HOT-ROLLED STEEL SHEET HAVING HIGH STRENGTH AND LOW YIELD RATIO FOR USE IN OIL WELL TUBE AND METHOD FOR PRODUCING THE SAME AND STEEL PIPE PREPARED BY THE SAME AND METHOD FOR PRODUCING THE SAME

摘要

PURPOSE: A hot-rolled steel sheet for an oil well pipe with high strength and low yield ratio and a manufacturing method thereof are provided to precisely control the components and structure of steel and to adjust manufacturing conditions. CONSTITUTION: A manufacturing method of a hot-rolled steel sheet for an oil well pipe with high strength and low yield ratio is as follows: a step of reheating a steel slab composed of 0.30-0.50 wt.% of C, 0.20-0.50 wt.% of Si, 1.3-2.5 wt.% of Mn, 0.02 wt.% or less of P, 0.01 wt.% or less of S, 0.001-0.006 wt.% of Ca, 0.01-0.06 wt.% of Al, 0.008 wt.% or less of N, 0.01-0.30 wt.% of Cr, and the rest of Fe and other inevitable impurities at 1000-1250C; a step of rough-rolling the reheated steel slab at 900-1100C; a step of finish-rolling the rough-rolled slab at 800-900C; a step of winding the finish-rolled steel sheet after the steel sheet is cooled to 560-610C at 10-20C/s. A relation between the Cr and the C satisfies next formula (9 = (C/12)/(Cr/52) = 30).