BLANK, FORMED ARTICLE, DIE ASSEMBLY, AND METHOD FOR PRODUCING BLANK

Abstract

A sheet-shaped blank (10) for press forming is produced by shearing a metal sheet (30). The blank (10) includes a sheared edge (14), which includes a sheared surface (14b) and a fractured surface (14c) in the sheet thickness direction and has a loop shape in plan view. In plan view, an edge of the sheared edge (14) includes concavely curved portions (20). The average of lengths of the fractured surface (14c) in the sheet thickness direction in the curved portions (20) is greater than the average of lengths of the fractured surface (14c) in the sheet thickness direction over the entire perimeter of the sheared edge (14).

Claims

1. A sheet-shaped blank for press forming produced by shearing a metal sheet, the blank comprising: a sheared edge comprising, in a sheet thickness direction, a sheared surface and a fractured surface, wherein the sheared edge has a loop shape in plan view, the sheared edge has an edge comprising, in plan view, a curved portion that is concavely curved, and an average of lengths of the fractured surface in the sheet thickness direction in the curved portion is greater than an average of lengths of the fractured surface in the sheet thickness direction over an entire perimeter of the sheared edge.

2. The blank according to claim 1, wherein, provided that a reference point of the curved portion is defined as a midpoint of the curved portion in a perimeter direction of the sheared edge or a point where a curvature of the curved portion in plan view is greatest, an average of lengths of the fractured surface in the sheet thickness direction within a region, which extends a predetermined length in the perimeter direction with the reference point as a center, is greater than the average of lengths of the fractured surface in the sheet thickness direction over the entire perimeter of the sheared edge.

3. The blank according to claim 2, wherein the average of lengths of the fractured surface in the sheet thickness direction within the region of the predetermined length is greater by 10% or more of the sheet thickness than the average of lengths of the fractured surface in the sheet thickness direction over the entire perimeter of the sheared edge.

4. The blank according to claim 2, wherein the sheared edge further comprises a shear droop portion positioned opposite from the fractured surface in the sheet thickness direction with the sheared surface interposed therebetween, and wherein an average of lengths of the shear droop portion in the sheet thickness direction within the region of the predetermined length is 20% or less of the sheet thickness.

5. The blank according to claim 2, wherein the predetermined length is a length of 50% of the sheet thickness of the blank.

6. The blank according to claim 2, wherein the predetermined length is a length of 2000% of the sheet thickness.

7. The blank according to claim 2, wherein the region of the predetermined length is a region where a curvature is 5 m.sup.−1 or more.

8. The blank according to claim 1, wherein the metal sheet comprises a hole formed by punching, and the sheared edge is formed along an edge of the hole.

9. The blank according to claim 1, wherein the metal sheet has an outer perimeter edge formed by blanking, and the sheared edge is formed along the outer perimeter edge.

10. The blank according to claim 1, wherein the curved portion is configured to stretch and deform during press forming.

11. A formed article produced by subjecting the blank according to claim 1 to press forming.

12. A die assembly comprising: a columnar punch; and a hollow die configured to receive the punch, the die assembly being configured to shear a metal sheet placed on the die by moving the punch in a predetermined direction, the punch comprising a bottom surface and an outer perimeter surface, the bottom surface comprising a cutting edge constituted by an outer perimeter edge of the bottom surface, the outer perimeter surface extending from the outer perimeter edge in a direction parallel to the predetermined direction, the outer perimeter edge comprising, in plan view, a curved portion that is convexly curved or concavely curved, the bottom surface comprising a planar portion and a cutout portion recessed with respect to the planar portion in the predetermined direction and configured to comprise the curved portion in plan view.

13. A die assembly comprising: a columnar punch; and a hollow die configured to receive the punch, the die assembly being configured to shear a metal sheet placed on the die by moving the punch in a predetermined direction, the die comprising a hollow support surface and an inner perimeter surface, the support surface being configured to support the metal sheet and comprising a cutting edge constituted by an inner perimeter edge of the die, the inner perimeter surface extending from the inner perimeter edge in a direction parallel to the predetermined direction, the inner perimeter edge comprising, in plan view, a curved portion that is convexly curved or concavely curved, the support surface comprising a planar portion and a cutout portion recessed with respect to the planar portion in the predetermined direction and configured to comprise the curved portion in plan view.

14. The die assembly according to claim 12, wherein a cutout depth of the cutout portion in a direction parallel to the predetermined direction is 0.1 times or more a sheet thickness of the metal sheet and 0.7 times or less the sheet thickness.

15. A method for producing a blank for press forming, the method using the die assembly according to claim 12, the method comprising the steps of: placing a metal sheet on the die of the die assembly, and shearing the metal sheet on the die using the punch of the die assembly.

16. A method for producing a blank comprising: a sheared edge comprising, in a sheet thickness direction, a sheared surface and a fractured surface, wherein the sheared edge has a loop shape in plan view, the sheared edge has an edge comprising, in plan view, a curved portion that is concavely curved, and an average of lengths of the fractured surface in the sheet thickness direction in the curved portion is greater than an average of lengths of the fractured surface in the sheet thickness direction over an entire perimeter of the sheared edge, the method using the die assembly according to claim 12, the method comprising the steps of: placing a metal sheet on the die of the die assembly, and shearing the metal sheet on the die using the punch of the die assembly, wherein, in the step of shearing, at least a portion of the curved portion of the blank is formed by cutting a portion of the metal sheet via the cutout portion of the punch or the cutout portion of the die.

17. The die assembly according to claim 13, wherein a cutout depth of the cutout portion in a direction parallel to the predetermined direction is 0.1 times or more a sheet thickness of the metal sheet and 0.7 times or less the sheet thickness.

18. A method for producing a blank for press forming, the method using the die assembly according to claim 13, the method comprising the steps of: placing a metal sheet on the die of the die assembly, and shearing the metal sheet on the die using the punch of the die assembly.

19. A method for producing a blank comprising: a sheared edge comprising, in a sheet thickness direction, a sheared surface and a fractured surface, wherein the sheared edge has a loop shape in plan view, the sheared edge has an edge comprising, in plan view, a curved portion that is concavely curved, and an average of lengths of the fractured surface in the sheet thickness direction in the curved portion is greater than an average of lengths of the fractured surface in the sheet thickness direction over an entire perimeter of the sheared edge, the method using the die assembly according to claim 13, the method comprising the steps of: placing a metal sheet on the die of the die assembly, and shearing the metal sheet on the die using the punch of the die assembly, wherein, in the step of shearing, at least a portion of the curved portion of the blank is formed by cutting a portion of the metal sheet via the cutout portion of the punch or the cutout portion of the die.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0053] FIG. 1 is a diagram for illustrating shearing.

[0054] FIG. 2 is a schematic cross-sectional view of an exemplary sheared edge of a metal sheet that has been cut by shearing.

[0055] FIG. 3 is a diagram for illustrating stretch flanging.

[0056] FIG. 4 is a schematic perspective view of a blank according to an embodiment of the present invention.

[0057] FIG. 5 is a schematic perspective view of a formed article according to an embodiment of the present invention.

[0058] FIG. 6 presents diagrams illustrating the blank according to the embodiment of the present invention.

[0059] FIG. 7 is an enlarged plan view of a curved portion of the blank.

[0060] FIG. 8 is a schematic perspective view of a die assembly according to an embodiment of the present invention.

[0061] FIG. 9 is a schematic perspective view of the die assembly according to the embodiment of the present invention.

[0062] FIG. 10 presents schematic diagrams of the punch.

[0063] FIG. 11 presents diagrams for illustrating a method for producing the blank.

[0064] FIG. 12 presents diagrams for illustrating the method for producing the blank.

[0065] FIG. 13 presents diagrams for illustrating the method for producing the blank.

[0066] FIG. 14 presents diagrams for illustrating the method for producing the blank.

[0067] FIG. 15 presents diagrams for illustrating the method for producing the blank.

[0068] FIG. 16 presents diagrams illustrating other configurations of a cutout portion.

[0069] FIG. 17 is a schematic perspective view of a die assembly according to another embodiment of the present invention.

[0070] FIG. 18 is a schematic perspective view of a blank according to another embodiment of the present invention.

[0071] FIG. 19 is a schematic perspective view of an exemplary die assembly for producing the blank of FIG. 18.

[0072] FIG. 20 is a plan view of a specimen.

[0073] FIG. 21 is a photograph of a sheared edge in a stretch flanged area of Comparative Example 1.

[0074] FIG. 22 is a photograph of a sheared edge in a stretch flanged area of Example 5.

[0075] FIG. 23 is a diagram for illustrating a stretch flanging test.

DESCRIPTION OF EMBODIMENTS

[0076] Hereinafter, blanks, formed articles, die assemblies, and methods for producing a blank, according to the present invention, will be described with reference to the drawings. There are no particular limitations on the material for blanks according to the present invention. Examples of the material for the blanks include metal materials such as steels. When a steel is used as the material for the blanks, there are no particular limitations on the type of steel. Also, there are no particular limitations on the thickness and strength of the blanks provided that the thickness and strength are sufficient for shearing.

(Configurations of Blank and Formed Article)

[0077] FIG. 4 is a schematic perspective view of a blank 10 according to an embodiment of the present invention. Referring to FIG. 4, the sheet-shaped blank 10 has an approximately rectangular shape in plan view and has a hole 10a at the center. The hole 10a is formed by shearing (punching, for example). Thus, in the present embodiment, the blank 10 has, at the center, a sheared edge that has a loop shape in plan view. In other words, the sheared edge having the loop shape forms the hole 10a. Methods for producing the blank 10 will be described later.

[0078] The blank 10 is subjected to, for example, press forming (e.g., burring or deep drawing) to be formed into parts for automobiles, home appliances, and others. Specifically, referring to FIG. 4 and FIG. 5, a formed article 12, which includes a flange portion 12a, is produced for example by performing stretch flanging on the blank 10 with the hole 10a being the center. In the following, the blank 10 will be described more specifically.

[0079] FIG. 6(a) is a plan view of the blank 10 and FIG. 6(b) is an enlarged cross-sectional view taken along line A-A in FIG. 6(a). In FIG. 6(b), the sheet thickness direction of the blank 10 is indicated by an arrow X. In the following description, the vertical direction of the blank 10 is defined as the sheet thickness direction of the blank 10.

[0080] Referring to FIG. 6, the blank 10 includes a front surface 10b and a back surface 10c that are approximately parallel to each other and extend perpendicular to the sheet thickness direction. The sheared edge 14 includes a shear droop portion 14a, a sheared surface 14b, and a fractured surface 14c positioned in this order from the front surface 10b side of the blank 10 in the sheet thickness direction. In the present embodiment, a burr 16 is formed on the back surface 10c side of the blank 10. In the present embodiment, the burr 16 is defined as a portion protruding downward from the back surface 10c of the blank 10. In the present embodiment, the sheared edge 14 is defined as a portion extending from the perimeter edge, on the front surface 10b side, of the hole 10a to the upper end of the burr 16. Thus, in the present embodiment, the length of the sheared edge 14 in the sheet thickness direction corresponds to a sheet thickness t of the blank 10 (the vertical distance between the front surface 10b and the back surface 10c).

[0081] Referring to FIG. 6(a), in plan view, the perimeter edge of the hole 10a (inner edge of the sheared edge 14) includes a plurality of straight portions 18 and a plurality of curved portions 20. In the present embodiment, the perimeter edge of the hole 10a (inner edge of the sheared edge 14) includes four straight portions 18 and four curved portions 20. In plan view, the curved portions 20 are located between the straight portions 18, and are concavely curved. In the present embodiment, the curved portions 20 are arcuately concavely curved. Referring to FIG. 5 and FIG. 6(a), each curved portion 20 is a portion that will stretch and deform during stretch flanging. The range of the curved portion is defined by assuming sites, in the curved portions, where the sign of the curvature changes or the curvature becomes zero to be boundaries. In other words, the two opposite ends of the concavely curved portion are the points where the sign of the curvature changes or the curvature becomes zero provided that the curvature of the inner edge of the sheared edge 14 is determined in plan view.

[0082] FIG. 7 is an enlarged plan view of the curved portion 20 (the portion encircled by the dashed line in FIG. 6(a)) of the blank 10. In FIG. 7, the perimeter direction of the sheared edge 14 is indicated by an arrow Y.

[0083] Referring to FIG. 6(b) and FIG. 7, in the blank 10, the average of lengths of the fractured surface 14c in the sheet thickness direction in the curved portion 20 is greater than the average of lengths of the fractured surface 14c in the sheet thickness direction over the entire perimeter of the sheared edge 14.

[0084] The average of lengths of the fractured surface 14c in the curved portion 20 in the sheet thickness direction is determined in the following manner. Firstly, the curved portion 20 is equally divided into five areas in the perimeter direction of the sheared edge 14. Then, the lengths of the fractured surface 14c in the sheet thickness direction are measured at the boundaries between adjacent areas. That is, in the curved portion 20, the length of the fractured surface 14c in the sheet thickness direction is measured at four points different in position in the perimeter direction of the sheared edge 14. Then, the average of the measured lengths at the four points is calculated and the result is designated as the average of lengths of the fractured surface 14c in the sheet thickness direction in the curved portion 20. The averages of lengths of the shear droop portion 14a and the sheared surface 1413 in the sheet thickness direction in the curved portion 20 can be determined in the same manner.

[0085] The average of lengths of the fractured surface 14c in the sheet thickness direction over the entire perimeter of the sheared edge 14 is determined in the following manner. Firstly, the sheared edge 14 is equally divided into a plurality of areas with a predetermined width in the perimeter direction of the sheared edge 14. Then, the lengths of the fractured surface 14c in the sheet thickness direction are measured at the boundaries between adjacent areas. That is, the length of the fractured surface 14c in the sheet thickness direction is measured at a plurality of points different in position in the perimeter direction of the sheared edge 14. Then, the average of the measured lengths at the plurality of points is calculated and the result is designated as the average of lengths of the fractured surface 14c in the sheet thickness direction over the entire perimeter of the sheared edge 14. The predetermined width is set to be closest to the width of the five areas of the curved portion 20 when equally divided in the perimeter direction. The averages of lengths of the shear droop portion 14a and the sheared surface 14b in the sheet thickness direction over the entire perimeter of the sheared edge 14 can be determined in the same manner.

[0086] Referring to FIG. 7, a region R is a region extending a predetermined length in the perimeter direction of the sheared edge 14 with a reference point 22, which is defined as described below, being the center of the predetermined length. It is preferred that the average of lengths of the fractured surface 14c (see FIG. 6(b)) in the sheet thickness direction within the region R be greater than the average of lengths of the fractured surface 14c in the sheet thickness direction over the entire perimeter of the sheared edge 14. The reference point 22 is defined as the midpoint of the curved portion 20 in the perimeter direction of the sheared edge 14 or as the point where the curvature of the curved portion 20 in plan view is greatest. The predetermined length of the region R is a length of, for example, 50%, 100%, 1000%, or 2000% of the sheet thickness of the blank 10. Alternatively, for example, a region where points having a curvature of 5 m.sup.−1 or more are continuous in the curved portion 20 may be designated as the region R having a predetermined length. In this case, the region R may be determined by measuring the curvature of the curved portion 20 using a radius gauge.

[0087] In the present embodiment, the average of lengths of the fractured surface 14c in the sheet thickness direction within the region R is greater than the average of lengths of the fractured surface 14c in the sheet thickness direction over the entire perimeter of the sheared edge 14, by 10% or more of the sheet thickness of the blank 10. Furthermore, in the present embodiment, the average of lengths of the shear droop portion 14a in the sheet thickness direction within the region R is 20% or less of the sheet thickness of the blank 10. The average of lengths of the fractured surface 14c in the sheet thickness direction within the region R is determined in the following manner. Firstly, the sheared edge 14 within the region R is equally divided into five areas in the perimeter direction. Then, the lengths of the fractured surface 14c in the sheet thickness direction are measured at the boundaries between adjacent areas. That is, in the region R, the length of the fractured surface 14c in the sheet thickness direction is measured at four points different in position in the perimeter direction of the sheared edge 14. Then, the average of the measured lengths at the four points is calculated and the result is designated as the average of lengths of the fractured surface 14c in the sheet thickness direction within the region R. The averages of lengths of the shear droop portion 14a and the sheared surface 14b in the sheet thickness direction within the region R can be determined in the same manner.

(Advantageous Effects of the Blank and Formed Article)

[0088] In the blank 10, the length of the fractured surface 14c in the sheet thickness direction is greater in the curved portion 20. In other words, in the portion, which tends to stretch and deform during press forming, the sheared surface 14b occupies a smaller fraction. With this configuration, the curved portion 20 can easily stretch and deform, and therefore, the occurrence of stretch flange cracking is suppressed at the curved portion 20 when the curved portion 20 is subjected to stretch flanging. Furthermore, in the areas other than the curved portion 20, the fractured surface 14c occupies a smaller fraction than in the curved portion 20. In other words, the sheared surface 14b, which is work hardened, occupies a larger fraction. As a result, sufficient fatigue strength is exhibited in the areas other than the curved portion 20. On the other hand, the fractured surface 14c occupies a larger fraction in the curved portion 20. Thus, in its condition before press forming, the curved portion 20 has reduced fatigue strength. However, during press forming, the curved portion 20 is work hardened by stretch flanging and therefore is increased in fatigue strength. As a result, the formed article 12 after press forming exhibits sufficient fatigue strength. As a result of these, the occurrence of stretch flange cracking is suppressed in production of the formed article 12 from the blank 10 while suppressing the decrease in fatigue strength of the formed article 12.

[0089] In the blank 10, for example, the average of lengths of the fractured surface 14c in the sheet thickness direction within the region R is set to be greater than the average of lengths of the fractured surface 14c in the sheet thickness direction over the entire perimeter of the sheared edge 14. This configuration suppresses the occurrence of stretch flange cracking at a central area (a positional center or an area where the curvature is large) of the curved portion 20.

[0090] In the blank 10, the average of lengths of the fractured surface 14c in the sheet thickness direction within the region R is greater than the average of lengths of the fractured surface 14c in the sheet thickness direction over the entire perimeter of the sheared edge 14, by 10% or more of the sheet thickness of the blank 10. This sufficiently suppresses the occurrence of stretch flange cracking at a central area of the curved portion 20.

[0091] In the blank 10, the average of lengths of the shear droop portion 14a in the sheet thickness direction within the region R is 20% or less of the sheet thickness of the blank 10. This suppresses the occurrence of stretch flange cracking more reliably.

[0092] In the blank 10, the predetermined length of the region R is set to a length of 50% of the sheet thickness of the blank 10, for example. This configuration more reliably suppresses the occurrence of stretch flange cracking at a central area of the curved portion 20. The predetermined length of the region R may be set to a length of 2000% of the sheet thickness of the blank 10, for example. This configuration suppresses the occurrence of stretch flange cracking over a sufficient range within the curved portion 20. Furthermore, the region R may be a region where the curvature is 5 m.sup.−1 or more, for example. This configuration sufficiently prevents the occurrence of stretch flange cracking in the curved portion 20, where larger stretch flanging deformation occurs during press forming.

[0093] Although the blank 10 includes the plurality of curved portions 20, it suffices if one of the curved portions 20 satisfies the requirements of the present invention. Accordingly, there may be a curved portion(s) 20 that does not satisfy the requirements of the present invention among the plurality of curved portions 20.

(Die Assembly for Producing Blank and Method for Producing Blank)

[0094] In the following, a die assembly for producing the above blank 10 and a method for producing the blank 10 using the die assembly will be described.

[0095] FIG. 8 and FIG. 9 are schematic perspective views of a die assembly 24 according to an embodiment of the present invention. Referring to FIG. 8, the die assembly 24 includes a columnar punch 26 and a hollow die 28, which has a hole 28a. The hole 28a is configured to receive the punch 26. Referring to FIG. 8, when the above blank 10 is to be produced, firstly a metal sheet 30, which has a rectangular shape in plan view, is placed on the die 28. Referring to FIG. 8 and FIG. 9, subsequently the punch 26 is moved in the sheet thickness direction (direction indicated by an arrow Z in FIG. 8) of the metal sheet 30 to press a central portion of the metal sheet 30 inward by the punch 26 in such a manner that the lower end of the punch 26 is inserted in the hole 28a. Accordingly, the central portion of the metal sheet 30 is cut off (sheared off) to form the hole 10a (see FIG. 4). That is, the above blank 10 (see FIG. 4) is produced. The details will be described later. Hereinafter, the punch 26 and the die 28 will be described specifically. In the following description, the direction of movement of the punch 26 in shearing of the metal sheet 30 (direction indicated by the arrow Z) is designated as the vertical direction. Also, the direction perpendicular to the vertical direction is designated as the lateral direction.

[0096] FIG. 10 presents schematic diagrams of the punch 26. FIG. 10(a) is a side view of the punch 26 and FIG. 10(b) is a bottom plan view of the punch 26.

[0097] Referring to FIG. 10, the punch 26 has a bottom surface 32 and an outer perimeter surface 34, which extends from an outer perimeter edge 32a of the bottom surface 32. In the punch 26, the outer perimeter edge 32a of the bottom surface 32 serves as the cutting edge. Accordingly, the outer perimeter edge 32a has an approximately rectangular shape in plan view as with the hole 10a so that the hole 10a (see FIG. 4) can be formed.

[0098] Referring to FIG. 10(b), the outer perimeter edge 32a of the bottom surface 32 includes a plurality of (four in the present embodiment) curved portions 36, which are convexly curved in bottom view (in plan view). In the present embodiment, the curved portions 36 are provided at the four respective corners of the approximately rectangular outer perimeter edge 32a.

[0099] Referring to FIGS. 10(a) and 10(b), the bottom surface 32 includes a planar portion 38 and a plurality of cutout portions 40, which are recessed upwardly (in a direction parallel to the direction of movement of the punch 26) with respect to the planar portion 38. Referring to FIG. 10(a), the cutout portions 40 have a rectangular shape in side view. More specifically, referring to FIGS. 10(a) and 10(b), the cutout portions 40 each include side walls 40a, 40b, 40c, which extend upwardly from the planar portion 38, and a ceiling 40d, which connects the upper edges of the side walls 40a, 40b, 40c. The side walls 40a, 40b, 40e are disposed in such a manner as to form an approximately U-shape in bottom view. In the present embodiment, each side wall 40a and each side wall 40b face each other, and each side wall 40c connects between one end of the side wall 40a and one end of the side wall 40b. The ceilings 40d are approximately parallel to the planar portion 38. Referring to FIG. 10(b), the cutout portions 40 are formed to include the center (apex) of the curved portions 36 in bottom view (in plan view).

[0100] Referring to FIG. 10(a), a cutout depth d of each cutout portion 40 is preferably set to 0.1 times or more the sheet thickness of the metal sheet 30 (see FIG. 9) and 0.7 times or less the sheet thickness. Referring to FIG. 10(b), a width w of the cutout portion 40 is appropriately set according to the dimensions of the curved portion 20 (see FIG. 6) of the blank 10 (see FIG. 6), but preferably, it is set to a size of 50 to 2000% of the sheet thickness of the metal sheet 30 and more preferably set to a size of 100 to 1000% of the sheet thickness. Furthermore, the die assembly 24 is preferably configured such that the centerline of the cutout portion 40 with respect to the width direction is positioned in alignment with the reference point 22 of the curved portion 20 of the blank 10 when the metal sheet 30 is to be cut. A length L of the cutout portion 40 is preferably equal to or greater than the sheet thickness of the metal sheet 30.

[0101] Referring to FIG. 8, the die 28 includes a hollow support surface 42 for supporting the metal sheet 30 and an inner perimeter surface 44, which extends downwardly from an inner perimeter edge 42a of the support surface 42. In the die 28, the inner perimeter edge 42a of the support surface 42 serves as the cutting edge. The inner perimeter edge 42a of the support surface 42 has a shape similar to the shape of the outer perimeter edge 32a of the bottom surface 32, and includes a plurality of curved portions 46, which correspond to the plurality of curved portions 36 of the outer perimeter edge 32a. The curved portions 46 have a concavely curved shape corresponding to the shape of the curved portions 36. The clearance between the punch 26 and the die 28 (i.e., the clearance between the outer perimeter edge 32a and the inner perimeter edge 42a) is set to, for example, a size of approximately 10% of the sheet thickness of the metal sheet 30.

[0102] In the following, a method for producing the blank 10 using the above die assembly 24 will be described specifically with reference to the drawings. FIGS. 11 to 15 are conceptual diagrams illustrating the relationships between the punch 26, the die 28, and the metal sheet 30 in the production of the blank 10. Specifically, in FIGS. 11 to 15, the figures labeled (a) are conceptual diagrams illustrating the relationships between the outer perimeter surface 34 (see FIG. 8) of the punch 26 in the vicinity of the curved portion 36 (see FIG. 8), the inner perimeter surface 44 (see FIG. 8) of the die 28 in the vicinity of the curved portion 46 (see FIG. 8), and the metal sheet 30, which is positioned between the curved portion 36 (see FIG. 8) and the curved portion 46 (see FIG. 8). In FIGS. 11 to 15, the figures labeled (b) are conceptual diagrams illustrating the relationships between the planar portion 38 of the punch 26, the support surface 42 of the die 28, and the metal sheet 30, which is positioned between the planar portion 38 and the support surface 42 (conceptual diagrams of the areas indicated by line b-b in FIG. 11(a)). In FIGS. 11 to 15, the figures labeled (c) are conceptual diagrams illustrating the relationships between the cutout portion 40 of the punch 26, the support surface 42 of the die 28, and the metal sheet 30, which is positioned between the cutout portion 40 and the support surface 42 (conceptual diagrams of the areas indicated by line c-c in FIG. 11(a)). In the figures labeled (a) in FIGS. 11 to 15, the metal sheet 30 is hatched to clarify the positional relationship.

[0103] Referring to FIG. 8, when the blank 10 is to be produced, firstly the metal sheet 30 is placed on the support surface 42 of the die 28 as described above. Then, as illustrated in FIG. 11 and FIG. 12, the punch 26 is moved to force the planar portion 38 of the punch 26 into the metal sheet 30. Accordingly, a sheared surface 48 (see FIG. 12) is formed on the front side of the metal sheet 30 by the outer edge of the planar portion 38. Furthermore, in the contact region between the die 28 and the back surface of the metal sheet 30, at the areas facing the outer edge of the planar portion 38, a sheared surface 50 is formed by the inner perimeter edge 42a of the support surface 42 of the die 28. As illustrated in FIGS. 12(a) and 12(c), while the amount of forcing of the punch 26 is still small, the ceilings 40d of the cutout portions 40 are not yet in contact with the metal sheet 30. Thus, in the metal sheet 30, at the areas facing the cutout portions 40, the sheared surface has not yet been formed. Also, in the contact region between the die 28 and the metal sheet 30, the area located below the cutout portion 40 has not yet received a large force, and therefore on the area as well, a sheared surface has not yet been formed.

[0104] As illustrated in FIGS. 13(a) and 13(b), when the punch 26 is further forced inward, cracks 52 occur in the front surface of the metal sheet 30 in areas in contact with the outer edge of the planar portion 38. Also, as illustrated in FIGS. 13(a) and 13(c), the ceilings 40d of the cutout portions 40 come into contact with the front surface of the metal sheet 30. Accordingly, a sheared surface 54 is formed in the metal sheet 30 in the contact regions between the ceilings 40d and the metal sheet 30. Furthermore, as illustrated in FIGS. 13(a) to 13(c), cracks 56 occur in the metal sheet 30 in the contact regions between the inner perimeter edge 42a of the support surface 42 of the die 28 and the metal sheet 30.

[0105] When the punch 26 is further forced inward, the cracks 52, 56 propagate in the sheet thickness direction of the metal sheet 30, so that fractured surfaces 58, 60 are formed on the front side and the back side of the metal sheet 30 as illustrated in FIGS. 14(a) and 14(b). As illustrated in FIGS. 14(a) and 14(c), cracks 52, 56 (see FIG. 13) propagate not only in the sheet thickness direction but also toward the contact regions between the metal sheet 30 and the cutout portions 40. As a result, fractured surfaces 58, 60 are also formed below the cutout portions 40. That is, before the cutout portions 40 are forced deeply into the metal sheet 30, the sufficiently large fractured surface 14c (see FIG. 6) is formed in the areas located below the cutout portions 40 in the metal sheet 30. Finally, when the punch 26 is further forced inward as illustrated in FIG. 15, the fractured surfaces 58, 60 propagate further so that a portion of the metal sheet 30 is cut off. In this manner, the blank 10 is produced.

(Advantageous Effects of Die Assembly and Production Method Using Die Assembly)

[0106] When the blank 10 is produced by the production method described above using the die assembly 24, the sufficiently large fractured surface 14c is formed in the areas located below the cutout portions 40 in the metal sheet 30 before the cutout portions 40 are forced deeply into the metal sheet 30. As a result, the lengths of the fractured surface 14c in the sheet thickness direction in the areas below the cutout portions 40 are greater than the lengths of the fractured surface 14c in the sheet thickness direction in the other areas. Thus, by cutting the areas that will undergo stretch flanging deformation during press forming via the cutout portions 40, stretch flange cracking is prevented. In addition, in the areas cut by the planar portion 38, the lengths of the fractured surface 14c in the sheet thickness direction are shorter, and therefore the decrease in fatigue strength is suppressed.

[0107] In the die assembly 24, the cutout depth of the cutout portions 40 is set to 0.1 times or more the sheet thickness of the metal sheet 30 and 0.7 times or less the sheet thickness, for example. This configuration makes it possible to appropriately delay the time at which the cutout portions 40 begin pressing the metal sheet 30 relative to the time at which the planar portion 38 begin pressing the metal sheet 30. As a result, in the areas cut by the cutout portions 40, the fractured surface 14c has appropriate lengths in the sheet thickness direction.

[0108] The die assembly 24 of the present invention can be produced merely by partially modifying the shape of the cutting edge (a portion corresponding to the outer perimeter edge 32a of the bottom surface 32) of conventional punches. As a result, the cost of die assembly production is reduced compared with the case in which a projection is provided in the punch (see for example Patent Document 1, described above). In addition, there is no need to consider the overall tool shape for shearing tools, which are of a variety of shapes, and therefore the die assembly is readily applicable to mass production facilities. Furthermore, when stretch flange cracking has occurred during press forming, a new cutout portion 40 can be added to the punch at a location corresponding to the location where the cracking occurred in the blank, by means such as an end mill. Thus, stretch flange cracking can be addressed on-site. In this regard as well, the die assembly is readily applicable to mass production facilities. The same applies to other punches to be described later and other dies including cutout portions to be described later.

[0109] It is preferred that sites that are prone to stretch flange cracking in the sheared edge of the blank be identified in advance by performing computation or conducting a stretch flanging test. Then, the die assembly may be configured to cut the identified sites by the cutout portions. This results in reduced costs of producing the die assembly and of processing the blank.

(Other Exemplary Die Assemblies)

[0110] Although, in the embodiment described above, the description refers to a case in which the punch 26 includes rectangular cutout portions 40 in side view, the shape of the cutout portions is not limited to the example described above. For example, the punch may include cutout portions 62, which have a trapezoidal shape in side view as illustrated in FIG. 16(a). The cutout portions 62 each include side walls 62a, 62b, 62c and a ceiling 62d as with the cutout portions 40. The side walls 62a, 62b are inclined such that the distance between them decreases toward the top in side view. Various studies by the present inventors reveal that the inclination angle of the side walls 62a, 62b with respect to a vertical plane is preferably not more than 30° in order to achieve efficient crack propagation by the cutout portions 62.

[0111] Alternatively, for example, the punch may include cutout portions 64, which have a semi-circular shape in side view as illustrated in FIG. 16(b). Alternatively, for example, the punch may include cutout portions 66, which have round corners 66c, 66d at boundaries between the planar portion 38 and side walls 66a, 66b as illustrated in FIG. 16(c). This configuration prevents damage at the boundaries between the cutout portions 66 and the planar portion 38. The radius of curvature of the radius corners 66c, 66d preferably ranges from 0.01 to 0.1 mm. Alternatively, for example, the punch may include cutout portions 68, which have beveled portions 68c, 68d at boundaries between the planar portion 38 and side walls 68a, 68b as illustrated in FIG. 16(d). This configuration also prevents damage at the boundaries between the cutout portions 68 and the planar portion 38.

[0112] In the embodiment described above, the description refers to the punch 26, which includes the plurality of cutout portions 40, but it is also possible to provide the cutout portions in the die instead of providing the cutout portions in the punch.

[0113] FIG. 17 is a schematic perspective view of a die assembly 24a according to another embodiment of the present invention. The die assembly 24a illustrated in FIG. 17 is different from the die assembly 24 illustrated in FIG. 8 in that a punch 70 is included in place of the punch 26 and a die 72 is included in place of the die 28.

[0114] The punch 70 is different from the punch 26 in that the plurality of cutout portions 40 (see FIG. 8) are not included. The die 72 is different from the die 28 in that the curved portions 46 include cutout portions 74, which have a shape similar to that of the cutout portions 40. Although not described in detail, in the case of using the die assembly 24a to produce the blank 10, advantageous effects similar to those of the case of using the above die assembly 24 to produce the blank 10 are achieved.

(Other Exemplary Blanks)

[0115] In the embodiment described above, the description refers to the blank 10, which has the hole 10a formed by punching, but the shape of the blank is not limited to the example described above. The present invention is also applicable to a blank in which a sheared edge is formed along the outer perimeter edge, e.g., a blank having a sheared edge formed by blanking along the outer perimeter edge.

[0116] FIG. 18 is a schematic perspective view of a blank according to another embodiment of the present invention. Referring to FIG. 18, a blank 76, which is sheet-shaped and elongate, has a shape such that the central portion in the longitudinal direction is narrower than the opposite end portions in the longitudinal direction. The blank 76 is produced by blanking for example and has a sheared edge 78 along the outer perimeter edge. The sheared edge 78 has a loop shape in plan view. In plan view, the outer edge of the sheared edge 78 includes a plurality of curved portions 80, which are concavely curved. Although not described in detail, the sheared edge 78 has a configuration similar to that of the sheared edge 14 of the blank 10 and the curved portions 80 have a configuration similar to that of the curved portions 20 of the blank 10. Thus, with the blank 76, advantageous effects similar to those of the above blank 10 are achieved.

[0117] Next, a die assembly for producing the above blank 76 will be described. FIG. 19 is a schematic perspective view of an exemplary die assembly for producing the blank 76. Referring to FIG. 19, the die assembly 82 includes a columnar punch 84 and a die 86, which has a hole 86a. The hole 86a is configured to receive the punch 84.

[0118] Referring to FIG. 19, the punch 84 includes a bottom surface 88 and an outer perimeter surface 90, which extends from an outer perimeter edge 88a of the bottom surface 88. In the punch 84, the outer perimeter edge 88a of the bottom surface 88 serves as the cutting edge. Accordingly, the outer perimeter edge 88a has a shape similar to that of the blank 76.

[0119] The outer perimeter edge 88a of the bottom surface 88 includes a plurality of (two in the present embodiment: only one curved portion 92 is illustrated in FIG. 19) curved portions 92, which are concavely curved in bottom view (in plan view). The bottom surface 88 includes a planar portion 94 and a plurality of (two in the present embodiment) cutout portions 96, which are recessed upwardly (in a direction parallel to the direction of movement of the punch 84) with respect to the planar portion 94, as with the above bottom surface 32 (see FIG. 10). Although not described in detail, the cutout portions 96 have a similar configuration to that of the above cutout portions 40, 62, 64, 66, or 68. The cutout portions 96 are formed to include the center (apex) of the curved portions 92 in bottom view (in plan view).

[0120] The die 86 includes a hollow support surface 98 for supporting the metal sheet (not illustrated) and an inner perimeter surface 100, which extends downwardly from an inner perimeter edge 98a of the support surface 98. In the die 86, the inner perimeter edge 98a of the support surface 98 serves as the cutting edge. The inner perimeter edge 98a of the support surface 98 has a shape similar to the shape of the outer perimeter edge 88a of the bottom surface 88, and includes a plurality of curved portions 102, which correspond to the plurality of curved portions 92 of the outer perimeter edge 88a. The curved portions 102 have a convexly curved shape corresponding to the shape of the curved portions 92. The clearance between the punch 84 and the die 86 is set to, for example, a size of approximately 10% of the sheet thickness of the metal sheet.

[0121] In the die assembly 82 as well, the punch 84 includes the cutout portions 96 as with the above punch 26. As a result, with the die assembly 82, advantageous effects similar to those of the above die assembly 24 are achieved. As with the die assembly 24a in FIG. 17, it is also possible to provide cutout portions in the curved portions 102 of the die 86 instead of providing the cutout portions 96 in the punch 84. This configuration produces advantageous effects similar to those of the die assembly 82.

EXAMPLE

[0122] In the following, the present invention will be described in more detail by way of examples, but the present invention is not limited to the examples described below.

First Example

[0123] Blanks for Examples 1 to 12 were produced by forming a hole in a 780 MPa class cold-roiled steel sheet of 1.6 mm sheet thickness (workpiece). The hole had a shape (30 mm×30 mm; the radius of curvature of the curved portions (radius corners) was 5 mm) similar to the shape of the hole 10a illustrated in FIG. 4. A punch illustrated in FIG. 8 was used (the shape of the cutout portions was rectangular. Opening width: 0 to 15 mm; length of cutout portion: 0 to entire punch bottom length; and corners, which are boundaries between the cutting edges and the cutout portions, had a roundness of R1.0). Furthermore, a blank for Comparative Example 1 was produced using a punch having a configuration similar to the punch of FIG. 8 except for the absence of cutout portions. Furthermore, a blank for Comparative Example 2 was produced using a punch disclosed in Patent Document 2. The clearance between the die and the punch was set to 10% of the sheet thickness of the workpiece.

[0124] The blanks produced in the above manner were subjected to burring using a truncated pyramid-shaped burring punch having a curved edge (not illustrated) to form a flange portion (burring portion) such as illustrated in FIG. 5 (burring test). In the burring test, the critical burring height at which cracking occurs in the sheared edge was measured to evaluate the stretch flanging properties.

[0125] To investigate the fatigue strength of the sheared portions, test specimens such as illustrated in FIG. 20 were cut and subjected to a plane bending fatigue test. The fatigue test specimens were cut by machining. The machined portions were subjected to grinding to increase the flatness. The sheared portions (portions corresponding to the holes formed by the punch) were not subjected to grinding. The maximum stress that could be applied to the outer layer of the test specimen (calculated from the bending moment) was used as the criterion, and the stress ratio was set to −1. The fatigue strength was evaluated by determining the stress at the failure limit at the point when ten million cycles of life was reached to be the fatigue limit.

[0126] Table 1 shows the configurations of the cutout portions of the punches used for punching and the results of the burring test. Table 2 shows the shear droop fraction, sheared surface fraction, and fractured surface fraction in the sheared edge at locations corresponding to stretch flanged areas and at locations not corresponding to the stretch flanged areas. It was assumed that portions (four corner portions) corresponding to the curved portions 20, which were described with reference to FIG. 7, were the stretch flanged areas. For reference, FIG. 21 and FIG. 22 show photographs of the exteriors of the sheared edges in stretch flanged areas of Comparative Example 1 and Example 5.

TABLE-US-00001 TABLE 1 Configuration of cutout portion Width/Sheet Depth/Sheet Length/Sheet Burring height Fatigue limit thickness (%) thickness (%) thickness (%) (mm) (MPa) Example 1 75 9.4 44 6 310 Example 2 313 12.5 Entire length 12 305 Example 3 313 31.3 Entire length 16 305 Example 4 313 50 Entire length 15 310 Example 5 313 62.5 Entire length 17 315 Example 6 625 62.5 Entire length 17 305 Example 7 938 62.5 Entire length 16 310 Example 8 313 62.5 62.5 15 310 Example 9 313 62.5 187.5 16 310 Example 10 313 62.5 625 16 310 Example 11 1875 62.5 Entire length 13 305 Example 12 313 62.5 18.8 14 310 Comparative — — — 9 310 Example 1 Comparative — — — 12 270 Example 2

TABLE-US-00002 TABLE 2 Sheared edge shape of Sheared edge shape of stretch flanged area non-stretch flanged area Difference Shear Sheared Fractured Shear Sheared Fractured in fractured droop surface surface droop surface surface surface fraction fraction fraction fraction fraction fraction fraction (%) (%) (%) (%) (%) (%) (%) Example 1 7.2 33.6 59.2 6.4 36.8 56.8 2.4 Example 2 7.36 25.6 67.04 6.4 36.8 56.8 10.24 Example 3 7.44 24 68.56 6.4 36.8 56.8 11.76 Example 4 7.6 14.4 78 6.4 36.8 56.8 21.2 Example 5 7.68 12.8 79.52 6.4 36.8 56.8 22.72 Example 6 7.68 16.8 75.52 6.56 35.2 58.24 17.28 Example 7 7.84 18.4 73.76 6.56 35.2 58.24 15.52 Example 8 7.6 17.6 74.8 6.56 36 57.44 17.36 Example 9 7.68 17.6 74.72 6.56 36 57.44 17.28 Example 10 7.68 17.6 74.72 6.56 36 57.44 17.28 Example 11 8 20 72 6.56 36 57.44 14.56 Example 12 7.52 22.4 70.08 6.56 36 57.44 12.64 Comparative 6.56 36 57.44 6.56 36 57.44 0 Example 1 Comparative 72 28 64.8 7.2 28 64.8 0 Example 2

[0127] The results of the burring test indicate that the blanks of Examples 2 to 12, in which the cutout depths of the cutout portions constitute a fraction (%) within a range of 10 to 70% of the sheet thickness of the blank, achieved larger burring heights than the blank of Comparative Example 1. Furthermore, the blank of Comparative Example 2, in which the fractured surface fraction was increased over the entire perimeter of the sheared edge, had cracks in the sheared edge at areas other than the stretch flanged areas and therefore exhibited a decreased fatigue strength. On the other hand, the blanks of Examples 1 to 12 did not have cracks also at areas other than the stretch flanged areas and therefore did not have a decrease in fatigue strength.

[0128] Although, in First Example, a 780 MPa class cold-rolled steel sheet of 1.6 mm sheet thickness was used, the present inventors empirically have found that other steel sheets having different thicknesses or strengths, when used, can achieve similar advantageous effects.

Second Example

[0129] Blanks for Examples 1 to 12 having a shape similar to that of the blank 76 illustrated in FIG. 18 were produced by shearing a 590 MPa class cold-rolled steel sheet of 1.6 mm sheet thickness (workpiece) using a punch 84 illustrated in FIG. 19. Furthermore, a blank for Comparative Example 1 was produced using a punch having a configuration similar to that of the punch 84 in FIG. 19 except for the absence of cutout portions. The clearance between the die and the punch was set to 10% of the sheet thickness of the workpiece.

[0130] FIG. 23(a) illustrates how the stretch flanging test was conducted and FIG. 23(b) illustrates the shape of a stretch flanged article. As illustrated in FIG. 23(a), in the stretch flanging test, a blank 108 was placed on a die 106 supported on a pad 104. Then, flanging was performed by pressing the blank 108 by a punch 110 to produce a stretch flanged article 112 illustrated in FIG. 23(b).

[0131] The stretch flanging test was conducted under various conditions including different stretch flange heights hl (5 mm, 10 mm, 15 mm, 20 mm, and 25 mm), i.e., under five conditions that are different from each other in the amount of plastic deformation in the sheared edge resulting from the stretch flanging test.

[0132] Table 3 shows the configurations of the cutout portions of the punches used for shearing and the results of the stretch flanging test. Table 4 shows the shear droop fraction, sheared surface fraction, and fractured surface fraction in the sheared edge at locations corresponding to the stretch flanged areas and at locations not corresponding to the stretch flanged areas.

TABLE-US-00003 TABLE 3 Stretch Configuration of cutout portion flange Width/Sheet Depth/Sheet Length/Sheet height thickness (%) thickness (%) thickness (%) (mm) Example 1 75 9.4 44 10 Example 2 313 12.5 Entire length 15 Example 3 313 31.3 Entire length 20 Example 4 313 50 Entire length 20 Example 5 313 62.5 Entire length 25 Example 6 625 62.5 Entire length 20 Example 7 938 62.5 Entire length 25 Example 8 313 62.5 62.5 20 Example 9 313 62.5 187.5 25 Example 10 313 62.5 625 25 Example 11 1875 62.5 Entire length 15 Example 12 313 62.5 18.8 15 Comparative — — — 10 Example 1

TABLE-US-00004 TABLE 4 Sheared edge shape of Sheared edge shape of stretch flanged area non-stretch flanged area Difference Shear Sheared Fractured Shear Sheared Fractured in fractured droop surface surface droop surface surface surface fraction fraction fraction fraction fraction fraction fraction (%) (%) (%) (%) (%) (%) (%) Example 1 12 38 SO 14 42 44 6 Example 2 13 30 57 14 42 44 13 Example 3 13 27 60 14 42 44 16 Example 4 14 26 60 14 42 44 16 Example 5 15 18 67 14 42 44 23 Example 6 15 22 63 14 42 44 19 Example 7 15 17 68 14 42 44 24 Example 8 14 20 66 14 42 44 22 Example 9 15 18 67 14 42 44 23 Example 10 15 18 67 14 42 44 23 Example 11 15 27 58 14 42 44 14 Example 12 12 28 60 14 42 44 16 Comparative 14 42 44 14 42 44 0 Example 1

[0133] The results of the stretch flanging test indicate that the blanks of Examples 1 to 12 did not have stretch flange cracking in the sheared edges. In contrast, the blank of Comparative Example 1 had stretch flange cracking.

INDUSTRIAL APPLICABILITY

[0134] The present invention provides a shearing method which achieves a reduction in the cost of producing the tool, which is readily applicable to mass production facilities, and which suppresses stretch flange cracking in the sheared edge. Thus, the present invention finds high applicability in the steel processing industry.