Duplex stainless steel and seamless stainless steel pipe

Abstract

The purpose of the present invention is to provide a duplex stainless steel and a seamless stainless steel pipe having high strength of yield strength of 448 MPa (65 ksi) or more, excellent corrosion resistance and excellent low-temperature toughness.　A duplex stainless steel having a component composition comprising, in mass %, C: 0.002-0.03%, Si: 0.05-1.0%, Mn: 0.1-1.7%, P: 0.040% or less, S: 0.020% or less, Cr: 20.0-28.0%, Ni: 4.0-10.0%, Mo: 2.0-5.0%, Al: 0.001-0.05%, N: 0.06-0.35%, Sb: 0.001-1.000% and O: 0.010% or less, with the balance being Fe and unavoidable impurities, the duplex stainless steel having a structure containing austenitic phase: 20-70% and ferrite phase: 30-80% in a volume ratio, and having a yield strength YS of 448 MPa or more.

Description

Duplex stainless steel and stainless steel seamless pipes

[0001] The present invention relates to duplex stainless steel and stainless steel seamless pipe suitable for use in oil and gas wells (hereinafter simply referred to as oil wells). ２ ), chloride ions (Cl － This invention relates to duplex stainless steels and stainless steel seamless pipes that have improved corrosion resistance in high-temperature, severely corrosive environments, including those containing uranium.

[0002] In recent years, in light of the expected depletion of energy resources in the near future, there has been active development of oil wells in severe corrosive environments that have not previously been considered, such as deep oil fields, environments containing carbon dioxide, and sour environments containing hydrogen sulfide. Oil well steel pipes used in such environments are required to have high strength and high corrosion resistance.

[0003] Conventionally, CO ２ and Cl － In oil and gas fields that are exposed to environments including the above, 13Cr martensitic stainless steel pipes have generally been used as oil well pipes for mining. However, in recent years, the development of oil wells that can be used at even higher temperatures (up to 230°C) has progressed, and the corrosion resistance of 13Cr martensitic stainless steel pipes has sometimes been insufficient. There is a demand for oil well pipes that have high corrosion resistance and can be used in such environments.

[0004] To meet this demand, duplex stainless steel is used for oil well pipes. Duplex stainless steel has excellent strength characteristics. However, in order to apply duplex stainless steel in severe corrosive environments containing large amounts of hydrogen sulfide, carbon dioxide, and chloride ions, such as deep oil wells, which have been developed in recent years, improved corrosion resistance is required.

[0005] In this regard, for example, Patent Document 1 discloses a duplex stainless steel having excellent corrosion resistance and a PREW value of 40 or more, which is achieved by controlling the contents of Cr, Mo, N, and W.

[0006] Furthermore, Patent Document 2 discloses a duplex stainless steel that is excellent in corrosion resistance and hot workability by controlling the contents of Cr, Mo, W, and N as well as the contents of B, Ta, etc.

[0007] In addition, in Patent Document 3, in order to reduce sulfide-based inclusions in steel that adversely affect hot workability and corrosion resistance, a CaO crucible and a CaO-CaF ２ -Al ２ O ３ This paper discloses a duplex stainless steel in which the S content is reduced to 3 ppm by weight or less by using slag of this type.

[0008] Furthermore, Patent Document 4 discloses a duplex stainless steel in which the total content of Ca and Mg and the content of S in the oxide-based inclusions are controlled, and the shape and density of the inclusions are further adjusted, as a technique for controlling oxide-based inclusions that can be the initiation points of pitting corrosion. Patent Document 4 also discloses a duplex stainless steel in which the size and number of the inclusions are controlled and the occurrence of localized corrosion is suppressed by optimally combining the slag basicity during reduction treatment, the killing temperature and time in the ladle, and the total processing ratio after casting, because even insoluble Al oxides that contain certain amounts of Ca, Mg, and S can be the initiation points of localized corrosion.

[0009] JP-A-5-132741 JP-A-8-170153 JP-A-3-291358 International Publication No. 2005 / 014872

[0010] As mentioned above, with the advancement of development of oil wells at even higher temperatures, there is a demand for oil well steel pipes to have high corrosion resistance. One of the methods for evaluating the corrosion resistance required when oil well steel pipes are used in oil wells at high temperatures up to 230°C is "a test using a 20 mass % NaCl aqueous solution (liquid temperature: 230°C, 10 MPa CO ２ When a test piece is immersed in a gas atmosphere for 336 hours, the corrosion rate may be 0.127 mm / y or less.

[0011] In addition to the problems mentioned above, when extracting oil, there are cases where the properties (mainly permeability) of the layer (reservoir) in which the oil is stored are poor, resulting in insufficient production, or where the expected production volume cannot be achieved due to clogging within the reservoir. Therefore, as one method for improving productivity, acidizing, in which an acid such as hydrochloric acid is injected into the reservoir, is sometimes carried out. In this case, the steel pipes used in oil wells are required to have excellent corrosion resistance in acid environments.

[0012] Furthermore, when used in cold regions, steel pipes used in oil wells are also required to have excellent low-temperature toughness. One of the methods for evaluating excellent low-temperature toughness is the "absorbed energy vE" test in a Charpy impact test at -40°C. －４０ In some cases, it may be required that the value be 100J or more.

[0013] In recent years, in order to realize a carbon-neutral society, CO generated from oil fields, gas fields, factories, etc. ２ Carbon Capture and Storage (CCS) technology, which injects and stores CO underground, is attracting attention. ２ Oil well tubular goods are used to inject CO ２ SOx, NOx, and O remaining in ２ The problem is that impurities such as these increase corrosiveness.

[0014] Patent Documents 1 to 4 disclose stainless steels with improved corrosion resistance. However, Patent Documents 1 to 4 sometimes fail to provide sufficient corrosion resistance at high temperatures, corrosion resistance in an acid environment, low-temperature toughness, and corrosion resistance in a CCS environment.

[0015] The present invention solves these problems of the prior art, and aims to provide a duplex stainless steel and a stainless steel seamless pipe that have high strength, with a yield strength of 448 MPa (65 ksi) or more, excellent corrosion resistance, and excellent low-temperature toughness.

[0016] The term "excellent corrosion resistance" as used herein refers to "excellent carbon dioxide corrosion resistance," "excellent corrosion resistance in an acid environment," and "excellent corrosion resistance in a CCS environment."

[0017] The term "excellent carbon dioxide gas corrosion resistance" used herein refers to a test liquid held in an autoclave: a 20 mass % NaCl aqueous solution (liquid temperature: 230°C, 10 MPa CO ２ The corrosion test is conducted by immersing a test piece in a gas atmosphere (aqueous solution) for 336 hours, and the corrosion rate is 0.127 mm / y or less, and no pitting corrosion of 0.2 mm or more occurs on the test piece after the corrosion test.

[0018] The term "excellent corrosion resistance in an acid environment" as used herein refers to a corrosion rate of 170 mm / y or less when the Cr content is 23.0% or less, or 95 mm / y or less when the Cr content exceeds 23.0%, when a test piece is immersed in a 15 mass % hydrochloric acid solution heated to 80°C for 2 minutes.

[0019] The term "excellent corrosion resistance in a CCS environment" refers to a test liquid held in an autoclave: a 15 mass % NaCl aqueous solution (liquid temperature: 120°C, CO ２ : 40 MPa, O ２ :100ppm, SO ２ : 50 ppm, NO ２ The corrosion rate is 0.025 mm / y or less and no pitting corrosion of 0.2 mm or more occurs when a test piece is immersed in an atmosphere containing 100 ppm of Cr, Cr, or 100 ppm of Cr, for 720 hours.

[0020] In addition, the term "excellent low-temperature toughness" as used herein refers to the absorbed energy vE at a test temperature of -40°C, obtained by subjecting a V-notch test piece (10 mm thick) to a Charpy impact test in accordance with the provisions of JIS Z 2242 (2018) so that the longitudinal direction of the test piece is the axial direction of the pipe. －４０ is 100 J or more.

[0021] In order to achieve the above-mentioned object, the present inventors have conducted extensive research into various factors that affect the corrosion resistance of stainless steel, particularly corrosion resistance in acid environments. As a result, by adding a predetermined amount or more of Sb in addition to Cr and Mo, excellent carbon dioxide corrosion resistance and excellent corrosion resistance in acid environments were obtained. Furthermore, by adding a predetermined amount or more of Ni and suppressing excessive addition of Mo, it was possible to simultaneously achieve excellent low-temperature toughness.

[0022] The present invention was completed based on these findings and further investigations. That is, the gist of the present invention is as follows. [1] A duplex stainless steel having a composition containing, by 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 unavoidable impurities; having a structure containing, by volume, 20 to 70% austenite phase and 30 to 80% ferrite phase; and having a yield strength YS of 448 MPa or more. [2] In addition to the above-mentioned component composition, the duplex stainless steel according to [1] further contains, in mass%, one or more groups selected from the following groups A to E. Group A: One or two types selected from W: 0.02 to 1.5%, Cu: 0.1 to 2.0%; Group B: One or two or more types selected from V: 0.20% or less, Nb: 0.20% or less, Ti: 0.20% or less; Group C: One or two types selected from Zr: 0.50% or less, B: 0.0100% or less; Group D: One or two or more types selected from REM: 0.08% or less, Ca: 0.010% or less, Sn: 0.20% or less, Mg: 0.0002 to 0.01%; Group E: One or two types selected from Ta: 0.01 to 0.15%, Co: 0.01 to 1.0%. [3] A duplex stainless steel having the component composition and structure described in [1] or [2] and having a yield strength YS of 758 MPa or more.[4] A stainless steel seamless pipe having a composition containing, by 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 unavoidable impurities; having a structure containing, by volume, 20 to 70% austenite phase and 30 to 80% ferrite phase; and having a yield strength YS of 448 MPa or more. [5] The stainless steel seamless pipe according to [4], wherein the component composition further contains, in mass%, one or more of the following groups A to E: Group A: One or two types selected from W: 0.02 to 1.5%, Cu: 0.1 to 2.0%; Group B: One or two or more types selected from V: 0.20% or less, Nb: 0.20% or less, Ti: 0.20% or less; Group C: One or two types selected from Zr: 0.50% or less, B: 0.0100% or less; Group D: One or two or more types selected from REM: 0.08% or less, Ca: 0.010% or less, Sn: 0.20% or less, Mg: 0.0002 to 0.01%; Group E: One or two types selected from Ta: 0.01 to 0.15%, Co: 0.01 to 1.0%. [6] A stainless steel seamless pipe having the chemical composition and structure described in [4] or [5] above, and having a yield strength YS of 758 MPa or more.

[0023] According to the present invention, it is possible to obtain a duplex stainless steel and a stainless steel seamless pipe having high strength of at least 448 MPa (65 ksi) yield strength, excellent corrosion resistance, and excellent low-temperature toughness.

[0024] The present invention will be described in detail below.

[0025] The duplex stainless steel (steel) and stainless steel seamless pipe (steel pipe) made from the duplex stainless steel of the present invention contain, by 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%, O: 0.010% or less, with the balance being Fe and unavoidable impurities. The duplex stainless steel (steel) and the stainless steel seamless pipe (steel pipe) made from the duplex stainless steel have a composition comprising, by volume, 20 to 70% austenite phase and 30 to 80% ferrite phase, and have a yield strength YS of 448 MPa or more.

[0026] Composition of duplex stainless steel and stainless steel seamless pipe The reasons for limiting the range of the composition of the duplex stainless steel and stainless steel seamless pipe of the present invention will be explained below. Note that % regarding the content of components is "% by mass".

[0027] C: 0.002 to 0.03% C is an element that stabilizes the austenite phase and improves strength and low-temperature toughness. To achieve high strength with a yield strength YS of 65 ksi (448 MPa), the C content is set to 0.002% or more. The C content is preferably 0.005% or more. More preferably, the C content is 0.010% or more. Even more preferably, the C content is 0.015% or more. On the other hand, if the C content exceeds 0.03%, excessive carbide precipitation may occur due to heat treatment, adversely affecting corrosion resistance. Therefore, the C content is set to 0.03% or less. The C content is preferably 0.027% or less. More preferably, the C content is 0.025% or less. Even more preferably, the C content is 0.023% or less, and most preferably, 0.020% or less.

[0028] Si: 0.05 to 1.0% Si is an element that functions as a deoxidizer, and to obtain this effect, the Si content is set to 0.05% or more. Preferably, the Si content is 0.1% or more. More preferably, the Si content is 0.2% or more, and even more preferably, 0.3% or more. On the other hand, if the Si content exceeds 1.0%, excessive precipitation of intermetallic compounds occurs due to heat treatment, deteriorating the corrosion resistance of the steel. Therefore, the Si content is set to 1.0% or less. Preferably, the Si content is 0.7% or less. More preferably, it is 0.6% or less. The Si content is even more preferably 0.5% or less, and most preferably 0.4% or less.

[0029] Mn: 0.1 to 1.7% Like Si, Mn is an effective deoxidizer and also improves hot workability by fixing the S inevitably contained in steel as sulfides. These effects are achieved with a Mn content of 0.1% or more. Therefore, the Mn content is set to 0.1% or more. Preferably, the Mn content is 0.15% or more, more preferably 0.2% or more. The Mn content is further preferably 0.25% or more, and most preferably 0.3% or more. On the other hand, a Mn content exceeding 1.7% not only reduces hot workability but also adversely affects corrosion resistance. Therefore, the Mn content is set to 1.7% or less. Preferably, the Mn content is 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.

[0030] P: 0.040% or less P is an element that reduces the corrosion resistance of duplex stainless steel, and if the P content exceeds 0.040%, the corrosion resistance will be significantly reduced. Therefore, the P content is set to 0.040% or less. The P content is preferably 0.030% or less. More preferably, the P content is 0.025% or less. Even more preferably, the P content is 0.020% or less. Most preferably, the P content is 0.015% or less. However, reducing the P content to less than 0.005% requires a long time for dephosphorization during the process of producing molten steel, which increases the manufacturing cost of duplex stainless steel. Therefore, the P content is preferably 0.005% or more. The P content is more preferably 0.007% or more, and even more preferably 0.010% or more.

[0031] S: 0.020% or less S is an element that reduces hot workability during the production process of duplex stainless steel, and if the S content exceeds 0.020%, it will hinder the production of duplex stainless steel. Therefore, the S content is set to 0.020% or less. Preferably, the S content is 0.010% or less. More preferably, the S content is 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 a long time for desulfurization treatment during the process of melting molten steel, which increases the production cost of duplex stainless steel. Therefore, the S content is preferably set to 0.0003% or more. The S content is more preferably 0.0005% or more, and even more preferably 0.0007% or more.

[0032] Cr: 20.0 to 28.0% Cr is a basic component effective for maintaining corrosion resistance and improving strength. To achieve these effects, the Cr content is set to 20.0% or more. To obtain even higher strength, the Cr content is preferably 22.0% or more, more preferably 23.0% or more. Most preferably 23.5% or more. On the other hand, if the Cr content exceeds 28.0%, the σ phase is likely to precipitate, deteriorating both corrosion resistance and toughness. Therefore, the Cr content is set to 28.0% or less. The Cr content is more preferably 27.5% or less, and even more preferably 27.0% or less. Furthermore, from the viewpoint of toughness, the Cr content is most preferably 25.8% or less.

[0033] Ni: 4.0 to 10.0% Ni is an element contained to stabilize the austenite phase and obtain a dual-phase structure. If the Ni content is less than 4.0%, the austenite phase becomes unstable and the volume fraction of the ferrite phase becomes excessive. Therefore, the Ni content is set to 4.0% or more. Preferably, the Ni content is set to 4.5% or more. More preferably, the Ni content is set to 5.3% or more. Even more preferably, the Ni content is set to 5.5% or more. On the other hand, if the Ni content exceeds 10.0%, the austenite phase becomes dominant and the volume fraction of the austenite phase becomes excessive. Furthermore, since Ni is an expensive element, economic efficiency is also impaired. Therefore, the Ni content is set to 10.0% or less. Preferably, the Ni content is set to 8.0% or less. More preferably, the Ni content is set to 7.5% or less. Even more preferably, the Ni content is set to 6.9% or less. Most preferably, the Ni content is set to 6.5% or less.

[0034] Mo: 2.0 to 5.0% Mo is an element that has the effect of improving the corrosion resistance of duplex stainless steel. －This contributes to preventing pitting corrosion caused by the above. If the Mo content is less than 2.0%, this effect cannot be obtained. Therefore, the Mo content is set to 2.0% or more. Preferably, the Mo content is 2.5% or more. More preferably, it is 3.0% or more. Even more preferably, it is 3.3% or more. On the other hand, if the Mo content exceeds 5.0%, the σ phase precipitates, reducing toughness and corrosion resistance. Therefore, the Mo content is set to 5.0% or less. Preferably, the Mo content is 4.5% or less. More preferably, the Mo content is 4.0% or less. Even more preferably, it is 3.5% or less.

[0035] Al: 0.001 to 0.05% Al is an element that functions as a deoxidizer during the process of melting molten steel, the raw material for duplex stainless steel. If the Al content is less than 0.001%, this effect cannot be obtained. Therefore, the Al content is set to 0.001% or more. Preferably, the Al content is 0.005% or more. More preferably, it is 0.005% or more. Even more preferably, it is 0.01% or more. On the other hand, if the Al content exceeds 0.05%, alumina-based inclusions are likely to precipitate, which reduces hot workability during the manufacturing process of duplex stainless steel and also deteriorates toughness. Therefore, the Al content is set to 0.05% or less. Preferably, the Al content is set to 0.045% or less. More preferably, the Al content is set to 0.039% or less. The Al content is even more preferably 0.035% or less, and most preferably 0.03% or less.

[0036] N: 0.06 to 0.35% N is known to improve pitting corrosion resistance and contribute to solid-solution strengthening in ordinary duplex stainless steels, and is actively added, with the N content set to 0.06% or more. To obtain the desired properties of the present invention, the N content is preferably 0.07% or more. More preferably, the N content is 0.08% or more. Even more preferably, the N content is 0.10% or more. Most preferably, the N content is 0.12% or more. However, excessive N content forms nitrides, which reduce toughness and corrosion resistance and also deteriorate hot workability. Therefore, the N content is set to 0.35% or less. Preferably, the N content is 0.30% or less, more preferably 0.20% or less, even more preferably 0.17% or less, and most preferably 0.14% or less.

[0037] Sb: 0.001 to 1.000% Sb is an important element in this patent because it improves corrosion resistance, particularly in acidic environments. To achieve the desired corrosion resistance, 0.001% or more of Sb is contained. Therefore, in the present invention, the Sb content is set to 0.001% or more. Furthermore, from the viewpoint of suppressing pitting corrosion in a CCS environment, the Sb content is preferably 0.005% or more. On the other hand, the effect saturates even if the Sb content exceeds 1.000%, so the Sb content is set to 1.000% or less. Furthermore, 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%. Sb also has the effect of improving fatigue resistance, hydrogen embrittlement resistance, hot workability, low-temperature toughness, machinability, and ductility, and increasing strength. Sb concentrates on the steel surface directly beneath the scale formed during hot rolling. Furthermore, since Sb is an element that does not dissolve in an acid treatment environment, it is believed to concentrate on the surface of the test piece. Therefore, even a small amount of Sb can be effective in both the actual service environment in which steel is used and the simulated environment evaluated using a test piece. In order to obtain better corrosion resistance in a CCS environment, the Sb content is preferably 0.005% or more, more preferably 0.010% or more, and even more preferably 0.012% or more. As mentioned above, the Sb content is most preferably less than 0.021%. Since Sb concentrates on the surface of the test piece and has protective properties, duplex stainless steel adjusted to the above range can be used in an autoclave using a test liquid: a 15 mass% NaCl aqueous solution (liquid temperature: 120°C, CO ２ : 40 MPa, O ２ :100ppm, SO ２ : 50 ppm, NO ２ When a test piece is immersed in an atmosphere containing 100 ppm of Cr, the immersion time is set to 720 hours, and the corrosion rate is 0.025 mm / y or less and no pitting corrosion of 0.2 mm or more occurs.

[0038] O (oxygen): 0.010% or less O (oxygen) exists as an oxide in steel and has an adverse effect on various properties. For this reason, it is desirable to reduce O as much as possible. In particular, if the O content exceeds 0.010%, both SSC resistance and SSC resistance in low-temperature environments will be significantly reduced. For this reason, the O content is set to 0.010% or less. The O content is preferably 0.007% or less. More preferably, the O content is 0.004% or less. The O content is even more preferably 0.003% or less, and 0.002% or less. Since excessive reduction leads to an increase in manufacturing costs, the O content is preferably 0.0005% or more. More preferably, the O content is 0.001% or more. Even more preferably, the O content is 0.0015% or more.

[0039] The steel according to one embodiment of the present invention may contain only the basic components and the balance, with the balance being Fe and unavoidable impurities.

[0040] In addition to the above basic components, the present invention may contain one or more elements selected from the following groups A to E, as needed. Group A: One or two elements selected from W: 0.02 to 1.5%, Cu: 0.1 to 2.0%; Group B: One or two elements selected from V: 0.20% or less, Nb: 0.20% or less, and Ti: 0.20% or less; Group C: One or two elements selected from Zr: 0.50% or less, and B: 0.0100% or less; Group D: One or two elements selected from REM: 0.08% or less, Ca: 0.010% or less, Sn: 0.20% or less, and Mg: 0.0002 to 0.01%; Group E: One or two elements selected from Ta: 0.01 to 0.15%, and Co: 0.01 to 1.0%.

[0041] Group A W: 0.02 to 1.5% W is an element that improves sulfide stress corrosion cracking resistance and sulfide stress cracking resistance. To achieve this effect, when W is contained, the W content is 0.02% or more. Preferably, the W content is 0.1% or more. On the other hand, if W is contained in a large amount exceeding 1.5%, low-temperature toughness may be reduced. Therefore, when W is contained, the W content is 1.5% or less. Preferably, the W content is 1.0% or less. More preferably, it is 0.7% or less. Even more preferably, it is 0.3% or less.

[0042] Cu: 0.1 to 2.0% Cu is an element that improves corrosion resistance and stabilizes the austenite phase. To achieve these effects, when Cu is contained, the content of each of these elements is set to 0.1% or more. It is preferably set to 0.2% or more. It is more preferably set to 0.4% or more, and even more preferably set to 0.6% or more. However, since an excessive Cu content deteriorates hot workability, when Cu is contained, the content is set to 2.0% or less. It is preferably set to 1.5% or less. It is more preferably set to 1.2% or less, and even more preferably set to 1.0% or less. It is most preferably set to 0.8% or less.

[0043] Group B V: 0.20% or less V is an element that improves the strength of steel by precipitation strengthening. To achieve this effect, the V content is preferably 0.02% or more. More preferably, the V content is 0.04% or more. On the other hand, if V is contained in an amount exceeding 0.20%, low-temperature toughness may be reduced. Furthermore, if V is contained in a large amount, sulfide stress cracking resistance may be reduced. Therefore, when V is contained, the V content is set to 0.20% or less. The V content is preferably 0.15% or less. More preferably, the V content is 0.08% or less. The V content is further preferably 0.06% or less. Most preferably, the V content is 0.04% or less.

[0044] Nb: 0.20% or less Nb is an element that improves the strength of steel through precipitation strengthening. Nb also has the effect of refining crystal grains and improving low-temperature toughness and sulfide stress corrosion cracking resistance. To achieve this effect, the Nb content is preferably 0.02% or more. More preferably, the Nb content is 0.04% or more. On the other hand, if Nb is contained in an amount exceeding 0.20%, coarse precipitates and intermetallic compounds may precipitate, reducing low-temperature toughness. Furthermore, if Nb is contained in a large amount, sulfide stress cracking resistance may be reduced. Therefore, when Nb is contained, the Nb content is set to 0.20% or less. The Nb content is preferably 0.15% or less, more preferably 0.10% or less. The Nb content is further preferably 0.08% or less. Most preferably 0.06% or less.

[0045] Ti: 0.20% or less Ti is an element that refines crystal grains and improves low-temperature toughness and sulfide stress corrosion cracking resistance. Ti also has the effect of improving the strength of steel through precipitation strengthening. To achieve this effect, the Ti content is preferably 0.02% or more. More preferably, the Ti content is 0.04% or more. On the other hand, if Ti is contained in an amount exceeding 0.20%, low-temperature toughness may be reduced. Furthermore, if Ti is contained in a large amount, sulfide stress cracking resistance may be reduced. Therefore, when Ti is contained, the Ti content is set to 0.20% or less. The Ti content is preferably 0.15% or less, more preferably 0.12% or less. Even more preferably, the Ti content is 0.08% or less. Most preferably, the Ti content is 0.06% or less.

[0046] Group C Zr: 0.50% or less Zr and B are both useful elements that contribute to increasing strength, and may be selectively contained as needed. Zr contributes to the above-mentioned increase in strength, and also contributes to improving sulfide stress corrosion cracking resistance. To achieve these effects, the Zr content is preferably 0.02% or more. More preferably, the Zr content is 0.05% or more. On the other hand, if Zr is contained in an amount exceeding 0.50%, low-temperature toughness may be reduced. Therefore, when Zr is contained, the Zr content is set to 0.50% or less. The Zr content is preferably 0.40% or less, more preferably 0.30% or less. Even more preferably, the Zr content is 0.20% or less. Most preferably, the Zr content is 0.10% or less.

[0047] B: 0.0100% or less B is useful as an element that contributes to the above-mentioned increase in strength and also contributes to improving hot workability. To achieve this effect, the B content is preferably 0.0005% or more. More preferably, the B content is 0.0010% or more. On the other hand, if B is contained in an amount exceeding 0.0100%, low-temperature toughness and hot workability may be reduced. Therefore, when B is contained, the B content is set to 0.0100% or less. Preferably, the B content is 0.0080% or less. More preferably, the B content is 0.0060% or less. Even more preferably, the B content is 0.0050% or less, and most preferably, the B content is 0.0040% or less.

[0048] Group D REM: 0.08% or less REM is useful as an element that contributes to improving sulfide stress corrosion cracking resistance and may be contained as needed. To ensure this effect, it is preferable to contain 0.01% or more of REM. More preferably, the REM content is 0.015% or more. On the other hand, if the REM content exceeds 0.08%, it may reduce low-temperature toughness and hot workability. Therefore, if REM is contained, the REM content is 0.08% or less. The REM content is preferably 0.06% or less, more preferably 0.04% or less. Even more preferably, the REM content is 0.03% or less. Most preferably, it is 0.02% or less. In the present invention, REM refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanides from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. The REM concentration in the present invention is the total content of one or more elements selected from the above-mentioned REMs.

[0049] Ca: 0.010% or less Ca is useful as an element that contributes to improving sulfide stress corrosion cracking resistance, and may be contained as needed. To ensure this effect, it is preferable to contain 0.001% or more of Ca. More preferably, the Ca content is 0.0015% or more. On the other hand, even if Ca is contained in an amount exceeding 0.010%, the effect saturates, and an effect commensurate with the content cannot be expected, which may be economically disadvantageous. Therefore, when Ca is contained, the Ca content is set to 0.010% or less. The Ca content is preferably 0.007% or less. More preferably, the Ca content is 0.005% or less. Even more preferably, the Ca content is 0.004% or less. Most preferably, the Ca content is 0.003% or less.

[0050] Sn: 0.20% or less Sn is useful as an element that contributes to improving corrosion resistance, and may be contained as needed. To ensure this effect, it is preferable to contain 0.0002% or more of Sn. More preferably, the Sn content is 0.0005% or more. On the other hand, even if Sn is contained in an amount exceeding 0.20%, the effect saturates, and an effect commensurate with the content cannot be expected, which may be economically disadvantageous. Therefore, when Sn is contained, the Sn content is set to 0.20% or less. More preferably, the Sn content is 0.15% or less. The Sn content is further preferably 0.10% or less, and most preferably 0.07% or less.

[0051] Mg: 0.0002 to 0.01% Mg is useful as an element that contributes to improving sulfide stress corrosion cracking resistance and may be contained as needed. To ensure this effect, it is preferable to contain 0.0002% or more of Mg. More preferably, the Mg content is 0.0005% or more. On the other hand, even if Mg is contained in an amount exceeding 0.01%, the effect saturates, and an effect commensurate with the content cannot be expected, which may be economically disadvantageous. Therefore, if Mg is contained, the Mg content is set to 0.01% or less. The Mg content is preferably 0.007% or less, more preferably 0.006% or less. Even more preferably, the Mg content is 0.005% or less. Most preferably, the Mg content is 0.003% or less.

[0052] Group E Ta: 0.01 to 0.15% Ta is useful as an element that contributes to improving carbon dioxide corrosion resistance, sulfide stress cracking resistance, and sulfide stress corrosion cracking resistance, and may be contained as needed. To ensure such effects, if Ta is contained, the Ta content is 0.01% or more. Preferably, the Ta content is 0.02% or more. On the other hand, if Ta is contained in an amount exceeding 0.15%, the effect saturates and an effect commensurate with the content may not be expected. Therefore, if Ta is contained, the Ta content is 0.15% or less. Preferably, the Ta content is 0.13% or less. More preferably, the Ta content is 0.10% or less. Even more preferably, the Ta content is 0.05% or less. Most preferably, the Ta content is 0.03% or less.

[0053] Co: 0.01 to 1.0% Co is useful as an element that contributes to improving carbon dioxide corrosion resistance, sulfide stress cracking resistance, and sulfide stress corrosion cracking resistance, and may be contained as needed. To ensure such effects, if Co is contained, the Co content is 0.01% or more. Preferably, the Co content is 0.02% or more. On the other hand, even if Co is contained in an amount exceeding 1.0%, the effect saturates and an effect commensurate with the content may not be expected. Therefore, if Co is contained, the Co content is 1.0% or less. Preferably, the Co content is 0.7% or less. More preferably, the Co content is 0.5% or less. The Co content is even more preferably 0.2% or less, and most preferably 0.1% or less.

[0054] Duplex Stainless Steel Structure: A structure containing, by volume fraction, 20-70% austenite phase and 30-80% ferrite phase. The duplex stainless steel of the present invention has a structure containing at least an austenite phase and a ferrite phase, and may have a structure consisting of an austenite phase and a ferrite phase. The volume fraction (%) of the austenite phase is 20-70%. The volume fraction (%) of the ferrite phase is 30-80%. If the austenite phase is less than 20%, the low-temperature toughness, sulfide stress cracking resistance, and sulfide stress corrosion cracking resistance may be poor. Therefore, the austenite phase is set to 20% or more. The austenite phase is preferably 30% or more, more preferably 35% or more, and even more preferably 40% or more. If the austenite phase is more than 70%, the strength may be poor. The volume fraction of each phase can be controlled by adjusting the steel composition and the solution treatment temperature. Specifically, the volume fraction of the austenite phase increases with an increase in the amount of austenite-forming elements (C, Mn, Ni, N, Cu, Co) or a decrease in the solution treatment temperature. Therefore, the austenite phase is set to 70% or less. The austenite phase is preferably set to 67% or less, more preferably 63% or less, and even more preferably 60% or less. The ferrite phase volume fraction increases with an increase in the amount of ferrite-forming elements (Si, Cr, Mo, W) or a increase in the solution treatment temperature. If the ferrite phase volume fraction is less than 30%, corrosion resistance may be poor. Therefore, the ferrite phase is set to 30% or more. The ferrite phase is preferably set to 33% or more, more preferably 37% or more, and even more preferably 40% or more. Furthermore, if the ferrite phase exceeds 80%, low-temperature toughness and corrosion resistance may be poor. Therefore, the ferrite phase is set to 80% or less. The ferrite phase is preferably set to 74% or less, more preferably 67% or less, and even more preferably 60% or less. The total volume fraction of the ferrite phase and the austenite phase is 99.5% or more. As will be described later in the Examples, the average area fractions of the ferrite phase and the austenite phase were calculated from the obtained structural photographs using an image analyzer, and these were used as the volume fractions (volume %) of each.

[0055] The remaining structure other than the above includes the σ phase, and the σ phase is acceptable if it is 0.5% or less. The σ phase is preferably 0.3% or less, and more preferably 0.1% or less. There is no particular lower limit, but the σ phase may be 0%.

[0056] In duplex stainless steel, the austenite and ferrite phases have different chemical properties, so when corrosion occurs, one of the phases dissolves preferentially. In an acid treatment simulation environment, the ferrite phase dissolves preferentially. Furthermore, during the rolling process of duplex stainless steel, the austenite phase precipitates from the ferrite phase, which is stable at high temperatures, so the ferrite phase is distributed in a mesh-like pattern and the austenite phase in an island-like pattern. When preferential dissolution occurs in the ferrite phase, if the ferrite phase is separated by the austenite phase, corrosion cannot progress further, improving corrosion resistance. Therefore, it is preferable for the austenite phase to be distributed in an island-like pattern within the ferrite phase.

[0057] To measure the volume fraction of each phase, a test specimen for microstructure observation is first taken from the cross section of the duplex stainless steel (or, if the duplex stainless steel is a seamless steel pipe, from the axial cross section of the pipe). The volume fractions of the ferrite and austenite phases are then determined by observing the observation surface with a scanning electron microscope (SEM). Specifically, the test specimen for microstructure observation is electrolytically etched in a KOH aqueous solution, and the microstructure is then imaged with a scanning electron microscope (1000x magnification). From the obtained microstructure photograph, an image analyzer is used to calculate the average area fractions of the ferrite and austenite phases, which are used as their respective volume fractions (volume %). In the image, the phase that is resistant to corrosion and therefore appears white when binarized is considered to be the austenite phase, while the phase that is susceptible to corrosion and therefore appears black when binarized is considered to be the ferrite phase. The above-mentioned method for observing each microstructure is also described in detail in the Examples below.

[0058] A preferred method for producing the duplex stainless steel and stainless steel seamless pipe of the present invention will be described below.

[0059] Molten steel having the above-described chemical composition is preferably produced by a conventional melting method such as a converter, and then processed into billets or other steel pipe materials by conventional methods such as continuous casting or ingot-making and blooming rolling. The heating temperature of the steel pipe material before hot working is preferably 1100°C or higher. The heating temperature is more preferably 1200°C or higher, and even more preferably 1250°C or higher. The heating temperature is preferably 1350°C or lower, more preferably 1330°C or lower, and even more preferably 1300°C or lower. This allows for both good hot workability during pipe-making and low-temperature toughness of the final product. The obtained steel pipe material is then processed by hot working, such as a commonly known pipe-making method, such as an extrusion pipe-making method such as the Ugine-Séjournet process or the Mannesmann pipe-making process, to produce a seamless steel pipe having the above-described composition and the desired dimensions. After hot working, a cooling treatment may be performed. This cooling treatment (cooling step) does not need to be particularly limited. If the composition range of the present invention is as described above, it is preferable to cool the hot-worked steel to room temperature at a cooling rate similar to that of air cooling.

[0060] The present invention also includes a method of producing duplex stainless steel by melting molten steel having the above-mentioned chemical composition using a conventional melting method such as a converter, forming billets, slabs, or other steel pipe materials using a conventional method such as continuous casting or ingot making-blooming, and then heating and then performing conventional hot rolling without the above-mentioned pipe-making process. It is preferable that the subsequent solution heat treatment and cold working of duplex stainless steel be carried out under the conditions described below.

[0061] Next, it is preferable to subject the steel pipe made to solution heat treatment. Specifically, the steel pipe is heated to a heating temperature of 1000°C or higher, and then cooled to a temperature of 300°C or lower at an average cooling rate faster than air cooling, more specifically, an average cooling rate of 1°C / s or higher. This allows intermetallic compounds, carbides, nitrides, sulfides, etc. that precipitated during pipe making or during cooling after pipe making to become solid-solution, thereby producing a seamless steel pipe with a structure containing the desired amounts of austenite and ferrite phases.

[0062] If the heating temperature of the solution heat treatment is less than 1000°C, the desired high toughness cannot be ensured. Therefore, the heating temperature of the solution heat treatment is preferably 1000°C or higher. More preferably, the heating temperature of the solution heat treatment is 1020°C or higher. The heating temperature of the solution heat treatment is even more preferably 1030°C or higher, and most preferably 1040°C or higher. Furthermore, from the viewpoint of preventing coarsening of the structure, the heating temperature of the solution heat treatment is preferably 1150°C or lower. More preferably, the heating temperature of the solution heat treatment is 1130°C or lower. The heating temperature of the solution heat treatment is even more preferably 1100°C or lower, and most preferably 1080°C or lower. In the present invention, the holding time at the heating temperature of the solution heat treatment is preferably 5 minutes or longer, from the viewpoint of uniform temperature distribution within the material. More preferably, it is 10 minutes or longer, even more preferably 15 minutes or longer, and most preferably 20 minutes or longer. Furthermore, the holding time at the heating temperature of the solution heat treatment is preferably 210 minutes or shorter. It is more preferably 180 minutes or less, even more preferably 120 minutes or less, and most preferably 60 minutes or less.

[0063] If the average cooling rate during solution heat treatment is less than 1°C / s, intermetallic compounds such as σ-phase and χ-phase will precipitate during cooling, significantly reducing low-temperature toughness and corrosion resistance. Therefore, the average cooling rate during solution heat treatment is preferably 1°C / s or more. The cooling rate during solution heat treatment is preferably 2°C / s or more, more preferably 5°C / s or more, even more preferably 10°C / s or more, and most preferably 15°C / s or more. The upper limit of the average cooling rate does not need to be particularly limited, but is preferably 500°C / s or less. It is preferably 200°C / s or less, more preferably 100°C / s or less, and even more preferably 50°C / s or less. Water cooling is preferred as the cooling method.

[0064] Cold Working To improve the yield strength of a material, strain is introduced by cold drawing, cold rolling, or cold working by tilt rolling with opposing rolls to increase strength. Preferably, reduction rolling or cold drawing is performed. The tilt rolling mill used for reduction rolling may be a two-roll tilt rolling mill or a three-roll tilt rolling mill with barrel-shaped rolls. Reduction rolling can be performed by adjusting the tilt angle, cross angle, and roll gap. Cold drawing may be performed using the cold pilger method. The processing temperature may be high to reduce deformation resistance. Specifically, the processing temperature is preferably in the range of 25 to 600°C, and it is desirable to avoid the 460 to 490°C temperature range where stainless steel becomes embrittled. That is, the processing temperature is preferably 25°C or higher, more preferably 50°C or higher, even more preferably 75°C or higher, and most preferably 100°C or higher. In either case, it is preferable not to perform the processing in the 460 to 490°C temperature range. The processing temperature is preferably 600°C or lower, more preferably 400°C or lower, and even more preferably 200°C or lower, and in any case, it is preferable not to carry out the processing in the temperature range of 460 to 490°C.

[0065] Pickling Pickling is preferably performed to remove scale formed on the surface of a steel pipe during hot rolling or solution heat treatment. When cold working is performed by cold drawing, the steel pipe surface needs to be lubricated, so pickling is preferably performed before cold working. On the other hand, when cold working is performed by tilt rolling, lubrication is not required, and cold working before pickling introduces strain into the steel pipe surface, mechanically destroying the scale and facilitating scale removal by pickling. Therefore, pickling is preferably performed after cold working. As for pickling conditions, a mixed solution of hydrofluoric acid and nitric acid is preferably used, and the 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 more, more preferably 20°C or more, and even more preferably 30°C or more.

[0066] As described above, the duplex stainless steel and stainless steel seamless pipe obtained by the present invention are high-strength steel pipes with a yield strength of 448 MPa or more, and have excellent corrosion resistance and low-temperature toughness. In particular, the duplex stainless steel and stainless steel seamless pipes that have been subjected to the above-mentioned cold working can be obtained as steel pipes with a yield strength of 758 MPa or more, which is considered to be of even higher strength. The stainless steel seamless pipe of the present invention can be made into a stainless steel seamless pipe for oil wells (high-strength stainless steel seamless pipe for oil wells).

[0067] The present invention will be described in more detail below based on examples, but the present invention is not limited to the following examples.

[0068] Steel pipe materials were cast using molten steel having the chemical compositions shown in Tables 1-1 and 1-2. The steel pipe materials were then heated and hot-formed using a model seamless rolling mill to produce seamless steel pipes with an outer diameter of 72 mm and a wall thickness of 11 mm, which were then air-cooled. The heating temperature of the steel pipe materials before hot-forming was 1250°C. After the steel pipe materials were cast and heated as described above, they were hot-rolled using a hot rolling mill to produce duplex stainless steel, which was then air-cooled. The heating temperature of the steel pipe materials before hot-rolling was 1250°C, and the final temperature of hot-rolling was 900°C or higher.

[0069] After air cooling, solution heat treatment was carried out at the temperature, soaking time, and cooling method shown in Table 2. The average cooling rate in the solution heat treatment was 7 to 80°C / s.

[0070] Furthermore, the above-mentioned duplex stainless steel was subjected to cold working in a cold rolling mill to strengthen the working strength, with the reduction ratio preferably being 10 to 90%.

[0071] Test specimens for microstructural observation were taken from the finally obtained seamless steel pipes, and quantitative evaluation of the microstructure, tensile tests, Charpy impact tests, corrosion tests, sulfide stress cracking resistance tests (SSC resistance tests), and sulfide stress corrosion cracking resistance tests (SCC resistance tests) were performed. The test methods were as follows. The results obtained from these tests are shown in Table 3.

[0072] (1) Measurement of the volume fraction (vol %) of each phase in the entire structure of the steel pipe. Test specimens for microstructure observation were taken from the seamless steel pipes subjected to the above-mentioned heat treatment so that the axial cross section of the pipe served as the 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 corroded in a KOH aqueous solution, and the microstructure was photographed with an optical microscope (magnification 400x). From the obtained microstructure photographs, an image analyzer was used to calculate the average area fractions of the ferrite phase and the austenite phase, and these were used as the respective volume fractions (vol %). In the photographed images, the phase that is resistant to corrosion and therefore appears white when binarized was defined as the austenite phase, and the phase that is easily corroded and therefore appears black when binarized was defined as the ferrite phase.

[0073] (2) Tensile Test [Evaluation of Tensile Properties] For seamless steel pipes that had not been subjected to cold working, tensile test pieces (Φ6.0 mm) according to JIS (Japan Industrial Standards) No. 14A were taken from the seamless steel pipes after solution heat treatment, and for seamless steel pipes that had been subjected to cold working, tensile tests were carried out in accordance with the provisions of JIS Z2241:2011 to determine the tensile properties (yield strength (YS), tensile strength (TS)).

[0074] (3) Charpy Impact Test V-notch test pieces (thickness 10 mm) were taken from the seamless steel pipes that were not subjected to cold working after solution heat treatment, and from the seamless steel pipes that were subjected to cold working, from the center of the wall thickness of the seamless steel pipes after cold working, in accordance with JIS Z 2242 (2018), so that the circumferential direction was the length of the test piece. A Charpy impact test was performed on the taken test pieces at a test temperature of -40°C, and the absorbed energy vE －４０ Three test pieces were taken from each steel pipe, and the Charpy impact test was carried out on these test pieces. The arithmetic mean values ​​of the values ​​obtained are shown in Table 2.

[0075] (4) Corrosion test (carbon dioxide corrosion resistance test) For seamless steel pipes that had not been subjected to cold working, corrosion test pieces of 3 mm thick x 30 mm wide x 40 mm long were prepared by machining after solution heat treatment from the seamless steel pipes that had not been subjected to cold working, and for seamless steel pipes that had been subjected to cold working, corrosion test pieces were prepared by machining from the seamless steel pipes after cold working, and a corrosion test was carried out on these test pieces to evaluate the carbon dioxide corrosion resistance. The corrosion test was carried out in an autoclave using a test liquid: a 20% NaCl aqueous solution (liquid temperature: 230°C, CO ２ The test specimens were immersed in a 10 MPa atmosphere (atmospheric pressure: 10 MPa) for 14 days (336 hours). After the test, the masses of the test specimens were measured, and the corrosion rate was calculated from the mass loss before and after the corrosion test. After the corrosion test, the test specimens were observed for the presence or absence of pitting corrosion on their surfaces using a 10x magnification loupe. Assuming that pitting corrosion is circular, pitting corrosion is considered to have occurred when pitting corrosion has a diameter of 0.2 mm or more. In the present invention, a corrosion rate of 0.127 mm / y or less and no pitting corrosion has occurred was evaluated as passing.

[0076] (5) Sulfide stress cracking resistance test (SSC resistance test) Round bar-shaped test pieces (diameter: 6.4 mm) were prepared from the above-mentioned cold-worked seamless steel pipe by machining in accordance with NACE TM0177 Method A, and SSC resistance tests were carried out on these test pieces. The SSC resistance test was carried out using a test liquid: a 20 mass % NaCl aqueous solution (liquid temperature: 25°C, H ２ S: 0.03MPa, CO ２ The test specimens were immersed in an aqueous solution prepared by adding acetic acid and sodium acetate to an atmosphere of 0.07 MPa (pH 3.5), and the immersion period was 720 hours, with an applied stress of 90% of the yield stress. After the test, the test specimens were visually inspected for cracks. Furthermore, after the test, the test specimens were inspected for the presence or absence of pitting corrosion on their surfaces using a magnifying glass with a magnification of 10x. In the present invention, a test specimen without cracks or pitting corrosion after the test was evaluated as passing. In Table 3, a case in which neither cracks nor pitting corrosion occurred is indicated by the symbol ○, and a case in which cracks and / or pitting corrosion occurred is indicated by the symbol ×.

[0077] (6) Sulfide Stress Corrosion Cracking Resistance Test (SCC Resistance Test) Four-point bending test pieces with a thickness of 3 mm, a width of 15 mm, and a length of 115 mm were machined from the seamless steel pipe subjected to the above-mentioned cold working, and an SCC resistance test was carried out on these test pieces. The SCC resistance test was carried out in an autoclave using a test liquid: a 10 mass % NaCl aqueous solution (liquid temperature: 80°C, H ２ S: 35kPa, CO ２ The test specimens were immersed in a 2 MPa atmosphere (atmospheric pressure: 2 MPa) for 720 hours, and an applied stress of 100% of the yield stress was applied. After the test, the test specimens were visually observed for the presence or absence of cracks on the surface. After the test, the test specimens were also observed for the presence or absence of pitting corrosion on the surface using a magnifying glass with a magnification of 10x. In the present invention, a test specimen in which no cracks or pitting corrosion occurred after the test was evaluated as passing. In Table 3, a case in which no cracks or pitting corrosion occurred is indicated by the symbol ○, and a case in which cracks and / or pitting corrosion occurred is indicated by the symbol ×.

[0078] (7) Corrosion Test in Acid Environment: The corrosion test to evaluate corrosion resistance in an acid environment was performed by immersing a test specimen in a 15% by mass hydrochloric acid solution heated to 80°C for 2 minutes. The weight of the test specimen after the test was measured, and the corrosion rate was calculated from the weight loss before and after the corrosion test. Here, when the Cr content was 23.0% or less, a corrosion rate of 170 mm / y or less was considered to be pass, and a corrosion rate exceeding 170 mm / y was considered to be fail. When the Cr content was more than 23.0%, a corrosion rate of 95 mm / y or less was considered to be pass, and a corrosion rate exceeding 95 mm / y was considered to be fail.

[0079] (8) Corrosion test in CCS environment The corrosion test in the CCS environment was carried out using the same test specimens as in the carbon dioxide corrosion test, with the test liquid held in an autoclave: 15% NaCl aqueous solution (liquid temperature: 120°C, CO ２ : 40 MPa, O ２ :100ppm, SO ２ : 50 ppm, NO ２The test specimens were immersed in a 100 ppm atmosphere (atmosphere containing 100 ppm of Cr, ...

[0080]

[0081]

[0082]

[0083]

[0084] All of the examples of the present invention have a high yield strength of 448 MPa or more and a high absorbed energy vE －４０ ≧100J, and furthermore, ２ and Cl － The stainless steel seamless pipes had excellent corrosion resistance (carbon dioxide corrosion resistance) in high-temperature corrosive environments of 230°C or higher, including those containing chlorine, and also had excellent corrosion resistance in acid environments and excellent corrosion resistance in CCS environments. On the other hand, the comparative examples outside the scope of the present invention either failed to achieve the high strength or high toughness aimed at by the present invention, or did not pass the carbon dioxide corrosion resistance test, the corrosion test in an acid environment, or the CCS simulated corrosion test. The same results as those of the above stainless steel seamless pipe were obtained with the duplex stainless steel obtained above.

Claims

1. A duplex stainless steel having a composition containing, by mass%, the following: C: 0.002-0.03%, Si: 0.05-1.0%, Mn: 0.1-1.7%, P: 0.040% or less, S: 0.020% or less, Cr: 20.0-28.0%, Ni: 4.0-10.0%, Mo: 2.0-5.0%, Al: 0.001-0.05%, N: 0.06-0.35%, Sb: 0.001-1.000%, and O: 0.010% or less, with the balance being Fe and unavoidable impurities; having a structure containing, by volume, 20-70% austenite phase and 30-80% ferrite phase; and having a yield strength YS of 448 MPa or more.

2. The duplex stainless steel according to claim 1, further comprising, in addition to the above-mentioned chemical composition, one or more elements selected from the following groups A to E, in mass percent: Group A: One or two types selected from W: 0.02-1.5%, Cu: 0.1-2.0%; Group B: One or two or more types selected from V: 0.20% or less, Nb: 0.20% or less, Ti: 0.20% or less; Group C: One or two types selected from Zr: 0.50% or less, B: 0.0100% or less; Group D: One or two or more types selected from REM: 0.08% or less, Ca: 0.010% or less, Sn: 0.20% or less, Mg: 0.0002-0.01%; Group E: One or two types selected from Ta: 0.01-0.15%, Co: 0.01-1.0%.

3. A duplex stainless steel having the chemical composition and structure described in claim 1 or 2 and having a yield strength YS of 758 MPa or more.

4. A stainless steel seamless pipe having a composition containing, by mass%, the following: C: 0.002-0.03%, Si: 0.05-1.0%, Mn: 0.1-1.7%, P: 0.040% or less, S: 0.020% or less, Cr: 20.0-28.0%, Ni: 4.0-10.0%, Mo: 2.0-5.0%, Al: 0.001-0.05%, N: 0.06-0.35%, Sb: 0.001-1.000%, and O: 0.010% or less, with the balance being Fe and unavoidable impurities; having a structure containing, by volume, 20-70% austenite phase and 30-80% ferrite phase; and having a yield strength YS of 448 MPa or more.

5. The stainless steel seamless pipe according to claim 4, wherein the composition further contains, in mass %, one or more elements selected from the following groups A to E: Group A: One or two types selected from W: 0.02-1.5%, Cu: 0.1-2.0%; Group B: One or two or more types selected from V: 0.20% or less, Nb: 0.20% or less, Ti: 0.20% or less; Group C: One or two types selected from Zr: 0.50% or less, B: 0.0100% or less; Group D: One or two or more types selected from REM: 0.08% or less, Ca: 0.010% or less, Sn: 0.20% or less, Mg: 0.0002-0.01%; Group E: One or two types selected from Ta: 0.01-0.15%, Co: 0.01-1.0%.

6. A stainless steel seamless pipe having the chemical composition and structure as set forth in claim 4 or 5 and having a yield strength YS of 758 MPa or more.

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