Patent ID: 7100410

Claim:
A method of manufacturing a seamless tube (a tube refers to a tube or pipe generically) in which a plug is provided along a pass line lying between a pair of cone-type inclined rolls that are put in place as opposed to each other with respect to the pass line, and the seamless tube is made by applying a piercing-rolling process while a billet as the work material is subjected to rotating and traveling, said method comprising of the step that said piercing-rolling process is applied so that both Dg/D 1 or the ratio of a roll diameter Dg (mm) at the gorge portion of an inclined roll to an inlet roll diameter D 1 (mm), and N/Df or the ratio of a billet revolution N, being given during the period from an onset engagement of said billet with rolls to the travel down to a plug nose, to a billet diameter reduction rate Df (%) satisfy either of following (1)–(3) formulas: in case of Dg/D 1 <1.1, 23 ≦N /( Df/ 100)≦40, (1) in case of 1.1≦Dg/D 1 <1.5, 20 ≦N /( Df/ 100)≦44, (2) in case of 1.5≦Dg/D 1 ≦1.8, 20 ≦N /( Df/ 100)≦48, (3) provided that following relationships are compatible, given by Ld: pass line lead (mm) from the position where the billet first engages with rolls to the plug nose, β: inclination angle (°) of the inclined rolls, and Rpg: opening clearance (mm) of the inclined rolls at the plug nose position, N=Ld/ (2 π·Bd· tan β), and Df={ ( Bd−Rpg )/ Bd}× 100.