Ni-based alloy tube

Abstract

The present invention provides an Ni-based alloy tube which is capable of achieving sufficient nitriding resistance in a high-temperature ammonia environment. An Ni-based alloy tube according to the present disclosure has a chemical composition that contains, in mass%, 0.150% or less of C, more than 1.00% but not more than 2.50% of Si, 0.01% to 1.00% of Mn, 0.0100% or less of P, 0.0100% or less of S, 1.50% to 3.00% of Cu, 25.00% to 35.00% of Cr, 2.00% to 6.00% of Fe, 1.00% to 3.00% of Mo, 0.10% to 1.00% of Ti, 0.0001% to 0.0100% of B, 0.0001% to 0.0050% of Ca, and 0.005% to 0.100% of REM, with the balance being made up of Ni and impurities, and the developed area ratio Sdr on the inner surface of the Ni-based alloy tube satisfies formula (1).　(1): Cr / ((2.0 + 0.8 × Fe + 6.0 × Ti) × (1.5 + 10.0 × Sdr)) ≥ 0.9 In formula (1), the content, in mass%, of the corresponding element is assigned to each of the atomic symbols.

Description

Ni-based alloy tube

[0001] The present disclosure relates to Ni-based alloy materials, and more particularly to Ni-based alloy pipes.

[0002] Recently, hydrogen has been attracting attention as a clean energy source that does not produce carbon dioxide. Hydrogen is a gas at room temperature and pressure, and when liquefied for transportation, it must be kept at an extremely low temperature of -253°C or below. This makes it difficult to transport hydrogen alone.

[0003] Therefore, the use of ammonia as a hydrogen carrier is being considered. Ammonia contains about 18% hydrogen by mass and liquefies at -33°C, which is higher than the boiling point of hydrogen. Therefore, studies are underway to transport ammonia as a hydrogen carrier and desorb hydrogen from the ammonia at the destination to use hydrogen as energy.

[0004] To desorb hydrogen from ammonia, a catalyst is used to decompose ammonia in a high-temperature environment of about 600°C under atmospheric pressure. In the following description, an environment of high temperature of about 600°C, atmospheric pressure, and an ammonia atmosphere is referred to as a "high-temperature ammonia environment." As described above, there is a growing need for plant components (e.g., reaction tubes and piping) that can withstand use in a high-temperature ammonia environment.

[0005] Plant components used in high-temperature ammonia environments are required to have high-temperature creep strength. Ni-based alloy materials used in thermal power plants, which are subjected to high-temperature environments similar to high-temperature ammonia environments, are considered as plant components having high high-temperature creep strength. Ni-based alloy materials for thermal power plants are disclosed, for example, in Japanese Patent Laid-Open Publication No. 2013-209721 (Patent Document 1). The Ni-based alloy disclosed in this document has a chemical composition, in mass %, of C: 0.007% or more but less than 0.020%, Si: 0.3% or less, Mn: 0.3% or less, Cr: 19.0 to 22.0%, Mo alone or Mo as an essential component, Mo + (½) × W: 9.0 to 12.0%, Al: 0.5 to 1.8%, Ti: 1.0 to 2.0%, Fe: 2.0% or less, Mg: 0.02% or less, and either or both of B: 0.02% or less and Zr: 0.2% or less, and further contains a value represented by Al / (Al + 0.56Ti) of 0.45 to 0.70, with the balance consisting of Ni and impurities. This Ni-based alloy also has an average grain size of 50 to 200 μm and a hardness of 250 HV or more. Patent Document 1 describes that this Ni-based alloy material exhibits excellent creep strength even when used in a high-temperature environment.

[0006] Japanese Patent Application Laid-Open No. 2013-209721

[0007] When a Ni-based alloy material is used in the above-mentioned high-temperature ammonia environment, nitriding in the surface layer of the alloy material may be accelerated in the high-temperature environment. As a result, a nitride layer consisting of a layer mainly composed of nitrides and a layer in which solute nitrogen is diffused at a high concentration is formed near the surface layer of the alloy material. If the nitride layer formed becomes thick, the portions of the alloy material other than the nitride layer become thin. In this case, the high-temperature creep strength is reduced. Therefore, Ni-based alloy materials used in high-temperature ammonia environments are required to have excellent nitriding resistance.

[0008] An object of the present disclosure is to provide a Ni-based alloy tube that has sufficient nitriding resistance in a high-temperature ammonia environment.

[0009] The Ni-base alloy pipe of the present disclosure has a chemical composition, in mass%, of C: 0.150% or less, Si: over 1.00% to 2.50%, Mn: 0.01 to 1.00%, P: 0.0100% or less, S: 0.0100% or less, Cu: 1.50 to 3.00%, Cr: 25.00 to 35.00%, Fe: 2.00 to 6.00%, Mo: 1.00 to 3.00%, Ti: 0.10 to 1.00%, B: 0.0001 to 0.0100%, Ca: 0.0001 to 0.0050%, REM: 0.005 to 0.100%, Co: 0 to 1.00%, W: 0 to 1.00%, N: 0 to 0.100%, V: 0 to 1.00%, Nb: 0 to 0.10%, Al: 0 to 0.10%, and the balance being Ni and impurities, wherein the developed area ratio Sdr on the inner surface of the Ni-based alloy pipe satisfies formula (1): Cr / ((2.0 + 0.8 × Fe + 6.0 × Ti) × (1.5 + 10.0 × Sdr)) ≧ 0.9 (1) where the content of the corresponding element in mass % is substituted for each element symbol in formula (1).

[0010] The Ni-based alloy tube of the present disclosure exhibits sufficient nitriding resistance in a high-temperature ammonia environment.

[0011] Fig. 1 is a front view of a test piece used in a nitriding resistance evaluation test, as viewed from the front of the surface consisting of the inner surface of an alloy pipe. Fig. 2 is a schematic diagram for explaining how to determine the nitride layer depth in the nitriding resistance evaluation test.

[0012] The present inventors first investigated, from the viewpoint of chemical composition, Ni-based alloy pipes that can provide sufficient nitriding resistance in high-temperature ammonia environments, and found that the chemical composition is, in mass %, C: 0.150% or less, Si: over 1.00 to 2.50%, Mn: 0.01 to 1.00%, P: 0.0100% or less, S: 0.0100% or less, Cu: 1.50 to 3.00%, Cr: 25.00 to 35.00%, Fe: 2.00 to 6.00%, Mo: 1.00 to 3.00%, Ti: 0.10 to 1.00%, and B: 0.0001 to 0.010%. The present inventors considered that a Ni-based alloy pipe consisting of 0%, Ca: 0.0001 to 0.0050%, REM: 0.005 to 0.100%, Co: 0 to 1.00%, W: 0 to 1.00%, N: 0 to 0.100%, V: 0 to 1.00%, Nb: 0 to 0.10%, Al: 0 to 0.10%, and the balance being Ni and impurities, would provide sufficient nitriding resistance in a high-temperature ammonia environment.

[0013] However, even Ni-based alloy pipes having the above-mentioned chemical composition sometimes fail to provide sufficient nitriding resistance in a high-temperature ammonia environment. Therefore, the present inventors have investigated means for further improving the nitriding resistance of Ni-based alloy pipes having the above-mentioned chemical composition in a high-temperature ammonia environment, from the viewpoint of suppressing the formation of a nitride layer. As a result, the present inventors have obtained the following findings.

[0014] (A) As described above, in a high-temperature ammonia environment, nitride formation is promoted in the surface layer of an alloy material. Nitrogen diffuses more easily in the generated nitride than in the base material of the alloy material. Therefore, nitride formation promotes nitrogen diffusion in the alloy material. When nitrides are generated in the surface layer, nitride formation is promoted further into the alloy material. In this way, a nitride layer is formed from the surface layer of the alloy material in the depth direction. In other words, the present inventors considered that reducing the content of elements that easily form nitrides is effective in suppressing the formation of a nitride layer. As a result of further investigation by the present inventors, it was found that Fe and Ti are more likely to form nitrides than other elements contained in an alloy material. Therefore, by reducing the content of Fe and Ti and adjusting them to an appropriate range, the formation of a nitride layer on the surface of an alloy material can be suppressed.

[0015] (B) In general, Cr, like Fe and Ti, is known to be an element that easily forms nitrides. Furthermore, in a high-temperature ammonia environment with an extremely low oxygen partial pressure, the formation of a Cr oxide film, known as a surface protective film, is difficult. Therefore, like Fe and Ti, reducing the Cr content is expected to be effective in suppressing the formation of a nitride layer. However, as a result of investigations, the present inventors have found that increasing the Cr content in a Ni-based alloy material with the above-mentioned chemical composition suppresses the formation of a nitride layer. The following mechanism is thought to be the reason for this: In a high-temperature environment such as a high-temperature ammonia environment, Cr carbides are dispersed and precipitated in the alloy material. The Cr carbides dispersed in the alloy material inhibit the diffusion of nitrogen in the alloy material. In a high-temperature ammonia environment, the precipitation of Cr carbides, which has this effect, takes precedence over the formation of Cr nitrides. Therefore, it is believed that increasing the Cr content in the alloy material inhibits the diffusion of nitrogen in the alloy material and suppresses the formation of a nitride layer.

[0016] (C) In a high-temperature ammonia environment, nitrogen that forms a nitride layer penetrates from the surface of the alloy material and diffuses in the depth direction of the alloy material. The penetration rate of nitrogen into the alloy material is correlated with the amount of nitrogen in contact with the surface of the alloy material. The amount of nitrogen in contact with the surface of the alloy material is correlated with the surface area of ​​the surface of the alloy material exposed to a nitriding atmosphere such as a high-temperature ammonia environment. In other words, the penetration rate of nitrogen into the alloy material is correlated with the surface area of ​​the surface of the alloy material exposed to a high-temperature ammonia environment. The developed area ratio Sdr defined in ISO25178-2:2012 is known as an index for evaluating the size of the surface area of ​​a certain surface. The developed area ratio Sdr is calculated by dividing the area of ​​a measurement region assumed to be flat by A ０ and the actual surface area (developed area) in the measurement area is defined as A １ When this is defined, it is expressed by the following formula: Sdr = (A １ -A ０ ) / A ０In other words, the developed area ratio Sdr represents the increase in the actual surface area of ​​the measurement region relative to the area of ​​the measurement region. In other words, when the developed area ratio Sdr of a certain surface is large, the surface area of ​​that surface increases at a greater rate compared to a flat surface. When the developed area ratio Sdr of an alloy material surface exposed to a high-temperature ammonia environment is large, the rate of nitrogen penetration into the alloy material is high. In this case, nitride formation progresses without sufficient precipitation of Cr carbides. As a result, in an alloy material with an increased Cr content as described above, the formation of Cr nitrides is promoted while the formation of Cr carbides is suppressed. Therefore, even the effect of suppressing nitrogen diffusion is impaired, and the formation of a nitride layer is greatly promoted. Therefore, to suppress the formation of a nitride layer, it is necessary to adjust the developed area ratio Sdr of the alloy material surface to a sufficiently small value depending on the chemical composition of the alloy material. In particular, for alloy pipes, the inner surface of the alloy pipe is generally exposed to a high-temperature ammonia environment, so it is important to adjust the developed area ratio Sdr of the inner surface.

[0017] Based on the findings of (A) to (C), the present inventors further investigated the relationship between the chemical composition of an Ni-based alloy pipe and the developed area ratio Sdr of the inner surface of the alloy pipe, which provides sufficient nitriding resistance in a high-temperature ammonia environment. As a result, the present inventors found that sufficient nitriding resistance in a high-temperature ammonia environment can be achieved if the developed area ratio Sdr of the inner surface of the alloy pipe satisfies the following formula (1), which was obtained by regression from the investigation results: Cr / ((2.0+0.8×Fe+6.0×Ti)×(1.5+10.0×Sdr))≧0.9 (1) Here, the content of the corresponding element in mass % is substituted for each element symbol in formula (1).

[0018] The Ni-base alloy pipe of this embodiment has been completed based on the above technical concept and has the following configuration. Note that the above mechanism is a conjecture. Therefore, the Ni-base alloy pipe of this embodiment may have sufficient nitriding resistance in a high-temperature ammonia environment through a mechanism different from that described above. However, as will be shown in the examples described later, a Ni-base alloy pipe having the following configuration can have sufficient nitriding resistance in a high-temperature ammonia environment.

[0019] The Ni-based alloy pipe of the first configuration has a chemical composition, in mass %, of C: 0.150% or less, Si: over 1.00% to 2.50%, Mn: 0.01 to 1.00%, P: 0.0100% or less, S: 0.0100% or less, Cu: 1.50 to 3.00%, Cr: 25.00 to 35.00%, Fe: 2.00 to 6.00%, Mo: 1.00 to 3.00%, Ti: 0.10 to 1.00%, B: 0.0001 to 0.0100%, Ca: 0.0001 to 0.0050%, REM: 0.005 to 0.100%, Co: 0 to 1.00%, W: 0 to 1.00%, N: 0 to 0.100%, V: 0 to 1.00%, Nb: 0 to 0.10%, Al: 0 to 0.10%, and the balance being Ni and impurities, wherein the developed area ratio Sdr on the inner surface of the Ni-based alloy pipe satisfies formula (1): Cr / ((2.0 + 0.8 × Fe + 6.0 × Ti) × (1.5 + 10.0 × Sdr)) ≧ 0.9 (1) where the content of the corresponding element in mass % is substituted for each element symbol in formula (1).

[0020] The Ni-based alloy pipe of the second configuration is the Ni-based alloy pipe of the first configuration, and the chemical composition contains, in mass %, one or more elements selected from the group consisting of Co: 0.01 to 1.00%, W: 0.01 to 1.00%, N: 0.001 to 0.100%, V: 0.01 to 1.00%, Nb: 0.01 to 0.10%, and Al: 0.01 to 0.10%.

[0021] The Ni-based alloy pipe according to this embodiment will be described in detail below. Note that "%" for elements means mass % unless otherwise specified.

[0022] [Features of the Ni-based alloy pipe of this embodiment] The Ni-based alloy pipe of this embodiment has the following features: (Feature 1) The chemical composition, in mass %, is: C: 0.150% or less, Si: over 1.00 to 2.50%, Mn: 0.01 to 1.00%, P: 0.0100% or less, S: 0.0100% or less, Cu: 1.50 to 3.00%, Cr: 25.00 to 35.00%, Fe: 2.00 to 6.00%, Mo: 1.00 to 3.00%, and Ti: 0.10 to 1.00%. 00%, B: 0.0001-0.0100%, Ca: 0.0001-0.0050%, REM: 0.005-0.100%, Co: 0-1.00%, W: 0-1.00%, N: 0-0.100%, V: 0-1.00%, Nb: 0-0.10%, and Al: 0-0.10%, with the balance consisting of Ni and impurities. (Feature 2) The developed area ratio Sdr on the inner surface of the alloy pipe satisfies formula (1): Cr / ((2.0 + 0.8 × Fe + 6.0 × Ti) × (1.5 + 10.0 × Sdr)) ≧ 0.9 (1) Here, the content of the corresponding element in mass % is substituted for each element symbol in formula (1). Each feature is described below.

[0023] [(Feature 1) Chemical Composition] The chemical composition of the Ni-based alloy pipe of this embodiment contains the following elements.

[0024] C: 0.150% or less Carbon (C) mainly combines with Cr to form Cr carbides in the alloy tube, improving high-temperature creep strength. Furthermore, Cr carbides suppress nitrogen diffusion in the alloy tube. As a result, nitriding resistance in a high-temperature ammonia environment is improved. Even if even a small amount of C is contained, the above effect can be achieved to some extent. Therefore, the C content is greater than 0%. On the other hand, if the C content exceeds 0.150%, excessive carbides are formed at grain boundaries. Therefore, even if the contents of other elements are within the ranges of this embodiment, the hot workability of the alloy material used to make the alloy tube is reduced. Therefore, the C content is less than 0.150%. The preferred lower limit of the C content is 0.001%, more preferably 0.005%, even more preferably 0.010%, even more preferably 0.020%, and even more preferably 0.030%. The upper limit of the C content is preferably 0.140%, more preferably 0.130%, even more preferably 0.120%, even more preferably 0.110%, and still more preferably 0.100%.

[0025] Si: More than 1.00% to 2.50% Silicon (Si) deoxidizes the alloy material used to make the alloy pipe. If the Si content is 1.00% or less, the above effect cannot be sufficiently achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Si content exceeds 2.50%, the hot workability of the alloy material used to make the alloy pipe decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Si content is more than 1.00% to 2.50%. A preferred lower limit of the Si content is 1.01%, more preferably 1.05%, even more preferably 1.10%, even more preferably 1.15%, and even more preferably 1.20%. A preferred upper limit of the Si content is 2.40%, even more preferably 2.20%, and even more preferably 2.00%.

[0026] Mn: 0.01 to 1.00% Manganese (Mn) improves the nitriding resistance of alloy tubes in high-temperature ammonia environments. If the Mn content is less than 0.01%, the above effect cannot be sufficiently achieved, even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Mn content exceeds 1.00%, the hot workability of the alloy material used to make the alloy tube decreases, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Mn content is 0.01 to 1.00%. The preferred lower limit of the Mn content is 0.02%, more preferably 0.03%, even more preferably 0.04%, and even more preferably 0.05%. The preferred upper limit of the Mn content is 0.90%, even more preferably 0.80%, even more preferably 0.70%, even more preferably 0.60%, and even more preferably 0.50%.

[0027] P: 0.0100% or less Phosphorus (P) is an impurity. That is, the lower limit of the P content is greater than 0%. If the P content exceeds 0.0100%, P segregates excessively at grain boundaries, reducing grain boundary strength. Therefore, even if the contents of other elements are within the ranges of this embodiment, the hot workability of the alloy material used to make the alloy pipe is reduced. Therefore, the P content is 0.0100% or less. The lower the P content, the more preferable it is. However, excessive reduction in the P content increases manufacturing costs. Therefore, considering industrial production, the lower limit of the P content is preferably 0.0001%, more preferably 0.0005%, even more preferably 0.0010%, and even more preferably 0.0020%. The upper limit of the P content is preferably 0.0090%, even more preferably 0.0080%, and even more preferably 0.0070%.

[0028] S: 0.0100% or less Sulfur (S) is an impurity. That is, the lower limit of the S content is greater than 0%. If the S content exceeds 0.0100%, coarse sulfide-based inclusions are formed. Therefore, even if the contents of other elements are within the ranges of this embodiment, the hot workability of the alloy material used to make the alloy pipe is reduced. Therefore, the S content is 0.0100% or less. The S content is preferably as low as possible. However, excessive reduction of the S content increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the S content is 0.0001%, more preferably 0.0002%, and even more preferably 0.0003%. The preferred upper limit of the S content is 0.0090%, more preferably 0.0080%, even more preferably 0.0070%, even more preferably 0.0060%, and even more preferably 0.0050%.

[0029] Cu: 1.50 to 3.00% Copper (Cu) forms precipitates during use of the alloy pipe in a high-temperature environment, thereby improving the high-temperature creep strength of the alloy pipe. If the Cu content is less than 1.50%, the above effect cannot be sufficiently achieved, even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Cu content exceeds 3.00%, the hot workability of the alloy material used to make the alloy pipe deteriorates, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Cu content is 1.50 to 3.00%. The preferred lower limit of the Cu content is 1.60%, more preferably 1.70%, and even more preferably 1.80%. The preferred upper limit of the Cu content is 2.90%, more preferably 2.70%, and even more preferably 2.50%.

[0030] Cr: 25.00 to 35.00% Chromium (Cr) combines with carbon in a high-temperature environment to form Cr carbides in the alloy tube. Cr carbides suppress the diffusion of nitrogen in the alloy tube. As a result, nitridation resistance in a high-temperature ammonia environment is improved. If the Cr content is less than 25.00%, the above effect cannot be fully achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Cr content exceeds 35.00%, excessive Cr nitrides are formed in a high-temperature ammonia environment. Cr nitrides promote the diffusion of nitrogen in the alloy tube. Therefore, nitridation resistance in a high-temperature ammonia environment is reduced even if the contents of other elements are within the ranges of this embodiment. Therefore, the Cr content is 25.00 to 35.00%. The preferred lower limit of the Cr content is 26.00%, more preferably 27.00%, and even more preferably 28.00%. The upper limit of the Cr content is preferably 34.00%, more preferably 33.00%, and even more preferably 32.00%.

[0031] Fe: 2.00 to 6.00% Iron (Fe) improves the hot workability of the alloy material used to make the alloy pipe. If the Fe content is less than 2.00%, the above effects cannot be fully achieved, even if the contents of other elements are within the ranges of this embodiment. Furthermore, Fe reduces the manufacturing cost of the alloy pipe. On the other hand, if the Fe content exceeds 6.00%, excessive Fe nitrides are formed in a high-temperature ammonia environment. Fe nitrides promote the diffusion of nitrogen in the alloy pipe. Therefore, even if the contents of other elements are within the ranges of this embodiment, nitriding resistance in a high-temperature ammonia environment decreases. Therefore, the Fe content is 2.00 to 6.00%. The preferred lower limit of the Fe content is 2.20%, more preferably 2.40%, and even more preferably 2.50%. The preferred upper limit of the Fe content is 5.50%, more preferably 5.00%, and even more preferably 4.50%.

[0032] Mo: 1.00 to 3.00% Molybdenum (Mo) forms precipitates during use of an alloy tube in a high-temperature environment, thereby improving the high-temperature creep strength of the alloy tube. If the Mo content is less than 1.00%, the above effect cannot be sufficiently achieved, even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Mo content exceeds 3.00%, the hot workability of the alloy material used to make the alloy tube deteriorates, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Mo content is 1.00 to 3.00%. The preferred lower limit of the Mo content is 1.10%, more preferably 1.30%, and even more preferably 1.50%. The preferred upper limit of the Mo content is 2.90%, more preferably 2.70%, and even more preferably 2.50%.

[0033] Ti: 0.10 to 1.00% Titanium (Ti) forms precipitates during use of an alloy tube in a high-temperature environment, thereby improving the high-temperature creep strength of the alloy tube. If the Ti content is less than 0.10%, the above effect is not sufficiently achieved, even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Ti content exceeds 1.00%, excessive Ti nitrides are formed in the alloy tube. Ti nitrides promote nitrogen diffusion in the alloy tube. Therefore, even if the contents of other elements are within the ranges of this embodiment, nitridation resistance in a high-temperature ammonia environment decreases. Therefore, the Ti content is 0.10 to 1.00%. The preferred lower limit of the Ti content is 0.15%, more preferably 0.18%, and even more preferably 0.20%. The preferred upper limit of the Ti content is 0.90%, more preferably 0.80%, even more preferably 0.70%, and even more preferably 0.50%.

[0034] B: 0.0001 to 0.0100% Boron (B) segregates at grain boundaries to strengthen them. As a result, the high-temperature creep strength of the alloy pipe is improved. If the B content is less than 0.0001%, the above effect cannot be fully achieved, even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the B content exceeds 0.0100%, the hot workability of the alloy material used to make the alloy pipe is reduced, even if the contents of other elements are within the ranges of this embodiment. Therefore, the B content is 0.0001 to 0.0100%. The preferred lower limit of the B content is 0.0003%, more preferably 0.0005%, and even more preferably 0.0010%. The preferred upper limit of the B content is 0.0080%, more preferably 0.0060%, and even more preferably 0.0040%.

[0035] Ca: 0.0001 to 0.0050% Calcium (Ca) deoxidizes the alloy material used to make the alloy pipe during the manufacturing process. If the Ca content is less than 0.0001%, the above effect cannot be fully achieved, even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Ca content exceeds 0.0050%, coarse inclusions form in the alloy pipe. As a result, the high-temperature creep strength of the alloy pipe decreases, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Ca content is 0.0001 to 0.0050%. The preferred lower limit of the Ca content is 0.0005%, more preferably 0.0010%, and even more preferably 0.0015%. The preferred upper limit of the Ca content is 0.0045%, more preferably 0.0042%, and even more preferably 0.0040%.

[0036] REM: 0.005 to 0.100% Rare earth elements (REM) segregate at grain boundaries to strengthen them. This increases the high-temperature creep strength of the alloy pipe. If the REM content of the alloy pipe is less than 0.005%, the above effect cannot be fully achieved, even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the REM content exceeds 0.100%, coarse inclusions form in the alloy pipe. Therefore, the high-temperature creep strength of the alloy pipe decreases, even if the contents of other elements are within the ranges of this embodiment. Therefore, the REM content is 0.005 to 0.100%. The preferred lower limit of the REM content is 0.010%, more preferably 0.015%, even more preferably 0.020%, and even more preferably 0.025%. The preferred upper limit of the REM content is 0.090%, even more preferably 0.080%, and even more preferably 0.070%.

[0037] 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, the REM content in this specification refers to the total content of these elements.

[0038] The balance of the chemical composition of the Ni-based alloy pipe of this embodiment is composed of Ni and impurities. The Ni content is, for example, 55.00 to 65.00%. The impurities in the chemical composition refer to substances that are mixed in from raw materials or the manufacturing environment during industrial production of the Ni-based alloy pipe, and are acceptable within a range that does not adversely affect the Ni-based alloy pipe of this embodiment.

[0039] [Optional Elements] The chemical composition of the Ni-based alloy pipe of this embodiment may further contain, in place of a portion of Ni, one or more elements selected from the group consisting of Co: 0-1.00%, W: 0-1.00%, N: 0-0.100%, V: 0-1.00%, Nb: 0-0.10%, and Al: 0-0.10%. All of these elements are optional elements. These optional elements will be described below.

[0040] [First Group: Co, W, and N] The chemical composition of the Ni-based alloy pipe of this embodiment may further contain elements of the first group in place of a portion of Ni. All elements of the first group dissolve in the alloy pipe to enhance the high-temperature creep strength of the alloy pipe. Each element of the first group will be described below.

[0041] Cobalt (Co) is an optional element and does not necessarily need to be contained. That is, the Co content may be 0%. When Co is contained, that is, when Co exceeds 0%, Co dissolves in the alloy pipe and enhances high-temperature creep strength. Even if even a small amount of Co is contained, the above effect can be obtained to some extent. However, if the Co content exceeds 1.00%, the hot workability of the alloy material used to make the alloy pipe deteriorates even if the contents of other elements are within the ranges of this embodiment. Therefore, the Co content is 0 to 1.00%, and if Co is contained, the Co content is 1.00% or less. The preferred lower limit of the Co content is 0.01%, more preferably 0.10%, and even more preferably 0.20%. The preferred upper limit of the Co content is 0.80%, more preferably 0.60%, and even more preferably 0.40%.

[0042] W: 0 to 1.00% Tungsten (W) is an optional element and does not necessarily need to be included. In other words, the W content may be 0%. When W is included, that is, when the W content exceeds 0%, W dissolves in the alloy pipe and enhances high-temperature creep strength. Even if even a small amount of W is included, the above effect can be achieved to some extent. However, if the W content exceeds 1.00%, the hot workability of the alloy material used to make the alloy pipe deteriorates even if the contents of other elements are within the ranges of this embodiment. Therefore, the W content is 0 to 1.00%, and when W is included, the W content is 1.00% or less. The preferred lower limit of the W content is 0.01%, more preferably 0.02%, and even more preferably 0.05%. The preferred upper limit of the W content is 0.80%, more preferably 0.60%, and even more preferably 0.40%.

[0043] N: 0 to 0.100% Nitrogen (N) is an optional element and does not necessarily need to be present. In other words, the N content may be 0%. When N is present, that is, when the N content exceeds 0%, N dissolves in the alloy tube and improves high-temperature creep strength. Even if even a small amount of N is present, the above effect can be achieved to some extent. However, when the N content exceeds 0.100%, excessive nitrides are formed in the alloy tube. Nitrides promote the diffusion of nitrogen in the alloy tube. Therefore, even if the contents of other elements are within the ranges of this embodiment, nitriding resistance in a high-temperature ammonia environment decreases. Therefore, the N content is 0 to 0.100%, and when present, the N content is 0.100% or less. The preferred lower limit of the N content is 0.001%, more preferably 0.010%, and even more preferably 0.020%. The preferred upper limit of the N content is 0.090%, more preferably 0.070%, and even more preferably 0.050%.

[0044] [Group 2: V and Nb] The chemical composition of the Ni-based alloy pipe of this embodiment may further contain elements of Group 2 in place of a portion of Ni. All elements of Group 2 form precipitates during use of the alloy pipe in a high-temperature environment, thereby improving the high-temperature creep strength of the alloy pipe. Each element of Group 2 will be described below.

[0045] V: 0 to 1.00% Vanadium (V) is an optional element and does not necessarily need to be contained. That is, the V content may be 0%. When vanadium (V) is contained, that is, when the V content exceeds 0%, V forms precipitates during use of the alloy tube in a high-temperature environment, thereby improving the high-temperature creep strength of the alloy tube. Even if even a small amount of V is contained, the above effect can be obtained to some extent. However, if the V content exceeds 1.00%, the hot workability of the alloy material used to make the alloy tube decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the V content is 0 to 1.00%, and when contained, the V content is 1.00% or less. The preferred lower limit of the V content is 0.01%, more preferably 0.05%, and even more preferably 0.10%. The preferred upper limit of the V content is 0.90%, more preferably 0.70%, and even more preferably 0.50%.

[0046] Nb: 0 to 0.10% Niobium (Nb) is an optional element and does not necessarily need to be contained. That is, the Nb content may be 0%. When Nb is contained, that is, when the Nb content exceeds 0%, Nb forms precipitates during use of the alloy pipe in a high-temperature environment, thereby improving the high-temperature creep strength of the alloy pipe. Even if even a small amount of Nb is contained, the above effect can be obtained to some extent. However, if the Nb content exceeds 0.10%, the hot workability of the alloy material used to make the alloy pipe deteriorates even if the contents of other elements are within the ranges of this embodiment. Therefore, the Nb content is 0 to 0.10%, and when contained, the Nb content is 0.10% or less. The preferred lower limit of the Nb content is 0.01%, more preferably 0.03%, and even more preferably 0.05%. The preferred upper limit of the Nb content is 0.09%, more preferably 0.07%, and even more preferably 0.05%.

[0047] [Group 3: Al] The chemical composition of the alloy pipe of this embodiment may further contain Al in place of a portion of Ni. Al: 0 to 0.10% Aluminum (Al) is an optional element and does not necessarily need to be contained. That is, the Al content may be 0%. When contained, i.e., when the Al content exceeds 0%, Al deoxidizes the alloy material used to make the alloy pipe during the manufacturing process. Even if even a small amount of Al is contained, the above effect can be achieved to some extent. However, if the Al content exceeds 0.10%, coarse inclusions will form in the alloy pipe. Therefore, even if the contents of other elements are within the ranges of this embodiment, the high-temperature creep strength of the alloy pipe will decrease. Therefore, the Al content is 0 to 0.10%, and if contained, the Al content is 0.10% or less. The preferred lower limit of the Al content is 0.01%, more preferably 0.03%, and even more preferably 0.05%. The upper limit of the Al content is preferably 0.09%, more preferably 0.07%, and even more preferably 0.05%.

[0048] [(Feature 2) Regarding Formula (1)] In the Ni-based alloy pipe of this embodiment, the developed area ratio Sdr on the inner surface of the Ni-based alloy pipe further satisfies Formula (1): Cr / ((2.0+0.8×Fe+6.0×Ti)×(1.5+10.0×Sdr))≧0.9 (1) Here, the content of the corresponding element in mass % is substituted for each element symbol in Formula (1).

[0049] F1 is defined as follows: F1 = Cr / ((2.0 + 0.8 × Fe + 6.0 × Ti) × (1.5 + 10.0 × Sdr)) F1 is an index that represents the difficulty of forming a nitride layer on the inner surface of an alloy pipe. As described above, Fe and Ti easily form nitrides in a high-temperature ammonia environment. Nitrides promote the diffusion of nitrogen in the alloy pipe. Therefore, the lower the Fe content and Ti content, the more difficult it is to form a nitride layer and the higher F1. On the other hand, Cr forms Cr carbides in a high-temperature ammonia environment. Cr carbides inhibit the diffusion of nitrogen in the alloy pipe. Therefore, the higher the Cr content, the more difficult it is to form a nitride layer and the higher F1.

[0050] The larger the developed area ratio Sdr on the inner surface of the alloy pipe, the faster the penetration rate of nitrogen into the alloy pipe. In this case, nitride formation is promoted and Cr carbide formation is suppressed. As a result, even in an alloy pipe with a high Cr content, the formation of a nitride layer is promoted. In other words, it is preferable that the developed area ratio Sdr on the inner surface is small enough to sufficiently suppress the nitride formation rate depending on the Fe content, Ti content, and Cr content. The smaller the developed area ratio Sdr on the inner surface, the higher F1 becomes.

[0051] The developed area ratio Sdr is the most suitable index for evaluating the surface area of ​​the inner surface of an alloy pipe. When the two-dimensional arithmetic mean roughness Ra or the three-dimensional arithmetic mean roughness Sa is used, a sufficient correlation with the rate of nitrogen penetration into the alloy pipe may not be obtained. Therefore, in order to more accurately adjust the difficulty of forming a nitride layer on the alloy pipe, it is necessary to evaluate the surface area of ​​the inner surface using the developed area ratio Sdr.

[0052] If F1 is 0.9 or more, the rate of penetration of nitrogen into the alloy tube is reduced, and the formation of nitrides is sufficiently suppressed. In this case, the Cr carbide also sufficiently suppresses the diffusion of nitrogen in the alloy tube. Therefore, a nitride layer is less likely to form on the inner surface of the alloy tube. As a result, sufficient nitriding resistance is obtained in a high-temperature ammonia environment.

[0053] The lower limit of F1 is preferably 1.0, more preferably 1.1, and even more preferably 1.2. The upper limit of F1 is not particularly limited, but is, for example, 5.0 when the alloy pipe satisfies Feature 1. The upper limit of F1 is preferably 4.5, and even more preferably 4.0. F1 is a value rounded to one decimal place.

[0054] [Method for Measuring the Developed Area Ratio Sdr of the Inner Surface] The developed area ratio Sdr of the inner surface of an alloy pipe is determined by the following method. Three test pieces are taken, each having the inner surface of the alloy pipe as the observation surface. The size of the observation surface is not particularly limited, but is, for example, 20 mm x 20 mm. The shape of the observation surface of each test piece is measured using, for example, a Keyence VR-3200 (trade name). The measurement magnification is 300 times, and the size of the measurement field of view is 1000 μm x 1000 μm. From the measured surface shape, the developed area ratio specified in ISO 25178-2:2012 is calculated using, for example, a Keyence VR-3000G2 analysis application. The arithmetic mean value of the developed area ratios of the three test pieces is taken as the developed area ratio Sdr of the inner surface of the alloy pipe. The analysis conditions are as follows: Reference plane settings: Vertical and horizontal lines in the center of the measurement field of view Surface shape correction: Quadratic curve correction Measurement area: Entire measurement field of view Filter type: Gaussian S-filter: None F-operation: None L-filter: None End effect correction: ON

[0055] [Effects of the Ni-based alloy pipe of this embodiment] As described above, the Ni-based alloy pipe of this embodiment satisfies Features 1 and 2. Therefore, the Ni-based alloy pipe of this embodiment has sufficient nitriding resistance in a high-temperature ammonia environment.

[0056] [Microstructure of Ni-based alloy pipe of this embodiment] The microstructure of the Ni-based alloy pipe of this embodiment is made of austenite, excluding precipitates and inclusions.

[0057] [Shape of Ni-based alloy pipe of this embodiment] The shape of the Ni-based alloy pipe of this embodiment is not particularly limited. The Ni-based alloy pipe of this embodiment is, for example, a seamless pipe.

[0058] [Applications of the Ni-base alloy tube of this embodiment] The Ni-base alloy tube of this embodiment is widely applicable to applications requiring nitriding resistance. In particular, the Ni-base alloy tube of this embodiment is suitable for high-temperature ammonia environments. An example of a high-temperature ammonia environment is an ammonia decomposition plant for the purpose of hydrogen extraction. However, the Ni-base alloy tube of this embodiment can also be applied to applications other than high-temperature ammonia environments.

[0059] [Method for manufacturing a Ni-base alloy pipe according to this embodiment] An example of a method for manufacturing a Ni-base alloy pipe according to this embodiment will be described. The method for manufacturing a Ni-base alloy pipe described below is one example for manufacturing a Ni-base alloy pipe according to this embodiment. Therefore, a Ni-base alloy pipe having the above-described configuration may be manufactured by a manufacturing method other than the manufacturing method described below. However, the manufacturing method described below is a preferred example of a method for manufacturing a Ni-base alloy pipe according to this embodiment. In this embodiment, a method for manufacturing a seamless pipe will be described as an example of a Ni-base alloy pipe.

[0060] An example of a method for manufacturing a Ni-based alloy pipe according to this embodiment includes the following steps: (Step 1) Material preparation step (Step 2) Hot working step (Step 3) Solution treatment step Each step will be described below.

[0061] [(Step 1) Material Preparation Step] In the material preparation step, a material for the Ni-based alloy pipe of this embodiment is prepared. Specifically, an alloy having a chemical composition that satisfies Feature 1 is produced. The refining method is not particularly limited, and any known method may be used.

[0062] The refined alloy is used to produce a cylindrical material by a well-known casting method. For example, a cylindrical ingot is produced by an ingot casting method. Alternatively, a billet may be produced by a continuous casting method. The produced ingot may be subjected to hot working to produce a billet. The hot working may be hot rolling or hot forging. The heating temperature of the material during hot working in the material preparation step is not particularly limited, but is, for example, 1000 to 1300°C.

[0063] [(Step 2) Hot Working Step] In the hot working step, the produced material (cylindrical ingot or billet) is subjected to hot extrusion to produce an intermediate alloy pipe. The hot working step includes the following steps: (Step 21) Hollow Material Preparation Step (Step 22) Hot Extrusion Step The hollow material preparation step and hot extrusion step in the hot working step will be described below.

[0064] [(Step 21) Hollow blank preparation step] In the hollow blank preparation step, the manufactured blank is hot pierced to produce a hollow blank. Specifically, the blank is machined to form a through hole (guide hole) along the central axis of the blank. The blank with the through hole formed is heated. The blank is heated to a temperature of, for example, 1000 to 1300°C. The heated blank is hot pierced using a plug to expand the inner diameter of the through hole in the blank. Hot piercing may be performed again on the blank after hot piercing. In this way, a hollow blank having a through hole with a predetermined inner diameter is prepared.

[0065] The hollow blank preparation step satisfies the following conditions: (Condition 1) The three-dimensional arithmetic mean roughness Sa of the surface of the plug used in the hot piercing that is finally performed is 10.0 μm or less.

[0066] In the hollow blank preparation process, if the surface roughness of the plug used in hot piercing of the blank is large, the surface area of ​​the through hole in the resulting hollow blank will be large. When hot piercing is performed multiple times, the surface roughness of the plug used in the last hot piercing has the greatest impact on the surface area of ​​the through hole in the hollow blank. When the surface area of ​​the through hole in the hollow blank is large, the surface area of ​​the inner surface of the alloy pipe manufactured using that hollow blank will also be large. If the three-dimensional arithmetic mean roughness Sa of the surface of the plug used in the last hot piercing exceeds 10.0 μm, the developed area ratio Sdr of the inner surface of the manufactured alloy pipe will be too large. In this case, the alloy pipe will not satisfy both Feature 1 and Feature 2. Therefore, the three-dimensional arithmetic mean roughness Sa of the surface of the plug used in the last hot piercing is 10.0 μm or less.

[0067] [(Step 22) Hot Extrusion Step] In the hot extrusion step, the produced hollow material is heated and subjected to hot extrusion, typically the Ugine-Séjournet process, to produce an intermediate alloy pipe. In the hot extrusion step, the following conditions are satisfied:

[0068] (Condition 2) When the heating temperature of the hollow material is defined as T1 (°C), the extrusion speed of the hot extrusion is defined as v1 (mm / sec), and the extrusion ratio of the hot extrusion is defined as Rd, the following formulas (A) and (B) are satisfied: -0.5 x (T1 - 900) + 1.3 x v1 + 5.0 x Rd < 120 (A) v1 ≥ 100 (B)

[0069] [Regarding formula (A)] FA is defined as -0.5 x (T1 - 900) + 1.3 x v1 + 5.0 x Rd. FA is an index that represents the effect of hot extrusion on the surface area of ​​the inner surface of the intermediate alloy pipe. The higher the FA, the larger the surface area of ​​the inner surface of the intermediate alloy pipe produced by hot extrusion. Therefore, the surface area of ​​the inner surface of the alloy pipe also increases. If the heating temperature T1 (°C) of the hollow material is low, the workability of the hollow material during hot extrusion decreases. Therefore, the surface area of ​​the inner surface of the intermediate alloy pipe increases, and FA also increases. If the extrusion speed v1 (mm / sec) of the hot extrusion is fast, or if the extrusion ratio Rd of the hot extrusion is large, the load applied to the hollow material during hot extrusion increases, making surface defects more likely to occur. Therefore, the surface area of ​​the inner surface of the intermediate alloy pipe increases, and FA also increases. If the FA is 120 or more, the developed area ratio Sdr of the inner surface of the alloy pipe manufactured from the intermediate alloy pipe will be too large. In this case, the alloy pipe will not be able to satisfy Feature 2 while satisfying Feature 1. Therefore, the FA is less than 120. There is no particular lower limit for the FA, but considering normal industrial production, it is, for example, 30.

[0070] [Regarding Formula (B)] If the extrusion speed v1 (mm / sec) is slow, the time required for hot extrusion increases. As a result, the temperature of the hollow material drops during hot extrusion, and workability deteriorates. If the extrusion speed v1 is less than 100 mm / sec, the time required for hot extrusion is too long. Therefore, the workability of the hollow material during hot extrusion deteriorates excessively. As a result, the developed area ratio Sdr of the inner surface of the alloy pipe becomes too large. In this case, the alloy pipe cannot satisfy both Feature 1 and Feature 2. Therefore, the extrusion speed v1 is 100 mm / sec or more. There is no particular upper limit for the extrusion speed v1, but considering normal industrial production and Formula (A), it is, for example, 180 mm / sec.

[0071] [(Step 3) Solution Treatment Step] In the solution treatment step, the intermediate alloy pipe produced in the hot working step is subjected to solution treatment. For example, the intermediate alloy pipe is loaded into a heat treatment furnace, maintained at a desired temperature, and then rapidly cooled. The solution treatment temperature is, for example, 1000 to 1300°C.

[0072] The Ni-based alloy pipe of this embodiment is manufactured by the above-described manufacturing method.

[0073] [Regarding Other Steps] The method for producing a Ni-based alloy pipe according to this embodiment may include steps other than those described above. For example, cold working may be performed on the intermediate alloy pipe after the hot working step and before the solution treatment step. The cold working may be cold rolling or cold drawing. In this case, the pipe can be worked to the desired dimensions. Even when cold working is performed after the hot working step, the conditions of the hot working step are the dominant factors determining the developed area ratio Sdr of the inner surface of the alloy pipe to be produced. Therefore, as long as the hot working step satisfies Conditions 1 and 2, the developed area ratio Sdr of the inner surface of the alloy pipe can be adjusted to a value that satisfies Feature 2, regardless of the conditions of the subsequent cold working.

[0074] The effects of the Ni-based alloy pipe of this embodiment will be explained more specifically using examples. The conditions in the following examples are one example of conditions adopted to confirm the feasibility and effects of the Ni-based alloy pipe of this embodiment. Therefore, the Ni-based alloy pipe of this embodiment is not limited to this one example of conditions.

[0075] Alloy tubes were manufactured having the chemical compositions shown in Tables 1A and 1B.

[0076]

[0077]

[0078] Specifically, a molten metal having the chemical composition of each test number was melted using a vacuum melting furnace. The molten metal was then used to produce a cylindrical ingot by ingot casting. The ingot was heated at 1220°C for 3 hours. The heated ingot was then hot forged to produce a billet.

[0079] The produced billets were machined to form through-holes along the central axes of the billets. The billets with the through-holes formed therein were heated to 1,200°C. Hot piercing was performed once on the heated billets to produce hollow blanks. The three-dimensional arithmetic mean roughness Sa (μm) of the surfaces of the plugs used for each test number is shown in Table 2.

[0080]

[0081] The produced hollow blanks were subjected to hot extrusion using the Ugine-Séjournet method. Table 2 shows the heating temperature T1 (°C), extrusion speed v1 (mm / sec), and extrusion ratio Rd for each test number.

[0082] The intermediate alloy pipe after hot extrusion was subjected to cold drawing. By cold drawing, the shape of the intermediate alloy pipe was worked to a diameter of 60 mm and a wall thickness of 8.0 mm. The intermediate alloy pipe after cold drawing was subjected to solution treatment. Specifically, the intermediate alloy pipe after cold drawing was heated to 1150°C and held at that temperature for 10 minutes. It was then cooled to room temperature. Alloy pipes of each test number were produced by the above manufacturing process.

[0083] [Evaluation Tests] The following evaluation tests were carried out on the manufactured alloy pipes with each test number: (Test 1) Measurement test of the developed area ratio Sdr of the inner surface (Test 2) Nitriding resistance evaluation test Each test will be described below.

[0084] [(Test 1) Measurement test of developed area ratio Sdr of inner surface] Based on the above-mentioned [Method for measuring developed area ratio Sdr of inner surface], the developed area ratio Sdr of the inner surface of the alloy pipe of each test number was measured. The obtained results are shown in Table 2.

[0085] [(Test 2) Nitridation Resistance Evaluation Test] The nitridation resistance of the alloy pipes of each test number in a high-temperature ammonia environment was evaluated by the following method. A test specimen having a surface consisting of the inner surface of the alloy pipe was taken from each alloy pipe of each test number. FIG. 1 is a front view of the test specimen, as seen from the front of the surface consisting of the inner surface of the alloy pipe. In the surface consisting of the inner surface of the alloy pipe, the side corresponding to the longitudinal direction L of the alloy pipe was defined as the long side, and the side corresponding to the circumferential direction C of the alloy pipe was defined as the short side. The long side of the surface consisting of the inner surface of the alloy pipe was 20 mm, and the short side was 15 mm. The normal direction of the surface consisting of the inner surface of the alloy pipe was defined as the thickness direction of the test specimen. The thickness of the test specimen corresponded to the wall thickness of the alloy pipe. A hanging through-hole with a diameter of 3 mm was formed in the thickness direction of the test specimen, centered at an arbitrary position 3 mm away from the short side of the surface consisting of the inner surface of the alloy pipe. The test specimen was hung on a jig and inserted into a tubular furnace. After sealing the tubular furnace, the pressure inside the furnace was reduced to 10 Pa or less. After reducing the pressure to 10 Pa or less, nitrogen was supplied into the furnace, and the pressure inside the furnace was returned to atmospheric pressure. After returning to atmospheric pressure, the temperature inside the furnace was heated to 600°C while supplying 100% ammonia gas at a flow rate of 500 ml / min. After the temperature inside the furnace reached 600°C, it was maintained at 600°C for 100 hours. Thereafter, the temperature inside the furnace was reduced to room temperature while supplying nitrogen, and the test piece was removed.

[0086] The removed test specimen was cut using a microcutter along a plane passing through the center of the surface consisting of the inner surface of the alloy pipe, including the long side direction and the thickness direction. The test specimen was embedded in resin so that the cross section became the observation surface. The observation surface of the resin-embedded test specimen was polished. On the observation surface, a line analysis of N concentration was performed using a field emission electron probe microanalyzer (FE-EPMA) on a line segment 110 μm in the depth direction (thickness direction of the test specimen) from an arbitrary position corresponding to the inner surface of the alloy pipe. In the line analysis using EPMA, the measurement pitch was 2 μm and the acceleration voltage was 15 kV. The above-mentioned line analysis was performed on three arbitrary measurement lines on the observation surface. For each measurement line segment, the average N concentration in mass% was determined at a depth of 100 to 110 μm from the inner surface of the alloy pipe. The obtained average N concentration in mass% at a depth of 100 to 110 μm was taken as the N concentration C of the base material of the alloy pipe.Ｎ The measurement data for each measurement line segment was plotted with the horizontal axis representing the depth position (μm) from the inner surface of the alloy pipe and the vertical axis representing the N concentration (%). The plotted points were then connected with straight lines to create a graph.

[0087] The graph created and the N concentration C of the base material of the alloy pipe Ｎ The nitride layer depth was calculated based on the above. Figure 2 shows a schematic diagram for explaining how to calculate the nitride layer depth. As shown in Figure 2, the graph created and the N concentration C of the base material of the alloy pipe are Ｎ All intersections between the measured line segment and a straight line L1 parallel to the horizontal axis corresponding to the line segment were determined. Of all the intersections obtained, the intersection point (intersection point P0 in FIG. 2) with the smallest value (value on the horizontal axis) of the depth position (μm) from the inner surface of the alloy pipe was determined. The depth position (μm) from the inner surface of the alloy pipe at the obtained intersection point was used as the nitride layer depth D obtained from the measured line segment. Ｎ The nitride layer depth D obtained from the three measurement lines was Ｎ The arithmetic mean value of the thickness (μm) was taken as the nitride layer depth (μm) of the test piece.

[0088] When the obtained nitride layer depth was 10 μm or less, the evaluation was "E (Excellent)" and it was determined that sufficient nitridation resistance was obtained in a high-temperature ammonia environment (indicated by "E" in the "Nitridation Resistance" column in Table 2). When the obtained nitride layer depth was more than 10 μm, the evaluation was "B (Bad)" and it was determined that sufficient nitridation resistance was not obtained in a high-temperature ammonia environment (indicated by "B" in the "Nitridation Resistance" column in Table 2).

[0089] [Test Results] Referring to Tables 1A, 1B and 2, the alloy pipes of test numbers 1 to 12 satisfied characteristics 1 and 2. Therefore, sufficient nitriding resistance was obtained in a high-temperature ammonia environment.

[0090] On the other hand, in test numbers 13 and 14, the Cr content was too low, and therefore F1 was too low, so sufficient nitriding resistance was not obtained in a high-temperature ammonia environment.

[0091] In test numbers 15 and 16, the Fe content was too high, and therefore F1 was too low, resulting in insufficient nitriding resistance in a high-temperature ammonia environment.

[0092] In test numbers 17 and 18, the Ti content was too high, and therefore F1 was too low, resulting in insufficient nitriding resistance in a high-temperature ammonia environment.

[0093] In Test Nos. 19 and 20, the three-dimensional arithmetic mean roughness Sa of the surface of the plug used in hot piercing in the hollow blank preparation process was too large. Therefore, F1 was too low. As a result, sufficient nitriding resistance was not obtained in a high-temperature ammonia environment.

[0094] In test numbers 21 and 22, the FA in the hot extrusion process was too high. Therefore, F1 was too low. As a result, sufficient nitriding resistance was not obtained in a high-temperature ammonia environment.

[0095] In test numbers 23 and 24, v1 in the hot extrusion process was too low, and therefore F1 was too low, resulting in insufficient nitriding resistance in a high-temperature ammonia environment.

[0096] 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 Ni-based alloy tube, having a chemical composition in mass %, C: 0.150% or less, Si: more than 1.00% to 2.50%, Mn: 0.01 to 1.00%, P: 0.0100% or less, S: 0.0100% or less, Cu: 1.50 to 3.00%, Cr: 25.00 to 35.00%, Fe: 2.00 to 6.00%, Mo: 1.00 to 3.00%, Ti: 0.10 to 1.00%, B: 0.0001 to 0.0100%, Ca: 0.0001 to 0.0050%, REM: 0.005 to 0.100%, Co: 0 to 1.00%, W: 0 to 1.00%, N: 0 to 0.100%, V: 0 to 1.00%, Nb: 0 to 0.10%, Al: 0 to 0.10%, and the balance consisting of Ni and impurities, wherein the developed area ratio Sdr on the inner surface of the Ni-based alloy tube satisfies the formula (1), Ni-based alloy tube. Cr / ((2.0 + 0.8×Fe + 6.0×Ti)×(1.5 + 10.0×Sdr)) ≥ 0.9 (1) Here, the content in mass % of the corresponding element is substituted for each element symbol in formula (1).

2. The Ni-based alloy tube according to claim 1, wherein the chemical composition contains, in mass %, Co: 0.01 to 1.00%, W: 0.01 to 1.00%, N: 0.001 to 0.100%, V: 0.01 to 1.00%, Nb: 0.01 to 0.10%, and Al: 0.01 to 0.10%, and contains one or more elements selected from the group consisting of these, Ni-based alloy tube.

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