| 摘要 |
PROBLEM TO BE SOLVED: To provide a large diameter welded steel pipe (UOE steel pipe) having little quality deterioration after being subjected to coating treatment, suitable to a line pipe mainly transporting crude oil and natural gas, and a manufacturing method therefor. SOLUTION: A steel composed by mass% of 0.03-0.1% C, 0.01-0.5% Si, 1.2-2.0% Mn, 0.05-0.4% Mo,≥0.1% Cu+Ni, 0.005-0.04% Ti, 0.005-0.07% Nb,≤0.08% Al,≤0.005% N, wherein≥3 Ti/N, and if necessary, one or more kinds from among V, Cr, Ca, B and the balance Fe with inevitable impurities, wherein formula (1) is satisfied is heated at 1,000-1,300°C, and after hot-rolling of the steel at Ar<SB>3</SB>temperature or higher to rolling-finishing temperature, an accelerated cooling is applied to the steel to 500-650°C at≥5°C/s cooling speed, and thereafter immediately, the steel is reheated to 550-750°C at≥0.5°C/s temperature raising speed, thereby a steel sheet is obtained in which the metallurgical structure is three-phase structure of ferrite, bainite and island-state martensite, and the volume fraction of the island-state martensite is 3-15% and the volume fraction of the retained austenite is≥2%, and the uniform elongation in the longitudinal direction is≥12%, the obtained steel sheet is formed into the pipe-shape by cold formation, and the butted part is welded to form into the steel pipe and a coating treatment is applied to the steel pipe at≤300°C heating temperature. The formula (1) is Ceql≤0.40, wherein, Ceql=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5. COPYRIGHT: (C)2009,JPO&INPIT
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