Abstract: A method of manufacturing pressure vessel steel having high strength and high toughness comprising the steps of heating steel to a temperature higher than 1200.degree. C., said steel containing from 0.03 to 0.12 wt. % C, 0.10 to 0.8 wt. % Si, 0.45 to 1.00 wt. % Mn, 0.80 to 3.50 wt. % Cr, 0.10 to 1.60 wt. % Mo, 0.10 to 0.53 wt. % Ni, 0.010 to 0.040 wt. % soluble aluminum, at least one of 0.05 to 0.40 wt. % V and 0.02 to 0.20 wt. % Nb, less than 0.010 wt. % Ti, 0.0002 to 0.0010 wt. % B, and less than 0.004 wt. % N, the balance of iron and impurities, the N and Ti satisfying the following equationN<14/48.times.Ti+0.0024%;rolling the heated steel at a temperature above 1050.degree. C. to obtain a total reduction of more than 30%; directly quenching the rolled steel; and tempering the rolled steel.

Abstract: A method of manufacturing a thick, high-strength steel pipe for low temperature service comprising the steps of forming into tubular shape a steel plate having a thickness of over 12 mm and a specific composition, of which the C.sub.eq value is adjusted to less than 0.50, when the Mn content is less than 1.0 % whereas the C.sub.eq value is adjusted to less than 0.45 when the Mn content is over 1.0 % and less than 2.0 %, and nextly of welding said tube form at high speeds and high efficiency by the process of single electrode or multiple electrode large-current gas-shielded arc welding with one pass per electrode on each side of the seam at a specific condition, each electrode preferably consisting of a solid wire essentially containing less than 0.3 % Ti with or without the addition of less than 0.