a surface perpendicular to the pressing direction, so that a distance from the second shoulder portion and the second side surface in the pressing direction is increased from an end edge of the second shoulder portion side toward an end edge on an opposite side of the second shoulder portion (a fifth configuration).
In the fifth configuration, the crest of the second lower die is inclined with respect to the surface perpendicular to the pressing direction. The crest of the second lower die is inclined with respect to the surface perpendicular to the pressing direction, so as to be lower on the shoulder portion side and higher on the opposite side of the shoulder portion. In this case, in the forming step, it becomes easier for the material to flow from the crest side of the second lower die to the side surface side through the shoulder portion. Thus, the generation of local distortion and cracks can be further suppressed in the continuous flange formed by the second lower die.
A structural member according to an embodiment includes a member body and a flange. The member body includes a first top plate, a first ridgeline portion, and a first vertical wall. The first ridgeline portion extends along an end edge of the first top plate. The first vertical wall is connected to the first top plate via the first ridgeline portion. The flange includes a second top plate, a second ridgeline portion, and a second vertical wall. The second ridgeline portion extends along an end edge of the second top plate. The second vertical wall is connected to the second top plate via the second ridgeline portion. The second ridgeline portion and the second vertical wall are connected to the first ridgeline portion and the first vertical wall via a corner portion. The second top plate is continuous with the first top plate on the bending outer side of the corner portion. In a cross section of the flange taken along a thickness direction of the second top plate at a border between the second top plate and the second ridgeline portion, a difference between a Vickers hardness measured at an end portion on an opposite side of the corner portion and a Vickers hardness measured at an end portion on the corner portion side is 30 Hv or less (a sixth configuration).
In the structural member according to the sixth configuration, the hardness of the flange (continuous flange) continuously provided to the member body via the corner portion is uniformized. More particularly, in a cross section of the continuous flange at the border between the top plate and the ridgeline portion, the difference between the Vickers hardness measured at the end portion on the opposite side of the corner portion and the Vickers hardness measured at the end portion on the corner portion side is 30 Hv or less. This means that the distortion generated at the time of forming of the structural member is dispersed along the ridgeline portion in the continuous flange. In this case, when load is input to the structural member, and additional distortion is generated in the edge of the continuous flange to which the distortion is relatively concentrated, it is possible to suppress the generation of cracks in the continuous flange starting from this edge. Accordingly, when the structural member is used for a vehicle body of an automobile, it is possible to allow the structural member to exhibit excellent collision resistance.
The structural member according to the sixth configuration may be formed of a steel sheet having a tensile strength of 590 MPa or more (a seventh configuration).
In the structural member according to the sixth or seventh configuration, the corner portion may have a curvature radius of 100 mm or less as viewed from the first top plate side (an eighth configuration).
In the structural member according to any one of the sixth to eighth configurations, the first top plate can include a flat surface on its surface. The second top plate may be inclined with respect to the flat surface, so that a distance from the second ridgeline portion and the second vertical wall in a direction perpendicular to the flat surface is increased from an end edge on the second ridgeline portion side toward an end edge on an opposite side of the second ridgeline portion (a ninth configuration).
Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. In the drawings, the same or corresponding configurations are denoted by the same numerals, and the same description will not be repeated.

[Configuration of Structural Member]

FIG. 1 is a perspective view of a structural member 10 according to a first embodiment. Fig. 2 is a plan view of the structural member 10. The structural member 10 is typically used for a vehicle body of an automobile. Although not particularly limited, the structural member 10 may be, for example, a rocker rear inner.
Referring to FIG. 1, the structural member 10 is formed of a metal sheet. The structural member 10 is formed of, for example, a steel sheet. The steel sheet preferably has a tensile strength of 590 MPa or more. The steel sheet more preferably has a tensile strength of 980 MPa or more, and further preferably has a tensile strength of 1180 MPa or more. The thickness of the structural member 10 is, for example, 0.8 mm or more, and is preferably 1.0 mm or more. The thickness of the structural member 10 is, for example, 4.0 mm or less, and preferably 3.0 mm or less.
The structural member 10 includes a member body 11, a flange 12, and a corner portion 13. The member body 11, the flange 12, and the corner portion 13 are integrally formed. The corner portion 13 is arranged between the member body 11 and the flange 12. That is, the flange 12 is continuously provided to the member body 11 via the corner portion 13. Hereinafter, the flange 12 is referred to as the continuous flange 12.
Referring to FIG. 1 and FIG. 2, the member body 11 has a transverse cross section of, for example, a substantially hat shape. The member body 11 includes a top plate 111, ridgeline portions 112 and 113, and vertical walls 114 and 115. The member body 11 further includes flange parts 116 and 117.
The top plate 111 includes a flat surface 111a on its surface. The flat surface 111a is a portion of the top plate 111 that serves as a reference surface for work, for example, when attaching or assembling the structural member 10 to the vehicle body of the automobile. The flat surface 111a has a flat shape, and does not substantially include a curved surface. In an example of the present embodiment, the periphery of a through-hole 111b penetrating the top plate 111 in a thickness direction serves as the flat surface 111a.
The ridgeline portions 112 and 113 are provided on both sides of the top plate 111. The ridgeline portion 112 extends along an end edge of the top plate 111. In the example of the present embodiment, the ridgeline portion 112 includes a curved portion in plan view of the structural member 10, that is, when the structural member 10 is viewed from the top plate 111 side. The ridgeline portion 113 extends along an end edge of the top plate 111 on the opposite side of the ridgeline portion 112. In the example illustrated in FIG. 1 and FIG. 2, the distance between the ridgeline portions 112 and 113 in plan view of the structural member 10, that is, the width of the top plate 111, is smaller on one side of the longitudinal direction of the member body 11, and is larger on the other side. Each of the ridgeline portions 112 and 113 can have a substantially arc shape as viewed in a cross section (transverse cross section) perpendicular to its extending direction.
The vertical wall 114 is connected to the top plate 111 via the ridgeline portion 112. The vertical wall 115 is connected to the top plate 111 via the ridgeline portion 113 on the opposite side of the vertical wall 114. The vertical walls 114 and 115 may stand perpendicular to the flat surface 111a of the top plate 111, or may be inclined with respect to a direction perpendicular to the flat surface 111a. The vertical walls 114 and 115 may be, for example, spaced apart from each other as they are distant from the top plate 111.
The flange part 116 is connected to one vertical wall 114 on the opposite side of the top plate 111. The flange part 117 is connected to the other vertical wall 115 on the opposite side of the top plate 111. The flange parts 116 and 117 protrude toward the outside of the structural member 10 from the vertical walls 114 and 115, respectively. The flange parts 116 and 117 extend along the vertical walls 114 and 115, respectively.
Still referring to FIG. 1 and FIG. 2, the continuous flange 12 is connected to the member body 11 via the corner portion 13. The corner portion 13 has an inwardly recessed curved shape in plan view of the structural member 10. The corner portion 13 may have an arc shape in plan view of the structural member 10. The curvature radius of the corner portion 13 is, for example, 100 mm or less. The curvature radius of the corner portion 13 is preferably 50 mm or less, and is more preferably 30 mm or less. The curvature radius of the corner portion 13 is preferably 1 mm or more, although the lower limit value is not particularly set.
The continuous flange 12 includes a top plate 121, a ridgeline portion 122, and a vertical wall 123. The continuous flange 12 further includes a flange part 124.
The top plate 121 is continuous with the top plate 111 of the member body 11 on the bending outer side of the corner portion 13. The ridgeline portion 122 extends along one end edge of this top plate 121. The ridgeline portion 122 can have a substantially arc shape as viewed in a cross section (transverse cross section) perpendicular to its extending direction. The ridgeline portion 122 is connected to one ridgeline portion 112 of the member body 11 via the corner portion 13. The ridgeline portion 122 of the continuous flange 12 is bent with respect to the ridgeline portion 112 of the member body 11 in plan view of the structural member 10.
At least a portion of the ridgeline portion 112 of the member body 11 adjacent to the corner portion 13 has a substantially straight shape in plan view of the structural member 10. Similarly, at least a portion of the ridgeline portion 122 of the continuous flange 12 adjacent to the corner portion 13 has a substantially straight shape in plan view of the structural member 10. In the present embodiment, "a substantially straight shape" includes not only a perfectly straight line, but also a curved line that can be considered to be a straight line due to its large curvature radius. For example, a curved line extending with a curvature radius of 200 mm or more is considered to be a straight line. Although not particularly limited, the extension length of the ridgeline portion 122 in plan view of the structural member 10 may be, for example, 15 mm or more.
The vertical wall 123 is connected to the top plate 121 via the ridgeline portion 122. The vertical wall 123 is connected to one vertical wall 114 of the member body 11 via the corner portion 13. The vertical wall 123 of the continuous flange 12 is bent with respect to the vertical wall 114 of the member body 11 in plan view of the structural member 10.
The flange part 124 is connected to the vertical wall 123 on the opposite side of the top plate 121. The flange part 124 protrudes toward the outside of the structural member 10 from the vertical wall 123. The flange part 124 is continuous with one flange part 116 of the member body 11.
FIG. 3 is a side view of the structural member 10. FIG. 3 illustrates the view of the structural member 10 as viewed from the continuous flange 12 side. As illustrated in FIG. 3, the top plate 121 of the continuous flange 12 is inclined with respect to a horizontal surface in side view of the structural member 10. The horizontal surface is the same flat surface as the flat surface 111a of the top plate 111 of the member body 11, or a flat surface parallel to the flat surface 111a. In addition, a direction perpendicular to such a flat surface is referred to as a vertical direction.
In the example of the present embodiment, the top plate 121 of the continuous flange 12 is inclined with respect to the horizontal surface, so that the distance from the ridgeline portion 122 and the vertical wall 123 in the vertical direction is increased from an end edge on the ridgeline portion 122 side toward an end edge (free end edge) on the opposite side. In side view of the structural member 10, the surface of the top plate 121 is an inclined surface that rises as it approaches the free end edge. However, the top plate 121 may be substantially horizontal in side view of the structural member 10.
Similar to the top plate 121 of the continuous flange 12, a portion of the top plate 111 of the member body 11 that is continuous with at least the top plate 121 of the continuous flange 12 can be inclined with respect to the horizontal surface. In the example of the present embodiment, the top plate 111 of the member body 11 is inclined with respect to the horizontal surface, so as to rise from the vicinity of the middle of its longitudinal direction toward the continuous flange 12 side. However, when the top plate 121 of the continuous flange 12 is substantially horizontal, the portion of the top plate 111 of the member body 11 that is continuous with the top plate 121 of the continuous flange 12 can also be substantially horizontal.

[Method for Producing Structural Member]

Hereinafter, a method for producing the structural member 10 will be described with reference to FIG. 4A to FIG. 4G. FIG. 4A to FIG. 4G are schematic diagrams for describing the method for producing the structural member 10. The method for producing the structural member 10 includes a preparing step and a forming step. The structural member 10 is produced by cold press forming.

(Preparing Step)

The preparing step is a step of preparing a starting material made of a metal sheet. The starting material may be a steel sheet having a tensile strength of, for example, 590 MPa or more, preferably 980 MPa or more, and more preferably 1180 MPa or more. The starting material may be, for example, a blank having a developed shape of the structural member 10 (FIG. 1 to FIG. 3). Such a blank can be formed by performing punching processing on a metal strip (coil) by using a die having a target shape. Alternatively, the blank may be formed by performing hollowing processing on a coil by laser. The starting material may be, for example, a preformed body formed by performing preforming on a blank.
Note that, it is possible to confirm that a steel sheet having a tensile strength of 590 MPa or more, preferably 980 MPa or more, and more preferably 1180 MPa or more has been used as the starting material, by taking a full thickness flat-sheet tensile test specimen from the top plate 111 of the structural member 10 (FIG. 1 to FIG. 3), and performing a tensile test. It is because, in the top plate 111 of the structural member 10, the influence of plastic deformation can be ignored, and the state of the starting material immediately before a producing step of the structural member 10 is exhibited.

(Forming Step)

In the forming step, forming is performed by subjecting the starting material to cold press working by using the press tooling 20 illustrated in FIG. 4A to FIG. 4C. First, the configuration of the press tooling 20 will be described.
In the present embodiment, a description will be mainly given of a portion of the press tooling 20 that forms one side (the continuous flange 12 side) of the structural member 10 in the width direction. A description will be omitted in the present embodiment for a portion of the press tooling 20 that forms the other side (the opposite side of the continuous flange 12) of the structural member 10 in the width direction, since the portion is not particularly different from a common press tooling for press forming of a structural member having, for example, a hat shape in transverse cross-sectional view.
FIG. 4A is a perspective view of the press tooling 20. As illustrated in FIG. 4A, the press tooling 20 includes a lower die 21, an upper die 22, and a pad 23. The lower die 21 is a punch, and the upper die 22 is a die corresponding to the lower die 21. When starting the forming step, the lower die 21 is arranged opposite the upper die 22 and the pad 23. The lower die 21 is arranged, for example, below the upper die 22 and the pad 23. The lower die 21, the upper die 22, and the pad 23 are attached to, for example, a known pressing machine (illustration is omitted). The upper die 22 and the pad 23 can relatively approach the lower die 21. Hereinafter, the direction in which the lower die 21 relatively approaches the upper die 22 and the pad 23 is referred to as a pressing direction.
The lower die 21 includes a first lower die 211 and a second lower die 212. The first lower die 211 is a portion of the lower die 21 for mainly forming the member body 11 of the structural member 10 (FIG. 1 to FIG. 3). The second lower die 212 is a portion of the lower die 21 for mainly forming the continuous flange 12 of the structural member 10 (FIG. 1 to FIG. 3). The second lower die 212 is a separate body from the first lower die 211.
The first lower die 211 includes a crest 211a, a shoulder portion 211b, and a side surface 211c. The first lower die 211 further includes a flange surface 211d.
The crest 211a is a surface that intersects the pressing direction. The crest 211a is a surface for mainly forming the top plate 111 of the member body 11 (FIG. 1 to FIG. 3). Therefore, the crest 211a has a shape corresponding to the top plate 111.
The shoulder portion 211b extends along an end edge of the crest 211a. The shoulder portion 211b is a surface for mainly forming the ridgeline portion 112 of the member body 11 (FIG. 1 to FIG. 3). Therefore, the shoulder portion 211b has a shape corresponding to the ridgeline portion 112.
The side surface 211c is connected to the crest 211a via the shoulder portion 211b. The side surface 211c is a surface for mainly forming the vertical wall 114 of the member body 11 (FIG. 1 to FIG. 3). Therefore, the side surface 211c has a shape corresponding to the vertical wall 114.
The flange surface 211d is connected to the side surface 211c on the opposite side of the crest 211a. The flange surface 211d is a surface for mainly forming the flange part 116 of the member body 11 (FIG. 1 to FIG. 3) and the flange part 124 of the continuous flange 12 (FIG. 1 to FIG. 3). Therefore, the flange surface 211d has a shape corresponding to the flange parts 116 and 124.
The second lower die 212 includes a crest 212a, a shoulder portion 212b, and a side surface 212c.
The crest 212a is a surface that intersects the pressing direction. The crest 212a is a surface for mainly forming the top plate 121 of the continuous flange 12 of the structural member 10 (FIG. 1 to FIG. 3). Therefore, the crest 212a has a shape corresponding to the top plate 121.
The shoulder portion 212b extends along an end edge of the crest 212a. The shoulder portion 212b is a surface for mainly forming the ridgeline portion 122 of the continuous flange 12 (FIG. 1 to FIG. 3). Therefore, the shoulder portion 212b has a shape corresponding to the ridgeline portion 122.
The side surface 212c is connected to the crest 212a via the shoulder portion 212b. The side surface 212c is a surface for mainly forming the vertical wall 123 of the continuous flange 12 (FIG. 1 to FIG. 3). Therefore, the side surface 212c has a shape corresponding to the vertical wall 123.
The lower die 21 further includes a corner portion 213. The corner portion 213 has a shape recessed inwardly of the lower die 21 as viewed from the pressing direction. The corner portion 213 is arranged between the shoulder portion 211b and the side surface 211c of the first lower die 211, and the shoulder portion 212b and the side surface 212c of the second lower die 212. The shoulder portion 211b and the side surface 211c of the first lower die 211 are adjacent to the corner portion 213. The shoulder portion 212b and the side surface 212c of the second lower die 212 are adjacent to the corner portion 213 on the opposite side of the shoulder portion 211b and the side surface 211c of the first lower die 211. When viewed from the pressing direction, the shoulder portion 212b and the side surface 212c of the second lower die 212 are arranged to be bent with respect to the shoulder portion 211b and the side surface 211c of the first lower die 211.
As viewed from the pressing direction, the corner portion 213 has a curvature radius of, for example, 100 mm or less. As viewed from the pressing direction, the curvature radius of the corner portion 213 is preferably 50 mm or less, and is more preferably 30 mm or less. The curvature radius of the corner portion 213 is preferably 1 mm or more, although the lower limit value is not particularly set. At least a portion of the shoulder portion 211b of the first lower die 211 adjacent to the corner portion 213 has a substantially straight shape as viewed from the pressing direction. Similarly, at least a portion of the shoulder portion 212b of the second lower die 212 adjacent to the corner portion 213 has a substantially straight shape as viewed from the pressing direction.
FIG. 4B is a diagram of the lower die 21 as viewed along the pressing direction from the crests 211a and 212a side. Referring to FIG. 4B, at least a part of the corner portion 213 is preferably included in the second lower die 212 corresponding to the continuous flange 12 (FIG. 1 to FIG. 3). In other words, when the lower die 21 is viewed along the pressing direction, a dividing line 214 between the first lower die 211 and the second lower die 212 is preferably positioned in the range from an end (R stop) on the second lower die 212 side to an end (R stop) on the first lower die 211 side of the corner portion 213. As viewed from the pressing direction, the dividing line 214 may, for example, intersect the corner portion 213 in the range from the center of the corner portion 213 in the extending direction to the end (R stop) on the first lower die 211 side. However, the dividing line 214 may intersect the corner portion 213 on the second lower die 212 side than the center of the extending direction of the corner portion 213 as viewed from the pressing direction. When viewed from the pressing direction, the dividing line 214 may, for example, extend substantially parallel to the edge of the continuous flange 12 (FIG. 1 to FIG. 3).
The lower die 21 is divided into the first lower die 211 and the second lower die 212 at the position of the dividing line 214. Before starting the forming step, the second lower die 212 is retreated with respect to the first lower die 211. More particularly, as viewed from the pressing direction, the first lower die 211 and the second lower die 212 are arranged so that the shoulder portion 212b and the side surface 212c of the second lower die 212 are at positions retreated with respect to the positions of the shoulder portion 211b and the side surface 211c of the first lower die 211.
Returning to FIG. 4A, the upper die 22 and the pad 23 are arranged so as to face the lower die 21 in the pressing direction. The upper die 22 is attached to, for example, a slide (illustration is omitted) that can be lifted and lowered in the pressing machine. The pad 23 is connected to the slide via, for example, a stretchable elastic member (illustration is omitted). The pad 23 mainly faces the crest 211a of the first lower die 211. A forming surface 221 of the upper die 22 mainly faces the crest 212a of the second lower die 212. The forming surface 221 of the upper die 22 has a shape corresponding to the shoulder portion 211b, the side surface 211c, and the flange surface 211d of the first lower die 211, the crest 212a, the shoulder portion 212b, and the side surface 212c of the second lower die 212, and the corner portion 213.
FIG. 4C is a diagram of the upper die 22 and the pad 23 as viewed along the pressing direction. Referring to FIG. 4C, when viewed from the pressing direction, the dividing line 24 between the upper die 22 and the pad 23 is preferably arranged at the position of the corner portion 213 (FIG. 4B) or in the vicinity of the corner portion 213. As viewed from the pressing direction, the dividing line 24 between the upper die 22 and the pad 23 may be at the same position as the dividing line 214 between the first lower die 211 and the second lower die 212 (FIG. 4B), or may be arranged to be shifted from the dividing line 214 to the first lower die 211 side or the second lower die 212 side. Similar to the dividing line 214 of the lower die 21, the dividing line 24 is, for example, positioned in the range from the end (R stop) on the second lower die 212 side to the end (R stop) on the first lower die 211 side of the corner portion 213. However, the dividing line 24 may be arranged on the second lower die 212 side slightly over the corner portion 213. The dividing line 24 may extend substantially in the normal direction of the corner portion 213 or the shoulder portion 212b of the second lower die 212 as viewed from the pressing direction.
In the forming step, forming is performed by subjecting the starting material to cold press working by using the press tooling 20 configured in this manner. The forming step includes a first step and a second step.
Referring to FIG. 4A to FIG. 4C, when starting the forming step, the upper die 22 and the pad 23 attached to the slide (illustration is omitted) of the pressing machine are located at a top dead center. At this time, the second lower die 212 is arranged at a position retreated from the first lower die 211 as viewed from the pressing direction.
FIG. 4D is a diagram schematically illustrating an IVD-IVD cross section of FIG. 4C. As illustrated in FIG. 4D, before starting the forming step, the second lower die 212 is arranged spaced apart from the flange surface 211d of the first lower die 211 in the horizontal direction. In cross-sectional view of the press tooling 20, the second lower die 212 is arranged with a gap G between the first lower die 211 and the second lower die 212. The second lower die 212 is attached to the pressing machine (illustration is omitted) via a cam mechanism 25. The cam mechanism 25 includes a cam driver 251 for moving the second lower die 212.
Referring to FIG. 4A to FIG. 4D, in the first step of the forming, in the arrangement state of the second lower die 212 as described above, the upper die 22 and the pad 23 are relatively brought closer to the lower die 21 in the pressing direction, and a starting material M arranged between the upper die 22 and the pad 23, and the lower die 21 is sandwiched between by the pad 23 and the first lower die 211 to be held. The pad 23 holds the starting material M prior to the upper die 22. While the pad 23 holds the starting material M with the crest 211a of the first lower die 211, the pad 23 does not substantially holds the starting material M between the pad 23 and the crest 212a of the second lower die 212. That is, although a portion of the starting material M located mainly on the crest 211a of the first lower die 211 is held by the pad 23, a portion of the starting material M located on the crest 212a of the second lower die 212 is not held by the pad 23 at all or hardly held by the pad 23.
In the second step of the forming, in the state where the starting material M is held by the pad 23 and the first lower die 211, the upper die 22 is further relatively brought closer to the lower die 21 in the pressing direction to sandwich the starting material M between the upper die 22 and the first lower die 211, and the second lower die 212 is moved so that the side surface 212C of the second lower die 212 is aligned with the side surface 211c of the first lower die 211 as viewed from the pressing direction, so as to sandwich the starting material M between the upper die 22 and the second lower die 212. The state where the side surface 212c of the second lower die 212 is aligned with the side surface 211c of the first lower die 211 refers to a state where the difference in height (step height) between the adjacent side surfaces 211c and 212c is, for example, 0.5 mm or less.
More particularly, while holding the starting material M on the crest 211a of the first lower die 211 by the pad 23, the upper die 22 is further relatively brought closer to the first lower die 211 and the second lower die 212 in the pressing direction. Accordingly, as illustrated in FIG. 4E, the starting material M is pressed down by the upper die 22. In an early stage of the second step, the starting material M is not sandwiched between the upper die 22 and the pad 23, and the second lower die 212. Therefore, at the position of the second lower die 212, since the starting material M is pressed down by the upper die 22, and is pulled to the flange surface 211d side, the inflow of the material from the crest 212a side to the side surface 212c side is facilitated.
As illustrated in FIG. 4F, at an end stage of the second step, the cam driver 251 of the cam mechanism 25 is actuated. For example, as the cam driver 251 itself is lowered in the pressing direction, the cam driver 251 moves the second lower die 212 in a direction perpendicular to the pressing direction. For example, immediately before the upper die 22 reaches a bottom dead center, the second lower die 212 starts movement by the cam mechanism 25. As illustrated in FIG. 4G, for example, at the same time that the upper die 22 reaches the bottom dead center, the second lower die 212 reaches the position of the first lower die 211 and is stopped, and the second lower die 212 sandwiches the starting material M with the upper die 22. At the bottom dead center, the gap between the flange surface 211d of the first lower die 211 and the second lower die 212 substantially disappears. In addition, at the bottom dead center, the gap between the upper die 22 and the starting material M, and the gap between the starting material M, and the first lower die 211 and the second lower die 212 substantially disappear, and the starting material M is pressed by the upper die 22 and the lower die 21. Accordingly, the structural member 10 (FIG. 1 to FIG. 3) is formed.
The moved distance (cam stroke) of the second lower die 212 in the second step of the forming is preferably 9.0 times or less of the thickness of the starting material M. The moved distance of the second lower die 212 may be, for example, 2.0 times or more of the thickness of the starting material M. The thickness of the starting material M is, for example, 0.8 mm or more and 4.0 mm or less, and is preferably 1.0 mm or more and 3.0 mm or less. The moved distance of the second lower die 212 is equal to the gap G between the second lower die 212 and the first lower die 211 at the time of starting the forming (FIG. 4D).
The forming step described in the present embodiment may be included in the producing process until the structural member 10 obtains the shape as a final product. When a plurality of forming steps are included in the producing process, the forming step described in the present embodiment may be the first forming step, may be a middle forming step, or may be the last forming step.

[Effects]

In the method for producing the structural member 10 according to the present embodiment, when starting the forming step, the first lower die 211 and the second lower die 212 are arranged, so that the position of the side surface 212c of the second lower die 212 is retreated with respect to the position of the side surface 211c of the first lower die 211 as viewed from the pressing direction. Therefore, when the upper die 22 and the pad 23 are relatively brought closer to the first lower die 211 and the second lower die 212, the starting material M is not restrained at the position of the second lower die 212. The second lower die 212 starts sandwiching of the starting material M with the upper die 22 in the later stage of the forming step to form the continuous flange 12. In this manner, since the continuous flange 12 is not restrained until the later stage of the forming step, the flow of material in the continuous flange 12 is promoted. More specifically, since the upper die 22 presses down the continuous flange 12 to apply tension in the state where the second lower die 212 does not restrain the continuous flange 12, the inflow of the material from the top plate 121 of the continuous flange 12 to the vertical wall 123 can be promoted. Thus, the distortion generated in the continuous flange 12 can be dispersed, and the generation of cracks in the continuous flange 12 can be suppressed.
In the present embodiment, in the second step of the forming, the second lower die 212 is moved from the position at which the side surface 212c of the second lower die 212 is retreated with respect to the side surface 211c of the first lower die 211 to the position at which the side surface 212c of the second lower die 212 is aligned with the side surface 211c as viewed from the pressing direction. The moved distance of this second lower die 212 is preferably 9.0 times or less of the thickness of the starting material M. Accordingly, in the second step of the forming, the generation of wrinkles in the structural member 10 can be suppressed.
In the present embodiment, the second lower die 212 for forming the continuous flange 12 preferably includes at least a part of the corner portion 213. That is, the dividing line 214 between the first lower die 211 and the second lower die 212 is preferably arranged on the first lower die 211 side with respect to the end (R stop) on the second lower die 212 side of the corner portion 213. By including at least a part of the corner portion 213 in the second lower die 212, the restraint of the starting material M at the position of the corner portion 213 is reduced. That is, since the part of the corner portion 213 that has become a part of the second lower die 212 does not restrain the starting material M until the later stage of the forming step, the bending of the continuous flange 12 in the middle of the forming step becomes gentle, and the distortion of the continuous flange 12 is reduced. Accordingly, it becomes further difficult for cracks to be generated in the continuous flange 12.
In the present embodiment, the pad 23 is preferably configured so as to hold the starting material M on the bending outer side of the corner portion 213 of the lower die 21. In this case, it is possible to prevent wrinkles from being generated in the starting material M at the position of the corner portion 213.
In the structural member 10 according to the present embodiment, the top plate 121 of the continuous flange 12 is inclined with respect to the flat surface 111a of the top plate 111 of the member body 11. In the press tooling 20 for forming this structural member 10, the crest 212a of the second lower die 212 corresponding to the top plate 121 of the continuous flange 12 is also inclined with respect to a surface perpendicular to the pressing direction. The crest 212a of the second lower die 212 is inclined with respect to the surface perpendicular to the pressing direction, so as to be lower on the shoulder portion 212b side, and higher on the opposite side of the shoulder portion 212b. In this case, in the forming step, it becomes further easier for the material to flow from the crest 212a side to the shoulder portion 212b and the side surface 212c side of the second lower die 212, and the generation of local distortion and cracks can be further suppressed in the continuous flange 12.
In the structural member 10 according to the present embodiment, the continuous flange 12 is provided continuously with the member body 11. By providing the continuous flange 12, the impact absorbing capacity of the structural member 10 can be improved. In addition, in the present embodiment, the continuous flange 12 is connected to the member body 11 by the corner portion 13 having a relatively small curvature radius of, for example, 100 mm or less. Accordingly, since a relatively large flat surface portion is secured in the continuous flange 12, for example, it becomes easy to form points for spot welding in the continuous flange 12, which is advantageous when joining the continuous flange 12 to another member. However, when the curvature radius of the corner portion 13 is small, cracks are likely to be generated in the continuous flange 12 at the time of forming the structural member 10. In response to this, in the method for producing according to the present embodiment, since the continuous flange 12 is not restrained until the later stage of the forming step as described above, and the distortion of the continuous flange 12 can be dispersed, the generation of cracks in the continuous flange 12 can be suppressed even when the curvature radius of the corner portion 13 is small.
FIG. 5 is an enlarged view of the continuous flange 12 of the structural member 10. Referring to FIG. 5, the structural member 10 produced through the method for producing according to the present embodiment has characteristics in the hardness distribution of the continuous flange 12. More specifically, in a cross section of the continuous flange 12 taken along the thickness direction of the top plate 121 at the border 125 between the top plate 121 and the ridgeline portion 122, when the Vickers hardness measured at an end portion E1 on the opposite side of the corner portion 13 is HV1 [Hv], and the Vickers hardness measured at an end portion E2 on the corner portion 13 side is HV2 [Hv], the difference between the two: HV1 - HV2 is 30 Hv or less (however, HV1 ≥ HV2). This means that the distortion of the continuous flange 12 generated at the time of forming of the structural member 10 is dispersed along the ridgeline portion 122. In this case, when load is input to the structural member 10, and additional distortion is generated in the continuous flange 12, it is possible to prevent, for example, the generation of cracks starting from the edge of the continuous flange 12. Therefore, when the structural member 10 is used for a vehicle body of an automobile, it is possible to allow the structural member 10 to exhibit excellent collision resistance. HV1 - HV2 is preferably 20 Hv or less.
The Vickers hardnesses HV1 and HV2 of the cross section of the continuous flange 12 can be measured as follows. That is, first, the structural member 10 is laser cut along the border 125 between the top plate 121 and the ridgeline portion 122, and a part of the structural member 10 is taken. Next, the taken part is cut with an underwater cutter, resin filling and polishing are performed so that the cross section (the cross section along the border 125 between the top plate 121 and the ridgeline portion 122) of the continuous flange 12 is arranged on the surface, and a test specimen for hardness measurement is prepared. Then, a Vickers hardness test is performed in conformity with JIS Z 2244, by using this test specimen and a commercially available measuring instrument (fully automatic Vickers hardness tester HV-100, produced by Mitutoyo Corporation). The Vickers hardness is measured, for example, with a test force of 294.2 N (the numerical value of HV30), and a test force holding time of 15 s. The Vickers hardness is measured at a position of 1/4 of the thickness from the surface of the top plate 121 in a cross section included in the test specimen. In the cross section of the continuous flange 12, the Vickers hardness measured at the end portion E2 on the corner portion 13 side is HV2 [Hv], and the Vickers hardness measured at the end portion E1 on the opposite side of the corner portion 13 (on the edge side of the continuous flange 12) is HV1 [Hv].

FIG. 6 is a perspective view of a structural member 10A according to a second embodiment. Referring to FIG. 6, the structural member 10A according to the present embodiment has a similar configuration to the structural member 10 according to the first embodiment (FIG. 1 to FIG. 3). However, the structural member 10A is different from the structural member 10 according to the first embodiment in that the member body 11 does not include the ridgeline portion 113, the vertical wall 115, and the flange part 117. In the structural member 10A, the member body 11 includes the ridgeline portion 112, the vertical wall 114, and the flange part 116 on the continuous flange 12 side.
Similar to the first embodiment, the member body 11 includes, in its top plate 111, the flat surface 111a and the through-hole 111b. However, unlike the first embodiment, the flat surface 111a is recessed with respect to the other portions of the top plate 111. Similar to the first embodiment, the flat surface 111a is a portion that serves as the reference surface for work, for example, when attaching or assembling the structural member 10A to a vehicle body of an automobile. The flat surface 111a is a surface perpendicular to the pressing direction.
FIG. 7 is a side view of the structural member 10A. FIG. 7 illustrates the view of the structural member 10A as viewed from the continuous flange 12 side. As illustrated in FIG. 7, the top plate 121 of the continuous flange 12 is inclined with respect to the horizontal surface in side view of the structural member 10A. The definition of the horizontal surface is as described in the first embodiment.
The structural member 10A according to the present embodiment can also be produced by the method for producing described in the first embodiment.
Although the embodiments according to the present disclosure have been described above, the present disclosure is not limited to the above-described embodiments, and various modifications can be made as long as the modifications do not depart from the spirit of the present disclosure.
For example, each of the structural members 10 and 10A according to the above-described embodiments includes the single continuous flange 12. However, each of the structural members 10 and 10A can include a plurality of continuous flanges 12. When producing the structural member 10 or 10A including the plurality of continuous flanges 12, the second lower die 212 movable by the cam mechanism 25 can be provided to the press tooling 20 for each of the continuous flanges 12.
When the structural member 10 or 10A including the plurality of continuous flanges 12 is formed with the press tooling 20, the structural member 10 or 10A may be divided after the forming. Accordingly, a plurality of structural members 10 or 10A including one or more continuous flanges 12 can be produced in a single forming step.
In the above-described embodiments, the example has been described in which, in the state where the press tooling 20 is attached to the pressing machine, the upper die 22 and the pad 23 are arranged above the lower die 21, and the upper die 22 and the pad 23 are moved toward the lower die 21. However, contrary to the above-described embodiments, the upper die 22 and the pad 23 may be arranged below the lower die 21. In addition, the upper die 22 and the pad 23 may be relatively brought closer to the lower die 21 in the pressing direction, by moving the lower die 21 toward the upper die 22 and the pad 23.
In the above-described embodiments, the example has been described in which, in the second step of the forming, the second lower die 212 is moved in the direction perpendicular to the pressing direction. However, the movement direction of the second lower die 212 is not limited to this. The movement direction of the second lower die 212 may be inclined with respect to the direction perpendicular to the pressing direction. The movement direction of the second lower die 212 can be set, for example, in a range of ±30 degrees on the basis of a flat surface perpendicular to the pressing direction.
In the above-described embodiments, the second lower die 212 is moved by the cam mechanism 25. However, the second lower die 212 may be moved by means other than the cam mechanism 25. The second lower die 212 may be configured to be movable from the position retreated with respect to the first lower die 211 to the position at which the second lower die 212 is aligned with the first lower die 211.

EXAMPLES

Hereinafter, the present disclosure will be described in more detail with examples. However, the present disclosure is not limited to the following examples.
In order to confirm the effects of the present disclosure, CAE analysis was performed on the cold press forming using the press tooling 20 described in the above-described embodiments, by using commercially available software (AutoForm Forming R8, produced by AutoForm Engineering GmbH). The conditions and results of the analysis are illustrated in Table 1.

[Table 1]

Table 1

	Thickness (mm)	TS (MPa)	Dividing Position	Cam Stroke (mm)	Cam Stroke/ Thickness	Cracks	Wrinkles	Maximum Thickness Reduction Rate (%)	Value of Wrinkle
Comparative Example 1	1.2	1180	-	-	-	Poor	-	12.4	-
Example 1	1.2	1180	R Center	3	2.5	Good	Good	10.9	0.025
Example 2	1.2	1180	R Center	5	4.2	Good	Good	10.4	0.053
Example 3	1.2	1180	R Center	8	6.7	Good	Good	10.1	0.136
Example 4	1.2	1180	R Center	10	8.3	Good	Fair	10.5	0.176
Example 5	1.2	1180	R Center	12	10.0	Good	Poor	11.5	0.216
Example 6	1.2	1180	R Start	3	2.5	Fair	Good	11.4	0.026
Example 7	1.2	1180	R Start	5	4.2	Fair	Good	11.9	0.030
Example 8	1.2	1180	R End	3	2.5	Good	Good	8.9	0.157
Example 9	1.2	1180	R End	5	4.2	Good	Good	9.2	0.145
Example 10	1.2	1180	R End	10	8.3	Good	Good	10.3	0.054
Comparative Example 2	1.5	1180	-	-	-	Poor	-	13.7	-
Example 11	1.5	1180	R Center	5	3.3	Good	Good	10.8	0.029
Comparative Example 3	2.0	1180	-	-	-	Poor	-	16.5	-
Example 12	2.0	1180	R Center	6	3.0	Fair	Good	11.7	0.042
Comparative Example 4	1.2	590	-	-	-	Poor	-	15.2	-
Example 13	1.2	590	R Center	10	8.3	Good	Good	13.4	0.120

"Dividing position" in Table 1 indicates the position of the dividing line 214 between the first lower die 211 and the second lower die 212. When the dividing position is "R center", it means that, as illustrated in FIG. 8, the dividing line 214 extends from exactly the middle between the both ends (the both R stops as viewed from the pressing direction) in the extending direction of the corner portion 213. When the dividing position is "R start", it means that, as illustrated in FIG. 9, the dividing line 214 extends from the end (R stop) on the second lower die 212 side of the both ends in the extending direction of the corner portion 213. When the dividing position is "R end", it means that, as illustrated in FIG. 10, the dividing line 214 extends from the end (R stop) on the first lower die 211 side of the both ends in the extending direction of the corner portion 213.
"Cam stroke" in Table 1 is the distance that the second lower die 212 is moved in the later stage of the forming step.
"Cracks" in Table 1 were evaluated by using the maximum thickness reduction rate of the edge of the continuous flange 12. For Examples and Comparative Examples with a tensile strength of 1180 MPa, when the maximum thickness reduction rate is 11.0% or less, it was rated good (no cracks), when the maximum thickness reduction rate is more than 11.0% and less than 12.0%, it was rated fair (few cracks), and when the maximum thickness reduction rate is 12.0% or more, it was rated poor (cracks exist). On the other hand, for Example and Comparative Example with a tensile strength of 590 MPa, when the maximum thickness reduction rate is 14.0% or less, it was rated good (no cracks), when the maximum thickness reduction rate is more than 14.0% and less than 15.0%, it was rated fair (few cracks), and when the maximum thickness reduction rate is 15.0% or more, it was rated poor (cracks exist).
"Wrinkles" in Table 1 was evaluated based on the value of wrinkles in the analysis software. When the value of wrinkles is 0.150 or less, it was rated good (no wrinkles), when the value of wrinkles is more than 0.150 and less than 0.200, it was rated fair (few wrinkles), and when the value of wrinkles is 0.200 or more, it was rated poor (many wrinkles).
As illustrated in Table 1, in each of Examples in which the first lower die 211 and the second lower die 212 were divided, and the continuous flange 12 was not restrained by the second lower die 212 until the later stage of the forming step, cracks in the continuous flange 12 were suppressed compared with Comparative Examples in which the first lower die 211 and the second lower die 212 were not divided, and the continuous flange 12 was restrained from a relatively early stage of the forming step.
As illustrated in Table 1, in Examples 1 to 4 and 6 to 13, the moved distance (cam stroke) of the second lower die 212 was 9.0 times or less of the thickness of the starting material. On the other hand, in Example 5, the cam stroke was more than 9.0 times of the thickness of the starting material. In Example 5, more wrinkles were generated in the structural member, as compared with Examples 1 to 4 and 6 to 13. That is, in this analysis, it was confirmed that the generation of wrinkles was suppressed when the cam stroke was 9.0 times or less of the thickness of the starting material.
While the evaluation of cracks was fair in each of Examples in which the dividing position between the first lower die 211 and the second lower die 212 was "R start", the evaluation of cracks was good in most Examples of "R center" and "R end", and the cracks in the continuous flange 12 were reduced. Accordingly, it can be said that, in order to more easily suppress the generation of cracks in the continuous flange 12, the dividing line 214 between the first lower die 211 and the second lower die 212 is preferably in the middle of the extending direction of the corner portion 213, or on the first lower die 211 side than the middle.
A Vickers hardness test was actually performed on a distorted test specimen in the procedure described in the above-described embodiments, and the correlation between the Vickers hardness measured in the Vickers hardness test and the distortion calculated from the thickness reduction rate of the test specimen was obtained. Then, based on the correlation, conversion to the Vickers hardness from the equivalent plastic strain obtained in the analysis was performed for Comparison Example 1, Example 1, Example 2, and Example 8 that are illustrated in Table 1. FIG. 11 is a graph illustrating the relationship between the Vickers hardness in a cross section at the border 125 between the top plate 121 and the ridgeline portion 122, and the distance from the edge (the end portion E1 illustrated in FIG. 5) of the continuous flange for Comparison Example 1, Example 1, Example 2, and Example 8. Comparison Example 1, Example 1, Example 2, and Example 8 were structural members each formed of a steel sheet having a tensile strength of 1180 MPa. As illustrated in FIG. 11, in Comparison Example 1, the Vickers hardness was decreased as the distance from the edge E1 of the continuous flange was increased. On the other hand, also in Example 1, Example 2, and Example 8, although the Vickers hardness was decreased as the distance from the edge E1 of the continuous flange was increased, the degree of change in the Vickers hardness for these Examples was significantly smaller compared with that for Comparison Example 1.
In Comparison Example 1, the Vickers hardness in the R Start (the end portion E2 illustrated in FIG. 5) of the corner portion 13 was notably smaller than the Vickers hardness in the edge E1 of the continuous flange. In Comparison Example 1, the difference in the Vickers hardness between the edge E1 of the continuous flange and the R start E2 of the corner portion 13 was 49 Hv.
On the other hand, in Example 1, Example 2, and Example 8, the Vickers hardness in the R start E2 of the corner portion 13 was not significantly decreased from the Vickers hardness in the edge E1 of the continuous flange. The difference in the Vickers hardness between the edge E1 of the continuous flange and the R start E2 of the corner portion 13 was 9 Hv in Example 1, 13 Hv in Example 2, and 25 Hv in Example 8.
In this manner, it was confirmed that while the difference in the Vickers hardness between the edge E1 of the continuous flange and the R start E2 of the corner portion 13 was well over 30 Hv in Comparison Example 1, it was 30 Hv or less in each of Examples and the distortion at the time of the forming was dispersed. In particular, in Example 1 and Example 2, the difference in the Vickers hardness between the edge E1 of the continuous flange and the R start E2 of the corner portion 13 was 20 Hv or less, and the distortion at the time of the forming was well dispersed.
The Vickers hardness is generally proportional to the material strength. For example, when the Vickers hardness is increased by 30 Hv, the material strength is increased by about 100 MPa. In Example 1, Example 2, and Example 8, the Vickers hardness was not significantly decreased from the edge E1 of the continuous flange toward the R start E2 of the corner portion 13, and the Vickers hardness in the R start E2 of the corner portion 13 was greater than that in Comparison Example 1. Accordingly, it can be said that, in each of Examples, the material strength is higher than that in Comparison Example 1, and better component performance (yield stress) can be exhibited compared with Comparison Example 1.

REFERENCE SIGNS LIST

10, 10A: structural member
11: member body
111: top plate
111a: flat surface
112, 113: ridgeline portion
114, 115: vertical wall
12: flange
121: top plate
122: ridgeline portion
123: vertical wall
13: corner portion
20: press tooling
21: lower die
211: first lower die
211a: crest
211b: shoulder portion
211c: side surface
212: second lower die
212a: crest
212b: shoulder portion
212c: side surface
22: upper die
23: pad

Claims

A method for producing a structural member, comprising:
a preparing step of preparing a starting material made of a metal sheet; and

a forming step of performing forming by subjecting the starting material to cold press working by using a press tooling that includes a pad, an upper die, and a lower die,

wherein the lower die includes:
a first lower die including a first crest that intersects a pressing direction, a first shoulder portion extending along an end edge of the first crest, and a first side surface connected to the first crest via the first shoulder portion;

a second lower die including a second crest that intersects the pressing direction, a second shoulder portion extending along an end edge of the second crest, and a second side surface connected to the second crest via the second shoulder portion, the second lower die being a separate body from the first lower die; and

a corner portion provided between the first shoulder portion and the first side surface, and the second shoulder portion and the second side surface, the corner portion having a shape recessed inwardly of the lower die as viewed from the pressing direction, and

the forming step includes:
a first step in which, in a state where the first lower die and the second lower die are arranged so that the second side surface is at a position retreated with respect to the first side surface as viewed from the pressing direction, the pad and the upper die are relatively brought closer to the lower die in the pressing direction, and the starting material is sandwiched between the pad and the first lower die to be held;
and

a second step in which, in a state where the starting material is held by the pad and the first lower die, the upper die is further relatively brought closer to the lower die in the pressing direction to sandwich the starting material between the upper die and the first lower die, and the second lower die is moved so that the second side surface is aligned with the first side surface as viewed from the pressing direction, so as to sandwich the starting material between the upper die and the second lower die.
The method for producing according to claim 1,
wherein a moved distance of the second lower die in the second step is 9.0 times or less of a thickness of the starting material.
The method for producing according to claim 1,
wherein the second lower die further includes at least a part of the corner portion, and

the second shoulder portion and the second side surface are continuous with the corner portion.
The method for producing according to claim 1,
wherein the pad further holds a portion of the starting material adjacent to the corner portion on a bending outer side of the corner portion.
The method for producing according to claim 1,
wherein the second crest is inclined with respect to a surface perpendicular to the pressing direction, so that a distance from the second shoulder portion and the second side surface in the pressing direction is increased from an end edge on the second shoulder portion side toward an end edge on an opposite side of the second shoulder portion.
A structural member comprising:
a member body including a first top plate, a first ridgeline portion extending along an end edge of the first top plate, and a first vertical wall connected to the first top plate via the first ridgeline portion; and

a flange including a second top plate, a second ridgeline portion extending along an end edge of the second top plate, and a second vertical wall connected to the second top plate via the second ridgeline portion,

wherein the second ridgeline portion and the second vertical wall are connected to the first ridgeline portion and the first vertical wall via a corner portion,

the second top plate is continuous with the first top plate on a bending outer side of the corner portion, and

in a cross section of the flange taken along a thickness direction of the second top plate at a border between the second top plate and the second ridgeline portion, a difference between a Vickers hardness measured at an end portion on an opposite side of the corner portion and a Vickers hardness measured at an end portion on the corner portion side is 30 Hv or less.
The structural member according to claim 6,
wherein the structural member is formed of a steel sheet having a tensile strength of 590 MPa or more.
The structural member according to claim 6,
wherein the corner portion has a curvature radius of 100 mm or less as viewed from the first top plate side.
The structural member according to claim 6,
wherein the first top plate includes a flat surface on its surface, and

the second top plate is inclined with respect to the flat surface, so that a distance from the second ridgeline portion and the second vertical wall in a direction perpendicular to the flat surface is increased from an end edge on the second ridgeline portion side toward an end edge on an opposite side of the second ridgeline portion.