JP2007332442A - High-toughness ultrahigh-strength stainless steel pipe for oil well having excellent corrosion resistance, and its production method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-toughness ultrahigh-strength stainless steel pipe for an oil well having excellent corrosion resistance, and to provide its production method. <P>SOLUTION: A steel stock having a composition comprising, by mass, ≤0.05% C, 0.20 to 1.80% Mn, 14.0 to 18.0% Cr, 5.0 to 8.0% Ni, 1.5 to 3.5% Mo, 0.5 to 3.5% Cu, 0.005 to 0.15% N and ≤0.006% O, also satisfying Cr+2Ni+1.1Mo+0.7Cu≤32.5, and further comprising one or two selected from ≤0.20% Nb and ≤0.20% V so as to satisfy the condition of Nb+V≥0.05% is made into a pipe. The pipe is cooled to ≤150°C, and is thereafter subjected to tempering treatment where heating temperature T is 500 to 580°C, and also, T(20+logt) satisfies 15,200 to 16,800. In this way, a steel pipe having a YS of ≥965 MPa and vE<SB>-40</SB>of ≥50 J, and having excellent corrosion resistance in a CO<SB>2</SB>-containing high-temperature corrosive environment heated at ≥200°C can be obtained. It is also possible that, before the tempering treatment, the pipe is subjected to quenching treatment so as to be heated to an Ac<SB>3</SB>transformation point or above, and, successively, to be cooled at a cooling rate of air cooling or above. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

The present invention relates to oil wells for crude oil or natural gas, and ultra-high strength stainless steel pipes for oil wells used in gas wells, especially in extremely severe corrosive environments containing carbon dioxide (CO ₂ ), chlorine ions (Cl ⁻ ), etc. Relates to the improvement of corrosion resistance and resistance to sulfide stress corrosion cracking. The term “ultra-high strength” used herein refers to a grade having a yield strength of 965 MPa (140 ksi) or higher.

In recent years, in order to cope with soaring crude oil prices and the depletion of petroleum resources expected in the near future, deep oil fields that have not been excluded in the past, or highly corrosive sour gas that had been abandoned once Developments for rice fields etc. are flourishing on a global scale. Such oil, gas fields are generally the depth is very deep, and its atmosphere and a high temperature, CO _2, Cl ^- has a severe corrosive environment and the like. Therefore, steel pipes for oil wells used for mining such oil fields and gas fields are required to have high strength and corrosion resistance. Recently, 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.

Conventionally, CO _2, Cl ^-, etc. oilfield environment including, in gas fields, as oil well steel pipes, common to 13% Cr martensitic stainless steel but having excellent CO ₂ corrosion resistance is used Met. However, ordinary martensitic stainless steel has a problem that if it has a high strength, the toughness decreases and it cannot be used. For this reason, cold-worked duplex stainless steel pipes have been used in oil wells that require high strength. However, the duplex stainless steel pipe has a problem that it has a large amount of alloying elements and is expensive, has poor hot workability and can be produced only by a special hot working method. In recent years, oil field development in cold regions has also become active, and it is often required to have excellent low temperature toughness in addition to high strength. For this reason, a high strength steel pipe for oil wells having excellent CO ₂ corrosion resistance and high toughness based on 13% Cr martensitic stainless steel, which is excellent in hot workability and inexpensive, has been strongly desired.

  In response to such demands, for example, Patent Document 1, Patent Document 2, and Patent Document 3 describe an improved 13% Cr martensitic stainless steel (steel pipe) in which the corrosion resistance of 13% Cr martensitic stainless steel is improved. Has been proposed. However, the improved 13% Cr martensitic stainless steel pipe does not have sufficient corrosion resistance in environments exceeding 170 ° C, and the strength obtained is up to the yield stress YS: 758 MPa grade. There was a problem that it was not possible to cope with the deep wells that started.

Further, Patent Document 4, CO _2, Cl ^- or the like for oil wells stainless steel tube having an excellent CO ₂ corrosion resistance even in a high-temperature corrosive atmosphere exceeding 180 ° C. containing proposed. The stainless steel pipe described in Patent Document 4 contains Cr: 14-18%, Ni: 5.0-8.0%, Mo: 1.5-3.5%, Cu: 0.5-3.5%, and Cr, Ni, Mo, Cu Steel pipe with a composition that satisfies the specific relationship consisting of C, and the specific relationship consisting of Cr, Mo, Si, C, Mn, Ni, Cu, and N, ensuring a high strength of yield strength of 654 MPa or more. It is said to be excellent in CO ₂ corrosion resistance.
JP-A-8-120345 JP-A-9-268349 Japanese Patent Laid-Open No. 10-1755 Republished patent WO2004 / 001082

However, although the steel pipe manufactured by the technique described in Patent Document 4 has excellent CO ₂ corrosion resistance, it can only ensure high strength up to a yield strength of 910 MPa. There was a problem that it could not cope with the deep oil well.
The present invention shows the above-mentioned resolved the problems of the prior art, and a basic composition of 13% Cr martensitic stainless steel, inexpensive, CO _2, Cl ^-, etc. excellent in severe corrosive environments than 170 ° C. containing were together shown resistance CO ₂ corrosion resistance, yield strength: high strength above 965MPa, Charpy absorbed energy vE _-40 to combine the more high toughness 50J at -40 ° C., excellent high toughness for oil well corrosion resistance An object is to provide an ultra-high strength stainless steel pipe and a method for producing the same.

In order to achieve the above-mentioned object, the inventors have conducted intensive research on the influence of various alloy elements on the yield strength and the corrosion resistance in an environment containing CO ₂ and Cl ⁻ with 13% Cr steel as a basic composition. . As a result, C is remarkably reduced as compared with the prior art, Ni, Mo, and Cu are further contained so that Cr, Ni, Mo, and Cu satisfy a specific relationship, and Nb and V satisfy a specific relationship. In addition, by combining each composition and applying an appropriate heat treatment, the yield strength is 965MPa or more, and the Charpy absorbed energy vE- _{40 at} -40 ° C is 50J or more. It has been found that a good hot workability and corrosion resistance can be secured.

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) In mass%, C: 0.05% or less, Si: 0.50% or less, Mn: 0.20 to 1.80%, P: 0.03% or less, S: 0.005% or less, Cr: 14.0 to 18.0%, Ni: 5.0 to 8.0 %, Mo: 1.5 to 3.5%, Cu: 0.5 to 3.5%, N: 0.005 to 0.15%, O: 0.006% or less, and Cr, Ni, Mo, Cu are represented by the following formula (1)
Cr + 2Ni + 1.1Mo + 0.7Cu ≤ 32.5 (1)
(Here, Cr, Ni, Mo, Cu: content of each element (mass%))
In addition, Nb: 0.20% or less, V: 0.20% or less, one or two selected from the following formula (2)
Nb + V ≧ 0.05% (2)
(Where Nb, V: content of each element (mass%))
And having a composition comprising the balance Fe and inevitable impurities, having a martensite phase as the main phase, and having a structure containing an austenite phase of 3 to 15% by volume as the second phase, A high toughness ultra-high strength stainless steel pipe for oil wells with excellent corrosion resistance, characterized by a yield strength YS of 965 MPa or more and a Charpy absorbed energy vE- _{40 at} −40 ° C. of 50 J or more.

(2) In (1), in addition to the above composition, it is further selected in mass% from Ti: 0.3% or less, Zr: 0.2% or less, B: 0.01% or less, W: 3.0% or less Alternatively, a high toughness ultra-high strength stainless steel pipe for oil wells containing two or more kinds.
(3) In (1) or (2), in addition to the above composition, it further contains Ca: 0.0005 to 0.01% in mass%, and a high toughness ultra-high strength stainless steel pipe for oil wells.

(4) In any one of (1) to (3), in addition to the above-mentioned composition, it further includes mass: Al: 0.05% or less, and a high toughness ultra-high strength stainless steel pipe for oil wells.
(5) In mass%, C: 0.05% or less, Si: 0.50% or less, Mn: 0.20 to 1.80%, P: 0.03% or less, S: 0.005% or less, Cr: 14.0 to 18.0%, Ni: 5.0 to 8.0 %, Mo: 1.5 to 3.5%, Cu: 0.5 to 3.5%, N: 0.005 to 0.15%, O: 0.006% or less, and Cr, Ni, Mo, Cu are represented by the following formula (1)
Cr + 2Ni + 1.1Mo + 0.7Cu ≤ 32.5 (1)
(Here, Cr, Ni, Mo, Cu: content of each element (mass%))
In addition to Nb: 0.20% or less, V: 0.20% or less selected from the following formula (2)
Nb + V ≧ 0.05% (2)
(Where Nb, V: content of each element (mass%))
After forming a steel material having a composition composed of the balance Fe and inevitable impurities and cooling it to a temperature of 150 ° C. or lower, the steel tube is heated to a heating temperature T of 500 to 580 ° C. The temperature is within the range, and the heating temperature T and heating holding time t
15200 ≦ T (20 + logt) ≦ 16800 ...... (3)
(Where T: heating temperature (K), t: heating holding time (h))
Tempering that satisfies the above requirements, and a stainless steel pipe with a yield strength YS of 965 MPa or more and a Charpy absorbed energy vE- _{40 at} -40 ° C of 50 J or more. Manufacturing method of high strength stainless steel pipe.

(6) In (5), after the pipe forming and before the tempering treatment, the steel pipe is heated to a temperature of 800 ° C. or higher and then cooled to a temperature of 150 ° C. or lower at a cooling rate of air cooling or higher. A method for producing a high toughness ultra-high strength stainless steel pipe for oil wells, characterized by performing treatment.
(7) In (5) or (6), in addition to the above composition, it is further selected in mass% from Ti: 0.3% or less, Zr: 0.2% or less, B: 0.01% or less, W: 3.0% or less A method for producing a high toughness ultra-high-strength stainless steel pipe for oil wells, characterized by containing one or more of them.

(8) In any one of (5) to (7), in addition to the above-described composition, the mass% further includes Ca: 0.0005 to 0.01%. Production method.
(9) In any one of (5) to (8), in addition to the above-described composition, it further includes mass: Al: 0.05% or less, and a high toughness ultra-high strength stainless steel pipe for oil wells.

According to the present invention, CO _2, Cl ^-, etc. also exhibit excellent resistance to CO ₂ corrosion under 170 ° C. or more severe corrosive environments containing, yield strength: and more high strength 965MPa, at -40 ℃ High-toughness ultra-high-strength stainless steel pipes for oil wells with excellent corrosion resistance, with Charpy absorbed energy vE- ₄₀ of 50J or higher, can be manufactured at low cost and have a remarkable industrial effect. Further, according to the present invention, there is an effect that an oil well pipe that can be used even in a severe environment can be supplied at low cost and stably.

First, the reason for the composition limitation of the high toughness ultra high strength stainless steel pipe for oil wells of this invention will be described. Hereinafter, unless otherwise specified, mass% is simply expressed as%.
C: 0.05% or less C is an important element related to the strength of martensitic stainless steel, but if it exceeds 0.05%, sensitization during Ni tempering tends to occur. From the viewpoint of preventing sensitization during tempering, C is limited to 0.05% or less in the present invention. Also, it is preferable that the amount is as small as possible in view of corrosion resistance. In addition, Preferably it is 0.01 to 0.03%.

Si: 0.50% or less
Si is an element that acts as a deoxidizer. In this invention, it is preferably contained in an amount of 0.05% or more. However, if it exceeds 0.50%, the CO ₂ corrosion resistance is reduced, and hot workability is also reduced. Let For this reason, Si was limited to 0.50% or less. In addition, Preferably it is 0.1 to 0.3%.
Mn: 0.20 to 1.80%
Mn is an element that increases the strength and needs to be contained in an amount of 0.20% or more in order to ensure the desired strength in the present invention. However, the content exceeding 1.80% adversely affects toughness. For this reason, Mn was limited to the range of 0.20 to 1.80%. In addition, Preferably it is 0.20 to 1.0%.

P: 0.03% or less P is an element that reduces both CO ₂ corrosion resistance, CO ₂ stress corrosion cracking resistance, pitting corrosion resistance and sulfide stress corrosion cracking resistance, and is reduced as much as possible in this invention. Although desirable, extreme reduction leads to increased manufacturing costs. P is limited to 0.03% or less as a range that can be implemented industrially at a relatively low cost and does not reduce CO ₂ corrosion resistance, CO ₂ stress corrosion cracking resistance, pitting corrosion resistance and sulfide stress corrosion cracking resistance. . In addition, Preferably it is 0.02% or less.

S: 0.005% or less S is an element that significantly deteriorates the hot workability of steel in the pipe manufacturing process. It is desirable to reduce it as much as possible, but if it is reduced to 0.005% or less, pipe manufacturing in the normal process Therefore, S is limited to 0.005% or less. In addition, Preferably it is 0.003% or less.
Cr: 14.0 to 18.0%
Cr is to form a protective coating is an element for improving corrosion resistance, particularly resistance CO ₂ corrosion is a major element contributing to the improvement of the resistance to CO ₂ stress corrosion cracking resistance, from the viewpoint of corrosion resistance, especially at elevated temperatures It needs to contain 14.0% or more. On the other hand, if the content exceeds 18.0%, the hot workability and strength are lowered. For this reason, Cr was limited to 14.0 to 18.0%.

Ni: 5.0-8.0%
Ni is to strengthen the protective film, resistance to CO ₂ corrosion resistance and CO ₂ stress corrosion cracking resistance, and has a function of improving pitting resistance and sulfide stress corrosion cracking resistance, further by solid solution strengthening of the steel strength Is an element that increases. Such an effect is obtained at a content of 5.0% or more under the usage environment targeted by the present invention, but a content exceeding 8.0% impairs the stability of the martensite structure and lowers the strength. For this reason, Ni was limited to the range of 5.0 to 8.0%.

Mo: 1.5-3.5%
Mo is, Cl ^- is an element that increases resistance to pitting, in the present invention, the content thereof needs to be 1.5% or more. If the Mo content is less than 1.5%, the corrosion resistance under the high-temperature and severe corrosive environment targeted by the present invention is not sufficient. On the other hand, if the content exceeds 3.5%, a large amount of ferrite is generated, CO ₂ corrosion resistance, CO ₂ stress corrosion cracking resistance and hot workability are deteriorated, and material costs are increased. For this reason, Mo was limited to the range of 1.5 to 3.5%.

Cu: 0.5-3.5%
Cu is an element that strengthens the protective film, suppresses hydrogen intrusion into the steel, and improves the resistance to sulfide stress corrosion cracking, and needs to be contained at 0.5% or more. On the other hand, if the content exceeds 3.5%, grain boundary precipitation of CuS occurs at a high temperature, and the hot workability decreases. For this reason, Cu was limited to the range of 0.5 to 3.5%. In addition, Preferably it is 0.5 to 2.5%.

N: 0.005-0.15%
N is an element that remarkably improves the pitting corrosion resistance. Such an effect is recognized when the content is 0.005% or more. However, when the content exceeds 0.15%, various nitrides are formed to deteriorate toughness. For this reason, N was limited to the range of 0.005 to 0.15%. In addition, Preferably it is 0.01 to 0.08%, More preferably, it is 0.02 to 0.06%.

O: 0.006% or less O exists as various oxides in steel and adversely affects various properties. Therefore, it is desirable to reduce as much as possible from the viewpoint of improving properties. In particular, if it exceeds 0.006%, hot workability, CO ₂ stress corrosion cracking resistance, pitting corrosion resistance, sulfide stress corrosion cracking resistance and toughness are significantly reduced. For this reason, O was limited to 0.006% or less.

One or two selected from Nb: 0.20% or less, V: 0.20% or less, the following formula (2)
Nb + V ≧ 0.05% …… (2)
Is contained so as to satisfy.
Nb and V are both elements that have the effect of increasing the strength of steel. In order to obtain the target high strength in the present invention, Nb and V are singly used so as to satisfy the above-described formula (2). Or, it is made to contain 0.05% or more in both. If each of Nb and V is contained in excess of 0.2%, the toughness is lowered and the high toughness that is the target of the present invention cannot be secured. For this reason, Nb and V are limited to 0.20% or less, respectively.

Further, in the present invention, Cr, Ni, Mo, and Cu are within the above-mentioned range and the following formula (1)
Cr + 2Ni + 1.1Mo + 0.7Cu ≤ 32.5 (1)
The content is adjusted so as to satisfy. In the present invention, in order to obtain a desired high strength, it is necessary to secure a structure having a martensite phase as a main phase, and Cr, Ni, Mo, and Cu are contained so that the formula (1) is satisfied. When the formula (1) is not satisfied, the structure does not become a structure having a martensite phase as a main phase, and a desired high strength cannot be obtained. Here, “a structure having a martensite phase as a main phase” means that the martensite phase is 80% or more by volume.

One or more selected from Ti: 0.3% or less, Zr: 0.2% or less, B: 0.01% or less, W: 3.0% or less
Ti, Zr, B, and W are all elements that have the effect of increasing the strength and improving the resistance to stress corrosion cracking, and can be selected to contain one or two as required. In order to obtain such effects, it is desirable to contain Ti: 0.01%, Zr: 0.01%, B: 0.0005%, W: 0.1% or more respectively, but Ti: 0.3%, Zr: 0.2%, B: Inclusions exceeding 0.01% and W: 3.0% respectively reduce toughness. For this reason, it is preferable to limit to Ti: 0.3% or less, Zr: 0.2% or less, B: 0.01% or less, and W: 3.0% or less.

Ca: 0.0005 to 0.01%
Ca is an element that has the action of fixing S as CaS and spheroidizing S-based inclusions to reduce the lattice strain of the matrix surrounding the inclusions and lower the hydrogen trapping ability. Can be contained depending on the case. In order to acquire such an effect, it is preferable to contain 0.0005% or more. On the other hand, a content exceeding 0.01% causes an increase in CaO and decreases the resistance to CO ₂ corrosion and pitting corrosion. For this reason, it is preferable to limit Ca to 0.0005 to 0.01%. In addition, More preferably, it is 0.001 to 0.005%. Further, REM may be contained instead of Ca.

Al: 0.05% or less
Al is an element having a strong deoxidizing action, and in order to secure such an effect, it is desirable to contain 0.005% or more, but inclusion exceeding 0.05% adversely affects toughness. For this reason, Al is preferably limited to 0.05% or less.
The balance other than the components described above consists of Fe and inevitable impurities.

The stainless steel pipe of the present invention has the above-described composition, and has a structure including a martensite phase as a main phase and a second phase as an austenite phase having a volume ratio of 3 to 15%. By making the structure having a martensite phase as a main phase, a steel pipe having a desired high strength is obtained. Here, the “main phase” means a phase occupying 80% or more by volume ratio.
The structure of the steel pipe of the present invention is a structure in which the second phase is an austenite phase and an austenite phase is precipitated in the martensite phase. If the austenite phase is less than 3%, the desired high toughness cannot be ensured. On the other hand, if the austenite phase exceeds 15%, the amount of austenite becomes too large to secure the desired strength. For this reason, the structure of the steel pipe of the present invention is a structure in which the martensite phase is the main phase and the second phase is an austenite phase having a volume ratio of 3 to 15%. In the present invention, a ferrite phase having a volume ratio of 5% or less may be included.

Below, the preferable manufacturing method of this invention steel pipe is demonstrated.
Molten steel having the above composition is melted by a normal melting method such as a converter, an electric furnace, a vacuum melting furnace, etc., and a steel pipe such as a billet by a normal method such as a continuous casting method or an ingot-bundling rolling method. It is preferable to use a raw material. Subsequently, these steel pipe materials are heated and hot-worked and piped using a normal Mannesmann-plug mill system or Mannesmann-Mandrel mill system manufacturing process to obtain seamless steel pipes of desired dimensions. After pipe making, the seamless steel pipe is preferably cooled to a temperature of 150 ° C. or lower at a cooling rate of air cooling or higher.

If it is a seamless steel pipe having the composition of the above-mentioned scope of the present invention, after hot working, it is cooled to a temperature of 150 ° C. or lower at a cooling rate of air cooling or higher, thereby forming a structure having a martensite phase as a main phase. However, after pipe forming, after cooling at a cooling rate higher than air cooling, it is further heated to a temperature of 800 ° C or higher and then cooled to a temperature of 150 ° C or lower at a cooling rate higher than air cooling. Is preferred. Thereby, high strength can be achieved.
It is also possible to produce ERW steel pipe and UOE steel pipe by using a steel material having the composition in the above-described range according to a normal process, and to make an oil well steel pipe. In this case, it is preferable to subject the steel pipe after pipe formation to reheating to a temperature of 800 ° C. or higher and then quenching to a temperature of 150 ° C. or lower at a cooling rate of air cooling or higher.

After the pipe making, the steel pipe cooled to a temperature of 150 ° C. or lower, or the steel pipe which has been subjected to quenching treatment and cooled to a temperature of 150 ° C. or lower is then heated to a temperature in the range of 500 to 580 ° C., and The heating temperature T and the heating holding time t are the following formula
15200 ≦ T (20 + logt) ≦ 16800 ...... (3)
(Where T: heating temperature (K), t: heating holding time (h))
A tempering treatment that satisfies

If the tempering process is performed with the steel pipe temperature after pipe forming or after quenching exceeding 150 ° C., the structure does not sufficiently martensite, and thus a desired high strength cannot be ensured. For this reason, a steel pipe shall be cooled until it becomes the temperature of 150 degrees C or less after pipe forming or at the time of hardening process.
If the heating temperature T in the tempering treatment is less than 500 ° C., the tempering is insufficient and the desired high toughness cannot be ensured. On the other hand, when the heating temperature T exceeds 580 ° C. and becomes a high temperature, the strength decreases, and a desired high strength cannot be secured. For this reason, it is preferable that the heating temperature T of the tempering treatment is set to a temperature in the range of 500 to 580 ° C.

  When the heating temperature T (K) in the tempering process is a temperature within the above-described range and the relational expression with the holding time t (h): T (20 + logt) is less than 15200, the precipitate is not sufficiently precipitated. The desired high strength cannot be ensured. On the other hand, if T (20 + logt) is larger than 16800, the strength of martensite is lowered, and a desired high strength cannot be secured. For this reason, the tempering treatment is performed within the range of the heating temperature and the holding time that satisfy the expression (3).

  Furthermore, based on an Example, this invention is demonstrated still in detail.

Molten steel having the composition shown in Table 1 is melted in a vacuum melting furnace, fully degassed, made into a 100 kg steel ingot, piped by a model seamless rolling mill for research, and after the pipe making, the temperatures shown in Table 2 Air-cooled to a seamless steel pipe (outer diameter 3.3in x wall thickness 0.5in).
Next, a test piece material (300 mm in length) was cut out from each steel pipe, and the test piece material was subjected to quenching treatment and tempering treatment under the conditions shown in Table 2. Some of the samples were not quenched.

Tensile test specimens (arc-shaped specimens) are collected from the specimen material that has been subjected to the above treatment in accordance with the API regulations, tensile tests are conducted in accordance with the API regulations, and tensile properties (yield) Strength YS and tensile strength TS) were determined.
In addition, Charpy impact test specimens (V-notch test specimens) are collected from the test specimen materials subjected to the above treatment in accordance with the provisions of JIS Z 2242 and −40 in accordance with the provisions of JIS Z 2242. Absorbed energy vE _-40 (J) at ° C was determined to evaluate toughness. In addition, vE- ₄₀ was made into the average value of three test pieces.

Further, corrosion test pieces (thickness 3 mm × width 30 mm × length 40 mm) were collected from the test piece material subjected to the above-described treatment by machining. The corrosion test was performed by immersing the corrosion test piece in a test solution: 20% NaCl aqueous solution (liquid temperature: 200 ° C., CO ₂ gas atmosphere of 30 atm) held in the autoclave, and setting the immersion period to 2 weeks. After the corrosion test, the weight of the test piece was measured, the weight loss by the corrosion test was determined, and the corrosion rate was calculated. In addition, the surface of the test piece after the corrosion test was observed with a magnifying glass 10 times to investigate the occurrence of pitting corrosion. The CO ₂ corrosion resistance was evaluated based on the obtained corrosion rate and the presence or absence of pitting corrosion.

  The obtained results are shown in Table 2.

Each of the inventive examples has an ultra-high strength of yield strength: 965 MPa or more, high toughness of vE- ₄₀ : 50 J or more, and corrosion rate even in a high temperature corrosion environment at 200 ° C. containing carbon dioxide gas. Is small, does not cause pitting, and has excellent CO ₂ corrosion resistance. On the other hand, in the comparative example out of the scope of the present invention, the yield stress is less than 965 MPa, the vE- ₄₀ is less than 50 J, the corrosion rate is high, the occurrence of pitting corrosion is observed, and the CO ₂ corrosion resistance is lowered. Is it?

Thus, the steel pipe according to the present invention has an ultra-high yield strength of 965 MPa or more, a high toughness of vE _-40 : 50 J or more, and is excellent even in a high-temperature corrosion environment up to 200 ° C. containing carbon dioxide gas. It is a steel pipe with CO ₂ corrosion resistance. Therefore, it can be seen that the steel pipe according to the present invention can be sufficiently used as an oil well steel pipe even in a high temperature oil well environment containing carbon dioxide gas up to 200 ° C.

Claims (9)

mass%
C: 0.05% or less, Si: 0.50% or less,
Mn: 0.20 to 1.80%, P: 0.03% or less,
S: 0.005% or less, Cr: 14.0 to 18.0%,
Ni: 5.0-8.0%, Mo: 1.5-3.5%,
Cu: 0.5 to 3.5%, N: 0.005 to 0.15%,
O: 0.006% or less, Cr, Ni, Mo, Cu is contained so as to satisfy the following formula (1), and Nb: 0.20% or less, V: 0.20% or less Or 2 types are contained so that the following (2) formula may be satisfied, it has the composition which consists of remainder Fe and an unavoidable impurity, makes a martensite phase the main phase and is 3 to 15% by volume as a 2nd phase. High-toughness ultra-high-strength stainless steel pipe for oil wells with excellent corrosion resistance, characterized by having a structure containing an austenite phase, yield strength YS of 965 MPa or more, and Charpy absorbed energy vE- _{40 at} -40 ° C of 50 J or more .
Record
Cr + 2Ni + 1.1Mo + 0.7Cu ≤ 32.5 (1)
Nb + V ≧ 0.05% (2)
Here, Cr, Ni, Mo, Cu, Nb, V: Content of each element (mass%)   In addition to the above-described composition, it may further contain at least one selected from mass%, Ti: 0.3% or less, Zr: 0.2% or less, B: 0.01% or less, W: 3.0% or less. The high-toughness ultra-high-strength stainless steel pipe for oil wells according to claim 1.   The high-toughness ultra-high-strength stainless steel pipe for oil wells according to claim 1 or 2, further comprising Ca: 0.0005 to 0.01% in mass% in addition to the composition.   The high toughness ultra-high-strength stainless steel pipe for oil wells according to any one of claims 1 to 3, further comprising, in addition to the composition, mass% and Al: 0.05% or less. mass%
C: 0.05% or less, Si: 0.50% or less,
Mn: 0.20 to 1.80%, P: 0.03% or less,
S: 0.005% or less, Cr: 14.0 to 18.0%,
Ni: 5.0-8.0%, Mo: 1.5-3.5%,
Cu: 0.5 to 3.5%, N: 0.005 to 0.15%,
O: 0.006% or less, Cr, Ni, Mo, Cu is contained so as to satisfy the following formula (1), and Nb: 0.20% or less, V: 0.20% or less Alternatively, two types are contained so as to satisfy the following formula (2), a steel material having a composition composed of the remaining Fe and unavoidable impurities is formed into a steel pipe, cooled to 150 ° C. or lower, and then, to the steel pipe, The heating temperature T is set to a temperature in the range of 500 to 580 ° C., and the tempering treatment is performed so that the heating temperature T and the heating holding time t satisfy the following expression (3). The yield strength YS is 965 MPa or more, −40 ° C. A stainless steel pipe having a Charpy absorbed energy vE- ₄₀ of 50 J or more in the production method of a high toughness ultra-high strength stainless steel pipe for oil wells excellent in corrosion resistance.
Record
Cr + 2Ni + 1.1Mo + 0.7Cu ≤ 32.5 (1)
Here, Cr, Ni, Mo, Cu: Content of each element (mass%)
Nb + V ≧ 0.05% (2)
Here, Nb, V: Content of each element (mass%)
15200 ≦ T (20 + logt) ≦ 16800 ...... (3)
Where T: heating temperature (K), t: heating holding time (h)   After the pipe making and before the tempering treatment, the steel pipe is subjected to a quenching treatment in which the steel pipe is heated to a temperature of 800 ° C. or higher and subsequently cooled to a temperature of 150 ° C. or lower at a cooling rate of air cooling or higher. The manufacturing method of the high toughness ultra-high-strength stainless steel pipe for oil wells of Claim 5.   In addition to the above composition, it may further contain at least one selected from mass%, Ti: 0.3% or less, Zr: 0.2% or less, B: 0.01% or less, W: 3.0% or less. The method for producing a high toughness ultra-high strength stainless steel pipe for oil wells according to claim 5 or 6.   The method for producing a high toughness ultra-high strength stainless steel pipe for oil wells according to any one of claims 5 to 7, further comprising Ca: 0.0005 to 0.01% in mass% in addition to the composition.   The high toughness ultra-high strength stainless steel pipe for oil wells according to any one of claims 5 to 8, further comprising, in addition to the composition, mass% and Al: 0.05% or less.