Ni-based alloy tube

Abstract

The present disclosure provides a Ni-based alloy tube that exhibits sufficient nitriding resistance in a high-temperature ammonia environment. The Ni-based alloy tube has a chemical composition, in mass %, of 0.150% or less C, more than 1.00% to 2.50% Si, 0.01 to 1.00% Mn, 0.0100% or less P, 0.0100% or less S, 1.50 to 3.00% Cu, 25.00 to 35.00% Cr, 2.00 to 6.00% Fe, 1.00 to 3.00% Mo, 0.10 to 1.00% Ti, 0.0001 to 0.0100% B, 0.0001 to 0.0050% Ca, and 0.005 to 0.100% REM, with the remainder being Ni and impurities. The developed area ratio Sdr of the inner surface of the Ni-based alloy tube 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).

Description

[Technical Field]

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

[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, which acts as a hydrogen carrier, and desorb hydrogen from the ammonia at its destination to use the hydrogen as energy.

[0004] To desorb hydrogen from ammonia, a catalyst is used to decompose the ammonia in a high-temperature environment of about 600°C at atmospheric pressure. In the following explanation, 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 mentioned 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. One example of a plant component with high high-temperature creep strength is a Ni-based alloy material used in thermal power plants, which are exposed to high-temperature environments similar to high-temperature ammonia environments. An example of a Ni-based alloy material for thermal power plants is disclosed in JP 2013-209721 A (Patent Document 1). The Ni-based alloy disclosed in this document has a chemical composition, in mass%, of C: 0.007% to less than 0.020%, Si: 0.3% or less, Mn: 0.3% or less, Cr: 19.0 to 22.0%, Mo alone or with Mo as essential component, Mo + (1 / 2) × 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 / or B: 0.02% or less and Zr: 0.2% or less, and further contains a value expressed as Al / (Al + 0.56Ti) of 0.45 to 0.70, with the balance being 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 states that this Ni-based alloy material exhibits excellent creep strength even when used in a high-temperature environment. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-209721 Summary of the Invention [Problem to be solved by the invention]

[0007] When Ni-based alloys are used in the high-temperature ammonia environment, nitriding of the surface layer of the alloy 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 of the alloy. If the nitride layer becomes thick, the portions of the alloy other than the nitride layer become thinner. In this case, the high-temperature creep strength decreases. Therefore, Ni-based alloys 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. [Means for solving the problem]

[0009] The Ni-based alloy tube of the present disclosure has: The chemical composition is, in mass%, C: 0.150% or less, Si: more than 1.00~2.50%, Mn: 0.01 to 1.00%, P: 0.0100% or less, S: 0.0100% or less, Cu: 1.50-3.00% Cr: 25.00~35.00%, Fe: 2.00-6.00%, Mo: 1.00-3.00%, Ti: 0.10 to 1.00%, B: 0.0001~0.0100%, Ca: 0.0001 to 0.0050%, REM: 0.005~0.100%, Co: 0 to 1.00%, W: 0~1.00%, N: 0~0.100%, V: 0~1.00%, Nb: 0 to 0.10% Al: 0 to 0.10%, and the balance being Ni and impurities, 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) Here, the content of the corresponding element in mass % is substituted for each element symbol in formula (1). [Effects of the Invention]

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

[0011] [Figure 1] 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. [Figure 2] FIG. 2 is a schematic diagram for explaining how to determine the nitride layer depth in the nitriding resistance evaluation test. DETAILED DESCRIPTION OF THE INVENTION

[0012] The present inventors first investigated, from the viewpoint of chemical composition, Ni-based alloy tubes 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 tube consisting of 0%, 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%, Al: 0-0.10%, and the balance being Ni and impurities, would have sufficient nitriding resistance in a high-temperature ammonia environment.

[0013] However, even Ni-based alloy tubes 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 ways to further improve the nitriding resistance of Ni-based alloy tubes 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, their generation 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 it 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, it is difficult to form a Cr oxide film, which is known as a surface protective film. Therefore, it is expected that, like Fe and Ti, reducing the Cr content will be effective in suppressing the formation of a nitride layer. However, as a result of investigation, the present inventors have found that increasing the Cr content in Ni-based alloys 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 high-temperature environments such as high-temperature ammonia environments, 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 high-temperature ammonia environments, the precipitation of Cr carbides, which have this effect, takes precedence over the formation of Cr nitrides. Therefore, it is thought 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, which forms a nitride layer, penetrates the surface of the alloy material and diffuses toward the depth of the alloy material. The rate at which nitrogen penetrates 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 alloy material exposed to a nitriding atmosphere such as a high-temperature ammonia environment. In other words, the rate at which nitrogen penetrates into the alloy material is correlated with the surface area of the alloy material exposed to a high-temperature ammonia environment. The developed area ratio Sdr, specified in ISO 25178-2:2012, is known as an index for evaluating the surface area of a surface. The developed area ratio Sdr is expressed by the following formula, where A0 is the area of a measurement area assumed to be flat and A1 is the actual surface area (developed area) of the measurement area. Sdr=(A1-A0) / A0 In 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 surface is large, the surface area of that surface increases at a larger rate than that of a flat surface. When the developed surface area ratio Sdr of an alloy material 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 alloy materials with a high Cr content as described above, the formation of Cr nitrides is promoted while the formation of Cr carbides is suppressed. This impairs the effect of suppressing nitrogen diffusion and instead significantly promotes the formation of a nitride layer. Therefore, to suppress the formation of a nitride layer, it is necessary to adjust the developed surface 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 adjusting the developed surface area ratio Sdr of the inner surface is important.

[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 that can provide sufficient nitriding resistance in a high-temperature ammonia environment and the developed area ratio Sdr of the inner surface of the alloy pipe. As a result, the present inventors found that sufficient nitriding resistance in a high-temperature ammonia environment can be obtained 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-based 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-based 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-based alloy pipe having the following configuration can have sufficient nitriding resistance in a high-temperature ammonia environment.

[0019] The Ni-based alloy tube of the first configuration is The chemical composition is, in mass%, C: 0.150% or less, Si: more than 1.00~2.50%, Mn: 0.01 to 1.00%, P: 0.0100% or less, S: 0.0100% or less, Cu: 1.50-3.00% Cr: 25.00~35.00%, Fe: 2.00-6.00%, Mo: 1.00-3.00%, Ti: 0.10 to 1.00%, B: 0.0001~0.0100%, Ca: 0.0001 to 0.0050%, REM: 0.005~0.100%, Co: 0 to 1.00%, W: 0~1.00%, N: 0~0.100%, V: 0~1.00%, Nb: 0 to 0.10% Al: 0 to 0.10%, and the balance being Ni and impurities, 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) Here, the content of the corresponding element in mass % is substituted for each element symbol in formula (1).

[0020] The second configuration of the Ni-based alloy tube is A Ni-based alloy tube having a first configuration, The chemical composition is, 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 It contains one or more elements selected from the group consisting of 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 tube 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%, Ti: 0.10 to 1. 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 remainder being 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 will be explained 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 inhibit the diffusion of nitrogen in the alloy tube. As a result, they improve nitriding resistance in a high-temperature ammonia environment. Even if even a small amount of C is included, the above effects 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%, carbides are excessively formed at the grain boundaries, and therefore 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 C content is 0.150% or less. The lower limit of the C content is preferably 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: over 1.00~2.50% Silicon (Si) deoxidizes the alloy material that is the raw material for the alloy pipe. If the Si content is 1.00% or less, 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 Si content exceeds 2.50%, the hot workability of the alloy material used to make the alloy pipe will decrease 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%. The lower limit of the Si content is preferably 1.01%, more preferably 1.05%, even more preferably 1.10%, even more preferably 1.15%, and even more preferably 1.20%. The upper limit of the Si content is preferably 2.40%, more preferably 2.20%, and even more preferably 2.00%.

[0026] Mn: 0.01 to 1.00% Manganese (Mn) enhances the nitriding resistance of the alloy tube in a high-temperature ammonia environment. If the Mn content is less than 0.01%, 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 Mn content exceeds 1.00%, the hot workability of the alloy material used to make the alloy pipe will be reduced 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 lower limit of the Mn content is preferably 0.02%, more preferably 0.03%, even more preferably 0.04%, and still more preferably 0.05%. The upper limit of the Mn content is preferably 0.90%, more preferably 0.80%, even more preferably 0.70%, even more preferably 0.60%, and still 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 more than 0%. If the P content exceeds 0.0100%, P segregates excessively at grain boundaries, reducing the 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 better. However, excessive reduction in the P content increases production costs. Therefore, in consideration of industrial production, the lower limit of the P content is preferably 0.0001%, more preferably 0.0005%, even more preferably 0.0010%, and still more preferably 0.0020%. The upper limit of the P content is preferably 0.0090%, 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 more 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 production costs. Therefore, in consideration of industrial production, the lower limit of the S content is preferably 0.0001%, more preferably 0.0002%, and even more preferably 0.0003%. The upper limit of the S content is preferably 0.0090%, more preferably 0.0080%, even more preferably 0.0070%, even more preferably 0.0060%, and even more preferably 0.0050%.

[0029] Cu: 1.50-3.00% 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 obtained 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 will be reduced 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 lower limit of the Cu content is preferably 1.60%, more preferably 1.70%, and even more preferably 1.80%. The upper limit of the Cu content is preferably 2.90%, more preferably 2.70%, and even more preferably 2.50%.

[0030] Cr: 25.00~35.00% Chromium (Cr) combines with carbon in a high-temperature environment to form Cr carbides in the alloy tube. Cr carbides inhibit the diffusion of nitrogen in the alloy tube. As a result, the nitriding resistance in a high-temperature ammonia environment is improved. If the Cr content is less than 25.00%, 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 Cr content exceeds 35.00%, excessive Cr nitrides are generated in a high-temperature ammonia environment. Cr 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, the nitriding resistance in a high-temperature ammonia environment decreases. Therefore, the Cr content is 25.00 to 35.00%. The lower limit of the Cr content is preferably 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 effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. In addition, Fe reduces the manufacturing cost of the alloy pipe. On the other hand, if the Fe content exceeds 6.00%, excessive Fe nitrides are generated in a high-temperature ammonia environment. The Fe 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, the nitriding resistance in a high-temperature ammonia environment decreases. Therefore, the Fe content is 2.00 to 6.00%. The lower limit of the Fe content is preferably 2.20%, more preferably 2.40%, and even more preferably 2.50%. The upper limit of the Fe content is preferably 5.50%, more preferably 5.00%, and even more preferably 4.50%.

[0032] Mo: 1.00-3.00% Molybdenum (Mo) forms precipitates during use of the 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 obtained 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 pipe will decrease 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 lower limit of the Mo content is preferably 1.10%, more preferably 1.30%, and even more preferably 1.50%. The upper limit of the Mo content is preferably 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 the alloy pipe in a high-temperature environment, thereby improving the high-temperature creep strength of the alloy pipe. If the Ti content is less than 0.10%, 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 exceeds 1.00%, excessive Ti nitrides are formed in the alloy tube. Ti 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, the nitriding resistance in a high-temperature ammonia environment decreases. Therefore, the Ti content is 0.10 to 1.00%. The lower limit of the Ti content is preferably 0.15%, more preferably 0.18%, and even more preferably 0.20%. The upper limit of the Ti content is preferably 0.90%, more preferably 0.80%, even more preferably 0.70%, and still more preferably 0.50%.

[0034] B: 0.0001 to 0.0100% Boron (B) segregates at grain boundaries to strengthen them, thereby improving the high-temperature creep strength of the alloy pipe. If the B content is less than 0.0001%, 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 B content exceeds 0.0100%, the hot workability of the alloy material used to make the alloy pipe will be 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 lower limit of the B content is preferably 0.0003%, more preferably 0.0005%, and even more preferably 0.0010%. The upper limit of the B content is preferably 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 of the alloy pipe. If the Ca content is less than 0.0001%, 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 exceeds 0.0050%, coarse inclusions are formed in the alloy pipe, and therefore the high-temperature creep strength of the alloy pipe is reduced 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 lower limit of the Ca content is preferably 0.0005%, more preferably 0.0010%, and even more preferably 0.0015%. The upper limit of the Ca content is preferably 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, thereby improving the high-temperature creep strength of the alloy pipe. If the REM content of the alloy pipe is less than 0.005%, 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 REM content exceeds 0.100%, coarse inclusions are formed in the alloy pipe, and therefore the high-temperature creep strength of the alloy pipe is reduced 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 lower limit of the REM content is preferably 0.010%, more preferably 0.015%, even more preferably 0.020%, and still more preferably 0.025%. The upper limit of the REM content is preferably 0.090%, 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), which has atomic number 21, yttrium (Y), which has atomic number 39, and the lanthanides lanthanum (La), which has atomic number 57, to lutetium (Lu), which has 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 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] [About optional elements] The chemical composition of the Ni-based alloy tube of this embodiment further includes, instead of a part of Ni, Co: 0 to 1.00%, W: 0~1.00%, N: 0~0.100%, V: 0~1.00%, Nb: 0 to 0.10%, and Al: 0 to 0.10%. These elements are all optional elements, and will be explained below.

[0040] [Group 1: Co, W, and N] The chemical composition of the Ni-based alloy pipe of this embodiment may further contain elements of Group 1 in place of a portion of Ni. All elements of Group 1 dissolve in the alloy pipe to improve the high-temperature creep strength of the alloy pipe. Each element of Group 1 will be described below.

[0041] Co: 0 to 1.00% Cobalt (Co) is an optional element and may not be contained, that is, the Co content may be 0%. When Co is contained, that is, when the Co content exceeds 0%, Co dissolves in the alloy pipe to increase the 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 will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the Co content is 0 to 1.00%, and when Co is contained, the Co content is 1.00% or less. The lower limit of the Co content is preferably 0.01%, more preferably 0.10%, and even more preferably 0.20%. The upper limit of the Co content is preferably 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 may not be contained, that is, the W content may be 0%. When W is contained, that is, when the W content is more than 0%, W dissolves in the alloy pipe to increase the high-temperature creep strength. Even if even a small amount of W is contained, the above effect can be obtained to some extent. However, if the W content exceeds 1.00%, the hot workability of the alloy material used to make the alloy pipe will decrease 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 contained, the W content is 1.00% or less. The lower limit of the W content is preferably 0.01%, more preferably 0.02%, and even more preferably 0.05%. The upper limit of the W content is preferably 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 may not be contained, that is, the N content may be 0%. When N is contained, that is, when the N content is more than 0%, N dissolves in the alloy pipe and increases the high-temperature creep strength. Even if even a small amount of N is contained, the above effect can be obtained to some extent. However, if the N content exceeds 0.100%, excessive nitrides are formed in the alloy tube. The 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, the nitriding resistance in a high-temperature ammonia environment decreases. Therefore, the N content is 0 to 0.100%, and if N is contained, the N content is 0.100% or less. The lower limit of the N content is preferably 0.001%, more preferably 0.010%, and even more preferably 0.020%. The upper limit of the N content is preferably 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 tube 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 tube in a high-temperature environment, thereby improving the high-temperature creep strength of the alloy tube. Each element of Group 2 will be described below.

[0045] V: 0 to 1.00% Vanadium (V) is an optional element and may not be contained, that is, the V content may be 0%. When V is contained, that is, when the V content is more than 0%, V 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 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 pipe will decrease 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 V is contained, the V content is 1.00% or less. The lower limit of the V content is preferably 0.01%, more preferably 0.05%, and even more preferably 0.10%. The upper limit of the V content is preferably 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 may not be contained, that is, the Nb content may be 0%. When Nb is contained, that is, when the Nb content is more than 0%, Nb forms precipitates during use of the alloy pipe in a high-temperature environment, thereby increasing 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 will decrease 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 Nb is contained, the Nb content is 0.10% or less. The lower limit of the Nb content is preferably 0.01%, more preferably 0.03%, and even more preferably 0.05%. The upper limit of the Nb content is preferably 0.09%, more preferably 0.07%, and even more preferably 0.05%.

[0047] [Group 3: Al] The chemical composition of the alloy tube 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 may not be contained, that is, the Al content may be 0%. When Al is contained, that is, when the Al content is more than 0%, Al deoxidizes the alloy material that is the raw material for the alloy pipe during the manufacturing process of the alloy pipe.Even if even a small amount of Al is contained, the above effect can be obtained to some extent. However, if the Al content exceeds 0.10%, coarse inclusions are formed in the alloy pipe, and therefore the high-temperature creep strength of the alloy pipe is reduced even if the contents of other elements are within the ranges of this embodiment. Therefore, the Al content is 0 to 0.10%, and when Al is contained, the Al content is 0.10% or less. The lower limit of the Al content is preferably 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)] Furthermore, in the Ni-based alloy pipe of this embodiment, the developed area ratio Sdr on the inner surface of the Ni-based alloy pipe satisfies the 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] Define F1 as follows: F1=Cr / ((2.0+0.8×Fe+6.0×Ti)×(1.5+10.0×Sdr)) F1 is an index that indicates the difficulty of forming a nitride layer on the inner surface of the alloy pipe. As mentioned above, Fe and Ti easily form nitrides in a high-temperature ammonia environment. Nitrides promote the diffusion of nitrogen in the alloy tube. Therefore, the lower the Fe and Ti contents, the less likely the nitride layer to form, 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 tube. Therefore, the higher the Cr content, the more difficult it is to form a nitride layer, and F1 also increases.

[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, the formation of nitrides is promoted and the formation of Cr carbides is suppressed. As a result, even in alloy pipes 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 rate of nitride formation 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 at which nitrogen penetrates into the alloy tube decreases, 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 more preferably 4.0. F1 is a value rounded off to the first decimal place.

[0054] [Method for measuring the inner surface developed area ratio Sdr] The developed area ratio Sdr on the inner surface of the 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. The measurement magnification is 300x, and the measurement field size is 1000 μm x 1000 μm. From the measured surface shape, the developed area ratio specified in ISO25178-2:2012 is calculated using, for example, a Keyence VR-3000G2 analysis application. The arithmetic mean value of the developed area ratios for the three test pieces is taken as the developed area ratio Sdr on the inner surface of the alloy pipe. The analysis conditions are as follows: Reference plane setting: Vertical and horizontal lines in the center of the measurement field Surface shape correction: quadratic surface correction Measurement area: Entire measurement field Filter type: Gaussian S-filter: None F-Operation: None L-filter: None End effect correction: ON

[0055] [Effects of the Ni-based alloy tube 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 the Ni-based alloy tube 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 the Ni-based alloy tube of this embodiment] The shape of the Ni-based alloy pipe of this embodiment is not particularly limited, and the Ni-based alloy pipe of this embodiment is, for example, a seamless pipe.

[0058] [Uses of the Ni-based alloy tube of this embodiment] The Ni-based alloy tube of this embodiment is widely applicable to applications requiring nitriding resistance. In particular, the Ni-based 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-based alloy tube of this embodiment is also applicable to applications other than high-temperature ammonia environments.

[0059] [Method for manufacturing Ni-based alloy pipe according to the present embodiment] An example of a method for manufacturing a Ni-based alloy pipe according to this embodiment will be described. The method for manufacturing a Ni-based alloy pipe described below is one example for manufacturing a Ni-based alloy pipe according to this embodiment. Therefore, a Ni-based 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-based 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-based alloy pipe.

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

[0061] [(Process 1) Material preparation process] In the material preparation step, a material for the Ni-based alloy pipe of this embodiment is prepared. Specifically, an alloy whose chemical composition 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] [(Process 2) Hot processing process] In the hot working process, the produced material (cylindrical ingot or billet) is subjected to hot extrusion to produce an intermediate alloy pipe. The hot working process 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 material preparation step] In the hollow blank preparation process, 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 material preparation process satisfies the following conditions: (Condition 1) The three-dimensional arithmetic mean roughness Sa of the surface of the plug used in the final hot piercing is 10.0 μm or less.

[0066] In the hollow blank preparation process, if the surface roughness of the plug used for hot piercing of the blank is high, 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. If 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 satisfy Feature 1 but not Feature 2. Therefore, the three-dimensional arithmetic mean roughness Sa of the surface of the plug used in the final hot piercing is 10.0 μm or less.

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

[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, formulas (A) and (B) are satisfied. -0.5×(T1-900)+1.3×v1+5.0×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 on the surface area of the inner surface of the intermediate alloy pipe by hot extrusion. 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 (℃) of the hollow material is low, the workability of the hollow material during hot extrusion will decrease, and as a result, the surface area of the inner surface of the intermediate alloy pipe will increase, and the FA will also increase. If the extrusion speed v1 (mm / sec) of hot extrusion is fast or the extrusion ratio Rd of hot extrusion is large, the load applied to the hollow material during hot extrusion becomes large, making it more likely for surface defects to occur. As a result, the surface area of the inner surface of the intermediate alloy pipe becomes larger, and the FA also becomes higher. 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 becomes too large. In this case, the alloy pipe cannot satisfy Feature 2 while satisfying Feature 1. Therefore, the FA is less than 120. The lower limit of FA is not particularly limited, but is, for example, 30 in consideration of normal industrial production.

[0070] [Regarding formula (B)] If the extrusion speed v1 (mm / sec) is slow, the time required for hot extrusion increases, which causes the temperature of the hollow material to drop during hot extrusion, resulting in poor workability. If the extrusion speed v1 is less than 100 mm / s, the time required for hot extrusion is too long. This results in excessively poor workability of the hollow material during hot extrusion. 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 Feature 2 while satisfying Feature 1. Therefore, the extrusion speed v1 is 100 mm / sec or more. The upper limit of the extrusion speed v1 is not particularly limited, but is, for example, 180 mm / sec, taking into consideration normal industrial production and formula (A).

[0071] [(Step 3) Solution treatment step] In the solution treatment process, the intermediate alloy pipe manufactured in the hot working process 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] [Other processes] 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 produced alloy pipe. 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. [Example]

[0074] The effects of the Ni-based alloy tube 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 tube of this embodiment. Therefore, the Ni-based alloy tube of this embodiment is not limited to this one example of conditions.

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

[0076] [Table 1A]

[0077] [Table 1B]

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

[0079] The manufactured billets were machined to form a through-hole along the central axis of the billet. The billet with the through-hole formed was heated to 1200°C. The heated billet was hot pierced once to produce a hollow blank. The three-dimensional arithmetic mean roughness Sa (μm) of the surface of the plugs used for each test number is shown in Table 2.

[0080] [Table 2]

[0081] The 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 processed 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 with each test number were manufactured using the above manufacturing process.

[0083] [About the evaluation test] The alloy pipes manufactured with each test number were subjected to the following evaluation tests. (Test 1) Measurement test of inner surface development area ratio Sdr (Test 2) Nitriding resistance evaluation test Each test will be explained below.

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

[0085] [(Test 2) Nitriding resistance evaluation test] The nitriding resistance of the alloy tubes of each test number in a high-temperature ammonia environment was evaluated by the following method. Test pieces having a surface consisting of the inner surface of the alloy pipe were taken from the alloy pipes of each test number. Figure 1 is a front view of the test piece, as seen from the front, showing 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 piece. The thickness of the test piece corresponded to the wall thickness of the alloy pipe. A 3 mm diameter through-hole for hanging was formed in the thickness direction of the test specimen, centered at an arbitrary position 3 mm from the short side on the inner surface of the alloy tube. 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 held at 600°C for 100 hours. After that, the temperature inside the furnace was reduced to room temperature while supplying nitrogen, and the test specimen was removed.

[0086] The removed test specimen was cut using a microcutter along a plane that passed through the center of the surface consisting of the inner surface of the alloy pipe and included the long side direction and 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 the N concentration was performed using a field emission electron probe microanalyzer (FE-EPMA) on a 110 μm line segment in the depth direction (thickness direction of the test specimen) from an arbitrary position corresponding to the inner surface of the alloy pipe. The line analysis using the EPMA was performed with a measurement pitch of 2 μm and an acceleration voltage of 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 calculated 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 used as the N concentration C of the base material of the alloy pipe. N The measurement data for each measurement line segment was then 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 to explain how the nitride layer depth was calculated. As shown in Figure 2, the graph created and the N concentration C of the base material of the alloy pipe are NAll the intersection points with the line L1 parallel to the horizontal axis corresponding to the line segment were determined. Of all the intersection points obtained, the intersection point with the smallest value (value on the horizontal axis) of the depth position (μm) from the inner surface of the alloy pipe (intersection point P0 in Figure 2) 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 measurement line segment. N The nitride layer depth D obtained from the three measurement lines was N The arithmetic mean value (μm) was taken as the nitride layer depth (μm) of the test piece.

[0088] When the nitride layer depth obtained was 10 μm or less, the evaluation was given as "E (Excellent)," and it was determined that sufficient nitriding resistance was obtained in a high-temperature ammonia environment (indicated by "E" in the "Nitriding Resistance" column in Table 2). When the nitride layer depth obtained was more than 10 μm, the evaluation was given as "B (Bad)," and it was determined that sufficient nitriding resistance was not obtained in a high-temperature ammonia environment (indicated by "B" in the "Nitriding 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, resulting in insufficient nitriding resistance 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, FA was too high in the hot extrusion process, and 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 was too low in the hot extrusion process, 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 pipe, The chemical composition, in mass%, is C: 0.150% or less, Si: more than 1.00 to 2.50%, Mn: 0.01-1.00%, P: 0.0100% or less, S: 0.0100% or less, Cu: 1.50-3.00%, Cr: 25.00-35.00%, Fe: 2.00-6.00%, Mo: 1.00-3.00%, Ti: 0.10-1.00%, B: 0.0001 to 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 to 0.10%, Al: 0 to 0.10%, and the balance being Ni and impurities; The developed area ratio Sdr on the inner surface of the Ni-based alloy pipe 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 of each element in mass % is substituted for each element symbol in formula (1).

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

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