WELDED STEEL PIPE HAVING EXCELLENT HYDROFORMABILITY AND METHOD FOR MAKING THE SAME

摘要

A welded steel pipe is formed by heating or soaking an untreated welded stee l pipe having a steel composition containing, on the basis of mass percent: about 0.05% to about 0.2% C; about 0.2% or less of Si; about 1.5% or less of Mn; about 0.1% or le ss of P; about 0.01 % or less of S; about 0.1 % or less of Al; and about 0.01 % or less of N; and by reduction- rolling the treated steel pipe at a cumulative reduction rate of at least about 35% and a final rolling temperature of about 500.degree.C to about 900.degree.C. The welded steel pipe exhibits excellent hydroformability, i.e., has a tensile strength of at least about 400 MPa and an n x r product of at least about 0.22. The treated steel pipe is preferably reduction-rolled a t a cumulative reduction rate of at least about 20% below the Ar3 transformation point. The welded steel pipe is suitable for forming structural components.