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Abstract

A welded steel pipe is formed by heating or soaking an untreated welded steel pipe having a steel composition containing, on the basis of mass percent: about 0.03% to about 0.2% C, about 2.0% or less of Si, not less than about 1.0% to about 1.5% Mn, about 0.1% or less of P, about 0.01% or less of S, about 1.0% or less of Cr, about 0.1% or less of Al, about 0.1% or less of Nb, about 0.1% or less of Ti, about 0.1% or less of V, 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 590 MPa and an n.times.r product of at least about 0.22. The treated steel pipe is preferably reduction-rolled at a cumulative reduction rate of at least about 20% below the Ar.sub.3 transformation point.

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