후강판

화학 조성이, 질량%로, C:0.01-0.20%, Si:0.10-0.25%, Mn:1.30-2.50%, P:0.01% 이하, S:0.0010-0.0100%, Ti:0.005-0.030%, Al:0.003% 이하, O:0.0010-0.0050%, N:0.0100% 이하, Cu:0-0.50%, Ni:0-1.50%, Cr:0-0.50%, Mo:0-0.50%, V:0-0.10%, Nb:0-0.05%, 및, 잔부:Fe 및 불순물이며, 또한, 강 중에, Ti 산화물의 주위에 MnS가 존재하는 복합 개재물을 포함하는 후강판이다. 복합 개재물의 단면에 있어서의 MnS의 면적률이 10% 이상 90% 미만이며, 복합 개재물의 둘레 길이에서 차지하는 MnS의 비율이 10% 이상이며, 입경 0.5-5.0㎛의 복합 개재물의 개수 밀도가, 10-100개/㎟이다. 이 후강판은, 대입열 용접시에 있어서, HAZ의 저온 특성이 뛰어나다. A thick steel plate is provided that has a chemical composition comprising, by mass%, C: 0.01-0.20%, Si: 0.10-0.25%, Mn: 1.30-2.50%, P: 0.01% or less, S: 0.0010-0.0100%, Ti: 0.005-0.030%, Al: 0.003% or less, O: 0.0010-0.0050%, N: 0.0100% or less, Cu: 0-0.50%, Ni: 0-1.50%, Cr: 0-0.50%, Mo: 0-0.50%, V: 0-0.10%, Nb: 0-0.05%, and the balance: Fe and impurities, and which contains in the steel a composite inclusion in which MnS is present around a Ti oxide. An area fraction of MnS in a cross-section of the composite inclusion is 10% or more and less than 90%, the proportion which MnS accounts for in the peripheral length of the composite inclusion is 10% or more, and the number density of composite inclusions that have a grain diameter in a range of 0.5-5.0 µm is in a range of 10-100 per mm 2 . The thick steel plate exhibits excellent HAZ low-temperature properties during high heat input welding.