JP2020082090A - Press molding method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce wrinkles in a top plate portion when forming a press-molded product having a hat-shaped cross section in which a top plate portion and a flange portion are curved in a concave shape in the longitudinal direction and wrinkles are generated in the top plate portion. To provide a press molding method capable of foam molding. SOLUTION: The first molding step includes a first molding step of molding an intermediate shape press-molded product having a bend in a flange portion and a second molding step of molding an intermediate shape press-molded product into a target shape. A punch 19 and a die 21 having top plate forming portions 19a and 21a, vertical wall forming portions 19b and 21b and flange forming portions 19c and 21c, and having a bottom dead point of 1 mm or more between the flange forming portions 19c and 21c. There is a gap of 5 mm or less, and the relationship between the radius of curvature R1 of the ridge line connecting the top plate molding portion 21a and the vertical wall molding portion 21b and the radius of curvature R2 of the ridge line connecting the vertical wall molding portion 21b and the flange molding portion 21c is R1 ≧ R2. A first molding die 7 that satisfies the above conditions is used. [Selection diagram] Fig. 1

Description

The present invention relates to a press molding method for a metal member.
Here, the metal member includes a hot-rolled steel sheet, a cold-rolled steel sheet, or a surface-treated steel sheet obtained by subjecting a steel sheet to a surface treatment (electrogalvanizing, hot-dip galvanizing, organic film treatment, etc.), stainless steel, aluminum, magnesium, etc. Alternatively, a plate made of various metals may be used.

In recent years, in order to reduce the weight of automobile bodies for the purpose of reducing CO ₂ emissions, application of high-strength thin steel sheets to automobile parts has been advanced. Further, in the manufacture of automobile parts, press molding which is excellent in manufacturing cost is often applied.
However, for example, when press-molding a hat-shaped cross-sectional part in which the top plate portion and the flange portion are curved in a concave shape over the longitudinal direction, there is a problem that wrinkles are likely to occur in the forming process of the member serving as the top plate portion. However, when a high-strength thin steel sheet is used, it is difficult to crush the generated wrinkles, which is a particular problem.

As a technique for suppressing the generation of wrinkles in the top plate portion curved in a concave shape along the longitudinal direction, in Patent Document 1, a bead (a bulge portion) is formed at a portion where wrinkles are expected to occur in an intermediate drawing process. There is disclosed a technique for suppressing the generation of wrinkles by molding a bead and extending the bead in the final step.

Further, in Patent Document 2, a wrinkle is formed by forming a scraping surface on at least one end of the saddle-shaped bent portion in the intermediate drawing step and molding an intermediate-shaped product having a height difference at both ends of the saddle-shaped bent portion. A method is disclosed in which generation is suppressed, and the shaving surface is swollen and disappeared in the finish drawing step, and the drawing shape is formed into a desired product outer shape.

Japanese Patent Publication No. 64-2452 Japanese Patent No. 5234262

The method disclosed in Patent Document 1 is to form a bead (a bulge portion) at a site where wrinkles are expected to occur in the intermediate drawing step. However, it is possible to accurately predict where wrinkles occur. When a high strength thin steel plate is used, it is difficult to completely erase the beads once formed.

Further, the method disclosed in Patent Document 2 is limited to the application to a molded product having a gutter-shaped cross section, and a molded product having a flange portion whose cross-sectional shape targeted by the present invention is a hat shape is formed. New problems are expected to occur during molding.

The present invention has been made in order to solve such a problem, the top plate portion and the flange portion are respectively curved in a concave shape in the longitudinal direction, the curvature radius of the top plate portion is smaller than the curvature radius of the flange portion. When forming a press-formed product with a hat-shaped cross-section that causes wrinkles on the top plate, the press that can form wrinkles on the top plate without forming bulging parts such as beads. It is intended to provide a molding method.

The press-molding method according to the present invention includes a top plate portion having a concave shape along the longitudinal direction, a pair of vertical wall portions continuing to both sides of the top plate portion, and a longitudinal direction continuing to the pair of vertical wall portions. A press forming method for a press-formed product having a hat-shaped cross section, which has a concave flange portion, and has a target shape having a curvature radius r1 of the top plate portion smaller than a curvature radius r2 of the flange portion. Forming step of forming an intermediate shape press-formed product having a portion, a vertical wall portion, and a flange portion, and the flange portion having bending caused by a material remainder, and the intermediate shape formed in the first forming step A second forming step of forming a press-formed product into a target shape, wherein the first forming step is a punch and a die having a top plate forming part, a vertical wall forming part and a flange forming part, and a bottom dead center. In the above, there is a gap of 1 mm or more and 5 mm or less along the entire length in the longitudinal direction between the punch and the flange forming portion of the die, and the curvature radius R1 of the ridge line connecting the top plate forming portion and the vertical wall forming portion in the die and It is characterized in that a first molding die is used in which the radius of curvature R2 of the ridgeline connecting the vertical wall molding portion and the flange molding portion in the die satisfies the relationship of R1≧R2.

In the press molding method according to the present invention, the first molding die is used to have a top plate portion, a vertical wall portion, and a flange portion, and the bending caused by a material remainder is caused over the entire length in the longitudinal direction of the flange portion. By including a first molding step of molding the intermediate shape press-molded product that has and a second molding step of molding the intermediate shape press-molded product molded in the first molding step into a target shape, the longitudinal direction When forming a press-molded product with a hat-shaped cross section in which the top plate and the flange curve in a concave shape over the entire length, the wrinkles of the top plate are reduced without forming bulging parts such as beads, and foam molding is performed. can do.

It is a figure explaining the press molding method which concerns on this invention, (a) is a figure used for the 1st molding process, (b) is a figure explaining the metal mold used for a 2nd molding process. It is a figure explaining the press-molded product shape|molded by the press-molding method which concerns on this invention, (a) is explanatory drawing of the press-formed product of the intermediate shape shape|molded by the 1st molding process, (b) is a 2nd molding. It is explanatory drawing of the press-formed product of the target shape shape|molded by the process. It is a figure which shows the shape of the 1st die|dye used by the 1st shaping|molding process which concerns on this invention. It is a figure explaining the process which suppresses the wrinkles of the top plate part concerning the present invention, (a) is before the start of shaping, (b) is the time when the 1st die contacts a blank, (c) is in the bottom dead center. It is a figure which shows a 1st die and a 1st punch. It is a figure explaining the cross-sectional shape of the metal mold|die and press-molded product in the bottom dead center of the 1st shaping|molding process which concern on this invention, (A) is an AA sectional view, (B) is a BB sectional view. .. It is a figure which shows the shape of the 2nd die|dye used by the 2nd shaping|molding process which concerns on this invention. It is a figure explaining the cross-sectional shape of the metal mold|die and press-molded product in the bottom dead center of the 2nd shaping|molding process which concern on this invention, (A) is an AA sectional view, (B) is a BB sectional view. .. It is a figure which shows an example of the press molded product made into a target shape in this invention. It is a figure explaining the problem in the conventional press molding method, (a) is a metal mold|die used for the conventional press molding, (b) is a figure which shows the press molded product by the conventional press molding method. It is a figure explaining the process which the wrinkle of the top plate part in the conventional press molding method generate|occur|produces, (a) is before a molding start, (b) is a time when a die contacts a blank, (c) is a bottom dead center. It is a figure which shows a die and a punch.

The press molding method according to the embodiment of the present invention forms a blank made of a metal plate into a press molded product 6 having a hat-shaped cross section as shown in FIG.
Hereinafter, after the press-formed product 6 to be molded in the present embodiment is described, each step of the press-molding method according to the present embodiment will be described in detail.

<Press-formed products>
As shown in FIG. 8, the press-formed product 6 having a target shape according to the present embodiment includes a top plate portion 1 having a concave shape along the longitudinal direction, a vertical wall portion 3 following the top plate portion 1, and a vertical wall. It is formed in a hat-shaped cross-section having a flange portion 5 which is concave along the longitudinal direction following the portion 3.
Here, as shown in FIG. 8, the curvature radius of the ridgeline connecting the top plate portion 1 and the vertical wall portion 3 (hereinafter referred to as the top plate side ridge line 15) is set to the top plate side curvature radius r1, and the vertical wall portion 3 and the flange The radius of curvature of the ridgeline connecting the portions 5 (hereinafter referred to as the flange side ridgeline 17) is defined as the flange side radius of curvature r2.
The present invention is directed to a target shape having a flange-side radius of curvature r2 larger than a top-plate-side radius of curvature r1.
Hereinafter, the press-molded articles formed by the conventional press-molding method and the press-molding method according to the present embodiment will be described by using the same reference numerals for the same parts.

In the press-formed product 6 as described above, a metal plate is sandwiched by a relative movement of the die 37 and the punch 39 by using a die 41 composed of a die 37 and a punch 39 as shown in FIG. 9A. It is formed by a one-step forming method of bending.
Note that FIG. 9A illustrates only the right half of the mold 41 and the surface of the mold 41 that is in contact with the blank (hereinafter referred to as the molding surface portion) in order to explain the shape of the mold 41 in an easy-to-understand manner. Illustrated. The same applies to the subsequent drawings relating to the mold.

In the press-formed product 43 formed by the conventional press-forming method, wrinkles 45 were generated on the top plate portion as shown in FIG. 9B.
The cause of such wrinkles 45 will be described below with reference to FIG.

FIG. 10 shows a state in the middle of forming in which the die 37 is vertically lowered with respect to the punch 39 in the conventional press forming method. FIG. 10(a) shows the state before the die 37 contacts the blank, and FIG. When the die 37 comes into contact with the blank, FIG. 10C is a diagram schematically showing the appearance of the forming surface portion viewed from the vertical wall forming portion side at the bottom dead center. The blank is not shown for the sake of simplicity.

When a target shape in which the flange-side radius of curvature r2 is larger than the top-plate-side radius of curvature r1 (r1<r2) is to be press-molded in one step, as shown in FIG. Then, the vertical wall portion 3 starts to be formed (see the broken line circle), and then the vertical wall portion 3 is sequentially formed toward the central portion.
When the molding of the vertical wall portion 3 starts, the material moves from the end portion in the longitudinal direction that is clamped first toward the central portion, and when the top plate portion 1 is molded at the bottom dead center, a wrinkle 45 is formed in the central portion in the longitudinal direction. Occur.
In particular, when the metal plate used is a high-strength steel plate with low ductility, it is difficult to crush the wrinkles 45 that have occurred, so a press forming method that reduces the occurrence of wrinkles 45 is required.

Therefore, in the press molding method according to the embodiment of the present invention, a blank is formed into an intermediate shape press-formed product 9 (see FIG. 2(a)) by using a first molding die 7 as shown in FIG. 1(a). The intermediate shape press-formed product 9 is transformed into the target shape press-formed product 13 (see FIG. 2(b)) by using the first forming step of forming into the shape 1 and the second forming die 11 as shown in FIG. 1(b). By providing the second molding step for molding, the occurrence of wrinkles 45 (see FIG. 9B) due to material movement is reduced.
Each step will be specifically described below.

<First molding step>
The first molding step uses the first molding die 7 having the first punch 19 and the first die 21 as shown in FIG. 1A, and the blank 23 as shown in FIG. 2A. This is a step of forming an intermediate shape press-formed product 9.

≪First molding die≫
The first punch 19 and the first die 21 of the first molding die 7 have top plate molding portions 19a and 21a, vertical wall molding portions 19b and 21b, and flange molding portions 19c and 21c, respectively.

At the bottom dead center, the first punch 19 and the first die 21 have a gap h of 1 mm or more over the entire length in the longitudinal direction between the flange forming portion 19c of the first punch 19 and the flange forming portion 21c of the first die 21. It is formed so that it can.
At the bottom dead center in the first forming step, the blank 23 is held between the top plate forming portion 19a of the first punch 19 and the top plate forming portion 21a of the first die 21.

FIG. 3 shows the shape of the molding surface of the first die 21.
In the first die 21, the radius of curvature of the ridgeline connecting the top plate forming portion 21a and the vertical wall forming portion 21b (hereinafter referred to as the top plate side ridge line 25) is defined as the top plate side curvature radius R1, and the vertical wall forming portion 21b and the flange forming. When the radius of curvature of the ridgeline connecting the portions 21c (hereinafter referred to as the flange side ridgeline 27) is the flange side radius of curvature R2, the flange side radius of curvature R2 is set to be smaller than the top side curvature radius R1 (R1> R2). In other words, in the first die 21, the flange side ridgeline 27 is sharper than the top plate side ridgeline 25.

Since the flange-side radius of curvature R2 of the first die 21 is smaller than the top-plate-side radius of curvature R1, the line length in the direction orthogonal to the longitudinal direction of the first die 21 (hereinafter, simply referred to as “longitudinal orthogonal direction”) is the longitudinal direction. The line length L _{B in} the vicinity of the end in the longitudinal direction is shorter than the line length L _A in the central portion in the direction (L _A >L _B ).

Further, the top plate side curvature radius R1 of the first die 21 is set to be substantially the same as the top plate side curvature radius r1 of the target shape, and further, the central portion in the longitudinal direction of the first die 21 (the height of the vertical wall is The height of the vertical wall forming portion 21b in the highest portion) is set to be substantially the same as the height of the vertical wall portion 3 of the target shape.

The shape of the first punch 19 conforms to the shape of the first die 21, and as shown in FIG. 4C, a gap h is formed between the flange forming portion 21c of the first die 21 and the entire length in the longitudinal direction. So, it is set as follows.
The top plate side ridge line and the flange side ridge line of the first punch 19 have the same radius of curvature as the corresponding top plate side ridge line 25 and the flange side ridge line 27 of the first die 21, respectively.
Further, the height of the vertical wall forming portion 19b of the first punch 19 is higher than the height of the vertical wall forming portion 21b of the first die 21 by the gap h over the entire length in the longitudinal direction.
Therefore, at the bottom dead center of the first forming step, a gap h is formed between the flange forming portions 19c and 21c over the entire length in the longitudinal direction.

≪Molding process and press-molded products≫
The molding process of the first molding process performed using the first molding die 7 as described above will be described below with reference to FIG.
FIG. 4 shows a state in the middle of the first forming step of vertically lowering the first die 21 with respect to the first punch 19 in the present embodiment, and FIG. 4A shows that the first die 21 is blank. Before contacting, FIG. 4(b) schematically shows the state of the molding surface portion seen from the vertical wall molding portion side at the bottom dead center, when the first die 21 contacts the blank. It is a figure. It should be noted that the blank is not shown for the sake of simplifying the description as in FIG. 10.

In contrast to the conventional press molding method described with reference to FIG. 10, in the first molding step of the present invention shown in FIG. 4, the flange side curvature radius R2 is smaller than the top plate side curvature radius R1 (R1>R2). Since the punch 19 and the first die 21 are used, the central portion in the longitudinal direction of the first die 21 comes into contact with the blank to start forming the vertical wall portion 3 (see the broken line circle), and then sequentially toward both end portions. The vertical wall portion 3 is formed.
As a result, the material moves from the central portion in the longitudinal direction, which is clamped first, toward both ends, so that wrinkles 45 do not occur in the central portion.

Further, as described above, the line length in the direction orthogonal to the longitudinal direction of the first die 21 is shorter in the line length L _{B in} the vicinity of the end in the longitudinal direction than in the line length L _A in the central portion in the longitudinal direction ( L _A >L _B ) (see FIG. 3), and as the molding in the first molding step progresses, a material surplus of the blank 23 is further generated in the vicinity of the longitudinal end of the blank 23.

As shown in FIG. 5, the top plate portion 1 and the vertical wall portion 3 of the blank 23 are sandwiched between the top plate forming portions 19a and 21a and the vertical wall forming portions 19b and 21b of the first punch 19 and the first die 21, respectively. Since the flange portion 5 of the blank 23 is not sandwiched by the gap h between the first punch 19 and the flange forming portions 19c, 21c of the first die 21, the material of the blank 23 generated near the end portion in the longitudinal direction. The surplus moves to the flange portion 5 and causes bending over the entire length of the flange portion 5 in the longitudinal direction.

In this way, in the first molding step, by starting the molding from the central portion in the longitudinal direction, it is possible to suppress the generation of wrinkles 45 in the top plate portion 3, which has occurred due to the material moving from the end portion to the central portion, In addition, an intermediate shape in which a material excess is generated at both ends in the longitudinal direction and a gap h is provided between the flange forming portions 19c and 21c, so that the flange 5 is bent by the material excessive over the entire length in the longitudinal direction. The press-formed product 9 can be formed (see FIG. 2A).

The gap h must be 1 mm or more in order to sufficiently bend the flange portion 5.

However, if the gap h is too large, the material surplus generated at both ends in the longitudinal direction of the blank 23 concentrates on the flange portion 5, and the material surplus concentrated on the flange portion 5 in the second molding step described later is removed by the vertical wall. In some cases, the wrinkles may remain on the flange portion 5 because they cannot be fully applied to the molding of the portion 3 (not shown).
Therefore, the gap h is preferably 5 mm or less.

<Second molding step>
The second molding step uses the second molding die 11 having the second punch 29 and the second die 31 as shown in FIG. 1B, and the intermediate shape press-formed product 9 molded in the first molding step. Is a step of forming a target shape press-formed product 13 as shown in FIG.

≪Second molding die≫
The second punch 29 and the second die 31 of the second molding die 11 (FIG. 1(b)) have top plate molding portions 29a and 31a, vertical wall molding portions 29b and 31b, and flange molding portions 29c and 31c, respectively. Have
The top plate molding portions 29a and 31a, the vertical wall molding portions 29b and 31b, and the flange molding portions 29c and 31c are set to have the same shape as the target shape.

FIG. 6 shows the shape of the molding surface of the second die 31.
In the second die 31, the radius of curvature r1 of the ridgeline that connects the top plate forming portion 31a and the vertical wall forming portion 31b (hereinafter referred to as the top plate side ridge line 33), the ridge line that connects the vertical wall forming portion 31b and the flange forming portion 31c (hereinafter , The flange side ridgeline 35), the flange side curvature radius r2 is larger than the top plate side curvature radius r1 (r1<r2). In other words, in the second die 31, the flange-side ridgeline 35 is more gently curved than the top-plate-side ridgeline 33.

Regarding the line length in the longitudinal orthogonal direction, in the first die 21, the line length L _B near the end in the longitudinal direction was shorter than the line length L _{A at the} center in the longitudinal direction (L _A >L _B), towards the line length l _b of the longitudinal ends near the line length l _a of the longitudinal center portion in the second die 31 is longer (l _a <l _b).
The length of the line length l _a of the longitudinal center of the longitudinal line length direction central portion L _A and the second die 31 of the first die 21 is the same (L _A = l _a), the second die 31 The line length l _b is longer than the line length L _B of the first die 21 (L _B <l _b ) (see FIGS. 3 and 6).

≪Press molded product≫
The press-molded product obtained by the second molding step using the second molding die 11 as described above will be described below.

As described above, regarding the line length in the longitudinal orthogonal direction in the vicinity of the end in the longitudinal direction, the line length l _b of the second die 31 is longer than the line length L _B of the first die 21 (L _B <l _b ) (see FIGS. 3 and 5).
Therefore, at the bottom dead center of the second molding step, in the blank 23, the material that has caused the bending over the entire length in the longitudinal direction of the flange portion 5 is the vertical wall portion near the longitudinal end portion where the height is increased. By moving to 3, the bending of the flange portion 5 is eliminated (see FIG. 7).

As described above, in the second molding step, the surplus material that causes the flange portion 5 of the intermediate-shape press-formed product 9 to bend is applied to the vertical wall portion 3 having a higher height, as shown in FIG. A target shape press-formed product 13 as shown in b) can be formed.
When the target shape press-formed product 13 shown in FIG. 2(b) formed by the press-forming method according to the present embodiment and the press-formed product 43 shown in FIG. 9(b) formed by the conventional press-forming method are compared, It can be seen that the wrinkles on the top plate are reduced.

A specific example of the case where the press molding is performed while satisfying the requirements in the above-described embodiment will be described below.

Nos. 1-1 to 1-8 in Table 1 are press-molded by using a cross-section hat-shaped press-formed product having a top plate side curvature radius r1=400 mm and a flange side curvature radius r2=10000 mm as a target shape.
In Nos. 2-1 to 2-4, press molding was carried out using a cross-section hat-shaped press-formed product having a top plate side radius of curvature r1=500 mm and a flange side radius of curvature r2=10000 mm as a target shape.
The above two types of target shapes are press-molded by the conventional press-molding method and the press-molding method of the present invention, and the plate thickness increase rate of the top plate portion in the obtained press-molded product is evaluated. It was

No. 1-1 and No. 2-1 are molded by the conventional press molding method consisting of one step. Further, Nos. 1-2, 1-3, 1-5 to 1-8 and Nos. 2-2 to 2-4 are molded by the press molding method of the present invention comprising two steps. No. 1-4 is based on two steps as in the present invention, but since the value of the gap h is outside the scope of the present invention, it is described as a comparative example.
R1 indicates the radius of curvature (mm) on the top plate side of the die, and R2 indicates the radius of curvature (mm) on the flange side of the die.
h indicates the gap (mm) between the flange forming portions 19c and 21c at the bottom dead center of the first forming step.
The top plate portion plate thickness increase rate indicates the plate thickness increase rate with respect to the blank before forming at the thickest part of the top plate portion of the press-formed product after forming.
In the evaluation, “◯” indicates that the increase rate of the plate thickness of the top plate portion was reduced and wrinkles were suppressed as compared with the conventional example, and the increase rate of the plate thickness of the top plate part was suppressed to less than 5%.

In Nos. 1-2 to 1-4, the value of R2 in the first forming step was set equal, the value of the gap h was changed, and the increase rate of the plate thickness of the top plate portion was measured.
In Nos. 1-2 and 1-3, which are examples of the present invention (1≦h≦5), the increase rate of the plate thickness of the top plate portion was reduced and wrinkles were suppressed as compared with the conventional example. In No. 1-4 (5<h), which is a comparative example, wrinkles remained on the flange portion, so the evaluation was evaluated as “x”.

For No. 1-5 to 1-8 and No. 2-2 to 2-4, the value of the gap h in the first forming step is set to be equal, and the value of R2 in the first forming step is changed to change the value of R2 in the top plate. The rate of increase in thickness was measured.
When R2 in the first molding step was set small, the rate of increase in plate thickness was low and there was a tendency to reduce wrinkles. However, even if R2 was increased until it became equal to R1 in the first molding step, the effect of reducing wrinkles was observed. Was given.

As described above, in the two types of target shapes, in the press-molded products obtained by the press-molding method of the present invention, the top plate thickness increase rate is decreased in both cases compared with the conventional press-molding method. Occurrence was reduced.

1 Top plate part 3 Vertical wall part 5 Flange part 6 Press-formed product (target shape)
7 1st molding die 9 Intermediate shape press molded product 11 2nd molding die 13 Target shape press molded product 15 Top plate side ridge (target shape)
17 Flange side ridge (target shape)
19 1st punch 19a Top plate forming part 19b Vertical wall forming part 19c Flange forming part 21 1st die 21a Top plate forming part 21b Vertical wall forming part 21c Flange forming part 23 Blank 25 Top plate side ridge line (1st die)
27 Ridge on the flange side (first die)
29 2nd punch 29a Top plate forming part 29b Vertical wall forming part 29c Flange forming part 31 2nd die 31a Top plate forming part 31b Vertical wall forming part 31c Flange forming part 33 Top plate side ridge line (2nd die)
35 Flange side ridge (2nd die)
37 die (conventional example)
39 punches (conventional example)
41 Mold (conventional example)
43 Press-formed product (conventional example)
45 wrinkle r1 Curvature radius on top side (target shape)
r2 Radius of curvature on the flange side (target shape)
R1 Top plate side radius of curvature (first die)
R2 Radius of curvature on flange side (first die)

Claims (1)

A top plate portion having a concave shape along the longitudinal direction, a pair of vertical wall portions continuing to both sides of the top plate portion, and a flange portion having a concave shape along the longitudinal direction continuing to the pair of vertical wall portions. A press-molding method for a press-molded product having a hat-shaped cross section in which the curvature radius r1 of the top plate portion is smaller than the curvature radius r2 of the flange portion as a target shape,
A first forming step of forming an intermediate shape press-formed product having a top plate portion, a vertical wall portion and a flange portion, and the flange portion having a bend caused by a material remainder;
A second molding step for molding the intermediate shape press-formed product formed in the first molding step into a target shape,
The first forming step is a punch and a die having a top plate forming portion, a vertical wall forming portion, and a flange forming portion, and at a bottom dead center, a full length in a longitudinal direction is provided between the punch and the flange forming portion of the die. There is a gap of 1 mm or more and 5 mm or less, and the radius of curvature R1 of the ridgeline connecting the top plate forming part and the vertical wall forming part in the die and the radius of curvature R2 of the ridgeline connecting the vertical wall forming part and the flange forming part in the die. Is used, wherein a first molding die satisfying the relation of R1≧R2 is used.