Manufacturing apparatus and manufacturing method for stretch-formed product

Abstract

There is provided a manufacturing apparatus and a manufacturing method for a stretch-formed product which can improve the yield of material in the stretch forming performed while clamping the blank by the lock beads. The manufacturing apparatus for a stretch-formed product includes a die and a blank holder which have clamping surfaces facing each other, a punch that, in a state where a margin of a blank of a sheet material is clamped by the clamping surfaces of the die and the blank holder, relatively presses a forming region of the blank into the die and thereby performs stretch forming on the forming region of the blank, and lock beads that are provided on the clamping surfaces of the die and the blank holder in mutually similar shapes and have first surfaces, second surfaces that intersect with the first surfaces, and third surfaces that intersect with the second surfaces from outer edges toward the centers of the die and the blank holder, the first surfaces each having a plurality of depression-protrusion parts.

Claims

1. A manufacturing apparatus for a stretch-formed product, the manufacturing apparatus comprising: a die and a blank holder which have clamping surfaces facing each other; a punch that, in a state where a margin of a blank of a sheet material is clamped by the clamping surfaces of the die and the blank holder, relatively presses a forming region of the blank into the die and thereby performs stretch forming on the forming region of the blank; and lock beads that are provided on the clamping surfaces of the die and the blank holder in mutually similar shapes and have first surfaces, second surfaces that intersect with the first surfaces, and third surfaces that intersect with the second surfaces from outer edges toward centers of the die and the blank holder, the first surfaces each having a plurality of depression-protrusion parts, wherein the plurality of depression-protrusion parts each have a fourth surface and a sixth surface facing each other and a fifth surface that intersects with the fourth surface and the sixth surface, the fourth surface, the fifth surface and the sixth surface intersect with the second surface, and when the plurality of depression-protrusion parts are viewed from the outer edge toward the center of the blank holder, the plurality of depression-protrusion parts have a trapezoidal shape, a rectangular shape or a combined shape thereof.

2. The manufacturing apparatus for a stretch-formed product according to claim 1, wherein in a case where the plurality of depression-protrusion parts each have the trapezoidal shape or the rectangular shape, a pitch interval of the plurality of depression-protrusion parts when a set of a protruded shape and a depressed shape is defined as one pitch is within a range of 5 to 50 mm and a height of the depression-protrusion part is within a range of 1.0 to 10.0 mm.

3. A manufacturing method for a stretch-formed product, the manufacturing method comprising the steps of: placing a blank of a sheet material between a die and a blank holder which include, on clamping surfaces facing each other, lock beads that have first surfaces, second surfaces that intersect with the first surfaces and third surfaces that intersect with the second surfaces from outer edges toward centers, the first surfaces each having a plurality of depression-protrusion parts, and the lock beads being provided in mutually similar shapes; clamping a margin of the blank by the die and the blank holder; and stretch-forming the blank by relatively pressing a forming region of the blank into the die by a punch in a state of clamping the margin of the blank, wherein the plurality of depression-protrusion parts each have a fourth surface and a sixth surface facing each other and a fifth surface that intersects with the fourth surface and the sixth surface, the fourth surface, the fifth surface and the sixth surface intersect with the second surface, and when the plurality of depression-protrusion parts are viewed from the outer edge toward the center of the blank holder, the plurality of depression-protrusion parts have a trapezoidal shape, a rectangular shape or a combined shape thereof.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a perspective view for explaining a configuration of a manufacturing apparatus for a stretch-formed product according to an embodiment of the present invention.

(2) FIG. 2 is a perspective view showing states before and after clamping a blank by lock beads according to the embodiment.

(3) FIG. 3 is a sectional view showing the lock beads, an outer peripheral part of the blank and a trim line.

(4) FIG. 4 is a top view showing the lock beads, the outer peripheral part of the blank and the trim line.

(5) FIG. 5 is a perspective view showing states before and after clamping the blank by other lock beads.

(6) FIG. 6 is a sectional view showing the other lock beads, the outer peripheral part of the blank and the trim line.

(7) FIG. 7 is a top view showing the other lock beads, the outer peripheral part of the blank and the trim line.

(8) FIG. 8 is an explanatory diagram showing a test procedure of an Evaluation 1 in an Example.

(9) FIG. 9 is a graph showing a result of the Evaluation 1 in the Example.

(10) FIG. 10 is photographs for explaining a difference in blank flowing mark due to a difference in decision on lock performance.

(11) FIG. 11 is a front view showing an outline shape of a blank of a stretch-formed product manufactured through an Evaluation 3 in the Example.

(12) FIG. 12 is a perspective view showing a dimension of each part of the stretch-formed product manufactured through the Evaluation 3 in the Example.

(13) FIG. 13 is an explanatory diagram showing a state of stretch forming.

(14) FIG. 14 is an explanatory diagram showing one example of the blank used in the stretch forming.

(15) FIG. 15 is a perspective view showing states before and after clamping the blank in the stretch forming using a die provided with conventional trapezoidal beads.

(16) FIG. 16 is a sectional view showing the vicinity of the conventional trapezoidal beads.

(17) FIG. 17 is a top view showing the vicinity of the conventional trapezoidal beads.

DESCRIPTION OF EMBODIMENTS

(18) Hereinafter, (a) preferred embodiment(s) of the present invention will be described in detail with reference to the appended drawings. In this specification and the appended drawings, structural elements that have substantially the same function and structure are denoted with the same reference numerals, and repeated explanation of these structural elements is omitted. Although in the following description, description will be made by taking a case where the stretch-formed product is a door outer panel by way of example, the stretch-formed product is not limited to the door outer panel. The present invention is equally applied also to other stretch-formed products such as a hood outer panel and a roof panel.

(19) <1. Manufacturing Apparatus for Stretch-Formed Product>

(20) FIG. 1 is a schematic explanatory diagram showing a configuration of a manufacturing apparatus 10 for a stretch-formed product according to the present embodiment. FIG. 1 is a perspective view showing the manufacturing apparatus 10 by partially omitting and simplifying it. As shown in FIG. 1, the manufacturing apparatus 10 has a die 11, a blank holder 12, and a punch 13. In FIG. 1, the contour of a blank 14 is shown by a two-dot chain line.

(21) (1-1. Basic Configuration)

(22) The die 11 has a clamping surface 11a which clamps the blank 14. The clamping surface 11a has a punch containing part in which the punch 13 is to be contained when forming, and a wedge-shaped bead 15 which is provided along the outside of a margin of the blank 14. The wedge-shaped bead 15 is one form of the lock bead. In order to make the drawing easily visible, in FIG. 1, the die 11 and the wedge-shaped bead 15 are shown by being simplified by a one-dot chain line. Details of the wedge-shaped bead 15 will be described later with reference to FIG. 2. In addition, the punch containing part is omitted in FIG. 1.

(23) The blank 14 is located at the center and has a forming region 14b corresponding to a part which will become a product (in the example in FIG. 1, a rear door outer panel), a clamp target part 14a to be clamped by the die 11 and the blank holder 12 and a trim line 14d. The clamp target part 14a and the outer peripheral part 14c are cut off along the trim line 14d and are discarded. Incidentally, in FIG. 1, the trim line 14d is omitted.

(24) The wedge-shaped bead 15 may be arranged on the entire circumference of the blank 14. Alternatively, in a case where a part to be subjected to drawing which involves material flowing-in and a part to be subjected to stretch forming which does not involve material flowing-in are present on the blank 14, the wedge-shaped bead 15 may be arranged only on the part to be subjected to the stretch forming. In this case, various types of known draw beads for the drawing can be provided on the part to be subjected to the drawing.

(25) The blank holder 12 is arranged so as to face the die 11. The blank holder 12 has the punch containing part in which the punch 13 is contained, and the clamping surface 12a which clamps the blank 14 in cooperation with the clamping surface 11a of the die 11. A wedge-shaped bead 16 is provided on the clamping surface 12a along the margin of the blank 14. The wedge-shaped bead 16 is arranged at a position corresponding to the wedge-shaped bead 15 provided on the die 11. Details of the wedge-shaped bead 16 will be described later with reference to FIG. 2.

(26) The punch 13 is arranged in the punch containing part of the blank holder 12 so as to face the punch containing part of the die 11. The punch 13 is relatively pressed into the die 11 at the time of forming. Thereby, the forming region 14b of the blank 14 is stretch-formed and the forming region 14b is formed into the door outer panel.

(27) Since the materials and functions of the die 11, the blank holder 12 and the punch 13 may be the same as the materials and functions which are known as those of dies, blank holders and punches of this type, and are well known to a person skilled in the art, further description on the die 11, the blank holder 12 and the punch 13 is omitted.

(28) (1-2. Lock Bead (Wedge-Shaped Bead))

(29) The wedge-shaped bead 15 to be provided on the die 11 and the wedge-shaped bead 16 to be provided on the blank holder 12 are provided so as to mutually correspond to each other in position and shape. When forming, the clamp target part 14a of the blank 14 is nipped and held, and clamped by the wedge-shaped beads 15 and 16 such that the blank 14 does not flow into the forming region 14b.

(30) FIG. 2 to FIG. 4 are explanatory diagrams showing the states of the wedge-shaped beads 15 and 16 at the time of stretch forming. FIG. 2 is a perspective view showing the states before and after clamping the blank 14. FIG. 3 is a sectional view showing the wedge-shaped beads 15 and 16, and the outer peripheral part 14c and the trim line 14d of the blank 14. FIG. 4 is a top view showing the wedge-shaped bead 15, and the outer peripheral part 14c and the trim line 14d of the blank 14. A region of the length L1 is a part to be clamped by the wedge-shaped beads 15 and 16 and to be discarded in most cases.

(31) Incidentally, in the following description, the wedge-shaped bead 15 provided on the die 11 will be mainly described. Since the wedge-shaped bead 16 provided on the blank holder 12 corresponds to the wedge-shaped bead 15 in position and shape, it can be understood by reading it through appropriate alteration.

(32) As shown in FIG. 2 and FIG. 3, the wedge-shaped bead 15 has a stepped shape including a first surface 15-1, a second surface 15-2 and a third surface 15-3 from the outer edge toward the center of the die 11 (from the left side to the right side in FIG. 3). That is, the first surface 15-1, the second surface 15-2 and the third surface 15-3 make a step from the outer edges toward the centers of the blank holder 12 and the die 11. The second surface 15-2 intersects with (in the shown example, is orthogonal to) the first surface 15-1. The third surface 15-3 intersects with (in the shown example, is orthogonal to) the second surface 15-2.

(33) As shown in FIG. 2 and FIG. 4, the first surface 15-1 of the wedge-shaped bead 15 has a plurality of depression-protrusion parts each configured by a fourth surface 15-4, a fifth surface 15-5 and a sixth surface 15-6. The fifth surface 15-5 intersects with the fourth surface 15-4. The sixth surface 15-6 intersects with the fifth surface 15-5 and faces the fourth surface 15-4. That is, the fourth surface 15-4 and the sixth surface 15-6 face each other.

(34) The fourth surface 15-4, the fifth surface 15-5 and the sixth surface 15-6 are continuously formed in order of the fourth surface 15-4, the fifth surface 15-5, the sixth surface 15-6 and the fifth surface 15-5 in an extending direction of the wedge-shaped bead 15, that is in the direction orthogonal to a direction going from the outer edge toward the center of the die 11. Thereby, depressed and protruded shapes are alternately arrayed on the first surface 15-1 in the extending direction of the wedge-shaped bead 15. In a section parallel to the second surface 15-2, the fourth surface 15-4, the fifth surface 15-5 and the sixth surface 15-6 of the wedge-shaped bead 15 form three sides of a substantial quadrilateral. Although they make three sides of a trapezoid in the shown example, they may make three sides of a rectangle.

(35) The height and pitch of this quadrilateral, the rising angles of the fourth surface 15-4 and the sixth surface 15-6, and the radius of curvature of a corner part that the fourth surface 15-4 or the sixth surface 15-6 and the fifth surface 15-5 make can be appropriately set. However, when the height of the quadrilateral is too low or the pitch is too large, it becomes difficult to obtain an effect of increasing the deformation resistance (hereinafter, also referred to as the unbending deformation resistance) caused when the depressed and protruded shapes are unbent to planar shapes, and the blank clamping force is lowered in some cases. Meanwhile, when the height of the quadrilateral is too high, it is feared that the blank 14 may be destroyed when clamping the blank 14. Meanwhile, when the rising angles of the fourth surface 15-4 and the sixth surface 15-6 are too small, it becomes difficult to obtain the effect of increasing the unbending deformation resistance, and the blank clamping force is lowered in some cases.

(36) By taking these points into consideration, in a case where the wedge-shaped bead 15 having a quadrilateral wedge shape (a trapezoidal one and a rectangular one are included) is to be formed, it is preferable that an interval per pitch that a protruded shape and a depressed shape of the quadrilateral are defined as one set be within a range of 5 to 50 mm and the height of the quadrilateral be within a range of 1.0 to 10.0 mm. Incidentally, the pitch interval in a case of the trapezoidal wedge shape and so forth means a pitch interval when a set of the protruded shape and the depressed shape of the trapezoid is defined as one pitch with a position where the height of the trapezoid is ? being set as a reference.

(37) In addition, when in relation to the height of the second surface 15-2, a level difference between the third surface 15-3 and the fifth surface 15-5 is too small, bending deformation of the blank 14 on the depression-protrusion part and bending deformation at the boundary between the second surface 15-2 and the third surface 15-3 cannot be individually generated and it is feared that the effect of increasing the unbending deformation resistance may not be obtained. In addition, when the level difference is too large, it is feared that the material yield of the blank 14 may be lowered. Accordingly, it is preferable that the level difference be within a range of 1.5 to 8.0 mm.

(38) A fourth surface 16-4, a fifth surface 16-5 and a sixth surface 16-6 of the wedge-shaped bead 16 provided on the blank holder 12 are arranged at positions respectively corresponding to the fourth surface 15-4, the fifth surface 15-5 and the sixth surface 15-6 of the wedge-shaped bead 15 provided on the die 11. Accordingly, in a state of clamping the blank 14 by the die 11 and the blank holder 12, the fourth surface 15-4, the fifth surface 15-5 and the sixth surface 15-6 of the wedge-shaped bead 15 respectively face the fourth surface 16-4, the fifth surface 16-5 and the sixth surface 16-6 of the wedge-shaped bead 16 via the blank 14.

(39) Incidentally, although in the above description, a case where the wedge-shaped bead 15, 16 has a stepped shape in which the first surface 15-1, 16-1 is located lower than the third surface 15-3, 16-3 has been taken by way of example, the stepped shape may be reversed. That is, the wedge-shaped bead 15, 16 may have a stepped shape in which the first surface 15-1, 16-1 is located higher than the third surface 15-3, 16-3.

(40) (1-3. Modified Examples of Lock Bead (Wedge-Shaped Bead))

(41) FIG. 5 to FIG. 7 are explanatory diagrams showing other wedge-shaped beads 17 and 18 used in the stretch forming as a modified example of the lock bead. FIG. 5 is a perspective view showing the states before and after clamping the blank 14. FIG. 6 is a sectional view showing the wedge-shaped beads 17 and 18, and the outer peripheral part 14c and the trim line 14d of the blank 14. FIG. 7 is a top view showing the wedge-shaped bead 17, and the outer peripheral part 14c and the trim line 14d of the blank 14. A region of the length L1 is a part to be clamped by the wedge-shaped beads 17 and 18 and to be discarded in most cases.

(42) Incidentally, similarly to the above description, the wedge-shaped bead 17 provided on the die 11 will be mainly described later. Since the wedge-shaped bead 18 provided on the blank holder 12 also corresponds to the wedge-shaped bead 17 in position and shape in this example, it can be understood by reading it through appropriate alteration.

(43) As shown in FIG. 5 and FIG. 6, the wedge-shaped bead 17 has a stepped shape including a first surface 17-1, a second surface 17-2 and a third surface 17-3 from the outer edge toward the center of the die 11 (from the left side to the right side in FIG. 6). That is, the first surface 17-1, the second surface 17-2 and the third surface 17-3 make a step from the outer edges toward the centers of the blank holder 12 and the die 11. The second surface 17-2 intersects with (in the shown example, is orthogonal to) the first surface 17-1. The third surface 17-3 intersects with (in the shown example, is orthogonal to) the second surface 17-2.

(44) As shown in FIG. 5 and FIG. 7, the first surface 17-1 of the wedge-shaped bead 17 has a plurality of depression-protrusion parts in which a fourth surface 17-4 and a fifth surface 17-5 are alternately and continuously formed in an extending direction of the wedge-shaped bead 17, that is in the direction orthogonal to the direction going from the outer edge toward the center of the die 11. Thereby, depressed and protruded shapes are alternately arrayed on the first surface 17-1 in the extending direction of the wedge-shaped bead 17. In a section parallel to the second surface 17-2, the fourth surface 17-4 and the fifth surface 17-5 of the wedge-shaped bead 17 form two sides of a triangle.

(45) The height and pitch of this triangle, the rising angles of the fourth surface 17-4 and the fifth surface 17-5, and the radius of curvature of a corner part that the fourth surface 17-4 and the fifth surface 17-5 make can be appropriately set. However, when the height of the triangle is too low or the pitch is too large, it becomes difficult to obtain the effect of increasing the unbending deformation resistance, and the blank clamping force is lowered in some cases. Meanwhile, when the rising angles of the fourth surface 17-4 and the fifth surface 17-5 are too small, the pitch of the triangle becomes large, and consequently it becomes difficult to obtain the effect of increasing the unbending deformation resistance and the blank clamping force is lowered in some cases. Meanwhile, when the rising angles of the fourth surface 17-4 and the fifth surface 17-5 are large and the height of the triangle becomes too high, it is feared that the blank 14 may be destroyed when clamping the blank 14 and wrinkles may be generated on the blank 14.

(46) By taking these points into consideration, in a case where the wedge-shaped bead 17 having a triangular wedge shape is to be formed, it is preferable that a pitch interval of the triangle be within a range of 5 to 50 mm and the rising angles of the fourth surface 17-4 and the fifth surface 17-5 be within a range of 10 to 40 degrees. Incidentally, the pitch interval in a case of the triangular wedge shape means the length of the base of the triangle.

(47) In addition, when in relation to the height of the second surface 17-2, a level difference between the third surface 17-3 and the apex of the triangle is too small, bending deformation of the blank 14 on the depression-protrusion part and bending deformation at the boundary between the second surface 17-2 and the third surface 17-3 cannot be individually generated and it is feared that the effect of increasing the unbending deformation resistance may not be obtained. In addition, when the level difference is too large, it is feared that the material yield of the blank 14 may be lowered. Accordingly, it is preferable that the level difference be within a range of 1.5 to 8.0 mm.

(48) A fourth surface 18-4 and a fifth surface 18-5 of the wedge-shaped bead 18 provided on the blank holder 12 are arranged at positions respectively corresponding to the fourth surface 17-4 and the fifth surface 17-5 of the wedge-shaped bead 17 provided on the die 11. Accordingly, in a state of clamping the blank 14 by the die 11 and the blank holder 12, the fourth surface 18-4 and the fifth surface 18-5 of the wedge-shaped bead 18 respectively face the fourth surface 17-4 and the fifth surface 17-5 of the wedge-shaped bead 17 via the blank 14.

(49) Incidentally, although in the above description, a case where the wedge-shaped bead 17, 18 has a stepped shape in which the first surface 17-1, 18-1 is located lower than the third surface 17-3, 18-3 has been taken by way of example, the stepped shape may be reversed. That is, the wedge-shaped bead 17, 18 may have a stepped shape in which the first surface 17-1, 18-1 is located higher than the third surface 17-3, 18-3.

(50) <2. Manufacturing Method for Stretch-Formed Product>

(51) Next, a manufacturing method for a stretch-formed product using the manufacturing apparatus for a stretch-formed product according to the present embodiment will be described together with an action of the lock bead. In the following example, description will be made with reference to FIG. 1 to FIG. 4 as appropriate by taking a case where the die 11 and the blank holder 12 are respectively provided with the wedge-shaped beads 15 and 16 shown in FIG. 2 to FIG. 4 by way of example. Also a case where the die 11 and the blank holder 12 are respectively provided with the wedge-shaped beads 17 and 18 shown in FIG. 5 to FIG. 7 is understood similarly.

(52) In the manufacturing method for a stretch-formed product according to the present embodiment, a known process of the stretch forming can be adopted as the entire process of the stretch forming. Briefly describing, first, the blank 14 is aligned and placed on the blank holder 12. Then, the margin of the blank 14 is clamped by the clamping surfaces 11a and 12a of the die 11 and the blank holder 12 on which the wedge-shaped beads 15 and 16 are provided by relatively moving the die 11 toward the blank holder 12.

(53) The forming region 14b located at the center of the blank 14 is relatively pressed toward the die 11 by relatively moving the punch 13 toward the die 11 in this state. At this time, the clamp target part 14a of the blank 14 is clamped by the wedge-shaped beads 15 and 16 provided on the clamping surfaces 11a and 12a such that the blank 14 does not flow from the outer edge part of the blank 14 toward the forming region 14b. Thereby, the stretch-formed product with no flowing of the blank 14 into the forming region 14b is formed.

(54) At this time, when clamping the blank 14, for example, a load which is about 200 t as a whole becomes necessary in some cases. Accordingly, a clamping function by the wedge-shaped beads 15 and 16 becomes important in order to prevent flowing of the blank 14 into the forming region 14b even while clamping the blank 14 by applying a large load.

(55) In the present embodiment, as shown in FIG. 2 to FIG. 4, the wedge-shaped bead 15, 16 has a stepped shape formed by the first surface 15-1, 16-1, the second surface 15-2, 16-2 and the third surface 15-3, 16-3 from the outer edge toward the center of each of the die 11 and the blank holder 12. The second surface 15-2, 16-2 intersects with the first surface 15-1, 16-1. The third surface 15-3, 16-3 intersects with the second surface 15-2, 16-2.

(56) The first surface 15-1, 16-1 has the fourth surface 15-4, 16-4 and sixth surface 15-6, 16-6 facing each other in the extending direction of the wedge-shaped bead 15, 16. Further, the first surface 15-1, 16-1 has the fifth surface 15-5, 16-5 which is arranged between the fourth surface 15-4, 16-4 and the sixth surface 15-6, 16-6 and intersects with the fourth surface 15-4, 16-4 and the sixth surface 15-6, 16-6.

(57) By configuring the wedge-shaped beads 15 and 16 in this way, the clamp target part 14a of the blank 14 which is clamped by the wedge-shaped beads 15 and 16 becomes non-linear in section in a direction orthogonal to a direction going from the outer edge part toward the forming region 14b of the blank 14. Thereby, the bending rigidity of the blank 14 clamped by the wedge-shaped beads 15 and 16 is improved and the unbending resistance when the blank 14 is about to flow toward the forming region 14b becomes large.

(58) In addition, on the clamp target part 14a of the blank 14, in a corner part corresponding to the boundary portion between the first surface 15-1, 16-1 and the second surface 15-2, 16-2 of the wedge-shaped bead 15, 16, the length of a part which becomes non-parallel to a corner part corresponding to the boundary portion between the second surface 15-2, 16-2 and the third surface 15-3, 16-3 of the wedge-shaped bead 15, 16 becomes long. Thereby, the unbending resistance when the blank 14 is about to flow toward the forming region 14b becomes large similarly.

(59) Accordingly, in cooperation with the deformation resistances of bending and unbending deformation at the corner part of the step which is formed by the first surface 15-1, 16-1, the second surface 15-2, 16-2 and the third surface 15-3, 16-3 of the wedge-shaped bead 15, 16 and the frictional resistances of the wedge-shaped bead 15, 16 with the blank 14, flowing of the blank 14 from the outer peripheral part 14c into the forming region 14b of the blank 14 can be effectively prevented.

(60) Accordingly, even in a case where the pressing length L1 by the wedge-shaped beads 15 and 16 along the direction going from the outer peripheral part 14c toward the forming region 14b of the blank 14 is made short, the blank clamping force required for the stretch forming is ensured. In the present embodiment, the material yield of the blank 14 can be improved by the amount that the pressing length L1 by the wedge-shaped beads 15 and 16 in the blank 14 can be shortened in this way.

EXAMPLES

(61) In the following, Examples of the present invention will be described.

(62) (Evaluation 1)

(63) In the Evaluation 1, a blank clamping performance (a blank locking force) of each bead was evaluated in accordance with a later described test procedure by using a material sliding test device which is equipped with a blank clamping part having the beads of each form formed on the die and the blank holder. The blank (a test material) used is an alloyed hot dip galvanized steel sheet which is 0.7 mm in sheet thickness and is at the 340 MPa level in tensile strength measured by a tensile test based on JIS Z 2241.

(64) In an Example 1, the blank clamping performance (the blank locking performance) of the wedge-shaped beads 15 and 16 was evaluated by using the material sliding test device which is equipped with the blank clamping part in which the wedge-shaped beads 15 and 16 shown in FIG. 2 to FIG. 4 had been formed. In an Example 2, the blank clamping performance of the wedge-shaped beads 17 and 18 was evaluated similarly to the Example 1 except that a material sliding test device which is equipped with a blank clamping part in which the wedge-shaped beads 17 and 18 shown in FIG. 5 to FIG. 7 had been formed is used.

(65) In a Comparative example 1, the blank clamping performance of the trapezoidal beads 2a and 3a was evaluated similarly to the Example 1 except that a material sliding test device which is equipped with a blank clamping part in which the conventional trapezoidal beads 2a and 3a shown in FIG. 15 to FIG. 17 had been formed is used. In addition, in a Comparative example 2, the blank clamping performance of stepped beads was evaluated similarly to the Example 1 except that a material sliding test device which is equipped with a blank clamping part in which the stepped beads constituted by only stepped shapes that the first surfaces 15-1, 16-1, 17-1, and 18-1 in FIG. 2 and FIG. 5 are flattened had been formed is used.

(66) [Test Procedure]

(67) As shown in FIG. 8, a blank 54 of 60 mm in sheet width is clamped by a die 61 and a blank holder 62 of each manufacturing apparatus with pressing force per unit length along the extending direction of the bead being set to 30 kgf/mm. The pressing length L1 of the blank 54 which is clamped by clamping surfaces of the die 61 and the blank holder 62 was set as follows.

(68) Example 1 (the wedge-shaped beads): L1=9.5 mm

(69) Example 2 (the wedge-shaped beads): L1=9.0 mm

(70) Comparative example 1 (the trapezoidal beads): L1=19.0 mm

(71) Comparative example 2 (the stepped beads): L1=9.0 mm

(72) The blank 54 is nipped and held by a chuck 58 at a position where the length up to an end of a clamp target part 55 to be clamped by the clamping surfaces of the die 61 and the blank holder 62 having the respective beads reaches 135 mm. The chuck 58 is moved from this state and the blank 54 is drawn out of the die 61 and the blank holder 62. In that occasion, the test was performed a plurality of times by changing a drawing-out length in a variety of ways and a flowing-in length was evaluated from a sliding mark generated on the blank 54 after each test.

(73) Here, time when the flowing-in length reaches about 1 mm was defined as a lock limit, that is, as a malfunction as the lock bead. Then, drawing-out force at the lock limit (the flowing-in length=1 mm), that is, the locking force was calculated and evaluated by interpolating data on drawing-out force?flowing-in length obtained in the above-mentioned tests. FIG. 9 is a graph showing a result of the evaluations. In FIG. 9, the locking force is indicated by a relative value with the locking force of the conventional trapezoidal beads 2a and 3a being set as 100%.

(74) As shown in FIG. 9, although the trapezoidal beads 2a and 3a in the Comparative example 1 exhibit a high locking force, the pressing length L1 by the die and the blank holder is 19.0 mm and it is difficult to miniaturize the blank 54 by shortening the pressing length L1. Accordingly, the Comparative example 1 is low in material yield of the blank 54. In addition, although the stepped bead in the Comparative example 2 is 9.0 mm in pressing length L1 and it is possible to shorten the pressing length L1, the locking force is about 70%. Accordingly, the Comparative example 2 cannot ensure the locking force required for the stretch forming.

(75) In contrast thereto, the wedge-shaped beads 15, 16, 17, and 18 in the Examples 1 and 2 are individually 9.5 mm and 9.0 mm in pressing length L1, it is possible to shorten the pressing lengths L1 thereof equally to the Comparative example 2 and the locking forces thereof reach 89% and 85% individually. In particular, since the wedge-shaped bead in the Example 1 has the sixth surface together with the fourth surface and the fifth surface, the locking force which is higher than that of the wedge-shaped bead in the Example 2 was ensured. The wedge-shaped beads 15, 16, 17, and 18 in the Examples 1 and 2 exhibit high locking performances required for the stretch forming in this way. In addition, since the wedge-shaped beads 15, 16, 17, and 18 in the Examples 1 and 2 can reduce the pressing lengths L1 much more than the conventional trapezoidal beads, the yield of material of the blank 54 can be improved much better.

(76) (Evaluation 2)

(77) In the Evaluation 2, the blank clamping performance (the blank locking force), the material yield, the appearance of the clamp target part and the influence on a product surface were each evaluated by drawing out the blank by using the chuck similarly to the Evaluation 1, while changing the shapes of the wedge-shaped beads 15 and 16 shown in FIG. 2 to FIG. 4 and the wedge-shaped beads 17 and 18 shown in FIG. 5 to FIG. 7. The blank (the test material) used is the alloyed hot dip galvanized steel sheet which is 0.7 mm in sheet thickness and is the 340 MPa level in tensile strength measured in the tensile test based on JIS Z 2241 similarly to the Evaluation 1. Table 1 indicates the shapes of the wedge-shaped beads and evaluation results. Examples 3 to 9 and Comparative examples 3 to 7 pertain to the wedge-shaped beads 15 and 16 which have a trapezoidal wedge shape (a square is included) and Examples 10 to 14 and Comparative examples 8 to 12 pertain to the wedge-shaped beads 17 and 18 which have a triangular wedge shape.

(78) TABLE-US-00001 TABLE 1 Evaluation Shape Clamp Wedge Wedge Target Part Presence/ Wedge Wedge Wall Corner Appearance Influence Absence Pitch Hight Angle Curvature Locking Material (Cracks and on Product Wedge Shape of Step (mm) (mm) (deg.) (mm) Performance Yield so forth) Surface Example 3 Trapezoid Present 30 2.5 90 1 good good good good Example 4 Present 30 3.0 90 1 good good good good Example 5 Present 20 2.5 90 1 good good good good Example 6 Present 30 2.5 90 2 good good good good Example 7 Present 5 2.0 90 1 good good good good Example 8 Present 8 1.0 90 0.5 good good good good Example 9 Present 50 10.0 90 5 good good good good Comparative Present 80 2.5 90 1 bad good good good example 3 Comparative Present 30 0.5 60 1 bad good good good example 4 Comparative Present 50 2.5 15 1 bad good good good example 5 Comparative Present 30 20.0 90 1 bad bad good example 6 Comparative Absent 50 2.5 90 1 bad good good bad example 7 Example 10 Triangle Present 10 3.0 ?34 1 good good good good Example 11 Present 20 3.0 ?17 1 good good good good Example 12 Present 10 1.0 ?12 0.5 good good good good Example 13 Present 5 1.5 ?38 1 good good good good Example 14 Present 50 10.0 ?22 1 good good good good Comparative Present 80 3.0 ?4 1 bad good good good example 8 Comparative Present 20 1.0 ?6 1 bad good good good example 9 Comparative Present 20 10.0 ?45 1 bad bad good example 10 Comparative Present 24 1.0 ?6 25 bad good good good example 11 Comparative Absent 80 3.0 ?4 1 bad good good bad example 12

(79) In the Comparative examples 7 and 12, Absence of Step indicates a form in which the second surfaces 15-2, 16-2, 17-2 and 18-2 and the third surfaces 15-3, 16-3, 17-3 and 18-3 in FIG. 2 and FIG. 5 are not provided and the wedge-shaped beads are formed on flat surfaces. The wedge pitch in a case of the trapezoidal wedge shape corresponds to the pitch interval when the set of the protruded shape and the depressed shape of the trapezoid is defined as one pitch with the position where the wedge height is ? being set as the reference. In addition, the wedge pitch in a case of the triangular wedge shape corresponds to the length of the base of the triangle. The wedge wall angle means the rising angles of the fourth surfaces 15-4 and 16-4 and the sixth surfaces 15-6 and 16-6 in the case of the trapezoidal wedge shape, and means the rising angles of the fourth surfaces 17-4 and 18-4 and the fifth surfaces 17-5 and 18-5 in the case of the triangular wedge shape.

(80) FIG. 10 shows the sliding marks on the blank individually in a case where decision on the locking performance is good and in a case where it is bad. A photograph shown at the top of FIG. 10 is a photograph obtained by photographing the blank in the case where decision on the locking performance is bad from the blank holder side, and a photograph shown at the bottom of FIG. 10 is a photograph obtained by photographing the blank in the case where decision on the locking performance is good from the blank holder side. In the case where decision on locking is bad, the sliding mark on the blank is observed from a corner of the boundary between the second surface and the third surface to the third surface side. In contrast thereto, in the case where decision on locking is good, few sliding marks are observed on the blank.

(81) As indicated on the Table 1, in case of the wedge-shaped bead having a trapezoidal wedge shape, the Comparative example 3 in which the pitch interval is 80 mm, the Comparative example 4 in which the wedge height is 0.5 mm, the Comparative example 5 in which the wedge wall angle is 15 degrees and the Comparative example 7 in which there is no step have lowered locking performance and it was not possible to ensure the locking force required for the stretch forming. Among them, in regard to the Comparative example 7, the influence on the product surface was also observed. In regard to the Comparative example 5, it is conceivable that it is affected by increase in the wedge pitch in combination with a small wedge wall angle. In addition, in the Comparative example 6 in which the wedge height is 20.0 mm, the blank of the clamp target part was destroyed, the locking force could not be evaluated and also the material yield was lowered.

(82) On the other hand, it was found that in the case of the wedge-shaped bead having the trapezoidal wedge shape, if the pitch interval is within a range of 5 to 50 mm and the wedge height is within a range of 1.0 to 10.0 mm, the locking force required for the stretch forming can be ensured and also the material yield of the blank can be improved.

(83) Meanwhile, in the case of the wedge-shaped bead having the triangular wedge shape, the Comparative examples 8, 9, 11, and 12 in which the wedge wall angle is 4 degrees or 6 degrees have lowered locking performance and it was not possible to ensure the locking force required for the stretch forming. In addition, in the Comparative example 10 in which the wedge height is 10.0 mm, the wedge wall angle reached 45 degrees, the blank of the clamp target part was destroyed, the wrinkles were generated on the blank, the locking force could not be evaluated and also the material yield was lowered.

(84) On the other hand, in the case of the wedge-shaped bead having the triangular wedge shape, it was found that, if the pitch interval is within a range of 5 to 50 mm and the wedge wall angle is within a range of 10 to 40 degrees, the locking force required for the stretch forming can be ensured and also the material yield of the blank can be improved.

(85) (Evaluation 3)

(86) In the Evaluation 3, in regard to the case where the wedge-shaped bead was used and the case where the conventional trapezoidal bead was used, the material yield of the blank was individually evaluated. FIG. 11 and FIG. 12 are diagrams showing the blank and an outline shape of the stretch-formed product manufactured in the Example and the Comparative example. FIG. 11 is a front view of the blank of the stretch-formed product and FIG. 12 is a perspective view showing the dimension of each part of the stretch-formed product. The stretch-formed product is a formed product which has been formed modeling after the door outer panel.

(87) In the Example, the stretch forming was performed by using the die and the blank holder having the wedge-shaped beads 15 and 16 shown in FIG. 2 to FIG. 4 and the stretch-formed product shown in FIG. 11 and FIG. 12 was manufactured. In addition, as the Comparative example, the stretch forming was performed by using the die and the blank holder having the conventional trapezoidal beads shown in FIG. 15 to FIG. 17 and the stretch-formed product shown in FIG. 11 and FIG. 12 was manufactured. In the both, the blank used was the alloyed hot dip galvanized steel sheet which is 0.7 mm in sheet thickness and is at the 340 MPa level in tensile strength measured in the tensile test based on JIS Z 2241. In both cases, the blank of a minimum area which would not hinder the stretch forming was used.

(88) The pressing length L1 of the clamp target part of the blank to be clamped by each lock bead was 9.5 mm in the Example and 19.0 mm in the Comparative example. Consequently, while the area of the blank in the Example was about 1.372 m.sup.2, the area of the blank in the Comparative example was 1.425 m.sup.2. Accordingly, in the case where the wedge-shaped beads of the Example were used, the yield of material in the stretch forming was improved by about 4% in comparison with the case where the trapezoidal beads of the Comparative example were used. At present, improvement of the yield of material in the stretch forming is in a situation that it almost reaches its limit and it is an extremely noticeable effect that the yield of the material can be improved by about 4%.

REFERENCE SIGNS LIST

(89) 10 manufacturing apparatus

(90) 1 die

(91) 11a clamping surface

(92) 12 blank holder

(93) 12a clamping surface

(94) 13 punch

(95) 14 blank

(96) 14a clamp target part

(97) 14b forming region

(98) 14c outer peripheral part

(99) 14d trim line

(100) 15, 16, 17, 18 wedge-shaped bead (lock bead)

(101) 15-1, 16-1, 17-1, 18-1 first surface

(102) 15-2, 16-2, 17-2, 18-2 second surface

(103) 15-3, 16-3, 17-3, 18-3 third surface

(104) 15-4, 16-4, 17-4, 18-4 fourth surface

(105) 15-5, 16-5, 17-5, 18-5 fifth surface

(106) 15-6, 16-6 sixth surface

(107) 54 blank

(108) 55 clamp target part

(109) 58 chuck

(110) 61 die

(111) 62 blank holder