Press forming apparatus and method for manufacturing press-formed product

Abstract

The present invention provides a press-forming apparatus for stretch-flange-formed members using sheet material having a tensile strength of 980 MPa or more, including a curved portion with a radius of curvature of 20 to 60 mm, a method for manufacturing a press-formed product, and a press-formed product. In the press-forming apparatus and method for manufacturing a press-formed product of the present invention, the punch-side edge of the die is convexly curved toward the punch with a radius of curvature of 20 to 60 mm, the gap G between the punch-side edge of the die and the blank holder varies along the die edge and is smallest at the curved portion, and the gap between the pressing surfaces of the die and the blank holder is controlled by a shim-like gap retainer with a thickness s satisfying 0.1t≦s<1.0t, where t is the blank thickness. The press-formed product of the present invention has a top plate portion with a tensile strength of 980 MPa or more, a concave outer peripheral edge portion with a radius of curvature of 20 mm to 60 mm, and a surface cross-sectional hardness of the concave vertical wall portion that is 1.13 times or more the top plate portion.

Description

[Technical Field]

[0001] The present invention relates to a press-forming apparatus, a method for manufacturing a press-formed product, and a press-formed product. In particular, the present invention relates to a manufacturing technology for a press-formed product having a stretch-flange-formed portion, in which a top plate has a concave outer peripheral edge portion where a portion of the outer peripheral edge is curved inward, and a flange portion continuing from the outer peripheral edge of the top plate is stretch-flanged. Examples of press-formed products having such stretch-flange-formed portions include press-formed products used as automobile suspension parts, which have an L-shaped or T-shaped portion in plan view. The present invention is particularly suitable for manufacturing press-formed products made from ultra-high tensile steel having a tensile strength of 980 MPa or higher. [Background technology]

[0002] For example, press-formed products are used as structural components for automobiles. Press-formed products are manufactured by press-forming a blank using a punch and a die. The blank is pressed into the die by the punch and formed into the desired shape.

[0003] Patent Document 1 discloses a method for manufacturing a press-formed product using a pad in addition to a punch and a die. In Patent Document 1, the side surface of the punch and the side surface of the die cooperating with the punch include curved surfaces. In Patent Document 1, a blank that has been given a step shape in advance is clamped between the punch and the pad, and the die is moved relative to the punch along the side surface of the punch. In this way, a press-formed product is formed from the blank.

[0004] Patent Document 2 also discloses a method for manufacturing a press-formed product using a punch, a die, and a pad. In Patent Document 2, the edge of the upper surface of the punch has a shape with a recess in the center in a plan view. The die has a shape corresponding to the punch. In Patent Document 2, a blank placed on the upper surface of the punch is pressed with a pad to form a bead on the blank. The blank is then bent by the punch and die to form a press-formed product.

[0005] In Patent Documents 1 and 2, the punch-side edge of the die includes a curved portion. The curved portion is a portion that curves convexly toward the punch side in a plan view of the die. In this case, when a blank is pressed by the punch during press forming and is drawn toward the punch-side edge of the die, the portion of the blank that is drawn toward the curved portion is stretched in the direction of extension of the curved portion. If the blank is stretched and its thickness is reduced, cracks may occur in the blank.

[0006] To address these issues, the present inventors disclosed in Patent Document 3 a method for manufacturing a press-formed product using a punch, a die, and a blank holder. In Patent Document 3, when viewed from the pressing direction, the punch-side edge of the die includes a die edge convex portion protruding toward the punch. The die edge convex portion includes a die edge curved portion that is convex toward the punch and extends with a curvature radius of 400 mm or less. In Patent Document 3, when viewed from the pressing direction, the distance between the edge of the blank holder and the edge of the blank sandwiched between the blank holder and the die, on a line perpendicular to the direction of extension of the edge of the blank, varies along the edge of the blank, and this distance is largest at the blank edge curved portion. Therefore, the blank edge curved portion that is drawn into the vicinity of the die edge convex portion is clamped between the die and the blank holder, allowing compressive stress in the thickness direction to be applied to the blank edge curved portion until the end of press forming. This allows the occurrence of cracks in the blank, more specifically, cracks at the edge of the blank, to be suppressed during the production of press-formed products. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Patent No. 6052478 [Patent Document 2] Patent No. 6202059 [Patent Document 3] Patent No. 7352123 Summary of the Invention [Problem to be solved by the invention]

[0008] However, even in the invention of Patent Document 3, there was room for improvement in the problem of press cracking when manufacturing components that use ultra-high tensile steel with a tensile strength of 980 MPa or higher and that involve severe stretch flange forming processes with a flange curvature radius of 20 mm to 60 mm.

[0009] The present invention aims to provide a press forming apparatus, a method for manufacturing a press-formed product, and a press-formed product that are capable of suppressing press cracks in stretch flange-formed parts made of ultra-high tensile steel and including curved portions with a curvature radius of 20 mm to 60 mm. [Means for solving the problem]

[0010] [1] A press forming apparatus comprising a punch, a die, and a blank holder, the die includes a curved portion, the edge of the die on the punch side being convexly curved toward the punch side and extending with a curvature radius of 20 mm or more and 60 mm or less when viewed from the pressing direction, the blank holder is disposed such that, when viewed from the press direction, a distance G between the punch-side edge of the blank holder and the edge of the die on a line perpendicular to an extension direction of the edge of the die varies along the edge of the die and is smallest at the curved portion; The distance between the pressing surface of the die and the pressing surface of the blank holder is controlled by a shim-shaped spacing retainer having a thickness s, The spacing retaining portion is provided on one or more surfaces of the pressing surface of the die, the pressing surface of the blank holder, or an extended surface of the pressing surface of the blank holder, within the range of the pressing surface of the die and excluding the clamping portion of the blank, when viewed from the press direction; The thickness s of the spacing retaining portion satisfies 0.1t≦s<1.0t, where t is the plate thickness of the blank. Press forming equipment.

[0011] [2] The press forming apparatus according to [1], wherein, when viewed from the pressing direction, the distance G increases as the distance from the curved portion increases.

[0012] [3] The press forming apparatus according to [1] or [2], wherein, when viewed from the press direction, the line length Lh of the section of the edge of the blank holder where the gap is the smallest Gmin is 0.5 to 2.5 times the line length Ld of the curved portion.

[0013] [4] A method for manufacturing a press-formed product having a stretch flange formed portion using a plate material having a tensile strength of 980 MPa or more in a blank before press forming, The method comprises: sandwiching the blank between a die and a blank holder; controlling a pressing surface distance between a pressing surface of the die and a pressing surface of the blank holder by a shim or a shim-like distance retaining portion; and pressing the blank in a pressing direction by a punch; the die includes a curved portion, the edge of the die on the punch side when viewed from the pressing direction, which is convexly curved toward the punch side and extends with a curvature radius of 20 mm or more and 60 mm or less; the blank holder is disposed such that, when viewed from the press direction, a distance G between the punch-side edge of the blank holder and the edge of the die on a line perpendicular to an extension direction of the edge of the die varies along the edge of the die and is smallest at the curved portion; When the pressing surface interval is controlled by the shim, the shim is arranged in a region where the pressing surface of the die and the pressing surface of the blank holder overlap, excluding the clamping portion of the blank, when viewed from the pressing direction; When the pressing surface interval is controlled by the interval maintaining unit, the interval maintaining unit is disposed on one or more surfaces of the pressing surface of the die, the pressing surface of the blank holder, or an extended surface of the pressing surface of the blank holder within the range of the pressing surface of the die and excluding the clamping portion of the blank when viewed from the pressing direction, The thickness s of the shim or the spacing retaining portion satisfies 0.1t≦s<1.0t, where t is the plate thickness of the blank. Manufacturing method for press-molded products.

[0014] [5] The method for manufacturing a press-molded product according to [4], wherein, when viewed from the pressing direction, the distance G increases as the distance from the curved portion increases.

[0015] [6] A method for manufacturing a press-formed product according to [4] or [5], wherein, when viewed from the press direction, the line length Lh of the section of the edge of the blank holder where the gap is smallest Gmin is 0.5 to 2.5 times the line length Ld of the curved portion.

[0016] [7] A press-formed product having a stretch flange-forming portion including a top plate portion having a concave outer peripheral edge portion curved so that a part of the outer peripheral edge is concave inward, and a concave vertical wall portion continuous with the concave outer peripheral edge portion of the top plate portion, The top plate portion has a tensile strength of 980 MPa or more, The radius of curvature of the concave outer peripheral edge portion is 20 mm or more and 60 mm or less, A press-molded product, wherein the surface cross-sectional hardness of the concave vertical wall portion is 1.13 times or more the surface cross-sectional hardness of the top plate portion.

[0017] [8] A press-molded product according to [7], wherein the end of the concave vertical wall portion is thinner than the plate thickness of the top plate portion at the maximum load in a tensile test.

[0018] [9] The press-molded product according to [7] or [8], wherein the concave vertical wall portion has a height of 20 mm or more.

[0019] According to the present invention, it is possible to provide a press forming apparatus, a method for manufacturing a press-formed product, and a press-formed product that are capable of suppressing press cracks in stretch flange-formed parts made of ultra-high tensile steel and including curved portions with a curvature radius of 20 mm to 60 mm. [Brief explanation of the drawings]

[0020] [Figure 1]This is a diagram that explains the direction in which the blank is pulled into the curved portion of the die hole and the direction in which the blank is stretched along the extension direction of the curved portion in a plane perpendicular to the press direction when a stretch flange forming portion including a curved portion is formed using a normal punch and die. [Figure 2] FIG. 1 is a diagram illustrating the positional relationship between the die and the blank holder in a plane perpendicular to the pressing direction, which was studied in the first preliminary study leading to the present invention. [Figure 3] FIG. 10 is a diagram showing the effect of the line length of the blank holder edge (ratio to the curved portion line length) in the closest section between the die edge and the blank holder edge on damage to the flange edge of a press-formed product (damage value I), which was investigated in the second preliminary study leading to the present invention. [Figure 4] FIG. 10 is a diagram showing the effect of the shim thickness (ratio to the blank thickness) on the plate thickness reduction rate of the flange edge of a press-formed product, which was investigated in the third preliminary investigation that led to the present invention. [Figure 5] FIG. 10 is a diagram showing the effect of shim thickness (ratio to blank thickness) on damage (damage value I) to the flange edge of a press-formed product, which was investigated in the third preliminary study leading to the present invention. [Figure 6] 1 is a perspective view schematically illustrating a press forming apparatus according to an embodiment of the present invention. [Figure 7] FIG. 7 is a cross-sectional view of the press-forming apparatus shown in FIG. [Figure 8] FIG. 7 is a diagram showing a schematic view of the relationship between the punch, die, blank holder, and shim-shaped spacing retainer included in the press forming apparatus shown in FIG. 6, and the edge of the blank placed on the press forming apparatus, when viewed from the pressing direction. [Figure 9A] 1 is a perspective view schematically illustrating a blank material to be supplied to a press-forming device in a method for manufacturing a press-formed product according to an embodiment of the present invention; FIG. [Figure 9B] 1 is a perspective view schematically showing a state in which a blank is placed on a press-forming apparatus at an early stage of press-forming in a method for manufacturing a press-formed product according to an embodiment of the present invention; FIG. [Figure 9C]1 is a cross-sectional view schematically showing a state in which a blank is clamped between a die and a blank holder immediately after the start of press forming in a manufacturing method of a press-formed product according to an embodiment of the present invention. FIG. [Figure 9D] 9C is a cross-sectional view schematically showing a situation in which the punch is moved relatively from the state of FIG. 9C to force the blank into the die hole in the middle of press forming in a method for manufacturing a press-formed product according to an embodiment of the present invention. [Figure 9E] 9D is a cross-sectional view schematically showing a state where the punch has been further moved relatively from the state of FIG. 9D to complete forming at the completion stage of press forming in the method for manufacturing a press-formed product according to an embodiment of the present invention. FIG. [Figure 9F] 9E is a perspective view schematically illustrating a press-formed product obtained in the press-formed product manufacturing method according to the embodiment of the present invention, in the state where forming is completed as shown in FIG. 9E. FIG. [Figure 9G] 9D is a perspective view schematically illustrating a press-formed product obtained when forming is completed in the state of FIG. 9D in the method for manufacturing a press-formed product according to the embodiment of the present invention. FIG. [Figure 10] FIG. 10 is a schematic perspective view showing a situation in which a blank placed on a press-forming device is clamped between a punch and a pad at the initial stage of press-forming in a method for manufacturing a press-formed product according to a modified embodiment of the present invention. [Figure 11] 11 is a cross-sectional view schematically showing a state in which the blank is further clamped between the die and the blank holder from the state shown in FIG. 10 immediately after the start of press forming in a method for manufacturing a press-formed product according to a modified embodiment of the present invention. [Figure 12] 1 is a perspective view schematically illustrating a press-formed product according to an embodiment of the present invention. FIG. [Figure 13] FIG. 10 is a diagram showing the circumferential distribution of the average hardness of the surface layer portion of the press-formed product according to the embodiment of the present invention in a curved portion in comparison with a comparative example. [Figure 14] FIG. 10 is a diagram showing the reduction rate of stress generated at the flange end depending on the flange height, which can be used to estimate the effect of improving the fatigue durability of a press-formed product by increasing the flange height of the press-formed product according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0021] Prior to arriving at the present invention, the inventors have been studying methods for manufacturing members that have a small radius of curvature in the flange portion continuing from the outer peripheral edge of the top plate portion and that involve severe stretch flange forming. Therefore, the following first to third preliminary study results that are particularly relevant to the present invention will be described.

[0022] (First Preliminary Study) FIG. 1 shows a punch 10 and a die 20 used in typical press forming, projected onto a plane perpendicular to the press direction. When press forming is performed using the punch 10 and the die 20, the blank is drawn toward the edge 24 of the die 20 on the punch 10 side, as indicated by arrow A1 in FIG. 1. When the edge 24 of the die 20 includes a curved portion 24a and straight portions 24b on both sides of the curved portion 24a as viewed from the press direction, the portion of the blank drawn toward the curved portion 24a is stretched in the direction of extension of the curved portion 24a, as indicated by double-headed arrow A2 in FIG. 1. Meanwhile, the portion of the blank drawn toward the straight portions 24b experiences virtually no tensile stress along the direction of extension of the straight portions 24b. Therefore, the blank thickness is thin at the curved portions 24a and relatively thick at the straight portions 24b. The dashed lines in FIG. 1 are examples of contour lines of the blank thickness during press forming. The isothickness line is a line that connects points of equal thickness in the blank during press forming. In Figure 1, the thicker the isothickness line, the thicker the blank is at that point.

[0023] When a blank is stretched and its thickness is reduced, cracks may occur in the portion of the blank where the thickness is reduced. Therefore, the inventors came up with the idea of ​​applying compressive stress in the thickness direction of the blank as one method of suppressing cracks in the blank.

[0024] That is, as described with reference to FIG. 1, when the punch-side edge 24 of the die 20 includes a curved portion 24a as viewed from the pressing direction, the blank thickness decreases as the blank is drawn toward the curved portion 24a during press forming. In contrast, as shown in FIG. 2, for example, the distance G between the punch-side edge 34 of the blank holder 30 and the punch-side edge 24 of the die 20 as viewed from the pressing direction varies along the edge 24 of the die 20 and is minimized at the curved portion 24a. This allows the die 20 and blank holder 30 to clamp the portion of the blank drawn toward the curved portion 24a, i.e., the reduced-thickness portion, up to the vicinity of the curved portion 24a, and apply compressive stress in the thickness direction to the reduced-thickness portion up to the vicinity of the curved portion 24a. This can suppress cracking of the blank during the production of press-formed products.

[0025] (Second preliminary study) The present inventors decided to conduct further analytical studies on the effect of suppressing the occurrence of cracks in a blank during press forming using a mold in which a blank holder 30 and a die 20 are arranged, as shown in Fig. 2. In the following description, the blank holder may be abbreviated to simply "holder."

[0026] Before conducting an analytical study, let us classify the deformation mechanisms of the blank during press forming when using the die configuration shown in Figure 2. Broadly speaking, the following two cases can be assumed. (a) When viewed from the press direction, if the closest section between the punch side edge 24 of the die 20 and the punch side edge 34 of the holder 30 is extremely short, it is thought that the effect of suppressing the occurrence of blank cracks will be small compared to when bending is performed without using the holder 30. (b) On the other hand, when viewed from the press direction, if the closest section between the punch side edge 24 of the die and the punch side edge 34 of the holder 30 is extremely long, it is thought that the effect of suppressing the occurrence of blank cracks will be less different from when normal drawing is performed.

[0027] Therefore, when carrying out press forming analysis, the inventors used the Cockcroft-Latham ductile fracture criterion (see Equation 1) to study the appropriate length of the section that minimizes the distance between the punch-side edge 24 of the die 20 and the punch-side edge 34 of the holder 30. Note that the Cockcroft-Latham ductile fracture criterion is described in "M. G. Cockcroft and D. J. Latham, "Ductility and the Workability of Metals," Journal Institute of Metals, Vol. 96, 1968, pp. 33-39," and therefore a detailed description thereof will be omitted here.

[0028] The magnitude of damage to the flange edge of a press-formed product after forming is expressed as a damage value I, which is given by the following mathematical formula 1. Here, the crack suppression effect confirmed in the first preliminary study was evaluated in terms of the reduction rate (%) from the damage value I for normal drawing.

[0029]

number

[0030] The positional relationship between the die 20 and the holder 30, which are the subject of this analysis, is outlined in the supplementary diagram in Figure 3. For example, the length Lh of the holder edge 34 in the section where the distance between the punch-side edge 24 of the die 20 and the punch-side edge 34 of the holder 30, as viewed from the press direction, is minimum (Gmin), and is expressed as the ratio Lh / Ld to the length Ld of the curved portion 24a of the punch-side edge 24 of the die 20. Furthermore, the curved portion 24a of the edge 24 of the die 20, as viewed from the press direction, is assumed to extend with a constant radius of curvature. Furthermore, the length Ld of the curved portion 24a of the edge 24 of the die 20 is calculated as the product of the angle (included angle θ (rad)) at which the tangents at both ends of the curved portion 24a of the edge 24 intersect and the radius of curvature of the curved portion 24a of the edge 24.

[0031] If Lh / Ld, which represents the length of the section of closest distance Gmin (assumed to be a substantially parallel section) between the punch-side edge 24 of the die 20 and the punch-side edge 34 of the holder 30, is less than 1.0, the edge 24a of the die and the edge 34 of the holder 30 will be closest in a portion of the curved portion of the die 20. On the other hand, if Lh / Ld, which represents the length of this section of closest distance Gmin (closest section), is 1.0, this means that the punch-side edge 34 of the holder 30 is closest in almost the entire curved portion 24a of the punch-side edge 24 of the die 20. Furthermore, if Lh / Ld, which represents the length of the section of closest distance Gmin, is greater than 1.0, the punch-side edge 34 of the holder 30 will be closest in the entire curved portion 24a of the punch-side edge 24 of the die 20 and in a portion of the straight portion 24b adjacent to this curved portion 24a.

[0032] The press forming analysis here was performed for the case where the edge of the blank is pulled out from between the die 20 and the holder 30 to complete the forming. The analysis was also performed for cases where the included angle θ of the curved portion 24a of the edge 24 of the die 20, as viewed from the press direction, was 60°, 90°, and 120°. Figure 3 shows the analysis results. As can be seen from Figure 3, when press forming is performed according to the results of the first preliminary study, the damage value I is reduced compared to conventional drawing. In particular, when Lh / Ld, which represents the length of the section (closest section) where the distance between the punch-side edge 24 of the die 20 and the punch-side edge 34 of the holder 30 is the minimum Gmin, is between 0.5 and 2.5, the reduction rate of the damage value I compared to conventional drawing is significantly improved. In other words, when viewed from the press direction, it is preferable that the line length Lh of the part of the punch side edge 34 of the holder 30 that is the smallest distance from the punch side edge 24 of the die 20 is between 0.5 and 2.5 times the line length Ld of the edge 24a of the curved portion provided on the edge 24 of the die 20.

[0033] (Third Preliminary Study) Furthermore, based on the results of the first preliminary study, it was found that when attempting to further increase the flange height of the stretch flange processing portion, there is an appropriate range for applying compressive stress by clamping the reduced thickness portion up to the vicinity of the curved portion 24a between the die 20 and the blank holder 30. Therefore, as a means for alleviating the compressive stress applied to the reduced thickness portion within an appropriate range, it was considered to limit the maximum compressive stress or compressive strain when the edge of the blank is pulled out from between the die and the blank holder by interposing a shim between the die and the blank holder.

[0034] Figures 4 and 5 show experimental results of press-forming a model part with a 30 mm radius of curvature of the vertical wall of the stretch flange from a blank with a tensile strength of 980 MPa and a thickness of 2.9 mm, using the press-forming apparatus shown in Figures 6 to 8 (described later). Specifically, Figure 4 plots the thickness reduction rate of the flanged part as a function of the shim thickness (ratio to thickness) s / t (%), where shim thickness s is non-dimensionalized by the blank thickness t. Figure 4 shows that the use of shims can effectively reduce the maximum compressive stress or compressive strain when the edge of the blank is pulled out from between the die and the blank holder.

[0035] Figure 5 shows the change in the reduction rate of the damage value I of Equation 1 when the shim thickness (s) is reduced relative to the blank thickness (t), with the damage value I of Equation 1 when the shim thickness (ratio to plate thickness) (s / t) is 100%. Figure 5 shows that the damage value I is reduced when the shim thickness (ratio to plate thickness) (s / t) is greater than 0% and less than 100%. However, when the shim thickness (ratio to plate thickness) (s / t) is greater than 0% and less than 10%, it becomes difficult to control the shim thickness, which is not practically desirable. Figure 5 also shows that the reduction rate of the damage value I can be maintained at a high level of approximately 20% when the shim thickness (ratio to plate thickness) (s / t) is between 10% and 50% (experimental value: less than 50%). Furthermore, it can be seen that the reduction rate of the damage value I can be maintained at 10% or more when the shim thickness (ratio to plate thickness) (s / t) is between approximately 60% and 75%.

[0036] The present invention was made by further studying the results of the first to third preliminary studies described above.

[0037] Hereinafter, embodiments of the present invention will be described with reference to the drawings. In each drawing, the same or equivalent components are designated by the same reference numerals, and the same description will not be repeated.

[0038] (Press-molded product) The press-formed product according to this embodiment has a stretch flange-formed portion including a top plate portion having a concave outer peripheral edge portion curved inwardly, and a concave vertical wall portion continuous with the concave outer peripheral edge portion of the top plate portion. A typical example of the press-formed product according to this embodiment is a suspension arm (see FIG. 12 ), a major structural component of a suspension system that supports and absorbs shocks between an automobile's wheels and the vehicle body. The suspension system, also known as a suspension, is primarily composed of three components: a spring, a shock absorber, and a suspension arm. Depending on the wheel suspension system, a front suspension arm may consist of an upper arm and a lower arm, or it may consist of only a lower arm. In the latter case, the suspension arm and the lower arm refer to the same component, just with different names. A rear suspension arm may consist of an upper arm, a lower arm, and a trailing arm.

[0039] The radius of curvature of the concave outer periphery of the press-formed product according to this embodiment is 20 mm or more and 60 mm or less, which means that the radius of curvature of the concave outer periphery of the press-formed product according to this embodiment is equivalent to the radius of curvature of the concave outer periphery used in a general suspension arm.

[0040] The tensile strength of the top plate portion of the press-formed product according to this embodiment is 980 MPa or higher. This is because, even if the radius of curvature of the concave outer peripheral edge portion is 20 mm or more and 60 mm or less, and the press-formed product is generally difficult to process, if the tensile strength of the top plate portion is less than 980 MPa, such a case is excluded from this embodiment.

[0041] (Press forming equipment) FIG. 6 is a schematic perspective view of a press forming apparatus 100 according to this embodiment. As shown in FIG. 6, the press forming apparatus 100 includes a punch 10, a die 20, a blank holder 30, and a shim-shaped spacing retainer 40. Although only one side of the die 20 is shown with respect to the imaginary line CL in FIG. 6, the die 20 may be disposed on both sides of the punch 10. These dies 20 may be separate bodies or may be integrally formed. In this embodiment, the spaces between these dies 20 may be referred to as die holes. A blank holder 30 is provided corresponding to each of the dies 20.

[0042] The punch 10 and the blank holder 30 are configured to be movable relative to the die 20 in the press direction P. In the press forming apparatus 100, the punch 10 and the blank holder 30 may be moved in the press direction P by a drive mechanism (not shown) such as a fluid pressure cylinder, or the die 20 may be moved in the press direction P. The blank holder 30 is movable relative to the punch 10 in the press direction P by a drive mechanism (not shown) such as a cushion. Although not particularly limited, the press direction P is, for example, a vertical direction. If the press direction P is a vertical direction, it is easy to place a blank 200 (see FIG. 9A ), which will be described later, between the punch 10 and the die 20.

[0043] FIG. 7 is a cross-sectional view of the press forming apparatus 100 shown in FIG. 6. The cross-section of the press forming apparatus 100 is a cross-section of the press forming apparatus 100 taken along a plane parallel to the press direction P and perpendicular to the longitudinal direction of the punch 10. The direction perpendicular to the press direction P on the cross-section of the press forming apparatus 100 is referred to as the width direction of the press forming apparatus 100. As with FIG. 6, FIG. 7 shows only one side of the press forming apparatus 100 relative to the imaginary line CL. The imaginary line CL is, for example, the center line of the press forming apparatus 100 in the width direction.

[0044] As shown in FIG. 7, the punch 10 includes a punch top surface 11, a punch shoulder 12, and a punch side surface 13.

[0045] The punch top surface 11 is a surface that intersects with the press direction P. The punch top surface 11 may be perpendicular to the press direction P. The punch top surface 11 extends substantially in the width direction in a cross-sectional view of the press forming apparatus 100. The punch shoulder 12 is continuous with an end of the punch top surface 11. The punch shoulder 12 has, for example, a substantially arc-shaped shape in a cross-sectional view of the press forming apparatus 100. If the punch shoulder 12 is sharp, there is a risk that the punch shoulder 12 will cut the blank 200 (see FIG. 9A) during press forming. The radius of curvature of the punch shoulder 12 may be 1.0 mm or more. The punch side surface 13 is connected to the punch top surface 11 via the punch shoulder 12. The punch side surface 13 extends from the punch shoulder 12 in the press direction P in a cross-sectional view of the press forming apparatus 100. In a cross-sectional view of the press forming apparatus 100, the punch side surface 13 may be parallel to the press direction P or may be inclined with respect to the press direction P.

[0046] The die 20 includes a pressing surface 21, a die shoulder 22, and a die side surface 23. The pressing surface 21 of the die 20 is, for example, a flat surface substantially perpendicular to the press direction P. The die shoulder 22 is continuous with the end of the pressing surface 21 on the punch 10 side in a cross-sectional view of the press forming apparatus 100. The die shoulder 22 has, for example, a substantially arc-shaped shape in a cross-sectional view of the press forming apparatus 100. If the die shoulder 22 is sharp, there is a risk that the die shoulder 22 will cut the blank 200 (see FIG. 9A ) during press forming. The radius of curvature of the die shoulder 22 may be 1.0 mm or more. The die side surface 23 is connected to the pressing surface 21 via the die shoulder 22. In a cross-sectional view of the press forming apparatus 100, the die side surface 23 extends from the die shoulder 22 in the press direction P. The die side surface 23 has a shape corresponding to the punch side surface 13.

[0047] The blank holder 30 is disposed outside the punch 10 in the width direction of the press forming apparatus 100. The blank holder 30 is disposed so as to face the die 20 in the pressing direction P. The blank holder 30 includes a pressing surface 31, a holder shoulder 32, and a holder side surface 33.

[0048] The pressing surface 31 of the blank holder 30 faces the pressing surface 21 of the die 20 in the pressing direction P. The pressing surface 31 is, for example, a flat surface that is substantially perpendicular to the pressing direction P. In a cross-sectional view of the press forming apparatus 100, the holder shoulder 32 is continuous with the end of the pressing surface 31 on the punch 10 side. The holder side surface 33 is connected to the pressing surface 31 via the holder shoulder 32. In a cross-sectional view of the press forming apparatus 100, the holder side surface 33 extends from the holder shoulder 32 in the pressing direction P. The holder side surface 33 faces the punch side surface 13 in the width direction of the press forming apparatus 100.

[0049] In FIG. 6 , the shim-like spacing retainer 40 is provided on the pressing surface of the blank holder 30 to maintain a predetermined pressing surface distance between the pressing surface of the die 20 and the pressing surface of the blank holder 30. This shim-like spacing retainer 40 may be provided on the pressing surface of the die 20, the pressing surface of the blank holder 30, or an extended surface of the pressing surface of the blank holder 30, in an area within the pressing surface of the die 20 but excluding the blank clamping portion when viewed from the press direction. Also, the shim-like spacing retainer 40 may be divided and disposed at multiple locations within the pressing surface of the die 20 but excluding the blank clamping portion when viewed from the press direction. Also, the shim-like spacing retainer 40 may be divided into a die side and a blank holder side in the press direction as well, and the combined pressing surface distance on both sides may maintain the pressing surface distance. Furthermore, when the shim-shaped spacing retaining portions 40 are provided on the extended surfaces of the blank holders 30, excluding the pressing surfaces thereof, the pressing surfaces of the blank holders 30 can be used for their original purpose of clamping the blank. In this case, the extended surfaces are configured to be structurally linked to the blank holders 30 in an integrated manner.

[0050] The press-forming apparatus 100 may have a shape that is symmetrical with respect to the width center line CL in a cross-sectional view, or may have a shape that is asymmetrical with respect to the width center line CL.

[0051] FIG. 8 is a diagram schematically illustrating the relationship between the punch 10, die 20, blank holder 30, shim-like spacing retainer 40, and edge 201 of the blank when viewed from the press direction P. In other words, FIG. 8 is a diagram illustrating the punch 10, die 20, blank holder 30, and edge 201 of the blank when projected onto a plane perpendicular to the press direction P. In FIG. 8, the edge 24 of the die 20 is indicated by a solid line, and the edge 34 of the blank holder 30 is indicated by a dashed line. The blank holder 30 is hatched to make it easier to distinguish between the die 20 and the blank holder 30. In FIG. 8, the edge 14 of the punch 10 is indicated by a dashed line. The edge 14 of the punch 10 in FIG. 8 is the punch side surface 13, which is flush with the pressing surface 21 of the die 20. In FIG. 8, the shim-like spacing retainer 40 is indicated by a dashed line, and the edge 201 of the blank is indicated by a dashed line.

[0052] As shown in FIG. 8 , the edge 14 of the punch 10 on the die 20 side extends along the edge 24 of the die 20 on the punch 10 side when viewed from the press direction P. The die edge 24 is a die shoulder 22. The edge 14 of the punch 10 has a shape corresponding to the edge 24 of the die 20. For example, when viewed from the press direction P, the edge 14 of the punch 10 is substantially parallel to the edge 24 of the die 20 over its entire length. When viewed from the press direction P, the clearance between the edge 14 of the punch 10 and the edge 24 of the die 20 is equal to or greater than the thickness of the blank, which is the material for press forming, so as not to hinder the relative movement of the punch 10 and the die 20 in the press direction P. When viewed from the press direction P, the edge 24 of the die 20 includes a curved portion 24a and a straight portion 24b.

[0053] As shown in FIG. 8 , the curved portion 24a of the die edge 24 is curved convexly toward the punch 10 when viewed from the press direction P. This curved portion 24a extends with a radius of curvature of less than 400 mm when viewed from the press direction P. In this embodiment, there is at least one curved portion extending with a radius of curvature of 20 mm to 60 mm, which makes press forming particularly difficult. The curved portion 24a may have a constant radius of curvature or a variable radius of curvature. In this embodiment, when viewed from the press direction P, the radius of curvature of an arc passing through each point located at 5.0 mm intervals on the die edge 24 is measured. As a result, a section with a radius of curvature of less than 400 mm is considered to be the curved portion 24a, and a section with a radius of curvature of 400 mm or more is considered to be the non-curved portion (straight portion) 24b.

[0054] The straight portions 24b of the die edge 24 are provided continuously to both ends of the curved portion 24a. When viewed from the pressing direction P, the straight portions 24b on both sides of the curved portion 24a extend from the curved portion 24a so as to become more distant from each other as they move away from the curved portion 24a. When viewed from the pressing direction P, the straight portions 24b may be located on tangents to both ends of the curved portion 24a.

[0055] The blank holder 30 is provided, for example, so as to correspond to only a partial region of the edge 24 of the die 20. When viewed from the press direction P, the blank holder 30 is disposed at a position corresponding to the curved portion 24a of the edge 24 of the die 20. In this embodiment, when viewed from the press direction P, the blank holder 30 has a shape that substantially tapers toward the punch 10. In the example shown in FIG. 8 , the blank holder 30 has a roughly triangular shape when viewed from the press direction P.

[0056] In this embodiment, based on the results of the first preliminary study, the distance G between the edge 34 of the blank holder 30 on the punch 10 side and the edge 24 of the die 20 is varied along the edge 24 of the die 20 when viewed from the press direction P. That is, the distance G between the edge 34 of the blank holder 30 and the edge 24 of the die 20 is not constant over the entire length of the edge 34 of the blank holder 30. The distance G is the distance between the edge 34 of the blank holder 30 and the edge 24 of the die 20 on a line perpendicular to the extension direction of the edge 24 of the die 20 when viewed from the press direction P. In other words, the distance G is the distance from the edge 34 of the blank holder 30 to the edge 24 of the die 20 in the normal direction of the curved portion 24a or the normal direction of the straight portion 24b when viewed from the press direction P. The perpendicular line in the extension direction of the edge 24 of the die 20 is a perpendicular line or normal line to the edge of the die shoulder 22 (see FIG. 7) on the punch 10 side when viewed from the press direction P. The perpendicular line in the extension direction of the edge 24 of the die 20 roughly coincides with the direction in which the blank is drawn toward the die hole when the blank is formed using the press forming apparatus 100 (see FIGS. 6 and 7). The edge 34 of the blank holder 30 is the edge of the holder shoulder 32 (see FIG. 7) on the punch 10 side when viewed from the press direction P.

[0057] Furthermore, in this embodiment, based on the results of the first preliminary study, the distance G between the edge 34 of the blank holder 30 and the edge 24 of the die 20 is set to be smallest at the curved portion 24a when viewed from the press direction P. That is, if a position on the edge 24 of the die 20 where the distance from the edge 34 of the blank holder 30 is smallest is selected and a perpendicular line is drawn to the edge 24 of the die 20, the perpendicular line will pass through the curved portion 24a. The minimum value of the distance G is, for example, smaller than the clearance between the edge 14 of the punch 10 and the edge 24 of the die 20. The minimum value of the distance G may be 0 mm. When the minimum value of the distance G is 0 mm, the edge 34 of the blank holder 30 and the edge 24 of the die 20 overlap at the point where they are closest to each other when viewed from the press direction P. At other points, the edge 24 of the die 20 is positioned closer to the punch 10 than the edge 34 of the blank holder 30. When the minimum value of the gap G is greater than 0 mm, the entire edge 34 of the blank holder 30 is positioned on the opposite side of the punch 10 from the edge 24 of the die 20 when viewed from the pressing direction P.

[0058] In the example shown in FIG. 8 , when viewed from the pressing direction P, the distance G between the edge 34 of the blank holder 30 and the edge 24 of the die 20 is smallest at the apex of the curved portion 24a that protrudes toward the punch 10. The distance G between the edge 34 of the blank holder 30 and the edge 24 of the die 20 increases with increasing distance from the apex of the curved portion 24a. In other words, the distance G varies within the range of the curved portion 24a. The distance G between the edge 34 of the blank holder 30 and the edge 24 of the die 20 becomes even larger at the straight portions 24b on both sides of the curved portion 24a. When viewed from the pressing direction P, the distance G increases with increasing distance from the curved portion 24a.

[0059] In addition, in the present embodiment, based on the results of the second preliminary study, when viewed from the press direction P, the line length of the portion of the edge 34 of the blank holder 30 where the distance G from the edge 24 of the die 20 is the smallest can be, for example, 0.5 times or more and 2.5 times or less the line length of the curved portion 24a. The line length of the curved portion 24a, that is, the length in the extending direction of the curved portion 24a, can be the product of the clamping angle θ (rad) and the radius of curvature of the curved portion 24a. The clamping angle θ is the angle formed by the tangents at both ends of the curved portion 24a when viewed from the press direction P. The clamping angle θ may be 1 / 3π (rad) or more and 2 / 3π (rad) or less. When the radius of curvature of the curved portion 24a changes, the radius of curvature of the curved portion 24a can be, for example, the average value of the radii of curvature measured at intervals of 5.0 mm as described above.

[0060] In the present embodiment, based on the results of the third preliminary study, when the thickness of the blank is t, the pressing surface interval between the pressing surface 21 of the die 20 and the pressing surface 31 of the blank holder 30 during press forming is controlled to the thickness s of the shim-like interval holding portion 40 that satisfies 0.1t ≦ s < 1.0t. This is because when the edge 201 of the blank is pulled out from between the die 20 and the blank holder 30, the reduction rate of the plate thickness of the edge 201 of the blank related to press cracking can be significantly reduced as compared with the case where there is no shim-like interval holding portion 40 (see FIG. 4). On the other hand, when the edge 201 of the blank is pulled out from between the die 20 and the blank holder 30, a shim-like interval holding portion 40 thinner than s / t = 100%, where no pressing force is applied to the edge 201 of the blank, can reduce the damage value I of Equation 1 (see FIG. 5).

[0061] In the present embodiment, as shown in FIGS. 4 and 5, considering the conflicting effects of the thickness s of the shim-like interval holding portion 40 on the plate thickness reduction rate and the damage value, when the thickness of the blank is t, the thickness s of the shim-like interval holding portion 40 that satisfies 0.1t ≦ s < 1.0t is controlled. Among these, it is preferable to set 0.1t ≦ s ≦ 0.6t because the reduction rate of the damage value I can be maintained high at about 20%. Also, it is preferable to set 0.6t < s ≦ 0.75t because the reduction rate of the damage value I can be maintained at 10% or more.

[0062] 10 and 11 , the press forming apparatus 100 according to this embodiment may further include a pad 50 that faces the punch 10 in the press direction P. In the production of a press-formed product, the pad 50 approaches the punch 10 in the press direction P relatively, and can press the blank 200 on the punch 10.

[0063] (Method of manufacturing press-molded products) Next, a method for manufacturing a press-formed product using the press-forming apparatus 100 will be described with reference to FIGS. 9A to 9G.

[0064] In manufacturing a press-formed product, a blank 200 is first prepared as shown in FIG. 9A . The blank 200 can be obtained, for example, by blanking a rolled steel sheet. The blank 200 is formed, for example, into the shape of a target press-formed product when it is unfolded. The unfolded shape of the target press-formed product can be obtained, for example, using a function of finite element analysis software for analyzing press forming that derives the shape of the blank before forming from the shape of the formed product. The blank 200 may be a steel sheet. The blank 200 is preferably a high-tensile steel sheet. The tensile strength of the blank 200 is 980 MPa or more. The tensile strength of the blank 200 may be 1180 MPa or more, or 1300 MPa or more. The sheet thickness of the blank 200 may be, for example, 0.4 mm or more and 6.0 mm or less, or 1.0 mm or more and 4.0 mm or less.

[0065] The manufacturing method of the press-formed product includes clamping a blank 200 between a die 20 and a blank holder 30, controlling the distance between the pressing surfaces of the die 20 and the blank holder 30 with a shim-shaped distance retaining portion (shim) 40, and pressing the blank 200 in the pressing direction P with a punch 10.

[0066] As shown in FIG. 9B, first, the blank 200 is placed between the punch 10 and blank holder 30 and the die 20. For example, the blank 200 is placed on the punch top surface 11 and the pressing surface 31 of the blank holder 30. Alternatively, the blank 200 may be placed on the pressing surface 21 of the die 20 (see FIG. 7). Then, as shown in FIG. 9C, the blank holder 30 and the die 20 are brought relatively close to each other in the pressing direction P, and the blank 200 is sandwiched and pressed between the pressing surface 21 of the die 20 and the pressing surface 31 of the blank holder 30. At this time, as shown in FIGS. 9B and 9C, a shim or shim-like spacing retainer 40 is disposed on the pressing surface of the blank holder 30, outside the edge of the blank 200. When the shim or shim-like spacing member 40 is a shim, its location is the area where the pressing surface 21 of the die and the pressing surface 31 of the blank holder overlap, excluding the clamping portion of the blank 200, when viewed from the press direction. Also, when the shim-like spacing member is a shim, its location is one or more of the pressing surface 21 of the die, the pressing surface 31 of the blank holder, and the extended surface of the pressing surface of the blank holder, within the range of the pressing surface 21 of the die and excluding the clamping portion of the blank 200, when viewed from the press direction.

[0067] As shown in FIG. 9D , while the blank 200 is sandwiched between the die 20 and the blank holder 30, the die 20 and the punch 10 are further moved relatively closer to each other in the pressing direction P. As a result, the blank 200 is pushed into the die 20 side (die hole) by the punch 10. As the blank 200 is pushed by the punch 10, it is drawn toward the die hole. In FIG. 9D , the direction in which the blank 200 is drawn is indicated by arrow D. In this embodiment, as shown in FIG. 9E , the pushing (forming) by the punch 10 is completed after the blank 200 is completely pulled out from between the die 20 and the blank holder 30. However, as shown in FIG. 9D , forming may be completed with a portion of the blank 200 still sandwiched between the die 20 and the blank holder 30.

[0068] In this embodiment, as shown in Figures 9D and 9E, when the blank 200 is pulled toward the die hole, the distance between the pressing surface 21 of the die and the pressing surface 31 of the blank holder is controlled to a thickness s of the shim or shim-like spacing retainer 40 that satisfies 0.1t≦s<1.0t.

[0069] As shown in Fig. 9E, when the pressing by the punch 10 is completed, the blank 200 is formed into, for example, a press-formed product 300 shown in Fig. 9F. The press-formed product 300 includes a top plate 301, a ridge portion 302, and a flange (vertical wall) 303. Incidentally, as shown in Fig. 9D, forming may be completed with the edge 201 of the blank 200 still sandwiched between the die 20 and the blank holder 30. In this case, the press-formed product 300 includes a top plate 301, a ridge portion 302, a vertical wall 303, and a horizontal flange 304, as shown in Fig. 9G.

[0070] In the example shown in FIG. 9F , in a plan view of the top plate 301 of the press-molded product 300 seen from the vertical direction, the side edges of the top plate 301 have recesses on the inside in the width direction. The top plate 301 is formed so that the width of the center part in the longitudinal direction is smaller than the width of both ends in the longitudinal direction. In other words, the top plate 301 is formed so that the center part in the longitudinal direction is constricted inward in a plan view. The top plate 301 may have a flat shape as a whole, or may have an uneven shape in part. The top plate 301 may have through holes formed therein.

[0071] A ridge portion 302 is provided continuously on the side edge in the width direction of the top plate 301. A flange 303 (vertical wall) is connected to the side edge of the top plate 301 via the ridge portion 302. The flange 303 (vertical wall) is provided continuous with the ridge portion 302 and standing upright from the top plate 301. The ridge portion 302 and the flange 303 (vertical wall) extend along the side edge of the top plate 301.

[0072] The press-formed product 300 has a tensile strength of 980 MPa or more. The press-formed product 300 may have a tensile strength of 1180 MPa or more, or 1300 MPa or more. The press-formed product 300 may have a thickness of 0.4 mm or more and 6.0 mm or less, or 1.0 mm or more and 4.0 mm or less, for example.

[0073] When the press-forming method according to this embodiment is used, processing marks such as pressing marks or abrasion marks remain at the locations of the press-formed product 300 that are locally pressed by the blank holder 30. These processing marks do not adversely affect the mechanical properties of the press-formed product. For example, in the press-formed product 300 shown in FIG. 9F, processing marks remain on the back side of the flange (vertical wall) 303, which is not exposed in the drawing.

[0074] In the manufacturing method of the press-formed product according to the present embodiment, based on the results of the first preliminary study, press forming is performed using a die 20 including a curved portion 24a on the edge 24 of the blank 200 facing the punch 10. At this time, compressive stress is applied by the blank holder 30 to the portion of the blank 200 where the sheet thickness is reduced, thereby suppressing press cracking. That is, when the blank 200 is forced into the die hole by the punch 10 to form it, the blank 200 is drawn toward the curved portion 24a on the edge 24 of the die 20 facing the punch 10. The portion of the blank 200 drawn toward the curved portion 24a is stretched in the direction of extension of the curved portion 24a as it is drawn toward the curved portion 24a, thereby reducing its sheet thickness. In contrast, the press-forming apparatus 100 according to the present embodiment is configured such that, when viewed from the pressing direction P, the distance G between the edge 34 of the blank holder 30 facing the punch 10 and the edge 24 of the die 20 facing the punch 10 is minimized at the curved portion 24a. Therefore, the portion of the blank 200 that is drawn toward the curved portion 24a and thus undergoes a thickness reduction can be clamped by the die 20 and the blank holder 30 up to the vicinity of the curved portion 24a, and compressive stress can be applied in the thickness direction. This makes it possible to suppress cracks in the blank 200 during the production of the press-formed product 300.

[0075] From the viewpoint of suppressing press cracks in this embodiment, when the blank 200 is formed into the press-formed product 300, it is desirable that the blank 200, which is being drawn toward the curved portion 24a, be pressed by the die 20 and the blank holder 30 up to the vicinity of the curved portion 24a. On the other hand, from the viewpoint of material flow, if the die 20 and the blank holder 30 do not press the periphery of the thickness-reduced portion of the blank 200, material will flow into the thickness-reduced portion of the blank 200, thereby mitigating the reduction in thickness of the blank 200. Therefore, in this embodiment, from the viewpoints of both suppressing press cracks and material flow, the distance G between the edge 34 of the blank holder 30 and the edge 24 of the die 20 is changed along the extension direction of the edge 24 of the die 20. That is, when viewed from the press direction P, the edge 34 of the blank holder 30 extends along the edge 24 of the die 20 but is not parallel to the edge 24 of the die 20. When viewed from the press direction P, the distance G between the edge 34 of the blank holder 30 and the edge 24 of the die 20 is smallest at a portion of the curved portion 24a and is larger at other portions. As a result, during press forming, the blank 200 is pressed by the die 20 and the blank holder 30 up to the vicinity of the edge 24 of the die 20 at the location where the distance G is smallest, while the blank 200 is not pressed up to the vicinity of the edge 24 of the die 20 at a location adjacent to the smallest location. This allows material to flow into the thickness-reduced portion of the blank 200 while applying compressive stress in the thickness direction. Therefore, when drawing is performed using a die 20 including a curved portion 24a at the edge 24 on the punch 10 side, cracking of the blank 200 can be suppressed.

[0076] If the blank 200 has a portion where the thickness is particularly reduced in the portion drawn into the curved portion 24a, the edge 24 of the die 20 and the edge 34 of the blank holder 30 can be positioned closest to each other in the portion where the thickness is reduced. For example, as in this embodiment, the distance G between the edge 34 of the blank holder 30 and the edge 24 of the die 20 can be minimized at the apex of the curved portion 24a that is convex toward the punch 10. In this case, compressive stress can be applied in the thickness direction to the portion of the blank 200 where the thickness is reduced significantly, and material flow can be promoted. This allows the thickness reduction of the blank 200 to be efficiently alleviated and cracking of the blank 200 to be effectively suppressed.

[0077] In this embodiment, when viewed from the pressing direction P, the distance G between the edge 34 of the blank holder 30 and the edge 24 of the die 20 increases as the distance from the curved portion 24a increases. This allows the blank holder 30 to press the entire portion of the blank 200 that is drawn into the curved portion 24a more firmly up to the vicinity of the die hole than the portion that is drawn into the straight portion 24b, while encouraging material to flow into the curved portion 24a from the periphery.

[0078] In this embodiment, based on the results of the second preliminary study, the line length of the edge 34 of the blank holder 30 at the portion where the distance G between the edge 24 of the die 20 and the edge 34 is the smallest is preferably 0.5 to 2.5 times the line length of the curved portion 24a when viewed from the pressing direction P. This makes it possible to effectively suppress cracking of the blank 200.

[0079] In the manufacturing method of a press-formed product according to this embodiment, the method of controlling the spacing between the pressing surfaces based on the results of the third preliminary study can be achieved by using a shim separate from the die, in addition to the control using the spacing retainer described for the press-forming apparatus above. That is, when the thickness of the blank is t, a shim with a thickness s satisfying 0.1t≦s<1.0t is placed in the area where the pressing surfaces of the die and the blank holder overlap, excluding the area where the blank is clamped, as viewed from the pressing direction. This achieves the same effect as the effect of the spacing retainer described for the press-forming apparatus above (see Figures 4 and 5).

[0080] (press-molded products) Next, a press-formed product manufactured by a method for manufacturing a press-formed product using the press-forming apparatus 100 will be described.

[0081] The press-formed product according to this embodiment is a press-formed product having a stretch flange-formed portion, which includes a top plate portion having a concave outer peripheral edge portion curved so that a portion of the outer peripheral edge is concave inward, and a concave vertical wall portion continuous with the concave outer peripheral edge portion of the top plate portion. As already described as a prerequisite, the press-formed product according to this embodiment has a top plate portion having a tensile strength of 980 MPa or higher, and a curvature radius of the concave outer peripheral edge portion of 20 mm to 60 mm.

[0082] 12 is a perspective view schematically showing a press-formed product 400 according to this embodiment, which is different from the press-formed product 300 (see FIG. 9F) according to this embodiment. The press-formed product 400 is, for example, a suspension arm part for an automobile.

[0083] 12, the press-formed product 400 includes a top plate portion (top plate) 402, a top plate outer peripheral edge portion (ridge portion) 404, and a vertical wall portion (flange) 406. The three vertical wall portions 406 are connected to the edge of the top plate portion 402 via the top plate outer peripheral edge portion 404, and are each continuous with the top plate outer peripheral edge portion 404 and are provided to stand up from the top plate portion 402.

[0084] 12 , the recessed outer peripheral edge portion 404a and the recessed vertical wall portion 406a are curved overall in a plan view seen from the perpendicular direction to the top plate portion 402 of the press-formed product 400. In addition, the recessed outer peripheral edge portion 404b and the recessed vertical wall portion 406b are also curved overall in a plan view seen from the perpendicular direction to the top plate portion 402 of the press-formed product 400. The radius of curvature of the curve of the latter recessed outer peripheral edge portion 404b and the recessed vertical wall portion 406b is smaller than the radius of curvature of the curve of the former recessed outer peripheral edge portion 404a and the recessed vertical wall portion 406a, making these portions more susceptible to press cracking and particularly advantageous in this embodiment.

[0085] The press-formed product according to this embodiment is manufactured by the method for manufacturing a press-formed product according to this embodiment. As shown in FIG. 13, the surface cross-sectional hardness of the concave vertical wall portion is 1.13 times (350 Hv; see the dashed line in FIG. 13) or more the surface cross-sectional hardness of the top plate portion (310 Hv). The press-formed products of the invention example and comparative example shown in FIG. 13 were manufactured under the following conditions. That is, the press-formed product according to this embodiment (invention example) had a blank tensile strength of 980 MPa, a blank thickness t of 2.9 mm, a radius of curvature of the concave vertical wall portion of the flange forming model mold of 30 mm, a shim thickness s of 1.4 mm, and s / t = 0.48. On the other hand, the comparative example used the same blank and flange forming model mold as the invention example, but did not use a blank holder or shim. The cross-sectional surface hardness of the concave vertical wall and top plate of the press-molded product was measured in accordance with JIS Z 2244 at a position 0.1 mm below the surface of the cross section of the cut-out sample, using micro Vickers hardness Hv.

[0086] Referring to Figure 13, it can be seen that in the stretch flange forming region, the closer to the center of the curve, the stronger the contact with the mold, so the hardness increases in both the inventive example and the comparative example, but the inventive example has an even greater hardness increase due to the localized pressing force.

[0087] Furthermore, in the press-formed product according to this embodiment, the end of the concave vertical wall portion may be thinner than the plate thickness of the top plate portion at the maximum load in a tensile test. This is because, as confirmed in the results of the first preliminary study, according to the manufacturing method for the press-formed product according to this embodiment, the end of the concave vertical wall portion is subjected to compressive strain, resulting in high local deformability.

[0088] Furthermore, the press-formed product according to this embodiment may have a height of 20 mm or more in the concave vertical wall portion. As confirmed in the results of the first to third preliminary studies, according to the manufacturing method of the press-formed product according to this embodiment, the compressive strain at the end of the concave vertical wall portion, the plate thickness reduction rate, the damage value I, etc. are optimized, thereby making it possible to obtain a high concave vertical wall portion (see Table 1). Generally, considering that the fatigue strength of a press-formed product can be increased as the height of the concave vertical wall portion increases, the press-formed product according to this embodiment is preferable because it can improve fatigue strength compared to the comparative example.

[0089] The press-formed products of the invention examples and comparative examples in Table 1 were manufactured under the following conditions. That is, the press-formed product according to this embodiment (invention example) has a blank tensile strength of 1180 MPa, a blank thickness t of 2.9 mm, a radius of curvature of the concave vertical wall portion of the flange-forming model mold of 30 mm, a shim thickness s of 1.4 mm, and s / t = 0.48. On the other hand, the comparative example uses the same blank and flange-forming model mold as the invention example, but is manufactured under forming conditions without using a blank holder or shim.

[0090] [Table 1]

[0091] Figure 14 shows the effect of flange height on the fatigue durability of press-formed parts. To estimate this effect, we performed computer simulations of the stress at the flange end for each flange height. The stress reduction rate was then compared across flange heights, based on a 10mm flange height. The computer simulations were performed on an L-shaped front lower arm model formed from a 2.9mm thick blank. This L-shaped front lower arm is connected to the outside at three locations: the first body-side connector at the middle, the second body-side connectors at both ends, and the wheel support. In the specific computer simulation, the first and second body-side connectors were fixed, and a 10kN load was applied to the wheel support. The stress at the end of the stretch flange formed area at the base of the first body-side connector at the middle was calculated for each formed flange height. Figure 14 shows that the stress at the flange end decreases with increasing flange height. This shows that increasing the flange height of the stretch flanged portion, where fatigue cracks occur, reduces the stress generated at that portion, thereby improving the fatigue durability of the press-formed product.

[0092] Although the embodiment according to the present invention has been described above, the present invention is not limited to the above embodiment, and various modifications are possible without departing from the spirit of the present invention. For example, in the press forming apparatus according to the present invention, various dies can be used within the scope of the present invention, not limited to the die 20, blank holder 30, shim-like spacing retainer (shim) 40, etc. described in the above embodiment. [Explanation of symbols]

[0093] 10 punches 11 Punch top surface 12 Punch Shoulder 13 Punch side 14 Punch edge 20 Die 21 Die pressing surface 22 Die Shoulder 23 Die side 24 Die Edge 24a Die edge curve 24b Straight part of die edge 30 Blank holder (holder) 31 Blank holder (holder) pressing surface 32 Holder shoulder 33 Holder side 34 Edge of blank holder (holder) 40 Shim-shaped spacing retainer (shim) 50 pads 100 Press forming equipment 200 blank 201 Blank Edge 300 Press-molded products 301 Top plate 302 Ridge 303 Flange (vertical wall) 304 Horizontal flange 400 Press-molded products 402 Top plate 404 Top plate outer edge (ridge) 404a Concave outer periphery 404b Concave outer periphery 406 Vertical wall (flange) 406a Concave vertical wall 406b Concave vertical wall A1 The direction in which the blank is drawn during press forming A2 Stretching direction at the part of the blank that is drawn into the curved part CL Virtual line (center line, width center line) D Direction in which the blank is pulled in G. Distance between edge 24 and edge 34 Gmin: The closest distance between edge 24 and edge 34 Ld Line length of the curved edge of the die Lh Line length of the blank holder edge in the section of the nearest interval Gmin P Press direction s Thickness of the shim-shaped spacing retainer (shim thickness) t Blank thickness θ: angle between the tangents at both ends of the curved portion 24a

Claims

1. A press forming apparatus comprising a punch, a die, and a blank holder, the die includes a curved portion, the edge of the die on the punch side being convexly curved toward the punch side and extending with a curvature radius of 20 mm or more and 60 mm or less when viewed from the pressing direction, the blank holder is disposed such that, when viewed from the press direction, a distance G between the punch-side edge of the blank holder and the edge of the die on a line perpendicular to an extension direction of the edge of the die varies along the edge of the die and is smallest at the curved portion; The distance between the pressing surface of the die and the pressing surface of the blank holder is controlled by a shim-shaped spacing retaining portion having a thickness s, The spacing retaining portion is provided on one or more surfaces of the pressing surface of the die, the pressing surface of the blank holder, or an extended surface of the pressing surface of the blank holder, within the range of the pressing surface of the die and excluding the clamping portion of the blank, when viewed from the press direction; The thickness s of the spacing portion satisfies 0.1t≦s<1.0t, where t is the plate thickness of the blank. Press forming equipment.

2. The press forming apparatus according to claim 1 , wherein the gap G increases as the distance from the curved portion increases when viewed from the pressing direction.

3. 3. The press forming apparatus according to claim 1, wherein, when viewed from the press direction, a line length Lh of a section of the edge of the blank holder where the gap G is smallest (Gmin) is 0.5 to 2.5 times the line length Ld of the curved portion.

4. A method for producing a press-formed product having a stretch flange formed portion using a plate material having a tensile strength of 980 MPa or more in a blank before press forming, The method comprises: sandwiching the blank between a die and a blank holder; controlling a pressing surface distance between a pressing surface of the die and a pressing surface of the blank holder by a shim or a shim-like distance retaining portion; and pressing the blank in a pressing direction by a punch; the die includes a curved portion, the edge of the die on the punch side being convexly curved toward the punch side and extending with a curvature radius of 20 mm or more and 60 mm or less when viewed from the pressing direction, the blank holder is disposed such that, when viewed from the press direction, a distance G between the punch-side edge of the blank holder and the edge of the die on a line perpendicular to an extension direction of the edge of the die varies along the edge of the die and is smallest at the curved portion; When the pressing surface interval is controlled by the shim, the shim is arranged in a region where the pressing surface of the die and the pressing surface of the blank holder overlap, excluding the clamping portion of the blank, when viewed from the pressing direction; When the pressing surface interval is controlled by the interval maintaining unit, the interval maintaining unit is disposed on one or more surfaces of the pressing surface of the die, the pressing surface of the blank holder, or an extended surface of the pressing surface of the blank holder within the range of the pressing surface of the die and excluding the clamping portion of the blank when viewed from the pressing direction, The thickness s of the shim or the spacing retaining portion satisfies 0.1t≦s<1.0t, where t is the plate thickness of the blank. Manufacturing method for press-molded products.

5. The method for manufacturing a press-formed product according to claim 4 , wherein the gap G increases as the distance from the curved portion increases when viewed from the pressing direction.

6. 6. The method for manufacturing a press-formed product according to claim 4 or 5, wherein, when viewed from the press direction, a line length Lh of a section of the edge of the blank holder where the gap G is smallest Gmin is 0.5 to 2.5 times the line length Ld of the curved portion.