THICK HIGH STRENGTH STEEL PLATE FOR HIGH HEAT INPUT WELDING HAVING REDUCED VARIATION IN BASE METAL LOW TEMPERATURE TOUGHNESS AND EXCELLENT HEAT AFFECTED ZONE TOUGHNESS, AND METHOD FOR PRODUCING THE SAME

摘要

PPROBLEM TO BE SOLVED: To provide a thick high strength steel plate in which the excellent low temperature toughness of a base metal is stably secured without variation, and further, the toughness of a weld heat affected zone upon high heat input welding is excellent as well. PSOLUTION: Disclosed is a thick high strength steel plate having a composition comprising 0.03 to 0.08% C, 0.05% (including 0%) Si, 1.4 to 1.7% Mn, 0.01 to 0.06% Al, 0.05% (excluding 0%) P, 0.01% (excluding 0%) S, 0.20 to 0.40% Cu, 0.20 to 0.60% Ni, 0.005 to 0.015% Nb, 0.005 to 0.03% Ti, 0.0005 to 0.0030% B and 0.0030 to 0.0090% N, respectively, and the balance iron with inevitable impurities, and also having a structure consisting essentially of a bainitic phase, and, at the position of a depth t/4 (t denotes sheet thickness) from the surface, provided that the region surrounded by a large-angle boundary in which the orientation difference of adjoining crystals is 15 is defined as a crystal grain, the maximum crystal grain size thereof is 20 m by a circle equivalent diameter. PCOPYRIGHT: (C)2009,JPO&INPIT