JP2017014543A - Stainless steel for oil well and stainless steel pipe for oil well - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a stainless steel for oil well having high strength and excellent in SCC resistance at high temperature, SSC resistance at ordinary temperature and low temperature toughness.SOLUTION: The stainless steel for oil well has a chemical composition containing, by mass%, C:0.04% or less, Si:0.05 to 1.0%, Mn:0.01 to 0.5%, Cr:16.0 to 18.0%, Mo:2.0 to 3.0%, Cu:0.7 to 3.5%, Ni:4.8 to 6.0%, sol.Al:0.001 to 0.1% and the balance Fe with impurities of which Ti, Nb and B are Ti:0.1% or less, Nb:0.1% or less and B:0.005% or less and having Fn1 defined by the formula (1) of 80 or more and a micro structure consisting of, by volume percentage, 10 to 50% of ferrite, 5 to 20% or retained austenite and the balance martensite. Fn1=576.5-2660.7×[C]-74.9×[Ni]-37.4×[Cu] (1).SELECTED DRAWING: None

Description

  The present invention relates to stainless steel and stainless steel pipes, and more particularly to stainless steel for oil wells and stainless steel pipes for oil wells.

With recent increases in crude oil prices and natural gas prices, oil wells and natural gas wells under more severe corrosive environments have been developed. Such oil wells and natural gas wells _{typically, CO 2, H 2 S,} Cl - is a corrosive environment containing such a. Therefore, the steel used as the material of the oil well steel pipe used in such oil wells and natural gas wells is required to have CO ₂ corrosion resistance. Under the above-mentioned corrosive environment, oil well steel pipes using martensitic stainless steel (so-called “13% Cr steel”) containing about 13% Cr by mass and excellent in CO ₂ corrosion resistance are used. Yes.

  By the way, the deep oil well that has been developed recently is a high-temperature corrosive environment exceeding 150 ° C. Stainless steel used in such a high-temperature corrosive environment is required to have high strength and excellent corrosion resistance. More specifically, stainless steel used in a high temperature corrosion environment such as a deep oil well is required to have excellent stress corrosion cracking (hereinafter referred to as SCC) resistance at high temperatures. . Furthermore, excellent resistance to sulfide stress cracking (hereinafter referred to as SSC) is required at room temperature. Fluid (crude oil or gas) produced from an oil well in a hot corrosive environment flows through the oil well pipe. When fluid production stops for any reason, the fluid temperature in the oil well pipe arranged near the ground surface falls to room temperature. In an oil well pipe that is in contact with a fluid at room temperature, SSC may occur. Therefore, stainless steel for oil wells used for deep oil wells is required to have not only SCC resistance at high temperatures but also SSC resistance at normal temperatures. Recently, stainless steel having a strength of 125 ksi class (862 MPa or more) has been demanded due to further deep well formation.

  The strength and corrosion resistance of 13% Cr steel are insufficient in such a high temperature corrosion environment. The duplex stainless steel has sufficient strength and corrosion resistance even in a high temperature corrosive environment. However, the duplex stainless steel has a problem that the content of alloy elements is high and the manufacturing cost is high.

  Therefore, a stainless steel having a lower alloy element content than a duplex stainless steel, a high strength, and a high corrosion resistance even in a high temperature corrosive environment is disclosed in JP-A-2005-105357 (Patent Document 1), JP 2012-149317 A (Patent Document 2), International Publication No. 2009/119048 (Patent Document 3), International Publication No. 2010/134498 (Patent Document 4) and International Publication No. 2011-136175 ( This is proposed in Patent Document 5).

  The high-strength stainless steel pipe for oil well disclosed in Patent Document 1 is C: 0.05% or less, Si: 0.50% or less, Mn: 0.10 to 1.80%, P: 0.03% or less, S: 0.005% or less, Cr: 14.0 to 17.0%, Ni: 5.0 to 8.0%, Mo: 1.0 to 3.5%, Cu: 0.5 to 3.5 %, Al: 0.05% or less, V: 0.20% or less, N: 0.03 to 0.15%, O: 0.006% or less, and Nb: 0.2% or less, Ti: Contains one or two selected from 0.3% or less, and the balance has a composition composed of Fe and inevitable impurities. In the structure of the steel pipe, MC type carbonitrides in the precipitates are present in an amount of 3.0% or more by mass% with respect to the total amount of precipitates. Thus, Patent Document 1 describes that high strength and excellent corrosion resistance can be obtained.

  The high-strength martensitic stainless steel seamless pipe disclosed in Patent Document 2 is 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: more than 15.5% and 17.5% or less, Ni: 2.5 to 5.5%, Mo: 1.8 to 3.5%, It has a composition containing Cu: 0.3 to 3.5%, V: 0.20% or less, Al: 0.05% or less, N: 0.06% or less, subjected to quenching and tempering treatment, yielding Strength: strength of 655 to 862 MPa and yield ratio: tensile properties of 0.90 or more. As a result, the yield strength is assured for oil wells and the yield ratio is 0.90 or more, resulting in a steel pipe with low tensile strength, which is resistant to carbon dioxide corrosion and sulfide stress corrosion. Patent Document 2 describes that corrosion resistance such as cracking is improved.

  The stainless steel disclosed in Patent Document 3 is used for oil well pipes. This stainless steel is in mass%, C: 0.001 to 0.05%, Si: 0.05 to 1%, Mn: 2% or less, P: 0.03% or less, S: less than 0.002% Cr: 16-18%, Ni: 3.5-7%, Mo: more than 2% and 4% or less, Cu: 1.5-4%, rare earth metal: 0.001-0.3%, sol. Al: 0.001 to 0.1%, Ca: 0.0001 to 0.01%, O: 0.05% or less and N: 0.05% or less, with the balance being Fe and impurities. Patent Document 3 describes that this stainless steel has a low corrosion rate and excellent SCC resistance in a high-temperature chloride aqueous solution environment containing carbon dioxide gas.

  The oil well stainless steel disclosed in Patent Document 4 has the following chemical composition and structure, and has a 0.2% offset proof stress of 758 MPa or more. Chemical composition is mass%, C: 0.05% or less, Si: 0.5% or less, Mn: 0.01 to 0.5%, P: 0.04% or less, S: 0.01% or less Cr: more than 16.0 to 18.0%, Ni: more than 4.0 to 5.6%, Mo: 1.6 to 4.0%, Cu: 1.5 to 3.0%, Al: 0 0.001 to 0.10%, N: 0.050% or less, and the balance is composed of Fe and impurities, satisfying the formulas (1) and (2). The structure includes a martensite phase and a ferrite phase having a volume ratio of 10 to 40%. When each of a plurality of virtual lines having a length of 50 μm in the thickness direction from the surface of the stainless steel and arranged in a row in a range of 200 μm at a pitch of 10 μm is arranged on the cross section of the stainless steel, The ratio of the number of imaginary line segments intersecting the ferrite phase to the total number of minutes is greater than 85%. Cr + Cu + Ni + Mo ≧ 25.5 (1), −8 ≦ 30 (C + N) +0.5 Mn + Ni + Cu / 2 + 8.2-1.1 (Cr + Mo) ≦ −4 (2), where in the formula (1) and the formula (2) The element content (mass%) is substituted for the element symbol. Patent Document 4 describes that this stainless steel has a high strength of 758 MPa or more with a 0.2% offset proof stress, and has excellent corrosion resistance and excellent SSC resistance in a high temperature environment.

  The high-strength stainless steel disclosed in Patent Document 5 is mass%, C: 0.05% or less, Si: 1.0% or less, Mn: 0.3% or less, P: 0.05% or less, S : Less than 0.002%, Cr: more than 16% and 18% or less, Mo: 1.5-3.0%, Cu: 1.0-3.5%, Ni: 3.5-6.5%, Al: 0.001 to 0.1%, N: 0.025% or less, and O: 0.01% or less, with the balance being a chemical composition composed of Fe and impurities, a martensite phase, and a volume fraction And a structure containing a ferrite phase of 10 to 48.5% and a residual austenite phase of 10% or less by volume, and having a yield strength of 758 MPa or more and a uniform elongation of 10% or more. This stainless steel has excellent corrosion resistance in a high temperature environment and excellent SSC resistance at room temperature. Furthermore, Patent Document 5 describes that it has a yield strength of 758 MPa or more and has a workability superior to 13% Cr steel.

JP 2005-105357 A JP 2012-149317 A International Publication No. 2009/119048 International Publication No. 2010/134498 International Publication No. 2011-136175

  By the way, development of deep oil wells is also conducted in cold regions. When oil well pipes are used in the development of deep oil wells in cold regions, the steels that make up oil well pipes have the above-mentioned high strength, excellent corrosion resistance (excellent SCC resistance at high temperatures and excellent SSC resistance at room temperature). Excellent low temperature toughness is required. Patent Documents 1 to 5 do not disclose a stainless steel having high strength such as 125 ksi class (862 MPa) or more and excellent corrosion resistance and excellent low temperature toughness.

  Furthermore, many of the stainless steels in these documents contain 10% or more of ferrite. In deep oil well development, oil well pipes with a wall thickness of 15 mm or more may be used. When the microstructure of such a thick oil well pipe contains 10% or more of ferrite, the low temperature toughness may be low.

  An object of the present invention is to provide an oil well stainless steel and an oil well stainless steel pipe having high strength, excellent SCC resistance at high temperature, SSC resistance at room temperature, and low temperature toughness.

The oil well stainless steel according to the present invention is, by mass%, C: 0.04% or less, Si: 0.05 to 1.0%, Mn: 0.01 to 0.5%, Cr: 16.0 to 18 0.0%, Mo: 2.0-3.0%, Cu: 0.7-3.5%, Ni: 4.8-6.0%, sol. Al: 0.001 to 0.1%, W: 0 to 2.0%, V: 0 to 0.5%, Ca: 0 to 0.01%, and Mg: 0 to 0.01% The balance consists of Fe and impurities, and among the impurities, P, S, O, N, Ti, Nb, and B are P: 0.05% or less, S: less than 0.002%, O: 0. 02% or less, N: 0.02% or less, Ti: 0.1% or less, Nb: 0.1% or less, and B: 0.005% or less, and Fn1 defined by the formula (1) is It has a chemical composition of 80 or more, a volume ratio of 10 to 50% ferrite, and 5 to 20% retained austenite, with the balance being martensite.
Fn1 = 576.5-2660.7 × [C] −74.9 × [Ni] −37.4 × [Cu] (1)
Here, the content (mass%) of the corresponding element is substituted for each element symbol in the formula (1).

  The oil well stainless steel pipe according to the present invention is manufactured from the above-described oil well stainless steel.

  The oil well stainless steel and oil well stainless steel pipe according to the present invention have high strength and are excellent in SCC resistance at high temperature, SSC resistance at normal temperature, and low temperature toughness.

  The present inventor investigated and examined stainless steel having high strength of 125 ksi class (862 MPa or more) and excellent SCC resistance at high temperature, SSC resistance at normal temperature and low temperature toughness. As a result, the present inventor obtained the following knowledge.

  Residual austenite enhances low temperature toughness. However, an excessive amount of retained austenite reduces the strength of the steel. On the other hand, martensite increases the strength of steel. Therefore, if the retained austenite amount can be appropriately adjusted while generating martensite in the microstructure by quenching and tempering, high strength can be obtained and excellent low temperature toughness can be obtained.

  Nickel (Ni) increases the amount of retained austenite after quenching and tempering, particularly the amount of retained austenite after tempering. Therefore, Ni can increase the amount of retained austenite and enhance low temperature toughness. Furthermore, copper (Cu) precipitates as fine Cu particles during tempering and increases the strength of the steel. Therefore, steel excellent in high strength and low temperature toughness can be obtained by adjusting the Ni content and the Cu content.

  However, Ni and Cu influence the martensitic transformation temperature together with C. More specifically, if the C content, Ni content, and Cu content are high, the martensitic transformation temperature of the steel is lowered. In this case, martensite transformation is less likely to occur, and the amount of retained austenite after quenching is excessively increased. As a result, the strength of the steel is reduced.

An index Fn1 of the amount of retained austenite is defined by equation (1).
Fn1 = 576.5-2660.7 × [C] −74.9 × [Ni] −37.4 × [Cu] (1)
The content (mass%) of the corresponding element is substituted for each element symbol in the formula (1).

  When Fn1 is 80 or more, the volume ratio of retained austenite is 5 to 20% and the yield strength is 862 MPa or more in the microstructure of the stainless steel having the above-described chemical composition. In this case, even if it is a stainless steel pipe for oil wells having a wall thickness of 15 mm or more, high strength and excellent low temperature toughness can be obtained.

  Ti, Nb and B all increase the strength of the steel, but lower the low temperature toughness of the steel. Therefore, in the present invention, Ti, Nb and B are impurities, and it is preferable that their contents be as low as possible.

The oil well stainless steel completed based on the above knowledge is in mass%, C: 0.04% or less, Si: 0.05-1.0%, Mn: 0.01-0.5%, Cr: 16.0 to 18.0%, Mo: 2.0 to 3.0%, Cu: 0.7 to 3.5%, Ni: 4.8 to 6.0%, sol. Al: 0.001 to 0.1%, W: 0 to 2.0%, V: 0 to 0.5%, Ca: 0 to 0.01%, and Mg: 0 to 0.01% The balance consists of Fe and impurities, and among the impurities, P, S, O, N, Ti, Nb, and B are P: 0.05% or less, S: less than 0.002%, O: 0. 02% or less, N: 0.02% or less, Ti: 0.1% or less, Nb: 0.1% or less, and B: 0.005% or less, and Fn1 defined by the formula (1) is It has a chemical composition of 80 or more, a volume ratio of 10 to 50% ferrite, and 5 to 20% retained austenite, with the balance being martensite.
Fn1 = 576.5-2660.7 × [C] −74.9 × [Ni] −37.4 × [Cu] (1)
Here, the content (mass%) of the corresponding element is substituted for each element symbol in the formula (1).

Preferably, the stainless steel for oil wells further has an Fn2 defined by the formula (2) of 2.5 or more.
Fn2 = [Ni] / [Cu] (2)
Here, the content (mass%) of the corresponding element is substituted for each element symbol in the formula (2).

  In this case, the low temperature toughness is further increased.

  The stainless steel for oil wells may contain W: 0.2 to 2.0%. The oil well stainless steel may also contain V: 0.01 to 0.5%. The stainless steel for oil wells may also contain one or more of Ca: 0.0005 to 0.01% and Mg: 0.0005 to 0.01%.

  The oil well stainless steel pipe according to the present invention is manufactured from the above-described oil well stainless steel.

Preferably, in the oil well stainless steel pipe, the Ni content is further higher than Fn3 defined by the formula (3).
Fn3 = 0.0211 × t + 4.4606 (3)
Here, t (mm) in Formula (3) means the thickness (mm) of the stainless steel pipe for oil wells.

  In this case, since Ni is sufficiently contained with respect to the thickness, further excellent low temperature toughness can be obtained.

  The above-mentioned stainless steel pipe for oil well has a wall thickness of 15 mm or more, for example. The above-mentioned stainless steel pipe has a yield strength of 862 MPa or more, for example.

  Hereinafter, the stainless steel for oil wells and the stainless steel pipe for oil wells according to the present invention will be described.

[Chemical composition]
The chemical composition of the stainless steel for oil wells of this embodiment contains the following elements. Unless otherwise specified, “%” for elements means “% by mass”.

C: 0.04% or less Carbon (C) is inevitably contained. C precipitates as a carbide during tempering. In this case, the CO ₂ corrosion resistance and SCC resistance at high temperatures of the steel are reduced. Furthermore, if the C content is too high, the amount of retained austenite produced during quenching increases. In this case, in order to reduce the amount of retained austenite produced, the Cu and Ni contents effective for strength and low temperature toughness must be reduced. Accordingly, a lower C content is preferred. The C content is 0.04% or less. The upper limit with preferable C content is 0.035%, More preferably, it is 0.03%. In actual operation, the lower limit of the C content is, for example, 0.001%.

Si: 0.05-1.0%
Silicon (Si) deoxidizes steel. If the Si content is too low, this effect cannot be obtained. On the other hand, if the Si content is too high, the hot workability of the steel decreases. Therefore, the Si content is 0.05 to 1.0%. A preferable lower limit of the Si content is 0.10%. The upper limit with preferable Si content is 0.75%, More preferably, it is 0.50%.

Mn: 0.01 to 0.5%
Manganese (Mn) deoxidizes steel. If the Mn content is too low, this effect cannot be obtained. On the other hand, if the Mn content is too high, austenite tends to remain excessively after quenching and tempering, and the volume ratio of residual austenite may exceed 20%. In this case, the strength of the steel after tempering decreases. Therefore, the Mn content is 0.01 to 0.5%. A preferable lower limit of the Mn content is 0.05%. The upper limit with preferable Mn content is 0.4%, More preferably, it is 0.3%, Especially preferably, it is less than 0.15%.

Cr: 16.0 to 18.0%
Chromium (Cr) enhances the SCC resistance in a high temperature corrosive environment. If the Cr content is too low, this effect cannot be obtained. On the other hand, since Cr is a ferrite-forming element, if the Cr content is too high, the amount of ferrite in the steel becomes excessive and the strength of the steel decreases. Therefore, the Cr content is 16.0 to 18.0%. A preferred lower limit of the Cr content is 16.5%. The upper limit with preferable Cr content is 17.5%.

Ni: 4.8-6.0%
Nickel (Ni) is an austenite-forming element and stabilizes austenite at high temperatures. Therefore, Ni increases the amount of martensite at room temperature and increases the strength of the steel. Ni further enhances the SCC resistance of the steel in a hot corrosive environment. Ni further promotes the generation of stable retained austenite at room temperature during tempering and enhances low temperature toughness. If the Ni content is too low, these effects cannot be obtained. On the other hand, if the Ni content is too high, the residual austenite produced during tempering will be excessive. Therefore, the Ni content is 4.8 to 6.0%. The minimum with preferable Ni content is 4.9%, More preferably, it is 5.0%. The upper limit with preferable Ni content is 5.9%.

  In addition, in the chemical composition of the steel of the present invention, austenite is generated in the tempering stage as described above in addition to the austenite existing in the stage after quenching, so-called retained austenite. Therefore, in the final product after tempering, the unified name of “residual austenite” is used, including residual austenite present from the stage after quenching and austenite generated after tempering.

Mo: 2.0-3.0%
When fluid production is suspended in the oil well, the temperature of the fluid in the oil well pipe decreases. In this case, the steel is more susceptible to sulfide stress cracking (SSC). Molybdenum (Mo) increases SSC resistance. Mo is further contained together with Cr, thereby enhancing the SCC resistance at high temperatures. If the Mo content is too low, these effects cannot be obtained. On the other hand, since Mo is a ferrite-forming element, if the Mo content is too high, an excessive amount of ferrite in the steel is generated, and the strength of the steel is reduced. Therefore, the Mo content is 2.0 to 3.0%. A preferable lower limit of the Mo content is 2.2%. The upper limit with preferable Mo content is 2.8%.

Cu: 0.7 to 3.5%
Copper (Cu) precipitates as fine Cu particles during tempering to increase the strength of the steel. Cu further reduces the elution rate of the steel in a high temperature corrosive environment, and increases the corrosion resistance of the steel. If the Cu content is too low, these effects cannot be obtained. On the other hand, if the Cu content is too high, martensitic transformation does not proceed sufficiently during quenching, and the amount of retained austenite becomes excessive. In this case, the strength of the steel decreases. Therefore, the Cu content is 0.7 to 3.5%. The minimum with preferable Cu content is 0.8%, More preferably, it is 0.9%. The upper limit with preferable Cu content is 3.0%, More preferably, it is 2.7%.

sol. Al: 0.001 to 0.1%
Aluminum (Al) deoxidizes steel. sol. If the Al content is too low, this effect cannot be obtained. On the other hand, sol. If the Al content is too high, the effect is saturated. sol. If the Al content is too high, more inclusions are generated, and the SSC resistance and low temperature toughness of the steel are reduced. Therefore, sol. The Al content is 0.001 to 0.1%. Here, sol. Al means acid-soluble Al. sol. The minimum with preferable Al content is 0.005%, More preferably, it is 0.010%, More preferably, it is 0.015%. sol. The upper limit with preferable Al content is less than 0.05%, More preferably, it is 0.035%.

  The balance of the stainless steel according to this embodiment is Fe and impurities. The impurities referred to here are those that are mixed from ore, scrap, or production environment as raw materials when industrially producing stainless steel, and do not adversely affect the stainless steel of this embodiment. Means what is allowed.

  Among impurities, the contents of P, S, O, N, Ti, Nb and B are as follows.

P: 0.05% or less Phosphorus (P) is an impurity. P segregates at the grain boundaries and reduces the CO ₂ corrosion resistance and SSC resistance of the steel. Therefore, the P content is 0.05% or less. The upper limit with preferable P content is 0.030%, More preferably, it is 0.020%, More preferably, it is 0.015%. The P content is preferably as low as possible.

S: Less than 0.002% Sulfur (S) is an impurity. S segregates at the grain boundaries and lowers the SSC resistance of the steel. S further reduces the hot workability of the steel. Therefore, the S content is less than 0.002%. The upper limit with preferable S content is 0.001%. The S content is preferably as low as possible.

O: 0.02% or less Oxygen (O) is an impurity. O forms coarse oxides and lowers the toughness and SSC resistance of the steel. Therefore, the O content is 0.02% or less. The upper limit with preferable O content is 0.01%, More preferably, it is 0.005%. The O content is preferably as low as possible.

N: 0.02% or less Nitrogen (N) is an impurity. N forms coarse nitrides. Coarse nitrides are the starting point for pitting corrosion and reduce the SSC resistance of the steel. Therefore, the N content is 0.02% or less. The upper limit with preferable N content is 0.015%, More preferably, it is 0.012%. The N content is preferably as low as possible.

Ti: 0.1% or less Titanium (Ti) is an impurity. In this chemical composition, Ti reduces the low temperature toughness of the steel. Therefore, it is preferable that the Ti content is as low as possible. Ti content is 0.1% or less. The preferable Ti content is 0.04% or less, more preferably 0.02% or less, and still more preferably 0.01% or less.

Nb: 0.1% or less Niobium (Nb) is an impurity. In this chemical composition, Nb reduces the low temperature toughness of the steel. Therefore, it is preferable that the Nb content is as low as possible. Nb content is 0.1% or less. The Nb content is preferably 0.04% or less, more preferably 0.02% or less, and still more preferably 0.01% or less.

B: 0.005% or less Boron (B) is an impurity. In this chemical composition, B decreases the low temperature toughness of the steel. For this reason, the B content is preferably as low as possible. The B content is 0.005% or less. The preferable B content is 0.003% or less, and more preferably 0.001% or less.

  The stainless steel of the present embodiment may further contain W instead of part of Fe.

W: 0 to 2.0%
Tungsten (W) is an optional element and may not be contained. When contained, W increases the SSC resistance of the steel in the same manner as Mo. Further, W is contained together with Cr, thereby increasing the SCC resistance of the steel at a high temperature. If W is contained even a little, the above effect can be obtained. On the other hand, since W is a ferrite forming element, if the W content is too high, excessive ferrite is generated in the steel. In this case, the strength of the steel decreases. Therefore, the W content is 0 to 2.0%. The minimum with preferable W content for acquiring the said effect more effectively is 0.2%, More preferably, it is 0.5%. The upper limit with preferable W content is 1.5%.

  The stainless steel of this embodiment may further contain V in place of part of Fe.

V: 0 to 0.5%
Vanadium (V) is an optional element and may not be contained. When contained, V precipitates as fine precipitates during tempering and increases the strength of the steel. If V is contained even a little, the above effect can be obtained. On the other hand, if the V content is too high, the SSC resistance and low temperature toughness of the steel will be reduced. Therefore, the V content is 0 to 0.5%. The minimum with preferable V content for acquiring the said effect more effectively is 0.01%. The upper limit with preferable V content is 0.2%, More preferably, it is 0.1%.

The stainless steel of this embodiment is further replaced with a part of Fe, and one of Ca and Mg.
It may contain seeds or more.

Ca: 0 to 0.01%
Mg: 0 to 0.01%
Calcium (Ca) and magnesium (Mg) are both optional elements and need not be contained. When included, all of these elements enhance the hot workability of the steel. If one or more of these elements are contained, the above effect can be obtained. On the other hand, if the content of these elements is too high, inclusions are excessively generated. In this case, the toughness and corrosion resistance of the steel are reduced. Therefore, the Ca content is 0 to 0.01%, and the Mg content is 0 to 0.01%. The preferable lower limit of the Ca content for obtaining the above effect more effectively is 0.0005%, and the preferable lower limit of the Mg content is 0.0005%.

[Fn1 defined by formula (1)]
The chemical composition of the stainless steel of this embodiment further has an Fn1 defined by the formula (1) of 80 or more.
Fn1 = 576.5-2660.7 × [C] −74.9 × [Ni] −37.4 × [Cu] (1)
Here, the content (mass%) of the corresponding element is substituted for each element symbol in the formula (1).

  Fn1 is an index of the amount of retained austenite generated during quenching. If Fn1 is less than 80, the martensitic transformation point is excessively lowered, and the amount of retained austenite of the steel after quenching is increased. In this case, the strength of the steel after tempering decreases. If Fn1 is 80 or more, the volume ratio of the retained austenite after tempering is appropriate as 5 to 20%, and excellent low temperature toughness is obtained. Therefore, the Fn1 value is 80 or more. The minimum with a preferable Fn1 value is 84, More preferably, it is 88. The upper limit of Fn1 is not particularly limited. However, in the case of the above chemical composition, the upper limit of Fn1 is 250 or less.

[Microstructure]
The microstructure of the stainless steel of this embodiment contains 10 to 50% ferrite and 5 to 20% retained austenite by volume ratio, and the balance is martensite.

  Ferrite in the microstructure increases the SCC resistance of the steel at high temperatures. If the volume fraction of ferrite (hereinafter referred to as ferrite fraction) is too low, the above effect cannot be obtained. On the other hand, if the ferrite fraction is too high, the strength and low temperature toughness of the steel decrease. Therefore, the ferrite fraction is 10 to 50%. A preferred lower limit of the ferrite fraction for increasing the SCC resistance is 15%. A preferable upper limit of the ferrite fraction for maintaining the strength and low temperature toughness of steel at a high level is 45%, and a more preferable upper limit is 40%.

  The retained austenite after tempering increases the low temperature toughness of the steel. If the volume fraction of retained austenite (hereinafter referred to as retained austenite fraction) is too low, the above effect cannot be obtained. On the other hand, if the retained austenite fraction is too high, the yield strength of the steel is lowered, and a yield strength of 862 MPa or more cannot be stably obtained. Accordingly, the retained austenite fraction is 5 to 20%. A preferable upper limit of the retained austenite fraction for further increasing the yield strength of steel is 15%.

[Method for measuring ferrite fraction and retained austenite fraction]
The ferrite fraction (vol.%), Retained austenite fraction (vol.%) And martensite fraction (vol.%) In the microstructure of the stainless steel for oil wells according to the present invention are measured by the following methods.

[Measurement method of ferrite fraction]
Samples for microstructural observation are taken from stainless steel for oil wells. Of the surface of the test piece, a cross section (hereinafter referred to as an observation surface) perpendicular to the plate width direction of the steel plate is polished. The observation surface after polishing is etched using a mixed solution of aqua regia and glycerin. In the etched observation plane, the ferrite is specified. The area ratio of the specified ferrite is measured by a point calculation method based on JIS G0555 (2003). Since the measured area ratio is considered to be equal to the volume fraction, this is defined as the ferrite fraction (vol%).

[Measurement method of retained austenite fraction]
The retained austenite fraction (volume fraction of retained austenite: unit is vol.%) Is determined using an X-ray diffraction method. A 15 mm × 15 mm × 2 mm test piece is collected from the oil well stainless steel. Using the collected specimens, X-rays of each of (200) plane and (211) plane of ferrite (α phase), (200) plane, (220) plane and (311) plane of austenite (γ phase) Measure the intensity and calculate the integrated intensity of each surface. After the calculation, the retained austenite fraction V _γ is obtained for each combination (6 sets in total) of each surface of the α phase and each surface of the γ phase using the following equation.
_{V γ = 100 / (1+ (} I α × R γ) / (I γ × R α))
Here, “I _α ” in the formula is the integrated intensity of the α phase, and “I _γ ” is the integrated intensity of the γ phase. “R _α ” is a crystallographic theoretical calculation value of the α phase, and “R _γ ” is a crystallographic theoretical calculation value of the γ phase. The average value of the volume fraction Vγ of each surface is defined as the retained austenite fraction (vol.%).

[Measurement method of martensite fraction]
Of the microstructure of the oil well stainless steel having the above chemical composition, the remainder other than ferrite and retained austenite is composed of martensite. The volume ratio of martensite (martensite fraction: unit is vol.%) Is obtained by the following formula.
Martensite fraction = 100− (ferrite fraction + residual austenite fraction)

[Fn2 defined by formula (2)]
Preferably, the stainless steel for oil wells according to the present invention further has an Fn2 defined by the formula (2) of 2.5 or more.
Fn2 = [Ni] / [Cu] (2)
Here, the content (mass%) of the corresponding element is substituted for each element symbol in the formula (2).

  Fn2 is an index of low temperature toughness. As described above, Ni increases the low temperature toughness of the steel. On the other hand, Cu increases the strength of steel. Therefore, when the ratio of Ni to Cu is small, the strength of the steel becomes too high, and the low temperature toughness of the steel is rather lowered. If Fn2 is 2.5 or more, the Ni content relative to the Cu content is sufficient, so that excellent low temperature toughness can be obtained. A preferred lower limit of Fn2 is 3.0.

[Production method]
As an example of the method for producing stainless steel of the present invention, a method for producing a seamless steel pipe will be described.

  A material having the above chemical composition is prepared. The raw material may be a slab manufactured by a continuous casting method (including round CC). Moreover, the steel piece manufactured by hot-working the ingot manufactured by the ingot-making method may be sufficient. It may be a steel piece manufactured from a slab.

  The prepared material is charged into a heating furnace or a soaking furnace and heated. Subsequently, the raw material is hot-worked to produce a raw tube. For example, the Mannesmann method is performed as hot working. Specifically, the material is pierced and rolled with a piercing machine to form a raw pipe. Subsequently, the base tube is further rolled by a mandrel mill or a sizing mill. Hot extrusion may be performed as hot working, or hot forging may be performed.

  The base tube after hot working is quenched and tempered, and the strength is adjusted so that the yield strength is 862 MPa or more (125 ksi class). The quenching temperature is 900 ° C. or higher. A preferable tempering temperature is 620 ° C. or less.

[Stainless steel pipe for oil well]
The oil well stainless steel pipe manufactured by the above steps has high strength, and preferably has a yield strength of 862 MPa or more. The oil well stainless steel pipe is further excellent in SCC resistance at high temperature, SSC resistance at room temperature, and low temperature toughness.

[Fn3 defined by Formula (3)]
Preferably, in the oil well stainless steel pipe, the Ni content is higher than Fn3 defined by the formula (3).
Fn3 = 0.0211 × t + 4.4606 (3)
Here, the wall thickness (mm) of the oil well stainless steel is substituted for t (mm) in the equation (3).

  In this case, sufficient Ni is contained to stop the deterioration of the low temperature toughness accompanying the increase in the wall thickness. Therefore, more excellent low temperature toughness can be obtained stably.

  Assuming the oil well stainless steel pipe described above, a plurality of stainless steel plates having various chemical compositions were manufactured, and their strength, toughness and low temperature toughness were investigated.

[Investigation method]
Molten steel having the chemical composition shown in Table 1 was melted in a vacuum high-frequency melting furnace to produce a 50 kg ingot.

  Each ingot was heated at 1250 ° C. for 2 hours. Hot forging was performed on the heated ingot to obtain a steel material having a thickness of 45 mm and a width of 60 mm. The steel was heated at 1230 ° C. for 1 hour. The steel material after heating was hot-rolled to produce a steel plate having a thickness corresponding to the thickness of the stainless steel pipe for oil wells of 12.7 to 31.8 mm. Quenching and tempering was performed on the steel sheet after hot rolling. In the quenching, the steel sheet was heated at 950 ° C. for 15 minutes and then water quenched. The steel sheet after quenching was tempered at 550 ° C. for 30 minutes and then air-cooled.

  Using the steel plates of each test number after quenching and tempering, a microstructure observation test, a tensile test, an impact test, an SCC resistance test at high temperature and an SSC resistance test at normal temperature were performed.

[Measurement of ferrite fraction, retained austenite fraction, martensite fraction]
Test specimens for microstructural observation were taken from the steel plates of each test number. Of the surface of the collected specimen, a cross section (hereinafter referred to as an observation surface) perpendicular to the plate width direction of the steel plate was polished. The observation surface after polishing was etched using a mixed solution of aqua regia and glycerin. Using the etched observation surface, the ferrite fraction (vol.%) Was determined by the measurement method described above.

  A test piece of 15 mm × 15 mm × 2 mm was taken from each test number steel plate. Using the collected test piece, the retained austenite fraction (vol.%) Was determined by the above-described measurement method.

  Based on the obtained ferrite fraction and retained austenite fraction, the martensite fraction (vol.%) Was determined by the above method.

[Tensile test]
A round bar tensile specimen was collected from the center of the thickness of each test number steel plate. The longitudinal direction of the round bar tensile test piece was a direction (L direction) parallel to the rolling direction of the steel sheet. The diameter of the parallel part of the round bar tensile test piece was 6 mm, and the distance between the gauge points was 40 mm. The collected round bar tensile test piece was subjected to a tensile test at room temperature to obtain a yield strength (0.2% yield strength).

[Charpy impact test]
Full-size test pieces conforming to ASTM E23 were collected from the center of the thickness of each test number steel plate. The longitudinal direction of the test piece was parallel to the plate width direction. Using this test piece, a Charpy impact test was performed at −60 ° C., and the absorbed energy (J) was measured.

[High temperature SCC resistance evaluation test]
Four-point bending specimens were collected from each test number steel plate. The length of the test piece was 75 mm, the width was 10 mm, and the thickness was 2 mm. Each test piece was given deflection by four-point bending. At this time, in accordance with ASTM G39, the amount of deflection of each test piece was determined so that the stress applied to the test piece was equal to the proof stress of the test piece.

An autoclave at 200 ° C. in which 30 bar CO ₂ and 0.01 bar H ₂ S were sealed under pressure was prepared for each test number. The test piece subjected to the above-described deflection was stored in an autoclave. In each autoclave, the test piece was immersed in an aqueous solution containing 25 mass% NaCl and 0.41 g / liter of CH ₃ COONa (pH is 4.5, CH ₃ COONa + CH ₃ COOH buffer system) for 720 hours.

  The test piece after immersion for 720 hours was observed for the presence or absence of stress corrosion cracking (SCC). Specifically, the cross section of the part to which the tensile stress was applied was observed with an optical microscope with a 100 × field of view, and the presence or absence of cracks was determined. Furthermore, the corrosion weight loss of each test piece was calculated | required based on the change amount of the weight of the test piece before a test, and the weight of the test piece after 720 hours immersion. From the obtained corrosion weight loss, the annual corrosion amount (mm / year) of each test number was calculated.

[SSC resistance evaluation test at room temperature]
A round bar test piece for NACE TM0177 METHOD A was collected from each test number steel plate. The diameter of the test piece was 6.35 mm, and the length of the parallel part was 25.4 mm. A tensile stress was applied in the axial direction of each test piece. At this time, based on NACA TM0177-2005, the stress applied to each test piece was adjusted so as to be 90% of the yield stress (actual measurement) of each test material.

The test bath used was a 25 mass% NaCl solution saturated with 0.01 bar H ₂ S and 0.99 bar CO ₂ . The test bath was adjusted to pH 4.0 with CH ₃ COONa + CH ₃ COOH buffer containing 0.41 g / liter CH ₃ COONa. Furthermore, the temperature of the test bath was adjusted to 25 ° C.

  A round bar test piece loaded with tensile stress was immersed in the test bath for 720 hours. The test piece after 720 hours immersion was observed for the presence or absence of sulfide stress cracking (SSC). Specifically, the parallel part of the test piece was observed with the naked eye with respect to the test piece that was broken during the test and the test piece that was not broken. As a result of observation, it was judged that the test piece in which the occurrence of cracks or pitting corrosion was confirmed was inferior in SSC resistance.

[Test results]
Table 2 shows the test results.

  The “plate thickness” in Table 2 describes the plate thickness (mm) of the steel plate manufactured with each test number. “Α” describes the ferrite fraction (vol.%). “Residual γ” describes the retained austenite fraction (vol.%). “M” describes the martensite fraction (vol.%). “YS” describes the yield strength (MPa). “AE” describes the absorbed energy (J) obtained in the Charpy impact test at −60 ° C. “Corrosion amount” describes the annual corrosion amount (mm / year) obtained in the high temperature SCC resistance evaluation test, and “SCC resistance” describes the evaluation result of the high temperature SCC resistance evaluation test. Yes. “◯” indicates that no crack was confirmed in the test piece and the SCC resistance was excellent. “SSC resistance” describes the results of an SSC resistance evaluation test at room temperature. “◯” indicates that no SSC was confirmed on the test piece and the SSC resistance was excellent. “X” indicates that SSC was confirmed in the test piece and the SSC resistance was low.

  With reference to Table 2, in the test numbers 4-12, 16-22, 26, and 27, the chemical composition was appropriate and Fn1 was 80 or more. Therefore, the microstructure of these test numbers consists of ferrite, retained austenite and martensite, and the ferrite fraction is 10 to 50 vol. % And the retained austenite fraction is 5 to 20 vol. %Met. As a result, in these test numbers, the yield strength was 862 MPa or more. Furthermore, the absorbed energy AE at −60 ° C. was 80 J or more, and the low temperature toughness was high. Furthermore, the annual corrosion amount was 0.01 mm / year, and the SCC resistance at high temperatures was high. Furthermore, no crack was observed in the SSC resistance evaluation test, and the SSC resistance was high.

  In particular, in test numbers 5, 6, 8, 9, 11, 12, 16-22, 26, and 27, despite the plate thickness being 15 mm or more, low temperature toughness, high temperature SCC resistance, and normal temperature SSC resistance was high.

  Furthermore, when test numbers 6 and 9 having the same thickness of 25.4 mm are compared with test number 26, Fn2 of test numbers 6 and 9 is 2.5 or more, whereas in test number 26 It was less than 2.5. Therefore, the absorbed energy AE at −60 ° C. of Test Nos. 6 and 9 was higher than that of Test No. 26.

  Furthermore, Fn2 of test number 9 was 3.0 or more, which was higher than test number 6. Therefore, the absorbed energy AE of test number 9 was higher than that of test number 6. Further, when test numbers 5 and 8 having the same plate thickness of 19.1 mm were compared, Fn2 of test number 8 was 3.0 or more, which was higher than test number 5. Therefore, the absorbed energy AE of test number 8 was higher than that of test number 5.

  Furthermore, when the test number 6 and the test number 27 were compared, the Ni content of the test number 6 was higher than Fn3, but the Ni content of the test number 27 was lower than Fn3. Therefore, the absorbed energy AE of test number 6 was higher than the absorbed energy AE of test number 27.

  On the other hand, in test numbers 1 to 3, the Ni content was too low. Therefore, the retained austenite fraction in the microstructure was low. As a result, in Test Nos. 2 and 3 where the plate thickness (corresponding to the wall thickness) was 15 mm or more, the absorbed energy AE was less than 80 J, and the low temperature toughness was low.

  In test number 13, the Cu content was too low. Therefore, the yield strength YS was less than 862 MPa. Furthermore, cracks (SSC) were confirmed in the SSC resistance evaluation test. In test numbers 14 and 15, Fn1 was less than 80. Therefore, the retained austenite fraction exceeded 20%. As a result, the yield strength YS was less than 862 MPa.

  In test number 23, the Ti content was too high. In test number 24, the Nb content was too high. In test number 25, the B content was too high. Therefore, the absorbed energy AE of test numbers 23 to 25 was less than 80 J, and the low temperature toughness was low.

  The embodiment of the present invention has been described above. However, the above-described embodiment is merely an example for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately changing the above-described embodiment without departing from the spirit thereof.

The oil well stainless steel according to the present invention has a high strength of 125 ksi class (862 MPa or more), and is excellent in SCC resistance at high temperature, SSC resistance at room temperature, and low temperature toughness. In particular, the above characteristics can also be obtained when used as an oil-based stainless steel pipe having a wall thickness of 15 mm or more. Therefore, the oil well stainless steel according to the present invention is a high temperature of 150 ° C. or higher, and can be widely applied to corrosive environments containing CO ₂ , H ₂ S, and Cl ^−. Is preferred.

Claims (9)

% By mass
C: 0.04% or less,
Si: 0.05 to 1.0%,
Mn: 0.01 to 0.5%,
Cr: 16.0 to 18.0%,
Mo: 2.0-3.0%,
Cu: 0.7 to 3.5%,
Ni: 4.8 to 6.0%,
sol. Al: 0.001 to 0.1%,
W: 0 to 2.0%,
V: 0 to 0.5%
Ca: 0 to 0.01%, and
Mg: 0 to 0.01% is contained, the balance consists of Fe and impurities,
Among the impurities, P, S, O, N, Ti, Nb and B are respectively
P: 0.05% or less,
S: less than 0.002%,
O: 0.02% or less,
N: 0.02% or less,
Ti: 0.1% or less,
Nb: 0.1% or less, and
B: 0.005% or less,
A chemical composition in which Fn1 defined by the formula (1) is 80 or more;
Stainless steel for oil wells having a volume ratio of 10 to 50% ferrite and 5 to 20% residual austenite, and the balance being martensite.
Fn1 = 576.5-2660.7 × [C] −74.9 × [Ni] −37.4 × [Cu] (1)
Here, the content (mass%) of the corresponding element is substituted for each element symbol in the formula (1). The oil well stainless steel according to claim 1, further comprising:
Stainless steel for oil wells, wherein Fn2 defined by formula (2) is 2.5 or more.
Fn2 = [Ni] / [Cu] (2)
Here, the content (mass%) of the corresponding element is substituted for each element symbol in the formula (2). The oil well stainless steel according to claim 1 or claim 2,
W: Stainless steel for oil wells containing 0.2 to 2.0%. The oil well stainless steel according to any one of claims 1 to 3,
V: Stainless steel for oil wells containing 0.01 to 0.5%. The oil well stainless steel according to any one of claims 1 to 4,
Ca: 0.0005 to 0.01%, and
Mg: Stainless steel for oil wells containing at least one selected from the group consisting of 0.0005 to 0.01%.   The stainless steel pipe for oil wells manufactured from the stainless steel for oil wells according to any one of claims 1 to 5. The oil well stainless steel pipe according to claim 6, further comprising:
The stainless steel pipe for oil wells, whose Ni content (% by mass) is higher than Fn3 defined by formula (3).
Fn3 = 0.0211 × t + 4.4606 (3)
Here, t (mm) in the formula (3) means the wall thickness (mm) of the oil well stainless steel. A stainless steel pipe for an oil well according to claim 6 or claim 7,
A stainless steel pipe for oil wells having a wall thickness of 15 mm or more. A stainless steel pipe for an oil well according to any one of claims 6 to 8,
A stainless steel pipe for oil wells having a yield strength of 862 MPa or more.