STAINLESS STEEL PIPE AND METHOD OF MANUFACTURING THE SAME

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.