Piercing-rolling method and piercing/rolling apparatus for seamless steel tube

摘要

A method for manufacturing seamless steel tubes by the Mannnesmann tube making process wherein a billet is passed through a piercer and the center of the billet is pierced through to obtain a hollow shell. A pair of disk rolls are disposed to incline by a skew angle &dgr; with respect to a pass line toward cone-type main roll located at the entry side, where a material to be rolled enters into the disk roll sliding face, thereby to become unparallel with the outlet face angle of the main roll, while the skew angle &dgr; is set to satisfy the following conditions related to the skew angle &dgr;, the inlet face angle &thgr; 1 and the outlet face angle &thgr;2 of the main roll, thereby to obtain expansion ratio of outer diameter of 1.15 or higher value.P P&thgr;2+2° &dgr; 9°, &dgr;+ &thgr;1 12°PPThe piercing/rolling operation with high expansion ratio of outer diameter is carried out stably without occurrence of such troubles as incompleted release from the main rolls, flaw on the outer surface and incompleted engagement.