Ni-based alloy tube

Abstract

Provided is a Ni-based alloy tube that can obtain sufficient nitridation resistance in a high-temperature ammonia environment. The Ni-based alloy tube according to the present disclosure has the chemical composition described in the specification, and the developed area ratio Sdr on the inner surface of the Ni-based alloy tube satisfies the formula (1). Cr / ((2.0 + 0.8×Fe + 6.0×Ti)×(1.5 + 10.0×Sdr)) ≧ 0.9 (1) Here, for each element symbol in the formula (1), the content in mass% of the corresponding element is substituted. When an element is not contained, "0" is substituted for the corresponding element symbol.

Description

Technical Field

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

Background Art

[0002] Recently, hydrogen has attracted attention as a clean energy that does not generate carbon dioxide. Hydrogen is a gas at normal temperature and pressure, and when liquefied for transportation, it needs to be in an extremely low temperature state of -253°C or lower. Therefore, it is difficult to transport hydrogen alone.

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

[0004] To desorb hydrogen from ammonia, ammonia is decomposed using a catalyst in a high-temperature environment of about 600°C at normal pressure. In the following description, an environment at about 600°C, normal pressure, and in an ammonia atmosphere is referred to as a "high-temperature ammonia environment". As described above, the need for plant members (for example, reaction tubes and pipes) that can withstand use in a high-temperature ammonia environment is increasing.

[0005] Plant members applied to a high-temperature ammonia environment are required to have high high-temperature creep strength. As a plant member having high high-temperature creep strength, a Ni-based alloy material used in a thermal power plant, which is a high-temperature environment equivalent to the high-temperature ammonia environment, can be considered. The Ni-based alloy material for a thermal power plant is disclosed, for example, in Japanese Patent Application Laid-Open No. 2013-209721 (Patent Document 1). The Ni-based alloy material disclosed in this document has a chemical composition of C: 0.007% or more and less than 0.020%, Si: 0.3% or less, Mn: 0.3% or less, Cr: 19.0 to 22.0%, Mo as an essential component and 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 either or both of B: 0.02% or less and Zr: 0.2% or less by mass%, with the balance being composed of Ni and impurities. This Ni-based alloy material further has a value represented by Al / (Al + 0.56×Ti) of 0.45 to 0.70, an average crystal 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 can obtain excellent creep strength even in use in a high-temperature environment.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0007] When using a Ni-based alloy material in the above-mentioned high-temperature ammonia environment, nitridation on the surface layer of the alloy material may be promoted under a high-temperature environment. As a result, a nitride layer composed of a layer mainly composed of nitrides and a layer in which solid-solution nitrogen diffuses at a high concentration is formed in the vicinity of the surface layer of the alloy material. If the formed nitride layer becomes thick, the portion other than the nitride layer in the alloy material becomes thin. In this case, the high-temperature creep strength decreases. Therefore, excellent nitridation resistance is required for the Ni-based alloy material used in a high-temperature ammonia environment.

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

Means for Solving the Problems

[0009] The Ni-based alloy tube of the present disclosure is The chemical composition is, by mass%, C: 0.001 to 0.150%, Si: 0.01 to 1.00%, Mn: 0.01 to 1.00%, P: 0.050% or less, S: 0.015% or less, Cr: 13.00 to 35.00%, Fe: 5.00 to 15.00%, Mo: 0 to 0.10%, Co: 0 to 2.00%, W: 0 to 0.10%, N: 0 to 0.100%, Cu: 0 to 1.00%, V: 0 to 1.00%, Ti: 0 to 1.00%, Nb: 0 to 0.10%, Al: 0 to 1.00%, Ca: 0 to 0.0050%, B: 0 to 0.0100%, and The balance consists of Ni and impurities, The developed area ratio Sdr on the inner surface of the Ni-based alloy tube satisfies the formula (1). Cr / ((2.0 + 0.8 × Fe + 6.0 × Ti) × (1.5 + 10.0 × Sdr)) ≧ 0.9 (1) Here, in the formula (1), the content of each element symbol in mass% of the corresponding element is substituted. When an element is not contained, "0" is substituted for the corresponding element symbol.

Advantages of the Invention

[0010] In the Ni-based alloy tube of the present disclosure, sufficient nitridation resistance can be obtained in a high-temperature ammonia environment.

Brief Description of the Drawings

[0011]

Figure 1

Figure 2

Embodiments for Carrying Out the Invention

[0012] First, the present inventors examined Ni-based alloy tubes that can obtain sufficient nitriding resistance in a high-temperature ammonia environment from the perspective of chemical composition. As a result, the present inventors considered that if the chemical composition is, in mass %, C: 0.001 to 0.150%, Si: 0.01 to 1.00%, Mn: 0.01 to 1.00%, P: 0.050% or less, S: 0.015% or less, Cr: 13.00 to 35.00%, Fe: 5.00 to 15.00%, Mo: 0 to 0.10%, Co: 0 to 2.00%, W: 0 to 0.10%, N: 0 to 0.100%, Cu: 0 to 1.00%, V: 0 to 1.00%, Ti: 0 to 1.00%, Nb: 0 to 0.10%, Al: 0 to 1.00%, Ca: 0 to 0.0050%, B: 0 to 0.0100%, and the balance is composed of Ni and impurities, sufficient nitriding resistance can be obtained in a high-temperature ammonia environment.

[0013] However, even Ni-based alloy tubes having the above-described chemical composition may not obtain sufficient nitriding resistance in a high-temperature ammonia environment. Therefore, the present inventors examined means for further improving the nitriding resistance of Ni-based alloy tubes having the above-described chemical composition from the perspective of suppressing the formation of the nitrided layer. As a result, the present inventors obtained the following findings.

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

[0015] (B) Generally, Cr is also known to be an element that easily forms nitrides, similar to Fe and Ti. Also, in a high-temperature ammonia environment with an extremely low oxygen partial pressure, it is difficult to form a Cr oxide film known as a surface protective film. Therefore, similar to Fe and Ti, it is expected that suppressing the content of Cr is effective for suppressing the formation of the nitride layer. However, as a result of the investigation, the inventors found that in the Ni-based alloy material with the above chemical composition, increasing the Cr content suppresses the formation of the nitride layer. The following mechanism is considered as the reason. 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 having such an effect is prioritized over the formation of Cr nitrides. Therefore, it is considered that as the Cr content in the alloy material increases, the diffusion of nitrogen in the alloy material is inhibited, and the formation of the nitride layer is suppressed.

[0016] (C) In a high-temperature ammonia environment, the nitrogen that forms the nitride layer penetrates from the surface of the alloy material and diffuses in the depth direction of the alloy material. The rate of nitrogen penetration 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. That is, the rate of nitrogen penetration into the alloy material is correlated with the surface area of the surface of the alloy material exposed to a high-temperature ammonia environment. As an index for evaluating the size of the surface area at a certain surface, the developed area ratio Sdr defined in ISO25178-2:2012 is known. The developed area ratio Sdr is expressed by the following formula when the area of the measurement region assumed to be flat is defined as A0 and the actual surface area (developed area) in the measurement region is defined as A1. Sdr=(A1 - A0) / A0 That is, the developed area ratio Sdr represents how much the actual surface area in the measurement region has increased 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 has a large increase rate of the surface area compared to a flat surface. When the developed area ratio Sdr of the surface of the alloy material exposed to a high-temperature ammonia environment is large, the rate of nitrogen penetration into the alloy material is fast. In this case, the formation of nitrides proceeds in a state where the precipitation of Cr carbides is insufficient. As a result, in the alloy material with an increased Cr content as described above, the formation of Cr nitrides is promoted and the formation of Cr carbides is suppressed. Therefore, not only the effect of suppressing the diffusion of nitrogen is impaired, but rather the formation of the nitride layer is greatly promoted. Therefore, in order to suppress the formation of the nitride layer, it is necessary to adjust the developed area ratio Sdr of the surface of the alloy material to be sufficiently small according to the chemical composition of the alloy material. Especially in the case of alloy pipes, since the inner surface of the alloy pipe is generally exposed to a high-temperature ammonia environment, it is important to adjust the developed area ratio Sdr of the inner surface.

[0017] Based on the findings of (A) to (C), the inventors further examined the Ni-based alloy tube that can obtain sufficient nitridation resistance in a high-temperature ammonia environment from the perspective of the relationship between the chemical composition of the alloy tube and the developed area ratio Sdr on the inner surface of the alloy tube. As a result, the inventors found that sufficient nitridation resistance can be obtained in a high-temperature ammonia environment if the developed area ratio Sdr on the inner surface of the alloy tube satisfies the following formula (1) obtained regressively from the examination results. Cr / ((2.0 + 0.8×Fe + 6.0×Ti) × (1.5 + 10.0×Sdr)) ≥ 0.9 (1) Here, for each element symbol in formula (1), the content in mass% of the corresponding element is substituted. When an element is not contained, "0" is substituted for the corresponding element symbol.

[0018] The Ni-based alloy tube of this embodiment is completed based on the above technical idea and has the following configuration. Note that the above mechanism is an estimation. Therefore, the Ni-based alloy tube of this embodiment may also obtain sufficient nitridation resistance in a high-temperature ammonia environment by a mechanism different from the above. However, as shown in the examples described later, the Ni-based alloy tube having the following configuration can obtain sufficient nitridation 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: 0.001 to 0.150%, Si: 0.01 to 1.00%, Mn: 0.01 to 1.00%, P: 0.050% or less, S: 0.015% or less, Cr: 13.00 to 35.00%, Fe: 5.00 to 15.00%, Mo: 0 to 0.10%, Co: 0 to 2.00%, W: 0 to 0.10%, N: 0 to 0.100%, Cu: 0 to 1.00%, V: 0 to 1.00%, Ti: 0 to 1.00%, Nb: 0 to 0.10%, Al: 0 to 1.00%, Ca: 0 to 0.0050%, B: 0 to 0.0100%, and, the balance consists of Ni and impurities, the developed area ratio Sdr on the inner surface of the Ni-based alloy tube satisfies Equation (1). Cr / ((2.0 + 0.8×Fe + 6.0×Ti) × (1.5 + 10.0×Sdr)) ≥ 0.9 (1) Here, for each element symbol in Equation (1), the content in mass % of the corresponding element is substituted. When an element is not contained, "0" is substituted for the corresponding element symbol.

[0020] The Ni-based alloy tube of the second configuration is the Ni-based alloy tube of the first configuration, the chemical composition is in mass %, Mo: 0.01 to 0.10%, Co: 0.01 to 2.00%, W: 0.01 to 0.10%, N: 0.001 to 0.100%, Cu: 0.01 to 1.00%, V: 0.01 to 1.00%, Ti: 0.01 to 1.00%, Nb: 0.01 to 0.10%, Al: 0.01 to 1.00%, Ca: 0.0001 to 0.0050%, and, contains one or more elements selected from the group consisting of B: 0.0001 to 0.0100%.

[0021] Hereinafter, the Ni-based alloy tube according to this embodiment will be described in detail. Note that "% " regarding elements means mass % unless otherwise specified.

[0022] [Features of the Ni-based Alloy Tube of this Embodiment] The Ni-based alloy tube of this embodiment includes the following features. (Feature 1) The chemical composition, in mass %, is as follows: C: 0.001 - 0.150%, Si: 0.01 - 1.00%, Mn: 0.01 - 1.00%, P: 0.050% or less, S: 0.015% or less, Cr: 13.00 - 35.00%, Fe: 5.00 - 15.00%, Mo: 0 - 0.10%, Co: 0 - 2.00%, W: 0 - 0.10%, N: 0 - 0.100%, Cu: 0 - 1.00%, V: 0 - 1.00%, Ti: 0 - 1.00%, Nb: 0 - 0.10%, Al: 0 - 1.00%, Ca: 0 - 0.0050%, B: 0 - 0.0100%, and the balance consists of Ni and impurities. (Characteristic 2) The developed area ratio Sdr on the inner surface of the Ni-based alloy tube satisfies the formula (1). Cr / ((2.0 + 0.8×Fe + 6.0×Ti)×(1.5 + 10.0×Sdr)) ≥ 0.9 (1) Here, for each element symbol in formula (1), the content in mass % of the corresponding element is substituted. When an element is not contained, "0" is substituted for the corresponding element symbol. Hereinafter, each characteristic will be described.

[0023] [(Characteristic 1) Regarding the chemical composition] The chemical composition of the Ni-based alloy tube of this embodiment contains the following elements.

[0024] C: 0.001 - 0.150% Carbon (C) mainly combines with Cr to form Cr carbides in the alloy tube, enhancing the high-temperature creep strength. Furthermore, the Cr carbides suppress the diffusion of nitrogen in the alloy tube. As a result, the nitridation resistance in a high-temperature ammonia environment is enhanced. If the C content is less than 0.001%, even if the contents of other elements are within the range of this embodiment, the above effects cannot be sufficiently obtained. On the other hand, if the C content exceeds 0.150%, carbides are excessively generated at the grain boundaries. Therefore, even if the contents of other elements are within the range of this embodiment, the hot workability of the alloy material used as the material of the alloy tube decreases. Therefore, the C content is 0.001 - 0.150%. The preferable lower limit of the C content is 0.010%, more preferably 0.020%, and even more preferably 0.030%. The preferable upper limit of the C content is 0.100%, more preferably 0.095%, even more preferably 0.090%, even more preferably 0.085%, even more preferably 0.080%, and even more preferably 0.075%.

[0025] Si: 0.01 - 1.00% Silicon (Si) deoxidizes the alloy material that becomes the material of the alloy pipe. If the Si content is less than 0.01%, even if the contents of other elements are within the range of this embodiment, the above effects cannot be sufficiently obtained. On the other hand, if the Si content exceeds 1.00%, even if the contents of other elements are within the range of this embodiment, the hot workability of the alloy material that becomes the material of the alloy pipe deteriorates. Therefore, the Si content is 0.01 - 1.00%. The preferable lower limit of the Si content is 0.05%, more preferably 0.10%, and even more preferably 0.30%. The preferable upper limit of the Si content is 0.80%, more preferably 0.70%, and even more preferably 0.60%.

[0026] Mn: 0.01 - 1.00% Manganese (Mn) enhances the nitridation resistance of the alloy pipe in a high-temperature ammonia environment. If the Mn content is less than 0.01%, even if the contents of other elements are within the range of this embodiment, the above effects cannot be sufficiently obtained. On the other hand, if the Mn content exceeds 1.00%, even if the contents of other elements are within the range of this embodiment, the hot workability of the alloy material that becomes the material of the alloy pipe deteriorates. Therefore, the Mn content is 0.01 - 1.00%. The preferable lower limit of the Mn content is 0.05%, more preferably 0.10%, and even more preferably 0.30%. The preferable upper limit of the Mn content is 0.80%, more preferably 0.70%, and even more preferably 0.60%.

[0027] P: Below 0.050% Phosphorus (P) is an impurity. That is, the lower limit of the P content is more than 0%. If the P content exceeds 0.050%, P will segregate excessively at the grain boundaries, resulting in a decrease in the grain boundary strength. Therefore, even if the contents of other elements are within the range of this embodiment, the hot workability of the alloy material used as the material of the alloy pipe will decrease. Therefore, the P content is 0.050% or less. It is preferable that the P content is as low as possible. However, excessive reduction of the P content will increase the manufacturing cost. Therefore, considering industrial production, the preferable lower limit of the P content is 0.001%, more preferably 0.002%, and even more preferably 0.005%. The preferable upper limit of the P content is 0.045%, more preferably 0.042%, and even more preferably 0.040%.

[0028] S: 0.015% 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 0.015%, coarse sulfide-based inclusions will be generated. Therefore, even if the contents of other elements are within the range of this embodiment, the hot workability of the alloy material used as the material of the alloy pipe will decrease. Therefore, the S content is 0.015% or less. It is preferable that the S content is as low as possible. However, excessive reduction of the S content will increase the manufacturing cost. Therefore, considering industrial production, the preferable lower limit of the S content is 0.001%, more preferably 0.002%, and even more preferably 0.005%. The preferable upper limit of the S content is 0.010%, more preferably 0.008%, even more preferably 0.006%, and even more preferably 0.005%.

[0029] Cr: 13.00 - 35.00% Chromium (Cr) combines with C 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, the nitride resistance in a high-temperature ammonia environment is enhanced. If the Cr content is less than 13.00%, even if the contents of other elements are within the range of this embodiment, the above effects cannot be fully obtained. 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, even if the contents of other elements are within the range of this embodiment, the nitride resistance in a high-temperature ammonia environment decreases. Therefore, the Cr content is 13.00 to 35.00%. The preferable lower limit of the Cr content is 15.00%, more preferably 18.00%, and even more preferably 20.00%. The preferable upper limit of the Cr content is 33.00%, more preferably 32.00%, even more preferably 31.00%, and even more preferably 30.00%.

[0030] Fe: 5.00 to 15.00% Iron (Fe) enhances the hot workability of the alloy material that serves as the material of the alloy tube. If the Fe content is less than 5.00%, even if the contents of other elements are within the range of this embodiment, the above effects cannot be fully obtained. Also, Fe reduces the manufacturing cost of the alloy tube. On the other hand, if the Fe content exceeds 15.00%, excessive Fe nitrides are formed in a high-temperature ammonia environment. Fe nitrides promote the diffusion of nitrogen in the alloy tube. Therefore, even if the contents of other elements are within the range of this embodiment, the nitride resistance in a high-temperature ammonia environment decreases. Therefore, the Fe content is 5.00 to 15.00%. The preferable lower limit of the Fe content is 6.00%, more preferably 7.00%, and even more preferably 8.00%. The preferable upper limit of the Fe content is 14.00%, more preferably 13.00%, and even more preferably 12.00%.

[0031] The balance of the chemical composition of the Ni-based alloy tube of the present embodiment consists of Ni and impurities. Here, the impurities in the chemical composition are those mixed in from raw materials or the manufacturing environment during the industrial production of the Ni-based alloy tube, and are those allowed within a range that does not adversely affect the Ni-based alloy tube of the present embodiment.

[0032] [Regarding Optional Elements] The chemical composition of the Ni-based alloy tube of the present embodiment may further contain, instead of a part of Ni, Mo: 0 to 0.10%, Co: 0 to 2.00%, W: 0 to 0.10%, N: 0 to 0.100%, Cu: 0 to 1.00%, V: 0 to 1.00%, Ti: 0 to 1.00%, Nb: 0 to 0.10%, Al: 0 to 1.00%, Ca: 0 to 0.0050%, and, B: 0 to 0.0100%, and may contain one or more selected from the group consisting of. All of these elements are optional elements. Hereinafter, these optional elements will be described.

[0033] [Group 1: Mo, Co, W, and N] The chemical composition of the Ni-based alloy tube of the present embodiment may further contain Group 1 instead of a part of Ni. The elements of Group 1 all dissolve in the alloy tube and increase the high-temperature creep strength of the alloy tube. Hereinafter, each element of Group 1 will be described.

[0034] Mo: 0 to 0.10% Molybdenum (Mo) is an optional element and may not be contained. That is, the Mo content may be 0%. When contained, that is, when the Mo content exceeds 0%, Mo dissolves in the alloy tube and increases the high-temperature creep strength. If even a little Mo is contained, the above effect can be obtained to some extent. However, if the Mo content exceeds 0.10%, even if the contents of other elements are within the range of this embodiment, the hot workability of the alloy material used as the material of the alloy tube will deteriorate. Therefore, the Mo content is 0 to 0.10%, and when contained, the Mo content is 0.10% or less. The preferable lower limit of the Mo content is 0.01%, more preferably 0.02%, and even more preferably 0.05%. The preferable upper limit of the Mo content is 0.08%, more preferably 0.06%, and even more preferably 0.05%.

[0035] Co: 0 to 2.00% Cobalt (Co) is an optional element and may not be contained. That is, the Co content may be 0%. When contained, that is, when Co exceeds 0%, Co dissolves in the alloy tube and increases the high-temperature creep strength. Even if a small amount of Co is contained, the above effect can be obtained to some extent. However, if the Co content exceeds 2.00%, even if the contents of other elements are within the range of this embodiment, the hot workability of the alloy material used as the material of the alloy tube will deteriorate. Therefore, the Co content is 0 to 2.00%, and when contained, the Co content is 2.00% or less. The preferable lower limit of the Co content is 0.01%, more preferably 0.10%, and even more preferably 0.50%. The preferable upper limit of the Co content is 1.80%, more preferably 1.50%, and even more preferably 1.20%.

[0036] W: 0 to 0.10% Tungsten (W) is an optional element and may not be contained. That is, the W content may be 0%. When contained, that is, when the W content exceeds 0%, W dissolves in the alloy tube and increases the high-temperature creep strength. Even if a small amount of W is contained, the above effect can be obtained to some extent. However, if the W content exceeds 0.10%, even if the contents of other elements are within the range of this embodiment, the hot workability of the alloy material used as the material of the alloy tube will deteriorate. Therefore, the W content is 0 to 0.10%, and when contained, the W content is 0.10% or less. The preferable lower limit of the W content is 0.01%, more preferably 0.02%, and even more preferably 0.05%. The preferable upper limit of the W content is 0.08%, more preferably 0.06%, and even more preferably 0.05%.

[0037] 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 contained, that is, when the N content exceeds 0%, N dissolves in the alloy tube and increases the high-temperature creep strength. Even if a small amount of N is contained, the above effects can be obtained to a certain extent. However, if the N content exceeds 0.100%, nitrides are excessively generated 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 range of this embodiment, the nitridation resistance in a high-temperature ammonia environment deteriorates. Therefore, the N content is 0 to 0.100%, and when contained, the N content is 0.100% or less. The preferable lower limit of the N content is 0.001%, more preferably 0.010%, and even more preferably 0.020%. The preferable upper limit of the N content is 0.090%, more preferably 0.070%, and even more preferably 0.050%.

[0038] [Group 2: Cu, V, Ti, and Nb] The chemical composition of the Ni-based alloy tube of this embodiment may further contain Group 2 in place of a part of Ni. Each element of Group 2 generates precipitates during the use of the alloy tube in a high-temperature environment and increases the high-temperature creep strength of the alloy tube. Hereinafter, each element of Group 2 will be described.

[0039] Cu: 0 to 1.00% Copper (Cu) is an optional element and may not be contained. That is, the Cu content may be 0%. When contained, that is, when the Cu content exceeds 0%, Cu forms precipitates during the use of the alloy tube in a high-temperature environment, enhancing the high-temperature creep strength of the alloy tube. Even if a small amount of Cu is contained, the above effect can be obtained to a certain extent. However, if the Cu content exceeds 1.00%, even if the contents of other elements are within the scope of this embodiment, the hot workability of the alloy material used as the material of the alloy tube will decrease. Therefore, the Cu content is 0 to 1.00%, and when contained, the Cu content is 1.00% or less. The preferable lower limit of the Cu content is 0.01%, more preferably 0.05%, and even more preferably 0.10%. The preferable upper limit of the Cu content is 0.90%, more preferably 0.70%, and even more preferably 0.50%.

[0040] 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 contained, that is, when the V content exceeds 0%, V forms precipitates during the use of the alloy tube in a high-temperature environment, enhancing the high-temperature creep strength of the alloy tube. Even if a small amount of V is contained, the above effect can be obtained to a certain extent. However, if the V content exceeds 1.00%, even if the contents of other elements are within the scope of this embodiment, the hot workability of the alloy material used as the material of the alloy tube will decrease. Therefore, the V content is 0 to 1.00%, and when contained, the V content is 1.00% or less. The preferable lower limit of the V content is 0.01%, more preferably 0.05%, and even more preferably 0.10%. The preferable upper limit of the V content is 0.90%, more preferably 0.70%, and even more preferably 0.50%.

[0041] Ti: 0 to 1.00% Titanium (Ti) is an optional element and may not be contained. That is, the Ti content may be 0%. When contained, that is, when the Ti content exceeds 0%, Ti forms precipitates during the use of the alloy tube in a high-temperature environment, increasing the high-temperature creep strength of the alloy tube. Even if a small amount of Ti is contained, the above effect can be obtained to a certain extent. However, 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 range of this embodiment, the nitridation resistance in a high-temperature ammonia environment decreases. Therefore, the Ti content is 0 to 1.00%, and when contained, the Ti content is 1.00% or less. The preferred lower limit of the Ti content is 0.01%, more preferably 0.05%, and even more preferably 0.10%. The preferred upper limit of the Ti content is 0.90%, more preferably 0.70%, and even more preferably 0.50%.

[0042] 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 contained, that is, when the Nb content exceeds 0%, Nb forms precipitates during the use of the alloy tube in a high-temperature environment, increasing the high-temperature creep strength of the alloy tube. Even if a small amount of Nb is contained, the above effect can be obtained to a certain extent. However, if the Nb content exceeds 0.10%, even if the contents of other elements are within the range of this embodiment, the hot workability of the alloy material used as the material of the alloy tube decreases. 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 preferable upper limit of the Nb content is 0.09%, more preferably 0.07%, and even more preferably 0.05%.

[0043] [Group 3: Al and Ca] The chemical composition of the Ni-based alloy tube of the present embodiment may further contain Group 3 in place of a part of Ni. Each element of Group 3 deoxidizes the alloy material that becomes the material of the alloy tube. Hereinafter, each element of Group 3 will be described.

[0044] Al: 0 to 1.00% Aluminum (Al) is an optional element and may not be contained. That is, the Al content may be 0%. When it is contained, that is, when the Al content exceeds 0%, Al deoxidizes the alloy material that becomes the material of the alloy tube in the manufacturing process of the alloy tube. If even a little Al is contained, the above effect can be obtained to some extent. However, if the Al content exceeds 1.00%, coarse inclusions are generated in the alloy tube. Therefore, even if the contents of other elements are within the range of the present embodiment, the high-temperature creep strength of the alloy tube decreases. Therefore, the Al content is 0 to 1.00%, and when it is contained, the Al content is 1.00% or less. The preferable lower limit of the Al content is 0.01%, more preferably 0.05%, and even more preferably 0.10%. The preferable upper limit of the Al content is 0.99%, more preferably 0.90%, even more preferably 0.80%, even more preferably 0.70%, and even more preferably 0.50%.

[0045] Ca: 0 to 0.0050% Calcium (Ca) is an optional element and may not be contained. That is, the Ca content may be 0%. When it is contained, that is, when the Ca content exceeds 0%, Ca deoxidizes the alloy material that becomes the material of the alloy tube in the manufacturing process of the alloy tube. If even a little Ca is contained, the above effect can be obtained to some extent. However, if the Ca content exceeds 0.0050%, coarse inclusions will form in the alloy tube. Therefore, even if the contents of other elements are within the scope of this embodiment, the high-temperature creep strength of the alloy tube will decrease. Therefore, the Ca content is 0 to 0.0050%, and when contained, the Ca content is 0.0050% or less. The preferable lower limit of the Ca content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The preferable upper limit of the Ca content is 0.0040%, more preferably 0.0030%, and even more preferably 0.0020%.

[0046] [Group 4: B] The chemical composition of the Ni-based alloy tube of this embodiment may further contain B instead of a part of Ni. B: 0 to 0.0100% Boron (B) is an optional element and may not be contained. That is, the B content may be 0%. When contained, that is, when the B content exceeds 0%, B segregates at the grain boundaries to strengthen the grain boundaries. Therefore, the high-temperature creep strength of the alloy tube increases. If even a little B is contained, the above effects can be obtained to a certain extent. However, if the B content exceeds 0.0100%, even if the contents of other elements are within the scope of this embodiment, the hot workability of the alloy material used as the material of the alloy tube will decrease. Therefore, the B content is 0 to 0.0100%, and when contained, the B content is 0.0100% or less. The preferable lower limit of the B content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The preferable upper limit of the B content is 0.0080%, more preferably 0.0060%, and even more preferably 0.0040%.

[0047] [(Feature 2) Regarding formula (1)] In the Ni-based alloy tube of the present embodiment, the developed area ratio Sdr on the inner surface of the Ni-based alloy tube further satisfies the formula (1). Cr / ((2.0 + 0.8×Fe + 6.0×Ti)×(1.5 + 10.0×Sdr)) ≧ 0.9 (1) Here, for each element symbol in the formula (1), the content in mass% of the corresponding element is substituted. When an element is not contained, "0" is substituted for the corresponding element symbol.

[0048] Define F1 as follows. F1 = Cr / ((2.0 + 0.8×Fe + 6.0×Ti)×(1.5 + 10.0×Sdr)) F1 is an index representing the difficulty of forming a nitride layer on the inner surface of the alloy tube. As described above, Fe and Ti are likely to generate nitrides in a high-temperature ammonia environment. Nitrides promote the diffusion of nitrogen in the alloy tube. Therefore, the lower the Fe content and Ti content, the more difficult it is to form a nitride layer, and F1 also becomes higher. On the other hand, Cr generates Cr carbides in a high-temperature ammonia environment. Cr carbides suppress 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 becomes higher.

[0049] The larger the developed area ratio Sdr on the inner surface of the alloy tube, the faster the nitrogen intrusion rate into the alloy tube. In this case, the generation of nitrides is promoted and the generation of Cr carbides is suppressed. As a result, even in an alloy tube with a high Cr content, the formation of a nitride layer is rather promoted. That is, depending on the Fe content, Ti content, and Cr content, it is preferable that the developed area ratio Sdr on the inner surface is smaller to such an extent that the generation rate of nitrides can be sufficiently suppressed. The smaller the developed area ratio Sdr on the inner surface, the higher F1 becomes.

[0050] As an index for evaluating the surface area of the inner surface of the alloy tube, the developed area ratio Sdr is optimal. When using the two-dimensional arithmetic mean roughness Ra or the three-dimensional arithmetic mean roughness Sa, there may be no sufficient correlation with the nitrogen intrusion rate into the alloy tube. Therefore, in order to more accurately adjust the difficulty of forming a nitride layer on the alloy tube, it is necessary to evaluate the surface area of the inner surface using the developed area ratio Sdr.

[0051] If F1 is 0.9 or more, the nitrogen intrusion rate into the alloy tube decreases, and the generation of nitrides is sufficiently suppressed. In this case, the diffusion of nitrogen in the alloy tube is also sufficiently suppressed by Cr carbide. Therefore, it is difficult to form a nitride layer on the inner surface of the alloy tube. As a result, sufficient nitridation resistance can be obtained in a high-temperature ammonia environment.

[0052] The preferable lower limit of F1 is 1.0, more preferably 1.1, and even more preferably 1.2. The upper limit of F1 is not particularly limited. However, when the alloy tube satisfies Feature 1, for example, it is 2.5. The preferable upper limit of F1 is 2.4, more preferably 2.3. Note that F1 is a value up to the first decimal place obtained by rounding the second decimal place.

[0053] [Measurement method of the developed area ratio Sdr of the inner surface] The developed area ratio Sdr of the inner surface of the alloy tube is obtained by the following method. Collect three test pieces having the inner surface of the alloy tube as the observation surface. The size of the observation surface is not particularly limited, for example, it is 20 mm × 20 mm. Measure the shape of the observation surface of each test piece using, for example, the product name: VR-3200 manufactured by Keyence. The measurement magnification is 300 times, and the size of the measurement field of view is 1000 μm × 1000 μm. From the measured surface shape, calculate the developed area ratio defined in ISO25178-2:2012 using, for example, the VR-3000G2 analysis application manufactured by Keyence. 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 tube. The analysis conditions are as follows. Reference plane setting: Vertical and horizontal lines at the center of the measurement field of view Surface shape correction: Quadratic surface correction Measurement target area: The entire measurement field of view Filter type: Gaussian S-filter: None F-operation: None L-filter: None Correction of end effect: ON

[0054] [Effect of the Ni-based alloy tube of the present embodiment] As described above, the Ni-based alloy tube of the present embodiment satisfies Feature 1 and Feature 2. Therefore, the Ni-based alloy tube of the present embodiment can obtain sufficient nitridation resistance in a high-temperature ammonia environment.

[0055] [Microstructure of the Ni-based alloy tube of the present embodiment] The microstructure of the Ni-based alloy tube of the present embodiment consists of austenite, excluding precipitates and inclusions.

[0056] [Shape of the Ni-based alloy tube of the present embodiment] The shape of the Ni-based alloy tube of the present embodiment is not particularly limited. The Ni-based alloy tube of the present embodiment is, for example, a seamless tube.

[0057] [Applications of the Ni-based alloy tube of the present embodiment] The Ni-based alloy tube of the present embodiment can be widely applied to applications where nitridation resistance is required. In particular, the Ni-based alloy tube of the present embodiment is suitable for a high-temperature ammonia environment. The high-temperature ammonia environment is, for example, an ammonia decomposition plant for the purpose of hydrogen extraction. However, the Ni-based alloy tube of the present embodiment can also be applied to other applications other than the high-temperature ammonia environment.

[0058] [Manufacturing method of the Ni-based alloy tube of the present embodiment] An example of the manufacturing method of the Ni-based alloy tube of the present embodiment will be described. The manufacturing method of the Ni-based alloy tube described hereinafter is an example for manufacturing the Ni-based alloy tube of the present embodiment. Therefore, the Ni-based alloy tube having the above-described configuration may be manufactured by other manufacturing methods other than the manufacturing method described hereinafter. However, the manufacturing method described hereinafter is a preferable example of the manufacturing method of the Ni-based alloy tube of the present embodiment. In the present embodiment, as an example of the Ni-based alloy tube, a manufacturing method of a seamless tube will be described.

[0059] An example of the manufacturing method of the Ni-based alloy tube of the present embodiment includes the following steps. (Step 1) Stock preparation step (Step 2) Hot working step (Step 3) Solution treatment step Hereinafter, each step will be described.

[0060] [(Step 1) Stock preparation step] In the stock preparation step, a stock of the Ni-based alloy tube of the present embodiment is prepared. Specifically, an alloy whose chemical composition satisfies Feature 1 is manufactured. The refining method is not particularly limited, and a well-known method may be used.

[0061] Using the refined alloy, a columnar stock is manufactured by a well-known casting method. For example, a columnar ingot is manufactured by the ingot casting method. Alternatively, a billet may be manufactured by the continuous casting method. A billet may be manufactured by performing hot working on the manufactured ingot. The hot working may be hot rolling or hot forging. The heating temperature of the stock in the hot working in the stock preparation step is not particularly limited, but is, for example, 1000 to 1300°C.

[0062] [(Step 2) Hot working step] In the hot working step, hot extrusion is performed on the manufactured stock (columnar ingot or billet) to manufacture an intermediate alloy tube. The hot working step includes the following steps. (Step 21) Hollow stock preparation step (Step 22) Hot extrusion step Hereinafter, the hollow material preparation process and the hot extrusion process in the hot working process will be described.

[0063] [(Process 21) Hollow Material Preparation Process] In the hollow material preparation process, the manufactured material is hot pierced to produce a hollow material. Specifically, machining is performed on the material to form a through hole (guide hole) along the central axis of the material. The material with the through hole formed is heated. The heating temperature of the material is, for example, 1000 to 1300 °C. Hot piercing using a plug is performed on the heated material to expand the inner diameter of the through hole of the material. Hot piercing may be performed again on the material after hot piercing. In this way, a hollow material having a through hole with a predetermined inner diameter is prepared.

[0064] In the hollow material preparation process, the following conditions are satisfied. (Condition 1) The three-dimensional arithmetic mean roughness Sa of the surface of the plug used for the last hot piercing is 10.0 μm or less.

[0065] In the hollow material preparation process, if the surface roughness of the plug used for hot piercing the material is large, the surface area of the through hole of the manufactured hollow material will increase. When hot piercing is performed multiple times, the surface roughness of the plug used for the last hot piercing has the greatest influence on the surface area of the through hole of the hollow material. When the surface area of the through hole of the hollow material is large, the surface area of the inner surface of the alloy tube manufactured using the hollow material also increases. If the three-dimensional arithmetic mean roughness Sa of the surface of the plug used for the last hot piercing exceeds 10.0 μm, the developed area ratio Sdr of the inner surface of the manufactured alloy tube will become too large. In this case, the alloy tube can satisfy Feature 1 but cannot satisfy Feature 2. Therefore, the three-dimensional arithmetic mean roughness Sa of the surface of the plug used for the last hot piercing is 10.0 μm or less.

[0066] [(Process 22) Hot Extrusion Process] In the hot extrusion process, the manufactured hollow material is heated, and hot extrusion typified by the Ugiène Sédurne method is carried out to manufacture an intermediate alloy tube. In the hot extrusion process, the following conditions are satisfied.

[0067] (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 formulas (A) and (B) are satisfied. -0.5×(T1 - 900) + 1.3×v1 + 5.0×Rd < 120 (A) v1 ≥ 100 (B)

[0068] [Regarding formula (A)] Define FA = -0.5×(T1 - 900) + 1.3×v1 + 5.0×Rd. FA is an index representing the influence on the surface area given to the inner surface of the intermediate alloy tube by hot extrusion. The higher FA is, the larger the surface area of the inner surface of the intermediate alloy tube manufactured by hot extrusion becomes. Therefore, the surface area of the inner surface of the alloy tube also becomes larger. When the heating temperature T1 (°C) of the hollow material is low, the workability of the hollow material in hot extrusion decreases. Therefore, the surface area of the inner surface of the intermediate alloy tube becomes larger, and FA also becomes higher. When the extrusion speed v1 (mm / sec) of the hot extrusion is fast, or when the extrusion ratio Rd of the hot extrusion is large, the load applied to the hollow material in hot extrusion becomes large, and surface defects are likely to occur. Therefore, the surface area of the inner surface of the intermediate alloy tube becomes larger, and FA also becomes higher. If FA is 120 or more, the developed surface area ratio Sdr of the inner surface of the alloy tube manufactured from the intermediate alloy tube becomes too large. In this case, while the alloy tube satisfies Feature 1, it cannot satisfy Feature 2. Therefore, FA is less than 120. The lower limit of FA is not particularly limited, but considering normal industrial production, it is, for example, 30.

[0069] [Regarding formula (B)] When the extrusion speed v1 (mm / sec) is slow, the time required for hot extrusion increases. Therefore, the temperature of the hollow material decreases during hot extrusion, and the workability decreases. If the extrusion speed v1 is less than 100 mm / s, the time required for hot extrusion is too long. Therefore, the workability of the hollow material in hot extrusion is excessively reduced. As a result, the developed area ratio Sdr of the inner surface of the alloy tube becomes too large. In this case, while the alloy tube satisfies Feature 1, it cannot satisfy Feature 2. Therefore, the extrusion speed v1 is 100 mm / s or more. The upper limit of the extrusion speed v1 is not particularly limited, but in view of normal industrial production and Equation (A), it is, for example, 180 mm / s.

[0070] [(Process 3) Solution treatment process] In the solution treatment process, solution treatment is performed on the intermediate alloy tube manufactured in the hot working process. For example, the intermediate alloy tube is loaded into a heat treatment furnace, held at a desired temperature, and then rapidly cooled. The solution treatment temperature is, for example, 1000 to 1300 °C.

[0071] By the above manufacturing method, the Ni-based alloy tube of the present embodiment is manufactured.

[0072] [Regarding other processes] The manufacturing method of the Ni-based alloy tube of the present embodiment may include processes other than the above-described processes. For example, cold working may be performed on the intermediate alloy tube after the hot working process and before the solution treatment process. The cold working may be cold rolling or cold drawing. In this case, it can be processed into a desired dimension. Even when cold working is performed after the hot working process, the conditions of the hot working process are dominant as factors for determining the developed area ratio Sdr of the inner surface of the manufactured alloy tube. Therefore, if the hot working process satisfies Condition 1 and Condition 2, the inner surface developed area ratio Sdr of the alloy tube can be adjusted to satisfy Feature 2 regardless of the conditions of the subsequent cold working.

Example

[0073] The effects of the Ni-based alloy tube of the present embodiment will be further specifically described by way of examples. The conditions in the following examples are one example of the conditions adopted to confirm the feasibility and effects of the Ni-based alloy tube of the present embodiment. Therefore, the Ni-based alloy tube of the present embodiment is not limited to this one example of conditions.

[0074] Alloy tubes having the chemical compositions shown in Table 1A and Table 1B were manufactured.

[0075]

Table 1A

[0076]

Table 1B

[0077] Specifically, the molten metal having the chemical composition of each test number was melted using a vacuum melting furnace. Using the molten metal, a cylindrical ingot was manufactured by the ingot casting method. The ingot was heated at 1220 °C for 3 hours. Hot forging was performed on the heated ingot to manufacture a billet.

[0078] Machining was performed on the manufactured billet to form a through hole along the central axis of the billet. The billet with the through hole formed was heated to 1200 °C. Hot piercing was performed once on the heated billet to manufacture a hollow material. The three-dimensional arithmetic mean roughness Sa (μm) of the surface of the plug used for each test number is shown in Table 2.

[0079]

Table 2

[0080] Hot extrusion by the Ugiène Séjourné method was performed on the manufactured hollow material. The heating temperature T1 (°C) of the hollow material, the extrusion speed v1 (mm / second) of the hot extrusion, and the extrusion ratio Rd of the hot extrusion for each test number are shown in Table 2.

[0081] Cold drawing was performed on the intermediate alloy tube after hot extrusion. By cold drawing, the shape of the intermediate alloy tube was processed to a diameter of 60 mm and a wall thickness of 8.0 mm. Solution treatment was performed on the intermediate alloy tube after cold drawing. Specifically, the intermediate alloy tube after cold drawing was heated to 1150 °C and held for 10 minutes. Then, it was cooled to room temperature. Alloy tubes with each test number were manufactured through the above manufacturing process.

[0082] [Regarding the evaluation test] The following evaluation tests were performed on the manufactured alloy tubes with each test number. (Test 1) Measurement test of the developed area ratio Sdr of the inner surface (Test 2) Nitriding resistance evaluation test The following describes each test.

[0083] [(Test 1) Measurement test of the developed area ratio Sdr of the inner surface] Based on the above [Measurement method of the developed area ratio Sdr of the inner surface], the developed area ratio Sdr on the inner surface of the alloy tubes with each test number was measured. The obtained results are shown in Table 2.

[0084] [(Test 2) Nitriding resistance evaluation test] The nitriding resistance of the alloy tubes with each test number in a high-temperature ammonia environment was evaluated by the following method. Test pieces having a surface composed of the inner surface of the alloy tube were sampled from the alloy tubes with each test number. Figure 1 is a front view of the test piece seen from the front of the surface composed of the inner surface of the alloy tube. On the surface composed of the inner surface of the alloy tube, the side corresponding to the longitudinal direction L of the alloy tube was taken as the long side, and the side corresponding to the circumferential direction C of the alloy tube was taken as the short side. The long side of the surface composed of the inner surface of the alloy tube was 20 mm, and the short side was 15 mm. The normal direction of the surface composed of the inner surface of the alloy tube was taken as the thickness direction of the test piece. The thickness of the test piece corresponded to the wall thickness of the alloy tube. In the test piece, a through-hole for suspension with a diameter of 3 mm was formed in the thickness direction centered at an arbitrary position 3 mm away from the short side on the surface consisting of the inner surface of the alloy tube. The test piece was suspended 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 to return the pressure inside the furnace to normal pressure. After returning to normal pressure, while supplying 100% ammonia gas at a flow rate of 500 ml / min, the temperature inside the furnace was heated to 600 °C. After the temperature inside the furnace reached 600 °C, it was held at 600 °C for 100 hours. Then, while supplying nitrogen, the temperature inside the furnace was lowered to room temperature, and the test piece was taken out.

[0085] The taken-out test piece was cut using a microcutter along a plane passing through the center position of the surface consisting of the inner surface of the alloy tube and including the long side direction and the thickness direction. The test piece was resin-embedded so that the cross-section became the observation surface. The observation surface of the resin-embedded test piece was polished. On a line segment 110 μm in the depth direction (the thickness direction of the test piece) from an arbitrary position corresponding to the inner surface of the alloy tube on the observation surface, line analysis of the N concentration was performed using a field emission-electron probe microanalyzer (FE-EPMA). In the line analysis using EPMA, the measurement pitch was 2 μm and the acceleration voltage was 15 kV. The above-described line analysis was performed on three arbitrary measurement line segments on the observation surface. For each measurement line segment, the average N concentration in mass% at a depth position of 100 to 110 μm from the inner surface of the alloy tube was determined. The obtained average N concentration in mass% at a depth position of 100 to 110 μm was defined as the N concentration C N (%) of the base material of the alloy tube. Then, for each measurement line segment, the measurement data was plotted with the depth position (μm) from the inner surface of the alloy tube on the horizontal axis and the N concentration (%) on the vertical axis. Further, the plotted points were connected by straight lines to create a graph.

[0086] Based on the created graph and the N concentration C N of the base material of the alloy tube, the nitrided layer depth was determined. Fig. 2 shows a schematic diagram for explaining how to determine the nitrided layer depth. As shown in Fig. 2, based on the created graph and the N concentration C NAll the intersections with the straight line L1 parallel to the horizontal axis corresponding thereto were determined. Among all the obtained intersections, the intersection (intersection P0 in FIG. 2) with the smallest value of the depth position (μm) from the inner surface of the alloy tube (value on the horizontal axis) was determined. The depth position (μm) from the inner surface of the alloy tube at the obtained intersection was defined as the nitrided layer depth D N (μm) obtained from the said measurement line segment. The arithmetic mean value of the nitrided layer depths D N (μm) obtained from the three measurement line segments was defined as the nitrided layer depth (μm) of the said test piece.

[0087] When the obtained nitrided layer depth was 10 μm or less, the evaluation was “E (Excellent)”, and it was judged that sufficient nitriding resistance was obtained in the high-temperature ammonia environment (denoted as “E” in the “Nitriding Resistance” column in Table 2). When the obtained nitrided layer depth exceeded 10 μm, the evaluation was “B (Bad)”, and it was judged that sufficient nitriding resistance was not obtained in the high-temperature ammonia environment (denoted as “B” in the “Nitriding Resistance” column in Table 2).

[0088] [Test Results] Referring to Table 1A, Table 1B and Table 2, the alloy tubes of test numbers 1 to 15 satisfied Feature 1 and Feature 2. Therefore, sufficient nitriding resistance was obtained in the high-temperature ammonia environment.

[0089] On the other hand, in test numbers 16 and 17, the Cr content was too low. Therefore, F1 was too low. As a result, sufficient nitriding resistance was not obtained in the high-temperature ammonia environment.

[0090] In test numbers 18 and 19, the Fe content was too high. Therefore, F1 was too low. As a result, sufficient nitriding resistance was not obtained in the high-temperature ammonia environment.

[0091] In test number 20, the Ti content was too high. Therefore, F1 was too low. As a result, sufficient nitriding resistance was not obtained in the high-temperature ammonia environment.

[0092] In Test Nos. 21 and 22, the three-dimensional arithmetic mean roughness Sa of the surface of the plug used in the hot piercing process in the hollow material preparation process was too large. Therefore, F1 was too low. As a result, sufficient nitridability could not be obtained in the high-temperature ammonia environment.

[0093] In Test Nos. 23 and 24, FA in the hot extrusion process was too high. Therefore, F1 was too low. As a result, sufficient nitridability could not be obtained in the high-temperature ammonia environment.

[0094] In Test Nos. 25 and 26, v1 in the hot extrusion process was too low. Therefore, F1 was too low. As a result, sufficient nitridability could not be obtained in the high-temperature ammonia environment.

[0095] 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 the above-described embodiments can be appropriately modified and implemented without departing from the spirit thereof.

Claims

1. A Ni-based alloy tube, The chemical composition, in mass%, is C: 0.001 to 0.150%, Si: 0.01-1.00%, Mn: 0.01-1.00%, P: 0.050% or less, S: 0.015% or less, Cr: 13.00-35.00%, Fe: 5.00 to 15.00%, Mo: 0 to 0.10%, Co: 0-2.00%, W: 0-0.10%, N: 0-0.100%, Cu: 0 to 1.00%, V: 0-1.00%, Ti: 0 to 1.00%, Nb: 0 to 0.10%, Al: 0-1.00%, Ca: 0-0.0050%, B: 0 to 0.0100%, and The balance is Ni and impurities, The development 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, each element symbol in formula (1) is substituted with the content of the corresponding element in mass %. When an element is not contained, the corresponding element symbol is substituted with "0."

2. The Ni-based alloy tube according to claim 1, The chemical composition is, in mass%, Mo: 0.01 to 0.10%, Co: 0.01-2.00%, W: 0.01-0.10%, N: 0.001-0.100%, Cu: 0.01 to 1.00%, V: 0.01-1.00%, Ti: 0.01 to 1.00%, Nb: 0.01 to 0.10%, Al: 0.01-1.00%, Ca: 0.0001 to 0.0050%, and B: 0.0001 to 0.0100%; Ni-based alloy tube.