HIGH-STRENGTH HOT-ROLLED STEEL SHEET EXCELLENT IN UNIFORM ELONGATION AFTER COLD WORKING AND PROCESS FOR PRODUCING THE SAME.

摘要

A hot-rolled steel sheet having the tensile strength of 34 to 62 kgf/mm<2> and being excellent in uniform elongation even after ordinary cold working into round or square steel pipe, section or sheet pile without lowering the productivity. The production process comprises heating to 1.000-1.300 DEG C a billet containing 0.04-0.25 % of carbon, 0.0050-0.150 % of nitrogen and 0.003-0.050 % of titanium, having 0.0008-0.015 % of TiN with the grain diameter exceeding 1 mu m dispersed in the matrix, and having a Ceq. (WES) value of 0.10 to 0.45 %, rolling the hot billet until the rolling is complete at a temperature above the Ar3 transformation point, and either air-cooling from the temperature above 500 DEG C or coiling at above 500 DEG C and air-cooling, thereby adjusting the area ration of the pearlite phase in the steel texture to 5-20 %.