Production method of seamless steel pipe

A production method of a seamless steel pipe, comprising:

• a heating step of heating an Nb-containing steel material to 800 to 1030° C., the Nb-containing steel material consisting of
• in mass %,
• C:
• 0.21 to 0.35%,
• Si:
• 0.10 to 0.50%,
• Mn:
• 0.05 to 1.00%,
• P:
• 0.025% or less,
• S:
• 0.010% or less,
• Al:
• 0.005 to 0.100%,
• N:
• 0.010% or less
• Cr:
• 0.05 to 1.50%,
• Mo:
• 0.10 to 1.50%,
• Nb:
• 0.01 to 0.05%,
• B:
• 0.0003 to 0.0050%,
• Ti:
• 0.002 to 0.050%,
• V:
• 0 to 0.30%,
• Ca:
• 0 to 0.0050%,
• rare earth metal:
• 0 to 0.0050%, and
• the balance being Fe and impurities;
• a pipe-making step of producing a hollow shell by performing piercing-rolling or elongation-rolling on the Nb-containing steel material, by using a piercing mill, the piercing mill comprising:
• a plurality of skewed rolls that are disposed around a pass line on which the Nb-containing steel material passes,
• a plug that is disposed between the plurality of skewed rolls and on the pass line, and
• a mandrel bar that extends rearward of the plug along the pass line from a rear end of the plug;
• and
• a cooling step immediately after rolling, of carrying out cooling by using a cooling liquid on a hollow shell portion that passes between rear ends of the plurality of skewed rolls, in the hollow shell, so as to reduce an outer surface temperature of the hollow shell portion to 700 to 1000° C. within 15.0 seconds after the hollow shell portion passes between the rear ends of the plurality of skewed rolls
• wherein in the cooling step immediately after rolling, the outer surface temperature of the hollow shell portion is reduced to 700 to 1000° C. within 15.0 seconds after the hollow shell portion passes between the rear ends of the plurality of skewed rolls, by ejecting the cooling liquid toward an outer surface and/or an inner surface of the hollow shell portion that passes between the rear ends of the plurality of skewed rolls,
• wherein the piercing mill includes an outer surface cooling mechanism that is disposed around the mandrel bar behind the plurality of skewed rolls, and includes a plurality of outer surface cooling liquid ejection holes capable of ejecting the cooling liquid toward an outer surface of the hollow shell during piercing-rolling or elongation rolling, and in the cooling step immediately after rolling, the outer surface of the hollow shell portion that passes between the rear ends of the plurality of skewed rolls is cooled by ejecting the cooling liquid from the outer surface cooling mechanism to reduce the outer surface temperature of the hollow shell portion to 700 to 1000° C. within 15.0 seconds after the hollow shell portion passes between the rear ends of the plurality of skewed rolls,
• wherein the outer surface cooling mechanism cools the outer surface of the hollow shell portion that passes in a cooling zone having a specific length in an axial direction of the mandrel bar, the piercing mill further includes
• a front outer surface damming mechanism that is disposed around the mandrel bar behind the plug and in front of the outer surface cooling mechanism, and
• in the cooling step immediately after rolling,
• wherein the cooling liquid is restrained from flowing to the outer surface of the hollow shell which is before entering the cooling zone by the front outer surface damming mechanism, when the hollow shell is being cooled by the outer surface cooling mechanism, and
• wherein
• the front outer surface damming mechanism includes a plurality of front damming fluid ejection holes that are disposed around the mandrel bar, and eject a front damming fluid toward the outer surface of the hollow shell, and
• in the cooling step immediately after rolling,
• the cooling liquid is dammed from flowing to the outer surface of the hollow shell that is before entering the cooling zone, by ejecting the front damming fluid toward an upper portion of the outer surface of the hollow shell that is located in a vicinity of an entrance side of the cooling zone, from the front outer surface damming mechanism, when the hollow shell is being cooled by the outer surface cooling mechanism.