Patent ID: 7578157

Claim:
A piercing and rolling method for seamless tubes using a piercing and rolling apparatus which is provided with a pair of cone-shaped main rolls and a pair of disk rolls, each pair being arranged in an opposing manner with a pass line therebetween as a center axis, and a plug whose center axis coincides with the pass line, wherein a billet to be pierced and rolled is advanced while being spirally rotated by a drive rotation of the main rolls; the method of which is characterized in each of the disk rolls being arranged in an inclined state at a fixed skew angle δ to the pass line, characterized in that an average thickness Dw 1 of the disk roll and an outside diameter d of the billet to be pierced and rolled satisfy the following inequality (1): Dw 1 /d >0.8 inequality (1) wherein Dw 1 and d are, respectively, the average thickness of the disk roll and an outside diameter of the billet to be pierced and rolled, while the average thickness of the disk roll is the average thickness of a disk-shaped body excluding a peripheral curved groove part of the disk roll.