Making method for seamless metallic tube

A making method for a seamless metallic tube of piercing rolling a solid round billet of an outer diameter BD (mm) with a tilting roll type piercing rolling mill by use of a plug having:

• a nose rolling portion comprising a cylindrical portion with an axial length of L2 (mm) whose outer diameter d (mm) is equalized in the axial direction or whose outer diameter d is increased toward a rear end of the cylindrical portion in the axial direction while having a half angle in a cone angle of 2° or less, and a nose spherical portion having a curvature radius of r (mm) and an axial length of L1 (mm),
• a working portion of an axial length of L3 (mm), continued to said nose rolling portion and formed by an arc rotating surface of a curvature radius of R (mm) so that an outer diameter is increased toward an axial rear end of the working portion, and
• a tapered cylindrical reeling portion of an axial length of L4 (mm), continued to said working portion and formed by a cone angle of 2 θ (°), so that an outer diameter is increased toward a maximum outer diameter D (mm) on the axial rear end of an reeling portion, and in which
• tensile strength of at least said nose rolling portion at 1100° C. is 50 MPa or more,
• wherein the relationships between the outer diameter d, the curvature radius R, and the axial lengths L1, L2 and L3 of said plug and the outer diameter BD of said solid billet satisfy all of the following expressions (2) to (4):
• 0.06≦d/BD≦=0.12 (4)
• 0.020≦(d/2BD)/(R/L3)≦0.046 (2)
• 0.5d≦L1+L2≦3d (3).