Method for producing seamless metal pipe

A method for producing a seamless metal pipe using a piercing machine including a plurality of skew rolls and a plug arranged between the plurality of skew rolls, the method comprising the steps of:

• preparing a billet having a diameter B (mm),
• heating the billet,
• forming, in a center part of a rear end of the heated billet, a hole including four grooves extending in an axial direction of the billet, the grooves each having a groove width D (mm) satisfying Formula (1), a groove height H (mm) satisfying Formula (2) and a groove depth L1 (mm) satisfying Formula (3), and
• subjecting the billet provided with the hole to piercing-rolling from a front end by means of the piercing machine, wherein
• the four grooves are provided in a cross shape as seen from an axial direction of the billet,
• the four grooves extend from the rear end face of the billet,
• the grooves are disposed at equal intervals around the axial centerline of the billet, and two grooves that are adjacent in the circumferential direction are orthogonal, and wherein
• each groove side-face of the groove is parallel with the radial direction of the billet, and among the two ends of the groove side-face, an end that is nearer to a rear end center connects with an end that is nearer to the rear end center of the groove side-face of another groove that is adjacent thereto:
• 0.12≤D/B≤0.25 (1)
• 0.10≤H/B≤0.20 (2)
• 0.05≤L1/B≤0.10 (3).