Duplex seamless stainless steel pipe

Abstract

Provided is a duplex seamless stainless steel pipe having high strength, excellent crevice corrosion resistance in a supercritical corrosive environment, and excellent hot workability. A duplex seamless stainless steel pipe according to the present disclosure has a chemical composition as described in the specification, satisfies formula (1), and has a yield strength of 550 MPa or more. A duplex seamless stainless steel pipe according to the present disclosure has a microstructure composed of ferrite and austenite, wherein the ferrite volume fraction FX in a thick-walled central part of the duplex seamless stainless steel pipe is 35.0-65.0%, and the ferrite volume fraction FY in a region of 100 µm in the depth direction from the outer surface of the duplex seamless stainless steel pipe is at least 0.90 FX.　(1): Cr +3.3 (Mo+0.5W)+16N-Mn≥30.0. Here, in the element symbol in formula (1), the content of the corresponding element is substituted in units of mass%. When the corresponding element is not contained, "0" is substituted for the element symbol.

Description

Duplex stainless steel seamless pipe

[0001] The present disclosure relates to duplex stainless steel materials, and more particularly to duplex stainless steel seamless pipes.

[0002] A known method for improving the corrosion resistance of steel materials is to increase the chromium (Cr) content in the chemical composition and form a passive film mainly composed of Cr oxides on the surface of the steel material. Therefore, duplex stainless steel materials with an increased Cr content are sometimes used in environments requiring excellent corrosion resistance. Here, duplex stainless steel materials having a two-phase structure of ferrite and austenite phases have excellent corrosion resistance against crevice corrosion (hereinafter referred to as "crevice corrosion resistance"), which is a problem in aqueous solutions containing chlorides. Therefore, duplex stainless steel materials are used, for example, as materials for steel pipes for seawater heat exchangers and umbilical cables for offshore development.

[0003] By the way, currently, carbon dioxide (CO ２ The rise in CO ２ Efforts to reduce CO emissions have been underway, and CCUS in particular has been attracting attention. CCUS is an abbreviation for Carbon Dioxide Capture, Utilization and Storage. ２ It includes three technologies: CO capture, utilization, and storage. ２ CO2 emitted from industrial facilities such as power plants and factories is stored as a technology. ２ and CO2 ２ Technology to inject and store gas has been attracting attention.

[0004] In other words, such CO ２ Steel materials that are expected to be applied to storage technology include those for CO storage in depleted oil wells. ２ Therefore, high strength is required. In other words, there is a demand for duplex stainless steel materials that combine high strength with excellent corrosion resistance.

[0005] Japanese Patent Laid-Open No. 2002-339042 (Patent Document 1) and Japanese Patent Laid-Open No. 2016-3377 (Patent Document 2) propose a duplex stainless steel material having high strength and excellent corrosion resistance.

[0006] The duplex stainless steel material disclosed in Patent Document 1 is a duplex stainless steel for shafts, and consists, in mass%, of C: 0.080% or less, Si: 0.10 to less than 1.50%, Mn: 2.0% or less, P: 0.03% or less, S: 0.01% or less, Ni: 4.0 to 10.0%, Cr: 22.0 to 30.0%, Mo: 1.0 to 3.0%, W: 0 to 1.5%, Mo+0.5×W: 1.0 to 3.0%, Cu: more than 1.0 to 3.5%, N: 0.30% or less, B: 0.0005 to 0.01%, and the balance being substantially Fe. Furthermore, the C value (= ((%Cr) + 2 x (%Si) + 1.5 x (%Mo) + 0.75 x (%W)) / ((%Ni) + 0.5 x (%Mn) + 0.3 x (%Cu) + 30 x (%C) + 25 x (%N))) is 2.00 to 2.60, the D value (= (%Cr) + 3.3 x (%Mo) + 1.65 x (%W) + (%Cu) + 30 x (%N)) is 36.0 or more, and the 0.2% proof stress at room temperature is 650 MPa or more. Patent Document 1 discloses that this duplex stainless steel material simultaneously satisfies high strength, excellent toughness, and excellent corrosion resistance.

[0007] The duplex stainless steel material disclosed in Patent Document 2 contains, in mass %, C: 0.03% or less, Si: 0.2 to 1%, Mn: 0.5 to 2.0%, P: 0.040% or less, S: 0.010% or less, Sol. A steel sheet having a chemical composition of Al: 0.040% or less, Ni: 4 to less than 6%, Cr: 20 to less than 25%, Mo: 2.0 to 4.0%, N: 0.1 to 0.35%, O: 0.003% or less, V: 0.05 to 1.5%, Ca: 0.0005 to 0.02%, B: 0.0005 to 0.02%, and the balance being Fe and impurities, the metal structure is composed of a two-phase structure of a ferrite phase and an austenite phase, there is no precipitation of a sigma phase, and the proportion of the ferrite phase in the metal structure in terms of area ratio is 50% or less, and ２The number of oxide particles with a particle size of 30 μm or more present in the field of view is 15 or less. This duplex stainless steel material is disclosed in Patent Document 2 to have high strength, excellent corrosion resistance, and excellent low-temperature toughness.

[0008] JP 2002-339042 A JP 2016-3377 A

[0009] By the way, the carbon dioxide storage technology mentioned above involves storing CO in depleted oil wells. ２ In order to inject CO into the steel pipe ２ is compressed and pressurized to produce CO ２ Furthermore, CO captured from industrial facilities such as power plants and factories is ２ In ２ Such O ２ Supercritical CO containing ２ creates an extremely severe corrosive environment. ２ Supercritical CO containing ２ is also called a "supercritical corrosion environment."

[0010] In other words, duplex stainless steel materials intended for application to CCUS are required to have excellent crevice corrosion resistance even in supercritical corrosion environments, whereas the duplex stainless steel materials disclosed in Patent Documents 1 and 2 are not intended for use in such supercritical corrosion environments.

[0011] When duplex stainless steel materials are manufactured into seamless steel pipes, hot working may be performed. However, in such duplex stainless steel seamless pipes with enhanced crevice corrosion resistance in supercritical corrosion environments, defects may be formed on the outer surface due to hot working. Herein, defects formed on the outer surface of seamless steel pipes are also referred to as "external defects." If deep external defects are formed on a duplex stainless steel seamless pipe, the seamless steel pipe may not exhibit the desired mechanical properties. Therefore, duplex stainless steel seamless pipes are required to have not only high strength and excellent crevice corrosion resistance in supercritical corrosion environments, but also excellent hot workability. However, Patent Documents 1 and 2 do not discuss the hot workability of duplex stainless steel seamless pipes.

[0012] An object of the present disclosure is to provide a duplex stainless steel seamless pipe having high strength, excellent crevice corrosion resistance in supercritical corrosion environments, and excellent hot workability.

[0013] The duplex stainless steel seamless pipe according to the present disclosure has, in mass %, C: 0.050% or less, Si: 1.00% or less, Mn: 0.10 to 1.50%, P: 0.050% or less, S: 0.0050% or less, Cr: 20.00 to 28.00%, Cu: 0.05 to 3.00%, Ni: 4.00 to 7.00%, Mo: 0.80 to 3.00%, Ti: 0.001 to 0.050%, V: 0.01 to 0.50%, Nb: 0.001 to 0.100%, Co: 0.01 to 1.00%, Sn: 0.001 to 0.050%, sol. Al: 0.050% or less, N: 0.001 to 0.350%, O: 0.0100% or less, B: 0.0010 to 0.0050%, Ca: 0.0005 to 0.0100%, W: 0 to 1.50%, Zr: 0 to 0.010%, Ta: 0 to 0.050%, Mg: 0 to 0.010%, rare earth elements: 0 to 0.050%, As: 0 to 0.010%, Sb: 0 to 0.010%, Zn: 0 to 0.010%, Pb: 0 to 0.010%, and the balance being Fe and impurities, satisfying formula (1), having a yield strength of 550 MPa or more, and a microstructure consisting of ferrite and austenite, The volume fraction FX of the ferrite in the center of the wall thickness of the duplex stainless steel seamless pipe is 35.0 to 65.0%, and the volume fraction FY of the ferrite in a region 100 μm deep from the outer surface of the duplex stainless steel seamless pipe is 0.90FX or more. Cr+3.3(Mo+0.5W)+16N-Mn≧30.0 (1) Here, the element symbols in formula (1) are substituted with the content of the corresponding element in units of mass%. When the corresponding element is not contained, "0" is substituted for that element symbol.

[0014] The duplex stainless steel seamless pipe according to the present disclosure has high strength, excellent crevice corrosion resistance in supercritical corrosion environments, and excellent hot workability.

[0015] Fig. 1 is a diagram showing the relationship between the ratio of the ferrite volume fraction of the outer surface layer region to that of the central portion of the wall thickness (=FY / FX) and the maximum depth (µm) of outer surface defects, which is an index of hot workability, in this example. Fig. 2 is a schematic diagram explaining the preparation position of a test piece for determining the ferrite volume fraction FY (%) in the outer surface layer region. Fig. 3 is a schematic diagram explaining a method for specifying the observation field and the outer surface layer region.

[0016] Specifically, the present inventors attempted to obtain a duplex stainless steel seamless pipe having a high strength of 550 MPa or more in yield strength, and therefore first investigated, from the standpoint of chemical composition, a duplex stainless steel seamless pipe having a yield strength of 550 MPa or more, excellent crevice corrosion resistance in a supercritical corrosion environment, and excellent hot workability. As a result, the following contents were found in mass %: C: 0.050% or less, Si: 1.00% or less, Mn: 0.10 to 1.50%, P: 0.050% or less, S: 0.0050% or less, Cr: 20.00 to 28.00%, Cu: 0.05 to 3.00%, Ni: 4.00 to 7.00%, Mo: 0.80 to 3.00%, Ti: 0.001 to 0.050%, V: 0.01 to 0.50%, Nb: 0.001 to 0.100%, Co: 0.01 to 1.00%, Sn: 0.001 to 0.050%, sol. The present inventors considered that a duplex stainless steel seamless pipe consisting of Al: 0.050% or less, N: 0.001 to 0.350%, O: 0.0100% or less, B: 0.0010 to 0.0050%, Ca: 0.0005 to 0.0100%, W: 0 to 1.50%, Zr: 0 to 0.010%, Ta: 0 to 0.050%, Mg: 0 to 0.010%, rare earth elements: 0 to 0.050%, As: 0 to 0.010%, Sb: 0 to 0.010%, Zn: 0 to 0.010%, Pb: 0 to 0.010%, and the balance being Fe and impurities, may have a yield strength of 550 MPa or more, excellent crevice corrosion resistance in supercritical corrosion environments, and excellent hot workability.

[0017] Next, the inventors investigated various methods for improving the crevice corrosion resistance of duplex stainless steel seamless pipes having the above-mentioned chemical composition in supercritical corrosion environments. As a result, the inventors discovered that the crevice corrosion resistance of duplex stainless steel seamless pipes in supercritical corrosion environments can be improved if the chemical composition of the duplex stainless steel seamless pipe further satisfies the following formula (1): Cr + 3.3(Mo + 0.5W) + 16N - Mn ≥ 30.0 (1) Here, the element symbols in formula (1) are substituted with the content of the corresponding element in mass %. If the corresponding element is not contained, "0" is substituted for that element symbol.

[0018] Fn1 is defined as Cr + 3.3(Mo + 0.5W) + 16N - Mn. Fn1 is an index relating to the crevice corrosion resistance of a duplex stainless steel seamless pipe having the above-described chemical composition in a supercritical corrosion environment. Increasing Fn1 to 30.0 or higher can improve the crevice corrosion resistance of the duplex stainless steel seamless pipe in a supercritical corrosion environment. Therefore, the duplex stainless steel seamless pipe according to this embodiment satisfies the above-described chemical composition and has Fn1 of 30.0 or higher. As a result, excellent crevice corrosion resistance can be obtained even in a supercritical corrosion environment, provided that the other configurations of this embodiment are satisfied.

[0019] On the other hand, duplex stainless steel seamless pipes having the above-mentioned chemical composition and satisfying an Fn1 of 30.0 or more do not exhibit excellent hot workability, and external defects may be formed by hot working. As described above, in this specification, "external defects" refers to defects formed on the outer surface of the seamless steel pipe. Furthermore, if deep external defects are formed in a duplex stainless steel seamless pipe, the desired mechanical properties may not be obtained. Therefore, the inventors have investigated various methods for improving hot workability and suppressing the formation of deep external defects in duplex stainless steel seamless pipes having the above-mentioned chemical composition and satisfying an Fn1 of 30.0 or more.

[0020] Specifically, the inventors have focused on the microstructure and investigated the hot workability of duplex stainless steel seamless pipes. Here, the microstructure of a duplex stainless steel seamless pipe having the above-mentioned chemical composition is composed of ferrite and austenite. In this specification, "composed of ferrite and austenite" means that the amount of phases other than ferrite and austenite in the microstructure is negligibly small.

[0021] As a result of detailed studies by the inventors focusing on the microstructure, it was revealed that in duplex stainless steel seamless pipes with deep external surface defects, there is a significant difference in the microstructure between the vicinity of the outer surface and the central portion of the wall thickness. Specifically, in duplex stainless steel seamless pipes having the above-mentioned chemical composition and an Fn1 of 30.0 or more, the microstructure in the central portion of the wall thickness consists of 35.0 to 65.0% ferrite by volume, with the remainder being austenite. On the other hand, in the region 100 μm deep from the outer surface of the duplex stainless steel seamless pipe, the volume fraction of ferrite may be significantly smaller than that in the central portion of the wall thickness. Herein, the region 100 μm deep from the outer surface of the duplex stainless steel seamless pipe is also referred to as the "outer surface region."

[0022] More specifically, the volume fraction of ferrite in the central portion of the wall thickness of a duplex stainless steel seamless pipe is defined as FX (%). Furthermore, the volume fraction of ferrite in a region 100 μm in the depth direction from the outer surface of the duplex stainless steel seamless pipe (outer surface layer region) is defined as FY (%). As a result of detailed studies by the inventors, it has become clear that the depth of outer surface defects can be reduced if the ferrite volume fraction FY (%) in the outer surface layer region is equal to or greater than the ferrite volume fraction FX (%) in the central portion of the wall thickness x 0.90. This point will be explained in more detail using the drawings.

[0023] FIG. 1 is a diagram showing the relationship between the ratio of the ferrite volume fraction of the outer surface layer region to the wall thickness central portion (=FY / FX) and the maximum depth of external surface defects (μm), which is an index of hot workability, in this example. FIG. 1 was created using the ratio of the ferrite volume fraction of the outer surface layer region to the wall thickness central portion (=FY / FX) and the maximum depth of external surface defects (μm) for a duplex stainless steel seamless pipe having the above-mentioned chemical composition, an Fn1 of 30.0 or more, and a yield strength of 550 MPa or more, among the examples described below. Hereinafter, in this specification, the ratio of the ferrite volume fraction of the outer surface layer region to the wall thickness central portion is also referred to as "FY / FX." The ferrite volume fraction and the maximum depth of external surface defects were determined by the method described below.

[0024] Referring to Figure 1, it can be seen that when the ratio of the ferrite volume fraction of the outer surface layer region to the central portion of the wall thickness (FY / FX) is 0.90 or more, the maximum depth of external defects is 200 µm or less. That is, in a seamless steel pipe having the above-described chemical composition and an Fn1 of 30.0 or more, when FY / FX is 0.90 or more, hot workability is improved and the formation of deep external defects is suppressed. Therefore, the duplex stainless steel seamless steel pipe according to this embodiment has the above-described chemical composition, an Fn1 of 30.0 or more, a yield strength of 550 MPa or more, a microstructure consisting of ferrite and austenite, a ferrite volume fraction FX of 35.0 to 65.0% in the central portion of the wall thickness, and FY / FX of 0.90 or more. As a result, the duplex stainless steel seamless steel pipe according to this embodiment has high strength, excellent crevice corrosion resistance in supercritical corrosion environments, and excellent hot workability.

[0025] The duplex stainless steel seamless pipe according to this embodiment, which was completed based on the above findings, has the following features.

[0026] [1] A duplex stainless steel seamless pipe, comprising, in mass%, C: 0.050% or less, Si: 1.00% or less, Mn: 0.10 to 1.50%, P: 0.050% or less, S: 0.0050% or less, Cr: 20.00 to 28.00%, Cu: 0.05 to 3.00%, Ni: 4.00 to 7.00%, Mo: 0.80 to 3.00%, Ti: 0.001 to 0.050%, V: 0.01 to 0.50%, Nb: 0.001 to 0.100%, Co: 0.01 to 1.00%, Sn: 0.001 to 0.050%, sol. Al: 0.050% or less, N: 0.001 to 0.350%, O: 0.0100% or less, B: 0.0010 to 0.0050%, Ca: 0.0005 to 0.0100%, W: 0 to 1.50%, Zr: 0 to 0.010%, Ta: 0 to 0.050%, Mg: 0 to 0.010%, rare earth elements: 0 to 0.050%, As: 0 to 0.010%, Sb: 0 to 0.010%, Zn: 0 to 0.010%, Pb: 0 to 0.010%, and the balance being Fe and impurities, satisfying formula (1), having a yield strength of 550 MPa or more, and a microstructure consisting of ferrite and austenite, A duplex stainless steel seamless pipe, wherein the volume fraction FX of the ferrite at the center of the wall thickness of the duplex stainless steel seamless pipe is 35.0 to 65.0%, and the volume fraction FY of the ferrite in a region 100 μm deep from the outer surface of the duplex stainless steel seamless pipe is 0.90FX or more. Cr+3.3(Mo+0.5W)+16N-Mn≧30.0 (1) Here, the element symbols in formula (1) are substituted with the content of the corresponding element in units of mass%. When the corresponding element is not contained, "0" is substituted for that element symbol.

[0027] [2] A duplex stainless steel seamless pipe according to [1], containing one or more elements selected from the group consisting of W: 0.01 to 1.50%, Zr: 0.001 to 0.010%, Ta: 0.001 to 0.050%, Mg: 0.001 to 0.010%, rare earth elements: 0.001 to 0.050%, As: 0.001 to 0.010%, Sb: 0.001 to 0.010%, Zn: 0.001 to 0.010%, and Pb: 0.001 to 0.010%.

[0028] The duplex stainless steel seamless pipe according to this embodiment will be described in detail below. In the following description, the duplex stainless steel seamless pipe will also be simply referred to as "steel material." Furthermore, "%" relating to elements means mass % unless otherwise specified.

[0029] [Chemical Composition] The chemical composition of the duplex stainless steel seamless pipe according to this embodiment contains the following elements.

[0030] C: 0.050% or less Carbon (C) is unavoidably contained. That is, the lower limit of the C content is greater than 0%. C stabilizes austenite. However, C forms Cr carbides at grain boundaries, increasing the corrosion susceptibility at the grain boundaries. Therefore, if the C content is too high, the crevice corrosion resistance of the steel material will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the C content is 0.050% or less. A preferred upper limit of the C content is 0.045%, more preferably 0.040%, even more preferably 0.035%, and even more preferably 0.030%. To more effectively obtain the above effects, a preferred lower limit of the C content is 0.001%, more preferably 0.003%, and even more preferably 0.005%.

[0031] Si: 1.00% or less Silicon (Si) is inevitably contained. That is, the lower limit of the Si content is more than 0%. Si deoxidizes steel. However, if the Si content is too high, the hot workability of the steel material will deteriorate even if the contents of other elements are within the ranges of this embodiment. Therefore, the Si content is 1.00% or less. A preferred upper limit of the Si content is 0.90%, more preferably 0.80%. A preferred lower limit of the Si content to more effectively obtain the above effects is 0.05%, more preferably 0.10%.

[0032] Mn: 0.10 to 1.50% Manganese (Mn) deoxidizes and desulfurizes steel. If the Mn content is too low, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Mn content is too high, FY / FX decreases, and the hot workability of the steel material decreases, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Mn content is 0.10 to 1.50%. A preferred lower limit of the Mn content is 0.11%, more preferably 0.15%, and even more preferably 0.20%. A preferred upper limit of the Mn content is 1.40%, and even more preferably 1.30%.

[0033] P: 0.050% or less Phosphorus (P) is unavoidably contained. That is, the lower limit of the P content is greater than 0%. P segregates at grain boundaries. Therefore, if the P content is too high, the crevice corrosion resistance of the steel material will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the P content is 0.050% or less. A preferred upper limit of the P content is 0.045%, more preferably 0.040%. The P content is preferably as low as possible. However, an extreme reduction in the P content significantly increases manufacturing costs. Therefore, considering industrial production, a preferred lower limit of the P content is 0.001%, more preferably 0.003%, even more preferably 0.005%, and even more preferably 0.010%.

[0034] S: 0.0050% or less Sulfur (S) is unavoidably contained. That is, the lower limit of the S content is greater than 0%. S segregates at grain boundaries. Therefore, if the S content is too high, the crevice corrosion resistance of the steel material will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the S content is 0.0050% or less. A preferred upper limit of the S content is 0.0040%, more preferably 0.0030%, and even more preferably 0.0020%. The S content should be as low as possible. However, an extreme reduction in the S content significantly increases manufacturing costs. Therefore, considering industrial production, a preferred lower limit of the S content is 0.0001%, more preferably 0.0002%.

[0035] Cr: 20.00 to 28.00% Chromium (Cr) forms a passive film as an oxide on the surface of a steel material, thereby improving the crevice corrosion resistance of the steel material. If the Cr content is too low, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Cr content is too high, the manufacturing cost will increase significantly even if the contents of other elements are within the ranges of this embodiment. Therefore, the Cr content is 20.00 to 28.00%. The preferred lower limit of the Cr content is 20.30%, more preferably 20.70%, and even more preferably 21.00%. The preferred upper limit of the Cr content is 27.80%, more preferably 27.50%, and even more preferably 27.00%.

[0036] Cu: 0.05 to 3.00% Copper (Cu) precipitates in the steel material and increases the yield strength of the steel material. If the Cu content is too low, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Cu content is too high, the strength of the steel material becomes too high, and hot workability deteriorates, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Cu content is 0.05 to 3.00%. A preferred lower limit of the Cu content is 0.08%, and more preferably 0.10%. A preferred upper limit of the Cu content is 2.90%, more preferably 2.85%, and even more preferably 2.80%.

[0037] Ni: 4.00 to 7.00% Nickel (Ni) stabilizes the austenite structure of the steel material and improves the crevice corrosion resistance of the steel material. If the Ni content is too low, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Ni content is too high, the austenite volume fraction becomes too high, even if the contents of other elements are within the ranges of this embodiment, and the strength of the steel material decreases. Therefore, the Ni content is 4.00 to 7.00%. The preferred lower limit of the Ni content is 4.20%, more preferably 4.40%, and even more preferably 4.50%. The preferred upper limit of the Ni content is 6.90%, and even more preferably 6.80%.

[0038] Mo: 0.80 to 3.00% Molybdenum (Mo) increases the volume fraction of ferrite and increases FY / FX. As a result, the hot workability of the steel material is improved. If the Mo content is too low, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Mo content is too high, the hot workability of the steel material will actually decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the Mo content is 0.80 to 3.00%. A preferred lower limit of the Mo content is 0.85%, more preferably 0.90%, and even more preferably 1.00%. A preferred upper limit of the Mo content is 2.90%, and even more preferably 2.80%.

[0039] Ti: 0.001 to 0.050% Titanium (Ti) forms nitrides to increase the strength of the steel material. If the Ti content is too low, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Ti content is too high, the strength variation of the steel material increases depending on the state of precipitate formation, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Ti content is 0.001 to 0.050%. A preferred lower limit of the Ti content is 0.002%, and more preferably 0.005%. A preferred upper limit of the Ti content is 0.045%, and more preferably 0.040%.

[0040] V: 0.01 to 0.50% Vanadium (V) forms carbonitrides to increase the strength of the steel material. If the V content is too low, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the V content is too high, the strength of the steel material becomes too high, even if the contents of other elements are within the ranges of this embodiment, and the hot workability of the steel material decreases. Therefore, the V content is 0.01 to 0.50%. A preferred lower limit of the V content is 0.03%, and more preferably 0.05%. A preferred upper limit of the V content is 0.40%, more preferably 0.35%, and even more preferably 0.30%.

[0041] Nb: 0.001 to 0.100% Niobium (Nb) forms carbonitrides to increase the strength of the steel material. If the Nb content is too low, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Nb content is too high, the strength of the steel material becomes too high, even if the contents of other elements are within the ranges of this embodiment, and the hot workability of the steel material decreases. Therefore, the Nb content is 0.001 to 0.100%. A preferred lower limit of the Nb content is 0.003%, and more preferably 0.005%. A preferred upper limit of the Nb content is 0.095%, more preferably 0.090%, and even more preferably 0.080%.

[0042] Co: 0.01 to 1.00% Cobalt (Co) forms a film on the surface of a steel material, improving the corrosion resistance of the steel material. If the Co content is too low, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Co content is too high, the manufacturing cost will increase dramatically even if the contents of other elements are within the ranges of this embodiment. Therefore, the Co content is 0.01 to 1.00%. A preferred lower limit of the Co content is 0.02%, and more preferably 0.05%. A preferred upper limit of the Co content is 0.80%, more preferably 0.60%, and even more preferably 0.50%.

[0043] Sn: 0.001 to 0.050% Tin (Sn) improves the corrosion resistance of steel materials. If the Sn content is too low, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Sn content is too high, the hot workability of the steel material decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Sn content is 0.001 to 0.050%. A preferred lower limit of the Sn content is 0.002%, and more preferably 0.003%. A preferred upper limit of the Sn content is 0.040%, more preferably 0.030%, and even more preferably 0.020%.

[0044] Sol. Al: 0.050% or less Aluminum (Al) is unavoidably contained. That is, the lower limit of the Al content is greater than 0%. Al deoxidizes steel. However, if the Al content is too high, coarse inclusions are formed, even if the contents of other elements are within the ranges of this embodiment, and the crevice corrosion resistance of the steel material is reduced. Therefore, the Al content is 0.050% or less. A preferred upper limit of the Al content is 0.045%, more preferably 0.040%, and even more preferably 0.035%. To more effectively obtain the above effects, a preferred lower limit of the Al content is 0.001%, more preferably 0.005%, and even more preferably 0.008%. Note that the Al content referred to in this specification refers to the content of "acid-soluble Al," that is, sol. Al.

[0045] N: 0.001 to 0.350% Nitrogen (N) stabilizes the austenite structure of the steel material. If the N content is too low, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the N content is too high, even if the contents of other elements are within the ranges of this embodiment, the FY / FX ratio decreases, and the hot workability of the steel material decreases. Therefore, the N content is 0.001 to 0.350%. A preferred lower limit of the N content is 0.002%, more preferably 0.005%, and even more preferably 0.010%. A preferred upper limit of the N content is 0.330%, and even more preferably 0.300%.

[0046] O: 0.0100% or less Oxygen (O) is unavoidably contained. That is, the lower limit of the O content is greater than 0%. O forms oxides. Therefore, if the O content is too high, the crevice corrosion resistance of the steel material will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the O content is 0.0100% or less. A preferred upper limit of the O content is 0.0090%, more preferably 0.0080%. The O content is preferably as low as possible. However, an extreme reduction in the O content increases manufacturing costs. Therefore, considering industrial production, a preferred lower limit of the O content is 0.0005%, more preferably 0.0010%, and even more preferably 0.0015%.

[0047] B: 0.0010 to 0.0050% Boron (B) suppresses the segregation of S to grain boundaries in steel and improves the hot workability of the steel. If the B content is too low, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the B content is too high, boron nitride (BN) is formed, reducing the toughness of the steel, even if the contents of other elements are within the ranges of this embodiment. Therefore, the B content is 0.0010 to 0.0050%. The preferred lower limit of the B content is 0.0013%, and more preferably 0.0015%. The preferred upper limit of the B content is 0.0040%, more preferably 0.0035%, and even more preferably 0.0030%.

[0048] Ca: 0.0005 to 0.0100% Calcium (Ca) neutralizes S in the steel material by fixing it as sulfides, thereby improving the hot workability of the steel material. If the Ca content is too low, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Ca content is too high, oxides in the steel material coarsen, reducing the corrosion resistance of the steel material, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Ca content is 0.0005 to 0.0100%. A preferred lower limit of the Ca content is 0.0008%, more preferably 0.0010%. A preferred upper limit of the Ca content is 0.0080%, more preferably 0.0060%, and even more preferably 0.0050%.

[0049] The balance of the chemical composition of the duplex stainless steel seamless pipe according to this embodiment is composed of Fe and impurities. Here, the impurities in the chemical composition refer to substances that are mixed in from raw materials such as ore and scrap, or from the manufacturing environment, during industrial production of duplex stainless steel seamless pipe, and are acceptable within a range that does not adversely affect the duplex stainless steel seamless pipe according to this embodiment.

[0050] [Optional Elements] The chemical composition of the above-mentioned duplex stainless steel seamless pipe may further contain one or more elements selected from the group consisting of W, Zr, and Ta in place of a portion of Fe. All of these elements are optional elements, and they increase the strength and corrosion resistance of the steel material.

[0051] W: 0 to 1.50% Tungsten (W) is an optional element and does not necessarily need to be contained. That is, the W content may be 0%. When contained, W forms carbonitrides, increasing the strength of the steel. W also forms stable oxides and / or sulfides in corrosive environments, improving the corrosion resistance of the steel. Even if even a small amount of W is contained, the above effects can be achieved to some extent. However, if the W content is too high, even if the contents of other elements are within the ranges of this embodiment, the strength of the steel will be too high and the toughness of the steel will decrease. If the W content is too high, the manufacturing cost will also increase significantly. Therefore, the W content is 0 to 1.50%. The preferred lower limit of the W content is more than 0%, more preferably 0.01%, and even more preferably 0.02%. The preferred upper limit of the W content is 1.40%, and even more preferably 1.30%.

[0052] Zr: 0 to 0.010% Zirconium (Zr) is an optional element and does not necessarily need to be contained. That is, the Zr content may be 0%. When contained, Zr forms carbonitrides, increasing the strength of the steel. Zr also forms stable oxides and / or sulfides in corrosive environments, improving the corrosion resistance of the steel. Even if even a small amount of Zr is contained, the above effects can be achieved to some extent. However, if the Zr content is too high, even if the contents of other elements are within the ranges of this embodiment, the strength of the steel will be too high and the toughness of the steel will decrease. Therefore, the Zr content is 0 to 0.010%. The preferred lower limit of the Zr content is more than 0%, more preferably 0.001%, and even more preferably 0.002%. The preferred upper limit of the Zr content is 0.008%, and even more preferably 0.006%.

[0053] Ta: 0 to 0.050% Tantalum (Ta) is an optional element and does not necessarily need to be contained. That is, the Ta content may be 0%. When contained, Ta forms carbonitrides, increasing the strength of the steel. Ta also forms stable oxides and / or sulfides in corrosive environments, improving the corrosion resistance of the steel. Even if even a small amount of Ta is contained, the above effects can be achieved to some extent. However, if the Ta content is too high, even if the contents of other elements are within the ranges of this embodiment, the strength of the steel will be too high and the toughness of the steel will decrease. Therefore, the Ta content is 0 to 0.050%. The preferred lower limit of the Ta content is more than 0%, more preferably 0.001%, even more preferably 0.003%, and even more preferably 0.005%. The preferred upper limit of the Ta content is 0.045%, even more preferably 0.040%, and even more preferably 0.035%.

[0054] The chemical composition of the duplex stainless steel seamless pipe according to this embodiment may further contain one or more elements selected from the group consisting of Mg and rare earth elements in place of a portion of Fe. All of these elements are optional elements and improve the hot workability of the steel material.

[0055] Mg: 0 to 0.010% Magnesium (Mg) is an optional element and does not necessarily need to be contained. That is, the Mg content may be 0%. When contained, Mg neutralizes S in the steel material by fixing it as sulfide, thereby improving the hot workability of the steel material. Even if even a small amount of Mg is contained, the above effects can be obtained to some extent. However, if the Mg content is too high, even if the contents of other elements are within the ranges of this embodiment, oxides in the steel material will coarsen, reducing the corrosion resistance of the steel material. Therefore, the Mg content is 0 to 0.010%. The preferred lower limit of the Mg content is more than 0%, more preferably 0.001%, and even more preferably 0.002%. The preferred upper limit of the Mg content is 0.008%, and even more preferably 0.007%.

[0056] Rare Earth Elements: 0 to 0.050% Rare earth elements (REM) are optional elements and do not necessarily need to be contained. That is, the REM content may be 0%. When contained, REM neutralizes S in the steel material by fixing it as sulfides, improving the hot workability of the steel material. Even if even a small amount of REM is contained, the above effects can be achieved to some extent. However, if the REM content is too high, even if the contents of other elements are within the ranges of this embodiment, oxides in the steel material will coarsen, reducing the corrosion resistance of the steel material. Therefore, the REM content is 0 to 0.050%. The preferred lower limit of the REM content is more than 0%, more preferably 0.001%, even more preferably 0.005%, and even more preferably 0.010%. The preferred upper limit of the REM content is 0.045%, even more preferably 0.040%.

[0057] In this specification, REM refers to one or more elements selected from the group consisting of scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanoids lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. In addition, in this specification, the REM content refers to the total content of these elements.

[0058] The chemical composition of the duplex stainless steel seamless pipe according to this embodiment may further contain one or more elements selected from the group consisting of As, Sb, Zn, and Pb in place of a portion of Fe. All of these elements are optional elements and enhance the corrosion resistance of the steel material.

[0059] As: 0 to 0.010% Arsenic (As) is an optional element and does not necessarily need to be contained. That is, the As content may be 0%. When contained, As enhances the corrosion resistance of the steel material. Even if even a small amount of As is contained, the above effect can be obtained to some extent. However, if the As content is too high, the corrosion resistance of the steel material may actually decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the As content is 0 to 0.010%. The preferred lower limit of the As content is more than 0%, more preferably 0.001%, and even more preferably 0.002%. The preferred upper limit of the As content is 0.008%, more preferably 0.006%, and even more preferably 0.004%.

[0060] Sb: 0 to 0.010% Antimony (Sb) is an optional element and does not necessarily need to be contained. That is, the Sb content may be 0%. When contained, Sb enhances the corrosion resistance of the steel material. Even if even a small amount of Sb is contained, the above effect can be obtained to some extent. However, if the Sb content is too high, the manufacturing cost will increase dramatically even if the contents of other elements are within the ranges of this embodiment. Therefore, the Sb content is 0 to 0.010%. The preferred lower limit of the Sb content is more than 0%, more preferably 0.001%, and even more preferably 0.002%. The preferred upper limit of the Sb content is 0.008%, more preferably 0.006%, and even more preferably 0.004%.

[0061] Zn: 0 to 0.010% Zinc (Zn) is an optional element and does not necessarily need to be contained. That is, the Zn content may be 0%. When contained, Zn enhances the corrosion resistance of the steel material. Even if even a small amount of Zn is contained, the above effect can be obtained to some extent. However, if the Zn content is too high, the corrosion resistance of the steel material may actually decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the Zn content is 0 to 0.010%. The preferred lower limit of the Zn content is more than 0%, more preferably 0.001%, and even more preferably 0.002%. The preferred upper limit of the Zn content is 0.008%, more preferably 0.006%, and even more preferably 0.004%.

[0062] Pb: 0 to 0.010% Lead (Pb) is an optional element and does not necessarily need to be contained. That is, the Pb content may be 0%. When contained, Pb enhances the corrosion resistance of the steel material. Even if even a small amount of Pb is contained, the above effect can be obtained to some extent. However, if the Pb content is too high, the hot workability of the steel material may be reduced even if the contents of other elements are within the ranges of this embodiment. Therefore, the Pb content is 0 to 0.010%. The preferred lower limit of the Pb content is more than 0%, more preferably 0.001%, and even more preferably 0.002%. The preferred upper limit of the Pb content is 0.008%, more preferably 0.006%, and even more preferably 0.005%.

[0063] [Formula (1)] Assuming that the duplex stainless steel seamless pipe according to this embodiment has the above-mentioned chemical composition, it satisfies the following formula (1): Cr + 3.3(Mo + 0.5W) + 16N - Mn ≥ 30.0 (1) Here, the element symbols in formula (1) are substituted with the content of the corresponding element in units of mass %. If the corresponding element is not contained, "0" is substituted for that element symbol.

[0064] Fn1 (=Cr+3.3(Mo+0.5W)+16N-Mn) is an index of crevice corrosion resistance in a supercritical corrosion environment of a duplex stainless steel seamless pipe having the above-mentioned chemical composition. If Fn1 is 30.0 or more, the crevice corrosion resistance of the duplex stainless steel seamless pipe in a supercritical corrosion environment can be improved, provided that the pipe has the other configurations of this embodiment. Therefore, in this embodiment, Fn1 is set to 30.0 or more.

[0065] The preferred lower limit of Fn1 is 30.1, more preferably 30.5, and even more preferably 31.0. A higher Fn1 is preferable. However, in the duplex stainless steel seamless pipe according to this embodiment having the above-mentioned chemical composition, the upper limit of Fn1 is substantially 45.9. The upper limit of Fn1 may further be 45.0, 42.0, 40.0, or 38.0. In this embodiment, Fn1 is calculated by rounding the obtained value to one decimal place.

[0066] [Yield strength] The yield strength of the duplex stainless steel seamless pipe according to this embodiment is 550 MPa or more. The upper limit of the yield strength is not particularly limited, but is, for example, 655 MPa. In this embodiment, the lower limit of the yield strength may be 552 MPa or 554 MPa. Furthermore, in this embodiment, the upper limit of the yield strength may be 641 MPa, 621 MPa, or 614 MPa.

[0067] In this embodiment, the yield strength (MPa) of the duplex stainless steel seamless pipe is defined as follows. First, a tensile test specimen is prepared from the central portion of the wall thickness of the seamless steel pipe according to this embodiment. The longitudinal direction of the tensile test specimen is parallel to the axial direction of the steel pipe. The tensile test specimen is, for example, a round bar test specimen with a parallel portion diameter of 8.9 mm and a gauge length of 35.6 mm. The tensile test specimen may also be an arc-shaped test specimen with a width of 25.4 mm, a gauge length of 50.8 mm, and a thickness of the entire wall thickness.

[0068] Using the prepared tensile test specimen, a tensile test is performed at room temperature (24±3°C) in accordance with ASTM E8 / E8M (2022). The 0.2% offset yield strength (MPa) obtained by the tensile test is defined as the yield strength (MPa). In this embodiment, the yield strength (MPa) is calculated by rounding the obtained value to one decimal place.

[0069] [Microstructure] The duplex stainless steel seamless pipe according to this embodiment has a microstructure composed of ferrite and austenite, with the ferrite volume fraction FX at the center of the wall thickness of the seamless pipe being 35.0 to 65.0%, and the ferrite volume fraction FY in the region 100 μm deep from the outer surface of the duplex stainless steel seamless pipe (outer surface region) being 0.90FX or more. As described above, in this specification, the ratio of the ferrite volume fraction in the outer surface region to the center of the wall thickness is also referred to as "FY / FX." In other words, in the duplex stainless steel seamless pipe according to this embodiment, FY / FX is 0.90 or more.

[0070] As described above, in this specification, the microstructure "consisting of ferrite and austenite" means that the amount of phases other than ferrite and austenite in the microstructure is negligibly small. For example, the volume fraction of precipitates and inclusions in a duplex stainless steel seamless pipe having the above-described chemical composition is negligibly small compared to the volume fraction of ferrite and austenite. In other words, the microstructure of the duplex stainless steel seamless pipe according to this embodiment may contain minute amounts of precipitates, inclusions, etc. in addition to ferrite and austenite.

[0071] As described above, the duplex stainless steel seamless pipe having the above-described chemical composition has a ferrite volume fraction FX of 35.0 to 65.0% at the center of the wall thickness. The preferred lower limit of the ferrite volume fraction FX at the center of the wall thickness is 36.0%, and more preferably 37.0%. The preferred upper limit of the ferrite volume fraction FX at the center of the wall thickness is 60.0%, and more preferably 55.0%, and even more preferably 53.0%, and even more preferably 50.0%.

[0072] In this embodiment, the volume fraction FY of ferrite in the outer surface layer region is not particularly limited as long as FY / FX satisfies 0.90 or more. However, in the duplex stainless steel seamless steel pipe according to this embodiment having the above-mentioned chemical composition, the volume fraction FY of ferrite in the outer surface layer region is substantially 31.5 to 65.0%.

[0073] As described above, in this embodiment, the ratio FY / FX of the ferrite volume fraction of the outer surface layer region to that of the central portion of the wall thickness is 0.90 or more. As FY / FX approaches 1.00, the ferrite volume fractions in the microstructure become more equivalent between the central portion of the wall thickness and the outer surface layer region. Therefore, a higher FY / FX is preferable, and it may be 1.00. The upper limit of FY / FX may be 0.99.

[0074] Here, when manufacturing a duplex stainless steel seamless pipe having the above-mentioned chemical composition, hot working may be performed as described above. In particular, when piercing rolling or elongation rolling is performed as hot working, stress is applied from the outer surface of the mother pipe. Meanwhile, in the temperature range in which hot rolling is performed, austenite tends to have a higher hardness than ferrite in the microstructure. In particular, when the pipe has the above-mentioned chemical composition and Fn1 is 30.0 or more, the difference in hardness between austenite and ferrite tends to be significant. As a result, there is a possibility that outer surface defects are easily formed at the interface between ferrite and austenite during hot rolling.

[0075] Therefore, in this embodiment, FY / FX is set to 0.90 or more to maintain the volume fraction of ferrite in the outer surface layer region. As a result, excessive generation of hard austenite is suppressed in the outer surface layer region, which may suppress the formation of deep outer surface defects. It is possible that the hot workability of the duplex stainless steel seamless pipe is improved as a result of FY / FX being 0.90 or more due to a mechanism other than the above. However, the fact that the hot workability of the duplex stainless steel seamless pipe is improved as a result of FY / FX being 0.90 or more is proven by the examples described below.

[0076] In this embodiment, the volume fraction FX (%) of ferrite in the central wall thickness portion is defined as follows. First, a test piece for observing the microstructure of the central wall thickness portion is prepared from the central wall thickness portion of a seamless steel pipe according to this embodiment. The test piece has an observation surface of 5 mm in the pipe axis direction and 5 mm in the pipe circumferential direction. The pipe circumferential direction is the direction perpendicular to the pipe axis direction and the pipe diameter direction. Note that there are no particular limitations on the size of the test piece as long as the above observation surface can be obtained.

[0077] The observation surface of the prepared test piece is mirror-polished. The mirror-polished observation surface is electrolytically etched in a 7% potassium hydroxide etchant to reveal the structure. The observation surface with the revealed structure is observed in 10 fields using an optical microscope. The area of ​​each field is, for example, 250 μm × 250 μm (magnification 400x). Ferrite is identified from the contrast in each field. The area ratio of the identified ferrite is determined using a point counting method in accordance with ASTM E562 (2019). In this embodiment, the arithmetic mean value of the obtained ferrite area ratios in the 10 fields is defined as the ferrite volume fraction FX (%) at the center of the wall thickness. Note that in this embodiment, the ferrite volume fraction FX (%) at the center of the wall thickness is determined by rounding the obtained value to one decimal place.

[0078] In this embodiment, the volume fraction FY (%) of ferrite in the outer surface layer region is defined as follows. First, test specimens for observing the microstructure of the outer surface layer region are prepared from the seamless steel pipe according to this embodiment. Six test specimens in total are prepared, two from each of the two pipe end regions and the central region of the seamless steel pipe. At this time, the two test specimens prepared from each region are prepared so as to be located at both ends of the diameter in a cross section perpendicular to the pipe axis direction of the seamless steel pipe.

[0079] The preparation position of the test specimen will be explained more specifically with reference to the drawings. Fig. 2 is a schematic diagram explaining the preparation position of the test specimen for determining the ferrite volume fraction FY (%) in the outer surface layer region. Fig. 2 is a diagram showing a longitudinal section of the seamless steel pipe 1 including the pipe axis CL of the seamless steel pipe 1. The left-right direction in Fig. 2 is the pipe axis direction of the seamless steel pipe 1, and the up-down direction in Fig. 2 is the pipe diameter direction of the seamless steel pipe 1.

[0080] Referring to Figure 2, the distance from one pipe end of the seamless steel pipe 1 (the left end of the seamless steel pipe 1 in Figure 2) is DL1, and the line segment parallel to the pipe diameter direction is T1. Further referring to Figure 2, the distance from the other pipe end of the seamless steel pipe 1 (the right end of the seamless steel pipe 1 in Figure 2) is DL2, and the line segment parallel to the pipe diameter direction is T2. In this specification, the region from one pipe end of the seamless steel pipe 1 (the left end of the seamless steel pipe 1 in Figure 2) to the line segment T1 is defined as "one pipe end region R1". Similarly, the region from the other pipe end of the seamless steel pipe 1 (the right end of the seamless steel pipe 1 in Figure 2) to the line segment T2 is defined as "the other pipe end region R2". Furthermore, the region of the seamless steel pipe 1 excluding one pipe end region R1 and the other pipe end region R2 (the region from line segment T1 to line segment T2 in FIG. 2) is defined as a "central region R3."

[0081] In this embodiment, the distance DL1 from the pipe end of the line segment T1 and the distance DL2 from the pipe end of the line segment T2 can be set according to the length of the seamless steel pipe 1. Specifically, when the axial length of the seamless steel pipe 1 is 10 m or more, the distances DL1 and DL2 are preferably 500 mm. Note that when the axial length of the seamless steel pipe 1 is less than 10 m, the distances DL1 and DL2 are also preferably 500 mm. Positions P1A and P1B for preparing test specimens are identified from the axial center of one pipe end region R1 of the seamless steel pipe 1. At this time, positions P1A and P1B are symmetrical with respect to the pipe axis CL. Similarly, positions P2A and P2B are identified from the axial center of the other pipe end region R2 of the seamless steel pipe 1. Similarly, positions P3A and P3B are identified from the axial center of the central region R3 of the seamless steel pipe 1.

[0082] From each of the identified points, a test piece is prepared having an observation surface including the outer surface of the seamless steel pipe and including the pipe axial and pipe radial directions. The size of the observation surface is not particularly limited, but is, for example, 1 mm in the pipe axial direction and 1 mm in the pipe radial direction. As long as the above observation surface can be obtained, the size of the test piece is also not particularly limited. The observation surface of the prepared test piece is mirror-polished. The mirror-polished observation surface is electrolytically etched in a 7% potassium hydroxide etching solution to reveal the structure. Three arbitrary fields of view of the observation surface where the structure is revealed are observed using an optical microscope to generate photographic images. The area of ​​each observation field is, for example, 250 μm × 250 μm (magnification 400x).

[0083] Here, a method for specifying the observation field will be specifically described with reference to the drawings. FIG. 3 is a schematic diagram illustrating a method for specifying the observation field and the outer surface layer region. The region 50 in FIG. 3 corresponds to the observation field using an optical microscope. The left-right direction in FIG. 3 is the axial direction of the seamless steel pipe 1, and the up-down direction is the radial direction of the seamless steel pipe 1. In other words, the observation field 50 includes the axial direction and the radial direction. Referring to FIG. 3, the observation field 50 also includes the outer surface 10 of the seamless steel pipe 1. The outer surface 10 of the duplex stainless steel seamless pipe according to this embodiment has some irregularities. Therefore, in this embodiment, the observation field is specified so that the irregularities on the outer surface 10 are 20 μm or less in the depth direction (radial direction) of the seamless steel pipe 1.

[0084] Specifically, with reference to Figure 3, in the observation field 50, L1 is defined as a straight line that passes through the outermost part in the pipe diameter direction of the outer surface 10 of the seamless steel pipe 1 (upper position in Figure 3) and is parallel to the pipe axis direction of the seamless steel pipe 1. With further reference to Figure 3, in the observation field 50, L2 is defined as a straight line that passes through the innermost part in the pipe diameter direction of the outer surface 10 of the seamless steel pipe 1 (lower position in Figure 3) and is parallel to the pipe axis direction of the seamless steel pipe 1. In this case, in observation using an optical microscope, the observation field 50 is specified so that the distance between the lines L1 and L2 is 20 µm or less.

[0085] The outer surface layer region is further identified from the observation field of view 50 identified by the above method. Referring to Figure 3, a line segment TE that passes through the center position between the above-mentioned lines L1 and L2 and is parallel to the tube axis direction is defined as the upper end of the outer surface layer region. Referring to Figure 3, a line segment BE that is 100 μm in the depth direction (tube diameter direction) from the line segment TE and is parallel to the tube axis direction is defined as the lower end of the outer surface layer region. In this embodiment, the region of the observation field of view 50 that is separated by the line segments TE and BE is defined as the "outer surface layer region." In other words, the outer surface layer region is defined as a rectangular region that is 250 μm in the tube axis direction and 100 μm in the tube diameter direction.

[0086] For the outer surface layer region identified by the above method, image analysis is performed on the generated photographic image to determine the ferrite area ratio. Specifically, a binarization process is performed on the photographic image to distinguish between void regions, ferrite, and austenite. Those skilled in the art would naturally be able to distinguish between void regions, ferrite, and austenite from the contrast using binarization process. Furthermore, image analysis is performed to derive the ferrite area ratio. The ferrite area ratio is determined based on the outer surface layer region excluding the void regions.

[0087] For all test specimens, the area ratio of ferrite is determined for each of three visual fields. The arithmetic mean value of the area ratios of ferrite in the obtained 18 visual fields is defined as the volume ratio FY (%) of ferrite in the outer surface layer region. In this embodiment, the volume ratio FY (%) of ferrite in the outer surface layer region is determined by rounding the obtained value to one decimal place.

[0088] Furthermore, the ratio of the ferrite volume fraction of the outer surface layer region to that of the wall thickness central portion (FY / FX) is calculated from the obtained ferrite volume fraction FX (%) in the wall thickness central portion and the ferrite volume fraction FY (%) in the outer surface layer region. In this embodiment, FY / FX is calculated by rounding the obtained value to two decimal places.

[0089] [Crevice corrosion resistance] The duplex stainless steel seamless pipe according to this embodiment has the above-described chemical composition, Fn1 is 30.0 or more, the microstructure is composed of ferrite and austenite, the ferrite volume fraction FX in the central portion of the wall thickness of the seamless steel pipe is 35.0 to 65.0%, and the ferrite volume fraction FY in the outer surface layer region is 0.90FX or more. As a result, the duplex stainless steel seamless pipe according to this embodiment has a yield strength of 550 MPa or more, excellent crevice corrosion resistance in supercritical corrosion environments, and excellent hot workability. In this embodiment, excellent crevice corrosion resistance in supercritical corrosion environments is defined as follows.

[0090] First, a test specimen having a metal-metal contact surface is prepared from the duplex stainless steel seamless pipe according to this embodiment. Specifically, a test specimen having a length of 30 mm, a width of 30 mm, and a thickness of 3 mm, and a test specimen having a length of 30 mm, a width of 15 mm, and a thickness of 3 mm are taken from the center of the wall thickness of the duplex stainless steel seamless pipe according to this embodiment. Furthermore, a through hole having a diameter of 5 mm is formed in the center of the length and width of each of the two test specimens, penetrating the thickness direction. Furthermore, the thickness direction of each test specimen is taken so that it is parallel to the diameter direction (wall thickness direction) of the seamless steel pipe.

[0091] The contact surface of each test piece is finish-polished using waterproof abrasive paper coated with silicon carbide abrasive grains (grain size: P600). Furthermore, two test pieces are fixed with metal bolts, nuts, and washers using the through holes formed in each test piece to prepare a test piece. Washers are sandwiched between the bolt and one test piece, and between the nut and the other test piece. The chemical composition of the metal bolts, nuts, and washers is a Ni-based alloy equivalent to NW0276 specified in JIS G 4902 (2019). A fluororesin bushing is inserted into the through hole of each test piece to prevent direct contact between the metal bolts and washers and the test piece. In this case, the fluororesin is, for example, polytetrafluoroethylene (PTFE). The bolt tightening torque of the test piece is 3 N·m. The sizes of the bolts, nuts, and washers are not particularly limited as long as the above tightening torque is ensured and there is no direct contact between the test piece and the bolts, etc. For example, it is preferable that the thread diameter of the bolt is about 3 mm.

[0092] The test specimen prepared by the above method is sealed in an autoclave. A 5.0 mass % aqueous solution of sodium chloride is poured into the autoclave so that the test specimen is immersed. ２ and CO ２ The mixed gas is pressurized and sealed in to saturate the test solution to form a test bath. At this time, the total pressure of the mixed gas is 130 bar, and the O ２ The concentration is set to 300 ppm. After sealing the autoclave, the test bath is maintained at 100° C. and the test specimen is immersed in the test bath for 96 hours while stirring the test bath.

[0093] After 96 hours, the bolts of the test specimens were loosened to expose the contact surfaces. The contact surfaces of each test specimen were observed with a magnifying glass at 10x magnification to check for the presence or absence of crevice corrosion. If crevice corrosion with a depth of 15 μm or more was confirmed through observation with the magnifying glass, it was determined that crevice corrosion was present. In this embodiment, if no crevice corrosion was confirmed as a result of the corrosion test under the above conditions, it was determined that the test specimen had excellent crevice corrosion resistance even in a supercritical corrosion environment.

[0094] [Hot workability] The duplex stainless steel seamless pipe according to this embodiment has the above-described chemical composition, Fn1 is 30.0 or more, the microstructure is composed of ferrite and austenite, the ferrite volume fraction FX in the central portion of the wall thickness of the seamless steel pipe is 35.0 to 65.0%, and the ferrite volume fraction FY in the outer surface layer region is 0.90FX or more. As a result, the duplex stainless steel seamless pipe according to this embodiment has a yield strength of 550 MPa or more, excellent crevice corrosion resistance in supercritical corrosion environments, and excellent hot workability. In this embodiment, excellent hot workability is defined as follows.

[0095] The maximum depth of external surface defects is measured for the manufactured duplex stainless steel seamless pipe. Specifically, the outer surface of the manufactured duplex stainless steel seamless pipe is inspected by ultrasonic flaw detection and visual inspection to confirm the presence or absence of defects. A test piece is cut out from the position where a defect is confirmed to obtain a cross section parallel to the pipe axis direction and pipe diameter direction. The cross section is observed with an optical microscope to measure the depth of the defect. The maximum value of the obtained defect depth is defined as the "maximum depth of external surface defects (μm)." In this embodiment, if the maximum depth of external surface defects is 200 μm or less, it is determined that the pipe has excellent hot workability.

[0096] [Manufacturing method] An example of a method for manufacturing a duplex stainless steel seamless pipe according to this embodiment having the above-mentioned configuration will be described. Note that the method for manufacturing a duplex stainless steel seamless pipe according to this embodiment is not limited to the manufacturing method described below. The method for manufacturing a duplex stainless steel seamless pipe according to this embodiment comprises, in mass %, C: 0.050% or less, Si: 1.00% or less, Mn: 0.10 to 1.50%, P: 0.050% or less, S: 0.0050% or less, Cr: 20.00 to 28.00%, Cu: 0.05 to 3.00%, Ni: 4.00 to 7.00%, Mo: 0.80 to 3.00%, Ti: 0.001 to 0.050%, V: 0.01 to 0.50%, Nb: 0.001 to 0.100%, Co: 0.01 to 1.00%, Sn: 0.001 to 0.050%, sol. a material preparing step of preparing a material consisting of Al: 0.050% or less, N: 0.001 to 0.350%, O: 0.0100% or less, B: 0.0010 to 0.0050%, Ca: 0.0005 to 0.0100%, W: 0 to 1.50%, Zr: 0 to 0.010%, Ta: 0 to 0.050%, Mg: 0 to 0.010%, rare earth elements: 0 to 0.050%, As: 0 to 0.010%, Sb: 0 to 0.010%, Zn: 0 to 0.010%, Pb: 0 to 0.010%, and the balance being Fe and impurities, and the material satisfying formula (1); and a hot working step of heating the material in a heating furnace and then piercing-rolling the material under conditions that satisfy formula (A) to produce a mother pipe. and a solution treatment step of quenching the mother pipe at 900 to 1100°C. Cr+3.3(Mo+0.5W)+16N-Mn≧30.0 (1) T / Δt≧400 (A) Here, the element symbols in formula (1) are substituted with the contents of the corresponding elements in units of mass%. When the corresponding element is not contained, "0" is substituted for the element symbol. Furthermore, the surface temperature of the material before piercing-rolling is substituted for T in formula (A) in units of °C, and the contact time of the material with the rolls of the piercing-rolling mill is substituted for Δt in formula (A) in units of seconds.

[0097] As described above, one example of the method for manufacturing a duplex stainless steel seamless pipe according to this embodiment may include a material preparation step, a hot working step, and a solution treatment step. Each manufacturing step will be described in detail below.

[0098] [Material Preparation Step] In the material preparation step according to this embodiment, a material having the above-described chemical composition is prepared. The material may be prepared by manufacturing or by purchasing from a third party. In other words, the method for preparing the material is not particularly limited.

[0099] When manufacturing a raw material, for example, it is manufactured by the following method. Molten steel having the above-mentioned chemical composition is manufactured. A cast piece (slab, bloom, or billet) is manufactured using the molten steel by a continuous casting method. A steel ingot (ingot) may be manufactured using the molten steel by an ingot-making method. If necessary, the slab, bloom, or ingot may be subjected to blooming to manufacture a billet. The raw material is manufactured by the above-mentioned steps.

[0100] [Hot Working Step] In the hot working step according to this embodiment, the material (billet) prepared in the material preparation step is hot worked to produce a mother pipe (seamless steel pipe). Specifically, the material is first heated in a heating furnace. The material removed from the heating furnace is hot worked to produce a mother pipe.

[0101] In this embodiment, it is preferable to produce a mother pipe by performing piercing-rolling by the Mannesmann process as the hot working. In this case, it is preferable that the surface temperature T (°C) of the raw material before piercing-rolling and the roll contact time Δt (seconds) of the raw material with the piercing-rolling mill satisfy the following formula (A): T / Δt≧400 (A)

[0102] It is defined that FnA = T / Δt. Here, the surface temperature T (°C) of the billet before piercing means the surface temperature (°C) of the billet measured at the inlet side of the piercing mill. The roll contact time Δt (seconds) of the piercing mill means the time (seconds) during which any position of the billet is in contact with the rolls of the piercing mill. Specifically, the roll contact time Δt (seconds) of the piercing mill corresponds to the time (seconds) from when the front end of the billet touches the rolls of the piercing mill until when the front end of the billet separates from the rolls of the piercing mill.

[0103] FnA is an index of the ferrite volume fraction on the surface of the mother pipe to be produced. If FnA is too small, the outer surface layer of the material is dissipated in heat by contact with the rolls during piercing-rolling, and the austenite volume fraction on the surface layer of the mother pipe increases. As a result, the ferrite volume fraction in the outer surface layer region of the produced seamless steel pipe may decrease, resulting in a decrease in FY / FX. On the other hand, if FnA is 400 or more, the FY / FX can be stably maintained at 0.90 or more in the produced seamless steel pipe.

[0104] In this case, the surface temperature T (°C) of the material before piercing-rolling and the roll contact time Δt (seconds) of the material in the piercing-rolling mill are not particularly limited as long as they satisfy formula (A). The surface temperature T (°C) of the material before piercing-rolling may be, for example, 1230 to 1290°C. More preferably, the surface temperature T (°C) of the material before piercing-rolling is 1250 to 1280°C. In this case, FY / FX can be more stably maintained at 0.90 or more.

[0105] The roll contact time Δt (seconds) of the material in the piercing-rolling mill may be, for example, 3.60 seconds or less. More preferably, the roll contact time Δt (seconds) of the material in the piercing-rolling mill is 0.80 to 3.20 seconds. In this case, FY / FX can be more stably maintained at 0.90 or more.

[0106] In this embodiment, other conditions for piercing-rolling can be well-known. Specifically, the piercing ratio in piercing-rolling is not particularly limited, but is, for example, 1.0 to 4.0. Preferably, the billet after piercing-rolling is subjected to elongation rolling using a mandrel mill. Furthermore, as necessary, the billet after elongation rolling is subjected to sizing rolling using a reducer or a sizing mill. A mother pipe is manufactured through the above steps.

[0107] [Solution Treatment Step] In the solution treatment step, the mother pipe produced in the hot working step is subjected to solution treatment. The solution treatment method is not particularly limited and may be a well-known method. For example, the mother pipe is loaded into a heat treatment furnace, maintained at a desired temperature, and then rapidly cooled. When the mother pipe is loaded into a heat treatment furnace, maintained at a desired temperature, and then rapidly cooled to perform the solution treatment, the solution treatment temperature refers to the temperature (°C) of the heat treatment furnace used to perform the solution treatment. In this case, the solution treatment time refers to the time during which the mother pipe is maintained at the solution treatment temperature.

[0108] Preferably, the solution temperature in the solution treatment step of this embodiment is 900 to 1100°C. If the solution temperature is too low, precipitates (for example, a σ phase, which is an intermetallic compound) may remain in the mother pipe after the solution treatment. In this case, the corrosion resistance of the manufactured duplex stainless steel pipe may be reduced. If the solution temperature is too low, the ferrite volume fraction of the mother pipe after the solution treatment may be too low. In this case, austenite may coarsen, and the machinability of the manufactured duplex stainless steel pipe may be reduced. On the other hand, if the solution temperature is too high, the ferrite volume fraction of the mother pipe after the solution treatment may be too high. In this case, the manufactured duplex stainless steel seamless pipe may not have the desired mechanical properties.

[0109] When the solution treatment is carried out by charging the mother pipe into a heat treatment furnace, holding it at a desired temperature, and then quenching it, the solution treatment time is not particularly limited and may be carried out under well-known conditions. The solution treatment time is, for example, 5 to 180 minutes. The quenching method is, for example, water cooling.

[0110] [Other Steps] The manufacturing method according to this embodiment may include manufacturing steps other than those described above. For example, the duplex stainless steel seamless pipe according to this embodiment may be subjected to an aging heat treatment. Aging heat treatment means that the manufactured duplex stainless steel seamless pipe is maintained at a desired temperature. In this case, the aging heat treatment may be performed by a well-known method, and is not particularly limited. For example, the duplex stainless steel seamless pipe according to this embodiment may further be subjected to a pickling treatment. In this case, the pickling treatment may be performed by a well-known method, and is not particularly limited. For example, the duplex stainless steel seamless pipe according to this embodiment may further be subjected to cold working. Furthermore, other well-known post-treatments may be performed.

[0111] The duplex stainless steel seamless pipe according to this embodiment can be manufactured by the above steps. As mentioned above, the duplex stainless steel seamless pipe according to this embodiment is not limited to the above manufacturing method. The duplex stainless steel seamless pipe according to this embodiment will be explained in more detail below by way of examples.

[0112] Molten steels having the chemical compositions shown in Tables 1A, 1B, and 1C were melted using a 50 kg vacuum melting furnace, and steel ingots were produced by an ingot casting method. Note that a "-" in Table 1C indicates that the content of the corresponding element was at the impurity level. Specifically, the W content of steel code A was rounded to two decimal places and was 0%. Similarly, the Zr, Ta, Mg, REM, As, Sb, Zn, and Pb contents of steel code A were rounded to three decimal places and were 0%.

[0113]

[0114]

[0115]

[0116] Furthermore, Fn1 (=Cr+3.3(Mo+0.5W)+16N-Mn) for each test number was calculated from the element contents of each steel symbol. Fn1 for each test number is shown in Table 2.

[0117]

[0118] Ingots of each steel grade were hot worked to produce mother pipes (seamless steel pipes). Piercing-rolling by the Mannesmann method was performed as the hot working, followed by elongation rolling using a mandrel mill to produce mother pipes of each test number. In piercing-rolling, the billet surface temperature T (°C) before piercing and the billet roll contact time Δt (seconds) of the piercing mill were set as shown in Table 2. The billet roll contact time Δt (seconds) was defined as the time during which the front end of the billet contacted the rolls of the piercing mill. Furthermore, for each test number, FnA (=T / Δt) was calculated from the billet surface temperature T (°C) before piercing and the billet roll contact time Δt (seconds) of the piercing mill, and formula (A). Table 2 shows FnA for each test number.

[0119] The produced mother pipes of each test number were subjected to solution treatment. The temperature of the heating furnace in which the solution treatment was carried out was 900 to 1100°C, and the time for which the solution treatment was carried out was 5 to 180 minutes. Through the above steps, seamless steel pipes of each test number were produced.

[0120] [Evaluation Tests] The seamless steel pipes manufactured with each test number were subjected to a tensile test, a microstructure observation test, an outer surface flaw observation test, and a crevice corrosion resistance evaluation test.

[0121] [Tensile Test] A tensile test was performed on each seamless steel pipe according to ASTM E8 / E8M (2022) to determine the yield strength. Specifically, a round bar test specimen with a parallel diameter of 8.9 mm and a gauge length of 35.6 mm was prepared from the center of the wall thickness of each seamless steel pipe according to ASTM E8 / E8M (2022). The longitudinal direction of the round bar test specimen was parallel to the axial direction of the seamless steel pipe. Using each round bar test specimen according to ASTM E8 / E8M (2022), a tensile test was performed at room temperature (25°C) in air to determine the 0.2% offset yield strength (MPa). The determined 0.2% offset yield strength was defined as the yield strength (MPa). The yield strength obtained for each test number is shown in the "YS (MPa)" column of Table 3.

[0122]

[0123] [Microstructure Observation Test] The ferrite volume fraction FX at the center of the wall thickness of each seamless steel pipe was determined using the method described above. Specifically, a test specimen having an observation surface 5 mm in the axial direction and 5 mm in the circumferential direction was prepared from the center of the wall thickness of each seamless steel pipe of each test number. The observation surface of the prepared test specimen was polished to a mirror finish and subjected to electrolytic corrosion in a 7% potassium hydroxide etching solution. The observation surface, on which the structure was revealed by electrolytic corrosion, was observed in 10 fields of view using an optical microscope. The area of ​​each field of view was 250 μm × 250 μm (magnification 400 times).

[0124] In each field of view, ferrite and austenite were identified based on the contrast. The area fraction of the identified ferrite was determined using the point counting method in accordance with ASTM E562 (2019). The arithmetic average value of the ferrite area fractions in 10 fields of view was defined as the ferrite volume fraction FX (%) at the center of the wall thickness. The ferrite volume fraction FX (%) at the center of the wall thickness for each test number is shown in the "Center of wall thickness FX" column of the "Ferrite volume fraction (%)" column in Table 3. Note that in the center of the wall thickness, the microstructure contained negligible amounts of phases other than ferrite and austenite. In other words, the seamless steel pipe of each test number had a microstructure consisting of ferrite and austenite.

[0125] The ferrite volume fraction FY in the outer surface layer region of each seamless steel pipe was further determined using the method described above. Specifically, six test specimens were prepared from each seamless steel pipe using the method described above, as shown in Figure 2. Each test specimen had an observation surface measuring 1 mm in the axial direction and 1 mm in the radial direction. The observation surface of the prepared test specimens was polished to a mirror finish and subjected to electrolytic corrosion in a 7% potassium hydroxide etching solution. The observation surface, on which the structure was revealed by electrolytic corrosion, was observed in three fields using an optical microscope. The area of ​​each field was 250 μm × 250 μm (magnification: 400x).

[0126] In the observation using an optical microscope, the observation field was specified as shown in FIG. 3, as described above. From the specified observation field, the outer surface layer region was specified using the method described above. For the specified outer surface layer region, image analysis was performed on the photographic image using the method described above to determine the ferrite area ratio. For all test specimens, the ferrite area ratio was determined for three fields of view, and the arithmetic mean value of the ferrite area ratios in the obtained 18 fields of view was defined as the ferrite volume ratio FY (%) in the outer surface layer region. The ferrite volume ratio FY (%) in the outer surface layer region obtained for each test number is shown in the "Outer surface layer region FY" column of the "Ferrite volume ratio (%)" column in Table 3. Furthermore, the ratio of the ferrite volume ratio of the outer surface layer region to the central part of the wall thickness (FY / FX) was determined. The obtained FY / FX for each test number is shown in the "Ferrite volume ratio ratio (= FY / FX)" column in Table 3.

[0127] [External surface defect observation test] The outer surface of each seamless steel pipe of each test number was inspected by ultrasonic testing and visual inspection to confirm the presence or absence of defects. Test specimens having cross sections parallel to the pipe axis direction and pipe diameter direction were cut out from the positions where defects were confirmed. The obtained cross sections were observed with an optical microscope, and the depth of the defects was measured. The maximum value of the obtained depth of the defects was defined as the "maximum depth of external surface defects (μm)." The obtained maximum depth of external surface defects (μm) for each test number is shown in the "maximum depth of external surface defects (μm)" column in Table 3.

[0128] [Crevice Corrosion Resistance Evaluation Test] A crevice corrosion resistance evaluation test was conducted on the seamless steel pipes of each test number to evaluate crevice corrosion resistance. Specifically, test specimens having a metal-metal contact surface were prepared from the seamless steel pipes of each test number using the method described above. Specifically, a test specimen measuring 30 mm in length, 30 mm in width, and 3 mm in thickness and a test specimen measuring 30 mm in length, 15 mm in width, and 3 mm in thickness were taken from the central portion of the wall thickness of the seamless steel pipes of each test number. Each test specimen had a through hole with a diameter of 5 mm formed in the center of the length and width directions, penetrating the thickness direction. In addition, the thickness direction of the test specimen was parallel to the diameter direction of the seamless steel pipe.

[0129] The contact surfaces of the two test pieces were polished using waterproof abrasive paper coated with silicon carbide abrasive grains (grain size: P600). PTFE bushings were inserted into the through holes of the two test pieces. Metal bolts, nuts, and washers were passed through the diameter of the inserted bushings, and the two test pieces were fixed to prepare a test specimen. The screw diameter of the bolt was 3 mm, and the tightening torque of the bolt was 3 N m.

[0130] The prepared test specimen was placed in an autoclave, and a 5.0 mass % aqueous solution of sodium chloride was poured into the autoclave so that the test specimen was immersed. ２ and CO ２ The mixed gas was pressurized and sealed in to saturate the test solution, forming a test bath. At this time, the total pressure of the mixed gas was 130 bar, and the O ２ The concentration was set to 300 ppm. After sealing the autoclave, the test bath was maintained at 100° C. and the test specimen was immersed in the test bath for 96 hours while stirring.

[0131] The test specimens after 96 hours were checked for the presence or absence of crevice corrosion using the method described above. Test numbers in which no crevice corrosion was observed are marked with "EX (Excellent)" in the "Crevice Corrosion Resistance" column of Table 3. Furthermore, test numbers in which crevice corrosion was observed are marked with "NA (Not Acceptable)" in the "Crevice Corrosion Resistance" column of Table 3.

[0132] [Evaluation Results] Referring to Tables 1A, 1B, 1C, 2, and 3, the seamless steel pipes of test numbers 1 to 22 had the above-described chemical compositions, Fn1 of 30.0 or more, a microstructure consisting of ferrite and austenite, a ferrite volume fraction FX of 35.0 to 65.0% in the wall thickness center portion of the seamless steel pipe, and a ferrite volume fraction FY of 0.90FX or more in the outer surface layer region. As a result, these seamless steel pipes had high strength, with a yield strength of 550 MPa or more. Furthermore, in a crevice corrosion resistance test, no crevice corrosion was observed in these seamless steel pipes, indicating excellent crevice corrosion resistance in supercritical corrosion environments. Furthermore, these seamless steel pipes had a maximum depth of outer surface defects of 200 μm or less, indicating excellent hot workability.

[0133] On the other hand, the seamless steel pipes of test numbers 23 to 25 had Fn1 of less than 30.0. As a result, crevice corrosion was confirmed in the crevice corrosion resistance test for these seamless steel pipes, and they did not have excellent crevice corrosion resistance in a supercritical corrosion environment.

[0134] The seamless steel pipe of test number 26 had an excessively low Cu content, and as a result, the seamless steel pipe had a yield strength of less than 550 MPa and did not have high strength.

[0135] The seamless steel pipe of test number 27 had an excessively high Mn content. As a result, the volume fraction FY of ferrite in the outer surface region of this seamless steel pipe was less than 0.90FX. As a result, the maximum depth of outer surface defects of this seamless steel pipe exceeded 200 μm, and the seamless steel pipe did not have excellent hot workability.

[0136] The seamless steel pipe of test number 28 had a Cu content that was too low, a Mo content that was too low, and an Fn1 of less than 30.0. As a result, the volume fraction FY of ferrite in the outer surface region of this seamless steel pipe was less than 0.90FX. As a result, crevice corrosion was confirmed in the crevice corrosion resistance test for this seamless steel pipe, and it did not have excellent crevice corrosion resistance in a supercritical corrosion environment. Furthermore, the maximum depth of outer surface defects exceeded 200 μm, and it did not have excellent hot workability.

[0137] The seamless steel pipe of test number 29 had an excessively low Mo content. As a result, the ferrite volume fraction FX at the center of the wall thickness of this seamless steel pipe was less than 35.0%, and the ferrite volume fraction FY in the outer surface layer region was less than 0.90FX. As a result, the seamless steel pipe had a maximum depth of outer surface defects exceeding 200 μm, and did not have excellent hot workability.

[0138] The seamless steel pipe of test number 30 had an excessively high N content. As a result, the volume fraction FY of ferrite in the outer surface layer region of this seamless steel pipe was less than 0.90FX. As a result, the maximum depth of outer surface defects of this seamless steel pipe exceeded 200 μm, and the seamless steel pipe did not have excellent hot workability.

[0139] The seamless steel pipes of test numbers 31 to 35 had too low FnA. As a result, the volume fraction FY of ferrite in the outer surface layer region of these seamless steel pipes was less than 0.90FX. As a result, the maximum depth of outer surface defects of these seamless steel pipes exceeded 200 μm, and they did not have excellent hot workability.

[0140] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and can be implemented by appropriately modifying the above-described embodiments within the scope of the present disclosure.

Claims

1. A duplex stainless steel seamless pipe having, in mass %, C: 0.050% or less, Si: 1.00% or less, Mn: 0.10 to 1.50%, P: 0.050% or less, S: 0.0050% or less, Cr: 20.00 to 28.00%, Cu: 0.05 to 3.00%, Ni: 4.00 to 7.00%, Mo: 0.80 to 3.00%, Ti: 0.001 to 0.050%, V: 0.01 to 0.50%, Nb: 0.001 to 0.100%, Co: 0.01 to 1.00%, Sn: 0.001 to 0.050%, sol. Al: 0.050% or less, N: 0.001 to 0.350%, O: 0.0100% or less, B: 0.0010 to 0.0050%, Ca: 0.0005 to 0.0100%, W: 0 to 1.50%, Zr: 0 to 0.010%, Ta: 0 to 0.050%, Mg: 0 to 0.010%, rare earth elements: 0 to 0.050%, As: 0 to 0.010%, Sb: 0 to 0.010%, Zn: 0 to 0.010%, Pb: 0 to 0.010%, and the balance being Fe and impurities, satisfying formula (1), having a yield strength of 550 MPa or more, and a microstructure consisting of ferrite and austenite, A duplex stainless steel seamless pipe, wherein the volume fraction FX of the ferrite at the center of the wall thickness of the duplex stainless steel seamless pipe is 35.0 to 65.0%, and the volume fraction FY of the ferrite in a region 100 μm deep from the outer surface of the duplex stainless steel seamless pipe is 0.90FX or more. Cr+3.3(Mo+0.5W)+16N-Mn≧30.0 (1) Here, the element symbols in formula (1) are substituted with the content of the corresponding element in units of mass%. When the corresponding element is not contained, "0" is substituted for that element symbol.

2. A duplex stainless steel seamless pipe according to claim 1, containing one or more elements selected from the group consisting of W: 0.01 to 1.50%, Zr: 0.001 to 0.010%, Ta: 0.001 to 0.050%, Mg: 0.001 to 0.010%, rare earth elements: 0.001 to 0.050%, As: 0.001 to 0.010%, Sb: 0.001 to 0.010%, Zn: 0.001 to 0.010%, and Pb: 0.001 to 0.010%.

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