JP2010209402A - High-strength stainless steel pipe having high toughness and excellent corrosion resistance for oil well - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an inexpensive high-strength martensitic stainless steel pipe for use in oil wells which has excellent hot workability and a high strength exceeding 654 MPa of YS, further exhibits excellent CO<SB>2</SB>corrosion resistance under severe high-temperature corrosive environment of &ge;170&deg;C containing CO<SB>2</SB>, Cl<SP>-</SP>, etc., and excellent SSC resistance even under H<SB>2</SB>S-containing environments, and stably has high toughness. <P>SOLUTION: The stainless steel pipe contains, by mass, &le;0.04% C, &le;0.50% Si, 0.20 to 1.80% Mn, &le;0.03% P, &le;0.005% S, 15.5 to 17.5% Cr, 2.5 to 5.5% Ni, &le;0.20% V, 1.5 to 3.5% Mo, 0.50 to 3.0% W, &le;0.05% Al, &le;0.15% N and &le;0.006% O so as to satisfy three correlational formulae, and has microstructure, wherein regarding the largest one of the crystal grains composing the microstructure, the distance between two points in the crystal grains is &le;200 &mu;m. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

TECHNICAL FIELD The present invention relates to oil wells for crude oil or natural gas, and steel pipes for oil wells used for gas wells. In particular, the present invention includes carbon wells (CO ₂ ), chlorine ions (Cl ⁻ ), etc. The present invention relates to a high-strength stainless steel pipe for oil wells having excellent corrosion resistance suitable for well use.

In recent years, in order to cope with soaring crude oil prices and the depletion of oil resources expected in the near future, deep oil fields that have not been excluded in the past, and highly corrosive that once abandoned development Development on sour gas fields and the like has become active worldwide. 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.

  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.

In response to such a request, Patent Document 1 describes that in hot severe corrosive environment of 170 ° C. or higher, which has excellent hot workability and high strength exceeding YS: 654 MPa, and contains CO ₂ , Cl and the like. Proposal of an inexpensive high-strength martensitic stainless steel pipe for oil wells that exhibits excellent CO ₂ corrosion resistance and also exhibits excellent SSC resistance and high toughness even in the presence of H ₂ S Has been.

  Its basic configuration is as follows.

In mass%, C: 0.04% or less, Si: 0.50% or less, Mn: 0.20 to 1.80%, P: 0.03% or less, S: 0.005% or less, Cr: 15.5-17.5%, Ni: 2.5-5.5%, V : 0.20% or less, Mo: 1.5 to 3.5%, W: 0.50 to 3.0%, Al: 0.05% or less, N: 0.15% or less, O: 0.006% or less, satisfying the following formulas (1) to (3) A high-strength stainless steel pipe for oil wells having a composition comprising the balance Fe and inevitable impurities and having high toughness and excellent corrosion resistance.
Cr + 3.2Mo + 2.6W-10C ≧ 23.4 (1)
Cr + Mo + 0.5W + 0.3Si-43.5C-0.4Mn-0.3Cu-Ni-9N ≧ 11.5 (2)
2.2 ≦ Mo + 0.8W ≦ 4.5 (3)
Here, Cr, Mo, W, Si, C, Mn, Cu, Ni, N: Content of each element (mass%)

JP 2008-081793 A

  However, in the high-strength stainless steel pipe for oil well described in Patent Document 1, the low-temperature toughness is unstable and a desired value may not be obtained.

The present invention has been made in view of the above circumstances, has excellent hot workability, has a high strength exceeding YS: 654 MPa, and is severe at 170 ° C. or higher containing CO ₂ , Cl _2- , ^{and the} like. For low-priced oil wells that exhibit excellent CO ₂ corrosion resistance in high-temperature corrosive environments, and excellent SSC resistance even in the presence of H ₂ S, and that have stable high toughness An object is to provide a high-strength martensitic stainless steel pipe.

  The “high-strength martensitic stainless steel pipe” referred to in the present invention refers to a martensitic stainless steel pipe having a yield strength exceeding 654 MPa (95 ksi). Further, “high toughness” as used in the present invention refers to the case where the absorbed energy at −40 ° C. in the Charpy impact test shows 20 J or more.

  In order to achieve the above-mentioned object, the present inventors diligently investigated the cause of unstable toughness in the high-strength stainless steel pipe for oil well described in Patent Document 1. As a result, in the high strength stainless steel pipe for oil well described in Patent Document 1 characterized in that the microstructure is a two-phase structure of a ferrite phase and a martensite phase, coarse ferrite grains may be generated. It was found that the toughness became unstable and the desired value could not be obtained.

  Therefore, in the high-strength stainless steel pipe for oil well described in Patent Document 1, in order to stabilize the low-temperature toughness, it is necessary to refine the crystal grains constituting the microstructure. It has been found that the crystal grain size (the distance between any two points in the crystal grain) needs to be 200 μm or less in order to obtain it stably.

  Based on the above findings, the present invention has the following characteristics.

[1] In mass%,
C: 0.04% or less, Si: 0.50% or less,
Mn: 0.20 to 1.80%, P: 0.03% or less,
S: 0.005% or less, Cr: 15.5-17.5%,
Ni: 2.5-5.5%, V: 0.20% or less,
Mo: 1.5-3.5%, W: 0.50-3.0%
Al: 0.05% or less, N: 0.15% or less,
O: 0.006% or less is contained so as to satisfy the following formulas (1) to (3), the composition is composed of the balance Fe and inevitable impurities, and the largest one among the crystal grains constituting the microstructure A high-strength stainless steel pipe for oil wells having high toughness and excellent corrosion resistance, wherein the distance between any two points in the crystal grains is 200 μm or less.
Record
Cr + 3.2Mo + 2.6W-10C ≧ 23.4 (1)
Cr + Mo + 0.5W + 0.3Si-43.5C-0.4Mn-0.3Cu-Ni-9N ≧ 11.5 (2)
2.2 ≦ Mo + 0.8W ≦ 4.5 (3)
Here, Cr, Mo, W, Si, C, Mn, Cu, Ni, N: Content of each element (mass%)
[2] The high-strength stainless steel pipe for oil wells according to [1], wherein the composition further includes, in mass%, Cu: 0.5 to 3.5% in addition to the composition.

  [3] In addition to the above-mentioned composition, at least one selected from mass%, Nb: 0.20% or less, Ti: 0.3% or less, Zr: 0.2% or less, B: 0.01% or less The high-strength stainless steel pipe for oil wells according to [1] or [2], wherein the composition contains.

  [4] The high-strength stainless steel for oil wells according to any one of [1] to [3], wherein in addition to the composition, the composition further includes mass% and Ca: 0.0005 to 0.01%. Steel pipe.

  [5] The oil well height according to any one of the above [1] to [4], wherein the martensite phase is a base phase and the ferrite phase has a structure containing 10 to 50% by volume. Strength stainless steel pipe.

In the present invention, it is excellent in hot workability, YS: has a high strength of more than 654MPa, CO _2, Cl ^-, etc. In severe hot corrosion environment over 170 ° C. containing, excellent CO ₂ corrosion It is possible to provide an inexpensive, high-strength martensitic stainless steel pipe for oil wells that exhibits excellent SSC resistance and has stable and high toughness even in an environment where H ₂ S is present. .

  First, the reasons for limiting the composition of the high-strength stainless steel pipe of the present invention will be described. Hereinafter, unless otherwise specified, mass% is simply written as%.

C: 0.04% or less C is an important element related to the strength of martensitic stainless steel. To ensure the desired strength, 0.005% or more is desirable, but more than 0.04% is contained. Then, the sensitization at the time of tempering by Ni containing tends to increase. Therefore, C is limited to 0.04% or less for the purpose of preventing sensitization during tempering. In addition, Preferably, it is 0.005 to 0.03% of range from a viewpoint of corrosion resistance.

Si: 0.50% or less
Si is an element that acts as a deoxidizer in the normal steelmaking process, and in the present invention, it is desirable to contain 0.05% or more, but if it exceeds 0.50%, the CO ₂ corrosion resistance decreases, Hot workability also decreases. For this reason, Si was limited to 0.50% or less. In addition, Preferably it is 0.10 to 0.35%.

Mn: 0.20 to 1.80%
Mn is an element that increases the strength, and in the present invention, it is necessary to contain 0.20% or more in order to ensure the strength required as a martensitic stainless steel pipe for oil wells, but if it exceeds 1.80%, Adversely affect. For this reason, Mn was limited to the range of 0.20 to 1.80%. In addition, Preferably it is 0.25 to 0.60%.

P: 0.03% or less P is an element that degrades CO ₂ corrosion resistance, CO ₂ stress corrosion cracking resistance, pitting corrosion resistance and sulfide stress corrosion cracking resistance, and should be reduced as much as possible in the present invention. However, 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 deteriorate 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 hot workability in the pipe manufacturing process, and it is desirable that it be as small as possible. However, if it is reduced to 0.005% or less, pipes can be manufactured in the normal process. Therefore, S is limited to 0.005% or less. In addition, Preferably it is 0.003% or less.

Cr: 15.5-17.5%
Cr is an element improving the corrosion resistance by forming a protective coating, in particular resistance to CO ₂ corrosion, as well as effectively contribute to the improvement of resistance to CO ₂ stress corrosion cracking resistance, resistance to sulfide stress corrosion cracking (SSC resistance ). In the present invention, the content of 15.5% or more is particularly required from the viewpoint of improving corrosion resistance at high temperatures. On the other hand, the content exceeding 17.5% decreases the strength. For this reason, in the present invention, Cr is limited to a range of 15.5-17.5%. In addition, Preferably it is 16.0 to 17.0%.

Ni: 2.5-5.5%
Ni has an effect of strengthening the protective film, and is an element that enhances CO ₂ corrosion resistance, CO ₂ stress corrosion cracking resistance, pitting corrosion resistance, and sulfide stress corrosion cracking resistance. In order to ensure such an effect under the severe corrosive environment targeted by the present invention, Ni content of 2.5% or more is required. On the other hand, if the content exceeds 5.5%, the stability of the martensite structure decreases and the strength decreases. 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%.

V: 0.20% or less V is an element that has the effect of increasing the strength and improving the SSC resistance. To obtain such an effect, it is desirable to contain 0.01% or more, but 0.20% If it is contained in excess, the toughness decreases. For this reason, V was limited to 0.20% or less. In addition, Preferably it is 0.05 to 0.08%.

Mo: 1.5-3.5%
Mo is, Cl ^- by an element having the effect of increasing the resistance to pitting, further effectively acts to improve the SSC resistance. In order to ensure such an effect under the severe corrosive environment targeted by the present invention, the content of 1.5% or more is required. On the other hand, if the content exceeds 3.5%, the strength is lowered and the material cost is increased. For this reason, Mo was limited to the range of 1.5 to 3.5%. In addition, Preferably it is 1.8 to 3.0%.

W: 0.50 ~ 3.0%
W, like Mo, is an element effective for improving the SSC resistance, and such an effect becomes remarkable when the content is 0.50% or more. On the other hand, the content exceeding 3.0% deteriorates toughness. For this reason, Mo was limited to the range of 0.50 to 3.0%. In addition, Preferably it is 0.7 to 2.0%.

In the present invention, the contents of Mo and W are within the above-described range, and the following formula (3)
2.2 ≦ Mo + 0.8W ≦ 4.5 (3)
(Where, Mo, W: content of each element (mass%))
It is adjusted to satisfy. By containing Mo and W in combination, the SSC resistance is significantly improved. With a Mo + 0.8 W 2.2 or more, CO _2, Cl covered by the present invention ^- comprises a further even under severe corrosive environment of the hot containing H ₂ S, can ensure excellent SSC resistance. On the other hand, if Mo + 0.8W exceeds 4.5, toughness and hot workability are reduced. Therefore, in the present invention, the Mo and W contents are adjusted so that Mo and W satisfy the formula (3), and Mo + 0.8W is in the range of 2.2 to 4.5.

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 0.002% 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: 0.15% or less N is an element that remarkably improves pitting corrosion resistance, and such an effect becomes remarkable when the content is 0.01% or more. On the other hand, the content exceeding 0.15% forms various nitrides and lowers the toughness. 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 present as an oxide in steel and adversely affects various properties, so it is desirable to reduce it as much as possible. In particular, when the O content exceeds 0.006%, the hot workability, the CO ₂ stress corrosion cracking resistance, the pitting corrosion resistance, the sulfide stress corrosion cracking resistance, and the toughness are significantly reduced. For this reason, O was limited to 0.006% or less.

  In the present invention, in addition to the above composition, Cu: 0.5 to 3.5% and / or Nb: 0.20% or less, Ti: 0.3% or less, Zr: 0.2% Hereinafter, one or more of B: 0.01% or less and / or Ca: 0.0005 to 0.01% may be selected and contained as necessary.

Cu: 0.5-3.5%
Cu is an element having an action of strengthening the protective film and suppressing the penetration of hydrogen into the steel and improving the resistance to sulfide stress corrosion cracking, and can be contained as required. In order to acquire such an effect, it is preferable to contain 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, when it contains, it is preferable to limit Cu to 0.5 to 3.5% of range. In addition, More preferably, it is 0.5 to 2.5%, More preferably, it is 0.8 to 1.5%.

One or more selected from Nb: 0.20% or less, Ti: 0.3% or less, Zr: 0.2% or less, B: 0.01% or less
Each of Nb, Ti, Zr, and B is an element that increases the strength, and can be selected as required and contained in one or more kinds. Nb has an effect of further improving toughness in addition to the above-described effects. Ti, Zr, and B have an effect of further improving the stress corrosion cracking resistance in addition to the above-described effects. In order to obtain such an effect, it is desirable to contain Nb: 0.01% or more, Ti: 0.01% or more, Zr: 0.01% or more, B: 0.0005% or more. On the other hand, if Nb: 0.20%, Ti: 0.3%, Zr: 0.2%, and B: more than 0.01%, the toughness decreases. For this reason, when it contains, it is preferable to limit to Nb: 0.20% or less, Ti: 0.3% or less, Zr: 0.2% or less, B: 0.01% or less. More preferably, Nb: 0.02 to 0.10%, Ti: 0.02 to 0.12%, Zr: 0.02 to 0.08%, B: 0.0005 to 0.003%.

Ca: 0.0005 to 0.01%
Ca is an element having an action of fixing S as CaS and spheroidizing sulfide inclusions. This has the effect of reducing the lattice strain of the matrix around the inclusions and reducing the hydrogen trapping ability of the inclusions. Such an effect becomes prominent when the content is 0.0005% or more. However, if the content exceeds 0.01%, CaO increases, and the resistance to CO ₂ corrosion and pitting corrosion decreases. For this reason, when it contains Ca, it is preferable to limit to 0.0005 to 0.01% of range.

In the present invention, each of the above components is included within the above range, and the following formula (1)
Cr + 3.2Mo + 2.6W-10C ≧ 23.4 (1)
And the following equation (2)
Cr + Mo + 0.5W + 0.3Si-43.5C-0.4Mn-0.3Cu-Ni-9N ≧ 11.5 (2)
(Here, Cr, Mo, W, Si, C, Mn, Cu, Ni, N: content of each element (mass%))
The content of each component is adjusted so as to satisfy the above.

  By adjusting the Cr, Mo, W, and C contents so as to satisfy the above-described expression (1), the SSC resistance is remarkably improved. If the formula (1) cannot be satisfied, sufficient SSC resistance cannot be secured. Further, in addition to reducing P, S, and O to the above ranges, Cr, Mo, W, Si, C, Mn, Cu, Ni, N, so as to satisfy the above-described formula (2). By adjusting the content, it is possible to ensure the necessary and sufficient hot workability for forming a martensitic stainless steel seamless pipe, and to maintain a desired strength. Only by reducing P, S, and O within the above ranges, it is not possible to ensure the hot workability necessary and sufficient for forming a martensitic stainless steel seamless pipe. Moreover, when the formula (2) cannot be satisfied, the hot workability for the production of martensitic stainless steel seamless pipes is insufficient.

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

  The high-strength stainless steel pipe according to the present invention has the above-described composition, and preferably has a structure containing a martensite phase as a base phase and further containing a ferrite phase in a volume ratio of 10 to 50%. In the high-strength stainless steel pipe according to the present invention, the structure is a structure having a martensite phase as a base phase in order to maintain high strength. And in this invention, it is preferable to set it as the structure | tissue which contains a ferrite phase 10% or more by volume for the purpose of improving corrosion resistance, without reducing intensity | strength. On the other hand, when the ferrite phase exceeds 50% by volume, the strength is significantly reduced. For this reason, in the steel pipe of the present invention, the ferrite phase fraction is preferably limited to a range of 10 to 50% by volume. More preferably, it is 12 to 30%. In addition, there is no problem even if the second phase other than the ferrite phase contains an austenite phase of 20% by volume or less.

  In addition, in the high-strength stainless steel pipe according to the present invention, the distance between any two points in the crystal grain is 200 μm or less in the largest crystal grain constituting the microstructure.

  Thus, the desired toughness (the absorbed energy at −40 ° C. in the Charpy impact test is 20 J or more) can be stably obtained by making the crystal grains fine.

  Next, a preferred method for producing a high-strength stainless steel pipe according to the present invention will be described using a seamless steel pipe as an example.

  First, molten steel having the above composition is melted by a generally known melting method such as a converter, an electric furnace, a vacuum melting furnace, etc., and billet is obtained by a conventional 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.

  Here, when heating a steel pipe raw material in the above, it is preferable to carry out on the heating conditions which suppress the growth of a crystal grain.

  After the pipe making, the seamless steel pipe is preferably cooled to room temperature at a cooling rate equal to or higher than air cooling. Thereby, the structure | tissue of a steel pipe can be made into the structure | tissue which makes a martensite phase a base phase. In addition, you may manufacture a seamless steel pipe by the hot extrusion by a press system.

  In addition, it is preferable to perform the hardening process which cools to predetermined temperature with the cooling rate more than air cooling, after re-heating to predetermined temperature after pipe | tube forming at the cooling rate more than air cooling. Thereby, it is possible to obtain a fine and high toughness martensite structure preferably containing an appropriate amount of ferrite phase.

  The seamless steel pipe that has been subjected to the quenching treatment is then preferably subjected to a tempering treatment that is heated to a predetermined temperature and cooled at a cooling rate equal to or higher than air cooling. By heating to a predetermined temperature and tempering, the structure becomes a structure composed of a tempered martensite phase and a small amount of ferrite phase, and has a desired high strength, a desired high toughness, and a desired excellent corrosion resistance. It becomes a steel-free pipe. In addition, you may give only the above-mentioned tempering process without quenching process.

  Note that the heat treatment is preferably performed under heat treatment conditions that suppress the growth of crystal grains.

  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 steel pipe material having a composition within the scope of the present invention described above, steel pipes other than seamless steel pipes obtained in accordance with a normal manufacturing process, such as ERW steel pipes and UOE steel pipes, are described above in steel pipes after pipe making. Quenching-tempering treatment, which is re-heated to a predetermined temperature and then cooled to a predetermined temperature at a cooling rate higher than air cooling, and then tempered to be heated to a predetermined temperature and cooled at a cooling rate higher than air cooling. Are preferably applied.

  After degassing the molten steel with the composition of steel Nos. A to R 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, and then air-cooled or water-cooled after pipe making. A seamless steel pipe having an outer diameter of 83.8 mm and a wall thickness of 12.7 mm (3.3 inches × wall thickness of 0.5 inches) was used.

  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 cooling after pipe making, and hot workability was evaluated. In addition, the case where the crack of 1 mm or more exists in the front-and-rear end surface of a steel pipe was made into hot workability favorable, and the other was made into hot workability defect.

  Further, from the obtained seamless steel pipe, a test piece material was cut out, heated at 920 ° C. for 1 h, and then subjected to quenching treatment with water cooling (average cooling rate from 800 to 500 ° C .: 10 ° C./s). Furthermore, the tempering process which hold | maintains for 30 minutes at the temperature of the range of 500-650 degreeC and air-cools was performed.

  Tissue observation specimens are collected from the specimen material treated in this way, and the tissue observation specimens are corroded with aqua regia and imaged with a scanning electron microscope (400x) for image analysis. Using the apparatus, the maximum value of the distance between any two points in the crystal grain (maximum distance in the crystal) of the largest crystal grain was calculated.

Further, the retained austenite phase structure fraction 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 / IγRγ))
Where Iα: Integral intensity of α
Rγ: Theoretical calculation value of α
Iγ: Integral intensity of γ
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 pieces were collected from the test piece materials subjected to the above treatment, and subjected to a tensile test to obtain tensile properties (yield strength YS, tensile strength TS).

In addition, a V-notch test piece (5 mm thick) was collected from the test piece material subjected to the above treatment in accordance with JIS Z 2242, a Charpy impact test was performed, and the absorbed energy vE at −40 ° C. _-40 was determined and toughness was evaluated. In addition, when the desired toughness (absorbed energy vE- _{40 at} −40 ° C. is 20 J or more) is obtained, the toughness is good (◯), and when the desired toughness is not obtained, the toughness is poor (×). did.

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

The corrosion test was carried out by immersing the corrosion test piece in a test solution held in an autoclave: 20% NaCl aqueous solution (liquid temperature: 220 ° C., 100 atm 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, the case where the pitting corrosion was 0.3 mm or more in diameter was regarded as having pitting corrosion (x), and the other was not pitting corrosion (◯).

  In addition, a round bar-shaped test piece (parallel part diameter: 6.4 mm) is made from the test piece material that has been subjected to the above treatment in accordance with NACE-TM0177 Method A, and subjected to an SSC resistance test. Carried out.

The SSC resistance test was conducted in a test solution held in a test vessel: 20% NaCl aqueous solution (liquid temperature: 25 ° C., H ₂ S: 0.1 atm, CO ₂ : 0.9 atm atmosphere, solution pH: 3.5 (0.5% CH ₃ The specimen is immersed in COOH + CH ₃ COONa), the immersion period is set to 30 days (720h), 100% SMYS (Specified Minimum Yield Strength) stress is applied to the specimen, and the specimen breaks. The presence or absence of was investigated. The case where it broke was evaluated as “poor” as being inferior in SCC resistance, and the case where it was not broken was evaluated as “O”, as being excellent in SCC resistance.

  First, the maximum distance in the crystal is shown in Table 1.

  As shown in Table 1, steel Nos. A to I having a maximum intra-crystal distance of 200 μm or less among steel Nos. A to R are examples of the present invention, and steel No. A having a maximum intra-crystal distance of more than 200 μm. J to R are comparative examples.

  Table 2 shows the evaluation results of the corrosion resistance, SSC resistance and toughness obtained for each.

  Although not shown in Table 2, all of steel Nos. A to R had good hot workability and had a desired value (yield strength exceeded 654 Mpa). Each of Steel Nos. A to R was a structure containing a martensite phase as a base phase and containing a ferrite phase in a volume ratio of 10 to 50%.

  As shown in Table 2, the corrosion resistance and SSC resistance were good (◯) in all of the inventive examples (steel Nos. A to I) and the comparative examples (steel Nos. J to R).

  However, with regard to toughness, in the present invention examples (steel Nos. A to I) in which the maximum distance in the crystal is 200 μm or less, the desired toughness is obtained (◯), whereas the maximum distance in the crystal is 200 μm. In the comparative example (steel Nos. A to I) exceeding the range, the desired toughness was not obtained (×).

  From this result, the effectiveness of the present invention could be confirmed.

Claims (5)

mass%
C: 0.04% or less, Si: 0.50% or less,
Mn: 0.20 to 1.80%, P: 0.03% or less,
S: 0.005% or less, Cr: 15.5-17.5%,
Ni: 2.5-5.5%, V: 0.20% or less,
Mo: 1.5-3.5%, W: 0.50-3.0%
Al: 0.05% or less, N: 0.15% or less,
O: 0.006% or less is contained so as to satisfy the following formulas (1) to (3), the composition is composed of the balance Fe and inevitable impurities, and the largest one among the crystal grains constituting the microstructure A high-strength stainless steel pipe for oil wells having high toughness and excellent corrosion resistance, wherein the distance between any two points in the crystal grains is 200 μm or less.
Record
Cr + 3.2Mo + 2.6W-10C ≧ 23.4 (1)
Cr + Mo + 0.5W + 0.3Si-43.5C-0.4Mn-0.3Cu-Ni-9N ≧ 11.5 (2)
2.2 ≦ Mo + 0.8W ≦ 4.5 (3)
Here, Cr, Mo, W, Si, C, Mn, Cu, Ni, N: Content of each element (mass%)   The high-strength stainless steel pipe for oil wells according to claim 1, wherein the composition further includes, in addition to the composition, mass% and Cu: 0.5 to 3.5%.   In addition to the above composition, the composition further contains one or more selected from mass%, Nb: 0.20% or less, Ti: 0.3% or less, Zr: 0.2% or less, B: 0.01% or less The high-strength stainless steel pipe for oil wells according to claim 1 or 2.   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% in mass%.   The high-strength stainless steel pipe for oil wells according to any one of claims 1 to 4, which has a structure containing a martensite phase as a base phase and further containing a ferrite phase in a volume ratio of 10 to 50%.