JP2015094004A - MARTENSITIC Cr-CONTAINING STEEL MATERIAL - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a martensitic Cr-containing steel material which can achieve a yield strength of 539 MPa or less by low-temperature and short-lasting tempering and which has COcorrosion resistance and sulfide stress cracking resistance under corrosive atmosphere containing CO, HS, Clor the like.SOLUTION: There is provided the martensitic Cr-containing steel material having a chemical composition which contains C≤0.15%, Si:0.05 to 1.0%, Mn:0.01 to less than 3.0%, Cr:over 9.0% and 12.0% or less, effective Cr>8.2% in [Cr-11×C], sol.Al:0.001 to 0.05% and the balance Fe with impurities containing P≤0.03%, S≤0.003%, Ni≤0.3%, N≤0.10% and O≤0.01%, and which forms a martensite single phase. The martensitic Cr-containing steel material has structure satisfying a dislocation density (m) of 5.3×10or less and carbide density (m) of 2.2×10or less in the 2 dimension and further has a yield strength of 539 MPa or less. Specific amount of one or more of Cu, V, Ca and Mg may be contained instead of a part of Fe.

Description

The present invention relates to a martensitic Cr-containing steel material, and more particularly, to a martensitic Cr-containing steel material excellent in resistance to sulfide stress cracking and CO ₂ corrosion. More specifically, the present invention relates to a steel pipe for oil wells used in oil wells or natural gas wells having severe corrosive environments containing carbon dioxide (CO ₂ ), hydrogen sulfide (H ₂ S), chloride ions (Cl ⁻ ) and the like. The present invention relates to a martensitic Cr-containing steel material that is suitable for use as a raw material and is relatively inexpensive and has excellent CO ₂ corrosion resistance and sulfide stress cracking resistance.

  The “oil well steel pipe” is a casing used for excavation of an oil well or a gas well, extraction of crude oil or natural gas, for example, as described in the definition column of number 3514 of JIS G 0203 (2009). , Tubing and drill pipe.

With the recent rise in crude oil prices, the development of oil wells and natural gas wells under more severe corrosive environments has been underway. Such oil wells and natural gas wells _{generally, CO 2, H 2 S,} Cl - often have a severe corrosive environment, including. Therefore, a steel material having both CO ₂ corrosion resistance and sulfide stress cracking resistance is required as a material for the oil well steel pipe used in such oil wells and gas wells.

In general, in an environment containing CO ₂ and Cl ⁻ , martensitic stainless steel (so-called “13% Cr steel”) having excellent CO ₂ corrosion resistance and containing about 13% by mass of Cr is used. Oil well steel pipes are used. However, when H ₂ S coexists in the environment, sulfide stress cracking (hereinafter referred to as “SSC”) tends to occur in “13% Cr steel”.

  For this reason, “Super 13Cr steel” with improved SSC resistance of “13% Cr steel” is used for oil well steel pipes used in such an environment, and “duplex stainless steel” is used for more severe corrosion environments. “Ni—Cr alloy” is used. However, it is difficult to say that “Super 13Cr steel” has sufficient SSC resistance, and “duplex stainless steel” and “Ni—Cr alloy” are very expensive.

On the other hand, CO _2, H ₂ S, Cl ^- severe corrosive environment, including in oil wells and gas wells, expensive "duplex stainless steel" and "Ni-Cr alloy" oil well with (steel) tube In order to avoid the use, take measures such as replacing oil well steel pipes made of inexpensive SSC-resistant steel in a short period of time or reducing the corrosion rate by applying inner coating to such oil well steel pipes. I must take it.

Against this background, CO _2, H ₂ S, Cl ^- in oil wells and gas wells in severe corrosive environments, including, available without applying an inner coating, the excellent CO ₂ corrosion resistance and SSC resistance There is a demand for steel pipes for oil wells made of inexpensive steel.

  In response to the above request, for example, in Patent Document 1, in mass%, C: 0.30% or less, Si: 0.60% or less, Mn: 0.30 to 1.50%, P: 0.03 %: S: 0.005% or less, Cr: 3.0 to 9.0%, Al: 0.005% or less, and the balance being Fe and unavoidable impurities "Contained steel pipe" is disclosed. In addition, this Cr-containing steel pipe for oil wells may further include (a) Nb: 0.30% or less and V: 0.50% or less, 0.01% or more, b) Along with Nb or / and V described above, one or more selected from Cu, Ni, Mo, Ca, Ti, Zr, B and W may be included.

Then, Patent Document 1, oil well Cr-containing steel described above, CO ₂ partial pressure: 1 MPa, temperature: in CO ₂ corrosion test at 100 ° C., less resistant CO ₂ corrosion corrosion rate 0.100 mm / year In addition, SSC does not occur in a constant load test under the conditions of “Solution A (pH: 2.7)” and applied stress: 551 MPa in accordance with “NACE-TM0177-96 method A”. It is shown. In addition, the said Cr containing steel pipe for oil wells in patent document 1 is adjusted to the yield strength of what is called "80 ksi class (551-655 MPa)."

Further, in Patent Document 2, in mass%, C: 0.1 to 0.3%, Si: <1.0%, Mn: 0.1 to 1.0%, P: <0.02%, S : <0.01%, Cr: 11-14%, Ni: <0.5%, if necessary, (a) N: 0.01-0.1% or (b) N : 0.01 to 0.1%, and steel containing one or more of Ca, Mg, and REM 0.001 to 0.3% and having a martensite-based structure as Ac ₃ cooling to the Ms point temperature below after heating to a temperature between Ac _1, martensitic excellent in SSC resistance, characterized in that cooling to room temperature after heating to further Ac ₁ temperature below Thereafter A method for producing stainless steel is disclosed.

  This method is a technique characterized by performing the “two-phase region heat treatment” of the following <2> in an intermediate step between quenching and tempering.

That is, the technique proposed in Patent Document 2 is for hot rolled material,
<1> First, normalization treatment (corresponding to the above-mentioned “quenching”) is performed to make the structure mainly composed of martensite phase.
<2> Next, the two-phase region is heated so that a small amount of austenite phase reversely transformed exists in the structure mainly composed of the tempered martensite phase, and then cooled, so that the small amount of austenite phase is again converted into martensite. Transformed into the site phase,
<3> Finally, tempering is performed to soften the small amount of martensite phase.
Thus, SSC resistance and toughness are improved, and a low strength of 507 MPa (51.7 kgf / mm ² ) or less is realized in yield strength (0.2% yield strength).

JP 2000-63994 A JP-A-7-76722

Nakajima Koichi et al .: CAMP-ISIJ, 17 (2004), 396 G. K. Williamson and W.W. H. Hall: Acta Metal. 1 (1953), 22 H. M.M. Rietveld: J.M. Appl. Cryst. 2 (1969), 65

  As will be described later, it is considered that the SSC resistance of martensitic Cr-containing steel deteriorates due to an increase in yield strength (0.2% proof stress). For this reason, when using a high-strength material having a yield strength equal to or higher than a so-called “110 ksi class (758 to 862 MPa)”, it is assumed that it is considerably difficult to secure the SSC resistance.

On the other hand, the Cr-containing steel pipe for oil wells proposed in Patent Document 1 is adjusted to a low yield strength of so-called “80 ksi class” as described above. For this reason, in the invention described in Patent Document 1, it is considered that good SSC resistance is easily secured. However, the above-mentioned Cr-containing steel pipe for oil wells has a problem in CO ₂ corrosion resistance because of its low Cr content.

  The martensitic stainless steel using the two-phase region heat treatment disclosed in Patent Document 2, as is clear from the production method, requires another heat treatment to be sandwiched between quenching and tempering. It is difficult to avoid increasing costs. In addition, the final structure formed consists of a low-strength martensite structure that has been tempered at high temperature and a high-strength martensite structure that has been formed by two-phase region heat treatment and subsequent tempering. Therefore, there are some concerns regarding the SSC resistance.

  Patent Document 2 includes the following [1] and [2].

  [1] Generally, in the field of carbon steel and low alloy steel, the sulfide cracking resistance of steel depends on the steel strength, and the lower the strength, the better. The same is considered for martensitic stainless steel.

[2] In the conventional manufacturing method (method of carrying out normalization-tempering treatment), the yield strength cannot be reduced to 55 to 60 kgf / mm ² (539 to 588 MPa, 78 to 85.3 ksi) or less, which is inevitably obtained. There is a limit to the resistance to sulfide cracking.

  The martensitic Cr-containing steel material is usually manufactured by heat treatment of quenching-tempering (normalization-tempering referred to in Patent Document 2), but the yield strength (0.2% yield strength) is 539 MPa by quenching-tempering. An industrial production method capable of realizing a low strength of (78 ksi) or less by tempering at a relatively low temperature and in a short time has not yet been clarified.

  For this reason, if it becomes possible to provide a martensitic Cr-containing steel material excellent in SSC resistance with a yield strength of 539 MPa or less highly economically by an industrial production method, it is inexpensive except for oil well steel pipes for high depths. This can be used as a material for steel pipes for oil wells with excellent SSC resistance.

The present invention has been made in view of the above-described situation, and has a CO ₂ corrosion resistance and an SSC resistance, and can be manufactured by a highly economical industrial production method. It aims at providing the martensitic Cr containing steel material suitable for using as.

In order to solve the above-mentioned problems, the present inventors secure the CO ₂ corrosion resistance by optimizing the Cr content of the martensitic Cr-containing steel, and reduce the yield strength as much as possible. Specifically, it was examined whether or not an alloy design capable of increasing the SSC resistance by setting the pressure to 539 MPa or less was possible.

That is, the present inventors first simulated an oil well environment containing high-pressure CO ₂ from the viewpoint of CO ₂ corrosion resistance, in an autoclave having a CO ₂ partial pressure of 3 MPa, 5% NaCl, and a temperature of 125 ° C. Based on the CO ₂ corrosion test, the appropriate range of Cr content of the martensitic Cr-containing steel was examined.

As a result, the CO ₂ corrosion resistance of the martensitic Cr-containing steel depends on the Cr content and the C content in the steel,
It has been found that the corrosion rate in the environment can be reduced as the effective Cr = Cr content−11 × C content increases. In addition, when considering the actual oil and gas well environment, in the above-mentioned corrosive environment, it is necessary that the corrosion rate be 0.25 mm / year or less. The lower limit is 8.2%.

  Next, the present inventors examined a method for realizing a yield strength of 539 MPa or less without impairing SSC resistance using martensitic Cr-containing steel containing effective Cr of 8.2% or more.

  As a result, even when tempering as a heat treatment after quenching was performed at a relatively low temperature and in a short time, it was possible to reduce the yield strength to 539 MPa or less by optimizing the chemical composition. However, it has also become clear that the target SSC resistance may not be ensured by simply reducing the yield strength to 539 MPa or less.

  Therefore, the present inventors further examined in detail a structure capable of ensuring sufficient SSC resistance when the yield strength of the martensitic Cr-containing steel material is reduced to 539 MPa or less.

As a result, as shown in FIG. 1, the martensitic Cr-containing steel material has a two-dimensional dislocation density (m ⁻² ) and carbide density (m ⁻² ) of the following [3] and [4] It was found that the target SSC resistance can be secured if the structure satisfies the formula.
Dislocation density ≦ 5.3 × 10 ¹⁴ [3],
Carbide density ≦ 2.2 × 10 ¹² [4].

  This invention is completed based on said content, The summary exists in the martensitic Cr containing steel material shown below.

(1) In mass%,
C: 0.15% or less, Si: 0.05 to 1.0%, Mn: 0.01% or more and less than 3.0%, Cr: more than 9.0% and 12.0% or less, and the following The effective Cr defined by the formula [1] is more than 8.2%. Al: 0.001 to 0.05%,
The balance consists of Fe and impurities,
P, S, Ni, N and O in the impurities are P: 0.03% or less, S: 0.003% or less, Ni: 0.3% or less, N: 0.10% or less, and O: 0.0. 01% or less,
And Fn1 represented by the following formula [2] has a chemical composition satisfying Fn1 <240,
The dislocation density (m ⁻² ) in the structure in two dimensions and the carbide density (m ⁻² ) in two dimensions satisfy the following [3] and [4] expressions, respectively:
Furthermore, the martensitic Cr containing steel material characterized by the yield strength being 539 MPa or less.
Effective Cr = Cr-11 × C [1]
Fn1 = 23 × (Cr + 5.757 × sol.Al + 0.504 × Si + 3.471 × V) −477 × (C + 0.565 × N + 0.0480 × Cu + 0.0412 × Mn + 0.0688 × Ni) ... [2]
Dislocation density ≦ 5.3 × 10 ¹⁴ [3]
Carbide density ≦ 2.2 × 10 ¹² [4]
However, in the formulas [1] and [2], the element symbol means the content in mass% of the element.

  (2) The martensitic Cr-containing steel material according to (1) above, which contains, by mass%, Cu: 3.0% or less instead of part of Fe.

  (3) The martensitic Cr-containing steel material according to the above (1) or (2), wherein V is 0.20% or less in mass% instead of part of Fe.

  (4) The above (1), characterized in that, in place of a part of Fe, at least one selected from Ca: 0.010% or less and Mg: 0.010% or less in mass% To the martensitic Cr-containing steel material according to any one of (3) to (3).

The martensitic Cr-containing steel material of the present invention is excellent in that the yield strength can be easily reduced to 539 MPa or less even when tempering as a heat treatment after quenching is performed at a relatively low temperature and in a short time. It has CO ₂ corrosion resistance and SSC resistance. For this reason, the martensitic Cr-containing steel material according to the present invention is suitable as a material for steel pipes for oil wells used in oil wells and gas wells in severe corrosive environments containing CO ₂ , H ₂ S, Cl ^{− and the} like.

In the martensitic Cr-containing steel material, the two-dimensional dislocation density (m ⁻² ) and carbide density (m ⁻² ) are the solution A (H ₂ S partial pressure: 1 bar) shown in NACE-TM0177-2005. (0.1 MPa), pH: 2.68) It is a figure which shows the influence which acts on SSC resistance in an environment. A martensitic Cr-containing steel having a low carbon content and different Cr content and effective Cr was filled in a 5% NaCl aqueous solution at a CO ₂ partial pressure of 30 bar (3 MPa) in an autoclave at 125 ° C. for 336 hours (2 (Week) It is a diagram showing the relationship between the corrosion rate and Cr content, and the relationship between the corrosion rate and effective Cr, calculated from the mass difference before and after immersion, and the evaluation results in the examples described later. It is a summary.

  Hereinafter, each requirement of the present invention will be described in detail. In addition, “%” of the content of each element means “mass%”.

(A) Chemical composition C: 0.15% or less C is an element that increases strength, but if its content exceeds 0.15%, it causes an increase in precipitation of carbides, resulting in resistance to CO ₂ corrosion. Cause lowering of the toughness. Therefore, the C content is 0.15% or less. In the martensitic Cr-containing steel material of the present invention, since it is desirable that the two-dimensional carbide density (m ⁻² ) is small, the upper limit of the C content is preferably 0.12%, more preferably 0.08. %, More preferably 0.04%. On the other hand, the lower limit of the C content is preferably 0.001%, more preferably 0.005% in terms of decarburization cost and the like. If the Cr content is high, δ ferrite may be generated at high temperatures, leading to a decrease in hot workability. In such a case, it is preferable to contain a large amount of C.

Si: 0.05-1.0%
Si acts as a deoxidizer in the normal steelmaking process. In order to obtain a deoxidizing effect, it is necessary to contain 0.05% or more of Si. However, excessive content of Si leads to a decrease in CO ₂ corrosion resistance and hot workability, so an upper limit is provided to make the content 1.0% or less. A preferable lower limit of the Si content is 0.10%. Moreover, the upper limit with preferable Si content is 0.75%, and a more preferable upper limit is 0.50%.

Mn: 0.01% or more and less than 3.0% Mn is an element effective for improving the hardenability of steel. In order to obtain this effect, it is necessary to contain 0.01% or more of Mn. On the other hand, if the Mn content is excessive, the austenite transformation temperature is lowered and the tempering temperature cannot be sufficiently increased. And the Mn content is 0.01% or more and less than 3.0%. The minimum with preferable Mn content is 0.10%, and a more preferable minimum is 0.20%. Moreover, the upper limit with preferable Mn content is 1.2%, and a more preferable upper limit is 0.80%.

Cr: more than 9.0% and not more than 12.0% and the effective Cr defined by the above formula [1] is over 8.2% Cr is an element necessary for ensuring CO ₂ corrosion resistance . The present inventors filled a 5% NaCl aqueous solution with a CO ₂ partial pressure of 30 bar (3 MPa) and a corrosion rate under an environment of a temperature of 125 ° C. From the Cr content and the effective Cr defined by the above formula [1]. Lower limits were determined to be greater than 9.0% and greater than 8.2%, respectively. That is, when the Cr content and the effective Cr are 9.0% or less and 8.2% or less, the corrosion rate in the environment exceeds 0.25 mm / year. Increasing the Cr content improves CO ₂ corrosion resistance, but decreases SSC resistance. Further, when the content of C or Mn is small, δ ferrite may be formed and the hot workability may be lowered. Therefore, an upper limit is set so that the Cr content is 12.0% or less. The upper limit of the Cr content is preferably 11.0%. The upper limit of effective Cr may be a value close to 12.0%.

sol. Al: 0.001 to 0.05%
Al has a deoxidizing action. However, the content of Al is sol. If it is less than 0.001% with Al, the effect is not sufficient. On the other hand, if Al is contained excessively, the effect is saturated and inclusions increase, which may lead to a decrease in SSC resistance. Therefore, an upper limit is provided to reduce the Al content to sol. 0.001 to 0.05% with Al. sol. The upper limit of the Al content in Al is preferably less than 0.03%, more preferably 0.02%, and still more preferably 0.01%. “Sol.Al” means so-called “acid-soluble Al”.

  The martensitic Cr-containing steel material according to the present invention is composed of the above-described elements, the balance being Fe and impurities, and P, S, Ni, N and O (oxygen) in the impurities are P: 0.03% Hereinafter, the chemical composition is S: 0.003% or less, Ni: 0.3% or less, N: 0.10% or less, and O: 0.01% or less. “Impurity” refers to what is mixed from ore as a raw material, scrap, or a manufacturing environment when industrially producing a martensitic Cr-containing steel material.

P: 0.03% or less P is an impurity contained in the steel, and segregates at the grain boundary to deteriorate the CO ₂ corrosion resistance and SSC resistance. For this reason, since it is desirable that the content of P in the impurities is small, an upper limit is set to 0.03% or less. The upper limit of the P content is preferably 0.020%, more preferably 0.015%.

S: 0.003% or less S is an impurity contained in the steel and segregates at the grain boundary to lower the SSC resistance. Further, excessive inclusion of S also adversely affects hot workability. Therefore, since it is preferable that the content of S in the impurities is small, an upper limit is set to 0.003% or less. The upper limit of the S content is preferably 0.002%, more preferably 0.001%.

Ni: 0.3% or less Ni is an impurity in the present invention and is an undesirable element that lowers the SSC resistance. In particular, when the Ni content is 0.3% or more, the SSC resistance is significantly lowered. Therefore, the content of Ni in the impurities is set to 0.3% or less. The upper limit of the Ni content is preferably 0.2%, more preferably 0.12%.

N: 0.10% or less N is an impurity contained in steel. Excessive N content forms coarse nitrides and becomes a starting point of pitting corrosion, which tends to cause a decrease in SSC resistance. Therefore, an upper limit is set for the content of N in the impurities, so that the content is 0.10% or less. The upper limit of the N content is preferably 0.06%, more preferably 0.04%.

O: 0.01% or less O (oxygen) is an impurity contained in steel, and if it is excessively contained, a coarse oxide is formed, and toughness and SSC resistance are lowered. For this reason, since it is preferable that the content of O in the impurities is small, an upper limit is set to 0.01% or less. The upper limit of the O content is preferably 0.005%.

  The martensitic Cr-containing steel material according to the present invention may contain one or more elements selected from the following <a> to <c> instead of part of Fe.

<a>: Cu: 3.0% or less <b>: V: 0.20% or less <c>: Ca: 0.010% or less, Mg: 0.010% or less Hereinafter, these optional elements are described in detail. Describe.

Cu: 3.0% or less Cu has an effect of improving the CO ₂ corrosion resistance and contributes as an austenite stabilizing element. For this reason, you may contain Cu as needed. However, if the content exceeds 3.0%, it becomes sensitive to hot cracking and the hot workability decreases, so an upper limit is provided and the amount of Cu when contained is 3.0% or less. When Cu is contained, the amount of Cu is preferably 1.2% or less, and more preferably 0.80% or less. On the other hand, the Cu content is preferably 0.02% or more and more preferably 0.10% or more in order to stably develop the effect of Cu described above.

V: 0.2% or less
Since V improves the temper softening resistance, tempering at a high temperature becomes possible. As a result, the dislocation density is reduced and the SSC resistance is improved. For this reason, you may contain V as needed. However, excessive V content increases the strength of the steel and causes a decrease in SSC resistance. Therefore, an upper limit is provided, and the V content when contained is 0.2% or less. When V is contained, the amount of V is preferably 0.1% or less, and more preferably 0.05% or less. On the other hand, the V content is preferably 0.005% or more in order to stably express the effect of V described above.

Ca: 0.010% or less, Mg: 0.010% or less Ca and Mg are elements that effectively work to improve hot workability and manufacturing stability in continuous casting. For this reason, you may contain Ca and / or Mg as needed. However, if the content of Ca and Mg exceeds 0.010%, it tends to exist as coarse inclusions, so that the SSC resistance is deteriorated and the toughness is easily lowered. Therefore, the amount in the case of containing these is set to 0.010% or less for each of Ca and Mg. The upper limit of the Ca content and the Mg content when contained is preferably 0.0040%, more preferably 0.0030%.

  On the other hand, in order to stably express the effects of Ca and Mg described above, each content is preferably 0.0003% or more, and more preferably 0.0005% or more. The more preferable each content in the case of making it contain is 0.0008% or more.

  The above Ca and Mg can be contained in only one of them or in a composite of two, but the total amount of these elements when combined and contained is 0.0040% or less. It is preferable.

Fn1: less than 240 The Cr-containing steel material according to the present invention further includes:
Fn1 = 23 × (Cr + 5.757 × sol.Al + 0.504 × Si + 3.471 × V) −477 × (C + 0.565 × N + 0.0480 × Cu + 0.0412 × Mn + 0.0688 × Ni) ... [2]
Must have a chemical composition satisfying Fn1 <240.
However, in the formula [2], the element symbol means the content in mass% of the element.

  Said Fn1 shows the balance of a ferrite stabilization element and an austenite stabilization element, and the structure | tissue of a martensite single phase is substantially obtained by satisfying Fn1 <240.

(B) Structure The martensitic Cr-containing steel material according to the present invention having the chemical composition described in the above section (A) is a martensite single-phase structure by satisfying Fn1 <240 as described above. The density (m ⁻² ) and carbide density (m ⁻² ) must satisfy the following formulas [3] and [4], respectively.
Dislocation density ≦ 5.3 × 10 ¹⁴ [3],
Carbide density ≦ 2.2 × 10 ¹² [4].

FIG. 2 shows that in a martensitic Cr-containing steel material, the two-dimensional dislocation density (m ⁻² ) and carbide density (m ⁻² ) are NACE-Solution A (H ₂ S partial pressure: 1 bar (0. 1 MPa), pH: 2.68) It is a figure which shows the influence which acts on SSC resistance in an environment. As is apparent from FIG. 2, the SSC resistance can be ensured within the range where the above equations [3] and [4] are satisfied at the same time.

In the martensitic Cr-containing steel material according to the present invention having the chemical composition described in the above item (A), the carbides are mainly M ₂₃ C ₆ type carbides (“M” refers to elements such as Cr and Fe). ). The observed circle equivalent diameter of the carbide is in the range of approximately 30 to 500 nm, although it varies depending on the heat treatment conditions. Carbide is determined by, for example, two-dimensional observation with a scanning electron microscope (SEM), and the density (m ^-2 ) and size are determined by computer image analysis using appropriate photographs of 5,000 to 20,000 times. Can do.

  The measurement of dislocation density can be performed using an evaluation method based on the Williamson-Hall method described in Non-Patent Document 2 proposed by Nakajima et al. In Non-Patent Document 1.

Specifically, for example, in the measurement of the X-ray diffraction profile, a Co tube is used as the cathode tube, and the profile is measured in the range of 40 ° to 130 ° at 2θ using the θ-2θ diffraction method. Then, for each diffraction of the {110} plane, {211} plane, and {220} plane of the BCC crystal structure, the half width obtained by fitting using the Rietveld method described in Non-Patent Document 3 is used. The strain [ε] is obtained, and ρ = 14.4ε ² / b ² expressed by the strain [ε] and the Burgers vector [b].
The dislocation density [ρ] in the unit of m ⁻² can be obtained by calculating the following formula.

For the profile measurement derived from the measuring apparatus, for example, a pure silicon powder sample is used, and the value of the Burgers vector [b] may be 0.24823 × 10 ⁻⁹ m.

(C) Yield Strength The martensitic Cr-containing steel material according to the present invention having the chemical composition described in the above section (A) and having the structure described in the section (B) has a yield strength (0.2% yield strength). Must be 539 MPa or less.

When the yield strength exceeds 539 MPa, the SSC resistance under the solution A (H ₂ S partial pressure: 1 bar (0.1 MPa), pH: 2.68) environment shown in NACE-TM0177-2005 is significantly reduced. There is a case. The yield strength is preferably 517 MPa (75 ksi) or less.

  The martensitic Cr-containing steel material according to the present invention can be manufactured, for example, as follows.

  First, it melts using an electric furnace, an AOD furnace, a VOD furnace, etc., and adjusts a chemical composition.

  The molten metal with the adjusted chemical composition may then be cast into an ingot and then processed into a so-called “steel piece” such as a slab, bloom, billet, etc. by hot working such as forging, or continuous casting. In addition, a so-called “steel piece” such as a slab, a bloom, or a billet may be directly used.

  Further, the above-mentioned “steel piece” is used as a raw material and hot-worked into a desired shape such as a plate material or a pipe material. For example, when processing into a plate material, it can be hot processed into a plate or a coil by hot rolling. Moreover, when processing into a pipe material, it can be hot-worked into a tubular shape by the Mannesmann-Mandrel manufacturing method, or the plate may be welded into a tubular shape.

After hot working, it is allowed to cool in the atmosphere and cooled, and then reheat quenching and tempering are performed. Alternatively, it may be tempered after directly quenching after hot working. In addition, after hot working, heat may be supplemented at a temperature of Ar ₃ or higher, and in-line quenching may be performed, followed by tempering treatment.

  The quenching temperature is preferably 900 to 970 ° C. When the quenching temperature is high, the crystal grains are coarsened and the SSC resistance may be lowered.

Tempering is performed at a temperature of Ac ₁ point or less. In addition, in the case of the martensitic Cr-containing steel material according to the present invention having the chemical composition described in the above item (A), for example, about 30 minutes at a temperature of 700 to 750 ° C., depending on the size of the steel material. Even under relatively low temperature and short time conditions, the yield strength can be reduced to 539 MPa or less.

  Hereinafter, the present invention will be described in more detail with reference to examples.

  Steel having the chemical composition shown in Table 1 was melted in a vacuum high-frequency melting furnace and cast into a 50 kg ingot.

  Steels A to R in Table 1 are all steels whose chemical compositions are within the range defined by the present invention. On the other hand, steel S has a C content, steel T and steel U have a Cr content, and steel V has a Ni content outside the range defined in the present invention.

  Each ingot was heat-treated at 1200 ° C. for 2 hours, and then hot forged to process a 50 mm × 50 mm plate. The plate material thus obtained was further heated at 1200 ° C. for 1 hour and then hot rolled to finish a plate material having a thickness of 15 mm.

  Next, the 15 mm-thick plate material obtained by hot rolling was heated at 950 ° C. for 30 minutes and then water-quenched to obtain a martensite single-phase structure. The martensite single-phase structure was tempered under the conditions shown in Table 2 and then water-cooled.

A test piece was cut out from each of the above-mentioned quenched and tempered plate materials and examined for tensile properties, dislocation density, carbide density, SSC resistance and CO ₂ corrosion resistance.

  Tensile properties are from a direction parallel to the rolling direction (hereinafter referred to as “L direction”) at the thickness center of each of the above plate materials, and a round bar tensile test piece having a parallel portion diameter of 6 mm and a distance between gauge points of 40 mm. Was collected and subjected to a tensile test at room temperature to determine the yield strength (0.2% yield strength).

  The dislocation density was determined by cutting out a test piece having a size of 20 mm in length and breadth from the center of the thickness of each plate and a thickness of 2 mm. It grind | polished and it measured using this test piece.

  The dislocation density was measured using an evaluation method based on the Williamson-Hall method described in Non-Patent Document 2 proposed by Nakajima et al.

Specifically, for the measurement of the X-ray diffraction profile, a Co tube was used as the cathode tube, and the profile was measured in the range of 40 ° to 130 ° at 2θ using the θ-2θ diffraction method. Then, for each diffraction of the {110} plane, {211} plane, and {220} plane of the BCC crystal structure, the half width obtained by fitting using the Rietveld method described in Non-Patent Document 3 is used. The strain [ε] is obtained, and ρ = 14.4ε ² / b ² expressed by the strain [ε] and the Burgers vector [b].
The dislocation density [ρ] when the unit is m ⁻² was calculated.

Note that pure silicon powder crystals were used for profile measurement derived from the measuring apparatus. In addition, 0.24823 × 10 ⁻⁹ m was used as the value of the Burgers vector [b].

  Carbide density is determined by cutting a test piece 20 mm long and 15 mm high from each plate, embedding it in a resin so that the cross section perpendicular to the width direction of the original plate is the test surface, mirror-polishing, and then using a Villera reagent. Corroded and measured by SEM.

Specifically, 10 views of appropriate photographs of 5000 to 20000 times were taken with SEM, and the number of carbides per 1 m ^{2 of} area was calculated with the image analysis software using the taken images.

The SSC resistance was evaluated by a “Method A” tensile test shown in NACE-TM0177-2005. The test piece sampled the sub-size test piece defined in the above test method from the L direction at the center of the thickness of each plate. Solution A (H ₂ S partial pressure: 1 bar (0.1 MPa), pH: 2.68) was used as the test solution, and the applied stress was 90% of the actual yield strength. In addition, it was evaluated that the SSC resistance was good because no crack occurred in the above test, and this was the target.

The CO ₂ corrosion resistance was obtained by cutting out a test piece having a length of 75 mm, a width of 10 mm, and a thickness of 2 mm along the L direction from the center of the thickness of each plate, and this test piece was filled with a 5% NaCl aqueous solution. The corrosion rate was calculated from the mass difference before and after immersion in an autoclave at a temperature of 125 ° C. for 336 hours (2 weeks) at a CO ₂ partial pressure of 30 bar (3 MPa). In the above test, when the corrosion rate was 0.25 mm / year or less, the CO ₂ corrosion resistance was evaluated as good, and this was the target.

  Table 2 also shows the results of the above investigations. In the “SSC resistance” column, “◯” indicates that no crack occurred in the test in the above environment, while “x” indicates that a crack occurred.

From Table 2, the steel materials of test codes 1 to 18 that satisfy the conditions specified in the present invention are based on an industrial production method with high economic efficiency that tempering as a heat treatment after quenching is relatively low temperature and short time. Nevertheless, it is clearly a martensitic Cr-containing steel material that has CO ₂ corrosion resistance and SSC resistance and is suitable for use as a material for steel pipes for oil wells.

The martensitic Cr-containing steel material of the present invention is excellent in that the yield strength can be easily reduced to 539 MPa or less even when tempering as a heat treatment after quenching is performed at a relatively low temperature and in a short time. It has CO ₂ corrosion resistance and SSC resistance. For this reason, the martensitic Cr-containing steel material according to the present invention is suitable as a material for steel pipes for oil wells used in oil wells and gas wells in severe corrosive environments containing CO ₂ , H ₂ S, Cl ^{− and the} like.

Claims (4)

% By mass
C: 0.15% or less, Si: 0.05 to 1.0%, Mn: 0.01% or more and less than 3.0%, Cr: more than 9.0% and 12.0% or less, and the following The effective Cr defined by the formula [1] is more than 8.2%. Al: 0.001 to 0.05%,
The balance consists of Fe and impurities,
P, S, Ni, N and O in the impurities are P: 0.03% or less, S: 0.003% or less, Ni: 0.3% or less, N: 0.10% or less, and O: 0.0. 01% or less,
And Fn1 represented by the following formula [2] has a chemical composition satisfying Fn1 <240,
The dislocation density (m ⁻² ) in the structure in two dimensions and the carbide density (m ⁻² ) in two dimensions satisfy the following [3] and [4] expressions, respectively:
Furthermore, the martensitic Cr containing steel material characterized by the yield strength being 539 MPa or less.
Effective Cr = Cr-11 × C [1]
Fn1 = 23 × (Cr + 5.757 × sol.Al + 0.504 × Si + 3.471 × V) −477 × (C + 0.565 × N + 0.0480 × Cu + 0.0412 × Mn + 0.0688 × Ni) ... [2]
Dislocation density ≦ 5.3 × 10 ¹⁴ [3]
Carbide density ≦ 2.2 × 10 ¹² [4]
However, in the formulas [1] and [2], the element symbol means the content in mass% of the element.   The martensitic Cr-containing steel material according to claim 1, wherein the martensitic Cr-containing steel material according to claim 1, wherein the steel material contains Cu: 3.0% or less in mass% instead of a part of Fe.   The martensitic Cr-containing steel material according to claim 1 or 2, characterized by containing V: 0.20% or less in mass% instead of part of Fe.   It replaces with a part of Fe, and contains 1 or more types selected from Ca: 0.010% or less and Mg: 0.010% or less in the mass%. The martensitic Cr containing steel material in any one.

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