SEAMLESS STEEL PIPE AND METHOD FOR PRODUCING SAME

摘要

Disclosed is a seamless steel pipe suitable for submarine flowlines which has high strength, stable toughness and excellent corrosion resistance. Also disclosed is a method for producing such a seamless steel pipe. Specifically disclosed is an as-quenched seamless steel pipe consisting of 0.03-0.08% of C, 0.3-2.5% of Mn, 0.001-0.10% of Al, 0.02-1.0% of Cr, 0.02-1.0% of Ni, 0.02-0.8% of Mo, 0.004-0.010% of Ti, 0.002-0.008% of N, 0.0005-0.005% of Ca, 0-0.08% of V, 0-1.0% of Cu and the balance of Fe and impurities. In this seamless steel pipe, not more than 0.25% of Si, not more than 0.05% of P, not more than 0.005% of S, less than 0.005% of Nb and less than 0.0003% of B are contained as impurities. The seamless steel pipe has a structure consisting of not more than 20% by volume of polygonal ferrite, not more than 10% by volume of a mixture of martensite and residual austenite, and the balance of bainite. Also specifically disclosed is a high-strength thick seamless steel pipe which is obtained by tempering such a steel pipe. The amount of B may be 0.0003-0.001%, and the seamless steel pipe may further contain Mg and one or more of REM. A method for producing such a seamless steel pipe is characterized by the cooling rate during the quenching.