METHOD FOR PRODUCING LOW YIELD RATIO STEEL MATERIAL FOR LOW TEMPERATURE, EXCELLENT IN TOUGHNESS IN WELDED PART

摘要

PROBLEM TO BE SOLVED: To provide a method for producing a low yield ratio steel material for low temperature, excellent in the toughness in a welded part.;SOLUTION: After molten steel is pre-deoxidized with Al, the deoxidation with Si and Mn is performed and REM is added, and soluble oxygen content is adjusted to 0.0010-0.0050% and this molten steel contains 0.05-0.12% C, ≤0.50% Si, 0.8-1.8% Mn, ≤0.02% P, 0.0005-0.0060% S, 0.0030-0.0200% REM and if necessary, after adding B, Nb, V, Cu, Ni, Cr, Mo or W, the steel blank is made to contain ≤0.004% each of Al and Ti. Further, after heating this steel blank to 1050-1200°C, hot-rolling of ≥30% accumulative rolling reduction ratio at ≥950°C of the temperature zone and ≥30% accumulative rolling reduction ratio at 900°C of the temperature zone, is applied and thereafter, the front-step cooling at 750-600°C cooling stop temperature, is performed at 10°C/s cooling speed and the rear-step cooling to 600°C from the front-step cooling stop temperature, is performed at ≥10°C/s cooling speed.;COPYRIGHT: (C)2006,JPO&NCIPI