To obtain a steel tube on the inside surface of which lap-ping marks are not generated even when the draft at the tip of a billet is reduced by forming the inlet face angle of a roll having a barrel-shaped body part in two steps and making the poststage face angle larger than the prestage face angle.
In the first half part 2 of the inlet body part of a piercing roll 1 for manufacturing the seamless steel tubes, its face angle α1 is taken as a face angle in such degree that the internal lapping marks are not generated on the inside surface and, in the second half part 3 of the inlet body-part, by making the face angle α2 larger than the face angle α1, the generation of biting failure is prevented. In this way, even when the draft at the tip is taken as larger to such a degree that the internal lapping marks is not generated, the biting failure is not generated. When the inlet face angle of the piercing roll 1 is larger, large rolling force is applied to the billet and the billet is easily bitten with the piercing rolls. By the arrival of the tip of the billet at the tip of a piercing plug, the billet is greatly reduced and the lapping marks are generated on the inside surface of the tube. When the inlet face angle of the piercing roll 1 is smaller, the large rolling force is not applied to the billet and the biting failure is generated with the piercing rolls.
TAKAOKA TATSUO
IKUI KENJI
