Patent ID: 8882942

Claim:
A Ni-added steel plate comprising, by mass %: C: 0.03% to 0.10%; Si: 0.02% to 0.40%; Mn: 0.3% to 1.2%; Ni: 5.0% to 7.5%; Cr: 0.4% to 1.5%; Mo: 0.02% to 0.4%; Al: 0.01% to 0.08%; T.O: 0.0001% to 0.0050%; P: limited to 0.0100% or less; S: limited to 0.0035% or less; N: limited to 0.0070% or less; and the balance consisting of iron and unavoidable impurities, wherein a Ni segregation ratio based on mass % at a position of ¼ of a plate thickness away from a plate surface in a thickness direction is 1.3 or less, a fraction of an austenite after a deep cooling is 2% or more, an austenite unevenness index after the deep cooling is 5.0 or less, and an average equivalent circle diameter of the austenite after the deep cooling is 1 μm or less, wherein the austenite unevenness index after the deep cooling is a value obtained by dividing a maximum area fraction by a minimum area fraction, in which, among data which are evaluated such that an evaluation of an area fraction of the austenite is carried out with each viewing areas thereof being defined as a 5×5 μm area and is continuously carried out in the thickness direction with being centered on the position of ¼ of the plate thickness away from the plate surface in the thickness direction, an average of the data of 5 largest area fractions of the austenite is defined to be the maximum area fraction and an average of the data of 5 smallest area fractions of the austenite is defined to be the minimum area fraction.