CN114829647A

CN114829647A - High-strength stainless steel seamless steel pipe for oil well

Info

Publication number: CN114829647A
Application number: CN202080087895.0A
Authority: CN
Inventors: 江口健一郎; 村井刚; 远藤麻美; 柚贺正雄
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2019-12-24
Filing date: 2020-11-20
Publication date: 2022-07-29
Also published as: US20230033540A1; MX2022007286A; AR120872A1; BR112022011761A2; EP4043591A4; WO2021131445A1; JP6950851B1; JPWO2021131445A1; EP4043591A1

Abstract

The purpose of the present invention is to provide a high-strength seamless stainless steel pipe for oil wells, which has high strength, excellent hot workability, excellent carbon dioxide corrosion resistance, and excellent SSC resistance in a low-temperature environment. A high-strength stainless steel seamless steel pipe for oil wells, which has a composition containing specific components, satisfying the following formulae (1) and (2), and the balance being Fe and unavoidable impurities, and has a major axis of 5 [ mu ] m or more and 0.5<Ti/(Ti+Al+Mg+Ca)<1.0 of inclusionsThe number density is 0.5 pieces/mm ² Above and 3 pieces/mm ² Hereinafter, the yield strength of the high-strength stainless steel seamless steel pipe for oil wells is 655MPa or more. (wherein, 0.5<Ti/(Ti+Al+Mg+Ca)<1.0 the contents (mass%) of Ti, Al, Mg and Ca in the inclusions were zero. ) Cr +0.65Ni +0.6Mo +0.55Cu-20C is more than or equal to 15.0 … … (1) Cr + Mo +0.3Si-43.3C-0.4Mn-Ni-0.3Cu-9N is more than or equal to 11.0 … … (2), wherein Cr, Ni, Mo, Cu, C, Si, Mn and N are the content (mass%) of each element, and the content of the elements is zero.

Description

High-strength stainless steel seamless steel pipe for oil well

Technical Field

The present invention relates to a stainless seamless steel pipe suitable for use in an oil well, a gas well (hereinafter simply referred to as an oil well), or the like of crude oil or natural gas. In particular to the use of carbon dioxide (CO) ₂ ) Chloride ion (Cl) ^- ) And excellent in carbon dioxide corrosion resistance in a severe corrosive environment at a temperature of 150 ℃ or higher and SSC resistance in a low-temperature environment.

Background

In recent years, from the viewpoint of the rising price of crude oil and the expected depletion of oil resources in the near future, development of deep oil fields, oil fields and gas fields in so-called severe corrosive environments containing hydrogen sulfide and the like, which have not been conventionally found, has been actively conducted. Such oil and gas fields are usually extremely deep, and their atmosphere is also high in temperature and contains CO ₂ 、Cl ^- And further contains H ₂ A severe corrosive environment of S. Oil well steel pipes used in such environments are required to have a desired high strength and to have excellent corrosion resistance.

Heretofore, carbon dioxide (CO) has been contained ₂ ) Chloride ion (Cl) ^- ) In oil and gas fields in such environments, 13Cr martensitic stainless steel is often used as an oil well pipe used for miningAnd (5) steel pipes. Recently, the use of improved 13Cr martensitic stainless steels, which have a reduced C content and increased component systems such as Ni and Mo, has also been increasing.

For example, patent document 1 describes a martensitic stainless steel having a yield strength of 758 to 862MPa, which contains, in mass%, C: 0.010-0.030%, Mn: 0.30-0.60%, P: 0.040% or less, S: 0.0100% or less, Cr: 10.00-15.00%, Ni: 2.50 to 8.00%, Mo: 1.00-5.00%, Ti: 0.050 to 0.250%, V: 0.25% or less, N: 0.07% or less, and Si: 0.50% or less, Al: 0.10% or less, and the balance of Fe and impurities, and satisfies 6.0. ltoreq. Ti/C.ltoreq.10.1 as formula (1).

Further, patent document 2 discloses a method for producing a seamless martensitic stainless steel pipe, which comprises the steps of adding, in terms of weight%, C: less than or equal to 0.050, Si: less than or equal to 0.5, Mn: less than or equal to 1.5, P: less than or equal to 0.03, S: less than or equal to 0.005, Cr: 11.0 to 14.0, Ni: 4.0 to 7.0, Mo: 1.0 to 2.5, Cu: 1.0-2.5, Al: less than or equal to 0.05, N: 0.01 to 0.10, and the balance being Fe and unavoidable impurities, is hot worked, cooled to a temperature of not more than Ms point, and then subjected to heating to a temperature of not less than 550 ℃ and Ac at an average heating rate of 500 to T ℃ of not less than 1.0 ℃/sec ₁ And (4) a heat treatment in which the substrate is cooled to a temperature of not more than the Ms point after a temperature T of not more than the Ms point.

Further, patent document 3 discloses a high-strength martensitic stainless steel excellent in stress corrosion cracking resistance, which contains, in wt%, C: 0.06% or less, Cr: 12-16%, Si: 1.0% or less, Mn: 2.0% or less, Ni: 0.5 to 8.0%, Mo: 0.1-2.5%, Cu: 0.3-4.0%, N: 0.05% or less, the area ratio of the delta-ferrite phase is 10% or less, and fine precipitates of Cu are dispersed in the matrix.

Documents of the prior art

Patent document

Patent document 1: WO2008/023702

Patent document 2: japanese laid-open patent publication No. 9-170019

Patent document 3: japanese laid-open patent publication No. 7-166303

Disclosure of Invention

Problems to be solved by the invention

Recently, with the development of oil fields, gas fields, and the like in severe corrosive environments, oil well steel pipes are required to have high strength and high temperature of 150 ℃ or higher and to contain CO ₂ 、Cl ^- Has excellent carbon dioxide corrosion resistance under the severe corrosive environment. In addition, along with the increasing development environment, excellent SSC resistance is required even in a low-temperature environment such as deep sea.

However, the techniques described in patent documents 1 to 3 have high strength and excellent resistance to carbon dioxide corrosion, but have insufficient SSC resistance in a low-temperature environment.

Accordingly, an object of the present invention is to solve the above problems of the prior art and to provide a high-strength seamless stainless steel pipe for oil wells which has high strength, excellent hot workability, excellent carbon dioxide corrosion resistance, and excellent SSC resistance in a low-temperature environment.

The term "high strength" as used herein means a case where the steel sheet has a yield strength YS of 95ksi (655MPa) or more.

The excellent carbon dioxide corrosion resistance means that: test pieces were immersed in the test solution held in the autoclave: 20% by mass NaCl aqueous solution (liquid temperature: 150 ℃ C., 10 atmospheres CO) ₂ Gas atmosphere) and the etching rate is 0.125 mm/year or less when the immersion time is 14 days.

Further, "superior SSC resistance in a low-temperature environment" means that the test is performed as follows and no crack is generated in the test piece after the test: the test piece was immersed in the test solution: 25% by mass NaCl aqueous solution (liquid temperature: 4 ℃ C., H) ₂ S：0.1bar、CO ₂ : 0.9bar) was added to the aqueous solution, the pH was adjusted to 4.5, the immersion time was 720 hours, and 90% of the yield stress was applied as the load stress.

Means for solving the problems

The inventors of the present invention have achieved the above objectThe effect on low temperature SSC resistance was studied intensively for stainless steel pipes of various compositions. As a result, it was found that SSC of stainless steel starts from pitting corrosion. Next, as a result of studies on the occurrence of pitting corrosion, it was found that oxides or sulfides containing Al, Ca, Mg, or the like as a main component among various inclusions most easily become starting points of pitting corrosion in a low-temperature environment. Therefore, in order to improve the SSC resistance in a low-temperature environment, it is important to reduce oxide-based or sulfide-based inclusions mainly containing Al, Ca, Mg, and the like as much as possible. However, oxide-based inclusions and sulfide-based inclusions are generated from oxygen and sulfur contained as impurities in steel, and therefore, it is industrially impossible to reduce them to zero. Therefore, it is thought that the oxide-based inclusions and the sulfide-based inclusions are made to be harmless by changing their structures. Specifically, it was found that: by coating the inclusion, which is likely to form pitting corrosion, with TiN, the inclusion is less likely to serve as a starting point of pitting corrosion, and the SSC resistance in a low-temperature environment can be improved. The reason is considered to be that, by covering the inclusion with TiN, when the inclusion is dissolved, N ions are released into the solution, which become NH ³⁺ This locally raises the pH around the inclusions, thereby suppressing the occurrence and growth of pitting corrosion.

The inventors also investigated the effect of tissues on low temperature SSC resistance. As a result, it was found that when the prior austenite grain diameter is reduced in a low temperature environment, the growth of pitting and the generation of cracks are suppressed, and the SSC resistance is improved. This is considered to be because P, S segregated in the prior austenite grain boundary promotes (1) selective dissolution of the prior austenite grain boundary during the growth of the pitting corrosion and (2) embrittlement of the grain boundary during the intrusion of hydrogen into the steel. It is considered that when the prior austenite grain size is small, the grain boundary area per unit volume becomes large, and therefore the concentration of P, S segregated at the prior austenite grain boundary is reduced, and the SSC resistance is improved. The reason why the prior austenite grain boundary significantly affects the SSC resistance in a low-temperature environment is considered to be as follows: the solubility of hydrogen sulfide, which promotes the entry of hydrogen into steel, in the test solution increases; vaporization of hydrogen is suppressed due to a decrease in temperature.

The present invention has been completed based on the above findings and through further studies. That is, the gist of the present invention is as follows.

[1] A high-strength stainless steel seamless steel pipe for oil wells, which comprises, in mass%, C: 0.002-0.05%, Si: 0.05 to 0.50%, Mn: 0.04-1.80%, P: 0.030% or less, S: 0.002% or less, Cr: 11.0 to 14.0%, Ni: 3.0-6.5%, Mo: 0.5 to 3.0%, Al: 0.005-0.10%, V: 0.005-0.20%, Ti: 0.01-0.20%, Co: 0.01-1.0%, N: 0.002-0.15%, O: 0.010% or less, satisfying the following formulae (1) and (2), and the balance being Fe and unavoidable impurities,

the major diameter is more than 5 μm and 0.5<Ti/(Ti+Al+Mg+Ca)<1.0 number density of inclusions of 0.5 pieces/mm ² Above and 3 pieces/mm ² In the following, the following description is given,

the yield strength of the high-strength stainless steel seamless steel pipe for oil wells is 655MPa or more.

(wherein 0.5< Ti/(Ti + Al + Mg + Ca) <1.0 wherein Ti, Al, Mg and Ca represent the contents (mass%) of the respective elements in the inclusions, and zero is the content of the elements not contained.)

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)

Wherein Cr, Ni, Mo, Cu, C, Si, Mn, and N are the contents (mass%) of the respective elements, and the elements not contained are zero.

[2] The seamless steel pipe of high strength stainless steel for oil well according to [1], which further comprises, in mass%, a metal selected from the group consisting of Cu: 0.05-3.0%, W: 0.05-3.0% of one or two.

[3] The seamless steel pipe of high strength stainless steel for oil well according to [1] or [2], further comprising a component selected from the group consisting of Nb: 0.01 to 0.20%, Zr: 0.01-0.20%, 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: 0.01-0.1%, Mg: 0.002-0.01% of one or more than two.

[4] The high-strength seamless stainless steel pipe for oil wells according to any one of [1] to [3], wherein the average prior austenite grain diameter is 40 μm or less.

Effects of the invention

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

Detailed Description

First, the reasons for the limitation of the composition of the high-strength seamless steel pipe for oil wells of the present invention will be explained. Hereinafter, unless otherwise specified, mass% is abbreviated as%.

C：0.002～0.05％

C is an important element for increasing the strength of the martensitic stainless steel. In the present invention, it is necessary to contain 0.002% or more of C in order to secure a desired strength. On the other hand, when C is contained in an amount exceeding 0.05%, the strength is rather lowered. Further, the SSC resistance in a low-temperature environment is also deteriorated. Therefore, in the present invention, the C content is set to 0.002 to 0.05%. From the viewpoint of carbon dioxide corrosion resistance, the C content is preferably set to 0.03% or less. More preferably 0.002% or more, and still more preferably 0.015% or less. More preferably 0.002% or more, and still more preferably 0.010% or less.

Si：0.05～0.50％

Si is an element that functions as a deoxidizer. This effect can be obtained by containing 0.05% or more of Si. On the other hand, when Si is contained in an amount exceeding 0.50%, hot workability is lowered and carbon dioxide corrosion resistance is lowered. Therefore, the Si content is set to 0.05 to 0.50%. The Si content is preferably 0.10% or more, and preferably 0.40% or less. More preferably 0.10% or more, and still more preferably 0.30% or less.

Mn：0.04～1.80％

Mn is an element that suppresses the generation of δ ferrite during hot working and improves hot workability, and in the present invention, Mn needs to be contained by 0.04% or more. On the other hand, excessive content may adversely affect toughness and SSC resistance in a low-temperature environment. Therefore, the Mn content is set to be in the range of 0.04 to 1.80%. The Mn content is preferably 0.04% or more, and preferably 0.80% or less. More preferably 0.05% or more, more preferably 0.50% or less, still more preferably 0.05% or more, and still more preferably 0.26% or less.

P: less than 0.030%

P is an element that reduces all of the carbon dioxide corrosion resistance, pitting corrosion resistance, and SSC resistance, and is preferably reduced as much as possible in the present invention, but extreme reduction leads to an increase in production cost. Therefore, the P content is set to 0.030% or less as a range that can be industrially implemented at a low cost without causing an extreme decrease in the characteristics. The P content is preferably 0.020% or less.

S: less than 0.002%

S significantly reduces hot workability, and because S is segregated to the prior austenite grain boundary and forms Ca-based inclusions, it is preferable to reduce the amount as much as possible because it deteriorates SSC resistance in a low-temperature environment. When the S content is 0.002% or less, the number density of Ca-based inclusions is reduced, and segregation of S to prior austenite grain boundaries is suppressed, whereby a desired SSC resistance can be obtained. Therefore, the S content is set to 0.002% or less. The S content is preferably 0.0015% or less.

Cr：11.0～14.0％

Cr is an element contributing to improvement of corrosion resistance by forming a protective film, and in order to ensure corrosion resistance at high temperatures, it is necessary to contain 11.0% or more of Cr in the present invention. On the other hand, when Cr is contained in an amount exceeding 14.0%, martensitic transformation does not occur, and retained austenite is easily formed, whereby the stability of the martensite phase is lowered, and a desired strength cannot be obtained. Therefore, the Cr content is set to 11.0 to 14.0%. The Cr content is preferably 11.5% or more, preferably 13.5% or less, more preferably 12.0% or more, and more preferably 13.0% or less.

Ni：3.0～6.5％

Ni is an element that strengthens the protective film and improves corrosion resistance. Further, Ni is dissolved to increase the strength of the steel. Such an effect can be obtained by containing 3.0% or more of Ni. On the other hand, if Ni is contained in an amount exceeding 6.5%, martensitic transformation does not occur, and retained austenite is easily formed, whereby the stability of the martensitic phase is lowered and the strength is lowered. Therefore, the Ni content is set to 3.0 to 6.5%. The Ni content is preferably 5.0% or more, and preferably 6.0% or less.

Mo：0.5～3.0％

Mo is a hetero atom represented by formula Cl ^- And the element having increased resistance to pitting corrosion by low pH, it is necessary in the present invention to contain 0.5% or more of Mo. When Mo is contained in an amount of less than 0.5%, the corrosion resistance in a severe corrosive environment is lowered. On the other hand, when Mo is contained in an amount exceeding 3.0%, δ ferrite is generated, resulting in a decrease in hot workability and corrosion resistance. Therefore, the Mo content is set to 0.5 to 3.0%. The Mo content is preferably 0.5% or more, and preferably 2.5% or less. More preferably 1.5% or more, and still more preferably 2.3% or less.

Al：0.005～0.10％

Al is an element that functions as a deoxidizer. This effect can be obtained by containing 0.005% or more of Al. On the other hand, when Al is contained in an amount exceeding 0.10%, the amount of oxide becomes excessive, and the toughness is adversely affected. Therefore, the Al content is set to 0.005 to 0.10%. The Al content is preferably 0.01% or more, and preferably 0.03% or less.

V：0.005～0.20％

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

Ti：0.01～0.20％

Ti is an element that forms TiN and improves SSC resistance in a low-temperature environment by covering oxide-based or sulfide-based inclusions with the TiN. Such an effect requires that 0.01% or more of Ti be contained. On the other hand, even if more than 0.20% of Ti is contained, the effect is saturated. Therefore, the Ti content is set to 0.01 to 0.20%. The Ti content is preferably 0.03% or more, more preferably 0.20% or less. More preferably 0.05% or more, and still more preferably 0.15% or less.

Co：0.01～1.0％

Co is an element that increases the Ms point, decreases the retained austenite fraction, and improves the 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 an amount exceeding 1.0%, hot workability is lowered. Therefore, the Co content is set to 0.01 to 1.0%. The Co content is preferably 0.05% or more, and preferably 0.15% or less. The Co content is more preferably 0.05% or more, and still more preferably 0.09% or less.

N：0.002～0.15％

N is an element that significantly improves pitting corrosion resistance. This effect can be obtained by containing 0.002% or more of N. On the other hand, if N is contained in an amount exceeding 0.15%, the low-temperature toughness is lowered. Therefore, the N content is set to 0.002 to 0.15%. The N content is preferably 0.002% or more, preferably 0.015% or less, more preferably 0.003% or more, and still more preferably 0.008% or less.

O (oxygen): 0.010% or less

O (oxygen) exists in steel in the form of oxides, and adversely affects various properties. Therefore, O is preferably reduced as much as possible. In particular, when the O content exceeds 0.010%, both the hot workability and the SSC resistance at low temperature are remarkably reduced. Therefore, the O content is set to 0.010% or less. The O content is preferably 0.006% or less. More preferably, the O content is 0.004% or less.

In the present invention, Cr, Ni, Mo, Cu, and C are contained within the above range so as to satisfy the following expression (1).

Cr+0.65Ni+0.6Mo+0.55Cu-20C≥15.0……(1)

(wherein Cr, Ni, Mo, Cu and C are the contents (% by mass) of the respective elements, and the elements not contained are zero.)

(1) When the left value of the formula is less than 15.0, the temperature is high above 150 ℃ and the composition contains CO ₂ 、Cl ^- The carbon dioxide corrosion resistance in the high-temperature corrosion environment of (2) is lowered. Therefore, in the present invention, Cr, Ni, Mo, Cu, and C are contained so as to satisfy the formula (1).

In the present invention, Cr, Mo, Si, C, Mn, Ni, Cu, and N are contained so as to satisfy the following expression (2).

Cr+Mo+0.3Si-43.3C-0.4Mn-Ni-0.3Cu-9N≤11.0……(2)

(wherein Cr, Mo, Si, C, Mn, Ni, Cu and N are the contents (mass%) of the respective elements, and the elements not contained are zero.)

(2) If the left value of the formula exceeds 11.0, sufficient hot workability required for producing a stainless seamless steel pipe cannot be obtained, and the productivity of the steel pipe is lowered. Therefore, in the present invention, Cr, Mo, Si, C, Mn, Ni, Cu, and N are contained so as to satisfy the formula (2).

In the present invention, the major axis is 5 μm or more and 0.5<Ti/(Ti+Al+Mg+Ca)<The number density of inclusions of 1.0 was set to 0.5 pieces/mm ² Above and 3 pieces/mm ² The following. The major diameter is more than 5 μm and 0.5<Ti/(Ti+Al+Mg+Ca)<1.0 number density of inclusions less than 0.5/mm ² In the case, the amount of inclusions not covered with TiN increases, and pitting corrosion is formed as a starting point of SSC, and therefore, desired SSC resistance in a low-temperature environment cannot be obtained. On the other hand, 0.5<Ti/(Ti+Al+Mg+Ca)<1.0 number density of inclusions more than 3/mm ² In this case, as the number density of inclusions increases, the size of inclusions also increases, which becomes a starting point of pitting corrosion, and the desired SSC resistance in a low-temperature environment cannot be obtained. Note that, 0.5<Ti/(Ti+Al+Mg+Ca)<1.0 the contents (mass%) of Ti, Al, Mg and Ca in the inclusions were zero.

Further, the inclusion having a major axis of 5 μm or more is targeted for the reason that the inclusion having a major axis of 5 μm or more easily becomes a starting point of pitting corrosion.

In the present invention, the balance other than the above components is composed of Fe and inevitable impurities.

The above components are essential components, and in addition to these essential components, may further contain, as necessary, a component selected from the group consisting of Cu: 0.05-3.0%, W: 0.05-3.0% of one or two of the above elements as optional elements. May further comprise one or more elements selected from Nb: 0.01 to 0.20%, Zr: 0.01-0.20%, 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: 0.01-0.1%, Mg: 0.002-0.01% of one or more than two.

Cu：0.05～3.0％

Cu is an element for strengthening the protective film to improve corrosion resistance, and may be contained as necessary. Such an effect can be obtained by containing 0.05% or more of Cu. On the other hand, when Cu is contained in an amount exceeding 3.0%, grain boundaries of CuS precipitate, and hot workability deteriorates. Therefore, when Cu is contained, the Cu content is set to 0.05 to 3.0%. The Cu content is preferably 0.5% or more, and preferably 2.5% or less. The Cu content is more preferably 0.5% or more, and still more preferably 1.1% or less.

W：0.05～3.0％

W is an element contributing to an increase in strength, and may be contained as necessary. Such an effect can be obtained by containing 0.05% or more of W. On the other hand, even if W is contained in an amount exceeding 3.0%, the effect is saturated. Therefore, when W is contained, the W content is set to 0.05 to 3.0%. The W content is preferably 0.5% or more, preferably 1.5% or less.

Nb：0.01～0.20％

Nb is an element for improving strength, and may be contained as necessary. 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%. The Nb content is preferably 0.05% or more, and preferably 0.15% or less. More preferably 0.07% or more, and still more preferably 0.13% or less.

Zr：0.01～0.20％

Zr is an element contributing to increase in strength, and may be contained as necessary. Such an effect can be obtained by containing 0.01% or more of Zr. On the other hand, even if more than 0.20% of Zr is contained, 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 contributing to increase in strength, and may be contained as necessary. Such an effect can be obtained by containing 0.0005% or more of B. On the other hand, if B is contained in an amount exceeding 0.01%, hot workability is deteriorated. Therefore, when B is contained, the content of B is set to 0.0005 to 0.01%.

REM：0.0005～0.01％

REM is an element contributing to improvement of corrosion resistance, and may be contained as necessary. Such an effect can be contained by containing REM in an amount of 0.0005% or more. On the other hand, even if REM is contained in an amount exceeding 0.01%, the effect is saturated, and the effect matching 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 contributing to improvement of hot workability, and may be contained as necessary. Such an effect can be obtained by containing 0.0005% or more of Ca. On the other hand, when Ca is contained in an amount exceeding 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 content of Ca is set to 0.0005 to 0.0025%.

Sn：0.02～0.20％

Sn is an element contributing to improvement of corrosion resistance, and may be contained as necessary. Such an effect can be obtained by containing 0.02% or more of Sn. On the other hand, even if Sn is contained in an amount exceeding 0.20%, the effect is saturated, and the effect matching 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. Ta is an element that brings about the same effect as Nb, and a part of Nb may be replaced with Ta. Such an effect can be obtained by containing 0.01% or more of Ta. On the other hand, if more than 0.1% of Ta is contained, 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 for improving corrosion resistance, and may be contained as necessary. Such an effect can be obtained by containing 0.002% or more of Mg. On the other hand, even if Mg is contained in an amount exceeding 0.01%, the effect is saturated, and the effect matching the content cannot be expected. Therefore, when Mg is contained, the Mg content is set to 0.002 to 0.01%.

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

In the present invention, it is preferable that the average prior austenite grain diameter is 40 μm or less from the viewpoint of obtaining the desired SSC resistance in a low-temperature environment.

The number density and the average prior austenite grain diameter of the inclusions having a major axis of 5 μm or more and 0.5< Ti/(Ti + Al + Mg + Ca) <1.0 in the present invention can be measured by the methods described in the examples described later.

Next, a preferred method for producing the high-strength stainless steel seamless pipe for oil wells according to the present invention will be described.

In the present invention, a steel pipe material having the above-described composition is used as a starting material. The method for producing the steel pipe material as the starting material is not particularly limited, and any generally known method for producing a seamless steel pipe can be applied. The molten steel having the above composition is preferably smelted by a usual smelting method such as a converter, and made into a steel pipe material such as a billet by a usual method such as a continuous casting method or an ingot-cogging rolling method. The number of inclusions of 0.5< Ti/(Ti + Al + Mg + Ca) <1.0 can be controlled to a desired value by measuring the oxygen amount on line in the steel-making process and changing the amounts of Ti and N added depending on the oxygen amount.

Next, these steel pipe materials are heated and hot worked by a pipe forming process of a Mannesmann automatic mill system (Mannesmann-plug mill process) or a Mannesmann mandrel mill system (Mannesmann-plug mill process), which is a commonly known pipe forming method, to form pipes, thereby forming seamless steel pipes having the above-described composition in desired dimensions. The seamless steel pipe may be produced by hot extrusion using a press system. The seamless steel pipe after pipe production is preferably cooled to room temperature at a cooling rate not less than that of air cooling. This ensures a steel pipe structure having a martensite phase as a main phase. In order to reduce the average prior austenite grain size, it is preferable to perform pipe forming under the condition that (the cross-sectional area of the steel pipe after pipe forming)/(the cross-sectional area of the steel pipe material) is 0.20 or less. Further, it is preferable to perform pipe forming under the condition that (the sectional area of the steel pipe after pipe forming)/(the sectional area of the steel pipe after piercing) is 0.40 or less.

Subsequently, the steel pipe is cooled to room temperature at a cooling rate not less than the cooling rate of air cooling after pipe making, and in the present invention, the steel pipe is further subjected to quenching treatment as follows: is heated again to Ac ₃ The temperature is preferably not less than the transformation point, preferably not less than 800 ℃, preferably not less than 5 minutes, and then the steel sheet is cooled to a temperature not more than 100 ℃ at a cooling rate not less than air cooling. This makes it possible to refine the martensite phase and increase the strength. From the viewpoint of preventing the coarsening of the structure, the heating temperature for the quenching treatment is preferably set to 800 to 950 ℃.

Here, the "cooling rate not less than air cooling" means not less than 0.01 ℃/sec.

The steel pipe subjected to the quenching treatment is then subjected to a tempering treatment. The tempering treatment was as follows: heating to 500 deg.C or higher and below Ac ₁ The temperature of the transformation point (tempering temperature) is kept for a predetermined time, preferably 10 minutes or more, and then air-cooled. Tempering temperature Ac ₁ Above the transformation point, a fresh martensite phase precipitates after tempering, and the desired high strength cannot be secured. Therefore, the tempering temperature is more preferably set to 500 ℃ or higher and lower than Ac ₁ A point of phase change. As a result, the structure becomes a structure having a tempered martensite phase as a main phase, and a seamless steel pipe having a desired strength and a desired corrosion resistance is obtained.

In addition, from the viewpoint of reducing the average prior austenite grain diameter, quenching-tempering is preferably repeated two or more times.

The above Ac is ₃ Transformation point and Ac ₁ The transformation point was an actual measurement value read from the change in the expansion ratio (linear expansion ratio) when the test piece (φ 3 mm. times.L 10mm) was heated at a rate of 15 ℃/min and cooled。

The seamless steel pipe has been described as an example, but the present invention is not limited to this. Electric resistance welded steel pipes and UOE steel pipes can be produced by ordinary processes using the steel pipe materials having the above-described compositions, and oil well steel pipes can be produced.

Examples

The present invention will be further described below based on examples.

Molten steel having the composition shown in Table 1 was melted, cast into a steel pipe material, subjected to pipe forming by hot working using a model seamless rolling mill, and then subjected to air cooling to form a seamless steel pipe having an outer diameter of 83.8mm and a wall thickness of 12.7 mm. In steel pipe No.13, the amount of oxygen was measured on-line in the steel-making process, and the amounts of Ti and N added were varied according to the oxygen amount, thereby controlling the major axis to be 5 μm or more and 0.5<Ti/(Ti+Al+Mg+Ca)<1.0 the number of inclusions exceeds 3/mm ² . Further, steel pipe No.14 was controlled to have a major axis of 5 μm or more and 0.5. mu.m, by measuring the oxygen content on line in the steel making step and changing the addition amounts of Ti and N based on the oxygen content<Ti/(Ti+Al+Mg+Ca)<The number of 1.0 inclusions is less than 0.5 inclusions/mm ² 。

Next, a test piece material was cut out of the obtained seamless steel pipe, and subjected to the following quenching treatment: after heating at the heating temperature (reheating temperature) and soaking time shown in table 2, air cooling was performed at the cooling stop temperature shown in table 2. Then, tempering treatment was further performed by heating and air-cooling at a tempering temperature and a soaking time shown in table 2.

Further, API (American Petroleum Institute) arc tensile test pieces were cut from the test piece raw materials subjected to the quenching and tempering treatment, and tensile tests were performed according to the specifications of API to obtain tensile characteristics (yield strength YS, tensile strength TS). A sample having a yield strength YS of 655MPa or more was judged as a pass, and a sample having a yield strength YS of less than 655MPa was judged as a fail.

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

Etching ofThe test was carried out as follows: test pieces were immersed in the test solution held in the autoclave: 20% by mass NaCl aqueous solution (liquid temperature: 150 ℃ C., 10 atmospheres CO) ₂ Atmosphere), the immersion time was set to 14 days. The test piece after the test was weighed, and the corrosion rate calculated from the weight loss before and after the corrosion test was determined. The samples having a corrosion rate of 0.125 mm/year or less were judged as good, and the samples having a corrosion rate exceeding 0.125 mm/year were judged as bad.

In addition, with respect to the test piece after the corrosion test, the presence or absence of the occurrence of pitting on the surface of the test piece was observed using a magnifying glass having a magnification of 10 times. The term "pitting" means a case where the diameter is 0.2mm or more. The samples without pitting were determined to be acceptable, and the samples with pitting were determined to be unacceptable.

SSC assays were performed according to NACE TM0177 Method A. The test environment used was 25 mass% NaCl aqueous solution (liquid temperature: 4 ℃ C., H) ₂ S：0.1bar、CO ₂ : 0.9bar) was added sodium acetate + hydrochloric acid to adjust the pH to an aqueous solution of 4.5. The test was carried out with the immersion time set at 720 hours and the load stress at 90% of the yield stress. The test piece after the test was judged as pass (No in Table 3) when no crack occurred, and as fail (Presence in Table 3) when a crack occurred.

In the evaluation of hot workability, a smooth test piece in the shape of a round bar having a parallel portion diameter of 10mm was heated to 1250 ℃ by a greenle tester and held for 100 seconds, then cooled to 1000 ℃ at 1 ℃/second and held for 10 seconds, and then stretched until fracture was performed, and the reduction of the fracture surface was measured. The reduction of the cross section of 70% or more was regarded as excellent hot workability and was regarded as acceptable. The case where the reduction rate of the cross section was less than 70% was regarded as a failure.

The number of inclusions was 500mm cut from any of the positions 1/4 and 3/4 having a wall thickness from the outer surface of the steel pipe in the circumferential direction of the end of the steel pipe ² The area (2) was used as a sample for a Scanning Electron Microscope (SEM) having a cross section perpendicular to the direction of the thickness of the tube wall. The inclusions were identified by SEM observation for each cut sample, and the samples were classified by characteristic X-ray attached to SEMThe analysis device analyzes the chemical composition. Calculate 0.5<Ti/(Ti+Al+Mg+Ca)<1.0, the number of inclusions per unit area was calculated. Further, the inclusions having a long diameter of 5 μm or more are determined by defining the outer periphery of the inclusions by binarizing the contrast of a reflected electron image by a scanning electron microscope and measuring the long diameter from the outer periphery of the inclusions.

The measurement sample of the average prior austenite grain diameter was taken from any position in the circumferential direction of the end of the steel pipe at 1/2 where the distance from the outer surface of the pipe in the cross section orthogonal to the longitudinal direction of the pipe was the thickness. The cut sample was observed for EBSD, and then the prior austenite grains were reconstructed from the EBSD observation data using reverse analysis software for the prior austenite grains. With respect to the reconstructed image of the prior austenite crystal grains obtained, three 300 μm straight lines were drawn at intervals of 500 μm in the circumferential direction of the tube, and the average prior austenite grain diameter was measured by the cutting method.

The obtained results are shown in table 3.

[ Table 3]

Underline is outside the scope of the invention

The inventive examples all had a yield strength YS of 655MPa or more and excellent hot workability, and contained CO ₂ And Cl-has excellent corrosion resistance (carbon dioxide corrosion resistance) in a high-temperature corrosion environment of 150 ℃ or higher, excellent SSC resistance in a low-temperature environment, and a reduction ratio of a cross section of 70% or more. On the other hand, in comparative examples outside the range of the present invention, the yield strength YS, hot workability, SSC resistance in low temperature environment, corrosion resistanceAt least one of the etching rate and the reduction ratio of the profile does not have a desired value.

Claims

1. A high-strength stainless steel seamless steel pipe for oil wells, which comprises, in mass%, C: 0.002-0.05%, Si: 0.05 to 0.50%, Mn: 0.04-1.80%, P: 0.030% or less, S: 0.002% or less, Cr: 11.0 to 14.0%, Ni: 3.0-6.5%, Mo: 0.5 to 3.0%, Al: 0.005-0.10%, V: 0.005-0.20%, Ti: 0.01-0.20%, Co: 0.01-1.0%, N: 0.002-0.15%, O: 0.010% or less, satisfying the following formulae (1) and (2), and the balance being Fe and unavoidable impurities,

the yield strength of the high-strength stainless steel seamless steel pipe for oil wells is 655MPa or more,

wherein Ti, Al, Mg and Ca in 0.5< Ti/(Ti + Al + Mg + Ca) <1.0 are the mass% contents of each element in the inclusion, and zero is set as the element not contained,

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)

wherein Cr, Ni, Mo, Cu, C, Si, Mn, and N are the mass% contents of the respective elements, and zero is an element not contained therein.

2. The high-strength stainless steel seamless pipe for oil wells according to claim 1, further comprising a component selected from the group consisting of Cu: 0.05-3.0%, W: 0.05-3.0% of one or two.

3. The high-strength stainless steel seamless pipe for oil wells according to claim 1 or 2, further comprising an additive selected from the group consisting of Nb: 0.01 to 0.20%, Zr: 0.01-0.20%, 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: 0.01-0.1%, Mg: 0.002-0.01% of one or more than two.

4. The high-strength seamless stainless steel pipe for oil wells according to any one of claims 1 to 3, wherein the average prior austenite grain diameter is 40 μm or less.