It is low with the thick steel sheet and its production manner null it is superior in super full house thermal welding HAZ Characteristic for yield ratio

摘要

PROBLEM TO BE SOLVED: To provide a thick steel plate with a low yield ratio excellent in HAZ (heat affected zone) toughness in super high-heat input welding over 400 kJ/cm, for building construction, and to provide a manufacturing method therefor.;SOLUTION: This thick steel plate has a composition comprising 0.03-0.15% C, 0.05-0.50% Si, 0.5-2.0% Mn, 0.03% or less P, 0.0005-0.0030% S, 0.005-0.1% Al, 0.004-0.02% Ti, 0.0020-0.0070% N, 0.0005-0.0030% Ca, and 0.0050% or less O, while having ACR (defined by equation (1) in Claim 1 for a relation among particular contents) so as to satisfy 0.3 or more but 0.8 or less, and the balance Fe with unavoidable impurities, and has a metallographic structure in which an average particle diameter of dispersed particles is 50 nm or less, and density of the number of the particles is 1×10⁶/mm² or more. The manufacturing method therefor is characterized by reheating it to 1,000-1,250°C, subsequently hot-rolling it at a finishing rolling temperature of the Ar₃ transformation temperature or higher, and then acceleration-cooling it to 600-250°C at a cooling rate of 1°C/s or higher, in order to make a low yield ratio to be 80% or less.;COPYRIGHT: (C)2003,JPO