EP4659876A1

EP4659876A1 - Method for manufacturing press-formed article, press device, press-formed article, automobile frame component, and automobile

Info

Publication number: EP4659876A1
Application number: EP23919956.5A
Authority: EP
Inventors: Atsushi Ono; Masanobu KAMO; Shigeki Yamashita; Hiroki Fujimoto; Koichi Hamada
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2023-02-02
Filing date: 2023-12-18
Publication date: 2025-12-10
Also published as: JPWO2024161821A1; WO2024161821A1; CN120603662A

Abstract

A method for producing a press-formed product (10) includes a forming process and a burring process. In the forming process, a starting material (M) is placed between a first die (21) and a second die (22), and the dies (21, 22) are brought close to each other relatively to clamp and form the starting material (M) by means of a first forming surface (211) and a second forming surface (221). In the burring process, in a state in which clamping of the starting material (M) by the forming surfaces (211, 221) is maintained, a piercing punch (23) is caused to protrude from the first forming surface (211) to form a through-hole (123) in the starting material (M). In the burring process, thereafter, in a state in which clamping of the starting material (M) by the forming surfaces (211, 221) is maintained, a burring punch (24) is caused to protrude from the first forming surface (211) to raise an outer circumferential portion of the through-hole (123) to form a burring portion (12) in the starting material (M).

Description

TECHNICAL FIELD

The present disclosure relates to a method for producing a press-formed product, a press apparatus, a press-formed product, a structural component for an automobile, and an automobile.

BACKGROUND ART

For example, hydroformed products or press-formed products have conventionally been used as structural components for automobiles and the like. In some cases, another component is assembled to a component that is a hydroformed product or a press-formed product.
Patent Literature 1 discloses a technique for performing piercing in a hydroformed product for the purpose of positioning or the like when assembling or when attaching another component. According to Patent Literature 1, piercing and burring are performed on a hydroformed product inside a hydroforming mold. More specifically, a metal pipe is placed between an upper die and a lower die, and pressure is applied from the inside of the metal pipe by a pressure medium while also performing axial pressing from both ends of the metal pipe to thereby obtain a hydroformed product. A first punch and a cylindrical second punch inside which the first punch slides are housed in the upper die. After the hydroformed product is obtained, the first punch is pushed out from the upper die in the direction towards the cavity of the hydroformed product to thereby perform piercing of the hydroformed product with the first punch. The second punch is pushed out from the upper die in the direction towards the cavity of the hydroformed product by the head of the first punch. The hydroformed product is subjected to burring by the second punch.

CITATION LIST

PATENT LITERATURE

Patent Literature 1: Japanese Patent Application Publication No. 2005-297060

SUMMARY OF INVENTION

TECHNICAL PROBLEM

In Patent Literature 1, it is suggested that it is easier to subject a press-formed product to burring in a separate process from a forming process in comparison to a hydroformed product that is a pipe. However, even in the case of a press-formed product, it is difficult to form a burring portion by piercing and burring.
For example, in a case where a press-formed product for which a forming process has been completed and which has been taken out from a die is subjected to burring in a separate process to piercing, it is necessary to align the central axis of the burring punch with the central axis of the piercing punch with high accuracy. Further, in a case where burring is performed simultaneously at multiple places on a press-formed product, it is necessary to position the press-formed product with extremely high accuracy so that all of a plurality of through-holes formed by piercing are coaxial with the burring punches. In particular, in a case where the burring height is small, the influence of a misalignment between the piercing punch or through-holes and the burring punch is large, thus making formation of the burring portion more difficult. If burring is performed sequentially one location at a time on a press-formed product, although the difficulty of forming burring portions will be somewhat reduced, the production efficiency will also decrease.
In the case of forming a burring portion in a starting material for a press-formed product before a forming process, the same problem of misalignment arises as in the case of forming a burring portion in a press-formed product after a forming process. In addition, in a case where a burring portion is formed in advance in a starting material before the forming process, there is also the problem that the structure of the die used in the forming process as well as the forming process itself become complicated.
An objective of the present disclosure is to form a burring portion with high accuracy in a press-formed product, and also to efficiently produce a press-formed product.

SOLUTION TO PROBLEM

A method for producing a press-formed product according to the present disclosure includes a preparation process, a forming process, and a burring process. In the preparation process, a starting material composed of a metal sheet is prepared. In the forming process, the starting material is placed between a first die including a first forming surface and a second die including a second forming surface, and the first die is brought close relatively to the second die to clamp and form the starting material by means of the first forming surface and the second forming surface. The second forming surface has a shape that corresponds to the first forming surface. In the burring process, while maintaining a clamped state of the starting material by the first forming surface and the second forming surface, a piercing punch housed inside the first die is caused to protrude from the first forming surface, and a through-hole is formed in the starting material by the piercing punch. In the burring process, thereafter, while maintaining a clamped state of the starting material by the first forming surface and the second forming surface, a burring portion is formed in the starting material by causing a burring punch which is arranged coaxially with the piercing punch inside the first die and which has a larger diameter than the piercing punch to protrude from the first forming surface to raise an outer circumferential portion of the through-hole by means of the burring punch.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present disclosure, a burring portion can be formed with high accuracy in a press-formed product, and the press-formed product can be efficiently produced.

BRIEF DESCRIPTION OF DRAWINGS

[FIG. 1] FIG. 1 is a perspective view of a press-formed product according to an embodiment.
[FIG. 2] FIG. 2 is a transverse sectional view of the press-formed product illustrated in FIG. 1.
[FIG. 3] FIG. 3 is a transverse sectional view of a press apparatus according to an embodiment.
[FIG. 4] FIG. 4 is a partial enlarged view of the press apparatus illustrated in FIG. 3.
[FIG. 5A] FIG. 5A is a schematic diagram for describing a method for producing a press-formed product according to an embodiment.
[FIG. 5B] FIG. 5B is a schematic diagram for describing the method for producing a press-formed product according to the embodiment.
[FIG. 5C] FIG. 5C is a schematic diagram for describing the method for producing a press-formed product according to the embodiment.
[FIG. 5D] FIG. 5D is a schematic diagram for describing the method for producing a press-formed product according to the embodiment.
[FIG. 5E] FIG. 5E is a schematic diagram for describing the method for producing a press-formed product according to the embodiment.
[FIG. 5F] FIG. 5F is a schematic diagram for describing the method for producing a press-formed product according to the embodiment.
[FIG. 5G] FIG. 5G is a schematic diagram for describing the method for producing a press-formed product according to the embodiment.
[FIG. 6] FIG. 6 is a transverse sectional view of a press-formed product to which a sheet-shaped member has been joined.
[FIG. 7] FIG. 7 is a perspective view of a press-formed product according to a modification of the aforementioned embodiment.
[FIG. 8] FIG. 8 is a transverse sectional view of a press apparatus according to a modification of the aforementioned embodiment.

DESCRIPTION OF EMBODIMENTS

A method for producing a press-formed product according to an embodiment includes a preparation process, a forming process, and a burring process. In the preparation process, a starting material composed of a metal sheet is prepared. In the forming process, the starting material is placed between a first die including a first forming surface and a second die including a second forming surface, and the first die is brought close relatively to the second die to clamp and form the starting material by means of the first forming surface and the second forming surface. The second forming surface has a shape that corresponds to the first forming surface. In the burring process, while maintaining a clamped state of the starting material by the first forming surface and the second forming surface, a piercing punch housed inside the first die is caused to protrude from the first forming surface, and a through-hole is formed in the starting material by the piercing punch. In the burring process, thereafter, while maintaining a clamped state of the starting material by the first forming surface and the second forming surface, a burring portion is formed in the starting material by causing a burring punch which is arranged coaxially with the piercing punch inside the first die and which has a larger diameter than the piercing punch to protrude from the first forming surface to raise an outer circumferential portion of the through-hole by means of the burring punch (first configuration).
In the production method according to the first configuration, after a starting material has been clamped (pressed) and formed by the first forming surface of the first die and the second forming surface of the second die, while the starting material remains clamped by these forming surfaces, the starting material is subjected to piercing and burring by the piercing punch and the burring punch which are housed inside the first die. The burring punch is arranged coaxially with the piercing punch in advance, and therefore when forming a burring portion in the starting material after subjecting the starting material to forming, an accuracy defect caused by a misalignment between the central axis of the burring punch and the central axis of the piercing punch does not occur. Further, piercing by the piercing punch and burring by the burring punch are successively performed in a state in which the starting material after forming is clamped and fixed by the first forming surface of the first die and the second forming surface of the second die. Therefore, a misalignment does not occur between a through-hole formed by the piercing and the burring punch, and an accuracy defect at the burring portion that is caused by such a misalignment does not occur. Therefore, the burring portion can be formed with high accuracy in the press-formed product.
In the production method according to the first configuration, piercing and burring are performed while the starting material remains clamped by the first forming surface of the first die and the second forming surface of the second die. In other words, after the first die and the second die have been closed to form the starting material, piercing and burring are performed before the first die and the second die open. In this case, the number of processes can be reduced in comparison to a case where piercing and burring are performed in a process that is completely separate from the forming process, and thus the press-formed product can be efficiently produced.
In the burring process, at least one part of the burring punch may enter inside the second die from the second forming surface (second configuration).
In the second configuration, when forming the burring portion by means of the burring punch, at least one part of the burring punch enters the inside of the second die from the second forming surface. In this case, the outer circumferential portion of the through-hole formed in the starting material by the piercing punch can be suitably raised by the burring punch, and the burring portion is easily formed in an appropriate shape.
In the first or second configuration, the burring punch may be formed integrally with the piercing punch (third configuration).
According to the third configuration, the piercing punch and the burring punch are integrated. In this case, the piercing punch and the burring punch can be driven by a single drive mechanism, and thus the drive mechanism can be simplified.
In the third configuration, the burring punch may be connected to the piercing punch via a transition portion. In this case, preferably a surface of the transition portion has a curved shape that is convex to an outer circumferential side of the transition portion (fourth configuration).
According to the fourth configuration, the surface of the transition portion from the piercing punch towards the burring punch is formed in a convex curved shape. By this means, the occurrence of a situation in which the starting material fractures when transitioning to burring performed by the burring punch after piercing by the piercing punch can be suppressed.
In any one of the first to fourth configurations, the first die can house a plurality of the piercing punches and a plurality of the burring punches. The plurality of the burring punches are provided in correspondence with the plurality of piercing punches. In this case, in the burring process, a plurality of the burring portions are formed in the starting material by the plurality of the piercing punches and the plurality of the burring punches (fifth configuration).
In the fifth configuration, the plurality of the piercing punches and the plurality of the burring punches that correspond to the plurality of the piercing punches are provided in the first die. In the burring process, in a state in which the first forming surface of the first die and the second forming surface of the second die clamp the starting material after forming, piercing by each piercing punch and burring by each burring punch are successively performed. Since each burring punch is arranged coaxially with the corresponding piercing punch, a misalignment does not occur between the piercing punches and through-holes formed by the piercing punches, and the burring punches. Therefore, a plurality of burring portions can be formed simultaneously and with high accuracy in the starting material after forming, and the production efficiency with respect to the press-formed product can be increased.
In any one of the first to fifth configurations, the metal sheet constituting the starting material may be a steel sheet. The production method may further include a heating process of, after the preparation process, heating and austenitizing the starting material before the forming process. In this case, after the forming process, martensitic transformation of the starting material can be caused by holding the starting material in a state in which the starting material remains clamped by the first forming surface and the second forming surface (sixth configuration).
In the sixth configuration, a press-formed product formed from the starting material is hardened by quenching. Therefore, a press-formed product that has high strength can be obtained. Increasing the strength of the press-formed product makes it possible to thin the walls and reduce the weight of the press-formed product.
In the sixth configuration, preferably the burring process finishes in a first half of a holding period. The holding period is a period from a time point at which clamping of the starting material by the first forming surface and the second forming surface is started until the first die and the second die separate and clamping of the starting material is released (seventh configuration).
The starting material that is clamped by the first forming surface of the first die and the second forming surface of the second die is hardened by heat dissipation by means of the first die and the second die while the clamping of the starting material is being maintained. In a case where the starting material is subjected to working after hardening, there is a possibility that a crack will occur in the starting material or that wear of a working tool will occur. In contrast, according to the seventh configuration, piercing by the piercing punch and burring by the burring punch are completed while the formed starting material is relatively soft, that is, during the first half of the holding period in which the starting material is held by the first die and the second die. It is therefore possible to suppress the occurrence of a crack in the starting material during the piercing or burring, as well as the occurrence of wear or damage to each punch.
In the sixth or seventh configuration, preferably the burring process finishes before martensitic transformation of the starting material is completed (eighth configuration).
In the eighth configuration, piercing by the piercing punch and burring by the burring punch are finished while the formed starting material is comparatively soft, that is, before martensitic transformation of the starting material is completed. It is therefore possible to suppress the occurrence of a crack in the starting material during the piercing or burring, as well as the occurrence of wear or damage to each punch.
In any one of the sixth to eighth configurations, the second die can include a receiving portion. The receiving portion is a space that is provided inside the second die, and opens to the second forming surface. In this case, in the burring process, the outer circumferential portion raised by the burring punch is received by the receiving portion. The outer circumferential portion may face a peripheral wall that defines the receiving portion in the second die with a gap therebetween (ninth configuration).
In the ninth configuration, in the starting material, a portion that is raised by the burring punch is received by the receiving portion of the second die. A gap is generated between the raised portion of the starting material and the peripheral wall of the receiving portion. In this case, the movement resistance of the rising portion of the starting material with respect to the second die can be reduced, and hence the starting material after formation of the burring portion can be easily removed from the second die. In the ninth configuration, because the raised portion of the starting material faces the peripheral wall of the receiving portion of the second die, the second die can be used to dissipate heat from the raised portion and harden the raised portion.
In any one of the sixth to eighth configurations, the second die can include a receiving portion. The receiving portion is a space that is provided inside the second die, and opens to the second forming surface. In this case, in the burring process, the outer circumferential portion raised by the burring punch is received by the receiving portion. The outer circumferential portion may come into contact with a peripheral wall that defines the receiving portion in the second die (tenth configuration).
In the tenth configuration, a raised portion of the starting material that is raised by the burring punch is received by the receiving portion of the second die and comes into contact with the peripheral wall of the receiving portion. In this case, the second die can more reliably dissipate heat from the raised portion and harden the raised portion.
A press apparatus according to an embodiment includes a first die, a second die, a piercing punch, and a burring punch. The first die includes a first forming surface. The second die includes a second forming surface. The second forming surface faces the first forming surface. The second forming surface has a shape that corresponds to the first forming surface. The second die is capable of coming close to the first die relatively. The piercing punch is housed inside the first die. The piercing punch is configured to protrude from the first forming surface. The burring punch is housed inside the first die. The burring punch is arranged coaxially with the piercing punch, on an opposite side to the first forming surface with respect to the piercing punch. The burring punch is configured to protrude from the first forming surface following the piercing punch. The burring punch has a larger diameter than the piercing punch. The second die includes a receiving portion. The receiving portion is a space provided inside the second die. The receiving portion is arranged at a position corresponding to the piercing punch and the burring punch. The receiving portion opens to the second forming surface (eleventh configuration).
In the eleventh configuration, the burring punch may be formed integrally with the piercing punch (twelfth configuration).
In the twelfth configuration, the burring punch may be connected to the piercing punch via a transition portion. In this case, preferably a surface of the transition portion has a curved shape that is convex to an outer circumferential side of the transition portion (thirteenth configuration).
In any one of the eleventh to thirteenth configurations, the press apparatus can include a plurality of the piercing punches and a plurality of the burring punches. The plurality of the burring punches are provided in correspondence with the plurality of the piercing punches (fourteenth configuration).
A press-formed product according to an embodiment is made of steel. The press-formed product includes a formed product body and a burring portion. The burring portion includes a hole portion and a cylindrical portion. The hole portion penetrates the formed product body in a sheet thickness direction. The cylindrical portion rises in the sheet thickness direction from a peripheral edge of the hole portion. In the microstructure of the formed product body and the burring portion, an area fraction of martensite is 85% or more (fifteenth configuration).
In the fifteenth configuration, a sheet thickness of the press-formed product may be 2.0 mm or less (sixteenth configuration).
In the fifteenth or sixteenth configuration, a diameter of the hole portion of the burring portion may be 15.0 mm or less (seventeenth configuration).
In any one of the fifteenth to seventeenth configurations, the press-formed product can have a plurality of the burring portions (eighteenth configuration).
In any one of the fifteenth to eighteenth configurations, the formed product body can include a top plate, two vertical walls, and two flanges. The two vertical walls are connected to both sides of the top plate and are arranged to face each other. The two flanges are connected to the two vertical walls on an opposite side to the top plate, respectively, and protrude outward from the vertical walls. The burring portion is provided in the flanges (nineteenth configuration).
A structural component for an automobile according to an embodiment includes the press-formed product according to any one of the fifteenth to nineteenth configurations (twentieth configuration).
In the twentieth configuration, the structural component can further include a sheet-shaped member. The sheet-shaped member is joined to the press-formed product by a joining member (twenty-first configuration).
An automobile according to an embodiment includes the structural component according to the twentieth or twenty-first configuration (twenty-second configuration).
An embodiment of the present disclosure is described hereunder while referring to the accompanying drawings. In the drawings, the same reference symbols are assigned to the same or equivalent parts, and a description thereof is not repeated.

[Press-formed product]

FIG. 1 is a perspective view of a press-formed product 10 according to the present embodiment. The press-formed product 10 is formed of a metal sheet. The press-formed product 10 may be formed of a steel sheet. The sheet thickness of the press-formed product 10 is, for example, 2.0 mm or less. The sheet thickness of the press-formed product 10 may be 0.8 mm or more and 2.3 mm or less.
Referring to FIG. 1, the press-formed product 10 includes a formed product body 11 and a burring portion 12. In the example of the present embodiment, the formed product body 11 has an elongated shape. The formed product body 11 has a substantially hat shape in a cross section (transverse section) perpendicular to the longitudinal direction. The formed product body 11 includes a top plate 111, vertical walls 112 and 113, and flanges 114 and 115.
The top plate 111 extends in the longitudinal direction of the formed product body 11. The vertical walls 112 and 113 are connected to both sides of the top plate 111. More specifically, the vertical wall 112 is connected to a side edge of the top plate 111 via a ridge portion 116. The vertical wall 113 is arranged on the opposite side to the vertical wall 112, and is connected to the other side edge of the top plate 111 via a ridge portion 117. The ridge portions 116 and 117 are corner portions between the vertical walls 112 and 113 and the top plate 111, respectively. The ridge portions 116 and 117 may have a convex arcuate shape on the outer side of the formed product body 11 as seen in transverse sectional view of the formed product body 11.
The vertical walls 112 and 113 are arranged so as to face each other. The vertical walls 112 and 113 may be parallel or may be non-parallel. The vertical walls 112 and 113, for example, may separate from each other progressively as they move away from the top plate 111.
The flanges 114 and 115 are connected to the vertical walls 112 and 113, respectively, on the opposite side to the top plate 111. As seen in transverse sectional view of the formed product body 11, one end of the vertical wall 112 and one end of the vertical wall 113 are connected by the top plate 111, and the flanges 114 and 115 are connected to the other end of the vertical wall 112 and the other end of the vertical wall 113, respectively. The flanges 114 and 115 protrude outward from the vertical walls 112 and 113.
In the present embodiment, the press-formed product 10 has a plurality of burring portions 12. More specifically, a plurality of burring portions 12 are provided in each of the flanges 114 and 115 of the formed product body 11. In each of the flanges 114 and 115, the burring portions 12 are arranged along the longitudinal direction of the press-formed product 10.
FIG. 2 is a cross-sectional view (transverse sectional view) of the press-formed product 10 when cut along a plane perpendicular to the longitudinal direction thereof. Each of the burring portions 12 includes a hole portion 121 and a cylindrical portion 122. The hole portion 121 penetrates the formed product body 11 in the sheet thickness direction. In the present embodiment, the hole portion 121 penetrates the flange 114 or the flange 115 in the sheet thickness direction.
The cylindrical portion 122 rises from the peripheral edge of the hole portion 121 in the sheet thickness direction of the formed product body 11. In the present embodiment, the cylindrical portion 122 rises in the sheet thickness direction of the flange 114 or the flange 115 from the peripheral edge of the hole portion 121. The cylindrical portion 122 is provided in the surface on the opposite side to the top plate 111 in the flange 114 or the flange 115. In the present embodiment, although the inner peripheral surface of the cylindrical portion 122 has surface properties caused by contact with a piercing punch 23 and a burring punch 24, which are described later, during press forming, the inner peripheral surface of the cylindrical portion 122 has a smooth shape as a whole. In other words, there is substantially no uneven portion on the inner peripheral surface of the cylindrical portion 122. The phrase "there is substantially no uneven portion" means that an uneven portion is not provided intentionally on the inner peripheral surface of the cylindrical portion 122.
The hole portion 121 is, for example, a round hole that has a circular shape in plan view. The hole portion 121 may be a long hole having an oval shape or an oval track shape in plan view. The diameter of the hole portion 121 is, for example, 15.0 mm or less. The diameter of the hole portion 121 may be 4.0 mm or more. When the hole portion 121 is a long hole, the diameter of the hole portion 121 is the length of the long axis of the long hole. The cylindrical portion 122 has a shape corresponding to the hole portion 121 in plan view. The length in the axial direction (the burring height) of the cylindrical portion 122 is, for example, 3.0 mm or less. The burring height may be 0.4 mm or more.
When using the press-formed product 10, the burring portion 12 is used for joining the press-formed product 10 to another component. The press-formed product 10 is, for example, a component for an automobile. The press-formed product 10 may be included in a structural component for an automobile. In other words, an automobile can have a structural component that includes the press-formed product 10. The press-formed product 10 can be joined with one or more other components to constitute a structural component for an automobile together with the one or more other components. Examples of a structural component for an automobile include a center pillar, a side sill, and a bumper. However, application of the press-formed product 10 is not limited to these uses.

[Press apparatus]

The press-formed product 10 is produced by press forming a metal sheet. Hereunder, a press apparatus 20 for producing the press-formed product 10 is described with reference to FIG. 3. FIG. 3 is a cross-sectional view (transverse sectional view) of the press apparatus 20 when cut along a plane perpendicular to the longitudinal direction thereof.
Referring to FIG. 3, the press apparatus 20 includes an upper die 21 as a first die, a lower die 22 as a second die, a plurality of piercing punches 23, and a plurality of burring punches 24.
The upper die 21 and the lower die 22 are capable of coming close to each other and separating from each other relatively. For example, the upper die 21 and the lower die 22 may come close to each other and separate from each other by adopting a configuration in which the upper die 21 is attached to a slide 25 of the press apparatus 20, and the upper die 21 moves upward and downward together with the slide 25. In the example illustrated in FIG. 3, the upper die 21 is attached to the slide 25 via, for example, an expansion and contraction member 26 such as a gas spring or a hydraulic cylinder. In a case where the expansion and contraction member 26 is an actuator such as a hydraulic cylinder, the upper die 21 and the lower die 22 may be controlled to come close to each other and to separate from each other by controlling expansion and contraction of the expansion and contraction member 26. Hereinafter, the direction in which the upper die 21 and the lower die 22 come close to each other and separate from each other is referred to as "pressing direction". The pressing direction is, for example, the vertical direction.
The upper die 21 includes a forming surface 211. The forming surface 211 is provided on the lower surface of the upper die 21 as viewed on the paper surface in FIG. 3. The forming surface 211 has a shape that is concave toward the inner side of the upper die 21 in the pressing direction. The forming surface 211 includes a bottom face 211a, side faces 211b and 211c, and flange faces 211d and 211e. The side faces 211b and 211c are disposed on both sides of the bottom face 211a. The flange faces 211d and 211e are disposed on the outer side of the bottom face 211a and the side faces 211b and 211c.
The lower die 22 includes a forming surface 221. The forming surface 221 is provided on the upper surface of the lower die 22 as viewed on the paper surface in FIG. 3. The forming surface 221 has a shape that is convex toward the upper die 21 side. The forming surface 221 faces the forming surface 211 of the upper die 21 in the pressing direction. The forming surface 221 has a shape that corresponds to the forming surface 211 of the upper die 21.
The forming surface 221 includes a top face 221a, side faces 221b and 221c, and flange faces 221d and 221e. The top face 221a is a face for forming the top plate 111 (FIG. 1 and FIG. 2) of the press-formed product 10 together with the bottom face 211a of the forming surface 211 of the upper die 21. The side faces 221b and 221c are disposed on both sides of the top face 221a. The side faces 221b and 221c are faces for forming the vertical walls 112 and 113 (FIG. 1 and FIG. 2) of the press-formed product 10 together with the side faces 211b and 211c of the forming surface 211 of the upper die 21, respectively. The flange faces 221d and 221e are disposed on the outer side of the top face 221a and the side faces 221b and 221c. The flange faces 221d and 221e are faces for forming the flanges 114 and 115 (FIG. 1 and FIG. 2) of the press-formed product 10 together with the flange faces 211d and 211e of the forming surface 211 of the upper die 21, respectively.
The plurality of piercing punches 23 and the plurality of burring punches 24 form the plurality of burring portions 12 (FIG. 1 and FIG. 2) in the press-formed product 10. Therefore, the piercing punches 23 and burring punches 24 are arranged at positions corresponding to the burring portions 12 (FIG. 1 and FIG. 2). The plurality of burring punches 24 are provided in correspondence with the plurality of piercing punches 23. In other words, one burring punch 24 is provided for each piercing punch 23.
In the present embodiment, the piercing punches 23 and the burring punches 24 are housed in the upper die 21. More specifically, a plurality of housing portions 212 are provided in the upper die 21, and one piercing punch 23 and one burring punch 24 that corresponds to the piercing punch 23 are arranged inside each housing portion 212. Each housing portion 212 is a space that is provided inside the upper die 21, and opens to the forming surface 211. In the present embodiment, each housing portion 212 penetrates the upper die 21 in the pressing direction, and opens to the flange face 211d or the flange face 211e.
FIG. 4 is an enlarged view of the piercing punch 23 and the burring punch 24. Referring to FIG. 4, in the housing portion 212 of the upper die 21, the burring punch 24 is arranged on the opposite side to the forming surface 211 with respect to the piercing punch 23. The burring punch 24 is arranged coaxially with the piercing punch 23.
The piercing punch 23 has a columnar shape. As viewed along the axial direction, the piercing punch 23 may have a substantially perfectly circular shape, or may have an elliptic shape such as an oval shape or an oval track shape. The piercing punch 23 includes an end face 231 and a peripheral face 232. The end face 231 is a surface that faces the forming surface 221 (FIG. 3) of the lower die 22. The peripheral face 232 rises in the axial direction of the piercing punch 23 from the peripheral edge of the end face 231. When viewed in a cross section including the central axis of the piercing punch 23, the peripheral face 232 may be substantially perpendicular to the end face 231. A corner portion between the end face 231 and the peripheral face 232 forms a sharp angle.
The burring punch 24 has a columnar shape. The burring punch 24 has a larger diameter than the piercing punch 23. In other words, when the piercing punch 23 and the burring punch 24 are viewed along the axial direction, a peripheral face 241 of the burring punch 24 is located further on the outer side than the piercing punch 23 throughout the entirety thereof. As viewed along the axial direction, the burring punch 24 may have a substantially perfectly circular shape, or may have an elliptic shape such as an oval shape or an oval track shape. In the present embodiment, the peripheral face 241 of the burring punch 24 has a smooth shape throughout. In other words, there is substantially no uneven portion on the peripheral face 241 of the burring punch 24. The phrase "there is substantially no uneven portion" means that an uneven portion is not provided intentionally on the peripheral face 241 of the burring punch 24.
In the present embodiment, the burring punch 24 is formed integrally with the piercing punch 23. The burring punch 24 is connected to the piercing punch 23 via a transition portion 27. When viewed in a cross section including the central axes of the piercing punch 23 and the burring punch 24, the transition portion 27 widens progressively from the piercing punch 23 toward the burring punch 24.
A surface 271 of the transition portion 27 has a curved shape that is convex to the outer circumferential side of the transition portion 27. The surface 271 of the transition portion 27 has, for example, a spherical shape. The surface 271 of the transition portion 27 is smoothly continuous to the peripheral face 241 so as not to form a sharp angle between the surface 271 of the transition portion 27 and the peripheral face 241 of the burring punch 24.
Returning to FIG. 3, each of the piercing punches 23 is configured to protrude from the forming surface 211 of the upper die 21. Each of the burring punches 24 is configured to protrude from the forming surface 211 following the corresponding piercing punch 23. In the present embodiment, the piercing punches 23 and the burring punches 24 are directly connected to the slide 25 of the press apparatus 20, and the upper die 21 is supported by the slide 25 via the expansion and contraction member 26. When the expansion and contraction member 26 supporting the upper die 21 contracts, the piercing punch 23 can protrude from the forming surface 211, and the burring punch 24 can also protrude from the forming surface 211.
The lower die 22 includes a plurality of receiving portions 28. The receiving portions 28 are spaces that are provided in the lower die 22 to receive the piercing punches 23 and the burring punches 24 protruding from the forming surface 211 of the upper die 21. The receiving portions 28 are arranged at positions that correspond to the piercing punches 23 and the burring punches 24. Each receiving portion 28 opens to the forming surface 221 of the lower die 22. In the present embodiment, each receiving portion 28 extends through the inside of the lower die 22 in the pressing direction, and opens to the flange face 221d or the flange face 221e.

[Method for producing press-formed product]

Next, a method for producing the press-formed product 10 using the press apparatus 20 is described while referring to FIG. 5A to FIG. 5G. In the present embodiment, an example of producing the press-formed product 10 made of steel by hot stamping is described. The production method according to the present embodiment includes a preparation process, a heating process, a forming process, and a burring process.

(Preparation process)

Referring to FIG. 5A, in the preparation process, a starting material M consisting of a metal sheet is prepared. In the present embodiment, the starting material M consists of a steel sheet. The starting material M is, for example, a blank having an opened-out shape of the press-formed product 10 (FIG. 1 and FIG. 2).

(Heating process)

Referring to FIG. 5B, in the heating process, the prepared starting material M is heated and austenitized. More specifically, the starting material M consisting of a steel sheet is heated to the A_c3 transformation point or higher so that all or almost all of the microstructure of the starting material M becomes austenite. The starting material M is, for example, heated to about 850°C to 900°C using a heating furnace 30.

(Forming process)

After the heating process, the starting material M is subjected to a forming process. As illustrated in FIG. 5C and FIG. 5D, in the forming process, the starting material M is placed between the upper die 21 and the lower die 22, and the upper die 21 is brought closer relatively to the lower die 22 to clamp the starting material M between the forming surface 211 of the upper die 21 and the forming surface 221 of the lower die 22 and form the starting material M into the press-formed product 10. The forming process is described in detail hereunder.
Referring to FIG. 5C, before the start of the forming process, the press apparatus 20 is at top dead center. That is, the slide 25 of the press apparatus 20 is at the uppermost point of its stroke, and the upper die 21, the piercing punches 23, and the burring punches 24 that are connected to the slide 25 are separated from the lower die 22. In this state, first, the starting material M which has been heated to a high temperature is placed between the forming surface 211 of the upper die 21 and the forming surface 221 of the lower die 22. In the example in FIG. 5C, the starting material M is placed on the top face 221a of the forming surface 221 of the lower die 22.
Referring to FIG. 5D, next, the upper die 21 is brought close to the lower die 22. More specifically, the slide 25 is lowered, and the upper die 21 is moved toward the lower die 22 together with the slide 25. The starting material M is clamped (pressed) between the forming surface 211 of the upper die 21 and the forming surface 221 of the lower die 22. By this means, the starting material M is formed into the press-formed product 10 including the top plate 111, the vertical walls 112 and 113, and the flanges 114 and 115. After the forming process, the starting material M turned into the press-formed product 10 is held while being clamped by the forming surface 211 of the upper die 21 and the forming surface 221 of the lower die 22. By this means, the starting material M is subjected to heat dissipation (quenching) by means of the upper die 21 and the lower die 22, and martensitic transformation of the microstructure of the starting material M occurs.

(Burring process)

The burring process is performed at a timing that is after the upper die 21 and the lower die 22 close and the starting material M has been formed into the press-formed product 10, and is before the upper die 21 and the lower die 22 open. As illustrated in FIG. 5E and FIG. 5F, by performing the burring process, the plurality of burring portions 12 are formed in the press-formed product 10 by the piercing punches 23 and the burring punches 24.
Referring to FIG. 5E, in the burring process, first, in a state in which clamping of the starting material M by the forming surface 211 of the upper die 21 and the forming surface 221 of the lower die 22 is maintained, each of the piercing punches 23 housed in the upper die 21 is caused to protrude from the forming surface 211 so that each of the piercing punches 23 forms a through-hole 123 in the starting material M.
More specifically, after the starting material M is clamped and subjected to forming by the forming surface 211 of the upper die 21 and the forming surface 221 of the lower die 22, when the slide 25 is lowered further, the expansion and contraction member 26 contracts, and the piercing punches 23 and the burring punches 24 are lowered together with the slide 25 in a state in which the upper die 21 is stopped. The piercing punches 23 protrude from the forming surface 211 of the stopped upper die 21 to the starting material M side without rotation. The term "without rotation" means that each piercing punch 23 is not intentionally caused to rotate around its central axis, for example, it means that each piercing punch 23 does not rotate by an amount equal to or greater than one rotation (360°) around its central axis. Because the corner portion between the end face 231 and the peripheral face 232 of each piercing punch 23 forms a sharp angle (FIG. 4), the starting material M is punched by the corner portion of each piercing punch 23. By this means, a plurality of through-holes 123 are formed in the starting material M. In the present embodiment, the plurality of through-holes 123 are formed in each of the flanges 114 and 115.
Referring to FIG. 5F, in the burring process, in a state in which clamping of the starting material M is maintained by the forming surface 211 of the upper die 21 and the forming surface 221 of the lower die 22, after the piercing punches 23 have each protruded from the forming surface 211 and formed the through-holes 123 in the starting material M (FIG. 5E), the burring punches 24 corresponding to the respective piercing punches 23 are caused to protrude without rotation from the forming surface 221. The term "without rotation" means that each burring punch 24 is not intentionally caused to rotate around its central axis, for example, it means that each burring punch 24 does not rotate by an amount equal to or greater than one rotation (360°) around its central axis. Then, by the outer circumferential portions of the through-holes 123 being raised by the burring punches 24, the plurality of burring portions 12 are formed in the starting material M.
More specifically, after each of the piercing punches 23 has punched the starting material M, when the slide 25 is lowered further, the expansion and contraction member 26 contracts further, and each of the piercing punches 23 and the corresponding burring punches 24 are further lowered together with the slide 25 while the upper die 21 remains stopped. Since each burring punch 24 is arranged at the rear in the protruding direction of the corresponding piercing punch 23, the burring punch 24 protrudes to the starting material M side from the forming surface 211 of the upper die 21 following the piercing punch 23. The burring punch 24 raises the outer circumferential portion of the through-hole 123 (FIG. 5E) formed by the piercing punch 23 toward the lower die 22 side. By this means, the plurality of burring portions 12 which each include the hole portion 121 and the cylindrical portion 122 are formed in the starting material M.
FIG. 5G is an enlarged view of the burring punch 24 protruding from the forming surface 211 of the upper die 21, and the vicinity of the burring punch 24. As illustrated in FIG. 5G, in the burring process, at least one part of the burring punch 24 enters the inside of the lower die 22 from the forming surface 221. In a state in which the slide 25 of the press apparatus 20 is located at the lowest point of its stroke, that is, at bottom dead center of the press apparatus 20 (FIG. 5F), the burring punch 24 penetrates the starting material M and enters the lower die 22 and stops. One part of the burring punch 24 is received by the receiving portion 28 of the lower die 22. The outer circumferential portion of the through-hole 123 (FIG. 5E) raised by the burring punch 24, that is, the cylindrical portion 122 of the burring portion 12, is also received by the receiving portion 28 of the lower die 22. In the state in which the press apparatus 20 is at bottom dead center, the clearance between a peripheral wall 29 of the receiving portion 28 and the burring punch 24 is, for example, equal to or greater than the clearance between the forming surface 211 of the upper die 21 and the forming surface 221 of the lower die 22. When at least one part of the burring punch 24 has entered the inside of the lower die 22, the piercing punch 23 is located entirely inside the lower die 22.
In the burring process, the cylindrical portion 122 of the burring portion 12 may come in contact with the peripheral wall 29 defining the receiving portion 28 in the lower die 22. This facilitates heat dissipation from the cylindrical portion 122 by means of the burring punch 24 and the peripheral wall 29 of the receiving portion 28, and it becomes easier for transformation from austenite to martensite to occur in the microstructure of the cylindrical portion 122. However, as illustrated in FIG. 5G, in the burring process, the cylindrical portion 122 may face the peripheral wall 29 of the receiving portion 28 with a gap therebetween. Even in such case, heat can be dissipated from the cylindrical portion 122 by means of the burring punch 24 and the peripheral wall 29 of the receiving portion 28, and the microstructure of the cylindrical portion 122 can be transformed from austenite to martensite.
After the end of the burring process, the state in which the press apparatus 20 is at bottom dead center may be maintained for a predetermined time. While the state in which the press apparatus 20 is at bottom dead center is maintained, heat of the starting material M is dissipated by means of the upper die 21 and the lower die 22 and also the piercing punches 23 and the burring punches 24.
After the end of the burring process, the upper die 21 and the lower die 22 are separated from each other. More specifically, the slide 25 of the press apparatus 20 is raised to move the upper die 21 and thereby separate the upper die 21 from the lower die 22. During the period from when the press apparatus 20 reaches bottom dead center until the slide 25 begins to rise, the piercing punches 23 and the burring punches 24 are held in the state in which the piercing punches 23 and the burring punches 24 protrude from the forming surface 211 of the upper die 21. As the slide 25 rises, the expansion and contraction member 26 expands and the piercing punches 23 and the burring punches 24 are housed inside the upper die 21. After the upper die 21 has housed the piercing punches 23 and the burring punches 24, the upper die 21 separates from the lower die 22.
Preferably, the burring process finishes before martensitic transformation of the starting material M is completed. In other words, preferably formation of the through-holes 123 by the piercing punches 23 and raising of the outer circumferential portion of each through-hole 123 by the burring punches 24 are completed before the temperature of the starting material M reaches the martensitic transformation end temperature (Mf point) as the result of heat dissipation by means of the upper die 21 and the lower die 22. Preferably, the burring process is started before the temperature of the starting material M reaches the martensitic transformation start temperature (Ms point).
The burring process, for example, finishes in the first half of the holding period in which the starting material M is held by the upper die 21 and the lower die 22. In other words, preferably formation of the through-holes 123 by the piercing punches 23 and raising of the outer circumferential portion of each through-hole 123 by the burring punches 24 are completed in the first half of the holding period. The term "holding period" refers to a period from a time point at which the relative approach of the upper die 21 and the lower die 22 to each other stops and clamping of the starting material M by the forming surface 211 of the upper die 21 and the forming surface 221 of the lower die 22 starts, until a time point at which the upper die 21 and the lower die 22 separate and clamping of the starting material M is released. Although not particularly limited, the holding period is, for example, 15 seconds or more.
Through this production method, the press-formed product 10 as illustrated in FIG. 1 and FIG. 2 is obtained. The press-formed product 10 produced by the production method according to the present embodiment is subjected to quenching throughout the entirety thereof. Therefore, the formed product body 11 and the burring portion 12 have substantially the same hardness (Hv) throughout the entire thickness. In other words, a variation in the hardness (Hv) in the thickness direction of the formed product body 11 and the burring portion 12 is 30% or less. The tensile strength of the burring portion 12 is substantially the same as the tensile strength of the formed product body 11. The press-formed product 10 subjected to quenching throughout the entirety thereof may have, on one or both of the front surface and back surface, a softened layer where the hardness is lower than at the central part of the sheet thickness. The softened layer is obtained, for example, by performing hot-stamping forming on the starting material M that is a steel sheet in which the material of the outer layer has been adjusted. In this case, the press-formed product 10 subjected to quenching throughout the entirety thereof has substantially the same hardness (Hv) in the center side region, which is a region that occupies 80% of the total thickness including the center in the sheet thickness direction. The tensile strength of the formed product body 11 and the burring portion 12 is, for example, 980 MPa or more, preferably 1180 MPa or more, and more preferably 1500 MPa or more. The tensile strength of the formed product body 11 and the burring portion 12 may be 2700 MPa or less.
In the example of the present embodiment, the press-formed product 10 made of steel is produced by hot stamping. In this case, in the microstructure of the formed product body 11 and the burring portion 12, the area fraction of martensite is 85% or more. The martensite also includes tempered martensite and not just fresh martensite. In the microstructure of the formed product body 11 and the burring portion 12, a portion (the balance) other than martensite is, for example, ferrite, pearlite, bainite, and retained austenite and the like.
The area fraction of martensite in the formed product body 11 and the burring portion 12 can be measured as follows. A sample including a cross section along the sheet thickness direction is taken at an arbitrary position from each of the formed product body 11 and the burring portion 12. The cross section along the sheet thickness direction of the taken sample is mirror-polished, and thereafter etching is performed by immersing the sample in a nital solution for about 10 seconds to reveal the microstructure. Next, a field emission scanning electron microscope (FE-SEM) equipped with a secondary electron detector is used to capture a secondary electron image at a magnification of 5000× with respect to an observation region that is centered on a position located at 1/4 of the sheet thickness from the observation surface (for example, a region from a position at a depth of 1/8 to a position at a depth of 3/8 of the sheet thickness, and has a length of 500 µm along the sheet thickness direction).
In the obtained photographic image, martensite and the balance are distinguished from each other. Martensite and bainite can be easily distinguished from ferrite and pearlite based on contrast. Tempered martensite can be distinguished from bainite based on the presence or absence of iron carbides in lathshaped grains and the elongation direction of the iron carbides (cementite). Fresh martensite is not sufficiently etched by nital etching, and therefore it is possible to distinguish fresh martensite from other structures that are etched. However, because retained austenite is also not sufficiently etched, it is difficult to distinguish fresh martensite from retained austenite in the photographic image. The combined area of martensite and retained austenite can be determined from the photographic image. The combined area fraction of martensite and retained austenite is obtained by dividing the area of martensite and retained austenite by the total area of the observation region in the photographic image. The area fraction of martensite can be obtained by subtracting the area fraction of retained austenite from the combined area fraction of martensite and retained austenite.
The area fraction of retained austenite is measured as follows. The observation surface of the sample is polished using silicon carbide paper with a grit size of 600 to 1500, and thereafter the observation surface is finished to a mirror finish using a liquid in which diamond powder with a particle size of 1 to 6 µm is dispersed in a diluent such as alcohol or in pure water. Next, the sample is polished for 8 minutes at room temperature using colloidal silica that does not contain an alkaline solution to thereby remove strain introduced into the outer layer of the observation surface. Thereafter, measurement by electron backscatter diffraction is performed at measurement intervals of 0.1 µm with respect to the same observation region as in the photographic image of the secondary electron image described above, to thereby obtain crystal orientation information. The measurement is performed using an apparatus including a thermal field emission scanning electron microscope (JSM-7001F, manufactured by JEOL) and an EBSD detector (DVC5 type detector, manufactured by TSL). The degree of vacuum in the apparatus is to be set to 9.6 × 10^-5 Pa or less, the acceleration voltage is to be set to 15 kv, the irradiation current level is to be set to 13, and the electron beam irradiation level is to be set to 62. Using the Phase Map function included in the software (OIM Analysis (registered trademark)) that comes with the EBSD analysis device, the area fraction of retained austenite, which has an fcc structure, can be calculated based on the obtained crystal orientation information.

[Advantageous effects]

In the present embodiment, after the starting material M is subjected to press forming by the forming surface 211 of the upper die 21 and the forming surface 221 of the lower die 22, in a state in which the starting material M remains clamped by the forming surfaces 211 and 221, the starting material M is subjected to piercing and burring by each of the piercing punches 23 as well as the respective burring punches 24 corresponding to the piercing punches 23. Because each burring punch 24 is arranged coaxially with the corresponding piercing punch 23 in advance, when forming the burring portions 12 in the starting material M after forming, an accuracy defect caused by misalignment between the central axes of the punches 23 and 24 does not occur. Further, the piercing by the piercing punches 23 and the burring by the burring punches 24 are successively performed in a state in which the forming surfaces 211 and 221 clamp and fix the starting material M after forming. Consequently, misalignment of the central axes also does not occur between the through-hole 123 formed by the piercing and the burring punch 24, and an accuracy defect of the burring portion 12 caused by such a misalignment does not occur. Therefore, the burring portions 12 can be provided with high accuracy in the press-formed product 10 formed from the starting material M.
In the present embodiment, after the starting material M has been subjected to press forming by the forming surface 211 of the upper die 21 and the forming surface 221 of the lower die 22, piercing and burring are performed before the upper die 21 and the lower die 22 open. In this case, the number of processes can be reduced in comparison to a case where piercing and burring are performed in a process that is completely separate from the forming process, and thus the press-formed product 10 can be efficiently produced. In addition, in the present embodiment the plurality of piercing punches 23 and the plurality of burring punches 24 are provided in the upper die 21. Thus, after the starting material M has been subjected to press forming by the forming surface 211 of the upper die 21 and the forming surface 221 of the lower die 22, during the period until the upper die 21 and the lower die 22 open, the plurality of burring portions 12 can be simultaneously formed in the starting material M after forming. Therefore, the press-formed product 10 that includes the plurality of burring portions 12 can be efficiently produced.
In the present embodiment, each of the plurality of burring punches 24 is arranged coaxially with one of the piercing punches 23. Therefore, in a state in which the forming surface 211 of the upper die 21 and the forming surface 221 of the lower die 22 clamp the starting material M after forming, burring by each burring punch 24 is performed following the piercing by the piercing punches 23. At such time, a misalignment does not occur between each burring punch 24 and the preceding piercing punch 23 and the through-hole 123 formed by the relevant piercing punch 23. Therefore, the plurality of burring portions 12 can be simultaneously formed with high accuracy in the starting material M after forming.
In the present embodiment, when forming the burring portions 12 in the starting material M using each burring punch 24, at least one part of the burring punch 24 enters the inside of the lower die 22 from the forming surface 221. Each burring punch 24 enters into the lower die 22 and is received by a corresponding receiving portion 28 and stops. In this case, the outer circumferential portion of the through-hole 123 formed in the starting material M by the piercing punch 23 can be suitably raised by the burring punch 24. Therefore, the burring portion 12 is easily formed in an appropriate shape.
In the present embodiment, each of the piercing punches 23 and the corresponding burring punch 24 are formed integrally with each other. In this case, the piercing punches 23 and the burring punches 24 can be driven by a single drive mechanism, and thus the drive mechanism can be simplified.
For example, in a case where the piercing punch 23 is slid inside a cylindrical burring punch 24, there is a possibility that the wall thickness of the burring punch 24 will decrease. Specifically, in a case where the burring height of each burring portion 12 of the press-formed product 10 is small, it is necessary to make the difference between the outer diameter of the piercing punch 23 and the outer diameter of the burring punch 24 small. Consequently, the wall thickness of the cylindrical burring punch 24 is reduced, and the rigidity of the burring punch 24 decreases. On the other hand, in the present embodiment, each burring punch 24 is solid and is formed integrally with the corresponding piercing punch 23. By this means, even in the case of forming burring portions 12 having a small burring height, rigidity of the burring punch 24 can be secured.
In the present embodiment, the surface 271 of the transition portion 27 from the piercing punch 23 to the burring punch 24 is formed in a convex curved shape. In this case, when transitioning to burring by the burring punch 24 after piercing by the piercing punch 23, the occurrence of a situation in which the starting material M fractures can be suppressed.
In the present embodiment, the starting material M is heated and austenitized at a timing that is after the preparation process and is before the forming process. The starting material M is subjected to the forming process in a state in which the microstructure thereof has become austenite. In the forming process, when the starting material M has been clamped by the upper die 21 and the lower die 22 and formed into the press-formed product 10, heat of the press-formed product 10 is dissipated and martensitic transformation occurs. By this means, the press-formed product 10 hardens, and thus the press-formed product 10 that has high strength can be obtained. Because the press-formed product 10 has high strength, it is possible to thin the walls and reduce the weight of the press-formed product 10.
In the present embodiment, the starting material M that is clamped by the forming surface 211 of the upper die 21 and the forming surface 221 of the lower die 22 hardens as a result of heat of the starting material M dissipating through the upper die 21 and the lower die 22 while the clamping is being maintained. If piercing and burring were to be performed on the starting material M after hardening, a crack might occur in the starting material M or wear of the piercing punches 23 and the burring punches 24 might occur. However, in the present embodiment, piercing by the piercing punches 23 and burring by the burring punches 24 are completed while the formed starting material M is relatively soft. For example, the burring process using the piercing punches 23 and the burring punches 24 finishes before martensitic transformation of the starting material M is completed. Preferably, the burring process is completed in the first half of the holding period in which the starting material M is clamped by the upper die 21 and the lower die 22. This makes it possible to suppress the occurrence of a crack in the starting material M during the piercing or burring, and to suppress the occurrence of wear or damage to the piercing punches 23 and the burring punches 24.
In the present embodiment, the portions of the starting material M that are raised by each of the burring punches 24 to become the cylindrical portions 122 of the burring portions 12 are received by the receiving portions 28 of the lower die 22. At such time, a gap may be formed between the starting material M and the peripheral wall 29 of each receiving portion 28. By this means, movement resistance of the starting material M with respect to the peripheral wall 29 can be reduced, which facilitates removal of the starting material M turned into the press-formed product 10 from the lower die 22. However, it is preferable that the starting material M faces the peripheral wall 29 of the receiving portion 28 in close proximity thereto. In such case, heat dissipation (rapid cooling) of the cylindrical portion 122 by means of the peripheral wall 29 will be facilitated, and the burring portion 12 can be hardened. The size of the gap between the starting material M and the peripheral wall 29 is, for example, 1.0 mm or less, and from the viewpoint of ensuring hardenability, the gap is preferably 0.4 mm or less. From the viewpoint of securing wear resistance of the lower die 22, the size of the gap between the starting material M and the peripheral wall 29 is preferably 0.1 mm or more.
In the present embodiment, each of the portions of the starting material M raised by each of the burring punches 24 may come in contact with the peripheral wall 29 of the corresponding receiving portion 28 of the lower die 22. By this means, dissipation of heat and hardening of the burring portions 12 by means of the lower die 22 can be performed more reliably.
In the case of producing the press-formed product 10 by hot stamping, if the starting material M is formed by the upper die 21 and the lower die 22 after the burring portions 12 have been formed in the starting material M by the punches 23 and 24, it will be necessary to move the punches 23 and 24 when forming of the starting material M. More specifically, because heat of the burring portions 12 must be dissipated by means of the burring punches 24, the upper die 21 and the lower die 22 are caused to approach each other relatively to perform press forming of the starting material M while the burring punches 24 remain inserted within the burring portions 12. Since the portions of the starting material M where the burring portions 12 have been formed move toward the center of the upper die 21 and the lower die 22 as forming of the starting material M proceeds, it is also necessary to move each burring punch 24 and the piercing punch 23 that is integrated therewith in the lateral direction in accordance with movement of the starting material M. In such case, there is a problem that the structure of the press apparatus 20 becomes very complicated. However, in the present embodiment, after the starting material M has been subjected to forming by the upper die 21 and the lower die 22, forming of the burring portions 12 is performed by the piercing punches 23 and the burring punches 24 inside the upper die 21 and the lower die 22. Therefore, it suffices to only cause the piercing punches 23 and the burring punches 24 to protrude from the forming surface 211 of the upper die 21, and there is no necessity to move the piercing punches 23 and the burring punches 24 in the lateral direction. Consequently, the structure of the press apparatus 20 can be simplified.
A structural component for an automobile can include a plurality of components that are joined together. In a structural component, the components are generally joined together by spot welding. In a case where a component included in a structural component has a hat shape in transverse sectional view, as in the press-formed product 10 according to the present embodiment, a flange of the relevant component is joined to another component by spot welding. However, in a case where, for example, each component is formed of a high-strength steel sheet having a tensile strength of more than 780 MPa, the cross tension strength (CTS) of the spot weld zone decreases. If the tensile strength of the steel sheet is more than 1500 MPa, the tensile shear strength (TSS) also tends to decrease, and not just the cross tension strength.
In contrast, in the present embodiment a plurality of the burring portions 12 are provided in each of the flanges 114 and 115 of the press-formed product 10. Therefore, the press-formed product 10 can be mechanically joined to another component using these burring portions 12. For example, by inserting a joining member having a shaft portion and a head through the burring portion 12 which has been inserted into a through-hole provided in another component and then plastically deforming the front end of the shaft portion, the press-formed product 10 and the other component can be joined by caulking. The burring height of the burring portion 12 is, for example, 0.8 times or more and 1.2 times or less the sheet thickness of the other component. The joining member is, for example, a rivet. This kind of mechanical joint has markedly higher cross tension strength in comparison to a spot weld zone. In addition, because the burring portion 12 can come into contact with the shaft portion of the joining member over a wide area and disperse the load in the shear direction, the tensile shear strength can be increased. Therefore, the press-formed product 10 can be suitably used in a structural component for an automobile. In other words, since the press-formed product 10 including the burring portions 12 is joined to another component by a mechanical joint that has high strength, the mechanical joint is unlikely to fracture when the joined body between the press-formed product 10 and the other component is deformed by, for example, a collision. Hence, a decrease in the load capacity of the joined body can be suppressed.
When joining the press-formed product 10 to another component, preferably the front end of the shaft portion that is to be plastically deformed in the joining member is disposed on the front end (free end) side of the burring portion 12. By this means, in a case where the burring height is equal to or less than the sheet thickness of the other component and the burring height is thus insufficient relative to the sheet thickness of the other component, the insufficient amount can be filled by the plastically deformed portion of the joining member. On the other hand, in a case where the burring height is greater than the sheet thickness of the other component, the front end of the burring portion 12 protruding from the other component can be enveloped by the plastically deformed portion of the joining member. Therefore, the joining strength between the press-formed product 10 and the other component can be improved.
FIG. 6 is a transverse sectional view of the press-formed product 10 to which a sheet-shaped member 40 has been joined. Referring to FIG. 6, a structural component for an automobile can include the sheet-shaped member 40 in addition to the press-formed product 10. The sheet-shaped member 40 is joined to the press-formed product 10 by at least one joining member 50. The sheet-shaped member 40 blocks off an opening of the main body 11 of the press-formed product 10, and forms a closed cross-section together with the press-formed product 10. The sheet-shaped member 40 extends in the longitudinal direction of the press-formed product 10. In the example illustrated in FIG. 6, the sheet-shaped member 40 has a substantially flat shape. However, the shape of the sheet-shaped member 40 is not limited thereto. The sheet-shaped member 40 may include, for example, a concave or convex bead. The bead may extend in the longitudinal direction of the press-formed product 10 or in a direction that intersects with the longitudinal direction.
The sheet-shaped member 40 includes at least one through-hole 41. In the example in FIG. 6, the sheet-shaped member 40 includes a plurality of the through-holes 41. Each of the through-holes 41 penetrates the sheet-shaped member 40 in the sheet thickness direction. Each of the through-holes 41 is provided in the sheet-shaped member 40 in correspondence with a burring portion 12 of the press-formed product 10. The through-holes 41 have, for example, a circular shape in plan view. The cylindrical portion 122 of the corresponding burring portion 12 is inserted into each through-hole 41. The joining member 50 is inserted into the cylindrical portion 122 of each burring portion 12.
The joining member 50 is, for example, a rivet. The joining member 50 includes a head 51, a head 52, and a shaft portion 53. The head 51 is arranged on the press-formed product 10 side. The head 52 is arranged on the sheet-shaped member 40 side. In the example in FIG. 6, the flanges 114 and 115 of the press-formed product 10 and the sheet-shaped member 40 are arranged between the heads 51 and 52. The shaft portion 53 connects the heads 51 and 52. The flanges 114 and 115 of the press-formed product 10 and the sheet-shaped member 40 are arranged between the head 51 and the head 52.
An embodiment according to the present disclosure has been described above. However, the present disclosure is not limited to the above embodiment, and various modifications may be made without departing from the gist of the present disclosure.
In the press-formed product 10 according to the above embodiment, the cylindrical portion 122 of each burring portion 12 rises on the opposite side to the top plate 111. However, as illustrated in FIG. 7, the cylindrical portion 122 of each burring portion 12 may rise on the top plate 111 side. The direction in which the cylindrical portion 122 rises can be appropriately changed according to the stress state in the burring portion 12.
In the above embodiment, in order to raise the cylindrical portion 122 of each burring portion 12 on the opposite side to the top plate 111, the piercing punches 23 and the burring punches 24 protrude from the upper die 21. In other words, the upper die 21 corresponds to a first die that houses the piercing punches 23 and the burring punches 24, and the lower die 22 corresponds to a second die that receives the piercing punches 23 and the burring punches 24. However, in the case of producing the press-formed product 10 in which the cylindrical portion 122 of each burring portion 12 rises on the top plate 111 side, as in the example in FIG. 7, the lower die 22 is the first die that houses the piercing punches 23 and the burring punches 24, and the upper die 21 is the second die that receives the piercing punches 23 and the burring punches 24. In this case, as illustrated in FIG. 8, the piercing punches 23 and the burring punches 24 protrude from the forming surface 221 of the lower die 22 in the burring process. The upper die 21 has receiving portions 28 at positions corresponding to the piercing punches 23 and the burring punches 24. In FIG. 8, each receiving portion 28 is a space provided in the upper die 21, and opens to the forming surface 211.
In the above embodiment and the modification illustrated in FIG. 8, the upper die 21 is arranged above the lower die 22. However, the upper die 21 may be arranged below the lower die 22. Alternatively, the upper die 21 and the lower die 22 do not have to be arranged in the vertical direction. In other words, the pressing direction does not necessarily have to be the vertical direction.
In the above embodiment and the modification illustrated in FIG. 8, when forming the starting material M into the press-formed product 10, the upper die 21 and the lower die 22 are brought close to each other by moving the upper die 21. However, the upper die 21 and the lower die 22 may be brought close to each other by moving the lower die 22. It suffices that the upper die 21 and the lower die 22 can be brought close to each other relatively.
In the press-formed product 10 according to the above embodiment, a plurality of the burring portions 12 are provided in each of the flanges 114 and 115. However, the burring portions 12 may be provided in a portion other than the flanges 114 and 115. The position of the burring portions 12 in the press-formed product 10, for example, can be changed depending on the mode of joining the press-formed product 10 and another component and the like. Further, the press-formed product 10 does not necessarily have to include a plurality of burring portions 12. It suffices that the press-formed product 10 includes at least one burring portion 12. Similarly, it suffices that the press apparatus 20 includes at least a set of a piercing punch 23 and a burring punch 24.
In the above embodiment, each of the piercing punches 23 is formed integrally with a corresponding burring punch 24. However, the piercing punch 23 may be a separate body from the burring punch 24.
In the above embodiment, the piercing punches 23 and the burring punches 24 are directly connected to the slide 25 of the press apparatus 20. However, the piercing punches 23 and the burring punches 24 do not have to be connected to the slide 25. In such case, the piercing punches 23 and the burring punches 24 can operate independently of the slide 25. The operation of the piercing punches 23 and the burring punches 24 may be controlled by an actuator.
The press-formed product 10 according to the above embodiment has a substantially hat shape in transverse sectional view. However, the shape of the press-formed product 10 that includes at least one burring portion 12 is not limited thereto. The press-formed product 10 may have any shape imparted by press forming. The press-formed product 10 may, for example, have a shape which is curved partially or as a whole, or may have a shape to which a bead has been imparted.
The method for producing the press-formed product 10 according to the above embodiment includes a process of heating the starting material M. In other words, the press-formed product 10 is produced by hot stamping. However, the press-formed product 10 may be produced by cold press forming. In such case, the method for producing the press-formed product 10 need not include a process of heating the starting material M.

REFERENCE SIGNS LIST

10: Press-formed Product
11: Formed Product Body
111: Top Plate
112, 113: Vertical Wall
114, 115: Flange
12: Burring Portion
121: Hole Portion
122: Cylindrical Portion
123: Through-hole
20: Press Apparatus
21: Upper Die
22: Lower Die
211, 221: Forming Surface
23: Piercing Punch
24: Burring Punch
27: Transition Portion
271: Surface
28: Receiving Portion
29: Peripheral Wall
40: Sheet-shaped Member
50: Joining Member

Claims

A method for producing a press-formed product, comprising:
a preparation process of preparing a starting material composed of a metal sheet;

a forming process of placing the starting material between a first die including a first forming surface, and a second die including a second forming surface having a shape corresponding to the first forming surface, and bringing the first die close to the second die relatively to clamp and form the starting material by means of the first forming surface and the second forming surface; and

a burring process of, while maintaining a clamped state of the starting material by the first forming surface and the second forming surface, forming a burring portion in the starting material by causing a piercing punch housed in the first die to protrude from the first forming surface to form a through-hole in the starting material by means of the piercing punch and thereafter causing a burring punch that is arranged coaxially with the piercing punch inside the first die and that has a larger diameter than the piercing punch to protrude from the first forming surface to raise an outer circumferential portion of the through-hole by means of the burring punch.
The production method according to claim 1, wherein:
in the burring process, at least one part of the burring punch enters inside the second die from the second forming surface.
The production method according to claim 1, wherein:
the burring punch is formed integrally with the piercing punch.
The production method according to claim 3, wherein:
the burring punch is connected to the piercing punch via a transition portion; and

a surface of the transition portion has a curved shape that is convex to an outer circumferential side of the transition portion.
The production method according to claim 1, wherein:
the first die houses a plurality of the piercing punches, and a plurality of the burring punches provided in correspondence with the plurality of the piercing punches; and

in the burring process, a plurality of the burring portions are formed in the starting material by the plurality of the piercing punches and the plurality of the burring punches.
The production method according to claim 1, wherein:
the metal sheet is a steel sheet;

the production method further comprises a heating process of, after the preparation process, heating and austenitizing the starting material before the forming process; and

after the forming process, martensitic transformation of the starting material is caused by holding the starting material in a state in which the starting material remains clamped by the first forming surface and the second forming surface.
The production method according to claim 6, wherein:
the burring process finishes in a first half of a holding period that is a period from a time point at which clamping of the starting material by the first forming surface and the second forming surface is started until the first die and the second die separate and clamping of the starting material is released.
The production method according to claim 6, wherein:
the burring process finishes before martensitic transformation of the starting material is completed.
The production method according to claim 6, wherein:
the second die includes a receiving portion that is a space that is provided inside the second die and that opens to the second forming surface; and

in the burring process, the outer circumferential portion raised by the burring punch is received by the receiving portion, and faces a peripheral wall defining the receiving portion in the second die, with a gap between the outer circumferential portion and the peripheral wall.
The production method according to claim 6, wherein:
the second die includes a receiving portion that is a space that is provided inside the second die and that opens to the second forming surface; and

in the burring process, the outer circumferential portion raised by the burring punch is received by the receiving portion, and comes in contact with a peripheral wall defining the receiving portion in the second die.
A press apparatus, comprising:
a first die including a first forming surface;

a second die that includes a second forming surface facing the first forming surface and having a shape corresponding to the first forming surface, and that is capable of coming close to the first die relatively;

a piercing punch that is housed inside the first die and that is configured to protrude from the first forming surface; and

a burring punch that is housed inside the first die and is arranged coaxially with the piercing punch on an opposite side to the first forming surface with respect to the piercing punch, and that is configured to protrude from the first forming surface following the piercing punch, the burring punch having a larger diameter than the piercing punch,

wherein the second die includes a receiving portion that is a space provided inside the second die and that is arranged at a position corresponding to the piercing punch and the burring punch and opens to the second forming surface.
The press apparatus according to claim 11, wherein:
the burring punch is formed integrally with the piercing punch.
The press apparatus according to claim 12, wherein:
the burring punch is connected to the piercing punch via a transition portion; and

a surface of the transition portion has a curved shape that is convex to an outer circumferential side of the transition portion.
The press apparatus according to claim 11, comprising:
a plurality of the piercing punches; and

a plurality of the burring punches provided in correspondence with the plurality of the piercing punches.
A press-formed product that is made of steel, comprising:
a formed product body; and

a burring portion that includes a hole portion that penetrates the formed product body in a sheet thickness direction, and a cylindrical portion that rises in the sheet thickness direction from a peripheral edge of the hole portion,

wherein, in a microstructure of the formed product body and the burring portion, an area fraction of martensite is 85% or more.
The press-formed product according to claim 15, wherein:
a sheet thickness of the press-formed product is 2.0 mm or less.
The press-formed product according to claim 15, wherein:
a diameter of the hole portion is 15.0 mm or less.
The press-formed product according to claim 15, wherein:
the press-formed product comprises a plurality of the burring portions.
The press-formed product according to claim 15, wherein:
the formed product body includes:
a top plate,

two vertical walls that are connected to both sides of the top plate and are arranged so as to face each other, and

two flanges that are connected to the vertical walls on an opposite side to the top plate, respectively, and that protrude outward from the vertical walls; and

the burring portion is provided in the flanges.
A structural component for an automobile, comprising:
the press-formed product according to any one of claims 15 to 19.
The structural component according to claim 20, further comprising:
a sheet-shaped member that is joined to the press-formed product by a joining member.
An automobile, comprising:
the structural component according to claim 20.