Method of manufacturing seamless tube

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/D1 or the ratio of a roll diameter Dg (mm) at the gorge portion of an inclined roll to an inlet roll diameter D1 (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/D1<1.1,
• 23≦N/(Df/100)≦40 (1)
• in case of 1.1≦Dg/D1<1.5,
• 20≦N/(Df/100)≦44 (2)
• in case of 1.5≦Dg/D1≦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/(2π·Bd·tan β), and
• Df={(Bd−Rpg)/Bd}×100