Patent ID: 8894080

Claim:
A method of press-forming a tubular part having a cross section of an irregular shape which crushes a steel tube between an upper die and a lower die in order to form a V-shaped cross section, wherein when the curvature radius of a tip of the upper die is defined as R 1 , the curvature radius of a bottom portion of the lower die corresponding to the tip of the upper die is defined as R 2 , and the wall thickness of the steel tube is defined as t; R 1 , R 2 , and t satisfy R 1 +2t=R 2 and 1.5t≦R 1 ≦3t, the method comprising: bending a V-groove surface in a bottom portion of an upper plate of the steel tube and a V-groove surface in a bottom portion of a lower plate of the steel tube under a compressive stress state by pressing the steel tube in a state where gaps are provided between the tip of the upper die and the steel tube and between the bottom portion of the lower die and the steel tube before the upper die arrives at a bottom dead point position thereof; bending back the V-groove surface in the bottom portion of the upper plate of the steel tube and the V-groove surface in the bottom portion of the lower plate of the steel tube under a tensile stress state by pressing the steel tube in a state where the gaps are completely crushed when the upper die arrives at the bottom dead point position in order to form the tubular part having the cross section of the irregular shape; and imparting a compressive residual stress to a V-groove surface in a bottom portion of the tubular part having the cross section of the irregular shape by a spring-back which is caused in the tubular part having the cross section of the irregular shape after the upper die and the lower die are opened.