Abstract: In the present invention, a steel product having a specified composition comprising by weight C: 0.02 to 0.25%, Si: 0.01 to 2.0%, Mn: 0.30 to 1.5%, Al: 0.003 to 0.10%, Nb: 0.005 to 0.10%, and Mo: 0.05 to 1.00% with the balance consisting of Fe and unavoidable impurities is used to prepare a steel plate having a thickness of 3 to 100 mm (the upper limits of the Nb and Mo contents each being 0.025% particularly for a plate thickness of 3 to 25 mm), and the ratio of the yield stress of the steel plate at a temperature T, .sigma..sub.yT, to the yield stress of the steel plate at room temperature, .sigma..sub.y, over the temperature range of room temperature to 600.degree. C. is brought to a value falling within the following range:1.00-1.083.times.10.sup.-3 T<(.sigma..sub.yT /.sigma..sub.y)<1.16-5.101.times.10.sup.-4 T.Further, the constituents of a welding material are specified to bring the .gamma..sub.3 transformation temperature to below 620.degree. C.