Abstract: A piercing machine includes a plurality of skewed rolls, a plug, a mandrel bar and an outer surface cooling mechanism. The outer surface cooling mechanism is disposed around the mandrel bar at a position that is rearward of the plug, and with respect to an outer surface of a hollow shell advancing through a cooling zone which has a specific length in an axial direction of the mandrel bar and which is located rearward of the plug, as seen from an advancing direction of the hollow shell, the outer surface cooling mechanism ejects a cooling fluid toward an upper part of the outer surface, a lower part of the outer surface, a left part of the outer surface and a right part of the outer surface of the hollow shell to cool the hollow shell inside the cooling zone.

Abstract: The production method of a seamless steel pipe includes a heating step of heating an Nb-containing steel material to 800 to 1030° C., 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 including a plurality of skewed rolls, a plug disposed between the plurality of skewed rolls, and a mandrel bar, and a cooling step immediately after rolling, of carrying out cooling 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.

Abstract: A mandrel bar of a piercing machine includes: a bar body; a coolant channel formed inside the bar body and through which a coolant flows; an inner surface cooling mechanism which is disposed in a cooling zone and is connected to the coolant channel and which, during piercing-rolling or during elongating rolling, ejects the coolant to outside of the bar body to cool an inner surface portion of a hollow shell inside the cooling zone; and an inner surface damming mechanism which is disposed adjacent to the cooling zone on a rearward side of the cooling zone and which, during piercing-rolling or during elongating rolling, suppresses the coolant that is ejected to outside of the bar body from contacting the inner surface portion of the hollow shell that is positioned rearward of the cooling zone.

Abstract: A mandrel bar of a piercing machine includes: a bar body; a coolant channel formed inside the bar body and through which a coolant flows; an inner surface cooling mechanism which is disposed in a cooling zone and is connected to the coolant channel and which, during piercing-rolling or during elongating rolling, ejects the coolant to outside of the bar body to cool an inner surface portion of a hollow shell inside the cooling zone; and an inner surface damming mechanism which is disposed adjacent to the cooling zone on a rearward side of the cooling zone and which, during piercing-rolling or during elongating rolling, suppresses the coolant that is ejected to outside of the bar body from contacting the inner surface portion of the hollow shell that is positioned rearward of the cooling zone.

Abstract: The production method of a seamless steel pipe includes a heating step of heating an Nb-containing steel material to 800 to 1030° C., 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 including a plurality of skewed rolls, a plug disposed between the plurality of skewed rolls, and a mandrel bar, and a cooling step immediately after rolling, of carrying out cooling 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.

Abstract: A piercing machine includes a plurality of skewed rolls, a plug, a mandrel bar and an outer surface cooling mechanism. The outer surface cooling mechanism is disposed around the mandrel bar at a position that is rearward of the plug, and with respect to an outer surface of a hollow shell advancing through a cooling zone which has a specific length in an axial direction of the mandrel bar and which is located rearward of the plug, as seen from an advancing direction of the hollow shell, the outer surface cooling mechanism ejects a cooling fluid toward an upper part of the outer surface, a lower part of the outer surface, a left part of the outer surface and a right part of the outer surface of the hollow shell to cool the hollow shell inside the cooling zone.