JPWO2018181404A1 - Martensitic stainless steel - Google Patents

Abstract

The martensitic stainless steel material is, by mass%, C: 0.030% or less, Si: 1.00% or less, Mn: 1.00% or less, P: 0.030% or less, S: 0.005% or less, Al: 0.0010 to 0.0100%, N: 0.0500% or less, Ni: 5.00 to 6.50%, Cr: 10.00 to 13.40%, Cu: 1.80 to 3.50 %, Mo: 1.00 to 4.00%, V: 0.01 to 1.00%, Ti: 0.050 to 0.300%, Co: 0.300% or less, with the balance being Fe And a chemical composition that satisfies the formulas (1) and (2), a yield strength of 724 to 860 MPa, and a structure having a martensite of 80% or more by volume. The size of each intermetallic compound and each Cr oxide in the structure is 5.0 μm 2 or less, and the total area ratio of the intermetallic compound and Cr oxide is 3.0% or less. 11.5 ≦ Cr + 2Mo + 2Cu−1.5Ni ≦ 14.3 (1) Ti / C ≧ 7.5 (2)

Description

  The present invention relates to a steel material, and more particularly to a martensitic stainless steel material mainly composed of a martensite structure.

  As wells with low corrosivity (oil wells and gas wells) are depleted, wells with high corrosiveness are being developed. The highly corrosive well is an environment containing a lot of corrosive substances, and the temperature of the highly corrosive well is about room temperature to about 200 ° C. depending on the depth of the well. Corrosive substances are, for example, corrosive gases such as hydrogen sulfide and carbon dioxide. In this specification, the environment of a highly corrosive well containing carbon dioxide and having a hydrogen sulfide partial pressure of 0.1 atm or more is referred to as a “highly corrosive environment”.

  It is known that chromium (Cr) is effective in improving the carbon dioxide gas corrosion resistance of steel. Therefore, in an environment containing a large amount of carbon dioxide, it contains about 13% by mass of Cr, represented by API L80 13Cr steel (ordinary 13Cr steel), super 13Cr steel, etc., depending on the partial pressure and temperature of carbon dioxide. Martensitic stainless steel (hereinafter referred to as 13Cr steel) or duplex stainless steel with a further increased amount of Cr is used.

  However, hydrogen sulfide causes sulfide stress cracking (hereinafter referred to as “SSC”), for example, in high strength 13Cr steel oil well steel pipes of 724 MPa or more. High strength 13Cr steel of 724 MPa or higher is more sensitive to SSC than low alloy steel, and SSC is generated even at a relatively low hydrogen sulfide partial pressure (for example, less than 0.1 atm). Therefore, 13Cr steel is not suitable for use in the above highly corrosive environment containing carbon dioxide and hydrogen sulfide. On the other hand, duplex stainless steel is more expensive than 13Cr steel. Therefore, an oil well steel pipe having a high yield strength of 724 MPa or more and high SSC resistance that can be used in a highly corrosive environment is demanded.

  JP-A-10-001755 (Patent Document 1), JP-T-10-503809 (Patent Document 2), JP-A 2003-003243 (Patent Document 3), International Publication No. 2004/057050 (Patent Document) 4), JP-A 2000-192196 (Patent Document 5), JP-A 11-310855 (Patent Document 6), JP-A 08-246107 (Patent Document 7) and JP 2012-136742 ( Patent Document 8) proposes a steel excellent in SSC resistance.

  The martensitic stainless steel of Patent Document 1 is in mass%, C: 0.005 to 0.05%, Si: 0.05 to 0.5%, Mn: 0.1 to 1.0%, P: 0 0.025% or less, S: 0.015% or less, Cr: 10-15%, Ni: 4.0-9.0%, Cu: 0.5-3%, Mo: 1.0-3%, Al : 0.005 to 0.2%, N: 0.005% to 0.1%, with the balance being Fe and inevitable impurities. It satisfies 40C + 34N + Ni + 0.3Cu-1.1Cr-1.8Mo ≧ -10, and comprises a tempered martensite phase, a martensite phase, and a retained austenite phase. The total fraction of the tempered martensite phase and the martensite phase is 60% or more and 90% or less, and the remainder is the retained austenite phase.

  The martensitic stainless steel of Patent Document 2 is, by weight, C: 0.005 to 0.05%, Si ≦ 0.50%, Mn: 0.1 to 1.0%, P ≦ 0.03%. , S ≦ 0.005%, Mo: 1.0 to 3.0%, Cu: 1.0 to 4.0%, Ni: 5 to 8%, Al ≦ 0.06%. Cr + 1.6Mo ≧ 13 and 40C + 34N + Ni + 0.3Cu-1.1Cr-1.8Mo ≧ −10.5 are satisfied. The balance is a tempered martensite structure consisting essentially of Fe.

  The martensitic stainless steel of Patent Document 3 is mass%, C: 0.001 to 0.04%, Si: 0.5% or less, Mn: 0.1 to 3.0%, P: 0.04%. Hereinafter, S: 0.01% or less, Cr: 10-15%, Ni: 0.7-8%, Mo: 1.5-5.0%, Al: 0.001-0.10% and N: It contains 0.07% or less, and the balance consists of Fe and impurities. Furthermore, Mo ≧ 1.5−0.89Si + 32.2C is satisfied. The metal structure is mainly composed of tempered martensite, carbide precipitated during tempering, and an intermetallic compound mainly composed of Laves phase precipitated during tempering. The martensitic stainless steel of Patent Document 3 has a high strength with a yield strength of 860 MPa or more.

  The martensitic stainless steel of Patent Document 4 is mass%, C: 0.005 to 0.04%, Si: 0.5% or less, Mn: 0.1 to 3.0%, P: 0.04%. Hereinafter, S: 0.01% or less, Cr: 10-15%, Ni: 4.0-8%, Mo: 2.8-5.0%, Al: 0.001-0.10% and N: It contains 0.07% or less, and the balance consists of Fe and impurities. Furthermore, Mo ≧ 2.3-0.89Si + 32.2C is satisfied. The metal structure mainly comprises tempered martensite, carbides precipitated during tempering, and intermetallic compounds such as Laves phase and σ phase finely precipitated during tempering. The martensitic stainless steel of Patent Document 4 has a high strength with a proof stress of 860 MPa or more.

  The martensitic stainless steel of Patent Document 5 is, by weight, C: 0.001 to 0.05%, Si: 0.05 to 1%, Mn: 0.05 to 2%, P: 0.025% Hereinafter, S: 0.01% or less, Cr: 9-14%, Mo: 3.1-7%, Ni: 1-8%, Co: 0.5-7%, sol. Al: 0.001 to 0.1%, N: 0.05% or less, O (oxygen): 0.01% or less, Cu: 0 to 5%, W: 0 to 5%, the balance being Fe And inevitable impurities.

  The martensitic stainless steel of Patent Document 6 contains C: 0.05% or less and Cr: 7-15%. Furthermore, Cu content of a solid solution state is 0.25 to 5%.

  The martensitic stainless steel of Patent Document 7 is, by weight, C: 0.005% to 0.05%, Si: 0.05% to 0.5%, Mn: 0.1% to 1.0%, P: 0.025% or less, S: 0.015% or less, Cr: 12-15%, Ni: 4.5% -9.0%, Cu: 1% -3%, Mo: 2% -3% , W: 0.1% to 3%, Al: 0.005 to 0.2%, N: 0.005% to 0.1%, with the balance being Fe and inevitable impurities. 40C + 34N + Ni + 0.3Cu + Co-1.1Cr-1.8Mo-0.9W≥-10 is satisfied.

  The martensitic stainless steel seamless pipe of Patent Document 8 is mass%, C: 0.01% or less, Si: 0.5% or less, Mn: 0.1-2.0%, P: 0.03% Hereinafter, S: 0.005% or less, Cr: 14.0 to 15.5%, Ni: 5.5 to 7.0%, Mo: 2.0 to 3.5%, Cu: 0.3 to 3 0.5%, V: 0.20% or less, Al: 0.05% or less, N: 0.06% or less, and the balance is Fe and inevitable impurities. The martensitic stainless steel seamless pipe of Patent Document 8 has a yield strength: 655 to 862 MPa and a yield ratio: 0.90 or more.

JP-A-10-001755 Japanese National Patent Publication No. 10-503809 JP 2003-003243 A International Publication No. 2004/057050 JP 2000-192196 A JP-A-11-310855 Japanese Patent Laid-Open No. 08-246107 JP 2012-136742 A

  However, the martensitic stainless steels described in Patent Document 1 and Patent Document 2 may have excessively high yield strength. In this case, the SSC resistance is lowered.

  The martensitic stainless steels described in Patent Literature 3 and Patent Literature 4 are 13Cr steel, and fine carbides and intermetallic compounds are precipitated in the steel. Further, the observed intermetallic compounds are not only fine but also coarse to some extent. Therefore, the yield strength of martensitic stainless steel is as high as 125 ksi class (860 MPa or more). Therefore, the SSC resistance may be low.

  In the martensitic stainless steel described in Patent Document 5, the Mo content and the Co content are high. For this reason, the strength becomes too high, and the SSC resistance may decrease. Furthermore, if the Mo content is too high, the stability of the martensite structure may be reduced.

  The martensitic stainless steel of Patent Document 6 is a quenched martensitic steel that is not tempered. Therefore, the strength may be too high and the SSC resistance may be low.

  The martensitic stainless steel of Patent Document 7 contains Cu and W and does not contain Ti. Therefore, the yield strength of martensitic stainless steel may be too high. In this case, the SSC resistance is lowered.

  The martensitic stainless steel seamless pipe of Patent Document 8 contains 14.0 to 15.5% by mass of Cr. Therefore, it may have a ferrite phase. In this case, the strength may be insufficient.

  An object of the present disclosure is to provide a martensitic stainless steel material having a yield strength of 724 MPa or more and having excellent SSC resistance.

The martensitic stainless steel material according to the present disclosure is, in mass%, C: 0.030% or less, Si: 1.00% or less, Mn: 1.00% or less, P: 0.030% or less, S: 0.005. % Or less, Al: 0.0010 to 0.0100%, N: 0.0500% or less, Ni: 5.00 to 6.50%, Cr: 10.00 to 13.40%, Cu: 1.80 3.50%, Mo: 1.00 to 4.00%, V: 0.01 to 1.00%, Ti: 0.050 to 0.300%, Co: 0.300% or less, and W: It contains 0 to 1.50%, the balance is composed of Fe and impurities, the chemical composition satisfying the formulas (1) and (2), the yield strength of 724 to 860 MPa, and the martensite of 80% or more by volume ratio And an organization having The size of each intermetallic compound and each Cr oxide in the structure is 5.0 μm ² or less, and the total area ratio of the intermetallic compound and Cr oxide is 3.0% or less.
11.5 ≦ Cr + 2Mo + 2Cu−1.5Ni ≦ 14.3 (1)
Ti / C ≧ 7.5 (2)
Here, the content (mass%) of a corresponding element is substituted for each element symbol in the formulas (1) and (2).

  The martensitic stainless steel material according to the present embodiment has a yield strength of 724 MPa or more and excellent SSC resistance.

FIG. 1 is a diagram showing the relationship among F1 = Cr + 2Mo + 2Cu-1.5Ni, yield strength YS (MPa), and SSC resistance. FIG. 2 is a TEM (transmission electron microscope) image obtained by observing the metal structure of the steel of test number 3 in the example according to the present embodiment. FIG. 3 is an SEM (scanning electron microscope) image obtained by observing the metal structure of steel of test number 9 in the examples.

  The present inventors investigated and examined the SSC resistance of the martensitic stainless steel material and obtained the following knowledge.

[Chemical composition]
It is generally known that Cr, Mo, Cu and Ni are effective in increasing the SSC resistance of steel. Specifically, it is considered that Cr, Mo, and Cu are dissolved to increase the SSC resistance of the steel. On the other hand, Ni is considered to enhance the SSC resistance by strengthening the film on the surface of the steel material and reducing the amount of hydrogen (hydrogen permeation) entering the steel material.

  However, as a result of the study by the present inventors, it has been found for the first time that film strengthening with Ni reduces the hydrogen diffusion coefficient in steel in the highly corrosive environment as described above. If the hydrogen diffusion coefficient in steel is reduced, hydrogen tends to stay in the steel. As a result, the SSC resistance of the steel material decreases.

Therefore, the inventors further investigated Cr, Mo, Cu, and Ni that affect SSC resistance. As a result, in mass%, C: 0.030% or less, Si: 1.00% or less, Mn: 1.00% or less, P: 0.030% or less, S: 0.005% or less, Al: 0 0.0010 to 0.0100%, N: 0.0500% or less, Ni: 5.00 to 6.50%, Cr: 10.00 to 13.40%, Cu: 1.80 to 3.50%, Mo : 1.00 to 4.00%, V: 0.01 to 1.00%, Ti: 0.050 to 0.300%, Co: 0.300% or less, W: 0 to 1.50%, and Remainder: In steel materials having a chemical composition comprising Fe and impurities, excellent SSC resistance can be obtained if the Cr content, the Mo content, the Cu content and the Ni content satisfy the following formula (1). I found it.
11.5 ≦ Cr + 2Mo + 2Cu−1.5Ni ≦ 14.3 (1)
Here, the content (mass%) of the corresponding element is substituted for each element symbol of the formula (1).

  It is defined as F1 = Cr + 2Mo + 2Cu-1.5Ni. FIG. 1 is a diagram showing the relationship among F1 = Cr + 2Mo + 2Cu-1.5Ni, yield strength YS (MPa), and SSC resistance. FIG. 1 was obtained by the examples described below. “◯” in FIG. 1 indicates that no SSC occurred in the SSC resistance evaluation test in Examples described later. “X” in FIG. 1 indicates that SSC occurred in the SSC resistance evaluation test in Examples described later. When F1 is less than 11.5, or when F1 exceeds 14.3, the SSC resistance decreases. Therefore, F1 is 11.5 to 14.3.

  The SSC resistance further depends on the strength of the steel material. Specifically, if the strength of the steel material is high, the SSC resistance decreases. Therefore, the present inventors further have a mass% of C: 0.030% or less, Si: 1.00% or less, Mn: 1.00% or less, P: 0.030% or less, S: 0.005. % Or less, Al: 0.0010 to 0.0100%, N: 0.0500% or less, Ni: 5.00 to 6.50%, Cr: 10.00 to 13.40%, Cu: 1.80 3.50%, Mo: 1.00 to 4.00%, V: 0.01 to 1.00%, Ti: 0.050 to 0.300%, Co: 0.300% or less, W: 0 to 0 1. In the steel material which has a chemical composition which consists of 1.50% and remainder: Fe and an impurity and satisfy | fills Formula (1), the relationship between the strength of steel materials and SSC resistance was examined. As a result, it has been found that the steel material having the above chemical composition satisfying the formula (1) has the optimum yield strength of the steel material in order to obtain the required strength and the excellent SSC resistance. Specifically, if the yield strength of the steel material having the above chemical composition satisfying the formula (1) is 724 to 860 MPa, the required strength can be obtained and the SSC resistance is improved.

  Referring to FIG. 1, if F1 is 11.5 to 14.3 and YS is 860 MPa or less, excellent SSC resistance can be obtained. Therefore, in this embodiment, the yield strength of the steel material is 724 to 860 MPa.

  As described above, in the chemical composition of the steel material of the present embodiment, Mo is 1.00 to 4.00%, Cu is 1.80 to 3.50%, and V is 0.01 to 1.00% by mass. contains. These elements are dissolved to improve the SSC resistance. However, Mo, Cu and V also increase the strength of the steel material. As described above, if the strength of the steel material having the chemical composition that satisfies the formula (1) is too high, the SSC resistance is lowered.

  Therefore, the present inventors have further studied a method for adjusting the strength of the steel material. As a result, it has been found that excessive strengthening can be suppressed by containing Ti and adjusting the Ti content relative to the C content.

  The reason for this is as follows. Among Mo, Cu, and V that increase the strength of the steel material described above, V forms carbide (VC) and increases the strength of the steel material. Ti, like V, combines with C in steel to form carbides. Therefore, if Ti is combined with C, C for forming VC is insufficient because Ti consumes C. As a result, formation of VC can be suppressed.

In general, the affinity of Ti for C in steel is equivalent to the affinity of C for V. Therefore, in the V and Ti-containing material, VC and TiC are precipitated simultaneously. Therefore, the ratio of the Ti content (mass%) to the C content (mass%) is increased so that TiC is preferentially precipitated over VC. That is, Ti content (mass%) and C content (mass%) satisfy Formula (2).
Ti / C ≧ 7.5 (2)

  Define F2 = Ti / C. When F2 is less than 7.5, Ti is consumed to form a nitride such as TiN, and TiC cannot be sufficiently formed. Therefore, C in steel is used for forming VC, and the strength of the steel material becomes too high. If F2 is 7.5 or more, the Ti content is sufficiently higher than the C content. Therefore, TiC is preferentially deposited over VC. Therefore, formation of VC is suppressed. As a result, the yield strength of the steel material having the chemical composition satisfying the formula (1) can be suppressed to 860 MPa or less, and excellent SSC resistance can be obtained.

[About the organization]
It is known that if a coarse intermetallic compound or coarse Cr oxide exists in the structure, it becomes a starting point of SSC and the SSC resistance is lowered. Therefore, conventionally, the coarsening of the Cr oxide was suppressed, and the intermetallic compound was finely precipitated to improve the SSC resistance. That is, it has been considered that fine Cr oxides and fine intermetallic compounds do not affect the SSC resistance. However, the present inventors have newly found that in a steel material having the above chemical composition satisfying the formulas (1) and (2), the fine Cr oxide and the fine intermetallic compound also reduce the SSC resistance. . As a result of further investigation, in the steel material having the above chemical composition satisfying the formulas (1) and (2), the size of each intermetallic compound and each Cr oxide in the structure is 5.0 μm ² or less, and the metal It has been found that when the total area ratio of the intermetallic compound and the Cr oxide is 3.0% or less, the SSC resistance is further enhanced in the steel material having the above chemical composition that satisfies the formulas (1) and (2).

Here, the intermetallic compound in this specification is a precipitate of an alloy element that is precipitated after tempering. Examples of the intermetallic compound include a Laves phase such as Fe ₂ Mo, a sigma phase (σ phase), and a chi phase (χ phase). The σ phase is FeCr, and the χ phase is Fe ₃₆ Cr ₁₂ Mo ₁₀ . In this specification, the Cr oxide is chromia (Cr ₂ O ₃ ).

Intermetallic compounds and Cr oxides can be identified by observing the structure using the extraction replica method. The total area of the specified intermetallic compound and the specified Cr oxide is defined as the total area (μm ² ) of the intermetallic compound and the Cr oxide. The ratio of the total area of the intermetallic compound and the Cr oxide to the area of the entire observation region is defined as the total area ratio (%) of the intermetallic compound and the Cr oxide.

If there is an intermetallic compound having an area exceeding 5.0 μm ² or a Cr oxide exceeding 5.0 μm ² in the metal structure, the coarse intermetallic compound or coarse Cr oxide becomes the starting point of SSC. , SSC resistance decreases. Therefore, in the structure, the size of each intermetallic compound is 5.0 μm ² or less, and the size of each Cr oxide is 5.0 μm ² or less. That is, in this embodiment, the microstructure observation described later, the intermetallic compound size (area) exceeds 5.0 .mu.m ^2, and, Cr oxides size (area) of more than 5.0 .mu.m ² is observed .

Further, in the metal structure, if the total area ratio of the intermetallic compound and the Cr oxide exceeds 3.0%, even if the size of each intermetallic compound and each Cr oxide is 5.0 μm ² or less, Fine intermetallic compounds and fine Cr oxides are present in excess. Also in this case, the SSC resistance is lowered. Therefore, the total area ratio of the intermetallic compounds in the structure is set to 3.0% or less.

Satisfying formula (1) and formula (2), the structure contains martensite of 80% or more by volume, and the size of each intermetallic compound and each Cr oxide in the structure is 5.0 μm ² or less. If the total area ratio of the intermetallic compound and Cr oxide in the structure is 3.0% or less, the strength of the steel material can be adjusted to 724 to 860 MPa.

The martensitic stainless steel material according to the present embodiment completed based on the above knowledge is mass%, C: 0.030% or less, Si: 1.00% or less, Mn: 1.00% or less, P: 0.00. 030% or less, S: 0.005% or less, Al: 0.0010 to 0.0100%, N: 0.0500% or less, Ni: 5.00 to 6.50%, Cr: 10.00 to 13. 40%, Cu: 1.80 to 3.50%, Mo: 1.00 to 4.00%, V: 0.01 to 1.00%, Ti: 0.050 to 0.300%, Co: 0 300% or less, W: 0 to 1.50%, and balance: Fe and impurities, and has a chemical composition satisfying the formulas (1) and (2). The yield strength of the martensitic stainless steel material is 724 to 860 MPa. The structure of the martensitic stainless steel material has martensite of 80% or more by volume ratio. The size of each intermetallic compound and each Cr oxide in the structure is 5.0 μm ² or less, and the total area ratio of the intermetallic compound and Cr oxide is 3.0% or less.
11.5 ≦ Cr + 2Mo + 2Cu−1.5Ni ≦ 14.3 (1)
Ti / C ≧ 7.5 (2)
Here, the content (mass%) of a corresponding element is substituted for each element symbol in the formulas (1) and (2).

  The chemical composition of the martensitic stainless steel material may include W: 0.10 to 1.50%.

  The martensitic stainless steel material is, for example, an oil well seamless steel pipe.

  In this specification, “steel pipe for oil well” means, for example, a steel pipe for oil well described in the definition column of number 3514 of JIS G 0203 (2009). Specifically, “steel pipe for oil well” means a general term for casings, tubing, and drill pipes used for drilling oil wells or gas wells, extracting crude oil or natural gas, and the like. “Oil well seamless steel pipe” means that the oil well steel pipe is a seamless steel pipe.

  Hereinafter, the martensitic stainless steel material of this embodiment will be described in detail. “%” Regarding an element means mass% unless otherwise specified.

[Chemical composition]
The chemical composition of the martensitic stainless steel material of this embodiment contains the following elements.

C: 0.030% or less Carbon (C) is unavoidably contained. That is, the C content is more than 0%. C increases the hardenability and increases the strength of the steel material. However, if the C content is too high, the strength of the steel material becomes too high and the SSC resistance decreases. Therefore, the C content is 0.030% or less. The C content is preferably as low as possible. However, if the C content is excessively reduced, the manufacturing cost increases. Therefore, in consideration of industrial production, the preferable lower limit of the C content is 0.0001%, more preferably 0.0005%. From the viewpoint of the strength of the steel material, the preferable lower limit of the C content is 0.002%, more preferably 0.005%. The upper limit with preferable C content is 0.020%, More preferably, it is 0.015%.

Si: 1.00% or less Silicon (Si) is unavoidably contained. That is, the Si content is more than 0%. Si deoxidizes steel. However, this effect is saturated if the Si content is too high. Therefore, the Si content is 1.00% or less. The minimum with preferable Si content is 0.05%, More preferably, it is 0.10%. The upper limit with preferable Si content is 0.70%, More preferably, it is 0.50%.

Mn: 1.00% or less Manganese (Mn) is unavoidably contained. That is, the Mn content is more than 0%. Mn increases the hardenability of the steel. However, if the Mn content is too high, Mn segregates at grain boundaries together with impurity elements such as P and S. In this case, the SSC resistance decreases. Therefore, the Mn content is 1.00% or less. The minimum with preferable Mn content is 0.15%, More preferably, it is 0.20%. The upper limit with preferable Mn content is 0.80%, More preferably, it is 0.50%.

P: 0.030% or less Phosphorus (P) is an unavoidable impurity. That is, the P content is more than 0%. P segregates at the grain boundaries and lowers the SSC resistance of the steel. Therefore, the P content is 0.030% or less. The upper limit with preferable P content is 0.025%, More preferably, it is 0.020%. The P content is preferably as low as possible. However, if the P content is excessively reduced, the manufacturing cost increases. Therefore, if industrial production is considered, the minimum with preferable P content is 0.0001%, More preferably, it is 0.0005%.

S: 0.005% or less Sulfur (S) is an unavoidable impurity. That is, the S content is more than 0%. S, like P, segregates at the grain boundaries and lowers the SSC resistance of the steel. Therefore, the S content is 0.005% or less. The upper limit with preferable S content is 0.003%, More preferably, it is 0.001%. The S content is preferably as low as possible. However, if the S content is excessively reduced, the manufacturing cost increases. Therefore, if industrial production is considered, the minimum with preferable S content is 0.0001%, More preferably, it is 0.0005%.

Al: 0.0010 to 0.0100%
Aluminum (Al) deoxidizes steel. If the Al content is low, this effect cannot be obtained. On the other hand, if the Al content is too high, this effect is saturated. Therefore, the Al content is 0.0010 to 0.0100%. The minimum with preferable Al content is 0.0020%, More preferably, it is 0.0030%. The upper limit with preferable Al content is 0.0070%, More preferably, it is 0.0050%. As used herein, the Al content is sol. It means the content of Al (acid-soluble Al).

N: 0.0500% or less Nitrogen (N) is an unavoidable impurity. That is, the N content is more than 0%. N forms nitrides and decreases the SSC resistance. Therefore, the N content is 0.0500% or less. The upper limit with preferable N content is 0.0300% or less, More preferably, it is 0.0200% or less. The N content is preferably as low as possible. However, if the N content is excessively reduced, the manufacturing cost increases. Therefore, considering industrial production, the preferable lower limit of the N content is 0.0001%, more preferably 0.0005%.

Ni: 5.00 to 6.50%
Nickel (Ni) is an austenite-forming element and martensites the structure after quenching. When the Ni content is too low, the structure after tempering contains a large amount of ferrite. On the other hand, when the Ni content is too high, in a highly corrosive well, Ni reduces the hydrogen diffusion coefficient in the steel by strengthening the film. If the hydrogen diffusion coefficient in steel decreases, the SSC resistance decreases. Therefore, the Ni content is 5.00 to 6.50%. The minimum with preferable Ni content is 5.20%, More preferably, it is 5.30%. The upper limit with preferable Ni content is 6.30%, More preferably, it is 6.20%.

Cr: 10.00 to 13.40%
Chromium (Cr) increases the carbon dioxide corrosion resistance of steel at high temperatures. If the Cr content is too low, this effect cannot be obtained. When the Cr content is 10.00% or more, excellent carbon dioxide gas corrosion resistance at high temperatures is exhibited. On the other hand, if the Cr content is too high, the metal compound and Cr oxide are excessively produced, or a coarse metal compound and / or coarse Cr oxide is produced, resulting in a decrease in SSC resistance. Therefore, the Cr content is 10.00 to 13.40%. The minimum with preferable Cr content is 11.00%, More preferably, it is 11.50%. The upper limit with preferable Cr content is 13.30%, More preferably, it is 13.00%.

Cu: 1.80 to 3.50%
Copper (Cu) is an austenite-forming element like Ni and martensite the structure after quenching. Cu further dissolves in the steel to enhance the SSC resistance. If the Cu content is too low, these effects cannot be obtained. On the other hand, if Cu content is too high, hot workability will fall. Therefore, the Cu content is 1.80 to 3.50%. The minimum with preferable Cu content is 1.90%, More preferably, it is 1.95%. The upper limit with preferable Cu content is 3.30%, More preferably, it is 3.10%.

Mo: 1.00 to 4.00%
Molybdenum (Mo) increases the SSC resistance and strength of the steel. If the Mo content is too low, these effects cannot be obtained. On the other hand, Mo is a ferrite forming element. Therefore, if the Mo content is too high, austenite is difficult to stabilize, and a martensite structure is difficult to be obtained stably. Therefore, the Mo content is 1.00 to 4.00%. The minimum with preferable Mo content is 1.20%, More preferably, it is 1.50%, More preferably, it is 1.80%. The upper limit with preferable Mo content is 3.50%, More preferably, it is 3.00%, More preferably, it is 2.70%.

V: 0.01-1.00%
Vanadium (V) dissolves in steel and suppresses intergranular cracking of steel in a highly corrosive environment. If the V content is too low, this effect cannot be obtained. On the other hand, V increases the hardenability of steel and easily forms carbides. Therefore, if the V content is too high, the strength of the steel material increases and the SSC resistance decreases. Therefore, the V content is 0.01 to 1.00%. The minimum with preferable V content is 0.02%, More preferably, it is 0.03%. The upper limit with preferable V content is 0.80%, More preferably, it is 0.70%.

Ti: 0.050-0.300%
Titanium (Ti) combines with C to form a carbide. Thereby, C for forming VC is consumed by Ti, and formation of VC can be suppressed. Therefore, the SSC resistance of the steel is increased. If the Ti content is too low, this effect cannot be obtained. On the other hand, if the Ti content is too high, the above effect is saturated, and further, the formation of ferrite is promoted. Accordingly, the Ti content is 0.050 to 0.300%. The minimum with preferable Ti content is 0.060%, More preferably, it is 0.070%, More preferably, it is 0.080%. The upper limit with preferable Ti content is 0.250%, More preferably, it is 0.200%, More preferably, it is 0.150%.

Co: 0.300% or less Cobalt (Co) is an unavoidable impurity. That is, the Co content is more than 0%. If the Co content is too high, ductility and toughness are reduced. Therefore, the Co content is 0.300% or less. The upper limit of the preferred Co content is 0.270%, more preferably 0.250%. The Co content is preferably as low as possible. However, if the Co content is excessively reduced, the manufacturing cost increases. Therefore, considering industrial production, the preferable lower limit of the Co content is 0.0001%, more preferably 0.0005%.

  The balance of the martensitic stainless steel material according to the present embodiment is composed of Fe and impurities. Here, an impurity is a thing mixed from the ore as a raw material, a scrap, or a manufacturing environment, when manufacturing steel industrially.

  The chemical composition of the martensitic stainless steel material according to the present embodiment may further contain W instead of part of Fe.

W: 0 to 1.50%
Tungsten (W) is an optional element and may not be contained. That is, the W content may be 0%. When contained, W stabilizes the passive film and enhances the corrosion resistance. However, if the W content is too high, W combines with C to form fine carbides. This fine carbide increases the strength of the steel material by fine precipitation hardening, resulting in a decrease in SSC resistance. Therefore, the W content is 0 to 1.50%. The minimum with preferable W content is 0.10%, More preferably, it is 0.20%. The upper limit with preferable W content is 1.00%, More preferably, it is 0.50%.

[Regarding Formula (1)]
The chemical composition further satisfies formula (1).
11.5 ≦ Cr + 2Mo + 2Cu−1.5Ni ≦ 14.3 (1)
Here, the content (mass%) of the corresponding element is substituted for each element symbol in the formula (1).

  It is defined as F1 = Cr + 2Mo + 2Cu-1.5Ni. F1 is an index of SSC resistance in a steel material having the above chemical composition. Referring to FIG. 1, if F1 is less than 11.5, the SSC resistance decreases. It is considered that the SSC resistance is lowered because the Ni content that lowers the diffusion coefficient of hydrogen in the steel is too high with respect to the contents of Cr, Mo, and Cu, which increase the SSC resistance by solid solution. On the other hand, even if F1 exceeds 14.3, the SSC resistance decreases. Since the Ni content that forms a film on the surface and suppresses the entry of hydrogen is too low relative to the contents of Cr, Mo, and Cu, which increase the SSC resistance, the amount of hydrogen penetration increases, and as a result, It is thought that SSC property falls. Therefore, F1 is 11.5 to 14.3. A preferred lower limit of F1 is 11.7. The preferable upper limit of F1 is 14.0.

  In addition, the value of F1 is a value obtained by rounding off the second decimal place.

[Regarding Formula (2)]
The chemical composition satisfies the formula (1) and further satisfies the formula (2).
Ti / C ≧ 7.5 (2)
Here, the content (mass%) of the corresponding element is substituted for each element symbol in the formula (2).

  Define F2 = Ti / C. F2 is an index for inhibiting VC precipitation by Ti, that is, an index for the strength of the steel material. As described above, generally, in the V and Ti-containing material, VC and TiC are precipitated simultaneously. Therefore, in the chemical composition satisfying the formula (1), the ratio of Ti content (% by mass) to C content (% by mass) is further increased so that TiC is preferentially precipitated over VC. . Thereby, C is consumed by Ti, and it suppresses that C couple | bonds with V and VC forms.

  When F2 is less than 7.5, the Ti content relative to the C content is too low. In this case, Ti is consumed to form a nitride such as TiN, and TiC cannot be formed sufficiently. Therefore, C is used for forming VC, and the strength of the steel material becomes too high. If F2 is 7.5 or more, the Ti content relative to the C content is sufficiently high. As a result, Ti consumes C and forms TiC preferentially over VC. Thereby, formation of VC can be suppressed. Therefore, it can suppress that the intensity | strength of steel materials becomes high too much. As a result, it can have excellent SSC resistance.

  The value of F2 is a value obtained by rounding off the second decimal place.

[Organization]
[Martensite volume ratio: 80% or more]
The structure of the above-described martensitic stainless steel material is mainly martensite. Here, martensite includes tempered martensite. The term “martensite mainly” means that the volume ratio of martensite is 80% or more in the structure. The balance of the tissue is retained austenite. The volume fraction of retained austenite is 0 to 20%. The volume fraction of retained austenite is preferably as low as possible. The minimum with the preferable volume ratio of the martensite in a structure | tissue is 85%, More preferably, it is 90%. More preferably, the metal structure is a martensite single phase.

  In the above-described structure, a small amount of retained austenite does not cause a significant decrease in strength and significantly increases the toughness of the steel. However, if the volume fraction of retained austenite is too high, the strength of the steel is significantly reduced. Therefore, the volume ratio of retained austenite is 0 to 20%. From the viewpoint of securing strength, a more preferable volume ratio of retained austenite is 0 to 10%. As described above, the martensitic stainless steel material of the present embodiment may be a martensite single phase. Therefore, in this case, the volume ratio of retained austenite is 0%. On the other hand, when residual austenite is present even a little, the volume ratio of residual austenite is more than 0 to 20%, more preferably more than 0 to 10%.

[Measurement method of volume fraction of martensite in the structure]
The volume ratio (vol.%) Of martensite is obtained by subtracting the volume ratio (vol.%) Of retained austenite obtained by the following method from 100%.

The volume fraction of retained austenite is determined by the X-ray diffraction method. Specifically, a sample is taken from an arbitrary position of the martensitic stainless steel material. The sample size is 15 mm × 15 mm × thickness 2 mm. Using samples, the X-ray intensities of the (200) plane and (211) plane of ferrite (α phase) and the (200) plane, (220) plane and (311) plane of retained austenite (γ phase) taking measurement. Then, the integrated intensity of each surface is calculated. After the calculation, the volume ratio Vγ (%) is calculated using the formula (A) for each combination (6 sets in total) of each surface of the α phase and each surface of the γ phase. And the average value of six sets of volume ratios Vγ is defined as the volume ratio (%) of retained austenite.
Vγ = 100 / (1+ (Iα × Rγ) / (Iγ × Rα)) (A)
Here, “Iα” is the integrated intensity of the α phase (ferrite). “Rα” is a crystallographically calculated value of the α phase. “Iγ” is the integrated intensity of the γ phase (austenite). “Rγ” is a crystallographically calculated value of the γ phase.

  A value obtained by subtracting the volume ratio of retained austenite obtained by the above method from 100% is defined as the volume ratio (vol.%) Of martensite in the structure. Note that the volume ratio of precipitates such as inclusions and the following intermetallic compounds is excluded from “100%” of the volume ratio in the structure.

[Yield strength]
The yield strength of the martensitic stainless steel of this embodiment is 724 to 860 MPa. If the yield strength is less than 724 MPa, the strength applicable to a highly corrosive environment is not satisfied. On the other hand, when the yield strength exceeds 860 MPa, as shown in FIG. Therefore, the yield strength of the martensitic stainless steel of this embodiment is 724 to 860 MPa. The upper limit with preferable yield strength is 850 MPa, More preferably, it is 840 MPa. The minimum with preferable yield strength is 730 MPa, More preferably, it is 740 MPa. In this specification, the yield strength means 0.2% yield strength (MPa).

[Intermetallic compounds and Cr oxides in the structure]
In the martensitic stainless steel material of the present embodiment, the size of each intermetallic compound and each Cr oxide is 5.0 μm ² or less in the structure, and the total of intermetallic compounds and Cr oxides in the structure. The area ratio is 3.0% or less. That is, in this embodiment, the intermetallic compound and Cr oxide whose size exceeds 5.0 μm ² are not observed.

Here, the intermetallic compound is a precipitate of an alloy element that precipitates after tempering. Examples of the intermetallic compound include a Laves phase such as Fe ₂ Mo, a sigma phase (σ phase), and a chi phase (χ phase). The size of the intermetallic compound means the area (μm ² ) of the intermetallic compound observed in the measurement described later. In the case of the above-described chemical composition of the present embodiment, since there are very few intermetallic compounds other than the Laves phase, the σ phase, and the χ phase, there is no problem. The Cr oxide is chromia (Cr ₂ O ₃ ). The magnitude | size of Cr oxide means the area (micrometer < ² >) of Cr oxide observed in the below-mentioned measurement.

Even if it is a steel material having a chemical composition satisfying the formulas (1) and (2), a martensite volume ratio of 80% or more, and a yield strength of 724 to 860 MPa, an intermetallic compound and Cr in the structure Among oxides, when an intermetallic compound or Cr oxide having a size exceeding 5.0 μm ² exists, or when the total area ratio of the intermetallic compound and Cr oxide exceeds 3.0%, the intermetallic compound And SSC resulting from the Cr oxide is generated, and the SSC resistance is lowered. As described above, in a steel material having a chemical composition satisfying the formulas (1) and (2), a martensite volume ratio of 80% or more, and a yield strength of 724 to 860 MPa, each intermetallic compound and If the size of each Cr oxide is 5.0 μm ² or less and the total area ratio of the intermetallic compound and the Cr oxide is 3.0% or less, the intermetallic compound and the Cr oxide are resistant to each other. Since the SSC property is not affected, excellent SSC resistance is maintained.

  A smaller total area ratio of intermetallic compounds and Cr oxides in the structure is preferable. The minimum with the preferable total area ratio of an intermetallic compound and Cr oxide is 2.5%, More preferably, it is 2.0%, More preferably, it is 1.5%. More preferably, the total area ratio of the intermetallic compound and the Cr oxide is 0%.

If the size of each intermetallic compound and each Cr oxide is 5.0 μm ² or less, the influence on the SSC resistance is small. Even if the size of each intermetallic compound and each Cr oxide is 1.0 μm ² , 2.0 μm ² or 5.0 μm ² , the influence on the SSC resistance is small. However, even if the size of each intermetallic compound and each Cr oxide is 5.0 μm ² or less, if the total area ratio exceeds 3.0%, the SSC resistance is significantly affected.

[Measurement method of size of each intermetallic compound and each Cr oxide, total area ratio of intermetallic compound and Cr oxide]
The size of each intermetallic compound, the size of each Cr oxide, and the total area ratio of the intermetallic compound and Cr oxide are measured by observing the structure using the extraction replica method. Specifically, the measurement is performed by the following method.

  A test piece of 15 mm × 15 mm × 15 mm is taken from the center position in the thickness direction of the martensitic stainless steel material. The center position in the thickness direction is the center position of the plate thickness when the martensitic stainless steel material is a steel plate, and the center position of the thickness when the martensite stainless steel material is a steel pipe. One test piece is collected from the front end (TOP portion) in the longitudinal direction of the steel material and one from the rear end (BOTTOM portion). The front end means a front end area when the steel material is divided into 10 equal parts in the longitudinal direction, and the rear end means a rear end area.

An extraction replica film is formed from the surface of the collected test piece based on the extraction replica method. Specifically, the surface of the test piece is electropolished. The surface of the test piece after electropolishing is corroded using a Villera reagent (ethanol solution containing 1 to 5 g of hydrochloric acid and 1 to 5 g of picric acid). Thereby, precipitates and inclusions are exposed from the surface. The surface after corrosion is covered with a carbon vapor deposition film (hereinafter referred to as an extraction replica film). The test piece whose surface is covered with the extraction replica film is immersed in a bromine methanol solution (bromomethanol) to dissolve the test piece, and the extraction replica film is peeled off from the test piece. The exfoliated extraction replica film has a disk shape with a diameter of 3 mm. Using a TEM (transmission electron microscope), in each extraction replica film, an arbitrary 10 μm ² region is observed at four locations (four fields of view) at a magnification of 20000 times. That is, eight regions are observed in one steel material.

Elemental concentration analysis (EDS point analysis) using the energy dispersive X-ray spectroscopy (hereinafter referred to as EDS) for precipitates or inclusions confirmed by reflected electron images in each observation region ). Based on the element concentration obtained from each precipitate or inclusion by EDS point analysis, the intermetallic compound and the Cr oxide are specified. The individual areas (μm ² ) of the identified intermetallic compound and Cr oxide are determined. The area (μm ² ) of each intermetallic compound and Cr oxide is the size of each intermetallic compound and each Cr oxide. The total of the area of the intermetallic compound and the area of the Cr oxide is defined as the total area (μm ² ) of the intermetallic compound and the Cr oxide. The ratio of the total area of the intermetallic compound and the Cr oxide to the total area (80 μm ² ) of the entire observation region is defined as the total area ratio (%) of the intermetallic compound and the Cr oxide.

In addition, the magnitude | size of the intermetallic compound and Cr oxide which can be observed with the above-mentioned method is 0.05 micrometer < ² > or more. Therefore, in this embodiment, the lower limit of the size (area) of the intermetallic compound and Cr oxide to be measured is set to 0.05 μm ² . The total area of 0.05 .mu.m ² below intermetallic compound, when compared with the total area of the intermetallic compound in the size of 0.05~5.0μm ² (area), negligibly small. The total area of 0.05 .mu.m ² below Cr oxide, when compared with the total area of the Cr oxide of the magnitude of 0.05~5.0μm ² (area), negligibly small.

Further, in the observation of an optical microscope and SEM (scanning electron microscope), if clearly 5.0 .mu.m ² or more large intermetallic compounds or 5.0 .mu.m ² or more Cr oxide is observed even one, with it Just judge.

FIG. 2 is a TEM (transmission electron microscope) image obtained by observing the metal structure of steel of test number 3 in the examples described later. In the martensitic stainless steel material according to the present embodiment of FIG. 2, there is no intermetallic compound and Cr oxide having a size of 5.0 μm ² or more, and the total area ratio of the intermetallic compound and Cr oxide is 3 0.0% or less.

  FIG. 3 is an SEM image obtained by observing the metal structure of steel of test number 9 which is a comparative example in the examples described later. In FIG. 3, the black or gray region in the white region (matrix) was an intermetallic compound, and the total area ratio of the intermetallic compound was 4.0%.

[Production method]
An example of the manufacturing method of the above-mentioned martensitic stainless steel material will be described. The manufacturing method of the martensitic stainless steel material includes a step of preparing the material (preparation step), a step of hot working the material to manufacture a steel material (hot working step), and quenching and tempering the steel material. A process (heat treatment process). Hereinafter, each process is explained in full detail.

[Preparation process]
The molten steel which has the above-mentioned chemical composition and satisfy | fills Formula (1) and Formula (2) is manufactured. The material is manufactured using molten steel. Specifically, a slab (slab, bloom, billet) is manufactured by continuous casting using molten steel. You may manufacture an ingot by the ingot-making method using molten steel. If necessary, billets may be produced by subjecting slabs, blooms or ingots to partial rolling or hot forging. The material (slab, bloom, or billet) is manufactured by the above process.

[Hot working process]
Heat the prepared material. A preferable heating temperature is 1000 to 1300 ° C. A preferred lower limit of the heating temperature is 1150 ° C.

  The heated material is hot worked to produce martensitic stainless steel. When the martensitic stainless steel material is a steel plate, for example, the raw material is hot-rolled using one or a plurality of rolling mills including a pair of roll groups to produce a steel plate. In the case where the martensitic stainless steel material is an oil well steel pipe, for example, the material is pierced, rolled, and fixed-diameter rolled by a well-known Mannesmann-mandrel mill method to produce a seamless steel pipe.

[Heat treatment process]
The heat treatment step includes a quenching step and a tempering step. In the heat treatment step, first, a quenching step is performed on the steel material manufactured in the hot working step. Quenching is performed by a known method. Quenching temperature is _{A C3} transformation point or higher, for example, 900 to 1000 ° C.. After holding the steel material at the quenching temperature, it is rapidly cooled (quenched). Although holding time is not specifically limited, For example, it is 10 to 60 minutes. The quenching method is, for example, water cooling.

Furthermore, a tempering process is implemented with respect to the steel materials after hardening. In the tempering step, the strength of the steel material is adjusted to 724 to 860 MPa. In the tempering step, the precipitation of intermetallic compounds is further suppressed. Therefore, the tempering temperature is 570 ° C. Ultra _{to A C1} transformation point. The minimum with a preferable tempering temperature is 580 degreeC, More preferably, it is 585 degreeC. The upper limit with preferable tempering temperature is 630 degreeC, More preferably, it is 620 degreeC.

In the tempering step, the tempering temperature T (° C.) and the holding time t (minutes) at the tempering temperature satisfy the formula (3).
10000 ≦ (T + 273) × (20 + log (t / 60)) × (t / 60 × (0.5Cr + 2Mo) / (Cu + Ni)) ≦ 40000 (3)
Here, the tempering temperature (° C.) is substituted for T in Equation (3), and the holding time (minutes) at the tempering temperature is substituted for t. The content (mass%) of the corresponding element in the steel material is substituted for each element symbol in the formula (3).

  In the case of the above chemical composition satisfying the formulas (1) and (2), the amount of heat given to the steel during tempering affects the precipitation of intermetallic compounds. Furthermore, in the chemical composition satisfying the formula (1) and the formula (2), Cr and Mo are alloy elements constituting the intermetallic compound. Therefore, Cr and Mo promote the generation of intermetallic compounds such as Laves phase, σ phase, χ phase, and Cr oxide. On the other hand, in the chemical composition satisfying the formulas (1) and (2), Cu and Ni suppress the formation of the above-described intermetallic compounds such as Laves phase, σ phase, χ phase, and Cr oxide. Accordingly, the Cr content, the Mo content, the Cu content, and the Ni content affect tempering conditions for suppressing the formation of intermetallic compounds.

Therefore, in the present embodiment, tempering is performed at a tempering temperature T (° C.) and a holding time t (minutes) that satisfy Expression (3). In this case, in the steel material having a chemical composition satisfying the formula (1) and the formula (2) and having a martensite volume fraction of 80% or more, the size of the intermetallic compound is 5.0 μm ² or less, and The total area ratio of the intermetallic compound and the Cr oxide can be 3.0% or less.

In addition, when F3 = (T + 273) × (20 + log (t / 60)) × (t / 60 × (0.5Cr + 2Mo) / (Cu + Ni)), if F3 is less than 10,000 or F3 exceeds 40000, In the steel material after tempering, even if the yield strength is 724 to 860 MPa, there are those in which the size of the intermetallic compound exceeds 5.0 μm ² , or the total area ratio of the intermetallic compound and the Cr oxide is 3 It will exceed 0.0%. Therefore, F3 is 10,000 to 40,000.

  The minimum with preferable F3 is 10300, More preferably, it is 10500, More preferably, it is 10700. The upper limit with preferable F3 is 38000, More preferably, it is 37000, More preferably, it is 36000, More preferably, it is 35500.

The tempering temperature T (° C.) is the furnace temperature (° C.) of the heat treatment furnace for tempering. The holding time t (minutes) means the time held at the tempering temperature T. The martensitic stainless steel material of this embodiment can be manufactured by the above manufacturing process. In addition, about Cr oxide, if the steel material of the chemical composition which satisfy | fills the above-mentioned Formula (1) and Formula (2) is manufactured, the magnitude | size of each Cr oxide can be 5.0 micrometers ² or less. And by satisfy | filling the above-mentioned tempering conditions, the total area ratio of an intermetallic compound and Cr oxide can be 3.0% or less.

In addition, the martensitic stainless steel material of this embodiment is not limited to the above-mentioned manufacturing method. It has a chemical composition satisfying the formulas (1) and (2), the yield strength is 724 to 860 MPa, the volume ratio of martensite in the structure is 80% or more, each intermetallic compound and each Cr in the structure If the size of the oxide is 5.0 μm ² or less and the total area ratio of the intermetallic compound and the Cr oxide is 3.0% or less, the manufacturing method of the martensitic stainless steel material of the present embodiment is particularly It is not limited.

  Molten steel having the chemical composition shown in Table 1 was produced.

  The molten steel was melted in a 50 kg vacuum furnace, and an ingot was produced by an ingot-making method. The ingot was heated at 1250 ° C. for 3 hours. A block was manufactured by performing hot forging on the heated ingot. The block after hot forging was soaked at 1230 ° C. for 15 minutes and subjected to hot rolling to produce a plate material having a thickness of 13 mm.

  Quenching was performed on the plate material. The quenching temperature (° C.) during quenching and the holding time (minutes) at the quenching temperature were as shown in Table 2. The rapid cooling method (quenching method) after elapse of the holding time was water cooling in any test number. Tempering was performed on the plate material after quenching. Table 2 shows the tempering temperature (° C.) during tempering, the holding time (minutes) at the tempering temperature, and the F3 value.

  Quenching and tempering were performed and adjustment was performed so that the yield strength YS was 724 to 860 MPa. The martensitic stainless steel material was manufactured with the above manufacturing method.

[Evaluation test]
[Martensite volume fraction measurement test]
A test piece of 15 mm × 15 mm × 2 mm thickness was collected from each plate. The volume ratio (%) of retained austenite was determined by the above-mentioned X-ray diffraction method, and the difference in volume ratio of retained austenite from 100% was defined as the volume ratio (%) of martensite.

[About the size and total area ratio of intermetallic compounds and Cr oxides in the structure]
A test piece of 15 mm × 15 mm × 15 mm was collected from the central position of the thickness of each plate. One test piece was taken from the front end (TOP part) in the longitudinal direction of the plate material and one from the rear end (BOTTOM part). The front end means a front end area when the steel material is divided into 10 equal parts in the longitudinal direction, and the rear end means a rear end area.

An extracted replica film was prepared from the surface of the two collected specimens based on the extraction replica method. The extracted replica membrane was disk-shaped with a diameter of 3 mm. Using a TEM (transmission electron microscope), in each extraction replica film, 4 regions (4 fields of view) of an arbitrary 10 μm ² region were observed at a magnification of 20000 times. That is, eight regions were observed in one steel material.

Based on the contrast determined by the reflected electron image of each observation region, the intermetallic compound and the Cr oxide were specified. Each area (μm ² ) of the identified intermetallic compound and Cr oxide was defined as the size of each intermetallic compound and the size of each Cr oxide. Furthermore, the sum total of the area of the specified intermetallic compound and Cr oxide was made into the total area (micrometer < ² >) of an intermetallic compound and Cr oxide. The ratio of the total area of the intermetallic compound and the Cr oxide to the total area (80 μm ² ) of the entire observation region was defined as the total area ratio (%) of the intermetallic compound and the Cr oxide.

In the “structure” column of Table 2, “TM” is a martensite volume fraction in the structure of 80% or more, and the size of each intermetallic compound in the structure is 5.0 μm ² or less, It shows that the size of each Cr oxide is 5.0 μm ² or less, and the total area ratio of the intermetallic compound and Cr oxide in the structure is 3.0% or less. “TM + I” means that although the volume ratio of martensite in the structure is 80% or more, intermetallic compounds or Cr oxides of 5.0 μm ² or more existed in the structure and / or between the metals in the structure It shows that the total area ratio of the compound and Cr oxide exceeded 3.0%.

[Tensile test]
Tensile specimens were collected from each test material. The tensile test piece was a round bar tensile test piece having a parallel part diameter of 6 mm and a parallel part length of 40 mm. The longitudinal direction of the parallel part of the test piece was the rolling direction of the plate material. Using this test piece, a tensile test was performed at room temperature (25 ° C.) to obtain a yield strength YS (MPa). The yield strength YS was 0.2% proof stress. The obtained yield strength YS is shown in Table 2.

[SSC resistance evaluation test]
A round bar test piece having a parallel part diameter of 6.3 mm and a parallel part length of 25.4 mm was collected from each test material. A constant load test of NACE TM0177 Method A was performed in a test solution containing hydrogen sulfide using a round bar test piece. Specifically, as a test solution, a solution adjusted to pH 3.5 by adding CH ₃ COOH to an aqueous solution containing 5 wt% NaCl and 0.4 g / L CH ₃ COONa while passing 1 atmosphere of CO ₂ gas. Got ready. The applied stress to the round bar specimen during the test was 90% of the actual yield stress. The test piece to which the applied stress was applied was immersed in the aqueous solution saturated with a mixed gas of 0.1 atm H ₂ S gas and 0.9 atm CO ₂ for 720 hours. The test temperature was 24 ± 3 ° C.

  After the test, the surface of the parallel part was visually observed (using a 10 × magnifier). “NG” in “SSC resistance” in Table 2 indicates that cracking was observed. “OK” in “SSC resistance” in Table 2 indicates that no cracks were observed.

[Test results]
Referring to Table 2, the chemical compositions of Test No. 1 to Test No. 6 were appropriate and satisfied Formula (1) and Formula (2). As a result, in the structure, the volume ratio of martensite is 80% or more, the size of each intermetallic compound and each Cr oxide in the structure is 5.0 μm ² or less, and the intermetallic compound and Cr in the structure. The total area ratio of the oxide was 3.0% or less. As a result, excellent SSC resistance was exhibited even in an environment where H ₂ S was 0.1 atm.

On the other hand, in test number 7, although the chemical composition was appropriate, F3 exceeded 40000. As a result, an intermetallic compound exceeding 5.0 μm ² was confirmed, and the total area ratio of the intermetallic compound and the Cr oxide exceeded 3.0%. As a result, the SSC resistance was low.

  In Test No. 8, the chemical composition was appropriate, but the tempering temperature was too low. As a result, the total area ratio of the intermetallic compound and the Cr oxide exceeded 3.0%. As a result, the SSC resistance was low.

  Test number 9 to test number 10 did not satisfy the formula (2). Therefore, the yield strength could not be adjusted to 860 MPa or less by tempering, and the yield strength exceeded 860 MPa. As a result, the SSC resistance was low.

  In test number 11, the Ni content was too high and Ti was not contained. Therefore, the yield strength could not be adjusted to 860 MPa or less by tempering, and the yield strength exceeded 860 MPa. As a result, the SSC resistance was low.

  In test number 12 to test number 14, F1 deviated from the upper limit of formula (1). Therefore, the total area ratio of the intermetallic compound and the Cr oxide exceeded 3.0%. As a result, the SSC resistance was low.

  In test number 15, the Ti content was low, and F2 did not satisfy Formula (2). Therefore, the yield strength could not be adjusted to 860 MPa or less by tempering, and the yield strength exceeded 860 MPa. In Test No. 15, F1 exceeded the upper limit of formula (1). Therefore, the total area ratio of the intermetallic compound and the Cr oxide exceeded 3.0%. As a result, the SSC resistance was low.

  In test number 16 to test number 18, F1 was less than the lower limit of formula (1). Therefore, the SSC resistance was low. Since the ratio of Ni content with respect to Cr, Mo, and Cu content was too much, the diffusion coefficient of hydrogen in steel became too low, and as a result, it was thought that SSC resistance fell.

  In test number 19, F1 was less than the lower limit of formula (1), and F2 did not satisfy formula (2). Therefore, the yield strength could not be adjusted to 860 MPa or less by tempering, and the yield strength exceeded 860 MPa. Furthermore, the SSC resistance was low.

  In Test No. 20 to Test No. 22, the Cu content was low and the tempering temperature was too low. As a result, the SSC resistance was low.

  Test No. 23 had a high Ni content and a low Cu content. As a result, the SSC resistance was low.

Test No. 24 had a high Ni content, a low Cu content, and a tempering temperature that was too low. As a result, an intermetallic compound exceeding 5.0 μm ² was confirmed, and the total area ratio of the intermetallic compound and the Cr oxide exceeded 3.0%. Therefore, the yield strength exceeded 860 MPa, and the SSC resistance was low.

  Test No. 25 to Test No. 28 had a low Cu content and did not contain Ti. As a result, the SSC resistance was low.

  Test No. 29 had a high Ni content and a low Cu content. As a result, the SSC resistance was low.

  Test No. 30 had too high Ni content and did not contain Ti. As a result, the yield strength was too high and the SSC resistance was low.

  In test number 31, F1 was less than the lower limit of formula (1). Therefore, the SSC resistance was low. Since the ratio of Ni content with respect to Cr, Mo, and Cu content was too much, the diffusion coefficient of hydrogen in steel became too low, and as a result, it was thought that SSC resistance fell.

  In the test number 32, the formula (2) was not satisfied. Therefore, the yield strength could not be adjusted to 860 MPa or less by tempering, and the yield strength exceeded 860 MPa. As a result, the SSC resistance was low.

In Test No. 33, although the chemical composition was appropriate, F3 exceeded 40000. As a result, an intermetallic compound exceeding 5.0 μm ² was confirmed, and the total area ratio of the intermetallic compound and the Cr oxide exceeded 3.0%. As a result, the SSC resistance was low.

  The embodiment of the present invention has been described above. However, the above-described embodiment is merely an example for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately changing the above-described embodiment without departing from the spirit thereof.

Claims (3)

% By mass
C: 0.030% or less,
Si: 1.00% or less,
Mn: 1.00% or less,
P: 0.030% or less,
S: 0.005% or less,
Al: 0.0010 to 0.0100%,
N: 0.0500% or less,
Ni: 5.00 to 6.50%,
Cr: 10.00 to 13.40%,
Cu: 1.80 to 3.50%,
Mo: 1.00 to 4.00%,
V: 0.01-1.00%,
Ti: 0.050-0.300%
Co: 0.300% or less, and
W: 0 to 1.50%, the balance is Fe and impurities,
A chemical composition satisfying the formulas (1) and (2);
Yield strength of 724-860 MPa,
And a structure having martensite of 80% or more by volume ratio,
The size of each intermetallic compound and each Cr oxide in the structure is 5.0 μm ² or less, and the total area ratio of the intermetallic compound and the Cr oxide is 3.0% or less. Sight stainless steel material.
11.5 ≦ Cr + 2Mo + 2Cu−1.5Ni ≦ 14.3 (1)
Ti / C ≧ 7.5 (2)
Here, the content (mass%) of a corresponding element is substituted for each element symbol in the formulas (1) and (2). The martensitic stainless steel material according to claim 1,
The chemical composition is
W: A martensitic stainless steel material containing 0.10 to 1.50%. The martensitic stainless steel material according to claim 1 or claim 2,
The martensitic stainless steel material is a martensitic stainless steel material that is a seamless steel pipe for oil wells.

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