JP6672620B2 - Stainless steel for oil well and stainless steel tube for oil well - Google Patents

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 the recent rise in crude oil prices and natural gas prices, development of oil wells and natural gas wells under more severe corrosive environments has been promoted. Such oil and natural gas wells are generally corrosive environments containing CO ₂ , H ₂ S, Cl ^{− and the} like. Therefore, steel used as the material of the steel pipe for oil wells used in such oil wells and natural gas wells is required to have resistance to CO ₂ corrosion. Under the above-mentioned corrosive environment, a steel pipe for an oil well using a martensitic stainless steel excellent in CO ₂ corrosion resistance and containing about 13% by mass of Cr (so-called “13% Cr steel”) is used. I have.

  By the way, a deep oil well which has been recently developed 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 further excellent corrosion resistance. More specifically, stainless steel used in a high-temperature corrosive environment such as a deep oil well is required to have excellent stress corrosion cracking (SCC) resistance at high temperatures as corrosion resistance. . Furthermore, excellent sulfide stress cracking (Sulfide Stress Cracking, hereinafter, referred to as SSC) resistance at room temperature is required. Fluid (crude oil or gas) produced from an oil well in a hot corrosive environment flows through the oil well tube. When the production of fluid stops for any reason, the temperature of the fluid in the oil well tube located near the surface of the earth drops to room temperature. SSC can occur in oil country tubular goods in contact with fluid at room temperature. Therefore, stainless steels for oil wells used in deep oil wells are required to have not only SCC resistance at high temperature but also SSC resistance at room temperature. Recently, with further deepening of wells, stainless steel having a strength of 125 ksi class (862 MPa or more) has been demanded.

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

  Accordingly, a stainless steel having a lower content of alloying elements than duplex stainless steel, having high strength, and having high corrosion resistance even in a high-temperature corrosion environment is disclosed in JP-A-2005-105357 (Patent Document 1). JP 2012-149317 A (Patent Document 2), WO 2009/119048 (Patent Document 3), WO 2010/134498 (Patent Document 4), and WO 2011/136175 ( Patent Document 5) has been proposed.

  The high-strength stainless steel pipe for oil wells disclosed in Patent Document 1 has 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 unavoidable 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% based on the total amount of the precipitates. It is described in Patent Document 1 that high strength and excellent corrosion resistance are thereby obtained.

  The high-strength martensitic stainless steel seamless steel pipe disclosed in Patent Literature 2 is, by mass%, C: 0.01% or less, Si: 0.5% or less, Mn: 0.1 to 2.0%, P : 0.03% or less, S: 0.005% or less, Cr: more than 15.5% and 17.5% or less, Ni: 2.5 to 5.5%, Mo: 1.8 to 3.5%, Cu: 0.3 to 3.5%, V: 0.20% or less, Al: 0.05% or less, N: 0.06% or less, quenching and tempering, yielding Strength: Tensile properties of 655-862 MPa and yield ratio: 0.90 or more. As a result, a steel pipe having a low tensile strength can be obtained by setting the yield ratio to 0.90 or more while securing a predetermined yield strength for oil wells, thereby achieving a carbon dioxide gas corrosion resistance and a sulfide stress corrosion resistance. Patent Literature 2 describes that corrosion resistance such as cracking property is improved.

  The stainless steel disclosed in Patent Document 3 is used for oil country tubular goods. In this stainless steel, 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 to 18%, Ni: 3.5 to 7%, Mo: more than 2% to 4% or less, Cu: 1.5 to 4%, rare earth metal: 0.001 to 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 Literature 4 has the following chemical composition and structure, and has a 0.2% offset proof stress of 758 MPa or more. Chemical composition is, in 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, with the balance being Fe and impurities, satisfying the equations (1) and (2). The structure includes a martensite phase and a ferrite phase of 10 to 40% by volume. When a plurality of imaginary line segments each 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 are arranged on the cross section of the stainless steel, The ratio of the number of virtual lines intersecting with the ferrite phase to the total number of minutes is more than 85%. Cr + Cu + Ni + Mo ≧ 25.5 (1), −8 ≦ 30 (C + N) + 0.5Mn + Ni + Cu / 2 + 8.2-1.1 (Cr + Mo) ≦ −4 (2) where, in the expressions (1) and (2) Is substituted for the content (% by mass) of each element. Patent Document 4 describes that this stainless steel has a high strength of 758 MPa or more at 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, by 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 to 3.0%, Cu: 1.0 to 3.5%, Ni: 3.5 to 6.5%, Al: 0.001 to 0.1%, N: 0.025% or less, O: 0.01% or less, the balance being a chemical composition 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 retained austenite phase of 10% or less by volume, 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 has excellent SSC resistance at room temperature. Further, Patent Document 5 describes that the steel has a proof stress of 758 MPa or more and has workability superior to 13% Cr steel.

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

  By the way, deep oil well development is also performed in cold regions. When the oil country tubular goods are used for the development of deep oil wells in cold regions, the steel constituting the oil country tubular goods has the above-mentioned high strength, excellent corrosion resistance (excellent SCC resistance at high temperature and excellent SSC resistance at room temperature). Excellent low-temperature toughness in addition to the low-temperature toughness). Patent Literatures 1 to 5 do not disclose a stainless steel having high strength of 125 ksi class (862 MPa) or more, and having excellent corrosion resistance and excellent low-temperature toughness.

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

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

The stainless steel for oil wells 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%. 2.0%, 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 is made up of Fe and impurities. Of the impurities, P, S, O, N, Ti, Nb and B are each P: 0.05% or less, S: less than 0.002%, and 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 not less than 80, a ferrite of 10 to 50% by volume, and a retained austenite of 5 to 20%, with a microstructure of martensite in the remainder.
Fn1 = 576.5-2660.7 × [C] −74.9 × [Ni] −37.4 × [Cu] (1)
Here, the content (% by mass) of the corresponding element is substituted for each element symbol in the formula (1).

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

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

  Retained austenite increases 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 amount of retained austenite can be appropriately adjusted while forming martensite in the microstructure by quenching and tempering, high strength 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 increase the low-temperature toughness. Further, copper (Cu) precipitates as fine Cu particles during tempering, and increases the strength of steel. Therefore, by adjusting the Ni content and the Cu content, a steel excellent in high strength and low-temperature toughness can be obtained.

  However, Ni and Cu together with C influence the martensitic transformation temperature. More specifically, the higher the C, Ni and Cu contents, the lower the martensitic transformation temperature of the steel. In this case, martensitic transformation hardly occurs, and the amount of retained austenite after quenching becomes excessively large. As a result, the strength of the steel decreases.

The 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 (% by 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 stainless steel microstructure having the above-described chemical composition. In this case, high strength and excellent low-temperature toughness can be obtained even with a stainless steel pipe for oil wells having a thickness of 15 mm or more.

  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 the content of these is preferably as low as possible.

The oil well stainless steel completed on the basis of the above findings 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%, 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 is made up of Fe and impurities. Of the impurities, P, S, O, N, Ti, Nb and B are each P: 0.05% or less, S: less than 0.002%, and 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 not less than 80, a ferrite of 10 to 50% by volume, and a retained austenite of 5 to 20%, with a microstructure of martensite in the remainder.
Fn1 = 576.5-2660.7 × [C] −74.9 × [Ni] −37.4 × [Cu] (1)
Here, the content (% by mass) of the corresponding element is substituted for each element symbol in the formula (1).

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

  In this case, the low-temperature toughness further increases.

  The oil well stainless steel 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%.

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

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

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

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

  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 oil well stainless steel of the present embodiment contains the following elements. Unless otherwise specified, percentages for elements refer to percentages by weight.

C: 0.04% or less Carbon (C) is inevitably contained. C precipitates as carbide during tempering. In this case, the CO ₂ corrosion resistance at high temperatures and the SCC resistance of the steel decrease. Further, if the C content is too high, the amount of retained austenite generated during quenching increases. In this case, in order to reduce the amount of retained austenite, the Cu and Ni contents effective for strength and low-temperature toughness must be reduced. Therefore, the C content is preferably low. C content is 0.04% or less. A preferred upper limit of the C content is 0.035%, and more preferably 0.03%. In an actual operation, the lower limit of the C content is, for example, 0.001%.

Si: 0.05 to 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 preferred lower limit of the Si content is 0.10%. A preferred upper limit of the Si content is 0.75%, and more preferably 0.50%.

Mn: 0.01-0.5%
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 fraction of retained austenite may exceed 20%. In this case, the strength of the tempered steel decreases. Therefore, the Mn content is 0.01 to 0.5%. A preferred lower limit of the Mn content is 0.05%. The preferred upper limit of the Mn content is 0.4%, more preferably 0.3%, and particularly preferably less than 0.15%.

Cr: 16.0 to 18.0%
Chromium (Cr) enhances SCC resistance under 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 excessively large, 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%. A preferable upper limit of the Cr content is 17.5%.

Ni: 4.8 to 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 steel. Ni further enhances the SCC resistance of the steel in hot corrosion environments. Ni further promotes the formation of retained austenite that is stable 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 amount of retained austenite generated during tempering becomes excessively large. Therefore, the Ni content is 4.8 to 6.0%. The preferred lower limit of the Ni content is 4.9%, and more preferably 5.0%. A preferred upper limit of the Ni content is 5.9%.

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

Mo: 2.0 to 3.0%
When the production of fluid in an oil well is suspended, the temperature of the fluid in the oil well tube decreases. In this case, the sulfide stress cracking (SSC) susceptibility of the steel increases. Molybdenum (Mo) enhances SSC resistance. Mo further improves SCC resistance at high temperatures by being contained together with Cr. 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, ferrite in the steel is excessively generated, and the strength of the steel decreases. Therefore, the Mo content is 2.0 to 3.0%. A preferred lower limit of the Mo content is 2.2%. A preferred upper limit of the Mo content is 2.8%.

Cu: 0.7-3.5%
Copper (Cu) precipitates as fine Cu particles during tempering and increases the strength of 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, the martensitic transformation does not proceed sufficiently during quenching, and the amount of retained austenite increases excessively. In this case, the strength of the steel decreases. Therefore, the Cu content is 0.7 to 3.5%. A preferred lower limit of the Cu content is 0.8%, and more preferably 0.9%. A preferred upper limit of the Cu content is 3.0%, and more preferably 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 saturates. sol. If the Al content is too high, further inclusions are generated excessively, and the SSC resistance and low-temperature toughness of the steel deteriorate. Therefore, sol. The Al content is 0.001 to 0.1%. Here, sol. Al means acid-soluble Al. sol. A preferred lower limit of the Al content is 0.005%, more preferably 0.010%, and still more preferably 0.015%. sol. The preferred upper limit of the Al content is less than 0.05%, more preferably 0.035%.

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

  Among the 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 preferable upper limit of the P content is 0.030%, more preferably 0.020%, and still more preferably 0.015%. The P content is preferably as low as possible.

S: less than 0.002% Sulfur (S) is an impurity. S segregates at 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%. A preferable upper limit of the 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 a coarse oxide and lowers the toughness and SSC resistance of the steel. Therefore, the O content is 0.02% or less. The preferable upper limit of the O content is 0.01%, and more preferably 0.005%. The O content is preferably as low as possible.

N: 0.02% or less Nitrogen (N) is an impurity. N forms a coarse nitride. Coarse nitrides become a starting point of pitting corrosion and lower the SSC resistance of steel. Therefore, the N content is 0.02% or less. The preferable upper limit of the N content is 0.015%, and more preferably 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 be as low as possible. The Ti content is 0.1% or less. The preferred Ti content is 0.04% or less, more preferably 0.02% or less, and further 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, the Nb content is preferably as low as possible. The Nb content is 0.1% or less. The preferred Nb content is 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 reduces the low temperature toughness of the steel. Therefore, the B content is preferably as low as possible. The B content is 0.005% or less. The preferred B content is 0.003% or less, 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 need not be contained. When included, W, like Mo, enhances the SSC resistance of the steel. W further increases the SCC resistance of the steel at high temperatures by being contained together with Cr. The above effect can be obtained as long as W is contained at all. On the other hand, since W is a ferrite forming element, if the W content is too high, ferrite is excessively generated in the steel. In this case, the strength of the steel decreases. Therefore, the W content is 0 to 2.0%. A preferable lower limit of the W content for more effectively obtaining the above effects is 0.2%, and more preferably 0.5%. A preferred upper limit of the W content is 1.5%.

  The stainless steel of the present embodiment may further contain V instead 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-mentioned 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 decrease. Therefore, the V content is 0 to 0.5%. A preferred lower limit of the V content for more effectively obtaining the above effects is 0.01%. The preferred upper limit of the V content is 0.2%, and more preferably 0.1%.

The stainless steel of the present embodiment further includes one of Ca and Mg instead of a part of Fe.
It may contain more than one species.

Ca: 0 to 0.01%
Mg: 0 to 0.01%
Both calcium (Ca) and magnesium (Mg) are optional elements and need not be contained. When present, all of these elements enhance the hot workability of the steel. If at least one of these elements is contained even a little, the above effects can be obtained. On the other hand, if the content of these elements is too high, excessive inclusions are generated. In this case, the toughness and corrosion resistance of the steel decrease. Therefore, the Ca content is 0-0.01%, and the Mg content is 0-0.01%. A preferred lower limit of the Ca content for more effectively obtaining the above effects is 0.0005%, and a preferred lower limit of the Mg content is 0.0005%.

[About Fn1 defined by Expression (1)]
In the chemical composition of the stainless steel of the present embodiment, Fn1 defined by the formula (1) is 80 or more.
Fn1 = 576.5-2660.7 × [C] −74.9 × [Ni] −37.4 × [Cu] (1)
Here, the content (% by 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. When Fn1 is less than 80, the martensitic transformation point is too low, and the amount of retained austenite of the steel after quenching increases. In this case, the strength of the tempered steel decreases. When Fn1 is 80 or more, the volume ratio of retained austenite after tempering becomes appropriate to be 5 to 20%, and excellent low-temperature toughness can be obtained. Therefore, the Fn1 value is 80 or more. A preferred lower limit of the Fn1 value is 84, more preferably 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 the present embodiment contains 10 to 50% of ferrite and 5 to 20% of retained austenite by volume, with the balance being martensite.

  Ferrite in the microstructure enhances 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 effects 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 preferable lower limit of the ferrite fraction for improving SCC resistance is 15%. A preferable upper limit of the ferrite fraction for keeping the strength and low-temperature toughness of the steel at a high level is 45%, and a further preferable upper limit is 40%.

  The retained austenite after tempering increases the low temperature toughness of the steel. If the volume ratio of the retained austenite (hereinafter, referred to as the 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 decreases, and a yield strength of 862 MPa or more cannot be obtained stably. Therefore, the retained austenite fraction is 5 to 20%. A preferred upper limit of the retained austenite fraction for further increasing the yield strength of steel is 15%.

[Method of 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.

[Method of measuring ferrite fraction]
A specimen for microstructure observation is collected from the stainless steel for oil well. A cross section (hereinafter, referred to as an observation surface) perpendicular to the width direction of the steel plate is polished on the surface of the test piece. The polished observation surface is etched using a mixed solution of aqua regia and glycerin. The ferrite is specified on the etched observation surface. The area ratio of the specified ferrite is measured by a point calculation method based on JIS G0555 (2003). Since the measured area fraction is considered to be equal to the volume fraction, this is defined as the ferrite fraction (vol%).

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

[Method of measuring martensite fraction]
In the microstructure of the oil well stainless steel having the above-described chemical composition, the remainder other than ferrite and retained austenite is composed of martensite. The volume fraction of martensite (martensite fraction: unit is vol.%) Is determined by the following equation.
Martensite fraction = 100-(Ferrite fraction + Retained austenite fraction)

[About Fn2 defined by Expression (2)]
Preferably, the stainless steel for oil wells according to the present invention further has Fn2 defined by the formula (2) of 2.5 or more.
Fn2 = [Ni] / [Cu] (2)
Here, the content (% by 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 enhances the low-temperature toughness of 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. When 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 the 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). Alternatively, a steel slab manufactured by hot working an ingot manufactured by an ingot-making method may be used. A steel slab manufactured from a slab may be used.

  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 raw material is pierced and rolled by a piercing machine to form a raw tube. Subsequently, the raw 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 raw tube after the hot working is quenched and tempered, and the strength is adjusted so that the yield strength becomes 862 MPa or more (125 ksi class). The quenching temperature is 900 ° C. or higher. The preferred tempering temperature is 620 ° C or lower.

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

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

  In this case, sufficient Ni is contained to keep the low-temperature toughness from deteriorating due to the increase in wall thickness. Therefore, more excellent low-temperature toughness can be stably obtained.

  Assuming the above-mentioned stainless steel pipe for oil wells, a plurality of stainless steel sheets having various chemical compositions were manufactured, and their strength, toughness and low-temperature toughness were investigated.

[Survey 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 the heating was hot-rolled to produce a steel plate having a thickness of 12.7 to 31.8 mm corresponding to the thickness of the stainless steel pipe for oil wells. The steel sheet after hot rolling was quenched and tempered. 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 sheets of each test number after quenching and tempering, a microstructure observation test, a tensile test, an impact test, an SCC resistance test at a high temperature, and an SSC resistance test at a normal temperature were performed.

[Ferrite fraction, retained austenite fraction, martensite fraction measurement]
Test specimens for microstructure observation were collected from the steel sheets of each test number. A cross section (hereinafter, referred to as an observation surface) perpendicular to the sheet width direction of the steel plate was polished from the surface of the collected test piece. The polished observation surface was etched using a mixed solution of aqua regia and glycerin. Using the etched observation surface, the ferrite fraction (vol.%) Was determined by the above-described measurement method.

  A test piece of 15 mm × 15 mm × 2 mm was collected from the steel plate of each test number. The residual austenite fraction (vol.%) Was determined by the above-described measurement method using the collected test pieces.

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

[Tensile test]
From the center of the thickness of the steel plate of each test number, a round bar tensile test piece was collected. 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 gauge points was 40 mm. A tensile test was performed at room temperature on the collected round bar tensile test pieces to determine the yield strength (0.2% proof stress).

[Charpy impact test]
From the center of the thickness of the steel plate of each test number, a full-size test piece based on ASTM E23 was collected. 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 test pieces were collected from the steel plates of each test number. 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 a deflection due to four-point bending. At this time, the amount of deflection of each test piece was determined in accordance with ASTM G39 such 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 of CO ₂ and 0.01 bar of H ₂ S were sealed under pressure was prepared for each test number. The test piece subjected to the above-mentioned bending was stored in an autoclave. In each autoclave, the test piece was immersed in an aqueous solution containing 25 mass% of NaCl and 0.41 g / liter of CH ₃ COONa (pH 4.5, CH ₃ COONa + CH ₃ COOH buffer system) for 720 hours.

  The test piece after immersion for 720 hours was observed for occurrence of stress corrosion cracking (SCC). Specifically, the cross section of the portion to which the tensile stress was applied was observed with an optical microscope having a 100-fold visual field, and the presence or absence of cracks was determined. Further, based on the weight of the test piece before the test and the amount of change in the weight of the test piece after immersion for 720 hours, the corrosion loss of each test piece was determined. 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 the steel plate of each test number. The diameter of the test piece was 6.35 mm, and the length of the parallel portion was 25.4 mm. A tensile stress was applied to each test piece in the axial direction. At this time, in accordance with 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 of H ₂ S and 0.99 bar of CO ₂ . The test bath was adjusted to pH 4.0 with CH ₃ COONa + CH ₃ COOH buffer containing 0.41 g / L CH ₃ COONa. Further, the temperature of the test bath was adjusted to 25 ° C.

  A round bar test piece subjected to a tensile stress was immersed in the test bath for 720 hours. The test piece after immersion for 720 hours was observed for occurrence of sulfide stress cracking (SSC). Specifically, the parallel part of the test piece was visually observed with respect to the test piece that broke during the test and the test piece that did not break. As a result of the observation, it was determined that the test piece in which the occurrence of cracks or pitting corrosion was confirmed had poor SSC resistance.

[Test results]
Table 2 shows the test results.

  In “Sheet thickness” in Table 2, the thickness (mm) of the steel sheet manufactured in each test number is described. "Α" describes the ferrite fraction (vol.%). “Residual γ” describes a 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. The “corrosion amount” describes the annual corrosion amount (mm / year) obtained in the high-temperature SCC resistance evaluation test, and the “SCC resistance” describes the evaluation results of the high-temperature SCC resistance evaluation test. I have. "O" indicates that no crack was observed in the test piece and the SCC resistance was excellent. The “SSC resistance” describes the results of an SSC resistance evaluation test at room temperature. “O” indicates that no SSC was observed in the test piece, indicating that the SSC resistance was excellent. “X” indicates that SSC was confirmed in the test piece and the SSC resistance was low.

  Referring to Table 2, in Test Nos. 4 to 12, 16 to 22, 26 and 27, the chemical composition was appropriate and Fn1 was 80 or more. Therefore, the microstructures of these test numbers consist of ferrite, retained austenite and martensite, and have a ferrite fraction of 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 absorption energy AE at −60 ° C. was 80 J or more, and the low-temperature toughness was high. Further, the annual corrosion amount was 0.01 mm / year, and the SCC resistance at high temperatures was high. Further, no crack was observed in the SSC resistance evaluation test, and the SSC resistance was high.

  In particular, in Test Nos. 5, 6, 8, 9, 11, 12, 16 to 22, 26 and 27, even though the plate thickness is 15 mm or more, low-temperature toughness, SCC resistance at high temperatures, and room temperature Had high SSC resistance.

  Furthermore, comparing Test Nos. 6 and 9 with the same plate thickness of 25.4 mm and Test No. 26, Fn2 of Test Nos. 6 and 9 is 2.5 or more, whereas Test No. 26 It was less than 2.5. Therefore, the absorption energy AE at −60 ° C. of Test Nos. 6 and 9 was higher than that of Test No. 26.

  Furthermore, Fn2 of Test No. 9 was 3.0 or more, which was higher than Test No. 6. Therefore, the absorption energy AE of Test No. 9 was higher than Test Energy No. 6. In addition, comparing Test Nos. 5 and 8 having the same plate thickness of 19.1 mm, Fn2 of Test No. 8 was 3.0 or more, which was higher than Test No. 5. Therefore, the absorption energy AE of Test No. 8 was higher than that of Test No. 5.

  Furthermore, comparing Test No. 6 and Test No. 27, the Ni content of Test No. 6 was higher than Fn3, but the Ni content of Test No. 27 was lower than Fn3. Therefore, the absorption energy AE of Test No. 6 was higher than the absorption energy AE of Test No. 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 in which 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 No. 13, the Cu content was too low. Therefore, the yield strength YS was less than 862 MPa. Further, a crack (SSC) was confirmed in the SSC resistance evaluation test. In Test Nos. 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 Nos. 23 to 25 was less than 80 J, and the low-temperature toughness was low.

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

The stainless steel for oil wells 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 temperatures, SSC resistance at room temperature, and low-temperature toughness. In particular, even when used as an oil-based stainless steel pipe having a thickness of 15 mm or more, the above characteristics can be obtained. Therefore, the oil well stainless steel according to the present invention has a high temperature of 150 ° C. or more and can be widely applied to a corrosive environment containing CO ₂ , H ₂ S, and Cl ⁻ , and is particularly suitable for use as an oil well stainless steel pipe. It is suitable.

Claims (8)

In mass%,
C: 0.04% or less,
Si: 0.05 to 1.0%,
Mn: 0.01-0.5%,
Cr: 16.0 to 18.0%,
Mo: 2.0-3.0%,
Cu: 0.70 to 3.5%,
Ni: 4.9 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 being 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: 0.10% or less,
Nb: 0.10% or less, and
B: 0.005% or less;
A chemical composition wherein Fn1 defined by the formula (1) is 80 or more;
The ferrite contains 10 to 50% by volume of ferrite and 5.0 to 20% of retained austenite, and the balance has a microstructure of martensite,
The yield strength is 862 MPa or more;
Absorption energy in a Charpy impact test of -60 ° C. conforming to ASTM E23 is Ri der least 80 J,
A 25 mass% NaCl solution saturated with 0.01 bar of H ₂ S and 0.99 bar of CO ₂ was treated with a CH ₃ COONa + CH ₃ COOH buffer containing 0.41 g / l of CH ₃ COONa at pH 4.0. In a test for evaluating SSC resistance in accordance with NACE TM0177 METHODA A using a test bath at 25 ° C. adjusted to 25 ° C., cracks are not observed after immersion for 720 hours with 90% of the actually measured yield stress added. to that, oil well for stainless steel.
Fn1 = 576.5-2660.7 × [C] −74.9 × [Ni] −37.4 × [Cu] (1)
Here, the content (% by mass) of the corresponding element is substituted for each element symbol in the formula (1). The stainless steel for an oil well according to claim 1, further comprising:
The stainless steel for oil wells, wherein Fn2 defined by the formula (2) is 2.5 or more.
Fn2 = [Ni] / [Cu] (2)
Here, the content (% by 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%. 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, wherein
Ca: 0.0005 to 0.01%, and
Mg: An oil well stainless steel containing at least one selected from the group consisting of 0.0005 to 0.01%.   An oil well stainless steel pipe manufactured from the oil well stainless steel according to any one of claims 1 to 5. The oil well stainless steel pipe according to claim 6, further comprising:
A stainless steel pipe for oil wells, wherein the Ni content (% by mass) is higher than Fn3 defined by the formula (3).
Fn3 = 0.0211 × t + 4.4606 (3)
Here, t (mm) in the formula (3) means the thickness (mm) of the stainless steel for oil wells. The oil well stainless steel pipe according to claim 6 or claim 7,
Stainless steel pipe for oil wells having a wall thickness of 15 mm or more.