Austenitic alloy material

Abstract

Provide is an austenitic alloy material having excellent SCC resistance. An austenitic alloy material disclosed herein contains 19.0-28.0 mass% of Cr, 28.0-37.0 mass% of Ni, and 2.00-6.00 mass% of Mo, wherein: the number density ND1 of fine low-S inclusions, which have an Fn1 defined by expression (1) of 5.0 or more and an equivalent circle diameter of 2.0–10.0 µm, is 3.0 number / mm2 or more; the number density ND2 of coarse low-S inclusions, which have an Fn1 of 5.0 or more and an equivalent circle diameter of greater than 10.0 µm, is 2.0 number / mm2 or less; and the number density ND3 of high-S inclusions, which have an Fn1 of less than 5.0 and an equivalent circle diameter of 2.0 µm or more, is 2.0 number / mm2 or less.　Fn1=(Mg+Al+Ca+Ti+V+Mn) / S(1) Here, in the element symbols in expression (1), the contents of the element in the corresponding inclusions is substituted by mass%.

Description

Austenitic alloy material

[0001] The present disclosure relates to alloys, and more particularly to austenitic alloys.

[0002] Many oil wells and gas wells (hereinafter, oil wells and gas wells are collectively referred to as "oil wells") are in sour environments containing highly corrosive hydrogen sulfide. Sour environments may contain not only hydrogen sulfide but also carbon dioxide. Therefore, materials used in sour environments are required to have excellent stress corrosion cracking resistance (hereinafter, referred to as SCC resistance).

[0003] Examples of materials with high SCC resistance include 18-8 stainless steel materials, such as SUS304H, SUS316H, SUS321H, and SUS347H, and austenitic alloy materials, such as Alloy 800H, which is specified as NCF800H in JIS G 4902:2019. Austenitic alloy materials have superior SCC resistance compared to 18-8 stainless steel materials. Austenitic alloy materials are also more economical than Ni-based alloy materials, such as Alloy 617. Therefore, austenitic alloy materials are suitable as alloy materials for use in sour environments.

[0004] WO 2018 / 225869 (Patent Document 1) proposes an austenitic alloy material having excellent SCC resistance.

[0005] The austenitic alloy material described in Patent Document 1 has a chemical composition, in mass %, of C: 0.004 to 0.030%, Si: 1.00% or less, Mn: 0.30 to 2.00%, P: 0.030% or less, S: 0.0020% or less, Al: 0.001 to 0.100%, Cu: 0.50 to 1.50%, Ni: 25.00 to 55.00%, Cr: 20.00 to 30.00%, Mo: 2.00 to 10.00%, N: 0.005 to 0.100%, Ti: 0 to 0.800%, W: 0 to 0.30%, Nb: 0 to 0.050%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Nd: 0 to 0.050%, and the balance being Fe and impurities, the grain size number of the austenite grains is 2.0 to 7.0, and the mixed grain ratio is 5% or less. In Patent Document 1, SCC resistance is improved by suppressing the mixed grain ratio.

[0006] International Publication No. 2018 / 225869

[0007] The austenitic alloy material disclosed in Patent Document 1 also has enhanced SCC resistance. However, good SCC resistance in sour environments may be obtained by other means.

[0008] An object of the present disclosure is to provide an austenitic alloy material having excellent SCC resistance.

[0009] The austenitic alloy material of the present disclosure has a chemical composition, in mass%, of C: 0.030% or less, Si: 0.10 to 1.00%, Mn: 0.01 to 2.00%, P: 0.040% or less, S: 0.0050% or less, Cr: 19.0 to 28.0%, Ni: 28.0 to 37.0%, Mo: 2.00 to 6.00%, Cu: 0.01 to 2.00%, Nb: 0.001 to 0.100%, V: 0.01 to 0.50%, Co: 0.01 to 2.00%, Sn: 0.001 to 0.100%, Al: 0.010 to 0.500%, Ca: 0.0001 to 0.0100%, N: 0.001 to 0.350%, and O. : 0.0100% or less, Ta: 0 to 0.100%, Ti: 0 to 0.400%, Zr: 0 to 0.100%, W: 0 to 0.300%, Zn: 0 to 0.010%, Sb: 0 to 0.1000%, As: 0 to 0.050%, Pb: 0 to 0.0100%, B: 0 to 0.0200%, Mg: 0 to 0.020%, rare earth elements: 0 to 0.200%, and the balance being Fe and impurities, wherein the austenitic alloy material contains S, and Fn1 defined by formula (1) is 5.0 or more, and the number density ND1 of fine low-S inclusions which are inclusions having an equivalent circle diameter of 2.0 to 10.0 μm is 3.0 pieces / mm ２ or more, containing S, Fn1 is 5.0 or more, and the number density ND2 of coarse low-S inclusions, which are inclusions with an equivalent circle diameter of more than 10.0 μm, is 2.0 pieces / mm ２ or less, and the number density ND3 of high-S inclusions, which contain S, have an Fn1 of less than 5.0, and an equivalent circle diameter of 2.0 μm or more, is 2.0 pieces / mm ２ The formula is as follows: Fn1=(Mg+Al+Ca+Ti+V+Mn) / S (1) where the element symbol in formula (1) is substituted with the content of the corresponding element in the inclusion in mass %. When an element is not contained, "0" is substituted for the corresponding element symbol.

[0010] The austenitic alloy material of the present disclosure has excellent SCC resistance.

[0011] FIG. 1 is a schematic diagram for explaining the positional relationship between the molten metal in a mold and the submerged entry nozzle in continuous casting.

[0012] The present inventors have conducted research into austenitic alloy materials that have excellent SCC resistance even in highly corrosive sour environments.

[0013] The present inventors first investigated austenitic alloy materials having excellent SCC resistance in sour environments from the viewpoint of chemical composition. As a result, the following contents were found in mass %: C: 0.030% or less, Si: 0.10 to 1.00%, Mn: 0.01 to 2.00%, P: 0.040% or less, S: 0.0050% or less, Cr: 19.0 to 28.0%, Ni: 28.0 to 37.0%, Mo: 2.00 to 6.00%, Cu: 0.01 to 2.00%, Nb: 0.001 to 0.100%, V: 0.01 to 0.50%, Co: 0.01 to 2.00%, Sn: 0.001 to 0.100%, Al: 0.010 to 0.500%, Ca: 0.0001 to 0.0100%, N: 0.001 to 0.3 The present inventors considered that an austenitic alloy material having a chemical composition consisting of: 0.50%, O: 0.0100% or less, Ta: 0-0.100%, Ti: 0-0.400%, Zr: 0-0.100%, W: 0-0.300%, Zn: 0-0.010%, Sb: 0-0.1000%, As: 0-0.050%, Pb: 0-0.0100%, B: 0-0.0200%, Mg: 0-0.020%, rare earth elements: 0-0.200%, and the balance being Fe and impurities, may have excellent SCC resistance in a sour environment.

[0014] The present inventors further investigated means for obtaining excellent SCC resistance in sour environments from the viewpoint of microstructure.

[0015] The present inventors first investigated the mechanism of SCC initiation in austenitic alloys in sour environments, and as a result, the following facts were discovered.

[0016] In a sour environment, some of the inclusions present in the surface layer of the austenitic alloy material may dissolve, forming pits on the surface of the alloy material. These pits are thought to promote the occurrence of SCC in a sour environment.

[0017] In a sour environment, among inclusions in the surface layer of an austenitic alloy material, inclusions with a high S content, such as Mn sulfides or Ca sulfides, tend to dissolve preferentially. In the following description, inclusions with a high S content will be referred to as "high S inclusions." A specific definition of high S inclusions will be given later.

[0018] High-S inclusions in the surface layer of an austenitic alloy material dissolve in a sour environment and form pits. The pits formed by the high-S inclusions promote the occurrence of SCC in a sour environment. Therefore, if the number density of high-S inclusions in an austenitic alloy material can be reduced, the occurrence of SCC in a sour environment can be suppressed.

[0019] In order to reduce the number density of high-S inclusions, it is effective to reduce the S content in the austenitic alloy material as much as possible. Therefore, the inventors have attempted to improve SCC resistance by reducing the S content in the austenitic alloy material as much as possible. However, there is a limit to how much S content can be reduced in the austenitic alloy material. Moreover, excessive reduction of the S content significantly increases the manufacturing cost.

[0020] Therefore, the present inventors have considered reducing the number density of high S inclusions by other means while allowing a certain amount of S content in an austenitic alloy material, rather than reducing the number density of high S inclusions by reducing the S content as much as possible.

[0021] Here, the inventors have focused on composite inclusions, which are inclusions formed by the aggregation of multiple types of inclusions. ２ O ３ and oxides typified by MgO, sulfides typified by MnS and CaS, nitrides typified by TiN, etc. Note that TiN may form a solid solution with V. Inclusions other than oxides, sulfides, and nitrides may combine with oxides, sulfides, nitrides, etc. to form composite inclusions.

[0022] The S content in composite inclusions is lower than that of high-S inclusions. Therefore, in a sour environment, composite inclusions are less likely to dissolve than high-S inclusions. Furthermore, even if composite inclusions dissolve, the S-containing portions of the composite inclusions dissolve locally. Therefore, the pits formed are very small. These very small pits are easily repassivated and do not serve as the starting point for SCC. Therefore, if S is incorporated into the composite inclusions, the pits can be kept small even after dissolution, improving SCC resistance. Furthermore, increasing the number density of composite inclusions allows S to be incorporated into the composite inclusions. Therefore, the amount of S available for the formation of high-S inclusions is reduced. As a result, the number density of high-S inclusions is reduced.

[0023] In inclusions, if Fn1 defined by formula (1) is 5.0 or more, the S content in the inclusions is sufficiently low. Such inclusions are considered to be composite inclusions. In the following explanation, inclusions containing S and having Fn1 of 5.0 or more will be referred to as "low S inclusions." Fn1 = (Mg + Al + Ca + Ti + V + Mn) / S (1) Here, the element symbols in formula (1) are substituted with the content of the corresponding element in the inclusion in mass %. If an element is not contained, "0" is substituted for the corresponding element symbol.

[0024] As described above, it is believed that increasing the number density of low-S inclusions in an austenitic alloy material can minimize the depressions caused by dissolution, thereby improving SCC resistance. Increasing the number density of low-S inclusions also allows more S to be incorporated into the low-S inclusions. Therefore, the amount of S available for generating high-S inclusions is reduced. As a result, it is believed that the number density of high-S inclusions can be sufficiently reduced, further improving SCC resistance.

[0025] Based on the above considerations, the present inventors attempted to increase the number density of low S inclusions in an austenitic alloy material, and as a result, found that if the number density of low S inclusions in an austenitic alloy material is increased, the number density of high S inclusions can be sufficiently reduced.

[0026] However, even when the number density of low-S inclusions is increased and the number density of high-S inclusions is reduced, sufficient SCC resistance may still not be obtained in highly corrosive sour environments. Therefore, the present inventors conducted further studies and found the following:

[0027] In a sour environment, even if the inclusions are low in S, coarse low inclusions with an equivalent circle diameter of more than 10.0 μm are easily dissolved. Therefore, by increasing the number density of fine low inclusions with an equivalent circle diameter of 10.0 μm or less and reducing the number density of high S inclusions, and further reducing the number density of coarse low S inclusions with an equivalent circle diameter of more than 10.0 μm, the SCC resistance of austenitic alloy materials in a sour environment can be improved.

[0028] Based on the above findings, the present inventors have further investigated and found that in an austenitic alloy material having the above-mentioned chemical composition, the number density ND1 of fine low-S inclusions, which contain S and have an Fn1 defined by formula (1) of 5.0 or more and have an equivalent circle diameter of 2.0 to 10.0 μm, is 3.0 pieces / mm ２ or more, containing S, Fn1 is 5.0 or more, and the number density ND2 of coarse low-S inclusions, which are inclusions with an equivalent circle diameter of more than 10.0 μm, is 2.0 pieces / mm ２ or less, and the number density ND3 of high-S inclusions, which contain S, have an Fn1 of less than 5.0, and an equivalent circle diameter of 2.0 μm or more, is 2.0 pieces / mm ２ The present inventors have found that excellent SCC resistance can be obtained even in a sour environment if the temperature is below 100°C.

[0029] The austenitic alloy material of this embodiment, which has been completed based on the above findings, has the following configuration.

[0030] The austenitic alloy material of the first embodiment has a chemical composition, in mass%, of C: 0.030% or less, Si: 0.10 to 1.00%, Mn: 0.01 to 2.00%, P: 0.040% or less, S: 0.0050% or less, Cr: 19.0 to 28.0%, Ni: 28.0 to 37.0%, Mo: 2.00 to 6.00%, Cu: 0.01 to 2.00%, Nb: 0.001 to 0.100%, V: 0.01 to 0.50%, Co: 0.01 to 2.00%, Sn: 0.001 to 0.100%, Al: 0.010 to 0.500%, Ca: 0.0001 to 0.0100%, N: 0.001 to 0.350%, An austenitic alloy material comprising O: 0.0100% or less, Ta: 0 to 0.100%, Ti: 0 to 0.400%, Zr: 0 to 0.100%, W: 0 to 0.300%, Zn: 0 to 0.010%, Sb: 0 to 0.1000%, As: 0 to 0.050%, Pb: 0 to 0.0100%, B: 0 to 0.0200%, Mg: 0 to 0.020%, rare earth elements: 0 to 0.200%, and the balance being Fe and impurities, wherein the austenitic alloy material contains S, has Fn1 defined by formula (1) of 5.0 or more, and has a number density ND1 of 3.0 pieces / mm ２ or more, containing S, Fn1 is 5.0 or more, and the number density ND2 of coarse low-S inclusions, which are inclusions with an equivalent circle diameter of more than 10.0 μm, is 2.0 pieces / mm ２ or less, and the number density ND3 of high-S inclusions, which contain S, have an Fn1 of less than 5.0, and an equivalent circle diameter of 2.0 μm or more, is 2.0 pieces / mm ２ The formula is as follows: Fn1=(Mg+Al+Ca+Ti+V+Mn) / S (1) where the element symbol in formula (1) is substituted with the content of the corresponding element in the inclusion in mass %. When an element is not contained, "0" is substituted for the corresponding element symbol.

[0031] The austenitic alloy material of the second embodiment is the austenitic alloy material of the first embodiment, and has a chemical composition containing one or more elements selected from the group consisting of Ta: 0.001 to 0.100%, Ti: 0.001 to 0.400%, Zr: 0.001 to 0.100%, W: 0.001 to 0.300%, Zn: 0.001 to 0.010%, Sb: 0.0001 to 0.1000%, As: 0.001 to 0.050%, Pb: 0.0001 to 0.0100%, B: 0.0001 to 0.0200%, Mg: 0.001 to 0.020%, and rare earth elements: 0.001 to 0.200%.

[0032] The austenitic alloy material of the third embodiment is the austenitic alloy material of the first or second embodiment, and is an alloy pipe.

[0033] The austenitic alloy material of this embodiment will be described in detail below. Note that the austenitic alloy material may also be simply referred to as "alloy material."

[0034] [Features of the Austenitic Alloy Material of the Present Embodiment] The austenitic alloy material of the present embodiment satisfies the following features 1 and 2. (Feature 1) The chemical composition, in mass %, is C: 0.030% or less, Si: 0.10 to 1.00%, Mn: 0.01 to 2.00%, P: 0.040% or less, S: 0.0050% or less, Cr: 19.0 to 28.0%, Ni: 28.0 to 37.0%, Mo: 2.00 to 6.00%, Cu: 0.01 to 2.00%, Nb: 0.001 to 0.100%, V: 0.01 to 0.50%, Co: 0.01 to 2.00%, Sn: 0.001 to 0.100%, and Al: 0.010 to 0.50. 0%, Ca: 0.0001 to 0.0100%, N: 0.001 to 0.350%, O: 0.0100% or less, Ta: 0 to 0.100%, Ti: 0 to 0.400%, Zr: 0 to 0.100%, W: 0 to 0.300%, Zn: 0 to 0.010%, Sb: 0 to 0.1000%, As: 0 to 0.050%, Pb: 0 to 0.0100%, B: 0 to 0.0200%, Mg: 0 to 0.020%, rare earth elements: 0 to 0.200%, and the balance being Fe and impurities.

[0035] (Feature 2) In an austenitic alloy material, S is contained, and Fn1 defined by formula (1) is 5.0 or more, and the number density ND1 of fine low-S inclusions, which are inclusions with an equivalent circle diameter of 2.0 to 10.0 μm, is 3.0 pieces / mm ２ or more, containing S, Fn1 is 5.0 or more, and the number density ND2 of coarse low-S inclusions, which are inclusions with an equivalent circle diameter of more than 10.0 μm, is 2.0 pieces / mm ２ or less, and the number density ND3 of high-S inclusions, which contain S, have an Fn1 of less than 5.0, and an equivalent circle diameter of 2.0 μm or more, is 2.0 pieces / mm ２ The formula is as follows: Fn1=(Mg+Al+Ca+Ti+V+Mn) / S (1) Here, the element symbol in formula (1) is substituted with the content of the element in the corresponding inclusion in mass %. When an element is not contained, "0" is substituted for the corresponding element symbol. Feature 1 and Feature 2 will be described below.

[0036] [(Feature 1) Chemical Composition] The austenitic alloy material of this embodiment contains the following elements.

[0037] C: 0.030% or less Carbon (C) is an unavoidable impurity. In other words, the C content is greater than 0%. If the C content exceeds 0.030%, Cr carbides will form at the grain boundaries even if the contents of other elements are within the ranges of this embodiment. Cr carbides increase the cracking susceptibility at the grain boundaries. As a result, the corrosion resistance of the alloy material decreases. Therefore, the C content is 0.030% or less. The lower the C content, the better. However, excessive reduction in the C content significantly increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the C content is 0.001%, and more preferably 0.003%. The preferred upper limit of the C content is 0.028%, more preferably 0.025%, even more preferably 0.020%, and even more preferably 0.015%.

[0038] Si: 0.10 to 1.00% Silicon (Si) deoxidizes the alloy. If the Si 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 Si content exceeds 1.00%, the hot workability of the alloy material will deteriorate even if the contents of other elements are within the ranges of this embodiment. Therefore, the Si content is 0.10 to 1.00%. A preferred lower limit of the Si content is 0.11%, more preferably 0.12%, and even more preferably 0.20%. A preferred upper limit of the Si content is 0.95%, more preferably 0.90%, even more preferably 0.85%, and even more preferably 0.80%.

[0039] Mn: 0.01 to 2.00% Manganese (Mn) deoxidizes and desulfurizes the alloy. If the Mn content is less than 0.01%, the above effects cannot be sufficiently obtained, even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Mn content exceeds 2.00%, the hot workability of the alloy material will deteriorate, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Mn content is 0.01 to 2.00%. A preferred lower limit of the Mn content is 0.10%, more preferably 0.20%, even more preferably 0.25%, and even more preferably 0.30%. A preferred upper limit of the Mn content is 1.95%, even more preferably 1.80%, even more preferably 1.60%, even more preferably 1.50%, and even more preferably 1.40%.

[0040] P: 0.040% or less Phosphorus (P) is an unavoidable impurity. In other words, the P content is greater than 0%. P segregates at grain boundaries. Therefore, if the P content exceeds 0.040%, the hot workability of the alloy material decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the P content is 0.040% or less. The P content is preferably as low as possible. However, excessive reduction of the P content significantly increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the P content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. The preferred upper limit of the P content is 0.038%, more preferably 0.035%, even more preferably 0.030%, and even more preferably 0.025%.

[0041] S: 0.0050% or less Sulfur (S) is an unavoidable impurity. In other words, the S content is greater than 0%. S segregates at grain boundaries. Therefore, if the S content exceeds 0.0050%, the hot workability of the alloy material decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the S content is 0.0050% or less. The S content is preferably as low as possible. However, excessive reduction of the S content significantly increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the S content is 0.0001%, more preferably 0.0003%, and even more preferably 0.0005%. The preferred upper limit of the S content is 0.0040%, more preferably 0.0035%, even more preferably 0.0030%, and even more preferably 0.0025%.

[0042] Cr: 19.0 to 28.0% Chromium (Cr) enhances the corrosion resistance of the alloy material. If the Cr content is less than 19.0%, the above effect cannot be fully achieved, even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Cr content exceeds 28.0%, the hot workability of the alloy material decreases, even if the contents of other elements are within the ranges of this embodiment. In this case, intermetallic compounds, such as the σ phase, are more likely to form, reducing the corrosion resistance of the alloy material. Therefore, the Cr content is 19.0 to 28.0%. The preferred lower limit of the Cr content is 19.5%, more preferably 20.0%, even more preferably 20.5%, even more preferably 21.0%, even more preferably 21.5%, and even more preferably 22.0%. The preferred upper limit of the Cr content is 27.5%, even more preferably 27.0%, and even more preferably 26.5%.

[0043] Ni: 28.0 to 37.0% Nickel (Ni) is an austenite-forming element and stabilizes austenite in the alloy material. If the Ni content is less than 28.0%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Ni content exceeds 37.0%, the solid solubility limit of Ni decreases, resulting in a decrease in the strength of the alloy material, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Ni content is 28.0 to 37.0%. A preferred lower limit of the Ni content is 28.5%, more preferably 29.0%, even more preferably 29.5%, and even more preferably 30.0%. A preferred upper limit of the Ni content is 36.5%, even more preferably 36.0%, even more preferably 35.5%, even more preferably 35.0%, and even more preferably 34.5%.

[0044] Mo: 2.00 to 6.00% Molybdenum (Mo) contributes to the stabilization of corrosion protective films and improves the corrosion resistance of alloy materials. Mo also increases the strength of alloy materials through solid solution strengthening. If the Mo content is less than 2.00%, the above effects cannot be fully achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Mo content exceeds 6.00%, the hot workability of the alloy material decreases even if the contents of other elements are within the ranges of this embodiment. In this case, the manufacturing cost further increases significantly. Therefore, the Mo content is 2.00 to 6.00%. The preferred lower limit of the Mo content is 2.05%, more preferably 2.10%, even more preferably 2.15%, even more preferably 2.20%, even more preferably 2.40%, even more preferably 2.50%, and even more preferably 2.70%. The upper limit of the Mo content is preferably 5.90%, more preferably 5.80%, even more preferably 5.70%, even more preferably 5.60%, even more preferably 5.50%, even more preferably 5.00%, and even more preferably 4.50%.

[0045] Cu: 0.01 to 2.00% Copper (Cu) contributes to the stabilization of the corrosion protective film and enhances the corrosion resistance of the alloy material. If the Cu content is less than 0.01%, the above effect cannot be fully achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Cu content exceeds 2.00%, the hot workability of the alloy material decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Cu content is 0.01 to 2.00%. The preferred lower limit of the Cu content is 0.02%, more preferably 0.05%, even more preferably 0.10%, even more preferably 0.15%, and even more preferably 0.20%. The preferred upper limit of the Cu content is 1.95%, even more preferably 1.90%, even more preferably 1.80%, even more preferably 1.70%, and even more preferably 1.50%.

[0046] Nb: 0.001 to 0.100% Niobium (Nb) forms carbonitrides and the like with C and N, enhancing the strength of the alloy material. If the Nb content is less than 0.001%, the above-mentioned effects cannot be fully achieved, even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Nb content exceeds 0.100%, even if the contents of other elements are within the ranges of this embodiment, excessive carbonitrides and the like are formed, reducing the ductility of the alloy material. Therefore, the Nb content is 0.001 to 0.100%. The preferred lower limit of the Nb content is 0.002%, more preferably 0.003%, even more preferably 0.005%, even more preferably 0.008%, and even more preferably 0.010%. The preferred upper limit of the Nb content is 0.095%, even more preferably 0.085%, even more preferably 0.080%, and even more preferably 0.075%.

[0047] V: 0.01 to 0.50% Vanadium (V) forms carbonitrides and the like with C and N, increasing the strength of the alloy material. If the V content is less than 0.01%, the above effect cannot be sufficiently achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the V content exceeds 0.50%, carbonitrides and the like are formed in excess. Therefore, even if the contents of other elements are within the ranges of this embodiment, the ductility of the alloy material decreases. Therefore, the V content is 0.01 to 0.50%. The preferred lower limit of the V content is 0.02%, more preferably 0.03%, and even more preferably 0.05%. The preferred upper limit of the V content is 0.45%, more preferably 0.40%, even more preferably 0.35%, and even more preferably 0.30%.

[0048] Cobalt (Co) stabilizes austenite in the alloy material. If the Co 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 Co content exceeds 2.00%, the manufacturing cost will increase significantly even if the contents of other elements are within the ranges of this embodiment. Therefore, the Co content is 0.01 to 2.00%. The preferred lower limit of the Co content is 0.02%, more preferably 0.03%, even more preferably 0.05%, even more preferably 0.10%, even more preferably 0.15%, and even more preferably 0.20%. The preferred upper limit of the Co content is 1.95%, even more preferably 1.90%, even more preferably 1.80%, even more preferably 1.70%, even more preferably 1.60%, and even more preferably 1.50%.

[0049] Sn: 0.001 to 0.100% Tin (Sn) enhances the corrosion resistance of the alloy material. If the Sn content is less than 0.001%, 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 Sn content exceeds 0.100%, the hot workability of the alloy material decreases, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Sn content is 0.001 to 0.100%. The preferred lower limit of the Sn content is 0.002%, more preferably 0.005%, even more preferably 0.010%, and even more preferably 0.020%. The preferred upper limit of the Sn content is 0.095%, even more preferably 0.090%, even more preferably 0.080%, even more preferably 0.075%, and even more preferably 0.070%.

[0050] Al: 0.010 to 0.500% Aluminum (Al) deoxidizes the alloy. Furthermore, Al forms oxides to fix oxygen, improving the hot workability of the alloy material. Furthermore, Al improves the impact resistance and corrosion resistance of the alloy material. If the Al content is less than 0.010%, the above effects cannot be fully achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Al content exceeds 0.500%, excessive Al oxides are formed. Therefore, even if the contents of other elements are within the ranges of this embodiment, the hot workability of the alloy material is actually reduced. Therefore, the Al content is 0.010 to 0.500%. The preferred lower limit of the Al content is 0.015%, more preferably 0.020%, even more preferably 0.030%, even more preferably 0.040%, and even more preferably 0.050%. The upper limit of the Al content is preferably 0.475%, more preferably 0.460%, even more preferably 0.450%, and still more preferably 0.400%. Note that the Al content in this specification refers to the content of "acid-soluble Al", that is, sol. Al.

[0051] Ca: 0.0001 to 0.0100% Calcium (Ca) neutralizes S in the alloy material by fixing it as sulfide, thereby improving the hot workability of the alloy material. If the Ca content is less than 0.0001%, the above effect cannot be fully achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Ca content exceeds 0.0100%, coarse oxides are generated in the alloy material. Therefore, even if the contents of other elements are within the ranges of this embodiment, the hot workability of the alloy material is actually reduced. Therefore, the Ca content is 0.0001 to 0.0100%. The preferred lower limit of the Ca content is 0.0002%, more preferably 0.0005%, even more preferably 0.0010%, even more preferably 0.0014%, and even more preferably 0.0020%. The upper limit of the Ca content is preferably 0.0095%, more preferably 0.0090%, and still more preferably 0.0080%.

[0052] N: 0.001 to 0.350% Nitrogen (N) increases the strength of the alloy material through solid solution strengthening. If the N content is less than 0.001%, the above effect cannot be fully achieved, even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the N content exceeds 0.350%, the corrosion resistance of the alloy material may decrease, even if the contents of other elements are within the ranges of this embodiment. Therefore, the N content is 0.001 to 0.350%. The preferred lower limit of the N content is 0.005%, more preferably 0.010%, even more preferably 0.015%, even more preferably 0.020%, and even more preferably 0.030%. The preferred upper limit of the N content is 0.340%, even more preferably 0.320%, even more preferably 0.300%, even more preferably 0.270%, and even more preferably 0.250%.

[0053] O: 0.0100% or less Oxygen (O) is an unavoidable impurity. In other words, the O content is greater than 0%. O forms oxides. If the O content exceeds 0.0100%, coarse oxides are formed in the alloy material. Therefore, even if the contents of other elements are within the ranges of this embodiment, the hot workability of the alloy material is reduced. In this case, the corrosion resistance of the alloy material is further reduced. Therefore, the O content is 0.0100% or less. The O content is preferably as low as possible. However, excessive reduction of the O content significantly increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the O content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The preferred upper limit of the O content is 0.0090%, more preferably 0.0085%, and even more preferably 0.0080%.

[0054] The balance of the chemical composition of the austenitic alloy material of this embodiment is composed of Fe and impurities, where the impurities refer to substances that are mixed in from raw materials such as ore or scrap or the manufacturing environment when industrially manufacturing the austenitic alloy material, and are acceptable within a range that does not adversely affect the effects of the austenitic alloy material of this embodiment.

[0055] [Regarding optional elements] The chemical composition of the alloy material of this embodiment may further contain one or more elements selected from the group consisting of Ta: 0-0.100%, Ti: 0-0.400%, Zr: 0-0.100%, W: 0-0.300%, Zn: 0-0.010%, Sb: 0-0.1000%, As: 0-0.050%, Pb: 0-0.0100%, B: 0-0.0200%, Mg: 0-0.020%, and rare earth elements (REM): 0-0.200%. All of these are optional elements. These optional elements will be described below.

[0056] [Regarding Ta, Ti, and Zr] The chemical composition of the austenitic alloy material of this embodiment may contain one or more elements selected from the group consisting of Ta, Ti, and Zr, instead of a portion of Fe. All of these elements increase the strength of the alloy material. Ta, Ti, and Zr will be described below.

[0057] Ta: 0 to 0.100% Tantalum (Ta) is an optional element and does not necessarily need to be contained. That is, the Ta content may be 0%. When contained, that is, when the Ta content exceeds 0%, Ta forms precipitates and increases the strength of the alloy material. Even if even a small amount of Ta is contained, the above effect can be obtained to some extent. However, if the Ta content exceeds 0.100%, the strength of the alloy material becomes too high even if the contents of other elements are within the ranges of this embodiment. In this case, the toughness of the alloy material decreases. Therefore, the Ta content is 0 to 0.100%. The preferred lower limit of the Ta content is 0.001%, more preferably 0.002%, and even more preferably 0.005%. The preferred upper limit of the Ta content is 0.096%, more preferably 0.090%, and even more preferably 0.080%.

[0058] Ti: 0 to 0.400% Titanium (Ti) is an optional element and does not necessarily need to be contained. In other words, the Ti content may be 0%. When contained, that is, when the Ti content exceeds 0%, Ti forms precipitates and increases the strength of the alloy material. Even if even a small amount of Ti is contained, the above effect can be achieved to some extent. However, if the Ti content exceeds 0.400%, the strength of the alloy material becomes too high even if the contents of other elements are within the ranges of this embodiment. In this case, the toughness of the alloy material decreases. Therefore, the Ti content is 0 to 0.400%. The preferred lower limit of the Ti content is 0.001%, more preferably 0.003%, even more preferably 0.010%, and even more preferably 0.020%. The preferred upper limit of the Ti content is 0.390%, even more preferably 0.383%, and even more preferably 0.350%.

[0059] Zr: 0 to 0.100% Zirconium (Zr) is an optional element and does not necessarily need to be contained. In other words, the Zr content may be 0%. When contained, that is, when the Zr content exceeds 0%, Zr forms precipitates and increases the strength of the alloy material. Even if even a small amount of Zr is contained, the above effect can be obtained to some extent. However, if the Zr content exceeds 0.100%, the strength of the alloy material becomes too high even if the contents of other elements are within the ranges of this embodiment. In this case, the toughness of the alloy material decreases. Therefore, the Zr content is 0 to 0.100%. The preferred lower limit of the Zr content is 0.001%, more preferably 0.005%, even more preferably 0.010%, and even more preferably 0.020%. The preferred upper limit of the Zr content is 0.097%, even more preferably 0.060%, and even more preferably 0.040%.

[0060] [Regarding W, Zn, Sb, As, and Pb] The chemical composition of the alloy material of this embodiment may contain one or more elements selected from the group consisting of W, Zn, Sb, As, and Pb, instead of a portion of Fe. All of these elements improve the SCC resistance of the alloy material. W, Zn, Sb, As, and Pb will be described below.

[0061] W: 0 to 0.300% Tungsten (W) is an optional element and does not necessarily need to be contained. That is, the W content may be 0%. When W is contained, that is, when the W content exceeds 0%, W enhances the SCC resistance of the alloy material. 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 0.300%, the strength of the alloy material becomes too high even if the contents of other elements are within the ranges of this embodiment. In this case, the toughness of the alloy material decreases. Therefore, the W content is 0 to 0.300%. The preferred lower limit of the W content is 0.001%, more preferably 0.004%, even more preferably 0.010%, and even more preferably 0.020%. The preferred upper limit of the W content is 0.297%, even more preferably 0.250%, and even more preferably 0.150%.

[0062] Zn: 0 to 0.010% Zinc (Zn) is an optional element and does not necessarily need to be contained. In other words, the Zn content may be 0%. When contained, that is, when the Zn content exceeds 0%, Zn enhances the SCC resistance of the alloy material. Even if even a small amount of Zn is contained, the above effect can be obtained to some extent. However, if the Zn content exceeds 0.010%, the SCC resistance of the alloy material may actually decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the Zn content is 0 to 0.010%. The preferred lower limit of the Zn content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. The preferred upper limit of the Zn content is 0.009%, more preferably 0.008%, and even more preferably 0.007%.

[0063] Sb: 0 to 0.1000% Antimony (Sb) is an optional element and does not necessarily need to be contained. That is, the Sb content may be 0%. When contained, that is, when the Sb content exceeds 0%, Sb enhances the SCC resistance of the alloy material. Even if even a small amount of Sb is contained, the above effect can be obtained to some extent. However, if the Sb content exceeds 0.1000%, the manufacturing cost increases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Sb content is 0 to 0.1000%. A preferred lower limit of the Sb content is 0.0001%, more preferably 0.0002%, and even more preferably 0.0010%. A preferred upper limit of the Sb content is 0.0950%, more preferably 0.0800%, and even more preferably 0.0600%.

[0064] As: 0 to 0.050% Arsenic (As) is an optional element and does not necessarily need to be contained. That is, the As content may be 0%. When contained, that is, when the As content exceeds 0%, As enhances the SCC resistance of the alloy material. Even if even a small amount of As is contained, the above effect can be obtained to some extent. However, if the As content exceeds 0.050%, the SCC resistance of the alloy material may actually decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the As content is 0 to 0.050%. The preferred lower limit of the As content is 0.001%, more preferably 0.002%, and even more preferably 0.005%. The preferred upper limit of the As content is 0.048%, more preferably 0.040%, and even more preferably 0.035%.

[0065] Pb: 0 to 0.0100% Lead (Pb) is an optional element and does not necessarily need to be contained. That is, the Pb content may be 0%. When contained, that is, when the Pb content is greater than 0%, Pb enhances the SCC resistance of the alloy material. Even if even a small amount of Pb is contained, the above effect can be obtained to some extent. However, if the Pb content exceeds 0.0100%, the hot workability of the alloy material will deteriorate even if the contents of other elements are within the ranges of this embodiment. Therefore, the Pb content is 0 to 0.0100%. The preferred lower limit of the Pb content is 0.0001%, more preferably 0.0004%, and even more preferably 0.0010%. The preferred upper limit of the Pb content is 0.0094%, more preferably 0.0070%, and even more preferably 0.0050%.

[0066] [Regarding B, Mg, and Rare Earth Elements (REM)] The chemical composition of the alloy material of this embodiment may contain one or more elements selected from the group consisting of B, Mg, and rare earth elements (REM) in place of a portion of Fe. All of these elements improve the hot workability of the alloy material. B, Mg, and REM will be described below.

[0067] B: 0 to 0.0200% Boron (B) is an optional element and does not necessarily need to be contained. In other words, the B content may be 0%. When contained, that is, when the B content exceeds 0%, B suppresses the segregation of S to grain boundaries in the alloy material and improves the hot workability of the alloy material. Even if even a small amount of B is contained, the above effect can be obtained to some extent. However, if the B content exceeds 0.0200%, coarse B nitrides are formed. In this case, even if the contents of other elements are within the ranges of this embodiment, the toughness of the alloy material decreases. Therefore, the B content is 0 to 0.0200%. The preferred lower limit of the B content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0007%. The preferred upper limit of the B content is 0.0196%, more preferably 0.0150%, and even more preferably 0.0100%.

[0068] Mg: 0 to 0.020% Magnesium (Mg) is an optional element and does not necessarily need to be contained. That is, the Mg content may be 0%. When contained, that is, when the Mg content exceeds 0%, Mg neutralizes S in the alloy material by fixing it as sulfide, thereby improving the hot workability of the alloy material. Even if even a small amount of Mg is contained, the above effect can be obtained to some extent. However, if the Mg content exceeds 0.020%, coarse oxides are generated. In this case, even if the contents of other elements are within the ranges of this embodiment, the SCC resistance of the alloy material decreases. Therefore, the Mg content is 0 to 0.020%. The preferred lower limit of the Mg content is 0.001%, more preferably 0.002%, and even more preferably 0.005%. The preferred upper limit of the Mg content is 0.019%, more preferably 0.017%, and even more preferably 0.014%.

[0069] Rare Earth Elements (REM): 0 to 0.200% Rare earth elements (REM) are optional elements and may not be present. That is, the REM content may be 0%. When present, that is, when the REM content exceeds 0%, the REM neutralizes the S in the alloy material by fixing it as sulfides, thereby improving the hot workability of the alloy material. Even if even a small amount of REM is present, the above effect can be achieved to some extent. However, if the REM content exceeds 0.200%, oxides in the alloy material will coarsen even if the contents of other elements are within the ranges of this embodiment. In this case, the toughness of the alloy material will decrease. Therefore, the REM content is 0 to 0.200%. The preferred lower limit of the REM content is 0.001%, more preferably 0.005%, and even more preferably 0.010%. The preferred upper limit of the REM content is 0.196%, more preferably 0.150%, and even more preferably 0.100%.

[0070] In this specification, REM refers to one or more elements selected from the group consisting of scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanoids lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. In addition, the REM content in this specification refers to the total content of these elements.

[0071] [(Feature 2) Number Density of Fine Low-S Inclusions, Coarse Low-S Inclusions, and High-S Inclusions] For inclusions containing S in the austenitic alloy material of this embodiment, Fn1 is defined by formula (1): Fn1 = (Mg + Al + Ca + Ti + V + Mn) / S (1) Here, the element symbol in formula (1) is substituted with the content of the element in the corresponding inclusion in mass %. If an element is not contained, "0" is substituted for the corresponding element symbol. Fn1 is a value obtained by rounding the obtained value to one decimal place.

[0072] Furthermore, inclusions in austenitic alloy materials are defined as follows: (A) Inclusions containing S, having an Fn1 of 5.0 or more, and an equivalent circle diameter of 2.0 to 10.0 μm are defined as "fine low-S inclusions." (B) Inclusions containing S, having an Fn1 of 5.0 or more, and an equivalent circle diameter of more than 10.0 μm are defined as "coarse low-S inclusions." (C) Inclusions containing S, having an Fn1 of less than 5.0, and an equivalent circle diameter of 2.0 μm or more are defined as "high-S inclusions."

[0073] At this time, the number density ND1 of the fine low-S inclusions in the austenitic alloy material is 3.0 pieces / mm ２ or more, and the number density ND2 of coarse low-S inclusions is 2.0 pieces / mm ２ The number density ND3 of high S inclusions is 2.0 pieces / mm ２ The following is the result.

[0074] In addition, even if an inclusion containing S and having an equivalent circle diameter of less than 2.0 μm dissolves and forms a dent, the dent quickly re-passivates. Therefore, in an austenitic alloy material having the chemical composition of Feature 1, an inclusion containing S and having an equivalent circle diameter of less than 2.0 μm does not affect SCC resistance.

[0075] As described above, in the austenitic alloy material of this embodiment, the S content in the chemical composition is not drastically reduced, but a certain amount of S content is allowed while increasing the number density ND1 of fine low-S inclusions, which are composite inclusions, thereby reducing the S content available for forming high-S inclusions. This makes it possible to reduce the number density ND3 of high-S inclusions that are easily dissolved in a sour environment.

[0076] Furthermore, among low-S inclusions with an Fn1 of 5.0 or more, coarse low-S inclusions are coarse even when the S content is low. Therefore, coarse low-S inclusions are likely to dissolve in a highly corrosive sour environment and form coarse pits. These coarse pits are difficult to repassivate and promote the occurrence of SCC. Therefore, the number density ND1 of fine low-S inclusions is increased, the number density ND3 of high-S inclusions is decreased, and the number density ND2 of coarse low-S inclusions is also decreased.

[0077] [Number density ND1 of fine low-S inclusions] The number density ND1 of fine low-S inclusions is 3.0 pieces / mm ２ If the S content is less than 1.0, the amount of fine low-S inclusions in the alloy material is insufficient. In this case, the S content available for forming high-S inclusions in the alloy material increases, and the number density ND3 of the high-S inclusions becomes 2.0 pieces / mm ２ As a result, sufficient SCC resistance in a sour environment cannot be obtained. Therefore, the number density ND1 of fine low-S inclusions is 3.0 pieces / mm ２ The preferred lower limit of the number density ND1 is 3.2 pieces / mm ２ and more preferably 3.5 pieces / mm ２ and more preferably 4.0 pieces / mm ２ and more preferably 4.5 pieces / mm ２ and more preferably 5.0 pieces / mm ２ and more preferably 5.5 pieces / mm ２ The upper limit of the number density ND1 is not particularly limited. However, in an alloy material satisfying the characteristic 1, the upper limit of the number density ND1 is, for example, 25.0 pieces / mm ２ For example, 20.0 pieces / mm ２ is.

[0078] [Number density ND2 of coarse low-S inclusions] The number density ND2 of coarse low-S inclusions is 2.0 pieces / mm ２ If the density ND2 of the coarse low-S inclusions exceeds 2.0 pieces / mm, excessive coarse low-S inclusions are formed. In this case, the coarse low-S inclusions are likely to dissolve and form coarse pits in a highly corrosive sour environment. These coarse pits are likely to promote the occurrence of SCC. Therefore, the number density ND2 of the coarse low-S inclusions is 2.0 pieces / mm ２ The preferred upper limit of the number density ND2 is 1.9 pieces / mm ２ and more preferably 1.8 pieces / mm ２ and more preferably 1.7 pieces / mm ２ and more preferably 1.6 pieces / mm ２ and more preferably 1.5 pieces / mm ２ It is preferable that the number density ND2 is as small as possible. In other words, the number density ND2 is 0 pieces / mm ２ However, if the number density ND2 is reduced too much, the manufacturing cost may increase. Therefore, the preferable lower limit of the number density ND2 is 0.1 pieces / mm ２ and more preferably 0.2 pieces / mm ２ is.

[0079] [Regarding the number density ND3 of high-S inclusions] As mentioned above, high-S inclusions, even if small in size, are easily dissolved in a highly corrosive sour environment and tend to form pits. Pits formed from high-S inclusions tend to promote the occurrence of SCC, even if they are small in size. Therefore, when the number density ND3 of high-S inclusions is 2.0 pieces / mm ２ If the number density ND3 of high-S inclusions exceeds 2.0 pieces / mm ２ The preferred upper limit of the number density ND3 is 1.9 pieces / mm ２ and more preferably 1.8 pieces / mm ２ and more preferably 1.7 pieces / mm ２ and more preferably 1.6 pieces / mm ２ and more preferably 1.5 pieces / mm ２It is preferable that the number density ND3 is as small as possible. In other words, the number density ND3 is 0 pieces / mm ２ However, if the number density ND3 is reduced too much, the manufacturing cost may increase. Therefore, the preferable lower limit of the number density ND3 is 0.1 pieces / mm ２ and more preferably 0.2 pieces / mm ２ is.

[0080] [Number density ND1 of fine low-S inclusions (pieces / mm ２ ), the number density of coarse low-S inclusions ND2 (pieces / mm ２ ) and the number density ND3 of high-S inclusions (number / mm ２ In this embodiment, the number density ND1 (number / mm ) of fine low-S inclusions in the austenitic alloy material is measured. ２ ), the number density of coarse low-S inclusions ND2 (pieces / mm ２ ) and the number density ND3 of high-S inclusions (number / mm ２ ) can be calculated in the following way:

[0081] A test piece is taken from the austenitic alloy material. When the austenitic alloy material is an alloy pipe, a test piece having an observation surface including the pipe axial direction and the wall thickness direction (pipe radial direction) is taken from the center of the wall thickness. When the austenitic alloy material is an alloy plate, a test piece having an observation surface including the rolling direction and the plate thickness direction is taken from the center of the plate thickness. When the austenitic alloy material is an alloy bar having a circular cross section perpendicular to the axial direction, a test piece having an observation surface including the axial and radial directions is taken from the R / 2 part of the alloy bar. The R / 2 part means the center of the radius R in the circular cross section perpendicular to the axial direction of the alloy bar.

[0082] The observation surface of the collected test piece is mirror-polished. An observation field is selected from the mirror-polished observation surface. At this time, three or more observation fields are selected, and the total area of ​​the observation fields is 500 mm. ２Using a scanning electron microscope equipped with a composition analysis function (SEM-EDS device), a backscattered electron image of each observation field is obtained at 800x magnification. The obtained backscattered electron images are observed, and particles are identified from the contrast. The position coordinates of all identified particles in the observation field are recorded.

[0083] Based on the position coordinates of the identified particles, element concentration analysis (EDS analysis) is performed on the particles. Specifically, based on the position coordinates, the identified particles are scanned with an electron beam to analyze the element concentration of the particles. At this time, the electron beam is not scanned only on a portion of the particle, but the entire particle. This allows the element concentration of the entire particle to be analyzed. In the EDS analysis, the acceleration voltage is set to 20 kV, and the target elements are quantified as N, O, Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, V, Mn, Cu, Zr, and Nb.

[0084] Among the identified particles, when the total content of N, O, Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, V, Mn, Cu, Zr, and Nb is taken as 100.0 mass%, particles with a Cl content of more than 10.0%, particles with a K content of more than 10.0%, particles with a Na content of more than 20.0%, particles with a Ca content of more than 75.0%, and particles with an O content of more than 70.0% are dust or abrasives that adhered during mirror polishing. Therefore, these particles are determined not to be inclusions and are excluded from the scope of the present invention. Furthermore, when the total content of N, O, Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, V, Mn, Cu, Zr, and Nb is taken as 100.0 mass%, among the identified particles, particles with an S content of less than 1.0% are not S-containing inclusions but oxides or nitrides. Therefore, particles with an S content of less than 1.0% are also excluded from the scope of the present invention. In other words, when the total content of N, O, Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, V, Mn, Cu, Zr, and Nb is taken as 100.0 mass%, among the identified particles, particles other than particles with a Cl content of more than 10.0%, particles with a K content of more than 10.0%, particles with a Na content of more than 20.0%, particles with a Ca content of more than 75.0%, particles with an O content of more than 70.0%, and particles with an S content of less than 1.0% are recognized as inclusions containing S.

[0085] The equivalent circle diameter (μm) of particles identified as inclusions containing S is determined. The equivalent circle diameter means the diameter (μm) of a circle with the same area as the particle. The equivalent circle diameter is the value obtained by rounding off the obtained value to one decimal place.

[0086] Based on the EDS analysis results and equivalent circle diameter of each particle, fine low-S inclusions, coarse low-S inclusions, and high-S inclusions are identified as follows: (A) Fine Low-S Inclusions Particles having an Fn1 value of 5.0 or more and an equivalent circle diameter of 2.0 to 10.0 μm, as defined by formula (1), based on the Mg, Al, Ca, Ti, V, Mn, and S contents in mass%, where the total content of N, O, Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, V, Mn, Cu, Zr, and Nb is taken as 100.0 mass%, are identified as "fine low-S inclusions." Fn1 = (Mg + Al + Ca + Ti + V + Mn) / S (1) Here, the element symbols in formula (1) are substituted with the contents of the corresponding elements in the particle (inclusion) in mass%. When an element is not contained, the corresponding element symbol is substituted with "0." (B) Coarse Low-S Inclusions Particles having an Fn1 defined by formula (1) of 5.0 or more and an equivalent circle diameter of more than 10.0 μm based on the Mg content, Al content, Ca content, Ti content, V content, Mn content, and S content in mass%, where the total content of N, O, Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, V, Mn, Cu, Zr, and Nb is taken as 100.0 mass%, are specified as "coarse low-S inclusions." (C) High-Sulfur Inclusions Particles having an Fn1 defined by formula (1) of less than 5.0 and an equivalent circle diameter of 2.0 μm or more based on the Mg content, Al content, Ca content, Ti content, V content, Mn content, and S content in mass% when the total content of N, O, Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, V, Mn, Cu, Zr, and Nb is taken as 100.0 mass%, are specified as "high-Sulfur inclusions."

[0087] The fine low-S inclusions, coarse low-S inclusions, and high-S inclusions identified by the above method are counted in each field of view.

[0088] Based on the total number of counted fine low-S inclusions in all observation fields and the total area of ​​all observation fields, the number density ND1 (numbers / mm ２ Based on the total number of coarse low-S inclusions counted in all observation fields and the total area of ​​all observation fields, the number density ND2 (numbers / mm ２ Based on the total number of high-S inclusions counted in all observation fields and the total area of ​​all observation fields, the number density ND3 (numbers / mm ２ ) is required.

[0089] The number density ND1 (number / mm ２ ), the number density of coarse low-S inclusions ND2 (pieces / mm ２ ), and the number density ND3 of high S inclusions (pieces / mm ２ ) is the value obtained by rounding off the obtained value to one decimal place. In addition, as the SEM-EDS device, for example, an automatic analyzer manufactured by FEI (ASPEX) under the trade name of Metals Quality Analyzer can be used.

[0090] [Effects of the Austenitic Alloy Material of the Present Embodiment] The austenitic alloy material of the present embodiment satisfies Features 1 and 2. Therefore, excellent SCC resistance can be obtained in a sour environment.

[0091] [Method for Evaluating SCC Resistance] The SCC resistance of the austenitic alloy material of this embodiment is evaluated by the following method.

[0092] Two uniaxial tensile test specimens are taken from the austenitic alloy material. The tensile test specimens are those specified in NACE TM0198 (2020), with a diameter of 3.81 mm at the parallel section and a distance between grades of 25.4 mm. When the austenitic alloy material is an alloy pipe, the test specimen is taken from the center of the wall thickness. In this case, the longitudinal direction of the test specimen is parallel to the axial direction of the alloy pipe. When the austenitic alloy material is an alloy plate, the test specimen is taken from the center of the plate thickness. In this case, the longitudinal direction of the test specimen is parallel to the rolling direction of the alloy plate. When the austenitic alloy material is an alloy bar, the test specimen is taken from the R / 2 portion. In this case, the longitudinal direction of the test specimen is parallel to the axial direction of the alloy bar.

[0093] The test specimen is placed in a test vessel (autoclave). A test liquid, which is a 25% by mass aqueous solution of NaCl (sodium chloride), is transferred to the test vessel, and the test specimen is immersed in the test liquid. The temperature inside the autoclave is then set to 170°C, and the gauge pressure is set to 100 psi. ２ S gas is introduced into the autoclave. －６ A tensile test is carried out at a strain rate of 1 / s to determine the reduction in area at fracture (%). A tensile test is carried out on two test pieces, and the arithmetic mean value of the reduction in area at fracture of the two test pieces is taken as the reduction in area at fracture (%) of the austenitic alloy material.

[0094] Furthermore, the two test pieces are observed with a magnifying glass at 10x magnification to see if any cracks (secondary cracks) other than those on the main fracture surface have occurred in the drawn portions. If the occurrence of secondary cracks is suspected based on the observation with the magnifying glass, the test pieces are further observed with an optical microscope at 100x magnification to confirm the presence or absence of secondary cracks. In the SSRT test, if the reduction of area at fracture is 60.0% or more and no secondary cracks are observed in the drawn portions of the two test pieces, the test pieces are judged to have excellent SCC resistance.

[0095] [Yield strength] The yield strength of the austenitic alloy material according to this embodiment is not particularly limited. The yield strength of the austenitic alloy material according to this embodiment is, for example, 758 to 1034 MPa. The yield strength of the alloy material according to this embodiment may be 758 to 965 MPa.

[0096] [Method for measuring yield strength] The yield strength of the austenitic alloy material of this embodiment can be determined by the following method. A tensile test is performed according to ASTM E8 / E8M (2022). First, a tensile test specimen is taken from the austenitic alloy material. When the austenitic alloy material is an alloy pipe, a round bar-shaped tensile test specimen or an arc-shaped tensile test specimen is taken from the center of the wall thickness. In this case, the longitudinal direction of the tensile test specimen is parallel to the axial direction of the alloy pipe. When the alloy material is an alloy pipe and a round bar-shaped tensile test specimen cannot be taken from the alloy pipe, an arc-shaped tensile test specimen is taken from the alloy pipe. When the austenitic alloy material is an alloy plate, a round bar-shaped tensile test specimen is taken from the center of the plate thickness. In this case, the longitudinal direction of the tensile test specimen is parallel to the rolling direction of the alloy plate. When the austenitic alloy material is an alloy bar, a round bar-shaped tensile test specimen is taken from the R / 2 portion. In this case, the longitudinal direction of the tensile test piece is parallel to the axial direction of the alloy bar.

[0097] The size of a round bar-shaped tensile test specimen is, for example, 6 mm in diameter at the parallel portion and 30 mm in gauge length. The size of a circular arc-shaped tensile test specimen is, for example, the total thickness, 25.4 mm in width, and 50.8 mm in gauge length. A tensile test is performed using the tensile test specimen at room temperature (24±3°C) in the atmosphere. In this embodiment, the 0.2% offset proof stress obtained from the tensile test is defined as the yield strength (MPa). In this embodiment, the yield strength (MPa) is an integer value obtained by rounding the obtained value to one decimal place.

[0098] [Regarding the Microstructure of the Austenitic Alloy Material of the Present Embodiment] The austenitic alloy material of the present embodiment has a microstructure consisting of austenite. Here, "a microstructure consisting of austenite" means that the amount of other structures other than austenite is negligibly small. Examples of other structures other than austenite include inclusions and precipitates.

[0099] [Shape of Austenitic Alloy Material] The shape of the austenitic alloy material of this embodiment is not particularly limited. The austenitic alloy material may be an alloy pipe, an alloy plate, or an alloy bar having a circular cross section perpendicular to the axial direction. Preferably, the alloy material of this embodiment is an alloy pipe, and more preferably a seamless alloy pipe.

[0100] [Manufacturing Method] An example of a manufacturing method for the austenitic alloy material of this embodiment having the above-described configuration will be described. Note that the manufacturing method for the austenitic alloy material of this embodiment is not limited to the manufacturing method described below. This example of a manufacturing method for the austenitic alloy material of this embodiment includes the following steps: (Step 1) Material preparation step (Step 2) Hot working step (Step 3) Solution treatment step (Step 4) Cold working step Each manufacturing step will be described in detail below.

[0101] [(Step 1) Material Preparation Step] In the material preparation step, a molten alloy, which is a liquid alloy having a chemical composition that satisfies Feature 1, is melted. The molten alloy may be melted in an electric furnace, and the Ar—O ２ The smelting may be carried out in a mixed gas bottom blown decarburization furnace (AOD furnace) or a vacuum decarburization furnace (VOD furnace).

[0102] Using the produced molten metal, cast pieces (slabs, blooms, or billets) are produced by continuous casting. In continuous casting, the molten metal is first poured from a ladle into a tundish. The molten metal, temporarily stored in the tundish, is then guided into the mold through an immersion nozzle. The molten metal gradually solidifies from the outer surface, which is cooled in the mold. The solidified portion is gradually pulled downward from the mold, and the outer surface of the cast piece is further cooled while cooling water is poured onto it, causing the cast piece to solidify. Through these manufacturing processes, cast pieces that serve as the raw material for alloy materials are produced.

[0103] In the material preparation step, the bloom may further be bloomed into a billet. In this case, for example, the bloom is heated to 1150 to 1300°C. The heated bloom is then bloomed into a billet. Through the above steps, an austenitic alloy material (slab, bloom, or billet) is produced.

[0104] The continuous casting during the material preparation process satisfies the following conditions: (Condition 1) The retention time t of the molten metal at a temperature of 1600 to 1500°C in the tundish is 5 to 100 minutes. (Condition 2) The vertical distance D from the liquid surface of the molten metal in the mold to the center of the discharge port on the outer surface of the submerged entry nozzle is 150 to 400 mm. Conditions 1 and 2 are explained below.

[0105] [Regarding Condition 1] Fine low-S inclusions are formed by the aggregation of sulfides, nitrides, etc. on oxides acting as nuclei. To form such fine low-S inclusions, it is effective to float coarse oxides in the molten metal in the tundish and prevent the coarse oxides from flowing into the mold.

[0106] The tundish is equipped with a heating device that maintains the temperature of the molten metal. The heating device is, for example, an induction heating device or a plasma heating device. In the tundish, the molten metal is maintained at 1600 to 1500°C for a holding time t. If the holding time t is less than 5 minutes, coarse oxides in the molten metal do not rise sufficiently, and the coarse oxides remain in the molten metal and flow into the mold. In this case, the number density ND2 of coarse low-S inclusions in the manufactured austenitic alloy material becomes excessive.

[0107] On the other hand, if the holding time t exceeds 100 minutes, although the coarse oxides in the molten metal will rise sufficiently, the fine oxides will also become coarse and rise. As a result, the amount of fine oxides that serve as nuclei for the fine low-S inclusions will be insufficient in the molten metal flowing from the submerged nozzle into the mold. As a result, in the manufactured austenitic alloy material, the number density ND1 of the fine low-S inclusions will be excessively low, and the number density ND3 of the high-S inclusions will be excessive. Therefore, the holding time t is set to 5 to 100 minutes.

[0108] [Condition 2] Fig. 1 is a schematic diagram illustrating the positional relationship between the molten metal in the mold and the submerged entry nozzle during continuous casting. Referring to Fig. 1, the molten metal 10 passes through the submerged entry nozzle 20 and flows from the discharge port 22 to the outside of the submerged entry nozzle 20 into the mold 30.

[0109] The submerged nozzle 20 includes a cylindrical main body 21 and two discharge ports 22. The two discharge ports 22 are arranged opposite each other on the side wall portion near the bottom of the cylindrical main body. More specifically, one of the discharge ports 22 is arranged at an angle of 180° around the central axis of the submerged nozzle 20 relative to the other discharge port 22. The discharge ports 22 are inclined upward at an angle θ of 5 to 35° with respect to the horizontal direction.

[0110] The vertical distance from the liquid level 11 of the molten metal 10 in the mold 30 to the center of the discharge port 22 on the outer surface of the submerged nozzle 20 is defined as distance D (mm). Here, the "center of the discharge port 22 on the outer surface of the submerged nozzle 20" refers to the position P22 of the central axis C22 of the discharge port 22 on the outer surface of the submerged nozzle 20. Note that during continuous casting, the liquid level 11 fluctuates within ±10 mm. Therefore, the position of the liquid level 11 (liquid level height) when calculating distance D is the liquid level height (target level) set during actual continuous casting operation.

[0111] The distance D forms a stirring region DA between the liquid surface 11 and the discharge port 22 in the molten metal 10. In the stirring region DA, the molten metal flowing out of the discharge port 22 of the submerged nozzle 20 into the molten metal 10 rises and is stirred vertically. At this time, the temperature of the molten metal gradually decreases, resulting in the formation of sulfides and nitrides. If the molten metal flowing out of the discharge port 22 contains a sufficient amount of fine oxides, sulfides and nitrides are formed in the stirring region DA using the fine oxides as nuclei, or the sulfides and nitrides formed in the molten metal collide with the fine oxides and agglomerate, forming low-S inclusions with an Fn1 of 5.0 or more. The formation of low-S inclusions suppresses the formation of high-S inclusions. Furthermore, among the low-S inclusions, coarse low-S inclusions float to the liquid surface 11. A molten slag layer is formed on the liquid surface 11 by the mold powder. The coarse low-S inclusions that float to the liquid surface 11 are absorbed into the molten slag layer. Therefore, although fine low-S inclusions remain in the molten metal, the amount of coarse low-S inclusions in the molten metal is reduced.

[0112] As described above, the stirring area DA affects the number density of fine low-S inclusions, coarse low-S inclusions, and high-S inclusions in the manufactured austenitic alloy material. The distance D is a factor that determines the size of the stirring area DA.

[0113] If the distance D is less than 150 mm, the stirring area DA is too narrow. In this case, low-S inclusions may not be generated to a degree that sufficiently reduces the number density ND3 of high-S inclusions. As a result, the number density ND3 of high-S inclusions becomes excessive, or the number density ND1 of fine low-S inclusions becomes excessively low.

[0114] On the other hand, if the distance D exceeds 400 mm, the stirring area DA becomes too wide. In this case, low-S inclusions are sufficiently generated, and the generation of high-S inclusions can be sufficiently suppressed. However, the coarse low-S inclusions are less likely to rise to the liquid surface 11. As a result, the number density ND2 of the coarse low-S inclusions in the manufactured alloy material becomes excessive. Therefore, the distance D is set to 150 to 400 mm.

[0115] Electromagnetic stirring may be performed on the molten metal 10 in the mold 30. Electromagnetic stirring stirs the molten metal 10 in the horizontal direction. Therefore, electromagnetic stirring does not easily form a stirring area DA that stirs the molten metal 10 in the vertical direction as shown in Figure 1. By setting the distance D (mm) from the liquid surface 11 of the molten metal 10 in the mold 30 to the center of the discharge port 22 on the outer surface of the submerged nozzle 20 to 150 to 400 mm, a stirring area DA of an appropriate range can be formed.

[0116] [(Step 2) Hot Working Step] In the hot working step, the material (slab, bloom, or billet) produced in the material preparation step is hot worked to produce an intermediate alloy material. In this specification, the intermediate alloy material refers to a mother pipe if the final product is an alloy pipe, a plate-shaped alloy material if the final product is an alloy plate, or a rod-shaped alloy material with a circular cross section perpendicular to the axial direction if the final product is an alloy bar. The hot working may be hot forging, hot extrusion, or hot rolling. The hot working method is not particularly limited and may be a well-known method.

[0117] When the intermediate alloy material is a mother pipe, for example, hot extrusion such as the Eugène-Séjournet method or the Erhardt push bench method may be performed. Furthermore, hot rolling such as piercing and rolling by the Mannesmann method may also be performed. Hot working may be performed only once or multiple times. For example, the material is heated in a heating furnace. The heating temperature is not particularly limited, but is, for example, 1100 to 1300°C. The material extracted from the heating furnace is subjected to the above-described piercing and rolling, and then subjected to elongation and rolling to produce a mother pipe, which is an intermediate alloy material.

[0118] When the intermediate alloy material is a plate-shaped alloy material, the material is first heated in a heating furnace. The heating temperature is not particularly limited, but is, for example, 1100 to 1300°C. The material extracted from the heating furnace is hot-rolled using a roughing mill and a tandem finishing mill to produce a plate-shaped alloy material that is the intermediate alloy material.

[0119] When the intermediate alloy material is a rod-shaped alloy material, the material is first heated in a heating furnace. The heating temperature is not particularly limited, but is, for example, 1100 to 1300°C. The material extracted from the heating furnace is subjected to hot working to produce a rod-shaped alloy material, which is the intermediate alloy material. The hot working is, for example, blooming using a blooming mill or hot rolling using a continuous rolling mill. The continuous rolling mill has horizontal stands each having a pair of grooved rolls arranged side by side in the vertical direction, and vertical stands each having a pair of grooved rolls arranged side by side in the horizontal direction, arranged alternately.

[0120] [(Step 3) Solution Treatment Step] In the solution treatment step according to this embodiment, the intermediate alloy material is subjected to solution treatment. The method of solution treatment is not particularly limited, and a well-known method may be used. For example, the intermediate alloy material is loaded into a heat treatment furnace, held at a desired temperature, and then rapidly cooled. In this case, the temperature at which the solution treatment is performed (heat treatment temperature) refers to the temperature (°C) of the heat treatment furnace for carrying out the solution treatment. The time (holding time) held at the solution temperature refers to the time (minutes) for which the intermediate alloy material is held at the heat treatment temperature.

[0121] The solution treatment temperature in the solution treatment step is set to 1050 to 1150°C. If the solution treatment temperature is 1050°C or higher, precipitates in the intermediate alloy material are sufficiently dissolved during the solution treatment. On the other hand, if the solution treatment temperature exceeds 1150°C, the effect saturates.

[0122] Therefore, the solution temperature is set to 1050 to 1150° C. The holding time at the solution temperature is not particularly limited, but is, for example, 5 to 180 minutes. The rapid cooling method is, for example, water cooling.

[0123] [(Step 4) Cold Working Step] In the cold working step, the solution-treated intermediate alloy material is cold worked to produce an alloy material. The cold working may be cold drawing or cold rolling. In the cold working step, well-known cold working may be performed under well-known conditions. The temperature of the intermediate alloy material during cold working may be, for example, room temperature to 300°C.

[0124] The cold working ratio (%) is, for example, 5 to 40%. Here, if the area of ​​the cross section (transverse cross section) perpendicular to the longitudinal direction of the intermediate alloy material before the cold working process is S0 and the area of ​​the cross section (transverse cross section) perpendicular to the longitudinal direction of the alloy material after the cold working process is S1, the cold working ratio (%) is defined by the following formula: Cold working ratio (%) = 100 × (1 - S1 / S0)

[0125] If the cold working ratio is 5 to 40%, the yield strength of the alloy material after production can be adjusted to the range of 758 to 1034 MPa.

[0126] The austenitic alloy material of this embodiment can be manufactured by the above steps. Note that the above-mentioned manufacturing method of the austenitic alloy material is one example, and an austenitic alloy material satisfying Features 1 and 2 may be manufactured by other methods. The austenitic alloy material of this embodiment will be described in more detail below with reference to examples.

[0127] Alloy materials having the chemical compositions shown in Tables 1A, 1B and 1C were produced.

[0128]

[0129]

[0130]

[0131] Specifically, the molten metal of each test number was produced, and a bloom was produced by continuous casting. In the continuous casting, the holding time t (minutes) at which the molten metal temperature was 1600 to 1500°C in the tundish and the vertical distance D (mm) from the liquid surface of the molten metal in the mold to the center of the discharge port on the outer surface of the submerged entry nozzle were as shown in the "Holding time t (minutes)" and "Distance D (mm)" columns in Table 2. The upward inclination angle θ of the discharge port of the submerged entry nozzle (see Figure 1) was 15°.

[0132]

[0133] The produced blooms were subjected to blooming to produce round billets. The bloom heating temperature during blooming was 1250°C. A through-hole was formed along the central axis of the round billet by machining. The round billets heated to 1250°C were then hot extruded by the Eugène-Séjournet method to produce mother pipes (seamless alloy pipes) with an outer diameter of 170.0 mm and a wall thickness of 14.5 mm. The mother pipes were then subjected to solution treatment. The solution treatment temperatures (°C) were as shown in the "Solution Treatment Temperature (°C)" column in Table 2. The holding time at the solution treatment temperature was 20 minutes for all test numbers. After the holding time, the mother pipes were water-cooled. The solution-treated mother pipes were then subjected to cold working (cold drawing). The cold working ratios (%) of the cold working were as shown in the "Cold Working Ratio (%)" column in Table 2. By the above steps, austenitic alloy materials (seamless alloy pipes) of each test number were produced. Note that each austenitic alloy material of each test number had a microstructure consisting of austenite.

[0134] [Evaluation Tests] The following evaluation tests were carried out on the austenitic alloy materials manufactured with each test number. (Test 1) Number density ND1 (number / mm ２ ), the number density of coarse low-S inclusions ND2 (pieces / mm ２ ) and the number density ND3 of high-S inclusions (number / mm ２ ) Measurement test (Test 2) Yield strength measurement test (Test 3) SCC resistance evaluation test Each test will be explained below.

[0135] [(Test 1) Number density ND1 of fine low-S inclusions (number / mm ２ ), the number density of coarse low-S inclusions ND2 (pieces / mm ２ ) and the number density ND3 of high-S inclusions (number / mm ２ Measurement test of the number density ND1 (number / mm ２ ), the number density of coarse low-S inclusions ND2 (pieces / mm ２ ) and the number density ND3 of high-S inclusions (number / mm ２ Based on the method described in [Method for measuring the number density ND1 (number / mm ２ ), the number density of coarse low-S inclusions ND2 (pieces / mm ２ ), and the number density ND3 of high S inclusions (pieces / mm ２ The observation field was set to five fields of 10 mm × 10 mm, and the total area of ​​the observation fields was 500 mm ２ The measured number densities ND1 to ND3 (pieces / mm ２ ) in Table 3, "Number density ND1 (pieces / mm ２ ) and "Number density ND2 (pieces / mm ２ ) and "Number density ND3 (pieces / mm ２ ) are shown below.

[0136]

[0137] [(Test 2) Yield Strength Measurement Test] The yield strength (MPa) of the alloy material of each test number was determined based on the method described in the above-mentioned [Method for measuring yield strength]. Furthermore, a round bar-shaped test piece with a parallel part diameter of 6 mm and a gauge length of 30 mm was taken from the alloy material of each test number as a tensile test piece. The obtained results are shown in the "Yield strength (MPa)" column in Table 3.

[0138] [(Test 3) SCC Resistance Evaluation Test] The SCC resistance of the austenitic alloy material of each test number was evaluated based on the method described in the above-mentioned [Method for Evaluating SCC Resistance]. When the obtained reduction of area was 60.0% or more and no secondary cracks were observed in either of the two test pieces, the "SSRT Test" column in Table 3 is marked with "pass." On the other hand, when the obtained reduction of area was less than 60.0% or when a secondary crack was observed in either of the two test pieces, the "SSRT Test" column in Table 3 is marked with "fail." When the obtained reduction of area was 60.0% or more and no secondary cracks were observed in either of the two test pieces, the test piece was evaluated as having excellent SCC resistance.

[0139] [Evaluation Results] Referring to Tables 1A, 1B, 1C, 2, and 3, the austenitic alloy materials of test numbers 1 to 21 satisfied characteristics 1 and 2. Therefore, excellent SCC resistance was obtained. The yield strengths of the austenitic alloy materials of these test numbers were all 758 to 1034 MPa.

[0140] On the other hand, in test numbers 22 and 23, the holding time t at the molten metal temperature of 1600 to 1500°C in the tundish during continuous casting in the material preparation process was too short. As a result, the number density ND2 of coarse low-S inclusions was 2.0 pieces / mm ２ As a result, excellent SCC resistance was not obtained.

[0141] In test numbers 24 and 25, the holding time t at the molten metal temperature of 1600 to 1500°C in the continuous casting tundish in the material preparation process was too long. As a result, the number density ND1 of fine low-S inclusions was 3.0 pieces / mm ２ Furthermore, the number density ND3 of high S inclusions is less than 2.0 pieces / mm ２ As a result, excellent SCC resistance was not obtained.

[0142] In test numbers 26 and 27, the distance D from the surface of the molten metal in the mold to the center of the outlet on the outer surface of the submerged entry nozzle during continuous casting in the material preparation process was too short. As a result, the number density ND1 of fine low-S inclusions was 3.0 pieces / mm ２Furthermore, the number density ND3 of high S inclusions is less than 2.0 pieces / mm ２ As a result, excellent SCC resistance was not obtained.

[0143] In test numbers 28 and 29, the distance D was too short in the continuous casting of the material preparation process. As a result, the number density ND3 of high-S inclusions was 2.0 pieces / mm ２ As a result, excellent SCC resistance was not obtained.

[0144] In test numbers 30 and 31, the distance D was too long in the continuous casting of the material preparation process. As a result, the number density ND2 of the coarse low-S inclusions was 2.0 pieces / mm ２ As a result, excellent SCC resistance was not obtained.

[0145] 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. An austenitic alloy material having a chemical composition, in mass%, of C: 0.030% or less, Si: 0.10 to 1.00%, Mn: 0.01 to 2.00%, P: 0.040% or less, S: 0.0050% or less, Cr: 19.0 to 28.0%, Ni: 28.0 to 37.0%, Mo: 2.00 to 6.00%, Cu: 0.01 to 2.00%, Nb: 0.001 to 0.100%, V: 0.01 to 0.50%, Co: 0.01 to 2.00%, Sn: 0.001 to 0.100%, Al: 0.010 to 0.500%, Ca: 0.0001 to 0.0100%, N: 0.001 to 0.350%, O: 0.0100% or less, Ta: 0 to 0.100%, Ti: 0 to 0.400%, Zr: 0 to 0.100%, W: 0 to 0.300%, Zn: 0 to 0.010%, Sb: 0 to 0.1000%, As: 0 to 0.050%, Pb: 0 to 0.0100%, B: 0 to 0.0200%, Mg: 0 to 0.020%, Rare earth elements: 0 to 0.200%, and the balance being Fe and impurities, wherein in the austenitic alloy material, The steel contains S, has an Fn1 defined by the formula (1) of 5.0 or more, and has a number density ND1 of 3.0 pieces / mm ２ or more, containing S, the Fn1 being 5.0 or more, and the number density ND2 of coarse low-S inclusions, which are inclusions having an equivalent circle diameter of more than 10.0 μm, is 2.0 pieces / mm ２ or less, and the number density ND3 of high-S inclusions, which contain S, have Fn1 of less than 5.0, and have an equivalent circle diameter of 2.0 μm or more, is 2.0 pieces / mm ２ An austenitic alloy material is as follows: Fn1=(Mg+Al+Ca+Ti+V+Mn) / S (1) where the element symbol in formula (1) is substituted with the content of the element in the corresponding inclusion in mass %. When an element is not contained, "0" is substituted for the corresponding element symbol.

2. An austenitic alloy material according to claim 1, wherein the chemical composition contains one or more elements selected from the group consisting of Ta: 0.001 to 0.100%, Ti: 0.001 to 0.400%, Zr: 0.001 to 0.100%, W: 0.001 to 0.300%, Zn: 0.001 to 0.010%, Sb: 0.0001 to 0.1000%, As: 0.001 to 0.050%, Pb: 0.0001 to 0.0100%, B: 0.0001 to 0.0200%, Mg: 0.001 to 0.020%, and rare earth elements: 0.001 to 0.200%.

3. An austenitic alloy material according to claim 1 or 2, wherein the austenitic alloy material is an alloy pipe.

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