Japanese Patent JPH0523842

B2

In a method of manufacturing seamless tubes according to the present invention, piercing from a solid billet can be accomplished through one pass by means of a piercing mill ao the present invention, piercing from a solid billet can be accomplished through one pass by means of a piercing mill at the piercing ratio above 4.0 or the expansion ratio above 1.15, or the wall thickness/outside diameter ratio below 6.5%, and the piercing mill having cone-shaped rolls is used with its feed angle beta and cross angle gamma satisfying the following relations, 8 DEG