JP5640762B2 - High strength martensitic stainless steel seamless pipe for oil wells - Google Patents

Description

  The present invention relates to a martensitic stainless steel seamless pipe suitable for oil wells used in crude oil or natural gas oil wells, in particular, YS has a high strength of 95 ksi (655 MPa) or more, and The present invention relates to a seamless steel pipe for oil wells having both carbon dioxide corrosion resistance and sulfide stress corrosion cracking resistance.

  In recent years, from the viewpoint of soaring crude oil prices and the depletion of petroleum resources expected in the near future, oil fields with deeper depths that were not previously excluded, and oil fields with severe corrosive environments including carbon dioxide, chloride ions, etc. Development of gas fields and so on is thriving. Such oil fields and gas fields are generally extremely deep, and the atmosphere is high temperature and is a harsh environment containing carbon dioxide, chlorine ions, hydrogen sulfide, etc. For oil wells used in such an environment Steel pipes and the like are required to have a material having high strength and excellent corrosion resistance.

Conventionally, 13% Cr martensitic stainless steel pipes are often used as oil well pipes used for mining in environmental oil fields and gas fields containing carbon dioxide CO ₂ , chlorine ions Cl ^{− and the} like. Furthermore, recently, the use of improved 13Cr martensitic stainless steels with a reduced content of 13Cr martensitic stainless steel and increased Ni, Mo, etc. has been expanded.
For example, Patent Documents 1 to 5 propose improved martensitic stainless steel (steel pipe) in which the corrosion resistance of 13% Cr martensitic stainless steel (steel pipe) is improved.

For example, 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 combined. Then, Mo is added at 0.5 to 3%, and Nieq is adjusted to 10.5 or more. After hot working, the composition is cooled at a rate higher than air cooling, or (Ac ₃ transformation point + 10 ° C.) to (Ac ₃ transformation point + 200 ° C), or further heating to a temperature of Ac ₁ transformation point to Ac ₃ transformation point, followed by cooling to room temperature at a cooling rate higher than air cooling and tempering, martens having excellent corrosion resistance This is a method for producing a site-based stainless steel seamless steel pipe. According to the technique described in Patent Document 1, a martensitic stainless steel seamless steel 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; It is going to be.

The technique 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 with the above composition is 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. Martensitic stainless steel with excellent resistance to sulfide stress corrosion cracking by cooling to the following temperature and tempering at a temperature of Ac ₁ point or less to make the structure a mixture of tempered martensite and 20% by volume or more of γ phase. It is a manufacturing method of steel. 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 martensitic stainless steel composition containing 10 to 15% Cr, C is limited to 0.005 to 0.05%, Ni: 4.0% or more, Cu: 0.5 to 3 The composition is composed of a tempered martensite phase, a martensite phase, and a retained austenite phase. This is a martensitic stainless steel having a total fraction of the martensite phase of 60 to 90% and excellent in corrosion resistance and sulfide stress corrosion cracking resistance. 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.

  Moreover, the technique described in Patent Document 4 contains Cr of more than 15% and 19% or less, including C: 0.05% or less, N: 0.1% or less, Ni: 3.5 to 8.0%, and Mo: 0.1 to It is a martensitic stainless steel material for oil wells with excellent sulfide stress cracking properties that contains 4.0% and has a steel composition that simultaneously satisfies 30Cr + 36Mo + 14Si−28Ni ≦ 455 (%) and 21Cr + 25Mo + 17Si + 35Ni ≦ 731 (%). It is said that the steel material has excellent corrosion resistance even in a harsh oil well environment in which chloride ions, carbon dioxide gas and a small amount of hydrogen sulfide gas exist.

Moreover, the technique described in patent document 5 contains 10.0 to 17% of Cr, C: 0.08% or less, N: 0.015% or less, Ni: 6.0 to 10.0%, Cu: 0.5 to 2.0%, Furthermore, a steel composition containing Mo: 0.5 to 3.0% is formed, and a precipitate having an average crystal grain size of 25 μm or less and a grain size of 5 × 10 ⁻² μm or more precipitated in the matrix by cold working and annealing of 35% or more. Precipitation hardening type martensitic stainless steel with excellent strength and toughness with a structure suppressed to 6 × 10 ⁶ pieces / mm ² or less. By making the structure with fine crystal grains and few precipitates, high strength It is said that precipitation hardening martensitic stainless steel that does not cause toughness reduction 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 each technique described in Patent Documents 1 to 5 contains CO ₂ , Cl ^{− and the} like, and is a severe corrosive environment at a high temperature exceeding 180 ° C. Below, there was a problem that the desired corrosion resistance was not stably exhibited. The usable temperature of the improved 13% Cr martensitic stainless steel is up to about 165 ° C in an environment containing high-pressure carbon dioxide gas, and does not exhibit sufficient corrosion resistance at temperatures higher than that. Furthermore, in an environment containing hydrogen sulfide H ₂ S, there is a problem that sulfide stress corrosion cracking is likely to occur.

The present invention advantageously solves such problems of the prior art, has a high yield strength of 655 MPa or more, contains carbon dioxide CO ₂ , chlorine ions Cl ^{− and the} like, and further contains hydrogen sulfide H ₂ S. The purpose is to provide a high strength martensitic stainless steel seamless steel pipe for oil wells that has excellent corrosion resistance (carbon dioxide corrosion resistance and sulfide stress corrosion cracking resistance) even in harsh environments of 170 ° C or higher. And

In order to achieve the above-mentioned object, the inventors of the present invention based on 13Cr steel, which is a typical martensitic stainless steel, contains CO ₂ , Cl ^−, etc., and further contains H ₂ S, 170 ° C. or higher. The effects of various alloying elements on carbon dioxide corrosion resistance and sulfide stress corrosion cracking resistance in harsh environments were investigated. As a result, C is reduced to 0.01% by mass or less, Cr is set to more than 15.5% by mass and 17% by mass or less, Ni and Mo are adjusted to an appropriate range, and an appropriate amount of Cu, V, or further It has been found that by containing an appropriate amount of W, the carbon dioxide corrosion resistance and the sulfide stress corrosion cracking resistance are remarkably improved while having a desired yield strength YS: 655 MPa or more. In particular, by reducing C to 0.01% or less, yield strength YS: While ensuring the desired high strength of 655 MPa or more, the yield ratio is 90% or more and the increase in tensile strength (increased hardness) is small. It was found that the desired corrosion resistance can be secured. Further, it has been found that the inclusion of W can stably improve the corrosion resistance even in a severe environment with a low pH. It was also found that W has a large effect of improving corrosion resistance compared to Mo and the like.

The present invention has been completed based on the above findings and further studies. That is, the gist of the present invention is as follows.
(1) 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%, 17.5% or less, Ni: 2.5 -5.5%, Mo: 1.8-3.5%, Cu: 0.3-3.5%, V: 0.20% or less, Al: 0.05% or less, N: 0.06% or less, and having a composition comprising the balance Fe and inevitable impurities , Having a structure comprising a ferrite phase of 15% or more by volume and a residual austenite phase of 25% or less and a tempered martensite phase of 50% or more, yield strength: strength of 655 to 862 MPa and yield ratio: A high-strength martensitic stainless steel seamless pipe for oil wells that has 0.90 or more and is excellent in carbon dioxide corrosion resistance and sulfide stress corrosion cracking resistance.
(2) A high-strength martensitic stainless steel seamless steel pipe for oil wells characterized in that, in addition to the above-described composition, the composition further comprises W: 0.25 to 2.0% in mass%.
(3) A high-strength martensitic stainless steel seamless pipe for oil wells characterized in that, in addition to the above-described composition, Nb: 0.20% or less in addition to the above composition .
(4) In any one of (1) to (3), in addition to the above composition, it is further selected by mass% from Ti: 0.3% or less, Zr: 0.2% or less, and B: 0.0005 to 0.01%. A high-strength martensitic stainless steel seamless pipe for oil wells, characterized in that the composition contains one or more.
(5) In any one of (1) to (4), in addition to the above-mentioned composition, the composition further contains Ca: 0.0005 to 0.01% by mass%, and is a high-strength martensite system for oil wells Stainless steel seamless steel pipe .

According to the present invention, yield strength YS: have more high strength 655 MPa, and CO _2, Cl ^- include such further comprises H _{2 S,} even in harsh environments above 170 ° C., excellent A high-strength martensitic stainless steel seamless pipe having carbon dioxide gas corrosion resistance and sulfide stress corrosion cracking resistance, suitable for oil well pipes, can be manufactured at low cost and stably, and has a remarkable industrial effect.

First, the reason for the composition limitation of the high-strength martensitic stainless steel seamless steel pipe for oil wells of the present invention will be described. Hereinafter, unless otherwise specified, mass% is simply referred to as%.
C: 0.01% or less C is an important element related to the strength of martensitic stainless steel, and in order to ensure the desired high strength, it is desirable to contain 0.003% or more, but more than 0.01% Makes it easy to form Cr carbide, and corrosion resistance tends to decrease. For this reason, in the present invention, C is limited to 0.01% or less. In addition, Preferably, it is 0.005% or more from a viewpoint of ensuring desired high intensity | strength.

Si: 0.5% or less
Si is an element that acts as a deoxidizing agent, and dissolves to increase the strength of the steel. In order to acquire such an effect, it is desirable to contain 0.05% or more, but inclusion exceeding 0.5% lowers hot workability. For this reason, Si was limited to 0.5% or less. In addition, Preferably it is 0.15-0.4%.

Mn: 0.1-2.0%
Mn is an element that increases the strength of the steel pipe through solid solution strengthening or hardenability improvement. It needs to be contained in an amount of 0.1% or more in order to ensure the desired steel pipe strength, but a large amount exceeding 2.0% Inclusion adversely affects toughness. For this reason, Mn was limited to the range of 0.1 to 2.0%. In addition, Preferably, it is 0.3 to 0.8%.

P: 0.03% or less P is an element that degrades both hot workability and resistance to sulfide stress corrosion cracking. In the present invention, it is desirable to reduce as much as possible, but extreme reduction increases the manufacturing cost. Invite. P is limited to 0.03% or less as a range that can be implemented relatively inexpensively industrially and does not deteriorate both hot workability and sulfide stress corrosion cracking resistance. In addition, Preferably it is 0.01% 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: Over 15.5% and below 17.5%
Cr is an element that improves the corrosion resistance by forming a protective film, and is an element that contributes effectively to maintaining the desired carbon dioxide gas corrosion resistance and sulfide stress corrosion cracking resistance. In particular, in order to maintain the desired excellent corrosion resistance even at high temperatures, the Cr content is determined to exceed 15.5%. On the other hand, the content exceeding 17.5% decreases the hot workability. For this reason, Cr was limited to the range of more than 15.5% and less than 17.5%. In addition, Preferably it is 16.0 to 17.0% from a viewpoint of structure | tissue stability.

Ni: 2.5-5.5%
Ni is an element that has a function of strengthening the protective coating and enhances corrosion resistance such as carbon dioxide corrosion resistance and sulfide stress corrosion cracking resistance. In order to obtain such an effect under the severe corrosive environment targeted by the present invention, a content of 2.5% or more is necessary. On the other hand, even if the content exceeds 5.5%, the effect of improving the corrosion resistance is saturated and the strength may be lowered, and an effect commensurate with the content cannot be expected, which is economically disadvantageous. For this reason, Ni was limited to the range of 2.5 to 5.5%. In addition, Preferably it is 3.0 to 5.0%.

Mo: 1.8-3.5%
Mo, along with stabilizing the passive film, Cl ^- by having the effect of increasing the resistance to pitting,耐炭acid gas corrosion resistance, in element contributing to the improvement of both the resistance to sulfide stress corrosion cracking is there. In order to obtain such an effect, the content of 1.8% or more is required, but the content exceeding 3.5% lowers the hot workability and raises the material cost. For this reason, Mo was limited to the range of 1.8 to 3.5%. In addition, Preferably it is 2.0 to 3.0%.

Cu: 0.3-3.5%
Cu is an element that strengthens the protective film and contributes to the improvement of carbon dioxide corrosion resistance and sulfide stress corrosion cracking resistance, and also has the effect of improving pitting corrosion resistance. Needs to contain 0.3% or more. On the other hand, if the content exceeds 3.5%, CuS precipitates at grain boundaries at high temperatures, and hot workability is reduced. For this reason, Cu was limited to the range of 0.3 to 3.5%. In addition, Preferably it is 0.7 to 2.5%.

V: 0.20% or less V is an element that forms a carbide (precipitate) and contributes to an increase in strength through precipitation strengthening and improves resistance to sulfide stress corrosion cracking. In order to acquire such an effect, 0.01% or more of content is required. On the other hand, the content exceeding 0.20% lowers toughness. For this reason, V was limited to 0.20% or less. In addition, Preferably it is 0.03-0.08%.

Al: 0.05% or less
Al is an element having a strong deoxidizing action, and in order to obtain such an effect, it is desirable to contain 0.001% or more, but inclusion 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.
N: 0.06% or less N is an element that remarkably improves the pitting corrosion resistance. Such an effect becomes remarkable when the content is 0.01% or more. On the other hand, the content exceeding 0.06% forms various nitrides and lowers the toughness. For this reason, N was limited to 0.06% or less. In addition, Preferably it is 0.01 to 0.03%.

  The above-described components are basic components. In the present invention, in addition to the basic composition, W: 0.25 to 2.0% and / or Nb: 0.20% or less and / or , Ti: 0.3% or less, Zr: 0.2% or less, B: One or more selected from 0.0005 to 0.01%, and / or Ca: 0.0005 to 0.01%, as necessary And may be contained.

W: 0.25-2.0%
W, like Mo, has the effect of stabilizing the passive film and increasing the resistance to pitting corrosion caused by Cl ⁻ , improving both carbon dioxide corrosion resistance and sulfide stress corrosion cracking resistance. Is an element that contributes to In particular, W dissolves in a corrosive environment to form W oxide, which stably delays the progress of corrosion. In order to acquire such an effect, it is necessary to contain 0.25% or more, but when it contains more than 2.0%, hot workability will fall. For this reason, when contained, W is preferably limited to a range of 0.25 to 2.0%. In addition, More preferably, it is 0.5 to 1.5%.

Nb: 0.20% or less
Nb is an element that precipitates as a precipitate and contributes to increase in strength, and also contributes to improvement in toughness through refinement of crystal grains by hot rolling by expanding the γ non-recrystallization temperature range. Depending on the content. In order to acquire such an effect, it is desirable to contain 0.01% or more, but inclusion exceeding 0.20% adversely affects toughness. For this reason, when it contains, it is preferable to limit Nb to 0.20% or less. More preferably, it is 0.15% or less.

One or more selected from Ti: 0.3% or less, Zr: 0.2% or less, B: 0.0005 to 0.01%
Ti, Zr, and B are all elements that have the effect of increasing the strength and improving the resistance to sulfide stress corrosion cracking, and can be selected as needed and contained in one or more. In order to obtain such effects, it is desirable to contain Ti: 0.01%, Zr: 0.01%, B: 0.0005% or more, but Ti: 0.3%, Zr: 0.2%, B: 0.01%, respectively. If it exceeds, the toughness decreases. For this reason, when it contains, it is preferable to limit to Ti: 0.3% or less, Zr: 0.2% or less, and B: 0.0005-0.01%, respectively. More preferably, Ti is 0.2% or less, Zr: 0.1% or less, and B: 0.005% or less.

Ca: 0.0005 to 0.01%
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, if the content exceeds 0.01%, CaO is increased, and the carbon dioxide 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.

The balance other than the components described above consists of Fe and inevitable impurities. As an inevitable impurity, O: 0.006% or less is acceptable.
Next, a preferable structure of the high-strength martensitic stainless steel seamless steel pipe for oil wells of the present invention will be described.
The high-strength martensitic stainless steel seamless pipe for oil wells of the present invention is mainly composed of a tempered martensite phase. The “main body” here means a case where the phase has a volume ratio of 50% or more. As the second phase other than the phase composed mainly, including the following residual austenite 25% (gamma) 15% or more of ferrite phase (alpha). When the ferrite phase (α) is precipitated in a small amount of less than 15%, the hot workability is lowered. This tendency is particularly strong with high Cr steel.
Further, the residual γ is 25% or less by volume, reduced as much as possible. The presence of the residual γ reduces the yield strength, making it difficult to ensure the desired strength.

Below, the preferable manufacturing method of the high intensity | strength martensitic stainless steel seamless pipe for oil wells of this invention is demonstrated. A stainless steel seamless steel pipe having the above composition is used as a starting material for quenching and tempering. Furthermore, you may perform a correction process in order to correct the defect of a steel pipe shape as needed.
In the present invention, the production method of the starting material having the above-described composition is not particularly limited. However, the molten steel having the above-described composition is usually converted into a known melting method such as a converter, an electric furnace, or a vacuum melting furnace. It is preferable to use a steel pipe material such as billet by a usual method such as continuous casting and ingot-splitting rolling. Next, these steel pipe materials are heated and hot-worked and piped using a normal Mannesmann-plug mill method or Mannesmann-Mandrel mill manufacturing process to produce seamless steel pipes of the desired dimensions, and used as starting materials. Is preferred. In addition, you may manufacture a seamless steel pipe by the hot extrusion by a press system. Moreover, it is preferable that a seamless steel pipe is cooled to room temperature at a cooling rate equal to or higher than air cooling after pipe making.

The starting material (seamless steel pipe) is first quenched.
The quenching treatment in the present invention is a treatment of reheating to a quenching temperature not lower than the Ac ₃ transformation point, preferably in the range of 800 to 1050 ° C., and then cooling from the quenching temperature to a temperature range of 100 ° C. or lower at a cooling rate higher than air cooling. And Thereby, it can be set as a fine martensitic structure. If the quenching heating temperature is less than the Ac ₃ transformation point, the austenite single-phase region cannot be heated, and a sufficient martensite structure cannot be obtained by subsequent cooling, so that a desired strength cannot be ensured. For this reason, the heating temperature of the quenching process is limited to the Ac ₃ transformation point or higher. In addition, Preferably it is 930 degreeC or more.

  Cooling from the quenching heating temperature is performed to a temperature range of 100 ° C. or lower at an air cooling or higher cooling rate. Since the starting material in the present invention has high hardenability, a sufficient quenched structure (martensitic structure) can be obtained by cooling to a temperature range of 100 ° C. or lower at a cooling rate of about air cooling. The holding time at the quenching temperature is preferably 10 min or more from the viewpoint of soaking.

In addition, since the starting material in this invention has high hardenability, if it cools to 100 degrees C or less with the cooling rate more than air cooling after pipe-forming by hot processing, it can be set as sufficient martensitic structure. For this reason, the quenching process which reheats and quenches can be omitted, and the tempering process can be performed.
The seamless steel pipe that has been subjected to the quenching process is subsequently subjected to a tempering process.

In the present invention, the tempering process is an important process for securing excellent low temperature toughness. The tempering treatment in the present invention is a treatment of heating to a tempering temperature of 530 ° C. or more, preferably the Ac ₁ transformation point or less, preferably holding for 5 min or more, and then preferably cooling to room temperature at a cooling rate of air cooling or more. And As a result, a seamless steel pipe having a high strength of YS: 655 MPa or more and 862 MPa or less and a tensile property of yield ratio: 0.90 or more is obtained.

If the tempering temperature is less than 530 ° C., the subsequent correction temperature must be lowered to the tempering temperature or less, and therefore, the yield strength YS tends to vary. On the other hand, when the tempering temperature exceeds the Ac ₁ transformation point, an austenite phase is generated and transformed into quenched martensite during cooling. Since the quenched martensite has many movable dislocations, the yield strength YS decreases when the quenched martensite is generated. The cooling from the tempering temperature is preferably air cooling or a cooling rate higher than that from the viewpoint of obtaining sufficient martensite.

  Moreover, in this invention, you may perform a correction process in order to correct the defect of a steel pipe shape following a tempering process as needed. The straightening treatment is preferably performed in a temperature range of 450 ° C. or higher. When the temperature of the straightening treatment is less than 450 ° C., processing strain is locally added to the steel pipe during the straightening treatment, and mechanical characteristics, in particular, yield strength YS, are likely to vary. For this reason, when performing a correction process, it is preferable to carry out in the temperature range of 450 degreeC or more.

Produced by the production method described above, seamless steel pipe has a structure that describes the upper and the above-described composition, yield strength YS: 655 MPa or more 862MPa or less and the yield ratio: high strength with 0.90 or more, more OCTG As a martensitic stainless steel seamless pipe that also has sufficient corrosion resistance. If the yield ratio is less than 0.90, the tensile strength is increased and the sulfide stress corrosion cracking resistance is reduced, so that the desired sulfide stress corrosion cracking resistance is ensured. For this purpose, it is important to ensure a desired range of tensile properties. If YS is less than 655 MPa, the desired strength for oil well pipes cannot be satisfied. On the other hand, if YS exceeds 862 MPa, the resistance to sulfide stress corrosion cracking decreases.

The molten steel having the composition shown in Table 1 was sufficiently degassed, and then made into a 100 kg steel ingot to obtain a steel pipe material. These steel pipe materials were heated, piped using a research model seamless rolling mill, and then air-cooled to obtain seamless steel pipes (outer diameter 3.3 ”(83.8 mm) φ × thickness 0.5” (12.7 mm)).
A test material (steel pipe) was collected from the obtained seamless steel pipe, and the test material (steel pipe) was quenched and tempered under the conditions shown in Table 2.

From the specimens (steel pipes) that had been quenched and tempered, API arc-shaped tensile specimens were collected and subjected to tensile tests to determine tensile properties (yield strength YS, tensile strength TS).
In addition, corrosion test specimens of thickness 3mm x width 30mm x length 40mm are made by machining from test materials that have been subjected to quenching and tempering treatments. -A round bar tensile test specimen was taken in accordance with the method A of TM0177 and a sulfide stress corrosion cracking test was conducted.

The carbon dioxide stress corrosion test is performed by immersing a corrosion test piece in a test solution retained in an autoclave: 20% NaCl aqueous solution (liquid temperature: 230 ° C., CO ₂ gas partial pressure: 3 MPa CO ₂ gas atmosphere) The immersion period was 2 weeks (336 hr). The test piece after the corrosion test was weighed, and the corrosion rate was calculated from the weight loss before and after the corrosion test. Moreover, about the test piece after a corrosion test, the presence or absence of pitting corrosion was observed with the magnifier 10 times, and it was set as (circle) and pitting corrosion with x. Corrosion rate: 0.1 mm / y or less and no pitting corrosion were evaluated as “good” as being excellent in carbon dioxide stress corrosion resistance. Other than that, it was evaluated as x because the carbon dioxide stress corrosion resistance was poor.

The sulfide stress corrosion cracking test is performed by immersing a round bar tensile test piece in a test solution held in an autoclave and applying a stress of 90% of YS in accordance with the method A of NACE-TM0177. A test was held for 720 hours. The case where it did not break after 720 hours passed was evaluated as “good” as being excellent in resistance to sulfide stress corrosion cracking. Otherwise, it was set as “x”. The test solution used was a 20% NaCl aqueous solution (solution temperature: 25 ° C), 0.3% acetic acid (CH ₃ COOH) and CH ₃ COONa were added, and the pH was adjusted to 3.5. Was carried out in an environment (pressure: 1 atm) containing 10% H ₂ S and flowing 90% CO ₂ gas.

  The obtained results are shown in Table 3.

  Each of the examples of the present invention is a seamless steel pipe having sufficient strength and excellent corrosion resistance as an oil well pipe, and it can be seen that it can be used sufficiently even in a high-temperature, high-pressure carbon dioxide atmosphere. On the other hand, comparative examples that are out of the scope of the present invention do not have sufficient strength or have not secured the desired excellent corrosion resistance.

Claims (5)

% 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%, 17.5% or less,
Ni: 2.5-5.5%, Mo: 1.8-3.5%,
Cu: 0.3 to 3.5%, V: 0.20% or less,
Al: 0.05% or less, N: 0.06% or less, with the balance consisting of Fe and inevitable impurities , including 15% or more ferrite phase by volume and 25% or less residual austenite phase, 50% or more It has a structure consisting of a tempered martensite phase, has a yield strength of 655 to 862 MPa and a yield ratio of 0.90 or more, and is excellent in carbon dioxide corrosion resistance and sulfide stress corrosion cracking resistance. High strength martensitic stainless steel seamless pipe for oil wells.   The high-strength martensitic stainless steel seamless steel pipe for oil wells according to claim 1, wherein the composition further contains W: 0.25 to 2.0% by mass% in addition to the composition.   The high-strength martensitic stainless steel seamless pipe for oil wells according to claim 1 or 2, further comprising Nb: 0.20% or less in mass% in addition to the composition.   In addition to the above composition, the composition further contains, by mass%, one or more selected from Ti: 0.3% or less, Zr: 0.2% or less, and B: 0.0005 to 0.01%. The high-strength martensitic stainless steel seamless steel pipe for oil wells according to any one of claims 1 to 3. The high-strength martensitic stainless steel seamless pipe for oil wells according to any one of claims 1 to 4, wherein the composition further contains Ca: 0.0005 to 0.01% by mass% in addition to the composition. .