Method for producing seamless tubes

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 d0 [mm] thereof so as to prevent a transverse flaw on an outer surface in a top portion of a tube:

• when d0<200:
• T≦1250+1.1487×A−7.838×ln(t0/t)−10.135×ln(d0/d) (1)
• when d0≧200:
• T≦1219+1.1487×A−7.838×ln(t0/t)−10.135×ln(d0/d) (2)
• where Formulae (1) and (2) are determined by Formulae (3) to (5):
• A=L/Vav×1000 (3)
• Vav=(V0+V0×ρ)/2 (4)
• ρ=(t0×(d0−t0)×π)/(t×(d−t)×π) (5)
• where
• d0:
• outside diameter of starting material to be extruded [mm]
• t0:
• 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]
• Vav:
• average extrusion speed of starting material to be extruded [mm/sec]
• V0:
• ram speed [mm/sec]
• ρ:
• extrusion ratio.