Process for manufacturing a seamless tube

A process for manufacturing a seamless tube characterized by comprising subjecting a billet to piercing to produce a hollow shell while rotating and advancing the billet, using a pair of cone-shaped skew rolls having a gorge portion and disposed opposite each other around a pass line, a pair of grooved disk rolls, and a plug disposed along the pass line between the skew rolls and the disk rolls, under such conditions that the ratio (Dg/d) of the diameter Dg of the gorge portion of the skew rolls and the outer diameter d of a billet which is a material being rolled, the ratio (Dd/d) of the diameter Dd of the groove bottom of the disk rolls and the outer diameter d of the billet, and the ratio (Dd/Dg) of the diameter Dg of the gorge portion of the skew rolls and the diameter Dd of the groove bottom of the disk rolls satisfy either the following Equations (1), (2), and (3) or the following Equations (1), (2), and (4), the skew rolls have an inlet face angle θ1 which satisfies the following Equation (5), and the square root of the product (Ns×Df)0.5 of the rotational frequency Ns of the billet in a transient region when the billet is gripped by the skew rolls and the reduction ratio Df of the outer diameter of the billet satisfies the following Equation (6) which is a function of the ratio (Dg/D1) of the diameter of the skew rolls in the gorge portion and the diameter D1 of the skew rolls at the location where they contact the billet in the inlet thereof:

• 3≦Dg/d≦7 (1)
• 9≦Dd/d≦16 (2)
• in the case of an expansion ratio Exp≧1.15
• 2<Dd/Dg≦3 (3)
• in the case of an expansion ratio Exp<1.15
• 1.5≦Dd/Dg≦3 (4)
• 2.5°≦θ1≦4.5° (5)
• 0.46×(Dg/D1)−0.31≦(Ns×Df)0.5≦1.19×(Dg/D1)−0.95 (6)
• wherein Ns=Ld×Vr/(0.5×π×d×Vf) and Df=(d−dp)/d, where Vf is the smallest speed of the billet in the direction of its advance in a transient region when the billet is gripped by the skew rolls, Vr is the average speed in the circumferential direction of the billet in the transient region when the billet is gripped by the skew rolls, dp is the roll gap of the skew rolls at the tip of the plug, and Ld is the length along the pass line from the point in which the front end of the billet gets to contact with the skew rolls to the tip of the plug, the length being determined in the manner of two dimensional geometry in a state of the skew rolls having an inclination angle of zero.