Abstract: A high tensile steel which has a satisfactory yield strength of 60 kg/mm.sup.2 or more and excellent resistances to sulfide corrosive cracking and corrosion, and which comprises, as indispensable components, 0.05 to 0.50 wt % of C, 0.1 to 1.0 wt % of Si, 0.1 to 2.0 wt % of Mn, 0.05 to 1.50 wt % of Co and the balance consisting of Fe, is produced by hot- or cold-rolling it, rapidly heating the rolled steel to austenitize it, quenching the austenitized steel and, finally, tempering the quenched steel at a temperature not higher than the Ac.sub.1 point of the steel.

Abstract: A solid square cross-section bloom or billet is pierced to form a hollow cylindrical piece with a closed end, using a combination of a mandrel carrying a plug or piercing point at its leading end and a set of rolls. The closed-end cylindrical workpiece and the piercing mandrel together, at the completion of piercing, are fed into a rotary elongating mill, with the closed end of the workpiece foremost. Rotary elongation is performed on the workpiece while pushing the mandrel until the plug has at least passed the point at which the closed end of the workpiece clears the delivery end of the elongation rolls.

Abstract: A tubular blank is continuously rolled into a pipe through a mill which has a plurality of roll sets each comprising grooved driven rolls providing substantially circular passes of progressively reduced diameters. While the blank is passing through the roll sets, its interior is supported by a mandrel. The tail end of the mandrel is held by a pushing device when rolling is started. Namely, rolling is started by pushing the mandrel into the blank. The pushing of the mandrel is stopped between the time when the front end of the blank has passed the last roll center line in the first half roll set group and the time when it has reached before the roll center line of the last roll set. The tail end of the mandrel remains unreleased while rolling is continued.