Manufacturing method of press-molded product and die design method

Abstract

Even when high tensile steel or the like is used, the method reduces dimensional accuracy fluctuations caused by twisting, camber back, wall opening, and material strength fluctuations with a simple die structure. This is a manufacturing method for press-formed products in which a metal plate (3) is formed into a target part shape (1) having a cross section with a top plate portion (1A) and a vertical wall portion (1B) and a curved portion (2) that curves along the longitudinal direction. The angle between the top plate portion (1A) and the vertical wall portion (1B) is defined as the vertical wall angle. The method includes a first step (10A) of press-forming an intermediate part (4) using a die having a forming surface with a first vertical wall angle smaller than the vertical wall angle of the target part shape (1), and a second step (10B) of press-forming the intermediate part (4) using a die having a forming surface with a second vertical wall angle equal to or greater than the vertical wall angle of the target part shape (1). The vertical wall angle of the intermediate part (4) after demolding is set to a first vertical wall angle smaller than the vertical wall angle of the target part shape (1).

Description

[Technical field]

[0001] The present invention relates to a method for manufacturing a press-molded product and a method for designing a die used for manufacturing the same. The present invention relates to a technology for manufacturing a part having a target part shape, which has a top plate portion and a vertical wall portion and has a cross-sectional shape such as a U-shape, an L-shape, a hat shape, etc., by press working. The target part shape has one or more curved portions curved along the longitudinal direction, which is a direction intersecting the cross-section. In this specification, the target part shape in which the curved portion is curved so as to be concave or convex toward the vertical wall portion in top view is also referred to as a “left-right curved shape.” Also, the target part shape in which the curved portion is curved so as to be concave or convex toward the top plate portion is also referred to as a “top-bottom curved shape.”

[0002] In addition, when the target part shape is a "left-right curved shape" and has a cross-sectional shape with vertical wall portions on both the left and right sides of the top plate in the width direction, the target part shape is as follows. That is, in the curved portion, if one of the left and right vertical wall portions is concave in top view, the other vertical wall portion is convex in top view. However, the vertical wall angles of the left and right vertical wall portions do not need to be the same. Here, the angle between the top plate and the vertical wall in the cross-sectional shape is called the vertical wall angle. In the mold, the angle (shoulder angle) between the top plate molding surface that molds the top plate and the vertical wall molding surface that molds the vertical wall is the vertical wall angle. The vertical wall angle is an angle on the inside of the bend that is less than 180 degrees. [Background technology]

[0003] In recent years, in order to simultaneously improve the collision safety of automobile bodies and reduce their weight, the use of high tensile steel of 590 MPa or more has been increasing for auto body structural parts. Furthermore, in recent years, the use of ultra-high tensile steel of 980 MPa or more for auto body structural parts has been increasing. High tensile steel and ultra-high tensile steel have high yield strength and tensile strength. For this reason, when high tensile steel and ultra-high tensile steel are used as materials for auto body structural parts, forming defects such as springback become an issue during press forming.

[0004] One example of a press-formed product used in a vehicle body structural part is a part having the following shape. The part has a cross-sectional shape in which a top plate portion and a vertical wall portion are continuous in the cross-sectional direction, and a part of the longitudinal direction is curved in the width direction of the top plate portion when viewed from above. The inventor has found that when such a part is press-formed, compressive stress occurs on the outer side (convex side) of the curved vertical wall portion at the bottom dead point of forming, and tensile stress occurs on the inner side (concave side) of the curved vertical wall portion. The inventor has also found that these stresses cause springback including twisting in the part after release from the mold. In particular, when an ultra-high tensile steel is used as the material for the part, the stress at the bottom dead point of forming becomes large, and problems such as increased springback occur.

[0005] The inventors also found that even in a part whose target shape is a vertically curved shape, tensile stress occurs in the top plate and compressive stress occurs in the flange at the bottom dead center of molding, and these stresses cause springback, including camber back, in the part after it is released from the mold. Therefore, the inventors found the same thing in a part whose target shape is a vertically curved shape. And the same problem exists in a part whose target shape is a vertically curved shape. Furthermore, the strength of ultra-high tensile steels varies considerably when they are mass-produced. In other words, the strength of ultra-high tensile steels is prone to variation between coils (production lots). Therefore, when ultra-high tensile steels are used, there is also the issue that the amount of springback changes, making it particularly prone to variation in the dimensional accuracy of parts.

[0006] Conventional press molding methods include the techniques described in Patent Documents 1-4, for example. In the press forming method described in Patent Document 1, a blank sheet is bent at a bending line that curves along the longitudinal direction. During the bending, the blank sheet is overbent beyond the desired bending angle, and then bent back to the desired bending angle. Patent Document 1 describes that this reduces the torsional stress to zero and suppresses torsion.

[0007] The press forming method described in Patent Document 2 is a method for forming a part that is curved in the longitudinal direction. In this case, in Patent Document 2, the part that undergoes shrink flange deformation is formed so that the longitudinal line length is shorter than the line length of the target part shape. Also, in Patent Document 2, the part that undergoes stretch flange deformation is formed so that the longitudinal line length is longer than the line length of the target part shape. After this forming, in Patent Document 2, the part is formed into the target part shape. Patent Document 2 describes that this method reduces the stress that causes torsion and reduces torsion.

[0008] The press forming method described in Patent Document 3 targets a formed product having a top plate portion curved in the longitudinal direction and two vertical wall portions extending from both ends along the longitudinal direction of the top plate portion toward the inside of the curve. Patent Document 3 also discloses that the curvature of the top plate portion in the previous process and the angle between the top plate portion and the vertical wall portions are changed. Patent Document 3 states that this reduces the stress generated in the subsequent process and suppresses springback.

[0009] Patent Document 4 also covers a product shape in which the top plate and flange are continuous in the width direction via a vertical wall, and the top plate and flange have a hat-shaped cross section that is curved vertically in a side view along the longitudinal direction. Patent Document 4 also describes that the product is formed to have a smaller radius of curvature than the product shape in a pre-process, and is formed to have a larger radius of curvature than the product shape in a post-process. Patent Document 4 describes that this reduces the stress generated in the post-process and suppresses springback. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] JP 2004-195535 A [Patent Document 2] Patent No. 5664810 [Patent Document 3] JP 2011-206789 A [Patent Document 4] JP 2019-25533 A Summary of the Invention [Problem to be solved by the invention]

[0011] The press forming method described in Patent Document 1 aims to reduce the torsional stress after bending back to zero. However, when a significant change in material strength occurs in ultra-high tensile steel, the amount of torsion changes due to the change in material strength. This may result in the product not being within the dimensional tolerance. In addition, the method described in Patent Document 1 requires a very complicated die structure. This may result in the die being insufficiently rigid when forming ultra-high tensile steel. For this reason, when the method described in Patent Document 1 is adopted, the costs of repair and maintenance of equipment may be high. Furthermore, in the target part having curved portions on the left and right sides when viewed from above, there is a risk that twisting and changes in the angle of the vertical wall may occur simultaneously during press molding. However, Patent Document 1 does not take into consideration a method for achieving the target vertical wall angle of the product.

[0012] In addition, the press molding method described in Patent Document 2 cannot be applied to a part that does not have a flange portion connected to one end of a vertical wall portion. In addition, the method in Patent Document 2 is performed in one step. For this reason, the method in Patent Document 2 requires a complicated die structure and has poor productivity. In addition, in the press molding method described in Patent Document 3, the vertical wall portion is sagged in the post-process molding. This makes the behavior of the plate unstable during molding. For these reasons, it may be difficult to achieve stable dimensional accuracy in mass production with the method described in Patent Document 3. In addition, in the press molding method described in Patent Document 4, the radius of curvature of each of the top plate portion and the flange portion is changed in the upstream process. Therefore, it cannot be applied to a part with a U-shaped cross section without a flange portion. Even when targeting a part with a hat-shaped cross section that has a flange portion, the curved shape of the press molded part in the upstream process and the curved shape of the mold in the downstream process will be different. Therefore, there is a risk that the product molded in the upstream process cannot be set correctly in the mold in the downstream process.

[0013] The present invention has been made with attention to the above points. One of the objects of the present invention is to reduce twist, camber back, or change in vertical wall angle with a simple die structure even when high tensile steel or ultra-high tensile steel is used. Another object of the present invention is to significantly reduce fluctuations in dimensional accuracy when material strength fluctuations occur. Another object of the present invention is to provide a press forming method with excellent shape freezing ability and material strength sensitivity, which can obtain high-precision parts close to the target part shape (target part shape). [Means for solving the problem]

[0014] In order to solve the problem, one aspect of the present invention is a manufacturing method for a press-molded product in which a metal plate is molded into a target part shape having a cross section with a top plate portion and a vertical wall portion connected to a widthwise end of the top plate portion and extending in a direction different from the top plate portion, and having one or more curved portions curved along a longitudinal direction that is a direction intersecting the cross section, the method comprising: a first step of press-molding a metal plate into an intermediate part having the top plate portion and the vertical wall portion using a mold having a molding surface with a first vertical wall angle smaller than the vertical wall angle of the target part shape at a position that will become the curved portion, when the angle between the top plate portion and the vertical wall portion in the cross section is defined as a vertical wall angle; and a second step of press-molding the intermediate part using a mold having a molding surface with a second vertical wall angle equal to or greater than the vertical wall angle of the target part shape at a position that will become the curved portion, the first vertical wall angle being set so that the vertical wall angle of the intermediate part after release in the first step is smaller than the vertical wall angle of the target part shape at the position that will become the curved portion. Effect of the Invention

[0015] In one embodiment of the present invention, an intermediate part having a smaller vertical wall angle than the press-molded product of the target part shape is manufactured in a first step for at least the portion that will become the curved portion. Furthermore, in a second step, the intermediate part is press-molded with a die having a vertical wall angle equal to or larger than that of the press-molded product for at least the portion that will become the curved portion. As a result, according to this embodiment of the present invention, for example, when manufacturing a press-molded product whose target part shape is a "left-right curved shape," it is possible to reverse and reduce the stress generated in the vertical wall of the curved portion in the second process. Furthermore, according to the aspects of the present invention, even in the case of manufacturing a press-formed product whose target part shape is an "upper and lower curved shape," it is possible to reduce springback during press forming and improve the dimensional accuracy of the press-formed product. Furthermore, according to the aspects of the present invention, the angle of the vertical walls of the mold can be adjusted to accommodate this, so that the mold structure can be simplified.

[0016] As described above, according to the aspects of the present invention, even when a high tensile steel having a strength of 590 MPa or more or an ultra-high tensile steel having a strength of 980 MPa or more is used as a material, the following effects can be obtained: That is, in the manufacture of a press-formed product having a "laterally curved shape," it is possible to suitably reduce the change in twist or camber back and vertical wall angle with respect to the target part shape. Furthermore, according to the present invention, in the case of manufacturing both press-formed products having a "left-right curved shape" and press-formed products having an "up-down curved shape," even if the material strength changes, parts with high dimensional accuracy can be obtained. Therefore, according to the present invention, it is possible to improve the yield rate during mass production. [Brief description of the drawings]

[0017] [Figure 1] 1A is a diagram showing the shape of a press-formed product according to a first embodiment of the present invention, where (a) is a top view, (b) is a bird's-eye view, and (c) is a cross-sectional view taken along line XX in (a). [Diagram 2] FIG. 2 is a diagram showing a pressing process according to the first embodiment of the present invention. [Diagram 3] FIG. 2 is a schematic diagram illustrating pressing by a die in a first step in the first embodiment. [Figure 4] FIG. 2 is a schematic cross-sectional view illustrating a mold in a first step. [Diagram 5]1A is a schematic diagram illustrating pressing with a die in the second step in the first embodiment, and FIG. 1B is a diagram illustrating the relationship between the punch (lower die), pad, and intermediate part during pressing, and FIG. 1B is a diagram illustrating the relationship between the punch (lower die) and die (upper die). [Figure 6] 11 is a schematic cross-sectional view illustrating the relationship between a mold and an intermediate part in a second step. FIG. [Figure 7] FIG. 13 is a schematic diagram illustrating a comparative molding method. [Figure 8] 1A and 1B are diagrams for explaining the problems with the comparative molding method, in which (a) shows a produced press-molded product, and (b) shows its AA′ cross-sectional view. [Figure 9] 1A and 1B are diagrams showing the distribution of longitudinal stress at the bottom dead point of the press in a comparative forming method, where (a) is a view from the convex side of the vertical wall, and (b) is a view from the concave side of the vertical wall. [Figure 10] FIG. 13 is a diagram showing the stress-strain relationship due to press molding to explain the fluctuation in dimensional accuracy in a comparative molding method. [Figure 11] 1A and 1B are diagrams illustrating an example of stress distribution after the first process, where (a) is a view from the convex side of the vertical wall, and (b) is a view from the concave side of the vertical wall. [Figure 12] 13A and 13B are diagrams illustrating an example of stress distribution after the second process, where (a) is a view from the vertical wall convex side, and (b) is a view from the vertical wall concave side. [Figure 13] FIG. 2 is a diagram showing the stress-strain relationship due to press molding, for illustrating the fluctuation in dimensional accuracy, according to the first embodiment based on the present invention. [Figure 14] 1A and 1B are diagrams showing an example of strain distribution after the first process, where (a) is a view from the convex side of the vertical wall, and (b) is a view from the concave side of the vertical wall. [Figure 15] 13A and 13B are diagrams illustrating an example of strain distribution after the second process, where (a) is a view from the convex side of the vertical wall, and (b) is a view from the concave side of the vertical wall. [Figure 16] FIG. 13 is a diagram showing the relationship between the amount of twist and (θ1 / θ2). [Figure 17] FIG. 13 is a diagram showing the relationship between the amount of twist and (θ1 / θ0). [Figure 18] FIG. 13 is a diagram showing the relationship between the amount of twist and (θ1′ / θ2). [Figure 19] FIG. 13 is a diagram showing the relationship between the amount of twist and |ε2 / ε1|. [Figure 20] FIG. 13 is a diagram showing the relationship between the amount of twist and |ε2′ / ε1′|. [Figure 21] 1A is a perspective view of a press-formed product according to a second embodiment of the present invention, and FIG. [Figure 22] FIG. 11 is a schematic diagram illustrating pressing by a die in a first step in the second embodiment. [Diagram 23] 11A is a schematic diagram illustrating pressing with a die in the second step in the second embodiment, and FIG. 11B is a diagram illustrating the relationship between the punch (lower die), the pad, and the intermediate part during pressing, and FIG. 11B is a diagram illustrating the relationship between the punch (lower die) and the die (upper die). [Figure 24] FIG. 4 is a side view illustrating the occurrence of camber back. [Diagram 25] FIG. 13 is a diagram showing the distribution of longitudinal stress at the bottom dead point of the press in a comparative forming method. [Figure 26] FIG. 13 is a diagram showing the stress-strain relationship due to press molding to explain the fluctuation in dimensional accuracy in a comparative molding method. [Figure 27] 10A and 10B are diagrams showing cross-sectional shapes after a first step and a second step in a second embodiment, where (a) shows the cross-sectional shape after the first step and (b) shows the cross-sectional shape after the second step. [Figure 28] FIG. 11 is a diagram showing an example of stress distribution after a first process. [Figure 29] FIG. 11 is a diagram showing an example of stress distribution after a second process. [Diagram 30] FIG. 11 is a diagram showing the stress-strain relationship due to press molding, for illustrating the fluctuation in dimensional accuracy, according to the second embodiment based on the present invention. [Diagram 31] FIG. 11 is a diagram showing an example of strain distribution after a first process. [Diagram 32] FIG. 13 is a diagram showing an example of strain distribution after a second process. [Diagram 33] FIG. 13 is a diagram showing the relationship between the height deviation and εt2 / εt1. [Diagram 34] FIG. 13 is a diagram showing the relationship between height deviation and εb2 / εb1. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] Hereinafter, examples of a press molding method and the like according to embodiments of the present invention will be described with reference to the drawings. "First embodiment" (Target part shape) In this embodiment, the target part shape is a "right-left curved shape." In this embodiment, the target part shape 1 of the part (press molded product) to be manufactured has a cross section having a top plate portion 1A and vertical wall portions 1B on both left and right sides, as shown in FIG. 1. The vertical wall portions 1B on both left and right sides are connected to both ends of the top plate portion 1A in the width direction, respectively, and extend in a direction different from that of the top plate portion 1A. That is, in this embodiment, the cross section is illustrated as a U-shaped cross section. However, the present invention is also applicable to the cross section of the target part shape 1 having a hat-shaped cross section or an L-shaped cross section. The hat-shaped cross section has a flange portion. The L-shaped cross section is a shape in which the vertical wall portion 1B exists only on one side of the width direction of the top plate portion 1A.

[0019] The present invention is also applicable to a target part shape that has both a "right-left curved shape" and a "top-bottom curved shape." In addition, the dimensions (unit: mm) used in the embodiment are also shown in Fig. 1. However, the present invention is not limited to these dimensions in any way. The same applies to the other drawings. Furthermore, as shown in Fig. 1(a) and Fig. 1(b), the target part shape 1 of this embodiment is exemplified as a shape having a curved portion 2 curved toward one vertical wall portion 1B in a top view along the longitudinal direction. The longitudinal direction is a direction intersecting the above-mentioned cross section. The target part shape 1 exemplified in this embodiment is assumed to be composed of two straight portions with a flat top plate portion 1A and a curved portion 2 interposed between the two straight portions in a top view along the longitudinal direction. Here, the symbol 2A indicates the top plate portion of the curved portion 2, and the symbol 2B indicates the vertical wall portion (concave side) of the curved portion 2. In the present invention, a target part shape 1 having a plurality of curved portions 2 along the longitudinal direction can also be manufactured. In this case, it is not necessary that a straight portion exists between adjacent curved portions 2.

[0020] As shown in FIG. 1, when the tabletop 1A has vertical wall portions 1B on both the left and right sides, the tabletop 1A has the following configuration. That is, the two vertical wall portions 1B on the left and right sides of one curved portion 2 are convex toward one vertical wall portion 1B side and concave toward the other vertical wall portion 1B side when viewed from above. In this specification, the convex vertical wall portion 1B side is also referred to as the outer side of the curved portion 2 when viewed from above or the vertical wall convex portion. The concave vertical wall portion 1B side is also referred to as the inner side of the curved portion 2 when viewed from above or the vertical wall concave portion. However, the vertical wall angle on one vertical wall portion 1B side and the vertical wall angle on the other vertical wall portion 1B side may be the same angle or may be different from each other.

[0021] (Manufacturing method) The method for manufacturing a press-formed product according to this embodiment is a method for manufacturing a metal plate 3 (blank material) into the above-mentioned target part shape 1 by press forming. The present invention is suitable for the case where the material of the metal plate 3 is a high tensile material or an ultra-high tensile material. A high tensile material is a steel plate having a tensile strength of 590 MPa or more. The present invention is also applicable to metal plates 3 such as mild steel plates and aluminum plates. One of the objectives of this embodiment is to reduce the variation in dimensional accuracy of the part due to springback caused by twisting and changes in vertical wall angle, and variation in material strength (differences in strength) when manufacturing a press-molded part of the target part shape 1. As shown in FIG. 2, the method for manufacturing a press-molded product of this embodiment includes a first step 10A and a second step 10B. As described above, the angle between the top plate 1A and the vertical wall 1B in cross section is called the vertical wall angle. On the molding surface of the mold, the angle (shoulder angle) between the top plate molding surface that molds the top plate 1A and the vertical wall molding surface that molds the vertical wall 1B is the vertical wall angle.

[0022] (First step 10A) In the first step 10A, an upper die 11 and a lower die 12 are used as shown in the schematic diagram of Fig. 4. The molding surfaces of the upper die 11 and the lower die 12 have a first vertical wall angle θ1 that is smaller than the vertical wall angle θ0 of the target part shape 1. The first step 10A is a step of press-molding, using the dies 11 and 12, into an intermediate part 4 having a top plate portion and vertical walls whose width matches the target part shape 1. The intermediate part 4 may have a portion that is cut in a subsequent trimming step.

[0023] The mold used in the first step 10A includes a die 11 (upper mold) and a punch 12 (lower mold), as shown in Figures 3 and 4. The forming surfaces 11A, 12A of the die 11 and punch 12 used in the first step 10A have forming surfaces whose vertical wall angle is different from that of the target part shape 1. The mold used in the first step 10A also includes a pad 13. The pad 13 is a component of the mold that holds the portion of the metal sheet 3 that will become the top plate portion 1A against the top plate forming surface 12Aa of the punch 12. Reference symbols 12Aa and 11Aa indicate top plate forming surfaces, and reference symbols 12Ab and 11Ab indicate vertical wall forming surfaces.

[0024] That is, in the first step 10A, pressing is performed with the die 11 and punch 12 while the metal sheet 3 is pressed by the top plate forming surface 12Aa of the punch 12 and the pad 13, as shown in Figs. However, as described above, the first vertical wall angle θ1 on the molding surfaces 11A, 12A of the molds 11, 12 used in the first step 10A is set to an angle smaller than the vertical wall angle θ0 in the target part shape 1.

[0025] Moreover, the first vertical wall angle θ1 is set according to the vertical wall angle θ1' of the intermediate part 4. The vertical wall angle θ1' is the vertical wall angle of the intermediate part 4 after it is produced by press-molding the metal plate 3 in the first process 10A and released from the mold. In other words, the first vertical wall angle θ1 is set so as to be smaller than the vertical wall angle θ0 in the target part shape 1. Then, the intermediate part 4 has a shape after it is released from the mold, that is, the shape of the intermediate part 4 after springback. This first vertical wall angle θ1 may be set, for example, by evaluation using CAE analysis or an actual press. The first vertical wall angle θ1 is set as described above for each of the vertical wall angles on the left and right vertical wall portions 1B.

[0026] Only one of the vertical wall angles on the left and right vertical wall sides, that of the vertical wall portion 1B, may be set to the first vertical wall angle θ1 as described above. In this case, the vertical wall angle on the other vertical wall portion 1B may be set to an angle equal to the vertical wall angle θ0 in the target part shape 1. Setting the first vertical wall angle on only one vertical wall portion 1B based on the present disclosure is effective. However, it is preferable to set the first vertical wall angle θ1 on both the left and right vertical wall portions 1B as described above.

[0027] (Second step 10B) In the second step 10B, the intermediate part 4 produced in the first step 10A is press-molded to produce a press-molded product having the target part shape 1. The molding surface of the mold used in the second step 10B has a second vertical wall angle θ2 that is equal to or greater than the vertical wall angle θ0 of the target part shape 1. The molding surfaces of the molds 14, 15 used in the second step 10B have molding surfaces 14A, 15A that conform to the target part shape 1 except for the vertical wall angle, as shown in Fig. 5 and the schematic diagram of Fig. 6. The molds 14, 15 are die 14, which is an upper mold, and punch 15, which is a lower mold.

[0028] It is preferable that the second vertical wall angle θ2 is larger than the vertical wall angle θ0 of the target part shape 1. That is, in the second process 10B, the vertical wall portion 1B is bent back. For this reason, it is preferable to design the molding surface shape of the die for the second process 10B so that the second vertical wall angle θ2 is larger than the vertical wall angle θ0 of the target part shape 1. Moreover, the mold used in the second step 10B includes a pad 16 for pressing the top plate portion of the intermediate part 4.

[0029] Here, the second vertical wall angle θ2 is set to an angle equal to or larger than the vertical wall angle θ0. Therefore, the second vertical wall angle θ2 is larger than the vertical wall angle θ1' of the intermediate part 4. Therefore, a problem occurs when the following molding method is adopted for molding in the second step 10B. That is, consider a case where the pad 16 is extended from the die 14 side and the top plate portion of the intermediate part 4 is sandwiched between the pad 16 and the punch 15 to mold the intermediate part 4. In this case, there is a problem that when the intermediate part 4 is set on the punch 16, the intermediate part 4 is likely to be formed in an unstable state without being placed at a predetermined position on the punch 16. For this reason, in the second step 10B of this embodiment, before forming, the pad 16 is extended from the top plate forming surface of the punch 15, and the intermediate part 4 is set on the pad 16. Thereafter, the top plate portion of the intermediate part 4 is pressed against the top plate forming surface of the die 14 by the pad 16 (see FIG. 6(a)). In this state, the intermediate part 4 is press-formed by the pad 16 and the die 14. The above-mentioned operations constitute the second step 10B of this embodiment.

[0030] (Regarding the relationship between each vertical wall angle) Here, the vertical wall angle in the target part shape 1 is described as θ0. The first vertical wall angle at the molding surfaces 11A, 12A of the dies 11, 12 in the first step 10A is described as θ1. The vertical wall angle in the intermediate part 4 is described as θ1'. After the intermediate part 4 is molded in the first step 10A, the angle after it is released from the mold and springs back becomes the first vertical wall angle θ1'. The second vertical wall angle at the molding surfaces 14A, 15A of the dies 14, 15 in the second step 10B is described as θ2. In this embodiment, the vertical wall portions 1B exist on the left and right, but it is preferable to satisfy the following conditions individually for each of the left and right. It is preferable to set the first vertical wall angle θ1 and the second vertical wall angle θ2 so as to satisfy the following formula (1). In other words, it is preferable to set the second vertical wall angle θ2 of the mold in the second step 10B to be larger than the vertical wall angle θ0 in the target part shape 1. θ1 < θ1′ < θ0 < θ2 (1) The second vertical wall angle θ2 is preferably less than 1.05 times the vertical wall angle θ0.

[0031] Next, preferred conditions for setting the first vertical wall angle θ1 and the second vertical wall angle θ2 will be described. Figure 16 shows the results of investigating the relationship between (θ1 / θ2) and the amount of twist after press forming for the press-formed product shown in Figure 1. Here, "◯" indicates the result when a steel plate with a tensile strength of 980 MPa class was used, and "△" indicates the result when a steel plate with a tensile strength of 1180 MPa class was used. The amount of twist on the vertical axis in Fig. 16 is the difference in angle between target part shape 1 and the shape after mold release, as shown in Fig. 8. Specifically, the angle is adjusted so that the inclination of the top plate part of target part shape 1 matches the inclination of the top plate part of the shape after mold release at the longitudinal center position. The inclination angle of the top plate part at the AA' cross section of the longitudinal end shown in Fig. 8(a) is taken as the amount of twist.

[0032] In this embodiment, the target range of the amount of twist to obtain a good shape is set to within ±2°. In this case, it can be seen from FIG. 16 that it is preferable to set the relationship between the first vertical wall angle θ1 and the second vertical wall angle θ2 so as to satisfy the following formula (2). 0.7 ≦ (θ1 / θ2) (2) Here, if (θ1 / θ2) is less than 0.7, there is a risk of excessive unbending in the second process (second process 10B). Excessive unbending may increase the amount of springback (opening and twisting) and the fluctuation in dimensional accuracy due to the fluctuation in material strength. To obtain a better effect, it is preferable that (θ1 / θ2) is 0.75 or more and less than 0.95.

[0033] Next, preferred conditions for setting the first vertical wall angle θ1 and the vertical wall angle θ0 in the target part shape 1 will be described. FIG. 17 shows the results of investigating the relationship between (θ1 / θ0) and the amount of twist after press molding for the press-molded product shown in FIG. In this embodiment, the target range of the amount of twist to obtain a good shape is set to within ±2°. In this case, it can be seen from Fig. 17 that it is preferable to set the first vertical wall angle θ1 to a value that satisfies the following formula (3). 0.75 ≦ (θ1 / θ0) ····(3) If (θ1 / θ0) is less than 0.75, θ1 becomes excessively small relative to θ0, and there is a risk of excessive unbending in the second process (second process 10B). Excessive unbending can increase the amount of springback (opening and twisting) and the fluctuation in dimensional accuracy due to the fluctuation in material strength. In the present invention, since θ1<θ0, θ1 / θ0 is less than 1. Moreover, in order to obtain a greater effect, (θ1 / θ0) is preferably equal to or greater than 0.8 and less than 0.97.

[0034] Next, preferred conditions for setting the vertical wall angle θ1' and the second vertical wall angle θ2 when the mold is released and springs back in the first step will be described. FIG. 18 shows the results of investigating the relationship between (θ1′ / θ2) and the amount of twist after press molding for the press-molded product shown in FIG. In this embodiment, the target range of the amount of twist to obtain a good shape is set to within ±2°. In this case, it can be seen from Fig. 18 that it is preferable to set the second vertical wall angle θ2 to a value that satisfies the following formula (4). 0.8 ≦ (θ1′ / θ2) ····(4) If (θ1' / θ2) is less than 0.8, θ1' becomes excessively small relative to θ2. This may result in excessive unbending in the second process (second process 10B). If unbending too much, the amount of springback (opening and twisting) and dimensional accuracy fluctuations due to material strength fluctuations may increase. In the present invention, since θ1'<θ2, θ1' / θ2 is less than 1. To obtain a greater effect, (θ1' / θ2) is preferably equal to or greater than 0.85 and less than 0.99.

[0035] Next, it is preferable that the second vertical wall angle θ2 is set to a value that satisfies the following formula (5). 1.0 < (θ2 / θ0) < 1.05 (5) If (θ2 / θ0) exceeds 1.05, there is a high possibility that the vertical wall angle θ0 in the target part shape 1 cannot be achieved after springback in the second step (second step 10B). Here, the above appropriate ranges (values) may vary depending on the part shape and material strength, so it is preferable to consider the conditions for each vertical wall angle for each part. In addition, the ranges of the vertical wall angles θ1 and θ2 may be set so as to satisfy one or more conditions selected from the above formulas (1) to (5). These formulas are set mainly based on the examples and other studies carried out by the inventors.

[0036] (Operation etc.) <Comparative molding methods> Here, as a comparative forming method, a method as shown in Fig. 7 is considered. That is, the entire surface of the metal sheet 3 that will become the top plate portion 1A is sandwiched between a punch 20 and a pad 22. In this state, the metal sheet 3 is press-formed with the punch 20 (lower die) and a die 21 (upper die). In the comparative forming method, the forming surfaces (die shape) of the punch 20 and the die 21 are shaped to imitate the target part shape 1. This comparative forming method is a press forming method that applies normal padded foam forming. At this time, the target part shape 1 and the shape after release from the die are aligned so that the inclination of the top plate at the longitudinal center cross section matches.

[0037] FIG. 8(b) shows the cross-sectional shape 6 of the part after pressing by the comparative molding method. Reference symbol 6C indicates a curved portion. This cross-sectional shape (see FIG. 8(b)) is the shape of the AA′ cross section at the longitudinal end in the top view shown in FIG. 8(a). In the comparative molding method, as shown in FIG. 8(b), a tilt occurred in the top plate portion 6A before and after springback (before and after demolding). In other words, it can be seen that when the comparative molding method is used, the part is twisted due to springback. It can also be seen that the vertical wall angle in the cross section of the molded part becomes larger than the vertical wall angle of the die due to springback.

[0038] FIG. 9 shows the longitudinal stress distribution at the center of the plate thickness at the bottom dead point in the comparative forming method. As can be seen from FIG. 9(a), compressive stress occurs over a wide range on the vertical wall portion 1B on the convex side when viewed from above. Also, as can be seen from FIG. 9(b), tensile stress occurs over a wide range on the vertical wall portion 1B on the concave side when viewed from above. From this state, it is considered that the twist of the part cross section after demolding occurs due to the release of these stresses when the mold is released. As the material strength increases, this residual stress increases and the amount of springback increases. Furthermore, as the material strength increases, the material strength fluctuation during mass production of the material also becomes relatively large. Therefore, as the material strength increases, the material strength fluctuates relatively greatly between the coils used as the material. As a result, the problem of fluctuation in dimensional accuracy occurs due to the change in the amount of springback.

[0039] This dimensional accuracy variation will be explained using a schematic diagram of the stress-strain relationship. Figure 10 shows the relationship between stress and strain on the vertical wall on the concave side as viewed from above during forming using a comparative forming method. In Figure 10, a schematic diagram is shown for two metal plates 3, one with higher strength and one with lower strength, assuming variation in material strength. With the comparative forming method, when the target part shape 1 is formed using a press, the material on the concave side as viewed from above is stretched at the curved portion 2. This generates tensile stress. Furthermore, as shown in Figure 10, there is a difference in the stress value at the bottom dead center of forming between high-strength material and low-strength material. When the dies 20, 21 are released from this state, springback occurs, resulting in twisting of the pressed part. In addition, the amount of springback differs between high-strength material and low-strength material, causing fluctuations in dimensional accuracy. The higher the material strength, the greater the fluctuations in dimensional accuracy, resulting in deviations from the dimensional tolerances of the product. This is cited as an important issue during press forming. In addition, the change in vertical wall angle due to springback is mainly caused by the stress difference between the front and back of the punch shoulder. The amount of springback increases with increasing material strength. Furthermore, the change in vertical wall angle due to springback also causes the problem of dimensional accuracy fluctuation.

[0040] <Molding method according to this embodiment> On the other hand, in this embodiment, as described above, the mold shape for the first process 10A and the mold shape for the second process 10B are designed. This reverses the stress that causes the twist in the second process 10B. As a result, in this embodiment, both the amount of twist and the variation in dimensional accuracy of the twist are reduced. In addition, the stress that causes the change in the vertical wall angle is suppressed by the following phenomenon. That is, by bending back in the second process 10B, the stress on the punch shoulder is reversed, or a stress in the opposite direction is applied around the punch shoulder. As a result, it is possible to reduce both the change in the vertical wall angle and the fluctuation in the dimensional accuracy. Here, for example, draw molding or foam molding may be applied in the first process 10A, and molding into the target part shape 1 may be applied in the second process 10B.

[0041] In this embodiment, as described above, in the second step 10B, pressing is performed with the pad 16 pressing the intermediate part 4 against the die 14. This aims to stabilize the pressing in the second step 10B. That is, as shown in Figs. 5 and 6, in the second step 10B, the pad 16 is extended from the top plate forming surface of the punch 15 before forming, and the intermediate part 4 is set on the pad 16. Thereafter, forming is performed with the top plate portion of the intermediate part 4 sandwiched between the die 14 and the pad 16. This enables stable forming.

[0042] Here, consider the case where, in the second step 10B, as in normal molding (comparative molding method), a pad 16 is extended from the die 14 side, and the intermediate part 4 is sandwiched between the pad 16 and punch 15 for molding. In this case, when the intermediate part 4 is set on the lower die (pad 16), the vertical wall angle θ1' of the intermediate part 4 is smaller than the vertical wall angle θ2 of the die. For this reason, the intermediate part 4 is supported and molded in an unstable state where it only partially contacts the punch 15. As a result, the part becomes misaligned, and the target shape cannot be obtained.

[0043] 11 and 12 show an example of the stress distribution at the center of the plate thickness in the longitudinal direction at the bottom dead center when a press product is formed by the method of this embodiment. The conditions are the same as those of the comparative forming method, except for forming in two steps and adjusting the vertical wall angle. As shown in Fig. 11, at the bottom dead point of forming in the first step 10A, compressive stress is generated over a wide range on the vertical wall portion 1B on the convex side as viewed from above, and tensile stress is generated over a wide range on the vertical wall portion 1B on the concave side as viewed from above.

[0044] On the other hand, as shown in Fig. 12, at the bottom dead center of forming in the second step 10B, tensile stress is generated in the vertical wall portion 1B on the convex side 2A when viewed from above. Also, compressive stress is generated in the vertical wall portion 1B on the concave side 2B when viewed from above. Thus, in this embodiment, it can be seen that the stress is reversed compared to the normal method (normal forming), and further the value of the stress and the area where the stress is generated are reduced. In this embodiment, the vertical wall portion 1B is formed so as to expand in the second step 10B. As a result, the vertical wall portion 1B on the convex side 2A as viewed from above is formed in a direction in which its line length extends, so that a tensile stress is applied to it. On the other hand, the vertical wall portion 1B on the concave side 2B as viewed from above is formed in a direction in which its line length contracts, so that a compressive stress is applied to it.

[0045] The fluctuation in dimensional accuracy due to the fluctuation in material strength when molding is performed by the method of this embodiment will be described with reference to a schematic diagram of the stress-strain relationship. 13 is a diagram showing the relationship between stress and strain on the concave vertical wall in top view in this embodiment, in which a schematic diagram of a high-strength material and a low-strength material is shown assuming variation in material strength. In this embodiment, when the intermediate part 4 is formed in the first step 10A with a smaller vertical wall angle than the target part shape 1, a larger strain and tensile stress are imparted than in the normal manufacturing method.

[0046] After that, when the material is molded to the vertical wall angle of the target part shape 1 in the second process 10B, the material on the concave side when viewed from above is molded in a shrinking direction. This causes the tensile stress to reverse to compressive stress. When the stress is reversed, a point appears where the history curves (respective stress-strain curves) of the high strength material and low strength material intersect in a low stress region. In Figure 13, this intersection point is the position of the bottom dead center in the second process. Aiming for this point, the vertical wall angle in the first process 10A is reduced and molding is performed. This reduces the stress and stress difference at the bottom dead center of the molding of the high strength material and low strength material. As a result, both the twist and the dimensional accuracy fluctuation of the twist are reduced. The position where the stress is zero is the state after demolding. The change in the vertical wall angle in the first step 10A can be applied to only the curved convex side or only the curved concave side when viewed from above. Therefore, it can be applied even if a flange portion is provided on one side or both sides of the curved portion 2.

[0047] In the second step 10B, the vertical wall portion of the intermediate part 4 formed in the first step 10A is formed so as to bend back. That is, a stress in the opposite direction to that in the first step 10A is applied. That is, the stress in the cross-sectional direction of the punch shoulder is reversed, or a stress in the opposite direction is applied around the punch shoulder. Therefore, the change in the vertical wall angle and the fluctuation in the dimensional accuracy thereof are reduced. At this time, consider a case where forming is performed under conditions that reduce the amount of twist and the fluctuation in the dimensional accuracy of the twist. In this case, the stress of the punch shoulder or the stress around the punch shoulder is excessively reversed. Therefore, in this case, springback occurs in a direction smaller than the vertical wall angle of the die in the second step 10B.

[0048] For this reason, the first vertical wall angle θ1 and the second vertical wall angle θ2 are set so that the vertical wall angle after springback in the second process 10B becomes the vertical wall angle θ0 in the target part shape 1. For example, it is preferable to design each vertical wall angle of the mold in the first process 10A and the second process 10B to satisfy the above formulas (1) to (5). The each vertical wall angle of the mold in the first process 10A and the second process 10B is the first vertical wall angle θ1 and the second vertical wall angle θ2.

[0049] 14 and 15 show the distribution of longitudinal strain at the center of the plate thickness at the bottom dead center when a part is formed by the forming method of this embodiment. As shown in Fig. 14, after the first step 10A, a large compressive strain occurs on the convex side of the vertical wall, and a large tensile strain occurs on the concave side of the vertical wall. On the other hand, after the second step 10B, as shown in Fig. 15, the compressive strain on the convex side is reduced, and the tensile strain on the concave side is also reduced. From the above, it is found that it is preferable to set the vertical wall angles θ1 and θ2 of the die in the first step 10A and the second step 10B so as to obtain the strain behavior shown in Fig. 13. It is also preferable to set the second vertical wall angle θ2 so that the stress applied at the bottom dead center of press in the second step 10B coincides or approximates within the strength range of the material used. Based on this consideration, it is preferable to set the vertical wall angles θ1 and θ2 as follows.

[0050] [Top view convex side 2A] Regarding the longitudinal strain distribution at the bottom dead point in the press forming in the second step 10B, the average value of the strain in the lower half of the vertical wall portion 1B that is convex in top view is defined as ε2. This definition is made at least at the position that will become the curved portion 2. In addition, regarding the longitudinal strain distribution at the bottom dead point in the press forming in the first step 10A, the average value of the strain in the lower half of the vertical wall portion 1B that is convex in top view is defined as ε1. This definition is made at least at the position that will become the curved portion 2.

[0051] FIG. 19 shows the results of investigating the relationship between |ε2 / ε1| and the amount of twist after press forming for the press-formed product shown in FIG. In this embodiment, the target range of the amount of twist that provides a good shape is set to within ±2°. In this case, it can be seen from Fig. 19 that |ε2 / ε1| is preferably in the range of 0.4 to 0.85. Therefore, it is preferable to set the first vertical wall angle θ1 and the second vertical wall angle θ2 so as to have values ​​that satisfy the following formula (6). 0.4×|ε1| ≦ |ε2| < 0.85×|ε1| ···(6) Here, ε2 is the above average value at the bottom dead point of pressing in the second step 10B, with the longitudinal strain before the first step 10A (before forming) set as a reference value of 0. In other words, ε2 is the sum of the longitudinal strain applied in the first step 10A and the longitudinal strain applied in the second step 10B.

[0052] If |ε2| is smaller than 0.4×|ε1|, the strain recovery amount shown in FIG. 13 may become too large. In this case, the difference in springback amount due to the difference in strength of the material may not be absorbed to a desired value or less. Therefore, |ε2| is preferably 0.4×|ε1| or more, and more preferably 0.6×|ε1| or more. On the other hand, if |ε2| is made larger than 0.85×|ε1|, the amount of strain recovery after the second step 10B may be insufficient. In this case, the difference in springback amount due to the difference in strength of the material may not be reduced to a desired value or less. Therefore, |ε2| is preferably less than 0.85×|ε1|, and more preferably less than 0.8×|ε1|.

[0053] [Top view concave side 2B] Regarding the longitudinal strain distribution at the bottom dead point in the press forming in the second step 10B, the average value of the strain in the lower half of the vertical wall portion 1B, which is concave when viewed from above, is defined as ε2′. This definition is made at least at the position that will become the curved portion 2. In addition, regarding the longitudinal strain distribution at the bottom dead point in the press forming in the first step 10A, the average value of the strain in the lower half of the vertical wall portion 1B, which is concave in top view, is defined as ε1′. This definition is made at least at the position that will become the curved portion 2.

[0054] Figure 20 shows the results of investigating the relationship between |ε2' / ε1'| and the amount of twist after press forming for the press-formed product shown in Figure 1. Note that the shape and dimensions of the press-formed product were assumed to be the parts with the dimensions shown in Figure 1. Note that the unit of dimensions is [mm]. In this embodiment, the target range of the amount of twist that provides a good shape is set to within ±2°. In this case, it can be seen from Fig. 20 that |ε2' / ε1'| is preferably in the range of 0.4 to 0.85. Therefore, it is preferable to set the first vertical wall angle θ1 and the second vertical wall angle θ2 so as to have values ​​that satisfy the following formula (7). 0.4×|ε1′| ≦ |ε2′| < 0.85×|ε1′| ···(7) Here, ε2' is the above average value at the bottom dead center of pressing in the second step 10B, with the longitudinal strain before the first step 10A (before forming) set as a reference value of 0. In other words, ε2' is the sum of the longitudinal strain applied in the first step 10A and the longitudinal strain applied in the second step 10B.

[0055] If |ε2'| is smaller than 0.4 × |ε1'|, the strain recovery amount shown in Figure 13 may become too large. In this case, there is a possibility that the difference in springback amount due to the difference in strength of the material cannot be absorbed to a desired value or less. Therefore, |ε2'| is preferably 0.4 × |ε1'| or more, and more preferably 0.45 × |ε1'| or more. On the other hand, if |ε2′| is made larger than 0.85×|ε1′|, the amount of strain recovery after the second step 10B may be insufficient. In this case, the difference in springback amount due to the difference in strength of the material may not be reduced to a desired value or less. Therefore, |ε2′| is preferably less than 0.85×|ε1′|, and more preferably less than 0.7×|ε1′|.

[0056] In the above description, the molds for the first step 10A and the second step 10B are set so that the first vertical wall angle θ1 and the second vertical wall angle θ2 satisfy the formula (1) over the entire length of the top plate portion 1A in the longitudinal direction. However, the present invention is not limited to this. For example, only the region that becomes the curved portion 2 when viewed from above is set so that the first vertical wall angle θ1 and the second vertical wall angle θ2 satisfy the formula (1) or the like. Then, the vertical wall angles of the dies in the first step 10A and the second step 10B in the straight portion other than the curved portion 2 may be set to the vertical wall angle of the target part shape 1, for example.

[0057] (About mold design) Consider the case where, when designing the mold shapes (shapes of the molding surfaces) for the first process 10A and the second process 10B, the design is started without any consideration of the vertical wall angle of the target part shape 1. Also consider the case where the design is started from the mold for the second process 10B. In these cases, it is possible that an appropriate mold shape cannot be obtained, or that it takes a very long time to obtain it. In contrast to this, in this embodiment, the vertical wall angles of the mold in the first process 10A and the second process 10B are designed as follows.

[0058] First, CAE analysis or actual pressing is performed with the vertical wall angle θ1 of the die in the first process 10A set to a value smaller than the vertical wall angle θ0 of the target part shape 1. From the evaluation of this execution, the conditions are found under which the vertical wall angle θ1' of the intermediate part 4 after springback in the first process 10A is smaller than the vertical wall angle θ0 of the target part shape 1. Then, in the second process 10B, CAE analysis or actual pressing is performed with a die shape having a vertical wall angle equal to or greater than the vertical wall angle θ0 of the target part shape 1. From the evaluation of the execution, the conditions are found under which the vertical wall angle after springback in the second process 10B becomes the vertical wall angle θ0 of the target part shape 1 and twisting is suppressed. According to such a die design, the die shape can be designed in a short time.

[0059] As described above, in the present embodiment based on the present disclosure, even when high tensile steel with a tensile strength of 590 PMa or more or ultra-high tensile steel with a tensile strength of 980 MPa or more is used, the simple die structure can significantly reduce the dimensional accuracy fluctuation when the material strength varies. Therefore, a highly accurate part close to the target part shape can be obtained.

[0060] "Second embodiment" Next, a second embodiment based on the present invention will be described. The basic configuration of this embodiment is similar to that of the first embodiment, and the same configurations as those of the first embodiment will be described with the same reference numerals.

[0061] (Target part shape) As in the first embodiment, this embodiment is an example applicable to cases where the cross section is U-shaped, hat-shaped with a flange, or L-shaped. Note that the L-shaped cross section is a shape in which the vertical wall portion 1B is present only on one side in the width direction of the top plate portion 1A. In addition, like the first embodiment, this embodiment targets a target part shape having one or more curved portions that curve along the longitudinal direction in a direction intersecting the longitudinal direction. However, the target part shape 1 in this embodiment is an example of a "vertically curved shape" in which a curved portion 2 is curved toward the top plate portion 1A side, as shown in Fig. 21. In the example shown in Fig. 21, the curved portion 2 is curved so as to be convex toward the top plate portion 1A. In addition, the dimensions used in the example are also shown in Fig. 21. However, the present invention is not limited to these dimensions. The dimensions are in mm.

[0062] 21 is an example in which the cross section is hat-shaped and one curved portion 2 is formed along the entire length in the longitudinal direction. However, the target part shape only needs to have a curved portion along the longitudinal direction. The target part shape may be, for example, a shape in which a flat portion (straight portion) of the top plate portion 1A is connected to at least one of the left and right sides of the curved portion along the longitudinal direction when viewed from the side. However, the present invention also targets a target part shape 1 having a plurality of curved portions 2 along the longitudinal direction. In this case, it is not necessary that there is a straight line portion between adjacent curved portions 2.

[0063] (Manufacturing method) The manufacturing method of the press-formed product of this embodiment is a method of manufacturing a metal plate 3 (blank material) by press forming into the above-mentioned target part shape 1. The present invention is suitable when the material of the metal plate 3 is a high tensile material or an ultra-high tensile material. High tensile material is a steel plate having a tensile strength of 590 MPa or more. The present invention is also applicable to metal plates 3 such as mild steel plates and aluminum plates. The method for producing a press-molded product of this embodiment includes a first step 10A and a second step 10B, similar to the first embodiment (see FIG. 2).

[0064] (First step 10A) In the first step 10A, as in the schematic diagram of Fig. 4, an upper die 11 and a lower die 12 are used which have a first vertical wall angle θ1 smaller than the vertical wall angle θ0 of the target part shape 1. The first step 10A is a step of press-forming a metal plate into an intermediate part 4 using the dies 11 and 12. The intermediate part 4 has a top plate portion and vertical wall portions whose width follows the target part shape 1, for example, by matching the width of the target part shape 1. The intermediate part 4 may have a portion to be cut in a subsequent trimming step.

[0065] The mold used in the first step 10A includes a die 11 (upper die) and a punch 12 (lower die) as shown in Fig. 22. The forming surfaces of the die 11 and punch 12 used in the first step 10A have forming surfaces whose vertical wall angle is different from the target part shape 1. The mold used in the first step 10A also includes a pad 13 on the top plate forming surface of the punch 12 for pressing down the portion of the metal plate 3 that will become the top plate portion 1A. That is, in the first step 10A, pressing is performed with the die 11 and punch 12 while the metal sheet 3 is pressed by the top forming surface of the punch 12 and the pad 13 as shown in FIG. However, as described above, the first vertical wall angle θ1 on the molding surfaces of the dies 11, 12 used in the first step 10A is set to an angle smaller than the vertical wall angle θ0 in the target part shape 1.

[0066] However, the first vertical wall angle θ1 is set according to the vertical wall angle θ1' of the intermediate part 4. The vertical wall angle θ1' is the vertical wall angle of the intermediate part 4 after it is produced by press-forming the metal plate 3 in the first process 10A and released from the mold. In other words, the first vertical wall angle θ1 is set so as to be smaller than the vertical wall angle θ0 in the target part shape 1. Then, the intermediate part 4 has a shape after it is released from the mold, that is, the shape of the intermediate part 4 after springback. This first vertical wall angle θ1 may be set, for example, by evaluation using CAE analysis or an actual press. The first vertical wall angle θ1 is set as described above for each of the vertical wall angles on the left and right vertical wall portions 1B.

[0067] Only one of the vertical wall angles on the left and right vertical wall sides, that of the vertical wall portion 1B, may be set to the first vertical wall angle θ1 as described above. In this case, the vertical wall angle on the other vertical wall portion 1B may be set to an angle equal to the vertical wall angle θ0 in the target part shape 1. Setting the first vertical wall angle on only one vertical wall portion 1B based on the present disclosure is effective. However, it is preferable to set the first vertical wall angle θ1 on both the left and right vertical wall portions 1B as described above.

[0068] (Second step 10B) In the second step 10B, the intermediate part 4 produced in the first step 10A is press-molded to produce a press-molded product having the target part shape 1. The molding surface of the mold used in the second step 10B has a second vertical wall angle θ2 that is equal to or greater than the vertical wall angle θ0 of the target part shape 1. The molding surfaces of the molds 14, 15 used in the second step 10B have molding surfaces 14A, 15A that conform to the target part shape 1 except for the vertical wall angle, as in Fig. 23(a) and the schematic diagram of Fig. 6. The molds 14, 15 are die 14, which is an upper mold, and punch 15, which is a lower mold.

[0069] It is preferable that the second vertical wall angle θ2 is larger than the vertical wall angle θ0 of the target part shape 1. That is, in the second process 10B, the vertical wall portion 1B is bent back. For this reason, it is preferable to design the molding surface shape of the die for the second process 10B so that the second vertical wall angle θ2 is larger than the vertical wall angle θ0 of the target part shape 1. Moreover, the mold used in the second step 10B includes a pad 16 for pressing the top plate portion of the intermediate part 4.

[0070] Here, the second vertical wall angle θ2 is set to an angle equal to or larger than the vertical wall angle θ0. Therefore, the second vertical wall angle θ2 is larger than the vertical wall angle θ1' of the intermediate part 4. Therefore, a problem occurs when the following molding method is adopted for molding in the second step 10B. That is, consider a case where the pad 16 is extended from the die 14 side and the top plate portion of the intermediate part 4 is sandwiched between the pad 16 and the punch 15 to mold the intermediate part 4. In this case, there is a problem that when the intermediate part 4 is set on the punch 16, the intermediate part 4 is likely to be formed in an unstable state without being placed at a predetermined position on the punch 16.

[0071] For this reason, in the second step 10B of this embodiment, before forming, the pad 16 is extended from the top plate forming surface of the punch 16, and the intermediate part 4 is set on the pad 16. Thereafter, the pad 16 presses the top plate portion of the intermediate part 4 against the top plate forming surface of the die 14 (see FIG. 6(a)). In this state, the pad 16 and the die 14 press-form the intermediate part 4. In this embodiment, the second step 10B is configured to perform the above-mentioned operations.

[0072] (Regarding the relationship between each vertical wall angle) Here, the vertical wall angle in the target part shape 1 is described as θ0. The first vertical wall angle on the molding surfaces of the dies 11 and 12 in the first process 10A is described as θ1. The vertical wall angle in the intermediate part 4 is described as θ1'. The vertical wall angle θ1' is the angle after release and springback. The second vertical wall angle on the molding surfaces of the dies 14 and 15 in the second process 10B is described as θ2. In this embodiment, the vertical wall portions 1B are present on the left and right, and it is preferable that the following conditions are satisfied for each of the left and right portions individually. It is preferable to set the first vertical wall angle θ1 and the second vertical wall angle θ2 so as to satisfy the following formula (1). In other words, it is preferable to set the second vertical wall angle θ2 of the mold in the second step 10B to be larger than the vertical wall angle θ0 in the target part shape 1. θ1 < θ1′ < θ0 < θ2 (1) The second vertical wall angle θ2 is preferably less than 1.05 times the vertical wall angle θ0.

[0073] Moreover, it is preferable to set the first vertical wall angle θ1 and the second vertical wall angle θ2 so as to have values ​​that satisfy the following formula (8). 0.4×εt1 ≦ εt2 < 0.9×εt1 ···(8) Here, for the longitudinal strain distribution at the bottom dead point in the press forming of the second step 10B, the absolute value of the average strain of the top plate portion at least at the position that will become the curved portion is defined as εt2. Also, for the longitudinal strain distribution at the bottom dead point in the press forming of the first step, the absolute value of the average strain of the top plate portion at least at the position that will become the curved portion is defined as εt1.

[0074] Furthermore, when the cross-sectional shape is a hat shape having a flange portion, it is preferable to set the first vertical wall angle θ1 and the second vertical wall angle θ2 so as to have values ​​that satisfy the following formula (9). 0.4×εb1 ≦ εb2 < 0.9×εb1 ···(9) Here, for the longitudinal strain distribution at the bottom dead point in the press forming of the second step, the absolute value of the average strain of the flange portion at least at the position that will become the curved portion is defined as εb2. Also, for the longitudinal strain distribution at the bottom dead point in the press forming of the first step, the absolute value of the average strain of the flange portion at least at the position that will become the curved portion is defined as εb1.

[0075] When the cross-sectional shape is a U-shape having no flange portions, the absolute values ​​of the average values ​​of the strain in the lower half of the vertical wall portions may be used as εb1 and εb2, instead of the absolute values ​​of the average values ​​of the strain in the flange portions. The reasons for the conditions of equations (8) and (9) will be described later. In addition, the above appropriate ranges (values) may vary depending on the part shape and material strength, so it is preferable to consider each condition for each part.

[0076] (Operation etc.) <Comparative molding methods> As a comparative forming method, a pad was used to press down the entire surface area of ​​the top plate portion, and padded foam forming was applied. Figure 24 shows the shapes before and after mold release (before and after springback). Here, the target part shape was the shape shown in Figure 21, and this is the case where the target part shape was formed in one press forming operation. That is, the entire surface area of ​​the metal sheet 3 that will become the top plate portion 1A was sandwiched between the punch and the pad. In this state, press forming was performed using the punch (lower die) and die (upper die). The forming surfaces of the punch and die (die shape) were shaped to imitate the target part shape 1. In this case, as can be seen from Fig. 24, the curvature in side view changes before and after springback. Specifically, it can be seen that the radius of curvature in side view increases along the longitudinal direction due to springback.

[0077] FIG. 25 shows the stress distribution in the longitudinal direction at the center of the plate thickness of part 6 at the bottom dead point when the comparative forming method is used. As shown in Figure 25, tensile stress occurs over a wide range in the top plate. In addition, compressive stress occurs over a wide range in the flange. The aforementioned springback in the longitudinal direction is thought to occur when these stresses are released when the die is released. The springback in the longitudinal direction is called camber back. And the higher the material strength, the greater this residual stress becomes, and the greater the amount of springback tends to be. Furthermore, as the material strength increases, the variation in material strength during mass production of the material also becomes relatively greater. For this reason, the material strength varies between the coils (between production lots of the plate) as mentioned above, and this also creates the problem of dimensional accuracy variation, where the amount of springback changes.

[0078] This variation in dimensional accuracy will be explained using a schematic diagram of the stress-strain relationship. Figure 26 shows the relationship between stress and strain in the flange section during normal forming, using a comparative forming method. Here, the stress in the flange section is longitudinal stress. Figure 26 also shows schematic diagrams of two types of material, high strength material and low strength material, assuming variations in material strength. In this example, when the material is formed into the product shape, the material in the flange section shrinks. This causes compressive stress to occur in the flange section, but as shown in Figure 26, there is a difference in the stress value at the bottom dead point of forming between high strength material and low strength material. In Figure 26, the position where the product shape will be formed is the bottom dead point of forming.

[0079] When the die is released from this bottom dead center state, camber back (spring back) occurs. This camber back causes a change in curvature in the longitudinal direction. In addition to this, the amount of camber back varies between high-strength materials and low-strength materials. In other words, variation in dimensional accuracy occurs between materials. As the material strength increases, this variation in dimensional accuracy becomes greater, resulting in deviation from the product's dimensional tolerance. For this reason, variation in dimensional accuracy is a major issue when using materials with high material strength.

[0080] In order to reduce this camber back and dimensional accuracy fluctuation, in this embodiment, the cross-sectional shape at the bottom dead center of forming shown in FIG. 27 is set. That is, in the press forming method of this embodiment, in the first step 10A, the vertical wall angle θ1 between the top plate portion and the vertical wall portion is formed to be smaller than the vertical wall angle of the target part shape (see FIG. 27(a)). In addition, in the second step 10B, the vertical wall angle θ2 is press formed to be larger than the vertical wall angle after springback in the first step 10A (see FIG. 27(b)). In this embodiment, as shown in FIG. 27, it is preferable to once form the line length L1 of the vertical wall portion to be equal to or larger than the line length of the vertical wall portion of the target part shape in the first step 10A. The line length of the vertical wall portion of the target part shape is equivalent to the line length L2. After that, in the second step 10B, it is preferable to form the line length L2 of the vertical wall portion to the line length of the vertical wall portion of the target part shape.

[0081] Here, consider the intermediate part 4 after being released from the die in the first step 10A. The vertical wall angle θ1′ of the intermediate part 4 is slightly larger than the vertical wall angle θ1 of the die in the first step due to springback. It is preferable to design the die in the first step 10A so that the vertical wall angle θ1′ after the springback in the first step is smaller than the vertical wall angle θ0 of the target part shape. In this embodiment, the metal plate 3 is basically made of high tensile material. However, the present invention does not need to be limited to high tensile material, and mild steel plate, aluminum plate, etc. may also be used. In addition, the vertical wall portion needs to be bent back in the second step 10B. For this reason, it is preferable to design the die for the second step 10B so that the vertical wall angle θ2 in the second step 10B is equal to or larger than the vertical wall angle θ0 of the target part shape.

[0082] In this embodiment, the target part shape 1 has a pair of left and right vertical wall portions 1B and flange portions 1C on both sides in the width direction of the top plate portion 1A, and has a hat-shaped cross section. The first step 10A and the second step 10B may be performed using a mold in which at least one of the left and right vertical wall portions has a first vertical wall angle θ1 and a second vertical wall angle θ2. In this embodiment, the die shape for the first step 10A and the die shape for the second step 10B are designed as described above. This reverses the stress that causes camber back in the second step 10B. As a result, the amount of camber back and the variation in dimensional accuracy of the camber back are reduced. In addition, the stress that causes the change in the vertical wall angle is also bent back in the second step 10B. This reverses the stress on the punch shoulder, or applies stress in the opposite direction to the periphery of the punch shoulder. As a result, in this embodiment, both the change in the vertical wall angle and the variation in dimensional accuracy are reduced.

[0083] Here, in the first step 10A, draw molding or form molding may be applied. In addition, in the second step 10B, molding into the target part shape may be applied. However, if in the second step 10B, as in normal molding, a pad is extended from the die side and the intermediate part is sandwiched between the pad and punch to mold, the following problem may occur. That is, when the intermediate part is set in the lower die, the vertical wall angle θ1' of the intermediate part 4 is smaller than the vertical wall angle θ2 of the die (product) in the second step. Therefore, the intermediate part is molded in an unstable state supported by the lower die (punch). This may cause misalignment, making it impossible to obtain the target shape. To avoid this, in this embodiment, as shown in Fig. 23, in the second step 10B, before forming, the intermediate part 4 is set on the pad 16 with the pad 16 extended from the top plate forming surface of the punch 15. Then, forming is performed with the top plate portion of the intermediate part 4 sandwiched between the die 14 and the pad 16. By adopting such a forming method, it is possible to prevent the intermediate part 4 from becoming unstable when supported by the punch 15 in the second step, and stable forming is possible.

[0084] 28 and 29 show the stress distribution at the center of the plate thickness in the longitudinal direction at the bottom dead point of forming when a press product is formed by the method of this embodiment. Here, θ1=95°, θ2=130° are set. FIG. 28 shows the stress distribution after the first process 10A, and FIG. 29 shows the stress distribution after the second process 10B. As shown in FIG. 28, at the bottom dead point of forming in the first process 10A, compressive stress occurs over a wide range in the flange portion. Also, tensile stress occurs over a wide range in the top plate portion. On the other hand, as shown in FIG. 29, at the bottom dead point of forming in the second process 10B, the flange portion is reversed to tensile stress, and the top plate portion is reduced in tensile stress. Therefore, by adopting the method of this embodiment, the stress is reversed or reduced compared to the normal method.

[0085] This is for the following reason. In the second process 10B, the flange portion is formed so as to be stretched in the longitudinal direction. As a result, the flange portion is formed in the direction in which the linear length extends, resulting in tensile stress. Influenced by this, the top plate portion is formed in the direction in which the linear length contracts, resulting in reduced tensile stress. In the case of a U-shaped cross section without a flange portion, the lower side of the vertical wall portion functions in the same way as the above-mentioned flange portion. The fluctuation in dimensional accuracy when molding is performed using the method of this embodiment will be described with reference to a schematic diagram of the stress-strain relationship. Figure 30 is a diagram showing the relationship between stress and strain in the flange portion using the method of this embodiment. Figure 30 shows a schematic diagram of two types of material, one with high strength and one with low strength, assuming fluctuations in material strength.

[0086] In this embodiment, in the first step 10A, the vertical wall angle θ1 is formed smaller than the vertical wall angle θ0 of the target part shape 1. This imparts a larger strain and stress than the normal method (comparative forming method). Then, in this embodiment, in the second step 10B, the vertical wall angle θ2 is formed larger than the vertical wall angle θ0 of the target part shape 1. This forms the material of the flange in the direction in which it is stretched. As a result, the compressive stress is reversed to a tensile stress. Here, as shown in FIG. 30, when the stress is reversed, the history curves of the high strength material and the low strength material of the material intersect in a low stress region. It is preferable to reduce the vertical wall angle in the first step 10A and form the material by setting the forming bottom dead center to the point where the two intersect. In this case, the stress and the stress difference at the forming bottom dead center of the high strength material and the low strength material of the material are particularly reduced, and the camber back and the dimensional accuracy fluctuation are further reduced. In addition, in the case where the curvature in the longitudinal direction of one curved portion is not constant, it is sufficient to reduce the vertical wall angle of only a part of the curved portion in the first step 10A. In the case of a part shape consisting of multiple curved portions, it is sufficient to reduce the vertical wall angle of at least one or more curved portions in the first step 10A.

[0087] In the second step 10B, the intermediate part 4 formed in the first step 10A is formed so that the vertical wall portion is bent back. As a result, the stress in the cross-sectional direction of the punch shoulder is reversed, or a stress in the opposite direction is applied around the punch shoulder. This reduces the change in the vertical wall angle and the fluctuation in dimensional accuracy thereof. However, when forming is performed under conditions that reduce the above-mentioned camber back amount and the fluctuation in dimensional accuracy of the camber back, the following phenomenon may occur. That is, the stress of the punch shoulder or the stress around the punch shoulder is reversed excessively. As a result, springback may occur in a direction smaller than the vertical wall angle of the die in the second step. Therefore, it is preferable to design the vertical wall angle after springback in the second step 10B to be the target part shape. In other words, it is preferable to design the vertical wall angle θ2 of the die in the second step 10B to be equal to or larger than the vertical wall angle θ0 of the product. In this embodiment, the strain in the top plate portion is described with t added, such as εt1 and εt2, and the strain in the flange portion is described with b added, such as εb1 and εb2.

[0088] 31 and 32 show the strain distribution at the center of the plate thickness in the longitudinal direction at the bottom dead point when a press product is formed by the manufacturing method of this embodiment. FIG. 31 shows the strain distribution in the first step. FIG. 32 shows the strain distribution in the second step. As shown in FIG. 31, in the first step 10A, a large compressive strain occurs particularly in the flange portion. On the other hand, as shown in FIG. 32, in the second step 10B, the compressive strain in the flange portion that occurred in the first step 10A is reduced. In this way, the method of this embodiment results in the behavior of strain as shown in Fig. 30. Here, for the longitudinal strain distribution occurring at the bottom dead point of pressing in the second process 10B, the absolute value of the average strain of the top plate portion is defined as εt2. Also, for the longitudinal strain distribution occurring at the bottom dead point of pressing in the first process 10A, the absolute value of the average strain of the top plate portion is defined as εt1.

[0089] Figure 33 shows the results of investigating the relationship between εt2 / εt1 and the height deviation from the product after press forming for the press-formed product shown in Figure 21. In Figure 33, "□" indicates the result when a steel plate with a tensile strength of 590 MPa class was used. "○" indicates the result when a steel plate with a tensile strength of 980 MPa class was used. "△" indicates the result when a steel plate with a tensile strength of 1180 MPa class was used. The height deviation from the product on the vertical axis in Fig. 33 is a value that represents the difference in height between the target part shape and the part shape after mold release, as shown in Fig. 24. The height deviation is calculated as follows. First, alignment is performed at the longitudinal center so that the position of the top plate part of the target part shape 1 and the position of the top plate part of the shape after mold release match. In this state, the difference in height between the longitudinal ends shown in Fig. 24 is taken as the height deviation from the product in this embodiment.

[0090] In this embodiment, the target range of the height deviation amount for obtaining a good shape is set to within ±2 mm. In this case, it can be seen from Fig. 33 that εt2 / εt1 is preferably in the range of 0.4 to 0.9. That is, it is preferable to set the values ​​of the vertical wall angles θ1, θ2 of the mold in the first step 10A and the second step 10B so as to satisfy the following formula (11). 0.4×εt1 ≦ εt2 < 0.9×εt1 ···(11)

[0091] Figure 34 shows the results of investigating the relationship between εb2 / εb1 and the height deviation from the product after press forming for the press-formed product shown in Figure 21. In Figure 34, "□" indicates the results when a steel plate with a tensile strength of 590 MPa class was used. "○" indicates the results when a steel plate with a tensile strength of 980 MPa class was used. "△" indicates the results when a steel plate with a tensile strength of 1180 MPa class was used. Here, the absolute value of the average strain in the flange part for the longitudinal strain distribution occurring at the bottom dead point of the press in the second process is defined as εb2, and the absolute value of the average strain in the flange part for the longitudinal strain distribution occurring at the bottom dead point of the press in the first process is defined as εb1.

[0092] In this embodiment, the target range of the height deviation amount for obtaining a good shape is set to within ±2 mm. In this case, it can be seen from Fig. 34 that εb2 / εb1 is preferably in the range of 0.4 to 0.9. That is, it is preferable to set the values ​​of the vertical wall angles of the mold in the first step and the second step so as to satisfy the following formula (12). 0.4×εb1 ≦ εb2 < 0.9×εb1 ···(12) Here, in this embodiment, the cross section of the target part shape is described as being hat-shaped. When the cross section of the target part shape is U-shaped, the phenomenon that occurs in the flange portion in the first step 10A and the second step 10B occurs in the lower part of the vertical wall portion. Therefore, when the press-molded product has a U-shaped cross section without a flange portion, the following setting may be used. That is, it is preferable to set the absolute value of the average value of the strain for the portion that is the lower half of the vertical wall portion to εb2 for the second step 10B and εb1 for the first step 10A. It is also preferable to set the values ​​of the vertical wall angles θ1 and θ2 of the mold in the first step 10A and the second step 10B so as to satisfy formula (11) or formula (12).

[0093] Here, if εt2 is smaller than 0.4×εt1, the amount of strain recovery in the second step 10B shown in FIG. 30 may be too large. In this case, the difference in springback amount due to the difference in strength of the material may not be reduced to a desired value or less. Therefore, εt2 is preferably 0.4×εt1 or more. It is more preferable that εt2 is 0.5×εt1 or more. On the other hand, if εt2 is greater than 0.9×εt1, the amount of strain recovery in the second step 10B may be insufficient. In this case, the difference in springback amount due to the difference in strength of the material may not be reduced to a desired value or less. Therefore, εt2 is preferably less than 0.9×εt1. It is more preferable that εt2 is equal to or less than 0.8×εt1.

[0094] Furthermore, if εb2 is smaller than 0.4×εb1, the amount of strain recovery in the second step 10B shown in FIG. 30 may be too large. In this case, the difference in springback amount due to the difference in strength of the material may not be reduced to a desired value or less. Therefore, εb2 is preferably 0.4×εb1 or more. It is more preferable that εb2 is 0.6×εb1 or more. On the other hand, if εb2 is greater than 0.9 × εb1, the amount of strain recovery in the second step 10B may be insufficient. In this case, the difference in springback amount due to the difference in strength of the material may not be reduced to a desired value or less. Therefore, εb2 is preferably less than 0.9 × εb1. It is more preferable that εb2 is equal to or less than 0.8 × εb1.

[0095] Furthermore, when designing the mold shapes for the first process 10A and the second process 10B, if the design is started without any consideration of the vertical wall angle θ0 of the product, or if the design is started from the mold for the second process 10B, the following may occur: That is, an appropriate mold shape may not be obtained, or it may take a very long time to obtain it. Therefore, in this embodiment, the die shapes for the first step 10A and the second step 10B may be designed as follows. First, CAE analysis or actual pressing is performed with the vertical wall angle θ1 of the die in the first step 10A set to a value smaller than the vertical wall angle θ0 of the product. Then, a condition is found under which the vertical wall angle θ1' after springback in the first step 10A is smaller than the vertical wall angle θ0 of the product. After that, in the second step 10B, CAE analysis or actual pressing is performed with a die shape having a forming surface with a vertical wall angle equal to or greater than the vertical wall angle θ0 of the target part shape. Then, a condition is found under which the vertical wall angle after springback in the second step 10B becomes the target part shape and dimensional accuracy fluctuations are suppressed. This design makes it possible to design the die shape in a short time.

[0096] For the above reasons, the method of the present embodiment can be suitably applied even when using high tensile steel with a tensile strength of 590 PMa or more, or ultra-high tensile steel with a tensile strength of 980 MPa or more. That is, according to the present disclosure, it is possible to significantly reduce the dimensional accuracy fluctuation caused by the occurrence of material strength fluctuation with a simple die structure. Therefore, according to the present disclosure, it is possible to obtain a highly accurate part close to the target part shape.

[0097] (others) The present disclosure may also have the following configuration. (1) Disclosure 1 is a manufacturing method of a press-molded product in which a metal plate is molded into a target part shape having a cross section including a top plate portion and a vertical wall portion connected to a width direction end portion of the top plate portion and extending in a direction different from the top plate portion, and having one or more curved portions curved along a longitudinal direction that is a direction intersecting the cross section, When the angle between the top plate portion and the vertical wall portion in the above cross section is defined as the vertical wall angle, a first step of press-molding a metal plate into an intermediate part having the top plate portion and the vertical wall portion, using a die having a molding surface with a first vertical wall angle smaller than the vertical wall angle of the target part shape, at a position that will become the curved portion; a second step of press-molding the intermediate part at a position that will become the curved portion using a mold having a molding surface with a second vertical wall angle that is equal to or greater than the vertical wall angle of the target part shape; The first vertical wall angle is set so that a vertical wall angle of the intermediate part after demolding in the first step is smaller than a vertical wall angle of the target part shape at a position that will become the curved portion. A manufacturing method for press-molded products. (2) In disclosure 2, the curved portion has a concave or convex curvature toward the vertical wall portion. (3) Disclosure 3 defines the vertical wall angle in the target part shape as θ0, the first vertical wall angle as θ1, the vertical wall angle in the intermediate part molded in the first step and released from the mold as θ1', and the second vertical wall angle as θ2, With respect to the first vertical wall angle θ1 and the second vertical wall angle θ2, press molding is performed using a mold having a molding surface that satisfies the following formula (1). θ1 < θ1′ < θ0 < θ2 (1) (4) Disclosure 4 defines the first vertical wall angle as θ1 and the second vertical wall angle as θ2, The first vertical wall angle θ1 and the second vertical wall angle θ2 are set so as to satisfy the following formula (2). 0.7 ≦ (θ1 / θ2) (2) (5) In Disclosure 5, when the vertical wall angle in the target part shape is defined as θ0 and the first vertical wall angle is defined as θ1, The first vertical wall angle θ1 is set so as to satisfy the following formula (3). 0.75 ≦ (θ1 / θ0) ····(3) (6) Disclosure 6 defines the second vertical wall angle as θ2 and the vertical wall angle of the intermediate part molded in the first step and released from the mold as θ1'. The second vertical wall angle θ2 is set so as to satisfy the following formula (4). 0.8 ≦ (θ1′ / θ2) ····(4) (7) In Disclosure 7, when the vertical wall angle in the target part shape is defined as θ0 and the second vertical wall angle is defined as θ2, The value of the second vertical wall angle θ2 is set so as to satisfy the following formula (5). (θ2 / θ0) < 1.05 (5) (8) Disclosure 8 is directed to a target part shape in which the curved portion is curved so as to be convex toward the vertical wall portion, Regarding the longitudinal strain distribution at the bottom dead point in the press forming of the second step, the average value of the strain in the lower half of the vertical wall portion that is convex in top view at the position that becomes the curved portion is defined as ε2; Regarding the longitudinal strain distribution at the bottom dead point of the press in the press forming of the first step, when the average value of the strain in the lower half of the vertical wall of the vertical wall portion that is convex in top view at the position that becomes the curved portion is defined as ε1, The first vertical wall angle and the second vertical wall angle are set so as to satisfy the following formula (6). 0.4×|ε1| ≦ |ε2| < 0.85×|ε1| ···(6) (9) Disclosure 9 is directed to a target part shape in which the curved portion is curved to be concave toward the vertical wall portion, Regarding the longitudinal strain distribution at the bottom dead point in the press forming of the second step, the average value of the strain in the lower half of the vertical wall portion that is concave in top view at the position that becomes the curved portion is defined as ε2'; Regarding the longitudinal strain distribution at the bottom dead point of the press in the press forming of the first step, when the average value of the strain in the lower half of the vertical wall portion that is concave in top view at the position that becomes the curved portion is defined as ε1', The first vertical wall angle and the second vertical wall angle are set so as to satisfy the following formula (7). 0.4×|ε1′| ≦ |ε2′| < 0.85×|ε1′| ···(7) (10) In disclosure 10, the curved portion has a concave or convex curvature toward the top plate portion. (11) Disclosure 11 defines the vertical wall angle in the target part shape as θ0, the first vertical wall angle as θ1, the vertical wall angle in the intermediate part molded in the first step and released from the mold as θ1', and the second vertical wall angle as θ2, With respect to the first vertical wall angle θ1 and the second vertical wall angle θ2, press molding is performed using a mold having a molding surface that satisfies the following formula (1). θ1 < θ1′ < θ0 < θ2 (1) (12) Disclosure 12 defines the absolute value of the average value of the strain of the top plate portion at the position that becomes the curved portion as εt2 with respect to the longitudinal strain distribution at the bottom dead point of the press in the press forming of the second step, Regarding the longitudinal strain distribution at the bottom dead point of the press in the press forming of the first step, when the absolute value of the average value of the top plate strain at the position of the curved portion is defined as εt1, The first vertical wall angle and the second vertical wall angle are set so as to satisfy the following formula (8). 0.4×εt1 ≦ εt2 < 0.9×εt1 ···(8) (13) Disclosure 13 is a cross section having a U-shape or a hat shape having a flange portion, Regarding the longitudinal strain distribution at the bottom dead point of the press in the press forming of the second step, in the position that becomes the curved portion, if the cross section is U-shaped, the absolute value of the average value of the strain in the lower half of the vertical wall portion is defined as εb2, and if the cross section is a hat shape having a flange portion, the absolute value of the average value of the strain in the flange portion is defined as εb2, Regarding the longitudinal strain distribution at the bottom dead point of the press in the press forming of the first step, in the position that becomes the curved portion, when the cross section is U-shaped, the absolute value of the average value of the strain in the lower half of the vertical wall portion is defined as εb1, and when the cross section is a hat shape having a flange portion, the absolute value of the average value of the strain in the flange portion is defined as εb1. The first vertical wall angle and the second vertical wall angle are set so as to satisfy the following formula (9). 0.4×εb1 ≦ εb2 < 0.9×εb1 ···(9) (14) Disclosure 14 discloses a method for manufacturing a mold used in the second step, the mold including a die constituting one of an upper mold and a lower mold, and a punch constituting the other of the upper mold and the lower mold; In the second step, press forming is performed in a state in which the top plate portion of the intermediate part is sandwiched between the top plate forming surface of the die and a pad extended from the top plate forming surface of the punch. (15) Disclosure 15 is a method for designing a die used in the method for producing a press-molded product according to any one of Disclosures 1 to 9, When designing the vertical wall angles of each die in the first and second steps, First, perform CAE analysis or actual pressing with the vertical wall angle of the die in the first process smaller than the vertical wall angle of the product, find the condition in which the vertical wall angle after springback in the first process is smaller than the vertical wall angle of the product, and set the above first vertical wall angle; Then, for the second process, a CAE analysis or an actual press is performed with a die shape having a molding surface with a vertical wall angle equal to or greater than the vertical wall angle of the target part shape, and a condition is found under which the part shape after springback in the second process satisfies the vertical wall angle and the torsion tolerance range of the target part shape, and the second vertical wall angle is set. How to design a mold. (16) Disclosure 16 is a method for designing a die used in the method for producing a press-molded product according to any one of Disclosures 10 to 13, When designing the vertical wall angles of each die in the first and second steps, First, perform CAE analysis or actual pressing with the vertical wall angle of the die in the first process smaller than the vertical wall angle of the product, find the condition in which the vertical wall angle after springback in the first process is smaller than the vertical wall angle of the product, and set the above first vertical wall angle; Then, for the second process, a CAE analysis or an actual press is performed with a die shape having a molding surface with a vertical wall angle equal to or greater than the vertical wall angle of the target part shape, and a condition is found under which the part shape after springback in the second process satisfies the allowable range of the vertical wall angle in the target part shape, and the second vertical wall angle is set. How to design a mold. EXAMPLES

[0098] The effect of reducing torsional springback by the press forming method according to the present invention was confirmed. To confirm this, press forming analysis and springback analysis were performed using the finite element method (FEM). The results are described below.

[0099] "Example of the First Embodiment" Example 1 In this embodiment, the target part shape 1 was a left-right curved shape curved in the width direction of the top plate part at the center along the longitudinal direction, as shown in Fig. 1. At this time, the metal plate 3 (blank material) used for press forming was two types of steel plate with tensile strengths of 980 MPa class and 1180 MPa class. This was to evaluate with two types of material strength so as to compare the dimensional accuracy fluctuation. The plate thickness t of both types of steel plate was 1.4 mm. The case where a blank material 3 (metal plate) is press-formed is the subject of the study. The first vertical wall angle θ1 of the die in the first process 10A is reduced. In this case, press forming analysis and springback analysis are performed, and the amount of twist of the part before and after springback is measured.

[0100] Here, the vertical wall angle θ2 of the mold in the second step 10B was selected and set from 105°, 110°, and 115°. Then, the vertical wall angle θ1 of the mold in the first step 10A was changed between 90° and 115°, and an FEM analysis for evaluation was performed. The amount of twist was found as follows. First, the target part shape 1 and the shape after mold release were aligned so that the inclination of the top plate in the longitudinal center cross section matched. Then, the amount of twist was evaluated based on the inclination angle of the top plate in the AA' cross section of the longitudinal end shown in Figure 8. Note that counterclockwise twist is indicated as a plus, and clockwise twist is indicated as a minus. In this example, a torsion amount of within ±2.0° was deemed acceptable, and a difference in the torsion amount between the 980 MPa material and the 1180 MPa material of within ±1.0° was deemed acceptable.

[0101] (First example) In the first example, the vertical wall angle θ2 of the mold in the second step 10B was fixed to 115°. The evaluation in Table 1 shows the results when the vertical wall angle θ1 of the mold in the first step 10A was selected and set from 90°, 95°, and 115°. In the first example, the vertical wall angle θ0 of the target part shape 1 was set to 110°. The evaluation results of the first example are shown in Table 1.

[0102] [Table 1]

[0103] <No.1-5、No.1-6> No. 1-5 and No. 1-6 in Table 1 are evaluations of the amount of twist for the 980 MPa material and the 1180 MPa material when the vertical wall angle θ1 in the first process 10A is set to 115°. As can be seen from No. 1-6, in the 1180 MPa material, the vertical wall angle after springback after the first step 10A, i.e., the vertical wall angle θ1' of the intermediate part 4, is 120° or more. This vertical wall angle θ1' is significantly greater than the vertical wall angle θ0: 110° of the target part shape 1 and the vertical wall angle θ2: 115° of the die in the second step 10B. In addition, the amount of twist after springback in the first step 10A and the second step 10B is large, at 3.6°, respectively.

[0104] Also, as can be seen from No. 1-5, even in the case of the 980 MPa material, the vertical wall angle after springback following the first step 10A, i.e., the vertical wall angle θ1' of the intermediate part 4, is 120° or more. This vertical wall angle θ1' is significantly greater than the vertical wall angle θ0: 110° of the target part shape 1 and the vertical wall angle θ2: 115° of the die in the second step 10B. In addition, the amount of twist after springback in the first step 10A and the second step 10B is 2.0°, respectively. And, from No.1-5 and No.1-6, the dimensional accuracy variation between material strengths was 1.6°.

[0105] Furthermore, the vertical wall angle θ2 of the die in the second step 10B was 115°, whereas the vertical wall angle after springback in the second step 10B of the 1180 MPa material of No. 1-6 was about 123°. In other words, a large springback occurred. In addition, in the case of the 980 MPa material No. 1-5, the vertical wall angle after the springback in the second process 10B was 122°. This shows that a dimensional accuracy variation of 1° or more occurred between No. 1-5 and No. 1-6.

[0106] <No.1-3、No.1-4> No.1-3 and No.1-4 are the results for the 980 MPa material and the 1180 MPa material when the vertical wall angle θ1 in the first process 10A was set to 95°. As can be seen from No.1-4, for the 1180MPa material, the vertical wall angle θ1' after springback in the first process 10A was about 105°. This vertical wall angle θ1' was smaller than the vertical wall angle θ0: 110° of the target part shape 1 and the vertical wall angle θ2: 115° of the die in the second process 10B. In addition, the amount of twist after springback in the second process 10B was significantly reduced to 0.0°.

[0107] Furthermore, as can be seen from No. 1-3, even in the 980 MPa material, the amount of twist after springback in the second process 10B was significantly reduced to -0.1°. Also, the dimensional accuracy variation between material strengths was reduced to 0.1°. In addition, the vertical wall angle was also as follows, compared to the vertical wall angle θ2 of the die in the second step 10B: 115°. In other words, for both the 1180 MPa material and the 980 MPa material, the vertical wall angle was about 110° after springback in the second step 10B, reducing the amount of springback. Also, the difference between the 1180 MPa material No. 1-4 and the 980 MPa material No. 1-3 was reduced to about 0.5°.

[0108] <No.1-1、No.1-2> No. 1-1 and No. 1-2 show the results for the 980 MPa material and the 1180 MPa material when the vertical wall angle θ1 in the first process 10A was set to 90°. As can be seen from No. 1-2, in the 1180 MPa material, the vertical wall angle θ1′ after springback in the first step 10A was about 100°. The vertical wall angle θ1′ was smaller than the vertical wall angle θ0: 110° of the target part shape 1 and the vertical wall angle θ2: 115° of the die in the second step 10B.

[0109] Furthermore, the amount of twist after springback in the second process 10B was −1.0°, meaning that twist occurred in the opposite direction. Furthermore, as can be seen from No.1-1, even with the 980MPa material, the amount of twist after springback in the second process 10B was -0.7°, and twisting occurred in the opposite direction. However, this was less than the amount of twisting in the normal method, and the dimensional accuracy variation between material strengths was also reduced to 0.3°. This shows that if the vertical wall angle θ1 in the first process 10A is reduced too much, the stress will be reversed too much and twist will occur in the opposite direction.

[0110] From the above, by forming the mold in the first step 10A with a vertical wall angle θ1 of 95° and the mold in the second step 10B with a vertical wall angle θ2 of 115°, as in No.1-3 and No.1-4, the amount of twist, the amount of change in the vertical wall angle, and the fluctuation in the dimensional accuracy of these were significantly reduced. In addition, the vertical wall angle after springback in the second step 10B under these conditions was approximately 110°, which was lower than the vertical wall angle θ2 of the mold in the second step 10B. From this, it was found that when the vertical wall angle after springback in the second process 10B is set to the target part shape 1, the vertical wall angle θ1 of the die in the first process 10A needs to be designed to be smaller than the target part shape 1. Furthermore, it was found that the vertical wall angle θ2 of the die in the second process 10B needs to be designed to be equal to or larger than the vertical wall angle θ0 of the target part shape 1.

[0111] (Second example) In the second example, the vertical wall angle θ2 of the mold in the second step 10B is fixed to 110°. The vertical wall angle θ1 of the mold in the first step 10A is selected and set from 90°, 92°, 95°, and 110°. In the second example, the vertical wall angle θ0 of the target part shape 1 was set to 106°. The evaluation results of the second example are shown in Table 2.

[0112] [Table 2]

[0113] <NO.2-7、No.2-8> No. 2-7 and No. 2-8 are the results of the amount of twist for the 980 MPa material and the 1180 MPa material when the vertical wall angle θ1 in the first process 10A is set to 110°. As can be seen from No. 2-8, in the 1180 MPa material, the vertical wall angle θ1' after springback in the first step 10A was approximately 120°. This vertical wall angle θ1' greatly exceeds the vertical wall angle θ0: 106° of the target part shape 1 and the vertical wall angle θ2: 110° of the die in the second step 10B. In addition, the amounts of twist after springback in the first step 10A and the second step 10B were large, at 4.0° and 3.9°, respectively. Also, as can be seen from No. 2-7, in the 980 MPa material, the amount of twist after springback in the first step 10A and the second step 10B was 2.3° and 2.4°, respectively. Therefore, the dimensional accuracy variation between the material strengths was 1.5° after the second step 10B.

[0114] Furthermore, the vertical wall angle θ2 of the die in the second step 10B was 110°, whereas the vertical wall angle after springback in the second step 10B of the 1180 MPa material of No. 2-8 was about 118°. Therefore, a large springback occurred. In addition, in the case of the 980 MPa material of No. 2-7, the vertical wall angle after the springback in the second step 10B was about 117°. Therefore, a dimensional accuracy variation of 1° or more occurred.

[0115] <No.2-5、No.2-6> No. 2-5 and No. 2-6 show the results for the 980 MPa material and the 1180 MPa material when the vertical wall angle θ1 in the first process 10A was set to 95°. As can be seen from No. 2-6, for the 1180 MPa material, the vertical wall angle θ1' after springback in the first step 10A was about 105°. This angle was smaller than the 110° of the die in the second step 10B. In addition, the amount of twist after springback in the second step 10B was reduced to 1.0°.

[0116] Furthermore, as can be seen from No. 2-5, even in the 980 MPa material, the amount of twist after springback in the second process 10B was reduced to 0.2°, and the dimensional accuracy variation between material strengths was also reduced to 0.8°. As for the vertical wall angle, the vertical wall angle θ2 of the die in the second step 10B was 110°, while the vertical wall angle after springback in the second step 10B of the 1180 MPa material was approximately 107°, reducing the amount of springback. In addition, the difference between the 1180 MPa material No. 2-6 and the 980 MPa material No. 2-5 was reduced to about 0.5°.

[0117] <No.2-3、No.2-4> No. 2-3 and No. 2-4 show the results for the 980 MPa material and the 1180 MPa material when the vertical wall angle θ1 in the first process 10A was set to 92°. As can be seen from No. 2-4, for the 1180 MPa material, the vertical wall angle θ1' after springback in the first process 10A was about 102°. This vertical wall angle θ1' was smaller than the vertical wall angle θ0: 106° of the target part shape 1 and the vertical wall angle θ2: 110° of the die in the second process 10B. In addition, the amount of twist after springback in the second process 10B was significantly reduced to 0.1°.

[0118] As can be seen from No. 2-3, even in the 980 MPa material, the amount of twist after springback in the second process 10B was significantly reduced to 0.0°. Therefore, the dimensional accuracy variation between material strengths was also reduced to 0.1°. As for the vertical wall angle, the vertical wall angle θ2 of the die in the second step 10B was 110°, while the vertical wall angle after springback in the second step 10B of the 1180 MPa material was approximately 106°, reducing the amount of springback. In addition, the difference between the 1180 MPa material No. 2-4 and the 980 MPa material No. 2-3 was reduced to about 0.7°.

[0119] <No.2-1、No.2―2> No. 2-1 and No. 2-2 show the results for the 980 MPa material and the 1180 MPa material when the vertical wall angle θ1 in the first process 10A was set to 90°. As can be seen from No. 2-2, for the 1180MPa material, the vertical wall angle θ1' after springback in the first process 10A was about 100°. This vertical wall angle θ1' was smaller than the vertical wall angle θ0: 106° of the target part shape 1 and the vertical wall angle θ2: 110° of the die in the second process 10B. In addition, the amount of twist after springback in the second process 10B was -0.9°, and twisting occurred in the opposite direction. Furthermore, as can be seen from No. 2-1, even in the 980 MPa material, the amount of twist after springback in the second process 10B was -0.4°, and twisting occurred in the opposite direction. However, this was less than the amount of twist in the normal method, and the dimensional accuracy variation between material strengths was also reduced to 0.5°. This shows that if the vertical wall angle θ1 in the first step 10A is reduced too much, the stress will be reversed too much and twist in the opposite direction.

[0120] From the above, by forming the vertical wall angle θ1 of the die in the first step 10A at 92° and the vertical wall angle θ2 of the die in the second step 10B at 110°, the amount of twist, the amount of change in the angle of the vertical wall portion, and the fluctuation in the dimensional accuracy of these were significantly reduced. Furthermore, under these conditions, the vertical wall angle after springback in the second step 10B was approximately 106°, which was lower than the vertical wall angle θ2 of the die in the second step 10B. From this, it was found that if the vertical wall angle after springback in the second process 10B is to be the target part shape 1, it is necessary to design it as follows: That is, the vertical wall angle θ1 of the die in the first process 10A needs to be smaller than the target part shape 1, and the vertical wall angle θ2 of the die in the second process 10B needs to be designed to be an angle equal to or larger than the vertical wall angle of the product.

[0121] (Third example) In the third example, the vertical wall angle θ2 of the mold in the second step 10B is fixed to 105°. The vertical wall angle θ1 of the mold in the first step 10A is selected and set from 90°, 95°, 100°, and 105°. In the third example, the vertical wall angle θ0 of the target part shape 1 was set to 103°. The evaluation results of the third example are shown in Table 3.

[0122] [Table 3]

[0123] <No.3-7、No.3-8> No. 3-7 and No. 3-8 are the results of the amount of twist for the 980 MPa material and the 1180 MPa material when the vertical wall angle in the first process 10A is set to 105°. As can be seen from No. 3-8, in the 1180 MPa material, the vertical wall angle θ1' after springback in the first step 10A is approximately 115°. This vertical wall angle θ1' is significantly greater than the vertical wall angle θ0: 103° of the target part shape 1 and the vertical wall angle θ2: 105° of the die in the second step 10B. In addition, the amounts of twist after springback in the first step 10A and the second step 10B are large, at 4.1° and 3.7°, respectively.

[0124] Also, as can be seen from No. 3-7, in the 980 MPa material, the amounts of twist after springback in the first step 10A and the second step 10B were 2.5° and 2.3°, respectively. Therefore, the dimensional accuracy variation between the material strengths was 1.4° after the second step 10B. Furthermore, the vertical wall angle θ2 of the die in the second step 10B is 105°, whereas the vertical wall angle of the 1180 MPa material No. 3-8 after springback in the second step 10B is about 113°. Thus, a large springback occurs. The 980 MPa material No. 3-7 has a vertical wall angle of about 112°. Therefore, a dimensional accuracy variation of 1° or more occurs between the 1180 MPa material No. 3-8 and the 980 MPa material No. 3-7.

[0125] <No.3-5、No.3-6とNo.3-3、No.3-4> No. 3-5 and No. 3-6 show the results for the 980 MPa material and the 1180 MPa material when the vertical wall angle θ1 in the first process 10A was set to 100°. No. 3-3 and No. 3-4 show the results of the 980 MPa material and the 1180 MPa material when the vertical wall angle θ1 in the first process 10A was set to 95°. Under all conditions, the vertical wall angle θ1′ after springback in the first process 10A was larger than the vertical wall angle θ0:103° of the target part shape 1. In addition, no significant reduction in the dimensional accuracy variation of the twist amount after springback in the second process 10B was observed.

[0126] <No.3-1、No.3-2> No. 3-1 and No. 3-2 show the results for the 980 MPa material and the 1180 MPa material when the vertical wall angle θ1 in the first process 10A was set to 90°. As can be seen from No.3-2, for the 1180MPa material, the vertical wall angle θ' after springback in the first process 10A was about 101°. This vertical wall angle θ' was smaller than the vertical wall angle θ0: 103° of the target part shape 1 and the vertical wall angle θ2: 105° of the die in the second process 10B. In addition, the amount of twist after springback in the second process 10B was significantly reduced to -0.1°.

[0127] Moreover, as can be seen from No. 3-1, even in the case of the 980 MPa material, the amount of twist after the springback in the second step 10B was significantly reduced to 0.0°. Therefore, the dimensional accuracy variation between the material strengths of No. 3-1 (980 MPa material) and No. 3-2 (1180 MPa material) was also reduced to 0.1°. In addition, the vertical wall angle θ2 of the die in the second step 10B was 105°, whereas after springback in the second step 10B for the 1180 MPa material, it was approximately 103°, reducing the amount of springback. In addition, the difference between the 1180 MPa material No. 3-2 and the 980 MPa material No. 3-1 was reduced to about 0.3°.

[0128] From the above, it was found that it is good to set the vertical wall angle θ1 of the die in the first step 10A to 90° and the vertical wall angle θ2 of the die in the second step 10B to 105°. In this case, the amount of twist and the amount of change in the angle of the vertical wall portion and the fluctuation in the dimensional accuracy of these were significantly reduced. In addition, under these conditions, the vertical wall angle after springback in the second step 10B was approximately 103°, which was lower than the vertical wall angle θ2 of the die in the second step 10B. From this, it was found that if the vertical wall angle after springback in the second process 10B is to be the target part shape 1, it is necessary to design it as follows: That is, the vertical wall angle θ1 of the die in the first process 10A needs to be smaller than the vertical wall angle θ0 of the target part shape 1, and the vertical wall angle θ2 of the die in the second process 10B needs to be designed to be equal to or larger than the vertical wall angle θ0 of the target part shape 1.

[0129] Similarly, for the 590 MPa material and the 780 MPa material, when the vertical wall angle after springback in the second process 10B is set to the target part shape 1 based on the present invention, the following was confirmed. That is, the vertical wall angle θ1 of the die in the first process 10A is designed to be smaller than the vertical wall angle θ0 of the target part shape 1, and the vertical wall angle θ2 of the die in the second process 10B is designed to be larger than the vertical wall angle θ0 of the target part shape 1. It was confirmed that this resulted in the vertical wall angle θ0 of the target part shape 1 and that twisting was suppressed. Furthermore, it was confirmed that the dimensional accuracy variation was significantly reduced between the 590 MPa material and the 780 MPa material. The relationship between the above springback and the change in the vertical wall angle after the first process 10A is considered to vary depending on the type of steel and shape, so it is necessary to design a shape that suits each case.

[0130] <Example of embodiment 2> In this example, the target part shape 1 was a vertically curved shape curved toward the top plate portion along the longitudinal direction, as shown in Fig. 21. At this time, the metal plate 3 (blank material) used for press forming was three types of steel plate with tensile strengths of 590 MPa class, 980 MPa class, and 1180 MPa class. This was to evaluate with multiple material strengths in order to compare the dimensional accuracy fluctuations. The thickness t of the metal plate used in press forming was 1.4 mm. The standard product shape dimensions were: width of top plate 60 mm, height of vertical wall 30 mm, vertical wall angle between top plate and vertical wall 130°, maximum distance between left and right flanges 150 mm, longitudinal length of product 400 mm, and radius of curvature of top plate at curved part 1600 mm. The angle of 130° between top plate and vertical wall and radius of curvature of 1600 mm of curved part are standard values ​​and are not necessarily the target values ​​after press forming. Then, press forming analysis and springback analysis were carried out for the case where the vertical wall angle in the first process was reduced, and the radius of curvature and the vertical wall angle before and after springback were measured.

[0131] The side view radius of curvature of each die in the first step 10A and the second step 10B of this embodiment was set to R1600 [mm]. In addition, the vertical wall angle θ1 of the die in the first step 10A was changed in the range of 90° to 130°, and the vertical wall angle θ2 of the die in the second step 10B was unified to 130°, and an FEM analysis was performed. Table 4 shows the evaluation results. Note that the amount of camber back was deemed acceptable if the value was within ±2.0 mm after the springback in the second step 10B. In addition, the difference in height between the three steel types was deemed acceptable if it was 1.0 mm or less. In Table 4, "mold vertical wall angle" refers to the "mold vertical wall angle."

[0132] [Table 4]

[0133] <No.4-1~4-3> No. 4-1 to 4-3 are cases where the vertical wall angle θ1 of the die in the first step is 130°. In addition, 590 MPa, 980 MPa, and 1180 MPa steel plates were used. No. 4-1 to 4-3 show the results of determining the relationship between the average strain of the top plate and flange at the bottom dead center of forming for each material, the longitudinal curvature radius of the top plate after springback, and the vertical wall angle. Here, the "longitudinal curvature radius of the top plate" is also referred to as "top plate R". In No. 4-1 to 4-3, the vertical wall angle θ1 after springback in the first step 10A of each material is larger than the vertical wall angle θ2 of the mold in the second step 10B, which is 130°. In the case of the 1180MPa material, the vertical wall angle θ1′ after springback in the first step 10A is 138.3°. In addition, the top plate R after springback in the second step 10B of the 1180MPa material was R1964[mm]. And the maximum variation ΔR of the 590MPa material to 1180MPa material was R221mm. Thus, the difference in height between the steel types was large at 1.22mm, and did not meet the target condition of a height difference of 1mm or less.

[0134] <No.4-4~4-6> In contrast, No. 4-4 to 4-6 are cases where the vertical wall angle θ1 of the die in the first step 10A is 100°. No. 4-4 to 4-6 show the results of determining the relationship between the average strain of the top plate and flange parts at the bottom dead center of forming for each material, the top plate R after springback, and the vertical wall angle. The vertical wall angle θ1′ after springback in the first step 10A of each material is smaller than the vertical wall angle θ2 of the die in the second step 10B, which is 130°. For example, in the case of the 1180MPa material, the vertical wall angle θ1′ after springback in the first step 10A is 111.7°. In addition, the top plate R after springback in the second step 10B in the 1180MPa material is R1611[mm]. And, the maximum variation ΔR of the 590MPa material to the 1180MPa material is R102mm. The difference in height between the steel types is 0.93mm, which meets the target condition of 1mm or less. In addition, the vertical wall angle after springback in the second step 10B is 127.6° in the 1180MPa material. Furthermore, the amount of springback from 130° of the vertical wall angle θ2 of the die in the second step 10B and the amount of fluctuation in the 590 MPa to 1180 MPa materials were reduced compared to the comparative examples No. 4-1 to No. 4-3.

[0135] <No.4-7~4-9> Next, No. 4-7 to 4-9 are the cases where the vertical wall angle θ1 of the die in the first step 10A is 95°. No. 4-7 to 4-9 show the results of determining the relationship between the average strain of the top plate and flange parts at the bottom dead center of each material, the top plate R after springback, and the vertical wall angle. The vertical wall angle θ1′ after springback in the first step 10A of each material is smaller than the vertical wall angle θ2 of the die in the second step 10B, which is 130°. For example, in the case of the 1180MPa material, the vertical wall angle θ′ after springback in the first step 10A is 105.8°. In addition, the top plate R after springback in the second step 10B in the 1180MPa material is R1506[mm]. And the maximum variation ΔR of the 590MPa material to 1180MPa material is R52mm. The difference in height between the steel types is 0.46mm, which meets the target condition of 1mm or less. In addition, the vertical wall angle after springback in the second step 10B is 127° in the 1180MPa material. In Nos. 4-7 to 4-9, the springback amount from 130° of the vertical wall angle θ2 of the die in the second step 10B and the fluctuation amount of the 590 MPa material to the 1180 MPa material were reduced compared to the comparative examples Nos. 4-1 to 4-3.

[0136] <No.4-10~4-12> Next, No. 4-10 to 4-12 are the cases where the vertical wall angle θ1 of the die in the first step 10A is 90°. No. 4-10 to 4-12 show the results of determining the relationship between the average strain of the top plate and flange parts at the bottom dead center of the forming of each material, the top plate R after springback, and the vertical wall angle. The vertical wall angle θ1′ after springback in the first step 10A of each material is smaller than the vertical wall angle θ2 of the die in the second step 10B, which is 130°. For example, in the case of the 1180MPa material, the vertical wall angle θ1′ after springback in the first step 10A is 102.1°. In addition, the top plate R after springback in the second step 10B in the 1180MPa material is R1402[mm]. The maximum variation ΔR of the 590MPa material to the 1180MPa material is R74mm. The difference in height between the steel types is 0.76mm, which meets the target condition of 1mm or less. In addition, the vertical wall angle after springback in the second step 10B is 125.8° in the 1180MPa material. In Nos. 4-10 to 4-12, the amount of springback from 130° of the vertical wall angle θ2 of the die in the second step 10B and the amount of fluctuation of the 590 MPa material to the 1180 MPa material were reduced compared to the comparative examples Nos. 4-1 to 4-3. [Explanation of symbols]

[0137] 1 Target part shape 1A Top plate 1B Vertical wall section 1C Flange part 2 Curved section 3 Metal plate (blank material) 4 Intermediate parts 10A First Step 10B Second step 11 First step die 12 Punch for the first process 13 First step pad 14 Second process die 15 Punch for the second process 16 Second process pad θ0 Vertical wall angle of the target part shape θ1 First vertical wall angle (vertical wall angle of the mold in the first process) θ1′ Vertical wall angle of intermediate part θ2 Second vertical wall angle (vertical wall angle of the mold in the second process)

Claims

1. A method for manufacturing a press-molded product by molding a metal plate into a target part shape having a cross section including a top plate portion and a vertical wall portion connected to a width direction end of the top plate portion and extending in a direction different from the top plate portion, and having one or more curved portions curved along a longitudinal direction that is a direction intersecting the cross section, When the angle between the top plate portion and the vertical wall portion in the above cross section is defined as the vertical wall angle, a first step of press-molding a metal plate into an intermediate part having the top plate portion and the vertical wall portion, using a die having a molding surface with a first vertical wall angle smaller than a vertical wall angle of the target part shape at a position that will become the curved portion; a second step of press-molding the intermediate part at a position that will become the curved portion using a mold having a molding surface with a second vertical wall angle that is equal to or greater than the vertical wall angle of the target part shape; setting the first vertical wall angle so that a vertical wall angle of the intermediate part after demolding in the first step is smaller than a vertical wall angle of the target part shape at a position that will become the curved portion; The curved portion has a concave or convex curvature toward the vertical wall portion, When the vertical wall angle in the target part shape is defined as θ0, the first vertical wall angle is defined as θ1, the vertical wall angle in the intermediate part molded in the first step and released from the mold is defined as θ1', and the second vertical wall angle is defined as θ2, For the first vertical wall angle θ1 and the second vertical wall angle θ2, press molding is performed using a mold having a molding surface that satisfies the following formula (1): A manufacturing method for press-molded products. θ1 < θ1' < θ0 < θ2 (1)

2. A method for manufacturing a press-molded product, in which a metal plate is molded into a target part shape having a cross section with a top plate portion and a vertical wall portion connected to a widthwise end of the top plate portion and extending in a direction different from the top plate portion, and having one or more curved portions curved along a longitudinal direction that intersects with the cross section, comprising: When the angle between the top plate portion and the vertical wall portion in the above cross section is defined as the vertical wall angle, a first step of press-molding a metal plate into an intermediate part having the top plate portion and the vertical wall portion, using a die having a molding surface with a first vertical wall angle smaller than a vertical wall angle of the target part shape at a position that will become the curved portion; a second step of press-molding the intermediate part at a position that will become the curved portion using a mold having a molding surface with a second vertical wall angle that is equal to or greater than the vertical wall angle of the target part shape; setting the first vertical wall angle so that a vertical wall angle of the intermediate part after demolding in the first step is smaller than a vertical wall angle of the target part shape at a position that will become the curved portion; The curved portion has a concave or convex curvature toward the vertical wall portion, When the first vertical wall angle is defined as θ1 and the second vertical wall angle is defined as θ2, The first vertical wall angle θ1 and the second vertical wall angle θ2 are set so as to satisfy the following formula (2). A manufacturing method for press-molded products. 0.7 ≦ (θ1 / θ2) ... (2)

3. A method for manufacturing a press-molded product, in which a metal plate is molded into a target part shape having a cross section with a top plate portion and a vertical wall portion connected to a widthwise end of the top plate portion and extending in a direction different from the top plate portion, and having one or more curved portions curved along a longitudinal direction that intersects with the cross section, comprising: When the angle between the top plate portion and the vertical wall portion in the above cross section is defined as the vertical wall angle, a first step of press-molding a metal plate into an intermediate part having the top plate portion and the vertical wall portion, using a die having a molding surface with a first vertical wall angle smaller than a vertical wall angle of the target part shape at a position that will become the curved portion; a second step of press-molding the intermediate part at a position that will become the curved portion using a mold having a molding surface with a second vertical wall angle that is equal to or greater than the vertical wall angle of the target part shape; setting the first vertical wall angle so that a vertical wall angle of the intermediate part after demolding in the first step is smaller than a vertical wall angle of the target part shape at a position that will become the curved portion; The curved portion has a concave or convex curvature toward the vertical wall portion, When the vertical wall angle in the target part shape is defined as θ0 and the first vertical wall angle is defined as θ1, The first vertical wall angle θ1 is set to a value that satisfies the following formula (3). A manufacturing method for press-molded products. 0.75 ≦ (θ1 / θ0) ... (3)

4. A method for manufacturing a press-molded product, in which a metal plate is molded into a target part shape having a cross section with a top plate portion and a vertical wall portion connected to a widthwise end of the top plate portion and extending in a direction different from the top plate portion, and having one or more curved portions curved along a longitudinal direction that intersects with the cross section, comprising: When the angle between the top plate portion and the vertical wall portion in the above cross section is defined as the vertical wall angle, a first step of press-molding a metal plate into an intermediate part having the top plate portion and the vertical wall portion, using a die having a molding surface with a first vertical wall angle smaller than a vertical wall angle of the target part shape at a position that will become the curved portion; a second step of press-molding the intermediate part at a position that will become the curved portion using a mold having a molding surface with a second vertical wall angle that is equal to or greater than the vertical wall angle of the target part shape; setting the first vertical wall angle so that a vertical wall angle of the intermediate part after demolding in the first step is smaller than a vertical wall angle of the target part shape at a position that will become the curved portion; The curved portion has a concave or convex curvature toward the vertical wall portion, When the second vertical wall angle is defined as θ2 and the vertical wall angle of the intermediate part molded in the first step and released from the mold is defined as θ1′, The second vertical wall angle θ2 is set so as to satisfy the following formula (4). A manufacturing method for press-molded products. 0.8 ≦ (θ1' / θ2) ... (4)

5. A method for manufacturing a press-molded product, in which a metal plate is molded into a target part shape having a cross section with a top plate portion and a vertical wall portion connected to a widthwise end of the top plate portion and extending in a direction different from the top plate portion, and having one or more curved portions curved along a longitudinal direction that intersects with the cross section, comprising: When the angle between the top plate portion and the vertical wall portion in the above cross section is defined as the vertical wall angle, a first step of press-molding a metal plate into an intermediate part having the top plate portion and the vertical wall portion, using a die having a molding surface with a first vertical wall angle smaller than a vertical wall angle of the target part shape at a position that will become the curved portion; a second step of press-molding the intermediate part at a position that will become the curved portion using a mold having a molding surface with a second vertical wall angle that is equal to or greater than the vertical wall angle of the target part shape; setting the first vertical wall angle so that a vertical wall angle of the intermediate part after demolding in the first step is smaller than a vertical wall angle of the target part shape at a position that will become the curved portion; The curved portion has a concave or convex curvature toward the vertical wall portion, When the vertical wall angle in the target part shape is defined as θ0 and the second vertical wall angle is defined as θ2, The value of the second vertical wall angle θ2 is set so as to satisfy the following formula (5). A manufacturing method for press-molded products. (θ2 / θ0) < 1.05 (5)

6. A method for manufacturing a press-molded product, in which a metal plate is molded into a target part shape having a cross section with a top plate portion and a vertical wall portion connected to a widthwise end of the top plate portion and extending in a direction different from the top plate portion, and having one or more curved portions curved along a longitudinal direction that is a direction intersecting the cross section, When the angle between the top plate portion and the vertical wall portion in the above cross section is defined as the vertical wall angle, a first step of press-molding a metal plate into an intermediate part having the top plate portion and the vertical wall portion, using a die having a molding surface with a first vertical wall angle smaller than a vertical wall angle of the target part shape at a position that will become the curved portion; a second step of press-molding the intermediate part at a position that will become the curved portion using a mold having a molding surface with a second vertical wall angle that is equal to or greater than the vertical wall angle of the target part shape; setting the first vertical wall angle so that a vertical wall angle of the intermediate part after demolding in the first step is smaller than a vertical wall angle of the target part shape at a position that will become the curved portion; The curved portion has a concave or convex curvature toward the vertical wall portion, The curved portion is a target part shape that is curved so as to be convex toward the vertical wall portion, Regarding the longitudinal strain distribution at the bottom dead point in the press forming of the second step, the average value of the strain in the lower half of the vertical wall portion that is convex in top view at the position that becomes the curved portion is defined as ε2; Regarding the longitudinal strain distribution at the bottom dead point of the press in the press forming of the first step, when the average value of the strain in the lower half of the vertical wall of the vertical wall portion that is convex in top view at the position that becomes the curved portion is defined as ε1, The first vertical wall angle and the second vertical wall angle are set so as to satisfy the following formula (6). A manufacturing method for press-molded products. 0.4×|ε1| ≦ |ε2| < 0.85×|ε1| ...(6)

7. A method for manufacturing a press-molded product, in which a metal plate is molded into a target part shape having a cross section with a top plate portion and a vertical wall portion connected to a widthwise end of the top plate portion and extending in a direction different from the top plate portion, and having one or more curved portions curved along a longitudinal direction that intersects with the cross section, comprising: When the angle between the top plate portion and the vertical wall portion in the above cross section is defined as the vertical wall angle, a first step of press-molding a metal plate into an intermediate part having the top plate portion and the vertical wall portion, using a die having a molding surface with a first vertical wall angle smaller than a vertical wall angle of the target part shape at a position that will become the curved portion; a second step of press-molding the intermediate part at a position that will become the curved portion using a mold having a molding surface with a second vertical wall angle that is equal to or greater than the vertical wall angle of the target part shape; setting the first vertical wall angle so that a vertical wall angle of the intermediate part after demolding in the first step is smaller than a vertical wall angle of the target part shape at a position that will become the curved portion; The curved portion has a concave or convex curvature toward the vertical wall portion, The curved portion is a curved target part shape that is concave toward the vertical wall portion, Regarding the longitudinal strain distribution at the bottom dead point in the press forming of the second step, the average value of the strain in the lower half of the vertical wall portion that is concave in top view at the position that becomes the curved portion is defined as ε2'; Regarding the longitudinal strain distribution at the bottom dead point in the press forming of the first step, when the average value of the strain in the lower half of the vertical wall portion that is concave in top view at the position that becomes the curved portion is defined as ε1', The first vertical wall angle and the second vertical wall angle are set so as to satisfy the following formula (7). A manufacturing method for press-molded products. 0.4×|ε1′| ≦ |ε2′| < 0.85×|ε1′| ...(7)

8. A method for manufacturing a press-molded product, in which a metal plate is molded into a target part shape having a cross section with a top plate portion and a vertical wall portion connected to a widthwise end of the top plate portion and extending in a direction different from the top plate portion, and having one or more curved portions curved along a longitudinal direction that is a direction intersecting the cross section, When the angle between the top plate portion and the vertical wall portion in the above cross section is defined as the vertical wall angle, a first step of press-molding a metal plate into an intermediate part having the top plate portion and the vertical wall portion, using a die having a molding surface with a first vertical wall angle smaller than a vertical wall angle of the target part shape at a position that will become the curved portion; a second step of press-molding the intermediate part at a position that will become the curved portion using a mold having a molding surface with a second vertical wall angle that is equal to or greater than the vertical wall angle of the target part shape; setting the first vertical wall angle so that a vertical wall angle of the intermediate part after demolding in the first step is smaller than a vertical wall angle of the target part shape at a position that will become the curved portion; The curved portion has a concave or convex curve toward the top plate portion. A manufacturing method for press-molded products.

9. When the vertical wall angle in the target part shape is defined as θ0, the first vertical wall angle is defined as θ1, the vertical wall angle in the intermediate part molded in the first step and released from the mold is defined as θ1', and the second vertical wall angle is defined as θ2, For the first vertical wall angle θ1 and the second vertical wall angle θ2, press molding is performed using a mold having a molding surface that satisfies the following formula (1): A method for producing a press-molded product according to claim 8. θ1 < θ1' < θ0 < θ2 (1)

10. Regarding the longitudinal strain distribution at the bottom dead point in the press forming of the second step, the absolute value of the average value of the strain of the top plate portion at the position that becomes the curved portion is defined as εt2, Regarding the longitudinal strain distribution at the bottom dead point in the press forming of the first step, when the absolute value of the average value of the top plate strain at the position of the curved portion is defined as εt1, The first vertical wall angle and the second vertical wall angle are set so as to satisfy the following formula (8). A method for producing a press-molded product according to claim 8 or 9. 0.4×εt1 ≦ εt2 < 0.9×εt1 (8)

11. The cross section is U-shaped or hat-shaped having a flange portion, Regarding the longitudinal strain distribution at the bottom dead point of the press in the press forming of the second step, in the position that becomes the curved portion, when the cross section is U-shaped, the absolute value of the average value of the strain in the lower half of the vertical wall portion is defined as εb2, and when the cross section is a hat shape having a flange portion, the absolute value of the average value of the strain in the flange portion is defined as εb2, Regarding the longitudinal strain distribution at the bottom dead point of the press in the press forming of the first step, in the position that becomes the curved portion, when the cross section is U-shaped, the absolute value of the average value of the strain in the lower half of the vertical wall portion is defined as εb1, and when the cross section is a hat shape having a flange portion, the absolute value of the average value of the strain in the flange portion is defined as εb1. The first vertical wall angle and the second vertical wall angle are set so as to satisfy the following formula (9): A method for producing a press-molded product according to claim 8 or 9. 0.4×εb1 ≦ εb2 < 0.9×εb1 (9)

12. The mold used in the second step includes a die constituting one of the upper mold and the lower mold, and a punch constituting the other of the upper mold and the lower mold, In the second step, press forming is performed in a state in which the top plate portion of the intermediate part is sandwiched between a top plate forming surface of the die and a pad extended from the top plate forming surface of the punch. A method for producing a press-molded product according to any one of claims 1 to 9.

13. A method for designing a die used in the method for manufacturing a press-molded product according to any one of claims 1 to 7, When designing the vertical wall angles of each die in the first and second steps, First, perform CAE analysis or actual pressing with a vertical wall angle of the die in the first step smaller than the vertical wall angle of the product, find a condition in which the vertical wall angle after springback in the first step is smaller than the vertical wall angle of the product, and set the first vertical wall angle; Then, for the second process, a CAE analysis or an actual press is performed with a mold shape having a molding surface with a vertical wall angle equal to or larger than the vertical wall angle of the target part shape, and a condition is found under which the part shape after springback in the second process satisfies the vertical wall angle and the torsion tolerance range of the target part shape, and the second vertical wall angle is set. How to design a mold.

14. A method for designing a die used in the method for manufacturing a press-molded product according to any one of claims 8 to 9, When designing the vertical wall angles of each die in the first and second steps, First, perform CAE analysis or actual pressing with a vertical wall angle of the die in the first step smaller than the vertical wall angle of the product, find a condition in which the vertical wall angle after springback in the first step is smaller than the vertical wall angle of the product, and set the first vertical wall angle; Then, for the second process, a CAE analysis or an actual press is performed with a mold shape having a molding surface with a vertical wall angle equal to or larger than the vertical wall angle of the target part shape, and a condition is found under which the part shape after springback in the second process satisfies the allowable range of the vertical wall angle in the target part shape, and the second vertical wall angle is set. How to design a mold.

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