High-strength stainless steel seamless steel pipe for oil well

Abstract

The purpose of the present invention is to provide a high-strength seamless stainless steel pipe for oil wells, which has high strength, excellent low-temperature toughness, and excellent crevice corrosion resistance in an untreated seawater environment. A high-strength seamless stainless steel pipe for oil wells, which has a component composition that contains specific components and satisfies formula (1) and formula (2), with the remainder comprising Fe and unavoidable impurities, and which has a yield strength of 758 MPa or more, and an absorption energy vE-10 at-10 DEG C in a Charpy impact test of 40 J or more. Cr + 0.22 * Ni + 0.38 * (Mo + 0.5 * W) + 0.89 * Cu + 0.09 * Co > = 21.4 (1) In formula (1), Cr, Ni, Mo, W, Cu, and Co are the content (mass%) of each element, and the content of the elements not included is 0. Co-Nb > = 0.13 (2) In formula (2), Co and Nb are the contents (mass%) of the respective elements.

Description

TECHNICAL FIELD

[0001] The present application relates to a high-strength stainless steel seamless pipe for oil wells, which is suitable for use in oil wells and gas wells (hereinafter referred to as "oil wells") and the like of crude oil or natural gas. BACKGROUND

[0002] In recent years, from the viewpoint of the high rise of the price of crude oil, the depletion of oil resources that is expected in the near future, and the like, development of oil fields, gas fields, and the like that are deep, that are in a severe corrosion environment under a so-called sour environment containing hydrogen sulfide and the like, and the like that have not been explored in the past is being actively carried out. Such oil fields and gas fields are generally extremely deep, and the atmosphere thereof is also a severe corrosion environment of high temperature and containing CO2, Cl - , and H2S. For a steel pipe for oil wells used in such an environment, a material that has both a desired high strength and corrosion resistance is required.

[0003] In the past, in oil fields and gas fields of an environment containing carbon dioxide (CO2), chloride ions (Cl - ), and the like, a 13Cr martensitic stainless steel pipe has been used as an oil well pipe used in production. In addition, in recent years, the use of a modified 13Cr martensitic stainless steel in which the C of the 13Cr martensitic stainless steel is reduced and the contents of Ni, Mo, and the like are increased is also expanding.

[0004] In response to such a demand, for example, the technologies described in Patent Documents 1 to 5 are cited.

[0005] Patent Document 1 discloses a stainless steel pipe for oil wells, which is composed of a steel containing, in mass %, C: 0.05% or less, Si: 0.50% or less, Mn: 0.20 to 1.80%, P: 0.03% 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: 0.05% or less, V: 0.20% or less, N: 0.01 to 0.15%, O: 0.006% or less, and satisfying a prescribed formula, and the balance consisting of Fe and unavoidable impurities, whereby corrosion resistance is improved.

[0006] In addition, Patent Document 2 discloses a high-strength stainless steel seamless pipe for oil wells having a composition containing, in mass%, C: 0.005 to 0.05%, Si: 0.05 to 0.50%, Mn: 0.20 to 1.80%, P: 0.030% or less, S: 0.005% or less, Cr: 12.0 to 17.0%, Ni: 4.0 to 7.0%, Mo: 0.5 to 3.0%, Al: 0.005 to 0.10%, V: 0.005 to 0.20%, Co: 0.01 to 1.0%, N: 0.005 to 0.15%, O: 0.010% or less, and satisfying a prescribed formula, with the balance consisting of Fe and unavoidable impurities, whereby the yield strength is 655 MPa or more.

[0007] In addition, Patent Document 3 discloses a high-strength stainless steel pipe for oil wells having a composition containing, in mass%, C: 0.05% or less, Si: 0.50% or less, Mn: 0.10 to 1.80%, P: 0.03% or less, S: 0.005% or less, Cr: 14.0 to 17.0%, Ni: 5.0 to 8.0%, Mo: 1.0 to 3.5%, Cu: 0.5 to 3.5%, Al: 0.05% or less, V: 0.20% or less, N: 0.03 to 0.15%, O: 0.006% or less, and containing one or both of Nb: 0.2% or less and Ti: 0.3% or less, with the balance consisting of Fe and unavoidable impurities, and a structure in which MC-type carbonitride in precipitates exists in 3.0% or more in mass% relative to the total amount of precipitates, whereby the strength is high and high corrosion resistance is obtained.

[0008] In addition, Patent Document 4 discloses a high-strength stainless steel seamless pipe for oil wells having a composition containing Cr and Ni and a structure in which a tempered martensite phase is a main phase, in which the composition satisfies Cr / Ni < 5.3, a phase that becomes white by etching using Vilella's etching solution has a thickness of 10 μm or more and 100 μm or less in the wall thickness direction from the outer surface of the pipe, and has a surface layer structure that is dispersed by 50% or more in area ratio of the outer surface of the pipe.

[0009] Further, Patent Document 5 discloses a high-strength martensitic stainless steel seamless pipe for oil wells having a composition containing, in mass%, C: 0.01% or less, Si: 0.5% or less, Mn: 0.1 to 2.0%, P: 0.03% or less, S: 0.005% or less, Cr: 14.0 to 15.5%, Ni: 5.5 to 7.0%, Mo: 2.0 to 3.5%, Cu: 0.3 to 3.5%, V: 0.20% or less, Al: 0.05% or less, N: 0.06% or less, and the balance consisting of Fe and unavoidable impurities, thereby having a strength with a yield strength of 655 to 862 MPa and a yield ratio of 0.90 or more, and improved resistance to carbon dioxide corrosion and resistance to sulfide stress corrosion cracking.

[0010] Prior Art Documents

[0011] Patent Documents

[0012] Patent Document 1: International Publication No. 2004 / 001082

[0013] Patent Document 2: International Publication No. 2017 / 168874

[0014] Patent Document 3: Japanese Patent Application Publication No. 2005-105357

[0015] Patent Document 4: International Publication No. 2015 / 178022

[0016] Patent Document 5: Japanese Patent Application Publication No. 2012-136742 SUMMARY

[0017] PROBLEMS TO BE SOLVED BY THE INVENTION

[0018] In recent years, in order to improve the recovery rate of crude oil, a method of injecting water into a formation using a seamless steel pipe, such as Water Injection, is being used. Since seawater is abundantly present, it is often used for water injection. Chloride ions, dissolved oxygen, microorganisms, and the like present in seawater increase the corrosiveness, and thus are sometimes removed, but this incurs cost, and thus untreated seawater is sometimes used for water injection. For a seamless steel pipe used in such an environment, high corrosion resistance is required, but in the technologies described in Patent Documents 1 to 5, although the resistance to carbon dioxide corrosion is good, the resistance to crevice corrosion in an untreated seawater environment is insufficient. Further, development in cold regions, deep seas, and the like is on the rise, and thus low-temperature toughness is also required.

[0019] Therefore, an object of the present application is to solve the problems of the prior art described above, and to provide a high-strength stainless steel seamless pipe for oil wells having high strength and excellent low-temperature toughness, and excellent resistance to crevice corrosion in an untreated seawater environment.

[0020] Note that "high strength" in the present application means a case where the yield strength YS is 110 ksi (758 MPa) or more.

[0021] Further, "excellent low-temperature toughness" means a case where, according to the provisions of JIS Z 2242, a V-notch test piece (10 mm thick) is cut in a manner such that the length direction of the test piece is orthogonal to the molding direction and the notch is parallel to the molding direction, and a Charpy impact test is performed, and the absorbed energy vE in the Charpy impact test at a test temperature of -10°C is 40 J or more. -10

[0022] Further, "excellent resistance to crevice corrosion in untreated seawater" in the present application means a case where a test piece with a crevice is immersed in artificial seawater (liquid temperature: 25°C, atmospheric saturation at 1 atm), and a 30-day immersion test is performed, and, at this time, for the test piece after the corrosion test, a 10-fold magnification magnifying glass is used to observe whether or not a crevice has been generated on the surface of the test piece, and no crevice corrosion having a depth of 0.1 mm or more has been generated.

[0023] Note that the method of each of the above tests is also described in detail in the Examples described later.

[0024] Method for solving the problem

[0025] In order to achieve the above object, the present inventors have intensively studied the effects of various component compositions on the resistance to crevice corrosion of stainless steel pipes in untreated seawater environments. As a result, the following insight was obtained: it is necessary to adjust the contents of Cr, Mo, Cu, Ni, W, and Co in the component composition of the stainless steel material to satisfy formula (1).

[0026] Cr + 0.22 x Ni + 0.38 x (Mo + 0.5 x W) + 0.89 x Cu + 0.09 x Co ≥ 21.4... (1)

[0027] Here, Cr, Ni, Mo, W, Cu, and Co in formula (1) are the contents (mass %) of each element, and elements not contained are set to have a content of 0.

[0028] Further, the following insight was obtained: in order to obtain a desired low-temperature toughness value on the basis of satisfying the resistance to crevice corrosion, it is necessary to adjust the contents of Nb and Co to satisfy formula (2).

[0029] Co - Nb ≥ 0.13... (2)

[0030] Here, Co and Nb in formula (2) are the contents (mass %) of each element.

[0031] The present application was completed on the basis of the above insight through further research. That is, the gist of the present application is as follows.​

[0032] [1] A high-strength stainless steel seamless pipe for oil wells, which has a composition consisting of, in mass %, C: 0.002 to 0.050 %, Si: 0.05 to 0.50 %, Mn: 0.04 to 1.80 %, P: 0.030 % or less, S: 0.0020 % or less, Cr: 16.0 to 20.0 %, Ni: 4.0 to 7.5 %, Mo: 1.5 to 3.7 %, Al: 0.005 to 0.10 %, N: 0.002 to 0.15 %, Co: 0.2 to 1.0 %, Nb: 0.005 to 0.20 %, O: 0.010 % or less, further containing one or both of Cu: 3.5 % or less and W: 3.5 % or less, and satisfying formula (1) and formula (2), and the balance consisting of Fe and unavoidable impurities,

[0033] The high-strength stainless steel seamless pipe for oil wells described above has a yield strength of 758 MPa or more, and an absorbed energy vE at a test temperature of -10 °C in a Charpy impact test -10 is 40 J or more.

[0034] Cr + 0.22 x Ni + 0.38 x (Mo + 0.5 x W) + 0.89 x Cu + 0.09 x Co ≥ 21.4... (1)

[0035] Here, Cr, Ni, Mo, W, Cu, and Co in formula (1) are the contents (mass %) of each element, and elements not contained are set to have a content of 0.

[0036] Co - Nb ≥ 0.13... (2)

[0037] Here, Co and Nb in formula (2) are the contents (mass %) of each element.

[0038] [2] The high-strength stainless steel seamless pipe for oil wells according to [1], wherein, on the basis of the above composition, one or two or more of V: 0.50 % or less, Ti: 0.20 % or less, Zr: 0.20 % or less, B: 0.01 % or less, REM: 0.01 % or less, Ca: 0.0100 % or less, Sn: 0.20 % or less, Sb: 0.50 % or less, Ta: 0.1 % or less, and Mg: 0.0100 % or less is further contained in mass %.

[0039] Effects of the Invention

[0040] According to the present application, it is possible to provide a high-strength stainless steel seamless pipe for oil wells which has high strength and excellent low-temperature toughness, and excellent resistance to crevice corrosion in untreated seawater. DETAILED DESCRIPTION

[0041] The present application will be described in detail below. Note that the present application is not limited to the following embodiments.

[0042] First, the composition of the high-strength stainless steel seamless pipe for oil wells of the present application and the reasons for the limitations will be described. In the following, the mass % will be simply referred to as "%" unless otherwise specified.

[0043] C: 0.002 to 0.050%

[0044] C is an important element for increasing the strength of the martensitic stainless steel. In the present application, in order to ensure the strength targeted in the present application, it is necessary to contain 0.002% or more of C. Therefore, the C content is set to 0.002% or more. The C content is preferably set to 0.010% or more, more preferably to 0.015% or more, and further preferably to 0.020% or more. The C content is most preferably set to 0.022% or more. On the other hand, when C exceeding 0.050% is contained, the strength is rather decreased. In addition, the resistance to crevice corrosion in the environment of untreated seawater is also deteriorated. Therefore, in the present application, the C content is set to 0.050% or less. Note that it is preferably set to 0.040% or less. It is more preferably set to 0.035% or less, and further preferably to 0.030% or less. The C content is most preferably set to 0.028% or less.

[0045] Si: 0.05 to 0.50%

[0046] Si is an element that functions as a deoxidizer. This effect can be obtained when Si of 0.05% or more is contained. Therefore, the Si content is set to 0.05% or more. The Si content is preferably set to 0.10% or more, more preferably to 0.15% or more. The Si content is further preferably set to 0.20% or more, and most preferably to 0.22% or more. On the other hand, when Si exceeding 0.50% is contained, the resistance to crevice corrosion in the environment of untreated seawater is deteriorated. Therefore, the Si content is set to 0.50% or less. The Si content is preferably set to 0.45% or less, more preferably to 0.40% or less, and further preferably to 0.30% or less. The Si content is most preferably set to 0.25% or less.

[0047] Mn: 0.04 to 1.80%

[0048] Mn is an element that suppresses the generation of δ ferrite at the time of hot working, and improves hot workability. In the present application, it is necessary to contain 0.04% or more of Mn. Therefore, the content of Mn is set to 0.04% or more. The content of Mn is preferably set to 0.10% or more, more preferably to 0.20% or more, further preferably to 0.25% or more. The content of Mn is most preferably set to 0.35% or more. On the other hand, when Mn is excessively contained, the resistance to intergranular corrosion in the environment of untreated seawater deteriorates. Therefore, the content of Mn is set to 1.80% or less. The content of Mn is preferably set to 1.60% or less, more preferably to 0.80% or less, further preferably to 0.60% or less, most preferably to 0.40% or less.

[0049] P: 0.030% or less

[0050] P is an element that deteriorates the resistance to intergranular corrosion in the environment of untreated seawater. In the present application, it is preferable to reduce P as much as possible, but extreme reduction leads to an increase in manufacturing cost. Therefore, as a range that can be implemented at a relatively low price in industry without leading to extreme deterioration of characteristics, the content of P is set to 0.030% or less. It is preferable that the content of P be 0.025% or less, more preferably 0.020% or less. It is further preferable that the content of P be 0.018% or less, most preferably 0.015% or less. Note that the lower limit of the content of P is not particularly limited. However, as described above, excessive reduction leads to an increase in manufacturing cost, and therefore it is preferable to set the content of P to 0.005% or more.

[0051] S: 0.0020% or less

[0052] S significantly deteriorates hot workability, and deteriorates low-temperature toughness due to segregation to prior austenite grain boundaries, and therefore it is preferable to reduce S as much as possible. When the content of S is 0.0020% or less, segregation of S to prior austenite grain boundaries can be suppressed, and low-temperature toughness targeted in the present application can be obtained. For this reason, the content of S is set to 0.0020% or less. It is preferable that the content of S be 0.0015% or less. It is more preferable that the content of S be 0.0010% or less, further preferably 0.0007% or less. Note that the lower limit of the content of S is not particularly limited. However, excessive reduction leads to an increase in manufacturing cost, and therefore it is preferable to set the content of S to 0.0005% or more.

[0053] Cr: 16.0 to 20.0%

[0054] Cr is an element that contributes to the resistance to crevice corrosion in the environment of untreated seawater by forming a protective coating film. In the present application, it is necessary to contain 16.0% or more of Cr. Therefore, the Cr content is set to 16.0% or more. The Cr content is preferably set to 16.5% or more, more preferably to 16.8% or more, further preferably to 17.0% or more. The Cr content is most preferably set to 17.5% or more. On the other hand, when Cr is contained in excess of 20.0%, martensitic transformation does not occur, and retained austenite is easily generated, whereby the stability of the martensite phase is reduced, and the strength targeted in the present application cannot be obtained. In addition to this, δ-ferrite phase is precipitated at the time of heating at high temperature, and hot workability is significantly reduced. Therefore, the Cr content is set to 20.0% or less. The Cr content is preferably set to 19.5% or less, more preferably to 19.0% or less, further preferably to 18.5% or less. The Cr content is most preferably set to 18.0% or less.

[0055] Ni: 4.0 to 7.5%

[0056] Ni is an element that has an effect of making the protective coating film firm and improving the resistance to crevice corrosion in the environment of untreated seawater. In addition, Ni suppresses the precipitation of δ-ferrite phase, and improves hot workability. In addition, Ni is solid-solved to increase the strength of the steel. Such an effect can be obtained when Ni is contained in 4.0% or more. Therefore, the Ni content is set to 4.0% or more. The Ni content is preferably set to 5.0% or more, more preferably to 6.0% or more, further preferably to 6.1% or more. The Ni content is most preferably set to 6.3% or more. On the other hand, when Ni is contained in excess of 7.5%, martensitic transformation does not occur, and retained austenite is easily generated, whereby the stability of the martensite phase is reduced, and the strength is reduced. Therefore, the Ni content is set to 7.5% or less. The Ni content is preferably set to 7.0% or less, further preferably to 6.5% or less.

[0057] Mo: 1.5 to 3.7%

[0058] Mo is an element that makes the protective coating film firm and improves the resistance to crevice corrosion in the environment of untreated seawater by suppressing the precipitation of δ-ferrite phase. In addition, Mo is solid-solved to increase the strength of the steel. Such an effect can be obtained when Mo is contained in 1.5% or more. Therefore, the Mo content is set to 1.5% or more. The Mo content is preferably set to 2.0% or more, more preferably to 2.5% or more, further preferably to 2.6% or more. The Mo content is most preferably set to 2.7% or more. On the other hand, when Mo is contained in excess of 3.7%, the protective coating film is not made firm, and the resistance to crevice corrosion in the environment of untreated seawater is not improved. In addition, the protective coating film is not made firm, and the resistance to crevice corrosion in the environment of untreated seawater is not improved. Therefore, the Mo content is set to 3.7% or less. The Mo content is preferably set to 3.5% or less, more preferably to 3.3% or less, further preferably to 3.2% or less. The Mo content is most preferably set to 3.0% or less. -, and an element that increases resistance to pitting caused by low pH. It is necessary in the present application to contain 1.5% or more of Mo. When less than 1.5% of Mo is contained, carbon dioxide corrosion resistance, crevice corrosion resistance under severe corrosion environments are reduced. Therefore, the Mo content is set to 1.5% or more. The Mo content is preferably set to 2.0% or more, more preferably to 2.2% or more, further preferably to 2.5% or more. The Mo content is most preferably set to 2.7% or more. On the other hand, when more than 3.7% of Mo is contained, δ-ferrite is generated, resulting in a decrease in hot workability and carbon dioxide corrosion resistance, SSC resistance under low temperature environments. Therefore, the Mo content is set to 3.7% or less. The Mo content is preferably set to 3.5% or less, more preferably to 3.3% or less, further preferably to 3.0% or less. The Mo content is most preferably set to 2.8% or less.

[0059] Al: 0.005 to 0.10%

[0060] Al is an element that functions as a deoxidizing agent. This effect can be obtained by containing 0.005% or more of Al. Therefore, the Al content is set to 0.005% or more. The Al content is preferably set to 0.01% or more, more preferably to 0.015% or more. The Al content is further preferably set to 0.017% or more, most preferably to 0.02% or more. On the other hand, when more than 0.10% of Al is contained, the amount of oxides becomes excessive, adversely affecting crevice corrosion resistance. Therefore, the Al content is set to 0.10% or less. The Al content is preferably set to 0.05% or less, more preferably to 0.04% or less, further preferably to 0.03% or less. The Al content is most preferably set to 0.025% or less.

[0061] N: 0.002 to 0.15%

[0062] N is an element that suppresses the generation of δ-ferrite at a low cost, improving hot workability. Such an effect can be obtained by containing 0.002% or more of N. Therefore, the N content is set to 0.002% or more. The N content is preferably set to 0.01% or more, more preferably to 0.02% or more. The N content is further preferably set to 0.03% or more, most preferably to 0.04% or more. On the other hand, when more than 0.15% of N is contained, coarse nitrides are generated, resulting in a decrease in crevice corrosion resistance. Therefore, the N content is set to 0.15% or less. The N content is preferably set to 0.12% or less, more preferably to 0.10% or less, further preferably to 0.08% or less. The N content is most preferably set to 0.06% or less.

[0063] Co: 0.2 to 1.0%

[0064] Co is an element that improves the resistance to intercrystalline corrosion. Such an effect can be obtained by containing 0.2% or more of Co. Therefore, the content of Co is set to 0.2% or more. The content of Co is preferably set to 0.25% or more. The content of Co is more preferably set to 0.3% or more, further preferably to 0.35% or more, and most preferably to 0.4% or more. On the other hand, even if Co exceeding 1.0% is contained, the effect is saturated. Therefore, in the case where Co is contained, the content of Co is set to 1.0% or less. The content of Co is preferably set to 0.8% or less, more preferably to 0.7% or less. The content of Co is further preferably set to 0.65% or less, and most preferably to 0.6% or less.

[0065] Nb: 0.005 to 0.20%

[0066] Nb is an element that increases the Ms point, and is an element required to balance the resistance to intercrystalline corrosion and high strength. Such an effect can be obtained by containing 0.005% or more of Nb. Therefore, the content of Nb is set to 0.005% or more. The content of Nb is preferably set to 0.01% or more, more preferably to 0.05% or more, and further preferably to 0.07% or more. The content of Nb is most preferably set to 0.09% or more. On the other hand, when Nb exceeding 0.20% is contained, the low-temperature toughness is deteriorated. Therefore, the content of Nb is set to 0.20% or less. The content of Nb is preferably set to 0.17% or less, more preferably to 0.15% or less, and further preferably to 0.13% or less. The content of Nb is most preferably set to 0.11% or less.

[0067] O (oxygen): 0.010% or less

[0068] O (oxygen) exists in the form of an oxide in steel, and adversely affects various characteristics. Therefore, O is preferably reduced as much as possible. In particular, when the content of O exceeds 0.010%, the resistance to intercrystalline corrosion is also significantly reduced. Therefore, the content of O is set to 0.010% or less. It is preferable that the content of O be 0.007% or less, and more preferably 0.004% or less. The content of O is further preferably 0.003% or less, and most preferably 0.002% or less. Excessive reduction leads to an increase in manufacturing cost, and therefore it is preferable that the content of O be set to 0.0005% or more.

[0069] one or both of Cu: 3.5% or less and W: 3.5% or less

[0070] Cu: 3.5% or less

[0071] Cu is an element that makes the protective coating film firm and improves the resistance to crevice corrosion, and can be contained as needed. Such an effect can be obtained by containing Cu at 0.5% or more, and therefore the Cu content is preferably set to 0.5% or more, more preferably to 0.7% or more. The Cu content is further preferably 1.0% or more, and most preferably 1.2% or more. On the other hand, when Cu is contained at more than 3.5%, CuS precipitates at the grain boundaries, and the hot workability is reduced. Therefore, the Cu content is set to 3.5% or less. The Cu content is preferably set to 3.0% or less, more preferably to 2.5% or less, further preferably to 2.0% or less. The Cu content is most preferably 1.5% or less.

[0072] W: 3.5% or less

[0073] W is an element that contributes to an increase in strength and improvement in the resistance to crevice corrosion, and can be contained as needed. Such an effect can be obtained by containing W at 0.05% or more, and therefore the W content is preferably set to 0.05% or more, more preferably to 0.2% or more, further preferably to 0.3% or more, and most preferably to 0.5% or more. On the other hand, even when W is contained at more than 3.5%, the effect is saturated. Therefore, the W content is set to 3.5% or less. The W content is preferably set to 3.0% or less, more preferably to 2.0% or less, and further preferably to 1.5% or less. The W content is most preferably 1.0% or less.

[0074] Note that one or both of Cu: 3.5% or less and W: 3.5% or less means, in the present application, Cu: 3.5% or less and W: 3.5% or less when Cu and W are contained, and one of Cu and W is more than 3.5% in the comparative example.

[0075] In addition, in the present application, Cr, Ni, Mo, W, Cu, and Co are contained in the above ranges and satisfy the following formula (1).

[0076] Cr + 0.22 x Ni + 0.38 x (Mo + 0.5 x W) + 0.89 x Cu + 0.09 x Co ≥ 21.4 (1)

[0077] Here, Cr, Ni, Mo, W, Cu, and Co in formula (1) are the contents (mass%) of the respective elements, and the content of an element not contained is set to 0.

[0078] (1) When the left side value of the formula ("the value of Cr + 0.22 x Ni + 0.38 x (Mo + 0.5 x W) + 0.89 x Cu + 0.09 x Co") is less than 21.4, the resistance to intergranular corrosion in an untreated seawater environment is reduced. Therefore, in the present application, with respect to Cr, Ni, Mo, W, Cu, and Co, they are contained in a manner so as to satisfy the formula (1). That is, the left side value of the formula (1) is set to 21.4 or more. The left side value of the formula (1) is preferably set to 21.6 or more, more preferably to 21.8 or more, and further preferably to 22.0 or more. The upper limit of the left side value of the formula (1) is not particularly set. From the viewpoint of suppressing cost increase due to excessive alloy addition and suppressing strength reduction, the left side value of the formula (1) is preferably set to 26.0 or less. More preferably, it is 24.0 or less, and further preferably, it is 23.8 or less.

[0079] In addition, in the present application, Co and Nb are contained in a range described above and in a manner so as to satisfy the following formula (2).

[0080] Co - Nb ≥ 0.13... (2)

[0081] Here, Co and Nb in the formula (2) are the contents (mass %) of each element.

[0082] As described above, by making the left side value of the formula (1) 21.4 or more, it is possible to obtain desired resistance to intergranular corrosion in an untreated seawater environment. For this reason, it is necessary to appropriately contain Cr, Ni, Mo, W, Cu, and Co, but among these elements, elements other than Co are elements that greatly reduce the Ms point, and when they are excessively contained, it is not possible to obtain desired high strength. On the other hand, in order to increase the Ms point, it is effective to add Nb, but when Nb is excessively contained, low-temperature toughness is deteriorated. Therefore, by containing Co, which is an element that does not reduce the Ms point but increases the resistance to intergranular corrosion, more than Nb by 0.13% or more, it is possible to balance excellent resistance to intergranular corrosion and high strength and low-temperature toughness. When the left side value of the formula (2) ("the value of Co - Nb") is less than 0.13, the low-temperature toughness value is reduced. Therefore, in the present application, with respect to Co and Nb, they are contained in a manner so as to satisfy the formula (2). The left side value of the formula (2) is preferably set to 0.13 or more. The left side value of the formula (2) is preferably set to 0.17 or more, more preferably to 0.20 or more, and further preferably to 0.30 or more. The upper limit of the left side value of the formula (2) is not particularly set. From the viewpoint of suppressing cost increase due to excessive alloy addition and suppressing strength reduction, the left side value of the formula (2) is preferably set to 1.00 or less. The left side value of the formula (2) is more preferably set to 0.80 or less.

[0083] In the present application, the balance other than the above-described components is constituted by iron (Fe) and unavoidable impurities.

[0084] The above components are basic components, and by the basic components, the oil well high-strength stainless steel seamless pipe of the present application can obtain the target properties. In the present application, in addition to the above basic components, the following optional elements can be contained as necessary. The following V, Ti, Zr, B, REM, Ca, Sn, Sb, Ta, Mg components can be contained as necessary, and therefore these components can also be 0%.

[0085] one or two or more selected from V: 0.50% or less, Ti: 0.20% or less, Zr: 0.20% or less, B: 0.01% or less, REM: 0.01% or less, Ca: 0.0100% or less, Sn: 0.20% or less, Sb: 0.50% or less, Ta: 0.1% or less, Mg: 0.0100% or less

[0086] V: 0.50% or less

[0087] V is an element that increases the strength of the steel by precipitation strengthening, and can be contained as necessary. The effect can be obtained by containing 0.005% or more of V, and therefore the V content is preferably set to 0.005% or more. The V content is more preferably set to 0.03% or more, and further preferably set to 0.04% or more. The V content is most preferably set to 0.05% or more. On the other hand, even if V exceeding 0.50% is contained, the low-temperature toughness decreases. Therefore, in the case where V is contained, the V content is set to 0.50% or less. The V content is preferably set to 0.40% or less, and more preferably set to 0.30% or less. The V content is further preferably set to 0.25% or less, and most preferably set to 0.20% or less.

[0088] Ti: 0.20% or less

[0089] Ti is an element that increases the chemical stability of inclusions in oxide or sulfide systems, thereby improving the crevice corrosion resistance in untreated seawater, and can be contained as necessary. Such an effect can be obtained by containing 0.002% or more of Ti, and therefore the Ti content is preferably set to 0.002% or more. The Ti content is more preferably set to 0.003% or more. On the other hand, when Ti exceeding 0.20% is contained, TiN is precipitated as inclusions, and the crevice corrosion resistance is rather deteriorated. Therefore, in the case where Ti is contained, the Ti content is set to 0.20% or less. The Ti content is preferably set to 0.15% or less, and more preferably set to 0.10% or less. The Ti content is further preferably set to 0.07% or less, and most preferably set to 0.05% or less.

[0090] Zr: 0.20% or less

[0091] Zr is an element that contributes to an increase in strength, and can be contained as needed. Such an effect can be obtained by containing 0.01% or more of Zr. Therefore, the Zr content is preferably set to 0.01% or more, and more preferably to 0.02% or more. On the other hand, even if Zr exceeding 0.20% is contained, the effect is saturated. Therefore, in the case where Zr is contained, the Zr content is set to 0.20% or less. The Zr content is preferably set to 0.17% or less, more preferably to 0.13% or less, further preferably to 0.10% or less. The Zr content is most preferably set to 0.07% or less.

[0092] B: 0.01% or less

[0093] B is an element that contributes to an increase in strength, and can be contained as needed. Such an effect can be obtained by containing 0.0005% or more of B, and therefore the B content is preferably set to 0.0005% or more. More preferably, it is set to 0.001% or more. Further preferably, it is 0.002% or more. On the other hand, when B exceeding 0.01% is contained, hot workability is reduced. Therefore, in the case where B is contained, the B content is set to 0.01% or less. The B content is preferably set to 0.007% or less, and more preferably to 0.005% or less. The B content is further preferably set to 0.003% or less.

[0094] REM: 0.01% or less

[0095] REM (rare earth metal) is an element that contributes to improvement in intergranular corrosion resistance, and can be contained as needed. Such an effect can be obtained by containing 0.0005% or more of REM, and therefore it is preferably set to 0.0005% or more. More preferably, it is set to 0.001% or more. The REM content is further preferably 0.0015% or more. On the other hand, even if REM exceeding 0.01% is contained, the effect is saturated, and an effect matching the content cannot be expected, and becomes economically disadvantageous. Therefore, in the case where REM is contained, the REM content is set to 0.01% or less. The REM content is more preferably set to 0.007% or less. The REM content is further preferably 0.005% or less, and most preferably 0.003% or less.

[0096] Ca: 0.0100% or less

[0097] Ca is an element that contributes to improvement in the crevice corrosion resistance, and can be contained as needed. Such an effect can be obtained by containing 0.0005% or more of Ca. Therefore, the Ca content is preferably set to 0.0005% or more. The Ca content is more preferably set to 0.0010% or more. The Ca content is further preferably 0.0015% or more. On the other hand, when Ca is contained in excess of 0.0100%, the number density of coarse Ca-based inclusions increases, and the desired crevice corrosion resistance cannot be obtained. Therefore, in the case where Ca is contained, the Ca content is set to 0.0100% or less. The Ca content is more preferably set to 0.0070% or less. The Ca content is further preferably set to 0.0050% or less, and most preferably to 0.0030% or less.

[0098] Sn: 0.20% or less

[0099] Sn is an element that contributes to improvement in the crevice corrosion resistance, and can be contained as needed. Such an effect can be obtained by containing 0.02% or more of Sn, and therefore the Sn content is preferably set to 0.02% or more, and more preferably to 0.05% or more. The Sn content is further preferably set to 0.07% or more. On the other hand, even if Sn is contained in excess of 0.20%, the effect is saturated, and an effect matching the content cannot be expected, and becomes economically disadvantageous. Therefore, in the case where Sn is contained, the Sn content is set to 0.20% or less. The Sn content is more preferably set to 0.15% or less. The Sn content is further preferably set to 0.13% or less, and most preferably to 0.10% or less.

[0100] Sb: 0.50% or less

[0101] Sb is an element that contributes to improvement in the crevice corrosion resistance, and can be contained as needed. Such an effect can be obtained by containing 0.02% or more of Sb, and therefore the Sb content is preferably set to 0.02% or more. More preferably, it is set to 0.05% or more. On the other hand, even if Sb is contained in excess of 0.50%, the effect is saturated, and an effect matching the content cannot be expected, and becomes economically disadvantageous. Therefore, in the case where Sb is contained, the Sb content is set to 0.50% or less. The Sb content is preferably set to 0.40% or less, more preferably to 0.30% or less, and further preferably to 0.15% or less. The Sb content is most preferably set to 0.10% or less.

[0102] Ta: 0.1% or less

[0103] Ta is an element that increases strength, and also has the effect of improving resistance to intercrystalline corrosion. In addition, Ta is an element that brings about the same effect as Nb, and a portion of Nb can be replaced with Ta. Such an effect can be obtained by containing 0.01% or more of Ta, and therefore the Ta content is preferably set to 0.01% or more. The Ta content is more preferably set to 0.03% or more. The Ta content is further preferably set to 0.04% or more. On the other hand, when Ta is contained in excess of 0.1%, low-temperature toughness decreases. Therefore, in the case where Ta is contained, the Ta content is set to 0.1% or less. The Ta content is preferably set to 0.09% or less, more preferably to 0.07% or less. The Ta content is further preferably set to 0.06% or less, and most preferably to 0.05% or less.

[0104] Mg: 0.0100% or less

[0105] Mg is an element that improves resistance to intercrystalline corrosion, and can be contained as needed. Such an effect can be obtained by containing 0.0002% or more of Mg, and therefore the Mg content is preferably set to 0.0002% or more, more preferably to 0.0004% or more. On the other hand, even if Mg is contained in excess of 0.0100%, the effect saturates, and an effect matching the content cannot be expected. Therefore, in the case where Mg is contained, the Mg content is preferably set to 0.0100% or less. The Mg content is preferably set to 0.0080% or less, more preferably to 0.0050% or less, further preferably to 0.0020% or less. The Mg content is most preferably set to 0.0010% or less.

[0106] Next, the steel pipe organization of the high-strength stainless steel seamless pipe for oil wells of the present application is not particularly limited, but is preferably set to an organization described below, for example.

[0107] The high-strength stainless steel seamless pipe for oil wells of the present application preferably has a steel pipe organization composed of a martensite phase (tempered martensite phase), a residual austenite phase, and a ferrite phase.

[0108] The presence of an excess of the residual austenite phase decreases strength, and therefore the residual austenite phase is preferably set to 32% or less in area ratio. The residual austenite phase is more preferably set to 30% or less in area ratio, further preferably to 28% or less in area ratio. The lower limit is preferably 1% or more. In a state where only a small amount of ferrite phase is present, strain concentrates on the ferrite phase during hot working, decreasing hot workability, and therefore the ferrite phase is preferably set to 14% or more in area ratio. The ferrite phase is more preferably set to 16% or more in area ratio, further preferably to 18% or more in area ratio. The upper limit is preferably 50% or less.

[0109] Each of the above organizations can be measured by the following method.

[0110] First, a test piece for microstructure observation is cut out from the center of the wall thickness of a section orthogonal to the pipe axis direction, etched with Vilella reagent (a reagent obtained by mixing picric acid, hydrochloric acid, and ethanol at a ratio of 2 g, 10 ml, and 100 ml, respectively), photographed with a scanning electron microscope (magnification: 1000 times), and the microstructure fraction (area %) of ferrite phase is calculated using an image analysis device.

[0111] Then, for the test piece for X-ray diffraction, grinding and polishing are performed in a manner such that a section orthogonal to the pipe axis direction (C section) is the measurement surface, and the amount of residual austenite (γ) is measured using an X-ray diffraction method. Regarding the amount of residual austenite, the diffraction X-ray integrated intensity of the (220) plane of γ and the (211) plane of α (ferrite) is measured, and converted using the following formula. Note that, here, the volume fraction of residual austenite is regarded as the area fraction.

[0112] γ (volume fraction) = 100 / (1 + (IaRy / IyRa))

[0113] Here, Ia: integrated intensity of α, Ra: crystallographically theoretically calculated value of α, Iy: integrated intensity of γ, Rγ: crystallographically theoretically calculated value of γ.

[0114] In addition, the fraction (area %) of the martensite phase (tempered martensite phase) is set as the balance other than the ferrite phase and the residual γ phase. The fraction of the martensite phase is preferably 18% or more in terms of area fraction. More preferably, it is 30% or more. In addition, it is preferably 85% or less. More preferably, it is 75% or less.

[0115] Next, one embodiment of the method for manufacturing the high-strength stainless steel seamless pipe for oil wells of the present application will be described, but is not particularly limited to the following content.

[0116] Note that, in the following description of the manufacturing method, the temperature (°C) is set to the surface temperature of the pipe raw material and the pipe (the seamless pipe after pipe manufacturing) unless otherwise specified. These surface temperatures can be measured using a radiation thermometer or the like.

[0117] In the present application, a pipe raw material having the above-described composition is used as the initial raw material. The method for manufacturing the pipe raw material as the initial raw material is not particularly limited. For example, molten steel having the above-described composition is preferably melted by a converter or the like melting method, and a pipe billet or the like pipe raw material is manufactured by a continuous casting method or an ingot-billet-rolling method or the like.

[0118] Next, the steel pipe raw material is heated (heating step), and after the heated steel pipe raw material is made into a hollow pipe blank by a Mannesmann-plug mill process or a Mannesmann-mandrel mill process using a piercer, hot working is performed, and pipe making (pipe making step) is performed. Thus, a seamless steel pipe having the above composition and a desired size (a prescribed shape) is made. Note that the seamless steel pipe can also be made by hot extrusion using a press method.

[0119] In the heating step of the above steel pipe raw material, the heating temperature is preferably set to a temperature in the range of 1100 to 1350°C. When the heating temperature is lower than 1100°C, the hot workability decreases, and defects often occur during pipe making. Therefore, the heating temperature is preferably set to 1100°C or higher, and more preferably set to 1150°C or higher. The heating temperature is further preferably set to 1170°C or higher, and most preferably set to 1200°C or higher. On the other hand, when the heating temperature exceeds 1350°C and becomes high, the grains become coarse, and the low-temperature toughness decreases. Therefore, the heating temperature in the heating step is preferably set to 1350°C or lower. The above heating temperature is more preferably set to 1300°C or lower. The heating temperature is further preferably set to 1280°C or lower, and most preferably set to 1250°C or lower.

[0120] The pipe-made seamless steel pipe is cooled to room temperature at a cooling rate of air cooling or higher. Thus, a steel pipe organization in which a martensite phase is the main phase can be ensured.

[0121] In the present application, it is preferable to perform heat treatment (quenching treatment, tempering treatment) on the steel pipe (pipe-made seamless steel pipe) after the cooling at a cooling rate of air cooling or higher after the pipe making.

[0122] Specifically, it is preferable to perform quenching treatment in which the steel pipe (pipe-made seamless steel pipe) is reheated to a temperature (heating temperature) in the range of 850°C or higher and 1120°C or lower and held for a prescribed time, and then cooled to a temperature at which the steel pipe surface temperature becomes 100°C or lower (cooling stop temperature) at a cooling rate of air cooling or higher. Here, the "cooling rate of air cooling or higher" means 0.01°C / s or higher.

[0123] Thus, the above martensite phase and high-strength can be achieved. Therefore, the reheating temperature is preferably set to 850°C or higher.

[0124] To prevent coarsening of the structure and to melt the intermetallic compound, the temperature of the reheating (heating temperature of the quenching treatment) is more preferably set to 870°C or higher. It is further preferably set to 900°C or higher. The temperature of the reheating is most preferably set to 950°C or higher. It is preferably set to a temperature in the range of 1120°C or lower. The temperature of the reheating is more preferably set to 1100°C or lower, and is further preferably set to 1050°C or lower. It is most preferably set to 1000°C or lower.

[0125] From the viewpoint of ensuring the uniformity of the temperature, it is preferable to keep the steel pipe at the above-mentioned temperature of the reheating for 5 minutes or more. The time of the keeping is more preferably set to 10 minutes or more, and is further preferably set to 15 minutes or more. In addition, the time of the keeping is preferably set to 30 minutes or less. The time of the keeping is more preferably set to 25 minutes or less, and is further preferably set to 20 minutes or less.

[0126] From the viewpoint of ensuring the yield strength (YS) targeted in the present application, the temperature of the cooling stop after the quenching treatment is preferably set to 100°C or lower. The temperature of the cooling stop is more preferably set to 75°C or lower, and is further preferably set to 50°C or lower. In addition, the temperature of the cooling stop is preferably set to 30°C or higher, and is more preferably set to 40°C or higher.

[0127] For the steel pipe subjected to the above-mentioned quenching treatment, a tempering treatment is then performed. The tempering treatment is preferably set to a treatment of heating to a temperature of 500°C or higher and 650°C or lower (tempering temperature) and keeping for a prescribed time, and then performing air cooling. Instead of all or a part of the air cooling, other cooling such as water cooling, oil cooling, spray cooling, etc. can be performed.

[0128] When the tempering temperature is lower than 500°C, the strength becomes too high, and it is difficult to ensure the desired low-temperature toughness. Therefore, the tempering temperature is preferably set to 500°C or higher. The tempering temperature is more preferably set to 530°C or higher. The tempering temperature is further preferably set to 550°C or higher, and is most preferably set to 570°C or higher. Thereby, the structure of the steel pipe easily becomes a structure in which a tempered martensite phase is a main phase, and thus becomes a seamless steel pipe having the strength and the resistance to crevice corrosion targeted in the present application. On the other hand, when the tempering temperature is too high, a fresh martensite phase is precipitated after the tempering, and it is not possible to ensure the desired high strength. Therefore, the tempering temperature is preferably 650°C or lower. The tempering temperature is more preferably set to 640°C or lower. It is further preferably set to 620°C or lower. The tempering temperature is most preferably set to 600°C or lower.

[0129] In addition, from the viewpoint of ensuring the uniformity of the material, it is preferable to keep the steel pipe at the above-mentioned tempering temperature for 10 minutes or more. The time of the keeping is preferably set to 90 minutes or less.

[0130] In addition, in the present application, the above-described quenching treatment and tempering treatment can be repeated two or more times. Thereby, the low-temperature toughness value is improved. The upper limit of the number of times of quenching treatment and tempering treatment is not particularly limited, but from the viewpoint of preventing an increase in manufacturing cost, it is preferably set to three or less.

[0131] The above has been described taking a seamless steel pipe as an example, but the present application is not limited thereto. A resistance-welded steel pipe, a UOE steel pipe can also be manufactured using the steel pipe raw material having the above-described composition as an oil well steel pipe. In this case, as long as the obtained oil well steel pipe is subjected to quenching treatment and tempering treatment under the above-described conditions, an oil well high-strength stainless seamless steel pipe of the present application can be obtained.

[0132] As described above, according to the present application, an absorbed energy vE at a test temperature of -10°C in a Charpy impact test of 40 J or more can be obtained. -10 an oil well high-strength stainless seamless steel pipe having a yield strength YS of 758 MPa or more.

[0133] an absorbed energy vE at a test temperature of -10°C in a Charpy impact test of 40 J or more. -10 an absorbed energy vE at a test temperature of -10°C in a Charpy impact test of 40 J or more. -10 is preferably 50 J or more, more preferably 60 J or more, and further preferably 70 J or more. The upper limit is not particularly limited, and can be 200 J or less.

[0134] In addition, the yield strength YS is 758 MPa or more. The yield strength YS is preferably 800 MPa or more, and more preferably 850 MPa or more. The upper limit is not particularly limited, and can be 1000 MPa or less.

[0135] In addition, the hot workability of an intermediate product (billet or the like) at an intermediate stage of manufacturing a product is excellent.

[0136] Note that the hot workability can be evaluated by the following method.

[0137] A round bar test piece having a round bar shape with a parallel portion diameter of 10 mm is cut from a steel pipe raw material (cast sheet), heated to 1250°C using a Gleeble tester, held for 100 seconds, then cooled to 1000°C at 1°C / sec, held for 10 seconds, and then stretched to break, and the cross-sectional reduction rate (%) is measured. The smaller the cross-sectional reduction rate, the more it can be judged that the hot workability is worse. The above-described cross-sectional reduction rate is preferably 60% or more, and more preferably 70% or more. In addition, the cross-sectional reduction rate is preferably 90% or less. The cross-sectional reduction rate is more preferably 85% or less.

[0138] Example

[0139] Hereinafter, the present application will be described based on Examples. Note that the present application is not limited to the following Examples.

[0140] A molten steel having the composition shown in Table 1 was melted with a vacuum melting furnace to obtain a cast slab (steel pipe raw material). The obtained cast slab was heated at 1250°C for all levels to perform hot working.

[0141] Next, a test piece raw material was cut out from the steel material obtained by the hot working. Here, the dimensions of the steel material were set to a length of 1100 mm, a width of 160 mm, and a thickness of 15 mm. Using each test piece raw material, quenching treatment was performed after heating at the heating temperature (temperature of reheating) shown in Table 2 and soaking for the time shown in Table 2 to air cool to the cooling stop temperature shown in Table 2. Further, tempering treatment was performed by heating at the tempering temperature shown in Table 2 and soaking for the time shown in Table 2, and air cooling. A part of the test pieces (steel pipes No. 2, 4) were repeatedly subjected to quenching treatment and tempering treatment twice under the conditions shown in Table 2. Note that the cut test pieces were subjected to quenching treatment and tempering treatment, but can be considered to be the same as the case where a seamless steel pipe is subjected to quenching treatment and tempering treatment.

[0142] Then, using each test piece raw material after the quenching treatment and the tempering treatment were performed, evaluation of the tensile properties, the Charpy impact test properties, the corrosion properties, and the measurement of the microstructure were performed by the methods described below. Regarding the evaluation of the hot workability, the above-described cast slab was used, and the evaluation was performed by the methods described below.

[0143] [Evaluation of Tensile Properties]

[0144] A JIS (Japanese Industrial Standards) No. 14A tensile test piece (Φ6.0 mm) was cut out from the test piece raw material after the quenching treatment and the tempering treatment were performed, and a tensile test was performed in accordance with the provisions of JIS Z2241:2011 to obtain the tensile properties (yield strength (YS), tensile strength (TS)). Here, a specimen having a yield strength (YS) of 758 MPa or more was set as acceptable, and a specimen having a yield strength of less than 758 MPa was set as unacceptable.

[0145] [Evaluation of Charpy Impact Test Properties]

[0146] A V-notch test piece (10 mm thick) was cut out from the test piece raw material after the quenching treatment and the tempering treatment were performed in such a manner that the length direction of the test piece was orthogonal to the molding direction, and a Charpy impact test was performed in accordance with the provisions of JIS Z 2242 (2018). The test temperature was set to -10°C, and the absorbed energy vE at -10°C was obtained. -10The low temperature toughness was evaluated. Note that three test pieces were prepared, and the arithmetic average of the values obtained was used as the absorbed energy (J) of the stainless steel member. Here, the absorbed energy vE -10 of -10°C was evaluated as high toughness for a test piece of 40 J or more, and was recorded as pass. On the other hand, a test piece of vE -10 less than 40 J was recorded as fail.

[0147] [evaluation of corrosion properties]

[0148] A corrosion test piece having a hole of Φ 12 mm, a size of thickness 3 mm, width 20 mm, and length 50 mm was prepared by machining a test piece raw material after quenching and tempering, and a corrosion test was performed.

[0149] In the corrosion test, a jig for making a crevice made of fluorine resin was inserted into the hole of the test piece, and a crevice was formed by pressing the surface of the test piece at a torque of 20 N / mm 2 . An artificial sea water (liquid temperature: 25°C) was used as a test liquid, and the corrosion test piece was immersed, and the immersion time was set to 30 days. In the test, bubbling of the atmosphere in the test liquid was performed. For the corrosion test piece after the test, a magnifying glass with a magnification of 10 times was used to observe the test piece surface for the presence or absence of crevice corrosion. Here, a test piece in which no crevice corrosion occurred (indicated as "none" in the column of "crevice corrosion" in Table 3) was recorded as pass, and a test piece in which crevice corrosion occurred (indicated as "yes" in the column of "crevice corrosion" in Table 3) was recorded as fail.

[0150] Note that the case where no crevice corrosion occurred was evaluated as "excellent in crevice corrosion resistance".

[0151] [evaluation of hot workability]

[0152] In the evaluation of hot workability, a round bar test piece of round bar shape with a parallel portion diameter of 10 mm was cut from a cast piece, and was heated to 1250°C using a Gleeble tester, was held for 100 seconds, was cooled to 1000°C at 1°C / sec, was held for 10 seconds, and was stretched to break, and the cross-sectional reduction rate (%) was measured. The smaller the cross-sectional reduction rate, the worse the hot workability.

[0153] [measurement of microstructure]

[0154] Test pieces for observation of microstructure were produced from the test piece raw material after the quenching and tempering treatment, and each microstructure was measured. The observation surface of the microstructure was set to a cross section orthogonal to the rolling direction (C cross section). First, the test piece for observation of microstructure was etched with Vilella reagent (a reagent obtained by mixing picric acid, hydrochloric acid, and ethanol at a ratio of 2 g, 10 ml, and 100 ml, respectively), and the microstructure was photographed using a scanning electron microscope (acceleration voltage: 15 kV, magnification: 1000 times). The fraction of ferrite phase (area %) was calculated using an image analysis device (Image-J).

[0155] Then, for the test piece for X-ray diffraction, grinding and polishing were performed so that a cross section orthogonal to the rolling direction (C cross section) was the measurement surface, and the amount of residual austenite (γ) was measured using an X-ray diffraction method. Regarding the amount of residual austenite, the diffraction X-ray integrated intensity of the (220) plane of γ and the (211) plane of α (ferrite) was measured, and conversion was performed using the following formula. Note that, here, the volume fraction of residual austenite was regarded as the area fraction.

[0156] γ (volume fraction) = 100 / (1 + (IaRy / IyRa))

[0157] Here, Ia: integrated intensity of α, Ra: crystallographic theoretical calculation value of α, Iy: integrated intensity of γ, Rγ: crystallographic theoretical calculation value of γ.

[0158] In addition, the fraction (area %) of the martensite phase (tempered martensite phase) was set to the balance other than the ferrite phase and the residual γ phase.

[0159] The results obtained are shown in Table 3.

[0160]

[0161]

[0162]

Claims

1. A high-strength stainless steel seamless pipe for oil wells, comprising, by mass%, C: 0.002-0.050%, Si: 0.05-0.50%, Mn: 0.04-1.80%, P: less than 0.030%, S: less than 0.0020%, Cr: 16.0-20.0%, Ni: 4.0-7.5%, Mo: 1.5-3.7%, Al: 0.005-0.10%, N: 0.002-0.15%, Co: 0.2-1.0%, Nb: 0.005-0.20%, O: less than 0.010%, and further comprising one or two selected from Cu: less than 3.5% and W: less than 3.5%, and satisfying formulas (1) and (2), with the balance being Fe and unavoidable impurities. The high-strength stainless steel seamless pipe for oil wells has a yield strength of over 758 MPa and an absorbed energy vE at a Charpy impact test temperature of -10℃. -10 40J or above Cr+0.22×Ni+0.38×(Mo+0.5×W)+0.89×Cu+0.09×Co≥21.4…(1) Here, Cr, Ni, Mo, W, Cu, and Co in equation (1) represent the content (mass%) of each element, and the content of elements not present is set to 0. Co-Nb≥0.13 …(2) Here, Co and Nb in equation (2) represent the content (mass %) of each element.

2. The high-strength stainless steel seamless pipe for oil wells according to claim 1, wherein, Based on the aforementioned composition, the material further contains, by mass%, one or more of the following: V: less than 0.50%, Ti: less than 0.20%, Zr: less than 0.20%, B: less than 0.01%, REM: less than 0.01%, Ca: less than 0.0100%, Sn: less than 0.20%, Sb: less than 0.50%, Ta: less than 0.1%, and Mg: less than 0.0100%.

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