Press molding method and manufacturing method of press molded product

Abstract

To provide a press forming method and a manufacturing method for a press formed product that can sufficiently suppress generation of wrinkles of a flange part due to shrink deformation of a flange, without necessarily performing a trimming process after press forming, and also can be applied to bending formation.SOLUTION: A press forming method and a manufacturing method for a press formed product according to the present invention, which form a press formed product 1 which has a top plate part 3 having a convex outer peripheral edge portion 3a whose outer peripheral edge or a portion of the edge is curved in a convex shape outwardly and a vertical wall part 5 extending from the top plate part 3 through a punch shoulder R portion 9, comprise 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 the press formed product 1 in a target shape. In the intermediate formed product 15, a position of an end portion of a punch shoulder R portion 27 at the top plate part 3 side is same as a position in the target shape, and a curvature radius of the punch shoulder R portion 27 extending to at least the convex outer peripheral edge portion 3a of the top plate 3 is larger than a curvature radius in the target shape.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a press molding method for molding a press-molded product having a top plate portion and a vertical wall portion, and in particular to a press molding method and a manufacturing method for a press-molded product that suppress the occurrence of wrinkles associated with shrinkage flange deformation when molding 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 caused by shrink flange deformation 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 flange at that part may shrink and deform during press forming, causing wrinkles at 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. Similarly, parts that do not have a flange and are composed of a top plate and a vertical wall are prone to wrinkles at the end of the vertical wall due to shrink and 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 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 the method of Patent Document 1, the slanted wall portion is formed in a state where a portion of the blank material that is closer to the end than the portion corresponding to the slanted wall portion is clamped between a die and a punch, thereby preventing buckling of the blank material in the plate thickness direction and suppressing wrinkles that occur in the slanted wall portion.

[0005] Patent Document 2 discloses a method for manufacturing a press-formed product having a hat-shaped cross section in which the top plate and flange are continuous in the width direction via a side wall, and the top plate and flange have a curved portion that is convexly curved toward the top plate along the longitudinal direction. The method of Patent Document 2 includes a step drawing process in which a wrinkle suppression region is set on the outer periphery of the flange position and forming is performed by step drawing, and further an additional region is set in part of the flange position to be pressed by the wrinkle suppressor, thereby suppressing wrinkles that occur in the flange. [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 inclined wall portion is formed in a state in which the portion of the blank material that is closer to the end than the portion corresponding to the inclined wall portion is clamped between the die and the punch, so that the portion clamped between the die and the punch needs to be trimmed in the next process.

[0008] Furthermore, although the press molding method described in Patent Document 2 can suppress the occurrence of flange wrinkles, it has a problem in that it cannot be applied to press molding by bending (forming) because it uses a wrinkle suppressor.

[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 do not require a trimming process after press molding, sufficiently suppress wrinkles caused by shrinkage flange deformation, and are also applicable 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, and a vertical wall portion continuing from the top plate portion via a punch shoulder R portion, and comprises a first forming process of forming a metal plate into an intermediate formed product, and a second forming process of forming the intermediate formed product formed in the first forming process into the press-formed product of a target shape, wherein the position of the punch shoulder R end portion on the top plate portion side is identical to the target shape, and at least the radius of curvature of the punch shoulder R portion continuing to the convex outer peripheral edge portion of the top plate portion is larger than 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.

[0013] (4) The manufacturing method of a press-formed product of the present invention is a method for manufacturing 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 continuing from the top plate portion via a punch shoulder R portion, and comprises a first shaping process of forming a metal plate into an intermediate formed product, and a second shaping process of forming the intermediate formed product formed in the first shaping process into the press-formed product of a target shape, wherein the position of the punch shoulder R end portion on the top plate portion side is identical to the target shape, and at least the radius of curvature of the punch shoulder R portion continuing to the convex outer peripheral edge portion of the top plate portion is larger than the target shape.

[0014] (5) Moreover, in the above-mentioned (4), 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.

[0015] (6) In addition, in the above (4) or (5), 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, an intermediate molded product having a smaller amount of shrink flange deformation than the target shape is molded in the first molding process, and the intermediate molded product is molded into the target shape in the second molding process, so that material movement due to shrink flange deformation is less likely to occur in the second molding process, and wrinkles are less likely to occur. Therefore, the present invention can suppress an increase in plate thickness of a molded product having a target shape, and can obtain press-molded products having a good shape without wrinkles, which leads to improved yields in press molding. Furthermore, since the present invention does not require clamping the end of the blank with a punch and a die, the conventional trimming step is not essential. Furthermore, since the present invention does not require a blank holder, it can also be applied to bending. [Brief description of the drawings]

[0017] [Figure 1] 1 is an explanatory diagram of a press molding method according to an embodiment of the present invention. [Diagram 2] 2A and 2B are explanatory diagrams of a part (target shape) targeted in the embodiment, where FIG. 2A is a perspective view and FIG. 2B is a plan view. [Diagram 3] FIG. 11 is a diagram showing a molding process in a second molding step in the embodiment (part 1). [Figure 4] FIG. 11 is a diagram showing a molding process in a second molding step in the embodiment (part 2). [Diagram 5] FIG. 4 is an explanatory diagram of the amount of material flowing in the first molding step and the second molding step. [Figure 6] FIG. 13 is a diagram showing a comparison between the cross-sectional shapes of three examples of intermediate formed products in which the radius of curvature of the punch shoulder R portion is changed and the cross-sectional shape of the target shape. [Figure 7] 1 is a graph showing the relationship between the radius of curvature of the punch shoulder R portion of an intermediate formed product and the amount of material flowing in each process. [Figure 8] FIG. 1 is a diagram showing a plate thickness increase rate distribution and a maximum plate thickness increase rate of a press-molded product molded by a conventional press molding method. [Figure 9] FIG. 1 is a diagram showing a forming process in a conventional press forming method (part 1). [Figure 10] FIG. 2 is a diagram showing the forming process in a conventional press forming method (part 2). [Figure 11] 11A and 11B are diagrams showing another example of a part (target shape) to which the present invention can be applied, where FIG. 11A is a perspective view and FIG. 11B is a plan view. [Figure 12] 12 is a diagram showing wrinkles that occur when the part in FIG. 11 is molded by a conventional press molding method. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] A press-molded product targeted by the press molding method and the manufacturing method of the press-molded product according to the present embodiment will be described with reference to the example of FIG. 2. Note that FIG. 2 shows the whole or a characteristic part of the press-molded product. The press-molded product 1 shown in FIG. 2 has a top plate portion 3, a vertical wall portion 5, and a flange portion 7, and has a portion of the outer periphery of the top plate portion 3 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 is, for example, limited to the R of the convex outer periphery portion 3a when the top plate portion 3 is viewed in plan. In addition, the angle between the top plate portion 3 and the vertical wall portion 5 of the press-molded product 1 of this example, and the angle between the vertical wall portion 5 and the flange portion 7 were both set to 90°.

[0019] The boundary 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 of the punch used in press forming, and therefore this portion is referred to as the "punch shoulder R portion 9." In addition, the boundary between the vertical wall portion 5 and the flange portion 7 has an R shape corresponding to the shape of the die shoulder of the die, and therefore this portion is referred to as the "die shoulder R portion 11." Hereinafter, when the terms "punch shoulder R portion 9" and "die shoulder R portion 11" are used simply in this specification, they refer to the above-mentioned portions on the press-formed product 1 side, not the die side.

[0020] First, prior to describing the press-molding method according to the present embodiment, a problem that occurs when a press-molded product 1 as shown in FIG. 2 is press-molded by a conventional method will be described. FIG. 8 shows the results of an FEM analysis of a case where a press-formed product 1 is press-formed by a conventional method, and the distribution of the plate thickness increase rate is shown by color shading. The plate thickness increase rate is calculated by calculating 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 expressed as a ratio (proportion) to the plate thickness of the blank, with a larger value indicating a greater increase in plate thickness. Furthermore, the more the plate thickness increases, the more likely wrinkles are to occur in that portion of the press-formed product 1. Furthermore, the more localized the plate thickness increase is, the more likely wrinkles are to occur.

[0021] When a press-formed product 1 as shown in FIG. 2 is formed by a conventional method, for example, a punch and a die having a shape corresponding to the target shape are used to form a flat blank into the target shape in one process. In this case, the vertical wall portion 5 connected to the convex outer peripheral edge portion 3a of the top plate portion 3 and the flange portion 7 connected to this vertical wall portion 5 are likely to shrink and deform, causing material to concentrate and wrinkles to occur. In the case of the press-formed product 1 shown in FIG. 2, the end portion of the flange portion 7 indicated by the arrow in FIG. 8 is where the plate thickness has increased the most, and the maximum plate thickness increase rate was +12.5%. In this way, the local increase in plate thickness causes wrinkles in the relevant portion, which has been a problem. The reason why the plate thickness of the flange portion 7 increases locally as shown in FIG. 8 will be explained using FIG. 9 and FIG. 10.

[0022] 9 and 10 show the forming process when the press-formed product 1 is formed by the above-mentioned conventional press forming method. Figure 9 shows the deformation process of blank 13 in a front view (the upper view of Figure 9, seen from the direction of the arrow in Figure 2(b)) and a cross-sectional view (the lower view of Figure 9, corresponding to the AA' cross section in Figure 2(b)). 10 shows a top view, a front view (same as the front view in FIG. 9), and a side view of the deformation process of the blank 13. In FIG. 10, the die 23 is omitted in order to make the shape of the blank 13 easier to understand. In addition, numerical values ​​such as "10mmup" in the figure indicate the distance in the press direction between the punch 21 and the die 23, taking into account the sheet thickness of the blank 13. Therefore, "10mmup" indicates that the gap in the press forming direction between the flange portion of the punch 21 and the flange portion of the die 23 is the sheet thickness of the blank 13 plus +10mm. Also, "0mmup" indicates the bottom dead center state of forming.

[0023] When the vertical wall portion 5 (see FIG. 2(a)) continuing from the convex outer peripheral edge portion 3a of the top plate portion 3 starts to be formed, for example, two large mountain-shaped wrinkles are generated at the end portion of the blank 13 due to shrink flange deformation, as shown in the front views of "10 mm up" in FIG. 9 and FIG. 10. As the shrink flange deformation progresses, these two large mountain-shaped wrinkles become concentrated in the center and take on a distinct shape (see the front views of "5 mm up" and "3 mm up" in FIG. 10).

[0024] As the forming process progresses, the die 23 moves relatively and when the bottom surface of the die 23 reaches the top of the wrinkle, the forming process proceeds as if the die 23 were squashing the wrinkle. However, when the forming process reaches "1 mm up", the blank 13 is restrained with the wrinkle remaining, and the blank 13 reaches the bottom dead point of the forming process (see "0 mm up").

[0025] As described above, in the conventional forming process, large wrinkles occur in the gap between the punch 21 and the die 23, and the flange portion 7 is formed without completely squeezing these wrinkles, so wrinkles remain in the press-formed product 1 and the plate thickness of the wrinkled portion increases locally.

[0026] One method for preventing wrinkles from occurring during the forming process is to use a wrinkle suppressor at the portion corresponding to the flange portion 7, but this cannot be applied to bending that does not use a wrinkle suppressor.

[0027] The above-mentioned wrinkles due to the shrink flange deformation also occur in the case of a press-molded product 14 as shown in Fig. 11. The press-molded product 14 in Fig. 11 does not have a flange portion, and is composed of a top plate portion 3 and a vertical wall portion 5, and has a portion (convex outer peripheral edge portion 3a) where part of the outer peripheral edge of the top plate portion 3 is curved outwardly in a convex shape, similar to the press-molded product 1 in Fig. 2.

[0028] When a press-formed product 14 as shown in Figure 11 is produced by the conventional method, i.e., by using a punch and die shaped corresponding to the target shape to form a flat blank into the target shape in one process, wrinkles will 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 a dashed circle in the figure), as shown in Figure 12.

[0029] When formed in a single process, wrinkles occur in the flange portion 7 of the press-formed product 1 in Figure 2 and in the vertical wall portion 5 of the press-formed product 14 in Figure 11 because the material moves toward the convex curved portion in a concentrated manner due to shrinkage flange deformation. Therefore, the inventors have studied a method for reducing the amount of shrink flange deformation in each step by using a two-step press forming method in which a target shape is formed via an intermediate product, and have devised a shape of the intermediate product that can form the product while suppressing the amount of shrink flange deformation, and can reduce the material movement in the shrink direction by generating an elongation material flow during the target forming. The press-molding method according to the present embodiment is based on the above idea. Hereinafter, a specific description will be given of the case where the press-molded product shown in FIG. 2 is molded as an example.

[0030] The press molding method of this embodiment is a method for molding a press-molded product 1 as shown in Figure 2, and includes a first molding process for molding a blank 13 into an intermediate molded product 15, and a second molding process for molding the intermediate molded product 15 into the press-molded product 1, as 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.

[0031] FIG. 1(a) is a perspective view of a punch 17, a die 19 and a blank 13 before forming in the first forming step, and FIG. 1(b) is a cross-sectional view taken along line B of FIG. 1(a). FIG. 1(c) is a perspective view of a punch 21, a die 23 and an intermediate formed product 15 before forming in the second forming step, and FIG. 1(d) is a cross-sectional view taken along line C of FIG. 1(c). In addition, in each of the dies in Figs. 1(a) to 1(d), only the shape of the molding surface portion is shown as a plate, ignoring the thick portions. In addition, in the intermediate product 15 shown in FIG. 1(d), the parts corresponding to those in the press-formed product 1 are given the same reference numerals. Each step will be described in detail below.

[0032] <1st molding process> The first forming step is a step of press-forming a blank 13, which is a metal plate, into an intermediate product 15, as shown in Figs. 1(a) and 1(b). The punch 17 used in the first forming step has a shoulder curvature radius R1 larger than a shoulder curvature radius R2 of a punch 21 having a shape corresponding to the target shape used in the second forming step (R1>R2).

[0033] In the first forming step, as shown in Fig. 1(b), the die 19 is moved relatively while a part of the blank 13 is clamped between the upper surface of the top plate forming surface of the punch 17 and the pad 25. This forms an intermediate formed product 15 in which the radius of curvature of the punch shoulder R portion 27 is larger than the radius of curvature of the punch shoulder R portion 9 of the target shape. The position of the punch shoulder R end portion on the top plate 3 side of the intermediate formed product 15 (the R start position on the top plate 3 side of the punch shoulder R portion 27) is the same as the target shape. In other words, the punch shoulder R portion 27 of the intermediate formed product 15 is formed so that the R start position on the top plate 3 side coincides with the R start position on the top plate 3 side of the punch shoulder R portion 9 of the target shape. Since the intermediate formed product 15 has the shape described above, it can be formed with a smaller amount of shrinkage flange deformation compared to when the flat blank 13 is formed into the target shape. Therefore, in the first forming step, the plate thickness of the end portion of the flange portion 33 of the intermediate formed product 15 is unlikely to increase, and wrinkles are unlikely to occur.

[0034] <Second forming process> The second forming process is a process for forming the intermediate product 15 formed in the first forming process into a press-formed product 1 having a target shape. The punch 21 and die 23 in the second forming process have shapes corresponding to the target shape and are similar to the conventional mold shown in Fig. 9, so they are denoted by the same reference numerals.

[0035] 1(d), the top plate 3 of the intermediate product 15 is set to match the upper surface of the punch 21. At this time, the R start position of the punch shoulder R portion 27 of the intermediate product 15 is aligned with the R start position of the shoulder of the punch 21. Then, with the top plate 3 of the intermediate product 15 being clamped between the upper surface of the punch 21 and the pad 25, the die 23 is moved relatively to the punch 21, and the intermediate product 15 is formed into a target shape. When the intermediate molded product 15 is set on the upper surface of the punch 21, the portion of the intermediate molded product 15 from the punch shoulder R portion 27 to the flange portion 33 is curved in the circumferential direction and separated from the vertical wall molding surface portion of the punch 21, resembling an open umbrella.

[0036] From this state, molding begins, and the molding process is shown in Figures 3 and 4. In Fig. 3, the deformation process of the intermediate molded product 15 is shown in a front view and a cross-sectional view, similar to Fig. 9. In Fig. 4, the deformation process of the intermediate molded product 15 is shown in a top view, a front view, and a side view, similar to Fig. 10. The meaning of numerical values ​​such as "15mm up" and the fact that the die 23 is not shown in the top view, front view, and side view are the same as in Fig. 10.

[0037] As shown in Fig. 3, when the shoulder of the die 23 comes into contact with the punch shoulder R portion 27 of the intermediate formed product 15 at "10 mm up," the vertical wall portion 5 of the target shape is formed up to "3 mm up." In the conventional example, shrinkage flange deformation occurs during the formation of the vertical wall portion 5, and large mountain-shaped wrinkles occur in the gap between the punch 21 and the die 23 (see "10 mm up" to "3 mm up" in Fig. 9). In contrast, in the second forming step of this embodiment, the flange portion 33 of the intermediate product 15 is located near the flange forming surface of the punch 21, and no wrinkles are formed, regardless of the size of the gap between the punch 21 and the die 23. This is because the intermediate product 15 has higher rigidity than the flat blank 13 due to work hardening, making it difficult for the material to move and therefore difficult for the flange portion 33 to deform.

[0038] When the die 23 moves "1 mm up" relative to the punch 21, the formation of the target shape of the die shoulder R portion 11 and the flange portion 7 connected thereto begins, and the flange portion 7 becomes more likely to deform. However, at this point, the gap in the press forming direction between the flange forming surface of the punch 21 and the flange forming surface of the die 23 is very small at the plate thickness + 1 mm, so the flange portion 7 hardly shrinks and reaches the bottom dead point without undergoing flange deformation. Therefore, wrinkles are less likely to occur in the flange portion 7 of the press-formed product 1 after forming is complete.

[0039] In addition, the second forming process has the effect of dispersing the strain generated during the forming process without concentrating it locally, which can further reduce the shrink flange deformation. This point will be explained with reference to FIG.

[0040] As shown in FIG. 4, in the “15 mm up” state, the die 23 has not yet come into contact with the intermediate molded product 15, and the portion of the intermediate molded product 15 from the punch shoulder R portion 27 to the flange portion 33 has an umbrella-like shape. When the die 23 is moved relatively from the "15 mm up" state, the die 23 comes into contact with the punch shoulder R portion 27 of the intermediate formed product 15 at "10 mm up," and as shown in the side view of "10 mm up," a bend occurs in the punch shoulder R portion 27, and the punch shoulder R portion 9 and vertical wall portion 5 begin to be formed into the target shape.

[0041] At this time, the die 23 presses against the curved punch shoulder R portion 27 and the vertical wall portion 29, which exerts a force that stretches the tip of the flange portion 33 in the circumferential direction, countering the shrink flange deformation and mitigating the material flow in the shrink direction. This disperses the strain and prevents it from concentrating locally, further mitigating the shrink flange deformation.

[0042] In this way, due to the high rigidity of the intermediate molded product 15, material movement is less likely to occur during the molding process in the second molding step, and further, a material flow occurs that alleviates shrinkage flange deformation, so that the plate thickness of the flange portion 7 of the press-molded product 1 after molding is completed is less likely to increase.

[0043] As described above, in this embodiment, an intermediate formed product 15 with a small amount of shrinkage flange deformation is formed in the first forming process, and the intermediate formed product 15 is formed into the target shape in the second forming process, thereby eliminating the problem of localized increase in plate thickness and suppressing wrinkles that occur in the flange portion 7 of the press-formed product 1. Furthermore, since there is no need to clamp the end of the blank with a punch and a die, there is no need for a trimming step as in the conventional example shown in Patent Document 1.

[0044] Furthermore, the press molding method of the present embodiment can suppress wrinkles in the flange portion 7 without using a blank holder, and is therefore applicable to press molding by bending (forming). That is, it is particularly effective when drawing or bending is applied in the first forming step for forming the intermediate product 15, and bending is applied in the second forming step for forming the target shape.

[0045] 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 deformation. For example, the blank metal sheet may be a steel sheet with a tensile strength of 590 MPa or more, and in that case too, a sufficient wrinkle reduction effect can be achieved. Furthermore, by performing the first and second molding steps of the above-mentioned press molding method, the desired press-molded product can be manufactured, and the manufactured press-molded product has reduced wrinkles, as described above.

[0046] The above has been explained using the example of molding a press-molded product 1 having a flange portion 7 as shown in Figure 2, but wrinkles can also be reduced by a similar effect when molding a press-molded product 14 that does not have a flange portion as shown in Figures 11 and 12.

[0047] As described above, the press forming method of the present embodiment is designed to reduce wrinkles occurring in the press-formed product 1 by using the intermediate product 15 having a smaller amount of shrink flange deformation than the target shape. The amount of shrink flange deformation of the intermediate product 15 becomes smaller as the radius of curvature of the punch shoulder R portion 27 increases. This point will be further explained.

[0048] In Fig. 5(a), the cross-sectional shape of the blank 13 before forming in the first forming step is shown by a dashed line, and the cross-sectional shape of the intermediate formed product 15 at the bottom dead center after forming is shown by a solid line. Here, the distance from the end of the blank 13 to the end of the flange portion 33 of the intermediate formed product 15 in Fig. 5(a) is defined as the amount of material flow in the first forming step. The amount of material flowing in this first forming step decreases as the radius of curvature of the punch shoulder R portion 27 of the intermediate formed product 15 increases. A specific example is shown in FIG.

[0049] 6 compares the cross-sectional shapes of the target shape (R4) in which the radius of curvature of the punch shoulder R portion 9 is 4 mm with the cross-sectional shapes of the intermediate formed product 15 (R6, R8, R10) in which the radius of curvature of the punch shoulder R portion 27 is 6 mm, 8 mm, and 10 mm, with the top plate portion 3 superimposed. The top plate portion 3 of the intermediate formed product 15 and the top plate portion 3 of the target shape are the same shape, and the end portion (R start position) of the punch shoulder R portion 27 of the intermediate formed product 15 on the top plate portion 3 side is in the same position as the target shape.

[0050] As shown in Fig. 6, the greater the radius of curvature of the punch shoulder R portion 27 of the intermediate product 15, the more the end of the flange portion 33 is located on the right side of the page. In other words, the greater the radius of curvature of the punch shoulder R portion 27 of the intermediate product 15, the smaller the amount of material inflow shown in Fig. 5(a) is. When the amount of material flow is small, the amount of shrinkage flange deformation in the first forming process is small, so by increasing the radius of curvature of the punch shoulder R portion 27 of the intermediate formed product 15, the increase in plate thickness in the first forming process (the increase in plate thickness from the blank 13) can be reduced.

[0051] 5(b) shows the cross-sectional shape of the intermediate product 15 at the bottom dead center in the first forming step by a dashed line, and the cross-sectional shape of the press-formed product 1 at the bottom dead center in the second forming step by a solid line. Here, the distance from the end of the flange portion 33 of the intermediate product 15 to the end of the flange portion 7 of the press-formed product 1 in FIG. 5(b) is defined as the amount of material flow in the second forming step. The amount of material flowing in the second forming step increases as the radius of curvature of the punch shoulder R portion 27 of the intermediate formed product 15 increases. If the amount of material flowing in is large, the amount of shrink flange deformation in the second forming step increases, so by increasing the radius of curvature of the punch shoulder R portion 27 of the intermediate formed product 15, the increase in plate thickness in the second forming step (the increase in plate thickness from the intermediate formed product 15) increases.

[0052] As described above, the larger the radius of curvature of the punch shoulder R portion 27 of the intermediate product 15, the smaller the amount of material flowing in the first forming process and the larger the amount of material flowing in the second forming process. The relationship between the radius of curvature of the punch shoulder R portion 27 of the intermediate product 15 and the amount of material flowing in each process in the four examples shown in Fig. 6 is shown in Fig. 7.

[0053] As shown in Fig. 7, the larger the radius of curvature of the punch shoulder R portion 27 of the intermediate formed product 15, the less the amount of material flowing in the first forming process and the less the increase in plate thickness can be reduced, but the more the amount of material flowing in the second forming process and the greater the increase in plate thickness. Also, the amount of material flowing in the first forming process is significantly greater than the amount of material flowing in the second forming process. Therefore, in order to suppress the increase in plate thickness in the target shape, it is preferable to reduce the amount of material flowing in the first forming process and to increase the radius of curvature of the punch shoulder R portion 27 of the intermediate formed product 15. Furthermore, it is preferable that the radius of curvature of the punch shoulder R portion 27 of the intermediate formed product 15 is about 2 to 5 times the radius of curvature of the target shape. This point will be specifically explained together with the effects of the present invention in the following examples. EXAMPLES

[0054] A specific study was conducted using FEM analysis on the effect of suppressing wrinkles due to shrink flange deformation 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 in FIG. 2 was press-formed to a target shape. FEM analysis was performed on a conventional example in which a steel sheet is formed into a target shape in one process and an example of the present invention in which a steel sheet is formed into a target shape in two processes, and the maximum sheet thickness increase rate of the shrink flange deformation part at the bottom dead center of forming was obtained. Note that the analysis results of the conventional example are as explained in Figure 8, so the analysis results of the example of the present invention will be explained below.

[0055] In the present invention, FEM analysis was performed on two examples (R8 and R10 in FIG. 6) in which the radii of curvature of the punch shoulder R portion 27 of the intermediate formed product 15 were 8 mm and 10 mm, respectively, for the target shape (R4 in FIG. 6) in which the radius of curvature of the punch shoulder R portion 9 was 4 mm. The results are shown in Table 1. Note that both the maximum thickness increase rate of the intermediate formed product and the maximum thickness increase rate of the target formed product shown in Table 1 indicate the increase rate based on the thickness of the blank.

[0056] [Table 1]

[0057] As shown in Table 1, the maximum thickness increase rate of the target formed product (press-formed product 1) in the conventional example (No. 1) was 12.5%, whereas the maximum thickness increase rate of the target formed products in the present invention examples (No. 2 and No. 3) was lower than that of the conventional example. As a result, as described above, this example showed that the present invention can suppress flange wrinkles due to shrink flange deformation more than the conventional example. As described above, wrinkles can be more effectively suppressed by setting the radius of curvature of the punch shoulder R portion 27 of the intermediate product 15 so as to minimize the increase in plate thickness of the target product after the second forming step. This point will be described in detail below.

[0058] As can be seen by comparing the maximum thickness increase rate of the intermediate formed products 15 of No. 2 and No. 3, the larger the radius of curvature of the punch shoulder R portion 27 of the intermediate formed product 15, the smaller the maximum thickness increase rate of the intermediate formed product 15 in the first forming step. This is because, as shown in Figure 6, the intermediate formed product 15 with a larger radius of curvature of the punch shoulder R portion 27 has an end of the flange portion 33 that protrudes to the right side of the paper, and the amount of material flowing in the first forming step (see Figure 5(a)) is smaller. If the amount of material flowing in the first forming step is smaller, the amount of shrinkage flange deformation of the intermediate formed product 15 is smaller, and the maximum thickness increase rate is also reduced.

[0059] In addition, as can be seen by comparing the maximum thickness increase rate of the target formed products of No. 2 and No. 3, the larger the curvature radius of the punch shoulder R portion 27 of the intermediate formed product 15, the smaller the maximum thickness increase rate of the target formed product in the second forming process. Here, the product with the largest curvature radius, No. 3, had the smallest maximum thickness increase rate.

[0060] In this embodiment, the example in which the radius of curvature of the punch shoulder R portion 27 of the intermediate product 15 was the largest reduced the plate thickness increase rate. In this way, the radius of curvature of the punch shoulder R portion 27 of the intermediate product 15 should be set so that the plate thickness increase of the target product after the second forming step is as small as possible, which is effective in maximizing the wrinkle suppression effect. [Explanation of symbols]

[0061] 1 Press-molded product (target shape) 3 Top plate 3a Convex outer periphery 5 Vertical wall section 7 Flange 9 Punch shoulder R part 11 Die shoulder R 13 Blank (metal plate) 14 Press-molded products (other examples of target shapes) 15 Intermediate molded products 17 Punch (first forming process) 19 Die (first molding process) 21 Punch (second forming process or conventional example) 23 Die (second molding process or conventional example) 25 Pad 27 Punch shoulder R part (intermediate formed product) 29 Vertical wall part (intermediate molded product) 31 Die shoulder R part (intermediate molded product) 33 Flange part (intermediate molded product)

Claims

1. A press-molding method for molding a press-molded product having a top plate portion having a convex outer peripheral edge portion in which an outer peripheral edge or a part thereof is curved outwardly in a convex shape, and a vertical wall portion continuing from the top plate portion via a punch shoulder R portion, A first forming step of forming a metal plate into an intermediate product; a second forming step of forming the intermediate product formed in the first forming step into the press-formed product having a target shape, In the intermediate formed product, the position of the punch shoulder R end portion on the top plate portion side is the same as the target shape, and at least the radius of curvature of the punch shoulder R portion continuous with the convex outer peripheral edge portion of the top plate portion is larger than the target shape, A press molding method characterized in that in a second molding process, the R start position of the punch shoulder R portion of the intermediate molded product is aligned with the R start position of the shoulder R of the punch in the second molding process, and the intermediate molded product is molded into a target shape.

2. A press molding method as described in claim 1, characterized in that the radius of curvature of the punch shoulder R portion of the intermediate molded product is 2 to 5 times the radius of curvature of the punch shoulder R portion of the target shape.

3. The first forming step applies drawing or bending, 3. The press forming method according to claim 1, wherein the second forming step is performed by applying bending.

4. A method for manufacturing a press-molded product, comprising the steps of: forming a press-molded product having a top plate portion having a convex outer peripheral edge portion in which an outer peripheral edge or a part thereof is curved outwardly in a convex shape; and a vertical wall portion continuing from the top plate portion via a punch shoulder R portion, A first forming step of forming a metal plate into an intermediate product; a second forming step of forming the intermediate product formed in the first forming step into the press-formed product having a target shape, In the intermediate formed product, the position of the punch shoulder R end portion on the top plate portion side is the same as the target shape, and at least the radius of curvature of the punch shoulder R portion continuous with the convex outer peripheral edge portion of the top plate portion is larger than the target shape, A manufacturing method for press-molded products, characterized in that in a second molding process, the R start position of the punch shoulder R portion of the intermediate molded product is aligned with the R start position of the shoulder R of the punch in the second molding process, and the intermediate molded product is molded into a target shape.

5. A method for manufacturing a press-molded product as described in claim 4, characterized in that the radius of curvature of the punch shoulder R portion of the intermediate molded product is 2 to 5 times the radius of curvature of the punch shoulder R portion of the target shape.

6. The first forming step applies drawing or bending, The method for producing a press-formed product according to claim 4 or 5, characterized in that the second forming step applies bending forming.

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