Title - A SEAMLESS STAINLESS STEEL PIPE SUITABLE FOR USE IN OIL AND GAS WELLS, AND A METHOD FOR MANUFACTURING SAID SEAMLESS STAINLESS STEEL PIPE

Abstract

A seamless stainless steel tube and a method for manufacturing it are provided. A seamless stainless steel tube of the present invention having a component composition containing, in terms of % by mass, C: 0.06% or less, Si: 1.0% or less, Mn: 0.01% or more and 0.90% or less, P: 0.05% or less, S: 0.005% or less, Cr: 15.70% or more and 18.00% or less, Mo: 1.60% or more and 3.80% or less, Cu: 1.10% or more and 4.00% or less, Ni: 3.0% or more and 6.0% or less, Al: 0.10% or less, N: 0.10% or less, O: 0.010% or less, V: 0.120% or more and 1.000% or less, C, Si, Mn, Cr, Ni, Mo, Cu and N conform to a predetermined formula, and a remainder consisting of Fe and unavoidable impurities, wherein the seamless stainless steel tube has a microstructure that includes, in terms of volume fraction, 30% or more of a martensitic phase, 60% or less of a ferritic phase and 40% or less of a retained austenitic phase, and a yield strength of 758 MPa or more.

Description

SEAMLESS STAINLESS STEEL TUBE AND ITS MANUFACTURING METHOD Technical Field The present invention relates to a seamless stainless steel tube suitable for use in oil and gas wells (hereinafter referred to simply as oil wells). The present invention relates in particular to a seamless stainless steel tube having improved corrosion resistance in highly corrosive, high-temperature environments containing carbon dioxide (CO2) or chlorine ions (Cl-), environments containing hydrogen sulfide (H2S), and the like, and having improved high-temperature resistance. Background In recent years, given the anticipated depletion of energy resources in the near future, the development of oil wells in highly corrosive environments is actively underway. These environments include deep oil wells, carbon dioxide-containing environments, and hydrogen sulfide-containing environments, which are known as acidic environments and have not been previously addressed in the relevant technology. Steel pipes used in such environments must possess high toughness and high corrosion resistance. In the related art, 13% Cr martensitic stainless steel tubes have normally been used as tubes 1 1428729 of 51 steel pipes for oil wells are used for extraction in oil and gas fields in environments containing CO2, Cl-, and similar compounds. However, recently, the development of oil wells operating at higher temperatures (up to 200°C) has begun, and there are instances where 13% Cr martensitic stainless steel pipes lack corrosion resistance. There is a demand for steel pipes for oil wells that have high corrosion resistance and can be used even in such environments. In response to this demand, there are, for example, techniques mentioned in Patent Bibliography 1 to 5. Patent Bibliography 1 describes a stainless steel for an oil well having a composition containing, in terms of % by mass, C: 0.05% or less, Si: 1.0% or less, Mn: between 0.01% and 1.0%, P: 0.05% or less, S: less than 0.002%, Cr: between 16% and 18%, Mo: between 1.8% and 3%, Cu: between 1.0% and 3.5%, Ni: between 3.0% and 5.5%, Co: between 0.01% and 1.0%, Al: between 0.001% and 0.1%, O: 0.05% or less, and N: 0.05% or less, wherein Cr, Ni, Mo, and Cu meet a specific ratio. Furthermore, Patent Bibliography 2 describes a high-strength stainless steel seamless pipe for an oil well having a composition containing, in terms of % by mass, C: 0.05% or less, Si: 1.0% or less, 2 1428729 of 51 Min: between 0.1% and 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: between 0.3% and 3.5%, Ni: 3.0% or more and less than 5.0%, W: between 0.1% and 3.0%, Nb: between 0.07% and 0.5%, V: between 0.01% and 0.5%, Al: between 0.001% and 0.1%, N: between 0.010% and 0.100%, and O: 0.01% or less, wherein Nb, Ta, C, N, and Cu meet a specific ratio and have a microstructure that includes, in terms of volume fraction, 45% or more of one phase tempered martensitic, between 20% and 40% ferritic phase, and more than 10% and 25% or less of retained austenitic phase. This is considered to make it possible to obtain a high-strength, seamless stainless steel pipe for an oil well that exhibits a yield strength (YS) of 862 MPa or more and sufficient corrosion resistance even in highly corrosive, high-temperature environments containing CO2, Cl-, and H2S. Furthermore, Patent Bibliography 3 describes a high-strength stainless steel seamless pipe for an oil well having a composition containing, in terms of % by mass, C: between 0.005% and 0.05%, Si: between 0.05% and 0.50%, Mn: between 0.20% and 1.80%, P: 0.030% or less, S: 0.005% or less, Cr: between 12.0% and 17.0%, Ni: between 4.0% and 7.0%, Mo: between 0.5% and 3.0%, Al: between 0.005% and 0.10%, V: between 0.005% and 0.20%, Co: between 0.01% and 1.0%, N: between 0.005% and 0.15%, and O: 0.010% or less, wherein Cr, Ni, Mo, 3 1428729 of 51 Cu, C, Si, Mn and N comply with a specific relationship. Furthermore, Patent Bibliography 4 describes a high-strength stainless steel seamless pipe for an oil well having a composition containing, in terms of mass percent, C: 0.05% or less, Si: 0.5% or less, Mn: between 0.15% and 1.0%, P: 0.030% or less, S: 0.005% or less, Cr: between 14.5% and 17.5%, Ni: between 3.0% and 6.0%, Mo: between 2.7% and 5.0%, Cu: between 0.3% and 4.0%, W: between 0.1% and 2.5%, V: between 0.02% and 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 a specific ratio and have a microstructure containing, in terms of volume fraction, more than 45% of a martensitic phase as the main phase, between 10% and 45% of a ferritic phase as a secondary phase, and 30% or less of a retained austenitic phase.This is considered to make it possible to obtain a high-strength stainless steel seamless pipe for an oil well that exhibits a strength of 862 MPa or more in terms of yield strength (YS) and sufficient corrosion resistance even in highly corrosive high-temperature environments containing CO2, Cl~ and H2S. Furthermore, Patent Bibliography 5 describes a high-strength stainless steel seamless pipe for an oil well having a composition containing, in terms of % by mass, C: 0.05% or less, Si: 0.5% or less, 4 1428729 of 51 Mn: between 0.15% and 1.0%, P: 0.030% or less, S: 0.005% or less, Cr: between 14.5% and 17.5%, Ni: between 3.0% and 6.0%, Mo: between 2.7% and 5.0%, Cu: between 0.3% and 4.0%, W: between 0.1% and 2.5%, V: between 0.02% and 0.20%, Al: 0.10% or less, N: 0.15% or less, and B: between 0.0005% and 0.0100%, where C, Si, Mn, Cr, Ni, Mo, Cu, N, and W meet a specific ratio and have a microstructure containing, in terms of volume fraction, more than 45% of a martensitic phase as the main phase, between 10% and 45% of a ferritic phase as a secondary phase, and 30% or less of a retained austenitic phase. This is considered to make it possible to obtain a high-strength, seamless stainless steel pipe for an oil well that exhibits a yield strength (YS) of 862 MPa or more and sufficient corrosion resistance even in highly corrosive, high-temperature environments containing CO2, Cl-, and H2S.List of references Patent bibliography. [PTL 1] Report of International Publication No. WO 2013 / 146046 [PTL 2] Report of International Publication No. WO 2017 / 138050 [PTL 3] Report of International Publication No. WO 2017 / 168874 [PTL 4] Report of International Publication No. WO 5 1428729 of 51 2018 / 020886 [PTL 5] Report of International Publication No. WO 2018 / 155041 Summary description of the invention Technical problem As described above, given the ongoing development of oil wells operating at higher temperatures, it is considered that, for steel pipes used in an oil well, there is a demand for high toughness, excellent resistance to carbon dioxide corrosion under highly corrosive, high-temperature environments containing CO2 and Cl⁻, and, in addition, excellent resistance to sulfide stress cracking (SSC resistance). Furthermore, when used at high temperatures, there is a case where steel pipes for an oil well are also required to have high-temperature resistance. Specifically, there is a case where the ratio between the yield strength (conventional yield strength of 0.2%) at a temperature of 200 °C and the yield strength (conventional yield strength of 0.2%) at room temperature is required to be 0.85 or greater. Patent Bibliography 1 to 5 discloses stainless steels that have improved corrosion resistance, 6 1428729 of 51 but there is one case where stainless steels are not satisfactory in terms of having all the high-temperature corrosion resistance, high sulfide stress cracking resistance and high elevated temperature strength. The present invention is intended to solve such a problem in the related art, and an object of the present invention is to provide a seamless stainless steel tube having a high yield strength of 758 MPa (110 ksi) or higher, excellent corrosion resistance, and excellent resistance to elevated temperatures, and a method for manufacturing the same. The expression "excellent corrosion resistance" mentioned herein refers to a case in which the seamless stainless steel tube has excellent resistance to carbon dioxide corrosion and excellent resistance to sulfide stress cracking. The expression "excellent resistance to carbon dioxide corrosion" mentioned herein refers to a case in which the corrosion rate is 0.127 mm / year or slower in a test in which a sample is immersed in a test solution maintained in an autoclave: 20% by mass aqueous NaCl solution (liquid temperature: 200 °C, CO2 gas atmosphere of 30 atm) for a time 7 1428729 of 51 immersion established in 336 hours. Furthermore, the expression "excellent resistance to sulfide stress cracking" mentioned herein refers to a case in which a sample does not break or crack after a test carried out by immersing the sample in an aqueous solution with a pH adjusted to 3.0 by adding acetic acid and sodium acetate to a test solution maintained in an autoclave: 0.165% by mass of aqueous NaCl solution (liquid temperature: 25°C, CO2 gas of 0.99 atm, H2S atmosphere of 0.01 atm) and exposing the sample for 720 hours in a state in which 90% of the yield strength is applied to the sample. Furthermore, the expression "excellent resistance to elevated temperatures" mentioned herein refers to a case in which the ratio between the yield strength (conventional yield strength of 0.2%) at a temperature of 200 °C and the yield strength (conventional yield strength of 0.2%) at room temperature is 0.85 or more after performing a tensile test based on JIS Z 2241 and an elevated temperature tensile test based on JIS G 0567. The method for each test established above is also described in detail in the examples mentioned below. Solution to the problem 1428729 of 51 In order to achieve the objective described above, the inventors hereof conducted intensive studies regarding a variety of factors affecting the high-temperature toughness and corrosion resistance of stainless steel. As a result, it was possible to obtain excellent high-temperature toughness by containing a predetermined or greater amount of vanadium. Furthermore, it was possible to obtain excellent corrosion resistance (excellent resistance to carbon dioxide corrosion and excellent resistance to sulfide stress cracking) by containing a predetermined or greater amount of Cr, Mo, and Cu, and by setting the Mn content at a certain amount or less. The present invention has been completed by carrying out further studies based on these findings. That is to say, the essence of the present invention is as follows. [1] A seamless stainless steel tube having a component composition containing, in terms of % by mass: C: 0.06% or less; Yes: 1.0% or less; Mn: 0.01% or more and 0.90% or less; P: 0.05% or less; S: 0.005% or less; Cr: 15.70% or more and 18.00% or less; 1428729 of 51 Mo: 1.60% or more and 3.80% or less; Cu: 1.10% or more and 4.00% or less; Nor: 3.0% or more and 6.0% or less; Al: 0.10% or less; N: 0.10% or less; O: 0.010% or less; V: 0.120% or more and 1.000% or less, C, Si, Mn, Cr, Ni, Mo, Cu, and N comply with Formula (1) shown below; and wherein the remainder consists of Fe and unavoidable impurities, wherein the seamless stainless steel tube has a microstructure that includes, in terms of volume fraction, 30% or more of a martensitic phase, 60% or less of a terrific phase, and 40% or less of a retained austenitic phase, and a yield strength of 758 MPa or more. 13.0 < -5.9 x (7.82 + 27 C - 0.91 Si + 0.21 Mn - 0.9 Cr + Ni - 1.1 Mo + 0.2 Cu + 11 N) < 50.0 ... (1) Here, each of C, Si, Mn, Cr, Ni, Mo, Cu and N is the content (% by mass) of each element and are considered zero in the case that they are not contained. [2] The seamless stainless steel tube according to section [1], wherein the volume fraction of phase 10 1428729 of 51 martensitic is 40% or more, the volume fraction of the retained austenitic phase is 30% or less and the yield strength is 862 MPa or more. [3] The seamless stainless steel tube according to section [1] or [2], in addition to the composition of components, in terms of % by mass, further contains one or two or more groups selected from the following Group A to Group D: Group A: W: 3.0% or less, Group B: Nb: less than 0.10% Group C: one or more selected from B: 0.010% or less, Ta: 0.3% or less, Co: 1.5% or less, Ti: 0.3% or less, and Zr: 0.3% or less, and Group D: one or more selected from Ca: 0.01% or less, REM (rare earth metals): 0.3% or less, Mg: 0.01% or less, Sn: 1.0% or less, and Sb: 1.0% or less. [4] A method for manufacturing the high-strength seamless steel tube in accordance with any one of sections [1] to [3], including the method: heating a steel pipe material having the composition of components to a temperature between 1100 °C and 1350 °C to form the steel pipe material into a seamless steel pipe by hot working; Subsequently, carry out a tempering treatment by reheating the seamless steel tube to a temperature of 11 1428729 of 51 within a heating temperature range of 850°C to 1150°C and cooling the seamless steel tube to a cooling stop temperature of 50°C or less at a cooling rate for air cooling or faster; and then, carrying out an annealing treatment by heating the seamless steel tube to a temperature within an annealing temperature range of 500°C to 650°C. Advantageous effects of the invention According to the present invention, it is possible to obtain a seamless stainless steel tube having a high yield strength of 758 MPa (110 ksi) or higher, excellent corrosion resistance, and excellent resistance to elevated temperatures, and a method for manufacturing the same. Description of the forms of implementation The present invention will now be described in detail. A seamless stainless steel tube of the present invention having a component composition containing, in terms of % by mass, C: 0.06% or less, Si: 1.0% or less, Mn: 0.01% or more and 0.90% or less, P: 0.05% or less, S: 0.005% or less, Cr: 15.70% or more and 18.00% or less, Mo: 1.60% or more and 3.80% or less, Cu: 1.10% or more and 4.00% or less, Ni: 3.0% or more and 6.0% or less, Al: 0.10% or less, N: 0.10% 12 1428729 of 51 or less, 0: 0.010% or less, V: 0.120% or more and 1.000% or less, C, Si, Mn, Cr, Ni, Mo, Cu and N comply with Formula (1) below, a remainder consisting of Fe and unavoidable impurities, wherein the seamless stainless steel tube has a microstructure that includes, in terms of volume fraction, 30% or more of a martensitic phase, 60% or less of a terrific phase and 40% or less of a retained austenitic phase, a yield strength of 758 MPa or more. 13.0 < -5.9 x (7.82 + 27 C - 0.91 Si + 0.21 Mn - 0.9 Cr + Ni - 1.1 Mo + 0.2 Cu + 11 N) < 50.0 ... (1) Here, each of C, Si, Mn, Cr, Ni, Mo, Cu and N is the content (% by mass) of each element and are considered zero in the case that they are not contained. First, the reasons for limiting the composition of components of the seamless stainless steel tube of the present invention will be described. Hereafter, unless otherwise specified, the expression % by mass will be expressed simply with the percent symbol (%). C: 0.06% or less Carbon is an element that is inevitably present in steelmaking processes. When it contains more than 0.06% C, corrosion resistance deteriorates. Therefore, 13 1428729 of 51 The C content is set at 0.06% or less. The C content is preferably 0.05% or less, more preferably 0.04% or less, and more preferably still 0.03% or less. When the cost of decarburization is taken into account, the lower limit of the C content is preferably 0.002% and more preferably 0.003% or more. Yes: 1.0% or less Silicon is an element that acts as a deoxidizing agent. However, when it contains more than 1.0% Si, hot workability and corrosion resistance deteriorate. Therefore, the Si content is set at 1.0% or less. The Si content is preferably 0.7% or less, more preferably 0.5% or less, and more preferably 0.4% or less. No lower limit is given, provided a deoxidizing effect can be 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 0.90% or less Manganese is an element that acts as a deoxidizing and desulfurizing agent and improves workability when heated. To achieve the deoxidizing and desulfurizing effects, and also to improve strength, the Mn content is set at 0.01% or more. On the other hand, 14 1428729 of 51 When it contains more than 0.90% Mn, the sulfide stress cracking resistance deteriorates. Therefore, the Mn content is set at 0.01% or more and 0.90% or less. The Mn content is preferably 0.03% or more and, more preferably, 0.05% or more. In addition, the Mn content is preferably 0.7% or less, more preferably 0.5% or less, and even more preferably 0.4% or less. P: 0.05% or less Phosphorus is an element that degrades resistance to carbon dioxide corrosion and resistance to sulfide stress cracking and, preferably, is reduced as much as possible in the present invention, but 0.05% or less of P is acceptable. Therefore, the P content is set at 0.05% or less. The P content is preferably 0.04% or less, more preferably 0.03% or less, and even more preferably 0.02% or less. S: 0.005% or less Sulfur is an element that significantly degrades hot workability and impairs stable performance in hot tube manufacturing processes. Furthermore, sulfur is present as a sulfide-based inclusion in steel and degrades its resistance to sulfide stress cracking. Therefore, sulfur is preferably reduced as much as possible, but it is 15 1428729 of 51 acceptable a content of 0.005% or less of S. Therefore, the S content is set at 0.005% or less. The S content is, preferably, 0.004% or less, more preferably 0.003% or less, and most preferably still 0.002% or less. Cr: 15.70% or more and 18.00% or less Chromium is an element that forms a protective film on the surface of steel pipes and contributes to improved corrosion resistance. When the Cr content is less than 15.70%, it is not possible to ensure the desired resistance to carbon dioxide corrosion and the desired resistance to sulfide stress cracking. Therefore, it is necessary that it contain 15.70% or more Cr. On the other hand, when it contains more than 18.00% Cr, the ferrite fraction increases excessively, making it impossible to ensure the desired resistance. Therefore, the Cr content is set at 15.70% or more and 18.00% or less. The Cr content is preferably 16.00% or more and, more preferably, 16.30% or more. Furthermore, the Cr content is preferably 17.50% or less and, more preferably, 17.00% or less. 1.60% or more and 3.80% or less Molybdenum stabilizes the protective film on the surface of the steel pipe, increases resistance to pitting corrosion caused by Cl~ or low pH, and improves resistance to carbon dioxide corrosion and 16 1428729 of 51 Sulfide stress cracking resistance. To obtain the desired corrosion resistance, a Mo content of 1.60% or more is required. On the other hand, when more than 3.80% Mo is added, the ferrite fraction increases excessively, making it impossible to ensure the desired resistance. Therefore, the Mo content is set at 1.60% or more and 3.80% or less. The Mo content is preferably 1.80% or more and, more preferably, 2.00% or more. In addition, the Mo content is preferably 3.5% or less, more preferably 3.0% or less, and even more preferably 2.8% or less. Cu: 1.10% or more and 4.00% or less Copper strengthens the protective film on the surface of steel tubes and improves their resistance to carbon dioxide corrosion and sulfide stress cracking. To achieve the desired toughness and corrosion resistance, particularly carbon dioxide corrosion resistance, a copper content of 1.10% or more is required. However, when the copper content is excessively high, the hot workability of the steel deteriorates, and therefore the copper content is set at 4.00% or less. The copper content is preferably 1.80% or more, and even more preferably 2.00% or more. Furthermore, the content 17 1428729 of 51 of Cu is, preferably, 3.20% or less, higher preference 3.00% or less, and even higher preference 2.7% or less. Nor: 3.0% or more and 6.0% or less Nickel increases the strength of steel by strengthening the solid solution and improving its toughness. To ensure the toughness required for oil well pipes, they must contain 3.0% or more Ni. On the other hand, when the Ni content exceeds 6.0%, the stability of the martensitic phase deteriorates, and the strength is reduced. Therefore, the Ni content is set at 3.0% or more and 6.0% or less. The Ni content is preferably 3.5% or more, more preferably 4.0% or more, and even more preferably 4.5% or more. Additionally, the Ni content is preferably 5.5% or less and more preferably 5.2% or less. Al: 0.10% or less Aluminum acts as a deoxidizing agent. However, when the Al content exceeds 0.10%, corrosion resistance deteriorates. Therefore, the Al content is set at 0.10% or less. The Al content is preferably 0.07% or less, more preferably 0.05% or less, and even more preferably 0.04% or less. No lower limit is specified, provided a deoxidizing effect can be achieved. 18 1428729 of 51 However, in order 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 is an element unavoidably present in steelmaking processes, and it also increases steel's strength. However, when the nitrogen content exceeds 0.10%, a nitride forms, which degrades corrosion resistance. Therefore, the nitrogen content is set at 0.10% or less. The preferred nitrogen content is 0.08% or less, more preferably 0.05% or less, and even more preferably 0.03% or less. A lower limit for nitrogen content is not specified, but excessively reducing the nitrogen content increases steelmaking costs. Therefore, the preferred nitrogen content is 0.002% or more, and even more preferably 0.003% or more. Alternatively, the maximum nitrogen content is 0.010% or less. Oxygen is present in steel as an oxide and thus adversely affects a variety of its characteristics. Therefore, in the present invention, oxygen is contained, ideally, in the smallest possible amount. In particular, when the O content exceeds 0.010%, hot workability and corrosion resistance deteriorate. Therefore, the O content is set at 19 1428729 of 51 0.010% or less. The content of 0 is preferably 0.005% or less. V: 0.120% or more and 1.000% or less Vanadium is an important element in the present invention that enhances resistance to high temperatures. Vanadium forms a carbonitride and allows for high resistance not only at room temperature but also at high temperatures through precipitation enhancement. To achieve the desired high-temperature resistance, the material contains 0.120% or more of vanadium. However, even when the vanadium content exceeds 1.000%, the effect becomes saturated. Therefore, in the present invention, the vanadium content is set at 0.120% or more and at 1.000% or less. Furthermore, the vanadium content is preferably 0.180% or more, more preferably 0.250% or more, and even more preferably 0.300% or more. Furthermore, the V content is preferably 0.500% or less, more preferably 0.400% or less, and even more preferably 0.300% or less. In the present invention, the seamless stainless steel tube contains the elements in such a way as to comply with the component composition described above and, in addition, C, Si, Mn, Cr, Ni, Mo, Cu and N comply with Formula (1) shown below. 13.0 < -5.9 x (7.82 + 27 C - 0.91 Si + 0.21 Mn - 0.9 Cr + Ni 1428729 of 51 - 1.1 Mo + 0.2 Cu + 11 N) < 50.0 ... (1) Here, each of C, Si, Mn, Cr, Ni, Mo, Cu and N is the content (% by mass) of each element and are considered zero in the case that they are not contained. The expression -5.9 x (7.82 + 27 C - 0.91 Si + 0.21 Mn - 0.9 Cr + Ni - 1.1 Mo + 0.2 Cu + 11 N) in Formula (1) (hereafter also referred to simply as the central polynomial expression of Formula (1) or central value) is obtained as an index indicating the tendency to form a ferritic phase. When the alloying element shown in Formula (1) is contained in a manner adjusted to comply with Formula (1), a stable multiphase microstructure can be achieved, including a martensitic phase and a ferritic phase, or a martensitic phase, a ferritic phase, and a retained austenitic phase. In the case where an alloying element shown in Formula (1) is not contained, the content of the corresponding element is treated as 0% in the value of the central polynomial expression of Formula (1). When the value of the central polynomial expression in Formula (1) is less than 13.0, a ferritic phase decreases within a hot working temperature range, and the yield decreases during the manufacture of the seamless stainless steel tube. On the other hand, when the value 21 If the value of the central polynomial expression in Formula (1) exceeds 50.0, the ferritic phase exceeds 60% in terms of volume fraction, and it becomes impossible to ensure the desired strength. Therefore, in Formula (1) specified in the present invention, the left-hand side value, which serves as the lower limit, is set at 13.0, and the right-hand side value, which serves as the upper limit, is set at 50.0. The left-hand side value, which serves as the lower limit, in Formula (1) specified in the present invention is preferably 15.0 and more preferably 20.0. Furthermore, the right-hand side value is preferably 45.0 and more preferably 40.0. That is, the value of the central polynomial expression in Formula (1) is set at 13.0 or more and set at 50.0 or less. The value is preferably set at 15.0 or more and is set at 45.0 or less.Preferably, the value is set at 20.0 or more and set at 40.0 or less. In the present invention, the remainder of the composition, other than the components described above, includes Fe and unavoidable impurities. The essential elements described above enable the seamless stainless steel tube of the present invention to achieve the desired characteristics. In the present invention, in order to further improve the characteristics, in addition to the component composition 22 The 1428729 of 51 basics described above, may contain one or more of the following selective elements (W, Nb, B, Ta, Co, Ti, Zr, Ca, REM, Mg, Sn and Sb), as needed. Specifically, in the present invention, it is possible to contain, in addition to the component composition described above, W: 3.0% or less. Furthermore, in the present invention, it is possible to contain, in addition to the composition of components described above, Nb: less than 0.10%. Furthermore, in the present invention, it is possible to contain, in addition to the composition of components described above, one or more selected from B: 0.010% or less, Ta: 0.3% or less, Co: 1.5% or less, Ti: 0.3% or less, and Zr: 0.3% or less. Furthermore, in the present invention, it is possible to contain, in addition to the component composition described above, one or more elements selected from the following: Ca: 0.01% or less, REM: 0.3% or less, Mg: 0.01% or less, Sn: 1.0% or less, and Sb: 1.0% or less. W: 3.0% or less Tungsten is an element that contributes to improving the toughness of steel and is capable of enhancing resistance to carbon dioxide corrosion and sulfide stress cracking by stabilizing the protective film on the surface of the steel tube. Tungsten significantly improves corrosion resistance, in particular, when contained in combination with molybdenum. Tungsten 23 1428729 of 51 may be contained as needed to achieve the effects described above. On the other hand, even when it contains more than 3.0% W, the effects are saturated. Therefore, in the case of containing W, the W content is preferably set at 3.0% or less. The W content is, more preferably, less than 1.5% and, more preferably still, 1.0% or less. Furthermore, in the case of containing W, the W content is, more preferably, 0.05% or more, and more preferably still, 0.10% or more. Nb: less than 0.10% Niobium is an element that increases toughness and also improves corrosion resistance and can be included as needed. However, when it contains 0.10% or more Nb, the desired high-temperature resistance cannot be achieved. Therefore, when Nb is present, the Nb content is preferably set at less than 0.10%. The Nb content is preferably 0.05% or less, and even more preferably 0.03% or less. Furthermore, the Nb content is preferably 0.005% or more, and even more preferably 0.010% or more. B: 0.010% or less Boron is an element that increases strength and can be included as needed. Additionally, boron also contributes to improved hot workability and 24 1428729 of 51 also has the effect of suppressing the occurrence of cracks or fissures in the tube manufacturing process. On the other hand, even when containing more than 0.010% B, the effect of improving hot workability rarely occurs, and low-temperature toughness deteriorates. Therefore, when containing B, the B content is preferably set at 0.010% or less. The B content is, more preferably, 0.008% or less, and even more preferably, 0.007% or less. Furthermore, the B content is, more preferably, 0.0005% or more, and even more preferably, 0.0010% or more. Ta: 0.3% or less Tantalum is an element that increases toughness and also improves corrosion resistance and can be included as needed. To achieve this effect, tantalum is preferably contained at 0.001% or more. However, even when the tantalum content exceeds 0.3%, the effect is saturated. Therefore, if tantalum is included, its content is preferably limited to 0.3% or less. The preferred content is 0.25% or less, even more preferably 0.06% or less, more preferably 0.050% or less, and even more preferably 0.025% or less. The preferred content is 0.005% or more. Co: 1.5% or less 1428729 of 51 Cobalt is a strength-enhancing element and its content may be adjusted as needed. In addition to the effect described above, Co also improves corrosion resistance. To achieve this effect, Co is preferably contained at 0.0005% or more. The Co content is preferably 0.005% or more, and even more preferably 0.010% or more. However, even if the Co content exceeds 1.5%, the effect becomes saturated. Therefore, if Co is included, its content is preferably limited to 1.5% or less. Ideally, the Co content is less than 0.150%. Ti: 0.3% or less Titanium is a strength-enhancing element and its content can be adjusted as needed. To achieve this effect, Ti is preferably contained at 0.0005% or more. However, even when the Ti content exceeds 0.3%, toughness deteriorates. Therefore, if Ti is present, its content is preferably limited to 0.3% or less. Zr: 0.3% or less Zirconium is a strength-enhancing element and can be included as needed. In addition to the effect described above, Zr also improves resistance to sulfide stress cracking. With 26 1428729 of 51 In order to obtain an effect like this, Zr is preferably contained at 0.0005% or more. On the other hand, even when the Zr content exceeds 0.3%, the effect is saturated. Therefore, if containing Zr, the Zr content is preferably limited to 0.3% or less. Ca: 0.01% or less Calcium is an element that contributes to improving the stress cracking resistance of sulfides by controlling the shape of a sulfide and can be included as needed. To achieve this effect, Ca is preferably contained at 0.0005% or more. However, even with more than 0.01% Ca, the effect becomes saturated, and it is impossible to expect an effect that corresponds to the content. Therefore, if Ca is included, the Ca content is preferably limited to 0.01% or less. REM: 0.3% or less Rare earth metals (REMs) are elements that contribute to improving the stress cracking resistance of sulfides by controlling the shape of a sulfide and can be included as needed. To achieve this effect, REMs are preferably included at 0.0005% or more. However, even at concentrations exceeding 0.3% REMs, the effect becomes saturated, making it impossible to expect the desired result.27 1428729 of 51 content. Therefore, if it contains REM, the REM content is preferably limited to 0.3% or less. In the present invention, the acronym REM refers to lanthanoid elements from scandium (Se) (atomic number: 21) and yttrium (Y= (atomic number: 39), and from lanthanum (La) (atomic number: 57) to lutetium (Lu) (atomic number: 71). The concentration of REM in the present invention is the total content of one or more elements selected from the REM described above. Mg: 0.01% or less Magnesium is an element that improves corrosion resistance and can be included as needed. To achieve this effect, magnesium is preferably contained at 0.0005% or more. However, even at concentrations exceeding 0.01%, the effect becomes saturated, making it impossible to expect a correspondingly high level of corrosion resistance. Therefore, when magnesium is included, its content is preferably limited to 0.01% or less. Sn: 1.0% or less Tin is an element that improves corrosion resistance and can be present as needed. To achieve this effect, Sn is preferably contained at 0.001% or more. However, even at concentrations exceeding 1.0% Sn, the effect becomes saturated, making it impossible to expect the desired result. 1428729 of 51 content. Therefore, if it contains Sn, the Sn content is preferably limited to 1.0% or less. Sb: 1.0% or less Antimony is an element that improves corrosion resistance and can be included as needed. To achieve this effect, antimony is preferably contained at 0.001% or more. However, even at concentrations exceeding 1.0% antimony, the effect becomes saturated, making it impossible to expect a result commensurate with the concentration. Therefore, when antimony is present, its content is preferably limited to 1.0% or less. The reasons for limiting the microstructure of the seamless stainless steel tube of the present invention will now be described. The seamless stainless steel tube of the present invention has a component composition described above and a microstructure that includes, in terms of volume fraction, 30% or more of a martensitic phase, 60% or less of a terrific phase, and 40% or less of a retained austenitic phase. In the seamless stainless steel tube of the present invention, to ensure a desired strength, a martensitic phase is established at 30% or more in terms of volume fraction. The martensitic phase is preferably established at 40% or more. 1428729 of 51 preference, at 70% or less and of higher preference, at 65% or less. Furthermore, in the present invention, the volume fraction of the included ferritic phase is 60% or less. When a ferritic phase is included, it is possible to suppress the propagation of sulfide stress cracking and achieve excellent corrosion resistance. On the other hand, when the volume fraction exceeds 60% and a large amount of a ferritic phase precipitates, it becomes impossible to ensure the desired resistance. The ferritic phase is preferably 5% or more by volume fraction. The ferritic phase is, more preferably, 10% or more. Additionally, the ferritic phase is preferably 50% or less by volume fraction. The ferritic phase is, more preferably, 45% or less. Furthermore, in the present invention, in addition to the martensitic and ferritic phases, 40% or less of an austenitic phase (retained austenitic phase) is included by volume fraction. The presence of the retained austenitic phase improves ductility and toughness. On the other hand, when a large amount of an austenitic phase is precipitated such that the volume fraction exceeds 40%, it becomes impossible to ensure the desired strength. Moreover, the retained austenitic phase is set at 40% or less by volume fraction. The retained austenitic phase is, of 30 1428729 of 51, preferably 5% or more in terms of volume fraction. In addition, the retained austenitic phase is 35% or less in terms of volume fraction. The retained austenitic phase is, more preferably, 30% or less in terms of volume fraction. Here, the microstructure described above of the seamless stainless steel tube of the present invention can be measured by the following method. First, a test sample for microstructure observation is corroded with a Vilella reagent (a reagent obtained by mixing picric acid, hydrochloric acid, and ethanol in fractions of 2 g, 10 ml, and 100 ml, respectively), and an image of the microstructure is captured with a scanning electron microscope (magnification: 1,000x). The fraction of the microstructure (area ratio in %) of the ferritic phase is then calculated using an image analyzer. This area ratio is defined as the volume fraction (%) of the ferritic phase. Furthermore, a sample for X-ray diffraction is ground and polished so that a cross-section orthogonal to the axial direction of the tube (cross-section C) becomes a measurement surface, and the structural fraction of the retained austenitic phase (γ) is measured using the X-ray diffraction method. The microstructural fraction of the retained austenitic phase is obtained by measuring the 31 1428729 of 51 integrated X-ray diffraction intensities of the (220) plane of y and the (211) plane of a (ferrite) and converting the integrated X-ray diffraction intensities using the following equation: Y (volume fraction) = 100 / (1 + (laRy / IyRa)) Here, Iα indicates the integrated intensity of a, Ra indicates the crystallographic theoretical calculation value of α, Iγ indicates the integrated intensity of γ and Ry indicates the crystallographic theoretical calculation value of γ. Furthermore, the remainder other than the ferritic phase and the retained γ phase obtained by the measurement method described above is defined as the martensitic phase fraction. The preferred manufacturing method for the seamless stainless steel tube of the present invention will now be described. It is preferred that molten steel having the component composition described above be melted by a common casting method, such as in a converter, and shaped into steel pipe material, such as a billet, by a common method, such as a continuous casting method or an ingot drawing method. The heating temperature of the steel pipe material 32 1428729 of 51 before hot working is preferably between 1100 °C and 1350 °C. In this case, it is possible to satisfy both the ease of hot handling during tube manufacturing and the low-temperature toughness of a final product. The steel pipe material obtained is then transformed into a tube by hot working using a closed-mandrel Mannesmann rolling mill or a Mannesmann mandrel rolling mill, which is a common and well-known method of tube manufacturing, thus producing a seamless steel tube having the desired dimensions and composition described above. After hot working, a quenching treatment may be performed. There is no particular need to limit this quenching treatment (quenching step). After hot working, it is preferable to cool the tube to ambient temperature at approximately an air-cooling rate, provided that the tube remains within the component composition range of the present invention described above. In the present invention, a heat treatment is also carried out on the seamless steel tube obtained, which includes a quenching treatment and an tempering treatment. The tempering treatment is a treatment in which the 33 1428729 of 51 seamless steel pipe is reheated to a temperature within the heating temperature range of 850 °C to 1150 °C and then cooled at an air-cooling rate or faster. The cooling stop temperature at this point is 50 °C or less in terms of the surface temperature of the seamless steel pipe. When the heating temperature is below 850 °C, the reverse transformation from martensite to austenite does not occur, and the transformation from austenite to martensite does not take place during cooling, making it impossible to guarantee the desired strength. On the other hand, when the heating temperature exceeds 1150 °C and the seamless steel tube reaches a high temperature, the crystal grains become coarse. Therefore, the heating temperature for the quenching treatment is set within the range of 850 °C to 1150 °C. The heating temperature for the quenching treatment is preferably 900 °C or higher. The heating temperature for the quenching treatment is preferably 1100 °C or lower.Furthermore, when the cooling arrest temperature exceeds 50 °C, the transformation of austenite to martensite does not occur sufficiently, and the fraction of retained austenite becomes excessive. Therefore, in the present invention, 34. 1428729 of 51 The cooling stop temperature during quenching in the quenching treatment is set at 50 °C or less. Here, the expression cooling rate for air quenching or faster means 0.01 °C / s faster. Furthermore, in the tempering treatment, the immersion time is preferably set between 5 and 30 minutes to unify the temperature in the direction of the wall thickness and avoid variations in the quality of the material. The tempering process involves heating a seamless steel tube that has undergone quenching to a tempering temperature between 500 °C and 650 °C. After this heating, the seamless steel tube can then be cooled with air. When the tempering temperature is below 500 °C, it is too low, making it impossible to achieve the desired tempering effect. Conversely, when the tempering temperature is too high, above 650 °C, an intermetallic compound precipitates, making it impossible to obtain excellent low-temperature toughness. Therefore, the tempering temperature is typically set between 500 °C and 650 °C. The tempering temperature is preferably 520 °C or higher. The tempering temperature is preferably 630 °C or lower. 1428729 of 51 Furthermore, in the tempering treatment, the holding time (holding time in immersion) is preferably set between 5 and 90 minutes to unify the temperature in the direction of the wall thickness and avoid variations in the quality of the material. When the heat treatment described above (quenching and tempering) is carried out, the microstructure of the seamless steel tube becomes one that includes a martensitic phase, a ferritic phase, and a retained austenitic phase, specified by predetermined volume fractions. This makes it possible to obtain a seamless stainless steel tube with the desired toughness and excellent corrosion resistance. Until now, seamless stainless steel pipes obtained by the present invention are high-strength steel pipes with a yield strength of 758 MPa or more and excellent corrosion resistance and resistance to high temperatures. The yield strength is preferably 862 MPa or more. The yield strength is preferably 1034 MPa or less. The seamless stainless steel pipe of the present invention can be converted into a seamless stainless steel pipe for an oil well (high-strength seamless stainless steel pipe for an oil well). 1428729 of 51 Examples The present invention will now be described by way of examples. The present invention is not limited to the following examples. The steel pipe materials were cast using steels with the component composition shown in Table 1-1 and Table 1-2. These steel pipe materials were then heated and hot-worked into pipes using a seamless rolling mill pattern, producing seamless steel pipes with an outside diameter of 83.8 mm and a wall thickness of 12.7 mm. The seamless steel pipes were then air-cooled. The pre-hot-working temperature of the steel pipe materials was set at 1250 °C. The sample materials were cut from the obtained seamless steel tubes and subjected to a quenching treatment by reheating the sample materials to a heating temperature of 960 °C, holding them in the quenching water for 20 minutes, and then cooling them to a stop-cooling temperature of 30 °C. In addition, an annealing treatment was performed by holding the sample materials at a heating temperature (annealing temperature) of 37 °C. 1428729 of 51 575 °C for an immersion holding time of 20 minutes, at a heating temperature (tempering temperature) of 525 °C for an immersion holding time of 20 minutes, or at a heating temperature (tempering temperature) of 620 °C for an immersion holding time of 40 minutes, and then air-cooling the sample materials. The cooling rate during water quenching in the quenching treatment was 11 °C / s, and the cooling rate during air quenching in the tempering treatment was 0.04 °C / s. The blank cells in Table 1-1 and Table 1-2 indicate that the corresponding element was not intentionally added, which includes not only a case where the element is not contained (0%) but also a case where the element is unavoidably contained. 1428729 of 51 Table 1-1 Steel No. Composition of components (% by mass) Formula (1) (*3) Remarks C Si Mn PS Cr Mo Cu Ni Al NOV Other Central value Suitability A 0.017 0.36 0.261 0.012 0.0009 17.03 2.77 2.15 4.87 0.026 0.020 0.001 0.283 28.6 O Example of present B 0.010 0.26 0.331 0.019 0.0010 16.98 2.47 2.77 4.15 0.022 0.019 0.002 0.332 30.5 O Example of present C 0.013 0.28 0.384 0.018 0.0013 17.26 2.75 3.16 4.99 0.027 0.017 0.002 0.369 28.0 O Example of a present D 0.059 0.30 0.271 0.014 0.0012 17.40 2.41 2.43 5.28 0.027 0.020 0.003 0.293 18.5 O Example of a present E 0.018 0.92 0.221 0.016 0.0015 16.49 2.54 2.82 5.14 0.026 0.010 0.003 0.316 25.4 O Example of a present F 0.011 0.33 0.837 0.017 0.0012 16.96 2.39 1.86 4.14 0.027 0.012 0.002 0.220 31.0 O Example of a present G 0.011 0.25 0.023 0.017 0.0010 16.62 2.61 3.13 4.23 0.025 0.015 0.001 0.313 29.0 O Example of a present H 0.008 0.32 0.148 0.049 0.0015 16.80 2.35 3.05 4.50 0.023 0.018 0.001 0.193 27.3 O Example of a present I 0.010 0,31 0.190 0.019 0.0041 16.74 2.48 1.85 4.95 0.025 0.016 0.004 0.325 26.3 O Example of a present J 0.014 0.26 0.283 0.015 0.0015 17.95 2.53 2.80 4.75 0.025 0.011 0.004 0.296 32.4 O Example of a present K 0.013 0.32 0.310 0.016 0.0014 15.76 2.56 1.86 5.39 0.029 0.022 0.002 0.272 18.0 O Example Example of present value L 0.008 0.29 0.203 0.014 0.0011 17.04 3.73 3.18 4.79 0.026 0.015 0.004 0.310 35.6 O Example of present value M 0.009 0.32 0.183 0.014 0.0010 17.32 1.66 2.68 4.53 0.027 0.022 0.004 0.189 25.3 O Example of present value N 0.011 0.30 0.234 0.013 0.0015 16.70 2.90 3.81 4.20 0.024 0.018 0.001 0.200 30.5 O Example of a present O 0.018 0.29 0.259 0.015 0.0010 16.80 2.32 3.15 5.09 0.025 0.012 0.004 0.300 22.0 O Example of a present P 0.017 0.23 0.374 0.015 0.0012 16.74 2.33 1.18 4.27 0.026 0.012 0.003 0.330 28.6 O Example of a present Q 0.012 0.36 0.171 0.016 0.0010 16.93 2.53 2.75 5.86 0.023 0.012 0.004 0.300 21.4 O Example of a present T 0.014 0.30 0.261 0.015 0.0009 16.91 3.27 2.26 3.14 0,026 0.018 0.002 0.320 41.6 O Example of a present u 0.015 0.29 0.374 0.016 0.0010 16.54 3.00 2.78 4.53 0.089 0.013 0.004 0.282 29.0 O Example of a present V 0.018 0.29 0.259 0.015 0.0010 16.80 2.32 2.80 5.09 0.025 0.085 0.003 0.279 17.6 O Example of a present w 0.011 0.31 0.206 0.014 0.0008 16.96 2.48 3.16 5.38 0.024 0.015 0.009 0.321 23.2 O Example of a present X 0.011 0.27 0.333 0.015 0.0008 16.76 2.23 2.07 5.51 0.024 0.014 0.002 0.913 20.8 O Example of a present Y 0.008 0.36 0.150 0.016 0.0006 16.52 3.24 3.18 4.82 0.025 0.011 0.003 0.389 30.2 O Example of a present z 0.010 0.32 0.258 0.015 0.0011 16.77 2.36 1.92 5.46 0.026 0.017 0.002 0.127 22.4 O Example of a present AA 0.012 0.35 0.288 0.017 0.0009 17.02 2.80 1.87 5.47 0.026 0.020 0.002 0.185 26.2 O Example of a present AB 0.002 0.90 0.267 0.015 0.0010 17.78 3.78 1.82 4.29 0.018 0.006 0.001 0.335 49.1 O Example of a present AC 0.034 0.06 0.426 0.015 0.0008 16.01 1.79 2.65 4.72 0.027 0.007 0.003 0.379 13,4 O Example of a present AD 0.005 0.30 0.380 0.016 0.0010 16.63 2.54 2.88 5.22 0.026 0.017 0.001 0.330 W:1.41 23.7 O Example of a present AE 0.008 0.29 0.253 0.016 0.0013 16.55 2.55 1.81 4.46 0.022 0.017 0.002 0.282 Nb:0.07 28.7 0 Example of a present, (Ί) The remainder, other than the components described above, is iron (Fe) and unavoidable impurities. (*2) The underlined values ​​are outside the scope of the invention (*3) Formula (1): 13.0 < -5.9 x (7.82 + 27 C - 0.91 Si + 0.21 Mn - 0.9 Cr + Ni -1.1 Mo + 0.2 Cu + 11 N)< 50.0 1428729 of 51 Table 1-2 Steel No. Composition of components (% by mass) Formula (1) (*3) Remarks C Si Mi P S Cr Mo Cu Ni Al NOV Other Central value Suitability AF 0.009 0.24 0.240 0.013 0.0014 16.70 2.53 3.00 4.11 0.027 0.020 0.003 0.282 B: 0.003, Ta: 0.25, Co: 1.30, Ti: 0.22, Zr: 0.24 29.4 O Example of the present AG 0.014 0.23 0.374 0.014 0.0011 17.32 3.23 3.07 4.22 0.028 0.015 0.003 0.250 Ca:0.009,REM:0.22, Mg:0.005, Sn:0.95, Sb:0.93 35.8 O Example of the present AH 0.013 0.30 0.160 0.017 0.0007 17.01 2.58 3.19 4.56 0.027 0.015 0.003 0.262 W:1.03, Nb:0.05 28.6 O Example of the present Al 0.010 0.30 0.381 0.015 0.0014 16.93 2.36 2.39 4.15 0.027 0.014 0.002 0.328 W:0.23, B:0.00 4 30.4 O Example of the present AJ 0.012 0.30 0.282 0.015 0.0012 16.44 2.86 2.79 4.83 0.027 0.016 0.001 0.379 W:0.25, Sn:0.22 26.2 O Example of the present AK 0.008 0.36 0.150 0.016 0.0006 16.52 3.24 3.18 4.82 0.025 0.011 0.003 0.338 Nb:0.03, Ta:0.06 30.2 O Example of the present AL 0.014 0.27 0.265 0.016 0.0008 16.96 3.11 2,76 4.54 0.024 0.011 0.002 0.214 Nb:0.07, Sb:0.27 32.2 O Example of the present AM 0.011 0.28 0.247 0.016 0.0012 16.64 2.47 3.12 5.31 0.028 0.015 0.002 0.259 T¡:0.25, Ca:0.004 21.7 O Example of the present AN 0.012 0.25 0.408 0.014 0.0012 16.79 2.45 2.45 4.37 0.025 0.016 0.003 0.231 W:1.35, Nb:0.04, Zr:0.22, REM:0.21 28.1 O Example of the present AO 0.066 0.33 0.248 0.015 0.0015 17.19 3.00 2.67 4.98 0.027 0.012 0.002 0.255 22.3 O Comparative example AP 0.008 1.28 0.302 0.015 0.0006 16.51 3.15 1.98 4.34 0.024 0.016 0.003 0.366 38.2 O Comparative example AQ 0.005 0.30 0.928 0.016 0.0010 16.63 2.54 2.88 5.22 0.026 0.017 0.001 0.322 23.0 O Comparative example AR 0.014 0.29 0.289 0.056 0.0012 16.53 2.77 1.83 5.17 0.027 0.020 0.002 0.240 24.6 O Comparative example AS 0.014 0.29 0.330 0.016 0.0058 16.60 2.65 2.58 4.63 0.028 0.021 0.003 0.240 26.4 O Comparative example AT 0.017 0.28 0.402 0.016 0.0014 18.13 3.24 3.12 4.64 0.027 0.014 0.003 0.399 37.5 O Comparative example AU 0.012 0.28 0.294 0,017 0.0009 15.52 2.66 2.08 4.77 0.027 0.018 0.003 0.224 21.0 O Comparative example, 1428729 of 51 Table 1-2 (cont.) Steel No. Composition of components (% by mass) Formula (1)(*3) Remarks C Si Mn PS Cr Mo Cu Ni Al NOV Other Central value Suitability AV 0.014 0.30 0.280 0.017 0.0007 17.36 4.03 2.55 4.30 0.023 0.017 0.003 0.249 41.7 O Comparative example AW 0.009 0.25 0.364 0.014 0.0010 16.50 1.47 1.93 5.23 0.024 0.014 0.003 0.181 16.4 O Comparative example AX 0.014 0.32 0.330 0.016 0.0010 16.85 2.99 1.02 5.40 0.027 0.021 0.002 0.350 27.4 O Comparative Example AY 0.014 0.30 0.297 0.015 0.0012 17.36 2.70 3.16 6.17 0.024 0.014 0.003 0.209 21.5 O Comparative Example AZ 0.011 0.26 0.388 0.013 0.0008 16.64 2.51 3.08 5.14 0.131 0.013 0.003 0.216 22.9 O Comparative Example BA 0.012 0.26 0.262 0.017 0.0011 17.04 2.76 1.84 4.41 0.028 0.115 0.001 0.398 25.8 O Comparative Example BB 0.014 0.36 0.274 0.016 0.0013 16.60 2.44 2.43 4.58 0.025 0.014 0.013 0.266 26.4 O Comparative Example BC 0.014 0.27 0.265 0.016 0.0008 16.96 3.11 2.76 4.54 0.024 0.011 0.002 0.098 32.2 O Comparative Example BD 0.004 0.92 0,299 0.017 0.0014 17.93 3.78 1.80 3.89 0.019 0.014 0.002 0.197 51.6 X Comparative Example BE 0.017 0.28 0.350 0.016 0.0010 16.50 2.55 1.65 4.12 0.025 0.013 0.003 0.383 Ca:0.005 29.3 O Example of the present BF 0.014 0.30 0.253 0.016 0.0007 16.85 2.53 1.88 4.56 0.026 0.014 0.002 0.251 Co:0.05 28.8 O Example of the present BG 0.012 0.25 0.240 0.015 0.0009 17.36 2.58 3.00 4.35 0.025 0.022 0.002 0.183 Sb: 0.02 31.3 O Example of the present BH 0.011 0.32 0.374 0.013 0.0014 16.64 2.36 3.07 4.83 0.025 0.019 0.002 0.153 W:1.01 23.7 O Example of the present, (Ί) The remainder, other than the components described above, is iron (Fe) and unavoidable impurities. (*2) The underlined values ​​are outside the scope of the invention (*3) Formula (1): 13.0 < -5.9 χ (7.82 + 27 C - 0.91 Si + 0.21 Mn - 0.9 Cr + Ni -1.1 Mo + 0.2 Cu + 11 N)< 50.0 1428729 of 51 Samples of the finished heat-treated sample materials (seamless steel tubes) were collected and subjected to microstructure analysis, tensile testing, elevated-temperature tensile testing, and corrosion resistance testing. The test methods are described below. (1) Observation of the microstructure From each of the heat-treated test materials, a sample was taken for microstructure observation, such that a cross-section orthogonal to the axial direction of the tube became an observation surface. The sample obtained for microstructure observation was corroded with Vilella's reagent (a reagent obtained by mixing picric acid, hydrochloric acid, and ethanol in fractions of 2 g, 10 mL, and 100 mL, respectively). An image of the microstructure was captured using a scanning electron microscope (magnification: 1000x), and the microstructural fraction (area ratio (%)) of the ferritic phase was calculated using an image analyzer. This area ratio was defined as the volume fraction (%) of the ferritic phase. In addition, a sample for X-ray diffraction was collected from the heat-treated finished sample, which was crushed and polished so that a cross-section 42 1428729 of 51 orthogonal to the axial direction of the tube (cross-section C) was converted into a measurement surface, and the structural fraction of the retained austenitic phase (y) was measured using the X-ray diffraction method. The microstructural fraction of the retained austenitic phase was obtained by measuring the integrated X-ray diffraction intensities of the (220) plane of γ and the (211) plane of a (ferrite) and converting the integrated X-ray diffraction intensities using the following equation: γ (volume fraction) = 100 / (1 + (ΙαΚγ / ΙγΚα)) Here, Iα indicates the integrated intensity of a, Ra indicates the crystallographic theoretical calculation value of α, Iγ indicates the integrated intensity of γ and Kγ indicates the crystallographic theoretical calculation value of γ. The fraction of a martensitic phase is the remainder other than the ferritic phase, and the retained γ phase. (2) Traction test From the heat-treated test materials obtained, rod-shaped samples were taken so that the axial direction of the tube became a tensile direction. Tensile tests were carried out according to the JIS Z 2241 (2011) standard, and the elastic limit (YS) was obtained as the conventional elastic limit. 1428729 of 51 of 0.2%. Here, samples that had a yield strength (YS) of 758 MPa or more were considered to be high strength and were determined to have passed the test, and samples that had a yield strength less than 758 MPa were determined not to have passed the test. (3) High temperature tensile test From the heat-treated test materials, rod-shaped samples were taken so that the axial direction of the tube became a tensile direction. Tensile tests were carried out at a temperature of 200 °C according to JIS G 0567 (2012), and the yield strength (YS) was obtained as the conventional yield strength of 0.2%. Here, the same heat treatment was carried out on the same steel. In cases where the ratio between the conventional yield strength of 0.2% at 200 °C and the yield strength (conventional yield strength of 0.2%) obtained in the tensile test (2) was 0.85 or higher, the samples were determined to have passed the test. In cases where the ratio was less than 0.85, the samples were determined to have failed the test.(4) Corrosion resistance test (carbon dioxide corrosion resistance test and sulfide stress cracking resistance test). From the test materials obtained finished by heat treatment, they were produced by machining 1428729 of 51 corrosion samples with a thickness of 3 mm, a width of 30 mm and a length of 40 mm. Corrosion tests were carried out using the corrosion samples and the resistance to carbon dioxide corrosion was evaluated. The corrosion test to evaluate the corrosion resistance of carbon dioxide was performed by immersing the corrosion sample in a test solution maintained in an autoclave: a 20% by mass aqueous NaCl solution (liquid temperature: 200 °C, CO2 gas atmosphere: 30 atm) for a set immersion period of 14 days (336 hours). The sample weight was measured after the test, and the corrosion rate was calculated from the reduced weight before and after the corrosion test. Samples with a corrosion rate of 0.127 mm / year or slower were determined to have passed the test, while samples exceeding 0.127 mm / year were deemed to have failed. In addition, round rod-shaped samples (diameter: 3.81 mm) were prepared from the materials of the samples obtained by machining, and sulfide stress cracking resistance tests (SSC resistance tests) were carried out. In the SSC resistance test, the samples were immersed in an aqueous solution with a pH adjusted to 3.0 by adding acetic acid and sodium acetate at a 45 1428729 of 51 test solution maintained in an autoclave: aqueous NaCl solution at 0.165% by mass (liquid temperature: 25 °C, CO2 gas of 0.99 atm, H2S atmosphere of 0.01 atm) and exposed for 720 hours in a state in which 90% of the yield strength was applied to the samples, and whether or not the samples were observed to break or crack after the test. Samples that did not break or crack were determined to have passed the test (indicated by a symbol O in Table 2-1 and Table 2-2), and samples that broke or cracked failed the test (indicated by a symbol X in Table 2-1 and Table 2-2). The results obtained are shown in Tables 2-1 and 2-2. 1428729 of 51 Table 2-1 Steel No. Steel Tube No. Tempering Temperature (°C) Holding Time in Immersion (minutes) Microstructure (% by volume) Yield Strength YS (MPa) High Temperature Strength (°C) Corrosion Rate (mm / year) SSC Resistance Remarks M (°C) F (°C) A (°C) A 1 575 20 53 34 13 929 0.87 0.028 O Example of this document A 2 620 40 35 36 29 802 0.86 0.030 O Example of this document B 3 575 20 58 31 11 944 0.88 0.026 O Example of this document B 4 620 40 37 33 30 827 0.87 0.022 O Example of this document C 5 575 20 50 32 18 915 0.86 0.025 O Example of the present C 6 620 40 34 35 31 793 0.90 0.029 O Example of the present D 7 575 20 44 29 27 880 0.89 0.106 O Example of the present E 8 575 20 57 32 11 935 0.89 0.058 O Example of the present F 9 575 20 54 24 22 961 0.90 0.027 O Example of the present G 10 575 20 60 33 7 888 0.90 0.025 O Example of the present H 11 575 20 62 30 8 948 0.86 0.079 O Example of the present I 12 575 20 60 32 8 956 0.89 0,025 O Example of the present J 13 575 20 47 32 21 893 0.88 0.020 O Example of the present K 14 575 20 70 24 6 996 0.88 0.098 O Example of the present L 15 575 20 42 34 24 870 0.90 0.021 O Example of the present M 16 575 20 61 28 11 959 0.87 0.079 O Example of the present N 17 575 20 56 32 12 936 0.88 0.016 O Example of the present 0 18 575 20 60 29 11 921 0.88 0.019 O Example of the present, 1428729 of 51 Table 2-1 Steel No. Steel Tube No. Tempering Temperature (°C) Holding Time in Immersion (minutes) Microstructure (% by volume) Yield Strength YS (MPa) High Temperature Strength (°C) Corrosion Rate (mm / year) SSC Resistance Remarks MX) F (°C) AX) P 19 575 20 62 34 4 937 0.90 0.037 O Example of this Q 20 575 20 50 31 19 887 0.88 0.023 O Example of this T 21 575 20 53 34 13 927 0.87 0.031 O Example of this U 22 575 20 56 30 14 936 0.86 0.062 O Example of this V 23 575 20 53 28 19 923 0.90 0.069 O Example of the present w 24 575 20 53 31 16 924 0.89 0.070 O Example of the present X 25 575 20 60 27 13 944 0.88 0.028 O Example of the present Y 26 575 20 61 28 11 947 0.90 0.036 O Example of the present z 27 575 20 40 47 13 875 0.85 0.027 O Example of the present AA 28 575 20 69 24 7 991 0.89 0.025 O Example of the present AB 29 575 20 40 56 4 947 0.88 0.027 O Example of the present AC 30 575 20 69 25 6 963 0.88 0,031 O Example of the present AD 31 575 20 56 33 11 945 0.90 0.031 O Example of the present AE 32 575 20 52 31 17 919 0.90 0.026 O Example of the present, The underlined values ​​are outside the scope of the invention (Ί) M: Martensitic phase, F: Ferritic phase; A: Retained austenitic pass (*2) High temperature strength refers to the ratio between the yield strength (conventional yield strength of 0.2%) at 200 °C and the yield strength (conventional yield strength of 0.2%) at room temperature. 1428729 of 51 Table 2-2 Steel No. Steel Tube No. Tempering Temperature (°C) Holding Time in Immersion (minutes) Microstructure (% by volume) Yield Strength YS (MPa) High Temperature Strength (°C) Corrosion Rate (mm / year) SSC Resistance Remarks M (°C) F (°C) A (°C) AF 33 575 20 46 36 18 901 0.89 0.024 O Example of this document AG 34 575 20 56 30 14 928 0.90 0.027 O Example of this document AH 35 575 20 62 31 7 954 0.88 0.024 O Example of this document Al 36 575 20 55 33 12 936 0.87 0.027 O Example of this document AJ 37 575 20 52 33 15 923 0.90 0.025 O Example of the present AK 38 575 20 55 31 14 918 0.90 0.023 O Example of the present AL 39 575 20 55 31 14 936 0.89 0.027 O Example of the present AM 40 575 20 51 32 17 922 0.88 0.024 O Example of the present AN 41 575 20 58 33 9 945 0.90 0.026 O Example of the present P 42 525 20 66 34 0 942 0.90 0.035 O Example of the present AO 43 575 20 42 28 30 869 0.88 0.136 X Comparative Example AP 44 575 20 56 34 10 932 0.90 0.138 X Comparative Example AQ 45 575 20 62 28 10 946 0.90 0.085 X Comparative Example AR 46 575 20 60 31 9 940 0.89 0.151 X Comparative Example AS 47 575 20 37 37 26 956 0.90 0.076 X Comparative Example AT 48 575 20 29 55 16 731 0.88 0.015 O Comparative Example AU 49 575 20 42 36 22 867 0.88 0.156 X Comparative Example AV 50 575 20 29 46 25 740 0.90 0.018 O Comparative Example AW 51 575 20 55 30 15 891 0.90 0.154 X Comparative Example AX 52 575 20 46 31 23 899 0.89 0.135 X Comparative Example AY 53 575 20 29 28 43 719 0.90 0.026 O Comparative Example AZ 54 575 20 53 29 18 909 0.88 0.159 X Comparative Example BA 55 575 20 60 29 11 947 0.87 0.152 X Comparative Example BB 56 575 20 58 31 11 949 0.90 0.137 X Comparative Example BC 57 575 20 53 32 15 946 0.73 0.045 0 Comparative example, 1428729 of 51 Table 2-2 Steel No. Steel Tube No. Tempering Temperature (°C) Holding Time in Immersion (minutes) Microstructure (% by volume) Yield Strength YS (MPa) High Temperature Strength (°C) Corrosion Rate (mm / year) SSC Resistance Remarks M (°C) F (°C) A (°C) BD 58 575 20 20 63 17 658 0.85 0.037 0 Comparative Example BE 59 575 20 56 29 15 897 0.88 0.035 0 Example of the present BF 60 575 20 52 30 18 888 0.87 0.039 0 Example of the present BG 61 575 20 61 27 12 925 0.86 0.027 0 Example of the present BH 62 575 20 64 22 14 941 0.86 0.040 0 Example of the present The underlined values ​​are outside the scope of the invention (Ί) M: Martensitic phase, F: Ferritic phase; A: Retained austenitic pass (*2) High temperature strength refers to the ratio between the yield strength (conventional yield strength of 0.2%) at 200 °C and the yield strength (conventional yield strength of 0.2%) at room temperature. As shown in Table 2-1 and Table 2-2, the examples of the present invention were all seamless stainless steel tubes having a high strength of 758 MPa or more in terms of the YS yield strength, excellent corrosion resistance (carbon dioxide corrosion resistance) under highly corrosive high temperature (200 °C) environments containing CO2 and Cl-, excellent sulfide stress cracking resistance, and excellent elevated temperature resistance. 1428729 of 51 G. BREUER - 30525624826 Digitally signed by PORTALTRAM ITES - INPI Date: 2021.07.02 11:23:47 -03:00 Reason: Digitally Signed by the INPI Location: Buenos Aires, Argentina 1428729

Claims

1. A seamless stainless steel pipe suitable for use in oil and gas wells, characterized in that it has a component composition consisting, in terms of % mass: C: 0.002 to 0.06%; Si: 0.03% to 1.0%; Mn: 0.01% or more and 0.90% or less; P: 0.05% or less; S: 0.005% or less; Cr: 15.70% or more and 18.00% or less; Mo: 1.60% or more and 3.80% or less; Cu: 1.10% or more and 4.00% or less; Ni: 3.0% or more and 6.0% or less; Al: 0.005% to 0.10%; N: 0.002% to 0.10%; O: 0.010% or less; V: 0.250% or more and 1.000% or less; W: 3.0% or less; Nb: less than 0.10%; B: 0.010% or less; Ta: 0.3% or less; Ti: 0.3% or less; Zr: 0.3% or less; Ca: 0.01% or less; REM: 0.3% or less; Mg: 0.01% or less; Sn: 1.0% or less; and Sb: 1.0% or less; C, Si, Mn, Cr, Ni, Mo, Cu, and N comply with Formula (1) shown below; 13.0 ≤ -5.9 x (7.82 + 27 C - 0.91 Si + 0.21 Mn - 0.9 Cr + Ni - 1.1 Mo + 0.2 Cu + 11 N) ≤ 50.0 ...(1) Herein, each of C, Si, Mn, Cr, Ni, Mo, Cu, and N is the mass % content of each element; and a remainder consisting of Fe and unavoidable impurities, wherein the seamless stainless steel tube has a microstructure that includes, in terms of volume fraction, 5% or more and 60% or less of a ferritic phase, 40% or less of a retained austenitic phase, the remainder of the microstructure being 30% or more of a martensitic phase; and the yield strength is 758 MPa or more; the ratio of the yield strength at 200°C to the yield strength at room temperature being equal to or greater than 0.85, the yield strength at 200°C being measured by a tensile test based on JIS G 0567 and the yield strength at room temperature being measured by a tensile test based on JIS Z 2241. 2 Claims follow.