DUPLEX STAINLESS STEEL AND METHOD FOR MANUFACTURING THE SAME, AND DUPLEX STAINLESS STEEL PIPE.

Abstract

The invention is intended to provide a duplex stainless steel and a method for manufacturing the same. A duplex stainless steel pipe is also provided. A duplex stainless steel of the present invention has a specific composition and microstructure containing an austenitic phase and a ferrite phase. The duplex stainless steel satisfies the following contents of C, Si, Mn, Cr, Mo, Ni, N, Cu, and W in formula (1) below, and has a yield strength YS of 655 MPa or more, and an energy absorption vE-10 of 40 J or more as measured by a Charpy impact test at a test temperature of -10°C. 0.55[%C] - 0.056[%Si] + 0.018[%Mn] - 0.020[%Cr] - 0.087[%Mo] + 0.16[%Ni] + 0.28[%N] - 0.506[%Cu] - 0.035[%W] + [%Cu*F] = 0.94 (1).

Description

The present invention relates to a high-strength, high-toughness duplex stainless steel with excellent corrosion resistance, suitable for oilfield tubular articles, and to a method for manufacturing such duplex stainless steel. Specifically, the present invention relates to a duplex stainless steel for use as steel tubing for oilfield tubular articles, and to a method for manufacturing such duplex stainless steel. The present invention also relates to a duplex stainless steel pipe utilizing duplex stainless steel. BACKGROUND Rising crude oil prices and an anticipated shortage of petroleum resources in the near future have spurred the active development of oilfield tubular products for applications previously unthinkable, such as deep-water oil fields and oil and gas fields in severely corrosive environments containing hydrogen sulfide, also known as sour environments. These oil and gas fields are typically located at great depths, creating a high-temperature atmosphere of a severely corrosive environment containing CO2, Cl-, and H2S. Steel pipes for oilfield tubular products used in such environments must possess high strength and toughness, along with desirable corrosion resistance (resistance to carbon dioxide corrosion, resistance to sulfide corrosion cracking, and resistance to sulfide cracking). In oil and gas fields with environments containing substances such as CO2 and Cl, a variety of duplex stainless steel pipes have traditionally been used as oilfield tubular products for mining operations. For example, PTL 1 describes a method for manufacturing a high-strength duplex stainless steel with improved corrosion resistance. The method involves hot-working a copper-containing austenite-ferrite duplex stainless steel by heating it to 1,000°C or higher, and then directly quenching the steel from a temperature of 800°C or higher before heat treatment. PTL 2 describes a method for manufacturing a precipitation-hardened duplex stainless steel resistant to seawater. The method includes solution treatment of a precipitation-hardened duplex stainless steel resistant to seawater at 1,000°C or higher, and subsequent wear heat treatment at 450 to 600°C. The stainless steel subjected to these processes is a stainless steel containing, in % by weight, C: 0.03% or less, Si: 1% or less, Mn: 1.5% or less, P: 0.04% or less, S: 0.01% or less, Cr: 20 to 26%, Ni: 3 to 7%, Sol-Al: 0.03% or less, N: 0.25% or less, and Cu: 1 to 4%, and additionally, at least one of Mo: 2 to 6% and W: 4 to 10%, all of Ca: 0 to 0.005%, Mg: 0 to 0.05%, B: 0 to 0.03%, and Zr: 0 to 0.3%, and a total of 0 to 0.03% of Y, La, and Ce, and meeting a PT value of PT > 35 as a water resistance index of sea and a G value of 70 > G > 30 as a fraction of austenite. PTL 3 describes a method for manufacturing a high-strength duplex stainless steel material that can be used in applications such as logging lines for tubular products in deep oil and gas wells. In this method, a copper-containing austenite-ferrite duplex stainless steel material, after solution treatment, is cold-worked to reduce the cross-sectional area by 35% or more. Following cold working, the steel is quenched by heating to a temperature range of 800 to 1150°C at a heating rate of 50°C / s or more. After hot working at 300 to 700°C, the steel is further cold-worked, with or without subsequent aeration, at 450 to 700°C. PTL 4 describes a method for manufacturing a duplex stainless steel for sour gas oil country tubular products. The method includes a solution heat treatment of 1000 to 1150°C, and a subsequent wear heat treatment at 450 to 500°C for 30 to 120 minutes, using a steel containing C: 0.02 wt% or less, Si: 1.0 wt% or less, Mn: 1.5 wt% or less, Cr: 21 to 28 wt%, Ni: 3 to 8 wt%, Mo: 1 to 4 wt%, N: 0.1 to 0.3 wt%, Cu: 2 wt% or less, W: 2 wt% or less, Al: 0.02 wt% or less, Ti, V, Nb, Ta: 0.1 wt% or less of each, Zr, B: 0.01 wt% or less of each, P: 0.02 wt% or less, and S: 0.005 wt% or less. PTL 5 describes a method for manufacturing a high-strength, high-toughness duplex stainless steel, using a steel containing C: 0.03% or less, Si: 1.0% or less, Mn: 0.10 to 1.5%, P: 0.030% or less, S: 0.005% or less, Cr: 20.0 to 30.0%, Ni: 5.0 to 10.0%, Mo: 2.0 to 5.0%, Cu: 2.0 to 6.0%, and N: less than 0.07%. The method includes a solution heat treatment in which the steel is heated to a temperature of 1,000°C or higher and cooled to a temperature of 300°C or lower at an average air-cooling rate or faster, and a subsequent wear heat treatment that heats the steel to 350°C to 600°C before cooling. APPOINTMENT LIST Patent Literature PTL 1: JP-A-S61-23713 PTL 2: JP-A-H10-60526 PTL3: JP-A-H07-207337 PTL 4: JP-A-S61-157626 PTL 5: National Re-publication of PCT Patent Application No. 2018-43214 BRIEF DESCRIPTION OF THE INVENTION Technical Problem The recent development of oil and gas fields in increasingly severe corrosive environments has demanded high-strength, high-toughness steel pipe for oilfield tubular products with excellent corrosion resistance. Here, excellent corrosion resistance means excellent resistance to carbon dioxide corrosion at high temperatures of 200°C and above, excellent resistance to sulfide stress corrosion cracking (SCC) at low temperatures of 80°C and below, and excellent resistance to sulfide stress cracking. IV l4 JO» (SSC resistance) at ordinary temperatures of 20 to 30°C, particularly in a severely corrosive environment containing CO2, Cl· and H2S. There is also a demand for improved economy (cost and efficiency). However, the steels described in PTL 1 to PTL 4 do not take into account resistance to low-temperature sulfide stress corrosion cracking at 80°C or below. Resistance to sulfide stress cracking is also not considered in these related technical documents. PTL 5 states that the steel described therein has desirable resistance to low-temperature sulfide stress corrosion cracking at 80°C or below, and desirable resistance to sulfide stress cracking. However, PTL 5 does not describe whether pitting corrosion is present or absent at low temperatures of 80°C and below. The present invention has been developed to provide a solution to the aforementioned problems, and it is an object of the present invention to provide a high-strength, high-toughness duplex stainless steel that has excellent corrosion resistance, and a method for manufacturing such duplex stainless steel. Here, excellent corrosion resistance means having excellent resistance to carbon dioxide corrosion, excellent resistance to sulfide stress corrosion cracking, and excellent resistance to sulfide stress bursting even in a severe corrosive environment such as the one described above. A pipe made of such duplex stainless steel is suitable for use in harsh environments, such as in crude oil or natural gas wells, and in gas wells. As used in this document, high strength means strength with a yield strength of 95 ksi (655 MPa) or greater. As used in this document, high toughness means low-temperature toughness, specifically an absorption energy vE-10 of 40 J or greater as measured by a Charpy impact test at -10°C. As used in this document, excellent resistance to carbon dioxide corrosion means that a test specimen immersed in a test solution (a 20 wt% aqueous NaCl solution; liquid temperature: 200°C; CO2 gas atmosphere of 3.0 MPa) maintained in an autoclave has a corrosion rate of 0.125 mm / y less with no pitting corrosion after 336 hours of immersion in the solution.As used herein, excellent sulfide stress corrosion cracking resistance means that a test specimen immersed in a test solution (a 10 wt% aqueous NaCl solution; liquid temperature: 80°C; an atmosphere of 2 MPa CO2 gas and 35 kPa H2S) maintained in an autoclave exhibits no cracking or pitting corrosion after 720 hours of immersion under an applied stress equal to 100% of the yield strength. As used herein, excellent sulfide stress corrosion cracking resistance means that a test specimen immersed in a test solution (an aqueous solution with a pH adjusted to 3.5 by the addition of acetic acid and sodium acetate to a 20 wt% aqueous NaCl solution (liquid temperature: 25°C; an atmosphere of 0.07 MPa CO2 gas and 0.07 kPa H2S)) maintained in an autoclave exhibits no cracking or pitting corrosion after 720 hours of immersion under an applied stress equal to 100% of the yield strength).03 MPa H2S)) maintained in a test cell has no cracks or pitting corrosion after 720 hours of immersion under an applied stress equal to 90% of the elastic limit. Solution to the Problem In order to achieve the above objective, the present inventors carried out studies JO» intensive studies of several factors affecting the strength, toughness, carbon dioxide corrosion resistance, sulfide stress corrosion cracking resistance, and sulfide stress cracking resistance of a duplex stainless steel. The studies led to the following findings: 1) In a duplex stainless steel containing 2.0% or more Cu, copper tends to assume a supersaturated state in the ferrite phase during cooling after hot rolling and creates coarse ε-Cu precipitates in the ferrite phase. 2) The coarse ε-Cu after hot rolling cannot be easily removed with ordinary solution treatment, and removal requires long hours of heating. 3) In a material subjected to solution treatment and wear, the coarse ε-Cu remaining in the ferrite phase becomes a corrosion starting point and tends to cause selective corrosion in the ferrite phase by providing a pitting corrosion starting point. 4) Copper supersaturation can be overcome by heat treatment that causes precipitation of the o phase, which barely dissolves copper. Brief heating during the heat treatment promotes copper migration from the ferrite phase to the austenitic phase, and the amount of coarse ε-Cu in the ferrite phase can be greatly reduced by subsequent solution treatment. 5) The coarse ε-Cu present or absent in the ferrite phase has a correlation with the degree of copper supersaturation, and the resistance to selective corrosion improves when C, Si, Mn, Cr, Mo, Ni, N, Cu and W meet the following content ranges to comply with the following formula (1). 0.55[%C] - 0.056[%S¡] + 0.018[%Mn] - 0.020[%Cr] - 0.087[%Mo] + 0.16[%N¡] + 0.28[%N] 0.506[%Cu] - 0.035[%W] + [%Cu*F] < 0.94 (1) In formula (1), [% element symbol] represents the content (% by mass) of the element in the steel, and [% element symbol * F] represents the content (% by mass) of the element in the ferrite phase. The content is zero for elements that are not contained. The present invention was completed based on these findings, and the essence of the invention is as follows. [1] A duplex stainless steel having a composition that includes, in % by mass, C: 0.03% or less, Si: 1.0% or less, Mn: 0.10 to 1.5%, P: 0.040% or less, S: 0.01% or less, Cr: 20.0 to 28.0%, Ni: 2.0 to 10.0%, Mo: 2.0 to 5.0%, Cu: 2.0 to 6.0%, Al: 0.001 to 0.05% and N: less than 0.070%, and wherein the remainder is Fe and incidental impurities, the duplex stainless steel having a microstructure containing an austenitic phase and a ferrite phase, and meeting the following contents for C, Si, Mn, Cr, Mo, Ni, N, Cu and W in formula (1) below, the stainless steel duplex having a yield strength YS of 655 MPa or more, and an absorption energy vE-10 of 40 J or more, as measured by a Charpy impact test at a test temperature of -10°C, 0.55[%C] - 0.056[%Si] + 0.018[%Mn] - 0.020[%Cr] - 0.087[%Mo] + 0.16[%Ni] + 0.28[%N] 0.506[%Cu] - 0.035[%W] + [%Cu*F] < 0.94 (1), where [% element symbol] represents the content (% by mass) of the element in the steel, [% element symbol * F] represents the content (% by mass) of the element in the ferrite phase, and the content is zero for elements that are not contained. [2] Duplex stainless steel in accordance with [1], wherein the composition further includes, in % by mass, one or two or more groups selected from the following groups A to E, group A: W: 1.5% or less, group B: V: 0.20% or less, group C: one or two selected from Zr: 0.50% or less, and B: 0.0030% or less, group D: one or two or more selected from REM: 0.005% or less, Ca: 0.005% or less, Sn: 0.20% or less, and Mg: 0.01% or less, group E: one or two or more selected from Ta: 0.1% or less, Co: 1.0% or less and Sb: 1.0% or less. [3] A duplex stainless steel pipe that uses duplex stainless steel from [1] or [2], [4] A method for manufacturing a duplex stainless steel, comprising subjecting a steel material of the composition of [1] or [2] to a σ phase precipitation treatment that heats the steel material to a temperature of 700°C or more and 950°C or less, and cools the heated steel material to a temperature of 300°C or less at an average cooling rate of air cooling or faster, a solution heat treatment that heats the steel material to a temperature of 1,000°C or more, and cools the heated steel material to a temperature of 300°C or less at an average cooling rate of air cooling or faster, and a wear heat treatment that heats the steel material to a temperature of 350 to 600°C, and cools the heated steel material. Advantageous Effects of the Invention With the present invention, a duplex stainless steel can be obtained that has high strength with a yield strength of 95 ksi or more (655 MPa or more), high toughness with an absorption energy vE-w of 40 J or more as measured by a Charpy impact test at -10°C, and excellent corrosion resistance, including excellent resistance to carbon dioxide corrosion, excellent resistance to sulfide stress corrosion cracking, and excellent resistance to sulfide stress cracking, even in a severe corrosive environment containing hydrogen sulfide. A duplex stainless steel manufactured according to the present invention is applicable to seamless stainless steel piping for oilfield tubular products. This makes the present invention very useful in the industry. DESCRIPTION OF MODALITIES Duplex Stainless Steel Composition The reasons for limiting the composition of a duplex stainless steel of the present invention are described first. The percentage used, along with the component content, is then a mass percentage. C: 0.03% or less Carbon is an element that improves strength and low-temperature toughness by stabilizing the austenitic phase. The carbon content is preferably 0.002% or more to achieve high strength with a yield strength of 95 ksi or more (655 MPa or more) and low-temperature toughness with a vE-10 absorption energy of 40 J or more in a Charpy impact test. More preferably, the carbon content is 0.005% or more. A carbon content above 0.03% can lead to excessive carbide precipitation during heat treatment and adversely affect corrosion resistance. For this reason, the carbon content is 0.03% or less. Preferably, the carbon content is 0.02% or less. More preferably, the carbon content is 0.015% or less, and even more preferably 0.012% or less. Yes: 1.0% or less Silicon (Si) is an element that acts as a deoxidizing agent. The Si content is preferably 0.05% or more to achieve this effect. More preferably, the Si content is 0.10% or more. However, a Si content above 1.0% leads to excessive precipitation of intermetallic compounds during heat treatment and impairs the corrosion resistance of the steel. For this reason, the Si content is 1.0% or less. The Si content is preferably 0.8% or less, more preferably 0.7% or less, and even more preferably 0.6% or less. Mn: 0.10 to 1.5% Manganese (Mn) is an effective deoxidizing agent, similar to silicon (Si). Mn improves hot workability by fixing the secondary element sulfur (S) in the steel as sulfide. These effects can be achieved when the Mn content is 0.10% or more. For this reason, the Mn content is 0.10% or more. The Mn content is preferably 0.15% or more, and more preferably 0.20% or more. A Mn content above 1.5% adversely affects corrosion resistance and also impairs hot workability. For this reason, the Mn content is 1.5% or less. The Mn content is preferably 1.0% or less, more preferably 0.8% or less, and even more preferably 0.5% or less. P: 0.040% or less Phosphorus (P) is an element that reduces the corrosion resistance of duplex stainless steel, and corrosion resistance is seriously compromised when the P content exceeds 0.040%. For this reason, the P content is 0.040% or less. Preferably, the P content is 0.020% or less. However, reducing the P content to less than 0.005% requires a lengthy processing time to remove the phosphorus during the refining of the molten iron, which increases the manufacturing cost of duplex stainless steel. Therefore, the P content is preferably 0.005% or more. S: 0.01% or less Sulfur (S) is an element that impairs hot workability in the production of duplex stainless steel and causes manufacturing problems when present in amounts exceeding 0.01%. For this reason, the S content is 0.01% or less. Preferably, the S content is 0.005% or less. To avoid increased manufacturing costs, the S content is preferably 0.0005% or more. Cr: 20.0 to 28.0% Chromium (Cr) is a basic component that is effective in maintaining corrosion resistance and improving toughness. A Cr content of 20.0% or more is ideal for these effects. For improved toughness, the Cr content is preferably 21.0% or more, and more preferably 23.0% or more. A Cr content above 28.0% promotes the precipitation of the σ phase and impairs both corrosion resistance and toughness. For this reason, the Cr content is 28.0% or less. From a toughness standpoint, the Cr content is preferably 27.0% or less. Ni: 2.0 to 10.0% Ni is an element included to stabilize the austenitic phase and create a duplex microstructure. This effect cannot be achieved when the Ni content is less than 2.0%. For this reason, the Ni content is 2.0% or more. The Ni content is preferably 3.0% or more. The Ni content is more preferably 4.0% or more. With a Ni content greater than 10.0%, the austenitic phase predominates, and the desired strength of the present invention cannot be obtained. Since Ni is an expensive element, a Ni content greater than 10.0% is also undesirable from an economic standpoint. For this reason, the Ni content is 10% or less. Preferably, the Ni content is 8% or less. Mo: 2.0 to 5.0% Molybdenum (Mo) is an element that improves the corrosion resistance of duplex stainless steel and helps prevent corrosion, particularly pitting corrosion due to Cl·. This effect cannot be achieved when the Mo content is less than 2.0%. For this reason, the Mo content is 2.0% or higher. Preferably, the Mo content is 2.5% or higher. A Mo content greater than 5.0% causes precipitation of the σ phase and impairs toughness and corrosion resistance. For this reason, the Mo content is 5.0% or lower. Preferably, the Mo content is 4.5% or lower. Cu: 2.0 to 6.0% Copper greatly improves strength by forming fine precipitates of ε-Cu during wear heat treatment. Copper also strengthens the protective layer to reduce hydrogen ingress into the steel and improves resistance to sulfide stress cracking and sulfide stress corrosion cracking. This makes copper a very important element in the present invention. The copper content is 2.0% or more to obtain these effects. Preferably, the copper content is 2.5% or more. A copper content of more than 6.0% impairs low-temperature toughness. For this reason, the copper content is 6.0% or less. The copper content is preferably 5.5% or less. The copper content is more preferably 5.0% or less. Al: 0.001 to 0.05% Aluminum (Al) acts as a deoxidizing agent in the refining process of raw cast iron for duplex stainless steel production. This effect is not achieved when the Al content is below 0.001%. Therefore, the Al content is 0.001% or higher. Preferably, the Al content is 0.005% or higher. An Al content above 0.05% promotes the precipitation of alumina inclusions and impairs hot workability in duplex stainless steel production, resulting in decreased toughness. For this reason, the Al content is 0.05% or lower. Preferably, the Al content is 0.04% or lower. N: less than 0.070% In typical duplex stainless steels, nitrogen (N) is known to improve pitting corrosion resistance and contribute to solid solution strengthening. For this purpose, N is actively added in an amount of 0.10% or more. However, in wear heat treatment, N forms various nitrides and decreases sulfide stress corrosion cracking resistance in the low temperature range of 80°C or less. This becomes more pronounced when the N content is 0.070% or more. For this reason, the N content is less than 0.070%. The N content is preferably 0.05% or less, more preferably 0.04% or less, more preferably 0.03% or less, and even more preferably 0.015% or less. The N content is preferably 0.001% or more to obtain the desired properties of the present invention. More preferably, the N content is 0.0.05% or more. The remainder is iron and incidental impurities. Examples of incidental impurities include oxygen (O), and an O content of 0.01% or less is acceptable. These represent the basic components. In addition to the basic components above, the composition may optionally contain one or two or more groups selected from the following groups A to E, as required. Group A: W: 1.5% or less Water (W) is useful as an element that improves resistance to sulfide stress corrosion cracking and sulfide stress cracking. Desirably, W is contained in an amount of 0.02% or more to achieve this effect. The W content is more preferably 0.3% or more, and even more preferably 0.8% or more. When contained in an excessively large amount of more than 1.5%, W can cause a decrease in low-temperature toughness. For this reason, W, when included, is contained in an amount of 1.5% or less. More preferably, the W content is 1.2% or less. Group B: V: 0.20% or less Zinc (V) is useful as an element that improves the strength of steel through precipitation hardening. Desirably, V is contained in an amount of 0.02% or more to achieve this effect. More preferably, the V content is 0.04% or more. When contained in an amount greater than 0.20%, V can cause a decrease in low-temperature toughness. An excessively high V content can result in a decrease in resistance to sulfide stress cracking. For this reason, V, when present, is contained in an amount of 0.20% or less. More preferably, the V content is 0.08% or less. Group C: one or two selected from Zr: 0.50% or less, and B: 0.0030% or less Zr and B are useful as elements that contribute to increasing strength and can be selectively contained as needed. Zinc (Zr) also contributes to improved resistance to sulfide stress corrosion cracking, in addition to increasing strength. Ideally, Zr is present at a concentration of 0.02% or more to achieve these effects. More preferably, the Zr content is 0.05% or more. When present at concentrations above 0.50%, Zr can lead to a decrease in low-temperature toughness. For this reason, when Zr is present, it is present at a concentration of 0.50% or less. More preferably, the Zr content is 0.20% or less. B is useful as an element that also contributes to improving hot workability, in addition to increasing strength. Desirably, B is contained in an amount of 0.0005% or more to achieve these effects. More preferably, the B content is 0.0010% or more. When contained in an amount greater than 0.0030%, B can cause a decrease in low-temperature toughness and hot workability. For this reason, B, when present, is contained in an amount of 0.0030% or less. More preferably, the B content is 0.0025% or less. Group D: One or two or more selected from REM: 0.005% or less, Ca: 0.005% or less, Sn: 0.20% or less and Mg: 0.01% or less REM, Ca, Sn, and Mg are all useful as elements that contribute to improving resistance to sulfide stress corrosion cracking and can be selectively contained as needed. The preferred contents for this effect are REM: 0.001% or more, Ca: 0.001% or more, Sn: 0.05% or more, and Mg: 0.0002% or more. More preferably, the contents are REM: 0.0015% or more, Ca: 0.0015% or more, Sn: 0.09% or more, and Mg: 0.0005% or more. When contents exceed REM: 0.005%, Ca: 0.005%, Sn: 0.20%, and Mg: 0.01%, the increased contents do not always produce the expected effect due to saturation of the effect, which can be an economic disadvantage. For this reason, when present, the contents of these elements are REM: 0.005% or less, Ca: 0.005% or less, Sn: 0.20% or less, and Mg: 0.01% or less. More preferably, the contents are REM: 0.004% or less, Ca: 0.004% or less, Sn: 0.15% or less, and Mg: 0.005% or less. Group E: One or two or more selected from Ta: 0.1% or less, Co: 1.0% or less, and Sb: 1.0% or less Ta, Co, and Sb are all useful as elements that contribute to improving carbon dioxide corrosion resistance, sulfide stress cracking resistance, and sulfide stress corrosion cracking resistance, and can be selectively contained as needed. When contained to produce this effect, the contents of these elements are Ta: 0.01% or more, Co: 0.01% or more, and Sb: 0.01% or more. More preferably, the contents are Ta: 0.02% or more, Co: 0.02% or more, and Sb: 0.02% or more. When the contents are greater than Ta: 0.1%, Co: 1.0%, and Sb: 1.0%, the increased contents do not always produce the expected effect due to saturation of the effect. For this reason, when contained, the contents of these elements are Ta: 0.1% or less, Co: 1.0% or less, and Sb: 1.0% or less. More preferably, the contents are Ta: 0.05% or less, Co: 0.5% or less and Sb: 0.5% or less. The contents of C, Si, Mn, Cr, Mo, Ni, N, Cu, and optionally W, are adjusted to comply with the following formula (1). In formula (1), [%element symbol] represents the content (% by mass) of the element in the steel, and [%element symbol * F] represents the content (% by mass) of the element in the ferrite phase. The content is zero for elements that are not present. 0.55[%C] - 0.056[%S¡] + 0.018[%Mn] - 0.020[%Cr] - 0.087[%Mo] + 0.16[%N¡] + 0.28[%N] 0.506[%Cu] - 0.035[%W] + [%Cu*F] < 0.94 (1) Pitting corrosion resistance improves when the contents of C, Si, Mn, Cr, Mo, Ni, N, Cu, and optionally W, and the Cu content in the ferrite phase conform to formula (1). On the left-hand side of formula (1), a value obtained by multiplying the value of the linear expression for the component contents (the left-hand side of formula (1) excluding [% Cu * F]) by -1 approximates the equilibrium value of the Cu content in the ferrite phase. That is, the value on the left-hand side of formula (1) represents the difference between the equilibrium value of the Cu content in the ferrite phase and the actual Cu content in the ferrite phase, and corresponds to the degree of copper supersaturation.The value on the left-hand side of formula (1) is an index of the amount of ε-Cu in the ferrite phase. As the amount of coarse ε-Cu increases, the pitting corrosion resistance decreases. To further improve pitting corrosion resistance, the value on the left-hand side of formula (1) is preferably 0.92 or less. The lower limit is not particularly restricted. To ensure stable resistance, the value on the left-hand side of formula (1) is preferably 0.80 or more. The copper content in the ferrite phase can be determined as follows, for example. When the duplex stainless steel of the present invention is a pointless steel pipe, a test sample is taken for microstructure observation from an axial cross-sectional surface, and the ferrite phase is identified by EBSP (Electron Backscatter Diffraction) analysis. The copper content in the identified ferrite phase in each test sample is then measured at 20 arbitrarily selected points using a Field Emission Electron Probe Microanalyzer (FE-EPMA). The average of these measured copper content values ​​is determined to be the copper content of the ferrite phase in the steel. Duplex Stainless Steel Microstructure The duplex stainless steel of the present invention has a microstructure containing an austenitic phase and a ferrite phase. The volume fraction (%) of the austenitic phase is preferably from 20 to 70%. The volume fraction (%) of the ferrite phase is preferably from 30 to 80%. Less than 20% austenitic phase may result in decreased low-temperature toughness, sulfide stress crack resistance, and sulfide stress corrosion crack resistance. More than 70% austenitic phase may result in decreased strength. The austenitic phase is more preferably 25% or more, and even more preferably 65% ​​or less. Less than 30% ferrite phase may result in decreased strength.More than 80% ferrite phase can result in decreased low-temperature toughness, sulfide stress crack resistance, and sulfide stress corrosion crack resistance. The ferrite phase is more preferably 35% or more, and even more preferably 75% or less. In the present invention, the volume fraction of each phase can be measured using the method described in the following Examples. Duplex Stainless Steel Manufacturing Method A method for manufacturing a duplex stainless steel pipe is described below as an exemplary method of manufacturing a duplex stainless steel of the present invention. The method described below is based on an example where the duplex stainless steel of the present invention is a spotless steel pipe. The present invention is applicable not only to spotless steel pipes but to a variety of other forms of steel, including, for example, thin steel sheets, thick steel sheets, UOE, ERW, spiral steel pipes, and butt-welded pipes. In the present invention, a steel material (for example, a billet) of the above composition is used as the starting material (hereinafter also referred to as steel pipe material). In the present invention, the method used to produce the starting material is not particularly restricted, and commonly known methods may be used. For example, in a preferred method of manufacturing a steel pipe material of the above composition, pig iron of the above composition is refined into steel by an ordinary steelmaking process, such as by using a converter, and processed into a steel pipe material by using a common method known as continuous casting and ingot making. The steel pipe material is then heated to produce a spotless steel pipe of the above composition and the desired dimensions, using a technique known as the Eugene Sejerne extrusion process or the Mannesmann pipe manufacturing process. The ideal heating temperature for steel pipe material is, for example, 1,100 to 1,300°C. A heating temperature below 1,100°C can negatively affect the material's workability and cause cracks on the outer surface of the steel pipe during rolling. A heating temperature above 1,300°C can result in the material melting due to the heat generated during working beyond its melting point, causing difficulties in subsequent rolling processes. From the perspective of introducing a greater number of dislocations and grain boundaries to provide a nuclei for copper precipitation and produce a high-strength material in subsequent wear heat treatment, it is preferable for the overall reduction of hot working to be 20 to 60% within a temperature range of, for example, 800 to 1,300°C. A temperature below 800°C can impair the workability of the material and cause cracking on the outer surface of the steel pipe during rolling. A temperature above 1,300°C can result in the material melting due to the heat generated during working beyond its melting point, causing difficulties in a subsequent rolling process.When the total reduction is less than 20% in the aforementioned temperature range, it may not be possible to produce a sufficient number of dislocations and grain boundaries to act as a nuclei for copper precipitation and achieve a sufficient level of high strength. Rolling with a total reduction exceeding 60% can generate excessive heat during operation, potentially leading to melting of the material beyond its melting point and causing difficulties in subsequent rolling processes. As used herein, total reduction refers to the reduction in the wall thickness of the steel pipe after rolling with an extender, plug mill, or similar equipment following perforation with a perforator. After manufacturing, the spotless steel pipe is cooled. Preferably, in the case of the above composition, the spotless steel pipe is cooled to room temperature at an average air cooling rate or faster. In this way, the spotless steel pipe can have the microstructure described above. In the present invention, the spot-cooled steel pipe is subjected to a phase precipitation treatment, a solution heat treatment, and an abrasion heat treatment, in that order, to produce the duplex stainless steel pipe. σ-phase precipitation treatment The dot-free steel pipe is then subjected to a phase precipitation treatment, an important process in the present invention. Specifically, in the present invention, a dot-free steel pipe of the above composition is heated to a heating temperature of 700°C or higher and 950°C or lower, and cooled to a temperature of 300°C or lower at an average air-cooling rate or faster, specifically, at an average cooling rate of 1°C / s or higher. This causes the σ phase to precipitate, exceeding the supersaturated state of copper in the ferrite phase. The degree of copper supersaturation in the ferrite phase corresponds to formula (1). The σ phase precipitation treatment can produce a duplex stainless steel pipe that meets formula (1).From the standpoint of promoting o-phase precipitation, the heating temperature in the o-phase precipitation treatment is preferably 900°C or lower. Preferably, the heating temperature for the o-phase precipitation treatment is 750°C or higher. From the standpoint of creating a uniform temperature in the material, the o-phase precipitation treatment retains the above heating temperature for preferably at least 5 minutes, and more preferably at least 10 minutes. Preferably, the o-phase precipitation treatment retains the above heating temperature for a maximum of 300 minutes, and more preferably a maximum of 100 minutes. The average cooling rate in the o-phase precipitation treatment is preferably 2°C / s or higher. Cooling can be, for example, air cooling or water cooling.The upper limit of the average cooling rate is not particularly restricted; however, the average cooling rate is preferably 50°C / s or less because the effect on material characteristics saturates with increasing average cooling rates. As used herein, average cooling rate means the average cooling rate from the heating temperature to a cooling stop temperature. When the cooling stop temperature of the σ-phase precipitation treatment is above 300°C, the added copper precipitates as coarse ε-Cu during cooling, and a considerably long heating time will be required to redissolve the copper into a solid solution in the subsequent solution treatment, resulting in decreased productivity.Failure to sufficiently redissolve the copper during the subsequent solution heat treatment results in reduced toughness due to the remaining coarse ε-Cu. For this reason, the cooling stop temperature in the σ-phase precipitation treatment is preferably 300°C or lower, more preferably 250°C or lower. Heat Treatment of the Solution In the present invention, the σ-phase precipitation treatment is followed by a solution heat treatment of the spotless steel pipe subjected to the σ-phase precipitation treatment. Specifically, the spotless steel pipe subjected to the σ-phase precipitation treatment is further heated to a temperature of 1,000°C or higher and cooled to a temperature of 300°C or lower at an average cooling rate of air cooling or faster, specifically, at an average cooling rate of 1°C / s or higher. In this way, the intermetallic compounds, carbides, nitrides, sulfides, and other similar precipitates formed before or during the σ-phase precipitation treatment can be dissolved into solid solutions, and the resulting spotless steel pipe can have a microstructure containing appropriate amounts of austenitic and ferrite phases. The desired high toughness cannot be ensured when the solution heat treatment temperature is below 1000°C. Preferably, the solution heat treatment temperature is 1020°C or higher. To prevent oiling of the microstructure, the solution heat treatment temperature is preferably 1150°C or lower. More preferably, the solution heat treatment temperature is 1130°C or lower. In the present invention, to create a uniform temperature in the material, the solution heat treatment retains the above heating temperature for preferably at least 5 minutes, more preferably at least 10 minutes. Preferably, the solution heat treatment retains the above heating temperature for JO» at most 210 minutes, more preferably at most 100 minutes. When the average cooling rate of solution heat treatment is less than 1°C / s, precipitation of intermetallic compounds such as the σ and χ phases occurs during the cooling process, and low-temperature toughness and corrosion resistance are severely reduced. The upper limit of the average cooling rate is not necessarily particularly restricted. The cooling rate for solution heat treatment is preferably 2°C / s or higher. When the solution heat treatment quenching temperature exceeds 300°C, the added copper precipitates into coarse ε-Cu during quenching, and the desired high strength, high toughness, and corrosion resistance cannot be guaranteed. Therefore, the solution heat treatment quenching temperature should be 300°C or lower, and preferably 250°C or lower. Wear Heat Treatment After solution heat treatment, the spotless steel pipe undergoes a wear heat treatment. Specifically, the solution-treated spotless steel pipe is heated to a temperature of 350 to 600°C and then cooled. The wear heat treatment contributes to strength by causing the added copper to form fine ε-Cu precipitates. The fine ε-Cu does not provide a starting point for selective ferrite phase corrosion and, consequently, does not serve as a starting point for pitting corrosion. The wear heat treatment of the spotless steel pipe produces a high-strength duplex stainless steel pipe that has the desired high strength and toughness, and excellent corrosion resistance. The thickening of ε-Cu occurs when the wear heat treatment is performed at a high heating temperature above 600°C. In this case, the stainless steel pipe of the product cannot achieve the desired high strength and high toughness, nor the desired corrosion resistance. Preferably, the heating temperature for the wear heat treatment is 550°C or lower. When the heating temperature for the wear heat treatment is below 350°C, the fine precipitation of ε-Cu is insufficient, and the desired high strength cannot be obtained. Preferably, the heating temperature for the wear heat treatment is 400°C or higher. In the present invention, from the standpoint of creating a uniform temperature in the material, the wear heat treatment is held at the above heating temperature for preferably at least 5 minutes.The microstructure cannot achieve the desired uniformity when the wear heat treatment is maintained for less than 5 minutes. More preferably, the wear heat treatment is maintained for at least 20 minutes. Preferably, the wear heat treatment is maintained for a maximum of 210 minutes. In the wear heat treatment, quenching means cooling from a temperature range of 350 to 600°C to ambient temperature at an average air-cooling rate or faster. Specifically, the average air-cooling rate or faster is 1°C / s or more. The quenching rate in the wear heat treatment is preferably 2°C / s or more. Example 1 Examples of the present invention are described below. It should be noted that the present invention is not limited to the following examples. The cast irons of the compositions shown in Table 1 were refined separately to produce steel using a converter and cast into a billet (steel pipe material) by continuous casting. After heating to 1,150–1,250°C, the steel pipe material was hot-worked using a hot-pointless rolling mill to produce a spotless steel pipe measuring 83.8 mm in outside diameter and 12.7 mm in wall thickness. After production, the spotless steel pipe was air-cooled. The hot working was performed with a total reduction of 20–60% in a temperature range of 800–1,300°C. IVIA / a / ZUZ I / U 14JO» Table 1 ML / a / ZUZ 1 / U 1 I Comcosición en rasa: 1 o • 0 023 | • 0 046 | 0 046 | 0 021 | o 0 041 | I 0 034 I <O 0 044 | «O m 0 028 | 0 022 • < 3 00024 I - | 8900 0 ¢0 y o 03 o - 6100 0 0 0039 - • 0 0026 0 0026 • • REM 0 0023 0 0035 0 0089 0 0019 ÍD • 0 0027 I r-- 0 0021 | • • I 0 0035I co C 3 < 3 • - C 3 • 0.061 | -1 | 980 0 • • | eso o < 0 38 045 4 0 88 | - CO • < - • LO LO 0018 0012 0 015 0 022 850 0 0 026 0 055 0 041 0 039 <P C-4 0014 0016 0016 8100 0018 0 022 870 0 0 042 0 036 I 0 024 I 0 073 «íj 0012 5100 0016 0011 0016 0018 0013 0 022 0 026 <O 0011 0013 0013 0016 0016 0016 0 022 0018 0016 I 0 062 I 0016 £ 03 OJ <o C’3 LO CO 03 <y>03 <o LO LO co <’3 CO CO ¢-4 <O co 04 ¢-4 θ ¢-4 ¢-4 n Ό ¢-4 LO ¢-4 LO co ¢-4 C.) mi cO ¢-4 O M •Γ· ¢ 5 <O co <o ‘O £ M f*3 CO m m <33 1-. IO <o U3 <£><C <O <33 LO io¿ 03 <£>a) m <o ω 224 ¿O 10 22 2 | 247 246 214 ¿57 <O <O CO 245 22 2 | 225 977 C£>299 | 242 254 co 264 LO LO C-4 co 0 0011 | | 0100 0 O | 8000 0 0 0011 | 0 0013 | 0 0014 | | 6000 8 | LLOCO g : 0013 0 0011 | 0 0013 | 0 0009 | 0 0009 | 0 0014 | 0 0011 | 0 0013 | 0 0011 10 0009I 0 0009 | Q-| HORSE 0 CO C 5 *n C 3 oon | < 3 10 C 3 C 3 C 3 < 3 C 3 ¢-4 < 3 < 3 C 3 LO C 3 LO C 3 LO < 3 < 3 | 110 0 ¢-4 C 3 •o < 3 cO C 3 5 en c 3 LO ( 3 CP C 3 CO < 3 O <P C 3 CO < 3 <33 -J < 3 < 3 CO CO < 3 CO C.3 <33 C 3 ±58 a > CO C 3 < 3 Ct> an < 3 lA < 3 LO < 3 M c 3 ¢33 C 3 o CJ u> < 3 C 3 W3 U> C 3 U3 C 3 <33 ( 3 a> co C 3 Ά C 3 < 3 U3 03 < 3 C 3 <33 C 3 CO < 3 Q3 a> C 3 LO LO C 3 - a < 3 < 3 LO CXJ C 3 C 3 C 3 ¢33 MC 3 o 0 012 770 0 oo C 3 0 016 < 3 CT> < 3 0 "21 910 0 810 0 r- C 3 0 028 <33 < 3 0 016 910 0 uoo 0 022 0 021 0011 «3 C 3 , LIO 0 I CO C 3 AceO For m ω O LU LK OI - ~3 —1 27 o CL O (X (0 1— =>. This was followed by σ phase precipitation treatment, where the spotless steel pipe was heated and cooled to a temperature of 300°C or less under the conditions shown in Table 2. After the σ phase precipitation treatment, the spotless steel pipe underwent solution heat treatment, where it was heated under the conditions shown in Table 2 and cooled to a temperature of 300°C or less. This was followed by abrasion heat treatment, where the solution-treated spotless steel pipe was further heated under the conditions shown in Table 2 and cooled in air at an average cooling rate of 1°C / s or more.In the σ phase precipitation treatment and solution heat treatment, the spotless steel pipe was cooled at an average cooling rate of 1°C / s more in the case of air cooling, and 10°C / s more in the case of water cooling. Table 2 Heat treatment of wear Retention time imim C*) C*) í > ν') < :> e > í :> e > C*) < :> ( > in e > in í :> C 5 C*) e > in < :> C'i < 5 m < :> < > e > < :> < > ( > < :> < > m < :> CN < :> • The underline means outside the range of the invention Heating temperature ¡n M | 005 I 550 | I 008 1 008 1 | 008 550 | LO 500 | I 008 I 008 | 500 | | 008 | 003 I 00- ín -.f I 008 550 | Lf> 500 | I 008 | 008 I 008 500 | | 008 500 | I 008 I 008 I 008 I 008 I S78 LO 500 | 500 | 700 | -50 | ín -í Heat treatment of the solution Temp stop cooling 1 :Ci o £ £ ín £ £ LO C £ o £ ín ín o £ ín O £ C l· CN C 5 CXJ C 5 CXJ £ ín £ ín ín ín C í ín e 5 N Cooling I En' do- water 1 En' oor water I En', do' water I En' 00- water En' oo' water I En', oo- water 1 En' oo- water En' oo' water I enbe ,oc ,u3 | En' oo' water En' oo' water 1 enoe <oc ,ua | En', oo’ agua En', oo’ agua En' oo’agua En', oo’ agua I En' oo’ agua I En' do’ agua En', oo’ agua m 3 Π3 O En' oo’agua | enbe <oc ;u3 | | En', oo' agua| Drinking water | enoe <oc ,ug | En' oo’ agua En', oo’ agua En' oo’ agua I En' oo’ agua | En', oo’ agua | | En' oo’ agua | En' oo’ agua I enbe ^oc ;u3 | | En' oo’ agua | En'oo’aire En', oo’ agua En' oo’ agua En', oo’ agua Tiempo de retención ímim CO < >< or < > C*) < :> < > C*) < :> < > e > in < or m < > CO e > in < > in < > r·) < > e > e > < > e > e > < > e > in < > C*) < > in Heating time i:c) θ C 5 1070 | | | | o / ot 1 ozcl I 1070 I ti C 5 I OZN r? e > I o / ot I OZN 1 OZN | 1070 | 1070 | mi o, I OZN r? and > 1070 | I ozci I 0Z01 I ozoi 1070 | | 0Z01 1070 | 1070 | | OZCL | ozci I ozoi 1 OZCL 1070 | I 0Z01 I OZCL I ozci I 0Z01 I ozoi | OZCL | OZCL | OZCL Treatment of σ- Temp phase irrigation t:Ci en en cf ΓΠ c 5 C ie ) f CDCC ó • en π LO UJ LO UJ í^ cn N Irrigation I Water irrigation 1 Water irrigation I Water irrigation I Water irrigation I water irrigation oor water En' oor agua I En' oor agua I En' oor agua En' oor agua I En' oor agua 1 En' oorwater In' gour water In' gour water • Π3 3 a In' gold water | En' gold water | | En' gold water | En' gold water I En' gold water | En' hour water En' hour water En' hour water I En' hour water | En' gold water | | En' gold water | En' oor water I En' oor water I Φ σ o LU In' oor water In' oor water In' oor water Start time i min: in £ in $ in £ in £ in £ in £ in £ in £ ín < en £ in £ in £ ÍR4 íR íR <4 in ín Heating temp i: C i I 800 1 800 | | 008 I | 008 1 008 I 750 II 08 / 750 | I 006 I | 006 1,006 1,006 1,800 | 800 I | 008 to 800 | I 008 800 | I 008 800 | | 008 | 008 I 008 I 1 008 I 089 I 086 I 008 I 008 I | 008 I 008 800 | | 008 | 008 ON eye* / < cu OQ l£l 11 O - in in in in in _J| - The CL ua: with 1- =1 < < < < < < < Steel pipe no. - is -rm <o r-· «X» σ. o - en UD «o r-. CO σ» 8 en ÍN LO Ín Ín CO CTi ín m ín m rn en LO <n en f-. en 00 en Table 3 The α <c Notes UJ uj UJ UJ UJ UJ UJ UJ UJ UJ ( ) UJ UJ UJ UJ <_) UJ ( ) UJ í) UJ <) UJ •) UJ •) UJ < ) LU ( ) UJ () UJ ( ) UJ ( ) UJ UJ ( ) LL UJ () UJ () UJ ( ) UJ UJ) UJ () UJ ( ) UJ Strength Test SCC Presence or absence of cracking and corrosion by chopping Strength Test 3SC Presence or absence of agnetation and corrosion by chopping O Φ P'presence or absence of corosicn by chopping H3 Φ Corrosion speed.0 imm010y | 0 010 | 1 cio o 1 knoo 1 knoo 1 knoo 0 010 | 0 010 1 | 010 0 I oioo 1 oio o 1 oio o 0 010 | 0 010 | | 010 0 1 oil or 1 oil o 2 0 010 | 0 010 | 1 oioo II oioo 1 crown 0 010 II crown | sleep 1 sleep 0 010 | 1 knot 1 knot 1 knot 1 knot 0 010 | 0 010 | 0 148 | | 010 0 1 ooio Tenacity Energy of absorption vE.: YES; a> the sso> with S Oí ίο Oí CO S<O a Oí m m io h Oí θ ai ín O CO I ¿el OO iO| 3 s OO S col Cl O> CO CO Traction ear -I Resist Traction TS ip co ip Ear <í> co <O Oí Oí ip Oí Oí lo co Oí co CO 40 co «í co Oí 40 Oí oí ai co < 5 Oí a><11 O> 5 Oí co Oí «1 co co £ Oí ó> o> LO Oí Oí Oí Oí Ol Ol ai Ol LO 01 LP Oí LO Ol Properties Elastic limit YS iMPai Oí U) <0 LO ip r·- Oí co '0 - LO Oí Oí Oí <0 Oí co <0 «1 <0 <0 M s Ol <o < 5 I 86? s C 1 <0 LO LO C 1 «í LO o, ai c 5| '0 Y2 | Ό C 5 LO C>h- 10 months Oí r-.Value on the left side of the key ;1 Ol c 5 Ol CO c 5 Oí c > Oí c 5 Oí c 5 LO a> c > Oí c 5 C1 Ol c > Olco C 5 Oí < 5 <11 < 5 <11 ai < 1 't co < ​​5 <71 < 5 Oí <1 <1 5 <H' < l\ Oí < 1 <11 < 1 Oí co < 1 Oí < 5 oí < >ai < > oi < 5 os ai < 5 Oí < 5 a> a> < 1 oí í 5 o> C 5 oi < 5 oi < 5 oí < 5 <11 < 5 oí < y Oí < y Ol C 5 o'ura Bulk Cu content l% by mass: CU 0 CM 'o CU> <0 CO CM CU CU o> co Jo CU CU CU CU s uy cu co LO Oí CU CM 3 82 | Oh σι <o CM CM CM CM CU CU CU oo φ o F-accion de volunte de fase austenitica (%i ιό ιό LO LO LO Oí ίο ró O IO LO Olí 3 < 5 »0 <11 LO co 05 CO ΓΟ ai <0 «O LO LO LO LO LO LO «y io oi LO oi LO <0 LO F-accion de volunte de fase de fernta ;%i (P ’.f ip ’.f £ Ί LO LO M <«í «í LO £ Sí oí a r— 2 <íí LO r— LO u o Q>Or •í. < < < < HE or HE <* O =E - with CQ with >1 ol a.o CU «1 Hl =>l < < < < < < < Steel Pipe Non - CU TO L0 c- CO O) 2 - 05 't WITH £ Λ LO >0 with m. • The underscore signifies the range of the invention ” PE Present Example: CE Conca-active Example After σ-phase precipitation treatment, solution heat treatment, and wear heat treatment (hereafter collectively referred to simply as heat treatment), a test sample was taken for observation of the microstructure of the spotless steel pipe (duplex stainless steel pipe) and examined in a microstructure quantification assessment, 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 (SCO resistance test). The tests were performed as described below. The test results are presented in Table 3. (1) Measurement of the volume fraction (% by volume) of each phase in the complete microstructure of the steel pipe. After heat treatment, a test sample of the spotless steel pipe (duplex stainless steel pipe) was taken for observation of its axial cross-section. The volume fraction of the ferrite and austenitic phases was determined by observing the cross-section with a scanning electron microscope. Specifically, the test sample for microstructure observation was etched with Vilella's solution (a mixed reagent containing 2 g of picric acid, 10 mL of hydrochloric acid, and 100 mL of ethanol), and the microstructure was photographed with a scanning electron microscope (1000x). From the micrograph of the microstructure, the average area ratio for the ferrite and austenitic phases was calculated using an image analyzer, and the calculated value was determined as the volume fraction (% by volume) of each phase. (2) Measurement of the Cu content in the ferrite phase. A test sample prepared in the same manner as for microstructure observation was examined for ferrite identification using EBSP analysis. For the phase identified as ferrite in each test sample, the Cu content was determined by measuring the sample at 20 arbitrarily selected points using FE-EPMA. The average of these measured Cu content values ​​was then determined as the Cu content (% by mass) of the ferrite phase in the steel. (3) Traction Test After heat treatment, a strip sample was taken from the non-pointed steel pipe (duplex stainless steel pipe) in an orientation such that the tensile direction was along the axial direction of the pipe, in accordance with API-5CT standards. In the tensile test performed in accordance with API standards, the yield strength YS (MPa) and tensile strength TS (MPa) were measured on each test sample as measures of the tensile properties. (4) Charpy Impact Test After heat treatment, a V-notch test sample (10 mm thick) with a length equal to the circumference of the voided steel pipe (duplex stainless steel pipe) was taken from the center of the wall thickness, in accordance with ISO 11960. The test sample was measured for absorption energy vE 10 (J) in a Charpy impact test performed at a test temperature of -10°C. The measurement was performed on three test samples taken from each steel pipe, and an arithmetic mean of the three test samples was calculated after the Charpy impact test. The results are presented in Table 3. (5) Corrosion Test (carbon dioxide gas corrosion resistance test) After heat treatment, the spotless steel pipe (duplex stainless steel pipe) was machined to prepare a corrosion test specimen measuring 3 mm thick, 30 mm wide, and 40 mm long. Each test specimen was then subjected to a corrosion test to evaluate its resistance to carbon dioxide gas corrosion. In the corrosion test, the test sample was immersed in a test solution (a 20% by mass aqueous NaCl solution; liquid temperature: 200°C; CO2 atmosphere of 3.0 MPa) maintained in an autoclave, and the sample weight was measured after 14 days (336 hours) of immersion in the solution. The corrosion rate was determined from the weight reduction relative to the pre-test weight. After the corrosion test, the presence or absence of pitting corrosion on a surface of the test sample was observed using a 10x magnifying glass. In this case, the presence of pitting corrosion meant the presence of pits with a diameter of 0.2 mm or more. In the present invention, samples were deemed acceptable when the corrosion rate was 0.125 mm / in or less and pitting corrosion was absent.In Table 3, the symbol o indicates that pitting corrosion is absent, and the symbol x indicates that pitting corrosion is present. (6) Sulfide stress cracking resistance test (SSC resistance test) After heat treatment, the spotless steel pipe (duplex stainless steel pipe) was machined to prepare a test specimen in the form of a round rod (diameter φ = 6.4 mm), in accordance with NACE TM0177, Method A, and the specimen was tested in an SSC strength test. In the SSC strength test, the test specimen was immersed in a test solution (an aqueous solution adjusted to pH 3.5 by adding acetic acid and sodium acetate to a 20 wt% aqueous NaCl solution (liquid temperature: 25°C; atmosphere of 0.03 MPa H₂S and 0.07 MPa CO₂)) for 720 hours under an applied stress equal to 90% of the yield strength. The test specimen was then visually inspected for the presence or absence of cracking. The presence or absence of pitting corrosion on the surface of the test specimen was also observed using a 10x magnifying glass. In the present invention, test specimens were deemed acceptable when cracking and pitting corrosion were absent after the test. In Table 3, the symbol "o" indicates that cracking and pitting corrosion are absent. JO» and the symbol x indicates that cracking and / or pitting corrosion are present. (7) Sulfide stress corrosion cracking resistance test (SCC strength test) After heat treatment, the spotless steel pipe (duplex stainless steel pipe) was machined to prepare a 4-point bend test specimen measuring 3 mm thick, 15 mm wide and 115 mm long, and the specimen was tested in an SCC strength test. In the SCC strength test, the test specimen was immersed in a test solution (a 10% by mass aqueous NaCl solution; liquid temperature: 80°C; an atmosphere of 35 kPa H₂S and 2 MPa CO₂) in an autoclave for 720 hours under an applied stress equal to 100% of the yield strength. The test specimen was then visually inspected for the presence or absence of surface cracking. The test specimen was also examined for the presence or absence of pitting corrosion on its surface using a 10x magnifying glass. In the present invention, test specimens were deemed acceptable when cracking and pitting corrosion were absent after the test. In Table 3, the symbol 'o' indicates that cracking and pitting corrosion are absent, and the symbol 'x' indicates that cracking and / or pitting corrosion are present. The duplex stainless steel pipes in the present examples all had high strength with a yield strength of 655 MPa or more, and high toughness with an absorption energy vE-w of 40 J or more as measured by a Charpy impact test. The duplex stainless steel pipes in the present examples also had excellent corrosion resistance (resistance to carbon dioxide gas corrosion) in a high-temperature corrosive environment containing CO2- and Cl· at 200°C or more, and excellent resistance to sulfide stress cracking and sulfide stress corrosion cracking, as demonstrated by the absence of cracking (in both SSC and SCC) in an environment containing H2S.Conversely, in comparative examples that did not fall within the range of the present invention, the desired levels of high strength or high toughness were not achievable, and the corrosion rate was excessively high, as evidenced by pitting corrosion occurring in a high-temperature corrosive environment containing CO2- and Cl· at 200°C or higher. The comparative examples also exhibited surface cracking (SSC or SCC, or both) in an environment containing H2S.< / y>

Claims

1. A duplex stainless steel having a composition comprising, in % by mass, C: 0.03% or less, Si: 1.0% or less, Mn: 0.10 to 1.5%, P: 0.040% or less, S: 0.01% or less, Cr: 20.0 to 28.0%, Ni: 2.0 to 10.0%, Mo: 2.0 to 5.0%, Cu: 2.0 to 6.0%, Al: 0.001 to 0.05% and N: less than 0.070%, and wherein the remainder is Fe and incidental impurities, the duplex stainless steel having a microstructure containing an austenitic phase and a ferrite phase, and meeting the following contents of C, Si, Mn, Cr, Mo, Ni, N, Cu and W in formula (1) below, the duplex stainless steel that has a yield strength YS of 655 MPa or more, and an absorption energy vE-10 of 40 J or more, measured by a Charpy impact test at a test temperature of -10°C, 0.55[%C] - 0.056[%S¡] + 0.018[%Mn] - 0.020[%Cr] - 0.087[%Mo] + 0.16[%N¡] + 0.28[%N] 0.506[%Cu] - 0.035[%W] + [%Cu*F] < 0.94 (1), wherein [%element symbol] represents the content (% by mass) of the element in the steel, [%element symbol * F] represents the content (% by mass) of the element in the ferrite phase, and the content is zero for elements that are not contained.

2. The duplex stainless steel according to claim 1, wherein the composition further comprises, in % by mass, one or two or more groups selected from the following groups A to E, group A: W: 1.5% or less, group B: V: 0.20% or less, group C: one or two selected from Zr: 0.50% or less, and B: 0.0030% or less, group D: one or two or more selected from REM: 0.005% or less, Ca: 0.005% or less, Sn: 0.20% or less, and Mg: 0.01% or less, group E: one or two or more selected from Ta: 0.1% or less, Co: 1.0% or less and Sb: 1.0% or less.

3. A duplex stainless steel pipe utilizing the duplex stainless steel of claim 1 or 2. ML / a / ZUZ 1 / UI 4 Jo» 4. A method for manufacturing a duplex stainless steel, comprising subjecting a steel material of the composition of claim 1 or 2 to a σ-phase precipitation treatment that heats the steel material to a temperature of 700°C or more and 950°C or less, and cools the heated steel material to a temperature of 300°C or less at an average cooling rate of air cooling or faster, a solution heat treatment that heats the steel material to a temperature of 1,000°C or more, and cools the heated steel material to a temperature of 300°C or less at an average cooling rate of air cooling or faster, and a wear heat treatment that heats the steel material to a temperature of 350 to 600°C, and cools the heated steel material.