HIGH STRENGTH STEEL PIPE USED FOR PIPELINE AND HAVING EXCELLENT DEFORMABILITY, AND ITS PRODUCTION M

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PROBLEM TO BE SOLVED : To provide a high strength steel pipe which has excellent deformability and an excellent strength of X 80 to 100 class in the API standards and is suitable for a pipeline, and to provide its production method. SOLUTION : In the high strength steel pipe for a pipeline having excellent deformability, the ratio between the yield strength YSL in the longitudinal direction of the steel pipe and the yield strength YSC in the circumferential direction, YSL/YSC, is 70 to 95%. The steel pipe comprises suitable amounts of C, Si, Mn, P, S, Nb, Ti, Al and N, and further comprises one or more kinds of metals selected from Ni, Mo, Cr, Cu, V, B, Ca, rare earth metals and Mg. The steel pipe has a microstructure consisting of 30 to 80% ferrite in an area ratio, and the balance martensite and/or bainite, and preferably has a YSC of ≥80 psi. In the production method, a slab is heated to ≥850°C, is hot-rolled, is air-cooled to ≤500°C, is thereafter reheated to 740 to 850°C, and is cooled to ≤400°C at ≥10°C/s. The pipe may be expanded by 0.8 to 3%.