METHOD FOR MANUFACTURING HEAVY GAUGE HIGH TENSILE STRENGTH HOT ROLLED STEEL SHEET WITH EXCELLENT HIC RESISTANCE

摘要

PROBLEM TO BE SOLVED: To provide a method for manufacturing a heavy gauge high tensile strength hot rolled steel sheet with excellent HIC resistance, that is well suited as a material for grade X65 or higher high-strength welded steel pipe.;SOLUTION: A steel material contains, by mass%, of 0.02-0.08% C, 0.50-1.85% Mn, 0.02-0.10% Nb, 0.001-0.05% Ti, ≤0.0005% B, and satisfies (Ti+Nb/2)/C4. The steel is subjected to the hot rolling. The steel material is successively subjected to: the first cooling process where after completing the finish-rolling, an acceleratively cooling is applied until the surface temperature becomes at most Ar₃ transforming point and at least Ms point in the average cooling speed of at least 20°C/s and lower than the martensite-generating cooling critical speed of the hot-rolled sheet surface; the second cooling process for rapidly cooling until the center of the sheet becomes the temperature zone of 350°C to 600°C thickness; and a third cooling process, in which the steel sheet is coiled in the form of coil at the coiling temperature zone of 350°C to 600°C the center temperature of the sheet thickness so as to hold or retain for ≥30 min in the temperature zone of 350-600°C in at least the position 1/4T-3/4T in the thickness direction of the coil. Then, the high tensile strength, hot rolled steel sheet excellent in HIC resistance and having ≥520 MPa tensile strength and ≤230 HV surface hardness, is obtained.;COPYRIGHT: (C)2010,JPO&INPIT