EP4632101A1

EP4632101A1 - Martensitic stainless seamless steel pipe

Info

Publication number: EP4632101A1
Application number: EP24806852.0A
Authority: EP
Inventors: Hiroyasu Ebina; Kenichiro Eguchi; Shinsuke Ide
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2023-05-18
Filing date: 2024-03-14
Publication date: 2025-10-15
Also published as: JP7574975B1; MX2025009620A; JPWO2024236897A1; CN120615135A

Abstract

There is provided a martensitic stainless steel seamless pipe having a yield stress of 758 MPa or more, an absorbed energy of 100 J or more at -80°C in a Charpy impact test, and an excellent corrosion resistance.

The martensitic stainless steel seamless pipe has a chemical composition including, in % 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, the balance being Fe and incidental impurities, the composition satisfying particular inequalities (1) and (2).

Description

Technical Field

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

Background Art

In a conventional oil or gas field, pipes of a martensitic stainless steel, such as a 13% Cr steel (steel containing about 13% by mass of Cr) or a 17% Cr steel (steel containing about 17% by mass of Cr), are frequently used as oil well pipes for extraction. In recent years, carbon dioxide capture and storage (CCS) technology has been promoted worldwide in order to achieve carbon neutrality. In a CCS process, a martensitic stainless steel pipe is used as an injection pipe (hereinafter also referred to as a CCS injection pipe) for injecting recovered carbon dioxide into an aquifer or a depleted oil field.
Oil well pipes or CCS injection pipes are connected to a depth of several thousand meters into the ground, and therefore are required to have a strength sufficient to support their own weight. Further, both oil well pipes and CCS injection pipes are required to be corrosion-resistant in an environment where high-temperature, high-pressure carbon dioxide and chloride ions are present. In particular, carbon dioxide containing a highly corrosive impurity such as oxygen, a sulfur oxide, or a nitrogen oxide is sometimes injected through CCS injection pipes. Therefore, a material having higher corrosion resistance is required for CCS injection pipes.
When oil well pipes or CCS injection pipes are used in a cold region, they need to be non-brittle even at low temperatures; they are required to have a low-temperature toughness. Especially in the case of CCS injection pipes, the pressure of a fluid passing through the steel pipes is high. Therefore, there is a risk that upon the occurrence of a sudden pressure drop e.g. due to leakage of the fluid, the temperature of the fluid may decrease due to the Joule-Thomson effect. Therefore, toughness at lower temperatures is sometimes required for CCS injection pipes.
Patent Literature 1 proposes a high-strength, high-toughness seamless steel pipe having a chemical composition with a Cr content of 14% by mass or less. Patent Literatures 2 to 4 propose a high-strength, high-toughness and high-corrosion resistance seamless steel pipe having a chemical composition with a Cr content of about 17% by mass and an Ni content of 6.0% by mass or less, and having a microstructure containing a certain amount of ferrite phase.

Citation List

Patent Literature

PTL 1: International Publication No. WO 2009/004741
PTL 2: Japanese Unexamined Patent Application Publication No. 2017-14543
PTL 3: International Publication No. WO 2019/035329
PTL 4: International Publication No. WO 2020/202957

Summary of Invention

Technical Problem

Patent Literature 1 reports a corrosion test conducted for one week in a 20 mass % aqueous NaCl solution at a solution temperature of 80°C in a 30-atm CO₂ atmosphere. However, the corrosion resistance of the steel pipe disclosed in this literature may sometimes be insufficient in a more corrosive environment in oil well pipes or CCS injection pipes.
Patent Literatures 2 and 3 describe, as an excellent low-temperature toughness, an absorbed energy of 80 J or more at -60°C and an absorbed energy of 40 J or more at - 40°C, respectively, in a Charpy impact test. Patent Literature 4 describes, as an excellent low-temperature toughness, an absorbed energy of 300 J or more at -10°C and a ductile-brittle transition temperature of -40°C or less in a Charpy impact test. However, toughness at lower temperatures is sometimes required for CCS injection pipes which can possibly be used in an environment at -60°C or less.
As described above, while a martensitic stainless steel containing about 13% by mass of Cr can achieve a high strength and a high toughness, it is sometimes insufficient in corrosion resistance. On the other hand, a martensitic stainless steel containing about 17% by mass of Cr and having a relatively high corrosion resistance has the problem of a reduction in the low-temperature toughness due to the inclusion of a ferrite phase.
It is an object of the present invention 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 at -80°C in a Charpy impact test, and an excellent corrosion resistance.
In the present invention, "having an excellent corrosion resistance" means a corrosion rate of 0.127 mm/y or less as determined in a test in which a test specimen is immersed for 336 hours in a 20 mass % aqueous NaCl solution at a solution temperature of 200°C in a 30-atm CO₂ gas atmosphere.

Solution to Problem

In order to achieve the above object, the present inventors made intensive studies on components of a martensitic stainless steel which achieves high strength, high toughness, and high corrosion resistance. As a result, it was revealed that by increasing Ni to 6 to 9% and decreasing the amount of a ferrite phase with respect to conventional 17% Cr steel, it becomes possible to achieve toughness at lower temperatures while maintaining high corrosion resistance. However, when the content of an alloy element such as Cr or Ni is increased, the Ms temperature, which is the temperature at which martensite transformation starts, can decrease to below room temperature. There is, therefore, a risk that a sufficient strength may not be achieved even after quenching of the steel. It has now been found that by formulating C, Cu, Ni, Cr, Mo, Nb, N, and W in an appropriate relational expression, and adjusting the chemical composition so that the value, calculated by the relational expression, satisfies a predetermined range, it becomes possible to provide a martensitic stainless steel seamless pipe having a desired strength, an excellent low-temperature toughness, and an excellent corrosion resistance.
The present invention was completed based on such findings and on additional studies. Thus, the present invention can be embodied as follows.

[1] A martensitic stainless steel seamless pipe having a chemical composition containing, in % 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,
- the balance being Fe and incidental impurities, the composition satisfying the following inequalities (1) and (2): 1001 - 2172C - 17.35Cu - 44.95Ni - 34.45Cr - 22.15Mo + 60.96Nb - 978.3N - 18.57W > 50.00 $Cr + 0.65 Ni + 0.6 Mo + 0.3 W + 0.55 Cu - 20 C > 20.00$
- where the element symbols each represent the content (mass %) of the corresponding element, and are each zero when the corresponding element is not contained.
[2] The martensitic stainless steel seamless pipe as described in [1], wherein the chemical composition further containing, in % by mass, one or two or more selected from the following:
- 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.
[3] The martensitic stainless steel seamless pipe as described in [1] or {2}, having a yield strength of 758 MPa or more and an absorbed energy of 100 J or more at -80°C in a Charpy impact test.

Advantageous Effects of Invention

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 at -80°C in a Charpy impact test, and an excellent corrosion resistance.
According to the present invention, a martensitic stainless steel seamless pipe can now be used as a CCS injection pipe that requires superior low-temperature toughness and higher corrosion resistance than the conventional oil well pipe. Further, according to the present invention, it becomes possible to use a martensitic stainless steel seamless pipe, which is less expensive and has superior toughness than a duplex stainless steel such as a 25% Cr steel, for applications where such a duplex stainless steel is commonly used.

Description of Embodiments

The martensitic stainless steel seamless pipe of the present invention (hereinafter also referred to simply as the seamless steel pipe of the present invention) has a chemical composition containing, in % 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, the balance being Fe and incidental impurities, the composition satisfying the following inequalities (1) and (2): 1001 - 2172C - 17.35Cu - 44.95Ni - 34.45Cr - 22.15Mo + 60.96Nb - 978.3N - 18.57W > 50.00 $Cr + 0.65 Ni + 0.6 Mo + 0.3 W + 0.55 Cu - 20 C > 20.00$ where the element symbols each represent the content (mass %) of the corresponding element, and are each zero when the corresponding element is not contained.
The following are the reasons for the limitations on the chemical composition of the seamless steel pipe of the present invention. In the following description, "% by mass" will be expressed simply as "%" unless otherwise specified.

C: 0.005 to 0.100%

C enhances the hardenability and increases the strength of the steel material. In order to achieve the effect, the C content is made 0.005% or more. On the other hand, when the C content is too high, the Ms temperature will decrease and martensite transformation will not occur at room temperature, resulting in a failure to ensure a sufficient strength. Therefore, the C content is made 0.100% or less. The C content is preferably 0.007% or more. Further, the C content is preferably 0.050% or less, more preferably 0.030% or less, and even more preferably 0.020% or less.

Si: 0.05 to 1.00%

Si acts as a deoxidizing agent. In order to achieve the effect, the Si content is made 0.05% or more. On the other hand, the Si content of more than 1.00% reduces the corrosion resistance and hot workability of the steel due to precipitation of δ-ferrite. Therefore, the Si content is made 1.00% or less. The Si content is preferably 0.50% or less. On the other hand, the Si content is preferably 0.10% or more from the viewpoint of ensuring stable strength.

Mn: 0.05 to 1.00%

Mn is an element which enhances the hot workability and the strength. In order to achieve the effect, the Mn content is made 0.05% or more. On the other hand, the inclusion of an excessive amount of Mn causes precipitation of MnS, resulting in a reduction in the resistance to sulfide stress corrosion cracking. Therefore, the Mn content is made 1.00% or less. The Mn content is preferably 0.50% or less. Further, the Mn content is preferably 0.10% or more.

P: 0.05% or less

P is an impurity which is inevitably contained in the steel. P is an element which reduces the corrosion resistance. Therefore, in the present invention, the amount of P is preferably reduced as much as possible. However, an extreme reduction of the amount of P increases the production cost. Therefore, the P content is limited to 0.05% or less as a range which does not cause an extreme deterioration in the properties and which enables a low-cost industrial production. The P content is preferably 0.03% or less. While there is no particular limitation on the lower limit of the P content, excessive removal of P involves an increase in the production cost. Therefore, the P content is preferably 0.005% or more from the viewpoint of the production cost.

S: 0.005% or less

S is an impurity which is inevitably contained in the steel. S is an element which significantly reduces the hot workability, and therefore its amount is preferably reduced as much as possible. By reducing the S content to 0.005% or less, it becomes possible to produce the steel pipe by the Mannesmann method. Therefore, the S content is limited to 0.005% or less in the present invention. The S content is preferably 0.002% or less. While there is no particular limitation on the lower limit of the S content, excessive removal of S involves an increase in the production cost. Therefore, the S content is preferably 0.0005% or more from the viewpoint of the production cost.

Ni: 6.0 to 9.0%

Ni is an element which strengthens a protective film and enhances the corrosion resistance and, through its dissolution in the steel, increases the strength of the steel. Further, Ni is an element which stabilizes an austenite phase at a quenching temperature. Therefore, the Ni content is made 6.0% or more. When the Ni content is less than 6.0%, the amount of ferrite increases and the toughness decreases. On the other hand, when the Ni content exceeds 9.0%, the Ms temperature decreases and the strength decreases. Therefore, the Ni content is limited to 6.0 to 9.0%. The Ni content is preferably 6.5% or more. Further, the Ni content is preferably 8.0% or less, more preferably 7.5% or less.

Cr: 15.0 to 17.5%

Cr is an element which forms a protective film and enhances the corrosion resistance. The inclusion of Cr in an amount of 15.0% or more can achieve the intended corrosion resistance. On the other hand, the inclusion of Cr in an excessive amount of more than 17.5% will decrease the Ms temperature and increase the amount of ferrite, resulting in a failure 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% or more, more preferably 16.0% or more. Further, the Cr content is preferably 17.0% or less.

Al: 0.001 to 0.10%

Al acts as a deoxidizing agent. In order to achieve the effect, the Al content is made 0.001% or more. However, the Al content of more than 0.10% adversely affects the toughness due to excessive precipitation of an oxide. Therefore, the Al content is limited to 0.10% or less in the present invention. The Al content is preferably 0.01% or more. Further, the Al content is preferably 0.04% or less.

Nb: 0.001 to 0.20%

Nb can reduce the amount of dissolved carbon through the formation of a carbide, thereby increasing the Ms temperature. In order to achieve the effect, the Nb content is made 0.001% or more. On the other hand, the Nb content of more than 0.20% may cause precipitation of a coarse carbide, resulting in a reduction in the toughness. Therefore, the Nb content is made 0.20% or less. The Nb content is preferably 0.01% or more. Further, the Nb content is preferably 0.10% or less.

N: 0.1% or less

N is an impurity which is inevitably contained in the steel. N increases the steel strength through its dissolution in the steel. However, when the N content exceeds 0.1%, a large amount of various nitride inclusions will be formed, resulting in a reduction in the corrosion resistance. Therefore, the N content is limited to 0.1% or less in the present invention. The N content is preferably 0.005% or more. Further, the N content is preferably 0.05% or less, more preferably 0.03% or less.

O: 0.01% or less

O is an impurity which is inevitably contained in the steel. O is present as an oxide in the steel, and therefore has an adverse effect on various properties. Therefore, in the present invention, the amount of O is preferably reduced as much as possible. In particular, when the O content exceeds 0.01%, the hot workability, the corrosion resistance, and the toughness will be reduced. Therefore, the O content is made 0.01% or less. The O content is preferably 0.005% or less. While there is no particular limitation on the lower limit of the O content, excessive removal of O involves an increase in the production cost. Therefore, the O content is preferably 0.001% or more from the viewpoint of the production cost.

Cu: 3.5% or less

Cu strengthens a protective film and enhances the corrosion resistance of the steel. However, the Cu content of more than 3.5% reduces the hot workability due to precipitation of metallic Cu. Therefore, the Cu content is made 3.5% or less. The Cu content is preferably 3.0% or less. The seamless steel pipe of the present invention need not necessarily contain Cu. The Cu content is preferably 0.5% or more.

Mo: less than 1.0%

Mo is an element which enhances the corrosion resistance. On the other hand, the inclusion of Mo in an excessive amount of 1.0% or more causes a decrease in the Ms temperature, whereby the hardenability is reduced and ferrite is likely to be formed. This may result in a reduction in the 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 need not necessarily contain Mo. The Mo content is preferably 0.1% or more.

W: 1.0% or less

W is an element which enhances the corrosion resistance. On the other hand, the inclusion of W in an excessive amount of more than 1.0% causes a decrease in the Ms temperature. Therefore, 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 need not necessarily contain W. The W content is preferably 0.1% or more.
Further, the chemical composition of the seamless steel pipe of the present invention contains the elements in such amounts as to satisfy the following inequality (1). 1001 - 2172C - 17.35Cu - 44.95Ni - 34.45Cr - 22.15Mo + 60.96Nb - 978.3N - 18.57W > 50.00 where the element symbols each represent the content (mass %) of the corresponding element, and are each zero when the corresponding element is not contained.
Inequality (1) is related to the Ms temperature: When the value calculated on the left side of inequality (1) is more than 50.00, martensite transformation occurs even by cooling to room temperature, and a sufficient strength can be ensured. Therefore, the value calculated on the left side of inequality (1) is made more than 50.00. The value calculated on the left side of inequality (1) is preferably 55.00 or more. Further, the value calculated on the left side of inequality (1) is preferably 300.00 or less.
Further, the chemical composition of the seamless steel pipe of the present invention contains the elements in such amounts as to satisfy the following inequality (2). $Cr + 0.65 Ni + 0.6 Mo + 0.3 W + 0.55 Cu - 20 C > 20.00$ where the element symbols each represent the content (mass %) of the corresponding element, and are each zero when the corresponding element is not contained.
Inequality (2) is related to the corrosion resistance: When the value calculated on the left side of inequality (2) is more than 20.00, it is possible to achieve a corrosion rate of 0.127 mm/y or less as determined in a test in which a test specimen is immersed for 336 hours in a 20 mass % aqueous NaCl solution at a solution temperature of 200°C in a 30-atm CO₂ gas atmosphere. Therefore, the value calculated on the left side of inequality (2) is made more than 20.00. The value calculated on the left side of inequality (2) is preferably 20.50 or more, more preferably 21.00 or more. On the other hand, excessive addition of an alloy element(s) promotes the precipitation of an intermetallic compound(s). Therefore, the value calculated on the left side of inequality (2) is preferably 30.00 or less.
The balance of the chemical composition can be Fe and incidental impurities.
The chemical composition of the seamless steel pipe of the present invention may further contain, as optional elements, one or two or more selected from the following: 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 chemical composition of the seamless steel pipe of the present invention need not contain any of such optional elements (content 0%).

Co: 1% or less

Co is an element which enhances the corrosion resistance and hardenability of the steel. On the other hand, the Co content of more than 1% reduces the low-temperature toughness. Therefore, when the steel contains Co, the Co content is made 1% or less. The Co content is preferably 0.5% or less. Further, when the steel contains Co, the Co content is preferably 0.01% or more.

V: 0.1% or less

V is an element which increases the strength of the steel through precipitation strengthening. On the other hand, the V content of more than 0.1% reduces the low-temperature toughness. Therefore, when the steel contains V, the V content is made 0.1% or less. The V content is preferably 0.08% or less. Further, when the steel contains V, the V content is preferably 0.01% or more.

Ti: 0.1% or less

Ti is an element which forms a carbonitride and increases the strength of the steel. On the other hand, the Ti content of more than 0.1% reduces the low-temperature toughness. Therefore, when the steel contains Ti, the Ti content is made 0.1% or less. The Ti content is preferably 0.02% or less. Further, when the steel contains Ti, the Ti content is preferably 0.001% or more.

Zr: 0.2% or less

Zr is an element which forms a carbonitride and increases the strength of the steel. On the other hand, the Zr content of more than 0.2% reduces the low-temperature toughness. Therefore, when the steel contains Zr, the Zr content is made 0.2% or less. The Zr content is preferably 0.15% or less. Further, when the steel contains Zr, the Zr content is preferably 0.01% or more.

Ta: 0.2% or less

Ta is an element which forms a carbonitride and increases the strength and corrosion resistance of the steel. On the other hand, the Ta content of more than 0.2% reduces the low-temperature toughness. Therefore, when the steel contains Ta, the Ta content is made 0.2% or less. The Ta content is preferably 0.15% or less. Further, when the steel contains Ta, the Ta content is preferably 0.01% or more.

Hf: 0.2% or less

Hf is an element which forms a carbonitride and increases the strength of the steel. On the other hand, the Hf content of more than 0.2% reduces the low-temperature toughness. Therefore, when the steel contains Hf, the Hf content is made 0.2% or less. The Hf content is preferably 0.15% or less. Further, when the steel contains Hf, the Hf content is preferably 0.01% or more.

Ca: 0.01% or less

Ca is an element which enhances the hot workability of the steel by fixing S as a sulfide in the steel material. On the other hand, the Ca content of more than 0.01% causes coarsening of inclusions, resulting in a reduction in the toughness and in the corrosion resistance. Therefore, when the steel contains Ca, the Ca content is made 0.01% or less. The Ca content is preferably 0.005% or less. Further, when the steel contains Ca, the Ca content is preferably 0.0001% or more.

REM: 0.01% or less

REM (rare earth metal) is an element which enhances the hot workability of the steel by fixing S as a sulfide in the steel material. On the other hand, the REM content of more than 0.01% causes coarsening of inclusions, resulting in a reduction in the toughness and in the corrosion resistance. Therefore, when the steel contains REM, the REM content is made 0.01% or less. The REM content is preferably 0.005% or less. Further, when the steel contains REM, the REM content is preferably 0.0001% or more. In the present invention, REM is a general term for a total of 17 elements consisting of Sc of atomic number 21, Y of atomic number 39, and 15 elements from La of atomic number 57 to Lu of atomic number 71. The REM content refers to the total content of these elements.

Mg: 0.01% or less

Mg is an element which enhances the hot workability of the steel by fixing S as a sulfide in the steel material. On the other hand, the Mg content of more than 0.01% causes coarsening of inclusions, resulting in a reduction in the toughness and in the corrosion resistance. Therefore, when the steel contains Mg, the Mg content is made 0.01% or less. The Mg content is preferably 0.005% or less. Further, when the steel contains Mg, the Mg content is preferably 0.0001% or more.

B: 0.01% or less

B is an element which enhances the hot workability of the steel by inhibiting segregation of S at grain boundaries in the steel material. On the other hand, the B content of more than 0.01% causes the formation of a coarse nitride, resulting in a reduction in the toughness of the steel. Therefore, when the steel contains B, the B content is made 0.01% or less. The B content is preferably 0.005% or less. Further, when the steel contains B, the B content is preferably 0.0001% or more.

Sn: 0.2% or less

Sn is an element which increases the corrosion resistance of the steel. On the other hand, the Sn content of more than 0.2% reduces the hot workability of the steel. Therefore, when the steel contains Sn, the Sn content is made 0.2% or less. The Sn content is preferably 0.1% or less. Further, when the steel contains Sn, the Sn content is preferably 0.01% or more.

Sb: 0.2% or less

Sb is an element which increases the corrosion resistance of the steel. On the other hand, the Sb content of more than 0.2% reduces the hot workability of the steel. Therefore, when the steel contains Sb, the Sb content is made 0.2% or less. The Sb content is preferably 0.1% or less. Further, when the steel contains Sb, the Sb content is preferably 0.01% or more.
A preferred method for producing the seamless steel pipe of the present invention will now be described. While a steel pipe material having the above-described chemical composition is used in the present invention, there is no need to place a particular limitation on a method for producing the seamless steel pipe; for example, any known seamless steel pipe production method can be used. In a preferred method, a molten steel having the above-described chemical composition is produced by a steelmaking process using a converter or the like, and the molten steel is formed into a steel pipe material, such as a billet, using a continuous casting method, an ingot casting-blooming method, or the like. Subsequently, the steel pipe material is heated, hot-worked and pipe-formed by the Mannesmann-plug mill method or the Mannesmann-mandrel mill method, which is a known pipe-forming method, followed by quenching and tempering to produce a seamless steel pipe having the above-described chemical composition.
The microstructure of the seamless steel pipe of the present invention contains martensite as the main phase. "Containing martensite as the main phase" herein means that the volume fraction of martensite is 60% or more in the microstructure. The balance can be one or two selected from 0 to 40% of retained austenite by volume fraction and 0 to 10% of ferrite by volume fraction. The volume fraction of martensite is preferably 80% or more, more preferably 90% or more. The microstructure of the seamless steel pipe of the present invention may consist of 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. There is no particular limitation on the upper limit of the yield strength; for 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 at -80°C in a Charpy impact test. The absorbed energy is preferably 150 J or more, more preferably 180 J or more.

EXAMPLES

Molten steels, having the chemical compositions shown in Table 1, were each produced in a 50-kg vacuum furnace, and each molten steel was formed into an ingot by an ingot casting method. The ingot was then subjected to the following treatments that simulated a process for producing a martensitic stainless steel seamless pipe. The ingot was heated at 1250°C for 60 minutes and rolled to a thickness of 105 mm. Thereafter, the rolled ingot was 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 subjected to a quenching treatment which was conducted by heating it at 1000°C for 20 minutes, and then water-cooling it to 25°C. Thereafter, the plate material was subjected to a tempering treatment which was conducted by heating it at 570°C for 30 minutes, and then naturally cooling it to room temperature to obtain a test material (plate material).

Tensile Test

A round bar tensile test specimen, having a parallel-portion diameter of 6 mm and a parallel-portion length of 25 mm, was taken from a central portion of each test plate in the thickness direction (corresponding to a central portion of a seamless steel pipe in the thickness direction) according to the API standard. The longitudinal direction of the parallel portion of the test specimen was made to coincide with the rolling direction of the plate material. Using the test specimen, a tensile test was conducted at room temperature (25°C) to determine the yield strength YS (MPa). 0.2% proof stress was taken as the yield strength YS. Test specimens with a yield strength YS of 758 MPa or more were evaluated as acceptable, and test specimens with a yield strength YS of less than 758 MPa were evaluated as unacceptable.

Impact Test

A V-notch test specimen (full size) was taken from a central portion of each test plate in the thickness direction according to JIS Z 2242:2018 in such a manner that the longitudinal direction of the test specimen was perpendicular to the rolling direction. The test specimen was subjected to a Charpy impact test at a temperature of - 80°C. Test specimens with an absorbed energy of 100 J or more were evaluated as acceptable, and test specimens with an absorbed energy of less than 100 J were evaluated as unacceptable.

Corrosion Resistance Test

A 30 mm × 40 mm × 3 mm test specimen was taken from each test material. The test specimen was subjected to a corrosion test which was conducted by immersing the test specimen in a test liquid, a 20 mass % aqueous NaCl solution (solution temperature 200°C, 30-atm CO₂ gas atmosphere) held in an autoclave, for an immersion period of 336 hours. The weight of the test specimen after the test was measured, and a corrosion rate was calculated from the difference in weight before and after the corrosion test. Test specimens with a corrosion rate of 0.127 mm/y or less were evaluated as acceptable, and test specimens with a corrosion rate of more than 0.127 mm/y were evaluated as unacceptable.

Microstructure Observation

A test specimen for microstructure observation and a test specimen for X-ray diffraction were taken from a central portion of each test plate in the thickness direction (corresponding to a central portion of a seamless steel pipe in the thickness direction). A cross-section, perpendicular to the longitudinal direction (rolling direction) of the steel material, of the test specimen for microstructure observation was etched using Vilella's etching solution. Thereafter, the cross-section was observed using an optical microscope at an appropriate magnification in the range of 100 to 1000. The image obtained was analyzed to calculate the area fraction of a ferrite phase. The area fraction was regarded as the volume fraction of the ferrite phase. The volume fraction of a retained austenite phase was determined by chemically polishing a cross-section, perpendicular to the thickness direction of the steel material, of the test specimen for X-ray diffraction, and subjecting the cross-section to X-ray diffraction. Using a Co-Kα ray source for the incident X-rays, the area fraction of the retained austenite phase was calculated from the intensity ratio between the (200), (211) planes of ferrite and the (200), (220), (311) planes of austenite. The area fraction was regarded as the volume fraction of the retained austenite phase. The results are shown in Table 2. In Table 2, the balance other than the ferrite phase and the retained austenite phase is a martensite phase. In the present invention, a martensite phase may contain a precipitate phase, other than a ferrite phase and a retained austenite phase, in a volume fraction of 5% or less.

[Table 1]

No.	Chemical composition (mass %)																	Remarks
No.	C	Si	Mn	P	S	Ni	Cr	Al	Nb	N	O	Cu	Mo	W	Other elements	Ineq. (1)	Ineq. (2)	Remarks
1	0.010	0.31	0.25	0.021	0.0009	7.1	16.5	0.03	0.08	0.0156	0.002	1.0	0.2	0.2		55.83	21.65	Example
2	0.008	0.36	0.26	0.015	0.0010	8.8	15.4	0.03	0.09	0.0091	0.005	0	0	0		54.12	20.96	Example
3	0.013	0.31	0.22	0.015	0.0010	6.1	16.2	0.03	0.08	0.0139	0.003	3.0	0.2	0.6		64.14	21.86	Example
4	0.008	0.14	0.90	0.024	0.0010	7.6	17.3	0.04	0.19	0.0074	0.004	0	0	0		50.36	22.08	Example
5	0.008	0.31	0.35	0.015	0.0010	7.3	15.1	0.03	0.08	0.0054	0.004	2.9	0.8	0.1		65.00	21.79	Example
6	0.012	0.42	0.48	0.015	0.0010	6.9	16.1	0.03	0.04	0.0147	0.004	0.8	0.3	0.4	Co:0.6	70.24	21.09	Example
7	0.008	0.11	0.29	0.019	0.0007	6.8	16.3	0.04	0.09	0.0249	0.003	1.5	0.2	0.3	V:0.06, Ti:0.01	61.53	21.60	Example
8	0.009	0.39	0.24	0.019	0.0008	7.2	16.2	0.03	0.08	0.0091	0.003	0.7	0.7	0	Zr:0.11	68.05	21.51	Example
9	0.012	0.36	0.37	0.018	0.0008	6.9	16.6	0.04	0.03	0.0069	0.003	0.5	0.1	0.9	Ta:0.12, Hf:0.09	60.39	21.45	Example
10	0.011	0.37	0.20	0.023	0.0012	6.7	15.3	0.03	0.09	0.0066	0.004	2.5	0.2	0.2	Ca:0.004	96.37	20.99	Example
11	0.013	0.41	0.29	0.024	0.0007	6.8	15.7	0.03	0.04	0.0261	0.005	0.2	0.7	0.3	REM:0.008, Sn:0.1	78.60	20.48	Example
12	0.011	0.38	0.29	0.016	0.0012	7.2	16.4	0.03	0.07	0.0058	0.004	1.4	0.1	0.5	Mg:0.003, Sb:0.09	51.29	21.84	Example
13	0.007	0.47	0.16	0.020	0.0008	7.1	16.5	0.03	0.09	0.0197	0.004	1.1	0.2	0	B:0.002	60.92	21.70	Example
14	0.010	0.16	0.46	0.014	0.0008	7.4	16.4	0.03	0.07	0.0107	0.003	0	0	0	Co:0.41, V:0.02, Ti:0.004, Ca:0.001	76.29	20.98	Example
15	0.007	0.41	0.38	0.021	0.0009	6.3	16.0	0.04	0.04	0.0191	0.005	2.9	0.6	0.2	Ta:0.06, REM:0.005, B:0.001	67.84	21.97	Example
16	0.015	0.16	0.23	0.016	0.0012	6.7	16.5	0.03	0.05	0.0106	0.002	1.1	0.1	0.1	Zr:0.04, Hf:0.13, Sn:0.05, Sb:0.13	68.35	21.25	Example
17	0.045	0.76	0.07	0.009	0.0006	6.0	17.1	0.05	0.19	0.0053	0.002	0	0	0	Ti:0.06, Ca:0.007	50.86	20.10	Example
18	0.012	0.08	0.84	0.024	0.0009	6.2	16.7	0.03	0.002	0.0703	0.003	0	0	0	Mg:0.008, B:0.006	52.28	20.49	Example
19	0.008	0.43	0.42	0.023	0.0010	9.1	15.1	0.03	0.04	0.0122	0.004	0	0.1	0		44.39	20.87	Comp. Example
20	0.008	0.30	0.30	0.017	0.0008	4.8	16.5	0.03	0.001	0.0058	0.003	2.4	0.9	0		131.85	21.33	Comp. Example
21	0.011	0.40	0.46	0.018	0.0008	6.4	17.8	0.03	0.07	0.0106	0.004	0.5	0.1	0.1		57.37	22.11	Comp. Example
22	0.030	0.25	0.32	0.014	0.0009	6.3	14.6	0.03	0.09	0.0484	0.003	1	0.9	0		69.64	19.20	Comp. Example
23	0.010	0.14	0.47	0.014	0.0008	7.0	16.6	0.04	0.07	0.0101	0.001	2.4	0	0		44.40	22.29	Comp. Example
24	0.021	0.24	0.37	0.018	0.0009	6.2	15.2	0.03	0.05	0.0306	0.002	0.5	0.2	0.1		111.21	19.24	Comp. Example
25	0.013	0.19	0.30	0.017	0.0007	5.3	17.3	0.01	0.001	0.0102	0.003	2.0	1.1	0.9	V:0.05, B:0.0002	52.19	22.51	Comp. Example

• Underline indicates that the value is outside the scope of the present invention.
• The balance of the chemical composition consists of Fe and incidental impurities.

[Table 2]

No.	Ferrite phase (vol. %)	Retained austenite phase (vol. %)	Yield strength YS (MPa)	vE-80(*) (J)	Corrosion rate (mm/y)	Remarks
1	3	10	888.4	193.1	0.103	Example
2	2	21	772.7	138.4	0.108	Example
3	2	8	901.4	193.1	0.070	Example
4	4	9	898.4	217.4	0.081	Example
5	3	16	802.3	226.9	0.086	Example
6	1	2	945.6	191.0	0.103	Example
7	2	4	947.4	176.5	0.107	Example
8	3	4	916.2	176.3	0.079	Example
9	2	9	898.0	242.9	0.111	Example
10	2	6	918.7	226.9	0.109	Example
11	1	5	938.3	188.5	0.122	Example
12	1	11	896.0	242.9	0.080	Example
13	0	10	840.6	155.2	0.092	Example
14	0	11	861.7	178.2	0.094	Example
15	4	2	981.9	191.0	0.096	Example
16	3	5	949.5	187.9	0.088	Example
17	1	22	762.3	118.5	0.123	Example
18	2	18	771.1	134.4	0.121	Example
19	3	42	660.3	235.6	0.116	Comp. Example
20	28	10	874.2	60.5	0.101	Comp. Example
21	12	9	818.0	31.5	0.077	Comp. Example
22	3	6	940.8	135.9	0.180	Comp. Example
23	1	78	427.4	206.4	0.098	Comp. Example
24	1	12	770.4	186.3	0.177	Comp. Example
25	29	10	861.2	40.1	0.082	Comp. Example

* Absorbed energy at -80°C in Charpy impact test

All the test materials of the examples according to the present invention had a high strength, in particular a yield strength YS of 758 MPa or more, a high toughness, in particular an absorbed energy of 100 J or more at -80°C in the Charpy impact test, and an excellent corrosion resistance in a high-temperature corrosive environment at 200°C, containing CO₂ and Cl^- ions. On the other hand, the test materials of the comparative examples, which fall outside the scope of the present invention, failed to achieve a desired value(s) for at least one of the yield strength YS, the absorbed energy at -80°C in the Charpy impact test, and the corrosion resistance.
While the embodiments of the present invention have been described, the embodiments are merely examples for carrying out the present invention. Thus, the present invention is not limited to the embodiments described above; various changes and modifications may be made without departing from the concept and scope of the present invention.

Industrial Applicability

The martensitic stainless steel seamless pipe of the present invention is useful as an oil well pipe or a CCS injection pipe which requires a high strength, an excellent low-temperature toughness, and an excellent corrosion resistance, and is particularly suitable for use as a CCS injection pipe.

Claims

A martensitic stainless steel seamless pipe having a chemical composition comprising, in % 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,

the balance being Fe and incidental impurities, the composition satisfying the following inequalities (1) and (2): 1001 - 2172C - 17.35Cu - 44.95Ni - 34.45Cr - 22.15Mo + 60.96Nb - 978.3N - 18.57W > 50.00 $Cr + 0.65 Ni + 0.6 Mo + 0.3 W + 0.55 Cu - 20 C > 20.00$

where the element symbols each represent the content (mass %) of the corresponding element, and are each zero when the corresponding element is not contained.
The martensitic stainless steel seamless pipe according to claim 1, wherein the chemical composition further comprises, in % by mass, one or two or more selected from the following:
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 martensitic stainless steel seamless pipe according to claim 1 or 2, having a yield strength of 758 MPa or more and an absorbed energy of 100 J or more at -80°C in a Charpy impact test.