Press forming method and method for manufacturing a press-formed product

Abstract

To provide a press molding method and a method for manufacturing a press molded article which do not require a trimming step after press molding, sufficiently suppress wrinkles of a flange part generated by shrinking flange deformation, and can be applied to bending molding.SOLUTION: A press molding method and a method for manufacturing a press molded article, which are methods for molding a press molded article 1 that has a top plate part 3 having a convex outer peripheral edge part 3a where an outer peripheral edge or a part thereof is curved outward in a convex shape, a vertical wall part 5 continuous to the top plate part 3, and a flange part 7 continuous to the vertical wall part 5, includes: a first molding step of molding a metal plate into an intermediate molded article 15; and a second molding step of molding the intermediate molded article 15 molded in the first molding step into the press molded article 1 having a target shape, wherein the intermediate molded article 15 has a height from the top plate part 3 in at least a part corresponding to the convex outer peripheral edge part 3a of the top plate part 3 to a flange part 25 lower than a height from the top plate part 3 to the flange part 7 of the press molded article 1 having the target shape.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a press forming method and a manufacturing method of a press-formed product having a top plate portion, a vertical wall portion, and a flange portion, and particularly relates to a press forming method and a manufacturing method of a press-formed product that suppresses the generation of wrinkles associated with shrink flange deformation when forming the press-formed product.

Background Art

[0002] Due to the stricter collision safety standards for automobiles, while the collision safety of vehicle bodies is improving, in response to carbon dioxide emissions regulations, weight reduction of vehicle bodies is also required for better fuel efficiency and electrification (EV). To achieve both improved collision safety and weight reduction of vehicle bodies, the application of high-strength steel sheets of 590 MPa or higher (also referred to as high-tensile materials) to vehicle body structural parts is progressing. When press-forming high-tensile materials into vehicle body structural parts, suppressing wrinkles caused by shrink flange deformation has become an issue.

[0003] For example, among automotive parts, there are parts having a top plate portion, a vertical wall portion, and a flange portion, such as the A-pillar upper, A-pillar lower, bumper parts, etc. In such parts, when the outer peripheral edge or a part of the top plate portion has a convexly curved shape outward, during press forming, the vertical wall portion and the flange portion of this part undergo shrink flange deformation, and wrinkles may occur at the ends of the vertical wall portion and the ends of the flange portion. Especially in the case of high-tensile materials, they are more likely to buckle due to increased strength and are more likely to generate wrinkles.

[0004] Therefore, Patent Document 1 discloses a method of preventing buckling of a blank material in the plate thickness direction and suppressing wrinkles generated in a slant wall portion by sandwiching a portion on the end side rather than the portion corresponding to the slant wall portion in the blank material between a die and a punch during forming when forming a press-formed product having a top plate portion, a slant wall portion continuous with at least one side of the top plate portion and having no flange at the tip, and the whole or a part of the slant wall portion being convexly curved toward the slant wall portion side in the longitudinal direction of the press-formed product in plan view, by forming the slant wall portion while sandwiching the end side portion.

[0005] Further, Patent Document 2 discloses a method for manufacturing a press-formed product by press-forming a metal plate into a product shape having a hat-shaped cross section in which a top plate portion and a flange portion are continuous in the width direction via a side wall portion, and the top plate portion and the flange portion have a curved portion that is convex toward the top plate portion along the longitudinal direction. When performing the forming by stepwise drawing, a wrinkle suppressing region for pressing the metal plate with a wrinkle press is set in an outer peripheral portion rather than the flange portion position, and a stepwise drawing process is performed. When performing the forming by stepwise drawing, an additional region for pressing with a wrinkle press is also set in a part of the flange portion position to suppress wrinkles generated in the flange portion.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0007] However, in the press-forming method described in Patent Document 1, since the inclined wall portion is formed in a state where a portion on the end side rather than a portion corresponding to the inclined wall portion in the blank material is clamped by the die and the punch, it is necessary to trim the portion clamped by the die and the punch in the next step. Furthermore, the method of Patent Document 1 also has a problem that it cannot be applied to the forming of a press-formed product having a flange portion continuous with the vertical wall portion (inclined wall portion).

[0008] In this regard, although the method for manufacturing a press-formed product described in Patent Document 2 can be applied to the forming of a press-formed product having a flange portion, since a wrinkle press is used, there is a problem that it cannot be applied to press-forming by bending (forming).

[0009] The present invention has been made to solve such problems, and an object thereof is to provide a press forming method and a method for manufacturing a press formed product that can sufficiently suppress wrinkles in a flange portion caused by shrink flange deformation without requiring a trimming process after press forming and can also be applied to bending forming.

Means for Solving the Problems

[0010] (1) The press forming method according to the present invention is a method for forming a press formed product having a top plate portion having a convex outer peripheral edge portion whose outer peripheral edge or a part thereof is curved convexly outward, a vertical wall portion continuous with the top plate portion, and a flange portion continuous with the vertical wall portion. The method includes a first forming step of forming a metal plate into an intermediate formed product, and a second forming step of forming the intermediate formed product formed in the first forming step into the press formed product having a target shape. The intermediate formed product is characterized in that the height from the top plate portion to the flange portion of at least a portion corresponding to the convex outer peripheral edge portion of the top plate portion is lower than the height from the top plate portion to the flange portion of the press formed product having the target shape.

[0011] (2) Further, in the above (1), the first forming step applies drawing forming or bending forming, and the second forming step applies bending forming.

[0012] (3) Further, in the above (1) or (2), the metal plate is a steel plate having a tensile strength of 590 MPa or more.

[0013] (4) Further, the method for manufacturing a press-formed product according to the present invention is a method for manufacturing a press-formed product having a top plate portion with a convex outer peripheral edge portion where the outer peripheral edge or a part thereof is curved convexly outward, a vertical wall portion continuous with the top plate portion, and a flange portion continuous with the vertical wall portion, comprising a first forming step of forming a metal plate into an intermediate formed product, and a second forming step of forming the intermediate formed product formed in the first forming step into the press-formed product having a target shape, wherein the height from the top plate portion to the flange portion of at least a portion corresponding to the convex outer peripheral edge portion of the top plate portion of the intermediate formed product is lower than the height from the top plate portion to the flange portion of the press-formed product having the target shape.

[0014] (5) Further, in the above (4), the first forming step applies drawing forming or bending forming, and the second forming step applies bending forming.

[0015] (6) Further, in the above (4) or (5), the metal plate is a steel plate having a tensile strength of 590 MPa or more.

Effects of the Invention

[0016] In the present invention, there are provided a first forming step of forming a metal plate into an intermediate formed product and a second forming step of forming the intermediate formed product into a press-formed product having a target shape. The height from the top plate portion to the flange portion of at least a portion corresponding to the convex outer peripheral edge portion of the top plate portion of the intermediate formed product is lower than the height from the top plate portion to the flange portion of the press-formed product having the target shape. Thus, in the first forming step, the amount of shrink flange deformation can be reduced to form the intermediate formed product. Also, in the second forming step, since the intermediate formed product having higher rigidity than a flat blank is formed into the target shape, material movement due to shrink flange deformation hardly occurs and wrinkles hardly occur. Therefore, an increase in the plate thickness of the formed product having the target shape can be suppressed, a press-formed product having a good shape without wrinkles can be obtained, leading to an improvement in the yield in press forming. Also, since it is not necessary to sandwich the end portion of the blank with a punch and a die, the conventional trimming process is not essential. Furthermore, since no wrinkle suppression is required, it is also applicable to bending forming.

Brief Description of the Drawings

[0017]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Modes for Carrying Out the Invention

[0018] Regarding the press-formed product targeted by the press-forming method and the manufacturing method of the press-formed product according to this embodiment, it will be described based on the example of FIG. 2. Note that FIG. 2 shows a part of the press-formed product. The press-formed product 1 shown in FIG. 2 has a top plate portion 3, a vertical wall portion 5, and a flange portion 7, and a part of the outer peripheral edge of the top plate portion 3 has a portion curved convexly outward (hereinafter referred to as "convex outer peripheral edge portion 3a"). Note that the boundary between the convex outer peripheral edge portion 3a and other portions is, for example, up to the R stop of the convex outer peripheral edge portion 3a when the top plate portion 3 is viewed in plan view. Also, in this example, the angle formed by the vertical wall portion 5 and the flange portion 7 of the press-formed product 1 is set to 90°.

[0019] The boundary portion between the top plate portion 3 and the vertical wall portion 5 in the press-formed product 1 has an R shape corresponding to the shape of the punch shoulder used in press forming, so this portion is referred to as "punch shoulder R portion 9". Also, since the boundary portion between the vertical wall portion 5 and the flange portion 7 has an R shape corresponding to the shape of the die shoulder, it is referred to as "die shoulder R portion 11". Hereinafter, when simply referred to as "punch shoulder R portion 9" and "die shoulder R portion 11" in this specification, it refers to the above-mentioned portions of the press-formed product 1 rather than the mold.

[0020] First, prior to describing the press-forming method and the manufacturing method of the press-formed product according to this embodiment, regarding the problems in the case of press-forming the press-formed product 1 as shown in FIG. 2 by the conventional method, an example will be given where the height from the top plate portion 3 to the flange portion 7 of the target shape is 16 mm. FIG. 9 shows the result of FEM analysis when the press-formed product 1 is press-formed by the conventional method, and shows the distribution of the sheet thickness increase rate by the shade of color. The sheet thickness increase rate is obtained by finding the difference (sheet thickness increment) between the sheet thickness of the press-formed product 1 after press forming and the sheet thickness of the blank before press forming, and is expressed as a ratio (percentage) to the sheet thickness of the blank, indicating that the larger the value, the more the sheet thickness has increased. Also, the more the sheet thickness increases, the more likely wrinkles are to occur in that portion of the press-formed product. Furthermore, the more locally the sheet thickness increase becomes, the more likely it is to wrinkle.

[0021] When forming the press-formed product 1 as shown in Fig. 2 by a conventional method, for example, a punch and a die having a shape corresponding to the target shape are used, and a flat blank is formed into the target shape in one step. In this case, the vertical wall portion 5 and the flange portion 7 corresponding to the convex outer peripheral edge portion 3a of the top plate portion 3 are likely to shrink and flange-deform, causing the material to concentrate and wrinkles to easily occur. In the case of the press-formed product 1 shown in Fig. 2, the part where the plate thickness increased the most was the end portion of the flange portion 7 indicated by the arrow in Fig. 9, and the maximum plate thickness increase rate was +12.5%. Thus, wrinkles occurred in this part due to the local increase in plate thickness, which was a problem. The reason for the local increase in the plate thickness of the flange portion 7 as shown in Fig. 9 will be explained with reference to Fig. 10.

[0022] Fig. 10 shows the forming process when forming the press-formed product 1 by the above-described conventional press-forming method. In Fig. 10, the process of the blank 13 deforming is shown by a front view (the upper view, the view seen from the arrow direction in Fig. 2(b)) and a cross-sectional view (the lower view) corresponding to the A-A' cross-section in Fig. 2(b), respectively. Note that numerical values such as "5mmup" in the figure indicate the distance in the press-forming direction between the punch 21 and the die 23 (excluding the plate thickness). Therefore, in the case of "5mmup", it shows that the gap in the press-forming direction between the portion of the punch 21 that forms the flange portion 7 of the press-formed product 1 and the portion of the die 23 that forms the flange portion 7 is a figure with +5mm added to the plate thickness of the blank 13. Also, "0mmup" indicates the state at the forming bottom dead center.

[0023] As shown in Fig. 10, when the vertical wall portion 5 corresponding to the convex outer peripheral edge portion 3a of the top plate portion 3 starts to be formed, as shown in the front view ("10mmup", the upper view), for example, two large mountain-shaped wrinkles occur at the end of the blank 13 due to the shrink flange deformation. These two large mountain-shaped wrinkles become more distinct as the shrink flange deformation progresses and concentrate at the center of the convex outer peripheral edge (see the front views of "5mmup" and "3mmup").

[0024] As the forming progresses and the lower surface of the die 23 reaches the top of the peak, the forming progresses such that the die 23 crushes the wrinkles. However, when the forming progresses up to "1 mm up", the blank 13 is constrained while leaving the wrinkles and reaches the bottom dead center of forming (see "0 mm up").

[0025] As described above, in the conventional forming process, large wrinkles are generated in the gap between the punch 21 and the die 23, and the flange portion 7 is formed while the wrinkles cannot be completely crushed. Therefore, wrinkles remain in the press-formed product 1, and the plate thickness of the portion where the wrinkles are generated locally increases.

[0026] As a means to prevent wrinkles from occurring during the forming process, it is advisable to use a wrinkle presser at the portion corresponding to the flange portion 7, but it cannot be applied in the bending forming because there is no wrinkle presser.

[0027] Therefore, the press-forming method of the present embodiment takes measures that can reduce the generation of wrinkles in the flange portion 7 more than before and can be applied even in bending forming.

[0028] Specifically, the press-forming method according to the present embodiment is a method of forming a press-formed product 1 having a top plate portion 3 having a convex outer peripheral edge portion 3a in which a part of the outer peripheral edge is curved convexly outward, a vertical wall portion 5 continuous with the top plate portion 3 via a punch shoulder R portion 9, and a flange portion 7 continuous with the vertical wall portion 5 via a die shoulder R portion 11. As shown in FIG. 1, it includes a first forming step of forming a blank 13, which is a metal plate, into an intermediate formed product 15, and a second forming step of forming the intermediate formed product 15 into a press-formed product 1 having a target shape. Note that by executing the press-forming method, the press-formed product 1 is manufactured. Therefore, the invention of the press-forming method can be configured as an invention of a manufacturing method of a press-formed product. Therefore, the embodiments of the press-forming method described below are common to the embodiments of the manufacturing method of the press-formed product.

[0029] FIG. 1(a) is a perspective view of the punch 17, die 19, and blank 13 before forming in the first forming process, and FIG. 1(b) is a cross-sectional view taken along line B of FIG. 1(a). Also, FIG. 1(c) is a perspective view of the punch 21, die 23, and intermediate formed product 15 before forming in the second forming process, and FIG. 1(d) is a cross-sectional view taken along line C of FIG. 1(c). Note that in FIGS. 1(a) to 1(d), each mold is shown in a plate shape only with the shape of the forming surface portion, ignoring the thickness portion. Also, in the intermediate formed product 15 shown in FIG. 1(d), the same parts as those of the press formed product 1 are denoted by the same reference numerals. Hereinafter, each process will be described in detail.

[0030] <First Forming Process> The first forming process is a process of press-forming the blank 13, which is a metal plate, into the intermediate formed product 15 as shown in FIGS. 1(a) and 1(b). The punch 17 and die 19 have a height hd1 from the forming surface portion corresponding to the top plate portion 3 of the intermediate formed product 15 to the forming surface portion corresponding to the flange portion 25, which is lower than the height hd0 from the forming surface portion corresponding to the top plate portion 3 of the press formed product 1 to the forming surface portion corresponding to the flange portion 7 in the punch 21 and die 23 used in the second forming process for forming into the target shape (hd1 < hd0).

[0031] In the first forming process, as shown in FIG. 1(b), with a part of the blank 13 sandwiched between the upper surface of the top plate forming surface portion of the punch 17 and the pad 27, the die 19 is lowered to form an intermediate formed product 15 (see FIG. 1(d)) in which the height hd1 from the top plate portion 3 to the flange portion 25 (hereinafter simply referred to as "the height of the intermediate formed product 15") is lower than the height hd0 from the top plate portion 3 to the flange portion 7 of the press formed product 1 having the target shape (hereinafter simply referred to as "the height of the press formed product 1"). Note that the angle formed by the vertical wall portion 29 and the flange portion 25 of the intermediate formed product 15 in this example is the same 90° as the angle formed by the vertical wall portion 5 and the flange portion 7 of the target shape. When forming the vertical wall portion 29 of the intermediate molded product 15 in the first forming step, shrink flange deformation occurs. However, as described above, since the height of the intermediate molded product 15 is lower than the height of the press molded product 1 of the target shape, the amount of shrink flange deformation is smaller compared to the case of forming the conventional flat blank 13 into the target shape. Therefore, in the first forming step, it is difficult for the thickness of the flange portion 25 of the intermediate molded product 15 to increase, and wrinkles are less likely to occur.

[0032] <Second Forming Step> The second forming step is a step of forming the intermediate molded product 15 formed in the first forming step into the press molded product 1 of the target shape. The punch 21 and the die 23 in the second forming step have shapes corresponding to the target shape, and are the same as the molds in the conventional example of FIG. 10, so the same reference numerals are given.

[0033] In the second forming step, as shown in FIG. 1(d), the punch shoulder R portion 9 of the intermediate molded product 15 is set to match the shoulder portion of the punch 21, and the die 23 is relatively moved while the top plate portion 3 of the intermediate molded product 15 is clamped between the punch 21 and the pad 27 to form the intermediate molded product 15 into the target shape. The state of the forming process in the second forming step is shown in FIG. 3. In FIG. 3, similar to FIG. 10, the deformation process of the blank 13 is shown by a front view (the upper figure, the figure viewed from the arrow direction in FIG. 2(b)) and a cross-sectional view (the lower figure) corresponding to the A-A' cross-section in FIG. 2(b). The meaning of numerical values such as "5mm up" is also the same as in FIG. 10. The height of the intermediate molded product 15 is set to 12 mm, and the height of the press molded product 1 (target shape) is set to 16 mm.

[0034] When the die 23 is lowered from the state of Fig. 1(d), the die 23 contacts the flange portion 25 of the intermediate molded product 15 at "5 mm up". Thereafter, the die 23 bends back the die shoulder R portion 31 of the intermediate molded product 15, and the lower part of the vertical wall portion 5 begins to be molded. Although this molding involves shrink flange deformation, since the intermediate molded product 15 is work-hardened, it has higher rigidity than the flat blank 13 and the material is less likely to move. Therefore, at "5 mm up", instead of the conventional two local mountain shapes, even if wrinkles occur as shown in the front view of "3 mm up", at this time, the gap in the press molding direction between the die 23 and the punch 21 is as small as the plate thickness + 3 mm, so the wrinkles are crushed by the die 23 without becoming large and reach the molding bottom dead center. Thus, due to the high rigidity of the intermediate molded product 15, wrinkles are less likely to occur during the molding process, and in the second molding step, it is also difficult for the plate thickness to increase at the flange portion 7 of the press molded product 1, and wrinkles are less likely to occur.

[0035] The distributions of the plate thickness increase rates of the intermediate molded product 15 and the press molded product 1 in the above-described example are shown in Fig. 4. Fig. 4(a) shows the distribution of the plate thickness increase rate of the intermediate molded product 15 at the molding bottom dead center of the first molding step. Fig. 4(b) shows the distribution of the plate thickness increase rate of the press molded product 1 at the molding bottom dead center of the second molding step. Note that the distributions of the plate thickness increase rates in Fig. 4(a) and Fig. 4(b) are shown in a smaller range than the conventional example of Fig. 9.

[0036] As can be seen by comparing Fig. 4(a) and Fig. 4(b), the plate thickness increase of the press molded product 1 molded in the second molding step is dispersed over a wider range than that of the intermediate molded product 15 molded in the first molding step. This indicates that the strain due to the shrink flange deformation during the second molding step is dispersed over a wider range than the strain due to the shrink flange deformation during the first molding step, and it can also be seen from this figure that it is difficult for the plate thickness to increase locally and for wrinkles to occur in the second molding step.

[0037] As described above, in the first forming step of the present embodiment, an intermediate molded product 15 having a shape in which the height from the top plate portion 3 to the flange portion 25 is lower than the target shape is formed, and in the second forming step, the intermediate molded product 15 is formed into the target shape, thereby solving the problem of local increase in plate thickness and suppressing wrinkles generated in the flange portion 7 of the press-molded product 1. Furthermore, since it is not necessary to sandwich the end portion of the blank with a punch and a die, a trimming process is not required as in the conventional example shown in Patent Document 1. Note that by executing the first forming step and the second forming step of the above-described press forming method, a target press-molded product can be manufactured, and the manufactured press-molded product has wrinkles suppressed as described above.

[0038] In addition, the press forming method and the method for manufacturing a press-molded product according to the present embodiment can suppress wrinkles in the flange portion 7 without using a wrinkle suppressor, so that it can be applied to press forming by bending (forming). That is, it is particularly effective when drawing or bending is applied in the first forming step of forming the intermediate molded product 15 and bending is applied in the second forming step of forming the target shape.

[0039] Furthermore, the press forming method and the method for manufacturing a press-molded product according to the present embodiment are particularly effective when using a high-strength steel sheet that is likely to generate wrinkles due to shrink flange deformation. For example, the metal sheet (blank) may be a steel sheet having a tensile strength of 590 MPa or more, and in that case, a sufficient effect of reducing wrinkles can also be achieved.

[0040] Note that the press forming method and the method for manufacturing a press-molded product according to the present embodiment are configured to be able to reduce wrinkles generated in the molded product having the target shape more than before by forming the intermediate molded product 15 having a height from the top plate portion 3 to the flange portion 25 lower than the target shape. This point will be further described.

[0041] Fig. 5(a) shows, by a dashed line, the cross-sectional shape of the blank 13 before forming in the first forming process, and, by a solid line, the cross-sectional shape of the intermediate formed product 15 at the bottom dead center after forming. Here, the distance from the end of the blank 13 to the end of the flange portion 25 of the intermediate formed product 15 in Fig. 5(a) is defined as the material inflow amount in the first forming process. The material inflow amount in this first forming process becomes smaller as the height of the intermediate formed product 15 is lower. When the material inflow amount is small, the amount of shrink flange deformation in the first forming process becomes small. Therefore, by reducing the height of the intermediate formed product 15, the increase in plate thickness (increase in plate thickness from the blank 13) due to the first forming process can be reduced.

[0042] Fig. 5(b) shows, by a dashed line, the cross-sectional shape of the intermediate formed product 15 at the bottom dead center of the first forming process, and, by a solid line, the cross-sectional shape of the press-formed product 1 at the bottom dead center of the second forming process. Here, the distance from the end of the flange portion 25 of the intermediate formed product 15 to the end of the flange portion 7 of the press-formed product 1 in Fig. 5(b) is defined as the material inflow amount in the second forming process. The material inflow amount in this second forming process becomes larger as the height of the intermediate formed product 15 is lower. When the material inflow amount is large, the amount of shrink flange deformation in the second forming process becomes large. Therefore, by reducing the height of the intermediate formed product 15, the increase in plate thickness (increase in plate thickness from the intermediate formed product 15) due to the second forming process becomes larger. Also, the lower the height of the intermediate formed product 15, the larger the gap in the press-forming direction between the punch 21 and the die 23 in the second forming process when the forming of the vertical wall portion 5 starts. Therefore, wrinkles are likely to increase during the forming process, which becomes a factor in the increase in plate thickness of the formed product with the target shape.

[0043] As described above, the lower the height of the intermediate formed product, the smaller the material inflow amount in the first forming process and the larger the material inflow amount in the second forming process. As an example of the above relationship, Fig. 6 shows the relationship between the height of the intermediate formed product 15 and the material inflow amount in each process when the height from the top plate portion to the flange portion of the target shape is 16 mm.

[0044] When the height of the intermediate formed product 15 is decreased, the amount of material flowing in during the first forming process can be reduced and an increase in plate thickness can be decreased. However, the amount of material flowing in during the second forming process may increase, resulting in an increase in the increase in plate thickness and it may not be possible to sufficiently reduce the final increase in plate thickness. Therefore, the present invention becomes more effective by setting the height of the intermediate formed product 15 so as to balance the reduction in the increase in plate thickness in both the first forming process and the second forming process. For example, it is preferable that the height of the intermediate formed product is about 50% to 90% of the height of the target shape, as this provides a good balance in the amount of material flowing in during each process. This point will be specifically described together with the effects of the present invention in the following examples.

Example

[0045] Regarding the effect of suppressing wrinkles in the shrink flange deformation in the press forming method and the method for manufacturing a press formed product of the present invention, specific examinations were conducted using FEM analysis, and the results will be described below. In this example, a steel sheet with a thickness of 1.0 mm and a tensile strength of 980 MPa grade was used as a blank, and the case of press forming with the press formed product 1 in FIG. 2 as the target shape was confirmed. The height of the target shape is 16 mm. FEM analysis was performed on a conventional example in which the steel sheet was formed into the target shape in one step and an example of the present invention in which the steel sheet was formed into the target shape in two steps, and the maximum plate thickness increase rate at the shrink flange deformation site was obtained. Since the analysis results of the conventional example are as described in FIG. 9, the analysis results of the example of the present invention will be described below.

[0046] In the example of the present invention, FEM analysis was performed on four cases where the height of the intermediate formed product 15 was 14 mm, 12 mm, 10 mm, and 8 mm. FIG. 7 shows the plate thickness increase rate distribution at the bottom dead center of forming the intermediate formed product 15 in the first forming process, and FIG. 8 shows the plate thickness increase rate distribution at the bottom dead center of forming the formed product (press formed product 1) of the target shape in the second forming process. In FIGS. 7 and 8, numerical values such as "14 mm" indicate the height of the intermediate formed product 15, and numerical values such as "7.4%" indicate the maximum plate thickness increase rate. Also, the maximum plate thickness increase rates in FIGS. 7 and 8 are both shown as increase rates based on the thickness of the blank 13. Table 1 shows a summary of the results in FIGS. 7 to 9.

[0047]

Table 1

[0048] As shown in FIGS. 7 to 9 and Table 1, in the conventional example, the maximum plate thickness increase rate of the molded product (press-molded product 1) with the target shape was 12.5%, whereas in the examples of the present invention (No. 2 to No. 5), the maximum plate thickness increase rates of the molded products with the target shape were all reduced compared to the conventional example. As a result, as described above, in the present embodiment, it was shown that the present invention can suppress flange wrinkles due to shrinkage flange deformation more effectively than before. As described above, by setting the height of the intermediate molded product 15 so that the increase in plate thickness can be reduced well in both the first molding step and the second molding step, wrinkles can be suppressed more effectively. This will be specifically described below.

[0049] When comparing the maximum plate thickness increase rates of the intermediate molded products 15 of No. 2 to No. 5, it can be seen that in the first molding step, the lower the height of the intermediate molded product 15, the smaller the maximum plate thickness increase rate of the intermediate molded product 15 in the first molding step. This is because the lower the height of the intermediate molded product 15, the lower the material inflow amount (see FIG. 5(a)) in the first molding step.

[0050] On the other hand, in the second molding step, when comparing the maximum plate thickness increase rates of the molded products with the target shape of No. 2 to No. 5, it can be seen that the smallest maximum plate thickness increase rate is No. 3, and for No. 4 and No. 5 in which the height of the intermediate molded product 15 is lower than that of No. 3, the maximum plate thickness increase rate is higher than that of No. 3. This is because the lower the height of the intermediate molded product 15, the earlier the start time of forming the vertical wall portion 5, and the greater the material inflow amount (see FIG. 5(b)) in the second molding step.

[0051] Therefore, as in the example of No. 3 in Table 1, when setting the height of the intermediate molded product 15 so as to minimize the increase in plate thickness after the second molding step, the effect of suppressing wrinkles can be maximized, which is effective.

Explanation of Reference Numerals

[0052] 1 Press-formed product (target shape) 3 Top plate part 3a Convex outer peripheral edge part 5 Vertical wall part (press-formed product) 7 Flange part (press-formed product) 9 Punch shoulder R part 11 Die shoulder R part (press-formed product) 13 Blank (metal plate) 15 Intermediate formed product 17 Punch (first forming process) 19 Die (first forming process) 21 Punch (second forming process, conventional example) 23 Die (second forming process, conventional example) 25 Flange part (intermediate formed product) 27 Pad 29 Vertical wall part (intermediate formed product) 31 Die shoulder R part (intermediate formed product)

Claims

1. A press forming method for forming a press-formed product having a top plate portion with a convex outer peripheral edge portion whose outer peripheral edge or a part thereof is curved convexly outward, a vertical wall portion continuous with the top plate portion, and a flange portion continuous with the vertical wall portion as a target shape, comprising: a first forming step of forming a metal plate into an intermediate formed product having a top plate portion with a convex outer peripheral edge portion whose outer peripheral edge or a part thereof is curved convexly outward, a vertical wall portion continuous with the top plate portion, and a flange portion continuous with the vertical wall portion; a second forming step of forming the intermediate formed product formed in the first forming step into the press-formed product having the target shape; in the intermediate formed product, the height from the top plate portion to the flange portion of at least the portion corresponding to the convex outer peripheral edge portion of the top plate portion is lower than the height from the top plate portion to the flange portion of the press-formed product having the target shape, and the angle formed by the vertical wall portion and the flange portion is the same as that of the target shape; In the second forming step, the punch shoulder R portion of the intermediate formed product is set in alignment with the shoulder portion of the punch, and the die shoulder R portion of the intermediate formed product is bent back to form the intermediate formed product into the target shape. The press forming method is characterized by this.

2. The press forming method according to claim 1, characterized in that the metal plate is a steel plate having a tensile strength of 590 MPa or more.

3. A method for manufacturing a press-formed product for manufacturing a press-formed product having a top plate portion with a convex outer peripheral edge portion whose outer peripheral edge or a part thereof is curved convexly outward, a vertical wall portion continuous with the top plate portion, and a flange portion continuous with the vertical wall portion as a target shape, comprising: a first forming step of forming a metal plate into an intermediate formed product having a top plate portion with a convex outer peripheral edge portion whose outer peripheral edge or a part thereof is curved convexly outward, a vertical wall portion continuous with the top plate portion, and a flange portion continuous with the vertical wall portion; a second forming step of forming the intermediate formed product formed in the first forming step into the press-formed product having the target shape; in the intermediate formed product, the height from the top plate portion to the flange portion of at least the portion corresponding to the convex outer peripheral edge portion of the top plate portion is lower than the height from the top plate portion to the flange portion of the press-formed product having the target shape, and the angle formed by the vertical wall portion and the flange portion is the same as that of the target shape; In the second forming step, the punch shoulder R portion of the intermediate formed product is set in alignment with the shoulder portion of the punch, and the die shoulder R portion of the intermediate formed product is bent back to form the intermediate formed product into the target shape. A method for manufacturing a press-formed product, characterized by this.

4. The method for manufacturing a press-formed product according to claim 3, characterized in that the metal plate is a steel plate having a tensile strength of 590 MPa or more.

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