CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Japanese Patent Application No. 2020-021787 filed on Feb. 12, 2020, incorporated herein by reference in its entirety.

BACKGROUND

1. Technical Field

The disclosure relates to method for manufacturing a pipe.

2. Description of Related Art

As disclosed in, e.g., Japanese Unexamined Patent Application Publication No. 2019-022912 (JP 2019-022912 A), UO forming is widely used as a method for manufacturing a pipe. UO forming is a press forming process in which a plate member is press-formed into a U shape (U forming) and then press-formed into an O shape (cylindrical shape) (O forming) by using a pair of dies each having a cavity with a semicircular section.

SUMMARY

The inventors examined use of UO forming to manufacture a pipe including a tapered part protruding from one end of a pipe body in such a manner that the tapered part is tilted radially inward from the one end of the pipe body, and found the following problem. For example, in the case where such a pipe is manufactured by merely using UO forming, the tapered part of the pipe protrudes along an extension of the longitudinal direction of the pipe body. When the tapered part is bent and tilted radially inward after UO forming, wrinkling or cracking may occur near the boundary between the pipe body and the tapered part.

The disclosure provides a method for manufacturing a pipe which reduces wrinkling and cracking near the boundary between a pipe body and a tapered part.

A method for manufacturing a pipe according to an aspect of the disclosure includes: press-forming a plate member into a U shape; and press-forming the plate member formed into the U shape into an O shape. The pipe includes a pipe body and a tapered part tilted radially inward and protruding from one end of the pipe body. Before the plate member is press-formed into the U shape, a boundary between a portion of the plate member corresponding to the pipe body and a portion of the plate member corresponding to the tapered part is pressed from a surface of the plate member corresponding to an inner peripheral surface of the pipe to bend the portion of the plate member corresponding to the tapered part toward the surface of the plate member corresponding to the inner peripheral surface of the pipe.

According to the above aspect, before the plate member is press-formed into the U shape, the boundary between the portion of the plate member corresponding to the pipe body and the portion of the plate member corresponding to the tapered part is pressed from the surface of the plate member corresponding to the inner peripheral surface of the pipe to bend the portion of the plate member corresponding to the tapered part toward the surface of the plate member corresponding to the inner peripheral surface of the pipe. This reduces wrinkling and cracking at the boundary between the portion corresponding to the pipe body and the portion corresponding to the tapered part as compared to the case where the portion corresponding to the tapered part is bent after U forming or O forming.

In the above aspect, a bend having a groove shape may be formed at the boundary when the boundary is pressed. According to the above configuration, since the portion where the bend has been formed is work-hardened by compression, a taper angle of the portion corresponding to the tapered part is maintained even during U forming and O forming that will be performed later, and springback is also reduced.

In the above aspect, the bend may have a curved section. With this configuration, the plate member is less likely to become excessively thin at the bend.

In the above aspect, when the boundary is pressed, the bend may be formed while bending a middle part of the portion corresponding to the pipe body into a U shape. The above configuration further reduces wrinkling and cracking at the boundary between the portion corresponding to the pipe body and the portion corresponding to the tapered part.

In the above aspect, the bend may be formed by pressing the boundary with a punch having a ridge corresponding to the bend. According to the above configuration, the bend is easily formed.

In the above aspect, a die on which the plate member is placed when the boundary is pressed may be provided with a groove at a position corresponding to the ridge of the punch.

In the above aspect, a die on which the plate member is placed when the boundary is pressed may be a die divided by a clearance that is predetermined. The clearance may be located at a position corresponding to the ridge of the punch. According to the above configuration, the plate member is less likely to become excessively thin at the bend, and the life of the ridge of the punch is increased.

In the above aspect, the ridge of the punch may have a curved section.

In the above aspect, a radius of curvature of the portion corresponding to the pipe body after the plate member is press-formed into the U shape may be smaller than a radius of curvature of the portion corresponding to the pipe body after the plate member formed into the U shape is press-formed into the O shape.

According to the above aspect of the disclosure, a method for manufacturing a pipe is provided which reduces wrinkling and cracking near the boundary between a pipe body and a tapered part.

