Press molding method and manufacturing method of press molded product

Abstract

To provide a press-molding method and a manufacturing method for a press-molded product which can sufficiently suppress wrinkles at a flange part that are generated by shrink flanging, in steps which are less than before and can be applied to bending-molding.SOLUTION: A press-molding method and a manufacturing method for a press-molded product mold a press-molded product 1 which has: a top plate part 3 having an outer peripheral edge or a convex-shape outer peripheral edge portion 3a a portion of which is curved outward in a convex shape; a vertical wall part 5 leading to the top plate part 3; and a flange part 7 leading to the vertical wall part 5. The methods comprise; a blanking step S1 of collecting a developed blank having a shape in which the press-molded product 1 is developed, from a metal plate; a first molding step S3 of molding the developed blank into an intermediate molded product 9; and a second molding step S5 of molding the intermediate molded product 9 into the press-molded product 1. In the first molding step S1, the metal plate is molded so that a bending radius of a die shoulder R portion 15 formed to correspond at least to the convex-shape outer peripheral edge part 3a of the top plate part 3 of the intermediate molded product 9 is larger than a bending radius of the die shoulder R portion 15 of the press-molded product 1.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a press molding method and a manufacturing method for a press-molded product having a top plate portion, a vertical wall portion, and a flange portion, and more particularly to a press molding method and a manufacturing method for a press-molded product that suppresses the generation of wrinkles associated with shrink flange molding when forming an expanded blank, which is a blank shape obtained by expanding the press-molded product, into the press-molded product. [Background technology]

[0002] As automobile crash safety standards become stricter, the crash safety of car bodies is being improved, but in response to carbon dioxide emission regulations, there is also a need to reduce the weight of car bodies in order to improve fuel efficiency and promote the shift to electric vehicles. In order to achieve both improved crash safety and reduced weight of car bodies, the use of high-strength steel sheets (also called high-tensile steel) of 590 MPa class or higher is becoming more common in car body structural parts. When press-forming high-tensile steel into car body structural parts, suppressing wrinkles that occur during shrink flange forming is an issue.

[0003] For example, some automobile parts have a top plate, a vertical wall, and a flange, such as an A-pillar upper, an A-pillar lower, or a bumper part. In such parts, when the outer periphery of the top plate or a part thereof is curved outwardly in a convex shape, the vertical wall or flange at that part shrinks during press forming, resulting in flange forming, and wrinkles may occur at the end of the vertical wall or the end of the flange. In particular, high-tensile steel is prone to buckling due to its high strength, and wrinkles are likely to occur.

[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 the entire or part of the slanted wall portion is curved convexly toward the slanted wall portion in the longitudinal direction of the press-formed product in a plan view. In Patent Document 1, when forming the press-formed product with a concave die and a convex punch, a portion of the blank material that is closer to the end than the portion corresponding to the slanted wall portion is clamped between the die and the punch during forming, and the slanted wall portion is formed in a state where the portion on the end side is clamped. This makes it possible to prevent buckling of the blank material in the plate thickness direction and suppress wrinkles that occur in the slanted wall portion.

[0005] Patent Document 2 also discloses a method for manufacturing a product shape having a hat-shaped cross section in which the top plate and the flange are continuous in the width direction via the side wall, and the top plate and the flange have a curved portion that is curved convexly toward the top plate along the longitudinal direction. Patent Document 2 discloses a step drawing process in which, when manufacturing a press-formed product by press-forming a metal plate, a wrinkle-holding region in which the metal plate is held down with a wrinkle holder is set on the outer periphery from the flange position, and forming is performed by step drawing. When forming by step drawing, an additional region to be held down with the wrinkle holder is set also in part of the flange position, so that wrinkles occurring in the flange can be suppressed. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2016-221558 A [Patent Document 2] JP 2018-034176 A Summary of the Invention [Problem to be solved by the invention]

[0007] However, in the press forming method described in Patent Document 1, the oblique wall portion is formed in a state where the portion of the blank material closer to the end side than the portion corresponding to the oblique wall portion is clamped between the die and punch, so that the portion clamped between the die and punch must be trimmed in the next process. As a result, at least four processes must be performed: a blanking process in which a blank is taken from a metal plate, a press forming process in which the blank is press-formed into a target shape, a restriking process in which the blank is pressed again with the same die to reduce springback, and the trimming process. Meanwhile, in the automobile industry, in order to reduce costs when press forming metal plates, it is required to improve productivity by eliminating as many processes as possible. Furthermore, the method of Patent Document 1 has the problem that it cannot be applied to the molding of a press-formed product having a flange portion continuous with a vertical wall portion (inclined wall portion).

[0008] Furthermore, the manufacturing method for press-formed products described in Patent Document 2 also requires at least four steps: a blanking step, a press forming step, a restriking step, and a trimming step, and there is a demand to reduce the number of steps, as in Patent Document 1. Furthermore, although the manufacturing method of Patent Document 2 can be applied to press-formed products having flange portions, there is a problem in that the method is a draw-formed product because a blank holder is used, and therefore cannot be applied to press forming by bending (forming) forming.

[0009] The present invention has been made to solve such problems, and aims to provide a press molding method and a manufacturing method for press-molded products that can sufficiently suppress wrinkles in the flange portion caused by shrink flange molding with fewer steps than conventional methods and that can also be applied to bending molding. [Means for solving the problem]

[0010] (1) The press forming method of 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 portion thereof is curved convexly outward, a vertical wall portion continuing to the top plate portion via a punch shoulder R portion, and a flange portion continuing to the vertical wall portion via a die shoulder R portion, and includes a blanking process for taking an expanded blank from a metal plate, which is a blank having a shape obtained by expanding the press-formed product of a target shape, a first forming process for forming the expanded blank into an intermediate formed product, and a second forming process for forming the intermediate formed product formed in the first forming process into a press-formed product of the target shape, wherein the first forming process is characterized in that the bending radius of the die shoulder R portion formed corresponding to at least the convex outer peripheral edge portion of the top plate portion of the intermediate formed product is formed so as to be larger than the bending radius of the die shoulder R portion of the press-formed product of the target shape.

[0011] (2) Moreover, in the above-mentioned (1), the first forming step is characterized in that drawing or bending is applied, and the second forming step is characterized in that bending is applied.

[0012] (3) In addition, in the above (1) or (2), the metal plate is a steel plate having a tensile strength of 590 MPa or more. (4) The manufacturing method of a press-formed product of the present invention is a manufacturing method of a press-formed product having a top plate portion having a convex outer peripheral edge portion whose outer peripheral edge or a portion thereof is curved convexly outward, a vertical wall portion continuing to the top plate portion via a punch shoulder R portion, and a flange portion continuing to the vertical wall portion via a die shoulder R portion, and the method includes a blanking process for taking an expanded blank, which is a blank having a shape obtained by expanding the press-formed product of a target shape, from a metal plate, a first shaping process for shaping the expanded blank into an intermediate formed product, and a second shaping process for shaping the intermediate formed product formed in the first shaping process into a press-formed product of the target shape, characterized in that the first shaping process is formed so that the bending radius of the die shoulder R portion formed corresponding to at least the convex outer peripheral edge portion of the top plate portion of the intermediate formed product is larger than the bending radius of the die shoulder R portion of the press-formed product of the target shape. Effect of the Invention

[0013] In the present invention, by forming the intermediate product in the first forming step so that the bending radius of the die shoulder R is larger than that of the press-formed product of the target shape, it is possible to sufficiently suppress wrinkles in the flange caused by shrink flange forming. This makes it possible to efficiently manufacture press-formed products of good shapes without wrinkles while omitting the trimming step that was conventionally performed, leading to improved productivity and yield in press forming. In addition, since no blank holder is required, it can also be applied to bending. [Brief description of the drawings]

[0014] [Figure 1] 1 is an explanatory diagram of a press molding method according to an embodiment of the present invention. [Diagram 2] FIG. 2 is a plan view of a part that is the subject of the embodiment. [Diagram 3] FIG. 1 is an explanatory diagram (part 1) of a first molding step according to an embodiment. [Figure 4] FIG. 2 is an explanatory diagram (part 2) of the first molding step according to the embodiment. [Diagram 5] FIG. 4 is an explanatory diagram of a second molding step according to the embodiment. [Figure 6] 6A and 6B are diagrams showing the distribution of sheet thickness increase rates of intermediate formed products press-formed in the first forming step, where FIG. 6A is a conventional example and FIG. 6B is an example of the invention. [Figure 7] 7A and 7B are diagrams showing the distribution of sheet thickness increase rates of a press-formed product press-formed into a target shape in the second forming step, where FIG. 7A is a conventional example and FIG. 7B is an example of the invention. [Figure 8] FIG. 8 is a diagram showing the distribution of sheet thickness increase rate at the bottom dead center of the second forming process in a partially enlarged view of a press-formed product of a target shape, where FIG. 8(a) is a conventional example and FIG. 8(b) is an example of the invention. [Figure 9] 3 is a diagram showing an intermediate product when the part shown in FIG. 2 is press-molded by a conventional method. FIG. [Figure 10]3A to 3C are diagrams illustrating problems that arise when the part shown in FIG. 2 is press-molded by a method according to a comparative example. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] The press-molded product targeted by the press molding method and the manufacturing method of the press-molded product according to the present embodiment has a top plate portion 3, a vertical wall portion 5, and a flange portion 7, such as the press-molded product 1 shown in Fig. 2. As shown by the arrow, part of the outer periphery of the top plate portion 3 has a portion that is convexly curved outward (hereinafter referred to as "convex outer periphery portion 3a"). Note that the boundary between the convex outer periphery portion 3a and other portions ends at the R of the convex portion when the top plate portion 3 is viewed in plan, for example. In addition, the boundary between the top plate portion 3 and the vertical wall portion 5 has an R shape corresponding to the shape of the shoulder of the punch used in press forming, and is referred to as the "punch shoulder R portion." Similarly, the boundary 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, and is referred to as the "die shoulder R portion." These portions will be described in detail later.

[0016] First, before describing the press forming method according to the present embodiment, problems that arise when press forming a press-formed product 1 as shown in Fig. 2 by a conventional method will be described with reference to Figs. 9 and 10. In Figs. 9 and 10, the distribution of the thickness increase rate of an intermediate product and a press-formed product is shown by different shades of color. The thickness increase rate is calculated by calculating the difference (thickness increment) between the thickness of the press-formed product after press forming and the thickness of the blank before press forming, and expressed as a ratio (proportion) to the thickness of the blank, with a larger value indicating a greater increase in thickness.

[0017] In a conventional press forming method, a rectangular blank (rectangular blank) is press formed into an intermediate formed product 19 as shown in Figure 9, and unnecessary portions are removed (trimmed) from the flange end of the intermediate formed product 19 to produce a press formed product 1 as shown in Figure 2. In this case, the processes required to manufacture the press-formed product 1 are four processes: a blanking process to obtain a rectangular blank; a press forming process to press the intermediate formed product 19; a restriking process to re-press the intermediate formed product 19; and a trimming process to remove unnecessary portions.

[0018] When an intermediate molded product 19 as shown in Fig. 9 is press molded, 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 shrunk and flange molded, which increases the plate thickness and makes it easy for wrinkles to occur. In the example of Fig. 9, the plate thickness increases the most at the end of the flange portion 7 indicated by the arrow in the figure (the maximum plate thickness increase rate is 17.6%). Therefore, this part is most likely to cause wrinkles, but since this part is removed in the trimming process described above, it is possible to obtain a press molded product 1 with a good shape and no wrinkles in the end. However, the above method requires a large number of steps, and as described above, there is a demand for reducing the number of steps from the viewpoints of reducing costs and improving productivity.

[0019] Therefore, as a method for forming the press-formed product 1 of Figure 2 in fewer steps than the conventional four-step press forming method, it can be considered to use a blank before press forming that is not a rectangular blank, but a blank (expanded blank) that has a shape obtained by expanding the press-formed product 1 of the target shape. In this case, the processes required to manufacture the press-formed product 1 are three: a blanking process to obtain an unfolded blank, a press-forming process to press the unfolded blank into the press-formed product 1, and a restriking process to re-press the press-formed product 1, and the trimming process can be omitted.

[0020] As described above, by using an expanded blank in which the portions that would have been removed in the conventional trimming process have been removed in advance, the number of processes can be reduced, thereby achieving significant cost reductions and productivity improvements. However, the three-step press forming method using an expanded blank has the following problems.

[0021] Figure 10 shows the distribution of thickness increase rates for the press-formed product 1 when formed through the three processes described above. As shown in Figure 10, when the unfolded blank is press-formed into the target shape, the thickness of the flange portion 7, the area that will become the shrink flange, increases by up to 47.4% (the area indicated by the arrow in the enlarged view in the figure). The reason for the significant increase in thickness compared to the example in Figure 9 is that the use of an unfolded blank shortens the width of the flange portion 7, making it easier for the material to move and for the material to concentrate in that area. Furthermore, because the wrinkled area is part of the target shape, it cannot be removed by a trimming process as in the conventional method.

[0022] In view of this, the press forming method of the present embodiment uses an expanded blank to reduce the number of steps compared to conventional press forming methods, while making it possible to suppress the occurrence of wrinkles in the flange portion 7.

[0023] Specifically, the press molding method of this embodiment is a method for molding a press-molded product 1, an example of which is shown in Figure 2, and includes a blanking process S1, a first molding process S3, and a second molding process S5 shown in Figure 1. In addition, since the press-molded product 1 is manufactured by carrying out the press-molding method, the invention of the press-molding method can be configured as the invention of the manufacturing method of the press-molded product. Therefore, the embodiment of the press-molding method described below is common to the embodiment of the manufacturing method of the press-molded product. Each step will be described in detail below.

[0024] <Blanking process> The blanking process S1 is a process for taking, from a metal plate, an expanded blank, which is a blank having a shape obtained by expanding the press-molded product 1 having a target shape. The expanded blank in this embodiment is the same as that in the comparative example described with reference to FIG. The shape of the blank taken in the blanking process S1 is not rectangular, but is a shape in which unnecessary parts are removed in advance as the target shape, thereby eliminating the need for the trimming process that was previously performed to remove unnecessary parts from the intermediate molded product 19, and reducing the number of processes.

[0025] <1st molding process> The first forming process S3 is a process in which the developed blank taken in the blanking process S1 is press-formed into an intermediate product. The intermediate product formed in the first forming process S3 has a shape corresponding to the target shape, but the shape of the die shoulder R part at the boundary between the vertical wall part 5 and the flange part 7 (part indicated by the arrow in FIG. 2) formed by shrink flange forming is different from the target shape. This point will be specifically explained below with reference to FIG. 3.

[0026] Fig. 3(a) shows the state of the forming bottom dead center of the portion corresponding to the AA' cross section in Fig. 2 when the unfolded blank is formed into a target shape as in the comparative example described in Fig. 10. Also, Fig. 3(b) shows the state of the forming bottom dead center of the portion corresponding to the AA' cross section in Fig. 2 when the unfolded blank is formed into an intermediate formed product 9 in the first forming step S3 of this embodiment. Note that dies are omitted from Figs. 3(a) and 3(b).

[0027] As shown in Figures 3(a) and 3(b), the boundary between the top plate 3 and the vertical wall 5 in the target shape (press-formed product 1) or intermediate product 9 is a part that becomes R-shaped along the shoulders of the punches 12, 11. Hereinafter, this part in the target shape or intermediate product 9 will be referred to as punch shoulder R part 13. Similarly, the boundary between the vertical wall 5 and the flange 7 is a part that becomes R-shaped along the shoulder of the die (not shown), so this part in the target shape or intermediate product 9 will be referred to as die shoulder R part 15. As can be seen by comparing Figures 3(a) and 3(b), in the first forming process of this embodiment, the intermediate molded product 9 is formed so that the bending radius of the die shoulder R portion 15 is larger than the bending radius of the die shoulder R portion 15 of the target shape.

[0028] By doing as described above, the distance a from the end of the R of the punch shoulder R portion 13 of the intermediate formed product 9 on the vertical wall portion 5 side to the blank end at the bottom dead center of forming becomes longer than the distance a of the target shape. Specifically, the distance a of the target shape in Fig. 3(a) is 5.9 mm, while the distance a of the intermediate formed product 9 in Fig. 3(b) is 8.1 mm, which is 2.2 mm longer.

[0029] Fig. 4 shows a plan view comparison of the difference between the target shape in Fig. 3(a) and the shape of the intermediate molded product 9 in Fig. 3(b). Fig. 4(b) shows the edge lines of the flange portion 7 in the area enclosed by the square in Fig. 4(a) when the target shape and the intermediate molded product 9 are overlapped with the top plate portion 3 aligned. As in FIG. 3, FIG. 4(b) also shows that the end of flange portion 7 in that region is positioned approximately 2.2 mm outward in intermediate molded product 9 (invention example) compared to the target shape (comparative example).

[0030] As described above, by increasing the bending radius of the die shoulder R portion 15 of the intermediate molded product 9, the deformation amount of the vertical wall portion 5 and the flange portion 7 corresponding to the convex outer peripheral edge portion 3a becomes smaller compared to the target shape, so that wrinkles due to shrink flange molding are less likely to occur in that portion of the intermediate molded product 9.

[0031] The bending radius of the die shoulder R portion 15 may be made larger than the bending radius of the die shoulder R portion 15 of the target shape over the entire length of the die shoulder R portion 15 (the entire length of the ridge line between the vertical wall portion 5 and the flange portion 7) or only in the portion that will be subjected to shrink flange forming. That is, it is sufficient to make the bending radius of the die shoulder R portion 15 formed continuously to at least the vertical wall portion 5 that is continuous to the convex outer peripheral edge portion 3a of the intermediate formed product 9 larger than the bending radius of the die shoulder R portion 15 of the target shape.

[0032] <Second molding process> The second forming process S5 is a process for forming the intermediate product 9 formed in the first forming process S3 into a press-formed product 1 having a target shape. Note that the mold used in the second forming process S5 has the same shape as the mold used in the first forming process S3 except for the portion for forming the die shoulder R portion 15 into the target shape, so this process also serves as the restriking process described in the conventional example and comparative example.

[0033] When the vertical wall portion 5 and flange portion 7 corresponding to the convex outer peripheral edge portion 3a (Figure 2) of the intermediate molded product 9 are molded into the target shape, the corresponding portion is subjected to shrink flange molding. The reason why the occurrence of wrinkles can be suppressed even in the shrink flange molding in the second molding process S5 will be explained based on Figure 5.

[0034] Figure 5(b) shows the change in appearance during the forming process of the part enclosed in a square in Figure 5(a), and the state of the forming process of the part corresponding to the AA' cross section. Values ​​such as "5mmup" are the distance to the bottom dead center of the die 17, taking into account the plate thickness of the intermediate formed product 9. Therefore, at "5mmup", the gap between the punch 12 and the die 17 in the press forming direction is the plate thickness of the intermediate formed product 9 + 5mm. The smaller the value, the closer the die 17 is to the punch 12, and the further the forming progresses, and at "0mmup", the bottom dead center of the forming is reached.

[0035] In the second forming process S5 of this embodiment, when the intermediate formed product 9 is set on the upper surface of the punch 12 and the die 17 is lowered, the die shoulder R portion 15 starts to be formed 1 mm before the bottom dead center of the forming (1 mm up), as shown in Figure 5 (b). When the die shoulder R portion 15 is formed into the target shape, the portion is subjected to shrink flange forming, but when the shrink flange forming starts, the gap between the punch 12 and the die 17 is very small, the plate thickness + 1 mm, and the gap becomes even smaller until the bottom dead center of forming. Therefore, wrinkles are unlikely to occur at the edge of the flange portion 7 even in the second forming process S5.

[0036] The effects of the present embodiment described above will be specifically described with reference to FIGS. Fig. 6(a) shows, as a comparative example, the distribution of the sheet thickness increase rate of the press-formed product 1 when the developed blank is press-formed into the target shape in the first press process. Fig. 6(b) shows the distribution of the sheet thickness increase rate of the intermediate formed product 9 formed in the first forming process S3 of this embodiment. The sheet thickness increase rate and color shading are the same as Figs. 9 and 10. As shown in Figures 6(a) and 6(b), the maximum thickness increase rate in the comparative example is 47.4%, while the maximum thickness increase rate of the intermediate molded product 9 in this embodiment is 15.2%, which shows that the increase in thickness in the area that will be subjected to shrink flange molding is significantly reduced.

[0037] Next, Figure 7(a) shows the distribution of the thickness increase rate when the press-formed product 1 in Figure 6(a) is restrike in the second press process, and Figure 7(b) shows the distribution of the thickness increase rate when the intermediate formed product 9 in Figure 6(b) is formed into the target shape in the second forming process S5. 7(a) and 7(b), the maximum thickness increase rate after restriking in the comparative example was 47.4%, whereas the maximum thickness increase rate after the second forming step S5 in this embodiment was 15.3%. Thus, even when comparing the final formed products, it is found that the press-formed product 1 press-formed by the press forming method of this embodiment sufficiently reduces the increase in thickness in the area that will become the shrink flange forming, and can suppress the occurrence of wrinkles in that area.

[0038] As explained above, when forming the die shoulder R portion 15 of the intermediate product 9 into the target bending radius in the second forming process S5, the gap between the punch 12 and the die 17 is very small, so that the material is less likely to concentrate locally. This effect has also been confirmed, and will be explained using Figure 8. Fig. 8 shows the distribution of the sheet thickness increase rate at the bottom dead center of forming in the part surrounded by a square in Fig. 7, in a smaller range than in Fig. 7. Fig. 8(a) shows the press-formed product 1 after restriking in the second press step in the comparative example shown in Fig. 7(a), and Fig. 8(b) shows the press-formed product 1 after the second forming step S5 in this embodiment.

[0039] As shown in Figures 8(a) and 8(b), the increase in thickness is localized in the comparative example in Figure 8(a), whereas the increase in thickness is distributed over a wide range in the press-formed product 1 of the present embodiment in Figure 8(b). This shows that the strain caused by the shrink flange forming in the second forming step S5 is distributed over a wide range, which prevents a local increase in thickness and suppresses the occurrence of wrinkles.

[0040] As described above, in this embodiment, by using an expanded blank, the trimming process that was conventionally performed can be omitted and the number of processes can be reduced. Furthermore, by forming the bending radius of the die shoulder R portion 15 to be larger than the target shape in the first forming process S3 and then forming it into the target shape in the second forming process S5, the problem of localized thickness increase caused by using an expanded blank can be solved and the occurrence of wrinkles can be suppressed.

[0041] As described above, the press forming method of the present embodiment can suppress wrinkles in the flange portion without using a blank holder, and is therefore applicable to press forming by bending (forming). That is, it is particularly effective when drawing or bending is applied in the first forming step S3 for forming the intermediate product 9, and bending is applied in the second forming step S5 for forming the target shape.

[0042] Furthermore, the press forming method of the present embodiment is particularly effective when using a high-strength steel sheet that is prone to wrinkles due to shrink flange forming. For example, the metal sheet from which the expanded blank is taken in the blanking step S1 may be a steel sheet having a tensile strength of 590 MPa or more, and in that case too, a sufficient wrinkle reduction effect can be achieved. By carrying out each step of the above-mentioned press molding method, a target press-molded product can be manufactured, and the manufactured press-molded product has reduced wrinkles as described above. EXAMPLES

[0043] A specific study was conducted using FEM analysis on the effect of suppressing wrinkles caused by shrink flange forming in the press forming method of the present invention, and the results are described below. In this example, a steel plate having a thickness of 1.0 mm and a tensile strength of 980 MPa was used as a blank, and the press-formed product 1 shown in Fig. 2 was press-formed to a target shape. The bending radius of the die shoulder R part 15 (see Fig. 3) of the target shape was set to 4 mm. As a conventional example, FEM analysis was performed on a case where a rectangular blank as described in FIG. 9 was formed into a target shape in four steps. As a comparative example, FEM analysis was also performed on a case where an expanded blank as described in FIG. 10 was formed into a target shape in three steps, and as an example of the present invention, FEM analysis was also performed on a case where an expanded blank as described in the embodiment was formed into a target shape in three steps. The maximum sheet thickness increase rate at the shrink flange forming portion in each example was obtained. Note that, as an example of the present invention, multiple examples were performed by changing the bending radius of the die shoulder R portion 15 of the intermediate formed product 9. The results are shown in Table 1.

[0044] [Table 1]

[0045] No. 1 is a conventional example that is performed in four steps: a blanking step to obtain a rectangular blank, a first pressing step to press the rectangular blank into an intermediate product 19 (see Fig. 9), a second pressing step (restriking step) to press again, and a trimming step to remove unnecessary parts from the intermediate product 19. In this conventional example, the bending radius of the die shoulder R part 15 of the intermediate product 19 was set to 4 mm, the same as the target shape.

[0046] No. 2 is a comparative example that involves three processes: a blanking process in which an expanded blank is extracted from a metal plate, a first press process in which the expanded blank is press-formed into the target shape, and a second press process (restriking process) in which the blank is pressed again using the same mold as in the first press process.

[0047] Nos. 3 to 5 are examples of the present invention, which are carried out in three steps: a blanking step to obtain an expanded blank, a first press step to press the expanded blank into an intermediate product 9 (Fig. 3(b)), and a second press step to press the intermediate product 9 into a target shape. The first and second press steps of Nos. 3 to 5 correspond to the first and second forming steps of the embodiment, respectively. In the examples of the present invention, the bending radius of the die shoulder R portion 15 of the intermediate product 9 was changed to three patterns: 6 mm, 10 mm, and 12 mm.

[0048] As shown in Table 1, in the conventional example No. 1, the maximum thickness increase rate during the first press process was 17.6%, and the maximum thickness increase rate during the second press process was 17.9%, and wrinkles occurred. In addition, although the comparative example No. 2 had fewer processes than the conventional example No. 1, the maximum plate thickness increase rate in both the first press process and the second press process was 47.4%, and large wrinkles occurred.

[0049] In contrast, in Example No. 3 of the present invention, in which the bending radius of the die shoulder R portion 15 in the first press process was 6 mm, the maximum thickness increase rate in the first press process was 16.1%, and the maximum thickness increase rate in the second press process was 16.3%. In Example No. 3, the number of processes was reduced compared to the conventional example No. 1, and the thickness increase rate was reduced compared to the conventional example No. 1 and the comparative example No. 2, and no wrinkles were generated. In addition, in Example No. 4 of the present invention, in which the bending radius of the die shoulder R portion 15 in the first press process was 10 mm, the maximum plate thickness increase rate during the first press process was 15.2%, and the maximum plate thickness increase rate during the second press process was 15.3%. As with Example No. 3, the plate thickness increase rate was reduced and wrinkles were prevented. Furthermore, in Example No. 5 of the present invention, in which the bending radius of the die shoulder R portion 15 in the first press process was 12 mm, the maximum thickness increase rate in the first press process was 13.3%, and the maximum thickness increase rate in the second press process was 13.9%, so the thickness increase rate was significantly reduced and no wrinkles occurred. In addition, in Nos. 3 to 5, the larger the bending radius of the die shoulder R portion 15 in the first press process was, the more the sheet thickness increase rate could be reduced.

[0050] From the results in Table 1, it was found that the bending radius of the die shoulder R portion of the intermediate molded product in the first molding process of the present invention is effective when R1 ≧ 1.5Ro, where Ro is the bending radius of the die shoulder R portion of the target shape and R1 is the bending radius of the die shoulder R portion of the intermediate molded product.

[0051] In addition, the larger the bending radius of the die shoulder R of the intermediate product, the smaller the amount of change in the first forming step and the greater the effect of reducing the increase in plate thickness, but on the other hand, the plate thickness is more likely to increase in the second forming step. This is because the larger the bending radius of the die shoulder R of the intermediate product, the earlier the start time of forming the die shoulder R in the second forming step, so the gap between the punch and the die at the start of forming the die shoulder R becomes larger, reducing the effect of suppressing wrinkles. Therefore, if the bending radius of the die shoulder R of the intermediate product is too large, forming of the die shoulder R may start before the gap between the punch and the die becomes small enough in the second forming process, making it difficult to suppress local concentration of material. Therefore, it is preferable to set the bending radius of the die shoulder R of the intermediate product so that a well-balanced wrinkle suppression effect is obtained in both the first and second forming processes. [Explanation of symbols]

[0052] 1 Press-molded product (target shape) 3 Top plate 3a Convex outer periphery 5 Vertical wall section 7 Flange 9 Intermediate molded products 11 Punch (first forming process) 12 Punch (second forming process, comparative example) 13 Punch shoulder R part 15 Die shoulder R 17 Die 19 Intermediate molded product (conventional example)

Claims

1. A press-molding method for molding a press-molded product having a top plate portion having a convex outer periphery portion in which an outer periphery or a part thereof is curved outwardly in a convex shape, a vertical wall portion continuing to the top plate portion via a punch shoulder R portion, and a flange portion continuing to the vertical wall portion via a die shoulder R portion, A blanking process in which an expanded blank, which is a blank having a shape obtained by expanding the press-molded product having a target shape, is taken from a metal plate; a first forming step of forming the developed blank into an intermediate product; a second forming step of forming the intermediate product formed in the first forming step into a press-formed product having the target shape, The first forming process is a press forming method characterized in that the bending radius of a die shoulder R portion formed corresponding to at least the convex outer peripheral edge portion of the top plate portion of the intermediate formed product is formed to be larger than the bending radius of the die shoulder R portion of the press formed product of the target shape, and with regard to the distance a from the R end of the vertical wall portion side of the punch shoulder R portion to the blank end, the distance a in the intermediate formed product is made longer than the distance a in the target shape.

2. A press molding method for forming a press-molded product having a top plate portion having a convex outer peripheral edge portion in which the outer peripheral edge or a part thereof is curved convexly outward, a vertical wall portion continuing to the top plate portion via a punch shoulder R portion, and a flange portion continuing to the vertical wall portion via a die shoulder R portion, A blanking process in which an expanded blank, which is a blank having a shape obtained by expanding the press-molded product having a target shape, is taken from a metal plate; a first forming step of forming the developed blank into an intermediate product; a second forming step of forming the intermediate product formed in the first forming step into a press-formed product having the target shape, The first forming step forms a bend radius of a die shoulder R portion formed corresponding to at least the convex outer peripheral edge portion of the top plate portion of the intermediate formed product so as to be larger than the bend radius of the die shoulder R portion of the press-formed product having the target shape, The first forming step applies drawing or bending, The press forming method, wherein the second forming step applies bending forming.

3. 3. 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.

4. A method for manufacturing a press-molded product having a top plate portion having a convex outer periphery portion in which an outer periphery or a part thereof is curved outwardly in a convex shape, a vertical wall portion continuing to the top plate portion via a punch shoulder R portion, and a flange portion continuing to the vertical wall portion via a die shoulder R portion, A blanking process in which an expanded blank, which is a blank having a shape obtained by expanding the press-molded product having a target shape, is taken from a metal plate; a first forming step of forming the developed blank into an intermediate product; a second forming step of forming the intermediate product formed in the first forming step into a press-formed product having the target shape, The first forming process is a method for manufacturing a press-molded product, characterized in that the bending radius of a die shoulder R portion formed corresponding to at least the convex outer peripheral edge portion of the top plate portion of the intermediate molded product is formed to be larger than the bending radius of the die shoulder R portion of the press-molded product of the target shape, and with regard to the distance a from the R end on the vertical wall portion side of the punch shoulder R portion to the blank end, the distance a in the intermediate molded product is made longer than the distance a in the target shape.

5. A method for manufacturing a press-molded product having a top plate portion having a convex outer peripheral edge portion in which the outer peripheral edge or a part thereof is curved convexly outward, a vertical wall portion continuing to the top plate portion via a punch shoulder R portion, and a flange portion continuing to the vertical wall portion via a die shoulder R portion, A blanking process in which an expanded blank, which is a blank having a shape obtained by expanding the press-molded product having a target shape, is taken from a metal plate; a first forming step of forming the developed blank into an intermediate product; a second forming step of forming the intermediate product formed in the first forming step into a press-formed product having the target shape, The first forming step forms a bend radius of a die shoulder R portion formed corresponding to at least the convex outer peripheral edge portion of the top plate portion of the intermediate formed product so as to be larger than the bend radius of the die shoulder R portion of the press-formed product having the target shape, The first forming step applies drawing or bending, A method for manufacturing a press-formed product, wherein the second forming step applies bending forming.

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