JP7574975B1 - Martensitic stainless steel seamless pipe - Google Patents

Abstract

The present invention provides a martensitic stainless steel seamless pipe having a yield stress of 758 MPa or more, an absorbed energy of 100 J or more in a Charpy impact test at -80°C, and excellent corrosion resistance. A martensitic stainless steel seamless pipe containing, by mass%, C: 0.005 to 0.100%, Si: 0.05 to 1.00%, Mn: 0.05 to 1.00%, P: 0.05% or less, S: 0.005% or less, Ni: 6.0 to 9.0%, Cr: 15.0 to 17.5%, Al: 0.001 to 0.10%, Nb: 0.001 to 0.20%, N: 0.1% or less, O: 0.01% or less, Cu: 3.5% or less, Mo: less than 1.0%, and W: 1.0% or less, with the balance being Fe and unavoidable impurities, and having a component composition that satisfies predetermined formulas (1) and (2).

Description

The present invention relates to a martensitic stainless steel seamless pipe.

In conventional oil and gas fields, martensitic stainless steel pipes such as 13% Cr steel (steel containing about 13% by mass of Cr) and 17% Cr steel (steel containing about 17% by mass of Cr) are widely used as oil well tubular goods for mining. In recent years, carbon dioxide capture and storage (CCS) technology has been promoted worldwide in order to achieve carbon neutrality. In CCS, martensitic stainless steel pipes are also used for injection pipes (hereinafter also referred to as CCS injection pipes) used to inject captured carbon dioxide into aquifers or depleted oil fields.

Oil well tubular goods and CCS injection pipes are steel pipes that are connected to depths of several thousand meters underground, so they must be strong enough to withstand their own weight. In addition, both oil well tubular goods and CCS injection pipes must be corrosion resistant in environments containing high-temperature, high-pressure carbon dioxide and chloride ions. In particular, with CCS, carbon dioxide containing highly corrosive impurities such as oxygen, sulfur oxides, and nitrogen oxides may be injected, so the material for the CCS injection pipes must be highly corrosion resistant.

Furthermore, when oil well tubular goods and CCS injection pipes are used in cold regions, they need to be able to withstand low temperatures without becoming brittle, and so low-temperature toughness is required. In particular, with CCS injection pipes, the pressure of the fluid passing through the steel pipe is high, so there is concern that if a sudden drop in pressure occurs due to a leak or other reason, the temperature will drop due to the Joule-Thomson effect. For this reason, there are cases where toughness at even lower temperatures is required.

Patent Document 1 proposes a seamless steel pipe with high strength and high toughness, having a composition with a Cr content of 14 mass% or less. Patent Documents 2 to 4 propose seamless steel pipes with high strength, high toughness and high corrosion resistance, having a composition with a Cr content of about 17 mass% and a Ni content of 6.0 mass% or less, and a structure containing a certain amount of ferrite phase.

International Publication No. 2009/004741 JP 2017-14543 A International Publication No. 2019/035329 International Publication No. 2020/202957

In the technology described in Patent Document 1, a corrosion test is conducted for one week in a 20 mass % NaCl aqueous solution at a liquid temperature of 80° C. and in a _CO2 atmosphere at 30 atmospheres. However, in the more severe corrosive environments of oil country tubular goods and CCS injection pipes, it may not be said that the corrosion resistance is sufficient.

In Patent Documents 2 and 3, excellent low-temperature toughness is defined as an absorbed energy of 80 J or more at -60°C and an absorbed energy of 40 J or more at -40°C in a Charpy impact test. In Patent Document 4, excellent low-temperature toughness is defined as an absorbed energy of 300 J or more at -10°C in a Charpy impact test and a ductile-brittle transition temperature of -40°C or less. However, in CCS press-in pipe applications, the usage environment may be -60°C or less, so toughness at even lower temperatures may be required.

As described above, martensitic stainless steels containing about 13% by mass of Cr can achieve high strength and toughness, but may not have sufficient corrosion resistance. On the other hand, martensitic stainless steels containing about 17% by mass of Cr, which have relatively high corrosion resistance, have the problem of reduced low-temperature toughness due to the inclusion of ferrite phase.

The present invention aims to provide a martensitic stainless steel seamless pipe having a yield stress of 758 MPa or more, an absorbed energy of 100 J or more in a Charpy impact test at -80°C, and excellent corrosion resistance.

In the present invention, "having excellent corrosion resistance" means that the corrosion rate when a test piece is immersed in a 20 mass % NaCl aqueous solution at a liquid temperature of 200°C for 336 hours in a _CO2 gas atmosphere at 30 atmospheres is 0.127 mm/y or less.

In order to achieve the above-mentioned object, the present inventors have intensively studied the components of martensitic stainless steel that achieve both high strength, high toughness, and high corrosion resistance. As a result, it has been revealed that by increasing the Ni content to 6-9 mass% and reducing the ferrite phase compared to conventional 17% Cr steel, it is possible to obtain toughness at lower temperatures while maintaining high corrosion resistance. However, if the content of alloys such as Cr and Ni increases, the Ms point, which is the temperature at which martensitic transformation begins, drops to below room temperature, and there is a risk that sufficient strength cannot be obtained even if quenching is performed. Therefore, it has been found that it is possible to obtain a martensitic stainless steel seamless pipe having the desired strength, excellent low-temperature toughness, and excellent corrosion resistance by defining C, Cu, Ni, Cr, Mo, Nb, N, and W in an appropriate relational formula and further adjusting the composition of the components so that the value calculated by the relational formula satisfies a predetermined range.

The present invention was completed based on these findings and further investigations.
[1] In mass%,
C: 0.005-0.100%,
Si: 0.05-1.00%,
Mn: 0.05-1.00%,
P: 0.05% or less,
S: 0.005% or less,
Ni: 6.0 to 9.0%,
Cr: 15.0-17.5%,
Al: 0.001-0.10%,
Nb: 0.001-0.20%,
N: 0.1% or less,
O: 0.01% or less,
Cu: 3.5% or less,
A martensitic stainless steel seamless pipe containing less than 1.0% Mo, 1.0% or less W, with the balance being Fe and unavoidable impurities, and having a chemical composition that satisfies the following formulas (1) and (2):
1001-2172C-17.35Cu-44.95Ni-34.45Cr-22.15Mo+60.96Nb-978.3N-18.57W> 50.00...(1)
Cr+0.65Ni+0.6Mo+0.3W+0.55Cu-20C> 20.00...(2)
Here, each element symbol in formula (1) and formula (2) indicates the content (mass%) of the corresponding element, and when the element is not contained, it is set to 0.
[2] The composition further includes, in mass%,
Co: 1% or less,
V: 0.1% or less,
Ti: 0.1% or less,
Zr: 0.2% or less,
Ta: 0.2% or less,
Hf: 0.2% or less,
Ca: 0.01% or less,
REM: 0.01% or less,
Mg: 0.01% or less,
B: 0.01% or less,
The martensitic stainless steel seamless pipe according to [1], containing one or more selected from Sn: 0.2% or less and Sb: 0.2% or less.
[3] A martensitic stainless steel seamless pipe according to [1] or [2], having a yield strength of 758 MPa or more and an absorbed energy in a Charpy impact test at -80 ° C. of 100 J or more.

According to the present invention, it is possible to provide a martensitic stainless steel seamless pipe having a yield stress of 758 MPa or more, an absorbed energy of 100 J or more in a Charpy impact test at -80°C, and excellent corrosion resistance.

According to the present invention, it is now possible to use martensitic stainless steel seamless pipes for CCS press-in pipe applications, which require superior low-temperature toughness and high corrosion resistance compared to conventional oil well pipe applications. Furthermore, according to the present invention, it is possible to provide martensitic stainless steel seamless pipes that are less expensive and have superior toughness even for applications where duplex stainless steels such as 25% Cr steel are used.

The martensitic stainless steel seamless pipe of the present invention (hereinafter, simply referred to as the seamless steel pipe of the present invention) contains, in mass%, C: 0.005 to 0.100%, Si: 0.05 to 1.00%, Mn: 0.05 to 1.00%, P: 0.05% or less, S: 0.005% or less, Ni: 6.0 to 9.0%, Cr: 15.0 to 17.5%, Al: 0.001 to 0.10%, Nb: 0.001 to 0.20%, N: 0.1% or less, O: 0.01% or less, Cu: 3.5% or less, Mo: less than 1.0%, and W: 1.0% or less, with the balance being Fe and unavoidable impurities, and has a component composition that satisfies the following formulas (1) and (2).
1001-2172C-17.35Cu-44.95Ni-34.45Cr-22.15Mo+60.96Nb-978.3N-18.57W> 50.00...(1)
Cr+0.65Ni+0.6Mo+0.3W+0.55Cu-20C> 20.00...(2)
Here, each element symbol in the above formula (1) and formula (2) indicates the content (mass%) of the corresponding element, and when the element is not contained, it is set to 0.

The reasons for limiting the composition of the seamless steel pipe of the present invention are explained below. In the following, unless otherwise specified, mass% will simply be written as %.

C: 0.005-0.100%
C improves the hardenability and strength of steel. To obtain this effect, the C content is set to 0.005% or more. On the other hand, if the C content is too high, the Ms point decreases and the room temperature Therefore, the C content is set to 0.100% or less. The C content is preferably set to 0.007% or more. The C content is set to The content is preferably 0.050% or less, more preferably 0.030% or less, and further preferably 0.020% or less.

Si: 0.05-1.00%
Silicon acts as a deoxidizer. To obtain this effect, the silicon content is set to 0.05% or more. On the other hand, if the silicon content exceeds 1.00%, the corrosion resistance is deteriorated due to the precipitation of δ-ferrite. and deteriorates hot workability. Therefore, the Si content is set to 1.00% or less. The Si content is preferably set to 0.50% or less. On the other hand, from the viewpoint of ensuring stable strength, the Si content is The amount is preferably 0.10% or more.

Mn: 0.05-1.00%
Mn is an element that improves hot workability and strength. To obtain this effect, the Mn content is set to 0.05% or more. On the other hand, excessive Mn content causes MnS to precipitate, which reduces the resistance. The Mn content is preferably 0.50% or less. It is more than 10%.

P: 0.05% or less P is an impurity that is inevitably contained. P is an element that reduces corrosion resistance, and it is desirable to reduce it as much as possible in the present invention. However, an extreme reduction in P increases manufacturing costs. Therefore, the P content is limited to 0.05% or less as a range that does not cause an extreme decrease in characteristics and is industrially feasible at low cost. The P content is preferably 0.03% or less. In addition, although there is no particular limit to the lower limit of the P content, since excessive de-P leads to an increase in manufacturing costs, from the viewpoint of manufacturing costs, the P content is preferably 0.005% or more.

S: 0.005% or less S is an impurity that is inevitably contained. Since S is an element that significantly reduces hot workability, it is desirable to reduce it as much as possible. By reducing the S content to 0.005% or less, pipe manufacturing by the Mannesmann method becomes possible, so the S content in the present invention is limited to 0.005% or less. The S content is preferably 0.002% or less. In addition, although there is no particular limit to the lower limit of the S content, since excessive de-S leads to an increase in manufacturing costs, from the viewpoint of manufacturing costs, the S content is preferably 0.0005% or more.

Ni: 6.0-9.0%
Ni is an element that strengthens the protective film to improve corrosion resistance, and further increases the strength of steel by dissolving in solid solution. In addition, Ni is an element that stabilizes the austenite phase at the quenching temperature, so The Ni content is set to 6.0% or more. If the Ni content is less than 6.0%, the amount of ferrite increases and the toughness decreases. On the other hand, if the Ni content exceeds 9.0%, the Ms point decreases. As a result, the strength decreases. Therefore, the Ni content is limited to 6.0 to 9.0%. The Ni content is preferably 6.5% or more. It is 8.0% or less, and more preferably 7.5% or less.

Cr: 15.0-17.5%
Cr is an element that forms a protective film to improve corrosion resistance, and a target corrosion resistance can be obtained by including 15.0% or more of Cr. On the other hand, if the Cr content is excessive, exceeding 17.5%, The decrease in Ms point and the increase in ferrite content make it impossible to achieve both strength and low-temperature toughness. Therefore, the Cr content is limited to 15.0 to 17.5%. The Cr content is preferably 15.5%. The Cr content is preferably 17.0% or less, and more preferably 16.0% or more.

Al: 0.001-0.10%
Al acts as a deoxidizer. To obtain this effect, the Al content is set to 0.001% or more. However, if the Al content exceeds 0.10%, the toughness is deteriorated due to the precipitation of excessive oxides. Therefore, the Al content in the present invention is limited to 0.10% or less. The Al content is preferably 0.01% or more. The Al content is preferably 0. It is less than 0.4%.

Nb: 0.001-0.20%
Nb forms carbides, thereby reducing solute carbon and improving the Ms point. To obtain this effect, the Nb content is set to 0.001% or more. On the other hand, if the Nb content exceeds 0.20%, The inclusion of Nb may cause the precipitation of coarse carbides and reduce toughness. Therefore, the Nb content is set to 0.20% or less. The Nb content is preferably 0.01% or more. The Nb content is preferably 0.10% or less.

N: 0.1% or less N is an impurity that is inevitably contained. N has the effect of dissolving in steel and increasing strength. However, if the N content exceeds 0.1%, various nitride-based inclusions are generated in large amounts, and corrosion resistance decreases. Therefore, the N content in the present invention is limited to 0.1% or less. The N content is preferably 0.005% or more. The N content is preferably 0.05% or less, and more preferably 0.03% or less.

O: 0.01% or less O is an impurity that is inevitably contained. Since O exists as an oxide in steel, it has a negative effect on various properties. Therefore, in the present invention, it is desirable to reduce it as much as possible. In particular, if the content of O exceeds 0.01%, the hot workability, corrosion resistance, and toughness decrease. Therefore, the O content is set to 0.01% or less. The O content is preferably 0.005% or less. Although the lower limit of the O content is not particularly limited, since excessive de-O leads to an increase in manufacturing costs, from the viewpoint of manufacturing costs, the O content is preferably set to 0.001% or more.

Cu: 3.5% or less Cu strengthens the protective coating and improves corrosion resistance. However, a Cu content exceeding 3.5% reduces hot workability due to precipitation of metallic Cu. Therefore, the Cu content is set to 3.5% or less. The Cu content is preferably 3.0% or less. The seamless steel pipe of the present invention does not need to contain Cu. The Cu content is preferably 0.5% or more.

Mo: less than 1.0% Mo is an element that improves corrosion resistance. On the other hand, excessive Mo content of 1.0% or more leads to a decrease in the Ms point, which reduces the hardenability and makes ferrite more likely to form, thereby reducing toughness. Therefore, in the present invention, the Mo content is limited to less than 1.0%. The Mo content is preferably 0.8% or less. The seamless steel pipe of the present invention does not need to contain Mo. The Mo content is preferably 0.1% or more.

W: 1.0% or less W is an element that improves corrosion resistance. On the other hand, an excessive W content of more than 1.0% leads to a decrease in the Ms point, so in the present invention, the W content is limited to 1.0% or less. The W content is preferably 0.8% or less. The seamless steel pipe of the present invention does not need to contain W. The W content is preferably 0.1% or more.

The chemical composition of the seamless steel pipe of the present invention further contains each element so as to satisfy the following formula (1).
1001-2172C-17.35Cu-44.95Ni-34.45Cr-22.15Mo+60.96Nb-978.3N-18.57W> 50.00...(1)
Here, each element symbol in formula (1) indicates the content (mass%) of the corresponding element, and when the element is not contained, the content is set to 0.
Formula (1) is a formula that correlates with the Ms point, and by satisfying the value calculated on the left side of formula (1) exceeding 50.00, martensitic transformation occurs even when cooled to room temperature, and sufficient strength can be ensured. Therefore, the value calculated on the left side of formula (1) is set to be greater than 50.00. The value calculated on the left side of formula (1) is preferably 55.00 or more. In addition, the value calculated on the left side of formula (1) is preferably 300.00 or less.

The chemical composition of the seamless steel pipe of the present invention further contains each element so as to satisfy the following formula (2).
Cr+0.65Ni+0.6Mo+0.3W+0.55Cu-20C> 20.00...(2)
Here, each element symbol in formula (2) indicates the content (mass%) of the corresponding element, and when the element is not contained, the content is set to 0.
Formula (2) is a formula related to corrosion resistance. By satisfying the condition that the value calculated on the left side of formula (2) is more than 20.00, the corrosion rate when the test piece is immersed in a 20 mass% NaCl aqueous solution at a liquid temperature of 200 ° C. for 336 hours under a _CO2 gas atmosphere at 30 atmospheres can be obtained as a value of 0.127 mm / y or less. Therefore, the value calculated on the left side of formula (2) is set to be more than 20.00. The value calculated on the left side of formula (2) is preferably 20.50 or more, more preferably 21.00 or more. In addition, since excessive alloy addition promotes the precipitation of intermetallic compounds, the value calculated on the left side of formula (2) is preferably 30.00 or less.

The remainder other than the above components may consist of Fe and unavoidable impurities.

The composition of the seamless steel pipe of the present invention may further contain one or more optional elements selected from Co: 1% or less, V: 0.1% or less, Ti: 0.1% or less, Zr: 0.2% or less, Ta: 0.2% or less, Hf: 0.2% or less, Ca: 0.01% or less, REM: 0.01% or less, Mg: 0.01% or less, B: 0.01% or less, Sn: 0.2% or less, and Sb: 0.2% or less. The composition of the seamless steel pipe of the present invention may not contain these optional elements (even 0%).

Co: 1% or less Co is an element that enhances corrosion resistance and hardenability. On the other hand, a Co content of more than 1% reduces low-temperature toughness. Therefore, when Co is contained, the Co content is set to 1% or less. The Co content is preferably 0.5% or less. Furthermore, when Co is contained, the Co content is preferably 0.01% or more.

V: 0.1% or less V is an element that increases the strength of steel by precipitation strengthening. On the other hand, a V content of more than 0.1% reduces low-temperature toughness. Therefore, when V is contained, the V content is set to 0.1% or less. The V content is preferably 0.08% or less. Furthermore, when V is contained, the V content is preferably 0.01% or more.

Ti: 0.1% or less Ti is an element that forms carbonitrides and increases the strength of steel. On the other hand, a Ti content of more than 0.1% reduces low-temperature toughness. Therefore, when Ti is contained, the Ti content is set to 0.1% or less. The Ti content is preferably 0.02% or less. Furthermore, when Ti is contained, the Ti content is preferably 0.001% or more.

Zr: 0.2% or less Zr is an element that forms carbonitrides and increases the strength of steel. On the other hand, a Zr content of more than 0.2% reduces low-temperature toughness. Therefore, when Zr is contained, the Zr content is set to 0.2% or less. The Zr content is preferably 0.15% or less. Furthermore, when Zr is contained, the Zr content is preferably 0.01% or more.

Ta: 0.2% or less Ta is an element that forms carbonitrides and enhances the strength and corrosion resistance of steel. On the other hand, a Ta content of more than 0.2% reduces low-temperature toughness. Therefore, when Ta is contained, the Ta content is set to 0.2% or less. The Ta content is preferably 0.15% or less. Furthermore, when Ta is contained, the Ta content is preferably 0.01% or more.

Hf: 0.2% or less Hf is an element that forms carbonitrides and increases the strength of steel. On the other hand, a Hf content of more than 0.2% reduces low-temperature toughness. Therefore, when Hf is contained, the Hf content is set to 0.2% or less. The Hf content is preferably 0.15% or less. Furthermore, when Hf is contained, the Hf content is preferably 0.01% or more.

Ca: 0.01% or less Ca is an element that improves the hot workability of steel by fixing S in the steel as sulfides. On the other hand, a Ca content of more than 0.01% coarsens inclusions and reduces toughness and corrosion resistance. Therefore, when Ca is contained, the Ca content is set to 0.01% or less. The Ca content is preferably 0.005% or less. Furthermore, when Ca is contained, the Ca content is preferably 0.0001% or more.

REM: 0.01% or less REM (rare earth metal) is an element that improves the hot workability of steel by fixing S in the steel as sulfide. On the other hand, the content of REM exceeding 0.01% coarsens the inclusions and reduces the toughness and corrosion resistance. Therefore, when REM is contained, the REM content is set to 0.01% or less. The REM content is preferably 0.005% or less. Furthermore, when REM is contained, the REM content is preferably 0.0001% or more. In the present invention, REM refers to a collective term for a total of 17 elements, including Sc with atomic number 21, Y with atomic number 39, and 15 elements from La with atomic number 57 to Lu with atomic number 71. Furthermore, the REM content refers to the total content of these elements.

Mg: 0.01% or less Mg is an element that improves the hot workability of steel by fixing S in the steel as sulfides. On the other hand, Mg content exceeding 0.01% coarsens inclusions and reduces toughness and corrosion resistance. Therefore, when Mg is contained, the Mg content is set to 0.01% or less. The Mg content is preferably 0.005% or less. Furthermore, when Mg is contained, the Mg content is preferably 0.0001% or more.

B: 0.01% or less B is an element that suppresses segregation of S in the steel to grain boundaries, thereby improving the hot workability of the steel. On the other hand, a B content of more than 0.01% forms coarse nitrides, reducing the toughness of the steel. Therefore, when B is contained, the B content is set to 0.01% or less. The B content is preferably 0.005% or less. Furthermore, when B is contained, the B content is preferably 0.0001% or more.

Sn: 0.2% or less Sn is an element that enhances the corrosion resistance of steel. On the other hand, if the content of Sn exceeds 0.2%, the hot workability of the steel is reduced. Therefore, when Sn is contained, the Sn content is set to 0.2% or less. The Sn content is preferably 0.1% or less. Furthermore, when Sn is contained, the Sn content is preferably 0.01% or more.

Sb: 0.2% or less Sb is an element that enhances the corrosion resistance of steel. On the other hand, if the content of Sb exceeds 0.2%, the hot workability of the steel is reduced. Therefore, when Sb is contained, the Sb content is set to 0.2% or less. The Sb content is preferably 0.1% or less. Furthermore, when Sb is contained, the Sb content is preferably 0.01% or more.

Next, a preferred method for manufacturing the seamless steel pipe of the present invention will be described. In the present invention, a steel pipe material having the above-mentioned composition is used, but the method for manufacturing the seamless steel pipe does not need to be particularly limited, and any known method for manufacturing seamless steel pipes can be applied. It is preferable to melt molten steel having the above-mentioned composition using a melting method such as a converter, and make a steel pipe material such as a billet using a method such as a continuous casting method or an ingot-making-blooming rolling method. Next, the steel pipe material is heated, and hot worked and pipe-made in a pipe-making process using the Mannesmann plug mill method or the Mannesmann mandrel mill method, which are known pipe-making methods, and then quenched and tempered to obtain a seamless steel pipe having the above-mentioned composition.

The microstructure of the seamless steel pipe of the present invention has martensite as the main phase. In this specification, having martensite as the main phase means that the volume fraction of martensite in the microstructure is 60% or more. The balance can be one or two selected from a volume fraction of 0 to 40% retained austenite and a volume fraction of 0 to 10% ferrite. The volume fraction of martensite is preferably 80% or more, and more preferably 90% or more. The microstructure of the seamless steel pipe of the present invention may be a single martensite phase.

The yield strength of the seamless steel pipe of the present invention is preferably 758 MPa or more. The yield strength is more preferably 792 MPa or more, and even more preferably 827 MPa or more. In addition, although not particularly limited, as an example, the yield strength is 1034 MPa or less.

The seamless steel pipe of the present invention preferably has an absorbed energy of 100 J or more in a Charpy impact test at -80°C. The absorbed energy is preferably 150 J or more, and more preferably 180 J or more.

50 kg of molten steel with the composition shown in Table 1 was melted in a vacuum furnace, and an ingot was produced by the ingot casting method. The following processing was then carried out to simulate the manufacturing process of martensitic stainless steel seamless steel pipes. That is, the ingot was heated at 1250°C for 60 minutes and rolled to a thickness of 105 mm. It was then heated again at 1250°C for 60 minutes and rolled to a thickness of 15 mm to produce a plate material. The plate material was heated at 1000°C for 20 minutes, quenched by water cooling to 25°C, and then heated at 570°C for 30 minutes and tempered by air cooling to room temperature to obtain the test material (plate material).

Tensile test: A round bar tensile test piece with a parallel part diameter of 6 mm and a parallel part length of 25 mm was taken from the center of the plate thickness of each test material (corresponding to the center of the plate thickness of a seamless steel pipe) in accordance with API regulations. The longitudinal direction of the parallel part of this 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 determine the yield strength YS (MPa). The yield strength YS was defined as 0.2% proof stress. A yield strength YS of 758 MPa or more was considered to be passed, and a yield strength YS of less than 758 MPa was considered to be failed.

Impact test: A V-notch test piece (full size) was taken from the center of the plate thickness of each test material so that the longitudinal direction of the test piece was perpendicular to the rolling direction, and a Charpy impact test was performed in accordance with the provisions of JIS Z 2242: 2018. The test temperature was -80 ° C., and those with an absorbed energy of 100 J or more were considered to have passed, and those with an absorbed energy of less than 100 J were considered to have failed.

Corrosion resistance test A test piece of 30 mm x 40 mm x 3 mm was taken from each test material. The test piece was immersed in a test liquid: 20 mass% NaCl aqueous solution (liquid temperature 200 ° C, _CO2 gas atmosphere at 30 atmospheres) held in an autoclave, and a corrosion test was performed with an immersion period of 336 hours. The weight of the test piece after the test was measured, and the corrosion rate was calculated from the weight difference before and after the corrosion test. A corrosion rate of 0.127 mm / y or less was considered to be passed, and a corrosion rate of more than 0.127 mm / y was considered to be failed.

Microstructure observation A test piece for microstructure observation and a test piece for X-ray diffraction were taken from each test material at the center position of the plate thickness (corresponding to the center position of the wall thickness of a seamless steel pipe). The cross section of the test piece for microstructure observation perpendicular to the longitudinal direction (rolling direction) of the steel material was etched using a Villela etching solution. The cross section was then observed using an optical microscope at an appropriate magnification of 100 to 1000 times. The obtained image was analyzed to calculate the area ratio of the ferrite phase. This area ratio was considered to be the volume ratio of the ferrite phase. The volume ratio of the retained austenite phase was determined by chemically polishing the cross section of the test piece for X-ray diffraction perpendicular to the thickness direction of the steel material, and performing X-ray diffraction. A Co-Kα radiation source was used for the incident X-rays, and the area ratio of the retained austenite phase was calculated from the intensity ratio of the (200), (211) planes of ferrite to the (200), (220), and (311) planes of austenite. This area ratio was regarded as the volume ratio of the retained austenite phase. The results are shown in Table 2. In Table 2, the remainder other than the ferrite phase and the retained austenite phase was regarded as the martensite phase. In the present invention, the martensite phase may contain a precipitate phase having a volume ratio of 5% or less in addition to the ferrite phase and the retained austenite phase.

All of the examples of the present invention had high strength with a yield strength YS of 758 MPa or more, high toughness with an absorbed energy of 100 J or more in a Charpy impact test at -80°C, and excellent corrosion resistance in a high-temperature corrosive environment of 200°C containing CO ₂ and Cl ^- ions. On the other hand, the comparative examples outside the scope of the present invention did not achieve the desired value in at least one of the yield strength YS, the absorbed energy in a Charpy impact test at -80°C, and the corrosion resistance.

The above describes an embodiment of the present invention. However, the above-described embodiment is merely an example for implementing the present invention. Therefore, the present invention is not limited to the above-described embodiment, and the above-described embodiment can be modified as appropriate within the scope of the spirit of the present invention.

The martensitic stainless steel seamless pipe of the present invention is useful as oil country pipes and CCS press-in pipes, which require high strength, excellent low-temperature toughness, and excellent corrosion resistance, and is particularly suitable as CCS press-in pipes.

Claims (3)

In mass percent,
C: 0.005-0.100%,
Si: 0.05-1.00%,
Mn: 0.05-1.00%,
P: 0.05% or less,
S: 0.005% or less,
Ni: 6.0 to 9.0%,
Cr: 15.0-17.5%,
Al: 0.001-0.10%,
Nb: 0.001-0.20%,
N: 0.1% or less,
O: 0.01% or less,
Cu: 3.5% or less,
A martensitic stainless steel seamless pipe containing less than 1.0% Mo, 1.0% or less W, with the balance being Fe and unavoidable impurities, and having a chemical composition that satisfies the following formulas (1) and (2):
1001-2172C-17.35Cu-44.95Ni-34.45Cr-22.15Mo+60.96Nb-978.3N-18.57W> 50.00...(1)
Cr+0.65Ni+0.6Mo+0.3W+0.55Cu-20C> 20.00...(2)
Here, each element symbol in formula (1) and formula (2) indicates the content (mass%) of the corresponding element, and when the element is not contained, it is set to 0. The composition further includes, in mass%,
Co: 1% or less,
V: 0.1% or less,
Ti: 0.1% or less,
Zr: 0.2% or less,
Ta: 0.2% or less,
Hf: 0.2% or less,
Ca: 0.01% or less,
REM: 0.01% or less,
Mg: 0.01% or less,
B: 0.01% or less,
2. The martensitic stainless steel seamless pipe according to claim 1, containing one or more selected from the group consisting of Sn: 0.2% or less and Sb: 0.2% or less. 3. The martensitic stainless steel seamless pipe according to claim 1 or 2, having a yield strength of 758 MPa or more and an absorbed energy in a Charpy impact test at -80°C of 100 J or more.