Steel excellent in toughness of weld heat-affected zone

摘要

The present invention realizes excellent HAZ toughness by further suppressing the coarsening of austenite grains when they are heated for a long time at a high temperature in order to remarkably improve HAZ properties. <??>The present invention is a steel excellent in the toughness of a weld heat-affected zone: having a specific chemical composition in mass, of C: 0.03 to 0.18%, Si: 0.5% or less, Mn: 0.4 to 2.0%, P: 0.02% or less, S: 0.02% or less, Al: 0.005 to 0.04%, Ti: 0.005 to 0.03%, Ca: 0.0005 to 0.003%, and N: 0.0005 to 0.007%, with the balance consisting of Fe and unavoidable impurities; satisfying the expression 0 </= EN </= 0.002 regarding the EN value expressed by an equivalent equation comprising (%N), (%Ti) and (%B); and containing oxide particles 0.005 to 2.0 mu m, preferably 0.1 to 2.0 mu m, in circle-equivalent diameter, which oxide particles contain at least Ca, Al and O in their composition and, in the mass percentage of the elements excluding O, contain Ca at 5% or more and Al at 5% or more with the balance consisting of other deoxidizing elements and/or unavoidable impurities, with the density of the oxide particles being 100 to 3,000 pieces/mm<2>.