WO2016079920A1

WO2016079920A1 - High-strength stainless steel seamless pipe for oil wells

Info

Publication number: WO2016079920A1
Application number: PCT/JP2015/005175
Authority: WO
Inventors: 江口　健一郎; 石黒　康英; 鈴木　健史
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2014-11-19
Filing date: 2015-10-13
Publication date: 2016-05-26
Also published as: AR102674A1; JPWO2016079920A1; JP6237873B2

Abstract

Provided is a high-strength stainless steel seamless pipe for oil wells, which has excellent corrosion resistance. This high-strength stainless steel seamless pipe for oil wells is configured to have a composition that contains 0.012-0.05% of C, 1.0% or less of Si, 0.1-0.5% of Mn, 0.05% or less of P, less than 0.005% of S, more than 16.0% but 18.0% or less of Cr, more than 2.0% but 4.0% or less of Mo, 3.0% or more but less than 5.0% of Ni, 0.5-2.0% of Cu, 0.01-3.0% of W, 0.02-0.5% of Nb, 0.001-0.1% of Al, 0.012-0.07% of N and 0.01% or less of O, with Nb, C, N and Mn satisfying Nb-6C-2N-0.04Mn ≥ -0.145(wherein Nb, C, N, and Mn represent the contents (mass%) of the respective elements), and with the balance made up of Fe and unavoidable impurities. This high-strength stainless steel seamless pipe for oil wells is also configured to have a structure that is composed of, in volume fractions, 50% or more of a tempered martensite phase, 20-40% of a ferrite phase and 10% or less of a residual austenite phase.

Description

High strength stainless steel seamless steel pipe for oil well

The present invention relates to a high-strength stainless steel seamless pipe suitable for use in oil wells or gas wells (hereinafter also simply referred to as oil wells) for crude oil or natural gas, and in particular, carbon dioxide (CO ₂ ), chlorine ions (Cl ⁻ ) In high-temperature corrosive environments and in environments containing hydrogen sulfide (H ₂ S).

In recent years, from the viewpoint of depletion of energy resources expected in the near future, oil fields with a deep corrosive environment in a so-called sour environment, such as high-depth oil fields and sulfides, which have not been conventionally excluded, The development of gas fields etc. is actively conducted. In such oil and gas fields, the depth is generally extremely deep, the atmosphere is also high temperature, and the environment is severely corrosive including CO ₂ , Cl ⁻ , and H ₂ S. The oil well steel pipe used in such an environment is required to have a material having both high strength and excellent corrosion resistance.

Conventionally, 13Cr martensitic stainless steel pipes have been generally used as oil well pipes used for mining in oil fields and gas fields in an environment containing CO ₂ , Cl ₂ ^{− and the} like. Recently, however, the development of oil wells under corrosive environments of even higher temperatures (up to 200 ° C) has been underway, and in such environments, 13Cr martensitic stainless steels sometimes lacked corrosion resistance. It was. For this reason, there has been a demand for an oil well steel pipe having excellent corrosion resistance that can be used in such an environment.

In response to such a request, for example, in Patent Document 1, in mass%, C: 0.005 to 0.05%, Si: 0.05 to 0.5%, Mn: 0.2 to 1.8%, P: 0.03% or less, S: 0.005% Contains Cr: 15.5-18%, Ni: 1.5-5%, Mo: 1-3.5%, V: 0.02-0.2%, N: 0.01-0.15%, O: 0.006% or less, Cr, Ni, Mo, Cu, and C satisfy a specific relational expression, and Cr, Mo, Si, C, Mn, Ni, Cu, and N are included so as to satisfy a specific relational expression, from the remaining Fe and inevitable impurities. A high-strength stainless steel with excellent corrosion resistance, and having a structure containing a martensite phase as a base phase and a ferrite phase in a volume ratio of 10 to 60%, or an austenite phase in a volume ratio of 30% or less. Steel pipes are described. Thus, CO _2, Cl ^- also exhibits sufficient corrosion resistance in high temperature harsh corrosive environments to 230 ° C. containing, yield strength: 654MPa for oil wells stainless steel tube further has a high toughness and high strength of greater than (95 ksi) Can be manufactured stably.

Patent Document 2 describes a high-strength stainless steel pipe for oil wells having high toughness and excellent corrosion resistance. In the technique described in Patent Document 2, in mass%, C: 0.04% or less, Si: 0.50% or less, Mn: 0.20 to 1.80%, P: 0.03% or less, S: 0.005% or less, Cr: 15.5 to 17.5 %, Ni: 2.5 to 5.5%, V: 0.20% or less, Mo: 1.5 to 3.5%, W: 0.50 to 3.0%, Al: 0.05% or less, N: 0.15% or less, O: 0.006% or less, and Cr, Mo, W and C satisfy a specific relationship, Cr, Mo, W, Si, C, Mn, Cu, Ni and N satisfy a specific relationship, and Mo and W satisfy a specific relationship. Thus, a steel pipe having a composition comprising the balance Fe and inevitable impurities and a structure containing a martensite phase as a base phase and a ferrite phase containing 10 to 50% by volume is obtained. As a result, a high-strength stainless steel pipe for oil wells with high strength exceeding yield strength: 654 MPa (95 ksi) and sufficient corrosion resistance in high-temperature severe corrosive environments containing CO ₂ , Cl ⁻ and H ₂ S. It is said that it can be manufactured stably.

Patent Document 3 describes a high-strength stainless steel pipe excellent in resistance to sulfide stress cracking and high-temperature carbon dioxide gas corrosion. In the technique described in Patent Document 3, in mass%, C: 0.05% or less, Si: 1.0% or less, Cr: 16% to 18%, Mo: 2% to 3%, Cu: 1 to 3.5% Ni: 3% or more and less than 5%, Al: 0.001 to 0.1%, O: 0.01% or less, Mn: 1% or less, N: 0.05% or less, Mn and N satisfy a specific relationship A composition composed of the balance Fe and inevitable impurities, a martensite phase as a main component, a ferrite phase of 10 to 40% by volume, and a residual austenite (γ) phase of 10% or less by volume The steel pipe has a structure including. As a result, the yield strength is 758 MPa (110 ksi) or higher, and it has sufficient corrosion resistance even in a high-temperature carbon dioxide environment of 200 ° C. Sufficient sulfide stress even when the environmental gas temperature is lowered. The stainless steel pipe is excellent in cracking and corrosion resistance.

Patent Document 4 describes a stainless steel pipe for oil wells. In the technique described in Patent Document 4, C: 0.05% or less, Si: 0.5% or less, Mn: 0.01 to 0.5%, P: 0.04% or less, S: 0.01% or less, Cr: more than 16.0% by mass% Contains 18.0%, Ni: more than 4.0 to 5.6%, Mo: 1.6 to 4.0%, Cu: 1.5 to 3.0%, Al: 0.001 to 0.10%, N: 0.050% or less, Cr, Cu, Ni and Mo specified In addition, (C + N), Mn, Ni, Cu and (Cr + Mo) satisfy a specific relationship, the composition comprising the balance Fe and unavoidable impurities, the martensite phase and the volume ratio of 10 to 40 % Of ferrite phase, the length of 50 μm in the thickness direction from the surface, and the ratio of the crossing of the phantom lines to a plurality of imaginary line segments arranged in a row in a range of 200 μm at a pitch of 10 μm is 85 The stainless steel pipe for oil wells has a structure with more than% and 0.2% proof stress: high strength of 758 MPa or more. As a result, the stainless steel pipe for oil wells has excellent corrosion resistance in a high temperature environment of 150 to 250 ° C. and excellent resistance to sulfide stress corrosion cracking at room temperature.

Patent Document 5 describes a high-strength stainless steel pipe for oil wells having high toughness and excellent corrosion resistance. In the technique described in Patent Document 5, in mass%, C: 0.04% or less, Si: 0.50% or less, Mn: 0.20 to 1.80%, P: 0.03% or less, S: 0.005% or less, Cr: 15.5 to 17.5 Ni: 2.5 to 5.5%, V: 0.20% or less, Mo: 1.5 to 3.5%, W: 0.50 to 3.0%, Al: 0.05% or less, N: 0.15% or less, O: 0.006% or less, Cr, Mo, W and C satisfy a specific relationship, Cr, Mo, W, Si, C, Mn, Cu, Ni and N, and Mo and W satisfy a specific relationship, respectively The steel pipe has a composition containing the remaining Fe and inevitable impurities, and having a structure in which the distance between any two points in the grain is 200 μm or less in the largest crystal grain. As a result, it has a high strength exceeding yield strength: 654 MPa (95 ksi), excellent toughness, and sufficient corrosion resistance in a high-temperature corrosive environment of 170 ° C or higher containing CO ₂ , Cl ⁻ , and H ₂ S. Trying to show.

Patent Document 6 describes a high-strength martensitic stainless steel seamless steel pipe for oil wells. In the technique described in Patent Document 6, C: 0.01% or less, Si: 0.5% or less, Mn: 0.1 to 2.0%, P: 0.03% or less, S: 0.005% or less, Cr: more than 15.5, 17.5% by mass %: Ni: 2.5-5.5%, Mo: 1.8-3.5%, Cu: 0.3-3.5%, V: 0.20% or less, Al: 0.05% or less, N: 0.06% or less, the balance Fe and inevitable A seamless steel pipe having a composition comprising impurities, preferably containing a ferrite phase of 15% or more by volume or a residual austenite phase of 25% or less, and the balance being a tempered martensite phase. Note that W: 0.25 to 2.0% and / or Nb: 0.20% or less may be contained. Thus, yield strength: 655 MPa or more 862MPa following high strength and yield ratio: has a 0.90 or more tensile properties, CO _2, Cl ^-, etc., further comprises H ₂ S, 170 ° C. or more high temperature severe corrosion It is said that a high-strength martensitic stainless steel pipe having sufficient corrosion resistance (carbon dioxide corrosion resistance, sulfide stress corrosion cracking resistance) even in the environment can be stably produced.

JP 2005-336595 A JP 2008-81793 A International Publication WO 2010/050519 International Publication WO 2010/134498 JP 2010-209402 A JP 2012-149317 A

However, with the recent development of severely corrosive oil and gas fields, oil well steel pipes have a high yield strength of 862 MPa (125 ksi) or higher and a high temperature of 200 ° C or higher. Excellent carbon dioxide corrosion resistance, excellent sulfide stress corrosion cracking resistance, and excellent sulfide stress crack resistance even under severe corrosive environments including CO ₂ , Cl ⁻ , and H ₂ S There is a demand for maintaining excellent corrosion resistance in combination.

However, the techniques described in Patent Documents 1 to 6 contain a large amount of alloy elements in order to improve corrosion resistance. Therefore, these techniques have a problem that the Ms transformation point becomes low, and the high strength of yield strength: 125 ksi class as described above cannot be stably secured together with excellent corrosion resistance.

Incidentally, the technique described in Patent Document 3 has a problem that it is necessary to substantially reduce N substantially in order to satisfy a specific relationship between Mn and N, resulting in an increase in manufacturing cost. In addition, the high-strength stainless steel pipe described in Patent Document 3 has a problem that the corrosion resistance is lowered under a severe corrosive environment and cannot be applied.

The present invention solves such a problem of the prior art, and has excellent carbon dioxide gas corrosion resistance, excellent sulfide stress corrosion cracking resistance, and excellent sulfide resistance even under severe corrosion environment with high strength and high temperature. An object of the present invention is to provide a high-strength stainless steel seamless steel pipe for oil wells that has stress cracking properties and excellent corrosion resistance. Here, “high strength” refers to a case where the yield strength is YS: a strength of 125 ksi (862 MPa) or higher.

“Excellent carbon dioxide corrosion resistance” as used herein refers to a test solution held in an autoclave: 20 mass% NaCl aqueous solution (liquid temperature: 200 ° C., 30 atmospheres CO ₂ gas atmosphere). When the piece is immersed and the immersion time is 336 hours, the corrosion rate is 0.127 mm / y or less.

In addition, “excellent sulfide stress corrosion cracking resistance” as used herein refers to a test solution retained in an autoclave: 20% by mass NaCl aqueous solution (liquid temperature: 100 ° C., 30 atm CO ₂ gas, 0.1 atm H ₂ S atmosphere), the test piece was immersed in an aqueous solution adjusted to pH: 3.3 by adding acetic acid + Na acetate, the immersion time was set to 720 hours, and 100% of the yield stress was applied as the applied stress. The case where cracks do not occur in the later test piece shall be said.

In addition, “excellent sulfide stress cracking resistance” as used herein refers to a test solution: 20 mass% NaCl aqueous solution (liquid temperature: 25 ° C., 0.9 atm CO ₂ gas, 0.1 atm H ₂ S atmosphere), The test piece is immersed in an aqueous solution adjusted to pH 3.5 by adding acetic acid + Na acetate, the immersion time is set to 720 hours, 90% of the yield stress is applied as the load stress, and the test piece after the test is cracked. The case where it does not occur shall be said.

In order to achieve the above-mentioned object, the present inventors have various effects on the Ms transformation point in order to obtain a high strength and corrosion resistance in a balanced and stable manner for a 17% Cr martensitic stainless steel pipe. The effects of alloying elements were studied earnestly. As a result, W and Nb are contained as essential components, and Nb, C, N and Mn are represented by the following formula (1): Nb-6C-2N-0.04Mn ≧ −0.145 (1)
(Where Nb, C, N and Mn: content of each element (mass%))
It has been found that by adjusting and containing so as to satisfy the requirements, excellent corrosion resistance can be maintained even under severe corrosive environment and desired strength can be stably obtained. The formula (1) is obtained experimentally by the present inventors, and adjusting the amounts of Nb, C, N and Mn so as to satisfy the formula (1) lowers the Ms transformation point. It has been found that it is important to stably obtain desired strength and further excellent corrosion resistance.

The present invention has been completed based on such findings and further studies. That is, the gist of the present invention is as follows.
(1) In mass%, C: 0.012 to 0.05%, Si: 1.0% or less, Mn: 0.1 to 0.5%, P: 0.05% or less, S: less than 0.005%, Cr: 16.0% to 18.0% or less, Mo: 2.0% to 4.0%, Ni: 3.0% to less than 5.0%, Cu: 0.5 to 2.0%, W: 0.01 to 3.0%, Nb: 0.02 to 0.5%, Al: 0.001 to 0.1%, N: 0.012 to 0.07% , O: 0.01% or less, and Nb, C, N and Mn as follows: (1) Formula Nb-6C-2N-0.04Mn ≧ -0.145 (1)
(Where Nb, C, N, Mn: content of each element (mass%))
And a composition comprising the balance Fe and inevitable impurities, a tempered martensite phase with a volume fraction of 50% or more, a ferrite phase with a volume fraction of 20 to 40%, and a volume fraction of 10 % High-strength stainless steel seamless pipe for oil wells having a structure consisting of a residual austenite phase of not more than% and a yield strength of YS: 862 MPa or more.
(2) In (1), in addition to the above composition, 1% selected from mass%, V: 0.5% or less, Ti: 0.3% or less, Zr: 0.2% or less, B: 0.0001 to 0.0050% High strength stainless steel seamless steel pipe for oil wells containing seeds or two or more.
(3) In the oil well according to (1) or (2), in addition to the above-mentioned composition, by mass%, Ca: 0.0050% or less, REM: 0.01% or less selected from 0.01% or less High strength stainless steel seamless pipe.
(4) In any one of (1) to (3), in addition to the above composition, one or two selected from Ta: 0.01 to 0.1% and Co: 0.01 to 1.0% by mass% High-strength stainless steel seamless steel pipe for oil wells.
(5) In any one of (1) to (4), in addition to the above-described composition, one or two selected from Sn: 0.20% or less and Mg: 0.0002-0.01% in mass% Contains high strength stainless steel seamless steel pipe for oil wells.

According to the present invention, the yield strength: high strength of 862 MPa or higher, high temperature of 200 ° C. or higher, and excellent carbon dioxide resistance even in a severe corrosive environment containing CO ₂ , Cl ⁻ , and H ₂ S. High-strength stainless steel seamless steel pipes for oil wells that combine corrosion resistance, excellent sulfide stress corrosion cracking resistance, and excellent sulfide stress crack resistance with excellent corrosion resistance can be obtained easily and stably. It has a remarkable industrial effect.

The high strength stainless steel seamless steel pipe for oil wells of the present invention is in mass%, C: 0.012 to 0.05%, Si: 1.0% or less, Mn: 0.1 to 0.5%, P: 0.05% or less, S: less than 0.005%, Cr : 16.0% to 18.0%, Mo: 2.0% to 4.0%, Ni: 3.0% to less than 5.0%, Cu: 0.5 to 2.0%, W: 0.01 to 3.0%, Nb: 0.02 to 0.5%, Al: 0.001 -0.1%, N: 0.012-0.07%, O: 0.01% or less, and Nb-6C-2N-0.04Mn ≧ -0.145 (where Nb, C, N and Mn are contained so as to satisfy the content (mass%) of each element), and have a composition composed of the balance Fe and inevitable impurities, and a tempered martens exceeding 50% by volume. It has a structure composed of a site phase, a ferrite phase having a volume ratio of 20 to 40%, and a retained austenite phase having a volume ratio of 10% or less, and has a yield strength of YS: 862 MPa or more.
First, the reason for limiting the composition of the seamless steel pipe of the present invention will be described. Hereinafter, unless otherwise specified, the mass% in the composition is simply expressed as%.

C: 0.012-0.05%
C is an element that lowers the Ms point and forms carbides to lower the corrosion resistance. For this reason, C content needs to be reduced to 0.05% or less. On the other hand, reduction of the C content to less than 0.012% leads to an increase in refining costs. Therefore, the C content is limited to 0.012 to 0.05%. Preferably, the C content is 0.020 to 0.040%.

Si: 1.0% or less Si is an element that acts as a deoxidizing agent. In order to obtain such an effect, it is desirable to contain 0.005% or more of Si. On the other hand, when a large amount of Si exceeds 1.0%, the hot workability decreases. For this reason, Si content was limited to 1.0% or less. Preferably, the Si content is 0.1 to 0.6%. More preferably, the Si content is 0.10 to 0.30%.

Mn: 0.1-0.5%
Mn is an element that increases the strength of martensitic stainless steel, and needs to contain 0.1% or more of Mn in order to ensure a desired strength. On the other hand, if the Mn content exceeds 0.5%, the toughness decreases. Therefore, the Mn content is limited to the range of 0.1 to 0.5%. More preferably, the Mn content is 0.15 to 0.30%.

P: 0.05% or less P is an element that lowers corrosion resistance such as carbon dioxide corrosion resistance and sulfide stress cracking resistance, and is preferably reduced as much as possible in the present invention, but the P content should be 0.05% or less. Acceptable. For these reasons, the P content is limited to 0.05% or less. Preferably, the P content is 0.02% or less.

S: Less than 0.005% S is an element that significantly reduces the hot workability and hinders the stable operation of the seamless steel pipe manufacturing process, and is preferably reduced as much as possible, but if the S content is less than 0.005% This makes it possible to produce seamless steel pipes in the normal process. For this reason, the S content is limited to less than 0.005%. Preferably, the S content is 0.002% or less.

Cr: 16.0% to 18.0% or less Cr is an element that contributes to improving corrosion resistance by forming a protective film. If it is 16.0% or less, the desired corrosion resistance cannot be ensured, so it is necessary to contain more than 16.0% Cr. To do. On the other hand, if the Cr content exceeds 18.0%, the ferrite fraction becomes too high and the desired strength cannot be ensured. For this reason, the Cr content is limited to the range of more than 16.0% and less than 18.0%. Preferably, the Cr content is 16.2 to 17.5%.

Mo: 2.0% to 4.0% Mo stabilizes the protective film, increases resistance to pitting corrosion due to Cl ^- and low pH, and improves resistance to sulfide stress cracking and resistance to sulfide stress corrosion It is an element. In order to acquire such an effect, it is necessary to contain Mo exceeding 2.0%. On the other hand, Mo is an expensive element. If Mo content exceeds 4.0%, the material cost increases, and the toughness and the resistance to sulfide stress corrosion cracking decrease. For this reason, the Mo content is limited to more than 2.0% and not more than 4.0%. More preferably, the Mo content is 2.2 to 3.0%.

Ni: 3.0% or more and less than 5.0% Ni is an element that strengthens the protective film and contributes to the improvement of corrosion resistance. Ni also increases the strength of the steel by solid solution strengthening. Such an effect becomes remarkable when the Ni content is 3.0% or more. On the other hand, the Ni content of 5.0% or more decreases the stability of the martensite phase and decreases the strength. For this reason, Ni content was limited to 3.0% or more and less than 5.0%. Preferably, the Ni content is 3.5 to 4.5%.

Cu: 0.5-2.0%
Cu is an element that strengthens the protective film and suppresses hydrogen intrusion into the steel and improves the resistance to sulfide stress cracking and the resistance to sulfide stress corrosion cracking. In order to acquire such an effect, 0.5% or more of content is required. On the other hand, if Cu is contained in excess of 2.0%, the effect is saturated, and an effect commensurate with the content cannot be expected, which is economically disadvantageous. Therefore, the Cu content is limited to the range of 0.5 to 2.0%. Preferably, the Cu content is 0.5 to 1.5%.

W: 0.01-3.0%
W is an important element in the present invention that contributes to improving the strength of the steel and stabilizes the protective film to increase the resistance to sulfide stress cracking and the resistance to sulfide stress corrosion cracking. By containing W in combination with Mo, the resistance to sulfide stress cracking is particularly improved. In order to obtain such an effect, it is necessary to contain 0.01% or more of W. On the other hand, the inclusion of a large amount of W exceeding 3.0% reduces toughness. For this reason, W is limited to the range of 0.01 to 3.0%. Preferably, the W content is 0.5 to 2.0%. More preferably, the W content is 0.8 to 1.2%.

Nb: 0.02 to 0.5%
Nb is an important element that fixes C and N as carbonitrides, reduces solid solution C and solid solution N affecting the Ms point, and suppresses a decrease in the Ms point, thereby contributing to high strength. In order to obtain such an effect, it is necessary to contain 0.02% or more of Nb. On the other hand, the inclusion of a large amount of Nb exceeding 0.5% causes a decrease in toughness and resistance to sulfide stress corrosion cracking. Therefore, the Nb content is limited to the range of 0.02 to 0.5%. Preferably, the Nb content is 0.02 to 0.3%. More preferably, the Nb content is 0.10 to 0.20%.

Al: 0.001 to 0.1%
Al is an element that acts as a deoxidizer. In order to obtain such an effect, it is necessary to contain 0.001% or more of Al. On the other hand, if the Al content exceeds 0.1%, the amount of oxide increases, the cleanliness decreases, and the toughness decreases. Therefore, the Al content is limited to the range of 0.001 to 0.1%. Preferably, the Al content is 0.01 to 0.07%.

N: 0.012-0.07%
N is an element that affects the Ms point, and is desirably reduced from the viewpoint of suppressing a decrease in the Ms point. However, reducing the N content by less than 0.012% raises the refining costs. On the other hand, when N is contained exceeding 0.07%, nitride is formed and toughness is lowered. For this reason, the N content is limited to 0.012 to 0.07%. Preferably, the N content is 0.02 to 0.06%.

O: 0.01% or less Since O exists as an oxide in steel, it adversely affects various properties. For this reason, in the present invention, it is desirable to reduce the O content as much as possible. In particular, when O exceeds 0.01%, hot workability, corrosion resistance, and toughness deteriorate. For this reason, the O content is limited to 0.01% or less. Preferably, the O content is 0.006% or less.

In the present invention, among the components described above, Nb, C, N, and Mn are within the above-described range, and the following formula (1) Nb-6C-2N-0.04Mn ≧ −0.145 (1)
(Where Nb, C, N and Mn: content of each element (mass%))
The content is adjusted so as to satisfy.

When the contents of Nb, C, N and Mn do not satisfy the formula (1), the Ms transformation point becomes low, the amount of retained austenite increases, and the desired strength cannot be obtained stably. For this reason, in this invention, it decided to adjust content of Nb, C, N, and Mn within the range of each above-mentioned component and satisfying (1) Formula.

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

In the present invention, in addition to the above-described composition, the element selected as necessary is selected from V: 0.5% or less, Ti: 0.3% or less, Zr: 0.2% or less, B: 0.0001 to 0.0050%. Species or two or more, and / or Ca: 0.0050% or less, REM: One or two selected from 0.01% or less, and / or Ta: 0.01 to 0.1%, Co: 0.01 to 1.0% And / or Sn: 0.20% or less and Mg: 0.0002 to 0.01% may be included.

V: 0.5% or less, Ti: 0.3% or less, Zr: 0.2% or less, B: One or more selected from 0.0001 to 0.0050% V, Ti, Zr and B all increase strength It is an element to be selected, and can be selected as necessary and contained in one or more kinds. V, Ti, Zr and B have an effect of improving the resistance to sulfide stress cracking in addition to the above-described effects. In order to obtain such an effect, V: 0.01% or more, Ti: 0.005% or more, Zr: 0.01% or more, B: 0.0001% or more are preferable, but V: 0.5%, Ti: 0.3 %, Zr: 0.2%, and B: 0.0050%, if included, the toughness is lowered. For this reason, when it is contained, it is preferable to limit to V: 0.5% or less, Ti: 0.3% or less, Zr: 0.2% or less, and B: 0.0001 to 0.0050%.

One or two selected from Ca: 0.0050% or less, REM: 0.01% or less Both Ca and REM are elements that contribute to the improvement of resistance to sulfide stress corrosion cracking through the control of sulfide morphology It can contain 1 type or 2 types as needed. In order to acquire such an effect, it is desirable to contain Ca: 0.0001% or more and REM: 0.001% or more. On the other hand, even if Ca: 0.0050% and REM: 0.01% are contained in excess, the effect is saturated and an effect commensurate with the content cannot be expected. For this reason, when it contains, it is preferable to limit to Ca: 0.0050% or less and REM: 0.01% or less, respectively.

One or two types selected from Ta: 0.01 to 0.1% and Co: 0.01 to 1.0% Ta and Co are elements that improve strength. Select one or two as required. Can be contained. In order to obtain such an effect, it is preferable to contain Ta: 0.01% or more and Co: 0.01% or more. On the other hand, even if the content exceeds Ta: 0.1% and Co: 1.0%, the effect is saturated and an effect commensurate with the content cannot be expected. For this reason, when it is contained, it is preferably limited to the ranges of Ta: 0.01 to 0.1% and Co: 0.01 to 1.0%, respectively.

Sn: 0.20% or less, Mg: One or two selected from 0.0002 to 0.01% Sn and Mg are both elements that improve corrosion resistance. Select one or two as required. Can be contained. In order to obtain such effects, it is desirable to contain Sn: 0.01% or more and Mg: 0.0002% or more. On the other hand, if the content exceeds Sn: 0.20% and Mg: 0.01%, the effect is saturated and an effect commensurate with the content cannot be expected. Therefore, when it is contained, it is preferable to limit to Sn: 0.20% or less and Mg: 0.0002 to 0.01%, respectively.

Next, the reason for limiting the structure of the high-strength stainless steel seamless steel pipe for oil wells of the present invention will be described.

The seamless steel pipe of the present invention has the above-described composition, and a tempered martensite phase having a volume ratio of 50% or more as a main phase, a ferrite phase having a volume ratio of 20 to 40%, and a volume ratio of 10% or less. Of the remaining austenite phase.

In the seamless steel pipe of the present invention, in order to ensure a desired strength, the tempered martensite phase is the main phase and the volume ratio is 50% or more. In the present invention, at least 20% or more of the ferrite phase is precipitated as the second phase by volume ratio. Thereby, progress of a crack can be suppressed and desired corrosion resistance can be secured. On the other hand, if a large amount of ferrite phase exceeds 40%, the strength decreases and the desired strength cannot be ensured. For this reason, the ferrite phase is limited to the range of 20 to 40% by volume.

Furthermore, in the present invention, a residual austenite phase is precipitated as a second phase in addition to the ferrite phase by 10% or less by volume. Due to the presence of the retained austenite phase, ductility and toughness are improved. In order to obtain such an effect, it is desirable to deposit 1% or more. On the other hand, a large amount of residual austenite phase exceeding 10% cannot secure the desired strength. For this reason, the residual austenite phase was limited to 10% or less by volume ratio. The residual austenite phase is preferably 4 to 8% by volume.

In the present invention as described above, the measurement of the structure having a tempered martensite phase of 50% or more, a ferrite phase of 20 to 40%, and a residual austenite phase of 10% or less in volume ratio, First, a specimen for tissue observation was collected from the specimen material so that the cross section in the tube axis direction became the observation surface, and the specimen for tissue observation was collected with 2 g, 10 ml, and 100 ml of Villera reagent (picric acid, hydrochloric acid, and ethanol, respectively). The structure is corroded with a ratio) and the structure is imaged with a scanning electron microscope (1000 times), and the structure fraction (volume%) of the ferrite phase is calculated using an image analyzer. And the test piece for X-ray diffraction is extract | collected from this test piece raw material so that C cross section may become a measurement surface, and a residual austenite phase fraction is measured using a X-ray diffraction method. The diffraction X-ray integrated intensity of the (220) plane of γ and the (211) plane of α is measured by X-ray diffraction, and the following formula γ (volume ratio) = 100 / (1+ (IαRγ / IγRα))
(Where Iα: α integral strength, Rα: α crystallographic theoretical calculation value, Iγ: γ integral strength, Rγ: γ crystallographic theoretical calculation value)
Convert using. The volume ratio of the tempered martensite phase can be calculated as the remainder other than these phases.

Further, this structure of the present invention can be controlled by setting a specific component composition, performing a tempering process at a specific temperature, performing a quenching process at a specific temperature, and the like.

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

In the present invention, a stainless steel seamless steel pipe having the above composition is used as a starting material.

The production method of the starting material is not particularly limited, and any known production method of seamless steel pipes can be applied. A preferred method for producing this starting material will be described below.

The molten steel having the composition described above can be melted by a conventional melting method such as a converter and used as a steel pipe material such as a billet by a normal casting method such as a continuous casting method. Subsequently, these steel pipe materials are heated and subjected to a Mannesmann-plug mill type or Mannesmann-Mandrel mill type hot pipe forming process to produce seamless steel pipes having the desired dimensions with the above-described composition, which can be used as starting materials. . In addition, there is no problem even if it is a seamless steel pipe by hot extrusion by a press method.

In addition, after pipe making, it is cooled to room temperature at a cooling rate higher than air cooling. Thereby, the structure of the steel pipe can be a structure having the martensite phase as the main phase (50% or more by volume ratio).

In the present invention, it is preferable to perform a quenching process and a tempering process following the cooling after the pipe making.

The quenching treatment is preferably a treatment in which the starting material is heated to a heating temperature of 850 ° C. or higher and then cooled to a cooling stop temperature of 50 ° C. or lower at a cooling rate of air cooling or higher. If the heating temperature is too low (less than 850 ° C.), the desired strength may not be ensured. This is because the reverse transformation from martensite to austenite does not occur during heating, and the transformation from austenite to martensite does not occur during cooling. On the other hand, when the heating temperature is higher than 1150 ° C., the crystal grains may be coarsened. For this reason, it is preferable to limit the heating temperature of the quenching process to 850 to 1150 ° C. More preferably, the temperature is 900 to 1050 ° C.

The tempering treatment is preferably a treatment in which a seamless steel pipe subjected to the quenching treatment is heated to a tempering temperature of 500 to 650 ° C. and allowed to cool or air cool. If the tempering temperature is less than 500 ° C., the desired tempering effect may not be expected because the temperature is too low. On the other hand, at a high temperature exceeding 650 ° C., an as-quenched martensite phase is generated, and it may not be possible to satisfy all of the desired strength, toughness, and excellent corrosion resistance. The tempering temperature is more preferably 550 to 600 ° C. In addition, the heating temperature and tempering temperature of said hardening process point out the temperature of the steel pipe outer surface.

As described above, by performing quenching treatment and tempering treatment under specific conditions, the structure of the seamless steel pipe becomes a structure having a tempered martensite phase, a ferrite phase and a retained austenite phase having a specific volume ratio. . Thereby, it becomes a stainless steel seamless steel pipe for oil wells having high strength, high toughness and excellent corrosion resistance.

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

Molten steel with the composition shown in Table 1 is melted in a converter, cast into a billet (steel pipe material) by a continuous casting method, piped using a model seamless rolling mill, and seamless steel pipe (outer diameter 83.8mmφ x wall thickness) 15.0 mm). In addition, it air-cooled after pipe making.

From the obtained seamless steel pipe, a test piece material was cut out, heated to the temperature shown in Table 2, subjected to quenching treatment with water and then cooled to the cooling stop temperature shown in Table 2, and then immediately shown in Table 2. And then tempered by air cooling after heating.

Specimens were collected from the specimen material (seamless steel pipe) that had been subjected to quenching and tempering, and subjected to structure observation, tensile test, impact test, and corrosion resistance test. The test method was as follows.

(1) Tissue observation A tissue observation test piece was collected from the obtained test piece material so that the cross section in the tube axis direction was an observation surface. The obtained specimen for tissue observation was corroded with Virella reagent (2g, 10ml and 100ml mixed with picric acid, hydrochloric acid and ethanol, respectively), and the tissue was imaged with a scanning electron microscope (1000x), and image analysis was performed. Using the apparatus, the structural fraction (volume%) of the ferrite phase was calculated.

In addition, an X-ray diffraction test piece was collected from the obtained test piece material so that the C cross section was a measurement surface, and the residual austenite phase fraction was measured using an X-ray diffraction method. The diffraction X-ray integrated intensity of the (220) plane of γ and the (211) plane of α is measured by X-ray diffraction, and the following formula γ (volume ratio) = 100 / (1+ (IαRγ / IγRα))
Here, Iα: α integral strength Rα: α crystallographic theoretical calculated value Iγ: γ integral strength Rγ: γ crystallographic theoretical calculated value.
The fraction of the tempered martensite phase was calculated as the remainder other than these phases.

(2) Tensile test API strip specimen specified by API standard 5CT is collected from the obtained specimen material so that the tube axis direction is the tensile direction, and is tensioned according to the API regulations. Tests were conducted to determine tensile properties (yield strength YS, tensile strength TS).

(3) Impact test V-notch test piece (5mm thick) is collected from the obtained specimen material so that the longitudinal direction of the specimen is perpendicular to the tube axis direction in accordance with JIS Z 2242. The Charpy impact test was conducted. The test temperature was −10 ° C., the impact value vE ₋₁₀ (J / cm ² ) at −10 ° C. was determined, and the toughness was evaluated. Three test pieces were used, and the arithmetic average of the obtained values was taken as the impact value (J / cm ² ) of the steel pipe.

(4) Corrosion resistance test A corrosion test piece having a thickness of 3 mm, a width of 30 mm and a length of 40 mm was produced from the obtained test piece material by machining, and a carbon dioxide gas corrosion test was performed.

The carbon dioxide corrosion test is performed by immersing the test piece in a test solution retained in an autoclave: 20 mass% NaCl aqueous solution (liquid temperature: 200 ° C., 30 atmospheres CO ₂ gas atmosphere), and the immersion period is 14 days ( 336 hours). About the test piece after a test, the weight was measured and the corrosion rate calculated from the weight loss before and behind a corrosion test was calculated | required. The case where the corrosion rate is 0.127 mm / y or less is acceptable. Further, the presence or absence of pitting corrosion on the surface of the test piece was observed using a magnifier with a magnification of 10 times for the test piece after the corrosion test. In addition, the presence of pitting means the case where the diameter is 0.2 mm or more. The case without pitting corrosion is acceptable.

Furthermore, in accordance with NACE TM0177 Method A, a round bar-like test piece (diameter: 6.4mmφ) is manufactured by machining from the obtained test piece material, and a sulfide stress cracking resistance test (SSC resistance test) is performed. did.

In addition, a four-point bending specimen with a thickness of 3 mm, a width of 15 mm and a length of 115 mm was collected from the obtained specimen material by machining, and in accordance with EFC17, a sulfide stress corrosion cracking resistance test (SCC resistance Test).

The anti-SCC test was performed by adding acetic acid + Na acetate to a test solution held in an autoclave: 20% by mass NaCl aqueous solution (liquid temperature: 100 ° C, H ₂ S: 0.1 atm, CO ₂ : 30 atm). The test piece was immersed in an aqueous solution adjusted to pH: 3.3, the immersion period was set to 720 hours, and 100% of the yield stress was applied as the applied stress. About the test piece after a test, the presence or absence of a crack was observed. The case where there is no crack is a pass.

The SSC resistance test is performed by adding acetic acid + Na acetate to a test solution held in an autoclave: 20% by mass NaCl aqueous solution (liquid temperature: 25 ° C, H ₂ S: 0.1 atm, CO ₂ : 0.9 atm). : The test piece was immersed in an aqueous solution adjusted to 3.5, the immersion period was 720 hours, and 90% of the yield stress was applied as the load stress. The test piece after the test was observed for cracks. The case where there is no crack is a pass.

Table 3 shows the obtained results.

Both Examples present invention, yield strength: and more high strength 862MPa, impact value vE _-10 at -10 ° C.: and 50 J / cm ² or more high toughness, CO _2, Cl ^- high as 200 ° C. containing the Excellent corrosion resistance (carbon dioxide corrosion resistance) in corrosive environments, and no cracking (SSC, SCC) in an environment containing H ₂ S. Excellent sulfide stress cracking resistance and sulfide stress corrosion cracking It is a high-strength stainless steel seamless steel pipe that combines properties. On the other hand, in the comparative example that is out of the scope of the present invention, at least one of Cu, W, Nb, the above-described formula (1), and the volume ratio of retained austenite is out of the scope of the present invention. At least one of sulfide stress cracking resistance (SSC resistance) and sulfide stress corrosion cracking resistance (SCC resistance) was inferior.

Claims

% By mass
C: 0.012 to 0.05%, Si: 1.0% or less,
Mn: 0.1 to 0.5%, P: 0.05% or less,
S: less than 0.005%, Cr: more than 16.0% and less than 18.0%,
Mo: 2.0% to 4.0%, Ni: 3.0% to less than 5.0%,
Cu: 0.5-2.0%, W: 0.01-3.0%,
Nb: 0.02 to 0.5%, Al: 0.001 to 0.1%,
N: 0.012 to 0.07%, O: 0.01% or less, and Nb, C, N, and Mn so as to satisfy the following formula (1), the balance Fe and an unavoidable impurity, It has a structure composed of a tempered martensite phase with a volume ratio of 50% or more, a ferrite phase with a volume ratio of 20-40%, and a retained austenite phase with a volume ratio of 10% or less, and a yield strength YS: High-strength stainless steel seamless steel pipe for oil wells that is 862 MPa or higher.
Nb-6C-2N-0.04Mn≥-0.145 (1)
Where Nb, C, N and Mn: content of each element (mass%)
In addition to the above composition, the composition further contains one or more selected from the group consisting of V: 0.5% or less, Ti: 0.3% or less, Zr: 0.2% or less, and B: 0.0001 to 0.0050%. The high strength stainless steel seamless steel pipe for oil wells according to claim 1.
The high-strength stainless steel joint for oil wells according to claim 1 or 2, further comprising one or two kinds selected from Ca: 0.0050% or less and REM: 0.01% or less in mass% in addition to the composition. Steel-free pipe.
4. The oil well according to claim 1, further comprising one or two selected from Ta: 0.01 to 0.1% and Co: 0.01 to 1.0% by mass% in addition to the composition. High strength stainless steel seamless steel pipe.
The oil well according to any one of claims 1 to 4, further comprising one or two kinds selected from Sn: 0.20% or less and Mg: 0.0002-0.01% by mass% in addition to the composition. High strength stainless steel seamless pipe.