METHOD FOR PRODUCING HIGH STRENGTH WELDED STEEL TUBE HAVING EXCELLENT LOW YIELD RATIO AND WELD ZONE

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PROBLEM TO BE SOLVED : To provide a method for producing a high strength welded steel tube of an X120 grade. SOLUTION : A steel having a composition comprising 0.03 to 0.12% C, ≤0.5% Si, 2.0 to 3.0% Mn, 0.01 to 0.08% Al, ≤0.8% Cu, 0.1 to 1.0% Ni, ≤0.8% Cr, ≤0.8% Mo, 0.01 to 0.08% Nb, ≤0.10% V, 0.005 to 0.025% Ti, 0.001 to 0.003% B, ≤0.01% Ca, ≤0.02% rare earth metals and 0.001 to 0.006% N, and satisfying 0.21≤Pcm≤0.30, and the balance Fe with inevitable impurities is hot-rolled at a cumulative rolling draft of ≥70% in the temperature range of ≤950°C. After the completion of the rolling, accelerated cooling at 20 to 80°C/s is started at ≥700°C. Immediately after the stop of the cooling in the temperature range of 450 to 650°C, reheating is performed at 600 to 700°C, and the steel sheet having a structure essentially consisting of bainite, and in which the area ratio of insular martensite as a second phase is 5 to 20% is formed into a tubular shape, is subjected to SAW (Submerged Arc Welding), and is thereafter expanded. COPYRIGHT : (C)2006, JPO&NCIPI