Method for producing high chromium seamless pipe or tube

Abstract

A billet containing, by mass %, 8.0 to 16.0% of Cr with a value represented by the formula of “Cr+4Si−(22C+0.5Mn+1.5Ni+30N)” of not more than 9.0%, wherein the symbols of the elements represent the contents, by mass %, of the elements in the steel, is heated by means of a heating furnace conditioned so as to have a soaking temperature of 1100 to 1300° C. and an in-furnace time (min) of not less than “0.5×Diameter of the billet (mm)”. Thereafter, by means of a centering machine capable of forming a center hole having a diameter of not less than “0.34×Diameter of the billet (mm)” at one end face of the said billet where piercing starts by punch of 60 to 110° punching angle, a center hole is formed at the said face of the billet, followed by piercing and rolling by an inclined roll type piercing mill of not more than 8.0% plug tip draft rate. The thus-produced high-Cr seamless pipe or tube has excellent internal surface properties with minimized internal surface defects.

1 . A method for producing a high-Cr seamless pipe or tube, comprising the steps of:
 heating a billet in a heating furnace;   forming a center hole, by means of a centering machine, at one end face of the heated billet where piercing starts; and   piercing and rolling the billet by using an inclined roll type piercing mill, wherein:   the said billet containing, by mass %, 8.0 to 16.0% of Cr, with the value of Cr* represented by the following formula (1) of not more than 9.0%;   the said heating step being carried out at a soaking temperature of 1100 to 1300° C., and an in-furnace time from the charging of the billet into the heating furnace to the discharging therefrom being set to satisfy the following formula (2);   the said center hole being formed by using a punch with a punching angle of 60 to 110° in the center hole forming step being set to have a diameter represented by the following formula (3); and   the said piercing and rolling step is carried out at a plug tip draft rate represented by the following formula (4) being set to not more than 8.0%:
   Cr*=Cr+4Si−(22C+0.5Mn+1.5Ni+30N)  (1), 
   In-furnace time (min)>0.5×Diameter of the round billet (mm)  (2), 
   Diameter of the center hole (mm)>0.34×Diameter of the round billet (mm)  (3), 
   Plug tip draft rate (%)={(Diameter of the round billet (mm)−Roll gap at the foremost end of the plug (mm))/Diameter of the round billet (mm)}×100  (4), 
   wherein, in the above formula (1), the symbols of the elements represent the contents, by mass %, of the elements in the high-Cr seamless pipe or tube.