WO2022009598A1

WO2022009598A1 - Seamless stainless steel pipe and production method therefor

Info

Publication number: WO2022009598A1
Application number: PCT/JP2021/022014
Authority: WO
Inventors: 祐一加茂; 正雄柚賀
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2020-07-06
Filing date: 2021-06-10
Publication date: 2022-01-13
Also published as: US20230340632A1; MX2022016143A; CN115917028A; AR122877A1; JP7226571B2; BR112022025826A2; EP4123037A1; JPWO2022009598A1

Abstract

Provided is a seamless stainless steel pipe and a production method therefor. The seamless stainless steel pipe of the present invention has a component composition containing, by mass%, not more than 0.06% C, not more than 1.0% Si, 0.01-0.90% Mn, not more than 0.05% P, not more than 0.005% S, 15.70-18.00% Cr, 1.60-3.80% Mo, 1.10-4.00% Cu, 3.0-6.0% Ni, not more than 0.10% Al, not more than 0.10% N, not more than 0.010% O, and 0.120-1.000% V, wherein the C, Si, Mn, Cr, Ni, Mo, Cu, and N satisfy a prescribed formula, and the remainder is composed of Fe and unavoidable impurities. The seamless stainless steel pipe has a structure including, by volume ratio, 30% or more of a martensite phase, 60% or less of a ferrite phase, and 40% or less of a retained austenite phase, and the yield strength of the seamless stainless steel pipe is 758 MPa or greater.

Description

Stainless steel seamless pipe and its manufacturing method

The present invention relates to a stainless seamless steel pipe suitable for use in oil wells and gas wells (hereinafter, simply referred to as oil wells). The present invention is particularly carbon dioxide (CO _2), chlorine ions - improvement under severe corrosive environment and high temperatures including, corrosion resistance under the environment like containing hydrogen sulfide (H 2 _S), and the strength at high temperatures ^(Cl) Regarding the stainless steel seamless steel pipe that was made.

In recent years, from the viewpoint of the depletion of energy resources expected in the near future, under the environment containing deep oil fields and carbon dioxide gas, which has not been omitted in the past, and the environment containing hydrogen sulfide called sour environment, etc. , Oil wells in severely corroded environments are being actively developed. Steel pipes for oil wells used in such an environment are required to have high strength and high corrosion resistance.

Conventionally, 13Cr martensitic stainless steel pipes have been generally used as steel pipes for oil wells used for oil wells in oil fields and gas fields in an environment containing _{CO 2} and Cl ^−. However, recently, the development of oil wells with higher temperatures (high temperatures up to 200 ° C.) has been promoted, and 13Cr martensitic stainless steel pipes may have insufficient corrosion resistance. There is a demand for steel pipes for oil wells that can be used even in such an environment and have high corrosion resistance.

In response to such a request, for example, there are technologies listed in Patent Documents 1 to 5. Patent Document 1 describes in terms of mass%, C: 0.05% or less, Si: 1.0% or less, Mn: 0.01 to 1.0%, P: 0.05% or less, S: 0.002. %, Cr: 16-18%, Mo: 1.8-3%, Cu: 1.0-3.5%, Ni: 3.0-5.5%, Co: 0.01-1.0 %, Al: 0.001 to 0.1%, O: 0.05% or less, and N: 0.05% or less, and Cr, Ni, Mo, Cu satisfy a specific relationship. The stainless steel for oil wells to have is described.

Further, in Patent Document 2, in mass%, C: 0.05% or less, Si: 1.0% or less, Mn: 0.1 to 0.5%, P: 0.05% or less, S: 0. Less than .005%, Cr: 15.0% or more and 19.0% or less, Mo: 2.0% or more and 3.0% or less, Cu: 0.3 to 3.5%, Ni: 3.0% or more 5 Less than 0.0%, W: 0.1 to 3.0%, Nb: 0.07 to 0.5%, V: 0.01 to 0.5%, Al: 0.001 to 0.1%, N : 0.010 to 0.100%, O: 0.01% or less, Nb, Ta, C, N, Cu have a composition satisfying a specific relationship, and 45% or more by volume. A high-strength stainless seamless steel tube for an oil well having a structure consisting of a tempered martensite phase, a ferrite phase of 20 to 40%, and a residual austenite phase of more than 10% and not more than 25% is described. As a result, it is possible to obtain a high-strength stainless steel seamless steel pipe for oil wells that has a yield strength (YS) of 862 MPa or more and _{sufficient corrosion resistance even in a high-temperature severe corrosion environment containing CO 2} , Cl ⁻ , and H _{2 S.} There is.

Further, in Patent Document 3, in terms of mass%, C: 0.005 to 0.05%, Si: 0.05 to 0.50%, Mn: 0.20 to 1.80%, P: 0.030. % Or less, S: 0.005% or less, Cr: 12.0 to 17.0%, Ni: 4.0 to 7.0%, Mo: 0.5 to 3.0%, Al: 0.005 to It contains 0.10%, V: 0.005 to 0.20%, Co: 0.01 to 1.0%, N: 0.005 to 0.15%, O: 0.010% or less, and Cr. , Ni, Mo, Cu, C, Si, Mn, N are described as high-strength stainless steel seamless pipes for oil wells having a composition satisfying a specific relationship.

Further, in Patent Document 4, in terms of mass%, C: 0.05% or less, Si: 0.5% or less, Mn: 0.15 to 1.0%, P: 0.030% or less, S: 0. .005% or less, Cr: 14.5 to 17.5%, Ni: 3.0 to 6.0%, Mo: 2.7 to 5.0%, Cu: 0.3 to 4.0%, W : 0.1 to 2.5%, V: 0.02 to 0.20%, Al: 0.10% or less, N: 0.15% or less, C, Si, Mn, Cr, Ni, Mo, Cu, N, W have a composition that satisfies a specific relationship, and in terms of volume ratio, martensite phase is more than 45% as the main phase, ferrite phase is 10 to 45% as the second phase, and residual austenite phase. A high-strength stainless steel seamless steel tube for oil wells having a structure containing 30% or less of is described. As a result, it is possible to obtain a high-strength stainless steel seamless steel pipe for oil wells that has a yield strength (YS) of 862 MPa or more and _{sufficient corrosion resistance even in a high-temperature severe corrosion environment containing CO 2} , Cl ⁻ , and H _{2 S.} There is.

Further, in Patent Document 5, in terms of mass%, C: 0.05% or less, Si: 0.5% or less, Mn: 0.15 to 1.0%, P: 0.030% or less, S: 0. .005% or less, Cr: 14.5 to 17.5%, Ni: 3.0 to 6.0%, Mo: 2.7 to 5.0%, Cu: 0.3 to 4.0%, W : 0.1 to 2.5%, V: 0.02 to 0.20%, Al: 0.10% or less, N: 0.15% or less, B: 0.0005 to 0.0100% , C, Si, Mn, Cr, Ni, Mo, Cu, N, W have a composition that satisfies a specific relationship, and in terms of volume ratio, the martensite phase as the main phase exceeds 45%, and the second phase Described are high-strength stainless seamless steel pipes for oil wells having a structure containing 10 to 45% of a ferrite phase and 30% or less of a retained austenite phase. As a result, it is possible to obtain a high-strength stainless steel seamless steel pipe for oil wells that has a yield strength (YS) of 862 MPa or more and _{sufficient corrosion resistance even in a high-temperature severe corrosion environment containing CO 2} , Cl ⁻ , and H _{2 S.} There is.

International Publication No. 2013/146046 International Publication No. 2017/138050 International Publication No. 2017/1687874 International Publication No. 2018-020886 International Publication No. 2018/155041

As described above, by the development of oil wells in a further high temperature proceeds, the oil well steel pipe, and high strength at elevated temperature, and, CO ₂ and Cl ^- Excellent耐炭acid under severe corrosive environments containing It is required to have both gas corrosion resistance and excellent sulfide stress cracking resistance (SSC resistance).

In addition, when used at high temperature, strength at high temperature (high temperature strength) may also be required. Specifically, it may be required that the ratio of the yield stress (0.2% proof stress) at 200 ° C. to the yield stress (0.2% proof stress) at room temperature is 0.85 or more.

Patent Documents 1 to 5 disclose stainless steels having improved corrosion resistance, but there are cases where it is not sufficient to combine high temperature corrosion resistance, high sulfide stress cracking resistance, and high high temperature strength. there were.

The present invention solves such a problem of the prior art, and has a high strength of yield strength: 758 MPa (110 ksi) or more, excellent corrosion resistance, and excellent high temperature strength. The purpose is to provide.

The term "excellent corrosion resistance" as used herein means a case of having "excellent carbon dioxide gas corrosion resistance" and "excellent sulfide stress cracking resistance".

The term "excellent carbon dioxide corrosion resistance" as used herein means that the test piece is placed in a test solution held in an autoclave: a 20 mass% NaCl aqueous solution (liquid temperature: 200 ° C., CO _{2 gas atmosphere at 30 atm).} It means a case where the corrosion rate is 0.127 mm / y or less when the immersion is carried out with the immersion time set to 336 hours.

The term "excellent sulfide stress cracking resistance" as used herein means a test solution held in an autoclave: a 0.165 mass% NaCl aqueous solution (liquid temperature: 25 ° C., 0.99 atm) CO ₂ gas. the _{H 2} S atmosphere) of 0.01 atm, the addition of acetic acid + sodium acetate pH: the test piece was immersed in an aqueous solution adjusted to 3.0, exposed for 720 hours in a state loaded with 90% of yield stress, It shall be the case where the test piece is not broken or cracked after the test.

The term "excellent high temperature strength" as used herein means a yield stress (0.2%) at room temperature after conducting a tensile test in accordance with JIS Z 2241 and a high temperature tensile test in accordance with JIS G 0567. The ratio of the yield stress (0.2% proof stress) at 200 ° C. to the proof stress) is 0.85 or more.
The method of each test described above is also described in detail in Examples described later.

The present inventors have diligently studied various factors affecting the high temperature strength and corrosion resistance of stainless steel in order to achieve the above-mentioned object. As a result, excellent high-temperature strength was obtained by containing V in a predetermined amount or more. Further, by containing Cr, Mo, and Cu in a predetermined amount or more and setting the Mn content to a certain amount or less, excellent corrosion resistance (excellent carbon dioxide gas corrosion resistance and excellent sulfide stress cracking resistance). was gotten.

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: 0.06% or less,
Si: 1.0% or less,
Mn: 0.01% or more and 0.90% or less,
P: 0.05% or less,
S: 0.005% or less,
Cr: 15.70% or more and 18.00% or less,
Mo: 1.60% or more and 3.80% or less,
Cu: 1.10% or more and 4.00% or less,
Ni: 3.0% or more and 6.0% or less,
Al: 0.10% or less,
N: 0.10% or less,
O: 0.010% or less,
V: Contains 0.120% or more and 1.000% or less,
Moreover, C, Si, Mn, Cr, Ni, Mo, Cu, and N satisfy the following formula (1).
The balance has a component composition consisting of Fe and unavoidable impurities,
It has a structure containing a martensite phase of 30% or more, a ferrite phase of 60% or less, and a retained austenite phase of 40% or less by volume.
Stainless steel seamless steel pipe with yield strength of 758 MPa or more.
Record
13.0 ≤ -5.9 x (7.82 + 27C-0.91Si + 0.21Mn-0.9Cr + Ni-1.1Mo + 0.2Cu + 11N) ≤ 50.0 ... (1)
Here, C, Si, Mn, Cr, Ni, Mo, Cu, and N: are the contents (mass%) of each element, and if they are not contained, they are set to zero.
[2] The stainless seamless steel pipe according to [1], wherein the martensite phase has a volume fraction of 40% or more, the retained austenite phase has a volume fraction of 30% or less, and the yield strength is 862 MPa or more.
[3] The stainless steel seamless according to [1] or [2], which further contains one group or two or more groups selected from the following groups A to D in mass% in addition to the component composition. Steel pipe.
Group A: W: 3.0% or less Group B: Nb: less than 0.10% Group C: B: 0.010% or less, Ta: 0.3% or less, Co: 1.5% or less, Ti: 0 .1 or 2 or more selected from 3% or less, Zr: 0.3% or less Group D: Ca: 0.01% or less, REM: 0.3% or less, Mg: 0.01% or less , Sn: 1.0% or less, Sb: 1.0% or less, one type or two or more types selected from [4] [1] to [3]. It is a manufacturing method of
The steel pipe material having the above-mentioned composition is heated at a heating temperature in the range of 1100 to 1350 ° C. and hot-worked to obtain a seamless steel pipe.
Next, the seamless steel pipe is reheated to a heating temperature in the range of 850 to 1150 ° C., and subjected to quenching treatment to cool it to a cooling stop temperature of 50 ° C. or lower at a cooling rate equal to or higher than air cooling.
Then, a method for manufacturing a stainless seamless steel pipe, which is subjected to a tempering process of heating to a temperature in the range of tempering temperature: 500 to 650 ° C.

According to the present invention, a stainless seamless steel pipe having a high yield strength of 758 MPa (110 ksi) or more, excellent corrosion resistance, and excellent high temperature strength, and a method for manufacturing the same can be obtained.

Hereinafter, the present invention will be described in detail.

The stainless seamless steel tube of the present invention has a mass% of C: 0.06% or less, Si: 1.0% or less, Mn: 0.01% or more and 0.90% or less, P: 0.05% or less, S: 0.005% or less, Cr: 15.70% or more and 18.00% or less, Mo: 1.60% or more and 3.80% or less, Cu: 1.10% or more and 4.00% or less, Ni: 3 It contains 0.0% or more and 6.0% or less, Al: 0.10% or less, N: 0.10% or less, O: 0.010% or less, V: 0.120% or more and 1.000% or less. Moreover, C, Si, Mn, Cr, Ni, Mo, Cu, and N satisfy the following formula (1), the balance has a component composition consisting of Fe and unavoidable impurities, and the volume ratio is 30% or more. It has a structure containing a martensite phase, a ferrite phase of 60% or less, and a retained austenite phase of 40% or less, and has a yield strength of 758 MPa or more.
13.0 ≤ -5.9 x (7.82 + 27C-0.91Si + 0.21Mn-0.9Cr + Ni-1.1Mo + 0.2Cu + 11N) ≤ 50.0 ... (1)
Here, C, Si, Mn, Cr, Ni, Mo, Cu, and N: are the contents (mass%) of each element, and if they are not contained, they are set to zero.

First, the reason for limiting the component composition of the stainless seamless steel pipe of the present invention will be described. Hereinafter, unless otherwise specified, "mass%" is simply referred to as "%".

C: 0.06% or less C is an element inevitably contained in the steelmaking process. If C is contained in excess of 0.06%, the corrosion resistance is lowered. Therefore, the C content is 0.06% or less. The C content is preferably 0.05% or less, more preferably 0.04% or less, still more preferably 0.03% or less. Considering the decarburization cost, the lower limit of the C content is preferably 0.002%, more preferably 0.003% or more.

Si: 1.0% or less Si is an element that acts as a deoxidizing agent. However, if Si is contained in excess of 1.0%, hot workability and corrosion resistance are deteriorated. Therefore, the Si content is set to 1.0% or less. The Si content is preferably 0.7% or less, more preferably 0.5% or less, still more preferably 0.4% or less. A lower limit is not set as long as the deoxidizing effect can be obtained, but the Si content is preferably 0.03% or more, more preferably 0.05% or more for the purpose of obtaining a sufficient deoxidizing effect. be.

Mn: 0.01% or more and 0.90% or less Mn is an element that acts as a deoxidizing material and a desulfurizing material and improves hot workability. In order to obtain the effects of deoxidation and desulfurization and to improve the strength, the Mn content is 0.01% or more. On the other hand, if Mn is contained in excess of 0.90%, the sulfide stress cracking resistance is lowered. Therefore, the Mn content is set to 0.01% or more and 0.90% or less. The Mn content is preferably 0.03% or more, more preferably 0.05% or more. The Mn content is preferably 0.7% or less, more preferably 0.5% or less, still more preferably 0.4% or less.

P: 0.05% or less P is an element that reduces carbon dioxide gas corrosion resistance and sulfide stress cracking resistance, and is preferably reduced as much as possible in the present invention, but 0.05% or less is acceptable. .. Therefore, the P content is set to 0.05% or less. The P content is preferably 0.04% or less, more preferably 0.03% or less, and further preferably 0.02% or less.

S: 0.005% or less S is an element that significantly reduces hot workability and hinders stable operation of the hot pipe making process. Further, S exists as a sulfide-based inclusion in steel and lowers the sulfide stress cracking resistance. Therefore, it is preferable to reduce S as much as possible, but it is acceptable if it is 0.005% or less. Therefore, the S content is set to 0.005% or less. The S content is preferably 0.004% or less, more preferably 0.003% or less, still more preferably 0.002% or less.

Cr: 15.70% or more and 18.00% or less Cr is an element that forms a protective film on the surface of steel pipes and contributes to improvement of corrosion resistance. If the Cr content is less than 15.70%, desired carbon dioxide gas corrosion resistance It is not possible to ensure resistance and sulfide stress cracking resistance. Therefore, the content of Cr of 15.70% or more is required. On the other hand, if the content of Cr exceeds 18.00%, the ferrite fraction becomes too high and the desired strength cannot be secured. Therefore, the Cr content is 15.70% or more and 18.00% or less. The Cr content is preferably 16.00% or more, more preferably 16.30% or more. The Cr content is preferably 17.50% or less, more preferably 17.00% or less.

Mo: 1.60% or more 3.80% or less Mo is a protective coating of the steel pipe surface is stabilized, Cl ^- and low pH increases the resistance to pitting,耐炭acid gas corrosion resistance and sulfide Increases stress corrosion cracking. In order to obtain the desired corrosion resistance, it is necessary to contain 1.60% or more of Mo. On the other hand, if Mo is added in excess of 3.80%, the ferrite fraction becomes too high and the desired strength cannot be secured. Therefore, the Mo content is 1.60% or more and 3.80% or less. The Mo content is preferably 1.80% or more, more preferably 2.00% or more. The Mo content is preferably 3.5% or less, more preferably 3.0% or less, and further preferably 2.8% or less.

Cu: 1.10% or more and 4.00% or less Cu has the effect of strengthening the protective film on the surface of the steel pipe and enhancing the carbon dioxide gas corrosion resistance and the sulfide stress cracking resistance. In order to obtain the desired strength and corrosion resistance, particularly carbon dioxide corrosion resistance, it is necessary to contain 1.10% or more of Cu. On the other hand, if the content is too large, the hot workability of the steel is lowered, so the Cu content is set to 4.00% or less. Therefore, the Cu content is 1.10% or more and 4.00% or less. The Cu content is preferably 1.80% or more, more preferably 2.00% or more. The Cu content is preferably 3.20% or less, more preferably 3.00% or less, still more preferably 2.7% or less.

Ni: 3.0% or more and 6.0% or less Ni increases the strength of steel by solid solution strengthening and improves the toughness of steel. In order to secure the toughness required for oil country tubular goods, it is necessary to contain 3.0% or more of Ni. On the other hand, if the content of Ni exceeds 6.0%, the stability of the martensite phase is lowered and the strength is lowered. Therefore, the Ni content is set to 3.0% or more and 6.0% or less. The Ni content is preferably 3.5% or more, more preferably 4.0% or more, still more preferably 4.5% or more. The Ni content is preferably 5.5% or less, more preferably 5.2% or less.

Al: 0.10% or less Al is an element that acts as a deoxidizing agent. However, if Al is contained in an amount of more than 0.10%, the corrosion resistance is lowered. Therefore, the Al content is set to 0.10% or less. The Al content is preferably 0.07% or less, more preferably 0.05% or less, still more preferably 0.04% or less. The lower limit is not set as long as the deoxidizing effect can be obtained, but the Al content is preferably 0.005% or more, more preferably 0.01% or more, for the purpose of obtaining a sufficient deoxidizing effect. be.

N: 0.10% or less N is an element that is inevitably contained in the steelmaking process, but it is also an element that enhances the strength of steel. However, if N is contained in excess of 0.10%, a nitride is formed and the corrosion resistance is lowered. Therefore, the N content is set to 0.10% or less. The N content is preferably 0.08% or less, more preferably 0.05% or less, still more preferably 0.03% or less. Although the lower limit of the N content is not particularly set, an extreme reduction in the N content leads to an increase in steelmaking cost. Therefore, the N content is preferably 0.002% or more, and more preferably 0.003% or more.

O: 0.010% or less O (oxygen) exists as an oxide in steel and therefore adversely affects various properties. Therefore, in the present invention, it is desirable to reduce the amount as much as possible. In particular, when the O content exceeds 0.010%, the hot workability and corrosion resistance deteriorate. Therefore, the O content is 0.010% or less. The O content is preferably 0.005% or less.

V: 0.120% or more and 1.000% or less V is an important element in the present invention for improving high temperature strength. V forms a carbonitride, and high strength can be obtained not only at room temperature but also at high temperature by strengthening precipitation. In order to obtain the desired high temperature strength, V is contained in an amount of 0.120% or more. On the other hand, even if V is contained in excess of 1.000%, the effect is saturated. Therefore, in the present invention, the V content is set to 0.120% or more and 1.000% or less. The V content is preferably 0.180% or more, more preferably 0.250% or more, and further preferably 0.300% or more. The V content is preferably 0.500% or less, more preferably 0.400% or less, and further preferably 0.300% or less.

In the present invention, while satisfying the above-mentioned component composition, C, Si, Mn, Cr, Ni, Mo, Cu, and N are further contained so as to satisfy the following formula (1).
13.0 ≤ -5.9 x (7.82 + 27C-0.91Si + 0.21Mn-0.9Cr + Ni-1.1Mo + 0.2Cu + 11N) ≤ 50.0 ... (1)
Here, C, Si, Mn, Cr, Ni, Mo, Cu, and N: are the contents (mass%) of each element, and if they are not contained, they are set to zero.
Eq. (1) "-5.9 x (7.82 + 27C-0.91Si + 0.21Mn-0.9Cr + Ni-1.1Mo + 0.2Cu + 11N)" (hereinafter, also simply referred to as "the central polynomial of Eq. (1)" or "median") Is obtained as an exponent indicating the formation tendency of the ferrite phase. If the alloying element represented by the formula (1) is adjusted and contained so as to satisfy the formula (1), a composite structure consisting of a martensite phase, a ferrite phase, or a retained austenite phase can be stably realized. Can be done. When the alloy element described in the formula (1) is not contained, the value of the polynomial in the center of the formula (1) shall treat the content of the element as 0%.

When the value of the polynomial in the center of the above equation (1) is less than 13.0, the ferrite phase is reduced in the hot working temperature range, and the yield at the time of manufacturing is lowered. On the other hand, if the value of the polynomial in the center of the above equation (1) exceeds 50.0, the ferrite phase exceeds 60% in volume fraction, and the desired strength cannot be secured. Therefore, in the equation (1) specified in the present invention, the lower limit of the lvalue is 13.0 and the upper limit of the rvalue is 50.0. The lvalue, which is the lower limit of the equation (1) defined in the present invention, is preferably 15.0, more preferably 20.0. The rvalue is preferably 45.0, more preferably 40.0. That is, the value of the polynomial in the center of Eq. (1) is 13.0 or more and 50.0 or less. It is preferably 15.0 or more and 45.0 or less. More preferably, it is 20.0 or more and 40.0 or less.

In the present invention, the balance other than the above-mentioned component composition is composed of Fe and unavoidable impurities.

With the above essential elements, the stainless seamless steel pipe of the present invention can obtain the desired characteristics. In the present invention, for the purpose of further improving the characteristics, if necessary, in addition to the above-mentioned basic composition, the following selective elements (W, Nb, B, Ta, Co, Ti, Zr, Ca) are further added. , REM, Mg, Sn, Sb) may be contained alone or in combination of two or more.

Specifically, in the present invention, W: 3.0% or less can be contained in addition to the above-mentioned component composition.
Further, in the present invention, Nb: less than 0.10% can be contained in addition to the above-mentioned component composition.
Further, in the present invention, in addition to the above-mentioned component composition, B: 0.010% or less, Ta: 0.3% or less, Co: 1.5% or less, Ti: 0.3% or less and Zr: 0. It can contain one or more selected from 3% or less.
Further, in the present invention, in addition to the above-mentioned component composition, Ca: 0.01% or less, REM: 0.3% or less, Mg: 0.01% or less, Sn: 1.0% or less, and Sb: 1. It can contain one or more selected from 0% or less.

W: 3.0% or less W is an element that contributes to improving the strength of steel and stabilizes the protective film on the surface of steel pipes to enhance carbon dioxide gas corrosion resistance and sulfide stress crack resistance. .. When W is contained in combination with Mo, the corrosion resistance is remarkably improved. W can be contained as needed in order to obtain the above effects. 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 preferably 3.0% or less. The W content is more preferably less than 1.5%, still more preferably 1.0% or less. When W is contained, the W content is more preferably 0.05% or more, still more preferably 0.10% or more.

Nb: Less than 0.10% Nb is an element that increases strength and improves corrosion resistance, and can be contained as needed. On the other hand, if 0.10% or more of Nb is contained, the desired high-temperature strength may not be obtained. Therefore, when Nb is contained, the Nb content is preferably less than 0.10%. The Nb content is more preferably 0.05% or less, still more preferably 0.03% or less. The Nb content is more preferably 0.005% or more, still more preferably 0.010% or more.

B: 0.010% or less B is an element that increases the strength and can be contained as needed. In addition, B also contributes to the improvement of hot workability and has the effect of suppressing the occurrence of cracks and cracks in the pipe making process. On the other hand, even if B is contained in an amount of more than 0.010%, not only the effect of improving the hot workability hardly appears, but also the low temperature toughness is lowered. Therefore, when B is contained, the B content is preferably 0.010% or less. The B content is more preferably 0.008% or less, still more preferably 0.007% or less. The B content is more preferably 0.0005% or more, still more preferably 0.0010% or more.

Ta: 0.3% or less Ta is an element that increases strength and improves corrosion resistance, and can be contained as needed. In order to obtain such an effect, it is preferable to contain 0.001% or more of Ta. On the other hand, even if Ta is contained in excess of 0.3%, the effect is saturated. Therefore, when Ta is contained, it is preferable to limit the Ta content to 0.3% or less. The Ta content is more preferably 0.25% or less, further preferably 0.06% or less, still more preferably 0.050% or less, still more preferably 0.025% or less. More preferably, it is 0.005% or more.

Co: 1.5% or less Co is an element that increases the strength and can be contained as needed. Co has an effect of improving corrosion resistance in addition to the above-mentioned effect. In order to obtain such an effect, it is preferable to contain 0.0005% or more of Co. The Co content is more preferably 0.005% or more, still more preferably 0.010% or more. On the other hand, even if Co is contained in excess of 1.5%, the effect is saturated. Therefore, when Co is contained, it is preferable to limit the Co content to 1.5% or less. The Co content is more preferably less than 0.150%.

Ti: 0.3% or less Ti is an element that increases the strength and can be contained as needed. In order to obtain such an effect, it is preferable to contain 0.0005% or more of Ti. On the other hand, if Ti is contained in excess of 0.3%, the toughness is lowered. Therefore, when Ti is contained, it is preferable to limit the Ti content to 0.3% or less.

Zr: 0.3% or less Zr is an element that increases the strength and can be contained as needed. In addition to the above-mentioned effects, Zr also has an effect of improving sulfide stress cracking resistance. In order to obtain such an effect, it is preferable to contain Zr in an amount of 0.0005% or more. On the other hand, even if Zr is contained in excess of 0.3%, the effect is saturated. Therefore, when Zr is contained, it is preferable to limit the Zr content to 0.3% or less.

Ca: 0.01% or less Ca is an element that contributes to the improvement of sulfide stress cracking resistance through morphological control of sulfide, and can be contained as needed. In order to obtain such an effect, it is preferable to contain 0.0005% or more of Ca. On the other hand, even if Ca is contained in an amount of more than 0.01%, the effect is saturated and the effect commensurate with the content cannot be expected. Therefore, when Ca is contained, it is preferable to limit the Ca content to 0.01% or less.

REM: 0.3% or less REM (rare earth metal) is an element that contributes to the improvement of sulfide stress cracking resistance through morphological control of sulfide, and can be contained as needed. In order to obtain such an effect, it is preferable to contain 0.0005% or more of REM. On the other hand, even if REM is contained in an amount of more than 0.3%, the effect is saturated and the effect commensurate with the content cannot be expected. Therefore, when REM is contained, it is preferable to limit the REM content to 0.3% or less.
The REM referred to in the present invention includes scandium (Sc) having an atomic number of 21 and yttrium (Y) having an atomic number of 39, and lanthanum (La) having an atomic number of 57 to lutetium (Lu) having an atomic number of 71. It is a lanthanoid. The REM concentration in the present invention is the total content of one or more elements selected from the above-mentioned REMs.

Mg: 0.01% or less Mg is an element that improves corrosion resistance and can be contained as needed. In order to obtain such an effect, it is preferable that Mg is contained in an amount of 0.0005% or more. On the other hand, even if Mg is contained in an amount of more than 0.01%, the effect is saturated and the effect commensurate with the content cannot be expected. Therefore, when Mg is contained, it is preferable to limit the Mg content to 0.01% or less.

Sn: 1.0% or less Sn is an element that improves corrosion resistance and can be contained as needed. In order to obtain such an effect, it is preferable to contain Sn in 0.001% or more. On the other hand, even if Sn is contained in an amount of more than 1.0%, the effect is saturated and the effect commensurate with the content cannot be expected. Therefore, when Sn is contained, it is preferable to limit the Sn content to 1.0% or less.

Sb: 1.0% or less Sb is an element that improves corrosion resistance and can be contained as needed. In order to obtain such an effect, it is preferable to contain 0.001% or more of Sb. On the other hand, even if Sb is contained in an amount of more than 1.0%, the effect is saturated and the effect commensurate with the content cannot be expected. Therefore, when Sb is contained, it is preferable to limit the Sb content to 1.0% or less.

Next, the reason for limiting the structure of the stainless seamless steel pipe of the present invention will be described.

The stainless seamless steel pipe of the present invention has the above-mentioned composition and has a structure containing a martensite phase of 30% or more, a ferrite phase of 60% or less, and a residual austenite phase of 40% or less in terms of volume ratio. Have.

In the stainless seamless steel pipe of the present invention, the martensite phase is set to 30% or more by volume in order to secure the desired strength. The martensite phase is preferably 40% or more. The martensite phase is preferably 70% or less, more preferably 65% or less.

Further, in the present invention, a ferrite phase having a volume fraction of 60% or less is contained. When a ferrite phase is contained, the growth of sulfide stress cracking can be suppressed and excellent corrosion resistance can be obtained. On the other hand, if a large amount of ferrite phase is deposited in excess of 60% by volume, it may not be possible to secure the desired strength. Preferably, the ferrite phase has a volume fraction of 5% or more. More preferably, it is 10% or more. Further, preferably, the ferrite phase has a volume fraction of 50% or less. More preferably, it is 45% or less.

Further, in the present invention, in addition to the martensite phase and the ferrite phase, an austenite phase (residual austenite phase) having a volume fraction of 40% or less is contained. The presence of the retained austenite phase improves ductility and toughness. On the other hand, if a large amount of austenite phase exceeding 40% by volume is precipitated, the desired strength cannot be secured. Therefore, the retained austenite phase is set to 40% or less by volume. Preferably, the retained austenite phase is 5% or more by volume. Further, preferably, the retained austenite phase has a volume fraction of 35% or less. More preferably, the retained austenite phase is 30% or less by volume.

Here, the above-mentioned structure of the stainless seamless steel pipe of the present invention can be measured by the following method. First, the tissue observation test piece is corroded with a bilera reagent (a reagent in which picrinic acid, hydrochloric acid and ethanol are mixed at a ratio of 2 g, 10 ml and 100 ml, respectively), and the tissue is imaged with a scanning electron microscope (magnification: 1000 times). , The microstructure fraction (area ratio (%)) of the ferrite phase is calculated using an image analyzer. This area ratio is defined as the volume fraction (%) of the ferrite phase.

Then, the X-ray diffraction test piece is ground and polished so that the cross section (C cross section) orthogonal to the tube axis direction becomes the measurement surface, and the structural component of the retained austenite (γ) phase is used by the X-ray diffraction method. Measure the rate. The structure fraction of the retained austenite phase is converted by measuring the diffraction X-ray integrated intensity of the (220) plane of γ and the (211) plane of α (ferrite) and using the following equation.
γ (volume fraction) = 100 / (1+ (IαRγ / IγRα))
Here, the integrated intensity of Iα: α, the crystallographic theoretically calculated value of Rα: α, the integrated intensity of Iγ: γ, and the crystallographic theoretically calculated value of Rγ: γ are used.

Further, the balance other than the ferrite phase and the residual γ phase obtained by the above measurement method is used as the fraction of the martensite phase.

Hereinafter, a suitable manufacturing method for the stainless seamless steel pipe of the present invention will be described.

It is preferable that the molten steel having the above-mentioned 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-block rolling method or the like. The heating temperature of the steel pipe material before hot working is preferably 1100 to 1350 ° C. This makes it possible to achieve both hot workability during pipe making and low temperature toughness of the final product.
Then, the obtained steel pipe material is hot-processed using a pipe-making process of a Mannesmann-plug mill method or a Mannesmann-mandrel mill method, which is a generally known pipe-making method, and the desired dimensions are obtained. A seamless steel pipe having the above-mentioned composition. After the hot working, a cooling treatment may be performed. This cooling process (cooling step) does not need to be particularly limited. Within the above-mentioned component composition range of the present invention, it is preferable to cool to room temperature at a cooling rate of about air cooling after hot working.

In the present invention, the obtained seamless steel pipe is further subjected to a heat treatment including a quenching treatment and a tempering treatment.

The quenching treatment is a treatment in which the heating temperature is reheated to a temperature in the range of 850 to 1150 ° C., and then the cooling is performed at a cooling rate higher than that of air cooling. The cooling stop temperature at this time is 50 ° C. or less at the surface temperature of the seamless steel pipe.

If the heating temperature is less than 850 ° C., the reverse transformation from martensite to austenite does not occur, and the transformation from austenite to martensite does not occur during cooling, so that the desired strength cannot be secured. On the other hand, when the heating temperature exceeds 1150 ° C. and becomes high, the crystal grains become coarse. Therefore, the heating temperature of the quenching treatment is set to a temperature in the range of 850 to 1150 ° C. Preferably, the heating temperature of the quenching treatment is 900 ° C. or higher. Preferably, the heating temperature of the quenching treatment is 1100 ° C. or lower. Further, when the cooling shutdown temperature exceeds 50 ° C., the transformation from austenite to martensite does not sufficiently occur, and the retained austenite fraction becomes excessive. Therefore, in the present invention, the cooling shutdown temperature during cooling in the quenching process is set to 50 ° C. or lower. Here, the "cooling rate of air cooling or higher" is 0.01 ° C./s or higher.

Further, in the quenching treatment, the soaking time is preferably 5 to 30 minutes in order to make the temperature uniform in the wall thickness direction and prevent the material from fluctuating.

The tempering process is a process in which a seamless steel pipe that has been quenched is heated to a tempering temperature of 500 to 650 ° C. Further, after this heating, it can be allowed to cool.

If the tempering temperature is less than 500 ° C, the temperature is too low and the desired tempering effect cannot be expected. On the other hand, at a high temperature where the tempering temperature exceeds 650 ° C., intermetallic compounds are precipitated and excellent low temperature toughness cannot be obtained. Therefore, the tempering temperature is set to a temperature in the range of 500 to 650 ° C. Preferably, the tempering temperature is 520 ° C. or higher. Preferably, the tempering temperature is 630 ° C. or lower.

Further, in the tempering treatment, the holding time (soaking heat holding time) is preferably 5 to 90 minutes in order to make the temperature uniform in the wall thickness direction and prevent the material from fluctuating.

By performing the above heat treatment (quenching treatment and tempering treatment), the structure of the seamless steel pipe becomes a structure containing a martensite phase, a ferrite phase and a retained austenite phase specified by a predetermined volume ratio. This makes it possible to obtain a stainless seamless steel pipe having desired strength and excellent corrosion resistance.

As described above, the stainless seamless steel pipe obtained by the present invention is a high-strength steel pipe having a yield strength of 758 MPa or more, and has excellent corrosion resistance and high-temperature strength. Preferably, the yield strength is 862 MPa or more. Preferably, the yield strength is 1034 MPa or less. The stainless seamless steel pipe of the present invention can be a stainless seamless steel pipe for oil wells (high-strength stainless seamless steel pipe for oil wells).

Hereinafter, the present invention will be further described based on Examples. The present invention is not limited to the following examples.

A steel pipe material was cast using molten steel having the composition shown in Table 1-1 and Table 1-2. Then, the steel pipe material was heated and formed by hot working using a model seamless rolling mill to obtain a seamless steel pipe having an outer diameter of 83.8 mm and a wall thickness of 12.7 mm, which was air-cooled. At this time, the heating temperature of the steel pipe material before hot working was set to 1250 ° C.

The test piece material was cut out from the obtained seamless steel pipe, reheated to a heating temperature of 960 ° C., the soaking time was set to 20 minutes, and quenching treatment was performed to cool (water-cool) to a cooling shutdown temperature of 30 ° C. .. Further, the soaking time is 20 minutes at the heating temperature (tempering temperature) 575 ° C., or the soaking time is 20 minutes at the heating temperature (tempering temperature) 525 ° C., or the heating temperature (tempering temperature) is 620 ° C. The soaking heat retention time was set to 40 minutes, and then an air-cooling tempering treatment was performed. The cooling rate for water cooling during the quenching treatment was 11 ° C./s, and the cooling rate for air cooling (leaving cooling) during the tempering treatment was 0.04 ° C./s. The blanks in Table 1-1 and Table 1-2 indicate that they are not intentionally added, and include not only the case where they are not contained (0%) but also the cases where they are unavoidably contained.

Each test piece was collected from the obtained heat-treated test piece material (seamless steel pipe) and subjected to microstructure observation, tensile test, high-temperature tensile test, and corrosion resistance test. The test method was as follows.

(1) Structure observation From the obtained heat-treated test material, test pieces for structure observation were collected so that the cross section orthogonal to the tube axis direction became the observation surface. The obtained tissue observation test piece was corroded with a bilera reagent (a reagent in which picrinic acid, hydrochloric acid and ethanol were mixed at a ratio of 2 g, 10 ml and 100 ml, respectively), and the tissue was imaged with a scanning electron microscope (magnification: 1000 times). Then, the microstructure fraction (area ratio (%)) of the ferrite phase was calculated using an image analyzer. This area ratio was defined as the volume fraction (%) of the ferrite phase.

Further, a test piece for X-ray diffraction is collected from the obtained heat-treated test material, ground and polished so that the cross section (C cross section) orthogonal to the tube axis direction becomes the measurement surface, and the X-ray diffraction method is performed. The tissue fraction of the retained austenite (γ) phase was measured using. The microstructure fraction of the retained austenite phase was converted by measuring the diffraction X-ray integrated intensity of the (220) plane of γ and the (211) plane of α (ferrite) and using the following equation.
γ (volume fraction) = 100 / (1+ (IαRγ / IγRα))
Here, the integrated intensity of Iα: α, the crystallographic theoretically calculated value of Rα: α, the integrated intensity of Iγ: γ, and the crystallographic theoretically calculated value of Rγ: γ are used.
The fraction of the martensite phase is the balance other than the ferrite phase and the residual γ phase.

(2) Tensile test From the obtained heat-treated test material, rod-shaped test pieces were collected so that the pipe axis direction was the tensile direction, and a tensile test was conducted in accordance with JIS Z 2241 (2011). The yield stress (YS) was determined as 0.2% proof stress. Here, those having a yield strength YS of 758 MPa or more were regarded as high strength and accepted, and those having a yield strength of less than 758 MPa were rejected.

(3) High-temperature tensile test From the obtained heat-treated test material, rod-shaped test pieces were collected so that the pipe axis direction was the tensile direction, and a tensile test at 200 ° C. was performed in accordance with JIS G 0567 (2012). Was carried out, and the yield stress (YS) was determined as 0.2% proof stress. Here, the same steel is subjected to the same heat treatment, and the ratio of the 0.2% proof stress at 200 ° C. to the yield stress (0.2% proof stress) obtained in the tensile test of (2) is 0.85 or more. In some cases, it was passed, and in the case of less than 0.85, it was rejected.

(4) Corrosion resistance test (carbon dioxide gas corrosion resistance test and sulfide stress cracking resistance test)
From the obtained heat-treated test material, 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. A corrosion test was carried out using the corrosion test piece, and carbon dioxide corrosion resistance was evaluated.

In the corrosion test to evaluate the carbon dioxide corrosion resistance, the corrosion test piece was immersed in a test solution held in an autoclave: a 20 mass% NaCl aqueous solution (liquid temperature: 200 ° C., CO _{2 gas atmosphere at 30 atm).} The immersion period was 14 days (336 hours). 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 obtained. Those having a corrosion rate of 0.127 mm / y or less were rejected, and those having a corrosion rate of more than 0.127 mm / y were rejected.

Furthermore, a round bar-shaped test piece (diameter: 3.81 mm) was manufactured from the obtained test piece material by machining, and a sulfide stress cracking resistance test (SSC (Sulfide Stress Cracking) test) was carried out.

The SSC resistance test was carried out by adding acetic acid to a test solution held in an autoclave: 0.165 mass% NaCl aqueous solution (liquid temperature: 25 ° C., 0.99 atm CO ₂ gas, 0.01 atm H ₂ S atmosphere). Immerse the test piece in an aqueous solution adjusted to pH: 3.0 by adding + sodium acetate, expose it for 720 hours with 90% of the breakdown stress applied, and observe the presence or absence of breakage or cracking in the test piece after the test. did. Those without breakage or cracking are accepted (indicated by the symbol "○" in Table 2-1 and Table 2-2), and those with breakage or cracking are rejected (in Table 2-1 and Table 2-2, the symbol "○"). It is indicated by "x").

The obtained results are shown in Table 2-1 and Table 2-2.

As shown in Table 2-1 and Table 2-2, all of the examples of the present invention have a high yield strength of YS: 758 MPa or more and a high temperature corrosion environment of 200 ° C. containing _{CO 2} and Cl ^−. It was a stainless seamless steel tube having excellent corrosion resistance (carbon dioxide corrosion resistance), excellent sulfide stress cracking resistance, and excellent high temperature strength.

Claims

By mass%,
C: 0.06% or less,
Si: 1.0% or less,
Mn: 0.01% or more and 0.90% or less,
P: 0.05% or less,
S: 0.005% or less,
Cr: 15.70% or more and 18.00% or less,
Mo: 1.60% or more and 3.80% or less,
Cu: 1.10% or more and 4.00% or less,
Ni: 3.0% or more and 6.0% or less,
Al: 0.10% or less,
N: 0.10% or less,
O: 0.010% or less,
V: Contains 0.120% or more and 1.000% or less,
Moreover, C, Si, Mn, Cr, Ni, Mo, Cu, and N satisfy the following formula (1).
The balance has a component composition consisting of Fe and unavoidable impurities,
It has a structure containing a martensite phase of 30% or more, a ferrite phase of 60% or less, and a retained austenite phase of 40% or less by volume.
Stainless steel seamless steel pipe with yield strength of 758 MPa or more.
13.0 ≤ -5.9 x (7.82 + 27C-0.91Si + 0.21Mn-0.9Cr + Ni-1.1Mo + 0.2Cu + 11N) ≤ 50.0 ... (1)
Here, C, Si, Mn, Cr, Ni, Mo, Cu, and N: are the contents (mass%) of each element, and if they are not contained, they are set to zero.
The stainless seamless steel pipe according to claim 1, wherein the martensite phase has a volume fraction of 40% or more, the retained austenite phase has a volume fraction of 30% or less, and the yield strength is 862 MPa or more.
The stainless seamless steel pipe according to claim 1 or 2, further comprising one group or two or more groups selected from the following groups A to D in mass% in addition to the component composition.
Group A: W: 3.0% or less Group B: Nb: less than 0.10% Group C: B: 0.010% or less, Ta: 0.3% or less, Co: 1.5% or less, Ti: One or more selected from 0.3% or less, Zr: 0.3% or less Group D: Ca: 0.01% or less, REM: 0.3% or less, Mg: 0.01% Hereinafter, one or more selected from Sn: 1.0% or less and Sb: 1.0% or less.
The method for manufacturing a stainless seamless steel pipe according to any one of claims 1 to 3.
The steel pipe material having the above-mentioned composition is heated at a heating temperature in the range of 1100 to 1350 ° C. and hot-worked to obtain a seamless steel pipe.
Next, the seamless steel pipe is reheated to a heating temperature in the range of 850 to 1150 ° C., and subjected to quenching treatment to cool it to a cooling stop temperature of 50 ° C. or lower at a cooling rate equal to or higher than air cooling.
Then, a method for manufacturing a stainless seamless steel pipe, which is subjected to a tempering process of heating to a temperature in the range of tempering temperature: 500 to 650 ° C.