EP4711484A1

EP4711484A1 - Duplex stainless steel and seamless stainless steel pipe

Info

Publication number: EP4711484A1
Application number: EP24842899.7A
Authority: EP
Inventors: Kenichiro Eguchi; Yuichi Kamo; Shinsuke Ide
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2023-07-18
Filing date: 2024-06-24
Publication date: 2026-03-18
Also published as: JPWO2025018103A1; WO2025018103A1; JP7816551B2; CN121335999A

Abstract

An object of the present invention is to provide a duplex stainless steel and a stainless steel seamless pipe that have high strength, particularly, a yield strength of 448 MPa (65 ksi) or greater, and have excellent corrosion resistance and excellent low-temperature toughness.

A duplex stainless steel having a chemical composition containing, in mass%, C: 0.002 to 0.03%, Si: 0.05 to 1.0%, Mn: 0.1 to 1.7%, P: 0.040% or less, S: 0.020% or less, Cr: 20.0 to 28.0%, Ni: 4.0 to 10.0%, Mo: 2.0 to 5.0%, Al: 0.001 to 0.05%, N: 0.06 to 0.35%, Sb: 0.001 to 1.000%, and O: 0.010% or less, with the balance being Fe and incidental impurities, wherein the duplex stainless steel includes microstructures containing an austenite phase in a volume fraction of 20 to 70% and a ferrite phase in a volume fraction of 30 to 80%, and the duplex stainless steel has a yield strength YS of 448 MPa or greater.

Description

Technical Field

The present invention relates to a duplex stainless steel and a stainless steel seamless pipe that are suitable for use in oil wells and gas wells (hereinafter simply referred to as "oil wells"). In particular, the present invention relates to a duplex stainless steel and a stainless steel seamless pipe that have improved corrosion resistance for high-temperature and severe corrosive environments containing carbon dioxide gas (CO₂) and chlorine ions (Cl^-).

Background Art

Oil wells in severe corrosive environments have attracted little attention in the past, but such oil wells have been actively developed in recent years because of expected depletion of energy resources in the near future. Severe corrosive environments include deep oil fields, environments containing carbon dioxide gas, and environments containing hydrogen sulfide, which are called sour environments. Oil well steel pipes used in such environments are required to have high strength and high corrosion resistance.
In oil fields and gas fields located in environments containing CO₂, Cl^-, and the like, oil well steel pipes that have been typically used for drilling to date are 13Cr martensitic stainless steel pipes. Recently, however, oil wells at even higher temperatures (high temperatures up to 230°C) have begun to be developed, and, in some cases, the corrosion resistance exhibited by 13Cr martensitic stainless steel pipes has been insufficient. A need exists for an oil well steel pipe that has high corrosion resistance and, therefore, can be used in such environments.
Regarding this need, steels that can be used in oil well steel pipes include duplex stainless steels. Duplex stainless steels have excellent strength properties. However, using a duplex stainless steel in severe corrosive environments containing large amounts of hydrogen sulfide, carbon dioxide gas, and chloride ions, such as deep oil wells that have been actively developed in recent years, requires improvement in corrosion resistance.
In this regard, Patent Literature 1, for example, discloses a duplex stainless steel having excellent corrosion resistance; the duplex stainless steel has a PREW value of 40 or greater as a result of controlling the contents of Cr, Mo, N, and W.
Patent Literature 2 discloses a duplex stainless steel having excellent corrosion resistance and hot workability, which is achieved by controlling the contents of B, Ta, and the like, in addition to the contents of Cr, Mo, W, and N.
Patent Literature 3 discloses a duplex stainless steel having a reduced S content of 3 weight-ppm or less, which is achieved by using a CaO crucible and CaO-CaF₂-Al₂O₃-based slag in a vacuum melting furnace and is intended to reduce sulfide-based inclusions in the steel, which have a negative influence on hot workability and corrosion resistance.
Patent Literature 4 relates to a technology for controlling oxide-based inclusions, which act as initiation sites for pitting corrosion, and discloses a duplex stainless steel in which a total content of Ca and Mg and a S content in oxide-based inclusions are controlled, and in addition, a morphology and a density of the inclusions are adjusted. Patent Literature 4 further discloses a duplex stainless steel in which the occurrence of local corrosion is inhibited by controlling a size and the number of the inclusions, which is achieved by optimally combining a basicity of slag for a reduction treatment, a temperature and a time for killing in a ladle, and a post-casting total working ratio, based on the fact that even insoluble Al oxides can be initiation sites for local corrosion if the Al oxides contain Ca, Mg, and S in certain amounts or greater amounts.

Citation List

Patent Literature

PTL 1: Japanese Unexamined Patent Application Publication No. 5-132741
PTL 2: Japanese Unexamined Patent Application Publication No. 8-170153
PTL 3: Japanese Unexamined Patent Application Publication No. 3-291358
PTL 4: International Publication No. 2005/014872

Summary of Invention

Technical Problem

As described above, the development of oil wells at even higher temperatures has created a need for oil well steel pipes to have high corrosion resistance. One method for evaluating corrosion resistance that is required in oil well steel pipes for their use in oil wells having a high temperature of up to 230°C is as follows: a corrosion test is conducted by immersing a test specimen in a 20 mass% NaCl aqueous solution (solution temperature: 230°C, atmosphere with 10-MPa CO₂ gas) for an immersion time of 336 hours, and a determination is made as to whether a corrosion rate is 0.127 mm/y or less.
Other problems that exist in addition to the problem described above are as follows. In the process of petroleum drilling, if the properties (mainly permeability) of the formations in which petroleum accumulates (reservoir formations) are poor, a sufficient production volume may not be achieved, and if, for example, reservoir formations are clogged, an anticipated production volume may not be achieved. One method for improving the productivity that may be employed is acidizing, which is a process of injecting acid, such as hydrochloric acid, into the reservoir formations. In this case, steel pipes for use in oil wells are required to have excellent corrosion resistance in acid environments.
Furthermore, in cases where steel pipes for use in oil wells are used in cold regions, they are additionally required to have excellent low-temperature toughness. One method for determining whether low-temperature toughness is excellent may be determining whether an absorbed energy vE_-40, which is determined by a Charpy impact test at -40°C, is 100 J or greater.
Furthermore, in recent years, carbon capture and storage (CCS) technology has drawn attention as a process for realizing a carbon-neutral society. The technology involves injecting and storing CO₂ emitted from oil fields, gas fields, plants, and the like underground. Regarding CCS, oil well pipes are used to inject CO₂ underground, and one problem that has arisen is that impurities remaining in CO₂, such as SOx, NOx, and O₂, increase corrosiveness.
Patent Literature 1 to 4 disclose stainless steels having improved corrosion resistance. Unfortunately, in the cases of Patent Literature 1 to 4, collectively achieving corrosion resistance at high temperatures, corrosion resistance in acid environments, low-temperature toughness, and corrosion resistance in CCS environments has not been sufficiently successful in some cases.
The present invention solves the problems of the related art, and an object of the present invention is to provide a duplex stainless steel and a stainless steel seamless pipe that have high strength, particularly, a yield strength of 448 MPa (65 ksi) or greater, and have excellent corrosion resistance and excellent low-temperature toughness.
As used herein, the expression "excellent corrosion resistance" means "excellent carbon dioxide gas corrosion resistance", "excellent corrosion resistance in acid environments", and "excellent corrosion resistance in CCS environments".
As used herein, the expression "excellent carbon dioxide gas corrosion resistance" refers to a case where a corrosion rate is 0.127 mm/y or less, and no pitting corrosion of 0.2 mm or greater has occurred in a postcorrosion-test test specimen, as determined by a corrosion test performed by immersing the test specimen in a test solution held in an autoclave, which is a 20 mass% NaCl aqueous solution (solution temperature: 230°C, atmosphere with 10-MPa CO₂ gas), for an immersion time of 336 hours.
As used herein, the expression "excellent corrosion resistance in acid environments" refers to a case where a corrosion rate in a case where a Cr content is 23.0% or less is 170 mm/y or less, and the corrosion rate in a case where the Cr content is greater than 23.0% is 95 mm/y or less, as determined by a corrosion test performed by immersing a test specimen in a 15 mass% hydrochloric acid solution heated at 80°C for an immersion time of 2 minutes.
As used herein, the expression "excellent corrosion resistance in CCS environments" refers to a case where a corrosion rate is 0.025 mm/y or less, and no pitting corrosion of 0.2 mm or greater has occurred, as determined by a corrosion test performed by immersing a test specimen in a test solution held in an autoclave, which is a 15 mass% NaCl aqueous solution (solution temperature: 120°C, atmosphere with 40-MPa CO₂, 100 ppm O₂, 50 ppm SO₂, and 100 ppm NO₂), for an immersion time of 720 hours.
As used herein, the expression "excellent low-temperature toughness" refers to a case where the absorbed energy vE_-40 at a test temperature of -40°C is 100 J or greater, as determined by a Charpy impact test performed in accordance with the specifications of JIS Z 2242 (2018), using a V-notch test specimen (10 mm thick) that is cut such that a longitudinal direction of the test specimen corresponds to a pipe axis direction.

Solution to Problem

To achieve the above-described object, the present inventors diligently conducted studies regarding various factors that affect the corrosion resistance of stainless steels, in particular, the corrosion resistance in acid environments. Consequently, in a case where Sb was contained in a predetermined amount or more, in addition to Cr and Mo, excellent carbon dioxide gas corrosion resistance and excellent corrosion resistance in acid environments were achieved. Furthermore, in a case where Ni was contained in a predetermined amount or more, and Mo was inhibited from being excessively added, excellent low-temperature toughness was additionally achieved.
The present invention was completed based on this discovery and with further studies that were conducted. Specifically, a summary of the present invention is as follows.

[1] A duplex stainless steel having a chemical composition containing, in mass%,
- C: 0.002 to 0.03%,
- Si: 0.05 to 1.0%,
- Mn: 0.1 to 1.7%,
- P: 0.040% or less,
- S: 0.020% or less,
- Cr: 20.0 to 28.0%,
- Ni: 4.0 to 10.0%,
- Mo: 2.0 to 5.0%,
- Al: 0.001 to 0.05%,
- N: 0.06 to 0.35%,
- Sb: 0.001 to 1.000%, and
- O: 0.010% or less, with a balance of Fe and incidental impurities, wherein
- the duplex stainless steel includes microstructures containing an austenite phase in a volume fraction of 20 to 70% and a ferrite phase in a volume fraction of 30 to 80%, and
- the duplex stainless steel has a yield strength YS of 448 MPa or greater.
[2] The duplex stainless steel according to [1], wherein the chemical composition further contains, in mass%, one or more selected from Groups A to E, listed below:
- Group A: one or two selected from W: 0.02 to 1.5% and Cu: 0.1 to 2.0%,
- Group B: one or more selected from V: 0.20% or less, Nb: 0.20% or less, and Ti: 0.20% or less,
- Group C: one or two selected from Zr: 0.50% or less and B: 0.0100% or less,
- Group D: one or more selected from REM: 0.08% or less, Ca: 0.010% or less, Sn: 0.20% or less, and Mg: 0.0002 to 0.01%, and
- Group E: one or two selected from Ta: 0.01 to 0.15% and Co: 0.01 to 1.0%.
[3] A duplex stainless steel having the chemical composition and the microstructures according to [1] or [2] and having a yield strength YS of 758 MPa or greater.
[4] A stainless steel seamless pipe having a chemical composition containing, in mass%,
- C: 0.002 to 0.03%,
- Si: 0.05 to 1.0%,
- Mn: 0.1 to 1.7%,
- P: 0.040% or less,
- S: 0.020% or less,
- Cr: 20.0 to 28.0%,
- Ni: 4.0 to 10.0%,
- Mo: 2.0 to 5.0%,
- Al: 0.001 to 0.05%,
- N: 0.06 to 0.35%,
- Sb: 0.001 to 1.000%, and
- O: 0.010% or less, with a balance of Fe and incidental impurities, wherein
- the stainless steel seamless pipe includes microstructures containing an austenite phase in a volume fraction of 20 to 70% and a ferrite phase in a volume fraction of 30 to 80%, and
- the stainless steel seamless pipe has a yield strength YS of 448 MPa or greater.
[5] The stainless steel seamless pipe according to [4], wherein the chemical composition further contains, in mass%, one or more selected from Groups A to E, listed below:
- Group A: one or two selected from W: 0.02 to 1.5% and Cu: 0.1 to 2.0%,
- Group B: one or more selected from V: 0.20% or less, Nb: 0.20% or less, and Ti: 0.20% or less,
- Group C: one or two selected from Zr: 0.50% or less and B: 0.0100% or less,
- Group D: one or more selected from REM: 0.08% or less, Ca: 0.010% or less, Sn: 0.20% or less, and Mg: 0.0002 to 0.01%, and
- Group E: one or two selected from Ta: 0.01 to 0.15% and Co: 0.01 to 1.0%.
[6] A stainless steel seamless pipe having the chemical composition and the microstructures according to [4] or [5] and having a yield strength YS of 758 MPa or greater. Advantageous Effects of Invention

The present invention can provide a duplex stainless steel and a stainless steel seamless pipe that have high strength, particularly, a yield strength of 448 MPa (65 ksi) or greater, and have excellent corrosion resistance and excellent low-temperature toughness.

Description of Embodiments

The present invention will be described in detail below.
According to the present invention, a duplex stainless steel (steel) and a stainless steel seamless pipe (steel pipe) produced from the duplex stainless steel, serving as a material, have a chemical composition containing, in mass%, C: 0.002 to 0.03%, Si: 0.05 to 1.0%, Mn: 0.1 to 1.7%, P: 0.040% or less, S: 0.020% or less, Cr: 20.0 to 28.0%, Ni: 4.0 to 10.0%, Mo: 2.0 to 5.0%, Al: 0.001 to 0.05%, N: 0.06 to 0.35%, Sb: 0.001 to 1.000%, and O: 0.010% or less, with the balance being Fe and incidental impurities. The duplex stainless steel and the stainless steel seamless pipe include microstructures containing an austenite phase in a volume fraction of 20 to 70% and a ferrite phase in a volume fraction of 30 to 80% and have a yield strength YS of 448 MPa or greater.

Chemical Composition of Duplex Stainless Steel and Stainless Steel Seamless Pipe

Reasons for the limitation imposed on the ranges of the chemical composition of the duplex stainless steel and the stainless steel seamless pipe of the present invention will be described below. Note that "%" used in the context of the content of components means "mass%".

C: 0.002 to 0.03%

C is an element that has an effect of stabilizing the austenite phase, thereby improving strength and low-temperature toughness. A C content is specified to be 0.002% or greater to realize high strength, particularly, a yield strength YS of 65 ksi (448 MPa). The C content is preferably 0.005% or greater. The C content is more preferably 0.010% or greater. The C content is even more preferably 0.015% or greater. On the other hand, if the C content is greater than 0.03%, an excessive amount of carbides precipitate under heat treatment, and, consequently, corrosion resistance may be adversely affected. Accordingly, the C content is specified to be 0.03% or less. The C content is preferably 0.027% or less. The C content is more preferably 0.025% or less. The C content is even more preferably 0.023% or less and most preferably 0.020% or less.

Si: 0.05 to 1.0%

Si is an element that serves as a deoxidizing agent. To produce this effect, a Si content is specified to be 0.05% or greater. The Si content is preferably 0.1% or greater. The Si content is more preferably 0.2% or greater and even more preferably 0.3% or greater. On the other hand, if the Si content is greater than 1.0%, an excessive amount of intermetallic compounds precipitate under heat treatment, and, consequently, the corrosion resistance of the steel is degraded. Accordingly, the Si content is specified to be 1.0% or less. The Si content is preferably 0.7% or less. The Si content is more preferably 0.6% or less. The Si content is even more preferably 0.5% or less and most preferably 0.4% or less.

Mn: 0.1 to 1.7%

Similar to Si, described above, Mn is an element effective as a deoxidizing agent. Mn also improves hot workability by fixing S, which is incidentally present in steel, by forming a sulfide. These effects are produced when a Mn content is 0.1% or greater. Accordingly, the Mn content is specified to be 0.1% or greater. The Mn content is preferably 0.15% or greater and more preferably 0.2% or greater. The Mn content is even more preferably 0.25% or greater and most preferably 0.3% or greater. On the other hand, if the Mn content is greater than 1.7%, hot workability decreases, and in addition, corrosion resistance is adversely affected. Accordingly, the Mn content is specified to be 1.7% or less. The Mn content is preferably 1.6% or less. The Mn content is more preferably 1.5% or less, even more preferably 1.4% or less, and most preferably 1.3% or less.

P: 0.040% or less

P is an element that reduces the corrosion resistance of the duplex stainless steel. If an amount of P is greater than 0.040%, corrosion resistance significantly decreases. Accordingly, the P content is specified to be 0.040% or less. The P content is preferably 0.030% or less. The P content is more preferably 0.025% or less. The P content is even more preferably 0.020% or less. The P content is most preferably 0.015% or less. However, reducing the P content to less than 0.005% requires dephosphorization to be performed for an extended period of time in the process of producing molten steel, and, consequently, the cost of manufacturing the duplex stainless steel increases. Accordingly, the P content is preferably 0.005% or greater. The P content is more preferably 0.007% or greater and even more preferably 0.010% or greater.

S: 0.020% or less

S is an element that reduces hot workability associated with the process of manufacturing the duplex stainless steel. If an amount of S is greater than 0.020%, the manufacture of the duplex stainless steel is hindered. Accordingly, the amount of S is specified to be 0.020% or less. The S content is preferably 0.010% or less. The S content is more preferably 0.002% or less. The S content is even more preferably 0.001% or less. However, reducing the S content to less than 0.0003% requires desulfurization to be performed for an extended period of time in the process of producing molten steel, and, consequently, the cost of manufacturing the duplex stainless steel increases. Accordingly, the S content is preferably 0.0003% or greater. The S content is more preferably 0.0005% or greater and even more preferably 0.0007% or greater.

Cr: 20.0 to 28.0%

Cr is a basic component effective for maintaining corrosion resistance and improving strength. To produce these effects, a Cr content is specified to be 20.0% or greater. The Cr content is preferably 22.0% or greater and even more preferably 23.0% or greater, so as to achieve even higher strength. The Cr content is most preferably 23.5% or greater. On the other hand, if the Cr content is greater than 28.0%, a σ phase tends to precipitate, which degrades both corrosion resistance and toughness. Accordingly, the Cr content is specified to be 28.0% or less. The Cr content is more preferably 27.5% or less and even more preferably 27.0% or less. Most preferably, in terms of toughness, the Cr content is 25.8% or less.

Ni: 4.0 to 10.0%

Ni is an element that is contained to stabilize the austenite phase to obtain duplex microstructures. If a Ni content is less than 4.0%, the austenite phase becomes unstable, which results in an excessively large volume fraction of the ferrite phase. Accordingly, the Ni content is specified to be 4.0% or greater. The Ni content is preferably 4.5% or greater. The Ni content is more preferably 5.3% or greater. The Ni content is even more preferably 5.5% or greater. On the other hand, if the Ni content is greater than 10.0%, the austenite phase becomes a principal constituent, with the volume fraction of the austenite phase becoming excessively large. Furthermore, since Ni is an expensive element, an economic advantage is lost. Accordingly, the Ni content is specified to be 10.0% or less. The Ni content is preferably 8.0% or less. The Ni content is more preferably 7.5% or less. The Ni content is even more preferably 6.9% or less. The Ni content is most preferably 6.5% or less.

Mo: 2.0 to 5.0%

Mo is an element that acts to improve the corrosion resistance of the duplex stainless steel and, in particular, contributes to preventing pitting corrosion due to Cl^-. If a Mo content is less than 2.0%, the effect is not produced. Accordingly, the Mo content is specified to be 2.0% or greater. The Mo content is preferably 2.5% or greater. The Mo content is more preferably 3.0% or greater. The Mo content is even more preferably 3.3% or greater. On the other hand, if the Mo content is greater than 5.0%, a σ phase precipitates, which reduces toughness and corrosion resistance. Accordingly, the Mo content is specified to be 5.0% or less. The Mo content is preferably 4.5% or less. The Mo content is more preferably 4.0% or less. The Mo content is even more preferably 3.5.0% or less.

Al: 0.001 to 0.05%

Al is an element that serves as a deoxidizing agent in the process of producing molten steel from raw materials of the duplex stainless steel. If an Al content is less than 0.001%, the effect is not produced. Accordingly, the Al content is specified to be 0.001% or greater. The Al content is preferably 0.005% or greater. The Al content is more preferably 0.005% or greater. The Al content is even more preferably 0.01% or greater. On the other hand, if the Al content is greater than 0.05%, alumina-based inclusions tend to precipitate, which reduces hot workability associated with the process of manufacturing the duplex stainless steel and also reduces toughness. Accordingly, the Al content is specified to be 0.05% or less. The Al content is preferably 0.045% or less. The Al content is more preferably 0.039% or less. The Al content is even more preferably 0.035% or less and most preferably 0.03% or less.

N: 0.06 to 0.35%

N is known as an element that improves pitting corrosion resistance and contributes to solid solution strengthening in typical duplex stainless steels. N is actively added, and a N content is specified to be 0.06% or greater. The N content is preferably 0.07% or greater so that the properties sought by the present invention can be obtained. The N content is more preferably 0.08% or greater. The N content is even more preferably 0.10% or greater. The N content is most preferably 0.12% or greater. If N is contained in an excessive amount, however, nitrides are formed, which reduces toughness and corrosion resistance and also degrades hot workability. Accordingly, the N content is specified to be 0.35% or less. The N content is preferably 0.30% or less and more preferably 0.20% or less. The N content is even more preferably 0.17% or less. The N content is most preferably 0.14% or less.

Sb: 0.001 to 1.000%

Sb improves corrosion resistance, particularly, corrosion resistance in acid environments and is, therefore, an important element in this patent. Achieving desired corrosion resistance requires the presence of Sb in an amount of 0.001% or greater. Accordingly, in the present invention, an Sb content is specified to be 0.001% or greater. The Sb content is preferably 0.005% or greater so that pitting corrosion in CCS environments can be inhibited. On the other hand, even if Sb is contained in an amount greater than 1.000%, the effect no longer increases, and, therefore, the Sb content is 1.000% or less. The Sb content is preferably 0.500% or less, more preferably, 0.100% or less, even more preferably 0.070% or less, and most preferably less than 0.021%. Note that Sb also has an effect of improving fatigue resistance, hydrogen embrittlement resistance, hot workability, low-temperature toughness, machinability, and ductility, thereby increasing strength. Sb becomes concentrated on a surface of the steel immediately below scales that are formed during hot rolling. Furthermore, in acidizing environments, Sb is an insoluble element and is, therefore, believed to become concentrated on a surface of a test specimen. Accordingly, Sb can provide the effects even if it is present in a small amount, both in actual use environments in which the steel material is used and in simulated environments in which test specimens are evaluated. The Sb content is preferably 0.005% or greater so that even better corrosion resistance in CCS environments can be achieved. The Sb content is more preferably 0.010% or greater. The Sb content is even more preferably 0.012% or greater. Furthermore, as stated above, the Sb content is most preferably less than 0.021%. Sb becomes concentrated on a surface of test specimens and, therefore, has protective properties, and, accordingly, in the case of duplex stainless steels having an Sb content adjusted to be within the above-described range, it is possible to achieve the state in which a corrosion rate is 0.025 mm/y or less, and no pitting corrosion of 0.2 mm or greater has occurred, as determined by a corrosion test performed by immersing a test specimen in a test solution held in an autoclave, which is a 15 mass% NaCl aqueous solution (solution temperature: 120°C, atmosphere with 40-MPa CO₂, 100 ppm O₂, 50 ppm SO₂, and 100 ppm NO₂), for an immersion time of 720 hour.

O (oxygen): 0.010% or less

In steel, O (oxygen) is present as an oxide and adversely affects various properties. Accordingly, it is desirable to reduce the amount of O as much as possible. In particular, if an O content is greater than 0.010%, SSC resistance in low-temperature environments significantly decreases. Accordingly, the O content is specified to be 0.010% or less. The O content is preferably 0.007% or less. The O content is more preferably 0.004% or less. The O content is even more preferably 0.003% or less or 0.002% or less. Since excessive reduction results in an increase in the manufacturing cost, the O content is preferably 0.0005% or greater. The O content is more preferably 0.001% or greater. The O content is even more preferably 0.0015% or greater.
The basic components described above are contained, and the balance is Fe and incidental impurities. Note that in a steel according to an embodiment of the present invention, only the basic components described above and the balance may be contained, with the balance being Fe and incidental impurities.
Alternatively, in the present invention, not only the above-described basic components but also one or more groups selected from Groups A to E, listed below, may be contained as necessary:

Group A: one or two selected from W: 0.02 to 1.5% and Cu: 0.1 to 2.0%,
Group B: one or more selected from V: 0.20% or less, Nb: 0.20% or less, and Ti: 0.20% or less,
Group C: one or two selected from Zr: 0.50% or less and B: 0.0100% or less,
Group D: one or more selected from REM: 0.08% or less, Ca: 0.010% or less, Sn: 0.20% or less, and Mg: 0.0002 to 0.01%, and
Group E: one or two selected from Ta: 0.01 to 0.15% and Co: 0.01 to 1.0%.

Group A

W: 0.02 to 1.5%

W is an element that improves sulfide stress corrosion cracking resistance and sulfide stress cracking resistance. In the case where W is contained, a W content is 0.02% or greater so as to produce the effect. The W content is preferably 0.1% or greater. On the other hand, if W is contained in a large amount of greater than 1.5%, low-temperature toughness may decrease. Accordingly, in the case where W is contained, the W content is 1.5% or less. The W content is preferably 1.0% or less. The W content is more preferably 0.7% or less. The W content is even more preferably 0.3% or less.

Cu: 0.1 to 2.0%

Cu is an element that improves corrosion resistance and stabilizes the austenite phase. In the case where Cu is contained, a content of each of these elements is specified to be 0.1% or greater to produce the effects. The content is preferably 0.2% or greater. The content is more preferably 0.4% or greater and even more preferably 0.6% or greater. If Cu is contained in an excessive amount, however, hot workability is degraded. Accordingly, in the case where Cu is contained, the content is specified to be 2.0% or less. The content is preferably 1.5% or less. The content is more preferably 1.2% or less and even more preferably 1.0% or less. The content is most preferably 0.8% or less.

Group B

V: 0.20% or less

V is an element that improves the strength of steel through precipitation strengthening. A V content is preferably 0.02% or greater so as to produce the effect. The V content is more preferably 0.04% or greater. On the other hand, if V is contained in an amount greater than 0.20%, low-temperature toughness may decrease. Furthermore, if a large amount of V is contained, sulfide stress cracking resistance may decrease. Accordingly, in the case where V is contained, the V content is specified to be 0.20% or less. The V content is preferably 0.15% or less. The V content is more preferably 0.08% or less. The V content is even more preferably 0.06% or less. The V content is most preferably 0.04% or less.

Nb: 0.20% or less

Nb is an element that improves the strength of steel through precipitation strengthening. Nb also has an effect of refining grains, thereby improving low-temperature toughness and sulfide stress corrosion cracking resistance. A Nb content is preferably 0.02% or greater so as to produce the effects. The Nb content is more preferably 0.04% or greater. On the other hand, if Nb is contained in an amount greater than 0.20%, coarse precipitates and intermetallic compounds may precipitate, which may reduce low-temperature toughness. Furthermore, if a large amount of Nb is contained, sulfide stress cracking resistance may decrease. Accordingly, in the case where Nb is contained, the Nb content is specified to be 0.20% or less. The Nb content is preferably 0.15% or less and more preferably 0.10% or less. The Nb content is even more preferably 0.08% or less. The Nb content is most preferably 0.06% or less.

Ti: 0.20% or less

Ti is an element that refines grains, thereby improving low-temperature toughness and sulfide stress corrosion cracking resistance. Ti also has an effect of improving the strength of steel through precipitation strengthening. A Ti content is preferably 0.02% or greater so as to produce the effects. The Ti content is more preferably 0.04% or greater. On the other hand, if Ti is contained in an amount greater than 0.20%, low-temperature toughness may decrease. Furthermore, if a large amount of Ti is contained, sulfide stress cracking resistance may decrease. Accordingly, in the case where Ti is contained, the Ti content is specified to be 0.20% or less. The Ti content is preferably 0.15% or less and more preferably 0.12% or less. The Ti content is even more preferably 0.08% or less. The Ti content is most preferably 0.06% or less.

Group C

Zr: 0.50% or less

Zr and B are both useful as elements that contribute to increasing strength and may be selected as necessary and contained. Zr not only contributes to increasing strength, as just mentioned, but also contributes to improving sulfide stress corrosion cracking resistance. A Zr content is preferably 0.02% or greater so as to produce the effects. The Zr content is more preferably 0.05% or greater. On the other hand, if Zr is contained in an amount greater than 0.50%, low-temperature toughness may decrease. Accordingly, in the case where Zr is contained, the Zr content is specified to be 0.50% or less. The Zr content is preferably 0.40% or less and more preferably 0.30% or less. The Zr content is even more preferably 0.20% or less. The Zr content is most preferably 0.10% or less.

B: 0.0100% or less

B is useful as an element that contributes to increasing strength, as just mentioned, and also contributes to improving hot workability. A B content is preferably 0.0005% or greater so as to produce the effects. The B content is more preferably 0.0010% or greater. On the other hand, if B is contained in an amount greater than 0.0100%, low-temperature toughness and hot workability may decrease. Accordingly, in the case where B is contained, the B content is specified to be 0.0100% or less. The B content is preferably 0.0080% or less. The B content is more preferably 0.0060% or less. The B content is even more preferably 0.0050% or less and most preferably 0.0040% or less.

Group D

REM: 0.08% or less

REMs are useful as elements that contribute to improving sulfide stress corrosion cracking resistance, and, therefore, REMs may be contained as necessary. Preferably, one or more REMs are contained in an amount of 0.01% or greater so that the effect can be ensured. The REM content is more preferably 0.015% or greater. On the other hand, if one or more REMs are contained in an amount greater than 0.08%, low-temperature toughness and hot workability may decrease. Accordingly, in the case where one or more REMs are contained, the REM content is specified to be 0.08% or less. The REM content is preferably 0.06% or less and more preferably 0.04% or less. The REM content is even more preferably 0.03% or less. The REM content is most preferably 0.02% or less. As referred to in the present invention, REMs are scandium (Sc) (atomic number 21), yttrium (Y) (atomic number 39), and lanthanides ranging from lanthanum (La) (atomic number 57) to lutetium (Lu) (atomic number 71). As referred to in the present invention, an REM concentration is a total content of one or more elements selected from the REMs.

Ca: 0.010% or less

Ca is useful as an element that contributes to improving sulfide stress corrosion cracking resistance, and, therefore, Ca may be contained as necessary. Preferably, Ca is contained in an amount of 0.001% or greater so that the effect can be ensured. The Ca content is more preferably 0.0015% or greater. On the other hand, even if Ca is contained in an amount greater than 0.010%, the effect no longer increases, thus, an effect comparable to the content cannot be expected, and, therefore, an economic disadvantage may occur. Accordingly, in the case where Ca is contained, the Ca content is specified to be 0.010% or less. The Ca content is preferably 0.007% or less. The Ca content is more preferably 0.005% or less. The Ca content is even more preferably 0.004% or less. The Ca content is most preferably 0.003% or less.

Sn: 0.20% or less

Sn is useful as an element that contributes to improving corrosion resistance, and, therefore, Sn may be contained as necessary. Preferably, Sn is contained in an amount of 0.0002% or greater so that the effect can be ensured. The Sn content is more preferably 0.0005% or greater. On the other hand, even if Sn is contained in an amount greater than 0.20%, the effect no longer increases, thus, an effect comparable to the content cannot be expected, and, therefore, an economic disadvantage may occur. Accordingly, in the case where Sn is contained, the Sn content is specified to be 0.20% or less. The Sn content is more preferably 0.15% or less. The Sn content is even more preferably 0.10% or less and most preferably 0.07% or less.

Mg: 0.0002 to 0.01%

Mg is useful as an element that contributes to improving sulfide stress corrosion cracking resistance, and, therefore, Mg may be contained as necessary. Preferably, Mg is contained in an amount of 0.0002% or greater so that the effect can be ensured. The Mg content is more preferably 0.0005% or greater. On the other hand, even if Mg is contained in an amount greater than 0.01%, the effect no longer increases, thus, an effect comparable to the content cannot be expected, and, therefore, an economic disadvantage may occur. Accordingly, in the case where Mg is contained, the Mg content is specified to be 0.01% or less. The Mg content is preferably 0.007% or less and more preferably 0.006% or less. The Mg content is even more preferably 0.005% or less. The Mg content is most preferably 0.003% or less.

Group E

Ta: 0.01 to 0.15%

Ta is useful as an element that contributes to improving carbon dioxide gas corrosion resistance, sulfide stress cracking resistance, and sulfide stress corrosion cracking resistance, and, therefore, Ta may be contained as necessary. In the case where Ta is contained, an amount of the Ta is 0.01% or greater so that the effect can be ensured. The Ta content is preferably 0.02% or greater. On the other hand, even if Ta is contained in an amount greater than 0.15%, the effect no longer increases, and thus, an effect comparable to the content may not be expected. Accordingly, in the case where Ta is contained, the Ta content is specified to be 0.15% or less. The Ta content is preferably 0.13% or less. The Ta content is more preferably 0.10% or less. The Ta content is even more preferably 0.05% or less. The Ta content is most preferably 0.03% or less.

Co: 0.01 to 1.0%

Co is useful as an element that contributes to improving carbon dioxide gas corrosion resistance, sulfide stress cracking resistance, and sulfide stress corrosion cracking resistance, and, therefore, Co may be contained as necessary. In the case where Co is contained, an amount of the Co is 0.01% or greater so that the effect can be ensured. The Co content is preferably 0.02% or greater. On the other hand, even if Co is contained in an amount greater than 1.0%, the effect no longer increases, and thus, an effect comparable to the content may not be expected. Accordingly, in the case where Co is contained, the Co content is specified to be 1.0% or less. The Co content is preferably 0.7% or less. The Co content is more preferably 0.5% or less. The Co content is even more preferably 0.2% or less and most preferably 0.1% or less.

Microstructures of Duplex Stainless Steel

Microstructures Including Austenite Phase in Volume Fraction of 20 to 70% and Ferrite Phase in Volume Fraction of 30 to 80%

The duplex stainless steel of the present invention may include microstructures including at least an austenite phase and a ferrite phase or may include microstructures consisting of the austenite phase and the ferrite phase. The austenite phase has a volume fraction (%) of 20 to 70%. The ferrite phase has a volume fraction (%) of 30 to 80%. If the austenite phase is present in a volume fraction of less than 20%, low-temperature toughness, sulfide stress cracking resistance, and sulfide stress corrosion cracking resistance may be low. Accordingly, the volume fraction of the austenite phase is specified to be 20% or greater. The volume fraction of the austenite phase is preferably 30% or greater, more preferably 35% or greater, and even more preferably 40% or greater. If the volume fraction of the austenite phase is greater than 70%, strength may be low. The volume fraction of each of the phases can be controlled by adjusting the chemical composition of the steel and a temperature for a solution treatment. Specifically, the volume fraction of the austenite phase increases with an increase in the amounts of austenite-phase-forming elements (C, Mn, Ni, N, Cu, and Co) or with a decrease in the temperature for the solution treatment. Accordingly, the volume fraction of the austenite phase is specified to be 70% or less. The volume fraction of the austenite phase is preferably 67% or less, more preferably 63% or less, and even more preferably 60% or less. The volume fraction of the ferrite phase increases with an increase in the amounts of ferrite-phase-forming elements (Si, Cr, Mo, and W) or with an increase in the temperature for the solution treatment. If the volume fraction of the ferrite phase is less than 30%, corrosion resistance may be low. Accordingly, the volume fraction of the ferrite phase is specified to be 30% or greater. The volume fraction of the ferrite phase is preferably 33% or greater, more preferably 37% or greater, and even more preferably 40% or greater. If the volume fraction of the ferrite phase is greater than 80%, low-temperature toughness and corrosion resistance may be low. Accordingly, the volume fraction of the ferrite phase is specified to be 80% or less. The volume fraction of the ferrite phase is preferably 74% or less, more preferably 67% or less, and even more preferably 60% or less. The sum of the volume fractions of the ferrite phase and the austenite phase is specified to be 99.5% or greater. As will be described later in the Examples section, area fractions of the ferrite phase and the austenite phase are determined from obtained micrographs of the microstructures by using an image analyzer, and averages of the area fractions are calculated and used as the respective volume fractions (vol.%).
The remaining microstructures, other than those described above, may include a σ phase. An amount of a σ phase of up to 0.5% is permissible. The amount of the σ phase is preferably 0.3% or less and more preferably 0.1% or less. The lower limit thereof is not particularly limited, and the amount of the σ phase may be 0%.
In the duplex stainless steel, since the austenite phase and the ferrite phase have different chemical properties, one of the phases is preferentially dissolved if corrosion occurs. In a simulated acidizing environment, the ferrite phase is preferentially dissolved. Furthermore, when a duplex stainless steel is rolled in a rolling step, an austenite phase precipitates from a ferrite phase that is stable at high temperatures; consequently, the ferrite phase is distributed in the form of a network, and the austenite phase is distributed in the form of islands. With this division of the ferrite phase by the austenite phase, corrosion cannot propagate further when preferential dissolution occurs in the ferrite phase, and thus, corrosion resistance is improved. Accordingly, the distribution of the austenite phase in the form of islands in the ferrite phase is preferable.
The volume fraction of each of the phases can be measured as follows. First, a test specimen for microstructure observation is cut from the duplex stainless steel such that a cross section thereof serves as an observation surface (when the duplex stainless steel is a seamless steel pipe, a cross section in a pipe axis direction serves as the observation surface). The volume fractions of the ferrite phase and the austenite phase are determined by observing the observation surface with a scanning electron microscope (SEM). Specifically, the test specimen for microstructure observation is subjected to electrolytic etching in an aqueous KOH solution, and subsequently, images of the microstructures are acquired with a scanning electron microscope (1000×). Area fractions of the ferrite phase and the austenite phase are determined from the obtained micrographs of the microstructures by using an image analyzer, and averages of the area fractions are calculated and used as the respective volume fractions (vol.%). In the acquired images, the phase that is white after binarization is determined to be the austenite phase because the white color is the result of low susceptibility to corrosion, and the phase that is black after binarization is determined to be the ferrite phase because the black color is the result of susceptibility to corrosion. This method for observing the microstructures will also be described in detail later in the Examples section.
Now, preferred methods for manufacturing the duplex stainless steel and the stainless steel seamless pipe of the present invention will be described.
Preferably, a molten steel having the above-described chemical composition is produced by using a common steelmaking method, for example, with a converter, and the molten steel is solidified into a steel pipe material such as a billet by using a common method, such as a continuous casting method or an ingot casting-blooming method. The heating temperature for the steel pipe material before hot working is preferably 1100 °C or greater. The heating temperature is more preferably 1200°C or greater and even more preferably 1250°C or greater. Furthermore, the heating temperature is preferably 1350°C or less, more preferably 1330°C or less, and even more preferably 1300°C or less. In these cases, hot workability for the pipe making and low-temperature toughness of the final product can both be achieved. Next, the obtained steel pipe material is subjected to hot working that is a commonly known pipe making process, examples of which include an extrusion pipe making method, such as the Ugine-Sejournet method, and the Mannesmann pipe-making method. Accordingly, a seamless steel pipe having a desired size and the above-described composition is produced. After hot working, a cooling process may be performed. The cooling process (cooling step) need not be particularly limited. It is preferable that, after the hot working, the steel pipe be cooled to room temperature at a cooling rate similar to that of natural cooling, provided that the chemical composition is within the above-described range of the present invention.
The present invention also includes a method for manufacturing a duplex stainless steel. This method includes producing a molten steel having the above-described chemical composition by using a common steelmaking method, for example, with a converter, and then solidifying the molten steel into a steel pipe material such as a billet or a slab by using a common method, such as a continuous casting method or an ingot casting-blooming method; thereafter, this method, unlike the method described above, does not perform any pipe making process but performs common hot rolling after heating. Note that regarding a subsequent solution heat treatment and cold working that are to be performed under the following conditions, the conditions are also preferably used for the duplex stainless steel.

Solution Heat Treatment

Subsequently, preferably, a solution heat treatment is performed on the produced steel pipe. Specifically, the steel pipe is heated to a heating temperature of 1000°C or greater and subsequently cooled at an average cooling rate greater than or equal to that of natural cooling; more specifically, the steel pipe is cooled at an average cooling rate of 1°C/s or greater to a temperature of 300°C or less. Consequently, intermetallic compounds, carbides, nitrides, sulfides, and the like that precipitate during the pipe making or during the cooling after the pipe making can be dissolved, and, therefore, a seamless steel pipe with microstructures containing an austenite phase and a ferrite phase in desired amounts can be produced.
If the heating temperature for the solution heat treatment is less than 1000°C, desired high toughness cannot be ensured. Accordingly, the heating temperature for the solution heat treatment is preferably 1000°C or greater. The heating temperature for the solution heat treatment is more preferably 1020°C or greater. The heating temperature for the solution heat treatment is even more preferably 1030°C or greater and most preferably 1040°C or greater. Furthermore, the heating temperature for the solution heat treatment is preferably 1150°C or less so that the coarsening of the microstructures can be prevented. The heating temperature for the solution heat treatment is more preferably 1130°C or less. The heating temperature for the solution heat treatment is even more preferably 1100°C or less and most preferably 1080°C or less. In the present invention, a holding time associated with the heating temperature for the solution heat treatment is preferably 5 minutes or more so that the temperature in the material can become uniform. The holding time is more preferably 10 minutes or more, even more preferably 15 minutes or more, and most preferably 20 minutes or more. Furthermore, the holding time associated with the heating temperature for the solution heat treatment is preferably 210 minutes or less. The holding time is more preferably 180 minutes or less, even more preferably 120 minutes or less, and most preferably 60 minutes or less.
If the average cooling rate for the solution heat treatment is less than 1°C/s, an intermetallic compound, such as a σ phase or a χ phase, precipitates during the cooling, and, consequently, low-temperature toughness and corrosion resistance significantly decrease. Accordingly, the average cooling rate for the solution heat treatment is preferably 1°C/s or greater. The cooling rate for the cooling for the solution heat treatment is preferably 2°C/s or greater. The average cooling rate is more preferably 5°C/s or greater, even more preferably 10°C/s or greater, and most preferably 15°C/s or greater. The upper limit of the average cooling rate need not be particularly limited, and the average cooling rate is preferably 500°C/s or less. The average cooling rate is preferably 200°C/s or less, more preferably 100°C/s or less, and even more preferably 50°C/s or less. Preferably, the cooling is performed by water cooling.

Cold Working

Cold working for improving the yield strength of the material is performed. The cold working is carried out by cold drawing, cold rolling, or skew rolling that uses opposing rolls, to introduce strains to increase strength. Preferably, diameter reduction rolling or cold drawing is performed. A skew rolling mill used for the diameter reduction rolling may be a 2-roll skew rolling mill or a 3-roll skew rolling mill, each including barrel-shaped rolls. The diameter reduction rolling can be carried out by adjusting a skew angle, a cross angle, and a roll gap. The cold drawing may be carried out by cold pilgering. A temperature during the process may be a high temperature so that deformation resistance can be reduced. Specifically, it is desirable that the process temperature be within a range of 25 to 600°C, and that a temperature range of 460 to 490°C, in which stainless steel becomes brittle, be avoided for performing the process. That is, the process temperature is preferably 25°C or greater, more preferably 50°C or greater, even more preferably 75°C or greater, and most preferably 100°C or greater, and in all the cases, it is preferable that the process not be performed within the temperature range of 460 to 490°C. The process temperature is preferably 600°C or less, more preferably 400°C or less, and even more preferably 200°C or less, and in all the cases, it is preferable that the process not be performed within the temperature range of 460 to 490°C.

Pickling

Preferably, pickling is performed to remove scales formed on a surface of the steel pipe during the hot rolling or the solution heat treatment. In a case where the cold working is performed by cold drawing, it is desirable that pickling be performed before the cold working because the surface of the steel pipe needs to be lubricated. On the other hand, in a case where skew rolling is used for the cold working, it is desirable that pickling be performed after the cold working. This is because lubrication is not necessary, and in addition, performing cold working prior to pickling causes strains to be introduced to the surface of the steel pipe and, thus, causes scales to be mechanically broken, which makes it easy to remove scales by pickling. Regarding the conditions for the pickling, a mixed solution of hydrofluoric acid and nitric acid is preferably used, and a pickling temperature is preferably 80°C or less, more preferably 60°C or less, and even more preferably 40°C or less. Furthermore, the pickling temperature is preferably 10°C or greater, more preferably 20°C or greater, and even more preferably 30°C or greater.
As described, the duplex stainless steel and the stainless steel seamless pipe that are provided by the present invention are high-strength steel pipes having a yield strength of 448 MPa or greater and have excellent corrosion resistance and excellent low-temperature toughness. In particular, in the case where cold working is performed on the duplex stainless steel and the stainless steel seamless pipe, the resulting steel pipe has a yield strength of 758 MPa or greater, which is considered to be a higher level of strength. The stainless steel seamless pipe of the present invention can be used as a stainless steel seamless pipe for oil wells (high-strength stainless steel seamless pipe for oil wells).

EXAMPLES

The present invention will now be described in more detail based on Examples. Note that the present invention is not limited to the Examples below.
Steel pipe materials were cast from molten steels having the chemical composition shown in Table 1-1 and Table 1-2. Subsequently, the steel pipe materials were each heated and formed into a pipe by hot working that used a model seamless rolling mill, to provide seamless steel pipes having an outside diameter of 72 mm and a wall thickness of 11 mm, and then, the seamless steel pipes were naturally cooled. In this process, the heating temperature for the steel pipe materials before hot working was 1250 °C. Furthermore, as described above, after the steel pipe materials were cast and heated, the steel pipe materials were each subjected to hot rolling that used a hot rolling mill, to provide duplex stainless steels, and then, the duplex stainless steels were naturally cooled. In this process, the heating temperature for the steel pipe materials before hot rolling was 1250 °C, and the final temperature for hot rolling was 900°C or greater.
After the natural cooling, a solution heat treatment was performed with the temperature, soaking time, and cooling method shown in Table 2. The average cooling rate for the solution heat treatment was 7 to 80°C/s.
Furthermore, for the duplex stainless steels, cold working was performed in a cold rolling mill for work strengthening. The rolling reduction ratio is preferably 10 to 90%.
Test specimens for microstructure observation were cut from the seamless steel pipes that were finally obtained, and quantitative evaluation of the microstructures, a tensile test, a Charpy impact test, a corrosion test, a sulfide stress cracking resistance test (SSC resistance test), and a sulfide stress corrosion cracking resistance test (SCC resistance test) were conducted. Methods for the tests were as follows. The results obtained from the tests are shown in Table 3.

(1) Measurement of Volume Fraction (vol.%) of Each of Phases in Entire Microstructures of Steel Pipe

Test specimens for microstructure observation were cut from the seamless steel pipes, which were treated by the above-described heat treatment, such that a cross section in a pipe axis direction could serve as an observation surface. The volume fractions of the ferrite phase and the austenite phase were determined by observing the observation surface with an optical microscope. Specifically, the test specimens for microstructure observation were electrolytically etched in an aqueous KOH solution, and images of the microstructures were acquired with the optical microscope (magnification: 400×). Area fractions of the ferrite phase and the austenite phase were determined from the obtained micrographs of the microstructures by using an image analyzer, and averages of the area fractions were calculated and used as the respective volume fractions (vol.%). In the acquired images, the phase that was white after binarization was determined to be the austenite phase because the white color was the result of low susceptibility to corrosion, and the phase that was black after binarization was determined to be the ferrite phase because the black color was the result of susceptibility to corrosion.

(2) Tensile Test

[Evaluation of Tensile Properties]

JIS (Japanese Industrial Standards) No. 14A tensile test specimens (φ 6.0 mm) were cut from the seamless steel pipes after the solution heat treatment in the case where the seamless steel pipes were not subjected to cold working, and from the seamless steel pipes after the cold working in the case where the seamless steel pipes were subjected to cold working. A tensile test was conducted in accordance with the specifications of JIS Z 2241:2011 to determine the tensile properties (yield strength (YS) and tensile strength (TS)).

(3) Charpy Impact Test

V-notch test specimens (10 mm thick) were cut from after the solution heat treatment in the case where the seamless steel pipes were not subjected to cold working, and from a wall-thickness middle portion of the seamless steel pipes after the cold working in the case where the seamless steel pipes were subjected to cold working. The V-notch test specimens were cut in accordance with JIS Z 2242 (2018) such that a length of the test specimens corresponded to a circumferential direction. A Charpy impact test was conducted on the cut test specimens at a test temperature of -40°C to measure the absorbed energy vE_-40 (J). Three test specimens were cut from each of the steel pipes. The Charpy impact test was conducted on these test specimens, and the arithmetic mean of the resulting values is shown in Table 2.

(4) Corrosion Test (carbon dioxide gas corrosion resistance test)

Corrosion test specimens 3 mm in thickness, 30 mm in width, and 40 mm in length were prepared with a machining process, from after the solution heat treatment in the case where the seamless steel pipes were not subjected to cold working, and from the seamless steel pipes after the cold working in the case where the seamless steel pipes were subjected to cold working. A corrosion test was conducted on these test specimens to evaluate the carbon dioxide gas corrosion resistance. In the corrosion test, the test specimens were immersed in a test solution held in an autoclave, which was a 20% NaCl aqueous solution (solution temperature: 230°C, atmosphere with 10-MPa CO₂), for an immersion period of 14 days (336 hours). After the test, the mass of the test specimens was measured, and the corrosion rate was determined by calculating the reduction in the mass from the masses before and after the corrosion test. In addition, an observation was performed on the test specimens after the corrosion test with a 10× magnifying glass, to see whether pitting corrosion had occurred on the surface of the test specimens. The expression "pitting corrosion has occurred" refers to a case where pitting corrosion with a diameter of 0.2 mm or greater is present on the assumption that the pitting corrosion has a circular shape. In the present invention, in cases where the corrosion rate was 0.127 mm/y or less, and there was no pitting corrosion, a rating of "pass" was given.

(5) Sulfide Stress Cracking Resistance Test (SSC resistance test)

Round bar test specimens (diameter: 6.4 mmφ) were prepared in accordance with NACE TM0177 Method A, with a machining process, from the seamless steel pipes that were subjected to cold working, and an SSC resistance test was conducted on these test specimens. The SSC resistance test was conducted as follows. The test specimens were immersed in a test solution for an immersion period of 720 hours, with an applied stress of 90% of a yield stress being applied; the test solution was an aqueous solution with an adjusted pH of 3.5 and was prepared by adding acetic acid and Na acetate to a 20 mass% NaCl aqueous solution (solution temperature: 25°C, atmosphere with 0.03-MPa H₂S and 0.07-MPa CO₂). After the test, the test specimens were visually observed to see whether there were cracks. In addition, an observation was performed on the test specimens after the test with a 10× magnifying glass, to see whether pitting corrosion had occurred on the surface of the test specimens. In the present invention, in cases where, after the test, the test specimen did not have cracks or pitting corrosion, a rating of "pass" was given. In Table 3, the symbol "o" indicates that there were no cracks or pitting corrosion, and the symbol "×" indicates that there was a crack and/or pitting corrosion.

(6) Sulfide Stress Corrosion Cracking Resistance Test (SCC resistance test)

Four-point bending test specimens 3 mm in thickness, 15 mm in width, and 115 mm in length were prepared with a machining process, from the seamless steel pipes that were subjected to cold working. An SCC resistance test was conducted on these test specimens. The SCC resistance test was conducted as follows. The test specimens were immersed in a test solution held in an autoclave, which was a 10 mass% NaCl aqueous solution (solution temperature: 80°C, atmosphere with 35-kPa H₂S and 2-MPa CO₂), for an immersion period of 720 hours, with an applied stress of 100% of a yield stress being applied. After the test, the test specimens were visually observed to see whether there were cracks in the surface of the test specimens. In addition, an observation was performed on the test specimens after the test with a 10× magnifying glass, to see whether pitting corrosion had occurred on the surface of the test specimens. In the present invention, in cases where, after the test, the test specimen did not have cracks or pitting corrosion, a rating of "pass" was given. In Table 3, the symbol "o" indicates that there were no cracks or pitting corrosion, and the symbol "×" indicates that there was a crack and/or pitting corrosion.

(7) Corrosion Test in Acid Environment

Furthermore, a corrosion resistance for evaluating the corrosion resistance in acid environments was conducted as follows. Test specimens were immersed in a 15 mass% hydrochloric acid solution heated at 80°C, for an immersion time of 2 minutes. After the test, the weight of the test specimens was measured, and the corrosion rate was determined by calculating the reduction in the weight from the weights before and after the corrosion test. In cases where the Cr content was 23.0% or less, test specimens with a corrosion rate of 170 mm/y or less were given a rating of "pass", and test specimens with a corrosion rate of greater than 170 mm/y were given a rating of "fail". In cases where the Cr content was greater than 23.0%, test specimens with a corrosion rate of 95 mm/y or less were given a rating of "pass", and test specimens with a corrosion rate of greater than 95 mm/y were given a rating of "fail".

(8) Corrosion Test in CCS Environment

Furthermore, a corrosion test in a CCS environment was conducted as follows, with the same test specimens as those used in the carbon dioxide gas corrosion resistance test. The test specimens were immersed in a test solution held in an autoclave, which was a 15% NaCl aqueous solution (solution temperature: 120°C, atmosphere with 40-MPa CO₂, 100 ppm O₂, 50 ppm SO₂, and 100 ppm NO₂), for an immersion period of 30 days (720 hours). After the test, the mass of the test specimens was measured, and the corrosion rate was determined by calculating the reduction in the mass from the masses before and after the corrosion test. In addition, an observation was performed on the test specimens after the corrosion test with a 10× magnifying glass, to see whether pitting corrosion had occurred on the surface of the test specimens. Note that "pitting corrosion is present" refers to a case where pitting corrosion with a diameter of 0.2 mm or greater is present on the assumption that the pitting corrosion has a circular shape. In the present invention, in cases where the corrosion rate was 0.025 mm/y or less, and there was no pitting corrosion, a rating of "pass" was given. In cases where only pitting corrosion with a diameter of less than 0.2 mm was observed, it was determined that pitting corrosion did not occur.

[Table 1-1]

Steel Grade No.	Chemical Composition (mass%)																									Classification
Steel Grade No.	C	Si	Mn	P	S	Cr	Ni	Mo	Al	N	Sb	O	W	Cu	V	Nb	Ti	Zr	B	REM	Ca	Sn	Mg	Ta	Co	Classification
A	0.0202	0.47	1.47	0.019	0.0009	22.35	6.43	3.21	0.025	0.0781	0.028	0.0048	0	0	0.007	0.021	0	0.03	0.0030	0	0.0030	0	0.0050	0.060	0	Conforming Steel
B	0.0201	0.49	1.48	0.017	0.0009	22.36	5.38	3.21	0.016	0.1404	0.005	0.0045	0	0	0.007	0.020	0.10	0	0.0024	0	0.0017	0	0	0	0	Conforming Steel
C	0.0207	0.47	1.48	0.017	0.0011	22.36	4.56	3.21	0.019	0.1977	0.028	0.0045	0	0	0.008	0.019	0.04	0	0.0026	0	0.0030	0	0	0	0	Conforming Steel
D	0.0211	0.47	1.49	0.017	0.0009	22.41	5.38	3.19	0.023	0.1399	0.028	0.0035	0	0	0.007	0.018	0.04	0	0.0025	0	0.0061	0	0.0040	0	0	Conforming Steel
E	0.0190	0.48	1.47	0.017	0.0009	22.16	5.39	3.20	0.016	0.1408	0.018	0.0047	0	0	0.007	0.001	0	0	0.0028	0	0.0030	0	0	0	0	Conforming Steel
F	0.0196	0.51	1.45	0.017	0.0010	22.40	5.42	3.22	0.015	0.1400	0.014	0.0042	0	0	0.007	0.012	0	0	0.0044	0	0.0015	0	0	0	0	Conforming Steel
G	0.0192	0.46	1.48	0.016	0.0009	22.14	5.31	3.18	0.018	0.1403	0.013	0.0041	0	0	0.007	0.030	0	0	0.0026	0	0.0030	0	0	0	0	Conforming Steel
H	0.0212	0.47	1.47	0.017	0.0009	22.32	4.46	3.20	0.018	0.1415	0.027	0.0051	0	0	0.008	0.020	0	0	0.0027	0	0.0025	0	0	0	0.490	Conforming Steel
I	0.0180	0.46	0.79	0.019	0.0008	24.95	7.49	3.22	0.024	0.0999	0.025	0.0062	0.29	0.52	0.009	0.020	0	0	0.0027	0	0.0023	0	0	0	0	Conforming Steel
J	0.0215	0.49	0.77	0.018	0.0007	25.09	6.82	3.19	0.015	0.1521	0.012	0.0053	0.29	0.52	0.010	0.018	0.07	0	0.0034	0	0.0026	0	0.0020	0	0.560	Conforming Steel
K	0.0191	0.47	0.80	0.018	0.0009	25.04	6.50	3.20	0.014	0.2069	0.023	0.0063	0.29	0.52	0.010	0.022	0	0	0.0028	0	0.0035	0	0	0.050	0.040	Conforming Steel
L	0.0204	0.28	0.78	0.018	0.0010	24.87	6.91	3.69	0.021	0.2481	0.024	0.0065	0	0	0.010	0.019	0	0	0.0030	0	0.0030	0	0	0.110	0	Conforming Steel
M	0.0217	0.29	0.78	0.017	0.0012	24.83	6.60	3.67	0.014	0.2920	0.021	0.0068	0	0	0.013	0.019	0	0	0.0027	0	0.0033	0.100	0	0.060	0	Conforming Steel
N	0.0230	0.28	0.78	0.017	0.0013	24.64	6.19	3.65	0.015	0.3216	0.027	0.0044	0	0	0.013	0.019	0	0	0.0028	0	0.0024	0	0	0	0	Conforming Steel
O	0.0197	0.45	1.67	0.024	0.0010	22.01	5.32	3.08	0.018	0.1747	0.028	0.0036	0.05	0.26	0.064	0.024	0	0	0.0021	0	0.0006	0	0	0	0	Conforming Steel
P	0.0185	0.32	0.50	0.021	0.0010	25.27	6.95	3.64	0.010	0.2713	0.030	0.0052	0.50	0.52	0.102	0.004	0	0	0.0020	0	0.0006	0	0.0050	0.040	0	Conforming Steel
Q	0.0150	0.49	1.47	0.028	0.0004	22.40	5.85	3.10	0.002	0.1720	0.017	0.0066	0.04	0.26	0.071	0.015	0.01	0	0.0030	0	0.0012	0.040	0	0	0.760	Conforming Steel
R	0.0140	0.52	0.63	0.029	0.0006	25.10	6.50	3.12	0.006	0.1440	0.018	0.0086	0.14	0.21	0.078	0.012	0	0	0.0024	0	0.0021	0	0	0	0	Conforming Steel
S	0.0100	0.50	1.13	0.023	0.0004	22.50	6.28	3.28	0.004	0.1700	0.034	0.0090	0.02	0.07	0.097	0.007	0	0	0	0.0010	0.0031	0	0	0	0	Conforming Steel
T	0.0200	0.38	1.08	0.023	0.0006	25.80	6.55	3.06	0.002	0.2050	0.007	0.0092	0.03	0.12	0.086	0.020	0	0.03	0.0024	0.0020	0.0018	0	0	0	0	Conforming Steel
U	0.0202	0.39	0.78	0.016	0.0008	25.18	6.42	2.92	0.010	0.1725	0.027	0.0050	0.21	0.49	0	0	0	0	0.0005	0	0	0.130	0	0	0	Conforming Steel
V	0.0206	0.39	0.79	0.016	0.0009	24.94	6.49	2.94	0.010	0.1712	0.009	0.0046	0.21	0.50	0	0	0	0	0.0028	0	0	0	0	0	0	Conforming Steel
W	0.0199	0.39	0.20	0.016	0.0009	25.03	6.49	2.94	0.010	0.1747	0.020	0.0044	0.21	0.50	0	0	0	0	0.0003	0	0	0	0	0	0	Conforming Steel
X	0.0243	0.12	0.79	0.016	0.0007	25.20	6.46	2.95	0.010	0.1779	0.024	0.0048	0.21	0.50	0	0	0	0	0.0004	0	0	0	0	0.110	0	Conforming Steel
Y	0.0213	0.12	0.19	0.017	0.0008	25.11	6.47	2.94	0.010	0.1692	0.010	0.0039	0.20	0.50	0	0	0.05	0	0.0003	0	0	0.050	0	0.060	0	Conforming Steel
Z	0.0225	0.11	0.19	0.016	0.0008	24.87	6.51	2.98	0.010	0.1701	0.010	0.0048	0.21	0.50	0	0	0	0	0.0029	0	0	0	0	0	0	Conforming Steel

Underline: outside the scope of the invention

[Table 1-2]

Steel Grade No.	Chemical Composition (mass%)																									Classification
Steel Grade No.	C	Si	Mn	P	S	Cr	Ni	Mo	Al	N	Sb	O	W	Cu	V	Nb	Ti	Zr	B	REM	Ca	Sn	Mg	Ta	Co	Classification
AA	0.0211	0.50	1.49	0.016	0.0008	22.42	5.46	3.34	0.024	0.1085	0	0.0042	0.39	0	0.010	0	0	0	0.0029	0	0.0024	0	0	0	0	Comparative Steel
AB	0.0186	0.49	1.51	0.015	0.0008	22.51	5.45	3.33	0.040	0.1298	0.018	0.0033	0	0.60	0.008	0	0	0.04	0.0064	0.0730	0	0	0	0	0	Conforming Steel
AC	0.0198	0.49	1.50	0.017	0.0009	22.38	5.46	3.38	0.019	0.1336	0	0.0052	0	1.09	0.008	0	0	0	0.0028	0	0	0.020	0	0	0	Comparative Steel
AD	0.0207	0.49	1.52	0.018	0.0007	22.44	5.44	3.30	0.022	0.1354	0.019	0.0018	0	1.09	0.008	0	0	0	0.0063	0	0	0	0	0.060	0	Conforming Steel
AE	0.0199	0.49	1.51	0.016	0.0007	22.32	5.44	3.31	0.022	0.1350	0	0.0028	0	1.09	0.008	0	0	0	0.0084	0.0635	0	0.011	0	0	0	Comparative Steel
AF	0.0184	0.52	1.51	0.015	0.0008	22.41	5.46	3.42	0.026	0.1270	0.009	0.0029	0	0	0.010	0	0.01	0.02	0.0029	0.0220	0	0	0	0	0	Conforming Steel
AG	0.0203	0.53	1.57	0.017	0.0009	23.58	5.65	3.60	0.030	0.1349	0	0.0018	0	0	0.010	0	0	0	0.0027	0	0	0	0	0	0	Comparative Steel
AH	0.0203	0.53	1.57	0.017	0.0009	23.58	5.65	3.60	0.030	0.1349	0	0.0018	0	0	0.010	0	0	0	0.0027	0	0	0	0	0	0	Comparative Steel
AI	0.0198	0.51	0.80	0.016	0.0008	25.45	7.44	3.26	0.027	0.1207	0	0.0045	0.29	0.59	0.009	0	0	0	0.0027	0	0	0	0	0	0	Comparative Steel
AJ	0.0198	0.51	0.80	0.016	0.0008	25.45	7.44	3.26	0.027	0.1207	0	0.0045	0.29	0.59	0.009	0	0	0	0.0027	0	0	0	0	0	0	Comparative Steel
AK	0.0220	0.50	0.82	0.018	0.0007	25.36	7.49	3.28	0.042	0.1136	0	0.0046	0.27	0.61	0.009	0	0	0	0.0059	0.0145	0	0.021	0	0	0	Comparative Steel
AL	0.0188	0.49	0.82	0.017	0.0007	25.36	7.48	3.25	0.027	0.1167	0	0.0044	0.26	1.09	0.008	0	0	0	0.0062	0	0	0.008	0	0	0	Comparative Steel
AM	0.0211	0.47	0.82	0.017	0.0009	25.37	7.51	3.27	0.021	0.1132	0.009	0.0047	0.28	1.09	0.008	0	0	0	0.0093	0	0	0	0	0	0	Conforming Steel
AN	0.0190	0.50	0.83	0.018	0.0007	25.24	7.49	3.28	0.034	0.1176	0.016	0.0028	0.28	1.09	0.008	0	0.02	0	0.0094	0.0220	0	0	0.0070	0	0.050	Conforming Steel
AO	0.0175	0.48	0.81	0.016	0.0010	25.20	7.47	3.22	0.027	0.1246	0.016	0.0048	0.28	1.59	0.007	0	0	0	0.0030	0.0130	0	0	0	0	0	Conforming Steel
AP	0.0193	0.49	0.82	0.018	0.0011	25.47	7.52	3.26	0.020	0.1123	0.010	0.0046	0.28	1.60	0.007	0	0.05	0.02	0.0058	0.0125	0	0	0	0	0	Conforming Steel
AQ	0.0168	0.50	0.77	0.015	0.0009	22.15	5.44	3.15	0.020	0.1702	0	0.0025	0	0	0.008	0	0	0	0.0032	0	0	0	0.0030	0.082	0.030	Comparative Steel
AR	0.0156	0.51	1.51	0.026	0.0008	22.28	5.45	3.41	0.023	0.1766	0	0.0049	0	0	0.010	0	0	0	0.0028	0	0	0	0	0.120	0	Comparative Steel
AS	0.0155	0.53	1.58	0.027	0.0008	23.42	5.66	3.39	0.031	0.0999	0	0.0020	0	0	0.010	0	0	0	0.0034	0	0	0	0	0.074	0.100	Comparative Steel
AT	0.0150	0.51	0.78	0.015	0.0008	22.34	5.43	3.40	0.024	0.0986	0	0.0035	0	0	0.009	0	0	0	0.0028	0	0	0	0.0030	0.123	0.040	Comparative Steel
AU	0.0155	0.50	0.76	0.016	0.0010	22.20	5.49	3.17	0.021	0.0944	0	0.0046	0	0	0.008	0	0	0	0.0035	0	0	0	0	0.008	0.070	Comparative Steel
AV	0.0160	0.50	1.48	0.026	0.0009	22.35	5.86	3.14	0.023	0.1763	0	0.0032	0.03	0.31	0.075	0.016	0	0	0.0031	0	0.0008	0	0.0050	0	0	Comparative Steel
AW	0.0202	0.47	1.47	0.019	0.0009	21.35	7.93	2.71	0.025	0.0541	0.030	0.0048	0	0	0.007	0.021	0	0.03	0.0030	0	0.0030	0	0.0050	0.060	0	Comparative Steel
AX	0.0142	0.57	0.77	0.017	0.0009	23.12	4.46	3.50	0.018	0.0615	0.030	0.0051	0.20	0	0.008	0.020	0	0	0.0027	0	0.0025	0	0	0	0.490	Conforming Steel
AY	0.0230	0.28	0.78	0.017	0.0013	23.94	6.29	2.95	0.015	0.3216	0.030	0.0044	0	0	0.013	0.019	0	0	0.0028	0	0.0024	0	0	0	0	Conforming Steel
AZ	0.0260	0.17	0.17	0.034	0.0018	21.40	4.87	2.97	0.025	0.0635	0.009	0.0041	0	0.49	0	0	0	0	0	0	0	0	0	0	0	Conforming Steel
BA	0.0260	0.18	0.16	0.035	0.0018	21.39	4.86	2.97	0.025	0.0645	0.009	0.0051	0	0	0	0	0	0	0	0	0	0	0	0	0	Conforming Steel

Underline: outside the scope of the invention

[Table 2]

Steel Pipe No.	Steel Grade No.	Heat Treatment			Cold Working
		Solution Treatment
		Heating Temperature (°C)	Soaking Time (min)	Cooling
1	A	1050	85	Water Cooling	Yes
2	B	1060	90	Water Cooling	Yes
3	C	1045	70	Water Cooling	Yes
4	D	1045	50	Water Cooling	No
5	E	1065	120	Water Cooling	Yes
6	F	1065	85	Water Cooling	No
7	G	1075	55	Water Cooling	Yes
8	H	1070	45	Water Cooling	No
9	I	1070	45	Water Cooling	Yes
10	J	1070	55	Water Cooling	Yes
11	K	1075	90	Water Cooling	Yes
12	L	1065	85	Water Cooling	Yes
13	M	1060	85	Water Cooling	Yes
14	N	1055	95	Water Cooling	Yes
15	O	1040	60	Water Cooling	Yes
16	P	1080	75	Water Cooling	Yes
17	Q	1070	80	Water Cooling	Yes
18	R	1080	55	Water Cooling	Yes
19	S	1040	45	Water Cooling	No
20	T	1060	115	Water Cooling	No
21	U	1070	120	Water Cooling	No
22	V	1055	40	Water Cooling	No
23	W	1080	95	Water Cooling	No
24	X	1080	35	Water Cooling	No
25	Y	1050	115	Water Cooling	No
26	Z	1030	120	Water Cooling	No
27	AA	1065	35	Water Cooling	No
28	AB	1080	75	Water Cooling	No
29	AC	1040	80	Water Cooling	No
30	AD	1080	50	Water Cooling	No
31	AE	1040	65	Water Cooling	No
32	AF	1060	105	Water Cooling	No
33	AG	1065	30	Water Cooling	No
34	AH	1065	100	Water Cooling	No
35	AI	1040	120	Water Cooling	No
36	AJ	1050	50	Water Cooling	No
37	AK	1065	55	Water Cooling	No
38	AL	1070	70	Water Cooling	No
39	AM	1055	120	Water Cooling	No
40	AN	1035	55	Water Cooling	No
41	AO	1060	45	Water Cooling	No
42	AP	1070	65	Water Cooling	No
43	AQ	1075	70	Water Cooling	No
44	AR	1060	65	Water Cooling	No
45	AS	1030	35	Water Cooling	No
46	AT	1050	35	Water Cooling	No
47	AU	1065	100	Water Cooling	No
48	AV	1030	100	Water Cooling	No
49	AW	1050	30	Water Cooling	No
50	AX	1050	30	Water Cooling	No
51	AY	1050	30	Water Cooling	No
52	AZ	1050	30	Water Cooling	Yes
53	BA	1050	30	Water Cooling	Yes

Underline: outside the scope of the present invention

[Table 3]

Steel Pipe No.	Steel Grade No.	Microstructures		Tensile Properties		Low-Temperature Toughness	Carbon Dioxide Gas Corrosion Resistance Test		SSC Resistance Test	SCC Resistance Test	Acid Environment	CCS-Simulated Corrosion Test			Notes
		Austenite	Ferrite	Yield Strength YS	Tensile Strength TS	Charpy Absorbed Energy at -40°C	Corrosion Rate	Pitting Corrosion			Corrosion Rate	Corrosion Rate	Pitting Corrosion of 0.2 mm or Greater	Pitting Corrosion of 0.1 mm or Greater and Less Than 0.2 mm
		(Area %)	(Area %)	(MPa)	(MPa)	(J)	(mm/v)				(mmlv)	(mm/v)
1	A	54	46	806	849	335	0.001	No	○	○	167	0.001	No	Yes	Invention Example
2	B	50	50	846	902	321	0.003	No	○	○	163	0.001	No	No	Invention Example
3	C	49	51	850	916	311	0.014	No	○	○	165	0.002	No	Yes	Invention Example
4	D	50	50	548	727	321	0.002	No	-	-	167	0.001	No	Yes	Invention Example
5	E	51	49	805	866	324	0.007	No	○	○	167	0.002	No	No	Invention Example
6	F	50	50	508	704	306	0.001	No	-	-	166	0.001	No	No	Invention Example
7	G	51	49	815	870	324	0.009	No	○	○	164	0.001	No	No	Invention Example
8	H	43	57	555	720	280	0.016	No	-	-	167	0.001	No	Yes	Invention Example
9	I	52	48	858	888	301	0.001	No	○	○	90	0.001	No	Yes	Invention Example
10	J	51	49	838	905	289	0.001	No	○	○	87	0.001	No	No	Invention Example
11	K	53	47	850	908	306	0.001	No	○	○	88	0.002	No	Yes	Invention Example
12	L	57	43	845	929	356	0.001	No	○	○	90	0.002	No	Yes	Invention Example
13	M	59	41	839	939	367	0.001	No	○	○	89	0.001	No	No	Invention Example
14	N	59	41	851	949	370	0.001	No	○	○	88	0.001	No	Yes	Invention Example
15	O	57	43	822	929	346	0.009	No	○	○	166	0.001	No	Yes	Invention Example
16	P	59	41	871	966	343	0.001	No	○	○	87	0.002	No	Yes	Invention Example
17	Q	58	42	922	979	346	0.001	No	○	○	165	0.002	No	No	Invention Example
18	R	45	55	893	956	281	0.001	No	○	○	87	0.002	No	No	Invention Example
19	S	58	42	507	762	375	0.001	No	-	-	163	0.002	No	Yes	Invention Example
20	T	49	51	567	800	306	0.001	No	-	-	90	0.002	No	No	Invention Example
21	U	51	49	560	749	313	0.001	No	-	-	89	0.002	No	Yes	Invention Example
22	V	52	48	559	748	323	0.001	No	-	-	89	0.002	No	No	Invention Example
23	W	52	48	555	745	319	0.001	No	-	-	89	0.002	No	No	Invention Example
24	X	53	47	546	738	326	0.001	No	-	-	87	0.001	No	Yes	Invention Example
25	Y	52	48	543	736	321	0.001	No	-	-	87	0.001	No	No	Invention Example
26	Z	54	46	548	740	330	0.001	No	-	-	89	0.001	No	No	Invention Example
27	AA	46	54	536	710	205	0.001	No	-	-	182	0.002	Yes	Yes	Comparative Example
28	AB	52	48	513	699	103	0.001	No	-	-	167	0.001	No	No	Invention Example
29	AC	56	44	526	726	284	0.001	No	-	-	174	0.002	Yes	Yes	Comparative Example
30	AD	56	44	515	718	170	0.001	No	-	-	166	0.002	No	No	Invention Example
31	AE	56	44	530	713	77	0.001	No	-	-	174	0.001	Yes	Yes	Comparative Example
32	AF	48	52	520	709	272	0.001	No	-	-	163	0.001	No	No	Invention Example
33	AG	44	56	521	744	282	0.001	No	-	-	96	0.001	Yes	Yes	Comparative Example
34	AH	44	56	530	738	282	0.001	No	-	-	96	0.002	Yes	Yes	Comparative Example
35	AI	51	49	538	756	292	0.001	No	-	-	103	0.001	Yes	Yes	Comparative Example
36	AJ	51	49	550	752	292	0.001	No	-	-	102	0.001	Yes	Yes	Comparative Example
37	AK	52	48	558	738	195	0.001	No	-	-	104	0.001	Yes	Yes	Comparative Example
38	AL	55	45	560	746	209	0.001	No	-	-	107	0.001	Yes	Yes	Comparative Example
39	AM	55	45	560	746	172	0.001	No	-	-	88	0.002	No	No	Invention Example
40	AN	55	45	555	744	176	0.001	No	-	-	90	0.002	No	No	Invention Example
41	AO	59	41	571	767	197	0.001	No	-	-	87	0.002	No	No	Invention Example
42	AP	57	43	585	767	152	0.001	No	-	-	88	0.002	No	No	Invention Example
43	AQ	54	46	520	726	335	0.007	No	-	-	175	0.002	Yes	Yes	Comparative Example
44	AR	53	47	529	742	332	0.001	No	-	-	177	0.002	Yes	Yes	Comparative Example
45	AS	42	58	554	717	267	0.001	No	-	-	176	0.001	Yes	Yes	Comparative Example
46	AT	45	55	524	688	288	0.001	No	-	-	175	0.001	Yes	Yes	Comparative Example
47	AU	47	53	512	679	295	0.005	No	-	-	173	0.002	Yes	Yes	Comparative Example
48	AV	59	41	514	733	347	0.001	No	-	-	177	0.002	Yes	Yes	Comparative Example
49	AW	19	81	439	668	352	0.001	No	-	-	164	0.001	No	Yes	Comparative Example
50	AX	25	75	599	787	195	0.001	No	-	-	87	0.002	No	Yes	Invention Example
51	AY	65	35	527	746	314	0.001	No	-	-	87	0.002	No	Yes	Invention Example
52	AZ	50	50	776	912	309	0.026	No	×	×	160	0.003	No	No	Invention Example
53	BA	51	49	794	929	292	0.027	No	×	×	150	0.002	No	No	Invention Example

Underline: outside the scope of the present invention or outside the scope of the property sought by the present invention
·In the CCS-simulated corrosion test, "Yes" regarding the "pitting corrosion of 0.1 mm or greater and less than 0.2 mm" means that pitting corrosion of 0.1 mm or greater and less than 0.2 mm was observed.

In all of Invention Examples, the obtained stainless steel seamless pipes had high strength, particularly, a yield strength of 448 MPa or greater, had high toughness, particularly, an absorbed energy vE_-40 of ≥100 J as determined by a Charpy impact test, had excellent corrosion resistance (carbon dioxide gas corrosion resistance) in corrosive environments containing CO₂ and Cl^- and having a high temperature of 230°C or greater, had excellent corrosion resistance in acid environments, and had excellent corrosion resistance in CCS environments. In contrast, Comparative Examples, which are outside the scope of the present invention, failed to achieve the high strength sought by the present invention, failed to achieve high toughness, failed to pass the carbon dioxide gas corrosion resistance test, failed to pass the corrosion test in acid environments, or failed to pass the CCS simulated corrosion test. Note that the duplex stainless steels obtained as described above had the same results as those of the stainless steel seamless pipes.

Claims

A duplex stainless steel comprising a chemical composition containing, in mass%,
C: 0.002 to 0.03%,

Si: 0.05 to 1.0%,

Mn: 0.1 to 1.7%,

P: 0.040% or less,

S: 0.020% or less,

Cr: 20.0 to 28.0%,

Ni: 4.0 to 10.0%,

Mo: 2.0 to 5.0%,

Al: 0.001 to 0.05%,

N: 0.06 to 0.35%,

Sb: 0.001 to 1.000%, and

O: 0.010% or less, with a balance of Fe and incidental impurities, wherein

the duplex stainless steel comprises microstructures containing an austenite phase in a volume fraction of 20 to 70% and a ferrite phase in a volume fraction of 30 to 80%, and

the duplex stainless steel has a yield strength YS of 448 MPa or greater.
The duplex stainless steel according to Claim 1, wherein the chemical composition further contains, in mass%, one or more selected from Groups A to E, listed below:
Group A: one or two selected from W: 0.02 to 1.5% and Cu: 0.1 to 2.0%,

Group B: one or more selected from V: 0.20% or less, Nb: 0.20% or less, and Ti: 0.20% or less,

Group C: one or two selected from Zr: 0.50% or less and B: 0.0100% or less,

Group D: one or more selected from REM: 0.08% or less, Ca: 0.010% or less, Sn: 0.20% or less, and Mg: 0.0002 to 0.01%, and

Group E: one or two selected from Ta: 0.01 to 0.15% and Co: 0.01 to 1.0%.
A duplex stainless steel comprising the chemical composition and the microstructures according to Claim 1 or 2 and having a yield strength YS of 758 MPa or greater.
A stainless steel seamless pipe comprising a chemical composition containing, in mass%,
C: 0.002 to 0.03%,

Si: 0.05 to 1.0%,

Mn: 0.1 to 1.7%,

P: 0.040% or less,

S: 0.020% or less,

Cr: 20.0 to 28.0%,

Ni: 4.0 to 10.0%,

Mo: 2.0 to 5.0%,

Al: 0.001 to 0.05%,

N: 0.06 to 0.35%,

Sb: 0.001 to 1.000%, and

O: 0.010% or less, with a balance of Fe and incidental impurities, wherein

the stainless steel seamless pipe comprises microstructures containing an austenite phase in a volume fraction of 20 to 70% and a ferrite phase in a volume fraction of 30 to 80%, and

the stainless steel seamless pipe has a yield strength YS of 448 MPa or greater.
The stainless steel seamless pipe according to Claim 4, wherein the chemical composition further contains, in mass%, one or more selected from Groups A to E, listed below:
Group A: one or two selected from W: 0.02 to 1.5% and Cu: 0.1 to 2.0%,

Group B: one or more selected from V: 0.20% or less, Nb: 0.20% or less, and Ti: 0.20% or less,

Group C: one or two selected from Zr: 0.50% or less and B: 0.0100% or less,

Group D: one or more selected from REM: 0.08% or less, Ca: 0.010% or less, Sn: 0.20% or less, and Mg: 0.0002 to 0.01%, and

Group E: one or two selected from Ta: 0.01 to 0.15% and Co: 0.01 to 1.0%.
A stainless steel seamless pipe comprising the chemical composition and the microstructures according to Claim 4 or 5 and having a yield strength YS of 758 MPa or greater.