| 摘要 |
PROBLEM TO BE SOLVED: To provide a steel material excellent in toughness in heat affected zone(HAZ) of small to large heat input welded joint, and its production. SOLUTION: The steel material excellent in toughness in heat affected zone has a composition containing, by weight, 0.03-0.2% C, 0.02-0.5% Si, 0.6-2% Mn, 0.00005-0.02% Al, 0.0005-O.02% Ti, and 0.001-0.01% O (oxygen) and satisfying the following (1), (2), (3): (1) 5-50atom% Mn, Al+Ti=50 to 95 atom%; (2) spinel or inverse-spinel crystal structure of 0.75-0.9nm lattice constant; (3) 1-10μm major axis. Further, oxides are dispersed in this steel material by >=4 pieces/mm on the average. This steel material can be produced by applying controlled rolling at the time of hot rolling and then accelerated cooling or direct quench- and-temper treatment to a cast slab of <=350mm shorter side or applying heat treatment after hot rolling.
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