| 摘要 |
PROBLEM TO BE SOLVED: To provide a welded steel pipe which has excellent hydroforming properties, and to provide its production method. SOLUTION: A welded steel pipe having a composition containing, by mass, 0.05 to 0.3% C, <=20.% Si, >1.5 to 5.0% Mn, <=0.1% P, <=0.01% S, <=0.1% Al, <=0.3% Ti, <=0.01% N, <=0.1% Nb and <=0.1% Cr is used as a steel pipe stock, and is heated or soaked. The steel pipe is thereafter subjected to reduction rolling at a cumulative reduction ratio of >=35% and at a rolling finishing temperature of 500 to 900 deg.C. Thus, the welded steel pipe having a tensile strength of >=780 MPa, n×r of >=0.15 and excellent hydroforming properties is obtained. In the reduction rolling, the cumulative reduction ratio in the temperature range of the Ar3 transformation point or below is preferably controlled to >=20%.
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