SEAMLESS STAINLESS STEEL PIPE

Abstract

This document provides a seamless stainless steel pipe that has high strength and excellent low-temperature toughness and corrosion resistance. The seamless stainless steel pipe has a composition comprising, in % by mass, C: 0.06% or less, Si: 1.0% or less, Mn: 0.01% or more and 1.0% or less, P: 0.05% or less, S: 0.005% or less, Cr: 14.0% or more and 17.0% or less, Mo: more than 3.80% and 6.0% or less, Cu: more than 1.03% and 3.5% or less, Ni: 3.5% or more and 6.0% or less, Al: 0.10% or less, N: 0.10% or less, and O: 0.010% or less, wherein C, Si, Mn, Cr, Ni, Mo, Cu and N meet a predetermined ratio, and the remainder is Fe and incidental impurities, seamless stainless steel pipe having a microstructure containing at least 40% martensitic phase, at most 60% ferrite phase and at most 30% austenite phase retained by volume, seamless stainless steel pipe having a yield strength of 862 MPa or more.

Description

SEAMLESS STAINLESS STEEL PIPE Technical field The present invention relates to a seamless martensitic stainless steel pipe suitable for tubular products in petroleum-producing countries for oil and gas wells (hereinafter referred to simply as oil wells). In particular, the invention relates to improved corrosion resistance in various corrosive environments, such as a severe high-temperature corrosive environment containing carbon dioxide (CO2) and chloride ions (Cl-), and an environment containing hydrogen sulfide (H2S), and to improved low-temperature toughness. Background of the technique The anticipated scarcity of energy resources in the near future has spurred the active development of tubular products for oil-producing countries for use in applications that were previously unthinkable, such as deep-sea oil fields, carbon dioxide environments, and hydrogen sulfide or acidic environments. Steel pipelines for petroleum products destined for these environments require exceptional strength and excellent corrosion resistance. Tubular products for oil-producing countries, used for oil and gas extraction in environments containing CO2, Cl, and similar substances, typically utilize 13Cr martensitic stainless steel pipes. Oilfield tubular products capable of withstanding higher temperatures (up to 200°C) have also been developed. However, the corrosion resistance of 13Cr martensitic stainless steel is not always sufficient for such applications. Therefore, steel pipes for petroleum products that exhibit excellent corrosion resistance are required, even when used in these environments. In relation to such demand, for example, PTL 1 describes a high-strength seamless stainless steel pipe for tubular products of petroleum countries 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: 13.5 to 15.4%, Ni: 3.5 to 6.0%, Mo: 1.5 to 5.0%, Cu: 3.5% or less, W: 2.5% or less, N: 0.15% or less, and wherein C, Si, Mn, Cr, Ni, Mo, W, Cu and N meet a specific ratio. In this related technique it is stated that a seamless high-strength stainless steel pipe for tubular products of petroleum countries can be manufactured that has a yield strength of 110 ksi (758 MPa) or more, and that shows sufficient corrosion resistance even in a severe high-temperature corrosive environment containing CO2, Cl, and H2S. PTL 2 describes a seamless, high-strength stainless steel pipe for tubular products in petroleum-producing countries that has excellent corrosion resistance. The seamless, high-strength stainless steel pipe has a composition comprising, by mass %, C: nocL Ln / Lznz / E / Yi 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: 15.5 to 17.5%, Ni: 3.0 to 6.0%, Mo: 1.5 to 5.0%, Cu: 4.0% or less, W: 0.1 to 2.5%, and N: 0.15% or less, wherein C, Si, Mn, Cr, Ni, Mo, Cu, N, and W are in a specific ratio. It is stated in this related art that a seamless, high-strength stainless steel pipe for tubular products in petroleum-producing countries can be manufactured having a yield strength of 110 ksi (758 MPa) or more, and exhibiting sufficient corrosion resistance even in a severe, high-temperature corrosive environment containing CO2, Cl-, and H2S. PTL 3 describes a seamless stainless steel pipe for tubular products in petroleum-producing countries having a composition comprising, by mass %, C: 0.05% or less, Si: 0.50% or less, Mn: 0.20 to 1.80%, P: 0.030% or less, S: 0.005% or less, Cr: 14.0 to 18.0%, Ni: 5.0 to 8.0%, Mo: 1.5 to 3.5%, Cu: 0.5 to 3.5%, Al: 0.10% or less, Nb: more than 0.20% and 0.50% or less, V: 0.20% or less, N: 0.15% or less, and O: 0.010% or less, and wherein Cr, Ni, Mo, Cu, C, Si, Mn, and N meet a specified ratio. This related technique states that a seamless stainless steel pipe for tubular products of petroleum countries can be manufactured that has a yield strength of 110 ksi (758 MPa) or more, and that shows sufficient corrosion resistance even in a severe high-temperature corrosive environment containing CO2, Cl·, and H2S. PTL 4 describes a seamless, high-strength stainless steel pipe for tubular products in petroleum-producing countries having a composition comprising, 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: more than 3.0% 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 wherein Nb, Ta, C, N and Cu fulfill a specific relationship, and have a microstructure that contains at least 45% tempered martensitic phase, between 20 and 40% ferrite phase, and more than 10% and at most 25% volume-retained austenite phase.In this related technique it is stated that a seamless high-strength stainless steel pipe can be manufactured for tubular products of petroleum countries that has a yield strength 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 pipe for tubular products in petroleum-producing countries having a composition comprising, by 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 are in a specified ratio, and having a microstructure containing more than 45% martensitic phase (a dominant phase), from 10 to 45% ferrite phase (a secondary phase), and at most 30% austenite phase retained by volume.In this related technique it is stated that a high-strength stainless steel seamless pipe can be manufactured for tubular products of petroleum countries that has a yield strength YS of 862 MPa or more, and that shows sufficient corrosion resistance even in a severe high-temperature corrosive environment containing CO2, CI, and H2S. PTL 6 describes a seamless, high-strength stainless steel pipe for tubular products in petroleum-producing countries having a composition comprising, by 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, N: 0.15% or less, and B: 0.0005 to 0.0100%, wherein C, Si, Mn, Cr, Ni, Mo, Cu, N, and W are in a specific ratio, and having a microstructure containing more than 45% martensitic phase (a dominant phase), 10 to 45% ferrite phase (a secondary phase), and at most 30% austenite phase retained by volume.In this related technique it is stated that a seamless high-strength stainless steel pipe for tubular products of petroleum countries can be manufactured that has a yield strength, YS, of 862 MPa or more, and that shows sufficient corrosion resistance even in a severe high-temperature corrosive environment containing CO2, Ch, and H2S. List of appointments Patent literature PTL 1: JP-A-2014-25145 PTL 2: JP-A-2015-110822 PTL 3: WO2014 / 112353 PTL 4: WO2017 / 138050 PTL 5: WO2018 / 020886 PTL 6: Japanese patent no. 6399259 Brief summary of the invention Technical problem In cold-weather applications, desirable low-temperature toughness is required. In the field of materials for tubular products in oil-producing countries, it is common practice to evaluate low-temperature toughness by measuring the absorption energy at -10 °C in a Charpy impact test, vE-10, and the test requires an absorption energy of at least 300 J. The steels disclosed in the above PTL 1 to PTL 6 contain a ferrite phase. The characteristic fracture mode of the ferrite phase is that the ferrite phase, which is ductile at high temperatures, abruptly becomes brittle once it reaches a certain temperature. This temperature is commonly known as the ductile-brittle transition temperature (hereafter also referred to as the transition temperature).When a Charpy impact test is performed at a test temperature close to the transition temperature, the Charpy absorption energy tends to vary. If absorption energy is used solely as an index for evaluating low-temperature toughness (Ln / Lznz / B / Yi), the test could give the false impression that the steel, when tested with a limited number of specimens, has desirable low-temperature toughness. An evaluation carried out at a single test temperature also raises concerns about toughness in a low-temperature environment. For this reason, low-temperature toughness is often evaluated from the perspective of the transition temperature, in addition to absorption energy, and a transition temperature of -40 °C or lower is required for optimal performance. It is stated in PTL 1 to PTL 6 above that the techniques described in these related art documents can produce a steel pipe that passes a sulfide stress cracking (SSC) test performed by immersing a test specimen in a test solution: a 20 wt% aqueous NaCl solution (liquid temperature 25 °C; H2S atmosphere 0.1 atm and CO2 atmosphere 0.9 atm) 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. However, these techniques cannot be considered satisfactory in achieving high strength with a yield strength, YS, of 862 MPa or more, while also providing excellent low-temperature toughness and ensuring resistance to sulfide stress cracking (SSC resistance) in an even more severe environment.The following are the possible explanations of the present inventors in this regard. Sulfide stress cracking (SSC) in stainless steel is caused by the generation of large amounts of hydrogen as a result of an increased corrosion rate due to pitting corrosion in defective portions of a passive film. To achieve desirable resistance to SSC, it is effective to add elements that improve pitting corrosion resistance, such as chromium (Cr), molybdenum (Mo), and tungsten (W). However, Cr, Mo, and W are ferrite phase stabilizers, and when added in large quantities, they accelerate grain growth in the ferrite phase when heat is applied to manufacture a steel pipe from raw steel pipe material. This seriously impairs the low-temperature toughness of the final product. Furthermore, when added in large quantities, Mo and W precipitate into intermetallic compounds during the quenching process, further decreasing low-temperature toughness.For the same purpose, PTL 5 discloses a technique that reduces the total amount of Cr, Mo, and W to 0.75% or less by mass in the precipitate. However, it remains difficult, with the technique disclosed in PTL 5, to achieve desirable low-temperature toughness while simultaneously ensuring desirable CSS resistance. As described above, the techniques of the related technique are not satisfactory in terms of providing a seamless stainless steel pipe that has high strength and desirable low-temperature hardness, while also ensuring desirable resistance to sulfide stress cracking (SSC resistance), desirable resistance to carbon dioxide corrosion, and desirable resistance to sulfide stress cracking (SCC resistance). The present invention aims to provide a solution to problems in the related art, and it is an object of the present invention to provide a seamless stainless steel pipe nocL Ln / Lznz / B / Yi having high strength with a yield strength of 862 MPa (125 ksi) or more, and excellent low-temperature toughness with an absorption energy at -10 °C, vE-w, of 300 J or more, and a ductile-brittle transition temperature of -40 °C or less as measured by a Charpy impact test, in addition to having excellent corrosion resistance. As used herein, excellent corrosion resistance means excellent resistance to carbon dioxide corrosion, excellent resistance to sulfide stress cracking (SCC resistance) and excellent resistance to sulfide stress cracking (SSC resistance). As used herein, excellent resistance to carbon dioxide corrosion means that a test specimen immersed in a test solution that is a 20% by mass aqueous solution of NaCl (liquid temperature of 200 °C; a CO2 gas atmosphere of 30 atm) in an autoclave has a corrosion rate of 0.127 mm / y less after 336 hours in the solution. As used herein, excellent sulfide stress corrosion cracking resistance means that a test specimen immersed in a test solution that is a 20% by mass aqueous NaCl solution (liquid temperature: 100 °C; an atmosphere of 30-atm CO2 gas and 0.1-atm H2S) maintained in an autoclave and having a pH adjusted to 3.3 by the addition of acetic acid and sodium acetate does not crack even after 720 hours under an applied stress equal to 100% of the yield stress in the solution. As used herein, excellent sulfide stress cracking resistance (SSC resistance) means that a test specimen immersed in a test solution that is a 20% by mass aqueous NaCl solution (liquid temperature: 25°C; an atmosphere of CO2 gas of 0.9 atm and H2S of 0.1 atm) maintained in an autoclave and having a pH adjusted to 3.0 by the addition of acetic acid and sodium acetate does not crack even after 720 hours under an applied stress equal to 90% of the yield stress in the solution. As used herein, the term yield strength means a yield strength measured in accordance with API (American Petroleum Institute) specifications for an API arc-shaped tensile test specimen taken from a heat-treated test material in an orientation such that the test specimen had a tensile direction along the pipe axis direction. As used herein, excellent low-temperature toughness is understood to mean having an absorption energy vE-w at -10 °C of 300 J or more, and a ductile-brittle transition temperature of -40 °C or less, measured at a test temperature of 50 °C to -120 °C in a Charpy impact test performed in accordance with JIS Z2242 specifications for a V-notch test specimen (10 mm thick) taken from a heat-treated test material in an orientation such that the test specimen had a longitudinal direction along the pipe axis direction. nocL Ln / Lznz / B / Yi Solution to the problem To achieve the aforementioned objectives, the present inventors conducted intensive research on various factors affecting the corrosion resistance and low-temperature toughness of a seamless stainless steel pipe with a Cr-containing composition of 14.0% by mass or more. The seamless stainless steel pipe was found to exhibit the desired corrosion resistance when it had a Mo content greater than 3.80% by mass and a Cu content greater than 1.03% by mass. The seamless stainless steel pipe also exhibited the desired low-temperature toughness when it contained no W, or contained W in a limited amount of 0.84% ​​or less. Possible explanations for these findings are discussed below. Molybdenum (Mo) is an element that improves resistance to pitting corrosion and can enhance resistance to cold corrosion syndrome (CCS) when present in higher quantities. Copper (Cu) reduces hydrogen ingress into the steel by strengthening the protective layer and can also improve CSS resistance. Wattle (W) is thought to precipitate more readily into an intermetallic compound during tempering than Mo and Cu. This is likely the reason for the desirable CSS resistance and low-temperature toughness obtained when the Mo content exceeds 3.80% by mass and the Cu content exceeds 1.03% by mass, and when W is absent or present in a limited amount of 0.84% ​​or less. The present invention is based on these findings and was completed after further studies. Specifically, the essence of the present invention is as follows. [1] A seamless stainless steel pipe having a composition comprising, by mass %, C: 0.06% or less, Si: 1.0% or less, Mn: 0.01% or more and 1.0% or less, P: 0.05% or less, S: 0.005% or less, Cr: 14.0% or more and 17.0% or less, Mo: more than 3.80% and 6.0% or less, Cu: more than 1.03% and 3.5% or less, Ni: 3.5% or more and 6.0% or less, Al: 0.10% or less, N: 0.10% or less, and O: 0.010% or less, wherein C, Si, Mn, Cr, Ni, Mo, Cu, and N satisfy the following formula (1), and the remainder being Fe and incidental impurities, the seamless stainless steel pipe has a microstructure containing at least 40% martensitic phase, at most 60% ferrite phase and at most 30% austenite phase retained by volume, seamless stainless steel pipe having a yield strength of 862 MPa or more, Formula (1) 13.0 < -5.9 x (7.82 + 27C -0.91SI + 0.21 Mn - 0.9Cr + Ni - 1.1Mo + 0.2Cu + 11N) < 50.0, where C, Si, Mn, Cr, Ni, Mo, Cu and N represent the content of each element in % by mass. [2] Seamless stainless steel pipe in accordance with [1], wherein the composition further comprises, in % by mass, W: 0.84% ​​or less. [3] Seamless stainless steel tubing in accordance with [1] or [2], wherein the composition further comprises, by mass %, one or two or more selected from Nb: 0.5% or less, V: 0.5% or less, and B: 0.01% or less. [4] Seamless stainless steel tubing conforming to any of [1] to [3], wherein the composition further comprises, in % by mass, one or two or more selected from Ti: 0.3% or less, Zr: 0.3% or less, Co: 1.5% or less, and Ta: 0.3% or less. [5] Seamless stainless steel tubing conforming to any of [1] to [4], wherein the composition further comprises, 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. Advantages of the invention The present invention has enabled the production of a seamless stainless steel pipe that has high strength with a yield strength of 862 MPa (125 ksi) or more, and excellent low-temperature toughness with an absorption energy at -10 °C, vE-w, of 300 J or more, and a ductile-brittle transition temperature of -40 °C or less, as measured by a Charpy impact test, in addition to having excellent corrosion resistance, including excellent resistance to carbon dioxide corrosion even in a severe high-temperature corrosive environment of 200 °C containing CO2 and Cl·, and excellent resistance to sulfide stress corrosion cracking. Description of the modalities A seamless stainless steel pipe of the present invention is a seamless stainless steel pipe having a composition comprising, by mass %, C: 0.06% or less, Si: 1.0% or less, Mn: 0.01% or more and 1.0% or less, P: 0.05% or less, S: 0.005% or less, Cr: 14.0% or more and 17.0% or less, Mo: more than 3.80% and 6.0% or less, Cu: more than 1.03% and 3.5% or less, Ni: 3.5% or more and 6.0% or less, Al: 0.10% or less, N: 0.10% or less, and O: 0.010% or less, wherein C, Si, Mn, Cr, Ni, Mo, Cu, and N satisfy the following formula (1), and the remainder is Fe and impurities. Incidental, seamless stainless steel pipe having a microstructure containing at least 40% martensitic phase, at most 60% ferrite phase, and at most 30% austenite phase retained by volume, seamless stainless steel pipe having a yield strength of 862 MPa or more. Formula (1) 13.0 < -5.9 x (7.82 + 27C - 0.91Si + 0.21Mn - 0.9Cr + Ni - 1.1Mo + 0.2Cu + 11N) <50.0, 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) for elements that are not contained. The reasons for specifying the composition of a seamless steel pipe of the present invention are described below. In what follows, % means percentage by mass, unless specifically stated otherwise. C: 0.06% or less Carbon (C) 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. Considering the cost of decarburization, the lower limit for the carbon content is preferably 0.002%, 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 is present in amounts exceeding 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. Mn: 0.01% or more and 1.0% or less Manganese (Mn) acts as a deoxidizing and desulfurizing agent, improving hot workability. Mn also increases the strength of steel. To achieve these effects, Mn is present at a concentration of 0.01% or higher. Toughness decreases when the Mn content exceeds 1.0%. Therefore, the Mn content is typically between 0.01% and 1.0%. The Mn content is preferably 0.03% or higher, and more preferably 0.05% or higher. The Mn content is also preferably 0.8% or lower, and more preferably 0.6% or lower. P: 0.05% or less Phosphorus (P) is an element that impairs corrosion resistance, including resistance to carbon dioxide corrosion and sulfide stress cracking. Therefore, P is preferably contained in as small an amount as possible 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, and 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 pipe 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: 14.0% or more and 17.0% or less Chromium (Cr) is an element that forms a protective layer on the surface of steel pipes and contributes to improved corrosion resistance. The desired corrosion resistance cannot be guaranteed when the Cr content is less than 14.0%. For this reason, the Cr content must be 14.0% or higher. When it exceeds 17.0%, the ferrite fraction becomes too high, and the desired resistance cannot be guaranteed. Therefore, the Cr content is 14.0% or higher and 17.0% or lower. The Cr content is preferably 14.2% or higher, and more preferably 14.5% or higher. The Cr content is preferably 16.3% or lower, and more preferably 16.0% or lower. Mo: More than 3.80% and 6.0% or less By stabilizing the protective coating on the surface of the steel pipe, Mo increases resistance to pitting corrosion due to Cl· and low pH, and improves resistance to sulfide stress cracking and sulfide stress corrosion cracking. This makes Mo an important element in the present invention. The Mo content must be greater than 3.80% to obtain the desired corrosion resistance. A Mo content greater than 6.0% results in a decrease in low-temperature toughness. For this reason, the Mo content is greater than 3.80% and less than 6.0%. The Mo content is preferably 3.85% or more, more preferably 3.90% or more. The Mo content is preferably 5.8% or less, more preferably 5.5% or less. Cu: More than 1.03% 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 toughness. Cu also reduces hydrogen ingress into the steel by strengthening the protective coating on the surface of the steel pipe, and improves resistance to sulfide stress cracking and sulfide stress corrosion cracking. A Cu content greater than 1.03% is required to achieve the desired strength and corrosion resistance. Excessively high Cu content leads to decreased hot malleability of the steel, and the Cu content should be 3.5% or less. For this reason, the Cu content is above 1.03% and below 3.5%. The Cu content is preferably 1.2% or more, more preferably 1.5% or more. The Cu content is preferably 3.2% or less, more preferably 3.0% or less. nocLLn / Lznz / E / Yii Ni: 3.5% or more and 6.0% or less Nickel (Ni) is an element that reinforces the protective coating on the surface of steel pipes and contributes to improved corrosion resistance. By strengthening the solid solution, Ni also increases the strength of the steel and improves its toughness. These effects are accentuated when the Ni content is 3.5% or higher. 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 3.5% or higher and 6.0% or lower. The Ni content is preferably 4.0% or higher, more preferably 4.5% or higher. The Ni content is preferably 5.8% or lower, more preferably 5.5% or lower. Al: 0.10% or less Aluminum (Al) acts as a deoxidizing agent. However, low-temperature toughness 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. No lower limit is necessary as long as 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. Nickel (Ni) is also an element that increases the strength of steel. However, when present in amounts greater than 0.10%, N forms nitrides and decreases toughness. 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% results in a decrease in hot malleability, corrosion resistance, and toughness. Therefore, the oxygen content is 0.010% 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. Formula (1) nocL Ln / Lznz / B / Yi 13.0 < -5.9 χ (7.82 + 27C - 0.91 Si + 0.21 Μη - 0.9Cr + Ni - 1.1Mo + 0.2Cu + 11N) < 50.0 In the formula, C, Si, Mn, Cr, Ni, Mo, Cu and N represent the content of each element in % by mass. 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 appearing in formula (1) is not present, the value of the mean polynomial of formula (1) is calculated considering the content of that element to be zero percent. When the average polynomial value in formula (1) is less than 13.0, the ferrite phase decreases, and defects and cracks are more likely to occur during hot working. Conversely, when the average polynomial value in formula (1) is greater than 50.0, the ferrite phase exceeds 60% by volume, and the desired strength cannot be guaranteed. 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 50.0 as the upper limit. In the present invention, the balance 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 (W, Nb, V, B, Ti, Zr, Co, Ta, Ca, REM, Mg, Sn, Sb), as indicated below. Specifically, in the present invention, the composition may additionally contain W: 0.84% ​​or less. In the present invention, the composition may additionally contain one or two or more selected from Nb: 0.5% or less, V: 0.5% or less, and B: 0.01% or less. In the present invention, the composition may additionally contain one or two or more selected from Ti: 0.3% or less, Zr: 0.3% or less, Co: 1.5% or less, and Ta: 0.3% or less. In the present invention, the composition may additionally contain one or two 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. W: 0.84% ​​or less Water (W) is an element that contributes to improving the strength of steel and can increase resistance to sulfide stress cracking and sulfide stress corrosion cracking by stabilizing the protective coating on the surface of the steel pipe. In particular, W improves resistance to sulfide stress cracking when contained with molybdenum (Mo). When present in excessively high amounts, W precipitates into an intermetallic compound and impairs low-temperature toughness. For this reason, W, when included, is present at a concentration of 0.84% ​​or less. The W content is preferably 0.001% or more, more preferably 0.005% or more. The W content is preferably 0.7% or less, more preferably 0.6% or less. Nb: 0.5% or less Nitrogen (Nb) is a strength-enhancing element and can be added as needed. An Nb content above 0.5% reduces toughness and sulfide stress crack resistance. For this reason, when Nb is present, it is at 0.5% or less. The Nb content is preferably 0.4% or less, more preferably 0.3% or less. The Nb content is preferably 0.02% or more, more preferably 0.05% or more. V: 0.5% or less Volatile (V) is a strength-enhancing element and can be added as needed. A V content above 0.5% reduces toughness and sulfide stress crack resistance. For this reason, when V is present, it is at 0.5% or less. The V content is preferably 0.4% or less, more preferably 0.3% or less. The V content is preferably 0.02% or more, more preferably 0.05% or more. B: 0.01% or less Boron (B) is a strength-enhancing element that can be added as needed. B also contributes to improved hot malleability and reduces fracture and cracking during the tube manufacturing process. However, a B content above 0.01% has little effect on improving hot malleability and leads to a decrease in low-temperature toughness. 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. Ti: 0.3% or less Titanium (Ti) is a strength-enhancing element that can be added as needed. 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 toughness. Therefore, when present, Ti is limited to 0.3% or less. nocL Ln / Lznz / B / Yi Zr: 0.3% or less Zinc (Zr) is a strength-enhancing element that can be added as needed. 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. A Zr content above 0.3% decreases toughness. For this reason, when Zr is present, it is limited to 0.3% or less. Co: 1.5% or less Cobalt (Co) is a strength-enhancing element and can be added as needed. In addition to this effect, Co also improves resistance to sulfide stress cracking. To achieve these effects, Co is preferably contained in an amount of 0.0005% or more. A Co content above 1.5% decreases toughness. For this reason, when Co is present, it is limited to 1.5% or less. Ta: 0.3% or less Ta is a strength-enhancing element that can be added as needed. In addition to this effect, Ta also improves resistance to sulfide stress cracking. To achieve these effects, Ta is preferably present at a concentration of 0.0005% or more. A Ta content above 0.3% decreases toughness. For this reason, when Ta is present, it is limited to 0.3% or less. Ca: 0.01% or less Calcium (Ca) is an element that contributes to improving the resistance to stress corrosion cracking of sulfides by controlling the sulfide's shape, and it can be added as needed. To achieve this effect, Ca is preferably present in an amount of 0.0005% or more. When Ca is present in an amount greater than 0.01%, the effect becomes saturated, and the Ca cannot produce the desired effect from the increased content. For this reason, when Ca is present, it is in a limited amount of 0.01% or less. REM: 0.3% or less REM is an element that contributes to improving the resistance to stress corrosion cracking of sulfides by controlling the sulfide's shape, and it can be added as needed. 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 desired effect from the increased content. For this reason, when REM is included, it is in a limited amount of 0.3% or less. nocL Ln / Lznz / B / Yi In this document, REM refers to scandium (Se; atomic number 21) and triium (Y; atomic number 39), as well as the lanthanides from lanthanum (La; atomic number 57) to lutetium (Lu; atomic number 71). As used herein, REM concentration refers to the total content of one or two or more elements selected from the aforementioned REM elements. Mg: 0.01% or less Magnesium (Mg) is an element that improves corrosion resistance and can be added as needed. To achieve this effect, Mg is preferably present at a concentration of 0.0005% or more. When Mg is present at a concentration greater than 0.01%, the effect becomes saturated, and the increased concentration will no longer produce the desired effect. For this reason, when Mg is present, it is limited to a concentration of 0.01% or less. Sn: 0.2% or less Tin (Sn) is an element that improves corrosion resistance and can be added as needed. To achieve this effect, Sn is preferably present in an amount of 0.001% or more. When Sn is present in an amount greater than 0.2%, the effect becomes saturated, and the Sn cannot produce the desired effect from the increased content. For this reason, when Sn is included, it is in a limited amount of 0.2% or less. Sb: 1.0% or less Antimony (Sb) is an element that improves corrosion resistance and can be added as needed. To achieve this effect, Sb is preferably present at a concentration of 0.001% or more. When Sb is present at a concentration greater than 1.0%, the effect becomes saturated, and the increased concentration will no longer produce the desired effect. For this reason, when Sb is present, it is limited to a concentration of 1.0% or less. The reason for limiting the microstructure in the seamless steel pipe of the present invention is described below. In addition to having the above composition, the seamless steel pipe of the present invention has a microstructure containing at least 40% martensitic phase, at most 60% ferrite phase, and at most 30% volume-retained austenite phase. To ensure the desired strength, the seamless steel pipe of the present invention contains at least 40% martensitic phase by volume. In the present invention, the ferrite phase is at most 60% by volume. With the ferrite phase, the progression of sulfide stress corrosion cracking and sulfide stress cracking can be reduced, and excellent corrosion resistance is achieved. If the ferrite phase precipitates in a large amount exceeding 60% by volume, it may not be possible to guarantee the desired strength. The ferrite phase is preferably 5% or more by volume. The ferrite phase is preferably 50% or less by volume. The seamless steel tube of the present invention contains a maximum of 30% austenitic 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 austenitic phase. If the austenitic phase precipitates in a large quantity, exceeding 30% by volume, the desired strength cannot be guaranteed. For this reason, the retained austenite phase is 30% or less by volume. The retained austenite phase is preferably 5% or more by volume. The retained austenite phase is 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 a Vilella 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 (1000x magnification). The ferrite phase fraction of the microstructure (volume ratio (%)) is then calculated using an image analyzer. Separately, an X-ray diffraction test specimen is sculpted and polished to have a measurement cross-section (C-section) orthogonal to the axial direction of the pipe, and the fraction of the retained austenite phase microstructure (γ) (volume ratio (%)) is measured using 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 (γ), and the (211) plane of the ferrite phase (a), and converting the calculated values ​​using the following formula. γ (volume ratio) = 100 / (1 + (laRy / lyRa)), where la is the integral intensity of a, Ra is the crystallographic theoretical value of α, Iγ is the integral intensity of γ, and Ry is the crystallographic theoretical value of γ. The martensitic phase fraction is the remainder other than the ferrite phase and retained phase fractions determined by the previous measurement method. As used here, the martensitic phase may contain at most 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 pipe of the present invention. Preferably, molten steel of the above composition is converted into steel by a smelting process, for example, using a converter, and formed into steel pipe material, for example, an ingot, using a conventional method such as continuous casting or ingot casting. The steel pipe material is then hot-worked to form a tube by a known tube-making process, for example, the Mannesmann plug mill process or the Mannesmann mandrel rolling process, to produce a seamless steel pipe of the desired dimensions having the above composition. The hot working may be followed by quenching. The quenching process is not particularly restricted.After hot working, the pipe 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 quenching and tempering. During quenching, the steel pipe is reheated to a temperature of 850 to 1,150 °C and then cooled at an air-cooling 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, the reverse transformation from martensite to austenite does not occur, and the austenite does not transform into martensite during quenching, thus compromising the desired strength. Furthermore, the crystal grains thicken when the heating temperature exceeds 1,150 °C. For this reason, the quenching heating temperature is 850 to 1,150 °C. The quenching heating temperature is preferably 900 °C or higher. The quenching heating temperature is preferably 1,100 °C or lower. When the cooling 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, the cooling stop temperature in the present invention is 50°C or lower. Here, the air cooling rate or faster means 0.01 °C / s more. During cooling, the soaking time is preferably 5 to 30 minutes, to achieve a uniform temperature along the direction of the wall thickness, and to avoid variation in the material. In quenching, the quenched seamless steel pipe is heated to a quenching temperature of 500 to 650 °C. This heating may be followed by natural cooling. A quenching temperature below 500 °C is too low to produce the desired quenching effect. When the quenching temperature exceeds 650 °C, intermetallic compounds precipitate, and it is not possible to achieve the desired hardness at a lower temperature. For this reason, the quenching temperature is between 500 and 650 °C. A quenching temperature of 520 °C or higher is preferred. A quenching temperature of 630 °C or lower is also preferred. In tempering, the holding time is preferably from 5 to 90 minutes, to achieve a uniform temperature along the direction of the wall thickness, and to avoid variation of the material. After heat treatment (quenching and tempering), seamless steel pipe has a microstructure in which the martensitic, ferrite, and retained austenite phases are contained in a predetermined specific volume ratio. This allows seamless stainless steel pipe to have the desired strength and toughness, as well as excellent corrosion resistance. The seamless stainless steel pipe obtained in the present invention as described above is a high-strength steel pipe having a yield strength of 862 MPa or higher, and excellent low-temperature toughness and excellent corrosion resistance. Preferably, the yield strength is 1,034 MPa or less. The seamless stainless steel pipe of the present invention can be used as a seamless stainless steel pipe for tubular products in petroleum-producing countries (a high-strength seamless stainless steel pipe for tubular products in petroleum-producing countries). Examples The present invention is described in more detail below through the Examples. Molten steels of the compositions indicated in Tables 1-1 and 1-2 were converted into steel by a converter and cast into ingots (steel pipe stock) by continuous casting. The steel pipe stock was heated, hot-worked to form a seamless pipe using a rolling mill pattern, and air-cooled to produce a seamless steel pipe with an outside diameter of 83.8 mm and a wall thickness of 12.7 mm. The heating of the steel pipe stock prior to hot-working was carried out at a heating temperature of 1,250 °C. Each seamless steel pipe was cut from a test specimen, which was then subjected to quenching processes that included reheating to 960 °C and quenching (water quenching) to a quenching stop temperature of 30 °C with a 20-minute soak. This was followed by tempering, which included heating to 575 °C and air quenching with a 20-minute soak. During quenching, the water quenching was performed at a rate of 11 °C / s. The air quenching (natural cooling) during tempering was performed at a rate of 0.04 °C / s. Steel was evaluated as Compliant when it met formula (1), and Did Not Comply when it did not meet formula (1), as shown in Tables 1-1 and 1-2. nocL Ln / Lznz / B / Yi Table 1-1 01 cu § «5 en 8 | Maple present | Maple preseas | Maple present | Maple present | Maple present | Maple present | Maple present | Maple present | Maple present | Maple present | Maple present | | v^ísssjcI ojssy Maple present | Maple present | £ Φ U1 2 Φ £J < Maple present-1 Maple present | Maple present | Maple present 1 Maple present | I Maple present · 1 | e^essjd oteoy rti Φ ω 2 o SA rti 03 40 Jü a S rj Maple present 1 Maple present | Maple present | Maple present | Maple present | Maple present | Maple present | rti £ 03 40 ω fe 0 1 | Maple present-1 Maple present | Maple present | Maple present | s ü ^j—η Q d 43 oc -Q •Q <5 y—, •0 O dd <3 •0 0 dd •o -Ti -ó •0 <3 -íX •o •Q 0 -N) d •Q -© -O tt¡ x 1 uu w. Gl U- a * U u 'l M. ua ui Mr íT* u M. 0. M. u Q. U 0. uu 0- ·“* u M. 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"í1 Ó IO OI K? in w O OI in ID Ιό Oí lO O § S eó with CN O-.J CN ÓJ en ΓΝ f 1 «J o ώ εα oí 05 eo oí 00 s í Xt NT XI1 N xf -d- xf O· Οι ΙΠ Qi rsi ¡Ó FN 1 - ifj o) 01 es tí A·S x uS KJ 0M .lí eM a, O oo OO o O. 01 OU 03 oos CS O o OO o «3 xf· id *r *rm ΙΟ X <05 o O © 05 xr 03 03 s ¢5 o O ¢5 ¢5 d 03 t CO1 til CO1 γ*ν γΊ tN r*) tN oj esj Γ0 <5 <5 í? is d t.3 e;i 0¡ or 04 rx un ) CN ¢5 « « t5 is rM O d 6 d 03 ¢5 O ¢5 0 Φ s with T ... | | nocLLn / Lznz / E / Yi (*1) The rest is Fe and incidental impurities (*3) Formula (1): 13.0 < -5.9x(7.82+27C-0.91 Si+0.21 Mn-0.9Cr+N¡-1.1 Mo+10.2Culor)(501.2Cu -5.9x(7.82+27C-0.91 Si+0.21 Mn-0.9Cr+N¡-1.1 Mo+0.2Cu+11N) Tables 1 -2 Observations Comparative Steel | p § § Comparative Steel 1 Comparative Steel | I Comparative Steel | O 3 !£ I uo | Comparative Steel | Comparative Steel 1 Comparative Steel | 1 Comparative Steel | Comparative Steel I 1 Comparative Steel | I Comparative Steel 1 | Comparative Steel | I Comparative Steel | I Comparative Steel I | Comparative Steel 1 o :S £ ! 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HI 826'0 I en cS 1 (Wl-Ό 1 1 0.900 1 | G.350 | I 0.G83 II S6P0 | I 973 0 I ¿ó fM 105. | di <3 ep 1 61Ό M tp « © 8 © I ΦΖ0 l sio Ri o 1 ISO 1 880 ¢5 n) Q 0.20 I ga 0.23 | u | 0.0650. | 1 ¿Elü'O 1 I sóido I | Ú.0ÚS1 | | 0.0194 | | 0.0157 | | 0 0153 | ra ooo | 0.0163 | IS£l00 II 0.0190 I | 0.0184 | I 0.0162 II 0800'0 | I «ΌΙΰ'Ο I 1 601-0'0 1 a tO OI 0.0535 IO oo I 0.0165 I | 0.0189 | G¡ rX OO .§3 δ z S AK 5 < A? 3 tn 5 c % $ $ y S cü m S LLi ω fe. nocLLn / Lznz / E / Yi (*1) The rest is Fe and incidental impurezas (*2) The subrayado means outside the rank of the present invention (*3) Formula (1): 13.0 < -5.9x(7.82+27C-0.91Si+0.21Mn-0.9Cr+Ni-1.1Mo+0.2Cu+11N) < 50.0 (*4) Value: -5.9x(7.82+27C-0.91 Si+0.21 Mn-0.9Cr+N¡-1.1 Mo+0.2Cu+11N) A test sample was taken from the heat-treated test material (seamless steel tube) and subjected to microstructure observation, a tensile test, an impact 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 from the heat-treated test material, oriented so that the observed cross-section was along the pipe axis. 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 visualized using a scanning electron microscope (at 1000x magnification). The percentage area of ​​the ferrite phase microstructure was then calculated using an image analyzer to determine the volume fraction (% by volume). Separately, an X-ray diffraction test sample was taken from the heat-treated test material. The test sample was ground and polished to have a measurement cross-section (cross-section C) orthogonal to the axial direction of the tube, and the fraction of the retained austenite phase microstructure (y) was measured using 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 (γ) and the (211) plane of the ferrite phase (a), and converting the calculated values ​​using the following formula. γ (volume ratio) = 100 / (1 + (laRy / lyRa)), where la is the integral intensity of a, Ra is the crystallographic theoretical value of α, 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 fraction and the retained phase y. (2) Traction test An API (American Petroleum Institute) arc tensile test specimen was taken from the heat-treated test material, oriented so 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, tensile strength TS). High-strength steel was determined to be acceptable when it had a yield strength YS of 862 MPa or higher, and unacceptable when it had a yield strength YS of less than 862 MPa. (3) Impact test A Charpy impact test was performed according to JIS Z 2242 specifications using a V-notched test specimen (10 mm thick) taken from the heat-treated nocL Ln / Lznz / B / Yi test material. The specimen was oriented longitudinally along the pipe axis. The test was conducted over a temperature range of 50 °C to -120 °C, and the absorption energy at -10 °C (vE-10) and the ductile-brittle transition temperature were determined to assess low-temperature toughness. Each test was performed on three test specimens, and the arithmetic mean of the measured values ​​was calculated as the absorption energy (J) of the steel pipe. Steel was determined to be acceptable when it had an absorption energy at -10 °C, vEio, of 300 J or more, and a ductile-brittle transition temperature of -40 °C or less, and unacceptable when it did not meet either of these conditions. (4) Corrosion resistance test A corrosion test sample 3 mm thick, 30 mm wide and 40 mm long was prepared from the heat-treated test material by machining, and subjected to a corrosion test to evaluate resistance to carbon dioxide corrosion. The corrosion test was performed by immersing the test specimen in a test solution: a 20% by mass aqueous NaCl solution (liquid temperature: 200 °C; CO2 gas atmosphere: 30 atm) in an autoclave for 14 days (336 hours). The corrosion rate was determined from the calculated reduction in the weight of the tested sample, measured before and after the corrosion test. Steel was considered acceptable when its corrosion rate was less than 0.127 mm / y and unacceptable when it was greater than 0.127 mm / y. A test specimen in the form of a round rod (diameter: 6.4 mm) was prepared from the test specimen material by machining according to NACE TM0177, method A, and was subjected to a sulfide stress cracking resistance test (SSC strength test). The CSE resistance test was performed by immersing a 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 3.0 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 sample was observed. The steel was determined to be acceptable when it had no cracks, and unacceptable when cracks were present. From the test sample material, a four-point bending specimen 3 mm thick, 15 mm wide and 115 mm long was extracted by machining and subjected to a sulfide stress cracking resistance test (sulfide corrosion resistance test) in accordance with the EFC (European Corrosion Federation) 17. The SCC resistance test was performed by immersing a test specimen in a test solution: a 20% by mass aqueous NaCl solution (liquid temperature: 100 °C; an atmosphere of 0.1 atm of H₂S and 30 atm of CO₂) maintained in an autoclave and with a pH adjusted to 3.3 by the addition of acetic acid and sodium acetate. A stress equal to 100% of the yield strength was applied for 720 hours in the solution. The presence or absence of cracks in the tested sample was observed. Steel was considered acceptable when it had no cracks, and unacceptable when cracks were present. The results are presented in Table 2. Table 2-1 nocLLn / Lznz / E / Yii Number of steel pipes Number of steel pipes WiCTüstrutura í% in volume Tensile strength YS iWa) vE.íü U) Transition temperature ΓO Corrosion index SSC scc Observations M ΓΌ F in i'1; A 1 63 24 13 953 323 -50 0011 Acceptable Acceptable Present Example B 2 7Q 20 10 966 311 -45 0.012 Acceptable Acceptable Present Example C 3 70 1® 11 959 322 -50 Acceptable Acceptable Present Example D 4 63 14 23 902 329 -55 0.065 Acceptable Acceptable Present Example E 5 42 40 18 888 333 -55 0.055 Acceptable Acceptable Present Example F 6 66 19 15 977 307 -45 9.019 Acceptable Acceptable Present Example G 7 63 28 9 878 339 0.015 Acceptable Acceptable Present Example H 8 60 36 10 940 31S -45 0.055 Acceptable Acceptable Present Example 1 9 63 28 9 948 329 -50 0.058 Acceptable Acceptable Present Example J 10 45 40 15 388 319 -50 0.006 Acceptable Acceptable Present Example K 11 73 16 11 971 308 -45 0.030 Acceptable Acceptable Present Example L 12 45 40 15 887 309 -45 0.008 Acceptable Acceptable Present Example 13 64 24 12 959 311 -45 0.046 Acceptable Acceptable Present Example N 14 62 23 15 921 317 -50 0.010 Acceptable Acceptable Present Example 0 15 60 32 S 379 331 -55 0.021 Acceptable Acceptable Present Example P 16 51 24 25 875 350 -65 0.019 Acceptable Acceptable Present Example Q 17 52 37 11 869 353 -65 0.071 Acceptable Acceptable Present Example R 18 64 24 12 953 305 -42 0.017 Acceptable Acceptable Present Example s 19 55 15 77 873 31O -45 0 015 Acceptable Acceptable Present Example T 2S 67 23 10 953 308 -45 0.060 Acceptable Acceptable Present Example u 21 49 49 2 866 309 -45 0.017 Acceptable Acceptable Present Example V 22 6S 2 29 875 339 -60 0.025 Acceptable Acceptable Present Example w 23 63 22 15 893 311 -45 0.022 Acceptable Acceptable Present Example X 24 68 20 11 931 307 -45 0.009 Acceptable Acceptable Present Example Y 25 64 24 12 945 320 -50 0.011 Acceptable Acceptable Present Example z 25 54 33 13 887 309 -45 0.016 Acceptable Acceptable Example present AA 27 60 30 10 965 318 -45 0.013 Acceptable Acceptable Example present AB 28 57 31 12 937 318 -50 0.020 Acceptable Acceptable Example present AC 29 64 24 12 953 305 -42 0.017 Acceptable Acceptable Example present AD 39 72 15 13 969 307 -42 0.016 Acceptable Acceptable Example present AE 31 57 27 16 943 315 -45 0.019 Acceptable Acceptable Example present AF 32 56 28 16 934 320 -50 0.018 Acceptable Acceptable Example present AG 33 18 11 953 309 -45 0.022 Acceptable Acceptable Present Example AH 34 50 32 18 910 322 -50 0.025 Acceptable Acceptable Present Example Al 35 45 40 15 930 322 -50 0.011 Acceptable Acceptable Present Example AJ 36 64 24 12 957 309 -45 0.016 Acceptable Acceptable Present Example 8G 59 51 34 15 943 320 -50 0.011 Acceptable Acceptable Present Example BH 59 64 28 8 963 311 -45 0.01Ü Acceptable Acceptable Present Example 81 61 54 36 10 944 309 -42 0.013 Acceptable Acceptable Present Example 84 62 47 31 22 909 333 -55 0.025 Acceptable Acceptable Present Example BK 63 50 30 20 934 328 -45 0.011 Acceptable Acceptable Present Example BL 64 62 27 11 955 315 -45 0.016 Acceptable Acceptable Present Example BM 65 61 26 13 971 331 -60 9.019 Acceptable Acceptable Present Example EN 66 40 33 19 888 329 -60 0.006 Acceptable Acceptable Present Example BO 67 59 32 9 943 302 -42 0.036 Acceptable Acceptable Present Example. (*1) M: quenched martensitic phase, F: ferrite phase, A: retained austenite phase [Table 2-2] N™. of steel Njh; of steel pipe Microstructure in volume) Resists FI'CIS 3 the tensile YS (MPa) vE^o U) Transition temperature Γθ) index ele CtHlOSSÓfl cmnVyj ssc scc Observations ft} F Π) rn AK 37 57 14 29 866 334 -55 0.135 Not acceptable Not acceptable Comparative example AL 38 55 24 21 86S 396 -45 0.138 Not acceptable Not acceptable Comparative example AM ​​39 46 34 28 1SS2 288 -35 9.019 Acceptable Acceptable Comparative example AN 4Q 67 27 5 848 345 0 016 Acceptable Acceptable Comparative example AO 41 53 26 11 S5S 3£¡9 -45 0.137 Not acceptable Not acceptable Comparative example AP 42 62 28 1S 945 322 -59 0.140 Not acceptable Not acceptable Comparative example AQ 43 38 48 22 848 315 -45 0 010 Acceptable Acceptable Comparative example AR 44 7S ^2 9 S9® 303 -42: 0.153 Not acceptable Not acceptable Comparative example AS 45 44 38 38 889 253 -29 0 614 Acceptable Acceptable Comparative example AT 46 es 24 13 966 31S -45 0.138 Not acceptable Not acceptable Comparative example AU 47 7δ 23 7 B6C 34S -65 0.147 Not acceptable Not acceptable Comparative example AV 43 3S 43 840 353 -79 9.020 Acceptable Acceptable Comparative example AW 49 49 44 ? 847 321 -59 0 089 Acceptable Acceptable Comparative Example AX 56 54 33 13 S47 286 -3B 0016 Acceptable Acceptable Comparative Example AY 51 43 22 29 &65 271 -25 0.015 Acceptable Acceptable Comparative Example AZ 52 53 31 11 944 276 -39 0.131 Not Acceptable Not Acceptable Comparative Example 8A 5.3 32 61 7 897 315 ​​-45 0018 Acceptable Acceptable Comparative Example 80 54 52 5 33 858 359 -65 0.031 Acceptable Acceptable Comparative Example SC 55 87 20 13 SIS 269 -25 0.008 Acceptable Acceptable Example Comparative 80 56 58 32 1>3 9S9 69 or 0.02.2 Not acceptable Acceptable Comparative example 8E 57 46 39 15 SS7 78 to 0.009 Not acceptable Acceptable Comparative example BF 58 53 34 13 932 SI -IB 0.011 Acspisbís· Acceptable Comparative example. (*1) M: quenched martensitic phase, F: ferrite phase, A: retained austenite phase (*2) The underline means outside the scope of the present invention nocL Ln / Lznz / B / Yi All the seamless stainless steel pipes in the present examples had high strength with a yield strength (YS) of 862 MPa or more, and high toughness with an absorption energy at -10 °C of 300 J or more, and a ductile-brittle transition temperature of -40 °C or less. The seamless stainless steel pipes in the present examples also had excellent corrosion resistance (carbon dioxide corrosion resistance) in a high-temperature corrosive environment of 200 °C containing CO2 and Cl, and excellent sulfide stress cracking resistance, as evidenced by the absence of cracking (SSC and SCC) in an environment containing H2S.< / s>

Claims

1. Seamless stainless steel pipe having a composition comprising, by mass %, C: 0.06% or less, Si: 1.0% or less, Mn: 0.01% or more and 1.0% or less, P: 0.05% or less, S: 0.005% or less, Cr: 14.0% or more and 17.0% or less, Mo: more than 3.80% and 6.0% or less, Cu: more than 1.03% and 3.5% or less, Ni: 3.5% or more and 6.0% or less, Al: 0.10% or less, N: 0.10% or less, and O: 0.010% 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, the seamless stainless steel pipe having a microstructure containing at least 40% martensitic phase, at most 60% ferrite phase and at most 30% austenite phase retained by volume, seamless stainless steel pipe having a yield strength of 862 MPa or more, Formula (1) 13.0 < -5.9 x (7.82 + 27C - 0.91 Si + 0.21 Mn - 0.9Cr + Ni - 1.1Mo + 0.2Cu + 11N) < 50.0, where C, Si, Mn, Cr, Ni, Mo, Cu and N represent the content of each element in % by mass.

2. The seamless stainless steel pipe according to claim 1, wherein the composition further comprises, in % by mass, W: 0.84% ​​or less.

3. The seamless stainless steel pipe according to claim 1 or 2, wherein the composition further comprises, by mass %, one or two or more selected from Nb: 0.5% or less, V: 0.5% or less, and B: 0.01% or less.

4. The seamless stainless steel pipe according to any of claims 1 to 3, wherein the composition further comprises, by mass %, one or two or more selected from Ti: 0.3% or less, Zr: 0.3% or less, Co: 1.5% or less, and Ta: 0.3% or less.

5. The seamless stainless steel pipe according to any of claims 1 to 4, wherein the composition further 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.