THICK STEEL SHEET

PROBLEM TO BE SOLVED: To provide a thick steel sheet excellent in low temperature toughness of a heat affected zone (HAZ) during large heat input weldment.SOLUTION: There is provided a thick steel sheet having sheet thickness of 50 to 100 mm, a chemical composition of C:0.01 to 0.10%, Si:0.10 to 0.25%, Mn:1.00 to 2.50%, P:0.0100% or less, S:0.0010 to 0.0100%, Ti:0.005 to 0.030%, Al:0.003% or less, Ni:0.50 to 1.50%, O:0.0010 to 0.0050%, N:0.0100% or less, Cu, Cr, Mo, V and Nb as an optional additive element and the balance Fe with impurities, containing a composite inclusion where MnS exists around Ti oxide in the steel, having an area ratio of the MnS in a cross section of the composite inclusion of 10% or more and less than 90%, a percentage of the MnS in a boundary surface of the composite inclusion of 10% or more, a number density of the composite inclusions with a particle diameter of 0.5 to 5.0 μm of 10 to 100/mmand X represented by the following (i) formula of 0.04 to 9.70.SELECTED DRAWING: None 【課題】大入熱溶接時において、溶接熱影響部（HAZ）の低温靭性に優れた厚鋼板の提供。【解決手段】板厚が50〜100ｍｍであって、化学組成がC:0.01〜0.10%、Si:0.10〜0.25%、Mn:1.00〜2.50%、P:0.0100%以下、S:0.0010〜0.0100%、Ti:0.005〜0.030%、Al:0.003%以下、Ni:0.50〜1.50%、O:0.0010〜0.0050%、N:0.0100%以下、任意添加元素としてCu,Cr,Mo,V及びNb並びに残部:Fe及び不純物であり、鋼中にTi酸化物の周囲にMnSが存在する複合介在物を含み、複合介在物の断面におけるMnSの面積率が10〜90%未満、複合介在物の界面におけるMnSの割合が10%以上、粒径0.5〜5.0μmの複合介在物の個数密度が10〜100個/mm2、下記(i)式に示すXが0.04〜9.70である、厚鋼板。【選択図】なし