JP7021838B2 - Press molding method for tubular parts and press molding dies used for this - Google Patents

Description

The present invention relates to a method for press-molding a tubular part, and more particularly to a method for press-molding a tubular part having an irregular cross section.

As a method of forming a tubular part having a circular cross section, a metal plate is press-molded and bent into a U-shape, and then this molded product is press-molded with different molds to hold the ends of the vertical wall portions of the molded product together. A so-called UO bending molding method of butt-molding into a tubular shape is known (see, for example, Patent Document 1 below). Specifically, a metal plate 101 bent into a U-shaped cross section is set on the lower mold 120 {see FIG. 16 (A)}, and the upper mold 110 is lowered to abut the ends of the pair of vertical wall portions 101a against each other. {See FIG. 16 (B)}. After that, by further lowering the upper mold 110, each vertical wall portion 101a is curved to the side that bulges outward {see FIG. 16 (C)}, and is pressed against the molding surfaces of the upper mold 110 and the lower mold 120 to cross-section. A circular tubular part 130 is formed {see FIG. 16 (D)}.

Japanese Unexamined Patent Publication No. 2014-4626

However, it is very difficult to form a tubular part having a deformed (non-round) cross section by the conventional UO bending molding method as described above. For example, FIG. 17 shows a procedure for molding a tubular part having a substantially quadrangular cross section by the above method. First, the metal plate 201 bent into a U shape is set on the lower mold 220 {see FIG. 17 (A)}, and the upper mold 210 is lowered so that the upper ends of the pair of vertical wall portions 201a are on the molding surface of the upper mold 210. Bring them closer to each other along {see Figure 17 (B)}. When the upper die 210 is further lowered, the upper ends of the pair of vertical wall portions 201a abut against each other and bend to an undesired position of the vertical wall portion 201a (a position different from the bending portion provided in the tubular part of the finished product). Part 201b is molded {see FIG. 17 (C)}. After that, when the upper mold 210 is further lowered, the bent portion 201b provided on the vertical wall portion 201a is brought into contact with the upper mold 210 and pushed down {see FIG. 17 (D)}, whereby the materials that are in contact with each other are abutted against each other. Since the ends are displaced up and down, it cannot be molded into a predetermined shape {see FIGS. 17 (E) and 17 (F)}.

Therefore, an object to be solved by the present invention is to provide a press molding method capable of forming a tubular part having a deformed cross section.

In order to solve the above problems, the present invention is a method for press-molding a tubular part having a different cross section, and comprises a U-bending step of forming a bottom portion and a pair of vertical wall portions rising from both ends of the bottom portion. An intermediate bending step of forming a bent portion on each vertical wall portion by pressing the end portions of the pair of vertical wall portions against a locking portion provided on the mold, and the pair of vertical wall portions. Provided is a method of press molding a tubular part having a final bending step of forming the tubular part by abutting the ends of the tubular part.

As a result of trial and error by the present inventors, an intermediate bending step is provided between the U bending step and the final bending step (for example, the O bending step), and in this intermediate bending step, the ends of the pair of vertical wall portions are molded. By pressing against the locking part of the vertical wall to form a bent part of the desired shape at the desired position of the vertical wall part, it is possible to avoid molding defects in the subsequent final bending process and to obtain a tubular part having a deformed cross section. It became clear that it can be molded by pressing. In particular, by matching the shape of the bent portion formed in the intermediate bending step with at least a part of the shape of the bent portion provided in the tubular part of the finished product, it is possible to reliably avoid molding defects in the subsequent final bending step. Can be done.

The above intermediate bending step is a press forming die for forming a bent portion into a pair of vertical wall portions of a molded product having a bottom portion and a pair of vertical wall portions rising from both ends of the bottom portion, and the molded product. It is provided with one mold in which the bottom of the mold is set and the other mold that can be approached and separated from the one mold, and as the other mold separates from the one mold. A pair of inclined surfaces that are close to each other, a locking portion that is provided between the pair of inclined surfaces and protrudes toward the one mold side, and an end portion of each inclined surface that is provided on the one mold side. This can be performed by using a press-molding die having a vertical wall portion of the molded product and a molding surface for molding the bent portion.

By the way, the intermediate beam connecting the left and right trailing arms of the rear suspension of an automobile usually has a substantially U-shaped cross section in order to allow torsional deformation. Such an intermediate beam having a substantially U-shaped cross section may be formed by press molding (also referred to as pipe crushing molding) of a tubular member. Specifically, the tubular part 301 having a circular cross section as shown in FIG. 18 (A) is crushed by the upper mold 310 and the lower mold 320 as shown in FIGS. A U-shaped molded product 330 is obtained. In this molded product 330, strain and stress are concentrated on the folded portion 330a {see FIG. 18 (D)}, so that an annealing step is required after press molding, which leads to high cost. Further, in the pipe crushing molding, the tubular part 301 before being crushed is required to have a circular cross section and a straight shape in the longitudinal direction, so that the product shape is limited.

Therefore, the bottom portion formed in the U-bending step is provided with a convex curved portion protruding in the direction in which the pair of vertical wall portions extend, and the molded product is subjected to the intermediate bending step and the final bending step. , A tubular part having a substantially U-shaped cross section can be formed. In this case, since the folded portion of the tubular part having a substantially U-shaped cross section is formed by bending, strain and stress concentration are alleviated in the folded portion, and post-treatment (annealing, etc.) for alleviating the stress is unnecessary. Become. Further, according to this method, even a tubular part having a different cross-sectional shape in the longitudinal direction can be molded into a substantially U-shaped cross section, so that the degree of freedom in the product shape is increased.

As described above, according to the press molding method of the present invention, a tubular part having an irregular cross section can be press molded.

(A) to (D) are sectional views showing the procedure of the press molding method which concerns on embodiment of this invention. It is a perspective view of a tubular part. (A) to (D) are sectional views showing the procedure of an intermediate bending process. It is an enlarged sectional view of the shoulder bending part of an intermediate molded product. (A) to (C) are sectional views showing the procedure of the O bending process. It is a side view of the tubular part which concerns on other embodiment. (A) is a cross-sectional view taken along line 7A-7A of the flat portion of the tubular part of FIG. 7, (B) is a cross-sectional view taken along line 7B-7B of the gradually changing portion of the tubular part, and (C) is the same tubular part. It is a cross-sectional view on the 7C-7C line of the substantially circular part of the part. FIG. 7 is a cross-sectional view showing a state in which an intermediate bending step is applied to the U-shaped molded product of FIG. 7, in which (A) is a flat portion, (B) is a gradual change portion, and (C) is a substantially circular portion. FIG. 7 is a cross-sectional view showing a state in which an intermediate bending step is applied to the U-shaped molded product of FIG. 7, in which (A) is a flat portion, (B) is a gradual change portion, and (C) is a substantially circular portion. FIG. 7 is a cross-sectional view showing a state in which an intermediate bending step is applied to the U-shaped molded product of FIG. 7, in which (A) is a flat portion, (B) is a gradual change portion, and (C) is a substantially circular portion. FIG. 7 is a cross-sectional view showing a state in which an intermediate bending step is applied to the U-shaped molded product of FIG. 7, in which (A) is a flat portion, (B) is a gradual change portion, and (C) is a substantially circular portion. It is a side view of the intermediate molded product. It is sectional drawing of the upper mold of the intermediate bending die which concerns on other embodiment. It is sectional drawing of the upper mold of the intermediate bending die which concerns on other embodiment. (A) to (D) are sectional views showing the procedure of the press molding method which concerns on other embodiment. (A) to (D) are sectional views showing the procedure of the conventional UO bending forming method. (A) to (F) are cross-sectional views showing a case where a tubular part having a deformed cross section is press-molded by a conventional UO bending molding method. (A) to (D) are sectional views showing the procedure of the conventional pipe crushing molding method.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

As shown in FIG. 1, the press forming method of the present invention is performed through a U bending step, an intermediate bending step, and a final bending step (for example, an O bending step). This press molding method is a method for molding a tubular part having a deformed cross section. In the present embodiment, as shown in FIG. 2, a tubular part 1 having a substantially square cross section and having the same cross section in the longitudinal direction is molded. Show the case. In the following description, for convenience, the side where the butt portion 1a between the material ends of the tubular component 1 is located is on the upper side, and the opposite side is on the lower side, and the directions orthogonal to the longitudinal direction and the vertical direction of the tubular component 1 (FIG. 1). And the left-right direction in FIG. 2) is the width direction.

In the U-bending step, the blank material 2 shown in FIG. 1 (A) is pressed with a U-bending die (not shown) to obtain a U-shaped molded product 3 having a substantially U-shaped cross section shown in FIG. 1 (B). The U-shaped molded product 3 has a bottom portion 11, a pair of vertical wall portions 12 rising upward from both ends in the width direction of the bottom portion 11, and bending portions provided between the bottom portion 11 and each vertical wall portion 12 (hereinafter referred to as a bending portion). It has a "bottom bent portion 13"). The shape of the lower portion (bottom portion 11 and bottom bent portion 13) of the U-shaped molded product 3 is substantially the same as the shape of the lower portion of the tubular part 1 of the finished product. In the illustrated example, the pair of vertical wall portions 12 form a flat plate shape parallel to each other.

In the intermediate bending step, the U-shaped molded product 3 is pressed by the intermediate bending die 20 to obtain the intermediate molded product 4 shown in FIG. 1 (C). As shown in FIG. 3, the intermediate bending die 20 includes an upper die 21 and a lower die 22. In the present embodiment, the lower mold 22 is a fixed type, and the upper mold 21 is a movable type that moves up and down with respect to the lower mold 22.

The upper die 21 is provided between a pair of inclined surfaces 21a that are inclined in a direction of approaching each other as they go upward, and a locking portion 21b that is provided between the upper ends of the pair of inclined surfaces 21a and protrudes downward, and a pair of inclined surfaces. It has a molding surface 21c provided below the 21a. The locking portion 21b has a locking surface 21d extending downward from the upper end of each inclined surface 21a. The locking portion 21b is provided integrally with or separately from the main body portion 21e of the upper die 21 (in the illustrated example, the locking portion 21b is provided separately). The molded surface 21c is smoothly continuous with the lower end of the inclined surface 21a. The lower mold 22 is provided with a recess 22a having the same shape as the lower portion of the U-shaped molded product 3.

In the intermediate bending step, first, as shown in FIG. 3A, the bottom portion 11 of the U-shaped molded product 3 is set in the recess 22a of the lower mold 22. Then, by lowering the upper die 21 and pushing down the upper end 12f of the pair of vertical wall portions 12 of the U-shaped molded product 3 with the upper die 21, the pair of vertical walls are as shown in FIG. 3 (B). The upper end 12f of the portion 12 is guided along the inclined surface 21a of the upper die 21, the pair of vertical wall portions 12 is inclined inward, and the upper end 12f is the locking portion 21b of the upper die 21 (the pair of locking surfaces 21d). ). As a result, the upper ends 12f of the pair of vertical wall portions 12 are further restricted from moving inward in the width direction (sides approaching each other) in a state of being separated in the width direction. In this state, when the upper die 21 is further lowered, the vertical wall portion 12 is compressed in the length direction to cause buckling, and the intermediate portion of the vertical wall portion 12 bulges outward {see FIG. 3 (C). }. After that, when the upper mold 21 is further lowered, the bulging portion of the vertical wall portion 12 is pressed against the molding surface 21c of the upper mold 21, whereby the bent portion (the upper end of the vertical wall portion 12) is pressed against the middle portion of the vertical wall portion 12. A bent portion adjacent to 12f; hereinafter referred to as a “shoulder bent portion 12a”) is formed {see FIG. 3 (D)}. As a result, the intermediate molded product 4 is molded.

The pair of vertical wall portions 12 of the intermediate molded product 4 thus formed include a shoulder bending portion 12a, a side portion 12b extending downward from the shoulder bending portion 12a, and an inclined portion 12c extending diagonally upward from the shoulder bending portion 12a. Is provided. As shown enlarged in FIG. 4, the shoulder bending portion 12a of the intermediate molded product 4 has the same shape as a part of the shoulder bending portion 1b of the tubular part 1 of the finished product, and in the illustrated example, an arc having the same curvature. Has a shape. The shoulder bending portion 12a of the intermediate molded product 4 is provided at the same position as the shoulder bending portion 1b of the tubular part 1 of the finished product. That is, the distance between the end portion 12f of the vertical wall portion 12 of the intermediate molded product 4 and the shoulder bending portion 12a (distance in the material extending direction in the cross section orthogonal to the longitudinal direction) is the butt portion 1a of the tubular component 1 and the shoulder bending portion. Equal to the distance to part 1b. The inclined portion 12c of the intermediate molded product 4 has a substantially flat plate shape, and in the illustrated example, extends in the tangential direction from the upper end of the shoulder bending portion 12a.

In the O-bending step, the intermediate molded product 4 is pressed by the O-bending die 30 to obtain the tubular part 1 shown in FIGS. 1 (D) and 2. Specifically, first, the bottom portion 11 of the intermediate molded product 4 is set on the lower mold 32 {see FIG. 5 (A)}, and the upper mold 31 is lowered to lower the upper end 12f of the inclined portion 12c of the intermediate molded product 4. The upper die 31 pushes it downward, further bends the shoulder bent portion 12a of the vertical wall portion 12, and abuts the ends of the materials against each other {see FIG. 5 (B)}. After that, by further lowering the upper mold 31 and tightening the mold, the ends of the materials are brought into contact with each other, and the entire molded product is projected outward and pressed against the mold, whereby the tubular part 1 having an irregular cross section is formed. {See Fig. 5 (C)}. After that, welding is performed on the butt portion 1a between the material ends of the tubular part 1.

As described above, in the intermediate bending step, the shoulder bending portion 12a having a desired shape is formed at the desired position of the vertical wall portion 12, and in the subsequent O bending step, the bending process is performed starting from the shoulder bending portion 12a. Since it can be applied, it is possible to avoid a situation in which a bent portion is formed at an undesired position of the vertical wall portion 12. This makes it possible to press-mold a tubular part having an irregular cross section while maintaining a state in which the ends of the material are butted against each other. In particular, as in the present embodiment, the shoulder bending portion 12a of the intermediate molded product 4 has the same shape as a part of the bending portion 1b of the tubular part 1 of the finished product, thereby ensuring molding defects in the O bending process. Can be prevented.

The present invention is not limited to the above embodiment. Hereinafter, other embodiments of the present invention will be described, but the same points as those of the above-described embodiments will be omitted.

In the above embodiment, the case where the present invention is applied to the press molding of a straight tubular part 1 having the same cross-sectional shape in the longitudinal direction is not limited to this, but the tubular component having a different cross-sectional shape in the longitudinal direction is not limited to this. The present invention can also be applied to press molding. For example, the tubular part 41 shown in FIGS. 6 and 7 has a flat portion 41a having a substantially square shape having a long cross-sectional shape in the width direction, and a substantially circular portion 41b having substantially the same width direction dimension and vertical dimension. It has a gradual change portion 41c in which the flat portion 41a and the substantially circular portion 41b are smoothly continuous.

First, a U-shaped molded product 50 having a substantially U-shaped cross section is molded in the U-bending step, and the U-shaped molded product 50 is set in the recess 22a of the lower mold 22 of the intermediate bending die 20 as shown in FIG. do. The U-shaped molded product 50 has different cross-sectional shapes in the longitudinal direction, and the lower portion (bottom portion 11 and bottom bent portion 13) of each region is a flat portion 41a, a substantially circular portion 41b, and a gradual change portion of the tubular part 41, respectively. It has substantially the same shape as the lower part of 41c.

Then, as shown in FIG. 9, the upper die 21 is lowered, and the upper end 12f of the pair of vertical wall portions 12 of the U-shaped molded product 50 is tilted inward on the inclined surface 21a of the upper die 21. At this time, in the region corresponding to the flat portion 41a of the tubular component 41 shown in FIG. 9A, the upper end 12f of the vertical wall portion 12 of the U-shaped molded product 50 is in contact with the inclined surface 21a of the upper die 21. On the other hand, in the region corresponding to the gradually changing portion 41c and the substantially circular portion 41b of the tubular part 41 shown in FIGS. 9B and 9C, the upper end 12f of the vertical wall portion 12 of the U-shaped molded product 50 corresponds to the upper mold 21. It does not touch and inclines inward following the region corresponding to the flat portion 41a.

After that, when the upper die 21 is further lowered, as shown in FIG. 10, the upper end 12f of the vertical wall portion 12 abuts on the locking portion 21b (pair of locking surfaces 21d) of the upper die 21. At this time, it is preferable to set the shape of the upper die 21 (particularly, the shape of the locking portion 21b) so that the upper end 12f of the vertical wall portion 12 simultaneously abuts on the locking portion 21b over the entire longitudinal direction.

When the upper mold 21 is further lowered, as shown in FIG. 11, the vertical wall portion 12 is compressed, and the intermediate portion of the vertical wall portion 12 is curved outward and pressed against the molding surface 21c of the upper mold 21. As a result, the shoulder bending portion 12a is formed over the entire area in the longitudinal direction of the intermediate portion of the vertical wall portion 12, and the intermediate molded product 51 is formed. After that, the intermediate molded product 51 is set in the O-bending die, and the upper die is lowered and pressed to form the tubular part 41 (not shown).

As shown in FIGS. 6 and 7, the tubular part 41 having a deformed cross section and whose cross-sectional shape changes in the longitudinal direction is very difficult to be molded by a press. Specifically, when the tubular part 41 having the flat portion 41a and the substantially circular portion 41b is press-molded as described above, the flat portion 41a is desired due to the influence of the bending process of the substantially circular portion 41b in the O bending step. There is a risk that the position will be curved and molding defects will occur. For this reason, conventionally, tubular parts having a complicated shape as described above have been manufactured by a high-cost method such as hydroforming. According to the verification by the present inventors, even if the tubular part has a deformed cross section and the cross-sectional shape changes in the longitudinal direction, as described above, the desired position of the vertical wall portion 12 in the intermediate bending step. It was clarified that by forming the shoulder bending portion 12a having a desired shape, it is possible to avoid molding defects in the subsequent O-bending step and to form the shoulder bending portion 12a by simple press molding.

In the above O-bending step, if the timing at which the O-bending die abuts on the upper end 12f of the vertical wall portion 12 of the intermediate molded product 51 differs in the longitudinal direction, the timing at which the end portions of the materials abut against each other differs in the longitudinal direction. .. In this case, due to the influence of the longitudinal region that abuts first (compressive stress), molding defects may occur in the longitudinal region that abuts after that.

In order to avoid such a defect, the height change amount in the longitudinal direction of the upper end 12f of the intermediate molded product 51, that is, the height change amount in the longitudinal direction of the locking surface 21d of the intermediate bending die is set to the tubular component of the finished product. It is preferable that the height change amount is the same as the height change amount in the longitudinal direction of the upper end of 41. In the illustrated example, the height change amount in the longitudinal direction of the upper end 12f of the intermediate molded product 51 shown in FIG. 12 (the shape of the upper end 12f of the intermediate molded product 51 in the side view) and the tubular part 41 of the finished product shown in FIG. The amount of change in height of the upper surface 41d in the longitudinal direction (the shape of the upper surface 41d of the tubular component 41 in the side view) is the same. In this case, the height difference X of the upper end 12f of the intermediate molded product 51 and the height difference Y of the upper surface 41d of the tubular component 41 are equal (X = Y). As a result, in the O-bending step, the entire longitudinal direction of the upper end 12f of the intermediate molded product 51 abuts on the O-bending die at the same time, and the ends of the material are simultaneously abutted against each other in the entire longitudinal direction to be molded. Molding defects in the bending process can be reliably prevented.

In the above intermediate bending step, if the angle of the inclined portion 12c of the intermediate molded product 51 (that is, the angle of the inclined surface 21a of the upper die 21 of the intermediate bending die 20) is raised and brought closer to the mold clamping direction, the intermediate bending is performed. Since the compressive force is likely to be applied to the vertical wall portion 12 in the process, the shoulder bending portion 12a is easily formed. However, if the inclined portion 12c is raised too much, the molding region of the shoulder bending portion 12a becomes too small, so that there is a possibility that molding defects may occur in the subsequent O-bending step.

Therefore, by arranging the shoulder bending portion 12a of the vertical wall portion 12 of the intermediate molded product 51 outside the shoulder bending portion of the tubular part 41 in the width direction, the inclined portion is secured while securing the molding region of the shoulder bending portion 12a. 12c can be launched. Specifically, for example, as shown in FIG. 13, the inclined surface 21a and the forming surface 21c of the upper die 21 of the intermediate bending die 20 have the lower end P of the forming surface 21c with respect to the above embodiment (see the chain line). Rotate outward in the width direction around the center (see arrow). As a result, while securing the molding region of the bent portion 12a in the same manner as in the above embodiment, the inclined surface 21a of the upper mold 21 can be raised and brought closer to the mold clamping direction (vertical direction in the figure). The shoulder bent portion 12a of the product 51 can be reliably molded.

Further, in the examples shown in FIGS. 8 to 11, the locking portion 21b provided on the upper die 21 of the intermediate bending die has a substantially V-shaped cross section, and the locking surface 21d is substantially perpendicular to the inclined surface 21a. It is extended. As a result, the upper end 12f of the vertical wall portion 12 abuts in a state substantially orthogonal to the locking surface 21d, so that the shape of the end portion of the material (upper end 12f of the vertical wall portion 12) is formed by the contact with the locking surface 21d. It is possible to prevent it from collapsing and prevent joint defects in subsequent welding.

Further, as shown in FIG. 14, a locking portion 21b having a substantially V-shaped cross section may be provided separately from the main body portion 21e of the upper die 21 so that the vertical position with respect to the main body portion 21e can be adjusted. In this case, the contact position between each locking surface 21d and the upper end 12f of the vertical wall portion 12 can be adjusted by adjusting the vertical position of the locking portion 21b having a substantially V-shaped cross section with respect to the main body portion 21e. can. For example, when the locking portion 21b is lowered from the position shown by the dotted line in FIG. 14 and arranged at the position shown by the solid line, the contact portion between the locking surface 21d and the upper end 12f of the vertical wall portion 12 moves outward in the width direction. Therefore, the compressive force applied to the vertical wall portion 12 increases. Therefore, by adjusting the vertical position of the locking portion 21b, the optimum conditions for forming the bent portion 12a can be easily set.

The shape of the tubular part 1 is not limited to the above, and for example, the above-mentioned press molding method can be applied to form a tubular part 61 having a substantially U-shaped cross section as shown in FIG. 15 (D). Specifically, first, as shown in FIG. 15 (A), the blank material 2 is pre-bent to form the convex curved portion 14, and then the U-bending step is performed as shown in FIG. 15 (B). The U-shaped molded product 3 is molded. As a result, the bottom portion 11 of the U-shaped molded product 3 is provided with a convex curved portion 14 projecting in the direction in which the vertical wall portion 12 extends (upper in the figure). Then, in the intermediate bending step, the U-shaped molded product 3 is pressed to form the bent portion 12a on the vertical wall portion 12 to obtain the intermediate molded product 4 {see FIG. 15 (C)}. Then, by pressing the intermediate molded product 4 in the O-bending step, the ends of the intermediate molded product 4 are abutted against each other and molded to obtain a tubular part 61 having a substantially U-shaped cross section {see FIG. 15 (D)}.

In this case, since the folded portion 61a at the lower part of the tubular part 61 having a substantially U-shaped cross section is formed by bending, the strain and stress generated in the folded portion 61a are relaxed, and after annealing or the like for removing the stress. No processing is required. Further, since the cross-sectional shape (particularly, the shape of the folded portion 61a) can be freely designed, for example, by reducing the curvature of the folded portion 61a, the concentration of strain and stress in the folded portion 61a can be further alleviated. Can be done. Further, in the above method, even a tubular part whose cross-sectional shape changes in the longitudinal direction can be molded, so that the degree of freedom in the product shape is increased.

In the above embodiments, the case where the intermediate bending step is one step is shown, but the intermediate bending step may be divided into a plurality of steps.

The above press forming method can be applied to the press forming of tubular parts having a deformed cross section, and is, for example, applied to the press forming of the left and right trailing arms of the rear suspension of an automobile and the intermediate beam connecting them. Can be done.

1 Tubular part 1a Butt part 1b Shoulder bending part 2 Blank material 3 U-shaped molded product 4 Intermediate molded product 11 Bottom 12 Vertical wall part 12a Shoulder bending part (bending part)
12b Side part 12c Inclined part 13 Bottom bending part 20 Intermediate bending mold 21 Upper mold 21a Inclined surface 21b Locking part 21c Molding surface 22 Lower mold 30 O Bending mold 31 Upper mold 32 Lower mold

Claims (2)

A method for press molding tubular parts with irregular cross sections.
A U-bending process for forming a pair of vertical wall portions rising from the bottom portion and both ends of the bottom portion, and
In the intermediate bending die, the ends of the pair of vertical wall portions are abutted against the locking portions provided in the intermediate bending die and pressed to bulge outward to be provided in the intermediate bending die. An intermediate bending step of forming a bent portion having a larger curvature than other regions of the vertical wall portion in the intermediate portion of each vertical wall portion by pressing against the formed molding surface .
In the final bending die, the pair of vertical wall portions are bulged outward to form the final bending die by pressing the ends of the pair of vertical wall portions against each other while further bending the bent portions. A method for press molding a tubular part, which comprises a final bending step of pressing the tubular part against a provided molding surface to form the tubular part. The press molding method according to claim 1, wherein a convex curved portion is provided on the bottom portion formed in the U-bending step so as to project in a direction in which the pair of vertical wall portions extend.

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