| 摘要 |
PROBLEM TO BE SOLVED: To provide a high tensile strength steel plate of 500 to 600 MPa class, having excellent toughness in HAZ even in the case of large heat input welding where the amount of welding heat input exceeds 20 kJ/mm. SOLUTION: The steel plate has a chemical composition consisting of, by weight, 0.03-0.2% C, <=0.4% Si, 0.5-2% Mn, <=0.015% P, 0.0005-0.006% S, 0.001-0.01% Al, 0.007-0.02% Ti, 0.0001-0.003% Mg, 0.001-0.004% O, 0.0025-0.006% N, and the balance iron with inevitable impurities. Moreover, TiN with a size of 0.01 to <0.5μm, containing oxides composed of Mg and Al, exists by >=10,000 pieces/mm2. Further oxides with a size of 0.5 to 5μm, where the sum of the average contents of Mg and Ti is >=15 wt.%, exist by >=30 pieces/mm2.
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