Patent ID: 8490452

Claim:
A method for producing a metal seamless tube, wherein when a hollow starting material to be extruded is hot extruded by providing a solid lubricating glass between the starting material to be extruded and a die after the hollow starting material has been heated, the starting material is hot extruded by being heated to a heating temperature T [° C.] satisfying the relationship of Formula (1) or Formula (2) depending on the outside diameter d 0 [mm] thereof so as to prevent a transverse flaw on an outer surface in a top portion of a tube: when d 0 <200: T≦ 1250+1.1487× A− 7.838×ln( t 0 /t )−10.135×ln( d 0 /d ) (1) when d 0 ≧200: T≦ 1219+1.1487× A− 7.838×ln( t 0 /t )−10.135×ln( d 0 /d ) (2) where Formulae (1) and (2) are determined by Formulae (3) to (5): A=L/V av ×1000 (3) V av =( V 0 +V 0 ×ρ)/2 (4) ρ=( t 0 ×( d 0 −t 0 )×π)/( t ×( d−t )×π) (5) where d 0 : outside diameter of starting material to be extruded [mm] t 0 : wall thickness of starting material to be extruded [mm] d: outside diameter of extruded tube [mm] t: wall thickness of extruded tube [mm] A: die passing time [msec (millisecond)] L: length of approach portion along extrusion direction from its inlet end to entry end of following bearing portion of die [mm] V av : average extrusion speed of starting material to be extruded [mm/sec] V 0 : ram speed [mm/sec] ρ: extrusion ratio.