Method of manufacturing stainless pipe for oil wells and stainless steel pipe for oil wells

1. A stainless steel pipe for oil wells having a chemical composition of, in mass %:

up to 0.05% C;

up to 1.0% Si;

0.01 to 1.0% Mn;

up to 0.05% P;

below 0.002% S;

0.001 to 0.1% Al;

16.0 to 18.0% Cr;

3.0 to 5.5% Ni;

1.8 to 3.0% Mo;

1.0 to 3.5% Cu;

up to 0.05% N;

up to 0.05% O;

0 to 0.3% Ti;

0 to 0.3% Nb;

0 to 0.3% V;

0 to 2.0% W;

0 to 0.01% Ca;

0 to 0.01% B; and

the balance Fe and impurities,

wherein the steel has a metal structure having:

an average retained-austenite ratio, V γAVE , of 2.5 to 15% in volume fraction and a standard deviation of retained-austenite ratio, σ(γ), of 1.0 or below;

a volume fraction of ferrite phase of not lower than 10% and lower than 60%; and

a volume fraction of martensite of not lower than 40%;

wherein Vγ AVE and σ(γ) are calculated as:

V γAVE =( V γI +V γM +V γO )/3

σ(γ)=(( V γI −V γAVE ) 2 +( V γM −V γAVE ) 2 +( V γO −V γAVE ) 2 )/2) 1/2

wherein

V γI is a volume fraction of retained austenite on an inner surface of the steel pipe,

V γM is a volume fraction of retained austenite in a middle section as determined along a wall thickness of the steel pipe, and

V γO is a volume fraction of retained austenite on an outer surface of the steel pipe.

2. The stainless steel pipe for oil wells according to claim 1 , wherein an average amount of absorbed energy in a Charpy impact test at 10° C. is not lower than 150 J, with a standard deviation not higher than 15 J.

3. The stainless steel pipe for oil wells according to claim 1 , wherein an average amount of absorbed energy in a Charpy impact test at −60° C. is not lower than 50 J.

4. The stainless steel pipe for oil wells according to claim 1 , wherein the chemical composition contains one or more elements selected from the group consisting of, in mass %:

0.005 to 0.3% Ti;

0.005 to 0.3% Nb; and

0.005 to 0.3% V.

5. The stainless steel pipe for oil wells according to claim 1 , wherein the chemical composition contains, in mass %:

0.01 to 2.0% W.

6. The stainless steel pipe for oil wells according to claim 1 , wherein the chemical composition contains one or both elements selected from the group consisting of, in mass %:

0.0002 to 0.01% Ca; and

0.0002 to 0.01% B.

7. The stainless steel pipe for oil wells according to claim 1 , wherein a yield strength is not lower than 862 MPa.

8. A method of manufacturing the stainless steel pipe for oil wells of claim 1 comprising:

the step of preparing a hollow shell having a chemical composition of, in mass %: up to 0.05% C; up to 1.0% Si; 0.01 to 1.0% Mn; up to 0.05% P; below 0.002% S; 0.001 to 0.1% Al; 16.0 to 18.0% Cr; 3.0 to 5.5% Ni; 1.8 to 3.0% Mo; 1.0 to 3.5% Cu; up to 0.05% N; up to 0.05% 0; 0 to 0.3% Ti; 0 to 0.3% Nb; 0 to 0.3% V; 0 to 2.0% W; 0 to 0.01% Ca; 0 to 0.01% B; and the balance Fe and impurities;

a first step for holding the hollow shell in a temperature range of 420 to 460° C. for a holding time of 60 to 180 minutes; and

a second step, after the first step, for holding the hollow shell in a temperature range of 550 to 600° C. for a holding time of 5 to 300 minutes.

9. The method of manufacturing a stainless steel pipe for oil wells according to claim 8 , wherein the holding time for the second step is 60 to 300 minutes.

10. The method of manufacturing a stainless steel pipe for oil wells according to claim 8 , wherein the chemical composition contains one or more elements selected from the group consisting of, in mass %:

0.005 to 0.3% Ti;

0.005 to 0.3% Nb; and

0.005 to 0.3% V.

11. The method of manufacturing a stainless steel pipe for oil wells according to claim 8 , wherein the chemical composition contains, in mass %:

0.01 to 2.0% W.

12. The method of manufacturing a stainless steel pipe for oil wells according to claim 8 , wherein the chemical composition contains one or both elements selected from the group consisting of, in mass %:

0.0002 to 0.01% Ca; and

0.0002 to 0.01% B.