STEEL MATERIAL SUPERIOR IN TOUGHNESS AT HEAT-AFFECTED ZONE IN HIGH-HEAT-INPUT WELD, AND MANUFACTURING METHOD THEREFOR

摘要

PROBLEM TO BE SOLVED: To provide a steel material superior in such a toughness as to stably show a high value in a Charpy test even for a high-heat-input weld zone, in a steel material to be used for a weld structure in a large-sized structure such as a building and a bridge, and to provide a manufacturing method therefor. SOLUTION: The steel material superior in the toughness at a heat-affected zone in the high-heat-input weld includes, by mass%, 0.01-0.2% C, 0.03-0.5% Si, 0.5-2.0% Mn, 0.02% or less P, less than 0.01% S, more than 0.005-0.08% Al, 0.0005-0.02% Ti, 0.0003-0.02% Ca, 0.002-0.009% N and 0.001-0.0035% O (oxygen), and the balance Fe with impurities; and has a ferrite area rate of 15% or more at a 1/4 deep position in a plate thickness direction. In the steel material, there exist CaO-Al2O3-based inclusions with particle diameters of 0.5-5μm, and the aspect ratio of the inclusion is 1.9 or less. The manufacturing method therefor is also disclosed. COPYRIGHT: (C)2010,JPO&INPIT