Press forming method

Abstract

The press-molding method according to the present invention is for molding a press-molded article 1 being curved so as to be recessed or so as to project in the height direction and having a top plate part 3, a vertical wall part 5 contiguous to the top plate part 3, and a flange part 7 contiguous to the vertical wall part 5, the method comprising: a first molding step for molding the top plate part 3 having the same shape as the target shape of the press-molded article 1, and molding the vertical wall part 5 and the flange part 7 so as to set the height of the vertical wall higher than the target shape; and a second molding step for remolding a flange-side ridge part 11 between the vertical wall part 5 and the flange part 7 so that the vertical wall part 5 molded in the first molding step will have the same vertical wall height as the target shape. The vertical wall height of the vertical wall part 5 molded in the first molding step is made to be greater than the target shape by having added a value not more than 1 / 2 of the curvature radius of the flange-side ridge part 11 as viewed in a longitudinal vertical cross-section of the target shape.

Description

Press forming method

[0001] The present invention relates to a press forming method for a metal thin-sheet, and particularly relates to a press forming method for a press formed product having a flange portion curved convexly or concavely in the height direction.

[0002] In the press forming of a press formed product having a top portion, a side wall portion, and a flange portion, and at least the flange portion being curved convexly or concavely in the height direction, due to the residual stress of the flange portion generated in the forming process, springback occurs after die release, and as a result, there has been a problem that the target flange angle cannot be obtained. Therefore, a press forming method for suppressing the springback of such press formed products is desired.

[0003] So far, as a technique for suppressing the springback of a press formed product in which at least the flange portion is curved convexly or concavely in the height direction, Patent Document 1 and Patent Document 2 disclose that by forming the flange portion at an angle different from the product shape in a plurality of forming steps, the residual stress in the direction parallel to the bending ridge line portion between the vertical wall portion and the flange portion is increased or decreased, and as a result, a method of obtaining shape accuracy by controlling the springback caused by the residual stress is disclosed.

[0004] Japanese Patent No. 5382281 Japanese Unexamined Patent Application Publication No. 2015-131306

[0005] When press-forming a press-formed product having a top plate portion, a vertical wall portion, and a flange portion, and in which at least the flange portion is curved convex or concave in the height direction, if the flange angle is changed during the forming process to reduce residual stress in the flange portion, the stress at the tip edge of the flange portion changes significantly, but the stress at the base of the flange portion does not change easily. Furthermore, when forming a press-formed product having a flange portion perpendicular to the forming direction, if a trimming process is to be inserted between processes for changing the flange angle, the cutting edge does not strike the workpiece perpendicularly during the trimming process, which carries the risk of defects such as damage to the tool of press forming. For this reason, there has been a need for a technology that can reduce residual stress in the flange portion and reduce springback without changing the flange angle during the process of forming the flange portion in multiple processes.

[0006] The present invention has been made in view of the above problems, and its object is to provide a press molding method for forming a press-formed product having a top plate portion, a vertical wall portion, and a flange portion, wherein at least the flange portion is curved in a convex or concave shape in the height direction, while suppressing springback during the molding process.

[0007] The inventor first investigated the cause of springback in the press-formed product 1, which is shown as an example in Figure 11.

[0008] The press-formed product 1 shown in Figure 11 has a top plate portion 3, a vertical wall portion 5 continuous with the top plate portion 3, and a flange portion 7 continuous with the vertical wall portion 5 (Figure 11(a)), and is curved in a convex shape in the height direction when viewed from the side (Figure 11(b)). The top plate portion 3 and the vertical wall portion 5 are continuous via a top plate side ridge portion 9, and the vertical wall portion 5 and the flange portion 7 are continuous via a flange side ridge portion 11, and the top plate side ridge portion 9 and the flange side ridge portion 11 are straight lines along the longitudinal direction when viewed from above (Figure 11(c)).

[0009] Such a press-formed product 1 is typically formed in a single process by crash forming using an upper tool 51, a lower tool 53, and a pad 55, as shown in Figure 12. In this case, the blank 41 is held between the pad 55 and the lower tool 53 and curved in a convex shape in the height direction (Figure 13(b)). The portion of the blank 41 corresponding to the flange 7 undergoes shrink flange deformation (Figure 13(c)), and compressive stress remains in the flange 7 at the bottom dead center of the forming process (Figure 13(d)).

[0010] Therefore, after demolding the press-formed product 1, the compressive stress remaining in the flange portion 7 is released, causing springback (elastic recovery) in which it stretches in the longitudinal direction. As a result, the flange portion 7 deforms so that its easily movable ends spring upward in the height direction, and the angle between the vertical wall portion 5 and the flange portion 7 decreases.

[0011] Furthermore, the inventors also investigated the cause of springback in the press-formed product 21, which is shown as an example in Figure 14.

[0012] The press-formed product 21 shown in Figure 14 has a top plate portion 23, a vertical wall portion 25 continuous with the top plate portion 23, and a flange portion 27 continuous with the vertical wall portion 25 (Figure 14(a)), and is curved in a concave shape in the height direction when viewed from the side (Figure 14(b)). The top plate portion 23 and the vertical wall portion 25 are continuous via the top plate side ridge portion 29, and the vertical wall portion 25 and the flange portion 27 are continuous via the flange side ridge portion 31, and the top plate side ridge portion 29 and the flange side ridge portion 31 are straight lines along the longitudinal direction when viewed from above (Figure 14(c)).

[0013] Such press-formed products 21 are typically formed in one step by foam molding using an upper die 61, a lower die 63, and a pad 65, as shown in Figure 15. In this case, as shown in Figure 16, the blank 41 is sandwiched between the pad 65 and the lower die 63 and curved concavely in the height direction (Figure 16(b)), and the portion of the blank 41 corresponding to the flange portion 27 undergoes stretch flange deformation (Figure 16(c)), leaving tensile stress in the flange portion 27 at the bottom dead center of molding (Figure 16(d)). Therefore, after demolding the press-formed product 21, the tensile stress remaining in the flange portion 27 is released, causing springback as it shrinks in the longitudinal direction. As a result, the flange portion 27 deforms so that its easily movable ends spring upward in the height direction, and the angle between the vertical wall portion 25 and the flange portion 27 becomes smaller.

[0014] As described above, when a press-formed product that curves convex or concave in the height direction is formed into the target shape in a single process, springback occurs due to residual stress in the flange portion. Therefore, to suppress such springback, it is important to reduce the stress generated in the flange portion during the forming process.

[0015] Therefore, the inventors diligently studied methods for reducing stress in the flange portion and found that by forming the press-formed product in two steps, and by changing the height of the vertical wall portion in the first and second steps, it is possible to control the stress in the flange portion and suppress springback caused by residual stress in the flange portion. The present invention is based on this finding. Its configuration will be described below.

[0016] The press forming method according to the present invention forms a press-formed product into a target shape, having a top plate portion, a vertical wall portion continuous from the top plate portion, and a flange portion continuous from the vertical wall portion via a ridge portion, wherein at least the flange portion is curved in a convex or concave shape in the height direction, and comprises a first forming step of forming the top plate portion having the same shape as the target shape of the press-formed product, and forming the vertical wall portion and the flange portion such that the vertical wall height is greater than that of the target shape, and a second forming step of reforming the ridge portion between the vertical wall portion and the flange portion such that the vertical wall portion formed in the first forming step has the vertical wall height of the target shape, wherein the vertical wall height of the vertical wall portion formed in the first forming step is greater than the vertical wall height of the target shape by adding a value of 1 / 2 or less of the curvature radius of the ridge portion in the longitudinal perpendicular cross section of the target shape.

[0017] In the present invention, a press-formed product having a top plate portion, a vertical wall portion continuous with the top plate portion, and a flange portion continuous with the vertical wall portion via a ridge portion, wherein at least the flange portion is curved in a convex or concave shape in the height direction, is formed into a target shape, comprising: a first forming step of forming the top plate portion having the same shape as the target shape of the press-formed product, and forming the vertical wall portion and the flange portion such that the vertical wall height is greater than that of the target shape; and a second forming step of reforming the ridge portion between the vertical wall portion and the flange portion so that the vertical wall portion formed in the first forming step has the vertical wall height of the target shape, wherein the vertical wall height of the vertical wall portion formed in the first forming step is made greater than the vertical wall height of the target shape by adding a value of 1 / 2 or less of the radius of curvature of the ridge portion in the longitudinal perpendicular cross section of the target shape, thereby reducing the stress generated in the flange portion during the forming process and suppressing springback after demolding of the press-formed product.

[0018] Figure 1 shows the process of forming a press-formed product that curves convexly in the height direction using a press-forming method according to an embodiment of the present invention, and the stress distribution during the forming process. Figure 2 is a diagram explaining the effects of the press-forming method according to an embodiment of the present invention (Part 1). Figure 3 is an explanatory diagram of the mechanism of the effects of the press-forming method according to an embodiment of the present invention. Figure 4 is a diagram showing an example of the effects of the press-forming method according to an embodiment of the present invention (Part 1). Figure 5 shows the process of forming a press-formed product that curves concavely in the height direction using a press-forming method according to an embodiment of the present invention, and the stress distribution during the forming process. Figure 6 is a diagram explaining the effects of the press-forming method according to an embodiment of the present invention (Part 2). Figure 7 is a diagram showing an example of the effects of the press-forming method according to an embodiment of the present invention (Part 2). Figure 8 shows the target shape of the press-formed product that curves convexly in the height direction to be formed in an embodiment of the present invention ((a) perspective view, (b) longitudinal vertical cross-sectional view). Figure 9 shows the curvature in the height direction of the press-formed product to be formed in an embodiment of the present invention. Figure 10 shows the target shape of a press-formed product that curves concavely in the height direction to be molded in an embodiment of the present invention ((a) perspective view, (b) longitudinal vertical cross-sectional view). Figure 11 shows an example of a press-formed product that curves convexly in the height direction to be molded in the present invention ((a) perspective view, (b) side view, (c) top view). Figure 12 shows the process of forming a press-formed product that curves convexly in the height direction by a conventional press forming method. Figure 13 shows the deformation of the blank and the stress distribution during the process of forming a press-formed product that curves convexly in the height direction by a conventional press forming method. Figure 14 shows an example of a press-formed product that curves concavely in the height direction to be molded in the present invention ((a) perspective view, (b) side view, (c) top view). Figure 15 shows the process of forming a press-formed product that curves concavely in the height direction by a conventional press forming method. Figure 16 shows the deformation of the blank and the stress distribution during the process of forming a press-formed product that curves concavely in the height direction by a conventional press forming method.

[0019] The press forming method according to an embodiment of the present invention forms a press-formed product 1 that is curved in a convex shape in the height direction along the longitudinal direction, as illustrated in Figure 11 above, into a target shape, and comprises a first forming step (Figures 1(a) to 1(b)) and a second forming step (Figures 1(b) to 1(c)), as shown in Figure 1. The first forming step and the second forming step will be described below.

[0020] <First Molding Process> The first molding process, as shown in Figures 1(a) to 1(b), involves forming a top plate portion 3 of the blank 41 that has the same shape as the target shape of the press-molded product 1, and forming the vertical wall portion 5 and the flange portion 7 so that the vertical wall height (=h1) of the vertical wall portion 5 is greater than the vertical wall height of the target shape (h2 in Figure 1(c)) (h1 > h2). The vertical wall height h1 of the vertical wall portion 5 is made greater than the vertical wall height h2 of the target shape by adding a value of 1 / 2 or less of the radius of curvature of the flange-side ridge portion 11 in the longitudinally perpendicular cross section of the target shape.

[0021] In the first molding process, a top plate portion 3 with the same shape as the target shape is molded, and in order to mold the vertical wall portion 5 and flange portion 7 so that the vertical wall height is greater than that of the target shape, the position of the top plate side ridge portion 9, which is the ridge portion between the top plate portion 3 and the vertical wall portion 5 in the blank 41, is set to the same position as the target shape, and the position of the flange side ridge portion 11, which is the ridge portion between the vertical wall portion 5 and the flange portion 7, is shifted from the target shape during molding.

[0022] In this embodiment, as shown in Figure 1, the distance between the top plate portion 3 and the flange portion 7 in the height direction of the press-formed product 1 is defined as the vertical wall height of the vertical wall portion 5. However, the vertical wall height of the vertical wall portion 5 may also be defined as the distance between the top plate portion 3 and the flange portion 7 in the in-plane direction of the vertical wall portion 5.

[0023] <Second Molding Process> As shown in Figures 1(b) to 1(c), the second molding process involves reshaping the flange-side ridge line 11 between the vertical wall portion 5 and the flange portion 7 so that the vertical wall portion 5 formed in the first molding process has a vertical wall height h2 of the target shape, thereby forming a press-molded product 1 of the target shape.

[0024] Next, the effects and advantages of the press forming method according to this embodiment will be explained based on Figures 2 to 4. Figure 2 is a side view of the process of forming a blank 41 into a press-formed product 1, where the first bottom dead center in Figure 2 refers to the forming bottom dead center in the first forming process, and the second bottom dead center refers to the forming bottom dead center in the second forming process.

[0025] As described above, the first molding process involves molding the blank 41 so that the vertical wall height h1 of the vertical wall portion 5 is greater than the vertical wall height h2 of the target shape, forming the vertical wall portion 5, the flange portion 7, and the flange-side ridge portion 11. However, the longitudinal length of the flange-side ridge portion 11 formed in the first molding process is shorter than the longitudinal length of the portion corresponding to the flange-side ridge portion 11 in the blank 41 before molding.

[0026] For example, in Figure 2, if points a0 and b0 on the blank 41 before molding move to points a1 and b1, respectively, at the bottom dead center of the molding process, the flange length between a1 and b1 becomes shorter than the flange length between a0 and b0. In this way, during the first molding process, the flange portion 7 (flange-side ridge portion 11) is molded under shrinkage flange deformation, which shortens its longitudinal length, and compressive stress is generated in the flange portion 7 in the longitudinal direction.

[0027] The subsequent second molding process involves reshaping the flange-side ridge portion 11 so that the vertical wall portion 5 has a vertical wall height h2 of the target shape. However, the longitudinal length of the flange-side ridge portion 11 at the bottom dead center of the second molding process is longer than the longitudinal length at the bottom dead center of the first molding process.

[0028] For example, in Figure 2, if points a1 and b1 at the bottom dead center (first bottom dead center) of the first molding process move to points a2 and b2, respectively, at the bottom dead center (second bottom dead center) of the second molding process, then the flange length between a2 and b2 will be longer than the flange length between a1 and b1.

[0029] Therefore, in the second molding process, the flange-side ridge line portion 11 is reshaped so that the longitudinal length of the flange portion 7 is increased, and tensile deformation acts on the flange portion 7 toward the longitudinal outward direction.

[0030] Thus, in the flange portion 7, the first molding process is performed to make the flange portion 7 shorter in the longitudinal direction than the target shape of the press-formed product 1, and the subsequent second molding process is performed to return it to the longitudinal direction of the target shape of the press-formed product 1. Therefore, in the first molding process, a large strain is generated in the flange portion 7, causing compressive stress, but by slightly reducing the strain in the second molding process, this compressive stress is significantly reduced. In other words, the second molding process utilizes the characteristic that stress changes significantly in response to even slight strain recovery.

[0031] This point will be explained with reference to Figure 3. Figure 3 is a longitudinal stress-strain diagram of the flange portion from the start of molding to the second bottom dead center. As shown in Figure 3, a large stress is accumulated in the flange portion at the first bottom dead center during the first molding process. However, the stress is significantly reduced by slightly reducing the strain from the first bottom dead center to the second bottom dead center during the second molding process. Thus, the present invention utilizes the characteristic that stress changes significantly in response to even slight strain reduction.

[0032] Therefore, as shown in Figure 4, the compressive stress of the flange portion 7 at the bottom dead center of the second molding step of the present invention (Figure 4(a)) is reduced compared to the compressive stress of the flange portion 7 generated by a conventional press molding method (Figure 4(b)). As a result, springback when the press-molded product 1 is demolded after the second molding step is suppressed, and the change in the angle between the vertical wall portion 5 and the flange portion 7 can be reduced.

[0033] Furthermore, in the press forming method according to this embodiment, not only is the compressive stress of the flange portion 7 reduced, but the tensile stress around the top plate side ridge portion 9 between the top plate portion 3 and the vertical wall portion 5 can also be reduced.

[0034] In other words, at the bottom dead center of the first molding process, tensile stress is generated near the top plate side ridge 9, as shown in Figure 1(b). Then, in the second molding process, when the flange side ridge 11 is reshaped to achieve the target shape of the vertical wall height, tensile deformation acts on the flange 7 and compressive deformation acts on the top plate side ridge 9. As a result, the tensile stress at the bottom dead center of the second molding process can be reduced near the top plate side ridge 9, as shown in Figure 1(c).

[0035] As described above, according to the press forming method of this embodiment, in addition to reducing the compressive stress of the flange portion 7, the tensile stress of the top plate side ridge portion 9 is reduced, thereby suppressing the springback of the flange portion 7. Furthermore, since the angle between the vertical wall portion 5 and the flange portion 7 can be changed between the first forming step and the second forming step, the flange portion 7 can be formed at a target angle, for example, horizontally (in a direction perpendicular to the forming direction).

[0036] As mentioned above, the first molding process involves increasing the vertical wall height of the vertical wall portion 5 by adding a value less than or equal to half the radius of curvature of the flange-side ridge portion 11 in the longitudinally perpendicular cross-section of the target shape, making it greater than the vertical wall height of the target shape. The effect of this value added to the vertical wall height will be verified in the embodiment described later.

[0037] The above description pertains to a press-formed product 1 (see Figure 11) that curves convexly in the height direction. However, the press-forming method according to the present invention may also be used to form a press-formed product 21 that curves concavely in the height direction, as illustrated in Figure 14 above.

[0038] Even when forming the press-formed product 21, the molding is carried out in two steps, as shown in Figure 5: a first molding step (Figures 5(a) to 5(b)) and a second molding step (Figures 5(b) to 5(c)).

[0039] First, the first molding process molds the blank 41 into a top plate portion 3 with the same shape as the target shape of the press-molded product 1, and molds the vertical wall portion 5, flange portion 7, and flange-side ridge portion 11 so that the vertical wall height h1 of the vertical wall portion 5 is greater than the vertical wall height h2 of the target shape (h1 > h2) (Figures 5(a) to 5(b)). Then, the vertical wall height h1 of the vertical wall portion 25 is made greater than the vertical wall height h2 of the target shape by adding a value of 1 / 2 or less of the radius of curvature of the flange-side ridge portion 31 in the longitudinal perpendicular cross section of the target shape.

[0040] In the subsequent second molding process, the flange-side ridge line 31 between the vertical wall portion 25 and the flange portion 27 is reshaped so that the vertical wall portion 25 formed in the first molding process has a vertical wall height h2 of the target shape, thereby forming a press-molded product 21 of the target shape (Figures 5(b) to 5(c)).

[0041] The effects and advantages of forming a press-formed product 21 that curves concavely in the height direction will be explained based on Figures 6 and 7. Figure 6 is a side view of the process of forming a blank 41 into a press-formed product 21. In Figure 6, the first bottom dead center refers to the bottom dead center of the first forming process, and the second bottom dead center refers to the bottom dead center of the second forming process.

[0042] First, in the first molding process, as shown in Figure 6, points c0 and d0 on the blank 41 before molding move to points c1 and d1, respectively, at the molding bottom dead center (first bottom dead center) of the first molding process. In this case, the flange length between c1 and d1 becomes longer than the flange length between c0 and d0. Thus, in the first molding process, the flange portion 27 (flange-side ridge portion 31) is molded under elongation flange deformation, which increases its longitudinal length, and tensile stress is generated in the flange portion 27 in the longitudinal direction.

[0043] The subsequent second molding process involves reshaping the flange-side ridge portion 31 so that the vertical wall portion 25 has a vertical wall height h2 of the target shape. However, the longitudinal length of the flange-side ridge portion at the bottom dead center of the second molding process is shorter than the longitudinal length at the bottom dead center of the first molding process.

[0044] For example, in FIG. 6, assuming that point c1 and point d1 at the forming bottom dead center (the first bottom dead center) of the first forming step move to point c2 and point d2 respectively at the forming bottom dead center (the second bottom dead center) of the second forming step, the flange length between c2 - d2 is shorter than the flange length between c1 - d1.

[0045] Therefore, in the second forming step, the flange side ridge line portion 31 is reformed so that the longitudinal length of the flange portion 27 becomes shorter, and compressive deformation acting inward in the longitudinal direction acts on the flange portion 27.

[0046] Thus, forming is performed in the first forming step such that the longitudinal length is longer than the target shape of the press-formed product 21, and then forming is performed in the subsequent second forming step to return to the longitudinal length of the target shape of the press-formed product 21. For this reason, in the first forming step, large strain occurs in the flange portion 27 and tensile stress is generated, but in the second forming step, by slightly reducing the strain, the tensile stress is significantly reduced. This is as shown in the reason in FIG. 3.

[0047] Therefore, as shown in FIG. 7, the tensile stress (FIG. 7(a)) of the flange portion 27 at the forming bottom dead center of the second forming step of the present invention is reduced compared to the tensile stress (FIG. 7(b)) of the flange portion 27 by the conventional press-forming method. As a result, after the second forming step, springback when the press-formed product 21 is脱模 is suppressed, and a change in the angle formed by the vertical wall portion 25 and the flange portion 27 can be made small.

[0048] Furthermore, when the press-formed product 21 is formed by the press-forming method according to the present invention, not only the tensile stress of the flange portion 27 can be reduced, but also the compressive stress around the top plate side ridge line portion 29 between the top plate portion 23 and the vertical wall portion 25 can be reduced.

[0049] It should be noted that the term "脱模" in the original text seems to be incorrect or incomplete. It might be a misspelling or an inappropriate expression. If this is a technical term specific to a certain field, it needs to be accurately determined and corrected for a more accurate translation. The above translation is based on the current text as much as possible.In other words, at the bottom dead center of the first molding process, compressive stress is generated near the top plate side ridge 29, as shown in Figure 5(b). Then, in the second molding process, when the flange side ridge 31 is reshaped to achieve the target shape of the vertical wall height, compressive deformation acts on the flange 27 and tensile deformation acts on the top plate side ridge 29. As a result, the compressive stress at the bottom dead center of the second molding process can be reduced near the top plate side ridge 29, as shown in Figure 5(c).

[0050] As described above, according to the press forming method of this embodiment, by reducing the tensile stress of the flange portion 27 and the compressive stress of the top plate side ridge portion 29, the springback in the height direction of the flange portion 27 is further suppressed. Furthermore, since the first forming step and the second forming step can be performed without changing the angle between the vertical wall portion 25 and the flange portion 27, the flange portion 27 can be formed at a target angle, for example, in the horizontal direction (a direction perpendicular to the forming direction).

[0051] The above description pertains to the case where the target of the press-formed product is one in which both the top plate portion and the flange portion are curved in a convex or concave shape in the height direction. However, in the present invention, it is sufficient that at least the flange portion is curved in a convex or concave shape in the height direction, and the top plate portion may be flat without curving in the height direction.

[0052] For example, in a press-formed product (not shown) in which the top plate is flat and the flange is curved convexly in the height direction, the top plate side ridge where the top plate and the vertical wall connect is straight along the longitudinal direction in a side view.

[0053] When such a press-formed product is formed using the press-forming method according to the present invention, compressive stress is generated in the flange portion and tensile stress is generated near the linear top plate side ridge portion, similar to the press-formed product 1 (see Figure 1) described above. However, according to the press-forming method according to the present invention, in the second forming step, the compressive stress in the flange portion can be reduced and the tensile stress near the linear top plate side ridge portion can be reduced, thereby suppressing springback after demolding.

[0054] Furthermore, in press-formed products where the top plate portion is flat and the flange portion is curved in a concave shape, similar to press-formed product 21 (see Figure 5), tensile stress is generated in the flange portion and compressive stress is generated in the straight top plate side ridge portion during the first forming process. However, according to the press forming method of the present invention, in the second forming process, the tensile stress in the flange portion can be reduced, and the compressive stress near the straight top plate side ridge portion can be reduced, thereby suppressing springback after demolding.

[0055] Furthermore, although the above description pertains to a press-formed product in which a vertical wall portion is continuous from one side of the top plate portion, the present invention may also pertain to a press-formed product with a hat-shaped cross-section in which a pair of vertical wall portions are continuous from two opposing sides of the top plate portion.

[0056] We conducted verification tests to confirm the effects and advantages of the press forming method according to the present invention, and these tests are described below.

[0057] In this embodiment, press forming analysis was performed on the press-formed product 1 shown in Figure 8, and springback analysis was performed using the analysis results of the press forming analysis. Then, based on the springback analysis results, the springback of the flange portion 7 of the press-formed product 1 was evaluated.

[0058] In the press forming analysis, a steel plate with a tensile strength of 980 MPa and a thickness of 1.2 mm was used as the blank. Figures 8 and 9 show the target shape of the press-formed product 1 to be formed. The target shape of the press-formed product 1 is as follows: as shown in Figure 9, the radius of curvature of the convex curve in the height direction (camber convex R in Figure 9) is 1000 mm or 500 mm; as shown in Figure 8(b), the height of the vertical wall portion 5 is 30 mm, the angle between the top plate portion 3 and the vertical wall portion 5 is 95°, the angle between the vertical wall portion 5 and the flange portion 7 is 95°, the top plate portion 3 and the flange portion 7 are parallel (the flange portion 7 is horizontal), the radius of curvature of the top plate side ridge portion 9 in the longitudinal vertical cross section of the target shape (cross section A-A' in Figure 8(a)) is 5 mm, and the radius of curvature of the flange side ridge portion 11 in the longitudinal vertical cross section of the target shape (cross section A-A' in Figure 8(a)) is 6.2 mm.

[0059] The press forming analysis was performed on the process of forming the press-formed product 1 in two steps: a first forming step in which the height of the vertical wall portion 5 is changed during forming, and a second forming step in which the height of the vertical wall portion is formed to the target shape. The springback analysis was performed on the springback behavior of the press-formed product 1 after demolding at the bottom dead center of the second forming step, which was determined by the press forming analysis. The amount of change in angle between the vertical wall portion 5 and the flange portion 7 before and after demolding was determined as the amount of springback.

[0060] In this example, a press-formed product 1 formed by the press-forming method according to the present invention was used as the inventive example. Furthermore, for comparison, a case in which the press-formed product 1 is formed in one step was used as the conventional example, and a case in which the press-formed product 1 is formed in two steps, a first molding step and a second molding step, with the vertical wall height of the vertical wall portion 5 formed in the first molding step being outside the range of the present invention was used as the comparative example.

[0061] Tables 1 and 2 show the vertical wall height h1 of the vertical wall portion formed in the first forming process, the angle θ1 at the bottom dead center of forming and the angle θ2 after demolding between the vertical wall portion 5 and the flange portion 7, and the angle change amount θ1 - θ2, which were determined by press forming analysis and springback analysis with changed vertical wall heights. Here, Table 1 shows the case where the radius of curvature (camber convex R) of the curvature in the height direction of the press-formed product 1 is 1000 mm, and Table 2 shows the case where the camber convex R of the press-formed product 1 is 500 mm.

[0062]

[0063]

[0064] In Tables 1 and 2, Conventional Example 1 and Conventional Example 2 show that the vertical wall portion 5 was formed to the target vertical wall height h2 in one step using a conventional press forming analysis method.

[0065] Comparative Examples 1 and 11 are modified in which the vertical wall height h1 of the vertical wall portion 5 formed in the first molding step is equal to the vertical wall height h2 of the target shape. The angle change amount θ1-θ2 after the second molding step was similar to or increased compared to Conventional Examples 1 and 2.

[0066] Comparative Examples 2 to 4 and Comparative Examples 12 to 14 are modified in which the vertical wall height h1 of the vertical wall portion 5 formed in the first molding step is smaller than the vertical wall height h2 (= 30 mm) of the target shape (h1 < h2). The angle change amount θ1 - θ2 after the second molding step is larger than that of Conventional Example 1 or Conventional Example 2, resulting in increased springback.

[0067] Examples 1 to 3 and Examples 11 to 13 are characterized in that the vertical wall height h1 of the vertical wall portion 5 formed in the first molding step is made larger than the vertical wall height h2 (=30mm) of the target shape by adding a value of 1 / 2 or less of the radius of curvature (=6.2mm) of the flange-side ridge portion 11 in the longitudinal vertical cross section of the target shape. The angle change amount θ1-θ2 after the second molding step is smaller than in the conventional example, resulting in suppressed springback.

[0068] Furthermore, in Comparative Examples 5 to 6 and Comparative Examples 15 to 16, the vertical wall height h1 of the vertical wall portion 5 formed in the first molding step was increased to a value exceeding 1 / 2 of the radius of curvature of the flange-side ridge portion 11 in the longitudinally perpendicular cross-section of the target shape, making it larger than the vertical wall height h2 of the target shape. The angle change amount θ1-θ2 after the second molding step was larger than that of Conventional Example 1 or Conventional Example 2, resulting in increased springback.

[0069] From the above results, it was shown that by forming a press-molded product 1 that curves convexly in the height direction in two processes, a first molding process and a second molding process, and by making the vertical wall height h1 of the vertical wall portion 5 in the first molding process greater than the vertical wall height h2 of the target shape by adding a value of 1 / 2 or less of the radius of curvature of the flange-side ridge portion 11 in the longitudinal vertical cross section of the target shape, the angular change between the vertical wall portion 5 and the flange portion 7 due to springback can be reduced.

[0070] Furthermore, in this embodiment, we also considered the case in which a press-formed product that curves concavely in the height direction is formed by the press-forming method according to the present invention.

[0071] Similar to the aforementioned press-formed product 1 which is curved in a convex shape, press-formed product 21 shown in Figure 10 was used as the subject of analysis, and springback analysis was performed using the analysis results of the press-formed product analysis. Then, based on the springback analysis results, the springback in the flange portion 27 of the press-formed product 21 was evaluated.

[0072] In the press forming analysis, a steel plate with a tensile strength of 980 MPa and a thickness of 1.2 mm was used as the blank. Figures 9 and 10 show the target shape of the press-formed product 21 to be formed. The target shape of the press-formed product 21 is as follows: as shown in Figure 9, the radius of curvature of the concave curve in the height direction (camber concave R in Figure 9) is 1000 mm or 500 mm; as shown in Figure 10(b), the height of the vertical wall portion 25 is 30 mm, the angle between the top plate portion 23 and the vertical wall portion 25 is 95°, the angle between the vertical wall portion 25 and the flange portion 27 is 95°, the top plate portion 23 and the flange portion 27 are parallel (the flange portion 27 is horizontal), the radius of curvature of the top plate side ridge portion 29 in the longitudinal vertical cross section of the target shape (cross section A-A' in Figure 10(a)) is 5 mm, and the radius of curvature of the flange side ridge portion 31 in the longitudinal vertical cross section of the target shape (cross section A-A' in Figure 10(a)) is 6.2 mm.

[0073] The press forming analysis was performed on the process of forming the press-formed product 21 in two steps: a first forming step in which the vertical wall height h1 of the vertical wall portion 25 is changed during forming, and a second forming step in which the flange-side ridge portion 31 is reshaped to achieve the vertical wall height h2 of the target shape. The springback analysis was performed on the springback behavior of the press-formed product 21 after demolding, and the amount of change in angle between the vertical wall portion 25 and the flange portion 27 before and after demolding was determined as the amount of springback.

[0074] For the press-formed product 21 that curves in a concave shape, the example of the invention was one formed by the press-forming method according to the present invention. Furthermore, for comparison, the case in which the press-formed product 21 is formed in one step was given as a conventional example, and the case in which the press-formed product 21 is formed in two steps, a first molding step and a second molding step, with the vertical wall height h1 of the vertical wall portion 25 formed in the first molding step being outside the range of the present invention was given as a comparative example.

[0075] Tables 3 and 4 show the vertical wall height h1 of the vertical wall portion 25 formed in the first molding process, the angle θ1 at the bottom dead center of molding and the angle θ2 after demolding between the vertical wall portion 25 and the flange portion 27, and the angle change amount θ1-θ2, as determined by press molding analysis and springback analysis with changed vertical wall height h1. Here, Table 3 shows the case where the radius of curvature (camber recess R) of the curvature in the height direction of the press-formed product 21 is 1000 mm, and Table 4 shows the case where the camber recess R of the press-formed product 21 is 500 mm.

[0076]

[0077]

[0078] In Tables 3 and 4, Conventional Examples 3 and 4 were formed in one step using a conventional press forming analysis method to achieve the target shape with a vertical wall height h2.

[0079] Comparative Examples 21 and 31 are modified in which the vertical wall height h1 of the vertical wall portion 25 formed in the first molding step is equal to the vertical wall height h2 of the target shape. The angle change amount θ1-θ2 after the second molding step was similar to or increased compared to Conventional Examples 3 and 4.

[0080] Comparative Examples 22 to 24 and Comparative Examples 32 to 34 are modified in which the vertical wall height h1 of the vertical wall portion 25 formed in the first molding step is smaller than the vertical wall height h2 (= 30 mm) of the target shape (h1 < h2). The angle change amount θ1 - θ2 after the second molding step is larger than that of Conventional Example 3 or Conventional Example 4, resulting in increased springback.

[0081] Examples 21 to 23 and Examples 31 to 33 increase the vertical wall height h1 of the vertical wall portion 25 formed in the first molding step by adding a value less than or equal to half of the radius of curvature of the flange-side ridge portion 31 (= 6.2 mm) in the longitudinally perpendicular cross section of the target shape, making it larger than the vertical wall height h2 (= 30 mm) of the target shape. The angle change amount θ1 - θ2 after the second molding step is smaller than in Conventional Example 3 or Conventional Example 4, resulting in suppressed springback.

[0082] Furthermore, in Comparative Examples 25 to 26 and Comparative Examples 35 to 36, the vertical wall height h1 of the vertical wall portion 25 formed in the first molding step was increased to a value exceeding 1 / 2 of the radius of curvature of the flange-side ridge portion 31 in the longitudinally perpendicular cross-section of the target shape, making it larger than the vertical wall height h2 of the target shape. The angle change amount θ1-θ2 after the second molding step was larger than that of Conventional Example 3 or Conventional Example 4, resulting in increased springback.

[0083] From the above results, it was shown that by forming a press-molded product 21 that curves concavely in the height direction in two processes, a first molding process and a second molding process, and by making the vertical wall height h1 of the vertical wall portion 25 in the first molding process greater than the vertical wall height h2 of the target shape by adding a value of 1 / 2 or less of the radius of curvature of the flange-side ridge portion 31 in the longitudinal vertical cross section of the target shape, the angular change between the vertical wall portion 25 and the flange portion 27 due to springback after demolding can be reduced.

[0084] According to the present invention, it is possible to provide a press molding method that suppresses springback when forming a press-formed product having a top plate portion, a vertical wall portion, and a flange portion, wherein at least the flange portion is curved in a convex or concave shape in the height direction.

[0085] 1 Press-formed product (convex curve) 3 Top plate section 5 Vertical wall section 7 Flange section 9 Top plate side ridge section 11 Flange side ridge section 21 Press-formed product (concave curve) 23 Top plate section 25 Vertical wall section 27 Flange section 29 Top plate side ridge section 31 Flange side ridge section 41 Blank 51 Upper die 53 Lower die 55 Pad 61 Upper die 63 Lower die 65 Pad h1 Vertical wall height (bottom dead center of the first molding process) h2 Vertical wall height (target shape)

Claims

1. The compression molding method shapes the product into the target shape of the compressed product, consisting of: the top section, the side walls connected to the top section, and the flange connected to the side walls via a ridge. The flange is shaped to be at least convex or concave in the height direction. The assembly method includes: the first molding process, which shapes the top section to be identical in shape to the top section of the target shape of the compressed product, and the side walls and flange in such a way that the height of the side wall of the compressed product is greater than the height of the side wall of the target shape; and the second molding process, which reshapes the ridge between the side walls and the flange in such a way that the height of the side wall of the side wall formed in the first molding process becomes the height of the side wall of the target shape.The height of the side wall of the side wall section formed in the first forming process is set to be greater than the height of the side wall of the target shape by adding half or less the radius of curvature of the ridge in the vertical cross-section in the longitudinal direction of the target shape;

Images