Manufacturing method of martensite stainless seamless steel pipe

Abstract

The present invention relates to a manufacturing method of martensite stainless seamless steel pipe for oil well pipes and couplings. The martensite stainless seamless steel pipe has the chemical components comprising, by mass percent, 0.15 to 0.22% of C, 0.18 to 1.0% of Si, 0.05 to 1.0% of Mn, 10.5 to 14.0% of Cr, and the balance of Fe and impurities that include, by mass percent, not greater than 0.20% of Ni, not greater than 0.05% of Al, not greater than 0.100% of N, not greater than 0.015% of S, and not greater than 0.020% of P. The method comprises the steps of piercing and rolling a billet having the said chemical components to obtain a seamless steel pipe having a cross-sectional area being 10 to 95% of that of said billet; then heating the seamless steel pipe at a temperature of 850 to 980° C., and then subjecting said seamless steel pipe to quenching and tempering in the condition that let P value in the follow expression be 18500 to 22200. P=T ·(17.7+log t ) wherein “T” represents soaking temperature (K), “t” represents soaking time (second), “log t” represents the value of common logarithm of “t”. The martensite stainless seamless steel pipe with high weatherability can be obtained according to the present invention.

1 . A manufacturing method of martensite stainless seamless steel pipe with chemical components comprising, by mass percent, 0.15 to 0.22% of C, 0.18 to 1.0% of Si, 0.05 to 1.0% of Mn, 10.5 to 14.0% of Cr, and the balance of Fe and impurities that include, by mass percent, not greater than 0.20% of Ni, not greater than 0.05% of Al, not greater than 0.100% of N, not greater than 0.015% of S, and not greater than 0.020% of P; 
 said method comprising the steps of piercing and rolling a billet having the said chemical components to obtain a seamless steel pipe having a cross-sectional area being 10 to 95% of that of said billet; then heating the seamless steel pipe at a temperature of 850 to 980° C., and then subjecting said seamless steel pipe to quenching and tempering in the condition that let P value in the follow expression be 18500 to 22200.        P=T ·(17.7+log  t )    wherein “T” represents soaking temperature (K), “t” represents soaking time (second), “log t” represents the value of common logarithm of “t”.    
   
   
       2 . A manufacturing method of martensite stainless seamless steel pipe according to  claim 1 , wherein said P value is 18500 to 215000, and said martensite stainless seamless steel pipe obtained thereby is used for couplings of oil well pipes.