| 简介 |
PROBLEM TO BE SOLVED : To provide a thick high strength steel plate having a Ceq of 0.35 to 0.40, and, e.g., having a plate thickness of 50 to 80 mm and a tensile strength in a class of 490 to 620 MPa, and having excellent HAZ (heat affected zone) toughness in high heat input welding at 20 to 100 kJ/mm.
SOLUTION : The steel plate has a composition comprising, by mass, 0.03 to 0.14% C, 0.01 to 0.30% Si, 0.8 to 2.0% Mn, ≤0.02% P, ≤0.005% S, 0.003 to 0.050% Nb, 0.005 to 0.040% Al, 0.005 to 0.030% Ti, 0.0010 to 0.0050% B, 0.0030 to 0.0080% N, and 0.0010 to 0.0050% O, and in which the Ceq in formula (1) is 0.35 to 0.40, and also, Nb, Ti, N, and B satisfy expression (2), and further comprising one or more kinds selected from 0.0005 to 0.0030% Ca, 0.0005 to 0.0050% Mg, and 0.005 to 0.030% rare earth metals, and comprising multiple oxides with a diameter of the equivalent circle of 0.005 to 0.5 μm by 100 to 3, 000 pieces/mm2; formula (1) : Ceq=C+Mn/6+(Ni+Cu)/15+(Cr+Mo+V)/5, and expression (2) : 0.29Ti+1.3B≤N≤0.29Ti+1.3B+0.15Nb.
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