THICK STEEL PLATE

To provide a thick steel plate with improved low-temperature toughness after a post-welding heat treatment, while suppressing a crack at the post-welding heat treatment.SOLUTION: A thick steel plate includes, by mass%, 0.080-0.170% C, 0.10-0.50% Si, 1.10-1.50% Mn, 0.020% or less P, 0.0050% or less S, 0.61-0.95% Ni, 0.10-0.90% Cr, 0.30-1.00% Mo, 0.005-0.050% V, 0.010-0.080% Al, 0.0010-0.0080% N, 0.0050% or less O, and a balance Fe with impurities. The thick steel plate has a chemical composition satisfying an expression 1 (in which each [symbol of element] denotes a content of each element.) and an average of the largest crystal grains up to 10th is 80 μm or less with regard to a circle equivalent diameter at a 1/4 position of a plate thickness from a surface thereof: 0.60≤VX≤1.90. Expression 1, VX=3.69-0.75×(2.7×[C]+0.4×[Si]+[Mn]+0.45×[Ni]+0.8×[Cr]+[Mo]).SELECTED DRAWING: None 【課題】溶接後熱処理後の低温靭性が改善され、溶接後熱処理時の割れが抑制される厚鋼板を提供する。【解決手段】質量％で、Ｃ：０．０８０～０．１７０％、Ｓｉ：０．１０～０．５０％、Ｍｎ：１．１０～１．５０％、Ｐ：０．０２０％以下、Ｓ：０．００５０％以下、Ｎｉ：０．６１～０．９５％、Ｃｒ：０．１０～０．９０％、Ｍｏ：０．３０～１．００％、Ｖ：０．００５～０．０５０％、Ａｌ：０．０１０～０．０８０％、Ｎ：０．００１０～０．００８０％、Ｏ：０．００５０％以下、残部：Ｆｅ及び不純物からなり、式１（各［元素記号］は各元素の含有量）を満たす化学組成を有し、表面から１／４板厚位置における結晶粒の円相当直径の上位１０個の平均が８０μｍ以下である厚鋼板。０．６０≦ＶＸ≦１．９０ ... 式１ＶＸ＝３．６９－０．７５×（２．７×［Ｃ］＋０．４×［Ｓｉ］＋［Ｍｎ］＋０．４５×［Ｎｉ］＋０．８×［Ｃｒ］＋［Ｍｏ］）【選択図】なし