METHOD FOR MANUFACTURING STEEL SHEET SUPERIOR IN DEFORMABILITY AND LOW-TEMPERATURE TOUGHNESS FOR ULTRAHIGH-STRENGTH LINE PIPE, AND METHOD FOR MANUFACTURING STEEL PIPE FOR ULTRAHIGH-STRENGTH LINE PIPE

摘要

PROBLEM TO BE SOLVED: To provide a method for manufacturing a steel sheet of which the base metal has superior strength, low-temperature toughness and deformability, for an ultrahigh-strength line pipe that can be easily welded on the actual place and has a tensile strength of 900 MPa or higher (X120 or higher in the API standards), and to provide a method for manufacturing a steel pipe for an ultrahigh-strength line pipe.;SOLUTION: The steel material comprises, by mass%, 0.03 to 0.07% C, 0.01 to 0.50% Si, 1.5 to 2.5% Mn, 0.01% P, 0.0030% S, 0.0001 to 0.2% Nb, 0.0005 to 0.03% Al, 0.003 to 0.030% Ti, 0.0001 to 0.0050% B, and the balance iron with unavoidable impurities. The method for manufacturing the steel sheet superior in deformability and low-temperature toughness for the ultrahigh-strength line pipe comprises the steps of: heating the steel material; hot-rolling it; cooling it with water to the / transformation starting point or higher; reheating it; and subsequently cooling it with water.;COPYRIGHT: (C)2009,JPO&INPIT