Stainless steel pipe and method of manufacturing the same

Abstract

A stainless steel pipe has a composition of, in mass %: C: up to 0.02%; Si: 0.05 to 1.00%; Mn: 0.1 to 1.0%; P: up to 0.030%; S: up to 0.002%; Ni: 5.5 to 8%; Cr: 10 to 14%; Mo: 2 to 4%; V: 0.01 to 0.10%; Ti: 0.05 to 0.3%; Nb: up to 0.1%; Al: 0.001 to 0.1%; N: up to 0.05%; Cu: up to 0.5%; Ca: 0 to 0.008%; Mg: 0 to 0.05%; B: 0 to 0.005%; and balance Fe and impurities. The pipe having a microstructure including martensite and, by volume fraction, 12 to 18% retained austenite. The martensite has prior austenite grains of a crystal grain size number lower than 8.0 in accordance with ASTM E112. The stainless steel pipe has a yield strength of 550 to 700 MPa.

1 . A stainless steel pipe having a chemical composition of, in mass %:
 C: up to 0.02%;   Si: 0.05 to 1.00%;   Mn: 0.1 to 1.0%;   P: up to 0.030%;   S: up to 0.002%;   Ni: 5.5 to 8%;   Cr: 10 to 14%;   Mo: 2 to 4%;   V: 0.01 to 0.10%;   Ti: 0.05 to 0.3%;   Nb: up to 0.1%;   Al: 0.001 to 0.1%;   N: up to 0.05%;   Cu: up to 0.5%;   Ca: 0 to 0.008%;   Mg: 0 to 0.05%;   B: 0 to 0.005%; and   balance Fe and impurities,   the stainless steel pipe having a microstructure including martensite and, by volume fraction, 12 to 18% retained austenite,   the martensite having prior austenite grains of a crystal grain size number lower than 8.0 in accordance with ASTM E112,   the stainless steel pipe having a yield strength of 550 to 700 MPa.   
     
     
         2 . The stainless steel pipe according to  claim 1 , wherein the chemical composition includes one or more elements selected from the group consisting of, in mass %:
 Ca: 0.001 to 0.008%;   Mg: 0.001 to 0.05%; and   B: 0.0005 to 0.005%.   
     
     
         3 . A method of manufacturing a stainless steel pipe comprising:
 hot-working a steel material having a chemical composition of, in mass %; C: up to 0.02%; Si: 0.05 to 1.00%; Mn: 0.1 to 1.0%; P: up to 0.030% S: up to 0.002%; Ni: 5.5 to 8%; Cr: 10 to 14%; Mo: 2 to 4%; V: 0.01 to 0.10%; Ti: 0.05 to 0.3%; Nb: up to 0.1%; Al: 0.001 to 0.1%; N: up to 0.05%; Cu: up to 0.5%; Ca: 0 to 0.008%; Mg: 0 to 0.05%; B: 0 to 0.005%; and balance Fe and impurities, to produce a hollow shell;   after the hot-working, quenching the hollow shell from a temperature between 940 and 980° C. in an in-line manner with respect to the hot-working; and   tempering the quenched hollow shell at a temperature between the Ac 1  point and Ac 3  point under the conditions described in Equation (1) below:
   680 ≤T +15.39 ln ( t )≤720   (1),
 
   where T is a tempering temperature in ° C., and t is a tempering time in minutes.