| 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, 0.03 to 0.2% C, <=2.0% Si, >1.0 to 1.5% Mn, <=0.01% S, <=0.1% Ti, <=0.1% Nb and <=1.0% Cr, and in which the contents of Al and P are controlled to the optimum ranges is used as a steel pipe stock, and is heated and 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 >=590 MPa, n×r of >=0.22 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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