WO2021131445A1

WO2021131445A1 - High-strength stainless steel seamless pipe for oil wells

Info

Publication number: WO2021131445A1
Application number: PCT/JP2020/043310
Authority: WO
Inventors: 江口　健一郎; 村井　剛; まみ遠藤; 正雄柚賀
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2019-12-24
Filing date: 2020-11-20
Publication date: 2021-07-01
Also published as: CN114829647A; JP6950851B1; JPWO2021131445A1; EP4043591A1; BR112022011761A2; EP4043591A4; AR120872A1; MX2022007286A; US20230033540A1

Abstract

The purpose of the present invention is to provide a high-strength stainless steel seamless pipe for oil wells, said high-strength stainless steel seamless pipe having excellent carbon dioxide gas corrosion resistance and excellent SSC resistance in a low temperature environment, while having high strength and excellent hot formability.　A high-strength stainless steel seamless pipe for oil wells, said high-strength stainless steel seamless pipe having a composition that contains a specific component composition with the balance being made up of Fe and unavoidable impurities, while satisfying formula (1) and formula (2). With respect to this high-strength stainless steel seamless pipe for oil wells, the number density of inclusions having a length of 5 μm or more and satisfying 0.5 < Ti/(Ti + Al + Mg + Ca) < 1.0 is from 0.5 per mm2 to 3 per mm2, and the yield strength is 655 MPa or more. (Meanwhile, Ti, Al, Mg and Ca in 0.5 < Ti/(Ti + Al + Mg + Ca) < 1.0 represent the respective contents (mass%) of the elements in the inclusions, while representing zero in cases where the corresponding element is not contained therein.)　Formula (1): Cr + 0.65Ni + 0.6Mo + 0.55Cu - 20C ≥ 15.0 Formula (2): Cr + Mo + 0.3Si - 43.3C - 0.4Mn – Ni - 0.3Cu - 9N ≤ 11.0 In the formulae, Cr, Ni, Mo, Cu, C, Si, Mn and N represent the respective contents (mass%) of the elements, while representing zero in cases where the corresponding element is not contained therein.

Description

High-strength stainless steel seamless steel pipe for oil wells

The present invention relates to a stainless seamless steel pipe that is suitably used for oil wells of crude oil or natural gas, gas wells (hereinafter, simply referred to as oil wells) and the like. In particular, carbon dioxide (CO _2), chlorine ions (Cl ^-) include, Mu excellent stainless seam SSC resistance in耐炭acid gas corrosion resistance and low-temperature environment under extremely severe corrosive environment temperature higher than 0.99 ° C. Regarding steel pipes.

In recent years, from the viewpoint of soaring crude oil prices and the depletion of petroleum resources expected in the near future, it is in a so-called sour environment containing deep oil fields and hydrogen sulfide, which have not been omitted in the past. The development of oil fields and gas fields in severely corroded environments is active. Such oil, gas fields are generally the depth is very deep, also and CO _2, Cl its atmosphere at high temperature ^- and is further a severe corrosive environment containing H _{2 S.} Steel pipes for oil wells used in such an environment are required to have a material having desired high strength and excellent corrosion resistance.

Conventionally, carbon dioxide (CO _2), chlorine ions (Cl ^-) oil field environment, and the like, in the gas field, 13Cr martensitic stainless steel pipe is often used as oil country tubular goods for use in mining. Furthermore, recently, the use of improved 13Cr martensitic stainless steel, which has a reduced C content and increased Ni, Mo, etc., of 13Cr martensitic stainless steel, is expanding.

For example, Patent Document 1 describes, in terms of mass%, C: 0.010 to 0.030%, Mn: 0.30 to 0.60%, P: 0.040% or less, S: 0.0100% or less, Cr: 10.00 to 15.00%, Ni: 2.50 to 8.00%, Mo: 1.00 to 5.00%, Ti: 0.050 to 0.250%, V: 0.25% Hereinafter, N: 0.07% or less, Si: 0.50% or less, Al: 0.10% or less, one or more of them are contained, and the balance is composed of Fe and impurities as the formula (1). A martensitic stainless steel that satisfies 6.0 ≦ Ti / C ≦ 10.1 and has a yield strength of 758 to 862 MPa is disclosed.

Further, in Patent Document 2, in% by weight, C: ≤0.050, Si: ≤0.5, Mn: ≤1.5, P: ≤0.03, S: ≤0.005, Cr: 11 .0 to 14.0, Ni: 4.0 to 7.0, Mo: 1.0 to 2.5, Cu: 1.0 to 2.5, Al: ≤0.05, N: 0.01 to A martensitic stainless steel containing 0.10, which has a composition of Fe and unavoidable impurities as a balance, is cooled to a temperature below the Ms point after hot working, and then cooled to a temperature T of _{550 ° C or higher and Ac 1 or lower.} Disclosed is a method for manufacturing a martensitic stainless seamless steel tube, which is subjected to a heat treatment of heating the temperature to an average heating rate of 500 to T ° C. of 1.0 ° C./sec or more and then cooling it to a temperature of Ms point or less. There is.

Further, in Patent Document 3, in terms of weight%, C: 0.06% or less, Cr: 12 to 16%, Si: 1.0% or less, Mn: 2.0% or less, Ni: 0.5 to 8 It contains 0.0%, Mo: 0.1 to 2.5%, Cu: 0.3 to 4.0%, N: 0.05% or less, and the area ratio of the δ-ferrite phase is 10% or less. A high-strength martensitic stainless steel having excellent stress corrosion cracking resistance in which fine precipitates of Cu are dispersed in a matrix is disclosed.

WO2008 / 023702 Japanese Unexamined Patent Publication No. 9-170019 Japanese Unexamined Patent Publication No. 7-166303

Recent, with the development of oil and gas fields in severe corrosive environments, the oil well steel pipe, and the high strength, at a high temperature of at least 0.99 ° C., and, CO _2, Cl ^- even in severe corrosive environments containing, It has been required to have both excellent carbon dioxide corrosion resistance. In addition, with the harsh development environment, it has been required to have excellent SSC resistance even in a low temperature environment such as the deep sea.

However, although the techniques described in Patent Documents 1 to 3 have high strength and excellent carbon dioxide corrosion resistance, the SSC resistance in a low temperature environment is not sufficient.

Therefore, the present invention solves the problems of the prior art, has high strength and excellent hot workability, has excellent carbon dioxide corrosion resistance, and has excellent SSC resistance in a low temperature environment. It is an object of the present invention to provide a strong stainless seamless steel pipe.

Note that the "high strength" here means the case where the yield strength is YS: 95ksi (655MPa) or more.

In addition, excellent carbon dioxide corrosion resistance means that the test piece is placed in a test solution held in an autoclave: a 20 mass% NaCl aqueous solution (liquid temperature: 150 ° C., CO _{2 gas atmosphere at 10 atm).} It means the case where the corrosion rate is 0.125 mm / y or less when the immersion is carried out with the immersion period set to 14 days.

In addition, "excellent SSC resistance in a low temperature environment" means that Na acetate + hydrochloric acid is added to a test solution: 25 mass% NaCl aqueous solution (liquid temperature: 4 ° C, H ₂ S: 0.1 bar, CO _{2: 0.9 bar).} In addition, the test piece was immersed in an aqueous solution adjusted to pH: 4.5, the immersion time was set to 720 hours, and 90% of the yield stress was added as the load stress to perform the test, and the test piece after the test cracked. It shall mean the case of not doing so.

In order to achieve the above-mentioned object, the present inventors have diligently studied the influence on the low temperature SSC resistance of stainless steel pipes having various compositions. As a result, it was found that all stainless steel SSCs originated from pitting corrosion. Next, when the occurrence of pitting corrosion was examined, among various inclusions, oxides or sulfides containing Al, Ca, Mg, etc. as the main components were most likely to be the starting point of pitting corrosion in a low temperature environment. Do you get it. Therefore, in order to improve the SSC resistance in a low temperature environment, it is important to reduce oxide-based or sulfide-based inclusions containing Al, Ca, Mg, etc. as main components as much as possible. However, oxide-based inclusions and sulfide-based inclusions are produced by oxygen and sulfur contained as impurities in steel, and therefore cannot be industrially reduced to zero. Therefore, we came up with the idea of detoxifying by changing the structure of oxide-based inclusions and sulfide-based inclusions. Specifically, it has been found that by coating the above-mentioned inclusions that are prone to pitting corrosion with TiN, it is difficult to become the starting point of pitting corrosion and the SSC resistance in a low temperature environment can be improved. The reason for this is that by covering the inclusions with TiN, when the inclusions dissolve, N ions are released into the solution, which ^{changes to NH 3+} , which locally raises the pH around the inclusions. This is thought to be due to the inhibition of pitting corrosion.

The present inventors also examined the effect of tissue on low temperature SSC resistance. As a result, it was found that in a low temperature environment, reducing the particle size of the old austenite suppresses the growth of pitting corrosion and the occurrence of cracks, and improves the SSC resistance. This is because P and S segregated at the old austenite grain boundaries (1) promote the selective dissolution of the old austenite grain boundaries during pitting corrosion growth, and (2) the grain boundaries when hydrogen invades the steel. It is considered that this is because it promoted the embrittlement of. It is considered that the smaller the old austenite grain size, the larger the grain boundary area per unit volume, so that the concentration of P and S segregated at the old austenite grain boundary decreases, and the SSC resistance is improved. The reason why the influence of the old austenite grain boundaries on the SSC resistance in a low temperature environment is remarkable is that the solubility of hydrogen sulfide in the test solution, which promotes the invasion of hydrogen into steel, increases, and the temperature It is considered that the cause is that the gasification of hydrogen is suppressed due to the decrease.

The present invention has been completed with further studies based on such findings. That is, the gist of the present invention is as follows.
[1] By mass%,
C: 0.002-0.05%, Si: 0.05-0.50%,
Mn: 0.04 to 1.80%, P: 0.030% or less,
S: 0.002% or less, Cr: 11.0 to 14.0%,
Ni: 3.0-6.5%, Mo: 0.5-3.0%,
Al: 0.005 to 0.10%, V: 0.005 to 0.20%,
Ti: 0.01-0.20%, Co: 0.01-1.0%,
It contains N: 0.002 to 0.15%, O: 0.010% or less, satisfies the following formulas (1) and (2), and has a composition consisting of the balance Fe and unavoidable impurities.
The major axis is 5 μm or more, 0.5 <Ti / (Ti + Al + Mg + Ca) <1.0, the number density of inclusions is 0.5 / mm ² or more and 3 / mm ² or less, and the yield strength is 655 MPa or more. High-strength stainless seamless steel pipe for oil wells.
(Here, in 0.5 <Ti / (Ti + Al + Mg + Ca) <1.0, Ti, Al, Mg, Ca: the content (mass%) of each element in the inclusions, and the elements not contained are zero. )
Note Cr + 0.65Ni + 0.6Mo + 0.55Cu-20C ≧ 15.0 ‥‥ (1)
Cr ＋ Mo ＋ 0.3Si-43.3C－0.4Mn－Ni－0.3Cu-9N ≦ 11.0 ‥‥‥ (2)
Here, Cr, Ni, Mo, Cu, C, Si, Mn, N: the content (mass%) of each element, and the element not contained is zero.
[2] The height for oil wells according to [1], which further contains one or two selected from Cu: 0.05 to 3.0% and W: 0.05 to 3.0% in mass% in addition to the above composition. Strong stainless seamless steel pipe.
[3] In addition to the above composition, in mass%, Nb: 0.01 to 0.20%, Zr: 0.01 to 0.20%, B: 0.0005 to 0.01%, REM: 0.0005 to 0.01%, Ca: 0.0005 to 0.0025%, Sn. The high-strength stainless steel seam for oil wells according to [1] or [2], which contains one or more selected from 0.02 to 0.20%, Ta: 0.01 to 0.1%, and Mg: 0.002 to 0.01%. Steelless pipe.
[4] The high-strength stainless seamless steel pipe for oil wells according to any one of [1] to [3], wherein the average old austenite particle size is 40 μm or less.

According to the present invention, for oil wells having excellent hot workability, excellent carbon dioxide corrosion resistance, excellent SSC resistance in a low temperature environment, and a high yield strength of YS: 655 MPa or more. A high-strength stainless seamless steel pipe can be obtained.

First, the reason for limiting the composition of the high-strength seamless steel pipe for oil wells of the present invention will be described. Hereinafter, unless otherwise specified, mass% is simply referred to as%.

C: 0.002-0.05%
C is an important element that increases the strength of martensitic stainless steel. In the present invention, it is necessary to contain 0.002% or more of C in order to secure the desired strength. On the other hand, if C is contained in excess of 0.05%, the strength is rather lowered. In addition, SSC resistance in a low temperature environment also deteriorates. Therefore, in the present invention, the C content is 0.002 to 0.05%. From the viewpoint of carbon dioxide corrosion resistance, the C content is preferably 0.03% or less. More preferably, it is 0.002% or more, and more preferably 0.015% or less. More preferably, it is 0.002% or more, and further preferably 0.010% or less.

Si: 0.05-0.50%
Si is an element that acts as an antacid. This effect is obtained with a Si content of 0.05% or more. On the other hand, if the content of Si exceeds 0.50%, the hot workability is lowered and the carbon dioxide corrosion resistance is lowered. Therefore, the Si content is set to 0.05 to 0.50%. Preferably, the Si content is 0.10% or more, preferably 0.40% or less. More preferably, it is 0.10% or more, and more preferably 0.30% or less.

Mn: 0.04 to 1.80%
Mn is an element that suppresses the formation of δ ferrite during hot working and improves hot workability, and the present invention requires the content of Mn to be 0.04% or more. On the other hand, excessive content adversely affects toughness and SSC resistance in low temperature environments. Therefore, the Mn content is in the range of 0.04 to 1.80%. Preferably, the Mn content is 0.04% or more, preferably 0.80% or less. It is more preferably 0.05% or more, more preferably 0.50% or less, still more preferably 0.05% or more, still more preferably 0.26% or less.

P: 0.030% or less P is an element that reduces carbon dioxide corrosion resistance, pitting corrosion resistance, and SSC resistance, and it is preferable to reduce it as much as possible in the present invention, but an extreme reduction causes an increase in manufacturing cost. .. Therefore, the P content is set to 0.030% or less as a range that can be industrially carried out at a relatively low cost without causing an extreme deterioration in characteristics. Preferably, the P content is 0.020% or less.

S: 0.002% or less S significantly reduces hot workability, and S deteriorates SSC resistance in a low temperature environment due to segregation to the old austenite grain boundaries and formation of Ca-based inclusions, so it is reduced as much as possible. It is preferable to do so. When the S content is 0.002% or less, the number density of Ca-based inclusions can be reduced, the segregation of S into the former austenite grain boundaries can be suppressed, and the desired SSC resistance can be obtained. Therefore, the S content is set to 0.002% or less. Preferably, the S content is 0.0015% or less.

Cr: 11.0 to 14.0%
Cr is an element that forms a protective film and contributes to the improvement of corrosion resistance, and in the present invention, the content of Cr is required to be 11.0% or more in order to secure the corrosion resistance at high temperature. On the other hand, if the content of Cr exceeds 14.0%, the stability of the martensite phase is lowered and the desired strength cannot be obtained by facilitating the formation of residual autostate without transforming the martensite. Therefore, the Cr content is set to 11.0 to 14.0%. Preferably, the Cr content is 11.5% or more, preferably 13.5% or less, more preferably 12.0% or more, and even more preferably 13.0% or less.

Ni: 3.0-6.5%
Ni is an element that has the effect of strengthening the protective film and improving corrosion resistance. In addition, Ni dissolves in solid solution to increase the strength of steel. Such an effect can be obtained with a Ni content of 3.0% or more. On the other hand, if the content of Ni exceeds 6.5%, the stability of the martensite phase is lowered and the strength is lowered by facilitating the formation of residual austate without transforming the martensite. Therefore, the Ni content is set to 3.0 to 6.5%. Preferably, the Ni content is 5.0% or more, preferably 6.0% or less.

Mo: 0.5-3.0%
Mo is ^{an element that increases resistance to pitting corrosion due to Cl −} and low pH, and the present invention requires a content of Mo of 0.5% or more. A Mo content of less than 0.5% reduces corrosion resistance in harsh corrosive environments. On the other hand, if the Mo content exceeds 3.0%, δ ferrite is generated, which causes deterioration of hot workability and corrosion resistance. Therefore, the Mo content is set to 0.5 to 3.0%. Preferably, the Mo content is 0.5% or more, preferably 2.5% or less. More preferably, it is 1.5% or more, and more preferably 2.3% or less.

Al: 0.005 to 0.10%
Al is an element that acts as an antacid. This effect is obtained by containing 0.005% or more of Al. On the other hand, if Al is contained in excess of 0.10%, the amount of oxide becomes too large and the toughness is adversely affected. Therefore, the Al content is set to 0.005 to 0.10%. Preferably, the Al content is 0.01% or more, preferably 0.03% or less.

V: 0.005 to 0.20%
V is an element that improves the strength of steel by strengthening precipitation. This effect is obtained by containing 0.005% or more of V. On the other hand, even if V is contained in excess of 0.20%, the low temperature toughness is lowered. Therefore, the V content is set to 0.005 to 0.20%. Preferably, the V content is 0.03% or more, preferably 0.08% or less.

Ti: 0.01-0.20%
Ti is an element that forms TiN, which improves SSC resistance in low temperature environments by covering oxide-based or sulfide-based inclusions. For such an effect, it is necessary to contain 0.01% or more of Ti. On the other hand, even if Ti is contained in excess of 0.20%, the effect is saturated. Therefore, the Ti content is set to 0.01 to 0.20%. Preferably, the Ti content is 0.03% or more, preferably 0.20% or less. More preferably, it is 0.05% or more, and more preferably 0.15% or less.

Co: 0.01-1.0%
Co is an element that increases the Ms point to reduce the retained austenite fraction and improve strength and SSC resistance. Such an effect can be obtained by containing 0.01% or more of Co. On the other hand, even if Co is contained in excess of 1.0%, the hot workability is lowered. Therefore, the Co content is set to 0.01 to 1.0%. Preferably, the Co content is 0.05% or more, preferably 0.15% or less. More preferably, the Co content is 0.05% or more, and more preferably 0.09% or less.

N: 0.002 to 0.15%
N is an element that significantly improves pitting corrosion resistance. This effect is obtained with an N content of 0.002% or more. On the other hand, if N is contained in excess of 0.15%, the low temperature toughness decreases. Therefore, the N content is set to 0.002 to 0.15%. Preferably, the N content is 0.002% or more, preferably 0.015% or less, more preferably the N content is 0.003% or more, and more preferably 0.008% or less.

O (oxygen): 0.010% or less O (oxygen) exists as an oxide in steel and adversely affects various properties. Therefore, it is desirable to reduce O as much as possible. In particular, when the O content exceeds 0.010%, both hot workability and SSC resistance at low temperatures are significantly reduced. Therefore, the O content should be 0.010% or less. Preferably, the O content is 0.006% or less. More preferably, the O content is 0.004% or less.

Further, in the present invention, Cr, Ni, Mo, Cu and C are contained within the above range and so as to satisfy the following equation (1).
Cr + 0.65Ni + 0.6Mo + 0.55Cu-20C ≧ 15.0 ‥‥ (1)
(Here, Cr, Ni, Mo, Cu, C: the content (mass%) of each element, and the element not contained is zero.)
When the lvalue of Eq. (1) is less than 15.0, the carbon dioxide gas corrosiveness in a high temperature corrosive environment containing _{CO 2} and Cl ^{− decreases at a high temperature of 150 ° C. or higher.} Therefore, in the present invention, Cr, Ni, Mo, Cu, and C are contained so as to satisfy the equation (1).

Further, in the present invention, Cr, Mo, Si, C, Mn, Ni, Cu and N are contained so as to satisfy the following equation (2).
Cr ＋ Mo ＋ 0.3Si-43.3C－0.4Mn－Ni－0.3Cu-9N ≦ 11.0 ‥‥‥ (2)
(Here, Cr, Mo, Si, C, Mn, Ni, Cu, N: the content (mass%) of each element, and the element not contained is zero.)
If the lvalue of Eq. (2) exceeds 11.0, the hot workability necessary and sufficient for forming a stainless seamless steel pipe cannot be obtained, and the manufacturability of the steel pipe deteriorates. Therefore, in the present invention, Cr, Mo, Si, C, Mn, Ni, Cu, and N are contained so as to satisfy the equation (2).

Further, in the present invention, the number density of inclusions having a major axis of 5 μm or more and 0.5 <Ti / (Ti + Al + Mg + Ca) <1.0 is 0.5 pieces / mm ² or more and 3 pieces / mm ² or less. When the major axis is 5 μm or more and the number density of inclusions with 0.5 <Ti / (Ti + Al + Mg + Ca) <1.0 is ^{less than 0.5 pieces / mm 2} , the amount of inclusions not covered by TiN increases and SSC Since pitting corrosion is the starting point of the above, the desired SSC resistance in a low temperature environment cannot be obtained. On the other hand, when the number density of inclusions with 0.5 <Ti / (Ti + Al + Mg + Ca) <1.0 is ^{higher than 3 pieces / mm 2} , the size of inclusions increases as the number density of inclusions increases, and conversely. It becomes the starting point of pitting corrosion, and the desired SSC resistance in a low temperature environment cannot be obtained. In addition, in 0.5 <Ti / (Ti + Al + Mg + Ca) <1.0, Ti, Al, Mg, Ca: the content (mass%) of each element in the inclusions, and the elements not contained are set to zero. ..

Also, the reason why inclusions with a major axis of 5 μm or more are targeted is that inclusions with a major axis of 5 μm or more are likely to be the starting point of pitting corrosion.

In the present invention, the balance other than the above-mentioned components consists of Fe and unavoidable impurities.

The above-mentioned components are the basic components, but in addition to these basic compositions, one or more selected from Cu: 0.05 to 3.0% and W: 0.05 to 3.0% as a selection element, if necessary, or Two kinds can be contained. Furthermore, Nb: 0.01 to 0.20%, Zr: 0.01 to 0.20%, B: 0.0005 to 0.01%, REM: 0.0005 to 0.01%, Ca: 0.0005 to 0.0025%, Sn: 0.02 to 0.20%, Ta: 0.01 to 0.1% , Mg: One or more selected from 0.002 to 0.01% may be contained.

Cu: 0.05-3.0%
Cu is an element that strengthens the protective film and enhances corrosion resistance, and can be contained as needed. Such an effect can be obtained by containing 0.05% or more of Cu. On the other hand, if the content of Cu exceeds 3.0%, the grain boundary of CuS is precipitated and the hot workability is lowered. Therefore, when Cu is contained, the Cu content is set to 0.05 to 3.0%. Preferably, the Cu content is 0.5% or more, preferably 2.5% or less. More preferably, the Cu content is 0.5% or more, and more preferably 1.1% or less.

W: 0.05-3.0%
W is an element that contributes to increasing the strength and can be contained as needed. Such an effect can be obtained by containing 0.05% or more of W. On the other hand, even if W is contained in excess of 3.0%, the effect is saturated. Therefore, when W is contained, the W content is set to 0.05 to 3.0%. Preferably, the W content is 0.5% or more, preferably 1.5% or less.

Nb: 0.01-0.20%
Nb is an element that enhances strength and can be contained as needed. Such an effect can be obtained by containing 0.01% or more of Nb. On the other hand, even if Nb is contained in excess of 0.20%, the effect is saturated. Therefore, when Nb is contained, the Nb content is set to 0.01 to 0.20%. Preferably, the Nb content is 0.05% or more, preferably 0.15% or less. More preferably, it is 0.07% or more, and more preferably 0.13% or less.

Zr: 0.01-0.20%
Zr is an element that contributes to increasing strength and can be contained as needed. Such an effect can be obtained by containing 0.01% or more of Zr. On the other hand, even if Zr is contained in excess of 0.20%, the effect is saturated. Therefore, when Zr is contained, the Zr content is set to 0.01 to 0.20%.

B: 0.0005-0.01%
B is an element that contributes to the increase in strength and can be contained as needed. Such an effect can be obtained by containing 0.0005% or more of B. On the other hand, if B is contained in excess of 0.01%, the hot workability is lowered. Therefore, when B is contained, the B content is set to 0.0005 to 0.01%.

REM: 0.0005-0.01%
REM is an element that contributes to the improvement of corrosion resistance and can be contained as needed. Such an effect can be obtained by containing 0.0005% or more of REM. On the other hand, even if REM is contained in excess of 0.01%, the effect is saturated and the effect commensurate with the content cannot be expected, which is economically disadvantageous. Therefore, when REM is contained, the REM content is set to 0.0005 to 0.01%.

Ca: 0.0005-0.0025%
Ca is an element that contributes to the improvement of hot workability and can be contained as needed. Such an effect can be obtained by containing 0.0005% or more of Ca. On the other hand, if Ca is contained in an amount of more than 0.0025%, the number density of coarse Ca-based inclusions increases, and the desired SSC resistance in a low temperature environment cannot be obtained. Therefore, when Ca is contained, the Ca content is set to 0.0005 to 0.0025%.

Sn: 0.02 to 0.20%
Sn is an element that contributes to the improvement of corrosion resistance and can be contained as needed. Such an effect can be obtained by containing 0.02% or more of Sn. On the other hand, even if Sn is contained in excess of 0.20%, the effect is saturated and the effect commensurate with the content cannot be expected, which is economically disadvantageous. Therefore, when Sn is contained, the Sn content is set to 0.02 to 0.20%.

Ta: 0.01-0.1%
Ta is an element that increases strength and also has the effect of improving sulfide stress cracking resistance. In addition, Ta is an element that has the same effect as Nb, and a part of Nb can be replaced with Ta. Such an effect can be obtained by containing 0.01% or more of Ta. On the other hand, if Ta is contained in excess of 0.1%, the toughness decreases. Therefore, when Ta is contained, the Ta content is set to 0.01 to 0.1%.

Mg: 0.002-0.01%
Mg is an element that improves corrosion resistance and can be contained as needed. Such an effect can be obtained by containing 0.002% or more of Mg. On the other hand, even if Mg is contained in excess of 0.01%, the effect is saturated and the effect commensurate with the content cannot be expected. Therefore, when Mg is contained, the Mg content is set to 0.002 to 0.01%.

In the high-strength stainless seamless steel pipe for oil wells of the present invention, the martensite phase (tempered martensite phase) is used as the main phase in order to secure the desired strength. The remainder other than the main phase contains at least one of a retained austenite phase and a ferrite phase. Here, the main phase means a volume ratio (area ratio) of 45% or more.

Further, in the present invention, it is preferable that the average austenite particle size is 40 μm or less from the viewpoint of obtaining the desired SSC resistance in a low temperature environment.

The number densities of inclusions having a major axis of 5 μm or more and 0.5 <Ti / (Ti + Al + Mg + Ca) <1.0 in the present invention, and the average austenite particle size are described in Examples described later. It can be measured by the method.

Next, a preferable manufacturing method of the high-strength stainless seamless steel pipe for oil wells of the present invention will be described.

In the present invention, a steel pipe material having the above composition is used as a starting material. The method for producing the steel pipe material as the starting material does not need to be particularly limited, and any of the generally known methods for producing the seamless steel pipe can be applied. It is preferable that the molten steel having the above composition is melted by a usual melting method such as a converter and used as a steel pipe material such as a billet by a usual method such as a continuous casting method, an ingot-incubation rolling method or the like. The number of inclusions with 0.5 <Ti / (Ti + Al + Mg + Ca) <1.0 described above is, for example, the amount of oxygen measured online in the steelmaking process, and the amount of Ti and N added according to the value. It can be controlled to a desired value by changing it.

Then, these steel pipe materials are heated and hot-processed by using a pipe-making process of a Mannesmann-Plug mill method or a Mannesmann-Mandrell mill method, which is a generally known pipe-making method, and the above-mentioned desired dimensions are obtained. A seamless steel pipe having the above composition. In addition, a seamless steel pipe may be obtained by hot extrusion by a press method. The seamless steel pipe after pipe formation is preferably cooled to room temperature at a cooling rate equal to or higher than air cooling. As a result, a steel pipe structure having the martensite phase as the main phase can be secured. In order to reduce the average old austenite grain size, it is preferable to make the pipe under the condition that (cross-sectional area of steel pipe after pipe making) / (cross-sectional area of steel pipe material) is 0.20 or less. Further, it is preferable to make the pipe under the condition that (cross-sectional area of the steel pipe after pipe making) / (cross-sectional area of the steel pipe after drilling) is 0.40 or less.

Following cooling to cool to room temperature at a cooling rate equal to or higher than air cooling after pipe formation, in the present invention, the steel pipe is further _{reheated to a temperature of Ac 3} transformation point or higher, preferably 800 ° C. or higher, and held for 5 minutes or longer. Then, a quenching process is performed in which the temperature is cooled to 100 ° C. or lower at a cooling rate equal to or higher than air cooling. As a result, the martensite phase can be made finer and stronger. The heating temperature of the quenching treatment is preferably 800 to 950 ° C. from the viewpoint of preventing coarsening of the structure.

Also, here, the "cooling rate of air cooling or higher" is 0.01 ° C / s or higher.

The hardened steel pipe is then tempered. The tempering treatment is a treatment of heating to a temperature of 500 ° C. or higher and lower than the Ac ₁ transformation point (tempering temperature), holding for a predetermined time, preferably 10 minutes or longer, and then air-cooling. When the tempering temperature becomes equal to _{or higher than the Ac 1} transformation point, the fresh martensite phase is precipitated after the tempering, and the desired high strength cannot be secured. Therefore, it is more preferable that the tempering temperature is 500 ° C. or higher _{and lower than the Ac 1 transformation point.} As a result, the structure becomes a structure having the tempered martensite phase as the main phase, and becomes a seamless steel pipe having a desired strength and a desired corrosion resistance.

Also, from the viewpoint of reducing the average old austenite particle size, it is desirable to repeat quenching and tempering twice or more.

The above Ac ₃ transformation point and Ac ₁ transformation point are actually measured from the change in expansion coefficient (linear expansion coefficient) when the test piece (φ3 mm × L10 mm) is heated and cooled at a speed of 15 ° C / min. Let it be a value.

Up to this point, the seamless steel pipe has been described as an example, but the present invention is not limited to this. Using the steel pipe material having the above composition, it is also possible to manufacture an electrosewn steel pipe and a UOE steel pipe according to a normal process to obtain a steel pipe for an oil well.

Hereinafter, the present invention will be described based on further examples.

The molten steel with the composition shown in Table 1 is melted, cast into a steel pipe material, piped by hot working using a model seamless rolling mill, air-cooled after pipe making, and seamless with an outer diameter of 83.8 mm and a wall thickness of 12.7 mm. It was a steel pipe. In addition, steel pipe No. For No. 13, the amount of oxygen is adjusted online in the steelmaking process so that the number of inclusions with a major axis of 5 μm or more and 0.5 <Ti / (Ti + Al + Mg + Ca) <1.0 ^{exceeds 3 / mm 2.} It was controlled by changing the amount of Ti and N added according to the value. In addition, the steel pipe No. For 14, the amount of oxygen is online in the steelmaking process so that the major axis is 5 μm or more and the number of inclusions with 0.5 <Ti / (Ti + Al + Mg + Ca) <1.0 is ^{less than 0.5 / mm 2.} It was controlled by changing the amount of Ti and N added according to the value.

Next, the test piece material was cut out from the obtained seamless steel pipe, heated at the heating temperature (reheating temperature) and soaking time shown in Table 2, and then air-cooled at the cooling stop temperature shown in Table 2. did. Then, a tempering treatment was further performed in which the material was heated at the tempering temperature and soaking time shown in Table 2 and air-cooled.

In addition, API (American Petroleum Institute) arc-shaped tensile test pieces are collected from the hardened-tempered test piece material, and a tensile test is conducted in accordance with the API regulations. Tensile characteristics (yield strength YS, Tensile strength TS) was determined. Those with a yield strength of YS of 655 MPa or more were accepted, and those with a yield strength of less than 655 MPa were rejected.

Furthermore, a corrosion test piece having a thickness of 3 mm, a width of 30 mm, and a length of 40 mm was prepared by machining from the material of the test piece that had been subjected to quenching-tempering treatment, and a corrosion test was carried out.

The corrosion test was carried out by immersing the test piece in a test solution held in an autoclave: a 20 mass% NaCl aqueous solution (liquid temperature: 150 ° C., CO ₂ gas atmosphere at 10 atm), and the immersion period was 14 days. .. The weight of the test piece after the test was measured, and the corrosion rate calculated from the weight loss before and after the corrosion test was determined. Those with a corrosion rate of 0.125 mm / y or less were accepted, and those with a corrosion rate of more than 0.125 mm / y were rejected.

In addition, the presence or absence of pitting corrosion on the surface of the test piece was observed using a loupe with a magnification of 10 times for the test piece after the corrosion test. Note that pitting corrosion refers to the case where the diameter is 0.2 mm or more. Those without pitting corrosion were accepted, and those with pitting corrosion were rejected.

The SSC test was performed in accordance with NACE TM0177 Method A. The test environment used was an aqueous solution adjusted to pH: 4.5 by adding sodium acetate + hydrochloric acid to a 25 mass% NaCl aqueous solution (liquid temperature: 4 ° C, H ₂ S: 0.1 bar, CO _{2: 0.9 bar).} The test was carried out with the immersion time as 720 hours and 90% of the yield stress as the load stress. The case where no cracks occurred in the test piece after the test was regarded as acceptable (in Table 3: none), and the case where cracks occurred was regarded as rejected (in Table 3: yes).

To evaluate the hot workability, a round bar-shaped smoothing test piece with a parallel part diameter of 10 mm was used, heated to 1250 ° C with a gleeble tester, held for 100 seconds, and then cooled to 1000 ° C at 1 ° C / sec. After holding for 10 seconds, it was pulled until it broke, and the cross-sectional reduction rate was measured. When the cross-sectional reduction rate was 70% or more, it was considered to have excellent hot workability and passed. If the cross-sectional reduction rate was less than 70%, it was rejected.

The number of inclusions is 1/4 of the wall thickness from the outer surface of the pipe as a scanning electron microscope (SEM) sample with a cross section orthogonal to the pipe wall thickness direction from any one location in the circumferential direction of the steel pipe pipe end, 3 / An area of ^{500 mm 2} was sampled from position 4. For each sample collected, inclusions were identified by SEM observation, and the chemical composition was analyzed by the characteristic X-ray analyzer attached to the SEM. The inclusions with 0.5 <Ti / (Ti + Al + Mg + Ca) <1.0 were calculated, and the number of inclusions per unit area was calculated. In addition, inclusions having a major axis of 5 μm or more are determined by binarizing the contrast of the reflected electron image of the scanning electron microscope to define the outer peripheral portion of the inclusions and measuring the major axis from the outer peripheral portion of the inclusions. went.

The average old austenite particle size measurement sample was taken from the outer surface of the pipe in a cross section orthogonal to the longitudinal direction of the pipe from any one location in the circumferential direction of the end of the steel pipe from a position of 1/2 the wall thickness. After observing the collected samples with EBSD, the old austenite grains were reconstructed from the observation data of the EBSD using the inverse analysis software of the old austenite grains. For the obtained reconstructed image of austenite grains, three straight lines of 300 μm were drawn in the circumferential direction of the tube at intervals of 500 μm, and the average austenite particle size was measured by a cutting method.

The results obtained are shown in Table 3.

All of the examples of the present invention have a yield strength of YS: 655 MPa or more and are excellent in hot workability, and also have excellent corrosion resistance (carbon dioxide gas corrosive resistance) in a high temperature corrosive environment of 150 ° C. or more containing _{CO 2} and Cl ^−. Furthermore, it was excellent in SSC resistance in a low temperature environment, and the cross-sectional reduction rate was 70% or more. On the other hand, in the comparative example outside the scope of the present invention, at least one of yield strength YS, hot workability, SSC resistance in a low temperature environment, corrosion rate, and cross-sectional reduction rate could not be obtained.

Claims

By mass%
C: 0.002-0.05%, Si: 0.05-0.50%,
Mn: 0.04 to 1.80%, P: 0.030% or less,
S: 0.002% or less, Cr: 11.0 to 14.0%,
Ni: 3.0-6.5%, Mo: 0.5-3.0%,
Al: 0.005 to 0.10%, V: 0.005 to 0.20%,
Ti: 0.01-0.20%, Co: 0.01-1.0%,
It contains N: 0.002 to 0.15%, O: 0.010% or less, satisfies the following formulas (1) and (2), and has a composition consisting of the balance Fe and unavoidable impurities.
The major axis is 5 μm or more, 0.5 <Ti / (Ti + Al + Mg + Ca) <1.0, the number density of inclusions is 0.5 / mm 2 or more and 3 / mm 2 or less, and the yield strength is 655 MPa or more. High-strength stainless seamless steel pipe for oil wells.
(Here, in 0.5 <Ti / (Ti + Al + Mg + Ca) <1.0, Ti, Al, Mg, Ca: the content (mass%) of each element in the inclusions, and the elements not contained are zero. )
Note Cr + 0.65Ni + 0.6Mo + 0.55Cu-20C ≧ 15.0 ‥‥ (1)
Cr ＋ Mo ＋ 0.3Si-43.3C－0.4Mn－Ni－0.3Cu-9N ≦ 11.0 ‥‥‥ (2)
Here, Cr, Ni, Mo, Cu, C, Si, Mn, N: the content (mass%) of each element, and the element not contained is zero.
The high-strength stainless steel seam for oil wells according to claim 1, further containing one or two selected from Cu: 0.05 to 3.0% and W: 0.05 to 3.0% in mass% in addition to the above composition. Steel pipe.
In addition to the above composition, in mass%, Nb: 0.01 to 0.20%,
Zr: 0.01-0.20%,
B: 0.0005-0.01%,
REM: 0.0005-0.01%,
Ca: 0.0005-0.0025%,
Sn: 0.02 to 0.20%,
Ta: 0.01-0.1%,
The high-strength stainless seamless steel pipe for oil wells according to claim 1 or 2, which contains one or more selected from 0.002 to 0.01% of Mg.
The high-strength stainless seamless steel pipe for oil wells according to any one of claims 1 to 3, wherein the average old austenite particle size is 40 μm or less.