PURPOSE: To obtain a low Ni alloy steel plate having low-temp. toughness easily by direct hardening and tempering after hot rolling by heating steel contg. specified contents of C, Si, Mn, Ni and subjecting the same to initial rolling then to skin pass rolling in a specific temp. range and draft.

CONSTITUTION: The steel contg. ≤0.20wt% C, 0.05W0.50% Si, ≤0.80% Mn, 2.0W10.0% Ni, and if necessary, contg. ≤1.00% Cr and ≤0.50% Mo is heated to temp. higher than the Ac3 point. Thereafter, the heated steel is subjected to initial rolling down to an intermediate thickness at ≥830°C. In succession, the steel is subjected to skin pass rolling at ≥30% draft in the temp. range wherein the temp. Tn at which the value of the equation determined by the cooling curve in the stage of rolling and the TTT curve of the steel does not exceed 1.0 and is nearest 1.0 is a lower limit and the temp. which is the lower of Tn+200°C and 830°C is an upper limit. The rolled steel is subjected to quick cooling then to direct hardening and further to tempering. By this method, the low-temp. structural material of high quality having excellent low-temp. toughness is obtained.