RU2718019C1 - Martensitic stainless steel product - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to the field of metallurgy, namely to a martensite stainless steel product made, in particular, in the form of a pipe and used in oil and gas wells. Steel has chemical composition consisting of, wt%: 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: from 5.00 to 6.50, Cr: from 10.00 to 13.40, Cu: from 1.80 to 3.50, Mo: from 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, the rest is iron and impurities. Steel has yield strength 724–860 MPa, and its composition satisfies the following ratios: 11.5≤Cr+2Mo+2Cu-1.5Ni≤14.3 and Ti/C≥7.5. Microstructure of steel contains martensite with volume fraction of 80 % or more, wherein size of each intermetallic compound and each Cr oxide in microstructure is 5.0 mcm² or less, and total fraction of area of intermetallic compounds and Cr oxides is 3.0 % or less.

EFFECT: manufactured product has high resistance to sulphide cracking under stress.

3 cl, 3 dwg, 2 tbl

Description

FIELD OF THE INVENTION

[0001]

The present invention relates to a steel product, and more particularly to a martensitic stainless steel product that has a mainly martensitic microstructure.

BACKGROUND

[0002]

As wells with a low corrosive environment (oil wells and gas wells) are exhausted, the operation of wells with high corrosivity is expanding. Wells with high corrosivity are located in an environment that contains a large amount of corrosive substances, and the temperatures of wells that have high corrosivity vary from normal temperature to approximately 200 ° C, depending on the depth of the well. Corrosive substances are, for example, corrosive gases such as hydrogen sulfide and gaseous carbon dioxide. In the present description, a well medium containing carbon dioxide gas and having high corrosivity, in which the partial pressure of hydrogen sulfide is 0.1 bar or more, is referred to as a “highly corrosive medium."

[0003]

It is known that chromium (Cr) is effective for improving the corrosion resistance of steel by gaseous carbon dioxide. Therefore, in an environment that contains a large amount of carbon dioxide gas, martensitic stainless steel containing approximately 13 wt.% Cr (hereinafter referred to as “13Cr steel”) represented by API steel L80-13Cr (ordinary 13Cr steel) or 13Cr super steel is used and the like, or two-phase stainless steel to which a large amount of Cr and the like are added, in accordance with the partial pressure or temperature of the gaseous carbon dioxide.

[0004]

However, hydrogen sulfide causes stress sulfide cracking (“SSC”) in steel pipes for an oil well made of 13Cr steel with high strength, for example, 724 MPa or more. 13Cr steel having a high strength of 724 MPa or more has a higher susceptibility to sulphide stress cracking as compared to low alloy steel, and in 13Cr steel, sulphide stress cracking occurs even at a relatively low partial pressure of hydrogen sulfide (for example, less than 0, 1 atm). Therefore, 13Cr steel is not suitable for use in the above highly corrosive environment containing gaseous carbon dioxide and hydrogen sulfide. In addition, two-phase stainless steel is expensive compared to 13Cr steel. Therefore, steel casing pipes for an oil well, tubing and drill pipes having a high yield strength of 724 MPa or more, as well as high resistance to sulfide stress cracking, which could be used in a highly corrosive environment, are required.

[0005]

Steels having excellent resistance to sulfide stress cracking are proposed in Japanese Patent Application No. 10-001755 (Patent Document 1), Japanese Patent Application No. 10-503809 (Patent Document 2), and Japanese Patent Application No. 2003-003243 (Patent document 3), in international patent application WO 2004/057050 (Patent document 4), in Japanese patent application No. 2000-192196 (Patent document 5), in Japanese patent application No. 11-310855 (Patent document 6), in Japanese patent application No. 08-246107 (Patent Document 7) and Japanese Patent Application No. 2012-136742 (Patent Document 8).

[0006]

The martensitic stainless steel disclosed in Patent Document 1 consists of: C: from 0.005 wt.% To 0.05 wt.%, Si: from 0.05 wt.% To 0.5 wt.%, Mn: from 0, 1 wt.% To 1.0 wt.%, P: 0.025 wt.% Or less, S: 0.015 wt.% Or less, Cr: from 10 wt.% To 15 wt.%, Ni: from 4.0 wt. wt.% to 9.0 wt.%, Cu: from 0.5 wt.% to 3 wt.%, Mo: from 1.0 wt.% to 3 wt.%, Al: from 0.005 wt.% to 0, 2 wt.%, And N: from 0.05 wt.% To 0.1 wt.%, With a residue representing Fe and inevitable impurities. This steel satisfies the ratio 40C + 34N + Ni + 0.3Cu-1.1Cr-1.8Mo≥-10, and consists of a tempering martensite phase, a martensite phase, and a residual austenite phase. The sum of the fractions of the tempering martensite phase and the martensite phase is 60% or more and 90% or less, and the remaining part is the residual austenite phase.

[0007]

The martensitic stainless steel disclosed in Patent Document 2 consists of: C: from 0.005 wt.% To 0.05 wt.%, Si ≤ 0.50 wt.%, Mn: from 0.1 to 1.0 wt.% , P≤0.03 wt.%, S≤0.005 wt.%, Mo: from 1.0 wt.% To 3.0 wt.%, Cu: from 1.0 wt.% To 4.0 wt.% Ni: from 5 wt.% To 8 wt.%, And Al≤0.06 wt.%. This steel satisfies the ratios Cr + 1.6Mo≥13 and 40C + 34N + Ni + 0.3Cu-1.1Cr-1.8Mo≥-10.5. The remainder is essentially Fe, and the microstructure is tempering martensite.

[0008]

The martensitic stainless steel disclosed in Patent Document 3 consists of: C: from 0.001 wt.% To 0.04 wt.%, Si: 0.5 wt.% Or less, Mn: from 0.1 wt.% To 3 , 0 wt.%, P: 0.04 wt.% Or less, S: 0.01 wt.% Or less, Cr: from 10 wt.% To 15 wt.%, Ni: from 0.7 wt.% up to 8 wt.%, Mo: from 1.5 wt.% to 5.0 wt.%, Al: from 0.001 wt.% to 0.10 wt.%, and N: 0.07 wt.% or less, with a residue representing Fe and impurities. This steel also satisfies the ratio Mo≥1.5-0.89Si + 32.2C. The microstructure of the metal consists mainly of tempering martensite, carbides precipitated during tempering, and intermetallic compounds based on the Laves phase, which are finely dispersed during tempering. The martensitic stainless steel of Patent Document 3 has high strength with a yield strength of 860 MPa or more.

[0009]

The martensitic stainless steel disclosed in Patent Document 4 consists of: C: from 0.005 wt.% To 0.04 wt.%, Si: 0.5 wt.% Or less, Mn: from 0.1 wt.% To 3 , 0 wt.%, P: 0.04 wt.% Or less, S: 0.01 wt.% Or less, Cr: from 10 wt.% To 15 wt.%, Ni: from 4.0 wt.% up to 8 wt.%, Mo: from 2.8 wt.% to 5.0 wt.%, Al: from 0.001 wt.% to 0.10 wt.%, and N: 0.07 wt.% or less, with a residue representing Fe and impurities. This steel also satisfies the ratio Mo≥2.3-0.89Si + 32.2C. The microstructure of the metal consists mainly of tempering martensite, carbides precipitated during tempering, and intermetallic compounds based on the Laves phase or σ phase, etc., which are finely dispersed during tempering. The martensitic stainless steel of Patent Document 4 has high strength with a yield strength of 860 MPa or more.

[0010]

The martensitic stainless steel disclosed in Patent Document 5 consists of: C: from 0.001 wt.% To 0.05 wt.%, Si: from 0.05 wt.% To 1 wt.%, Mn: from 0.05 wt. wt.% up to 2 wt.%, P: 0.025 wt.% or less, S: 0.01 wt.% or less, Cr: from 9 wt.% to 14 wt.%, Mo: from 3.1 wt.% up to 7 wt.%, Ni: from 1 wt.% to 8 wt.%, Co: from 0.5 wt.% to 7 wt.%, dissolved Al: from 0.001 wt.% to 0.1 wt.%, N: 0.05 wt.% Or less, O: 0.01 wt.% Or less, Cu: from 0 wt.% To 5 wt.% And W: from 0 wt.% To 5 wt.%, With the remainder representing Fe and inevitable impurities.

[0011]

The martensitic stainless steel disclosed in Patent Document 6 contains C: 0.05 wt.% Or less, and Cr: from 7 wt.% To 15 wt.%. In addition, the content of Cu in the dissolved state is from 0.25 to 5 wt.%.

[0012]

The martensitic stainless steel disclosed in Patent Document 7 consists of: C: from 0.005 wt.% To 0.05 wt.%, Si: from 0.05 wt.% To 0.5 wt.%, Mn: from 0, 1 wt.% To 1.0 wt.%, P: 0.025 wt.% Or less, S: 0.015 wt.% Or less, Cr: from 12 wt.% To 15 wt.%, Ni: from 4.5 wt. wt.% to 9.0 wt.%, Cu: from 1 wt.% to 3 wt.%, Mo: from 2 wt.% to 3 wt.%, W: from 0.1 wt.% to 3 wt.% Al: from 0.005 wt.% To 0.2 wt.%, And N: from 0.05 wt.% To 0.1 wt.%, With the remainder being Fe and unavoidable impurities. This steel satisfies the ratio 40C + 34N + Ni + 0.3Cu + Co-1.1Cr-1.8Mo-0.9W≥-10.

[0013]

The seamless martensitic stainless steel pipe disclosed in Patent Document 8 consists of: C: 0.01 mass% or less, Si: 0.5 mass% or less, Mn: 0.1 mass% to 2, 0 wt.%, P: 0.03 wt.% Or less, S: 0.005 wt.% Or less, Cr: from 14.0 wt.% To 15.5 wt.%, Ni: from 5.5 wt. % to 7.0 wt.%, Mo: from 2.0 wt.% to 3.5 wt.%, Cu: from 0.3 wt.% to 3.5 wt.%, V: 0.20 wt. % or less, Al: 0.05 wt.% or less, and N: 0.06 wt.% or less, with a residue representing Fe and unavoidable impurities. The seamless martensitic stainless steel pipe disclosed in Patent Document 8 has a strength of 655-862 MPa as yield strength and a yield strength to strength ratio of 0.90 or more.

LIST OF REFERENCES

PATENT LITERATURE

[0014]

Patent Document 1: Japanese Published Patent Application No. 10-001755

Patent Document 2: Japanese Published Patent Application No. 10-503809

Patent Document 3: Japanese Published Patent Application No. 2003-003243

Patent Document 4: International Patent Application No. WO 2004/057050

Patent Document 5: Japanese Published Patent Application No. 2000-192196

Patent Document 6: Japanese Published Patent Application No. 11-310855

Patent Document 7: Japanese Published Patent Application No. 08-246107

Patent Document 8: Japanese Published Patent Application No. 2012-136742

SUMMARY OF THE INVENTION

TECHNICAL PROBLEM

[0015]

However, the yield strength of martensitic stainless steels described in Patent Document 1 and Patent Document 2 sometimes becomes too high. In this case, the resistance to sulfide stress cracking is low.

[0016]

The martensitic stainless steels described in Patent Document 3 and Patent Document 4 are 13Cr steel, and thin carbides and intermetallic compounds are precipitated in this steel. Observed intermetallic compounds and the like. are not only subtle compounds, and some of the intermetallic compounds present, and the like. are somewhat rude. Consequently, the yield strength of martensitic stainless steel corresponds to a grade of 125 ksi (thousand psi) (yield strength of 860 MPa or more). Therefore, the resistance to sulfide stress cracking is low in some cases.

[0017]

In the martensitic stainless steel described in Patent Document 5, the Mo content and Co content are high. Therefore, in some cases, the strength is too high and the resistance to sulfide stress cracking is reduced. In addition, if the Mo content is too high, the stability of the martensitic microstructure in some cases decreases.

[0018]

The martensitic stainless steel disclosed in Patent Document 6 is a hardened martensitic steel that is not tempered. Therefore, in some cases, the strength is too high and the resistance to sulfide stress cracking becomes low.

[0019]

The martensitic stainless steel disclosed in Patent Document 7 contains Cu and W, and does not contain Ti. Therefore, the yield strength of this martensitic stainless steel may in some cases be too high. In this case, the resistance to sulfide stress cracking becomes lower.

[0020]

The seamless martensitic stainless steel pipe disclosed in Patent Document 8 contains 14.0-15.5 wt.% Cr. Therefore, in some cases, this steel has a ferritic phase. In this case, the strength may be insufficient.

[0021]

It is an object of the present invention to provide a martensitic stainless steel product having a yield strength of 724 MPa or more and having excellent resistance to sulfide stress cracking.

SOLUTION TO THE PROBLEM

[0022]

The martensitic stainless steel product in accordance with the present invention has the following chemical composition in mass percent: C: 0.030 wt.% Or less, Si: 1.00 wt.% Or less, Mn: 1.00 wt.% Or less, P: 0.030 wt.% Or less, S: 0.005 wt.% Or less, Al: from 0.0010 wt.% To 0.0100 wt.%, N: 0.0500 wt.% Or less, Ni: from 5.00 wt.% up to 6.50 wt.%, Cr: from 10.00 wt.% to 13.40 wt.%, Cu: from 1.80 wt.% to 3.50 wt.%, Mo: from 1, 00 wt.% To 4.00 wt.%, V: from 0.01 wt.% To 1.00 wt.%, Ti: from 0.050 wt.% To 0.300 wt.%, Co: 0.300 wt.% Or less , and W: from 0 to 1.50 wt.%, with a residue of Fe and impurities, and satisfies Expression (1) and Expression (2), s an yield strength of 724-860 MPa, and its microstructure contains martensite with a volume fraction of 80 vol.% or more. The size of each intermetallic compound and the size of each Cr oxide in the microstructure is not more than 5.0 μm ² , and the fraction of the total area of the intermetallic compounds and Cr oxides is 3.0% or less.

11.5≤Cr + 2Mo + 2Cu-1.5Ni≤14.3 (1)

Ti / C≥7.5 (2)

where each element symbol in Expression (1) and Expression (2) corresponds to the content (in mass percent) of the corresponding element.

USEFUL EFFECTS OF THE INVENTION

[0023]

The martensitic stainless steel product in accordance with the present embodiment has a yield strength of 724 MPa or more, and also has excellent resistance to sulfide stress cracking.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024]

[FIG. 1] FIG. 1 illustrates the relationship between F1 = Cr + 2Mo + 2Cu-1.5Ni, yield strength YS (MPa) and resistance to sulfide stress cracking.

[FIG. 2] FIG. 2 is an image taken using a TEM (transmission electron microscope) when observing the microstructure of the metal steel of Test No. 3 in the Examples in accordance with the present embodiment.

[FIG. 3] FIG. 3 is an image obtained using SEM (scanning electron microscope) when observing the microstructure of the metal steel of Test No. 9 in the Examples.

DESCRIPTION OF EMBODIMENTS

[0025]

The authors of the present invention conducted studies of resistance to sulfide cracking under stress of the product of martensitic stainless steel and found the following.

[0026]

[Regarding the chemical composition]

It is well known that Cr, Mo, Cu, and Ni are effective in increasing the resistance of steel to sulfide stress cracking. In particular, it is believed that Cr, Mo and Cu dissolve, increasing resistance to sulfide stress cracking. On the other hand, it is believed that Ni increases the resistance to sulfide stress cracking by hardening the film on the surface of the steel product, thereby reducing the amount of hydrogen that penetrates the steel product (permeability to hydrogen).

[0027]

However, as a result of research, the authors of the present invention first discovered that in a highly corrosive medium, such as described above, hardening the film with Ni reduces the diffusion coefficient of hydrogen in steel. If the diffusion coefficient of hydrogen in steel decreases, more hydrogen remains in the steel product. Therefore, the resistance of the steel product to sulfide stress cracking is reduced.

[0028]

Therefore, the authors of the present invention conducted additional studies regarding Cr, Mo, Cu and Ni, which are elements that affect the resistance to sulfide stress cracking. As a result, the inventors of the present invention found that excellent resistance to sulfide stress cracking is obtained when a steel product having the following chemical composition: C: 0.030 wt.% Or less, Si: 1.00 wt.% Or less, Mn : 1.00 wt.% Or less, P: 0.030 wt.% Or less, S: 0.005 wt.% Or less, Al: from 0.0010 wt.% To 0.0100 wt.%, N: 0.0500 wt.% or less, Ni: from 5.00 wt.% to 6.50 wt.%, Cr: from 10.00 wt.% to 13.40 wt.%, Cu: from 1.80 wt.% to 3.50 wt.%, Mo: from 1.00 wt.% To 4.00 wt.%, V: from 0.01 wt.% To 1.00 wt.%, Ti: from 0.050 wt.% To 0.300 wt.%, Co: 0,300% or less, and W: from 0 to 1,50 ma C.%, with the remainder of Fe and impurities, has such contents of Cr, Mo, Cu and Ni, which satisfy the following Expression (1):

11.5≤Cr + 2Mo + 2Cu-1.5Ni≤14.3 (1)

where each symbol of the element in Expression (1) corresponds to the content (in mass percent) of the corresponding element.

[0029]

F1 is defined as Cr + 2Mo + 2Cu-1,5Ni. FIG. 1 illustrates the relationship between F1 = Cr + 2Mo + 2Cu-1.5Ni, yield strength YS (MPa) and resistance to sulfide stress cracking. The graph shown in FIG. 1 was obtained by way of examples, which will be described later. The symbol "Ο" in FIG. 1 means that sulfide stress cracking did not occur in the quality assessment test for sulfide stress cracking resistance in the examples that will be described later. The symbol "×" in FIG. 1 means that sulfide stress cracking occurred in the test for evaluating the quality of sulfide stress cracking resistance in the examples that will be described later. When F1 is less than 11.5 or when F1 is greater than 14.3, resistance to sulfide stress cracking decreases. Accordingly, the value of F1 is from 11.5 to 14.3.

[0030]

Resistance to sulfide stress cracking also depends on the strength of the steel product. In particular, the higher the strength of the steel product, the more the resistance to sulfide stress cracking decreases. Therefore, the authors of the present invention also conducted studies of the relationship between resistance to sulfide cracking under stress and the strength of a steel product having the following chemical composition: C: 0.030 wt.% Or less, Si: 1.00 wt.% Or less, Mn: 1, 00 wt.% Or less, P: 0.030 wt.% Or less, S: 0.005 wt.% Or less, Al: from 0.0010 wt.% To 0.0100 wt.%, N: 0.0500 wt.% or less, Ni: from 5.00 wt.% to 6.50 wt.%, Cr: from 10.00 wt.% to 13.40 wt.%, Cu: from 1.80 wt.% to 3.50 wt.%, Mo: from 1.00 wt.% to 4.00 wt.%, V: from 0.01 wt.% to 1.00 wt.%, Ti: from 0.050 wt.% to 0.300 wt.% , Co: 0.300 wt.% Or less, and W: o t 0 wt.% up to 1.50 wt.%, with a residue of Fe and impurities, which satisfies Expression (1). As a result, in a steel product having the above chemical composition that satisfies Expression (1), the present inventors have found that there is an optimum yield strength for the steel product so that it has excellent resistance to sulfide stress cracking, while obtaining the necessary strength. In particular, if the yield strength of a steel product having the above chemical composition, which satisfies Expression (1), is from 724 to 860 MPa, the necessary strength can be obtained, while the resistance to sulfide stress cracking can be increased.

[0031]

As shown in FIG. 1, if the F1 value is in the range of 11.5 to 14.3 and the YS value is 860 MPa or less, excellent resistance to sulfide stress cracking is obtained. Therefore, in the present embodiment, the yield strength of the steel product is from 724 to 860 MPa.

[0032]

As described above, the chemical composition of the steel product of the present embodiment contains, in wt.%, From 1.00 to 4.00% Mo, from 1.80 to 3.50% Cu and from 0.01 to 1.00% V. These elements dissolve and increase resistance to sulfide stress cracking. However, Mo, Cu, and V also increase the strength of the steel product. As described above, if the strength of the steel product having the above chemical composition that satisfies Expression (1) is too high, the resistance to sulfide stress cracking is reduced.

[0033]

Therefore, the authors of the present invention conducted further studies of methods for controlling the strength of a steel product. As a result, the present inventors have found that by adding Ti and adjusting its content with respect to the C content, excessive hardening can be suppressed.

[0034]

The reasons for this are as follows. Among the above-mentioned elements Mo, Cu and V, which increase the strength of the steel product, V forms carbides (VC) and thereby increases the strength of the steel product. Similarly, V, Ti binds to C in steel, forming carbides. Therefore, if Ti binds to C, then the amount of C to form VC will be insufficient due to the consumption of C by titanium. As a result, the formation of VC can be suppressed.

[0035]

Typically, the affinity of Ti to C in steel is equal to the affinity of V to C. Therefore, in a material containing V and Ti, VC and TiC are released simultaneously. Therefore, the ratio of the Ti content (wt.%) To the C content (wt.%) Is increased in order to thereby adjust the chemical composition so that TiC stands out with priority over VC. In other words, the Ti content (wt.%) And the content of C (wt.%) Satisfy Expression (2).

Ti / C≥7.5 (2)

[0036]

F2 is defined as Ti / C. When the F2 value is less than 7.5, Ti is consumed to form nitrides such as TiN, and TiC cannot be formed sufficiently. Therefore, C is used to form VC in steel, and the strength of the steel product becomes too high. If the value of F2 is 7.5 or more, the Ti content is sufficiently high relative to the content C. Therefore, TiC is allocated with priority over VC. Therefore, the formation of VC is suppressed. As a result, the yield strength of the steel product having a chemical composition that satisfies Expression (1) can be suppressed to 860 MPa or less, and excellent resistance to sulfide stress cracking can be obtained.

[0037]

[Regarding microstructure]

It is known that if large intermetallic compounds or large Cr oxides are present in the microstructure, large intermetallic compounds and large Cr oxides become the starting points for sulfide stress cracking, and resistance to sulfide stress cracking decreases. Therefore, coarsening of Cr oxides is traditionally suppressed, and intermetallic compounds are forced to precipitate in a thin form, thereby increasing the resistance to sulfide stress cracking. Thus, it was previously believed that finely dispersed Cr oxides and finely dispersed intermetallic compounds did not affect the resistance to sulfide stress cracking. However, the inventors of the present invention have found that in a steel product having the aforementioned chemical composition that satisfies Expression (1) and Expression (2), small Cr oxides and small intermetallic compounds also reduce the resistance to sulfide stress cracking. As a result of further studies, the authors of the present invention found that in a steel product having the above chemical composition that satisfies Expression (1) and Expression (2) if the size of each intermetallic compound and the size of each Cr oxide in the microstructure is 5.0 μm ² or less, and the total area fraction of intermetallic compounds and Cr oxides is 3.0% or less, resistance to sulfide stress cracking in a steel product having the above chemical composition, which d satisfy the expression (1) and Expression (2) is further increased.

[0038]

As used herein, the term “intermetallic compound” refers to the release of an alloying element that forms after tempering. Intermetallic compounds are, for example, a Laves phase, such as the sigma phase Fe ₂ Mo (σ phase) or the chi phase (χ phase). The σ phase is FeCr, and the χ phase is Fe ₃₆ Cr ₁₂ Mo ₁₀ . As used herein, the term “Cr oxide” refers to chromium oxide (Cr ₂ O ₃ ).

[0039]

Intermetallic compounds and Cr oxides can be identified by observing the structure using a replica extraction method. The sum of the areas of the identified intermetallic compounds and the identified Cr oxides is taken as the total area (μm ² ) of the intermetallic compounds and Cr oxides. The fraction of the total area of intermetallic compounds and Cr oxides in the total observation area is taken as the fraction of the total area (%) of intermetallic compounds and Cr oxides.

[0040]

If an intermetallic compound having an area of more than 5.0 μm ² or Cr oxide having an area of more than 5.0 μm ^{2 is} present in the metal microstructure of the metal, such coarse (large) intermetallic compounds or coarse Cr oxides become the starting points for sulfide cracking under stress, and resistance to sulfide stress cracking is reduced. Accordingly, the size of each intermetallic compound in the microstructure is 5.0 μm ² or less, and the size of each Cr oxide in the microstructure is 5.0 μm ² or less. In particular, in the present embodiment, intermetallic compounds having a size (area) greater than 5.0 μm ² and Cr oxides having a size (area) greater than 5.0 μm ² are not observed when observing the microstructure as will be described later.

[0041]

In addition, if the total area fraction of intermetallic compounds and Cr oxides in the metal microstructure is more than 3.0%, the resistance to sulfide stress cracking is reduced because there are too many intermetallic compounds and Cr oxides having a size of 5.0 μm ² or less. Accordingly, the total area fraction of intermetallic compounds and Cr oxides in the microstructure is 3.0% or less.

[0042]

If the chemical composition of the steel product satisfies Expression (1) and Expression (2), the microstructure contains martensite having a region fraction of 80% or more, the size of each of the intermetallic compounds and the size of each of the Cr oxides in the microstructure is 5.0 μm ² or less, and the total area fraction of intermetallic compounds and Cr oxides in the microstructure is 3.0% or less, the strength of the steel product can be adjusted to a range of 724-860 MPa.

[0043]

The martensitic stainless steel product in accordance with the present invention, which was obtained on the basis of the above findings, has the following chemical composition: C: 0.030 wt.% Or less, Si: 1.00 wt.% Or less, Mn: 1.00 wt. % or less, P: 0.030 wt.% or less, S: 0.005 wt.% or less, Al: from 0.0010 wt.% to 0.0100 wt.%, N: 0.0500 wt.% or less, Ni: from 5.00 wt.% To 6.50 wt.%, Cr: from 10.00 wt.% To 13.40 wt.%, Cu: from 1.80 wt.% To 3.50 wt.% , Mo: from 1.00 wt.% To 4.00 wt.%, V: from 0.01 wt.% To 1.00 wt.%, Ti: from 0.050 wt.% To 0.300 wt.%, Co: 0.300 wt.% Or less, and W: from 0 wt.% To 1.50 wt.%, With a residue of Fe and impurities, and satisfactory creates Expression (1) and Expression (2). The yield strength of the aforementioned martensitic stainless steel product is 724-860 MPa. The microstructure of the above martensitic stainless steel product contains martensite having a volume fraction of 80% or more. The size of each intermetallic compound and the size of each Cr oxide in the microstructure is 5.0 μm ² or less, and the total area fraction of the intermetallic compounds and Cr oxides is 3.0% or less.

where each element symbol in Expression (1) and Expression (2) corresponds to the content (in mass percent) of the corresponding element.

[0044]

The chemical composition of the above martensitic stainless steel product may contain W: from 0.10 wt.% To 1.50 wt.%.

[0045]

The aforementioned martensitic stainless steel product is, for example, a seamless steel pipe for casing, tubing and drill pipe.

[0046]

In the present description, the term "steel pipe for casing, tubing and drill pipe" means, for example, a steel pipe for casing, tubing and drill pipe, which is described in determination column No. 3514 of JIS G 0203 (2009). In particular, the term “steel pipe for casing, tubing and drill pipe” is the generic name for casing, tubing and drill pipe that are used to drill oil wells or gas wells, as well as to extract crude oil and natural gas and etc. The term “seamless steel pipe for casing, tubing and drill pipe” means that the steel pipe for casing, tubing and drill pipe is a seamless steel pipe.

[0047]

The following describes in detail the martensitic stainless steel product of the present embodiment. The symbol "%" in relation to the composition means "wt.%", Unless explicitly stated otherwise.

[0048]

[Chemical composition]

The chemical composition of the martensitic stainless steel product of the present embodiment contains the following elements.

[0049]

C: 0.030 wt.% Or less

Carbon (C) is an inevitably contained element. Thus, the content of C is more than 0 wt.%. C increases hardenability and increases the strength of the steel product. However, if the C content is too high, the strength of the steel product will be too high, and the resistance to sulfide stress cracking will decrease. Accordingly, the content of C is 0.030 wt.% Or less. It is preferable to make the C content as low as possible. However, if the C content decreases excessively, manufacturing costs increase. Therefore, taking into account industrial production, the preferred lower limit of the C content is 0.0001 wt.%, And more preferably 0.0005 wt.%. From the point of view of the strength of the steel product, the preferred lower limit of the content of C is 0.002 wt.%, And more preferably 0.005 wt.%. A preferred upper limit of the C content is 0.020 wt.%, And more preferably 0.015 wt.%.

[0050]

Si: 1.00 wt.% Or less

Silicon (Si) is an unavoidable impurity. Thus, the Si content is more than 0 wt.%. Si deoxidizes steel. However, if the Si content is too high, this effect is saturated. Accordingly, the Si content is 1.00 wt.% Or less. The preferred lower limit of the Si content is 0.05 wt.%, And more preferably 0.10 wt.%. The preferred upper limit of the Si content is 0.70 wt.%, And more preferably 0.50 wt.%.

[0051]

Mn: 1.00 wt.% Or less

Manganese (Mn) is an inevitably contained element. Thus, the Mn content is more than 0 wt.%. Mn improves the hardenability of steel. However, if the Mn content is too high, Mn segregates at the grain boundaries together with contaminants such as P and S. In this case, the resistance to sulfide stress cracking decreases. Accordingly, the Mn content is 1.00 wt.% Or less. A preferred lower limit of the Mn content is 0.15 wt.%, And more preferably 0.20 wt.%. A preferred upper limit for the Mn content is 0.80 wt.%, And more preferably 0.50 wt.%.

[0052]

P: 0.030 wt.% Or less

Phosphorus (P) is an impurity that is inevitably contained. Thus, the content of P is more than 0 wt.%. P segregates at the boundaries of crystalline grains and reduces the resistance of steel to sulfide stress cracking. Accordingly, the phosphorus content is 0.030 wt.% Or less. A preferred upper limit of the P content is 0.025 wt.%, And more preferably 0.020 wt.%. Preferably, the content of P is as low as possible. However, if the sulfur content decreases excessively, production costs increase. Therefore, taking into account industrial production, the preferred lower limit of the P content is 0.0001 wt.%, And more preferably 0.0005 wt.%.

[0053]

S: 0.005 wt.% Or less

Sulfur (S) is an impurity that is inevitably contained. Thus, the content of S is more than 0 wt.%. Similarly, P, S also segregates at the boundaries of crystalline grains and reduces the resistance of steel to sulfide stress cracking. Accordingly, the sulfur content is 0.005 wt.% Or less. A preferred upper limit of the S content is 0.003 wt.%, And more preferably 0.001 wt.%. Preferably, the S content is as low as possible. However, if the sulfur content decreases excessively, production costs increase. Therefore, taking into account industrial production, the preferred lower limit for the sulfur content is 0.0001 wt.%, And more preferably 0.0005 wt.%.

[0054]

Al: from 0.0010 wt.% To 0.0100 wt.%

Aluminum (Al) deoxidizes steel. If the Al content is low, this effect cannot be achieved. On the other hand, if the Al content is too high, this effect is saturated. Accordingly, the Al content is from 0.0010 to 0.0100 wt.%. The preferred lower limit of the aluminum content is 0.0020 wt.%, And more preferably 0.0030 wt.%. A preferred upper limit of the aluminum content is 0.0070 wt.%, And more preferably 0.0050 wt.%. As used herein, the term “Al content” means the content of soluble Al (acid-soluble Al).

[0055]

N: 0.0500 wt.% Or less

Nitrogen (N) is an impurity that is inevitably contained. Thus, the nitrogen content is more than 0 wt.%. N forms nitrides and reduces the resistance to sulfide stress cracking. Accordingly, the N content is 0.0500 wt.% Or less. A preferred upper limit of the N content is 0.0300 wt.% Or less, and more preferably 0.0200 wt.% Or less. Preferably, the N content is as low as possible. However, if the nitrogen content decreases excessively, production costs increase. Therefore, taking into account industrial production, the preferred lower limit of the nitrogen content is 0.0001 wt.%, And more preferably 0.0005 wt.%.

[0056]

Ni: from 5.00 wt.% To 6.50 wt.%

Nickel (Ni) is an austenite-forming element and martensitizes the microstructure after quenching. If the Ni content is too low, the microstructure after tempering will contain a large amount of ferrite. On the other hand, if the Ni content is too high, in a highly corrosive well Ni will reduce the diffusion coefficient of hydrogen in steel due to the hardening of the film. If the diffusion coefficient of hydrogen in steel decreases, the resistance to sulfide stress cracking will decrease. Therefore, the nickel content is from 5.00 wt.% To 6.50 wt.%. A preferred lower limit of the Ni content is 5.20 wt.%, And more preferably 5.30 wt.%. A preferred upper limit of the Ni content is 6.30 wt.%, And more preferably 6.20 wt.%.

[0057]

Cr: from 10.00 wt.% To 13.40 wt.%

Chromium (Cr) enhances the corrosion resistance of steel to gaseous carbon dioxide at high temperature. If the Cr content is too low, this effect cannot be obtained. If the Cr content is 10.00 wt.% Or more, the steel gains excellent corrosion resistance to carbon dioxide gas at high temperatures. On the other hand, if the Cr content is too high, coarse intermetallic compounds and coarse oxides of Cr are formed, and the resistance to sulfide stress cracking decreases. Accordingly, the Cr content is from 10.00 to 13.40 wt.%. A preferred lower limit of the Cr content is 11.00 wt.%, And more preferably 11.50 wt.%. A preferred upper limit of the Cr content is 13.30 wt.%, And more preferably 13.00 wt.%.

[0058]

Cu: 1.80 wt.% To 3.50 wt.%

Copper (Cu) is an austenite-forming element, similar to Ni, and martensitizes the microstructure after quenching. Cu also dissolves in steel and increases resistance to sulfide stress cracking. If the Cu content is too low, these effects cannot be obtained. On the other hand, if the Cu content is too high, the hot workability is reduced. Therefore, the copper content is from 1.80 wt.% To 3.50 wt.%. The preferred lower limit of the copper content is 1.90 wt.%, And more preferably 1.95 wt.%. A preferred upper limit for the Cu content is 3.30 wt.%, And more preferably 3.10 wt.%.

[0059]

Mo: from 1.00 wt.% To 4.00 wt.%

Molybdenum (Mo) increases the resistance to sulfide stress cracking and the strength of steel. If the Mo content is too low, these effects cannot be obtained. Mo, on the other hand, is a ferrite forming element. Therefore, if the Mo content is too high, it becomes difficult to stabilize austenite and to stably obtain a martensitic microstructure. Accordingly, the Mo content is from 1.00 to 4.00 wt.%. The preferred lower limit of the Mo content is 1.20 wt.%, More preferably 1.50 wt.%, And most preferably 1.80 wt.%. The preferred upper limit of the Mo content is 3.50 wt.%, More preferably 3.00 wt.%, And most preferably 2.70 wt.%.

[0060]

V: from 0.01 wt.% To 1.00 wt.%

Vanadium (V) dissolves in steel and inhibits 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 product increases, and the resistance to sulfide stress cracking decreases. Accordingly, the content of vanadium is from 0.01 wt.% To 1.00 wt.%. A preferred lower limit of the V content is 0.02 wt.%, And more preferably 0.03 wt.%. A preferred upper limit of the V content is 0.80 wt.%, And more preferably 0.70 wt.%.

[0061]

Ti: from 0.050 wt.% To 0.300 wt.%

Titanium (Ti) binds to C to form carbides. This means that C for VC formation is consumed by titanium, and VC formation can be suppressed. Consequently, the resistance of steel to sulfide stress cracking of steel increases. This effect is not obtained if the titanium content is too low. On the other hand, if the Ti content is too high, the aforementioned effect is saturated and, in addition, the formation of ferrite is accelerated. Accordingly, the Ti content is from 0.050 wt.% To 0.300 wt.%. The preferred lower limit of the Ti content is 0.060 wt.%, More preferably 0.070 wt.%, And most preferably 0.080 wt.%. The preferred upper limit of the Ti content is 0.250 wt.%, More preferably 0.200 wt.%, And most preferably 0.150 wt.%.

[0062]

Co: 0.300 wt.% Or less

Cobalt (Co) is an impurity that is inevitably contained. Thus, the Co content is more than 0 wt.%. Toughness and toughness are reduced if the Co content is too high. Accordingly, the Co content is 0.300 wt.% Or less. A preferred upper limit for Co is 0.270 wt.%, And more preferably 0.250 wt.%. Preferably, the Co content is as low as possible. However, if the Co content decreases excessively, production costs increase. Therefore, in view of industrial production, the preferred lower limit of Co content is 0.0001 wt.%, And more preferably 0.0005 wt.%.

[0063]

The remainder of the chemical composition of the martensitic stainless steel product in accordance with this embodiment is Fe and impurities. Here, the term "impurities" refers to elements that, during the industrial production of steel, enter it from ore or scrap used as raw material, or from a production environment, etc.

[0064]

The chemical composition of the martensitic stainless steel product in accordance with the present embodiment may further comprise W instead of part Fe.

[0065]

W: from 0 wt.% To 1.50 wt.%

Tungsten (W) is an optional element and may not be contained. Thus, the content of W can be 0 wt.%. If W is contained, it stabilizes the passive film and improves corrosion resistance. However, if the W content is too high, W binds to C to form small carbides. These small carbides increase the strength of the steel product due to dispersion hardening, and as a result reduce the resistance to sulfide stress cracking. Accordingly, the W content is from 0 wt.% To 1.50 wt.%. A preferred lower limit of the W content is 0.10 wt.%, And more preferably 0.20 wt.%. A preferred upper limit of the W content is 1.00 wt.%, And more preferably 0.50 wt.%.

[0066]

[Regarding expression (1)]

The above chemical composition also satisfies Expression (1):

where each symbol of the element in Expression (1) corresponds to the content (in mass percent) of the corresponding element.

[0067]

F1 is defined as Cr + 2Mo + 2Cu-1,5Ni. F1 is the sulfide stress crack resistance index of a steel product having the above chemical composition. As shown in FIG. 1, resistance to sulfide stress cracking is reduced if F1 has a value of less than 11.5. It is believed that the reason that the resistance to sulfide stress cracking decreases, in this case, is that the Ni content, which reduces the diffusion coefficient of hydrogen in the steel, is too high relative to the content of Cr, Mo and Cu, which dissolve and increase the resistance to sulfide stress cracking. On the other hand, when the F1 value is more than 14.3, the resistance to sulfide stress cracking also decreases. It is believed that the reason that the resistance to sulfide stress cracking is reduced in this case is because the content of Ni, which forms a film on the surface and suppresses hydrogen infiltration, is too low with respect to the contents of Cr, Mo and Cu, which increase the resistance to sulfide stress cracking, and therefore, the amount of hydrogen that is infiltrated into the steel increases, and as a result, resistance to sulfide stress cracking decreases. Therefore, the value of F1 is from 11.5 to 14.3. The preferred lower limit of F1 is 11.7. The preferred upper limit of F1 is 14.0.

[0068]

It should be noted that the value F1 is the value obtained by rounding to two decimal places.

[0069]

[Regarding expression (2)]

The above chemical composition satisfies Expression (1), and also satisfies Expression (2):

where each element symbol in Expression (1) and Expression (2) corresponds to the content (in mass percent) of the corresponding element.

[0070]

F2 is defined as Ti / C. F2 is an index of suppressing the release of VC by titanium, i.e. an index of strength of a steel product. As described above, in most cases in a material containing V and Ti, VC and TiC are released simultaneously. Therefore, in a chemical composition that satisfies Expression (1), the ratio of the Ti content (wt.%) To the content of C (wt.%) Is increased, thereby adjusting the chemical composition so that TiC is released with higher priority than VC. This means that C is consumed by titanium, and the binding of C to V to form VC is suppressed.

[0071]

If the F2 value is less than 7.5, the Ti content is too low with respect to C. In this case, Ti is consumed to form nitrides such as TiN, and TiC cannot be formed sufficiently. Therefore, C is used to form the VC, and the strength of the steel product is too high. When the value of F2 is 7.5 or more, the Ti content is sufficiently high relative to the content of C. As a result, Ti consumes C and forms TiC with higher priority than VC. This means that VC formation can be suppressed. Therefore, an excessive increase in the strength of the steel product can be prevented. As a result, the steel product can have excellent resistance to sulfide stress cracking.

[0072]

It should be noted that the value of F2 is the value obtained by rounding to two decimal places.

[0073]

[Microstructure]

[Volume fraction of martensite: 80% or more]

Martensite is the main component of the microstructure of the above martensitic stainless steel product. In this case, the term "martensite" also includes vacation martensite. The phrase “martensite is the main microstructure” means that the volume fraction of martensite in the microstructure is 80% or more. The remainder of the microstructure is residual austenite. The volume fraction of residual austenite is from 0 to 20%. Preferably, the volume fraction of residual austenite is as low as possible. The preferred lower limit of the volume fraction of martensite in the microstructure is 85%, and more preferably 90%. Even more preferably, the microstructure of the metal is a single-phase martensitic structure.

[0074]

In the aforementioned microstructure, residual austenite, which is contained in a small amount, does not lead to a significant decrease in strength, and significantly increases the toughness of steel. However, if the volume fraction of residual austenite is too high, the strength of the steel is noticeably reduced. Accordingly, the volume fraction of residual austenite is from 0 to 20%. From the point of view of providing strength, the volume fraction of residual austenite is more preferably from 0 to 10%. As described above, the microstructure of the martensitic stainless steel product of the present embodiment may be a single-phase martensitic structure. Accordingly, in this case, the volume fraction of residual austenite will be 0%. On the other hand, when even a small amount of residual austenite is present, the volume fraction of residual austenite is from more than 0 to not more than 20%, and more preferably from more than 0 to 10%.

[0075]

[Method for measuring the volume fraction of martensite in the microstructure]

The volume fraction (vol.%) Of martensite is determined by subtracting the volume fraction (vol.%) Of residual austenite, which is determined as described below, from 100%.

[0076]

The volume fraction of residual austenite is determined using x-ray diffraction analysis. In particular, the sample is taken from an arbitrary position of the martensitic stainless steel product. The sample size is set equal to 15 mm × 15 mm × 2 mm of thickness. Using this sample, the corresponding X-ray intensities of the (200) plane and the (211) plane of ferrite (α-phase), as well as the (200) plane, (220) plane and (311) plane of residual austenite (γ-phase) are measured. Then, the integrated intensity of each plane is calculated. After calculating the integrated intensities, the volume fraction Vγ (%) is calculated using Formula (A) for each combination of the corresponding α-phase planes with the corresponding γ-phase planes (six combinations in total). Then, the average of these six combinations of volume fraction Vγ is determined as the volume fraction (%) of residual austenite.

Vγ = 100 / (1+ (Iα × Rγ) / (Iγ × Rα)) (A)

where, “Iα” is the integrated intensity of the α-phase (ferrite). "Rα" is the crystallographic theoretical calculated value of the α phase. “Iγ” is the integrated intensity of the γ phase (austenite). “Rγ” is the crystallographic theoretical calculated value of the γ phase.

[0077]

The value obtained by subtracting from 100% volume fraction of residual austenite obtained by the above method is taken as the volume fraction (vol.%) Of martensite in the microstructure. It should be noted that the volume fraction of inclusions and sediments, such as intermetallic compounds, which are described below, is excluded from the mentioned “100%” volume fraction of the microstructure.

[0078]

[Yield strength]

The yield strength of martensitic stainless steel of the present embodiment is 724-860 MPa. If the yield strength is less than 724 MPa, then the strength will not be sufficient for a steel product that can be used in a highly corrosive environment. On the other hand, if the yield strength is more than 860 MPa, as illustrated in FIG. 1, resistance to sulfide stress cracking in a steel product having the above chemical composition is reduced. Accordingly, the yield strength of martensitic stainless steel of the present embodiment is 724-860 MPa. A preferred upper yield strength is 850 MPa, and more preferably 840 MPa. A preferred lower yield strength is 730 MPa, and more preferably 740 MPa. In the present description, the term "yield strength" means the yield strength at plastic deformation of 0.2%.

[0079]

[Intermetallic compounds and Cr oxides in the microstructure]

In the martensitic stainless steel product of the present embodiment, the size of each intermetallic compound and the size of each Cr oxide in the microstructure is 5.0 μm ² or less, and the total area fraction of the intermetallic compounds and Cr oxides is 3.0% or less. In other words, in the present embodiment, intermetallic compounds having a size greater than 5.0 μm ² and Cr oxides having a size greater than 5.0 μm ² are not observed.

[0080]

As used herein, the term “intermetallic compound” refers to the release of an alloying element formed after tempering. Intermetallic compounds are, for example, a Laves phase, such as a sigma phase Fe ₂ Mo (σ phase) or a chi phase (χ phase). In addition, the term “intermetallic compound size” refers to the area (μm ² ) of the intermetallic compound that is observed in the measurement, which will be described later. In the case of the chemical composition of the above embodiment, intermetallic compounds other than the Laves phase, σ phase and χ phase are extremely small, so ignoring them is not a problem. As used herein, the term “Cr oxide” refers to chromium oxide (Cr ₂ O ₃ ). In addition, the term “Cr oxide size” refers to the area (μm ² ) of Cr oxide that is observed in a measurement that will be described later.

[0081]

Even when the steel product has a chemical composition that satisfies Expression (1) and Expression (2), it is a steel product in which the volume fraction of martensite is 80% or more, and has a yield strength of 724-860 MPa, if the intermetallic compounds are larger than 5.0 mm ² or Cr oxides with a size greater than 5.0 micron ² are present among the intermetallic compounds and the Cr oxides in the microstructure, or the total area ratio of the intermetallic compounds and the Cr oxides is more than 3.0%, cracking occurs sulfide n conjugate, which is attributed to intermetallic compounds and oxides of Cr, and resistance to sulfide stress cracking is reduced. As described above, in a steel product having a chemical composition satisfying Expression (1) and Expression (2), and in which the volume fraction of martensite is 80% or more, and the yield strength is 724-860 MPa, if the size of each intermetallic compound and the size of each oxide Cr ² is 5.0 microns or less, and the total area fraction of intermetallic compounds and oxides of Cr is 3.0% or less, because these intermetallic compounds and oxides of Cr do not affect the resistance to sulfide stress cracking is maintained n evoskhodnaya resistance to sulfide stress cracking.

[0082]

Preferably, the total area fraction of intermetallic compounds and Cr oxides in the microstructure is small. A preferred lower limit of the total area ratio of intermetallic compounds and Cr oxides is 2.5%, more preferably 2.0%, and most preferably 1.5%. More preferably, the total area ratio of the intermetallic compounds and Cr oxides is 0%.

[0083]

It should be noted that if the size of each intermetallic compound and the size of each Cr oxide is 5.0 μm ² or less, these intermetallic compounds and Cr oxides do not affect the resistance to sulfide stress cracking. In other words, even if the size of the intermetallic compounds and the size of each Cr oxide is 1.0 μm ² , 2.0 μm ² , or even 5.0 μm ² , these intermetallic compounds and Cr oxides do not affect the resistance to sulfide stress cracking. However, even if the size of each intermetallic compound and Cr oxide is 5.0 μm ² or less, if the total area fraction of the intermetallic compounds and Cr oxides is greater than 3.0%, these intermetallic compounds and Cr oxides noticeably affect sulfide cracking resistance energized.

[0084]

[Method for measuring the size of each intermetallic compound and the total area fraction of intermetallic compounds]

The size of each intermetallic compound, the size of each Cr oxide, and the total area fraction of the intermetallic compounds and Cr oxides are measured by observing the structure using a replica extraction method. In particular, the size and total area fraction are measured in the following way.

[0085]

A test sample having dimensions of 15 mm × 15 mm × 15 mm is removed from a central position in the direction of the thickness of the martensitic stainless steel product. In the case where the martensitic stainless steel product is a steel sheet, the term "central position in the thickness direction" refers to the central position relative to the thickness of the plate, and in the case where the martensitic stainless steel product is a steel pipe, the term "central position in thickness direction "refers to a central position relative to the wall thickness. One test sample is removed from the front end portion (upper part), and one test sample is removed from the rear end portion (lower part) in the longitudinal direction of the steel product. The term “front end portion” when the steel product is divided into equal 10 sections in the longitudinal direction, refers to a section at the front end, and the term “rear end portion” refers to a section at the rear end.

[0086]

Films of the extracted replica are created based on the method of extracting the replica from the surfaces of the extracted test samples. In particular, the surface of the test sample is electropolished. The surface of the test sample after electro-polishing is etched using a Wheeler reagent (ethanol solution containing 1-5 g of hydrochloric acid and 1-5 g of picric acid). As a result, the isolation and inclusion are exposed on the surface of the test sample. The surface of the test sample after etching is covered with a carbon film by deposition (hereinafter referred to as the film of the extracted replica). A test sample with a surface coated with a film of the extracted replica is immersed in a solution of bromine in methanol (bromomethanol) to dissolve the test sample, and the film of the extracted replica is cleaned of the test sample. The film of the extracted replica has a disk shape with a diameter of 3 mm. An arbitrary region of size 10 μm ² is observed in four places (four fields of view) at a magnification of 20,000 × on each film of the extracted replica using a TEM (transmission electron microscope). In other words, for each steel product, areas are observed in eight places.

[0087]

Element concentration analysis (point analysis of EDS) using energy dispersive X-ray spectrometry (hereinafter referred to as EDS) is performed on precipitates or inclusions confirmed by reflected electronic images of each observation area. Based on the element concentration obtained for each isolation or inclusion by EDS spot analysis, a particular intermetallic compound and a specific Cr oxide are determined. Then, the corresponding areas (μm ² ) of the identified intermetallic compounds and Cr oxides are determined. The corresponding areas (μm ² ) of the identified intermetallic compounds and Cr oxides are taken as the dimensions of the corresponding intermetallic compounds and the corresponding Cr oxides. The sum of the areas of the identified intermetallic compounds and Cr oxides is taken as the total area (μm ² ) of the intermetallic compounds and Cr oxides. The ratio of the total area of intermetallic compounds and Cr oxides to the total area of the entire observation area (80 μm ² ) is determined as the total area fraction (%) of intermetallic compounds and Cr oxides.

[0088]

It should be noted that the size of the intermetallic compounds and Cr oxides that can be observed by the aforementioned method is 0.05 μm ² or more. Accordingly, in the present embodiment, the lower limit of the size (area) of the intermetallic compounds and Cr oxides that are taken as measurement objects is 0.05 μm ² . It should be noted that when compared with the total area of intermetallic compounds and Cr oxides having a size (area) of 0.05-5.0 μm ² , the total area of intermetallic compounds and Cr oxides having a size (area) of 0.05 μm ² or less is small enough and can be neglected.

[0089]

In addition, in the case when at least one of the coarse intermetallic compounds, which obviously have a size of not less than 5.0 μm ² , or at least one of the Cr oxides, which obviously have a size of not less than 5.0 μm ² are observed when observed under an optical microscope or SEM (scanning electron microscope), a decision can be made taking into account the circumstances regarding coarse intermetallic compounds or coarse Cr oxides.

[0090]

FIG. 2 is an image obtained using TEM (transmission electron microscope) when observing the microstructure of the metal steel of Test No. 3 in the Examples that will be described later. In a martensitic stainless steel product in accordance with the present embodiment shown in the image in FIG. 2, there are no intermetallic compounds and Cr oxides having a size of 5.0 μm ² or more, and the total area ratio of the intermetallic compounds and Cr oxides is 3.0% or less.

[0091]

FIG. 3 is an SEM image obtained by observing the microstructure of the steel metal of Test No. 9, which is a Comparative Example among the examples that will be described later. In FIG. 3 black or gray areas among white areas (maternal phase) are intermetallic compounds, and the total area ratio of intermetallic compounds is 4.0%.

[0092]

[Mode of production]

Next, one example of a method for producing the above-described martensitic stainless steel product will be described. A method for producing a martensitic stainless steel product includes a process for preparing a starting material (preparation process), a process for hot processing a raw material for producing a steel product (a hot processing process), and a process for subjecting this steel product to hardening and tempering (a heat treatment process). Each of these processes is described in detail below.

[0093]

[Preparation process]

Molten steel is produced having the above chemical composition and satisfying expressions (1) and (2). The starting material is produced using this molten steel. In particular, casting (slab, bloom or billet) is carried out using a continuous casting process using molten steel. The ingot can also be produced using a process for manufacturing a metal ingot using molten steel. If necessary, a slab, bloom or ingot can be processed into a workpiece or hot forged. The source material (slab, bloom or billet) is produced using the above process.

[0094]

[Hot process]

The prepared starting material is heated. The preferred heating temperature 1 is from 1000 ° C to 1300 ° C. A preferred lower limit for the heating temperature is 1150 ° C.

[0095]

The heated starting material is hot worked to produce a martensitic stainless steel product. In the case where the martensitic stainless steel product is a steel sheet, the raw material is hot rolled using, for example, one or a plurality of rolling mills that include a pair of roll groups to thereby produce a steel sheet. In the case where the martensitic stainless steel product is a steel pipe for casing, tubing and drill pipe, for example, the raw material is pierced, elongated and calibrated using the Mannesman rolling process on a mandrel to produce a seamless steel pipe.

[0096]

[Heat treatment process]

The heat treatment process includes a hardening process and a tempering process. In the heat treatment process, the hardening process of the steel product that was produced using the hot processing process is first performed. Quenching is performed using a known method. The quenching temperature is equal to or higher than the Ac ₃ conversion point, and is, for example, from 900 to 1000 ° C. After the steel product is held at the hardening temperature, it quickly cools (hardens). Although the holding time is not particularly limited, it is, for example, from 10 to 60 minutes. The quenching method is, for example, water cooling.

[0097]

After hardening, the steel product undergoes a tempering process. During tempering, the strength of the steel product is adjusted so as to achieve a yield strength of 724 to 860 MPa. The tempering process also inhibits the release of intermetallic compounds. Therefore, the tempering temperature is set in the range from greater than 570 ° C to the conversion point A _C1 . A preferred lower temperature limit for tempering is 580 ° C, and more preferably 585 ° C. A preferred upper temperature limit for tempering is 630 ° C, and more preferably 620 ° C.

[0098]

During tempering, tempering temperature T (° C) and holding time t (min) at tempering temperature satisfy Expression (3).

where T is the tempering temperature (° C), and t is the exposure time (min). The designations of the elements in the Expression (3) mean the content (wt.%) Of the corresponding element in the steel product.

[0099]

In the case of the aforementioned chemical composition, which satisfies Expression (1) and Expression (2), the release of intermetallic compounds depends on the amount of heat that the steel product receives during tempering. In addition to this, in a chemical composition that satisfies Expression (1) and Expression (2), Cr and Mo are alloying elements that make up the intermetallic compounds. Therefore, Cr and Mo contribute to the formation of intermetallic compounds such as the Laves phase, σ phase and χ phase. On the other hand, in a chemical composition that satisfies Expression (1) and Expression (2), Cu and Ni inhibit the formation of the aforementioned intermetallic compounds, such as the Laves phase, σ phase and χ phase. Accordingly, the Cr content, Mo content, Cu content and Ni content influence the tempering conditions to suppress the formation of intermetallic compounds.

[0100]

Therefore, in the present embodiment, tempering is performed using tempering temperature T (° C) and holding time t (min) that satisfy Expression (3). In this case, in a steel product that has a chemical composition satisfying Expression (1) and Expression (2), and in which the volume fraction of martensite is 80% or more, the size of the intermetallic compounds can be made equal to 5.0 μm ² or less, and the total area fraction of intermetallic compounds and Cr oxides can be made equal to 3.0% or less.

[0101]

It should be noted that when F3 is (T + 273) × (20 + log (t / 60)) × (t / 60 × (0.5Cr + 2Mo) / (Cu + Ni)), if the value of F3 is less than 10000 or if the F3 value is greater than 40,000, even if the yield strength in the steel product after tempering is 724-860 MPa, there are a number of intermetallic compounds whose size exceeds 5.0 μm ² , or the total area fraction of the intermetallic compounds and Cr oxides is more than 3% Therefore, the F3 value is from 10,000 to 40,000.

[0102]

The preferred lower limit of F3 is 10300, more preferably 10500, and even more preferably 10700. The preferred upper limit of F3 is 38000, more preferably 37000, even more preferably 36000, and most preferably 35500.

[0103]

Tempering temperature T (° C) is the temperature (° C) of the heat treatment furnace in which the tempering is carried out. Holding time t (min) means holding time at tempering temperature T. Using the above-described manufacturing process, martensitic stainless steel material of this embodiment can be produced. As for Cr oxide, if a steel product is produced having a chemical composition satisfying the above formulas (1) and (2), the size of each Cr oxide can be made equal to 5.0 μm ² or less. In addition, by satisfying the above-described tempering conditions, the total area ratio of the intermetallic compound and Cr oxides can be made equal to 3.0% or less.

[0104]

The martensitic stainless steel product according to the present embodiment is not limited to the production method described above. If the manufactured steel product has a chemical composition that satisfies Expression (1) and Expression (2), in which the volume fraction of martensite is 80% or more and the yield strength is 724-860 MPa, the size of each intermetallic compound and the size of each Cr oxide is 5 , 0 μm ² or less, and the total area fraction of intermetallic compounds and Cr oxides is 3.0% or less, the method for producing the martensitic stainless steel product of the present embodiment is not particularly limited.

EXAMPLES

[0105]

Molten steels having the chemical compositions shown in Table 1 were produced.

[0106]

[Table 1]

TABLE 1 Test No. Chemical composition (wt.%, With the remainder of Fe and impurities) F1 F2 C Si Mn P S Al N Ni Cr Cu Mo V Ti Co W one 0.011 0.25 0.40 0.017 <0.001 0.0047 0.0090 6.43 13.31 1.97 2,53 0.05 0,094 0.222 0.05 12.7 8.5 2 0.011 0.25 0.40 0.015 <0.001 0.0039 0.0094 5.42 11.91 1.97 2.01 0.05 0,096 0.187 - 11.7 8.7 3 0.012 0.25 0.40 0.017 <0.001 0.0043 0,0100 5.94 11.90 1.97 2.49 0.05 0,094 0.212 - 11.9 7.8 4 0.012 0.25 0.40 0.017 <0.001 0.0040 0.0087 5.44 11.92 2.94 2.01 0.05 0,097 0.175 0.02 13.7 8.1 5 0.012 0.25 0.40 0.017 <0.001 0.0044 0.0092 6.38 11.84 2.92 2.48 0.05 0,094 0.220 0.04 13.1 7.8 6 0.011 0.24 0.41 0.015 <0.001 0.0043 0.0094 5.46 12.05 2.01 2.48 0.05 0,082 0.195 - 12.8 7.5 7 0.011 0.25 0.40 0.015 <0.001 0.0045 0.0098 5.67 12.01 2.10 2,51 0.05 0,086 0.208 - 12.7 7.8 eight 0.010 0.25 0.40 0.017 <0.001 0.0047 0.0087 5.49 12.03 2.00 2,50 0.05 0,083 0.212 - 12.8 8.3 9 0.011 0.24 0.40 0.017 <0.001 0.0047 0.0090 6.41 13.29 1.98 2,53 0.05 0,063 0.210 0.05 12.7 5.7 ten 0.012 0.24 0.40 0.017 <0.001 0.0047 0.0098 6.36 13.29 2,34 2,55 0.05 0.051 0.210 0.05 13.5 4.3 eleven 0.011 0.24 0.40 0.016 0,0007 0.0021 0.0071 6.52 13.35 1.95 2,52 0.04 - 0.225 - 12.5 0,0 12 0.012 0.25 0.40 0.017 <0.001 0.0090 0.0040 5.46 12.00 2.94 2.48 0.05 0,095 0.188 - 14.7 7.9 thirteen 0.011 0.25 0.40 0.017 <0.001 0.0090 0.0099 5.96 13.34 2.97 2.54 0.05 0,095 0.188 - 15.4 8.6 14 0.010 0.25 0.40 0.017 <0.001 0.0063 0.0089 5.97 13.33 2.47 2,55 0.05 0,096 0.198 - 14,4 9.6 15 0.011 0.25 0.40 0.017 <0.001 0.0042 0.0099 5.95 13.31 2.98 2,53 0.05 0,043 0.222 0.05 15.4 3.9 sixteen 0.011 0.26 0.40 0.017 <0.001 0.0041 0.0084 6.45 11.92 1.96 2.03 0.05 0,098 0.222 0.02 10,2 8.9 17 0.011 0.25 0.40 0.015 <0.001 0.0039 0.0099 5.96 11.94 1.97 2.02 0.05 0,096 0.206 0.02 11.0 8.7 18 0.011 0.25 0.41 0.017 <0.001 0.0045 0.0098 6.44 11.94 1.97 2,53 0.05 0,098 0.195 0.01 11.3 8.9 19 0.011 0.24 0.41 0.017 <0.001 0.0045 0.0098 6.48 11.96 1.98 2,52 0.05 0.051 0.195 - 11.2 4.6 twenty 0.011 0.24 0.41 0.014 0,0005 0.0040 0.0074 5.44 12.01 0.21 1.94 0.05 0,100 0.188 0.01 8.2 9.1 21 0.010 0.22 0.42 0.015 0,0005 0.0031 0.0063 5.41 12.08 0.17 1.93 0.06 0,096 0,200 - 8.2 9.6 22 0.007 0.22 0.44 0.018 0,0005 0.0032 0.0092 5.89 12.05 0.06 1.91 0.06 0.104 0,203 - 7.2 14.9 23 0.008 0.27 0.46 0.015 0,0006 0.0034 0.0075 6.59 12.07 0.05 2.45 0.05 0.104 0.244 - 7.2 13.0 24 0.007 0.23 0.40 0.013 0,0006 0.0026 0.0090 6.90 11.80 0.02 2.99 0.04 0,092 0.238 - 7.5 13.1 25 0.011 0.24 0.40 0.015 0,0007 0.0031 0.0074 6.48 13.37 0.20 2.00 0.04 - 0.223 - 8.1 0,0 26 0.011 0.24 0.40 0.015 0,0008 0.0029 0.0083 6.51 13.35 0.99 2.00 0.04 - 0.224 - 9.6 0,0 27 0.011 0.24 0.40 0.015 0,0008 0.0026 0.0086 6.50 13.32 0.22 2,51 0.04 - 0.224 - 9.0 0,0 28 0.012 0.24 0.40 0.015 0,0008 0.0020 0.0082 6.53 13.34 0.99 2,52 0.04 - 0.251 - 10.6 0,0 29th 0.008 0.19 0.40 0.011 0,0005 0.0025 0.0069 7.06 12.02 0.04 3.00 0,03 0,093 0.243 - 7.5 11.6 thirty 0.011 0.24 0.40 0.016 0,0007 0.0021 0.0071 6.52 13.35 1.95 2,52 0.04 - 0.226 - 12.5 0,0 31 0.012 0.33 0.42 0.017 0,0005 0.0035 0.0071 6.21 11.92 2.10 2.07 0.04 0,098 0.227 - 10.9 8.2 32 0.016 0.25 0.42 0.013 0,0005 0.0031 0.0087 6.48 12.80 1.98 2,52 0.05 0.103 0.251 - 12.1 6.4 33 0.013 0.20 0.41 0.017 <0.001 0.0031 0.0072 5.46 12.69 2.13 2,51 0.05 0.113 0.247 - 13.8 8.7

[0107]

The corresponding molten steels described above were melted in a 50 kg vacuum furnace, and ingots were produced using a metal ingot manufacturing process. Each ingot was heated at 1250 ° C for 3 hours. The heated ingot was hot forged to produce a block. After hot forging, the block was held at 1230 ° C for 15 minutes, and then hot rolled in order to produce a plate having a thickness of 13 mm.

[0108]

The corresponding plate was hardened. The quenching temperature (° C) and holding time (min) at the quenching temperature during quenching are shown in Table 2. For the sheet material of each test, the rapid cooling method (quenching method) after the exposure time expired was water cooling. After hardening, each plate was tempered. The tempering temperature (° C) and the holding time (min) at the tempering temperature, as well as the F3 value are shown in Table 2.

[0109]

[Table 2]

TABLE 2 Test No. Content (wt.%) F1 F2 Hardening process Vacation process F3 Microstructure YS (MPa) Resistance to sulfide stress cracking Cr Mo Cu Ni Quenching temperature (° C) The duration of exposure (min) Tempering temperature (° C) The duration of exposure (min) one 13.31 2,53 1.97 6.43 12.7 8.5 950 15 620 thirty 12267 TM 747 Ok 2 11.91 2.01 1.97 5.42 11.7 8.7 950 15 610 thirty 11739 TM 860 Ok 3 11.90 2.49 1.97 5.94 11.9 7.8 950 15 620 thirty 12154 TM 777 Ok 4 11.92 2.01 2.94 5.44 13.7 8.1 950 15 620 thirty 10475 TM 789 Ok 5 11.84 2.48 2.92 6.38 13.1 7.8 950 15 620 thirty 10290 TM 730 Ok 6 12.05 2.48 2.01 5.46 12.8 7.5 950 15 580 80 33659 TM 855 Ok 7 12.01 2,51 2.10 5.67 12.7 7.8 950 15 580 one hundred 40792 TM + I 840 NG eight 12.03 2,50 2.00 5.49 12.8 8.3 950 15 560 80 32872 TM + I 853 NG 9 13.29 2,53 1.98 6.41 12.7 5.7 950 15 600 thirty 11996 TM 981 NG ten 13.29 2,55 2,34 6.36 13.5 4.3 950 15 600 thirty 11608 TM 972 NG eleven 13.35 2,52 1.95 6.52 12.5 0,0 950 15 600 thirty 11893 TM 966 NG 12 12.00 2.48 2.94 5.46 14.7 7.9 950 15 620 thirty 11476 TM + I 837 NG thirteen 13.34 2.54 2.97 5.96 15.4 8.6 950 15 620 thirty 11573 TM + I 860 NG 14 13.33 2,55 2.47 5.97 14,4 9.6 950 15 620 thirty 12261 TM + I 857 NG 15 13.31 2,53 2.98 5.95 15.4 3.9 950 15 590 thirty 11151 TM + I 980 NG sixteen 11.92 2.03 1.96 6.45 10,2 8.9 950 15 610 thirty 10362 TM 839 NG 17 11.94 2.02 1.97 5.96 11.0 8.7 950 15 610 thirty 10978 TM 852 NG 18 11.94 2,53 1.97 6.44 11.3 8.9 950 15 620 thirty 11536 TM 739 NG 19 11.96 2,52 1.98 6.48 11.2 4.6 950 15 585 thirty 11008 TM 980 NG twenty 12.01 1.94 0.21 5.44 8.2 9.1 910 18 540 thirty 14010 TM 829 NG 21 12.08 1.93 0.17 5.41 8.2 9.6 910 15 565 thirty 14644 TM 766 NG 22 12.05 1.91 0.06 5.89 7.2 14.9 910 15 570 thirty 13739 TM 752 NG 23 12.07 2.45 0.05 6.59 7.2 13.0 910 15 615 thirty 14404 TM 779 NG 24 11.80 2.99 0.02 6.90 7.5 13.1 950 thirty 560 40 18900 TM + I 958 NG 25 13.37 2.00 0.20 6.48 8.1 0,0 950 15 620 thirty 14069 TM 766 NG 26 13.35 2.00 0.99 6.51 9.6 0,0 950 15 625 thirty 12589 TM 724 NG 27 13.32 2,51 0.22 6.50 9.0 0,0 950 15 630 thirty 15459 TM 766 NG 28 13.34 2,52 0.99 6.53 10.6 0,0 950 15 630 thirty 13850 TM 772 NG 29th 12.02 3.00 0.04 7.06 7.5 11.6 910 15 585 55 26558 TM 856 NG thirty 13.35 2,52 1.95 6.52 12.5 0,0 950 15 630 thirty 12302 TM 965 NG 31 11.92 2.07 2.10 6.21 10.9 8.2 910 15 610 thirty 29838 TM 814 NG 32 11.96 2,52 1.98 6.48 11.2 6.4 910 15 610 40 34471 TM 888 NG 33 12.69 2,51 2.13 5.46 13.8 8.7 910 15 610 60 47469 TM + I 807 NG

[0110]

Regulation was carried out by performing quenching and tempering so that the yield strength YS was from 724 to 860 MPa. Martensitic stainless steel products were produced using the production method described above.

[0111]

[Assessment Test]

[Test for measuring the volume fraction of martensite]

A test sample having dimensions of 15 mm × 15 mm × 2 mm in thickness was removed from each sheet material. The volume fraction (%) of residual austenite was determined using the aforementioned X-ray diffraction method, and the value obtained by subtracting the volume fraction of residual austenite from 100% was determined as the volume fraction (%) of martensite.

[0112]

[Regarding the size and total area fraction of intermetallic compounds and Cr oxides in the microstructure]

A test sample having dimensions of 15 mm × 15 mm × 15 mm was removed from a central position in the thickness direction of each plate. One of the aforementioned test samples was removed from a portion of the front end (upper part), and one of the test samples was removed from a part of the rear end (lower part) in the longitudinal direction of the plate. The term “front end portion” when the steel product is divided into equal 10 sections in the longitudinal direction, refers to a section at the front end, and the term “rear end portion” refers to a section at the rear end.

[0113]

Films of the extracted replica were created based on the method of extracting the replica from the surfaces of two extracted test samples. Each film of the extracted replica had a disk shape with a diameter of 3 mm. An arbitrary region of 10 μm ² was observed in four places (four fields of view) at 20,000 × magnification on each film of the extracted replica using a TEM (transmission electron microscope). In other words, for each steel product, areas were observed in eight places.

[0114]

Intermetallic compounds were identified based on the contrast of the images of the corresponding observation regions obtained by electron backscattering. The corresponding areas (μm ² ) of the identified intermetallic compounds and the corresponding Cr oxides were taken as the sizes of the corresponding intermetallic compounds and the corresponding Cr oxides. In addition, the sum of the areas of the identified intermetallic compounds and the identified Cr oxides was taken as the total area (μm ² ) of the intermetallic compounds and Cr oxides. The ratio of the total area of intermetallic compounds and Cr oxides to the total area (80 μm ² ) of the entire observation area was determined as the total area ratio (%) of intermetallic compounds and Cr oxides.

[0115]

In the column "Microstructure" in Table 2, "TM" means that the volume fraction of martensite in the microstructure was 80% or more, the size of each intermetallic compound in the microstructure was 5.0 μm ² or less, the size of each Cr oxide in the microstructure was 5 , 0 μm ² or less, and the total area fraction of intermetallic compounds and Cr oxides in the microstructure was 3.0% or less. In addition, “TM + I” means that although the volume fraction of martensite in the microstructure was 80% or more, intermetallic compounds or Cr oxides having a size greater than 5.0 μm ² and / or the total area fraction of intermetallic compounds were present in the microstructure and Cr oxides in the microstructure was more than 3.0%.

[0116]

[Tensile test]

A tensile test sample was taken from each test material. A tensile test piece in the form of a round bar having a diameter of the parallel portion of 6 mm and a length of the parallel portion of 40 mm was used as a tensile test specimen. The direction of plate rolling was used as the longitudinal direction of the parallel part of the test sample. The tensile test was carried out at normal temperature (25 ° C) using this test sample, and the yield strength YS (MPa) was determined. The maximum elongation stress of 0.2% was used as the yield strength YS. The obtained YS yield strengths are shown in Table 2.

[0117]

[SSC Resistance Test]

A test sample in the form of a round bar having a diameter of the parallel part of 6.3 mm and a length of the parallel part of 25.4 mm was taken from each test material. The constant load test was performed using test samples in the form of a round bar based on the NACE TM0177 Method A standard in a test solution including hydrogen sulfide. In particular, the test solution was an aqueous solution containing 5 wt.% NaCl and 0.4 g / l CH ₃ COONa, to which CH ₃ COOH was added while passing through it gaseous CO ₂ at a pressure of 1 atm, and whose pH value brought up to 3.5. The actual yield strength of 90% was set as the additional stress applied to the test specimen in the form of a round bar during the test. The test sample, to which the aforementioned additional voltage was applied, was immersed for 720 hours in the aforementioned aqueous solution saturated with a gaseous mixture of H ₂ S with a pressure of 0.1 atm and CO ₂ with a pressure of 0.9 atm. The test temperature was 24 ± 3 ° C.

[0118]

After the test, the surface of the parallel part was visually observed using a magnifier with a magnification of 10x. In Table 2, “NG” in the column “Resistance to sulfide stress cracking” indicates that cracking was observed. In Table 2, “OK” in the column “Resistance to sulfide stress cracking” means that no cracking was observed.

[0119]

[Test results]

As shown in Table 2, the chemical compositions of Tests Nos. 1-6 were suitable and satisfied Expression (1) and Expression (2). Therefore, the volume fraction of martensite in the microstructure was 80% or more, the size of each intermetallic compound and each Cr oxide in the microstructure was 5.0 μm ² or less, and the total area fraction of intermetallic compounds and Cr oxides in the microstructure was 3.0% or less . As a result, these steel products showed excellent resistance to sulfide stress cracking even in a medium containing H ₂ S at a pressure of 0.1 atm.

[0120]

In contrast, in Test No. 7, although the chemical composition was suitable, the F3 value was greater than 40,000. Therefore, intermetallic compounds with a size greater than 5.0 μm ² were observed, and the total area fraction of the intermetallic compounds and Cr oxides was greater than 3.0% As a result, the resistance to sulfide stress cracking was low.

[0121]

In Test No. 8, although the chemical composition was suitable, the tempering temperature was too low. Therefore, the total area fraction of intermetallic compounds and Cr oxides was greater than 3.0%. As a result, the resistance to sulfide stress cracking was low.

[0122]

In Tests No. 9 and No. 10, expression (2) was not satisfied. Therefore, the yield strength could not be adjusted to a value of 860 MPa or less by tempering, and the yield strength was more than 860 MPa. As a result, the resistance to sulfide stress cracking was low.

[0123]

In Test No. 11, the Ni content was too high, and Ti was not contained in the chemical composition. Therefore, the yield strength could not be adjusted to a value of 860 MPa or less by tempering, and the yield strength was more than 860 MPa. As a result, the resistance to sulfide stress cracking was low.

[0124]

In Tests No. 12-14, the value of F1 was greater than the upper limit of expression (1). Therefore, the total area fraction of intermetallic compounds and Cr oxides was greater than 3.0%. As a result, the resistance to sulfide stress cracking was low.

[0125]

In Test No. 15, the Ti content was low and the F2 value did not satisfy Expression (2). Therefore, the yield strength could not be adjusted to a value of 860 MPa or less by tempering, and the yield strength was more than 860 MPa. In addition, in Test No. 15, the value of F1 was also greater than the upper limit of Formula (1). Therefore, the total area fraction of intermetallic compounds and Cr oxides was greater than 3.0%. As a result, the resistance to sulfide stress cracking was low.

[0126]

In Tests No. 16-18, the value of F1 was less than the lower limit of expression (1). Therefore, the resistance to sulfide stress cracking was low. It is believed that since the ratio of Ni to Cr, Mo and Cu was too high, the diffusion coefficient of hydrogen in steel became too low, and as a result, the resistance to sulfide stress cracking decreased.

[0127]

In Test No. 19, the value of F1 was less than the lower limit of expression (1), and in addition, the value of F2 did not satisfy Expression (2). Therefore, the yield strength could not be adjusted to a value of 860 MPa or less by tempering, and the yield strength was more than 860 MPa. In addition, the resistance to sulfide stress cracking was low.

[0128]

In Tests No. 20-22, the Cu content was low and the tempering temperature was too low. Therefore, the resistance to sulfide stress cracking was low.

[0129]

In Test No. 23, the Ni content was high and the Cu content was low. Therefore, the resistance to sulfide stress cracking was low.

[0130]

In Test No. 24, the Ni content was high, the Cu content was low, and the tempering temperature was too low. Therefore, intermetallic compounds with a size greater than 5.0 μm ² were observed, and the total area ratio of the intermetallic compounds and Cr oxides was greater than 3.0%. As a result, the yield strength was 860 MPa or more, and the resistance to sulfide stress cracking was low.

[0131]

In Tests No. 25-28, the Cu content was low, and the chemical compositions did not contain Ti. Therefore, the resistance to sulfide stress cracking was low.

[0132]

In Test No. 29, the Ni content was high and the Cu content was low. Therefore, the resistance to sulfide stress cracking was low.

[0133]

In Test No. 30, the Ni content was too high and the chemical composition did not contain Ti. Therefore, the yield strength was too high, and the resistance to sulfide stress cracking was low.

[0134]

In Test No. 31, the value of F1 was less than the lower limit of expression (1). Therefore, the resistance to sulfide stress cracking was low. It is believed that since the ratio of Ni to Cr, Mo and Cu was too high, the diffusion coefficient of hydrogen in steel became too low, and as a result, the resistance to sulfide stress cracking decreased.

[0135]

In Test No. 32, expression (2) was not satisfied. Therefore, the yield strength could not be adjusted to a value of 860 MPa or less by tempering, and the yield strength was more than 860 MPa. As a result, the resistance to sulfide stress cracking was low.

[0136]

In Test No. 33, although the chemical composition was suitable, the F3 value was greater than 40,000. As a result, intermetallic compounds with a size greater than 5.0 μm ² were observed, and the total area ratio of the intermetallic compounds was greater than 3.0%. Therefore, the resistance to sulfide stress cracking was low.

[0137]

One embodiment of the present invention has been described above. However, the above embodiment is merely an example for implementing the present invention. Accordingly, the present invention is not limited to the above embodiment, and the above embodiment can be suitably modified within a range that does not deviate from the gist of the present invention.

Claims (28)

1. The product of martensitic stainless steel having a chemical composition consisting of, wt.%: 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: from 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: from 0.01 to 1.00; Ti: from 0.050 to 0.300; Co: 0.300 or less and W: from 0 to 1.50; the rest is iron and impurities, and satisfying expression (1) and expression (2); yield strength 724–860 MPa; and a microstructure containing martensite with a volume fraction of 80% or more; in which the size of each intermetallic compound and each Cr oxide in the microstructure is 5.0 μm ² or less, and the total area fraction of the intermetallic compounds and Cr oxides is 3.0% or less, 11.5 ≤ Cr + 2Mo + 2Cu - 1.5Ni ≤ 14.3 (1), Ti / C ≥ 7.5 (2), where each symbol of the element in expression (1) and expression (2) corresponds to the content (wt.%) of the corresponding element. 2. The product of martensitic stainless steel according to claim 1, in which the chemical composition contains, wt.%: W: 0.10 to 1.50. 3. A martensitic stainless steel product according to claim 1 or 2, which is a seamless steel pipe for casing, tubing and drill pipe.

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