SEAMLESS STAINLESS STEEL TUBE AND METHOD FOR MANUFACTURING THE SAME

Abstract

This document provides a seamless stainless steel tube that has high strength and excellent corrosion resistance. The seamless stainless steel tube has a composition that includes, in % by mass, C: 0.06% or less, Si: 1.0% or less, P: 0.05% or less, S: 0.005% or less, Cr: more than 15.7% and 18.0% or less, Mo: 1.8% or more and 3.5% or less, Cu: 1.5% or more and 3.5% or less, Ni: 2.5% or more and 6.0% or less, Al: 0.10% or less, N: 0.10% or less, O: 0.010% or less, W: 0.5% or more and 2.0% or less, and Co: 0.01% or more and 1.5% or less, wherein C, Si, Mn, Cr, Ni, Mo, Cu, and N conform to the predetermined formula, and the remainder is Fe and impurities. Incidentally, the seamless stainless steel tube has a microstructure containing at least 25% martensitic phase, at most 65% ferrite phase and at most 40% retained austenite phase by volume, and the seamless stainless steel tube has an elongation resistance of 758 MPa or more.

Description

SEAMLESS STAINLESS STEEL TUBE AND METHOD FOR MANUFACTURING IT FIELD OF INVENTION The present invention relates to a seamless martensitic stainless steel tube suitable for tubular products for the petroleum industry intended for oil and gas wells (hereinafter referred to simply as “oil wells”). In particular, the invention relates to improving corrosion resistance in various corrosive environments, such as a severe high-temperature corrosive environment containing carbon dioxide (CO2) and chloride ions (ClJ), and an environment containing hydrogen sulfide (H2S). BACKGROUND OF THE INVENTION The anticipated scarcity of energy resources in the near future has spurred the active development of oil wells that were unthinkable in the past, such as those in deep oil fields, environments containing carbon dioxide gas, and environments containing hydrogen sulfide, also known as sour environments. Steel pipes for the oil industry intended for these environments require high strength and excellent corrosion resistance. Tubular products for the petroleum industry used in oil and gas field development in environments containing CO2, Cl·, and similar compounds typically utilize 13Cr martensitic stainless steel tubing. Oil wells operating at higher temperatures (up to 200°C) have also been developed. However, the corrosion resistance of 13Cr martensitic stainless steel is not always sufficient for these applications. Therefore, steel tubing is needed for petroleum industry tubular products that exhibits excellent corrosion resistance even when used in such environments. In relation to this demand, for example, PTL 1 describes that it is possible to produce a stainless steel for tubular products for the petroleum industry having a composition comprising C: 0.05% or less, Si: 1.0% or less, Mn: 0.01 to 1.0%, P: 0.05% or less, S: less than 0.002%, Cr: 16 to 18%, Mo: 1.8 to 3%, Cu: 1.0 to 3.5%, Ni: 3.0 to 5.5%, Co: 0.01 to 1.0%, Al: 0.001 to 0.1%, O: 0.05% or less, and N: 0.05% or less, and in which Cr, Ni, Mo and Cu meet specific ratios. PTL 2 describes a seamless, high-strength stainless steel tube for tubular products for the petroleum industry having a composition comprising, in % by mass, C: 0.05% or less, Si: 1.0% or less, Mn: 0.1 to 0.5%, P: 0.05% or less, S: less than 0.005%, Cr: more than 15.0% and 19.0% or less, Mo: more than 2.0% and 3.0% or less, Cu: 0.3 to 3.5%, Ni: 3.0% or more and less than 5.0%, W: 0.1 to 3.0%, Nb: 0.07 to 0.5%, V: 0.01 to 0.5%, Al: 0.001 to 0.1%, N: 0.010 to 0.100%, and O: 0.01% or less, and in which Nb, Ta, C, N, and Cu meet a specific ratio, and which has a microstructure containing at least 45% tempered martensitic phase, 20 to 40% ferrite phase, and more than 10% and at most 25% volume-retained austenite phase.This related technical document states that this allows the production of a seamless high-strength stainless steel tube for tubular products for the petroleum industry that has an elongation resistance YS of 862 MPa or more, and that shows sufficient corrosion resistance even in a severe high-temperature corrosive environment containing CO2, Cl, and H2S. PTL 3 describes that it is possible to manufacture a high-strength seamless stainless steel tube for tubular products for the petroleum industry having a composition comprising C: 0.005 to 0.05%, Si: 0.05 to 0.50%, Mn: 0.20 to 1.80%, P: 0.030% or less, S: 0.005% or less, Cr: 14.0 to 17.0%, Ni: 4.0 to 7.0%, Mo: 0.5 to 3.0%, Al: 0.005 to 0.10%, V: 0.005 to 0.20%, Co: 0.01 to 1.0%, N: 0.005 to 0.15%, and O: 0.010% or less, and wherein Cr, Ni, Mo, Cu, C, SI, Mn and N comply with specific relationships. PTL 4 describes a seamless, high-strength stainless steel tube for tubular products for the petroleum industry having a composition comprising, in mass percent, C: 0.05% or less, Si: 0.5% or less, Mn: 0.15 to 1.0%, P: 0.030% or less, S: 0.005% or less, Cr: 14.5 to 17.5%, Ni: 3.0 to 6.0%, Mo: 2.7 to 5.0%, Cu: 0.3 to 4.0%, W: 0.1 to 2.5%, V: 0.02 to 0.20%, Al: 0.10% or less, and N: 0.15% or less, wherein C, Si, Mn, Cr, Ni, Mo, Cu, N, and W meet specified ratios, and having a microstructure containing more than 45% of martensitic phase as the primary phase, 10 to 45% ferrite phase and a maximum of 30% retained austenite phase as the secondary phase, by volume.This related technical document states that this allows the production of a seamless high-strength stainless steel tube for tubular products for the petroleum industry that has an elongation resistance YS of 862 MPa or more, and that shows sufficient corrosion resistance even in a severe high-temperature corrosive environment containing CO2, Cl· and H2S. PTL 5 describes a seamless, high-strength stainless steel tube for tubular products for the petroleum industry having a composition comprising, in % by mass, C: 0.05% or less, Si: 0.5% or less, Mn: 0.15 to 1.0%, P: 0.030% or less, S: 0.005% or less, Cr: 14.5 to 17.5%, Ni: 3.0 to 6.0%, Mo: 2.7 to 5.0%, Cu: 0.3 to 4.0%, W: 0.1 to 2.5%, V: 0.02 to 0.20%, Al: 0.10% or less, N: 0.15% or less, and B: 0.0005 to 0.0100%, and wherein C, Si, Mn, Cr, Ni, Mo, Cu, N, and W meet with specific relationships, and which has a microstructure containing more than 45% martensitic phase as the primary phase, 10 to 45% ferrite phase and at most 30% retained austenite phase as the secondary phase, by volume.This related technical document states that this allows the production of a seamless high-strength stainless steel tube for tubular products for the petroleum industry that has an elongation resistance YS of 862 MPa or more, and that shows sufficient corrosion resistance even in a severe high-temperature corrosive environment containing CO2, Cl· and H2S. List of appointments Patent Literature PTL 1: WO2013 / 146046 PTL 2: WO2017 / 138050 PTL 3: WO2017 / 168874 PTL 4: WO2018 / 020886 PTL 5: WO2018 / 155041 BRIEF DESCRIPTION OF THE INVENTION Technical problem Aside from the issues mentioned above, oil extraction also involves a number of problems, such as low production resulting from deficient oil-trapping layers (reservoirs) (particularly permeability), and the inability to achieve projected oil production volumes due to problematic events such as reservoir blockage. Acidizing is a technique used to pump hydrochloric acid or other acids into a reservoir to enhance productivity. Steel pipes for petroleum tubular products require acid resistance when used in this process. PTL 1 through PTL 5 describe stainless steels that have desirable corrosion resistance; however, these are insufficient in terms of corrosion resistance in an acidic environment. The present invention aims to provide a solution to problems in the related art, and one objective of the present invention is to provide a seamless stainless steel tube that has excellent corrosion resistance and high strength with an elongation resistance of 758 MPa (110 ksi) or more. Another objective of the present invention is to provide a method for manufacturing the seamless stainless steel tube. As used in this document, “excellent corrosion resistance” means “excellent resistance to carbon dioxide gas corrosion”, “excellent resistance to sulfide stress cracking” and “excellent resistance to acidic corrosion”. As used herein, “excellent resistance to carbon dioxide gas corrosion” means that a test specimen immersed in a test solution (a 20% by mass aqueous NaCl solution; liquid temperature of 200°C; CO2 gas atmosphere of 30 atm) held in an autoclave has a corrosion rate of 0.127 mm / year or less after 336 hours in the solution. As used herein, “excellent sulfide stress cracking resistance (SSC resistance)” means that a test specimen immersed in a test solution (a 20% by mass aqueous NaCl solution; liquid temperature: 25°C; an atmosphere of 0.1 atm of H2S and 0.9 atm of CO2) maintained in an autoclave and having a pH adjusted to 3.5 with the addition of acetic acid and sodium acetate does not crack even after 720 hours of immersion under an applied stress equal to 90% of the yield strength. As used herein, “excellent corrosion resistance in acidic environments” means that a test sample immersed in a 15% by mass hydrochloric acid solution heated to 80°C has a corrosion rate of 600 mm / year or less after 40 minutes of immersion. Solution to the problem To achieve the aforementioned objectives, the present inventors conducted intensive research on various factors affecting the corrosion resistance of stainless steel, particularly in an acidic environment. The studies showed that a stainless steel containing at least a predetermined amount of Co in addition to Cr, Mo, Ni, Cu, and W can develop sufficient corrosion resistance in an acidic environment. The present invention was completed after further studies based on these findings. Specifically, the essence of the present invention is as follows. [1] A seamless stainless steel tube having a composition that includes, in % by mass, C: 0.06% or less, Si: 1.0% or less, P: 0.05% or less, S: 0.005% or less, Cr: more than 15.7% and 18.0% or less, Mo: 1.8% or more and 3.5% or less, Cu: 1.5% or more and 3.5% or less, Ni: 2.5% or more and 6.0% or less, Al: 0.10% or less, N: 0.10% or less, O: 0.010% or less, W: 0.5% or more and 2.0% or less, and Co: 0.01% or more and 1.5% or less, wherein C, Si, Mn, Cr, Ni, Mo, Cu, and N satisfy the following formula (1), and the remainder is Fe and incidental impurities, pyQrnn / zznz / e / YiAi the seamless stainless steel tube has a microstructure containing at least 25% martensitic phase, at most 65% ferrite phase and at most 40% austenite phase retained by volume, the seamless stainless steel tube having an elongation resistance of 758 MPa or more, 13.0 <-5.9 x (7.82+ 27C - 0.91SI + 0.21 Mn - 0.9Cr + Ni - 1.1 Mo + 0.2Cu + 11 N) < 55.0 (1), where C, Si, Mn, Cr, Ni, Mo, Cu and N represent the content of each element in % by mass, and the content is 0 (zero; % by mass) for elements that are not contained. [2] The seamless stainless steel tube according to [1], wherein the composition further includes, in % by mass, one or two selected from Mn: 1.0% or less, and Nb: 0.30% or less. [3] The seamless stainless steel tube according to [1] or [2], wherein the seamless stainless steel tube of the composition in [1] or [2] has a microstructure containing at least 40% martensitic phase, at most 60% ferrite phase and at most 30% volume-retained austenite phase and has an elongation resistance of 862 MPa or more. [4] Seamless stainless steel tubing according to any of [1] to [3], wherein the composition further includes, in % by mass, one or two or more selected from V: 1.0% or less, B: 0.01% or less, and Ta: 0.3% or less. [5] The seamless stainless steel tube according to any of [1] to [4], wherein the composition further includes, in % by mass, one or two selected from Ti: 0.3% or less, and Zr: 0.3% or less. [6] Seamless stainless steel tubing according to any of [1] to [5], wherein the composition further includes, in % by mass, one or two or more selected from Ca: 0.01% or less, REM: 0.3% or less, Mg: 0.01% or less, Sn: 0.2% or less, and Sb: 1.0% or less. [7] A method for manufacturing the seamless stainless steel tube of any of [f] to [6], the method includes: to form a seamless steel tube of predetermined dimensions from a steel pipe material; Rapid tempering of the seamless steel tube to a temperature ranging from 850°F to 150°C, and cooling the seamless steel tube to a surface temperature of 50°C or less at an air cooling rate or faster; and tempering of the seamless steel tube by rapid tempering to a temperature of 500 to 650°C. Advantageous effects of the invention The present invention can provide a seamless stainless steel tube that has excellent corrosion resistance and high strength with an elongation resistance of 758 MPa (110 ksi) or more. DETAILED DESCRIPTION OF THE INVENTION A seamless stainless steel tube of the present invention is a seamless stainless steel tube having a composition that includes, in % by mass, C: 0.06% or less, Si: qj Qrnn / zznz / e / YiAi .0% or less, P: 0.05% or less, S: 0.005% or less, Cr: more than 15.7% and 18.0% or less, Mo: 1.8% or more and 3.5% or less, Cu: 1.5% or more and 3.5% or less, Ni: 2.5% or more and 6.0% or less, Al: 0.10% or less, N: 0.10% or less, O: 0.010% or less, W: 0.5% or more and 2.0% or less, and Co: 0.01% or more and 1.5% or less, and wherein C, Si, Mn, Cr, Ni, Mo, Cu and N comply with the following formula (1), and the remainder is Fe and incidental impurities, the seamless stainless steel tube has a microstructure containing at least 25% martensitic phase, at most 65% ferrite phase and at most 40% volume-retained austenite phase, the seamless stainless steel tube having an elongation resistance of 758 MPa or more, 13.0 < -5.9 x (7.82 + 27C - 0.91 Si + 0.21 Mn - 0.9Cr + Ni - 1.1 Mo + 0.2Cu + 11N) < 55.0 (1), where C, Si, Mn, Cr, Ni, Mo, Cu and N represent the content of each element in % by mass, and the content is 0 (zero; % by mass) for elements that are not contained. The reasons for specifying the composition of a seamless steel tube of the present invention are described below. In what follows, “%” means percentage by mass, unless specifically stated otherwise. C: 0.06% or less Carbon is an element that is incidentally included in the steelmaking process. Corrosion resistance decreases when the carbon content exceeds 0.06%. For this reason, the carbon content is 0.06% or less. The carbon content is preferably 0.05% or less, and more preferably 0.04% or less. Taking into account the cost of decarburization, the carbon content is preferably 0.002% or more, and more preferably 0.003% or more. Yes: 1.0% or less Silicon (Si) acts as a deoxidizing agent. However, hot workability and corrosion resistance decrease when Si content exceeds 1.0%. For this reason, the Si content is 1.0% or less. The Si content is preferably 0.7% or less, and more preferably 0.5% or less. It is not particularly necessary to establish a lower limit, provided the deoxidizing effect is achieved. However, to obtain a sufficient deoxidizing effect, the Si content is preferably 0.03% or more, and more preferably 0.05% or more. P: 0.05% or less Phosphorus (P) is an element that impairs corrosion resistance, including resistance to carbon dioxide gas corrosion and resistance to sulfide stress cracking. Therefore, P is preferably contained in the lowest possible amount in the present invention. However, a P content of 0.05% or less is acceptable. For this reason, the P content is 0.05% or less. The P content is preferably 0.04% or less, more preferably 0.03% or less. S: 0.005% or less Sulfur (S) is an element that seriously impairs hot workability and interferes with stable hot working operations in the tube manufacturing process. S exists in the form of sulfide inclusions in steel and impairs corrosion resistance. Therefore, S should preferably be contained in the lowest possible amount. However, an S content of 0.005% or less is acceptable. For this reason, the S content is 0.005% or less. The S content is preferably 0.004% or less, and more preferably 0.003% or less. Cr: More than 15.7% and 18.0% or less Chromium (Cr) forms a protective layer on the surface of steel pipes, contributing to improved corrosion resistance. The desired resistance to carbon dioxide gas corrosion, acid corrosion, and sulfide stress cracking cannot be achieved with a Cr content of 15.7% or less. Therefore, a Cr content above 15.7% is necessary. With a Cr content above 18.0%, the ferrite fraction increases excessively, preventing the achievement of the desired resistance. For this reason, the Cr content is typically between 15.7% and 18.0%. The Cr content is preferably 16.0% or more, and more preferably 16.3% or more. The Cr content is preferably 17.5% or less, and more preferably 17.2% or less, and more preferably 17.0% or less. Mo: 1.8% or more and 3.5% or less By stabilizing the protective coating on the surface of the steel pipe, Mo increases resistance to pitting corrosion due to chloride ions (Cl) and low pH, and increases resistance to carbon dioxide gas corrosion and corrosion in acidic environments. Mo also increases resistance to sulfide stress cracking. Mo should be present at a concentration of 1.8% or more to achieve the desired corrosion resistance. The effects become saturated at Mo content levels above 3.5%. For this reason, the Mo content is 1.8% or more and 3.5% or less. The Mo content is preferably 2.0% or more, more preferably 2.2% or more. The Mo content is preferably 3.3% or less, more preferably 3.0% or less, more preferably 2.8% or less, and even more preferably less than 2.7%. Cu: 1.5% or more and 3.5% or less Copper (Cu) increases retained austenite and contributes to improving the yield strength by forming a precipitate. This allows for high strength without reducing low-temperature hardness. Copper also strengthens the protective coating on the surface of the steel tube and improves resistance to carbon dioxide gas corrosion and acid corrosion. A copper content of 1.5% or more is required to achieve the desired hardness and corrosion resistance, particularly carbon dioxide gas corrosion. Excessively high copper content leads to a decrease in the hot workability of the steel, and the ideal copper content is 3.5% or less. For this reason, the copper content is 1.5% or more and 3.5% or less. The copper content is preferably 1.8% or more, and more preferably 2.0% or more. The copper content is preferably 3.2% or less, more preferably 3.0% or less. Ni: 2.5% or more and 6.0% or less Nickel (Ni) is an element that strengthens the protective coating on the surface of steel tubes and contributes to improved corrosion resistance, particularly resistance to corrosion in acidic environments. By strengthening solid solutions, Ni also increases the strength of the steel and improves its hardness. These effects are more pronounced when Ni is present in an amount of 2.5% or more. A Ni content above 6.0% leads to a decrease in the stability of the martensitic phase and reduces strength. For this reason, the Ni content is 2.5% or more and 6.0% or less. The Ni content is preferably more than 3.3%, more preferably 3.5% or more, more preferably 4.0% or more, and even more preferably 4.2% or more. The Ni content is preferably 5.5% or less, more preferably 5.2% or less, and even more preferably 5.0% or less. Al: 0.10% or less Aluminum (Al) acts as a deoxidizing agent. However, corrosion resistance decreases when the Al content exceeds 0.10%. For this reason, the Al content is 0.10% or less. The Al content is preferably 0.07% or less, and more preferably 0.05% or less. It is not particularly necessary to establish a lower limit, provided the deoxidizing effect is achieved. However, to obtain a sufficient deoxidizing effect, the Al content is preferably 0.005% or more, and more preferably 0.01% or more. N: 0.10% or less Nitrogen (N) is an element that is incidentally included in the steelmaking process. N also increases the strength of steel. However, when present in amounts greater than 0.10%, N forms nitrides and decreases corrosion resistance. For this reason, the N content is 0.10% or less. The N content is preferably 0.08% or less, more preferably 0.07% or less. There is no specific lower limit for the N content. However, an excessively low N content leads to an increase in the cost of steelmaking. For this reason, the N content is preferably 0.002% or more, more preferably 0.003% or more. O: 0.010% or less Oxygen (O₂) exists as an oxide in steel and causes adverse effects on several properties. For this reason, the oxygen content in the present invention is preferably kept to a minimum. An oxygen content greater than 0.010% reduces hot workability and corrosion resistance. Therefore, the oxygen content is 0.010% or less. W: 0.5% or more and 2.0% or less Water (W) is an element that contributes to improving the strength of steel and can increase resistance to carbon dioxide gas corrosion and acid corrosion by stabilizing the protective coating on the surface of steel pipes. W also improves resistance to sulfide stress cracking. In particular, W greatly improves corrosion resistance when contained with molybdenum (Mo). With a W content of 0.5% or more, the desired resistance to carbon dioxide gas corrosion and acid corrosion can be achieved. The effects become saturated at a W content above 2.0%. For this reason, W, when included, is present at 2.0% or less. The W content is preferably 0.8% or more, more preferably 1.0% or more. The W content is preferably 1.8% or less, more preferably 1.5% or less. Co: 0.01% or more and 1.5% or less Cobalt (Co) is an element that increases strength and improves corrosion resistance. To achieve the desired corrosion resistance in acidic environments, Co is contained in an amount of 0.01% or more. The effects are saturated with a Co content greater than 1.5%. For this reason, the Co content in the present invention is 0.01% or more and 1.5% or less. The Co content is preferably 0.05% or more, and more preferably 0.10% or more. The Co content is preferably 1.0% or less, and more preferably 0.5% or less. In the present invention, C, Si, Mn, Cr, Ni, Mo, Cu and N are contained in such a way as to comply with the following formula (1), in addition to complying with the above composition. 13.0<-5.9x (7.82 + 27C -0.91 Si + 0.21 Mn - 0.9Cr + Ni - 1.1Mo + 0.2Cu + 11N) < 55.0 (1) In the formula C, Si, Mn, Cr, Ni, Mo, Cu and N represent the content of each element in % by mass, and the content is 0 (zero; % by mass) for elements that are not contained. In formula (1), the expression -5.9 x (7.82 + 27C - 0.91 Si + 0.21 Mn - 0.9Cr + Ni - 1.1 Mo + 0.2Cu + 11N) (hereafter also referred to as the “mean polynomial of formula (1)”, or simply the “mean value”) is determined as an index indicating the probability of ferrite phase formation. With the alloying elements of formula (1) contained in quantities adjusted to comply with formula (1), it is possible to stably produce a microstructure composed of a martensitic phase and a ferrite phase, or a microstructure composed of a martensitic phase, a ferrite phase, and a retained austenite phase. When any of the alloying elements produced in formula (1) is not present, the value of the mean polynomial of formula (1) is calculated considering the element content to be zero percent. When the average polynomial value of formula (1) is less than 13.0, the ferrite phase decreases and the manufacturing yield decreases. On the other hand, when the average polynomial value of formula (1) is greater than 55.0, the ferrite phase becomes more than 65% by volume and the desired strength cannot be provided. For this reason, the formula (1) specified in the present invention establishes a left value of 13.0 as the lower limit, and a right value of 55.0 as the upper limit. The lower left value of the formula (1) specified in the present invention is preferably 15.0, more preferably 20.0. The right value is preferably 50.0, more preferably 45.0, even more preferably 40.0. In the present invention, the remainder in the above composition is Fe and incidental impurities. In the present invention, in addition to the basic components above, the composition may also contain one or two or more optional elements (Mn, Nb, V, B, Ta, Ti, Zr, Ca, REM, Mg, Sn and Sb), as indicated below. Specifically, in the present invention, the composition may additionally contain Mn: 1.0% or less, and Nb: 0.30% or less. In the present invention, the composition may additionally contain one or two or more selected from V: 1.0% or less, B: 0.01% or less, and Ta: 0.3% or less. In the present invention, the composition may additionally contain one or two selected from Ti: 0.3% or less, and Zr: 0.3% or less. In the present invention, the composition may additionally contain one or two or more selected from Ca: 0.01% or less, REM: 0.3% or less, Mg: 0.01% or less, Sn: 0.2% or less, and Sb: 1.0% or less. Mn: 1.0% or less Manganese (Mn), an optional element, acts as a deoxidizing and desulfurizing agent and improves workability and heat resistance. Mn is preferably present at a concentration of 0.001% or more, and more preferably 0.01% or more, to achieve these effects. The effects are saturated at a Mn content above 1.0%. Therefore, when present, Mn is present at a concentration of 1.0% or less. The Mn content is preferably 0.8% or less, and more preferably 0.6% or less. Nb: 0.30% or less Nitrogen (Nb), an optional element, increases strength and improves corrosion resistance. Its effects are saturated at Nb contents above 0.30%. Therefore, when Nb is present, it is at a concentration of 0.30% or less. The Nb content is preferably 0.25% or less, more preferably 0.2% or less. The Nb content is preferably 0.01% or more, more preferably 0.05% or more, and even more preferably 0.10%. V: 1.0% or less The V, an optional element, is a resistance-increasing element. The effect saturates with a V content above 1.0%. For this reason, when V is present, it is at 1.0% or less. The V content is preferably 0.5% or less, more preferably 0.3% or less. The V content is preferably 0.01% or more, more preferably 0.03% or more. B: 0.01% or less Boron (B), an optional element, is a strength enhancer. B also contributes to improved hot workability and reduces fracture and cracking during tube manufacturing. However, a B content above 0.01% has little effect on improving hot workability and leads to a decrease in low-temperature hardness. For this reason, when B is present, it is in an amount of 0.01% or less. The B content is preferably 0.008% or less, more preferably 0.007% or less. The B content is preferably 0.0005% or more, more preferably 0.001% or more. Ta: 0.3% or less Ta, an optional element, improves corrosion resistance and increases strength. To achieve these effects, Ta is preferably present at a concentration of 0.001% or more. The effects are saturated at a Ta content above 0.3%. Therefore, when Ta is present, it is limited to 0.3% or less. Ti: 0.3% or less Titanium (Ti), an optional element, increases strength. In addition to this effect, Ti also improves resistance to sulfide stress cracking. To achieve these effects, Ti is preferably present at a concentration of 0.0005% or higher. A Ti content above 0.3% decreases hardness. Therefore, when present, Ti is limited to 0.3% or less. Zr: 0.3% or less of Qrnn / zznz / e / YiAi Zinc (Zr), an optional element, increases strength. In addition to this effect, Zr also improves resistance to sulfide stress cracking. To achieve these effects, Zr is preferably present at a concentration of 0.0005% or higher. The effects are saturated at a Zr content above 0.3%. Therefore, when Zr is present, it is limited to 0.3% or less. Ca: 0.01% or less Calcium (Ca), an optional element, contributes to improved resistance to sulfide stress corrosion cracking by controlling the sulfide morphology. To achieve this effect, Ca is preferably present at a concentration of 0.0005% or higher. When Ca is present at concentrations exceeding 0.01%, the effect becomes saturated, and the increased concentration will no longer produce the desired effect. Therefore, when present, Ca is limited to 0.01% or less. REM: 0.3% or less REM, an optional component, is an element that contributes to improved resistance to sulfide stress corrosion cracking by controlling the sulfide morphology. To achieve this effect, REM is preferably contained in an amount of 0.0005% or more. When REM is present in an amount greater than 0.3%, the effect becomes saturated, and the REM cannot produce the intended effect with increasing content. For this reason, when REM is included, it is in a limited amount of 0.3% or less. In this document, “REM” means scandium (Se; atomic number 21) and triium (Y; atomic number 39), as well as lanthanides from lanthanum (La; atomic number 57) to lutetium (Lu; atomic number 71). In this document, “REM concentration” means the total content of one or two or more elements selected from the aforementioned REM elements. Mg: 0.01% or less Magnesium (Mg), an optional element, improves corrosion resistance. To achieve this effect, Mg is preferably present at a concentration of 0.0005% or higher. When Mg is present at concentrations exceeding 0.01%, the effect becomes saturated, and the increased concentration will no longer produce the desired effect. Therefore, when present, Mg is limited to 0.01% or less. Sn: 0.2% or less Tin (Sn), an optional element, improves corrosion resistance. To achieve this effect, Sn is preferably present at a concentration of 0.001% or more. When Sn is present at a concentration exceeding 0.2%, the effect becomes saturated, and the increased concentration will no longer produce the desired effect. Therefore, when present, Sn is limited to 0.2% or less. Sb: 1.0% or less Antimony (Sb), an optional element, improves corrosion resistance. To achieve this effect, Sb is preferably present at a concentration of 0.001% or less. When Sb is present at concentrations exceeding 1.0%, the effect becomes saturated, and the increased concentration will no longer produce the desired effect. Therefore, when present, Sb is limited to 1.0% or less. The reason for limiting the microstructure in the weld-free steel tube of the present invention is described below. In addition to having the above composition, the seamless steel tube of the present invention has a microstructure containing at least 25% martensitic phase, at most 65% ferrite phase, and at most 40% volume-retained austenite phase. To provide the desired strength, the seamless steel tube of the present invention contains at least 25% martensitic phase by volume. Preferably, the martensitic phase is at least 40% by volume. In the present invention, the ferrite is at most 65% by volume. With the ferrite phase, the progression of sulfide stress corrosion cracking and sulfide stress cracking can be reduced, and excellent corrosion resistance can be achieved. If the ferrite phase precipitates in a large amount exceeding 65% by volume, it may not be possible to provide the desired strength. The ferrite phase is preferably 5% or more by volume. The ferrite phase is preferably 60% or less, more preferably 55% or less, and even more preferably 50% or less by volume. The seamless steel tube of the present invention contains a maximum of 40% austenite phase (retained austenite phase) by volume, in addition to the martensitic and ferrite phases. Ductility and toughness are improved by the presence of the retained austenite phase. If the austenite phase precipitates in a large amount, exceeding 40% by volume, the desired strength cannot be achieved. For this reason, the retained austenite phase is 40% or less by volume. The retained austenite phase is preferably 5% or more by volume. The retained austenite phase is preferably 30% or less, more preferably 25% or less by volume. To measure the microstructure of the seamless steel pipe of the present invention, a test sample is corroded with Vilella's solution (a mixed reagent containing 2 g of picric acid, 10 mL of hydrochloric acid, and 100 mL of ethanol), and an image of the structure is obtained using a scanning electron microscope (1,000x magnification). Subsequently, the ferrite phase fraction of the microstructure (area ratio (%)) is calculated using an image analyzer. The area ratio is defined as the volume ratio (%) of the ferrite phase. Individually, an X-ray diffraction test specimen is ground and polished to have a measurement cross-section (C-section) orthogonal to the axial direction of the tube, and the fraction of the retained austenite phase microstructure (y) is measured by an X-ray diffraction method. The fraction of the retained austenite phase microstructure is determined by measuring the integral X-ray diffraction intensity for the (220) plane of the austenite phase (y), and the (211) plane of the ferrite phase (a), and converting the calculated values ​​using the following formula. y (volume ratio) = 100 / (1+(laRy / lyRa)), where la is the integral intensity of a, Ra is the crystallographic theoretical value of a, ly is the integral intensity of y, and Ry is the crystallographic theoretical value of y. The martensitic phase fraction is the remainder other than the ferrite phase and retained phase fractions determined by the measurement method described above. As used herein, the “martensitic phase” may contain a maximum of 5% precipitated phase by volume, other than the martensitic phase, ferrite phase, and retained austenite phase. The following describes a preferred method for manufacturing a seamless stainless steel tube of the present invention. qj Qrnn / zznz / e / Yi / u Preferably, molten steel of the above composition is manufactured by a steelmaking process, such as using a converter, and formed into a steel tube material, for example, a billet, using a standard method such as continuous casting or billet casting. The steel tube material is then hot-worked to form a tube by a known tubemaking process, for example, the Mannesmann closed mandrel rolling process or the Mannesmann mandrel rolling process, to produce a seamless steel tube of the desired dimensions having the above composition. After hot working, the quenching process may be carried out. The quenching process is not specifically limited.After hot working, the tube is cooled to room temperature at a cooling rate approximately equal to that of air cooling, provided that the composition is within the range of the present invention. In the present invention, this is followed by a heat treatment that includes rapid quenching and tempering. In rapid quenching, the steel tube is reheated to a temperature of 850 to 1,150°C and cooled at an air quenching rate or faster. The quench stop temperature is 50°C or less in terms of surface temperature. When the heating temperature is below 850°C, no reverse transformation from martensite to austenite occurs, and the austenite does not transform into martensite during quenching, resulting in the inability to provide the desired strength. Furthermore, the crystal grains become coarser when the heating temperature exceeds 1,150°C. For this reason, the heating temperature for rapid quenching is 850 to 1,150°C. The quenching temperature is preferably 900°C or higher. The quenching temperature is preferably 1,100°C or lower. When the quenching stop temperature exceeds 50°C, the austenite does not sufficiently transform into martensite, and the fraction of retained austenite becomes excessively high. For this reason, in the present invention, the quenching stop temperature in rapid quenching is 50°C or less. In this case, “air cooling rate or faster” means 0.01 °C / s more. In rapid tempering, the soaking time is preferably 5 to 30 minutes, to achieve a uniform temperature along the direction of the wall thickness and avoid material variation. In tempering, the seamless, rapid-hardened steel tube is heated to a tempering temperature of 500 to 650°C. This heating may be followed by natural cooling. A tempering temperature below 500°C is too low to produce the desired tempering effect. When the tempering temperature exceeds 650°C, intermetallic compounds precipitate, and the desired hardness cannot be achieved at a lower temperature. For this reason, the tempering temperature is between 500 and 650°C. The tempering temperature is preferably 520°C or higher. The tempering temperature is preferably 630°C or lower. In tempering, the soaking time is preferably from 5 to 90 minutes, to achieve a uniform temperature along the direction of the wall thickness and avoid material variation. qj Qrnn / zznz / e / YiAi After heat treatment (rapid quenching and tempering), the seamless steel tube has a microstructure in which the martensitic, ferrite, and retained austenite phases are contained in a predetermined specific volume ratio. This allows the seamless stainless steel tube to achieve the desired strength and excellent corrosion resistance. The seamless stainless steel tube obtained in the present invention as described above is a high-strength steel tube having an elongation of 758 MPa or more and excellent corrosion resistance. Preferably, the elongation is 862 MPa or more. The seamless stainless steel tube of the present invention can be used as a seamless stainless steel tube for tubular products in the petroleum industry (a high-strength seamless stainless steel tube for tubular products in the petroleum industry). Examples The present invention is then described in more detail through the Examples. The molten steels of the compositions indicated in Table 1-1 and Table 1-2 (Steel Nos. A to BJ) were melted into steel tube stock. The steel tube stock was heated and hot-worked to obtain a seamless steel tube with an outside diameter of 83.8 mm and a wall thickness of 12.7 mm, using a seamless pattern rolling mill. Subsequently, the seamless steel tube was air-cooled. The steel tube stock was heated to a temperature of 1,250°C prior to hot working. Each seamless steel tube was cut from a test sample, which was then subjected to rapid quenching. This involved reheating to 960°C and water quenching to a stop-quench temperature of 30°C with a 20-minute soak. The sample was then tempered by heating to either 575°C or 620°C and air quenching with a 20-minute soak. Steel tubes No. 1 to 65 were thus obtained. During rapid quenching, water quenching was performed at a rate of 11°C / s. Air quenching (natural cooling) during tempering was performed at a rate of 0.04°C / s. The tempering temperature was 575°C for steel tubes No. 1 to 65. 1 to 62, and 620°C for steel tubes no. 63 to 65. q; Qrnn / zznz / e / YiAi > Q h oh 1\cc G 0 Table 1-1 0 Sidewalk No. Composition (% by mass) Formula (1) (*3) Observations ('4) C Si Mn PS Cr Mo Cu Ni Nb Al NO w Co Other Average value Result A 0.015 0.37 0.318 0.016 0.0013 16.69 2.48 2.51 4.52 0.107 0.026 0.029 0.002 1.06 0.499 26.3 Satisfactory PS B 0.008 0.32 0.360 0.017 0.0011 17.38 2.61 2.54 5.24 0.196 0.026 0.024 0.003 1.10 0.496 27.6 Satisfactory PS C 0.009 0.29 0.302 0.017 0.0012 16.95 2.46 2.57 5.05 0.204 0.025 0.019 0.002 1.12 0.026 25.6 Satisfactory PS D 0.057 0.31 0.342 0.017 0.0012 17.20 2.46 2.61 5.21 0.062 0.027 0.016 0.002 1.35 0.200 18.5 Satisfactory PS E 0.009 0.93 0.296 0.017 0.0011 16.77 2.48 2.58 5.16 0.069 0.027 0.026 0.002 1.07 0.031 27.0 Satisfactory PS F 0.014 0.28 0.940 0.015 0.0010 16.92 2.59 2.64 4.60 0.086 0.027 0.019 0.002 1.29 0.510 - 27.2 Satisfactory PS G 0.014 0.36 0.012 0.016 0.0010 16.73 2.57 2.53 4.97 0.101 0.026 0.032 0.003 1.07 0.109 24.7 Satisfactory PS H 0.011 0.31 0.329 0.043 0.0009 16.98 2.45 2.60 4.97 0.126 0.027 0.026 0.002 1.32 0.212 25.4 Satisfactory PS 1 0.013 0.37 0.372 0.016 0.0042 17.13 2.55 2.61 4.34 0.136 0.025 0.032 0.003 1.28 0.046 30.0 Satisfactory PS J 0.010 0.34 0.275 0.017 0.0009 17.41 2.57 2.56 4.96 0.098 0.025 0.032 0.002 1.40 0.492 28.5 Satisfactory PS K 0.013 0.29 0.293 0.017 0.0011 15.76 2.60 2.60 4.79 0.171 0.025 0.023 0.002 1.09 0.174 - 20.8 Satisfactory PS L 0.015 0.33 0.324 0.017 0.0013 16.68 3.43 2.49 4.44 0.203 0.025 0.015 0.003 1.23 0.396 - 33.5 Satisfactory PS M 0.015 0.28 0.366 0.015 0.0010 16.96 1.84 2.64 5.09 0.122 0.026 0.025 0.002 1.20 0.080 19.8 Satisfactory PS N 0.009 0.37 0.286 0.015 0.0009 16.84 2.49 3.45 5.12 0.086 0.027 0.019 0.003 1.19 0.121 24.3 Satisfactory PS 0 0.012 0.32 0.348 0.017 0.0012 16.67 2.56 1.55 4.86 0.201 0.028 0.029 0.003 1.16 0.450 26.0 Satisfactory PS P 0.014 0.35 0.300 0.015 0.0011 16.43 2.57 2.56 5.48 0.118 0.024 0.031 0.002 1.42 0.274 19.7 Satisfactory PS Q 0.013 0.32 0.359 0.016 0.0011 16.95 2.51 2.46 3.38 0.246 0.026 0.018 0.002 1.17 0.502 35.3 Satisfactory PS R 0.013 0.36 0.304 0.016 0.0012 16.91 2.63 2.47 4.73 0.280 0.027 0.025 0.002 1.29 0.263 27.7 Satisfactory PS S 0.009 0.34 0.346 0.015 0.0011 17.33 2.48 2.55 4.39 0.020 0.025 0.035 0.002 1.30 0.522 30.7 Satisfactory PS T 0.009 0.34 0.362 0.016 0.0009 17.20 2.62 2.48 4.82 0.228 0.092 0.024 0.002 1.35 0.507 29.3 Satisfactory PS. > Q 1\ o σι o σι o σι oh 1\ cc u 0.010 0.28 0.286 0.015 0.0011 17.15 2.57 2.46 4.33 0.187 0.027 0.093 0.002 1.28 0.027 26.7 Satisfactory PS V 0.015 0.35 0.339 0.017 0.0010 16.98 2.51 2.51 4.61 0.055 0.026 0.019 0.009 1.26 0.339 - 28.0 Satisfactory PS w 0.008 0.31 0.375 0.014 0.0010 17.33 2.57 2.50 4.78 0.238 0.025 0.021 0.002 1.92 0.416 30.0 Satisfactory PS X 0.008 0.31 0.375 0.014 0.0010 17.33 2.57 2.50 4.78 0.238 0.025 0.021 0.002 0.88 0.416 30.0 Satisfactory PS Y 0.008 0.31 0.375 0.014 0.0010 17.33 2.57 2.50 4.78 0.238 0.025 0.021 0.002 1.26 1.323 30.0 Satisfactory PS z 0.008 0.30 0.311 0.015 0.0013 16.82 2.56 2.56 4.77 0.076 0.024 0.023 0.002 1.22 0.020 27.0 Satisfactory PS AA 0.006 0.90 0.050 0.016 0.0011 16.33 3.48 1.54 3.37 0.050 0.023 0.009 0.002 1.07 0.187 44.7 Satisfactory PS AB 0.032 0.02 0.520 0.013 0.0009 16.09 2.29 2.58 4.98 0.110 0.024 0.039 0.003 1.23 0.396 13.6 Satisfactory PS AC 0.015 0.30 0.347 0.016 0.0013 16.70 2.50 2.64 4.81 0.075 0.025 0.025 0.002 1.15 0.364 V:0.05. B:0.005 24.5 Satisfactorio PS. (Ί) El resto es Fe e impurezas incidentales ('2) Los valores subrayados significan fuera del intervallio de la presente invención (*3) Fórmula (1): 13.0 < -5.9 χ (7.82+ 27C 0.91 Si + 0.21 Mn 0.9Cr + Ni 1.1 Mo + 0.2Cu +11N) < 55.0 σι (*4) PS: Acero actual, CS: Acero comparativo ^ωωΐΏΜ^-^σι ο σι ο σι ο σι o Table 1-2 Steel No. Composition (% by mass) Formula (1)(*3) Remarks ('4) C Si Mn PS Cr Mo Cu Ni Nb Al N O w Co Other Average Value Result AD 0.012 0.30 0.338 0.018 0.0012 16.64 2.62 2.58 4.45 0.135 0.028 0.027 0.003 1.34 0.054 V:0.70 27.4 Satisfactory PS AE 0.011 0.33 0.316 0.016 0.0013 17.16 2.48 2.49 4.94 0.204 0.027 0.034 0.002 1.09 0.118 Ta:0.1 26.4 Satisfactory PS AF 0.012 0.31 0.361 0.017 0.0009 16.45 2.47 2.50 4.89 0.091 0.025 0.035 0.003 1.22 0.246 Ti:0.131.Zr:0.161 22.4 Satisfactory PS AG 0.014 0.31 0.374 0.017 0.0010 17.33 2.57 2.62 5.09 0.151 0.027 0.024 0.003 1.12 0.583 Ca:0.006, Mg:0.0050 26.8 Satisfactory PS AH 0.017 0.35 0.336 0.018 0.0013 17.11 2.63 2.58 4.99 0.058 0.025 0.026 0.003 1.09 0.088 REM:0.181 26.3 Satisfactory PS Al 0.013 0.28 0.366 0.017 0.0011 16.51 2.55 2.51 4.28 0.104 0.026 0.019 0.003 1.05 0.239 Sb:0.77 27.6 Satisfactory PS AJ 0.014 0.29 0.304 0.017 0.0013 16.91 2.57 2.51 4.50 0.101 0.026 0.015 0.002 1.35 0.213 B:0.007,T¡:0.102,Zr:0.201 28.8 Satisfactory PS AK 0.014 0.33 0.346 0.017 0.0011 17.33 2.45 2.50 5.24 0.126 0.025 0.032 0.002 1.07 0.198 V:0.06,REM:0.183 25.0 Satisfactory PS AL 0.011 0.28 0.362 0.017 0.0012 17.20 2.55 2.50 5.05 0.136 0.025 0.016 0.002 1.29 0.638 B:0.004,TI:0.218,Sn:0.143 27.2 Satisfactory PS AM 0.013 0.37 0.286 0.015 0.0012 17.15 2.57 2.50 5.21 0.098 0.028 0.022 0.003 1.07 0.819 Zr:0.198,Mg:0.0019 26.1 Satisfactory PS AN 0.068 0.30 0.284 0.015 0.0009 16.44 2.46 2.63 4.53 0.240 0.027 0.019 0.002 1.32 0.396 16.5 Satisfactory es AO 0.013 1.08 0.316 0.016 0.0011 17.06 2.54 2.64 5.09 0.204 0.028 0.023 0.003 1.19 0.517 29.7 Satisfactory is AP 0.012 0.37 0.004 0.016 0.0010 16.56 2.54 2.60 4.64 0.173 0.027 0.032 0.003 1.37 0.032 25.9 Satisfactory PS AQ 0.016 0.29 0.366 0.055 0.0010 16.91 2.64 2.49 4.70 0.159 0.027 0.015 0.002 1.20 0.313 27.7 Satisfactory is AR 0.015 0.37 0.325 0.014 0.0055 17.30 2.48 2.47 5.21 0.058 0.025 0.032 0.002 1.44 0.097 25.3 Satisfactory is AS 0.009 0.32 0.292 0.014 0.0009 17.56 2.54 2.50 4.89 0.181 0.026 0.016 0.002 1.08 0.428 30.6 Satisfactory PS AT 0.009 0.29 0.296 0.016 0.012 2.053 2.53 5.19 0.074 0.026 0.022 0.003 1.36 0.291 16.7 Satisfactory is AU 0.011 0.35 0.327 0.015 0.0009 17.18 1.72 0.024. 0.025 0.020 0.003 1.42 0.038 23.8 Satisfactory is AV 0.016 0.35 0.298 0.016 0.0009 17.02 2.57 1.42 4.62 0.129 0.027 0.027 0.027. 1.40 0.063 29.3 Satisfactory is AW 0.014 0.37 0.305 0.015 0.0010 16.42 2.59 2.50 5.59 0.131 0.028 0.016 0.004 20.2099 Satisfactory PS AX 0.012 0.30 0.319 0.016 0.0010 16.41 2.49 2.58 2.90 0.073 0.026 0.025 0.002 1.22 0.108 34.6 Satisfactory PS AZ20.0 0.35 0.351 0.015 0.0011 16.87 2.59 2.63 5.22 0.124 0.107 0.023 0.002 1.07 0.487 24.3 Satisfactory es BA 0.01 7 0.036 0.366 0.0012 16.65 2.54 2.57 5.04 0.109 0.024 0.109 0.002 1.05 0.074 17.5 Satisfactory es. > Q 1\ ch 1\ cc G o o σι o σι o σι o BB 0.010 0.37 0.333 0.016 0.0010 16.79 2.49 2.49 4.79 0.176 0.025 0.033 0.015 1.05 0.333 25.8 Satisfactory CS BC 0.010 0.37 0.333 0.016 0.0010 16.79 2.49 2.49 4.79 0.176 0.025 0.033 0.002 0.42 0.333 25.8 Satisfactory CS BD 0.011 0.28 0.346 0.017 0.0009 16.99 2.63 2.51 4.52 0.148 0.026 0.028 0.002 1.39 0.004 29.0 Satisfactory CS BE 0.007 0.94 0.020 0.016 0.0011 17.45 3.40 1.63 3.36 0.060 0.027 0.008 0.002 1.43 0.444 50.2 Satisfactory PS BF 0.016 0.35 0.337 0.014 0.0011 17.04 2.59 2.60 4.28 0.026 0.016 0.003 1.11 0.486 30.7 Satisfactory PS BG 0.009 0.32 0.292 0.014 0.0009 18.13 2.54 2.50 4.89 0.181 0.026 0.016 0.002 1.08 0.428 33.7 Satisfactory CS BH 0.014 0.37 0.305 0.015 0.0010 16.42 2.59 2.50 6.09 0.131 0.028 0.016 0.002 1.41 0.599 17.3 Satisfactory CS Bl 0.012 0.30 0.319 0.016 0.0010 16.41 2.49 2.58 2.41 0.073 0.026 0.025 0.002 1.22 0.108 37.5 Satisfactory CS BJ 0.006 0.95 0.022 0.016 0.0011 17.93 3.40 1.63 2.93 0.059 0.025 0.008 0.002 1.15 0.402 55.5 Unsatisfactory CS (Ί) The remainder is Fe and incidental impurities f 2) The underlined values ​​signify outside the range of the present invention (*3) Formula (1): 13.0 < -5.9 χ (7.82+ 27C 0.91 Si + 0.21 Mn 0.9Cr + Ni 1.1 Mo + 0.2Cu +11N) < (55.0) (*4) PS: Actual steel, CS: Comparative steel > Q 1\ ch 1\ ccooo A test sample was taken from the heat-treated test material (seamless steel tube) and subjected to microstructure observation, a tensile test, and a corrosion resistance test. The test methods are as follows. (1) Observation of the microstructure A test sample was taken for microstructure observation of the heat-treated test material, oriented so that a cross-section orthogonal to the tube axis was exposed for observation. The test sample was corroded with Vilella's solution (a mixed reagent containing 2 g of picric acid, 10 mL of hydrochloric acid, and 100 mL of ethanol), and the structure was imaged using a scanning electron microscope (1000x magnification). The ferrite phase fraction (area ratio (%)) of the microstructure was then calculated using an image analyzer. In this case, the area ratio is defined as the volume ratio (%) of the ferrite phase. Individually, an X-ray diffraction test sample was taken from the heat-treated test material. The test sample was ground and polished to obtain a measurement cross-section (C-section) orthogonal to the axial direction of the tube, and the fraction of the retained austenite phase microstructure (y) was measured by an X-ray diffraction method. The fraction of the retained austenite phase microstructure was determined by measuring the integral X-ray diffraction intensity for the (220) plane of the austenite phase (y) and the (211) plane of the ferrite phase (a), and converting the calculated values ​​using the following formula. γ (volume ratio) = 100 / (1 +(IαRγ / Iγβα)), where γ is the integral intensity of α, Ra is the theoretical crystallographic value of α, Iγ is the integral intensity of γ, and Ry is the theoretical crystallographic value of γ. The martensitic phase fraction is the remainder other than the ferrite phase and retained γ phase fractions. (2) Traction test An API (American Petroleum Institute) arc tensile test specimen was taken from the heat-treated test material, oriented such that the test specimen had a tensile direction along the pipe axis. The tensile test was performed according to API specifications to determine tensile properties (yield strength, YS). Steel was determined to be high-strength and acceptable when it had a yES of 758 MPa or greater, and unacceptable when it had a yES of less than 758 MPa. (3) Corrosion resistance test (carbon dioxide gas corrosion resistance test and acid environment corrosion resistance test) From the heat-treated test material, a corrosion test sample measuring 3 mm thick, 30 mm wide, and 40 mm long was prepared and subjected to corrosion tests to evaluate resistance to carbon dioxide gas corrosion and resistance to corrosion in an acidic environment. The corrosion test to evaluate resistance to carbon dioxide gas corrosion was performed by immersing the corrosion test sample in a test solution (a 20 wt% aqueous NaCl solution; liquid temperature: 200°C; CO2 gas atmosphere of 30 atm) in an autoclave for 14 days (336 hours). The corrosion rate was determined from the calculated reduction in the weight of the analyzed sample, measured before and after the corrosion test. Steel was considered acceptable when it had a corrosion rate of 0.127 mm / year or less, and unacceptable when it had a corrosion rate greater than 0.127 mm / year. The corrosion test to evaluate corrosion resistance in an acidic environment was performed by immersing the test sample for 40 minutes in a 15% by mass hydrochloric acid solution heated to 80°C. The corrosion rate was determined from the calculated reduction in the weight of the sample, measured before and after the corrosion test. Steel was considered acceptable when it had a corrosion rate of 600 mm / year or less, and unacceptable when it had a corrosion rate greater than 600 mm / year. (4) Sulfide stress cracking resistance test (SSC resistance test) A test specimen in the form of a round rod (diameter 0: 6.4 mm) was prepared from the test specimen material by machining, in accordance with NACE TM0177, Method A, and subjected to a sulfide stress cracking resistance test (SSC resistance test). In this case, “NACE” stands for National Association for Corrosion Engineering. The SSC strength test was performed by immersing the test specimen in a test solution (a 20% by mass aqueous NaCl solution; liquid temperature: 25°C; an atmosphere of 0.1 atm of H₂S and 0.9 atm of CO₂) maintained in an autoclave and with a pH adjusted to 3.5 by the addition of acetic acid and sodium acetate, and applying a stress equal to 90% of the yield strength for 720 hours in the solution. The presence or absence of cracks in the tested specimen was observed. Steel was considered acceptable when no cracks were present after the test. In Table 2, an open circle (o) indicates no cracks, and a cross (x) indicates cracks are present. The results are presented in Table 2. pyQrnn / zznz / e / YiAi Table 2 Steel No. Steel Tube No. Microstructure (% by volume) Elongation Strength YS (MPa) Corrosion Rate (mm / yr) Acid Corrosion Rate (mm / yr) SSC Remarks M (*1) F ('1) A (*1) A 1 59 29 12 964 0.030 550.7 Acceptable Current Example B 2 53 32 15 931 0.020 500.2 Acceptable Current Example C 3 60 29 11 968 0.025 525.7 Acceptable Current Example D 4 52 19 29 927 0.078 589.1 Acceptable Current Example E 5 56 29 15 945 0.110 579.4 Acceptable Current Example F 6 54 31 15 976 0.027 533.7 Acceptable Current Example G 7 58 27 15 903 0.027 534.9 Acceptable Current Example H 8 56 29 15 949 0.095 567.2 Acceptable Current Example I 9 51 35 14 922 0.088 581.1 Acceptable Current Example J 10 48 33 19 887 0.021 506.8 Acceptable Current Example K 11 72 22 6 1031 0.081 576.3 Acceptable Current Example L 12 47 41 12 901 0.026 530.6 Acceptable Current Example M 13 66 21 13 1000 0.093 584.6 Acceptable Current Example N 14 58 26 16 991 0.020 507.9 Acceptable Current example 0 15 61 30 9 892 0.103 562.2 Acceptable Current Example P 16 63 21 16 887 0.027 532.8 Acceptable Current Example Q 17 53 45 2 888 0.045 575.9 Acceptable Current Example R 18 57 34 9 950 0.027 534.0 Acceptable Current Example S 19 48 35 17 879 0.026 527.6 Acceptable Current Example T 20 49 39 12 911 0.099 584.3 Acceptable Current Example u 21 53 31 16 951 0.091 575.9 Acceptable Current Example V 22 54 31 15 934 0.072 578.9 Acceptable Current Example w 23 48 39 13 903 0.023 516.3 Acceptable Current Example X 24 54 35 11 941 0.086 578.4 Acceptable Current Example Y 25 53 36 11 927 0.023 516.3 Acceptable Current Example z 26 56 30 14 950 0.083 577.6 Acceptable Current Example AA 27 42 55 3 870 0.040 579.0 Acceptable Current Example AB 28 71 12 17 988 0.033 566.0 Acceptable Current Example AC 29 59 26 15 963 0.028 539.5 Acceptable Current Example AD 30 57 32 11 952 0.030 550.0 Acceptable Current Example AE 31 57 30 13 949 0.024 521.3 Acceptable Current Example AF 32 63 24 13 985 0.030 551.3 Acceptable Current Example AG 33 53 30 17 929 0.021 505.4 Acceptable Current Example AH 34 53 28 19 930 0.023 516.5 Acceptable Current Example Al 35 61 30 9 971 0.033 562.8 Acceptable Current Example AJ 36 59 29 12 964 0.030 550.7 Acceptable Current Example AK 37 51 32 17 931 0.020 500.2 Acceptable Current Example AL 38 55 31 14 968 0.021 521.9 Acceptable Current Example AM ​​39 55 30 15 968 0.019 540.9 Acceptable Current Example AN 40 63 18 19 982 0.143 618.3 Unacceptable Comparative Example AO 41 51 35 14 921 0.139 616.4 Unacceptable Comparative Example AP 42 58 31 11 858 0.030 549.5 Acceptable Current Example AQ 43 55 32 13 944 0.135 605.3 Unacceptable Comparative Example AR 44 51 28 21 918 0.139 611.9 Unacceptable Comparative Example AS 45 25 61 14 850 0.018 504.6 Acceptable Current Example AT 46 76 16 8 1051 0.144 617.3 Unacceptable Comparative Example AU 47 61 27 12 976 0.151 623.1 Unacceptable Comparative Example AV 48 53 35 12 858 0.140 618.7 Unacceptable Comparative Example. pyQrnn / zznz / e / YiAi Steel No. Steel Tube No. Microstructure (% by volume) Elongation Strength YS (MPa) Corrosion Rate (mm / yr) Acid Corrosion Rate (mm / yr) SSC Remarks M (*1) F (*1) A (*D) AW 49 63 22 15 860 0.026 531.3 Acceptable Current Example AX 50 57 40 3 858 0.073 587.9 Acceptable Current Example AZ 52 55 30 15 940 0.130 631.2 Unacceptable Comparative Example BA 53 61 17 22 982 0.129 608.6 Unacceptable Comparative Example BB 54 60 29 11 969 0.132 613.1 Unacceptable Example Comparative example BC 55 63 27 10 981 0.136 618.5 Unacceptable Comparative example BD 56 53 34 13 930 0.027 638.1 Acceptable Comparative example BE 57 29 61 10 805 0.032 558.5 Acceptable Current example BF 58 52 30 18 896 0.051 579.4 Acceptable Current example BG 59 23 51 26 705 0.015 502.7 Acceptable Comparative example BH 60 32 22 46 721 0.029 536.9 Acceptable Comparative example Bl 61 42 40 18 706 0.036 561.9 Acceptable Comparative example BJ 62 6 67 27 641 0.011 502.1 Acceptable Comparative Example A 63 38 29 33 831 0.029 548.9 Acceptable Current Example B 64 37 32 31 821 0.018 505.1 Acceptable Current Example C 65 39 29 32 840 0.026 526.7 Acceptable Current Example. qj Qrnn / zznz / e / YiAi The underlined values ​​mean outside the range of the present invention (1) M: Martensitic phase, F: Ferrite phase, A: Retained austenite phase

[0082] All the seamless stainless steel tubes in the examples presented had high strength with an elongation YS of 758 MPa or more. The seamless stainless steel tubes in the examples presented also had excellent corrosion resistance (resistance to carbon dioxide gas corrosion) in a high-temperature corrosive environment containing CCte and CL of 200°C, excellent corrosion resistance in an acidic environment, and excellent resistance to sulfide stress cracking.

Claims

1. A seamless stainless steel tube of a composition, characterized in that it comprises, in % by mass, C: 0.06% or less, Si: 1.0% or less, P: 0.05% or less, S: 0.005% or less, Cr: more than 15.7% and 18.0% or less, Mo: 1.8% or more and 3.5% or less, Cu: 1.5% or more and 3.5% or less, Ni: 2.5% or more and 6.0% or less, Al: 0.10% or less, N: 0.10% or less, O: 0.010% or less, W: 0.5% or more and 2.0% or less, and Co: 0.01% or more and 1.5% or less, and wherein C, Si, Mn, Cr, Ni, Mo, Cu and N comply with the following formula (1), and the remainder It is Fe and incidental impurities, the seamless stainless steel tube has a microstructure containing at least 25% martensitic phase, at most 65% ferrite phase and at most 40% austenite phase retained by volume, the seamless stainless steel tube having an elongation resistance of 758 MPa or more, 13.0 <-5.9 x (7.82 + 27C - 0.91 Si + 0.21 Mn - 0.9Cr + Ni - 1.1 Mo + 0.2Cu + 11N) < 55.0 (1), wherein C, Si, Mn, Cr, Ni, Mo, Cu and N represent the content of each element in % by mass, and the content is 0 (zero; % by mass) for elements that are not contained.

2. The seamless stainless steel tube according to claim 1, further characterized in that the composition additionally comprises, in % by mass, one or two selected from Mn: 1.0% or less, and Nb: 0.30% or less.

3. The seamless stainless steel tube according to either of claim 1 or 2, further characterized in that the seamless stainless steel tube of the composition of claim 1 or 2 has a microstructure containing at least 40% martensitic phase, at most 60% ferrite phase and at most 30% volume-retained austenite phase, and has an elongation resistance of 862 MPa or more.

4. The seamless stainless steel tube according to any one of claims 1 to 3, further characterized in that the composition additionally includes, in % by mass, one or two or more selected from V: 1.0% or less, B: 0.01% or less, and Ta: 0.3% or less.

5. The seamless stainless steel tube according to any one of claims 1 to 4, further characterized in that the composition additionally comprises, in % by mass, one or two selected from Ti: 0.3% or less, and Zr: 0.3% or less.

6. The seamless stainless steel tube according to any one of claims 1 to 5, further characterized in that the composition additionally comprises, in % by mass, one or two or more selected from Ca: 0.01% or less, REM: 0.3% or less, Mg: 0.01% or less, Sn: 0.2% or less, and Sb: 1.0% or less.

7. A method for manufacturing the seamless stainless steel tube according to any one of claims 1 to 6, the method being characterized in that it comprises: forming a seamless steel tube of predetermined dimensions from a steel tube material; rapidly quenching the seamless steel tube to a temperature ranging from 850 to 1,150°C, and cooling the seamless steel tube to a surface temperature of 50°C or less at an air-cooling rate or faster; and tempering the rapidly quenched seamless steel tube to a temperature of 500 to 650°C.