Press molding method, method for manufacturing press-molded article, intermediate molded article, and press molding die

Abstract

Provided is a technology capable of preventing the occurrence of wrinkles in a press-formed product that includes a top plate portion and a vertical wall portion, in which the top plate portion has a stepped surface in the longitudinal direction. Specifically, provided is a press forming method for obtaining a press-formed product including a top plate portion and a vertical wall portion coupled to the top plate portion in the width direction thereof via a ridge line portion, the top plate portion having a first top plate surface on the upper side, a stepped surface, and a second top plate surface on the lower side formed by bending for forming convex and concave portions in the longitudinal direction, the method including a first forming step of press-forming a metal sheet blank into an intermediate formed product with an intermediate shape, and a second forming step of forming the intermediate formed product into the press-formed product, characterized in that the intermediate formed product has an additional shape such that a first intersection point of the press-formed product is shifted toward the first top plate surface in the pressing direction, the first intersection point being a point of intersection between a lower ridge line as a boundary between the second top plate surface and the vertical wall portion and a baseline of the stepped surface.

Description

Technical Field

[0001] The present invention relates to a press forming method for a press-formed product including a top plate portion and a vertical wall portion, in which the top plate portion has a step formed by concave and convex bending in the longitudinal direction, as well as a method for producing the press-formed product, an intermediate formed product, and a press-forming die for use in press forming.Background Art

[0002] In press forming a metal part that includes a top plate surface, a vertical wall, and convex and concave portions at their boundary, wrinkles are a common drawback that occur in the vicinity of the convex and concave portions. For example, as illustrated in Figs. 4 and 5, when a metal part, namely, a press-formed product 1 having a top plate portion 10 with a stepped surface 12 is press-formed, the forming is carried out such that a vertical wall portion 20 and then a flange 30 are formed while the top plate portion 10 is pressed with a pad. At this time, as illustrated in an enlarged view of Fig. 4(b), the material bent along the boundary between an upper top plate surface 11 and the vertical wall portion 20, namely, along an upper ridge line 11a, and the material bent along the boundary between the stepped surface 12 and the vertical wall portion 20, namely, along a lateral ridge line 21a, converge on the vertical wall portion 20 around a convex portion 2 and a concave portion 3. The material flow-in direction M is indicated by an arrow. Therefore, a wrinkle D occurs in the vertical wall portion 20. The same applies to a metal part without a flange, as shown in Fig. 5, such as a press-formed product 1.

[0003] When press forming a metal part having a convex portion 2 and a concave portion 3 seamlessly formed across a top plate portion 10 and a vertical wall portion 20, as shown in Fig. 6, namely, a press-formed product 1, drawing is often employed to form the top plate portion 10 and the vertical wall portion 20 while a flange portion 30 is pinched with a die in advance. At this time, the material bent along the boundary between an upper top plate surface 11 and a stepped surface 12, namely, along an apex line 12a, and the material bent along the boundary between a front-side vertical wall surface 21 and the stepped surface 12, namely, along the lateral ridge line 21a, converge on the stepped surface 12 around the convex portion 2 and the concave portion 3. The material flow-in direction M is indicated by an arrow. Therefore, a wrinkle D occurs in the stepped surface 12.

[0004] Technologies have been developed to prevent the occurrence of such wrinkles. For example, Patent Literature 1 discloses a technology in which a bead is formed in advance at the boundary between a top plate surface and a vertical wall, thereby increasing the rigidity of the top plate surface and preventing material from converging at the portion where wrinkles are likely to occur. In addition, Patent Literature 2 discloses a technology for preventing wrinkles during press forming of a metal sheet by pinching the portion where wrinkles are likely to occur with a die. Patent Literature 3 discloses a technology for preventing wrinkles by providing a convex shape at a portion where wrinkles are likely to occur so as to absorb excess material, followed by squashing the convex shape in the next step.Citation ListPatent Literature

[0005] Patent Literature 1: International Publication No. WO 2021 / 181982 Patent Literature 2: International Publication No. WO 2020 / 105647 Patent Literature 3: JP-A-2019-013952 Summary of InventionTechnical Problem

[0006] However, the conventional technologies have the following problems.

[0007] That is, the technology described in Patent Literature 1 is designed to prevent wrinkles by increasing the rigidity of a portion where the deformation of a material is unnecessary, such as a top plate surface and flanges, thereby preventing material from converging on that portion. However, using this technology is not an effective measure because the wrinkles targeted by the present invention occur during the formation of a vertical wall or a stepped surface, which involves deforming material.

[0008] According to the technology described in Patent Literature 2, a portion with a relatively flat shape, such as a flange, can be easily formed by pinching and constraining the material with a die. However, since the target wrinkles of the present invention occur on a vertical wall or a stepped surface having a three-dimensional shape, it is not possible to constrain the material during press forming.

[0009] The technology described in Patent Literature 3 is effective, provided that a convex shape can be provided at a portion where wrinkles are likely to occur, as this allows excess material to be absorbed. However, it may be difficult to squash the convex shape into a flat one. A metal sheet with a thickness of several millimeters is easily buckled when subjected to in-plane compressive deformation. Therefore, to squash the convex shape into a flat shape, the absorbed material needs to be drawn outward. Note that conventional press forming involves shaping a metal sheet by sliding a die in the product's height direction. Due to its structural configuration, it is easy to move the material in the height direction. Meanwhile, it is difficult to move the material in a direction other than the product's height direction. As a result, when excess material is formed in a direction different from the height direction, such as in the case of the target wrinkles in the present invention, it becomes challenging to squash the shape into a flat one.

[0010] The present invention has been made in view of the foregoing circumstances, and an object thereof is to provide a technology capable of preventing the occurrence of wrinkles in a press-formed product including a top plate portion, a vertical wall portion, and a stepped surface formed in the longitudinal direction of the top plate portion.Solution to Problem

[0011] The press forming method according to the present invention, which advantageously solves the foregoing problems, is a method for press forming a press-formed product, the press-formed product including a top plate portion and a vertical wall portion coupled to the top plate portion in a width direction of the top plate portion via a ridge line portion, wherein the top plate portion includes a first top plate surface on an upper side, a stepped surface, and a second top plate surface on a lower side, these surfaces being formed by bending for forming convex and concave portions in the longitudinal direction, the method including a first forming step of press forming a metal sheet blank into an intermediate formed product with an intermediate shape, and a second forming step of forming the intermediate formed product into the press-formed product, characterized in that the intermediate formed product has an additional shape, in which a first intersection point of the press-formed product is shifted toward the first top plate surface in a pressing direction, the first intersection point being defined by a lower ridge line serving as a boundary between the second top plate surface and the vertical wall portion, and by a baseline of the stepped surface.

[0012] Note that in this specification, the first top plate surface on the upper side shall also be referred to as an upper top plate surface. In addition, the second top plate surface on the lower side shall also be referred to as a lower top plate surface.

[0013] The press forming method according to the present invention may include the following features that are considered to be more preferable solution means. (a) In the intermediate formed product, the shifted first intersection point lies on a lateral ridge line formed by the stepped surface and the vertical wall surface. (b) The vertical wall portion of the press-formed product includes a first vertical wall surface on a front side, a stepped surface that is common to the top plate portion, and a second vertical wall surface on a rear side, these surfaces being formed by bending for forming convex and concave portions, respectively, which are performed continuously to bending for forming the convex and concave portions of the top plate portion, and the intermediate formed product has an additional shape such that a first intersection point between the lower ridge line, serving as a boundary between the second top plate surface and the second vertical wall surface, and the baseline of the stepped surface lies either at a position on the stepped surface along an extension of the second vertical wall surface, or at a position closer to the first vertical wall surface than the said position on the stepped surface along the extension of the second vertical wall surface. (c) The additional shape has a second intersection point on the lower ridge line and a third intersection point on the baseline. (d) A ratio ΔH of a shifting distance h of the first intersection point of the intermediate formed product in the pressing direction relative to a distance H between the first top plate surface and the second top plate surface around the stepped surface in the pressing direction is in a range of 9 to 100%.

[0014] Note that in this specification, the first vertical wall surface on the front side shall also be referred to as a front-side vertical wall surface, while the second vertical wall surface on the rear side shall also be referred to as a rear-side vertical wall surface.

[0015] A method for producing a press-formed product according to the present invention that advantageously solves the foregoing problems includes obtaining a press-formed product by press forming a metal sheet blank using any one of the foregoing press forming methods.

[0016] The intermediate formed product according to the present invention, which advantageously solves the foregoing problems, is an intermediate formed product of a press-formed product, including a top plate portion, and a vertical wall portion coupled to the top plate portion in a width direction of the top plate portion via a ridge line portion, the top plate portion including a first top plate surface on an upper side, a stepped surface, and a second top plate surface on a lower side, these surfaces being formed by bending for forming convex and concave portions in the longitudinal direction, characterized in that the intermediate formed product has an additional shape such that a first intersection point of the press-formed product is shifted in a pressing direction toward the first top plate surface, the first intersection point being defined by a lower ridge line, which serves as a boundary between the second top plate surface and the vertical wall portion, and by a baseline of the stepped surface.

[0017] The intermediate formed product according to the present invention may include the following features that are considered to be more preferable solution means.

[0018] (e) The shifted first intersection point lies on a lateral ridge line formed by the stepped surface and the vertical wall surface.

[0019] (f) The vertical wall portion of the press-formed product includes a first vertical wall surface on a front side, a stepped surface that is common to the top plate portion, and a second vertical wall surface on a rear side, these surfaces being formed by bending for forming convex and concave portions, respectively, which are performed continuously to bending for forming the convex and concave portions of the top plate portion, and the intermediate formed product has an additional shape such that the first intersection point between the lower ridge line, defined by the second top plate surface and the second vertical wall portion, and the baseline of the stepped surface lies either at a position on the stepped surface along an extension of the second vertical wall surface, or at a position closer to the first vertical wall surface than the said position on the stepped surface along the extension of the second vertical wall surface.

[0020] The press-forming die according to the present invention, which advantageously solves the foregoing problems, is a press-forming die for press-forming a metal sheet blank into an intermediate formed product with an intermediate shape, when producing a press-formed product, the press-formed product including a top plate portion and a vertical wall portion coupled to the top plate portion in a width direction of the top plate portion via a ridge line portion, the top plate portion including a first top plate surface on an upper side, a stepped surface, and a second top plate surface on a lower side, these surfaces being formed by bending for forming convex and concave portions in the longitudinal direction, characterized in that the intermediate formed product has an additional shape such that a first intersection point of the press-formed product is shifted in a pressing direction toward the first top plate surface, the first intersection point being defined by a lower ridge line, serving as a boundary between the second top plate surface and the vertical wall portion, and a baseline of the stepped surface.

[0021] The press-forming die according to the present invention may include the following features, which are considered to be a more preferable solution means.

[0022] (g) In the intermediate formed product, the shifted first intersection point lies on a lateral ridge line formed by the stepped surface and the vertical wall surface.

[0023] (h) The vertical wall portion of the press-formed product includes a first vertical wall surface on a front side, a stepped surface that is common to the top plate portion, and a second vertical wall surface on a rear side, these surfaces being formed by bending for forming convex and concave portions, respectively, which are performed continuously to bending for forming the convex and concave portions of the top plate portion, and the intermediate formed product has an additional shape such that a first intersection point between a lower ridge line, serving as a boundary between the second top plate surface and the second vertical wall surface, and the baseline of the stepped surface lies either at a position on the stepped surface along an extension of the second vertical wall surface, or at a position closer to the first vertical wall surface than the said position on the stepped surface along the extension of the second vertical wall surface.Advantageous Effects of Invention

[0024] The press forming method, the method for producing a press-formed product, the intermediate formed product, and the press-forming die according to the present invention are capable of preventing the occurrence of wrinkles in the vertical wall portion around a stepped portion where the height of the top plate portion changes, as well as in the stepped surface.Brief Description of Drawings

[0025] [Fig. 1] is a schematic view illustrating a press forming method for a product shape 1 according to an embodiment of the present invention, in which (a) is a schematic perspective view of an intermediate formed product with the product shape 1 obtained after a first forming step, (b) is a side view thereof, (c) is a partially enlarged view of the side surface, and (d) is a schematic perspective view of a press-formed product with the product shape 1 obtained after a second forming step. [Fig. 2] is a schematic view illustrating a press forming method for pattern 1 to obtain a product shape 3 according to the foregoing embodiment, in which (a) is a schematic perspective view of an intermediate formed product of the product shape 3 obtained after a first forming step of pattern 1, (b) is a side view thereof, and (c) is a schematic perspective view of a press-formed product with the product shape 3 obtained after a second forming step. [Fig. 3] is a schematic view illustrating a press forming method for pattern 2 to obtain the product shape 3 according to the foregoing embodiment, in which (a) is a schematic perspective view of an intermediate formed product of the product shape 3 obtained after a first forming step of pattern 2, (b) is a side view thereof, and (c) is a schematic perspective view of a press-formed product with the product shape 3 obtained after a second forming step. [Fig. 4] is a schematic view illustrating a press forming method to obtain the product shape 1 with a conventional method, in which (a) is a schematic perspective view of a press-formed product with the product shape 1, (b) is a partially enlarged perspective view thereof, and (c) is a side view. [Fig. 5] is a schematic perspective view of a press-formed product with a product shape 2. [Fig. 6] is a schematic view illustrating a press forming method for obtaining the product shape 3 with a conventional method, in which (a) is a schematic perspective view of a press-formed product with the product shape 3, (b) is a partially enlarged perspective view thereof, and (c) is a side view. [Fig. 7] is a schematic view illustrating the effect of suppressing the occurrence of wrinkles with the press forming method for obtaining the product shape 1 according to the present embodiment. [Fig. 8] is a perspective view illustrating examples of the dimensions of press-formed products according to an Example, in which (a) illustrates a case of the product shape 1, and (b) illustrates a case of the product shape 3. [Fig. 9] is a perspective view illustrating distributions of the sheet thicknesses of press-formed products according to an Example, in which (a) illustrates a case of test No. 1, and (b) illustrates a case of test No. 2. Description of Embodiments

[0026] Hereinafter, an embodiment of the present invention will be specifically described with reference to the drawings. Note that the following embodiment only illustrates examples of a shape and a method for embodying the technical idea of the present invention. Thus, the configuration of the present invention is not limited thereto. That is, the technical idea of the present invention may be changed in various ways within the technical scope recited in the claims.

[0027] Fig. 1 is a schematic view illustrating a press forming method for a product shape 1 according to the present embodiment. Fig. 2 is a schematic view illustrating a press forming method for pattern 1 of a product shape 3 according to the present embodiment. Fig. 3 is a schematic view illustrating a press forming method for pattern 2 of the product shape 3 according to the present embodiment. A product shape 2 corresponds to the product shape 1 without flange portions 30, as illustrated in Fig. 5. In addition, the product shape 3 without flange portions 30 is also targeted by the present invention.

[0028] In the present embodiment, a press-formed product 1 with the product shape 1 includes a top plate portion 10, vertical wall portions 20 each coupled to the top plate portion 10 in the width direction of the top plate portion 10 via a ridge line portion, and flange portions 30 each coupled to an edge different from a top plate portion of each vertical wall portion 20 and extending therefrom. The top plate portion 10 includes an upper top plate surface 11, a stepped surface 12, and a lower top plate surface 13, formed by bending for forming a convex portion 2 and a concave portion 3 in the longitudinal direction thereof. The boundary between the upper top plate surface 11 and the stepped surface 12 has an apex line 12a, and the boundary between the stepped surface 12 and the lower top plate surface 13 has a baseline 12b. The boundary between the upper top plate surface 11 and each vertical wall portion 20 has an upper ridge line 11a. The boundary between the stepped surface 12 and each vertical wall portion 20 has a lateral ridge line 21a. The boundary between the lower top plate surface 13 and each vertical wall portion 20 has a lower ridge line 13a.

[0029] In the present embodiment, a press-formed product 1 of the product shape 3 includes a top plate portion 10 with a shape similar to that of the product shape 1. Moreover, each vertical wall portion 20 includes a front-side vertical wall surface 21, a common stepped surface 12, and a rear-side vertical wall surface 22, formed by bending for forming the convex portion 2 and the concave portion 3, respectively, which are performed continuously to bending for forming the convex portion 2 and the concave portion 3 of the top plate portion 10. The boundary between the front-side vertical wall surface 21 and the stepped surface 12 has a lateral ridge line 21a, and the boundary between the stepped surface 12 and the rear-side vertical wall surface 22 has a valley line 22a. The upper ridge line 11a corresponds to the boundary between the upper top plate surface 11 and the front-side vertical wall surface 21. The lower ridge line 13a corresponds to the boundary between the lower top plate surface 13 and the rear-side vertical wall surface 22.

[0030] According to the press forming method of the present embodiment, a metal sheet blank is subjected to press forming including two steps: an intermediate forming step, that is, a first forming step, and a product forming step, that is, a second forming step. Then, the press-formed product 1 with the product shape is produced. Figs. 1(a), 2(a), and 3(a) each show an example of an intermediate formed product 100 with an intermediate shape.

[0031] In each of the drawings illustrating the present embodiment, a line is used to indicate each boundary between two faces representing the shape. Meanwhile, the actual press-formed product 1 has a rounded shape. Such roundness may be appropriately determined by considering the curvature of the product shape, for example, by setting a small curvature radius for a curved surface corresponding to a boundary, provided that no cracks occur in the metal sheet. For example, regarding a metal sheet with a tensile strength of 590 MPa or less, the curvature radius of the inner surface of a curved surface is preferably set to 2 mm or greater because such a curvature radius is unlikely to cause cracks. Meanwhile, regarding a metal sheet with a tensile strength exceeding 590 MPa, the curvature radius is preferably set to 4 mm or greater, as this is also unlikely to cause cracks. The upper limit of the curvature radius is not limited to a particular value. However, if the curvature radius exceeds 10 mm, the intermediate shape changed from the shape of the metal part, that is, the press-formed product 1, namely, an additional shape 9 of the intermediate formed product 100 may have a wide range. In such a case, the additional shape 9 may interfere with a member, such as a seating face, that is functionally required.

[0032] In the present embodiment, the intersection point between the lower ridge line 13a and the baseline 12b of the press-formed product 1 is defined as a first intersection point 5. The first intersection point 5 undergoes bending for forming a convex portion in the width direction and a concave portion in the longitudinal direction. The intermediate formed product 100 has the additional shape 9 including a first intersection point 6 that corresponds to the intersection point 5, which has been shifted toward the upper top plate surface 11 in the pressing direction, i.e., in the height direction HD of the product. Specifically, in the case of the product shape 1, the additional shape 9 is formed as illustrated in Figs. 1(a) to (c). The additional shape 9 can be formed with a line segment connecting the first intersection point 6, a second intersection point 7, and a third intersection point 8. The first intersection point 6 of the intermediate formed product 100 corresponds to the first intersection point 5 of the press-formed product 1, which has been shifted toward the upper top plate surface 11 along the lateral ridge line 21a. The second intersection point 7 lies on the lower ridge line 13a. The third intersection point 8 lies on the baseline 12b. Note that the line segment may be either a straight line or a curved line, but straight is preferable, as it allows for easier design and management of a die shape. The first intersection point 6 may be located on the stepped surface 12 near the lateral ridge line. The product shape 2 differs from the product shape 1 only in that it does not have a flange. Thus, the additional shape 9 of the intermediate formed product 100 can be formed similarly to the product shape 1.

[0033] In pattern 1 for the product shape 3, the additional shape 9 is formed as illustrated in Figs. 2(a) and 2(b). The additional shape 9 can be defined by a line segment connecting the first intersection point 6, the second intersection point 7, and the third intersection point 8. The first intersection point 6 of the intermediate formed product 100 corresponds to the first intersection point 5 of the press-formed product 1. The first intersection point 5 has been shifted toward the upper top plate surface 11 and toward the front-side vertical wall surface 21 within the stepped surface 12. The first intersection point 6 may lie on the lateral ridge line 21a. The second intersection point 7 lie on the lower ridge line 13a. The third intersection point 8 lies on the baseline 12b. Note that the line segment may be either a straight line or a curved line, but a straight line is preferable as it allows for the easier design and management of the die shape. In addition, the additional shape 9 is formed by the concave portion 3 on the stepped surface 12, including the first intersection point 6 and running parallel with the front-side vertical wall surface 21, and the concave portion 3 on the rear-side vertical wall surface 22, including the second intersection point 7 and running parallel with the stepped surface.

[0034] Further, in pattern 2 for the product shape 3, the additional shape 9 is formed as illustrated in Figs. 3(a) and 3(b). The additional shape 9 can be formed with a line segment connecting the first intersection point 6, the second intersection point 7, and the third intersection point 8. The first intersection point 6 of the intermediate formed product 100 corresponds to the first intersection point 5 of the press-formed product 1, with the first intersection point 5 having been shifted toward the upper top plate surface 11 within the stepped surface 12 in parallel with the rear-side vertical wall surface 22. The second intersection point 7 lies on the lower ridge line 13a. The third intersection point 8 lies on the baseline 12b. The line segment may be either a straight line, a curved line, or a combination of the two, but a straight line is preferable as it allows for easier design and management of a die shape.

[0035] The ratio ΔH(%) of the shifting distance h of the first intersection point 6 from the first intersection point 5 in the product height direction HD, i.e., the pressing direction, relative to the distance H between the top plate surfaces 11 and 13 around the stepped surface 12 in the pressing direction is preferably in the range of 9 to 100%. ΔH of 100% indicates that the first intersection point 6 of the additional shape 9 is located at a ridge-line convex portion 4 of the upper top plate surface 11, that is, at the intersection between the upper ridge line 11a and the apex line 12a. Herein, the distance H between the top plate surfaces 11 and 13 in the pressing direction may be the difference in height between the apex line 12a and the baseline 12b of the intermediate formed product 100.

[0036] Even if the distance H between the top plate surfaces 11 and 13 in the pressing direction of the press-formed product 1 differs from that of the intermediate formed product 100, this difference is acceptable as long as there are no wrinkles or cracks in the press-formed product. If H of the intermediate formed product 100 is too large as compared to H of the press-formed product 1, wrinkles are likely to occur during the product forming step. Meanwhile, if H of the intermediate formed product 100 is too small as compared to H of the press-formed product 1, cracks are likely to occur.

[0037] In the present embodiment, the height position of a flange portion 30 of the intermediate formed product 100 can be selected from the range of a height position at the same level as the lower top plate surface of the intermediate formed product 100 to the height position of the flange portion 30 of the press-formed product 1, in the product height direction. The actually pressed part has a more complex shape. Therefore, it can be selected as appropriate to distribute the pressing load for each pressing or to form articles with other shapes such as beads or seating surfaces near the flange portion without causing wrinkles or cracks.

[0038] The inventors consider a mechanism for suppressing the occurrence of wrinkles according to the present embodiment as follows.<Product shape 1>

[0039] A metal part with the product shape 1 illustrated in Fig. 1(d), that is, the press-formed product 1 will be described as an example. In this configuration, the intermediate formed product 100 with the intermediate shape, as illustrated in Fig. 1(a), includes a ridge line portion connecting the first intersection point 6 and the second intersection point 7 of the additional shape 9, replacing the concave portion at the first intersection point 5 of the press-formed product 1 formed by bending for forming the concave portion 3. Accordingly, when the press-formed product 1 is formed from a metal sheet blank in a single step, the material flow-in direction M, which causes wrinkles in the vertical wall portion 20, is pulled toward the ridge line portion of the additional shape 9 during the intermediate forming step. Consequently, the material flow-in direction M is distributed to the ridge line portion of the additional shape 9. During the subsequent product forming step, the additional shape 9 is subjected to bending for forming the concave portion 3 to reach the first intersection point 5 of the press-formed product 1. Forming the press-formed product from a metal sheet blank in two steps, as described above, has the effect of suppressing the occurrence of wrinkles, compared to forming in a single step. This will be described later.

[0040] First, comparison will be made between the intermediate formed product 100 and the press-formed product 1, with regard to the length of the material along the ridge line, referring to Fig. 7. The length Y1-Y1' of the ridge line of the additional shape 9 of the intermediate formed product 100 is shorter than the corresponding length Y2-X2-Y2' of the press-formed product 1 after forming. Therefore, the material is stretched without generating any excess material. The direction of the ridge line is a direction different from the height direction, i.e., the pressing direction HD, where it is difficult to take measures against the occurrence of wrinkles with the conventional process, as described with reference to the problem of Patent Literature 3. Meanwhile, when comparison is made based on a cross-section in the pressing direction HD, the line segment X1-X1"-X1' of the additional shape 9 of the intermediate formed product 100 is longer than X2-X2' of the press-formed product 1. However, in this direction, the material can easily be pulled during press forming, and excess material is easily dispersed. Such advantages also apply to a metal part with the product shape 2 having no flanges, as shown in Fig. 5, namely, the press-formed product 1.<Product shape 3>

[0041] The same applies to a metal part with the product shape 3 as shown in Fig. 2(c) or 3(c), namely, the press-formed product 1. Forming the intermediate formed product 100 with the intermediate shape as illustrated in Fig. 2(a) or 3(a) in the intermediate forming step can suppress the occurrence of wrinkles. The first intersection point 6 of the additional shape 9 in the intermediate formed product 100 may be positioned closer to the upper top plate surface 11 than the first intersection point 5 of the press-formed product 1 in the height direction of the metal part, that is, the pressing direction HD. Either pattern 1, in which the first intersection point 6 of the additional shape is shifted to a position on a line connecting the first intersection point 5 and the ridge-line convex portion 4 of the press-formed product 1, or pattern 2, in which the first intersection point 6 of the additional shape is simply shifted to a position above the first intersection point 5 of the press-formed product 1, namely, a position on the stepped surface in the height direction of the metal part, may be used, for example. Patterns 1 and 2 have the same effect as the product shape 1 in suppressing the occurrence of wrinkles. Pattern 1 is preferable as a measure against wrinkles. Meanwhile, it should be noted that pattern 1 also involves changes to the shapes of the rear-side vertical wall surface 22 and the flange portion 30. For example, when the flange portion has spot welding points and thus requires a high level of dimensional accuracy, it is preferable to avoid forming the flange portion through two steps. If possible, the metal part, namely, the press-formed product 1 may have the intermediate shape of the pattern 1 or 2 as it is.<Steel grade and sheet thickness>

[0042] Although a metal sheet used in the present embodiment preferably has a tensile strength in the range of 270 to 1800 MPa and a sheet thickness in the range of 0.6 to 4.0 mm, a metal material with a tensile strength of less than 270 MPa or a sheet thickness of less than 0.6 mm is also employed. However, there are not many metal parts having a tensile strength and a sheet thickness that are less than the foregoing ranges among parts of common automobiles and home electrical appliances, for example. A metal sheet having a tensile strength of over 1800 MPa has low ductility. Therefore, cracks are likely to occur while a ridge line portion is reformed to obtain a product shape during the second forming step of the present embodiment. A metal sheet with a sheet thickness of over 4.0 mm is subjected to a high workload during the second forming step of the present embodiment. Thus, wear damage, such as dents in the dies, may occur.Example

[0043] Fig. 8 illustrates examples of the dimensions of the press-formed products used for review. For the purpose of preventing cracks in the metal sheet, each boundary between two faces was formed as a curved surface. Specifically, the boundary protruding toward the outer side of the product had a curvature radius of 5 mm with respect to the inner surface of the product, while a recessed boundary had a curvature radius of 6 mm with respect to the outer surface of the product. Note that each boundary between two faces of the intermediate formed product was also formed to have a similar curvature radius. Table 1 shows the sheet thickness and mechanical properties of each material used. [Table 1]Material symbolSheet thicknessYPTSElmmMPaMPa%SA0.615229545SB1.2710108012SC1.6105014907

[0044] In Comparative Examples, a metal sheet blank was formed into a press-formed product with a product shape in one forming step. In Invention Examples, a press-formed product with a product shape was formed in two forming steps including the intermediate forming step and the product forming step described in the foregoing embodiment. The intermediate forming step involves forming for the product shapes 1 and 2, and drawing for the product shape 3. The product forming step employs forming for all the product shapes.

[0045] Table 2 shows the results of the review of the product shapes 1 and 2. A step in the top plate portion, namely, the distance H between the upper top plate surface 11 and the lower top plate surface 13 in the pressing direction was set to two levels: 10 mm and 8 mm. Press forming was performed by varying the value of ΔH(%), based on the values shown in Table 2, where ΔH(%) is defined as the ratio of the shifting distance h, in the pressing direction, between the first intersection point 6 of the intermediate formed product 100 and the first intersection point 5 of the press-formed product 1, with respect to H. In Table 2, LA represents the distance between the first intersection point 5 of the press-formed product 1 and the second intersection point 7 of the intermediate formed product, while LB represents the distance between the first intersection point 5 of the press-formed product 1 and the third intersection point 8 of the intermediate formed product.

[0046] Wrinkles were evaluated based on two items. The first item is an increase ratio in sheet thickness where wrinkles are likely to occur after the final step. An increase ratio in sheet thickness of 35% or greater was considered unacceptable. When forming a metal sheet using a press die, it is common to provide a gap (clearance) between the upper and lower dies that corresponds to the sheet thickness of the metal sheet, to avoid compression in the sheet thickness direction of the metal sheet. An increase in sheet thickness may result in the sheet becoming the same size as, or larger than the gap provided between the dies. If the sheet thickness becomes too large, failures may occur to the dies. If the increase ratio in sheet thickness is less than 35%, failures, such as dents in the dies or damage to the dies, are unlikely to occur as the strength of the dies is sufficiently greater than the force of compressing the metal sheet. Meanwhile, if the increase ratio in sheet thickness is 35% or greater, such failures are likely to occur. The second item was the evaluation for the appearance of the product after the final step. Cases where the metal sheet buckled outward from the sheet plane and wrinkles overlapped, or where buckling occurred without folding of wrinkles, were evaluated as "Poor". All other cases were evaluated as "Good." [Table 2]No.Height HShapeMaterialHeight ratio ΔHLALBIncrease ratio of thicknessEvaluationRemarksmm%mmmm%1101SA000PoorComparative Example2101SA40101025GoodInvention Example3101SA76252519GoodInvention Example4101SA100303012GoodInvention Example582SC000PoorComparative Example682SC174433GoodInvention Example782SC308822GoodInvention Example882SC63202013GoodInvention Example

[0047] In test Nos. 1 and 5, significant wrinkles occurred and overlapped one another. Therefore, neither the measurement of the increase ratio in sheet thickness nor the evaluation of the appearance was performed. In Invention Examples, the appearance was deemed acceptable, with only minor wrinkles observed. A greater ΔH indicates a lower increase ratio in sheet thickness, and in such cases, the evaluations in terms of wrinkles were better.

[0048] Fig. 9 shows the results of the analysis of the distribution of the increase-decrease rate (%) of the sheet thickness for each of test Nos. 1 and 2, using the FEM (finite element method). Fig. 9(a) illustrates the distribution of the sheet thickness of test No. 1, where wrinkles D overlap one another. Fig. 9(b) illustrates the distribution of the sheet thickness of test No. 2, where no wrinkles occurred and the maximum increase ratio in sheet thickness was found to be 25%.

[0049] Tables 3 and 4 summarize the results of the review of the product shape 3. Table 3 shows a case where the intermediate shape corresponds to pattern 1, and Table 4 shows a case where the intermediate shape corresponds to pattern 2. Features in each table are similar to those in Table 2. In Tables 3 and 4, LC represents the distance between the first intersection point 5 of the press-formed product 1 and the first intersection point 6 of the intermediate formed product projected onto the baseline 12b. [Table 3]No.Height HShapeMaterialHeight ratio ΔHLALBLCIncrease ratio in sheet thicknessEvaluationRemarksmm%mmmmmm%973SB000045PoorComparative Example1073SB2155231GoodInvention Example1173SB552020511GoodInvention Example1273SB80303088GoodInvention Example [Table 4] No.Height HShapeMateri alHeight ratio ΔHLALBLCIncrease ratio in sheet thicknessEvaluationRemarksmm%mmmmmm%1373SB000036PoorComparative Example1473SB922030GoodInvention Example1573SB1533020GoodInvention Example

[0050] In each of test Nos. 9 and 13, ΔH was 0 and increase ratio in sheet thickness was as high as 35% or greater, with wrinkles occurred. Meanwhile, in tests conducted by setting ΔH to 9% or greater, the increase ratio in sheet thickness was able to be reduced, with no wrinkles occurred.Industrial Applicability

[0051] With the press forming method, the method for producing a press-formed product, the intermediate formed product, and the press-forming die according to the present invention, it is possible to prevent the occurrence of wrinkles in a vertical wall portion around a stepped portion where the height of a top plate portion changes and in a stepped surface. This increases the yields of press-formed products, and thus increases productivity. Thus, the present invention is industrially advantageous.Reference Signs List

[0052] 1press-formed product 100intermediate formed product 10top plate portion 11upper top plate surface 11aupper ridge line 12stepped surface 12aapex line 12bbaseline 13lower top plate surface 13alower ridge line 20vertical wall portion 21front-side vertical wall surface 21alateral ridge line 22rear-side vertical wall surface 22avalley line 30flange portion 2convex portion 3concave portion 4ridge-line convex portion (intersection between upper ridge line and apex line) 5first intersection point (between baseline and lower ridge line of press-formed product) 6first intersection point (of additional shape of intermediate formed product) 7second intersection point (on lower ridge line of additional shape of intermediate formed product) 8third intersection point (on baseline of additional shape of intermediate formed product) 9additional shape (of intermediate formed product) Dwrinkle Mflow-in direction (of material) HDproduct height direction (i.e., pressing direction)

Claims

1. A method for press forming a press-formed product, the press-formed product comprising a top plate portion and a vertical wall portion coupled to the top plate portion in a width direction of the top plate portion via a ridge line portion, wherein the top plate portion comprises a first top plate surface on an upper side, a stepped surface, and a second top plate surface on a lower side, these surfaces being formed by bending for forming convex and concave portions in a longitudinal direction, the method comprising a first forming step of press forming a metal sheet blank into an intermediate formed product with an intermediate shape, and a second forming step of forming the intermediate formed product into the press-formed product, characterized in that the intermediate formed product has an additional shape, in which a first intersection point of the press-formed product is shifted toward the first top plate surface in a pressing direction, the first intersection point being defined by a lower ridge line serving as a boundary between the second top plate surface and the vertical wall portion, and by a baseline of the stepped surface.

2. The press forming method according to claim 1, wherein in the intermediate formed product, the shifted first intersection point lies on a lateral ridge line formed by the stepped surface and the vertical wall surface.

3. The press forming method according to claim 1, wherein the vertical wall portion of the press-formed product comprises a first vertical wall surface on a front side, a stepped surface that is common to the top plate portion, and a second vertical wall surface on a rear side, these surfaces being formed by bending for forming convex and concave portions, respectively, which are performed continuously to bending for forming the convex and concave portions of the top plate portion, and the intermediate formed product has an additional shape such that a first intersection point between the lower ridge line, serving as a boundary between the second top plate surface and the second vertical wall surface, and the baseline of the stepped surface lies either at a position on the stepped surface along an extension of the second vertical wall surface, or at a position closer to the first vertical wall surface than the said position on the stepped surface along the extension of the second vertical wall surface.

4. The press forming method according to any one of claims 1 to 3, wherein the additional shape has a second intersection point on the lower ridge line and a third intersection point on the baseline.

5. The press forming method according to any one of claims 1 to 3, wherein a ratio ΔH of a shifting distance h of the first intersection point of the intermediate formed product in the pressing direction relative to a distance H between the first top plate surface and the second top plate surface around the stepped surface in the pressing direction is in a range of 9 to 100%.

6. A method for producing a press-formed product, comprising obtaining a press-formed product by press-forming a metal sheet blank using the press forming method according to any one of claims 1 to 3.

7. An intermediate formed product of a press-formed product, comprising a top plate portion, and a vertical wall portion coupled to the top plate portion in a width direction of the top plate portion via a ridge line portion, the top plate portion comprising a first top plate surface on an upper side, a stepped surface, and a second top plate surface on a lower side, these surfaces being formed by bending for forming convex and concave portions in the longitudinal direction, characterized in that the intermediate formed product has an additional shape such that a first intersection point of the press-formed product is shifted in a pressing direction toward the first top plate surface, the first intersection point being defined by a lower ridge line, which serves as a boundary between the second top plate surface and the vertical wall portion, and by a baseline of the stepped surface.

8. The intermediate formed product according to claim 7, wherein the shifted first intersection point lies on a lateral ridge line formed by the stepped surface and the vertical wall surface.

9. The intermediate formed product according to claim 7, wherein the vertical wall surface of the press-formed product comprises a first vertical wall surface on a front side, a stepped surface that is common to the top plate portion, and a second vertical wall surface on a rear side, these surfaces being formed by bending for forming convex and concave portions, respectively, which are performed continuously to bending for forming the convex and concave portions of the top plate portion, and the intermediate formed product has an additional shape such that the first intersection point between the lower ridge line, defined by the second top plate surface and the second vertical wall portion, and the baseline of the stepped surface, lies either at a position on the stepped surface along an extension of the second vertical wall surface, or at a position closer to the first vertical wall surface than the said position on the stepped surface along the extension of the second vertical wall surface.

10. A press-forming die for press-forming a metal sheet blank into an intermediate formed product with an intermediate shape, when producing a press-formed product comprising a top plate portion and a vertical wall portion coupled to the top plate portion in a width direction of the top plate portion via a ridge line portion, the top plate portion comprising a first top plate surface on an upper side, a stepped surface, and a second top plate surface on a lower side, these surfaces being formed by bending for forming convex and concave portions in a longitudinal direction, characterized in that the intermediate formed product has an additional shape such that a first intersection point of the press-formed product is shifted in a pressing direction toward the first top plate surface, the first intersection point being defined by a lower ridge line, serving as a boundary between the second top plate surface and the vertical wall portion, and a baseline of the stepped surface.

11. The press-forming die according to claim 10, wherein in the intermediate formed product, the shifted first intersection point lies on a lateral ridge line formed by the stepped surface and the vertical wall surface.

12. The press-forming die according to claim 10, wherein the vertical wall portion of the press-formed product comprises a first vertical wall surface on a front side, a stepped surface that is common to the top plate portion, and a second vertical wall surface on a rear side, these surfaces being formed by bending for forming convex and concave portions, respectively, which are performed continuously to bending for forming the convex and concave portions of the top plate portion, and the intermediate formed product has an additional shape such that a first intersection point between a lower ridge line, serving as a boundary between the second top plate surface and the second vertical wall portion, and the baseline of the stepped surface lies either at a position on the stepped surface along an extension of the second vertical wall surface, or at a position closer to the first vertical wall surface than the said position on the stepped surface along the extension of the second vertical wall surface.

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