Method of manufacturing flaring-processed metal pipe

1. A method of manufacturing a flaring-processed metal pipe having a pipe expanded section from a hollow shell including a plurality of portions having different deformation resistances when viewed in a circumferential direction, the method comprising:

among the plurality of portions spaced in the circumferential direction, specifying one of the plurality of portions as a low deformation resistance section having a first deformation resistance, and another of the plurality of portions as a high deformation resistance section having a second deformation resistance, the second deformation resistance being greater than the first deformation resistance; and

press-fitting a pipe expansion punch into the hollow shell and expanding the hollow shell, so that a thickness reduction rate of the low deformation resistance section is smaller than a thickness reduction rate of the high deformation resistance section.

2. The method of manufacturing a flaring-processed metal pipe according to claim 1 , wherein the pipe expansion punch includes a first abutment surface which abuts the low deformation resistance section of the hollow shell, and a second abutment surface which abuts the high deformation resistance section of the hollow shell, and an inclination angle of the first abutment surface with respect to the central axis of the pipe expansion punch is smaller than an inclination angle of the second abutment surface with respect to the central axis, and

wherein in the press-fitting and the expanding, the pipe expansion punch is press-fitted into the hollow shell while the first abutment surface of the pipe expansion punch abuts the low deformation resistance section of the hollow shell and the second abutment surface of the pipe expansion punch abuts the high deformation resistance section of the hollow shell.

3. The method of manufacturing a flaring-processed metal pipe according to claim 2 , wherein the inclination angle of the first abutment surface of the pipe expansion punch is 0°.

4. The method of manufacturing a flaring-processed metal pipe according to claim 3 , wherein the press-fitting and the expanding include:

press-fitting the pipe expansion punch into the hollow shell to obtain an intermediate formed product from the hollow shell; and

press-fitting a forming punch having a shape which coincides with an inner surface of the pipe expanded section of the flaring-processed metal pipe into the intermediate formed product.

5. The method of manufacturing a flaring-processed metal pipe according to claim 4 , wherein the hollow shell is an electric resistance welded steel pipe or a seamless steel pipe.

6. The method of manufacturing a flaring-processed metal pipe according to claim 4 , wherein in the press-fitting of the pipe expansion punch, the pipe expansion punch is press-fitted into the hollow shell such that a diameter expansion amount of the low deformation resistance section of the hollow shell is less than 0.5 times a diameter expansion amount of the high deformation resistance section of the hollow shell.

7. The method of manufacturing a flaring-processed metal pipe according to claim 6 , wherein the hollow shell is an electric resistance welded steel pipe or a seamless steel pipe.

8. The method of manufacturing a flaring-processed metal pipe according to claim 3 , wherein the hollow shell is an electric resistance welded steel pipe or a seamless steel pipe.

9. The method of manufacturing a flaring-processed metal pipe according to claim 2 , wherein the press-fitting and the expanding include:

press-fitting the pipe expansion punch into the hollow shell to obtain an intermediate formed product from the hollow shell; and

press-fitting a forming punch having a shape which coincides with an inner surface of the pipe expanded section of the flaring-processed metal pipe into the intermediate formed product.

10. The method of manufacturing a flaring-processed metal pipe according to claim 9 , wherein the hollow shell is an electric resistance welded steel pipe or a seamless steel pipe.

11. The method of manufacturing a flaring-processed metal pipe according to claim 9 , wherein in the press-fitting of the pipe expansion punch, the pipe expansion punch is press-fitted into the hollow shell such that a diameter expansion amount of the low deformation resistance section of the hollow shell is less than 0.5 times a diameter expansion amount of the high deformation resistance section of the hollow shell.

12. The method of manufacturing a flaring-processed metal pipe according to claim 11 , wherein the hollow shell is an electric resistance welded steel pipe or a seamless steel pipe.

13. The method of manufacturing a flaring-processed metal pipe according to claim 2 , wherein the hollow shell is an electric resistance welded steel pipe or a seamless steel pipe.

14. The method of manufacturing a flaring-processed metal pipe according to claim 2 , wherein the first abutment surface and the second abutment surface are sections of a conical surface.

15. The method of manufacturing a flaring-processed metal pipe according to claim 1 , wherein the hollow shell is an electric resistance welded steel pipe or a seamless steel pipe.

16. The method of manufacturing a flaring-processed metal pipe according to claim 1 , wherein the low deformation resistance section has a smaller thickness than the high deformation resistance section.