JP2006016637A - High-strength stainless steel pipe for oil well superior in corrosion resistance to carbon dioxide gas - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an inexpensive high-strength stainless steel pipe for an oil well, which has a high YS exceeding 654 MPa, and shows superior corrosion resistance to CO<SB>2</SB>gas in a severe corrosive environment which does not include hydrogen sulfide, includes carbon dioxide gas (CO<SB>2</SB>) and chlorine ions (Cl<SP>-</SP>), and is in a high temperature exceeding 170°C. <P>SOLUTION: This stainless steel pipe has a composition including, by mass%, 0.05% or less C, 0.5% or less Si, 0.2-1.8% Mn, 0.03% or less P, 0.005% or less S, 15.5-18.5% Cr, 1.5-5% Ni, 0.05% or less Al, 0.2% or less V, 0.15% or less N, and 0.006% or less O, so as to satisfy Cr+0.65Ni+0.6Mo+0.55Cu-20C≥18.0 and Cr+Mo+0.3Si-43.5C-0.4Mn-Ni-0.3Cu-9N≥11.5; and further has a structure including a martensitic phase as a base phase, and besides, a ferritic phase of 5 to 70 vol.%, or further including an austenitic phase of 30 vol.% or less. Furthermore, the stainless steel pipe may include one or two elements selected from less than 1.0% Mo and 3.5% or less Cu, one or more elements among Nb, Ti, Zr, B and W, or Ca. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

The present invention relates to oil wells for crude oil or natural gas, and steel pipes for oil wells used for gas wells, and is particularly suitable for oil wells and gas wells with severe corrosive environments containing carbon dioxide (CO ₂ ) and chlorine ions (Cl ⁻ ). The present invention relates to a high-strength stainless steel pipe for oil wells having excellent corrosion resistance. The “high strength stainless steel pipe” in the present invention refers to a stainless steel pipe having a high strength of yield strength: 654 MPa (95 ksi) or more.

In recent years, in order to cope with soaring crude oil prices and the depletion of oil resources expected in the near future, development of deep oil fields that have not been excluded in the past has become active on a global scale. Such oil, gas fields are generally the depth is very deep, and the atmosphere at a high temperature and, CO _2, Cl ^- has a severe corrosive environment and the like. Accordingly, steel pipes for oil wells used for mining such oil fields and gas fields are required to have high strength and excellent corrosion resistance.

Conventionally, 13% Cr martensitic stainless steel pipes with excellent resistance to CO ₂ corrosion have been used as oil well steel pipes in oil fields and gas fields in environments containing CO ₂ , Cl ^−, etc. there were. However, the conventional 13% Cr martensitic stainless steel pipe has a problem that if the yield strength exceeds 654 MPa, the toughness is remarkably lowered and it cannot be used. For this reason, cold-worked duplex stainless steel pipes have been used in wells that require high strength. However, the duplex stainless steel pipe has a problem that it has a large amount of alloy elements, is inferior in hot workability, and can be produced only by a special hot working method, and is expensive.

  In recent years, oil fields have been actively developed in cold regions, and it is often required to have excellent low temperature toughness in addition to high strength.

For this reason, it is based on 13% Cr martensitic stainless steel, which is excellent in hot workability and inexpensive, and has high yield strength exceeding 654 MPa (95 ksi) and excellent CO ₂ corrosion resistance. High-strength 13Cr martensitic stainless steel pipes for oil wells that have excellent properties and high toughness have been strongly desired.

  In response to such a demand, for example, Patent Document 1, Patent Document 2, Patent Document 3, Patent Document 4, and Patent Document 5 include an improvement in which the corrosion resistance of 13% Cr martensitic stainless steel (steel pipe) is improved. Type martensitic stainless steel (steel pipe) has been proposed.

The technology described in Patent Document 1 is a composition of 13% Cr martensitic stainless steel pipe, C is limited to 0.005 to 0.05%, and Ni: 2.4 to 6% and Cu: 0.2 to 4% are added in combination. Further, Mo is added in an amount of 0.5 to 3%, and Nieq is adjusted to 10.5 or more. After hot working, the composition is cooled at a speed higher than that of air cooling, or further (Ac ₃ transformation point + 10 ° C.) to (Ac ₃ transformation). Martensite system with excellent corrosion resistance that is heated to a temperature of the point + 200 ° C. or further heated to a temperature of Ac ₁ transformation point to Ac ₃ transformation point, then cooled to room temperature at a cooling rate higher than air cooling, and tempered. It is a manufacturing method of a stainless steel seamless steel pipe. According to the technique described in Patent Document 1, a martensitic stainless steel seamless pipe having high strength of API-C95 grade or higher, corrosion resistance in an environment containing CO _{2 of} 180 ° C. or higher, and SCC resistance, and It is going to be.

The technology described in Patent Document 2 includes C: 0.005 to 0.05%, N: 0.005 to 0.1%, Ni: 3.0 to 6.0%, Cu: 0.5 to 3%, and Mo: 0.5 to 3%. 13% Cr martensitic stainless steel was hot-worked and allowed to cool naturally to room temperature, then heated to (Ac ₁ point + 10 ° C) to (Ac ₁ point + 40 ° C) and held for 30 to 60 minutes. Of martensitic stainless steel with excellent resistance to sulfide stress corrosion cracking, which is cooled to a temperature and tempered at a temperature of Ac ₁ point or less, and the structure is a mixture of tempered martensite and γ phase of 20% by volume or more. It is a manufacturing method. According to the technique described in Patent Document 2, the resistance to sulfide stress corrosion cracking is remarkably improved by forming a tempered martensite structure containing 20% by volume or more of the γ phase.

  The technique described in Patent Document 3 is a composition of martensitic stainless steel containing 10 to 15% Cr, C is limited to 0.005 to 0.05%, Ni: 4.0% or more, Cu: 0.5 to 3% Combined addition, Mo added 1.0 to 3.0%, and Nieq adjusted to -10 or more, the structure consists of tempered martensite phase, martensite phase, residual austenite phase, tempered martensite phase, martensite It is a martensitic stainless steel excellent in corrosion resistance and sulfide stress corrosion cracking resistance, wherein the total fraction of phases is 60 to 90%. As a result, the corrosion resistance and sulfide stress corrosion cracking resistance in a wet carbon dioxide environment and a wet hydrogen sulfide environment are improved.

  The technique described in Patent Document 4 contains more than 15% and 19% or less of Cr, C: 0.05% or less, N: 0.1% or less, Ni: 3.5-8.0%, and Mo: 0.1-4.0% Is a martensitic stainless steel material for oil wells with excellent sulfide stress cracking properties that has 30Cr + 36Mo + 14Si-28Ni ≦ 455 (%) and 21Cr + 25Mo + 17Si + 35Ni ≦ 731 (%) at the same time. The steel material has excellent corrosion resistance even in a harsh oil well environment where ions, carbon dioxide gas, and a small amount of hydrogen sulfide gas exist.

The technique described in Patent Document 5 contains 10.0 to 17% Cr, includes C: 0.08% or less, N: 0.015% or less, Ni: 6.0 to 10.0%, Cu: 0.5 to 2.0%, and Mo. : A steel composition containing 0.5 to 3.0%, and by cold working and annealing of 35% or more, an average crystal grain size of 25 μm or less and a precipitate having a grain size of 5 × 10 ⁻² μm or more deposited on the matrix is 6 × 10 Precipitation hardening type martensitic stainless steel with a structure suppressed to ⁶ pieces / mm ² or less and excellent in strength and toughness. By making the structure with fine crystal grains and few precipitates, high strength and toughness It is said that precipitation hardening martensitic stainless steel that does not cause a decrease can be provided.
Japanese Patent Laid-Open No. 8-120345 JP-A-9-268349 Japanese Patent Laid-Open No. 10-1755 Japanese Patent No. 2814528 Japanese Patent No. 3251648

However, the improved 13% Cr martensitic stainless steel pipe manufactured by the techniques described in Patent Documents 1 to 5 contains CO ₂ , Cl ^−, etc., under a severe corrosive environment at a high temperature exceeding 170 ° C. There was a problem that the desired corrosion resistance was not stably exhibited. In addition, improved 13% Cr martensitic stainless steel pipes often contain a large amount of Mo in consideration of resistance to sulfide stress corrosion cracking. It was.

The present invention advantageously solves such problems of the prior art, does not contain hydrogen sulfide, contains carbon dioxide (CO ₂ ), chlorine ions (Cl ⁻ ), etc., and is severe at high temperatures exceeding 170 ° C. An object of the present invention is to provide an inexpensive high-strength stainless steel pipe for oil wells that exhibits excellent CO ₂ corrosion resistance even in a corrosive environment and has excellent corrosion resistance.

In order to achieve the above-mentioned problems, the present inventors based on a 13% Cr martensitic stainless steel pipe, maintained a high strength of yield strength: 654 MPa (95 ksi) or higher, at a high temperature up to 200 ° C. Moreover, the influence of the steel composition on the corrosion resistance in a corrosive environment containing CO ₂ up to 150 atm, Cl ^−, etc. was studied earnestly. As a result, C is remarkably reduced as compared with the prior art, and the Ni content, and further the Si, Mn, V, Al, N, and O contents are adjusted to an appropriate range, or an appropriate amount of Mo, Cu, or Nb, Ti. It was found that by adding the above, it was possible to ensure high strength and good hot workability and corrosion resistance, and the present invention was completed.

That is, the gist of the present invention is as follows.
(1) In mass%, C: 0.05% or less, Si: 0.5% or less, Mn: 0.2-1.8%, P: 0.03% or less, S: 0.005% or less, Cr: 15.5-18.5%, Ni: 1.5-5 %, Al: 0.05% or less, V: 0.2% or less, N: 0.15% or less, O: 0.006% or less, the following formulas (1) and (2)
Cr + 0.65Ni + 0.6Mo + 0.55Cu-20C ≧ 18.0 (1)
Cr + Mo + 0.3Si-43.5C-0.4Mn-Ni-0.3Cu-9N ≧ 11.5 (2)
(Here, Cr, Ni, Mo, Cu, C, Si, Mn, N: content of each element (mass%))
A high-strength stainless steel pipe for oil wells having excellent carbon dioxide corrosion resistance, characterized in that it has a composition composed of Fe and inevitable impurities.
(2) In (1), in addition to the composition described above, the composition further includes, in mass%, one or two selected from Mo: less than 1.0% and Cu: 3.5% or less. High strength stainless steel pipe for oil wells.
(3) In (1) or (2), in addition to the above-mentioned composition, it is mass%, Nb: 0.2% or less, Ti: 0.3% or less, Zr: 0.2% or less, B: 0.01% or less, W: 3.0 % High-strength stainless steel pipe for oil wells, characterized in that the composition contains one or more selected from the following.
(4) In any one of (1) to (3), in addition to the above-described composition, the composition further includes mass: Ca: 0.0005 to 0.01% or less. .
(5) In any one of (1) to (4), a high-strength stainless steel pipe for oil wells having a structure containing a martensite phase as a base phase and further containing a ferrite phase in a volume ratio of 5 to 70%. .
(6) A high-strength stainless steel pipe for oil wells according to (5), further containing an austenite phase of 30% or less by volume in addition to the structure.

According to the present invention, but does not contain hydrogen sulfide, CO _2, Cl ^- has excellent resistance to CO ₂ corrosion resistance even in a severe high-temperature corrosive environments comprising a sufficient corrosion resistance, it has a high toughness, high strength The oil well stainless steel pipe can be manufactured inexpensively and stably, and has a remarkable industrial effect.

  First, the reasons for limiting the components of the steel pipe of the present invention will be described. Hereinafter, mass% in the composition is simply referred to as%.

C: 0.05% or less C is an important element related to an increase in the strength of martensitic stainless steel. In the present invention, it is necessary to contain 0.01% or more. Sensitization during tempering increases. In order to prevent sensitization during tempering, C is limited to 0.05% or less in the present invention. Further, from the viewpoint of corrosion resistance, it is preferable that C is as small as possible. In addition, Preferably it is 0.01 to 0.03%.

Si: 0.5% or less
Si is an element that acts as a deoxidizing agent. In the present invention, Si is preferably contained in an amount of 0.1% or more. However, if it exceeds 0.5%, the CO ₂ corrosion resistance is reduced, and hot workability is further improved. Reduce. For this reason, Si was limited to 0.5% or less.

Mn: 0.2-1.8%
Mn is an element that increases the strength and needs to be contained in an amount of 0.2% or more in order to ensure the desired strength in the present invention, but if it exceeds 1.8%, the toughness is adversely affected. For this reason, Mn was limited to 0.2 to 1.8%. In addition, Preferably it is 0.3 to 1.0%.

P: 0.03% or less P is an element that deteriorates both CO ₂ corrosion resistance, CO ₂ stress corrosion cracking resistance and pitting corrosion resistance. In the present invention, it is desirable to reduce as much as possible, but extreme reduction Increases the manufacturing cost. P was limited to 0.03% or less as a range that can be industrially implemented at a relatively low cost and does not deteriorate the CO ₂ corrosion resistance, the CO ₂ stress corrosion cracking resistance, and the pitting corrosion resistance. In addition, Preferably it is 0.02% or less.

S: 0.005% or less S is an element that significantly deteriorates hot workability in the pipe manufacturing process, and is preferably as small as possible. However, if it is reduced to 0.005% or less, pipe manufacturing by a normal process becomes possible. Therefore, S is limited to 0.005% or less. In addition, Preferably it is 0.003% or less.

Cr: 15.5-18.5%
Cr is to form a protective coating is an element for improving corrosion resistance, particularly resistance CO ₂ corrosion resistance, a major element contributing to the improvement of the resistance to CO ₂ stress corrosion cracking resistance. In the present invention, the content of 15.5% or more is required particularly from the viewpoint of improving corrosion resistance at high temperatures. On the other hand, the content exceeding 18.5% decreases the strength. For this reason, Cr was limited to the range of 15.5-18.5%. In addition, Preferably it is 16.5-18.0%.

Ni: 1.5-5%
Ni is an element having an effect of strengthening the protective film and enhancing the resistance to CO ₂ corrosion resistance, the resistance to CO ₂ stress corrosion cracking and the pitting corrosion resistance. The effect is not recognized under the environment. On the other hand, when the content exceeds 5%, the stability of the martensite structure is lowered and the strength is lowered. For this reason, Ni was limited to the range of 1.5 to 5%. In addition, Preferably it is 2.0 to 4.5%.

Al: 0.05% or less
Al acts as a strong deoxidizer, but in order to obtain such an effect, it is desirable to contain 0.001% or more. On the other hand, a content exceeding 0.05% adversely affects toughness. For this reason, Al was limited to 0.05% or less. In addition, Preferably it is 0.03% or less.

V: 0.2% or less V has the effect of increasing strength and improving stress corrosion cracking resistance. Such an effect becomes remarkable when the content is 0.01% or more. On the other hand, if the content exceeds 0.2%, the toughness deteriorates. For this reason, V was limited to 0.2% or less. In addition, Preferably it is 0.02 to 0.08%.

N: 0.15% or less N is an element that remarkably improves pitting corrosion resistance. In the present invention, N is preferably contained in an amount of 0.01% or more. However, if it exceeds 0.15%, various nitrides are formed to deteriorate toughness. Let For this reason, N was limited to 0.15% or less. In addition, Preferably it is 0.02 to 0.08%.

O: 0.006% or less O is an element that exists as an oxide in steel and adversely affects various properties, and it is extremely important to reduce it as much as possible in order to sufficiently exhibit the performance of the steel pipe of the present invention. That is, when the O content is large, various oxides are formed, and hot workability, resistance to CO ₂ stress corrosion cracking, pitting corrosion resistance and toughness are significantly reduced. Therefore, in the present invention, O is limited to 0.006% or less.

  In addition to the basic composition described above, if necessary, one or two selected from Mo and Cu, and / or one selected from Nb, Ti, Zr, B and W or Two or more kinds and / or Ca can be contained.

    Mo and Cu are elements that improve corrosion resistance, and can be selected and contained as necessary. Mo is an element that improves corrosion resistance, particularly pitting corrosion resistance. If included, it is desirable to contain 0.3% or more, but even if 1.0% or more is contained, the effect is saturated in the corrosive environment targeted by the present invention. However, an effect commensurate with the content cannot be expected, which is economically disadvantageous. For this reason, it is preferable to limit Mo to less than 1.0%. In addition, More preferably, it is 0.3 to 0.7%.

  Cu is an element that strengthens the protective coating and improves the corrosion resistance. If it is contained, it is desirable to contain 0.2% or more, but if it exceeds 3.5%, CuS precipitates at the grain boundaries at high temperatures and heat Interworkability is reduced. For this reason, it is preferable to limit Cu to 3.5% or less. In addition, More preferably, it is 0.5 to 2.0%.

  Nb, Ti, Zr, B, and W are all elements that increase the strength, and can be selected and contained as necessary. Ti, Zr, B, and W are elements that improve the stress corrosion cracking resistance in addition to increasing the strength. Nb improves toughness in addition to increasing strength. Such an effect becomes remarkable when Nb: 0.03% or more, Ti: 0.03% or more, Zr: 0.03% or more, B: 0.0005% or more, and W: 0.2% or more. On the other hand, if Nb: 0.2%, Ti: 0.3%, Zr: 0.2%, B: 0.01%, W: more than 3.0%, the toughness decreases. For this reason, it is preferable to limit to Nb: 0.2% or less, Ti: 0.3% or less, Zr: 0.2% or less, B: 0.01% or less, and W: 3.0% or less, respectively.

Ca has the effect of fixing S as CaS and spheroidizing sulfide inclusions, thereby reducing the lattice strain of the matrix around the inclusions and reducing the hydrogen trapping ability of the inclusions. Have. Such an effect becomes remarkable when the content is 0.0005% or more. However, when the content exceeds 0.01%, CaO is increased, and the CO ₂ corrosion resistance and pitting corrosion resistance are lowered. For this reason, it is preferable to limit Ca to 0.0005 to 0.01% of range.

In the present invention, each of the above-described components is within the above-described range, and the following formulas (1) and (2)
Cr + 0.65Ni + 0.6Mo + 0.55Cu-20C ≧ 18.0 (1)
Cr + Mo + 0.3Si-43.5C-0.4Mn-Ni-0.3Cu-9N ≧ 11.5 (2)
The composition is adjusted so as to satisfy the above, and the balance is composed of Fe and inevitable impurities. Here, Cr, Ni, Mo, Cu, C, Si, Mn, and N are the contents (mass%) of each element. In addition, when calculating the left side of the formulas (1) and (2), the elements not included are calculated as 0%.

Corrosion resistance of the containing, in a high-temperature corrosive environment ^{- Cr, Ni, Mo, Cu} , and C content, (1) by adjusting so as to satisfy the formula, at elevated temperatures up to 200 ° C., and CO _2, Cl Is significantly improved.

  Moreover, by adjusting the Cr, Mo, Si, C, Mn, Ni, Cu, and N contents so as to satisfy the formula (2), the hot workability is improved, and for the production of seamless steel pipes. Sufficient hot workability can be secured and the strength is increased. In the present invention, P, S, and O are remarkably reduced in order to improve hot workability. However, by simply reducing P, S, and O, respectively, a martensitic stainless steel seamless steel pipe is formed. The hot workability necessary and sufficient for pipe making cannot be ensured. In order to secure the hot workability necessary and sufficient for making seamless steel pipes, Cr, Mo, Si, It is important to adjust the C, Mn, Ni, Cu, and N contents.

  The balance other than the above components is Fe and inevitable impurities.

  In addition to the above-described composition, the high-strength stainless steel pipe for oil wells of the present invention preferably has a structure containing a martensite phase as a base phase and further containing a ferrite phase in a volume ratio of 5 to 70%. The steel pipe of the present invention is based on a martensite structure in order to ensure high strength. In order to improve toughness and corrosion resistance without reducing the strength, it is preferable to have a structure containing a martensite phase as a base phase and a ferrite phase as a second phase in a volume ratio of 5 to 70%. If the ferrite phase is less than 5% by volume, the intended purpose cannot be achieved. On the other hand, when the ferrite phase is contained exceeding 70% by volume, the strength is lowered. For this reason, it is preferable to limit a ferrite phase to the range of 5-70% by a volume ratio. In addition, Preferably it is 10-50 volume%. Note that there is no problem even if the second phase other than the ferrite phase contains an austenite phase of 30% by volume or less.

    Next, a preferred method for producing the steel pipe of the present invention will be described using a seamless steel pipe as an example.

  The steel pipe of the present invention can be manufactured without any modification to the normal manufacturing process by using the steel material having the above composition.

  First, the molten steel having the above composition is melted by a generally known melting method such as a converter, electric furnace, vacuum melting furnace, etc., and billet is obtained by a generally known method such as a continuous casting method or an ingot-bundling rolling method. It is preferable to use a steel pipe material such as. Subsequently, these steel pipe materials are heated and hot-worked and formed using a normal Mannesmann-plug mill system or Mannesmann-Mandrel mill process to obtain seamless steel pipes of desired dimensions. The seamless steel pipe after pipe making is preferably cooled to room temperature at a cooling rate of at least 0.5 ° C./s on average, preferably on average from 800 to 500 ° C. In addition, you may manufacture a seamless steel pipe by the hot extrusion by a press system.

  If it is a seamless steel pipe having a composition within the scope of the present invention as described above, after hot working, it is cooled to room temperature at a cooling rate of air cooling or more, preferably 0.5 ° C./s or more on average from 800 to 500 ° C. According to the above, it is possible to obtain a structure having a martensite phase as a base phase, but after tube forming, cooling at an air cooling rate or more, preferably at an average of 0.5 ° C./s to 800 ° C. to 500 ° C., followed by cooling at a cooling rate Further, after reheating to a temperature of 800 ° C. or higher, it is preferably air-cooled or higher, preferably a quenching treatment of cooling to 800 ° C. to 500 ° C. at an average of 0.5 ° C./s or higher, cooling to 100 ° C. or lower, preferably room temperature . Thereby, it can be set as the fine and high toughness martensitic structure containing a suitable quantity of a ferrite phase.

  When the heating temperature in the quenching treatment is less than 800 ° C., the structure cannot be made into a sufficient martensite structure, and the strength tends to decrease. For this reason, it is preferable that the heating temperature of the quenching treatment is 800 ° C. or higher.

  The seamless steel pipe that has been subjected to the quenching treatment is then preferably tempered by being heated to a temperature of 650 ° C. or lower and cooled at a cooling rate of air cooling or higher. By heating to 650 ° C or less, preferably 400 ° C or more, and tempering, the structure becomes a structure composed of a tempered martensite phase or a smaller amount of ferrite phase, and the desired high strength and further desired high toughness. It becomes a seamless steel pipe having desired excellent corrosion resistance. In addition, it is preferable that the cooling after the heating in a tempering process shall be more than air cooling from a viewpoint of ensuring intensity | strength.

In addition, you may give only the above-mentioned tempering process without quenching process.
So far, the seamless steel pipe has been described as an example, but the steel pipe of the present invention is not limited to this. Using a steel pipe material having a composition within the scope of the present invention as described above, it is possible to produce an electric-welded steel pipe and a UOE steel pipe in accordance with a normal process to obtain a steel pipe for an oil well.

  Using the steel pipe material having the composition within the scope of the present invention as described above, the steel pipe other than the seamless steel pipe obtained according to the normal manufacturing process, for example, the ERW steel pipe, the UOE steel pipe, the steel pipe after the pipe making is described above. In the quenching-tempering process, after reheating to a temperature of 800 ° C. or higher, air cooling or higher, preferably 800 ° C. to 500 ° C. on average, 0.5 ° C./s or higher, and cooling rate of 100 ° C. or lower, preferably room temperature It is preferable to perform a quenching treatment and then a tempering treatment in which the temperature is 650 ° C. or lower, preferably 400 ° C. or higher and cooled at a cooling rate of air cooling or higher.

  Below, based on an Example, this invention is demonstrated further in detail.

  After degassing the molten steel having the composition shown in Table 1, it is cast into a 100kg steel ingot (steel pipe material), piped by hot working with a model seamless rolling mill, air cooled after pipe making, outer diameter 83.8mm x wall thickness 12.7 It was a seamless steel pipe of mm (3.3 in x 0.5 in thickness).

  About the obtained seamless steel pipe, the presence or absence of the crack generation | occurrence | production of the inner and outer surface was visually examined with air cooling after pipe forming, and hot workability was evaluated. No cracking was indicated by ○, and cracking was indicated by ×.

  Further, a test piece material was cut out from the obtained seamless steel pipe, heated under the conditions shown in Table 2, cooled with water, and further tempered under the conditions shown in Table 2.

  The specimen for tissue observation is collected from the specimen material that has been quenched and tempered in this way, and the specimen is corroded with aqua regia and imaged with a scanning electron microscope (1000x). Then, using the image analysis apparatus, the structure fraction (volume%) of the ferrite phase was calculated.

Moreover, the structural fraction of the retained austenite (γ) phase was measured using an X-ray diffraction method. Test specimens are taken from the quenched and tempered test specimen material, and the X-ray diffraction intensity of γ (220) plane and α (211) plane is measured by X-ray diffraction. Formula γ (volume ratio) = 100 / {1+ (IαRγ / IγRα)}
Where Iα: Integral intensity of α
Iγ: Integral intensity of γ
Rα: Calculated crystallographic theory of α
Rγ: Conversion was performed using a crystallographic theoretical calculation value of γ. The fraction of the martensite phase was calculated as the remainder other than these phases.

  In addition, API arc-shaped tensile test specimens were collected from the specimen material subjected to quenching and tempering treatment, and tensile tests were performed to determine tensile properties (yield strength YS, tensile strength TS).

  Furthermore, a corrosion test piece having a thickness of 3 mm, a width of 30 mm, and a length of 40 mm was produced from the specimen material subjected to quenching and tempering treatment by machining, and a corrosion test was performed.

The corrosion test was conducted by immersing the corrosion test piece in a test solution kept in an autoclave: 20% NaCl aqueous solution (liquid temperature: 200 ° C, 150 atmospheres CO ₂ gas atmosphere), and the immersion period was 2 weeks. . The test piece after the corrosion test was weighed, and the corrosion rate calculated from the weight loss before and after the corrosion test was obtained. Moreover, about the corrosion test piece after a test, the presence or absence of pitting corrosion on the test piece surface was observed using a magnifying glass with a magnification of 10 times.

In addition, a V-notch test piece (thickness: 5 mm) is taken from the specimen material that has been quenched and tempered in accordance with JIS Z 2202, and Charpy impact is applied in accordance with JIS Z 2242. The test was carried out to determine the absorbed energy vE- ₄₀ (J) at -40 ° C.

  The obtained results are shown in Table 2.

In all of the examples of the present invention, the occurrence of cracks on the surface of the steel pipe was not observed, the yield strength was YS: 654 MPa or more, the corrosion rate was low, no pitting corrosion occurred, hot workability and CO _2, Cl ^- comprises, even in harsh corrosive environment at a high temperature of 200 ° C., showed excellent耐炭acid gas corrosion resistance, and has a steel pipe having excellent corrosion resistance. Furthermore, by including a ferrite phase of 5% or more, it has a high strength of YS: 654 MPa or more, which has excellent corrosion resistance in a severe corrosive environment at a high temperature of 200 ° C. containing CO ₂ and Cl ^−. ing.

  On the other hand, in the comparative example that is out of the scope of the present invention, cracks are generated on the surface and the hot workability is lowered, or the corrosion rate is high, pitting corrosion occurs and the corrosion resistance is lowered. In particular, in the comparative example not satisfying the formula (2), the hot workability was lowered and the surface of the steel pipe was wrinkled. In addition, when the ferrite content is outside the preferred range of the present invention, the strength is lowered, and the high strength of yield strength YS: 654 MPa or more cannot be satisfied.

Claims (6)

mass%
C: 0.05% or less, Si: 0.5% or less,
Mn: 0.2 to 1.8%, P: 0.03% or less,
S: 0.005% or less, Cr: 15.5-18.5%,
Ni: 1.5-5%, Al: 0.05% or less,
V: 0.2% or less, N: 0.15% or less,
O: 0.006% or less is contained so as to satisfy the following formulas (1) and (2), and the balance is excellent carbon dioxide corrosion resistance characterized by having a composition comprising Fe and inevitable impurities Has high strength stainless steel pipe for oil well.
Record
Cr + 0.65Ni + 0.6Mo + 0.55Cu-20C ≧ 18.0 (1)
Cr + Mo + 0.3Si-43.5C-0.4Mn-Ni-0.3Cu-9N ≧ 11.5 (2)
Here, Cr, Ni, Mo, Cu, C, Si, Mn, N: Content of each element (mass%)   2. The composition according to claim 1, wherein in addition to the composition, the composition further includes at least one selected from mass: Mo: less than 1.0% and Cu: 3.5% or less. High strength stainless steel pipe for oil wells.   In addition to the above-mentioned composition, mass type, Nb: 0.2% or less, Ti: 0.3% or less, Zr: 0.2% or less, B: 0.01% or less, W: 3.0% or less The high-strength stainless steel pipe for oil wells according to claim 1 or 2, wherein the composition contains a seed or more.   The high-strength stainless steel pipe for oil wells according to any one of claims 1 to 3, wherein, in addition to the composition, the composition further includes Ca: 0.0005 to 0.01% or less in mass%.   5. The high-strength stainless steel pipe for oil wells according to claim 1, wherein the high-strength stainless steel pipe for oil wells has a structure containing a martensite phase as a base phase and further containing a ferrite phase in a volume ratio of 5 to 70%.   The high-strength stainless steel pipe for oil wells according to claim 5, further comprising an austenite phase having a volume ratio of 30% or less in addition to the structure.