WO2024247508A1 - High-strength stainless steel seamless pipe for oil wells - Google Patents

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 pipes for oil wells

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

In recent years, in view of the soaring crude oil prices and the expected depletion of petroleum resources in the near future, development of deep oil fields and oil and gas fields in severe corrosive environments, so-called sour environments containing hydrogen sulfide, which were not considered in the past, has been thriving. Such oil and gas fields are generally very deep, and the atmosphere is high temperature and contains CO ₂ , Cl ^- , and H ₂ S, creating a severe corrosive environment. Oil well steel pipes used in such environments are required to be made of materials that combine the desired high strength and corrosion resistance.

Conventionally, 13Cr martensitic stainless steel pipes have been widely used as oil country tubular goods for mining in oil and gas fields in environments containing carbon dioxide gas (CO ₂ ), chlorine ions (Cl ^- ), etc. Furthermore, in recent years, the use of improved 13Cr martensitic stainless steel with a composition system in which the C content of 13Cr martensitic stainless steel is reduced and the Ni, Mo, etc. are increased has also become widespread.

In response to such demands, there are technologies listed in Patent Documents 1 to 5, for example.

Patent Document 1 discloses a stainless steel pipe for oil wells that has improved corrosion resistance by having a steel composition that contains, by mass%, C: 0.05% or less, Si: 0.50% or less, Mn: 0.20-1.80%, P: 0.03% or less, S: 0.005% or less, Cr: 14.0-18.0%, Ni: 5.0-8.0%, Mo: 1.5-3.5%, Cu: 0.5-3.5%, Al: 0.05% or less, V: 0.20% or less, N: 0.01-0.15%, O: 0.006% or less, and satisfies a specified formula, with the remainder being Fe and unavoidable impurities.

Patent Document 2 also discloses a high-strength stainless steel seamless pipe for oil wells that contains, by mass%, C: 0.005-0.05%, Si: 0.05-0.50%, Mn: 0.20-1.80%, P: 0.030% or less, S: 0.005% or less, Cr: 12.0-17.0%, Ni: 4.0-7.0%, Mo: 0.5-3.0%, Al: 0.005-0.10%, V: 0.005-0.20%, Co: 0.01-1.0%, N: 0.005-0.15%, and O: 0.010% or less, and that satisfies a specific formula, with the remainder being Fe and unavoidable impurities, and thus has a yield strength of 655 MPa or more.

Patent Document 3 also describes the following composition by mass: C: 0.05% or less, Si: 0.50% or less, Mn: 0.10-1.80%, P: 0.03% or less, S: 0.005% or less, Cr: 14.0-17.0%, Ni: 5.0-8.0%, Mo: 1.0-3.5%, Cu: 0.5-3.5%, Al: 0.05% or less, V: 0.20% or less, N: 0.03-0.15%, O: 0 A high-strength stainless steel pipe for oil wells is disclosed that has high strength and high corrosion resistance due to its composition containing 0.006% or less of C, one or two selected from 0.2% or less of Nb and 0.3% or less of Ti, the balance being Fe and unavoidable impurities, and having a structure in which MC-type carbonitrides in the precipitates account for 3.0% or more of the total precipitate amount by mass%.

Patent Document 4 also discloses a high-strength stainless steel pipe for oil wells that has a composition containing Cr and Ni and a structure with tempered martensite as the main phase, the composition of which satisfies Cr/Ni≦5.3, and has a surface structure in which a phase that turns white when etched with a Virela 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.

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: 0.1-2.0%, P: 0.03% or less, S: 0.005% or less, Cr: 14.0-15.5%, Ni: 5.5-7.0%, Mo: 2.0-3.5%, Cu: 0.3-3.5%, V: 0.20% or less, Al: 0.05% or less, N: 0.06% or less, with the balance being Fe and unavoidable impurities, and has a yield strength of 655-862 MPa and a yield ratio of 0.90 or more, and has improved resistance to carbon dioxide corrosion and sulfide stress corrosion cracking.

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

In recent years, a method called water injection has been used to inject water into geological formations using seamless steel pipes in order to improve the recovery rate of crude oil. Seawater is often used for water injection because it is abundant. Chloride ions, dissolved oxygen, microorganisms, etc. present in seawater increase corrosiveness, so they are sometimes removed, but due to the cost, untreated seawater is sometimes used for water injection. High corrosion resistance is required for seamless steel pipes used in such environments, and the technologies described in Patent Documents 1 to 5, although having good resistance to carbon dioxide corrosion, did not have sufficient crevice corrosion resistance in untreated seawater environments. Furthermore, low-temperature toughness is also required due to the active development in cold regions and deep seas.

The present invention aims to solve the problems of the conventional technology and provide a high-strength stainless steel seamless pipe for oil wells that has high strength, excellent low-temperature toughness, and excellent resistance to crevice corrosion in untreated seawater environments.

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

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

In addition, in the present invention, "excellent crevice corrosion resistance in untreated seawater" refers to a case where a test piece with crevice is immersed in artificial seawater (liquid temperature: 25°C, atmospheric saturation at 1 atmospheric pressure) for 30 days, and the test piece after the corrosion test is observed for the presence or absence of crevice on the surface of the test piece using a 10x magnifying glass, and no crevice corrosion has occurred to a depth of 0.1 mm or more.

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

In order to achieve the above object, the present inventors have conducted extensive research into the effects of various compositions of stainless steel pipes on crevice corrosion resistance in untreated seawater environments, and have found that the contents of Cr, Mo, Cu, Ni, W and Co in the composition of stainless steel materials 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)
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.
In addition, the inventors have found that in order to obtain a desired low-temperature toughness value while satisfying crevice corrosion resistance, it is necessary to adjust the contents of Nb and Co so as to satisfy formula (2).
Co-Nb≧0.13...(2)
Here, Co and Nb in formula (2) are the contents (mass %) of each element.

The present invention has been completed based on these findings and through further investigation.
[1] In 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 to 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;
Further, it contains one or two selected from Cu: 3.5% or less and W: 3.5% or less,
and has a composition that satisfies formula (1) and formula (2), with the balance being Fe and unavoidable impurities,
A high-strength stainless steel seamless pipe for oil wells having a yield strength of 758 MPa or more and an absorbed energy vE _-10 of 40 J or more at a test temperature of -10°C in a Charpy impact test.
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) 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] In addition to the above-mentioned component composition, further, in 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,
Mg: 0.0100% or less.

The present invention provides a high-strength stainless steel seamless pipe for oil wells that has high strength, excellent low-temperature toughness, and excellent resistance to crevice corrosion in untreated seawater.

The present invention will be described in detail below. Note that the present invention is not limited to the following embodiments.

First, we will explain the composition of the high-strength stainless steel seamless pipe for oil wells of the present invention and the reasons for its limitations. Hereinafter, unless otherwise specified, mass% will simply be written as "%".

C: 0.002-0.050%
C is an important element for increasing the strength of martensitic stainless steel. In the present invention, it is necessary to contain 0.002% or more of C in order to ensure the desired strength of the present invention. For this reason, 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 further preferably set to 0.020% or more. The most preferable C content is 0.022% or more. On the other hand, if the C content exceeds 0.050%, the strength is rather reduced. Also, the resistance to crevice corrosion in an untreated seawater environment is also deteriorated. Therefore, in the present invention, the C content is set to 0.050% or less, preferably 0.040% or less, more preferably 0.035% or less, and further preferably 0.030% or less. The C content is most preferably 0.028% or less.

Si: 0.05-0.50%
Silicon is an element that acts as a deoxidizer. This effect can be obtained with a silicon content of 0.05% or more. Therefore, the silicon content is set to 0.05% or more. The silicon content is preferably The Si content is set to 0.10% or more, more preferably 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. %, the crevice corrosion resistance in the untreated seawater environment is deteriorated. Therefore, the Si content is set to 0.50% or less. The Si content is preferably set to 0.45% or less. The Si content is preferably 0.40% or less, more preferably 0.30% or less, and most preferably 0.25% or less.

Mn: 0.04-1.80%
Mn is an element that suppresses the formation of δ-ferrite during hot working and improves hot workability. In the present invention, the Mn content must be 0.04% or more. The Mn content is preferably 0.10% or more, more preferably 0.20% or more, and further preferably 0.25% or more. The Mn content is most preferably The Mn content is set to 0.35% or more. On the other hand, if Mn is contained excessively, the crevice corrosion resistance in an untreated seawater environment deteriorates. Therefore, the Mn content is set to 1.80% or less. is preferably 1.60% or less, more preferably 0.80% or less, further preferably 0.60% or less, and most preferably 0.40% or less.

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 it as much as possible, but an extreme reduction leads to an increase in manufacturing costs. For this reason, the P content is set to 0.030% or less as a range that can be implemented industrially at a relatively low cost without causing an extreme decrease in characteristics. 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. The lower limit of the P content is not particularly limited. However, as described above, an excessive reduction leads to an increase in manufacturing costs, so it is preferably 0.005% or more.

S: 0.0020% or less S significantly reduces hot workability and deteriorates low-temperature toughness by segregation to prior austenite grain boundaries, so it is preferable to reduce it as much as possible. If the S content is 0.0020% or less, the segregation of S to prior austenite grain boundaries can be suppressed, and the low-temperature toughness targeted in the present invention can be obtained. For this reason, the S content is set to 0.0020% or less. Preferably, the S content is set to 0.0015% or less. More preferably, the S content is set to 0.0010% or less, and further preferably, the S content is set to 0.0007% or less. The lower limit of the S content is not particularly limited. However, since excessive reduction leads to an increase in manufacturing costs, it is preferably set to 0.0005% or more.

Cr:16.0~20.0%
Cr is an element that forms a protective film and contributes to crevice corrosion resistance in an untreated seawater environment. In the present invention, the Cr content is required to be 16.0% or more. For this reason, the Cr content is The Cr content is preferably 16.5% or more, more preferably 16.8% or more, and further preferably 17.0% or more. The Cr content is most preferably On the other hand, if the Cr content exceeds 20.0%, the martensite transformation is not caused and the residual austenite is easily generated, which reduces the stability of the martensite phase. In addition, when heated to high temperatures, the δ ferrite phase precipitates, significantly reducing hot workability. For this reason, the Cr content is set to 20.0% or less. Cr Content The Cr content is preferably 19.5% or less, more preferably 19.0% or less, and further preferably 18.5% or less. The Cr content is most preferably 18.0% or less.

Ni: 4.0-7.5%
Ni is an element that strengthens the protective film and improves crevice corrosion resistance in an untreated seawater environment. Ni also inhibits the precipitation of the δ-ferrite phase and improves hot workability. In addition, Ni increases the strength of steel by dissolving in solid solution. This effect can be obtained with a Ni content of 4.0% or more. For this reason, the Ni content is set to 4.0% or more. The Ni content is preferably 5.0% or more, more preferably 6.0% or more, and further preferably 6.1% or more. The Ni content is most preferably 6.3% or more. On the other hand, if the Ni content exceeds 7.5%, the martensite transformation is not carried out and the residual austenite is easily generated, which reduces the stability of the martensite phase and decreases the strength. The Ni content is preferably 7.0% or less, and more preferably 6.5% or less.

Mo: 1.5-3.7%
Mo is an element that increases resistance to pitting corrosion caused by ^Cl- or low pH. In the present invention, the Mo content must be 1.5% or more. If the Mo content is less than 1.5%, severe corrosion will occur. However, Mo content is not sufficient to prevent deterioration of carbon dioxide corrosion resistance and crevice corrosion resistance in a severe corrosive environment. Therefore, the Mo content is set to 1.5% or more. The Mo content is preferably set to 2.0% or more, and more preferably 2.0% or more. The Mo content is preferably 2.2% or more, and more preferably 2.5% or more. The Mo content is most preferably 2.7% or more. On the other hand, the Mo content of more than 3.7% increases δ It generates ferrite, which leads to deterioration of hot workability, carbon dioxide corrosion resistance, and SSC resistance in a low temperature environment. For this reason, the Mo content is set to 3.7% or less. The Mo content is preferably The Mo content is set to 3.5% or less, more preferably 3.3% or less, and further preferably 3.0% or less. The Mo content is most preferably set to 2.8% or less.

Al: 0.005-0.10%
Al is an element that acts as a deoxidizer. This effect can be obtained by including 0.005% or more of Al. Therefore, the Al content is set to 0.005% or more. The Al content is The Al content is preferably 0.01% or more, more preferably 0.015% or more. The Al content is further preferably 0.017% or more, and most preferably 0.02% or more. If the Al content exceeds 10%, the amount of oxides becomes too large, adversely affecting the crevice corrosion resistance. Therefore, the Al content is set to 0.10% or less. The Al content is preferably The Al content is set to 0.05% or less, more preferably 0.04% or less, and further preferably 0.03% or less. The Al content is most preferably set to 0.025% or less.

N: 0.002-0.15%
N is an inexpensive element that suppresses the formation of δ-ferrite and improves hot workability. Such effects can be obtained with an N content of 0.002% or more. For this reason, the N content is The N content is 0.002% or more. The N content is preferably 0.01% or more, more preferably 0.02% or more. The N content is further preferably 0.03% or more, and most preferably On the other hand, if the N content exceeds 0.15%, coarse nitrides are formed, which reduces the crevice corrosion resistance. Therefore, the N content is set to 0.15%. The N content is preferably 0.12% or less, more preferably 0.10% or less, and further preferably 0.08% or less. The N content is most preferably 0.06% or less. % or less.

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

Nb: 0.005-0.20%
Nb is an element that increases the Ms point and is necessary to achieve both crevice corrosion resistance and high strength. Such an effect can be obtained by including 0.005% or more of Nb. For this reason, the Nb content is set to 0.005% or more, preferably 0.01% or more, more preferably 0.05% or more, and further preferably 0.07% or more. The Nb content is most preferably 0.09% or more. On the other hand, if the Nb content exceeds 0.20%, the low temperature toughness deteriorates. Therefore, the Nb content is set to 0.20% or less. The Nb content is preferably 0.17% or less, more preferably 0.15% or less, and further preferably 0.13% or less. The Nb content is most preferably 0.11% or less. do.

O (oxygen): 0.010% or less O (oxygen) exists as an oxide in steel and has a detrimental effect on various properties. For this reason, it is desirable to reduce O as much as possible. In particular, when the O content exceeds 0.010%, the crevice corrosion resistance is significantly reduced. 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 an increase in manufacturing costs, it is preferably set to 0.0005% or more.

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 film and enhances crevice corrosion resistance, and can be contained as necessary. Since such an effect can be obtained by containing 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, the Cu content exceeding 3.5% leads to grain boundary precipitation of CuS, and the hot workability is reduced. For this reason, the Cu content is 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.

W: 3.5% or less W is an element that contributes to increasing strength and enhances crevice corrosion resistance, and can be contained as necessary. Since such effects can be obtained by containing 0.05% or more of 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 W is contained in an amount exceeding 3.5%, the effect is saturated. For this reason, the W content is 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 addition, in the present invention, one or two selected from Cu: 3.5% or less and W: 3.5% or less means, when Cu and W are contained, Cu: 3.5% or less and W: 3.5% or less, and when one of Cu and W exceeds 3.5%, it is a comparative example.

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) are the contents (mass %) of each element, and the content of elements that are not contained is zero.
The left side value of the formula (1) ("Cr + 0.22 × Ni + 0.38 × (Mo + 0.5 × W) +
If the value of "0.89 x Cu + 0.09 x Co" is less than 21.4, the 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 left side value of formula (1) is 21.4 or more. The left side value 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 left side value of formula (1). From the viewpoint of suppressing the increase in cost and the decrease in strength due to the excessive addition of alloys, the left side value of formula (1) is preferably 26.0 or less. More preferably, it is 24.0 or less, and even more preferably, it is 23.8 or less.

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

As described above, the desired crevice corrosion resistance in an untreated seawater environment can be obtained by setting the value of the left side of formula (1) to 21.4 or more. To achieve this, it is necessary to appropriately contain Cr, Ni, Mo, W, Cu, and Co, but 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 if Nb is contained in excess, low-temperature toughness deteriorates. Therefore, by containing Co, an element that improves crevice corrosion resistance without lowering the Ms point, in an amount of 0.13% or more more than Nb, it is possible to achieve both excellent crevice corrosion resistance, high strength, and low-temperature toughness. 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 on the left side of formula (2) is preferably 0.13 or more. The value on 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 on the value on the left side of formula (2). From the viewpoint of suppressing the increase in cost due to the excessive addition of alloy and suppressing the decrease in strength, it is preferable that the value on the left side of formula (2) is 1.00 or less. It is more preferable that the value on the left side of formula (2) is 0.80 or less.

In the present invention, the remainder other than the above components consists of iron (Fe) and unavoidable impurities.

The above-mentioned components are the basic components, and the high-strength stainless steel seamless pipe for oil wells of the present invention can obtain 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 necessary. Each of the following components V, Ti, Zr, B, REM, Ca, Sn, Sb, Ta, and Mg can be contained as necessary, so these components may be 0%.
One or more selected from the group consisting 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.

V: 0.50% or less V is an element that improves the strength of steel by precipitation strengthening, and can be contained as necessary. This effect can be obtained by containing V at 0.005% or more, 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, even if V is contained at more than 0.50%, the low temperature toughness decreases. Therefore, when V is contained, the V content is 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.

Ti: 0.20% or less Ti is an element that exists in oxide-based or sulfide-based inclusions and improves the chemical stability of the inclusions to improve crevice corrosion resistance in an untreated seawater environment, and can be contained as necessary. Since such an effect can be obtained by containing 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 Ti is contained in an amount exceeding 0.20%, TiN precipitates as an inclusion, and the crevice corrosion resistance is deteriorated conversely. Therefore, when Ti is contained, the Ti content is 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.

Zr: 0.20% or less Zr is an element that contributes to increasing strength, and can be contained as necessary. Such an 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 Zr is contained in an amount exceeding 0.20%, the effect is saturated. Therefore, when Zr is contained, the Zr content is 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.

B: 0.01% or less B is an element that contributes to increasing strength, and can be contained as necessary. Since such an effect can be obtained by containing 0.0005% or more of B, 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 excess of 0.01%, the hot workability decreases. Therefore, when B is contained, the B content is 0.01% or less. The B content is preferably 0.007% or less, more preferably 0.005% or less. The B content is even more preferably 0.003% or less.

REM: 0.01% or less REM (rare earth metal) is an element that contributes to improving crevice corrosion resistance, and can be contained as necessary. Since such an effect can be obtained by containing 0.0005% or more of REM, the content is preferably 0.0005% or more. More preferably, the content is 0.001% or more. The REM content is further preferably 0.0015% or more. On the other hand, even if the REM content exceeds 0.01%, the effect is saturated and the effect commensurate with the content cannot be expected, which is economically disadvantageous. Therefore, when REM is contained, the REM content is 0.01% or less. The REM content is more preferably 0.007% or less. The REM content is further preferably 0.005% or less, and most preferably 0.003% or less.

Ca: 0.0100% or less Ca is an element that contributes to improving crevice corrosion resistance, and can be contained as necessary. Such an effect can be obtained by containing 0.0005% or more of Ca. For this reason, 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 Ca is contained in an amount exceeding 0.0100%, the number density of coarse Ca-based inclusions increases, and the desired crevice corrosion resistance cannot be obtained. For this reason, when Ca is contained, the Ca content is 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.

Sn: 0.20% or less Sn is an element that contributes to improving crevice corrosion resistance, and can be contained as necessary. Since such an effect can be obtained by containing 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 further preferably 0.07% or more. On the other hand, even if the Sn content exceeds 0.20%, the effect is saturated, and the effect commensurate with the content cannot be expected, which is economically disadvantageous. Therefore, when Sn is contained, the Sn content is 0.20% or less. The Sn content is more preferably 0.15% or less. The Sn content is further preferably 0.13% or less, and most preferably 0.10% or less.

Sb: 0.50% or less Sb is an element that contributes to improving crevice corrosion resistance, and can be contained as necessary. Since such an effect can be obtained by containing 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 Sb is contained in an amount exceeding 0.50%, the effect is saturated, and the effect commensurate with the content cannot be expected, which is economically disadvantageous. Therefore, when Sb is contained, the Sb content is 0.50% or less. The Sb content is preferably 0.40% or less, more preferably 0.30% or less, and even more preferably 0.15% or less. The Sb content is most preferably 0.10% or less.

Ta: 0.1% or less Ta is an element that increases strength and has the effect of improving crevice corrosion resistance. In addition, Ta is an element that brings about the same effect as Nb, and part of Nb can be replaced with Ta. Since such an effect can be obtained by containing 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 further preferably 0.04% or more. On the other hand, if Ta is contained in an amount exceeding 0.1%, the low temperature toughness decreases. Therefore, when Ta is contained, the Ta content is 0.1% or less. The Ta content is preferably 0.09% or less, more preferably 0.07% or less. The Ta content is further preferably 0.06% or less, and most preferably 0.05% or less.

Mg: 0.0100% or less Mg is an element that improves crevice corrosion resistance and can be contained as necessary. Since such an effect can be obtained by containing 0.0002% or more of Mg, the Mg content is preferably 0.0002% or more, more preferably 0.0004% or more. On the other hand, even if the Mg content exceeds 0.0100%, the effect is saturated and an effect commensurate with the content cannot 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.

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

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.

Since the inclusion of 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 is concentrated 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.

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

The test piece for X-ray diffraction 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 regarded as the area fraction here.
γ (volume ratio) = 100/(1+(IαRγ/IγRα))
Here, Iα is the integrated intensity of α, Rα is the theoretically calculated value of α, Iγ is the integrated intensity of γ, and Rγ is the theoretically calculated value of γ.

The fraction (area %) of the martensite phase (tempered martensite phase) is the remainder other than the ferrite phase and the residual gamma phase. The fraction of the martensite phase is preferably 18% or more in area percentage. It is more preferably 30% or more. It is also preferably 85% or less. It is more preferably 75% or less.

Next, an embodiment of the method for producing a high-strength stainless steel seamless pipe for oil wells according to the present invention will be described, although the embodiment is not limited thereto.
In the following description of the manufacturing method, the temperature (°C) refers to the surface temperature of the steel pipe material and the steel pipe (seamless steel pipe after pipe making) unless otherwise specified. These surface temperatures can be measured with a radiation thermometer or the like.

In the present invention, the starting material is a steel pipe material having the above-mentioned composition. There are no particular limitations on the manufacturing method of the starting steel pipe material. For example, it is preferable to melt molten steel having the above-mentioned composition using a melting method such as a converter, and turn it into a steel pipe material such as a billet using a method such as continuous casting or ingot casting and blooming rolling.

These steel pipe materials are then heated (heating process), and the heated steel pipe materials are shaped into hollow blanks using a piercing machine using the Mannesmann plug mill method or the Mannesmann mandrel mill method, after which they are hot worked and made into pipes (pipe-making process). This produces seamless steel pipes with the desired dimensions (prescribed shape) and the above-mentioned chemical composition. Note that seamless steel pipes may also be made by hot extrusion using a press method.

In the heating process of the steel pipe material described above, the heating temperature is preferably in the range of 1100 to 1350°C. If the heating temperature is less than 1100°C, the hot workability decreases and many defects 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.

After pipe making, the seamless steel pipe is cooled to room temperature at a cooling rate faster than air cooling. This ensures that the steel pipe structure has martensite as the main phase.

In the present invention, following the above-mentioned cooling at a cooling rate equal to or faster than the air cooling after pipe making, the steel pipe (seamless steel pipe after pipe making) is preferably subjected to heat treatment (quenching treatment, tempering treatment).
Specifically, it is preferable to perform a quenching treatment on the steel pipe (seamless steel pipe after pipe making) by reheating to a temperature (heating temperature) in the range of 850°C to 1120°C, holding the temperature for a predetermined time, and then cooling at a cooling rate faster than air cooling until the surface temperature of the steel pipe reaches a temperature of 100°C or less (cooling stop temperature). Here, the "cooling rate faster than air cooling" is 0.01°C/s or more.
This makes it possible to achieve the above-mentioned martensite phase and high strength. For this reason, the reheating temperature is preferably set to 850° C. or higher.
The reheating temperature (heating temperature of the quenching treatment) is more preferably 870°C or higher in order to prevent coarsening of the structure and dissolve the intermetallic compounds. It is even more preferably 900°C or higher. The reheating temperature is most preferably 950°C or higher. It is preferable that the temperature be in the range of 1120°C or lower. The reheating temperature is more preferably 1100°C or lower, even more preferably 1050°C or lower, and most preferably 1000°C or lower.

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

From the viewpoint of ensuring the yield strength (YS) targeted in the present invention, it is preferable that the cooling stop temperature after quenching is 100°C or less. The cooling stop temperature is more preferably 75°C or less, and even more preferably 50°C or less. In addition, the cooling stop temperature is preferably 30°C or more, and more preferably 40°C or more.

The steel pipe that has been subjected to the above-mentioned quenching treatment is then subjected to a tempering treatment. The tempering treatment is preferably a process 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 period of time, and then air-cooled. In place of all or part of the air cooling, other cooling methods such as water cooling, oil cooling, mist cooling, etc. may also be used.

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. For this reason, 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 tempered martensite phase as the main phase, resulting in a seamless steel pipe with 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. For this reason, 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.

In addition, from the viewpoint of ensuring uniform heating of the material, it is preferable to hold the steel pipe at the above-mentioned tempering temperature for 10 minutes or more. The holding time is preferably 90 minutes or less.

In addition, in the present invention, the above quenching and tempering treatments may be repeated two or more times. This improves the low-temperature toughness value. There is no particular upper limit on the number of quenching and tempering treatments, but it is preferable to keep it to three times or less in order to prevent an increase in manufacturing costs.

　The above has been explained using seamless steel pipes as an example, but the present invention is not limited to this. It is also possible to manufacture electric resistance welded steel pipes and UOE steel pipes using steel pipes for oil wells using steel pipes with the above-mentioned composition. In this case, by subjecting the obtained steel pipes for oil wells 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.

As described above, according to the present invention, a high-strength stainless steel seamless pipe for oil wells can be obtained, which has an absorbed energy vE _-10 of 40 J or more at a test temperature of -10°C in a Charpy impact test, excellent crevice corrosion resistance in untreated seawater, and high strength of a yield strength YS of 758 MPa or more.

The absorbed energy vE _-10 at a test temperature of -10°C in the Charpy impact test is 40 J or more. The absorbed energy vE- ₁₀ at a test temperature of -10°C in the Charpy impact test is preferably 50 J or more, more preferably 60 J or more, and even more preferably 70 J or more. The upper limit is not particularly limited, but may be 200 J or less.

Furthermore, 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.

Furthermore, the intermediate products (billets, etc.) produced during the manufacturing process of the product have excellent hot workability.
The hot workability can be evaluated by the following method.
A round bar test piece having a parallel section diameter of 10 mm taken from a steel pipe material (cast piece) 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 pulled until fractured to measure the reduction in area (%). The smaller the reduction in area, the worse the hot workability. The reduction in area is preferably 60% or more. More preferably, it is 70% or more. The reduction in area is preferably 90% or less. More preferably, it is 85% or less.

The present invention will be explained below based on examples. Note that the present invention is not limited to the following examples.

Molten steel with the composition shown in Table 1 was melted in a vacuum melting furnace to obtain slabs (steel pipe material). The resulting slabs were heated to 1250°C for all levels and hot worked.

Next, test specimen materials were cut out from the steel obtained by hot working. Here, the dimensions of the steel were length: 1100 mm, width: 160 mm, and thickness: 15 mm. Each test specimen material was heated at the heating temperature (reheating temperature) and soaking time shown in Table 2, and then quenched by air cooling to the cooling stop temperature shown in Table 2. Furthermore, tempering was performed by heating at the tempering temperature and soaking time shown in Table 2, and then air cooling. Some test specimens (steel pipes No. 2 and 4) were quenched and tempered twice under the conditions shown in Table 2. Note that the cut test specimens were quenched and tempered, but this can be considered to be the same as when a seamless steel pipe is quenched and tempered.

Then, using each test piece material that had been quenched and tempered, the tensile properties, Charpy impact test properties, and corrosion properties were evaluated, and the structure was measured, using the methods described below. The hot workability was evaluated using the above-mentioned cast pieces, using the methods described below.

[Evaluation of tensile properties]
A JIS (Japanese Industrial Standards) No. 14A tensile test piece (Φ6.0 mm) was taken from the test piece material that had been subjected to quenching and tempering, and a tensile test was 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 have passed, and a specimen with a yield strength of less than 758 MPa was deemed to have failed.

[Evaluation of Charpy impact test characteristics]
From the test piece material that had been subjected to quenching and tempering, a V-notch test piece (10 mm thick) was taken so that the longitudinal direction of the test piece was perpendicular 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 _-10 at -10°C was obtained to evaluate the low-temperature toughness. Note that three test pieces were used for each test piece, and the arithmetic average of the obtained values was taken as the absorbed energy (J) of the stainless steel member. Here, a test piece with an absorbed energy vE _-10 of 40J or more at -10°C was evaluated as having high toughness and was deemed to have passed. On the other hand, a test piece with a vE _-10 of less than 40J was deemed to have failed.

[Evaluation of corrosion characteristics]
From the test piece material that had been subjected to quenching and tempering treatment, 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, and a corrosion test was performed.
The corrosion test was performed by fitting a jig made of fluororesin for making a gap into the hole of the test piece, and pressing the surface of the test piece with a torque of 20 N/ ^mm2 to form a gap. The test liquid was artificial seawater (liquid temperature: 25°C), and the corrosion test piece was immersed in the test liquid for 30 days. The test was performed while bubbling air into the test liquid. After the test, the corrosion test piece was observed for the presence or absence of crevice corrosion on the surface of the test piece using a magnifying glass with a magnification of 10 times. Here, the test piece without crevice corrosion (shown as "absent" in the "crevice corrosion" column in Table 3) was considered to have passed, and the test piece with crevice corrosion (shown as "present" in the "crevice corrosion" column in Table 3) was considered to have failed.
In addition, cases where no crevice corrosion occurred were evaluated as "excellent crevice corrosion resistance."

[Evaluation of hot workability]
For the evaluation of hot workability, round bar test pieces with a parallel part diameter of 10 mm taken from the cast slab were 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 pulled until fractured to measure the reduction in area (%). The smaller the reduction in area, the worse the hot workability.

[Tissue Measurements]
A test piece for microstructure observation was prepared from the test piece material that had been subjected to quenching and tempering, and each structure was measured. The observation surface of the structure was a cross section (C cross section) perpendicular to the rolling direction. First, the test piece for microstructure observation was corroded with Villela's reagent (a reagent made by mixing picric acid, hydrochloric acid, and ethanol in the ratio of 2 g, 10 ml, and 100 ml, respectively), and the structure was imaged with a scanning electron microscope (accelerating voltage: 15 kV, magnification: 1000 times), and the structure fraction (area %) of the ferrite phase was calculated using an image analyzer (Image-J).
The test piece for X-ray diffraction was ground and polished so that the cross section (C cross section) perpendicular to the rolling direction was the measurement surface, and the amount of retained austenite (γ) was measured using an X-ray diffraction method. The amount of retained austenite was determined by measuring the integrated intensity of the diffracted X-rays 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 the retained austenite was regarded as the area fraction.
γ (volume ratio) = 100/(1+(IαRγ/IγRα))
Here, Iα is the integrated intensity of α, Rα is the theoretically calculated value of α, Iγ is the integrated intensity of γ, and Rγ is the theoretically 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 residual γ phase.

The results are shown in Table 3.

Claims (2)

In mass percent,
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 to 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;
Further, it contains one or two selected from Cu: 3.5% or less and W: 3.5% or less,
and has a composition that satisfies formula (1) and formula (2), with the balance being Fe and unavoidable impurities,
A high-strength stainless steel seamless pipe for oil wells having a yield strength of 758 MPa or more and an absorbed energy vE _-10 of 40 J or more at a test temperature of -10°C in a Charpy impact test.
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) 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. In addition to the above-mentioned component composition, further, in 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,
2. A high-strength stainless steel seamless pipe for oil wells according to claim 1, which contains one or more selected from the following: Mg: 0.0100% or less.