THICK STEEL PLATE

Disclosed is a thick steel plate with the composition satisfying predetermined C, Si, Mn, P, S, Al, Nb, Ti, N and Ca, with the balance consisting of Fe and inevitable impurities, the composition satisfying Di + 10Nb: 1.20 to 2.50 determined from the equation (1) below, wherein a total area fraction SA of crystal grains having an equivalent circle diameter of 7.5 µm or less, among crystal grains surrounded by high-angle grain boundaries having a crystal misorientation of 15° or more, is 34% or more at the 1/4 position of the plate thickness, and 27% or more at the 1/2 position of the plate thickness: Di=1.16×C/100.5×0.7×Si+1×5.1×Mn−1.2+5×0.35×Cu+1×0.36×Ni+1×2.16×Cr+1×3×Mo+1×1.75×V+1×200×B+1 where, in the equation (1), [element] indicates the content of each element expressed in % by mass, and the content of an element not included is zero.