High-strength hot-rolled steel sheet with low yield strength decreased after molding, the pipes and the manufacturing method of the high-strength hot-rolled steel sheet molding using it

摘要

PROBLEM TO BE SOLVED: To produce a high strength hot rolled steel sheet for the stock for a steel pipe excellent in yield strength by subjecting a carbon steel slab under heating under specified temp. conditions to form into a hot rolled steel sheet, therefore executing a two-stage cooling at specified cooling rates and coiling the same. ;SOLUTION: A steel slab contg., by weight, 0.02 to 0.25% C, 0.6% Si, 2.5% Mn, 0.05% P, 0.01% S, 0.01 to 0.10% Al, 0.1% Ti, 0.1% Nb and 0.1% V or furthermore contg. specified small amount of at least one kind among Ni, Cu, Mo, Cr, B, Ca and rare earth metals is heated at 1000 to 1400°C and is subjected to hot rolling at 70 to 900°C finishing temp. to form into a sheet material, which is immediately cooled for 2 to 20sec at the average cooling rate of ≤30°C/sec, is successively cooled to the Ar₁ point to 300°Cat the average cooling rate twice the above and is coiled at ≥300°C to produce the high strength hot rolled steel sheet composed of ferritic phases of ≥50% area rate (F) and secondary phases having ≤9μm average grain diameter other than the ferritic phases, in which the ratio of d/F is regulated to ≤0.15, and small in the yield strength of a steel pipe in the case of being formed into a steel pipe.;COPYRIGHT: (C)1997,JPO