High-strength stainless steel seamless pipe for oil wells

Abstract

To provide a high-strength stainless steel seamless pipe for oil wells, the high-strength stainless steel seamless pipe having high strength and excellent low-temperature toughness and being excellent in crevice corrosion resistance in an untreated seawater environment.　There is provided a high-strength stainless steel seamless steel pipe for oil wells having a component composition that comprises specific components and that satisfies relationship (1) and relationship (2), with the balance being Fe and unavoidable impurities, having a yield strength of 758 MPa or above, and having an absorption energy vE-10 of 40 J or above at a test temperature of -10°C in a Charpy impact test. Relationship (1): Cr + 0.22 × Ni + 0.38 × (Mo + 0.5 × W) + 0.89 × Cu + 0.09 × Co ≥ 21.4 Cr, Ni, Mo, W, Cu and Co in relationship (1) are the contained amounts (mass %) of the respective elements, and the contained amount of elements that are not contained is deemed to be zero. Relationship (2): Co-Nb ≥ 0.13 Co and Nb in relationship (2) are the contained amounts (mass %) of the respective elements.

Description

High-strength stainless steel seamless pipe for oil wells

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

[0002] In recent years, due to the soaring crude oil prices and the expected depletion of petroleum resources in the near future, there has been active development of oil fields and gas fields that have not been considered in the past, such as deep oil fields and oil fields and gas fields that are in a sour environment containing hydrogen sulfide and other substances and are highly corrosive. Such oil and gas fields are generally located at extremely deep depths, and their atmospheres are high in temperature and contain CO2, Cl, etc. － Furthermore, the environment is severely corrosive, containing H2S. Steel pipes for oil wells used in such environments are required to be made of a material that combines the desired high strength and corrosion resistance.

[0003] Conventionally, carbon dioxide (CO2), chloride ions (Cl － In oil and gas fields in environments containing heavy metals, 13Cr martensitic stainless steel pipes are widely used as oil country tubular goods for mining. Furthermore, recently, the use of improved 13Cr martensitic stainless steel, which has a chemical composition in which the C content of 13Cr martensitic stainless steel is reduced and the Ni, Mo, etc. is increased, has also become widespread.

[0004] To meet such demands, there are techniques such as those described in Patent Documents 1 to 5.

[0005] Patent Document 1 discloses a stainless steel pipe for oil wells having improved corrosion resistance, which contains, by 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%, and O: 0.006% or less, and which satisfies a predetermined formula, with the balance being Fe and unavoidable impurities.

[0006] Patent Document 2 discloses a high-strength stainless steel seamless pipe for oil wells 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 predetermined formula, with the remainder being Fe and unavoidable impurities. This discloses a high-strength stainless steel seamless pipe for oil wells having a yield strength of 655 MPa or more.

[0007] Furthermore, Patent Document 3 describes the following components 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. The present invention discloses a high-strength stainless steel pipe for oil wells that has high strength and high corrosion resistance, and that has a composition containing 0.006% or less of Nb, one or two selected from 0.2% or less of Nb and 0.3% or less of Ti, with the balance being Fe and unavoidable impurities, and that has a structure in which MC-type carbonitrides exist in the precipitates in an amount of 3.0% or more by mass relative to the total amount of precipitates.

[0008] Patent Document 4 discloses a high-strength stainless steel pipe for oil wells having a composition containing Cr and Ni and a structure in which the main phase is a tempered martensite phase, the composition satisfying Cr / Ni≦5.3, and the high-strength stainless steel seamless pipe for oil wells having a surface structure in which a phase that turns white when etched with a Vilela etching solution has a thickness of 10 μm to 100 μm from the outer surface of the pipe in the wall thickness direction and is dispersed in an area ratio of 50% or more of the outer surface of the pipe.

[0009] Patent Document 5 discloses a high-strength martensitic stainless steel seamless pipe for oil wells that contains, by 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, with the balance being Fe and unavoidable impurities. This gives the pipe a yield strength of 655 to 862 MPa and a yield ratio of 0.90 or more, and has improved carbon dioxide corrosion resistance and sulfide stress corrosion cracking resistance.

[0010] International Publication No. 2004 / 001082 International Publication No. 2017 / 168874 JP 2005-105357 A International Publication No. 2015 / 178022 JP 2012-136742 A

[0011] In recent years, a technique called water injection, in which water is injected into geological formations using seamless steel pipes, has been used to improve crude oil recovery rates. Seawater is often used for water injection because it is abundant. Chloride ions, dissolved oxygen, microorganisms, and other substances present in seawater increase corrosiveness, so they are sometimes removed. However, due to the cost involved, untreated seawater is sometimes used for water injection. Seamless steel pipes used in such environments are required to have high corrosion resistance. However, the technologies described in Patent Documents 1 to 5, while providing good resistance to carbon dioxide corrosion, lack sufficient crevice corrosion resistance in untreated seawater environments. Furthermore, with the increasing popularity of development in cold regions and deep seas, low-temperature toughness is also required.

[0012] Therefore, an object of the present invention is to solve the problems of the conventional technology and to provide a high-strength stainless steel seamless pipe for oil wells which has high strength and excellent low-temperature toughness, as well as excellent crevice corrosion resistance in an untreated seawater environment.

[0013] In the present invention, "high strength" refers to a steel having a yield strength YS of 110 ksi (758 MPa) or more.

[0014] In addition, "excellent low-temperature toughness" refers to a Charpy impact test performed on a V-notch test piece (10 mm thick) in accordance with the provisions of JIS Z 2242, with the longitudinal direction of the test piece perpendicular to the molding direction and the notch parallel to the molding direction. The test temperature in the Charpy impact test is -10 ° C. The absorbed energy vE －１０ is 40J or more.

[0015] Furthermore, in the present invention, "excellent crevice corrosion resistance in untreated seawater" refers to a case in which a test piece having a crevice formed therein is immersed in artificial seawater (liquid temperature: 25°C, atmospheric saturation at 1 atmospheric pressure) for a period of 30 days, and the test piece after the corrosion test is observed for the presence or absence of crevice formation on the surface of the test piece using a magnifying glass with a magnification of 10x, and no crevice corrosion of a depth of 0.1 mm or more is found to have occurred.

[0016] The methods for each of the above tests are also described in detail in the examples below.

[0017] To achieve the above-mentioned objective, the present inventors conducted extensive research into the effects of various chemical compositions of stainless steel pipes on crevice corrosion resistance in untreated seawater environments. As a result, they discovered that the contents of Cr, Mo, Cu, Ni, W, and Co in the chemical composition of the stainless steel material must be adjusted to satisfy formula (1): Cr + 0.22 × Ni + 0.38 × (Mo + 0.5 × W) + 0.89 × Cu + 0.09 × Co ≥ 21.4 (1) where Cr, Ni, Mo, W, Cu, and Co in formula (1) represent the content (mass%) of each element, and elements that are not present are represented by a zero content. They also discovered that in order to obtain a desired low-temperature toughness value while still satisfying crevice corrosion resistance, the contents of Nb and Co must be adjusted to satisfy formula (2). Co-Nb≧0.13 (2) Here, Co and Nb in formula (2) are the contents (mass %) of each element.

[0018] The present invention was completed based on these findings and further studies. That is, the gist of the present invention is as follows: [1] A steel sheet containing, by 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%, and O: 0.010% or less, and further containing one or two selected from Cu: 3.5% or less and W: 3.5% or less, and satisfying formula (1) and formula (2), with the balance consisting of Fe and unavoidable impurities, and having a yield strength of 758 MPa or more and an absorbed energy vE －１０ High-strength stainless steel seamless pipe for oil wells having a strain rate of 40J or more. 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 formula (1) represent the content (mass%) of each element, and the content of elements that are not contained is zero. Co - Nb ≧ 0.13 (2) Here, Co and Nb in formula (2) represent the content (mass%) of each element. [2] A high-strength stainless steel seamless pipe for oil wells according to [1], which further contains, in addition to the above-mentioned chemical composition, one or more selected from, by mass%, 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.

[0019] According to the present invention, it is possible to provide a high-strength stainless steel seamless pipe for oil wells which has high strength and excellent low-temperature toughness, as well as excellent crevice corrosion resistance in untreated seawater.

[0020] The present invention will be described in detail below, but the present invention is not limited to the following embodiments.

[0021] First, the chemical composition of the high-strength stainless steel seamless pipe for oil well use according to the present invention and the reasons for limiting it will be explained. Unless otherwise specified, % by mass will be simply referred to as "%".

[0022] C: 0.002 to 0.050% C is an important element that increases the strength of martensitic stainless steel. In the present invention, a C content of 0.002% or more is necessary to ensure the desired strength. Therefore, the C content is set to 0.002% or more. The C content is preferably set to 0.010% or more, more preferably set to 0.015% or more, and even more preferably set to 0.020% or more. The C content is most preferably set to 0.022% or more. On the other hand, if the C content exceeds 0.050%, the strength decreases. Furthermore, crevice corrosion resistance in an untreated seawater environment also deteriorates. Therefore, in the present invention, the C content is set to 0.050% or less. Preferably, it is set to 0.040% or less. More preferably, it is set to 0.035% or less, and even more preferably, it is set to 0.030% or less. The C content is most preferably set to 0.028% or less.

[0023] Si: 0.05 to 0.50% Si is an element that acts as a deoxidizer. This effect is achieved with a Si content of 0.05% or more. Therefore, the Si content is set to 0.05% or more. The Si content is preferably set to 0.10% or more, more preferably set to 0.15% or more. The Si content is further preferably set to 0.20% or more, and most preferably set to 0.22% or more. On the other hand, a Si content of more than 0.50% deteriorates crevice corrosion resistance in an untreated seawater environment. Therefore, the Si content is set to 0.50% or less. The Si content is preferably set to 0.45% or less, more preferably set to 0.40% or less, and even more preferably set to 0.30% or less. The Si content is most preferably set to 0.25% or less.

[0024] Mn: 0.04 to 1.80% Mn is an element that suppresses the formation of δ-ferrite during hot working and improves hot workability. The present invention requires a Mn content of 0.04% or more. Therefore, the Mn content is set to 0.04% or more. The Mn content is preferably set to 0.10% or more, more preferably set to 0.20% or more, and even more preferably set to 0.25% or more. The Mn content is most preferably set to 0.35% or more. On the other hand, excessive Mn content deteriorates crevice corrosion resistance in untreated seawater environments. Therefore, the Mn content is set to 1.80% or less. The Mn content is preferably set to 1.60% or less, more preferably set to 0.80% or less, even more preferably set to 0.60% or less, and most preferably set to 0.40% or less.

[0025] P: 0.030% or less P is an element that reduces crevice corrosion resistance in an untreated seawater environment. In the present invention, it is preferable to reduce the P content as much as possible, but extreme reduction leads to increased production costs. Therefore, the P content is set to 0.030% or less, which is a range that can be implemented industrially at relatively low cost without causing an extreme deterioration in properties. Preferably, the P content is 0.025% or less, more preferably 0.020% or less. The P content is further preferably 0.018% or less, and most preferably 0.015% or less. Note that there is no particular lower limit for the P content. However, as mentioned above, excessive reduction leads to increased production costs, so the P content is preferably 0.005% or more.

[0026] S: 0.0020% or less S significantly reduces hot workability and deteriorates low-temperature toughness by segregating to prior austenite grain boundaries, so it is preferable to reduce the content as much as possible. If the S content is 0.0020% or less, segregation of S to prior austenite grain boundaries is suppressed, and the low-temperature toughness targeted in the present invention can be obtained. For these reasons, the S content is set to 0.0020% or less. Preferably, the S content is 0.0015% or less. More preferably, the S content is 0.0010% or less, and even more preferably, 0.0007% or less. Note that there is no particular lower limit for the S content. However, since excessive reduction leads to increased manufacturing costs, the S content is preferably set to 0.0005% or more.

[0027] Cr: 16.0 to 20.0% Cr is an element that forms a protective film and contributes to crevice corrosion resistance in untreated seawater environments. The present invention requires a Cr content of 16.0% or more. Therefore, the Cr content is set to 16.0% or more. The Cr content is preferably set to 16.5% or more, more preferably 16.8% or more, and even more preferably 17.0% or more. The Cr content is most preferably set to 17.5% or more. On the other hand, a Cr content exceeding 20.0% prevents martensitic transformation and makes it easier for retained austenite to form, reducing the stability of the martensite phase and preventing the desired strength from being achieved. In addition, δ-ferrite phase precipitates during high-temperature heating, significantly reducing hot workability. Therefore, the Cr content is set to 20.0% or less. The Cr content is preferably set to 19.5% or less, more preferably 19.0% or less, and even more preferably 18.5% or less. The Cr content is most preferably 18.0% or less.

[0028] Ni: 4.0 to 7.5% Ni is an element that strengthens the protective coating and improves crevice corrosion resistance in untreated seawater environments. Ni also suppresses the precipitation of the δ-ferrite phase, improving hot workability. Ni also dissolves and increases the strength of the steel. These effects are achieved with a Ni content of 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 6.0% or more, and even more preferably 6.1% or more. The Ni content is most preferably set to 6.3% or more. On the other hand, a Ni content exceeding 7.5% prevents martensitic transformation and makes it easier for retained austenite to form, reducing the stability of the martensitic phase and decreasing strength. Therefore, the Ni content is set to 7.5% or less. The Ni content is preferably set to 7.0% or less, and even more preferably 6.5% or less.

[0029] Mo: 1.5 to 3.7% Mo is Cl －Mo is an element that increases resistance to pitting corrosion due to low pH or low temperature. The present invention requires a Mo content of 1.5% or more. A Mo content of less than 1.5% reduces carbon dioxide corrosion resistance and crevice corrosion resistance in severe corrosive environments. Therefore, the Mo content is set to 1.5% or more. The Mo content is preferably set to 2.0% or more, more preferably set to 2.2% or more, and even more preferably set to 2.5% or more. The Mo content is most preferably set to 2.7% or more. On the other hand, a Mo content exceeding 3.7% generates δ-ferrite, resulting in a decrease in hot workability, carbon dioxide corrosion resistance, and SSC resistance in 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 set to 3.3% or less, and even more preferably set to 3.0% or less. The Mo content is most preferably set to 2.8% or less.

[0030] Al: 0.005 to 0.10% Al is an element that acts as a deoxidizer. This effect can be achieved by including 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 set to 0.015% or more. The Al content is further preferably set to 0.017% or more, and most preferably set to 0.02% or more. On the other hand, if the Al content exceeds 0.10%, the amount of oxides becomes too large, 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 set to 0.04% or less, and even more preferably set to 0.03% or less. The Al content is most preferably set to 0.025% or less.

[0031] N: 0.002 to 0.15% N is an inexpensive element that suppresses the formation of δ-ferrite and improves hot workability. This effect can be achieved with an N content of 0.002% or more. Therefore, the N content is set to 0.002% or more. The N content is preferably set to 0.01% or more, more preferably set to 0.02% or more. The N content is further preferably set to 0.03% or more, and most preferably set to 0.04% or more. On the other hand, if the N content exceeds 0.15%, coarse nitrides are formed, reducing 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 set to 0.10% or less, and even more preferably set to 0.08% or less. The N content is most preferably set to 0.06% or less.

[0032] Co: 0.2 to 1.0% Co is an element that improves crevice corrosion resistance. This effect can be achieved by including 0.2% or more of Co. Therefore, the Co content is set to 0.2% or more. The Co content is preferably set to 0.25% or more. The Co content is more preferably set to 0.3% or more, even more preferably set to 0.35% or more, and most preferably set to 0.4% or more. On the other hand, even if the Co content exceeds 1.0%, the effect saturates. Therefore, when Co is included, the Co content is set to 1.0% or less. The Co content is preferably set to 0.8% or less, more preferably set to 0.7% or less. The Co content is further preferably set to 0.65% or less, and most preferably set to 0.6% or less.

[0033] Nb: 0.005 to 0.20% Nb is an element that increases the Ms point and is necessary for achieving both crevice corrosion resistance and high strength. This effect can be achieved by including 0.005% or more of Nb. Therefore, the Nb content is set to 0.005% or more. The Nb content is preferably set to 0.01% or more, more preferably set to 0.05% or more, and even more preferably set to 0.07% or more. The Nb content is most preferably set to 0.09% or more. On the other hand, if the Nb content exceeds 0.20%, low-temperature toughness deteriorates. Therefore, the Nb content is set to 0.20% or less. The Nb content is preferably set to 0.17% or less, more preferably set to 0.15% or less, and even more preferably set to 0.13% or less. The Nb content is most preferably set to 0.11% or less.

[0034] O (oxygen): 0.010% or less O (oxygen) exists as an oxide in steel and has a negative effect on various properties. For this reason, it is desirable to reduce the O content as much as possible. In particular, if the O content exceeds 0.010%, crevice corrosion resistance and other properties will decrease significantly. For this reason, the O content is set to 0.010% or less. Preferably, the O content is set to 0.007% or less, more preferably 0.004% or less. The O content is further preferably 0.003% or less, and most preferably 0.002% or less. Since excessive reduction leads to increased manufacturing costs, the O content is preferably set to 0.0005% or more.

[0035] One or two selected from Cu: 3.5% or less and W: 3.5% or less Cu: 3.5% or less Cu is an element that strengthens the protective coating and improves crevice corrosion resistance, and can be contained as needed. Since this effect is achieved by including 0.5% or more of Cu, the Cu content is preferably 0.5% or more, more preferably 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, a Cu content exceeding 3.5% leads to grain boundary precipitation of CuS, resulting in reduced hot workability. For this reason, the Cu content is set to 3.5% or less. The Cu content is preferably 3.0% or less, more preferably 2.5% or less, and even more preferably 2.0% or less. The Cu content is most preferably 1.5% or less.

[0036] W: 3.5% or less W is an element that contributes to increasing strength and enhancing crevice corrosion resistance, and can be added as needed. Since this effect is achieved by including 0.05% or more W, the W content is preferably 0.05% or more, more preferably 0.2% or more, even more preferably 0.3% or more, and most preferably 0.5% or more. On the other hand, even if the W content exceeds 3.5%, the effect saturates. Therefore, the W content is set to 3.5% or less. The W content is preferably 3.0% or less, more preferably 2.0% or less, and even more preferably 1.5% or less. The W content is most preferably 1.0% or less. In the present invention, when Cu and W are included, one or two selected from Cu: 3.5% or less and W: 3.5% or less refers to Cu: 3.5% or less and W: 3.5% or less. When one of Cu and W exceeds 3.5%, the steel is considered a comparative example.

[0037] In the present invention, Cr, Ni, Mo, W, Cu, and Co are contained within the above-mentioned ranges and so as to satisfy the following formula (1): 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 formula (1) represent the content (mass%) of each element, and the content of elements that are not contained is zero. If the value of the left side of formula (1) (the value of "Cr + 0.22 × Ni + 0.38 × (Mo + 0.5 × W) + 0.89 × Cu + 0.09 × Co") is less than 21.4, crevice corrosion resistance in an untreated seawater environment is reduced. For this reason, in the present invention, Cr, Ni, Mo, W, Cu, and Co are contained so as to satisfy formula (1). That is, the value of the left side of formula (1) is 21.4 or more. The value of the left side of formula (1) is preferably 21.6 or more, more preferably 21.8 or more, and even more preferably 22.0 or more. There is no particular upper limit for the value of the left side of formula (1). From the viewpoint of suppressing cost increases and strength reductions due to excessive alloy addition, the value of the left side of formula (1) is preferably 26.0 or less, more preferably 24.0 or less, and even more preferably 23.8 or less.

[0038] In the present invention, Co and Nb are contained within the above ranges and so as to satisfy the following formula (2): Co-Nb≧0.13 (2) where Co and Nb in formula (2) are the contents (mass%) of each element.

[0039] As described above, by setting the value of the left side of formula (1) to 21.4 or more, desired crevice corrosion resistance in an untreated seawater environment can be achieved. To achieve this, it is necessary to appropriately contain Cr, Ni, Mo, W, Cu, and Co. However, among these elements, all elements other than Co significantly lower the Ms point, and if contained in excess, the desired high strength cannot be obtained. On the other hand, adding Nb is effective for raising the Ms point, but excessive Nb content deteriorates low-temperature toughness. Therefore, by adding Co, an element that improves crevice corrosion resistance without lowering the Ms point, in an amount of 0.13% or more greater than Nb, excellent crevice corrosion resistance, high strength, and low-temperature toughness can be achieved. If the value of the left side of formula (2) (the value of "Co-Nb") is less than 0.13, the low-temperature toughness value decreases. For this reason, in the present invention, Co and Nb are contained so as to satisfy formula (2). The value of the left side of formula (2) is preferably 0.13 or more. The value of the left side of formula (2) is preferably 0.17 or more, more preferably 0.20 or more, and even more preferably 0.30 or more. There is no particular upper limit for the value of the left side of formula (2). From the viewpoint of suppressing cost increases and strength reductions due to excessive alloy addition, the value of the left side of formula (2) is preferably 1.00 or less. It is more preferable that the value of the left side of formula (2) is 0.80 or less.

[0040] In the present invention, the balance other than the above components consists of iron (Fe) and unavoidable impurities.

[0041] The above-mentioned components are the basic components, and the high-strength stainless steel seamless pipe for oil wells of the present invention can achieve the desired properties with these basic components. In addition to the above-mentioned basic components, the present invention can contain the following optional elements as needed. Each of the following components, V, Ti, Zr, B, REM, Ca, Sn, Sb, Ta, and Mg, can be contained as needed, so these components may be 0%. One 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, and Mg: 0.0100% or less

[0042] V: 0.50% or less V is an element that improves the strength of steel through precipitation strengthening and can be contained as needed. This effect is achieved by including 0.005% or more of V, so the V content is preferably 0.005% or more. The V content is more preferably 0.03% or more, and even more preferably 0.04% or more. The V content is most preferably 0.05% or more. On the other hand, if the V content exceeds 0.50%, the low-temperature toughness decreases. Therefore, when V is contained, the V content is set to 0.50% or less. The V content is preferably 0.40% or less, and more preferably 0.30% or less. The V content is more preferably 0.25% or less, and most preferably 0.20% or less.

[0043] Ti: 0.20% or less Ti is present in oxide- or sulfide-based inclusions and improves the chemical stability of the inclusions, thereby improving crevice corrosion resistance in untreated seawater environments. This element can be added as needed. Since this effect can be achieved by including 0.002% or more of Ti, the Ti content is preferably 0.002% or more. The Ti content is more preferably 0.003% or more. On the other hand, if the Ti content exceeds 0.20%, TiN precipitates as inclusions, which in turn deteriorates crevice corrosion resistance. Therefore, when Ti is included, the Ti content is set to 0.20% or less. The Ti content is preferably 0.15% or less, more preferably 0.10% or less. The Ti content is further preferably 0.07% or less, and most preferably 0.05% or less.

[0044] Zr: 0.20% or less Zr is an element that contributes to increasing strength and can be contained as needed. This effect can be obtained by containing 0.01% or more of Zr. Therefore, the Zr content is preferably 0.01% or more, and more preferably 0.02% or more. On the other hand, even if the Zr content exceeds 0.20%, the effect saturates. Therefore, when Zr is contained, the Zr content is set to 0.20% or less. The Zr content is preferably 0.17% or less, more preferably 0.13% or less, and even more preferably 0.10% or less. The Zr content is most preferably 0.07% or less.

[0045] B: 0.01% or less B is an element that contributes to increasing strength and can be contained as needed. Such an effect can be obtained by containing 0.0005% or more of B, so the B content is preferably 0.0005% or more. More preferably, it is 0.001% or more. Even more preferably, it is 0.002% or more. On the other hand, if B is contained in an amount exceeding 0.01%, hot workability decreases. Therefore, when B is contained, the B content is set to 0.01% or less. The B content is preferably set to 0.007% or less, more preferably 0.005% or less. The B content is even more preferably set to 0.003% or less.

[0046] REM: 0.01% or less REM (rare earth metal) is an element that contributes to improving crevice corrosion resistance and can be contained as needed. This effect can be achieved by containing 0.0005% or more of REM, so the REM content is preferably 0.0005% or more. More preferably, it is 0.001% or more. The REM content is even more preferably 0.0015% or more. On the other hand, if the REM content exceeds 0.01%, the effect saturates, and no effect commensurate with the content can be expected, which is economically disadvantageous. Therefore, when 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 even more preferably 0.005% or less, and most preferably 0.003% or less.

[0047] Ca: 0.0100% or less Ca is an element that contributes to improving crevice corrosion resistance and can be contained as needed. This effect can be achieved by containing 0.0005% or more of Ca. Therefore, the Ca content is preferably 0.0005% or more. The Ca content is more preferably 0.0010% or more. The Ca content is even more preferably 0.0015% or more. On the other hand, if the Ca content exceeds 0.0100%, the number density of coarse Ca-based inclusions increases, making it impossible to obtain the desired crevice corrosion resistance. Therefore, when Ca is contained, the Ca content is set to 0.0100% or less. The Ca content is more preferably 0.0070% or less. The Ca content is even more preferably 0.0050% or less, and most preferably 0.0030% or less.

[0048] Sn: 0.20% or less Sn is an element that contributes to improving crevice corrosion resistance and can be contained as needed. Since this effect can be achieved by including 0.02% or more of Sn, the Sn content is preferably 0.02% or more, more preferably 0.05% or more. The Sn content is even more preferably 0.07% or more. On the other hand, even if the Sn content exceeds 0.20%, the effect saturates, and no effect commensurate with the content can be expected, which is economically disadvantageous. Therefore, when 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 even more preferably set to 0.13% or less, and most preferably set to 0.10% or less.

[0049] Sb: 0.50% or less Sb is an element that contributes to improving crevice corrosion resistance and can be contained as needed. Since this effect can be achieved by including 0.02% or more of Sb, the Sb content is preferably 0.02% or more. More preferably, it is 0.05% or more. On the other hand, even if the Sb content exceeds 0.50%, the effect saturates, and no effect commensurate with the content can be expected, which is economically disadvantageous. Therefore, when 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 0.30% or less, and even more preferably 0.15% or less. The Sb content is most preferably set to 0.10% or less.

[0050] Ta: 0.1% or less Ta is an element that increases strength and also has the effect of improving crevice corrosion resistance. Ta also has the same effect as Nb, and can partially replace Nb. Since this effect can be achieved by including 0.01% or more of Ta, the Ta content is preferably 0.01% or more. The Ta content is more preferably 0.03% or more. The Ta content is even more preferably 0.04% or more. On the other hand, if Ta is included in an amount exceeding 0.1%, low-temperature toughness decreases. Therefore, when Ta is included, the Ta content is set to 0.1% or less. The Ta content is preferably 0.09% or less, more preferably 0.07% or less. The Ta content is even more preferably 0.06% or less, and most preferably 0.05% or less.

[0051] Mg: 0.0100% or less Mg is an element that improves crevice corrosion resistance and can be contained as needed. Since this effect can be achieved by including 0.0002% or more of Mg, the Mg content is preferably 0.0002% or more, and more preferably 0.0004% or more. On the other hand, even if the Mg content exceeds 0.0100%, the effect saturates and no effect commensurate with the content can be expected. Therefore, when Mg is contained, the Mg content is preferably 0.0100% or less. The Mg content is preferably 0.0080% or less, more preferably 0.0050% or less, and even more preferably 0.0020% or less. The Mg content is most preferably 0.0010% or less.

[0052] Next, the steel pipe structure of the high-strength stainless steel seamless pipe for oil well use of the present invention is not particularly limited, and it is preferable that the steel pipe structure be, for example, as follows.

[0053] The high-strength stainless steel seamless pipe for oil wells of the present invention preferably has a steel pipe structure consisting of a martensite phase (tempered martensite phase), a retained austenite phase, and a ferrite phase.

[0054] Since an excessive amount of the retained austenite phase reduces strength, the area fraction of the retained austenite phase is preferably 32% or less. The area fraction of the retained austenite phase is more preferably 30% or less, and even more preferably 28% or less. The lower limit is preferably 1% or more. If only a small amount of ferrite phase is present, strain concentrates in the ferrite phase during hot working, reducing hot workability, so the area fraction is preferably 14% or more. The area fraction of the ferrite phase is more preferably 16% or more, and even more preferably 18% or more. The upper limit is preferably 50% or less.

[0055] The above-mentioned structures can be measured by the following method: First, a test piece for structure observation is taken from the center of the wall thickness of a cross section perpendicular to the tube axis direction, and corroded with Virela's reagent (a mixture of picric acid, hydrochloric acid, and ethanol in proportions of 2 g, 10 ml, and 100 ml, respectively), and the structure is imaged with a scanning electron microscope (magnification: 1000 times), and the structure fraction (area %) of the ferrite phase is calculated using an image analyzer.

[0056] The X-ray diffraction test piece is then ground and polished so that the cross section (C cross section) perpendicular to the tube axis direction becomes the measurement surface, and the amount of retained austenite (γ) is measured using X-ray diffraction. The amount of retained austenite is determined by measuring the integrated intensity of diffracted X-rays from the (220) plane of γ and the (211) plane of α (ferrite), and converting it using the following formula. Note that the volume fraction of retained austenite is considered to be the area fraction: γ (volume fraction) = 100 / (1 + (IαRγ / IγRα)) where Iα is the integrated intensity of α, Rα is the crystallographically calculated value of α, Iγ is the integrated intensity of γ, and Rγ is the crystallographically calculated value of γ.

[0057] The fraction (area %) of the martensite phase (tempered martensite phase) is the remainder other than the ferrite phase and the residual γ phase. The fraction of the martensite phase is preferably 18% or more in area percentage, more preferably 30% or more, and more preferably 85% or less, and more preferably 75% or less.

[0058] Next, one embodiment of the method for producing a high-strength stainless steel seamless pipe for oil well use according to the present invention will be described, although it is not limited to the following. In the following description of the production method, temperatures (°C) refer to the surface temperatures of the steel pipe material and the steel pipe (seamless steel pipe after pipemaking) unless otherwise specified. These surface temperatures can be measured using a radiation thermometer or the like.

[0059] In the present invention, a steel pipe material having the above-described chemical composition is used as a starting material. The method for producing the steel pipe material as the starting material is not particularly limited. For example, it is preferable to produce molten steel having the above-described chemical composition by a melting method such as a converter, and then produce a steel pipe material such as a billet by a method such as a continuous casting method or an ingot-making and blooming rolling method.

[0060] Next, these steel pipe materials are heated (heating step), and the heated steel pipe materials are pierced by a piercing machine using a Mannesmann plug mill or a Mannesmann mandrel mill to form a hollow blank, which is then hot worked and made into a pipe (pipe-making step). This results in a seamless steel pipe of the desired dimensions (predetermined shape) and having the above-mentioned chemical composition. Alternatively, the seamless steel pipe may be made by hot extrusion using a press method.

[0061] In the above-mentioned heating process of the steel pipe material, the heating temperature is preferably in the range of 1100 to 1350°C. If the heating temperature is less than 1100°C, hot workability decreases and defects frequently occur during pipe making. Therefore, the heating temperature is preferably 1100°C or higher, more preferably 1150°C or higher. The heating temperature is even more preferably 1170°C or higher, and most preferably 1200°C or higher. On the other hand, if the heating temperature exceeds 1350°C and becomes too high, the crystal grains become coarse and the low-temperature toughness decreases. Therefore, the heating temperature in the heating process is preferably 1350°C or lower. The heating temperature is more preferably 1300°C or lower. The heating temperature is even more preferably 1280°C or lower, and most preferably 1250°C or lower.

[0062] After being formed, the seamless steel pipe is cooled to room temperature at a cooling rate faster than air cooling, thereby ensuring a steel pipe structure with martensite as the main phase.

[0063] In the present invention, following the cooling at a cooling rate equal to or greater than air cooling after pipe-making, it is preferable to subject the steel pipe (seamless steel pipe after pipe-making) to heat treatment (quenching treatment, tempering treatment). Specifically, it is preferable to reheat the steel pipe (seamless steel pipe after pipe-making) to a temperature (heating temperature) in the range of 850°C to 1120°C, hold the temperature for a predetermined time, and then perform quenching treatment by cooling at a cooling rate equal to or greater than air cooling until the steel pipe surface temperature reaches a temperature of 100°C or less (cooling stop temperature). Here, the "cooling rate equal to or greater than air cooling" is 0.01°C / s or greater. This allows the aforementioned martensite phase and high strength to be achieved. For this reason, the reheating temperature is preferably 850°C or greater. The reheating temperature (heating temperature of the quenching treatment) is more preferably 870°C or greater in order to prevent coarsening of the structure and dissolve intermetallic compounds. It is even more preferably 900°C or greater. The reheating temperature is most preferably 950°C or greater. The temperature is preferably in the range of 1120° C. or less. The reheating temperature is more preferably 1100° C. or less, further preferably 1050° C. or less, and most preferably 1000° C. or less.

[0064] From the viewpoint of ensuring uniform heating, it is preferable to hold the steel pipe at the above-mentioned reheating temperature for 5 minutes or more. The holding time is more preferably 10 minutes or more, and even more preferably 15 minutes or more. The holding time is preferably 30 minutes or less. The holding time is more preferably 25 minutes or less, and even more preferably 20 minutes or less.

[0065] From the viewpoint of ensuring the yield strength (YS) targeted in the present invention, the cooling stop temperature after quenching is preferably 100° C. or lower. The cooling stop temperature is more preferably 75° C. or lower, and even more preferably 50° C. or lower. The cooling stop temperature is preferably 30° C. or higher, and more preferably 40° C. or higher.

[0066] The steel pipe that has been subjected to the above-mentioned quenching treatment is then subjected to tempering treatment. The tempering treatment is preferably a treatment in which the pipe is heated to a temperature (tempering temperature) of 500°C or higher and 650°C or lower, held for a predetermined time, and then air-cooled. Other cooling methods such as water cooling, oil cooling, and mist cooling may be used instead of all or part of the air cooling.

[0067] If the tempering temperature is less than 500°C, the strength will be excessively high, making it difficult to ensure the desired low-temperature toughness. Therefore, the tempering temperature is preferably 500°C or higher. The tempering temperature is more preferably 530°C or higher. The tempering temperature is even more preferably 550°C or higher, and most preferably 570°C or higher. This makes it easier for the steel pipe structure to have a tempered martensite phase as the main phase, resulting in a seamless steel pipe having the strength and crevice corrosion resistance desired in the present invention. On the other hand, if the tempering temperature is too high, fresh martensite phase will precipitate after tempering, making it impossible to ensure the desired high strength. Therefore, the tempering temperature is preferably 650°C or lower. The tempering temperature is more preferably 640°C or lower. Even more preferably, it is 620°C or lower. The tempering temperature is most preferably 600°C or lower.

[0068] In order to ensure uniform heating of the material, it is preferable to hold the steel pipe at the tempering temperature for 10 minutes or more, and the holding time is preferably 90 minutes or less.

[0069] In the present invention, the quenching and tempering treatments may be repeated two or more times, thereby improving the low-temperature toughness value. There is no particular upper limit to the number of quenching and tempering treatments, but it is preferable to repeat the quenching and tempering treatments three or less times to prevent an increase in manufacturing costs.

[0070] Although the above description has been given using a seamless steel pipe as an example, the present invention is not limited to this. It is also possible to manufacture electric resistance welded steel pipes and UOE steel pipes as oil well steel pipes using the steel pipe material having the above-mentioned chemical composition. In this case, by subjecting the obtained oil well steel pipes to quenching and tempering treatments under the above-mentioned conditions, the high-strength stainless steel seamless steel pipes for oil wells of the present invention can be obtained.

[0071] As described above, according to the present invention, the absorbed energy vE at a test temperature of −10° C. in the Charpy impact test －１０ It is possible to obtain a high-strength stainless steel seamless pipe for oil wells having a yield strength YS of 40 J or more, excellent crevice corrosion resistance in untreated seawater, and high strength of 758 MPa or more.

[0072] Absorbed energy vE at test temperature -10°C in Charpy impact test －１０ The absorbed energy vE at a test temperature of -10°C in the Charpy impact test is 40J or more. －１０ is preferably 50 J or more, more preferably 60 J or more, and even more preferably 70 J or more. There is no particular upper limit, but it may be 200 J or less.

[0073] 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, but may be 1000 MPa or less.

[0074] Furthermore, intermediate products (billets, etc.) produced during the manufacturing process of a product have excellent hot workability. The hot workability can be evaluated by the following method. A round bar test specimen with a parallel section diameter of 10 mm taken from a steel pipe material (billette) was heated to 1250°C in a Gleeble tester, held for 100 seconds, cooled to 1000°C at 1°C / sec, held for 10 seconds, and then stretched to fracture, and the reduction in area (%) was measured. The smaller the reduction in area, the worse the hot workability. The reduction in area is preferably 60% or more, more preferably 70% or more. The reduction in area is preferably 90% or less, and more preferably 85% or less.

[0075] The present invention will be described below based on examples, but the present invention is not limited to the following examples.

[0076] Molten steel having the chemical composition shown in Table 1 was melted in a vacuum melting furnace to obtain cast slabs (steel pipe materials). All of the resulting cast slabs were heated to 1250°C and hot worked.

[0077] Next, test specimen materials were cut out from the steel material obtained by hot working. Here, the dimensions of the steel material were length: 1100 mm, width: 160 mm, and thickness: 15 mm. Using each test specimen material, a quenching treatment was performed by heating at the heating temperature (reheating temperature) and soaking time shown in Table 2, and then air-cooling to the cooling stop temperature shown in Table 2. Furthermore, a tempering treatment was performed by heating at the tempering temperature and soaking time shown in Table 2, and air-cooling. Some test specimens (steel pipe Nos. 2 and 4) were quenched and tempered twice under the conditions shown in Table 2. Note that although the cut-out test specimens were quenched and tempered, this can be considered to be the same as when a seamless steel pipe is quenched and tempered.

[0078] Then, using each test piece material that had been subjected to quenching and tempering, evaluations of tensile properties, Charpy impact test properties, corrosion properties, and measurement of microstructure were carried out by the methods described below. Evaluation of hot workability was carried out by the method described below using the above-mentioned cast pieces.

[0079] [Evaluation of Tensile Properties] JIS (Japanese Industrial Standards) No. 14A tensile test pieces (Φ6.0 mm) were taken from the quenched and tempered test piece materials, and tensile tests were carried out in accordance with the provisions of JIS Z2241:2011 to determine the tensile properties (yield strength (YS), tensile strength (TS)). Here, a specimen with a yield strength (YS) of 758 MPa or more was deemed to pass, and a specimen with a yield strength of less than 758 MPa was deemed to fail.

[0080] [Evaluation of Charpy Impact Test Characteristics] V-notch test specimens (10 mm thick) were taken from the quenched and tempered test specimen material so that the longitudinal direction of the test specimen was perpendicular to the manufacturing direction, and a Charpy impact test was carried out in accordance with the provisions of JIS Z 2242 (2018). The test temperature was -10°C, and the absorbed energy vE －１０ The absorbed energy (J) of the stainless steel member was determined by measuring the absorbed energy vE at -10°C. －１０Those with a vE of 40J or more were evaluated as having high toughness and were considered to have passed the test. －１０ Those with a value of less than 40J were deemed to have failed.

[0081] [Evaluation of corrosion characteristics] A corrosion test piece having a thickness of 3 mm, a width of 20 mm, and a length of 50 mm and a hole of Φ12 mm was machined from the quenched and tempered test piece material, and a corrosion test was carried out. In the corrosion test, a jig for creating a gap made of fluororesin was fitted into the hole of the test piece, and the surface of the test piece was pressed against the jig with a torque of 20 N / mm. ２ The corrosion test specimens were immersed in artificial seawater (liquid temperature: 25°C) for 30 days. Air was bubbled through the test liquid during the test. After the test, the corrosion test specimens were observed for the presence or absence of crevice corrosion on the surface using a 10x magnifying glass. Here, specimens without crevice corrosion (shown as "absent" in the "crevice corrosion" column in Table 3) were evaluated as passing, and specimens with crevice corrosion (shown as "present" in the "crevice corrosion" column in Table 3) were evaluated as failing. Incidentally, specimens without crevice corrosion were evaluated as having "excellent crevice corrosion resistance."

[0082] [Evaluation of Hot Workability] For the evaluation of hot workability, round bar test pieces having a parallel portion diameter of 10 mm taken from the cast slab were used, and the test pieces were heated to 1250°C in a Gleeble tester, held for 100 seconds, cooled to 1000°C at a rate of 1°C / sec, held for 10 seconds, and then stretched to fracture to measure the reduction in area (%). The smaller the reduction in area, the worse the hot workability.

[0083] [Microstructure Measurement] Test specimens for microstructure observation were prepared from the quenched and tempered test specimen material, and each microstructure was measured. The microstructure observation surface was a cross section (C-section) perpendicular to the rolling direction. First, the test specimen for microstructure observation was corroded with Virrella's reagent (a mixture of 2 g of picric acid, 10 ml of hydrochloric acid, and 100 ml of ethanol, respectively), and the microstructure was imaged using a scanning electron microscope (accelerating voltage: 15 kV, magnification: 1000x). The microstructure fraction (area %) of the ferrite phase was calculated using an image analyzer (Image-J). Next, the test specimen for X-ray diffraction was ground and polished so that the cross section (C-section) perpendicular to the rolling direction served as the measurement surface, and the amount of retained austenite (γ) was measured using X-ray diffraction. The amount of retained austenite was calculated by measuring the integrated diffracted X-ray intensity of the (220) plane of γ and the (211) plane of α (ferrite) and converting it using the following formula: Here, the volume fraction of retained austenite was considered to be the area fraction. γ (volume fraction) = 100 / (1 + (IαRγ / IγRα)), where Iα is the integrated intensity of α, Rα is the crystallographically calculated value of α, Iγ is the integrated intensity of γ, and Rγ is the crystallographically calculated value of γ. The fraction (area %) of the martensite phase (tempered martensite phase) was defined as the remainder other than the ferrite phase and the retained γ phase.

[0084] The results obtained are shown in Table 3.

[0085]

[0086]

[0087]

Claims

1. The alloy contains, by mass%, C: 0.002-0.050%, Si: 0.05-0.50%, Mn: 0.04-1.80%, P: 0.030% or less, S: 0.0020% or less, 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: 0.010% or less, and further contains one or two selected from Cu: 3.5% or less and W: 3.5% or less, The composition satisfies formula (1) and formula (2), with the balance being Fe and unavoidable impurities. The yield strength is 758 MPa or more, and the absorbed energy vE －１０ A high-strength stainless steel seamless pipe for oil wells having a strain rate of 40J or more. Cr + 0.22 x Ni + 0.38 x (Mo + 0.5 x W) + 0.89 x Cu + 0.09 x Co ≧ 21.4 ... (1) Here, Cr, Ni, Mo, W, Cu, and Co in formula (1) are the contents (mass%) of each element, and the content of elements that are not contained is zero. Co - Nb ≧ 0.13 ... (2) Here, Co and Nb in formula (2) are the contents (mass%) of each element.

2. A high-strength stainless steel seamless pipe for oil wells as described in claim 1, which in addition to the above-mentioned chemical composition further contains, by mass%, one or more elements selected from the following: 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.

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