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

Abstract

To provide a press forming method which does not require a trimming step after press forming, which sufficiently suppresses wrinkles generating by a shrink flange deformation, and which can be applied to bending forming, and to provide a method of manufacturing a press formed product.SOLUTION: A press forming method and a method of manufacturing a press formed product are for forming a press formed product 1 including a top plate part 3 having a convex outer peripheral edge part 3a in which an outer peripheral edge or part of it is bent in a convex manner outward, and a vertical wall part 5 connected to the top plate part 3. The press forming method and the method of manufacturing the press formed product include: a first forming step of forming a metal plate into an intermediate formed product 15; and a second forming step of forming the intermediate formed product 15 formed in the first forming step into a press formed product 1 of a target shape. In the intermediate formed product 15, at least an angle formed by the top plate part 3 of a portion corresponding to the convex outer peripheral edge part 3a of the top plate part 3 and a vertical wall part 25 is larger than an angle formed by the top plate part 3 of the press formed product 1 of a target shaped and the vertical wall part 5.SELECTED DRAWING: Figure 1

Description

Technical Field

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

Background Art

[0002] With the tightening of automotive crash safety standards, as the crash safety of vehicle bodies is improving, due to carbon dioxide emissions regulations, weight reduction of vehicle bodies is also required for improving fuel efficiency and electrification. In order to achieve both improvement in crash safety and weight reduction of the vehicle body, 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 generated 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 an A-pillar upper, an A-pillar lower, and bumper parts. When the outer peripheral edge or a part thereof of the top plate portion has a convexly curved shape outward in such parts, the flange portion of the part undergoes shrink flange deformation during press forming, and wrinkles may occur at the end of the flange portion. Particularly in the case of high-tensile materials, they are more likely to buckle due to increased strength and are more likely to generate wrinkles. Also, for parts composed of a top plate portion and a vertical wall portion without a flange portion, wrinkles are likely to occur at the end of the vertical wall portion due to shrink flange deformation.

[0004] Therefore, Patent Document 1 discloses a method for forming a press-formed product having a top plate portion and a slanted wall portion that is continuous with at least one side of the top plate portion and has no flange at its tip, and where the entire or a part of the slanted wall portion is convexly curved toward the slanted wall portion side in the longitudinal direction of the press-formed product in plan view. When forming the product using a concave die and a convex punch, a portion on the end side of the blank material rather than the portion corresponding to the slanted wall portion is clamped between the die and the punch during the forming process, and the slanted wall portion is formed while the end-side portion is being clamped. This prevents buckling of the blank material in the thickness direction and suppresses wrinkles generated in the slanted wall portion.

[0005] In addition, 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 continuously connected 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 side along the longitudinal direction. When forming the product, a wrinkle suppressing region is set where the metal plate is pressed by a wrinkle suppressor against the outer peripheral portion rather than the flange portion position, and a drawing process with stepped drawing is performed. When performing stepped drawing, an additional region where the metal plate is pressed by the wrinkle suppressor is set at 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 slanted wall portion is formed while a portion on the end side of the blank material rather than the portion corresponding to the slanted wall portion is clamped between the die and the punch, it is necessary to trim the portion clamped by the die and the punch in the next process.

[0008] In addition, although the method for manufacturing a press-formed product described in Patent Document 2 can suppress the generation of flange wrinkles, there is a problem that it cannot be applied to press forming by bending (forming) because a wrinkle suppressor is used.

[0009] The present invention has been made to solve such problems, and without requiring a trimming process after press forming, sufficiently suppresses wrinkles generated by shrink flange deformation, and is applicable to bending forming. An object is to provide a press forming method and a method for manufacturing a press formed product.

Means for Solving the Problems

[0010] (1) The press forming method according to the present invention is a press forming 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 and a vertical wall portion continuous with the top plate portion, 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 intermediate formed product has at least the top plate portion and the vertical wall portion of the portion corresponding to the convex outer peripheral edge portion of the top plate portion. It is characterized in that the angle formed by the vertical wall portion is larger than the angle formed by the top plate portion and the vertical wall 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) The manufacturing method of the press-formed product according to the present invention is a manufacturing method of 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, and a vertical wall portion continuous with the top plate 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 the target shape, wherein in the intermediate formed product, the angle formed by at least the top plate portion and the vertical wall portion corresponding to the convex outer peripheral edge portion of the top plate portion is larger than the angle formed by the top plate portion and the vertical wall portion of the press-formed product having the target shape.

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

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

Effect 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 the target shape. In the intermediate formed product, the angle formed by at least the top plate portion and the vertical wall portion corresponding to the convex outer peripheral edge portion of the top plate portion is larger than the angle formed by the top plate portion and the vertical wall portion of the press-formed product having the target shape. As a result, in the first forming step, the shrinkage flange deformation amount can be reduced to form the intermediate formed product, and since the intermediate formed product having higher rigidity than a flat blank is formed into the target shape in the second forming step, material movement due to shrinkage flange deformation hardly occurs during the forming process of the second forming step, and at the time of forming in the second forming step, the material flow in the elongation direction is added to the material flow in the shrinkage direction, and an increase in the plate thickness of the target formed product can be suppressed. As a result, a press-formed product with a good shape without wrinkles can be obtained, leading to an improvement in the yield in press forming. In addition, since it is not necessary to clamp the end of the blank with a punch and a die, the conventional trimming process is not essential. Furthermore, since no wrinkle presser is required, it is also applicable to bending forming.

Brief Description of the Drawings

[0017]

Figure 1

Figure 2

Figure 3

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Figure 8

Figure 9

Figure 10

Figure 11

Modes for Carrying Out the Invention

[0018] The press-formed product targeted by the press-forming method and the manufacturing method of the press-formed product according to this embodiment will be described with reference to 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 that curves 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. In addition, the angle formed by the top plate portion 3 and the vertical wall portion 5 of the press-formed product 1 in this example, and the angle formed by the vertical wall portion 5 and the flange portion 7 are both 90°.

[0019] Since 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 shoulder of the punch used for press-forming, 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 shoulder of the die, it is referred to as "die shoulder R portion 11". Hereinafter, when simply referred to as "punch shoulder R portion 9" or "die shoulder R portion 11" in this specification, it refers to the above-mentioned portions on the side of the press-formed product 1 rather than the die side.

[0020] First, prior to describing the press-forming method according to this embodiment, the problems in the case of press-forming the press-formed product 1 as shown in FIG. 2 by a conventional method will be described. FIG. 8 shows the result of FEM analysis when the press-formed product 1 is press-formed by a conventional method, and shows the distribution of the plate thickness increase rate by the shade of color. The plate thickness increase rate is obtained by finding the difference (plate thickness increment) between the plate thickness of the press-formed product 1 after press-forming and the plate thickness of the blank before press-forming, and is expressed as a ratio (percentage) to the plate thickness of the blank. The larger the value, the more the plate thickness has increased.

[0021] When forming the press-formed product 1 as shown in Fig. 2 by a conventional method, for example, using a punch and a die having shapes corresponding to the target shape, 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 have wrinkles due to shrink flange deformation and material concentration. In the case of the press-formed product 1 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. 8, 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. 8 will be explained with reference to Fig. 9.

[0022] Fig. 9 shows the forming process when forming the press-formed product 1 by the above-described conventional press-forming method. In Fig. 9, the process of deformation of the blank 13 corresponding to the portion where the top plate portion 3 is curved convexly is shown in a top view, a front view, and a side view, respectively. In each figure, the illustration of the die is omitted to make the shape of the blank 13 easier to understand. Note that numerical values such as "15mm up" in the figure indicate the distance between the punch 21 and the die (excluding the plate thickness). Therefore, in the case of "15mm up", 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 that forms the flange portion 7 is a figure obtained by adding +15 mm to the plate thickness of the blank 13.

[0023] When the vertical wall portion 5 corresponding to the convex outer peripheral edge portion 3a of the top plate portion 3 shown in Fig. 8 starts to be formed as shown in Fig. 9, as shown in the front view of "10mm up", for example, two mountain-shaped wrinkles are generated at the end of the blank 13 due to shrink flange deformation. These two mountain-shaped wrinkles become clearer as the forming progresses and the shrink flange deformation increases (see the front view of "5mm up").

[0024] As the forming progresses as described above, when the die descends and the lower surface of the die reaches the top of the wrinkle mountain, the forming progresses with the die crushing the wrinkle (see "3mm up", "1mm up"), and the forming bottom dead center is reached.

[0025] As described above, in the conventional forming process, wrinkles became distinct in the gap between the punch 21 and the die, and in an attempt to crush these enlarged wrinkles to form the flange portion 7, the thickness of the wrinkled portion locally increased and wrinkles remained in the press-formed product 1.

[0026] As a means of preventing wrinkles from occurring during the forming process, it is conceivable to use a wrinkle presser at a portion corresponding to the flange portion 7. However, since the use of a wrinkle presser is for drawing forming, it cannot be applied to bending forming.

[0027] Also, the wrinkles caused by the shrink flange deformation described above occur in the same manner in the case of the press-formed product 14 shown in FIG. 10. The press-formed product 14 in FIG. 10 has no flange portion and is composed of a top plate portion 3 and a vertical wall portion 5, and like the press-formed product 1 in FIG. 2, a part of the outer peripheral edge of the top plate portion 3 has a portion (convex outer peripheral edge portion 3a) that is curved convexly outward.

[0028] When the press-formed product 14 as shown in FIG. 10 is formed into the target shape in one step using a conventional method, that is, a punch and a die having a shape corresponding to the target shape and a flat blank, as shown in FIG. 11, wrinkles occur at the end of the vertical wall portion 5 corresponding to the convex outer peripheral edge portion 3a of the top plate portion 3 (the portion surrounded by the broken line circle in the figure).

[0029] When forming in one step, the reason why wrinkles occur in the flange portion 7 of the press-formed product 1 in FIG. 2 and the vertical wall portion 5 of the press-formed product 14 in FIG. 10 is that due to the shrink flange deformation, the material concentrates and moves to the convexly curved portion. Therefore, the inventor examined a method for reducing the shrink flange deformation amount in each step using a two-step press-forming method of forming the target shape via an intermediate formed product. Then, the inventor invented the shape of an intermediate formed product that can suppress the shrink flange deformation amount during forming and can generate a stretching material flow during target forming to reduce the material movement in the shrinking direction. The press forming method according to this embodiment is based on the above invention. Hereinafter, taking the case of forming the press-formed product shown in FIG. 2 as an example, it will be specifically described.

[0030] The press forming method according to this embodiment is a method for forming a press-formed product 1 having a top plate portion 3 with a convex outer peripheral edge portion 3a in which a part of the outer peripheral edge is curved convexly outward as shown in FIG. 2, 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. Accordingly, each step and operational effect of the press forming method described below are common to each step and operational effect of the manufacturing method of the press-formed product.

[0031] FIG. 1(a) is a perspective view of a punch 17, a die 19, and a blank 13 in a state before the first forming step, and FIG. 1(b) is a cross-sectional view taken along line A of FIG. 1(a). Further, FIG. 1(c) is a perspective view of a punch 21, a die 23, and an intermediate formed product 15 in a state before the second forming step, and FIG. 1(d) is a cross-sectional view taken along line B of FIG. 1(c). Note that each die in FIGS. 1(a) to 1(d) shows only the shape of the forming surface portion. Also, in the intermediate formed product 15 shown in FIG. 1(d), the same reference numerals are given to the portions corresponding to the press-formed product 1. Hereinafter, each step will be described in detail.

[0032] <First Forming Step> The first forming step is a step of press-forming a blank 13, which is a metal plate, into an intermediate formed product 15 as shown in FIGS. 1(a) and 1(b). The punch 17 and the die 19 are shaped such that the angle θ1 formed by the forming surface portion corresponding to the top plate portion 3 of the intermediate molded product 15 and the forming surface portion corresponding to the vertical wall portion 25 is greater than the angle θ2 formed by the forming surface portion corresponding to the top plate portion 3 of the press-molded product 1 and the forming surface portion corresponding to the vertical wall portion 5 in the punch 21 and the die 23 used in the second forming step (θ1 > θ2).

[0033] In the first forming step, 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 relatively moved to form an intermediate molded product 15 in which the angle formed by the top plate portion 3 and the vertical wall portion 25 is greater than the angle formed by the top plate portion 3 and the vertical wall portion 5 of the press-molded product 1 having the target shape. Since the intermediate molded product 15 has the above-described shape, it can be molded with a smaller amount of shrink flange deformation compared to the case of molding the flat blank 13 into the target shape. Therefore, in the first forming step, the end portion of the vertical wall portion 25 of the intermediate molded product 15 is less likely to have an increase in plate thickness and is less likely to generate wrinkles.

[0034] <Second Forming Step> The second forming step is a step of molding the intermediate molded product 15 molded in the first forming step into the press-molded product 1 having 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 mold of the conventional example in FIG. 9, so the same reference numerals are given.

[0035] In the second forming step, as shown in FIG. 1(d), the top plate portion 3 of the intermediate molded product 15 is set in alignment with the upper surface of the punch 21, and with the top plate portion 3 of the intermediate molded product 15 sandwiched between the punch 21 and the pad 27, the die 23 is relatively moved to mold the intermediate molded product 15 into the target shape. When the intermediate molded product 15 is set on the upper surface of the punch 21, the vertical wall portion 25 of the intermediate molded product 15 is curved in the circumferential direction and inclined downward, and is in a state of being separated from the vertical wall forming surface portion of the punch 21, and is in a state like an open umbrella. From this state, the forming will start, and the state of the forming process is shown in FIG. 3. In FIG. 3, similar to FIG. 9, the deformation process of the blank 13 is shown in a top view, a front view, and a side view respectively. The illustration of the die 23 is omitted, and the meaning of numerical values such as "15 mm up" is the same as in FIG. 9.

[0036] In the state of "15 mm up", the die 23 has not yet contacted the vertical wall portion 25 of the intermediate molded product 15, and the vertical wall portion 25 has a shape like an umbrella spread as shown in FIG. 1(d). When the die 23 is lowered from the state of "15 mm up", the die 23 contacts the vertical wall portion 25 of the intermediate molded product 15 at the time of "10 mm up". As shown in the side view of "10 mm up", a bent portion is formed in the vertical wall portion 25 and it starts to be formed into the vertical wall portion 5 of the target shape from the punch shoulder R portion 9 side. Although this forming involves shrink flange deformation, since the intermediate molded product 15 is work-hardened, it has higher rigidity than the flat blank 13 and material movement is less likely to occur. Also, by the die 23 pressing the curved vertical wall portion 25, a stretching flange direction force that pulls the tip of the vertical wall portion 25 in the circumferential direction, that is, a force like spreading an umbrella, acts to relieve the material flow in the shrinking direction against the shrink flange deformation. In this way, due to the high rigidity of the intermediate molded product 15, material movement is less likely to occur during the forming process. Furthermore, since a material flow that relieves the shrink flange deformation occurs, the flange portion 7 of the press-formed product 1 is also less likely to increase in thickness in the second forming process.

[0037] As described above, in the first forming process of the present embodiment, an intermediate molded product 15 is formed in which the angle formed by the top plate portion 3 and the vertical wall portion 25 is larger than the target shape. By forming the intermediate molded product 15 into the target shape in the second forming process, the problem of thickness increase due to shrink flange deformation is solved, and wrinkles generated in the flange portion 7 of the press-formed product 1 can be suppressed. Furthermore, since it is not necessary to sandwich the end of the blank with the punch and the die, a trimming process is not required as in the conventional example of Patent Document 1.

[0038] In addition, since the press forming method of the present embodiment can suppress wrinkles in the flange portion 7 without using a wrinkle presser, it can also 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 of the present embodiment is particularly effective when using a high-strength steel sheet that is likely to wrinkle due to shrink flange deformation. For example, the metal sheet (blank) may be a steel sheet with a tensile strength of 590 MPa or more, and in that case, a sufficient wrinkle reduction effect can also be achieved.

[0040] The above has been described by taking the case of forming the press-formed product 1 having the flange portion 7 as shown in FIG. 2 as an example. However, when forming the press-formed product 14 having no flange portion as shown in FIGS. 10 and 11, wrinkles can be reduced by the same action. By performing the first forming step and the second forming step of the above-described press forming method, the target press-formed product can be manufactured. The manufactured press-formed product has wrinkles suppressed as described above.

[0041] In addition, the press forming method and the manufacturing method of the press-formed product of the present embodiment form the intermediate molded product 15 in which the angle formed by the top plate portion 3 and the vertical wall portion 25 is larger than the target shape, so that wrinkles generated in the target molded product can be reduced more than before. However, if the angle of the intermediate molded product 15 is made too large, the wrinkle reduction effect in the target molded product may decrease. Therefore, this point will be explained.

[0042] FIG. 4(a) shows the cross-sectional shape of the blank 13 before forming in the first forming step by a broken line and the cross-sectional shape of the intermediate molded product 15 at the forming bottom dead center after forming by a solid line. Here, the distance from the end of the blank 13 in FIG. 4(a) to the end of the vertical wall portion 25 of the intermediate molded product 15 is defined as the material inflow amount in the first forming step. In this first forming step, the larger the angle formed by the top plate portion 3 and the vertical wall portion 25 of the intermediate formed product 15, the smaller the material inflow amount. When the material inflow amount is small, the shrink flange deformation amount in the first forming step becomes small. Therefore, by increasing the angle of the intermediate formed product 15, the increase in plate thickness (increase in plate thickness from the blank 13) due to the first forming step can be reduced.

[0043] FIG. 4(b) shows, in broken lines, the cross-sectional shape of the intermediate formed product 15 at the bottom dead center of the first forming step, and, in solid lines, the cross-sectional shape of the press-formed product 1 at the bottom dead center of the second forming step. Here, the distance from the end of the vertical wall portion 25 of the intermediate formed product 15 to the end of the flange portion 7 of the press-formed product 1 in FIG. 4(b) is defined as the material inflow amount in the second forming step. In this second forming step, the larger the angle formed by the top plate portion 3 and the vertical wall portion 25 of the intermediate formed product 15, the larger the material inflow amount. When the material inflow amount is large, the shrink flange deformation amount in the second forming step becomes large. Therefore, by increasing the angle 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 step becomes large.

[0044] Note that the example in FIG. 4 is an example where no flange portion is formed on the intermediate formed product 15. However, when the angle formed by the top plate portion 3 and the vertical wall portion 25 of the intermediate formed product 15 is close to the target shape, a flange portion may be formed on the intermediate formed product 15. In that case, the distance from the end of the blank 13 to the end of the flange portion of the intermediate formed product 15 becomes the material inflow amount in the first forming step, and the distance from the end of the flange portion of the intermediate formed product 15 to the end of the flange portion 7 of the press-formed product 1 becomes the material inflow amount in the second forming step.

[0045] As described above, the larger the angle formed by the top plate portion 3 and the vertical wall portion 25 of the intermediate formed product 15, the smaller the material inflow amount in the first forming step and the larger the material inflow amount in the second forming step. As an example of the above relationship, FIG. 5 shows the relationship between the angle of the intermediate formed product 15 and the material inflow amount in each step when the angle formed by the top plate portion 3 and the vertical wall portion 5 of the target shape is 90°.

[0046] When the angle of the intermediate molded product 15 is increased too much, the increase in plate thickness in the first molding step becomes small, but the increase in plate thickness in the second molding step becomes large, and there are cases where the final increase in plate thickness cannot be sufficiently reduced. Therefore, the present invention becomes more effective by setting the angle of the intermediate molded product 15 so as to reduce the increase in plate thickness well in both the first molding step and the second molding step. For example, it is preferable that the angle formed by the top plate portion 3 and the vertical wall portion 25 of the intermediate molded product 15 is about 1.1 times to 1.8 times the target shape. This point will be specifically described in the following examples.

Examples

[0047] Regarding the effect of suppressing wrinkles by shrink flange deformation in the press molding method and the manufacturing method of press molded products of the present invention, a specific examination was conducted using FEM analysis, and the results will be described below. In this example, a steel plate with a thickness of 1.0 mm and a tensile strength of 980 MPa class was used as a blank, and the case of press molding with the press molded product 1 in FIG. 2 as the target shape was confirmed. FEM analysis was performed on a conventional example in which the steel plate was formed into the target shape in one step and an example of the present invention in which the steel plate 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. 8, the analysis results of the example of the present invention will be described below.

[0048] In the example of the present invention, FEM analysis was performed on four cases in which the angle formed by the top plate portion 3 and the vertical wall portion 5 was 100°, 110°, 120°, and 130° with respect to the target shape of 90°. FIG. 6 shows the plate thickness increase rate distribution of the intermediate molded product 15 in the first molding step, and FIG. 7 shows the plate thickness increase rate distribution of the target molded product (press molded product 1) in the second molding step. In FIGS. 6 and 7, numerical values such as "100°" indicate the angle formed by the top plate portion 3 and the vertical wall portion 25 of the intermediate molded product 15, and numerical values such as "12.1%" indicate the maximum plate thickness increase rate. Also, the maximum plate thickness increase rates in FIGS. 6 and 7 are both shown as increase rates based on the plate thickness of the blank. Table 1 shows the results of FIGS. 6 to 8 summarized.

[0049]

Table 1

[0050] As shown in FIGS. 6 to 8 and Table 1, in the conventional example, the maximum plate thickness increase rate of the target molded product (press-molded product 1) was 12.5%, whereas in the examples of the present invention (No. 2 to No. 5), the maximum plate thickness increase rates of the final molded products were all reduced compared to the conventional example. 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. Note that, as described above, in the present invention, by setting the angle formed by the top plate portion 3 and the vertical wall portion 25 of the intermediate molded product 15 so that the plate thickness increase can be reduced well in both the first molding step and the second molding step, wrinkles can be suppressed more effectively. This point will be specifically described below.

[0051] 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 the larger the angle formed by the top plate portion 3 and the vertical wall portion 25 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 larger the angle, the smaller the material inflow amount (see FIG. 4(a)) in the first molding step and the reduction amount of the shrinkage deformation.

[0052] Also, when comparing the maximum plate thickness increase rates of the target molded products of No. 2 to No. 5, it can be seen that the larger the angle formed by the top plate portion 3 and the vertical wall portion 25 of the intermediate molded product 15, the smaller the maximum plate thickness increase rate of the target molded product in the second molding step. Here, the one with the smallest maximum plate thickness increase rate is No. 5 in which the angle is the largest.

[0053] In the present embodiment, the example in which the angle formed by the top plate portion 3 and the vertical wall portion 25 of the intermediate molded product 15 is the largest has the most reduced plate thickness increase rate. Thus, the angle of the intermediate molded product 15 may be set so that the plate thickness increase of the target molded product after the second molding step becomes as small as possible, whereby the wrinkle suppression effect can be maximized and it is effective.

Explanation of Symbols

[0054] 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 9 Punch shoulder R part 11 Die shoulder R part 13 Blank (metal plate) 14 Press-formed product (another example of target shape) 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) 25 Vertical wall part (intermediate formed product) 27 Pad

Claims

1. A press forming method for forming 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 convexly curved outward, and a vertical wall portion continuous with the top plate portion, comprising: a first forming step of forming a metal plate into an intermediate formed product; 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 in the intermediate formed product, the angle formed by the top plate portion and the vertical wall portion at least at a portion corresponding to the convex outer peripheral edge portion of the top plate portion is larger than the angle formed by the top plate portion and the vertical wall portion of the press-formed product having the target shape, the vertical wall portion is curved in the circumferential direction and inclined downward, and the tip of the vertical wall portion does not have a flange portion.

2. The press forming method according to claim 1, wherein 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 having a top plate portion with a convex outer peripheral edge portion where the outer peripheral edge or a part thereof is convexly curved outward, and a vertical wall portion continuous with the top plate portion, comprising: a first forming step of forming a metal plate into an intermediate formed product; 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 in the intermediate formed product, the angle formed by the top plate portion and the vertical wall portion at least at a portion corresponding to the convex outer peripheral edge portion of the top plate portion is larger than the angle formed by the top plate portion and the vertical wall portion of the press-formed product having the target shape, the vertical wall portion is curved in the circumferential direction and inclined downward, and the tip of the vertical wall portion does not have a flange portion.

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

Images