Duplex stainless steel and seamless stainless steel pipe

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

[0001] 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 (CO 2 ) and chlorine ions (Cl -< ).Background Art

[0002] 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.

[0003] In oil fields and gas fields located in environments containing CO 2 , 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.

[0004] 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.

[0005] 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.

[0006] 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.

[0007] 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-CaF 2 -Al 2 O 3 -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.

[0008] 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 ListPatent Literature

[0009] 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 InventionTechnical Problem

[0010] 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 CO 2 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.

[0011] 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.

[0012] 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.

[0013] 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 CO 2 emitted from oil fields, gas fields, plants, and the like underground. Regarding CCS, oil well pipes are used to inject CO 2 underground, and one problem that has arisen is that impurities remaining in CO 2 , such as SOx, NOx, and O 2 , increase corrosiveness.

[0014] 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.

[0015] 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.

[0016] 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".

[0017] 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 CO 2 gas), for an immersion time of 336 hours.

[0018] 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.

[0019] 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 CO 2 , 100 ppm O 2 , 50 ppm SO 2 , and 100 ppm NO 2 ), for an immersion time of 720 hours.

[0020] 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

[0021] 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.

[0022] 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

[0023] 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

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

[0025] 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

[0026] 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%

[0027] 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%

[0028] 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%

[0029] 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

[0030] 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

[0031] 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%

[0032] 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%

[0033] 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%

[0034] 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%

[0035] 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%

[0036] 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%

[0037] 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 CO 2 , 100 ppm O 2 , 50 ppm SO 2 , and 100 ppm NO 2 ), for an immersion time of 720 hour.O (oxygen): 0.010% or less

[0038] 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.

[0039] 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.

[0040] 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 AW: 0.02 to 1.5%

[0041] 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%

[0042] 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 BV: 0.20% or less

[0043] 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

[0044] 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

[0045] 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 CZr: 0.50% or less

[0046] 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

[0047] 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 DREM: 0.08% or less

[0048] 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

[0049] 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

[0050] 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%

[0051] 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 ETa: 0.01 to 0.15%

[0052] 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%

[0053] 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 SteelMicrostructures Including Austenite Phase in Volume Fraction of 20 to 70% and Ferrite Phase in Volume Fraction of 30 to 80%

[0054] 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.%).

[0055] 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%.

[0056] 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.

[0057] 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.

[0058] Now, preferred methods for manufacturing the duplex stainless steel and the stainless steel seamless pipe of the present invention will be described.

[0059] 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.

[0060] 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

[0061] 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.

[0062] 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.

[0063] 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

[0064] 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

[0065] 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.

[0066] 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

[0067] 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.

[0068] 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.

[0069] 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.

[0070] 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%.

[0071] 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

[0072] 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]

[0073] 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

[0074] 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)

[0075] 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 CO 2 ), 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)

[0076] 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 H 2 S and 0.07-MPa CO 2 ). 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)

[0077] 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 H 2 S and 2-MPa CO 2 ), 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

[0078] 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

[0079] 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 CO 2 , 100 ppm O 2 , 50 ppm SO 2 , and 100 ppm NO 2 ), 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%)ClassificationCSiMnPSCrNiMoAlNSbOWCuVNbTiZrBREMCaSnMgTaCoA0.02020.471.470.0190.000922.356.433.210.0250.07810.0280.0048000.0070.02100.030.003000.003000.00500.0600Conforming SteelB0.02010.491.480.0170.000922.365.383.210.0160.14040.0050.0045000.0070.0200.1000.002400.00170000Conforming SteelC0.02070.471.480.0170.001122.364.563.210.0190.19770.0280.0045000.0080.0190.0400.002600.00300000Conforming SteelD0.02110.471.490.0170.000922.415.383.190.0230.13990.0280.0035000.0070.0180.0400.002500.006100.004000Conforming SteelE0.01900.481.470.0170.000922.165.393.200.0160.14080.0180.0047000.0070.001000.002800.00300000Conforming SteelF0.01960.511.450.0170.001022.405.423.220.0150.14000.0140.0042000.0070.012000.004400.00150000Conforming SteelG0.01920.461.480.0160.000922.145.313.180.0180.14030.0130.0041000.0070.030000.002600.00300000Conforming SteelH0.02120.471.470.0170.000922.324.463.200.0180.14150.0270.0051000.0080.020000.002700.00250000.490Conforming SteelI0.01800.460.790.0190.000824.957.493.220.0240.09990.0250.00620.290.520.0090.020000.002700.00230000Conforming SteelJ0.02150.490.770.0180.000725.096.823.190.0150.15210.0120.00530.290.520.0100.0180.0700.003400.002600.002000.560Conforming SteelK0.01910.470.800.0180.000925.046.503.200.0140.20690.0230.00630.290.520.0100.022000.002800.0035000.0500.040Conforming SteelL0.02040.280.780.0180.001024.876.913.690.0210.24810.0240.0065000.0100.019000.003000.0030000.1100Conforming SteelM0.02170.290.780.0170.001224.836.603.670.0140.29200.0210.0068000.0130.019000.002700.00330.10000.0600Conforming SteelN0.02300.280.780.0170.001324.646.193.650.0150.32160.0270.0044000.0130.019000.002800.00240000Conforming SteelO0.01970.451.670.0240.001022.015.323.080.0180.17470.0280.00360.050.260.0640.024000.002100.00060000Conforming SteelP0.01850.320.500.0210.001025.276.953.640.0100.27130.0300.00520.500.520.1020.004000.002000.000600.00500.0400Conforming SteelQ0.01500.491.470.0280.000422.405.853.100.0020.17200.0170.00660.040.260.0710.0150.0100.003000.00120.040000.760Conforming SteelR0.01400.520.630.0290.000625.106.503.120.0060.14400.0180.00860.140.210.0780.012000.002400.00210000Conforming SteelS0.01000.501.130.0230.000422.506.283.280.0040.17000.0340.00900.020.070.0970.0070000.00100.00310000Conforming SteelT0.02000.381.080.0230.000625.806.553.060.0020.20500.0070.00920.030.120.0860.02000.030.00240.00200.00180000Conforming SteelU0.02020.390.780.0160.000825.186.422.920.0100.17250.0270.00500.210.4900000.0005000.130000Conforming SteelV0.02060.390.790.0160.000924.946.492.940.0100.17120.0090.00460.210.5000000.0028000000Conforming SteelW0.01990.390.200.0160.000925.036.492.940.0100.17470.0200.00440.210.5000000.0003000000Conforming SteelX0.02430.120.790.0160.000725.206.462.950.0100.17790.0240.00480.210.5000000.000400000.1100Conforming SteelY0.02130.120.190.0170.000825.116.472.940.0100.16920.0100.00390.200.50000.0500.0003000.05000.0600Conforming SteelZ0.02250.110.190.0160.000824.876.512.980.0100.17010.0100.00480.210.5000000.0029000000Conforming SteelUnderline: outside the scope of the invention [Table 1-2] Steel Grade No.Chemical Composition (mass%)ClassificationCSiMnPSCrNiMoAlNSbOWCuVNbTiZrBREMCaSnMgTaCoAA0.02110.501.490.0160.000822.425.463.340.0240.108500.00420.3900.0100000.002900.00240000Comparative SteelAB0.01860.491.510.0150.000822.515.453.330.0400.12980.0180.003300.600.008000.040.00640.073000000Conforming SteelAC0.01980.491.500.0170.000922.385.463.380.0190.133600.005201.090.0080000.0028000.020000Comparative SteelAD0.02070.491.520.0180.000722.445.443.300.0220.13540.0190.001801.090.0080000.006300000.0600Conforming SteelAE0.01990.491.510.0160.000722.325.443.310.0220.135000.002801.090.0080000.00840.063500.011000Comparative SteelAF0.01840.521.510.0150.000822.415.463.420.0260.12700.0090.0029000.01000.010.020.00290.022000000Conforming SteelAG0.02030.531.570.0170.000923.585.653.600.0300.134900.0018000.0100000.0027000000Comparative SteelAH0.02030.531.570.0170.000923.585.653.600.0300.134900.0018000.0100000.0027000000Comparative SteelAI0.01980.510.800.0160.000825.457.443.260.0270.120700.00450.290.590.0090000.0027000000Comparative SteelAJ0.01980.510.800.0160.000825.457.443.260.0270.120700.00450.290.590.0090000.0027000000Comparative SteelAK0.02200.500.820.0180.000725.367.493.280.0420.113600.00460.270.610.0090000.00590.014500.021000Comparative SteelAL0.01880.490.820.0170.000725.367.483.250.0270.116700.00440.261.090.0080000.0062000.008000Comparative SteelAM0.02110.470.820.0170.000925.377.513.270.0210.11320.0090.00470.281.090.0080000.0093000000Conforming SteelAN0.01900.500.830.0180.000725.247.493.280.0340.11760.0160.00280.281.090.00800.0200.00940.0220000.007000.050Conforming SteelAO0.01750.480.810.0160.001025.207.473.220.0270.12460.0160.00480.281.590.0070000.00300.013000000Conforming SteelAP0.01930.490.820.0180.001125.477.523.260.0200.11230.0100.00460.281.600.00700.050.020.00580.012500000Conforming SteelAQ0.01680.500.770.0150.000922.155.443.150.0200.170200.0025000.0080000.00320000.00300.0820.030Comparative SteelAR0.01560.511.510.0260.000822.285.453.410.0230.176600.0049000.0100000.002800000.1200Comparative SteelAS0.01550.531.580.0270.000823.425.663.390.0310.099900.0020000.0100000.003400000.0740.100Comparative SteelAT0.01500.510.780.0150.000822.345.433.400.0240.098600.0035000.0090000.00280000.00300.1230.040Comparative SteelAU0.01550.500.760.0160.001022.205.493.170.0210.094400.0046000.0080000.003500000.0080.070Comparative SteelAV0.01600.501.480.0260.000922.355.863.140.0230.176300.00320.030.310.0750.016000.003100.000800.005000Comparative SteelAW0.02020.471.470.0190.000921.357.932.710.0250.05410.0300.0048000.0070.02100.030.003000.003000.00500.0600Comparative SteelAX0.01420.570.770.0170.000923.124.463.500.0180.06150.0300.00510.2000.0080.020000.002700.00250000.490Conforming SteelAY0.02300.280.780.0170.001323.946.292.950.0150.32160.0300.0044000.0130.019000.002800.00240000Conforming SteelAZ0.02600.170.170.0340.001821.404.872.970.0250.06350.0090.004100.4900000000000Conforming SteelBA0.02600.180.160.0350.001821.394.862.970.0250.06450.0090.00510000000000000Conforming Steel Underline: outside the scope of the invention [Table 2] Steel Pipe No.Steel Grade No.Heat TreatmentCold WorkingSolution TreatmentHeating Temperature (°C)Soaking Time (min)Cooling1A105085Water CoolingYes2B106090Water CoolingYes3C104570Water CoolingYes4D104550Water CoolingNo5E1065120Water CoolingYes6F106585Water CoolingNo7G107555Water CoolingYes8H107045Water CoolingNo9I107045Water CoolingYes10J107055Water CoolingYes11K107590Water CoolingYes12L106585Water CoolingYes13M106085Water CoolingYes14N105595Water CoolingYes15O104060Water CoolingYes16P108075Water CoolingYes17Q107080Water CoolingYes18R108055Water CoolingYes19S104045Water CoolingNo20T1060115Water CoolingNo21U1070120Water CoolingNo22V105540Water CoolingNo23W108095Water CoolingNo24X108035Water CoolingNo25Y1050115Water CoolingNo26Z1030120Water CoolingNo27AA106535Water CoolingNo28AB108075Water CoolingNo29AC104080Water CoolingNo30AD108050Water CoolingNo31AE104065Water CoolingNo32AF1060105Water CoolingNo33AG106530Water CoolingNo34AH1065100Water CoolingNo35AI1040120Water CoolingNo36AJ105050Water CoolingNo37AK106555Water CoolingNo38AL107070Water CoolingNo39AM1055120Water CoolingNo40AN103555Water CoolingNo41AO106045Water CoolingNo42AP107065Water CoolingNo43AQ107570Water CoolingNo44AR106065Water CoolingNo45AS103035Water CoolingNo46AT105035Water CoolingNo47AU1065100Water CoolingNo48AV1030100Water CoolingNo49AW105030Water CoolingNo50AX105030Water CoolingNo51AY105030Water CoolingNo52AZ105030Water CoolingYes53BA105030Water CoolingYes Underline: outside the scope of the present invention [Table 3] Steel Pipe No.Steel Grade No.MicrostructuresTensile PropertiesLow-Temperature ToughnessCarbon Dioxide Gas Corrosion Resistance TestSSC Resistance TestSCC Resistance TestAcid EnvironmentCCS-Simulated Corrosion TestNotesAusteniteFerriteYield Strength YSTensile Strength TSCharpy Absorbed Energy at -40°CCorrosion RatePitting CorrosionCorrosion RateCorrosion RatePitting Corrosion of 0.2 mm or GreaterPitting Corrosion of 0.1 mm or Greater and Less Than 0.2 mm(Area %)(Area %)(MPa)(MPa)(J)(mm / v)(mmlv)(mm / v)1A54468068493350.001No○○1670.001NoYesInvention Example2B50508469023210.003No○○1630.001NoNoInvention Example3C49518509163110.014No○○1650.002NoYesInvention Example4D50505487273210.002No--1670.001NoYesInvention Example5E51498058663240.007No○○1670.002NoNoInvention Example6F50505087043060.001No--1660.001NoNoInvention Example7G51498158703240.009No○○1640.001NoNoInvention Example8H43575557202800.016No--1670.001NoYesInvention Example9I52488588883010.001No○○900.001NoYesInvention Example10J51498389052890.001No○○870.001NoNoInvention Example11K53478509083060.001No○○880.002NoYesInvention Example12L57438459293560.001No○○900.002NoYesInvention Example13M59418399393670.001No○○890.001NoNoInvention Example14N59418519493700.001No○○880.001NoYesInvention Example15O57438229293460.009No○○1660.001NoYesInvention Example16P59418719663430.001No○○870.002NoYesInvention Example17Q58429229793460.001No○○1650.002NoNoInvention Example18R45558939562810.001No○○870.002NoNoInvention Example19S58425077623750.001No--1630.002NoYesInvention Example20T49515678003060.001No--900.002NoNoInvention Example21U51495607493130.001No--890.002NoYesInvention Example22V52485597483230.001No--890.002NoNoInvention Example23W52485557453190.001No--890.002NoNoInvention Example24X53475467383260.001No--870.001NoYesInvention Example25Y52485437363210.001No--870.001NoNoInvention Example26Z54465487403300.001No--890.001NoNoInvention Example27AA46545367102050.001No--1820.002YesYesComparative Example28AB52485136991030.001No--1670.001NoNoInvention Example29AC56445267262840.001No--1740.002YesYesComparative Example30AD56445157181700.001No--1660.002NoNoInvention Example31AE5644530713770.001No--1740.001YesYesComparative Example32AF48525207092720.001No--1630.001NoNoInvention Example33AG44565217442820.001No--960.001YesYesComparative Example34AH44565307382820.001No--960.002YesYesComparative Example35AI51495387562920.001No--1030.001YesYesComparative Example36AJ51495507522920.001No--1020.001YesYesComparative Example37AK52485587381950.001No--1040.001YesYesComparative Example38AL55455607462090.001No--1070.001YesYesComparative Example39AM55455607461720.001No--880.002NoNoInvention Example40AN55455557441760.001No--900.002NoNoInvention Example41AO59415717671970.001No--870.002NoNoInvention Example42AP57435857671520.001No--880.002NoNoInvention Example43AQ54465207263350.007No--1750.002YesYesComparative Example44AR53475297423320.001No--1770.002YesYesComparative Example45AS42585547172670.001No--1760.001YesYesComparative Example46AT45555246882880.001No--1750.001YesYesComparative Example47AU47535126792950.005No--1730.002YesYesComparative Example48AV59415147333470.001No--1770.002YesYesComparative Example49AW19814396683520.001No--1640.001NoYesComparative Example50AX25755997871950.001No--870.002NoYesInvention Example51AY65355277463140.001No--870.002NoYesInvention Example52AZ50507769123090.026No××1600.003NoNoInvention Example53BA51497949292920.027No××1500.002NoNoInvention 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.

[0080] 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 CO 2 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.

Examples

examples

[0067]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.

[0068]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 ...

Technical Field

[0001] 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 (CO 2 ) and chlorine ions (Cl -< ).Background Art

[0002] 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.

[0003] In oil fields and gas fields located in environments containing CO 2 , 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.

[0004] 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.

[0005] 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.

[0006] 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.

[0007] 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-CaF 2 -Al 2 O 3 -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.

[0008] 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 ListPatent Literature

[0009] 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 InventionTechnical Problem

[0010] 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 CO 2 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.

[0011] 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.

[0012] 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.

[0013] 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 CO 2 emitted from oil fields, gas fields, plants, and the like underground. Regarding CCS, oil well pipes are used to inject CO 2 underground, and one problem that has arisen is that impurities remaining in CO 2 , such as SOx, NOx, and O 2 , increase corrosiveness.

[0014] 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.

[0015] 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.

[0016] 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".

[0017] 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 CO 2 gas), for an immersion time of 336 hours.

[0018] 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.

[0019] 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 CO 2 , 100 ppm O 2 , 50 ppm SO 2 , and 100 ppm NO 2 ), for an immersion time of 720 hours.

[0020] 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

[0021] 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.

[0022] 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

[0023] 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

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

[0025] 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

[0026] 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%

[0027] 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%

[0028] 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%

[0029] 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

[0030] 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

[0031] 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%

[0032] 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%

[0033] 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%

[0034] 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%

[0035] 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%

[0036] 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%

[0037] 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 CO 2 , 100 ppm O 2 , 50 ppm SO 2 , and 100 ppm NO 2 ), for an immersion time of 720 hour.O (oxygen): 0.010% or less

[0038] 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.

[0039] 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.

[0040] 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 AW: 0.02 to 1.5%

[0041] 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%

[0042] 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 BV: 0.20% or less

[0043] 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

[0044] 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

[0045] 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 CZr: 0.50% or less

[0046] 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

[0047] 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 DREM: 0.08% or less

[0048] 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

[0049] 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

[0050] 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%

[0051] 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 ETa: 0.01 to 0.15%

[0052] 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%

[0053] 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 SteelMicrostructures Including Austenite Phase in Volume Fraction of 20 to 70% and Ferrite Phase in Volume Fraction of 30 to 80%

[0054] 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.%).

[0055] 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%.

[0056] 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.

[0057] 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.

[0058] Now, preferred methods for manufacturing the duplex stainless steel and the stainless steel seamless pipe of the present invention will be described.

[0059] 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.

[0060] 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

[0061] 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.

[0062] 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.

[0063] 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

[0064] 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

[0065] 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.

[0066] 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

[0067] 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.

[0068] 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.

[0069] 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.

[0070] 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%.

[0071] 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

[0072] 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]

[0073] 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

[0074] 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)

[0075] 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 CO 2 ), 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)

[0076] 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 H 2 S and 0.07-MPa CO 2 ). 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)

[0077] 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 H 2 S and 2-MPa CO 2 ), 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

[0078] 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

[0079] 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 CO 2 , 100 ppm O 2 , 50 ppm SO 2 , and 100 ppm NO 2 ), 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%)ClassificationCSiMnPSCrNiMoAlNSbOWCuVNbTiZrBREMCaSnMgTaCoA0.02020.471.470.0190.000922.356.433.210.0250.07810.0280.0048000.0070.02100.030.003000.003000.00500.0600Conforming SteelB0.02010.491.480.0170.000922.365.383.210.0160.14040.0050.0045000.0070.0200.1000.002400.00170000Conforming SteelC0.02070.471.480.0170.001122.364.563.210.0190.19770.0280.0045000.0080.0190.0400.002600.00300000Conforming SteelD0.02110.471.490.0170.000922.415.383.190.0230.13990.0280.0035000.0070.0180.0400.002500.006100.004000Conforming SteelE0.01900.481.470.0170.000922.165.393.200.0160.14080.0180.0047000.0070.001000.002800.00300000Conforming SteelF0.01960.511.450.0170.001022.405.423.220.0150.14000.0140.0042000.0070.012000.004400.00150000Conforming SteelG0.01920.461.480.0160.000922.145.313.180.0180.14030.0130.0041000.0070.030000.002600.00300000Conforming SteelH0.02120.471.470.0170.000922.324.463.200.0180.14150.0270.0051000.0080.020000.002700.00250000.490Conforming SteelI0.01800.460.790.0190.000824.957.493.220.0240.09990.0250.00620.290.520.0090.020000.002700.00230000Conforming SteelJ0.02150.490.770.0180.000725.096.823.190.0150.15210.0120.00530.290.520.0100.0180.0700.003400.002600.002000.560Conforming SteelK0.01910.470.800.0180.000925.046.503.200.0140.20690.0230.00630.290.520.0100.022000.002800.0035000.0500.040Conforming SteelL0.02040.280.780.0180.001024.876.913.690.0210.24810.0240.0065000.0100.019000.003000.0030000.1100Conforming SteelM0.02170.290.780.0170.001224.836.603.670.0140.29200.0210.0068000.0130.019000.002700.00330.10000.0600Conforming SteelN0.02300.280.780.0170.001324.646.193.650.0150.32160.0270.0044000.0130.019000.002800.00240000Conforming SteelO0.01970.451.670.0240.001022.015.323.080.0180.17470.0280.00360.050.260.0640.024000.002100.00060000Conforming SteelP0.01850.320.500.0210.001025.276.953.640.0100.27130.0300.00520.500.520.1020.004000.002000.000600.00500.0400Conforming SteelQ0.01500.491.470.0280.000422.405.853.100.0020.17200.0170.00660.040.260.0710.0150.0100.003000.00120.040000.760Conforming SteelR0.01400.520.630.0290.000625.106.503.120.0060.14400.0180.00860.140.210.0780.012000.002400.00210000Conforming SteelS0.01000.501.130.0230.000422.506.283.280.0040.17000.0340.00900.020.070.0970.0070000.00100.00310000Conforming SteelT0.02000.381.080.0230.000625.806.553.060.0020.20500.0070.00920.030.120.0860.02000.030.00240.00200.00180000Conforming SteelU0.02020.390.780.0160.000825.186.422.920.0100.17250.0270.00500.210.4900000.0005000.130000Conforming SteelV0.02060.390.790.0160.000924.946.492.940.0100.17120.0090.00460.210.5000000.0028000000Conforming SteelW0.01990.390.200.0160.000925.036.492.940.0100.17470.0200.00440.210.5000000.0003000000Conforming SteelX0.02430.120.790.0160.000725.206.462.950.0100.17790.0240.00480.210.5000000.000400000.1100Conforming SteelY0.02130.120.190.0170.000825.116.472.940.0100.16920.0100.00390.200.50000.0500.0003000.05000.0600Conforming SteelZ0.02250.110.190.0160.000824.876.512.980.0100.17010.0100.00480.210.5000000.0029000000Conforming SteelUnderline: outside the scope of the invention [Table 1-2] Steel Grade No.Chemical Composition (mass%)ClassificationCSiMnPSCrNiMoAlNSbOWCuVNbTiZrBREMCaSnMgTaCoAA0.02110.501.490.0160.000822.425.463.340.0240.108500.00420.3900.0100000.002900.00240000Comparative SteelAB0.01860.491.510.0150.000822.515.453.330.0400.12980.0180.003300.600.008000.040.00640.073000000Conforming SteelAC0.01980.491.500.0170.000922.385.463.380.0190.133600.005201.090.0080000.0028000.020000Comparative SteelAD0.02070.491.520.0180.000722.445.443.300.0220.13540.0190.001801.090.0080000.006300000.0600Conforming SteelAE0.01990.491.510.0160.000722.325.443.310.0220.135000.002801.090.0080000.00840.063500.011000Comparative SteelAF0.01840.521.510.0150.000822.415.463.420.0260.12700.0090.0029000.01000.010.020.00290.022000000Conforming SteelAG0.02030.531.570.0170.000923.585.653.600.0300.134900.0018000.0100000.0027000000Comparative SteelAH0.02030.531.570.0170.000923.585.653.600.0300.134900.0018000.0100000.0027000000Comparative SteelAI0.01980.510.800.0160.000825.457.443.260.0270.120700.00450.290.590.0090000.0027000000Comparative SteelAJ0.01980.510.800.0160.000825.457.443.260.0270.120700.00450.290.590.0090000.0027000000Comparative SteelAK0.02200.500.820.0180.000725.367.493.280.0420.113600.00460.270.610.0090000.00590.014500.021000Comparative SteelAL0.01880.490.820.0170.000725.367.483.250.0270.116700.00440.261.090.0080000.0062000.008000Comparative SteelAM0.02110.470.820.0170.000925.377.513.270.0210.11320.0090.00470.281.090.0080000.0093000000Conforming SteelAN0.01900.500.830.0180.000725.247.493.280.0340.11760.0160.00280.281.090.00800.0200.00940.0220000.007000.050Conforming SteelAO0.01750.480.810.0160.001025.207.473.220.0270.12460.0160.00480.281.590.0070000.00300.013000000Conforming SteelAP0.01930.490.820.0180.001125.477.523.260.0200.11230.0100.00460.281.600.00700.050.020.00580.012500000Conforming SteelAQ0.01680.500.770.0150.000922.155.443.150.0200.170200.0025000.0080000.00320000.00300.0820.030Comparative SteelAR0.01560.511.510.0260.000822.285.453.410.0230.176600.0049000.0100000.002800000.1200Comparative SteelAS0.01550.531.580.0270.000823.425.663.390.0310.099900.0020000.0100000.003400000.0740.100Comparative SteelAT0.01500.510.780.0150.000822.345.433.400.0240.098600.0035000.0090000.00280000.00300.1230.040Comparative SteelAU0.01550.500.760.0160.001022.205.493.170.0210.094400.0046000.0080000.003500000.0080.070Comparative SteelAV0.01600.501.480.0260.000922.355.863.140.0230.176300.00320.030.310.0750.016000.003100.000800.005000Comparative SteelAW0.02020.471.470.0190.000921.357.932.710.0250.05410.0300.0048000.0070.02100.030.003000.003000.00500.0600Comparative SteelAX0.01420.570.770.0170.000923.124.463.500.0180.06150.0300.00510.2000.0080.020000.002700.00250000.490Conforming SteelAY0.02300.280.780.0170.001323.946.292.950.0150.32160.0300.0044000.0130.019000.002800.00240000Conforming SteelAZ0.02600.170.170.0340.001821.404.872.970.0250.06350.0090.004100.4900000000000Conforming SteelBA0.02600.180.160.0350.001821.394.862.970.0250.06450.0090.00510000000000000Conforming Steel Underline: outside the scope of the invention [Table 2] Steel Pipe No.Steel Grade No.Heat TreatmentCold WorkingSolution TreatmentHeating Temperature (°C)Soaking Time (min)Cooling1A105085Water CoolingYes2B106090Water CoolingYes3C104570Water CoolingYes4D104550Water CoolingNo5E1065120Water CoolingYes6F106585Water CoolingNo7G107555Water CoolingYes8H107045Water CoolingNo9I107045Water CoolingYes10J107055Water CoolingYes11K107590Water CoolingYes12L106585Water CoolingYes13M106085Water CoolingYes14N105595Water CoolingYes15O104060Water CoolingYes16P108075Water CoolingYes17Q107080Water CoolingYes18R108055Water CoolingYes19S104045Water CoolingNo20T1060115Water CoolingNo21U1070120Water CoolingNo22V105540Water CoolingNo23W108095Water CoolingNo24X108035Water CoolingNo25Y1050115Water CoolingNo26Z1030120Water CoolingNo27AA106535Water CoolingNo28AB108075Water CoolingNo29AC104080Water CoolingNo30AD108050Water CoolingNo31AE104065Water CoolingNo32AF1060105Water CoolingNo33AG106530Water CoolingNo34AH1065100Water CoolingNo35AI1040120Water CoolingNo36AJ105050Water CoolingNo37AK106555Water CoolingNo38AL107070Water CoolingNo39AM1055120Water CoolingNo40AN103555Water CoolingNo41AO106045Water CoolingNo42AP107065Water CoolingNo43AQ107570Water CoolingNo44AR106065Water CoolingNo45AS103035Water CoolingNo46AT105035Water CoolingNo47AU1065100Water CoolingNo48AV1030100Water CoolingNo49AW105030Water CoolingNo50AX105030Water CoolingNo51AY105030Water CoolingNo52AZ105030Water CoolingYes53BA105030Water CoolingYes Underline: outside the scope of the present invention [Table 3] Steel Pipe No.Steel Grade No.MicrostructuresTensile PropertiesLow-Temperature ToughnessCarbon Dioxide Gas Corrosion Resistance TestSSC Resistance TestSCC Resistance TestAcid EnvironmentCCS-Simulated Corrosion TestNotesAusteniteFerriteYield Strength YSTensile Strength TSCharpy Absorbed Energy at -40°CCorrosion RatePitting CorrosionCorrosion RateCorrosion RatePitting Corrosion of 0.2 mm or GreaterPitting Corrosion of 0.1 mm or Greater and Less Than 0.2 mm(Area %)(Area %)(MPa)(MPa)(J)(mm / v)(mmlv)(mm / v)1A54468068493350.001No○○1670.001NoYesInvention Example2B50508469023210.003No○○1630.001NoNoInvention Example3C49518509163110.014No○○1650.002NoYesInvention Example4D50505487273210.002No--1670.001NoYesInvention Example5E51498058663240.007No○○1670.002NoNoInvention Example6F50505087043060.001No--1660.001NoNoInvention Example7G51498158703240.009No○○1640.001NoNoInvention Example8H43575557202800.016No--1670.001NoYesInvention Example9I52488588883010.001No○○900.001NoYesInvention Example10J51498389052890.001No○○870.001NoNoInvention Example11K53478509083060.001No○○880.002NoYesInvention Example12L57438459293560.001No○○900.002NoYesInvention Example13M59418399393670.001No○○890.001NoNoInvention Example14N59418519493700.001No○○880.001NoYesInvention Example15O57438229293460.009No○○1660.001NoYesInvention Example16P59418719663430.001No○○870.002NoYesInvention Example17Q58429229793460.001No○○1650.002NoNoInvention Example18R45558939562810.001No○○870.002NoNoInvention Example19S58425077623750.001No--1630.002NoYesInvention Example20T49515678003060.001No--900.002NoNoInvention Example21U51495607493130.001No--890.002NoYesInvention Example22V52485597483230.001No--890.002NoNoInvention Example23W52485557453190.001No--890.002NoNoInvention Example24X53475467383260.001No--870.001NoYesInvention Example25Y52485437363210.001No--870.001NoNoInvention Example26Z54465487403300.001No--890.001NoNoInvention Example27AA46545367102050.001No--1820.002YesYesComparative Example28AB52485136991030.001No--1670.001NoNoInvention Example29AC56445267262840.001No--1740.002YesYesComparative Example30AD56445157181700.001No--1660.002NoNoInvention Example31AE5644530713770.001No--1740.001YesYesComparative Example32AF48525207092720.001No--1630.001NoNoInvention Example33AG44565217442820.001No--960.001YesYesComparative Example34AH44565307382820.001No--960.002YesYesComparative Example35AI51495387562920.001No--1030.001YesYesComparative Example36AJ51495507522920.001No--1020.001YesYesComparative Example37AK52485587381950.001No--1040.001YesYesComparative Example38AL55455607462090.001No--1070.001YesYesComparative Example39AM55455607461720.001No--880.002NoNoInvention Example40AN55455557441760.001No--900.002NoNoInvention Example41AO59415717671970.001No--870.002NoNoInvention Example42AP57435857671520.001No--880.002NoNoInvention Example43AQ54465207263350.007No--1750.002YesYesComparative Example44AR53475297423320.001No--1770.002YesYesComparative Example45AS42585547172670.001No--1760.001YesYesComparative Example46AT45555246882880.001No--1750.001YesYesComparative Example47AU47535126792950.005No--1730.002YesYesComparative Example48AV59415147333470.001No--1770.002YesYesComparative Example49AW19814396683520.001No--1640.001NoYesComparative Example50AX25755997871950.001No--870.002NoYesInvention Example51AY65355277463140.001No--870.002NoYesInvention Example52AZ50507769123090.026No××1600.003NoNoInvention Example53BA51497949292920.027No××1500.002NoNoInvention 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.

[0080] 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 CO 2 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.

Examples

examples

[0067]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.

[0068]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 ...

Claims

1. 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.

2. 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%.

3. 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.

4. 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.

5. 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%.

6. 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.

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