PRODUCING METHOD AND PRODUCING APPARATUS OF PRESS FORMED PRODUCT

Abstract

A producing method of the press formed product includes a preparation step of preparing a metal sheet as a starting material, and a press working step of performing press working on the starting material by using a punch, and a pad and a die that are paired with the punch. The press working step includes a first step of forming, by the punch and the pad, the entire region of a top panel part, the entire region of each of a ridge line parts including the shape-changing part, and a region up to a predetermined height from the border between a vertical wall part and the ridge line part of a region of each of the vertical wall parts, and a second step of forming, subsequent to the first step, by the punch and the die, the remaining regions with the starting material sandwiched between the punch and the pad.

Claims

1. A method for producing a press formed product from a starting material, the press formed product comprising a top panel part, vertical wall parts respectively extending from both ends of the top panel part, and ridge line parts between the vertical wall parts and the top panel part, and including a shape-changing part in a part of the ridge line parts, the producing method comprising: a preparation step of preparing a metal sheet as the starting material; and a press working step of performing press working on the starting material by using a punch, and a pad and a die that are paired with the punch, and the press working step including: a first step of forming, by the punch and the pad, at least a region adjacent to the shape-changing part of a region of the top panel part, at least a region of the shape-changing part of a region of each of the ridge line parts, and at least a region adjacent to the shape-changing part of a region of each of the vertical wall parts and up to a predetermined height from a border between the vertical wall part and the ridge line part; and a second step of forming, subsequent to the first step, by the punch and the die, the remaining regions, with the starting material sandwiched between the punch and the pad.

2. The producing method of the press formed product according to claim 1, wherein the predetermined height is 2 mm or more from the border between the vertical wall part and the ridge line part, and is equal to or less than a half of the total height of the vertical wall part.

3. The producing method of the press formed product according to claim 1, wherein, in the first step, the entire region of the top panel part, the entire region of each of the ridge line parts, and a region up to the predetermined height from the border between the vertical wall part and the ridge line part of a region of each of the vertical wall parts are formed by the punch and the pad, and in the second step, a region beyond the predetermined height of the region of each of the vertical wall parts is formed by the punch and the die.

4. The producing method of the press formed product according to claim 1, wherein the shape-changing part of the ridge line parts is at least one of (a) to (f) as follows: (a) the height of the ridge line part is changed; (b) the arc length in a cross section of the ridge line part is changed; (c) the ridge line part is twisted; (d) the ridge line part is bent in a width direction; (e) the ridge line part protrudes or is hollowed in the width direction; and (f) the radius of curvature in the cross section of the ridge line part is changed.

5. An apparatus for producing a press formed product from a starting material, the press formed product comprising a top panel part, vertical wall parts respectively extending from both ends of the top panel part, and ridge line parts between the vertical wall parts and the top panel part, and including a shape-changing part in a part of the ridge line parts, the producing apparatus comprising a punch, and a pad and a die that are paired with the punch, the punch including a front end surface corresponding to a region of the top panel part, side surfaces corresponding to regions of the respective vertical wall parts, and punch shoulder parts corresponding to regions of the respective ridge line parts, and the pad including a bottom surface corresponding to at least a region adjacent to the shape-changing part of a region of the top panel part, a corner part corresponding to at least a region of the shape-changing part of a region of each of the ridge line parts, and an inner side surface corresponding to at least a region adjacent to the shape-changing part of a region of each of the vertical wall parts and up to a predetermined height from a border between the vertical wall part and the ridge line part.

6. A producing apparatus of a press formed product, comprising: a punch including a front end surface, side surfaces, and punch shoulder parts provided between the front end surface and the side surfaces, the punch shoulder parts being provided with a shape-changing part in a part of a longitudinal direction of the punch; a pad including a bottom surface opposing the front end surface of the punch, inner side surfaces opposing a part of the side surfaces of the punch, and corner parts provided between the bottom surface and the inner side surfaces and opposing the punch shoulder parts; and a die including inner side surfaces opposing regions of the side surfaces of the punch, except for regions opposing the inner side surfaces of the pad of the regions of the side surfaces of the punch.

7. The producing apparatus of the press formed product according to claim 5, wherein the predetermined height is 2 mm or more from the border between the vertical wall part and the ridge line part, and is equal to or less than a half of the total height of the vertical wall part.

8. The producing apparatus of the press formed product according to claim 5, wherein the bottom surface of the pad corresponds to the entire region of the top panel part, each of the corner parts of the pad corresponds to the entire region of each of the ridge line parts, and each of the inner side surfaces of the pad corresponds to a region up to the predetermined height from the border between the vertical wall part and the ridge line part of a region of each of the vertical wall parts, and wherein the die includes inner side surfaces corresponding to regions beyond the predetermined height of regions of the respective vertical wall parts.

9. The producing apparatus of the press formed product according to claim 8, wherein the die is divided to correspond to each of the vertical wall parts, and wherein the producing apparatus comprises a die moving mechanism that moves each of the dies toward a respective one of the side surfaces of the punch after completion of pushing-in of the starting material into the pad by the punch.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0065] FIG. 1 is a cross-sectional view schematically showing an exemplary structure of a press forming apparatus performing general bending forming.

[0066] FIG. 2 is a perspective view showing an example of a press formed product having a relatively complex shape.

[0067] FIG. 3 is a perspective view showing an example of a press formed product having a relatively complex shape.

[0068] FIG. 4 is a perspective view showing an example of a press formed product having a relatively complex shape.

[0069] FIG. 5 is a perspective view showing an example of a press formed product having a relatively complex shape.

[0070] FIG. 6 is a perspective view showing an example of a press formed product having a relatively complex shape.

[0071] FIG. 7 is a perspective view showing an example of a press formed product having a relatively complex shape.

[0072] FIG. 8 is a perspective view showing an example of a press formed product having a relatively complex shape.

[0073] FIG. 9 is a perspective view showing an example of the structure of a producing apparatus used in producing a press formed product according to a first embodiment of the present invention.

[0074] FIG. 10 is a perspective view showing an example of the structure of a producing apparatus used in producing the press formed product according to the first embodiment of the present invention.

[0075] FIG. 11 is a perspective view showing an example of the structure of a producing apparatus used in producing the press formed product according to the first embodiment of the present invention.

[0076] FIG. 12A is a diagram showing the status of a press working step by the producing apparatus of the first embodiment, and showing the state before starting the forming.

[0077] FIG. 12B is a diagram showing the status of the press working step by the producing apparatus of the first embodiment, and showing the state of the early phase of the forming.

[0078] FIG. 12C is a diagram showing the status of the press working step by the producing apparatus of the first embodiment, and showing the state at the completion of the forming.

[0079] FIG. 13A is a diagram showing the status of a press working step by a producing apparatus of a second embodiment, and showing the state before starting the forming.

[0080] FIG. 13B is a diagram showing the status of the press working step by the producing apparatus of the second embodiment, and showing the state of the early phase of the forming.

[0081] FIG. 13C is a diagram showing the status of the press working step by the producing apparatus of the second embodiment, and showing the state at the completion of the forming.

[0082] FIG. 14A is a diagram showing the status of a press working step by a producing apparatus of a third embodiment, and showing the state before starting the forming.

[0083] FIG. 14B is a diagram showing the status of the press working step by the producing apparatus of the third embodiment, and showing the state of the early phase of the forming.

[0084] FIG. 14C is a diagram showing the status of the press working step by the producing apparatus of the third embodiment, and showing the state at the completion of the forming.

[0085] FIG. 15 is a graph showing the study result of an example.

DESCRIPTION OF EMBODIMENTS

[0086] As a result of a great deal of considerations for solving the above-described objects, the inventors have obtained the following findings.

[0087] When forming, by the pad bending forming, a press formed product having a channel cross-section or a hat-shaped cross-section including a shape-changing part in a part of a ridge line part, a region of a material metal sheet, which region is formed into a top panel part, is restrained by a punch and a pad from beginning to end. On the other hand, the region formed into the ridge line part and the region formed into the vertical wall part are processed sequentially with the pushing-in of the punch into the die, after the region of the top panel part is restrained by the punch and the pad. Then, at the time when the pushing-in of the punch into the die reaches the bottom dead point, the regions of the ridge line part and the vertical wall part are restrained by the punch and the die. Consequently, when press working the ridge line part including the shape-changing part, the material gathers to the region of the shape-changing part and the region in the vicinity thereof from the peripheral regions, and it is likely that an excessive material excess is generated. This excessive material excess causes wrinkles in the region of the shape-changing part and the region in the vicinity thereof.

[0088] In view of above, it can be said that the generation of wrinkles can be reduced by suppressing the excessive material excess in the region of the shape-changing part and the region in the vicinity thereof. In order to do so, the region restrained by the punch and the pad from the early phase of the press working (hereinafter referred to as the “restrained region”) should be increased. The restrained region is not only the region of the top panel part, but also the region of the shape-changing part and a part of the region of the vertical wall part adjacent to the shape-changing part, where a wrinkle is likely to be generated. In this manner, at the time of press working, there is no room for a material excess to be generated in the region of the shape-changing part and the region in the vicinity thereof, and the generation of wrinkles is reduced.

[0089] The present invention has been completed on the basis of the above-described findings. Hereinafter, a description will be given of embodiments of the present invention with reference to the drawings.

First Embodiment

[0090] FIGS. 9 to 1 are perspective views showing an example of the structure of a producing apparatus used in producing a press formed product in a first embodiment of the present invention. Each of the producing apparatuses 10 shown in FIGS. 9 to 11 is a press forming apparatus. The press forming apparatus 10 shown in FIG. 9 is an apparatus that produces the first exemplary press formed product 7 having the hat-shaped cross-section shown in the above-described FIG. 2. The press forming apparatus 10 shown in FIG. 10 is an apparatus that produces the second exemplary press formed product 7 having the hat-shaped cross-section shown in the above-described FIG. 3. The press forming apparatus 10 shown in FIG. 11 is an apparatus that produces the sixth exemplary press formed product 7 having the hat-shaped cross-section shown in the above-described FIG. 7.

[0091] FIGS. 12A to 12C are diagrams showing the statuses of the press working step by the producing apparatus of the first embodiment. In these figures, FIG. 12A shows the state before starting the forming. FIG. 12B shows the state of the early phase of the forming. FIG. 12C shows the state at the completion of the forming. These figures show the cross sections of the region(s) of the shape-changing part(s).

[0092] As shown in FIGS. 9 to 12C, the press forming apparatus 10 includes a punch 11 as a lower die, and includes a die 12 and a pad 13 as an upper die. That is, the punch 11 makes a pair with the die 12 and the pad 13. Note that FIGS. 9 to 11 show the die 12 by a dashed line, so as to facilitate the understanding of the structure.

[0093] The punch 11 includes a top surface 11a (a front end surface), a pair of side surfaces 11b, and punch shoulder parts 11c provided between the top surface 11a and the side surfaces 11b. The punch shoulder parts 11c are regions connecting the top surface 11a to the side surfaces 11b. The shape of the top surface 11a is the shape corresponding to the region of the top panel part 7a of the press formed product 7. The shapes of the side surfaces 11b are the shapes corresponding to the region of the vertical wall parts 7c of the press formed product 7. The shape of the punch shoulder part 11c is the shape corresponding to the region of the ridge line part 7b (including the shape-changing part 9) of the press formed product 7. The punch shoulder part 11c is provided with a shape-changing part in a part of the longitudinal direction of the punch 11. The shape of the shape-changing part of the punch shoulder part 11c is the shape corresponding to the region of the shape-changing part 9 that exists in a part of the ridge line part 7b of the press formed product 7. In other words, the punch shoulder part 11c includes a portion where the cross-section shape changes in the longitudinal direction of the punch 11, or where the extending direction of the punch shoulder part 11c is bent.

[0094] The pad 13 is arranged opposed to the top surface 11a of the punch 11. The pad 13 includes a bottom surface 13a, a pair of inner side surfaces 13b, and corner parts 13c. The corner parts 13c are portions that connect the bottom surface 13a to the inner side surfaces 13b. The shape of the bottom surface 13a is the shape corresponding to the region of the top panel part 7a of the press formed product 7. That is, the bottom surface 13a opposes the top surface 11a of the punch 11, and its shape is the shape conforming to the top surface 11a of the punch 11. The shape of the corner part 13c is the shape corresponding to the region of the ridge line part 7b (including the shape-changing part 9) of the press formed product 7. That is, the corner part 13c opposes the punch shoulder part 11c, and its shape is the shape conforming to the punch shoulder part 11c. As used herein, “the shape conforming” means the shape where the convexity and concavity of the opposing portions with the material metal sheet sandwiched therebetween is inverted. The inner side surface 13b opposes a part of the side surface 11b of the punch 11. Specifically, the shape of the inner side surface 13b is the shape corresponding to the region having a predetermined height h from the border between the vertical wall part 7c and the ridge line part 7b of the region of the vertical wall part 7c of the press formed product 7 (see FIG. 12C).

[0095] The pad 13 is supported by an upper die holder integrally operated with a slide via a pressure member. The pressure member is a hydraulic cylinder, a gas cylinder, a spring, a rubber, etc., and applies a downward force (a force directed to the punch 11) to the pad 13. As long as the pad 13 is supported via the pressure member, the pad 13 may be supported by a slide that operates integrally with the upper die holder.

[0096] In the first embodiment, the die 12 is divided into two, and each of the dies 12 is arranged on each of the outer sides of the pad 13. Each of the dies 12 includes an inner side surface 12a. The shape of the inner side surface 12a is the shape corresponding to the region beyond the predetermined height h of the region of the vertical wall part 7c of the press formed product 7. That is, the inner side surface 12a opposes a region of the regions of the side surface 11b of the punch 11 except for the regions opposing the inner side surface 13b of the pad 13. Each of the dies 12 is connected to the upper die holder or slide via a die moving mechanism such as a cam. After the pushing-in of the punch 11 into the pad 13 reaches the bottom dead point, and the pushing-in of the metal sheet 14 into the pad 13 by the punch 11 is completed, the die moving mechanism moves each of the dies 12 obliquely downward to a respective one of the side surfaces 11b of the punch 11 (see the outlined arrows in FIG. 12B).

[0097] The producing method of the press formed product 7 using such press forming apparatus 10 according to the first embodiment includes a preparation step and a press working step as follows.

[Preparation Step]

[0098] As shown in FIGS. 9 to 12A, a metal sheet 14 is prepared as a starting material. For example, a high-strength steel sheet having a tensile strength of 440 MPa or more can be used as the metal sheet 14. The metal sheet 14 may be a 590 MPa-class high-strength steel sheet, a 980 MPa-class high-strength steel sheet, or even a 1180 MPa-class high-strength steel sheet. Additionally, a stainless steel sheet, an aluminum sheet, a copper sheet, etc. may also be used as the metal sheet 14.

[Press Working Step]

[0099] As shown in FIGS. 9 to 12C, in the press working step, press working by bending forming is performed on the metal sheet 14 by using the press forming apparatus 10, so as to produce the press formed product 7. Hereinafter, a specific description will be given of the statuses in the press working step.

[0100] As shown in FIG. 12A, after the metal sheet 14 is placed on the punch 11, the slide is descended. In this manner, the pad 13 and the dies 12 descend. Then, the pushing-in of the metal sheet 14 into the pad 13 by the punch 11 is started, and the bending forming of the metal sheet 14 is begun.

[0101] When the descending of the slide is further continued, the pushing-in by the punch 11 with respect to the pad 13 reaches the bottom dead point, and the working by the punch 11 and the pad 13 is completed. In this manner, as shown in FIG. 12B, the entire region of the top panel part 7a is formed. Also, the entire regions of the ridge line parts 7b including the shape-changing parts 9 are formed. Further, the region having the predetermined height h is formed in the region of each of the vertical wall parts 7c.

[0102] After the working by the punch 11 and the pad 13 is completed as described above, subsequently, the descending of the slide is continued to the bottom dead point with the metal sheet 14 sandwiched and restrained between the punch 11 and the pad 13. Then, each of the dies 12 is moved, by the die moving mechanism, obliquely downward toward a respective one of the side surfaces 11b of the punch 11, such that the dies 12 are become close to each other (see the outlined arrows in FIG. 12B). In this manner, the working by the punch 11 and the dies 12 is begun, and the region beyond the predetermined height h is formed in the region of each of the vertical wall parts 7c (see FIG. 12C). That is, the remaining regions are formed by the punch 11 and the dies 12. Further, in this embodiment, the flange parts 7d, which are connected to the vertical wall parts 7c, are also formed by the punch 11 and the dies 12. In this way, the press formed product 7 is obtained.

[0103] According to the press working step using the press forming apparatus 10 as described above, the region of the shape-changing part 9 and a part of the region of the vertical wall part 7c adjacent to the shape-changing part 9, where a wrinkle is likely to be generated, are restrained by the punch 11 and the pad 13 from the early phase of the working. Therefore, at the time of press working, there is no room for a material excess to be generated in the region of the shape-changing part 9 and the region in the vicinity thereof. As a result, even when a high-strength steel sheet is used as the starting material, it is possible to suppress the generation of wrinkles in the region of the shape-changing part 9 and the region in the vicinity thereof. Accordingly, in the case where the press formed product having a relatively complex shape is produced, it is possible to expedite the increasing of the strength, and it is also possible to increase the degree of freedom in designing.

[0104] As described above, the region of each of the vertical wall parts 7c adjacent to the shape-changing part 9 is formed such that, first, the region having the predetermined height h is formed by the pad 13 and the punch 11, and thereafter, the remaining regions are formed by the dies 12 and the punch 11. It is preferable that the predetermined height h is 2 mm or more from the border between the vertical wall part 7c and the ridge line part 7b, and is equal to or less than a half of the total height H (H/2) of the vertical wall part 7c. The reason is as follows. In a case where the predetermined height h is less than 2 mm from the border between the vertical wall part 7c and the ridge line part 7b, at the time of the subsequent working by the dies 12 and the punch 11, the working regions of the vertical wall parts 7c become broad, and a wrinkle is likely to be generated. On the other hand, when the predetermined height h is more than a half of the total height H of the vertical wall part 7c, at the time of the aforementioned working by the pad 13 and the punch 11, the working regions of the vertical wall parts 7c become broad, and also in this case, a wrinkle is likely to be generated. However, the lower limit for the predetermined height h is preferably 3 mm, and more preferably 5 mm. On the other hand, the upper limit for the predetermined height h is preferably 40 mm, and more preferably, 20 mm.

[0105] The press formed product 7 to be produced in the first embodiment is not limited to the first, second and sixth exemplary press formed product 7 having the hat-shaped cross-section shown in FIGS. 2, 3 and 7. That is, the press formed product 7 may be the third to fifth and seventh exemplary press formed products 7 shown in the above-described FIGS. 4 to 6 and FIG. 8, as long as a part of the ridge line part 7b includes a shape-changing part 9. In short, the shape-changing part 9 of the ridge line part 7b of the press formed product 7 is at least one of (a) to (f) as follows:

[0106] (a) the height of the ridge line part is changed;

[0107] (b) the arc length in the cross section of the ridge line part is changed;

[0108] (c) the ridge line part is twisted;

[0109] (d) the ridge line part is bent in the width direction;

[0110] (e) the ridge line part protrudes or is hollowed in the width direction; and

[0111] (f) the radius of curvature in the cross section of the ridge line part is changed.

[0112] Note that, in the first exemplary press formed product 7 shown in the above-described FIG. 2, cracks are likely to be generated in the regions of the shape-changing parts 9 and in the region in the vicinity thereof. According to the first embodiment, it is also possible to suppress such cracks.

[0113] Additionally, the pair of ridge line parts of the press formed product does not have to be parallel. For example, a pair of ridge line parts may be crossed at their ends. Additionally, the cross-section shape of the press formed product may be a channel shape without the flange parts.

Second Embodiment

[0114] FIGS. 13A to 13C are diagrams showing the statuses of the press working step by a producing apparatus according to a second embodiment. Among these figures, FIG. 13A shows the state before starting the forming. FIG. 13B shows the state of the early phase of the forming. FIG. 13C shows the state at the completion of the forming. These figures show the cross sections of the region(s) of the shape-changing part(s), as in FIGS. 12A to 12C.

[0115] A second embodiment is based on the first embodiment, and modifies a part of the first embodiment. In the second embodiment, the height of the inner side surface 13b of the pad 13 is larger compared to the above-described first embodiment. Each of the dies 12 is arranged adjacent to the either side of the pad 13, and is fixed directly to the upper die holder. That is, no special die moving mechanism is provided. Further, in the case of the second embodiment, the die 12 may be integral without being divided into two.

[0116] As in the above-described first embodiment, in the press working step of the second embodiment, first, the working by the punch 11 and the pad 13 is completed with the descending of the upper die holder (see FIG. 13B). Then, the descending of the upper die holder is subsequently continued. Consequently, each of the dies 12 descends directly, and reaches to the bottom dead point (see the outlined arrows in FIG. 13B). In this manner, in each of the regions of the vertical wall parts 7c, the region beyond the predetermined height h is formed by the punch 11 and the die 12 (see FIG. 13C). In this way, the press formed product 7 is obtained.

[0117] According to the press working step of the second embodiment, it is also possible to reduce the generation of wrinkles in the region of the shape-changing part 9 and the region in the vicinity thereof, as in the above-described first embodiment. Additionally, in the second embodiment, it is unnecessary to provide a special die moving mechanism. Therefore, the apparatus configuration is simple.

Third Embodiment

[0118] FIGS. 14A to 14C are diagrams showing the statuses of the press working step by a producing apparatus according to a third embodiment. Among these figures, FIG. 14A shows the state before starting the forming. FIG. 14B shows the state of the early phase of the forming. FIG. 14C shows the state at the completion of the forming. These figures show the cross sections of the region(s) of the shape-changing part(s), as in FIGS. 12A to 12C, and FIGS. 13A to 13C.

[0119] A third embodiment is based on the second embodiment, and modifies a part of the second embodiment. In the above-described second embodiment, the substantial working regions of the vertical wall parts 7c by the dies 12 and the punch 11 are front end parts (lower bottom parts) of the vertical wall parts 7c, and are significantly narrow. Therefore, the addition of a restriking step for fine adjusting the shape may be required. The third embodiment improves such a disadvantage (an increase in the steps) of the second embodiment.

[0120] Each of the dies 12 in the third embodiment is arranged on either outer side of the punch 11. Each of the dies 12 is connected to a lower die holder or bolster via a die moving mechanism such as a cam. Each of the dies 12 may be connected to the upper die holder or slide via the die moving mechanism such as a cam, as in the above-described first embodiment. However, after the pushing-in of the punch 11 with respect to the pad 13 reaches the bottom dead point, and the pushing-in of the metal sheet 14 into the pad 13 by the punch 11 is completed, the die moving mechanism in the third embodiment moves each of the dies 12 horizontally toward a respective one of the side surfaces 11b of the punch 11 (see outlined arrows in FIG. 14B).

[0121] As in the above-described first embodiment, in the press working step of the third embodiment, first, the working by the punch 11 and the pad 13 is completed with the descending of the upper die holder (see FIG. 14B). Then, the descending of the upper die holder is subsequently continued to the bottom dead point. Consequently, each of the dies 12 is moved, by the die moving mechanism, horizontally toward a respective one of the side surfaces 11b of the punch 11, such that the dies 12 become closer to each other (see the outlines arrows in FIG. 14B). In this manner, in the region of each of the vertical wall parts 7c, the region beyond the predetermined height h is formed (see FIG. 14C). In this way, the press formed product 7 is obtained.

[0122] According to the press working step of the third embodiment, it is also possible to reduce the generation of wrinkles in the region of the shape-changing part 9 and the region in the vicinity thereof, as in the above-described first and second embodiments. Additionally, in the third embodiment, it is possible to suppress the addition of the restriking step, which may occur in the second embodiment, because the substantial working regions of the vertical wall parts 7c by the dies 12 and the punch 11 are broad.

[0123] Also, needless to say, the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the present invention. For example, though the press forming apparatus of the above-described embodiments has the structure that includes the punch as the lower die, and includes the die and the pad as the upper die, the press forming apparatus of the above-described embodiments may have the structure in which the arrangement of the upper and lower dies is inverted.

[0124] Additionally, the restrained region of the top panel part by the punch and the pad may be at least a region adjacent to the shape-changing part of the region of the top panel part. The restrained region of the ridge line part by the punch and the pad may be at least a region of the shape-changing part in the region of each of the ridge line parts. The restrained region of the vertical wall part by the punch and the pad may be at least a region adjacent to the shape-changing part and up to the predetermined height h in each of the vertical wall parts. For example, in the region of the top panel part, those regions that are not adjacent to the shape-changing part(s) may be formed by the pad, or may be formed by a separate pad. In the regions of the ridge line parts, those regions except for the shape-changing part(s) may be formed by the pad, or may be formed by the dies. In the regions of the vertical wall parts, those regions that are not adjacent to the shape-changing part(s) may be formed by the pad, or may be formed by the dies.

Example

[0125] The degree of generation of wrinkles was studied in the case of producing the second exemplary press formed product 7 shown in the above-described FIG. 3. As the Inventive Example of the present invention, an FEM analysis was conducted assuming the press working according to the first embodiment shown in the above-described FIGS. 12A to 12C. As a Comparative Example, an FEM analysis was conducted assuming the pad bending forming shown in the above-described FIG. 1. In each of the analyses, a DP steel having a 590 MPa-class tensile strength and a sheet thickness of 1.2 mm was used as the starting material. Then, in each of the Inventive Example of the present invention and the Comparative Example, the sheet thickness increase rate was calculated for the shape-changing part, where it was feared that a wrinkle might be generated. The higher the sheet thickness increase rate, the larger the material excess becomes, and a wrinkle is likely to be generated.

[0126] As used herein, the sheet thickness increase rate A is expressed by the following Formula (1).

A=(t1−t0)/t0×100[%]  (1)

[0127] In Formula (1), t0 represents the sheet thickness before the press working, and t1 represents the sheet thickness after the press working.

[0128] FIG. 15 is a graph showing the study result of the example. As shown in FIG. 15, in the Comparative Example, the sheet thickness increase rate in the shape-changing part was more than 15%, and the generation of wrinkles was expected. In contrast, in the Inventive Example of the present invention, the sheet thickness increase rate in the shape-changing part was suppressed to about 4%, which was the level at which a wrinkle was not generated.

REFERENCE SIGNS LIST

[0129] 7: press formed product [0130] 7a: top panel part [0131] 7b: ridge line part [0132] 7c: vertical wall part [0133] 7d: flange part [0134] 8: step height part [0135] 9: shape-changing part [0136] 10: press forming apparatus [0137] 11: punch [0138] 11a: front end surface [0139] 11b: side surface [0140] 11c: punch shoulder part [0141] 13: pad [0142] 13a: bottom surface [0143] 13b: inner side surface [0144] 13c: corner part [0145] 12: die [0146] 12a: inner side surface [0147] 14: starting material (metal sheet)