Stainless-steel pipe for oil well and process for producing the same

A method for manufacturing a corrosion-resistant seamless stainless steel pipe for oil country tubular goods comprising:

• 1) forming a seamless steel pipe from a steel pipe material having a composition;
• 2) quenching the steel pipe by heating the steel pipe to a temperature of the AC3 transformation point thereof or more;
• 3) subsequently cooling to room temperature at air-cooling speed or more;
• and 4) tempering the steel pipe at a temperature of the AC1 transformation point thereof or less, wherein the composition comprises on a mass basis:
• 0.05% or less of C;
• 0.50% or less of Si;
• 0.20% to 1.80% of Mn;
• 0.03 or less of P;
• 0.005% or less of S;
• 14.0% to 18.0% of Cr;
• 5.0% to 8.0% of Ni;
• 1.5% to 3.5% of Mo;
• 0.5% to 3.5% of Cu;
• 0.05% or less of Al;
• 0.03% to 0.20% of V;
• 0.01% to 0.15% of N;
• 0.006% or less of O, and
• the balance being Fe and incidental impurities,
• the composition satisfies expressions (1) and (2):
• Cr+0.65Ni+0.6Mo+0.55Cu+20C≧18.5 (1);
• Cr+Mo+0.3Si−43.5C−0.4Mn−Ni−0.3Cu−9N≦11 (2),
• where Cr, Ni, Mo, Cu, C, Si, Mn, and N represent the respective contents thereof on a mass % basis, and the structure of the stainless steel pipe includes 5 to 25 percent by volume of a residual austenite phase and the balance being a martensite phase.

A method for manufacturing a corrosion-resistant seamless stainless steel pipe for oil country tubular goods comprising:

• forming a steel pipe from a steel pipe material having a composition by hot working;
• cooling the steel pipe to room temperature at air-cooling speed or more, or quenching the steel pipe by further heating to a temperature of the AC3 transformation point thereof or more and cooling to room temperature at air-cooling speed or more;
• and tempering the steel pipe at a temperature of the AC1 transformation point thereof or less, wherein the composition comprises on a mass basis:
• 0.05% or less of C;
• 0.50% or less of Si;
• 0.20% to 1.80% of Mn;
• 0.03 or less of P;
• 0.005% or less of S;
• 14.0% to 18.0% of Cr;
• 5.0% to 8.0% of Ni;
• 1.5% to 3.5% of Mo;
• 0.5% to 3.5% of Cu;
• 0.05% or less of Al;
• 0.03% to 0.20% of V;
• 0.01% to 0.15% of N;
• 0.006% or less of O, and
• the balance being Fe and incidental impurities, and
• the composition satisfies expressions (1) and (2):
• Cr+0.65Ni+0.6Mo+0.55Cu+20C≧18.5 (1);
• Cr+Mo+0.3Si−43.5C−0.4Mn−Ni−0.3Cu−9N≦11 (2),
• where Cr, Ni, Mo, Cu, C, Si, Mn, and N represent the respective contents thereof on a mass % basis, and the structure of the stainless steel pipe includes 5 to 25 percent by volume of a residual austenite phase and the balance being a martensite phase.