PRESS-FORMING METHOD, METHOD OF MANUFACTURING COMPONENT WITH THE PRESS-FORMING METHOD AND COMPONENT MANUFACTURED WITH THE PRESS-FORMING METHOD

Abstract

A press-forming method for a component having a hat-shaped or U-shaped cross section and comprising a curved portion curved in a widthwise direction along with a longitudinal direction and straight side portions connecting to both ends of the curved portion from a sheet-shaped blank, wherein the blank is drawn to a hat-shaped or U-shaped cross section through a hat-shaped cross-sectional form having a top portion, vertical wall portions connecting at their upper end parts to both end parts of the top portion through fillet portions, and flange portions connecting at their internal end parts to the lower end parts of the vertical wall portions through fillet portions, and a material movement in the flange portion of the curved portion is caused in the drawing to mitigate tensile deformation or compression deformation in the circumferential direction generated in the flange portion of the curved portion.

Claims

1-10. (canceled)

11. A method for press-forming a component having a hat-shaped or U-shaped cross section and comprising a curved portion curved in a widthwise direction along with a longitudinal direction and straight side portions connecting to the both ends of the curved portion from a sheet-shaped blank, characterized in that the blank is drawn to a hat-shaped or U-shaped cross section through a hat-shaped cross-sectional form having a top portion, vertical wall portions connecting at their upper end parts to both end parts of the top portion through fillet portions, and flange portions connecting at their internal end parts to the lower end parts of the vertical wall portions through fillet portions, and a material movement in the flange portion of the curved portion is caused in the drawing to mitigate tensile deformation or compression deformation in the circumferential direction generated in the flange portion of the curved portion.

12. The press-forming method according to claim 11, wherein the material movement in the flange portion of the curved portion mitigating tensile deformation or compression deformation in the circumferential direction generated in the flange portion of the curved portion is caused by setting a balance position of the material flowed in the straight side portion to the vertical wall portion on the side of the stretch flanging deformation in the curved portion or the fillet portion between the vertical wall portion and the flange portion and increasing the material flowed from the side of the shrink flanging deformation in the straight side portion to an extent exceeding the top portion.

13. The press-forming method according to claim 11, wherein the material movement in the flange portion of the curved portion mitigating tensile deformation or compression deformation in the circumferential direction generated in the flange portion of the curved portion is caused by setting a balance position of the material flowed in the curved portion to the vertical wall portion on the side of the shrink flanging deformation or the fillet portion between the vertical wall portion and the flange portion and suppressing the material flowed from the side of the shrink flanging portion in the curved portion and increasing the material flowed from the side of the stretch flanging deformation to an extent exceeding the top portion.

14. The press-forming method according to claim 11, wherein a bending radius of the fillet portion between the top portion and the vertical wall portion is set to 1.1-10 times of a bending radius of the fillet portion between the vertical wall portion and the flange portion in the cross section.

15. The press-forming method according to claim 11, wherein a bending radius of a fillet portion farther from the balance position is made larger than a bending radius of a fillet portion near to the balance position among the fillet portions between the top portion and the vertical wall portions in the cross section.

16. The press-forming method according to claim 12, wherein the material movement in the flange portion of the curved portion mitigating tensile deformation or compression deformation in the circumferential direction generated in the flange portion of the curved portion is caused by setting a balance position of the material flowed in the curved portion to the vertical wall portion on the side of the shrink flanging deformation or the fillet portion between the vertical wall portion and the flange portion and suppressing the material flowed from the side of the shrink flanging portion in the curved portion and increasing the material flowed from the side of the stretch flanging deformation to an extent exceeding the top portion.

17. The press-forming method according to claim 12 wherein a bending radius of the fillet portion between the top portion and the vertical wall portion is set to 1.1-10 times of a bending radius of the fillet portion between the vertical wall portion and the flange portion in the cross section.

18. The press-forming method according to claim 13, wherein a bending radius of the fillet portion between the top portion and the vertical wall portion is set to 1.1-10 times of a bending radius of the fillet portion between the vertical wall portion and the flange portion in the cross section.

19. The press-forming method according to claim 12, wherein a bending radius of a fillet portion farther from the balance position is made larger than a bending radius of a fillet portion near to the balance position among the fillet portions between the top portion and the vertical wall portions in the cross section.

20. The press-forming method according to claim 13, wherein a bending radius of a fillet portion farther from the balance position is made larger than a bending radius of a fillet portion near to the balance position among the fillet portions between the top portion and the vertical wall portions in the cross section.

21. The press-forming method according to claim 12, wherein the flange portion is provided at a side near to the balance position in the cross section with beads.

22. The press-forming method according to claim 13, wherein the flange portion is provided at a side near to the balance position in the cross section with beads.

23. The press-forming method according to claim 12, wherein the vertical wall portion has a curved shape at a side near to the balance position in the cross section.

24. The press-forming method according to claim 13, wherein the vertical wall portion has a curved shape at a side near to the balance position in the cross section.

25. The press-forming method according to claim 12, wherein a metal sheet having a tensile strength of 440-1470 MPa is used as the blank.

26. The press-forming method according to claim 13, wherein a metal sheet having a tensile strength of 440-1470 MPa is used as the blank.

27. The press-forming method according to claim 14, wherein a metal sheet having a tensile strength of 440-1470 MPa is used as the blank.

28. The press-forming method according to claim 15, wherein a metal sheet having a tensile strength of 440-1470 MPa is used as the blank.

29. A method of manufacturing a component, wherein a sheet-shaped blank is drawn into a component of a preliminary shape having a hat-shaped and curved in a widthwise direction along with a longitudinal direction by using the press-forming method as claimed claim 11 and the component of the preliminary shape is subjected to at least one of a restriking for rendering a bending radius of the fillet portions into a predetermined radius and a trimming for rendering a contour shape thereof into a predetermined shape to manufacture a component having a hat-shaped or U-shaped cross section and curved in a widthwise along with a longitudinal direction.

30. A formed component having a hat-shaped or U-shaped cross section and curved in a widthwise direction along with a longitudinal direction, wherein the component is formed by drawing from a sheet-shaped blank by using the press-forming method as claimed in claim 11 and formed into a predetermined shape by using at least one of a restriking for rendering a bending radius of the fillet portions into a predetermined radius and a trimming for rendering a contour shape thereof.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] FIG. 1 is a perspective view illustrating a component comprising a curved portion curved in a widthwise direction along with a longitudinal direction and straight side portions connecting to both ends of the curved portion.

[0022] FIG. 2 is a plane view illustrating a component shown in FIG. 1 comprising a curved portion curved in a widthwise direction along with a longitudinal direction and straight side portions connecting to both ends of the curved portion.

[0023] FIG. 3 is a cross-sectional view taken along with a line A-A of a component shown in FIGS. 1 and 2.

[0024] FIG. 4 is a sectional view illustrating a structure of a die set for drawing a component comprising a curved portion curved in a widthwise direction along with a longitudinal direction and straight side portions connecting to both ends of the curved portion.

[0025] FIG. 5 is a sectional view illustrating a material movement in the conventional drawing.

[0026] FIG. 6 is a sectional view illustrating a material movement state in a drawing by an embodiment of the drawing method according to the invention.

[0027] FIG. 7 is a plane view illustrating a material movement state in a drawing by the drawing method according to the above embodiment.

[0028] FIG. 8 is a plane view illustrating a shape of a component causing a material movement pattern MA in a drawing by the drawing method according to the above embodiment.

EMBODIMENTS FOR CARRYING OUT THE INVENTION

[0029] An embodiment of the invention will be described in detail by means of an example with reference to the drawings. When a sheet-shaped blank is drawn into a component as shown in FIGS. 1-3 with a die set as shown in FIG. 4, a hat shape as shown in FIG. 5 is often formed in the conventional method by preferentially moving a material of the blank from a flange portion F to a vertical wall portion W, in which the material flowed from the top portion T to the vertical wall portion W is small as compared to the material flowed from the flange portion F.

[0030] This is due to the fact that the die set is designed so as to cause a balance of tension in the top portion P. When the material passes through a shoulder portion of a punch 2 forming a punch shoulder fillet portion P (portions having bending radii R1 and R2) and a shoulder portion of a die member 4 forming a die shoulder fillet portion D (portions having bending radii R3 and R4), respectively, as the punch shoulder fillet portion P (R surface portion) is shown on the left side of FIG. 3 by an enlarged scale, two R-stop positions E are existent in the shoulder of the punch 2 and die member 4, and the material is subjected to resistances (bending and unbending resistances) associated with bending deformation B and unbending deformation BR at these R-stop positions E as shown in FIG. 5.

[0031] Also, the material is subjected to a friction resistance from the shoulder portion of the punch 2 at a position of contacting the punch shoulder fillet portion P with the punch shoulder. The material passing through a portion forming the flange portion F or a portion forming the vertical wall portion W is subjected to an inflow resistance DR due to a friction caused by contacting with any one of the blank holder 1, the punch 2, and the die member 4 as shown in FIG. 5. When the die set has a bead shape, an emboss shape or the like, a resisting force deforming the material is caused along with such a shape. Further, if a shrink flanging deformation occurs in the flange portion F, shrink deformation is generated when the material flows from the flange portion F to the vertical wall portion W, so that the inflow resistance is increased. Since a position X balancing these resistances exists on the top portion T, the material flowed from the top portion T is small, and hence the material is moved preferentially from the flange portion F to the vertical wall portion W.

[0032] The inventors have got an idea that the material can be moved so as to mitigate the cracks due to the stretch flanging deformation and/or the wrinkles due to the shrink flanging deformation by changing the tension balance position X and made studies thereon. As a result, it has been found out that the balance position X can be freely determined by defining the shape of the die set and/or the friction resistance so that F1 and F2 are equal, wherein F1 is a total amount of the resistances on one hand with respect to a certain point and F2 is a total amount of resistances on the other hand.

[0033] FIG. 6 is a sectional view illustrating a material movement state in the drawing by an embodiment of the drawing method according to the invention. As shown in FIG. 6, when a component having a hat-shaped or U-shaped cross section and comprising a curved portion C curved in a widthwise direction along with a longitudinal direction and straight side portions S connecting to the both ends of the curved portion as shown in FIG. 3 is press-formed from a sheet-shaped blank, the following method can be used for moving a balance position X from the top portion T according to the embodiment of the drawing method.

[0034] In the drawing method according to the embodiment, when a component having a hat-shaped or U-shaped cross section and comprising a curved portion C curved in a widthwise direction along with a longitudinal direction and straight side portions S connecting to the both ends of the curved portion as shown in FIG. 3 is press-formed, for example, by using a die set having a structure shown in FIG. 4, it is possible to decrease the bending and unbending resistances by increasing a bending radius, so that the balance position X can be easily moved from the top portion T by setting bending radii R1 and R2 in the shoulder portions of the punch 2 to 1.1-10 times of a bending radius R3 of the shoulder portion of the die member 4 positioned at the F1 side shown on the left side of FIG. 6 or on the left side of FIG. 4.

[0035] Further, the balance position X is moved toward the vertical wall portion W by using a method of decreasing an inflow resistance at the F2 side on the right side of FIG. 6 or a method of increasing an inflow resistance from the flange portion F at the F1 side on the left side of FIG. 6. As the method of decreasing the inflow resistance at the F2 side, there are a method wherein a bending radius R4 in the shoulder portion of the die member 4 positioned at the F2 side on the right side of FIG. 6 or on the right side of FIG. 4 is set to 1.1-10 times of the bending radius R3 in the shoulder portion of the die member 4 positioned on the F1 side of the left side in FIG. 6 or on the left side in FIG. 4, a method of decreasing the friction resistance by weakly holding the flange portion F at the F2 side on the right side of FIG. 6 with the blank holder 1 and the die member 4, and so on.

[0036] On the other hand, as the method of increasing the inflow resistance at the F1 side, there are a method wherein beads and/or embosses (not shown) formed in the blank holder 1 and/or the die member 4, i.e. beads and/or embosses formed in the flange portion F at the F1 side on the left side of FIG. 6 have a bending radius smaller than that of beads and/or embosses formed in the flange portion F at the F2 side on the right side of FIG. 6, a method wherein beads and/or embosses (not shown) are formed only in a portion of the blank holder 1 and/or the die member 4 holding the flange portion F on the F1 side on the left side of FIG. 6, a method of generating shrink resistance by curving the vertical wall portion W at the balance position X in a direction perpendicular to the wall surface of the vertical wall portion W to intentionally cause shrink flanging deformation in the flange portion F, a method of increasing the friction resistance by strongly holding the flange portion F at the F1 side with the blank holder 1 and the die member 4, and so on. The balance position X can be moved more easily by using the above methods in combination.

[0037] The reason why the bending radius is preferable to be set to 1.1-10 times is due to the fact that when it is less than 1.1 times, the difference of the resistances is so small and it is difficult to move the balance position X, while when it is more than 10 times, the deformation amount of the material is increased in the restriking from the fillet portion of a preliminary shape into the fillet portion of a predetermined shape, so that the shortage in the ductility of the material is caused to increase the possibility of causing cracks.

[0038] When a component of a preliminary shape is press-formed in the drawing method according to the aforementioned embodiment by making the bending radius in the shoulder portions of the punch 2 and/or the die member 4 larger than that of a component of a predetermined shape, the component of the preliminary shape is re-struck by bending or drawing to make the bending radius in the fillet portions P and/or the fillet portions D of the component smaller, whereby a component provided with fillet portions having a predetermined radius can be manufactured.

[0039] If a predetermined contour shape cannot be obtained by the drawing or the subsequent restriking, it is possible to manufacture a component having the predetermined contour shape by conducting a trimming for rendering the contour shape into the predetermined shape after or together with the restriking.

[0040] The balance position X can be determined by conducting an experiment of the drawing or a numerical analysis by a finite element method in the target component. Since the influence by the bendingunbending resistance is larger than the influence by the friction resistance or the shrinking resistance in the flange portion F, the shape of the component may be simply determined so that the bendingunbending resistance counterbalances at the vertical wall portion W or the flange portion F.

[0041] The bendingunbending resistance Fb can be calculated from the following formula using yield strength e and thickness t of the material and bending radius R:

Fb=et/(2*(0.5*t+R))

[0042] As a result of finding the above method for determining the balance position X, the inventors could find a method of generating an ideal material movement for suppressing one or both of the stretch flanging deformation and shrink flanging deformation as shown in FIG. 7

[0043] In this method, the material movement pattern MA is caused at least by determining the balance position X in the vertical wall portion W on the side of the stretch flanging deformation EF or in the fillet portion between the vertical wall W and the flange portion F at the straight side portions S connecting to the both ends of the curved portion C in the middle part of the vertical wall portion W curved in a widthwise direction of a component to be formed. Moreover, it is preferable that the material movement pattern MB is caused by determining the balance position X in the vertical wall portion W on the side of the shrink flanging deformation CF or in the fillet portion between the vertical wall portion W and the flange portion F at the curved portion C of the middle part.

[0044] In the usual drawing, when the material flowed from the flange portion F is suppressed, the forming at the shoulder portions of the punch 2 or the shoulder portions of the die member 4 becomes difficult. In this embodiment, however, the forming at the shoulder portions of the punch 2 and the shoulder portions of the die member 4 are mitigated because the material flowed from the top portion T is caused.

[0045] In the material movement pattern MA, since the material movement is increased in the flange portion F, the vertical wall portion W, and the punch shoulder fillet portion P on the side of shrink flanging deformation CF shown on the left side of FIG. 7, tensile deformation is caused in the flange portion F on the side of the shrink flanging deformation CF of the curved portion C shown on the left side of FIG. 7. On the contrary, the material movement from the flange portion F on the side of the stretch flanging deformation EF shown on the right side of FIG. 7 is decreased in the material movement pattern MA, so that the flange portion F on the side of the stretch flanging deformation EF is hardly stretched in the curved portion C shown on the right side of FIG. 7.

[0046] In the material movement pattern MB, the material in the flange portion F, the vertical wall portion W, and the top portion T on the side of the stretch flanging deformation EF shown on the right side of FIG. 7 are largely moved toward the side of the shrink flanging deformation CF shown on the left side of FIG. 7 while drawing the material in the circumferential direction along with the curving shape of the curved portion C, and hence the stretch flanging deformation is mitigated. Further, the material movement toward the flange portion F on the side of the shrink flanging portion CF is decreased, so that the occurrence of the shrink flanging deformation is suppressed.

[0047] According to the press-forming method of this embodiment, not only a component having a hat-shaped cross section and curved in a widthwise direction but also a component having a U-shaped cross section and curved in a widthwise direction can be formed by using all the material located in the flange portion F for forming the vertical wall portion W.

[0048] It is preferable that a metal sheet as a raw material for the blank has a tensile strength of 440-1470 MPa. Since a metal sheet having a tensile strength of less than 440 MPa is excellent in the ductility and drawability, it is less in the merit using the drawing method of this embodiment. On the other hand, since a metal sheet having a tensile strength exceeding 1470 MPa is poor in the ductility, cracks are easily caused at the shoulder portions of the punch 2 and/or the shoulder portions of the die member 4 which are not targeted in the drawing method of this embodiment, so that the drawing of the component may be difficult.

[0049] Table 1 shows various specifications of steel sheets of 270, 440, 980, 1180, and 1470 MPa grade. Table 2 shows results examined on comparative examples of the conventional method and examples of the method according to this embodiment in components having a hat-shaped cross section shown in FIGS. 1-3 made by using steel sheets of 270, 440, 980, 1180, and 1470 MPa grades as a sample material.

[0050] In order to change the balance position X, the radius of the fillet portion is set to a value as shown in Table 3 at the curved portion C and in Table 4 at the straight side portions S connecting to the curved portion, respectively. Here, the radius of the fillet portion in the punch shoulder and the die shoulder is R1 and R3 on the side near to the shrink flanging deformation CF, and R2 and R4 on the side near to the stretch flanging deformation EF, respectively. A round bead having a bending radius of 8 mm is used as a bead.

[0051] When the material movement pattern MA is caused by using the shrink flanging deformation CF in the above examination, there is used a shape of a component wherein additional curved portions AC causing shrink flange deformation CF are further connected to the straight side portions S as shown in FIG. 8. In this example, a vertical wall of the additional curved portion AC causing the shrink flanging deformation CF has a curved shape having a bending radius of 200 mm. However, a shape causing the material movement pattern MA is not particularly limited to the above shape.

TABLE-US-00001 TABLE 1 Sym- Thickness YP TS El bol Steel type (mm) (MPa) (MPa) (%) 270 270 MPa grade steel sheet 0.7 160 293 50 440 440 MPa grade steel sheet 1.2 310 465 38 980 980 MPa grade steel sheet 1.4 650 985 16 1180 1180 MPa grade steel sheet 1.6 950 1200 10 1470 1470 MPa grade steel sheet 1.6 1290 1520 8

TABLE-US-00002 TABLE 2 Balance position X Straight side portion S connecting to Evaluation Material Curved portion C curved portion Wrinkle Crack 270 Top portion Top portion Comparative Example Top portion Flange on stretch flanging side Comparative Example 440 Top portion Top portion X X Comparative Example Top portion Flange on stretch flanging side Comparative Example Vertical wall on shrink flanging side Vertical wall on stretch flanging side Inventive Example Vertical wall on shrink flanging side Die shoulder on stretch flanging side Inventive Example Vertical wall on shrink flanging side Flange on stretch flanging side Inventive Example Die shoulder on shrink flanging side Vertical wall on stretch flanging side Inventive Example Die shoulder on shrink flanging side Die shoulder on stretch flanging side Inventive Example Die shoulder on shrink flanging side Flange on stretch flanging side Inventive Example Flange on shrink flanging side Vertical wall on stretch flanging side Inventive Example Flange on shrink flanging side Die shoulder on stretch flanging side Inventive Example Flange on shrink flanging side Flange on stretch flanging side Inventive Example 980 Top portion Top portion X X Comparative Example Top portion Flange on stretch flanging side Comparative Example Vertical wall on shrink flanging side Vertical wall on stretch flanging side Inventive Example Vertical wall on shrink flanging side Die shoulder on stretch flanging side Inventive Example Vertical wall on shrink flanging side Flange on stretch flanging side Inventive Example Die shoulder on shrink flanging side Vertical wall on stretch flanging side Inventive Example Die shoulder on shrink flanging side Die shoulder on stretch flanging side Inventive Example Die shoulder on shrink flanging side Flange on stretch flanging side Inventive Example Flange on shrink flanging side Vertical wall on stretch flanging side Inventive Example Flange on shrink flanging side Die shoulder on stretch flanging side Inventive Example Flange on shrink flanging side Flange on stretch flanging side Inventive Example 1180 Top portion Top portion X X Comparative Example Top portion Flange on stretch flanging side Comparative Example Vertical wall on shrink flanging side Vertical wall on stretch flanging side Inventive Example Vertical wall on shrink flanging side Die shoulder on stretch flanging side Inventive Example Vertical wall on shrink flanging side Flange on stretch flanging side Inventive Example Die shoulder on shrink flanging side Vertical wall on stretch flanging side Inventive Example Die shoulder on shrink flanging side Die shoulder on stretch flanging side Inventive Example Die shoulder on shrink flanging side Flange on stretch flanging side Inventive Example Flange on shrink flanging side Vertical wall on stretch flanging side Inventive Example Flange on shrink flanging side Die shoulder on stretch flanging side Inventive Example Flange on shrink flanging side Flange on stretch flanging side Inventive Example 1470 Top portion Top portion X X Comparative Example Top portion Flange on stretch flanging side Comparative Example Vertical wall on shrink flanging side Vertical wall on stretch flanging side Inventive Example Vertical wall on shrink flanging side Die shoulder on stretch flanging side Inventive Example Vertical wall on shrink flanging side Flange on stretch flanging side Inventive Example Die shoulder on shrink flanging side Vertical wall on stretch flanging side Inventive Example Die shoulder on shrink flanging side Die shoulder on stretch flanging side Inventive Example Die shoulder on shrink flanging side Flange on stretch flanging side Inventive Example Flange on shrink flanging side Vertical wall on stretch flanging side Inventive Example Flange on shrink flanging side Die shoulder on stretch flanging side Inventive Example Flange on shrink flanging side Flange on stretch flanging side Inventive Example

TABLE-US-00003 TABLE 3 Additional Balance curved position X R3 R1 R2 R4 Bead portion AC Top portion 5 5 5 5 Absence Absence Vertical wall on 5 5.5 5.5 5 Absence Absence shrink flanging side Die shoulder on 5 50 40 20 Absence Presence shrink flanging side Flange on shrink 10 20 20 10 Presence Absence flanging side (flange on shrink flanging side)

TABLE-US-00004 TABLE 4 Additional Balance curved position X R3 R1 R2 R4 Bead portion AC Top portion 5 5 5 5 Absence Absence Vertical wall on 20 30 30 5 Absence Absence stretch flanging side Die shoulder on 30 50 50 5 Absence Presence stretch flanging side Flange on stretch 10 30 30 10 Presence Absence flanging side (flange on stretch flanging side)

[0052] The conventional method is a usual drawing wherein the balance position X is on the top portion T. The evaluation of the formed products is performed visually wherein winkles and cracks generated in the flange portion are evaluated by three stages , and x based on standards shown in Table 5 and Table 6, respectively.

TABLE-US-00005 TABLE 5 Visual judgement on winkles No wrinkle Wrinkles generated in a portion other than an evaluated portion such as top portion X Wrinkles

TABLE-US-00006 TABLE 6 Visual judgement on cracks No crack Some cracking X Cracks

[0053] As a result of the above examination, the 270 MPa grade steel sheet can be formed without generating wrinkles or cracks by either one of the conventional drawing method and the drawing method according to this embodiment. In the case of using the steel sheets of not less than 440 MPa, predominant cracks and wrinkles are generated in the conventional drawing method, whereas the generation of cracks and wrinkles can be prevented in the drawing method according to this embodiment.

[0054] Although the above is described with reference to the illustrated examples, the invention is not limited to the above examples and can be properly changed within a scope disclosed in the claims. For example, the die set used in the drawing may have a structure that the right and left shoulder portions and the concave portion of the die member 4 are made of different members instead of the structure shown in FIG. 4, or the drawing may be conducted by making the bending radius of the shoulder portions in the punch 2 so as to connect the upper end parts of the left and right fillet portions P to the curved top portion T.

INDUSTRIAL APPLICABILITY

[0055] According to the press-forming method of the invention, the method of manufacturing a component by using the press-forming method, and the component manufactured by using the press-forming method, when a component having a hat-shaped or U-shaped cross section and comprising a curved portion curved in a widthwise direction along with a longitudinal direction and straight side portions connecting to the both ends of the curved portion is press-formed from a sheet-shaped blank, the drawing is performed so as to have a hat-shaped or U-shaped cross section through a hat-shaped cross-sectional form comprising a top portion, vertical wall portions connecting at their upper end parts to both end parts of the top portion through fillet portions and flange portions connecting at their internal end parts thereof to the lower end parts of the vertical wall portions through fillet portions, and the material movement toward the flange portions of the curved portion is caused in the drawing to mitigate tensile deformation in the circumferential direction or compression deformation in the circumferential direction generated in the flange portion of the curved portion, whereby one or both of the generation of winkles due to the shrink flanging deformation and the generation of cracks due to the stretch flanging deformation can be suppressed.

DESCRIPTION OF REFERENCE SYMBOLS

[0056] 1 blank holder [0057] 2 punch [0058] 3 lower die [0059] 4 die member [0060] 5 upper die [0061] 6 cushion pin [0062] C curved portion [0063] CF shrink flanging deformation [0064] D die shoulder fillet portion [0065] EF stretch flanging deformation [0066] F flange portion [0067] P punch shoulder fillet portion [0068] R1-R4 bending radius [0069] S straight side portion [0070] T top portion [0071] W vertical wall portion [0072] X balance position