| 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 deg.C and is subjected to hot rolling at 70 to 900 deg.C finishing temp. to form into a sheet material, which is immediately cooled for 2 to 20sec at the average cooling rate of <=30 deg.C/sec, is successively cooled to the Ar1 point to 300 deg.Cat the average cooling rate twice the above and is coiled at >=300 deg.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.
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