LOW YIELD RATIO HIGH-TENSILE STEEL MATERIAL EXCELLENT IN TOUGHNESS OF HEAVY HEAT INPUT WELDING HEAT-AFFECTED ZONE AND METHOD FOR MANUFACTURING THE SAME

摘要

PROBLEM TO BE SOLVED: To provide a steel material having a tensile strength of 590 MPa or higher and a yield ratio of 80% or less and excellent in toughness of a welding heat-affected zone without requring special acceleration cooling or heat treatment even when heavy heat input welding exceeding 500 kJ/cm is practiced.;SOLUTION: The manufacturing method includes heating a steel slab to 1,000°C or higher and 1,250°C or under, hot rolling it, and then cooling it by air cooling, wherein the steel slab comprises, by mass; 0.025-0.050% C, 0.01-0.40% or less Si, 1.20-2.50% Mn, 0.003-0.050% P, 0.0004-0.0100% S, 1.50-3.50% Cr, 0.10-0.50% Mo, 0.010-0.050% Al, 0.005-0.050% Ti, 0.0015-0.0060% N, while limiting B to 0.0003% or below, and Mn+0.4Cr to 2.50-3.00%, and the balance iron with inevitable impurities.;COPYRIGHT: (C)2013,JPO&INPIT