Austenitic alloy

Abstract

The present invention provides an austenitic alloy material with excellent resistance to SCC (Steel Chloride Crushing). The austenitic alloy material of this disclosure contains, by mass%, Cr: 19.0-28.0%, Ni: 28.0-37.0%, and Mo: 2.00-6.00%, with an Fn1 defined by formula (1) of 5.0 or higher, and a number density ND1 of fine low-S inclusions with an equivalent circular diameter of 2.0-10.0 μm of 3.0 inclusions / mm². 2 The above conditions are met, and the number density ND2 of coarse, low-S inclusions, which are inclusions with an Fn1 of 5.0 or higher and an equivalent circular diameter greater than 10.0 μm, is 2.0 inclusions / mm³. 2 The following conditions apply to high-sulfide inclusions: Fn1 is less than 5.0, and the equivalent circular diameter is 2.0 μm or larger; the number density ND3 is 2.0 inclusions / mm². 2 The following applies: Fn1=(Mg+Al+Ca+Ti+V+Mn) / S (1) Here, the elemental symbols in equation (1) are substituted with the mass percentage of the elemental content in the corresponding inclusion.

Description

Technical Field

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

Background Art

[0002] Many oil wells and gas wells (hereinafter, oil wells and gas wells are collectively referred to as "oil wells") are sour environments containing highly corrosive hydrogen sulfide. The sour environment 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 (Stress Corrosion Cracking resistance: hereinafter referred to as SCC resistance).

[0003] Examples of materials with high SCC resistance include 18-8 series stainless steel materials typified by SUS304H, SUS316H, SUS321H, SUS347H, etc., and austenitic alloy materials typified by Alloy800H defined as NCF800H in JIS G 4902:2019. Austenitic alloy materials have excellent SCC resistance compared to 18-8 series stainless steel materials. Austenitic alloy materials are also more economical compared to Ni-based alloy materials typified by Alloy617. Therefore, austenitic alloy materials are suitable as alloy materials used in sour environments.

[0004] International Publication No. 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 percent of: C: 0.004~0.030%, Si: 1.00% or less, Mn: 0.30~2.00%, P: 0.030% or less, S: 0.0020% or less, Al: 0.001~0.100%, Cu: 0.50~1.50%, Ni: 25.00~55.00%, Cr: 20.00~30.00%, Mo: The composition is 2.00-10.00%, N:0.005-0.100%, Ti:0-0.800%, W:0-0.30%, Nb:0-0.050%, Ca:0-0.0100%, Mg:0-0.0100%, Nd:0-0.050%, with the remainder being Fe and impurities, the austenite grain size number being 2.0-7.0, and the mixed grain ratio being 5% or less. Patent Document 1 improves SCC resistance by suppressing the mixed grain ratio. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] International Publication No. 2018 / 225869 [Overview of the project] [Problems that the invention aims to solve]

[0007] The austenitic alloy material disclosed in Patent Document 1 also exhibits improved SCC resistance. However, excellent SCC resistance in a sour environment may be obtained by other means.

[0008] The purpose of this disclosure is to provide an austenitic alloy material having excellent resistance to SCC (Steel Chloride Crushing). [Means for solving the problem]

[0009] The austenitic alloy material disclosed herein has a chemical composition in mass percent of: C: 0.030% or less, Si: 0.10~1.00%, Mn: 0.01~2.00%, P: 0.040% or less, S: 0.0050% or less, Cr: 19.0~28.0%, Ni: 28.0~37.0%, Mo: 2.00~6.00%, Cu: 0.01~2.00%, Nb: 0.001~0.100%, V: 0.01~0.50%, Co: 0.01~2.00%, Sn: 0.001~0.100%, Al: 0.010~0.500%, Ca: 0.0001~0.0100%, N: 0.001~0.350%, O The alloy consists of: 0.0100% or less of each element, 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 remainder being Fe and impurities. In this austenitic alloy, the number density ND1 of fine low-S inclusions, which contain S, have an Fn1 defined by formula (1) of 5.0 or more, and have an equivalent circular diameter of 2.0-10.0 μm, is 3.0 inclusions / mm³. 2 The above conditions are met, and the number density ND2 of coarse, low-S inclusions containing S, with an Fn1 of 5.0 or higher and an equivalent circular diameter greater than 10.0 μm is 2.0 inclusions / mm². 2 The following conditions apply to high-sulfide inclusions: they contain sulfur, have an Fn1 of less than 5.0, and have an equivalent circular diameter of 2.0 μm or more; their number density ND3 is 2.0 inclusions / mm³. 2 The following applies: Fn1=(Mg+Al+Ca+Ti+V+Mn) / S (1) Here, the elemental symbols in equation (1) are substituted with the mass percentage of the element's content in the corresponding inclusion. If no element is present, "0" is substituted for the corresponding elemental symbol. [Effects of the Invention]

[0010] The austenitic alloy material disclosed herein has excellent resistance to SCC (Steel Chloride Crust Crash). [Brief explanation of the drawing]

[0011] [Figure 1] Figure 1 is a schematic diagram illustrating the positional relationship between the molten metal and the immersion nozzle in the mold during continuous casting. [Modes for carrying out the invention]

[0012] The inventors of this invention investigated austenitic alloy materials that exhibit excellent resistance to SCC even in highly corrosive sour environments.

[0013] The inventors first investigated austenitic alloy materials that have excellent resistance to SCC in a sour environment from the perspective of chemical composition. As a result, in mass%, C: 0.030% or less, Si: 0.10~1.00%, Mn: 0.01~2.00%, P: 0.040% or less, S: 0.0050% or less, Cr: 19.0~28.0%, Ni: 28.0~37.0%, Mo: 2.00~6.00%, Cu: 0.01~2.00%, Nb: 0.001~0.100%, V: 0.01~0.50%, Co: 0.01~2.00%, Sn: 0.001~0.100%, Al: 0.010~0.500%, Ca: 0.0001~0.0100%, N: 0.001~0.3 The inventors believe that an austenitic alloy material having a chemical composition of 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 remainder being Fe and impurities, may have excellent resistance to SCC in a sour environment.

[0014] The inventors further investigated methods for obtaining excellent resistance to SCC in a sour environment from the perspective of microstructure.

[0015] The inventors first investigated the mechanism of SCC (Scalding Critical Carbide) formation in austenitic alloy materials under a sour environment. As a result, the following was discovered.

[0016] In a sour environment, some of the inclusions present on the surface of austenitic alloy materials may dissolve, forming depressions on the alloy surface. These depressions are thought to promote the occurrence of SCC (Scalding Crusted Carbide) in a sour environment.

[0017] In a sour environment, among the surface inclusions of austenitic alloys, those with a high sulfur content, particularly Mn sulfide or Ca sulfide, tend to dissolve preferentially. In the following explanation, inclusions with a high sulfur content will be referred to as "high-sulfur inclusions." A specific definition of high-sulfur inclusions will be given later.

[0018] High-sulfide (S) inclusions on the surface of austenitic alloys dissolve in a sour environment, forming depressions. These depressions, formed from S&Cs (Scaling Crushed Cells), promote the occurrence of S&Cs in a sour environment. Therefore, reducing the number density of S&Cs in austenitic alloys can suppress the occurrence of S&Cs in a sour environment.

[0019] To reduce the number density of high-sulfur inclusions, it is effective to minimize the sulfur content in austenitic alloy materials. Therefore, the inventors considered improving SCC resistance by minimizing the sulfur content in austenitic alloy materials. However, there are limits to how much the sulfur content in austenitic alloy materials can be reduced. Furthermore, excessive reduction of the sulfur content significantly increases manufacturing costs.

[0020] Therefore, the inventors considered reducing the number density of high-sulfur inclusions by means other than reducing the sulfur content as much as possible.

[0021] Here, the inventors focused on composite inclusions from a different perspective than conventional methods. A composite inclusion is an inclusion formed by the aggregation of multiple types of inclusions. Examples of multiple types of inclusions include oxides represented by Al2O3 and MgO, sulfides represented by MnS and CaS, and nitrides represented by TiN. Note that TiN may also form a solid solution of V. Other inclusions other than oxides, sulfides, and nitrides may also combine with oxides, sulfides, nitrides, etc., to form a composite inclusion.

[0022] The sulfur (S) content in composite inclusions is lower compared to high-S inclusions. Therefore, in a sour environment, composite inclusions are less likely to dissolve than high-S inclusions. Furthermore, even if composite inclusions do dissolve, only the portion containing S dissolves locally. As a result, the resulting depressions are very small. Such extremely small depressions are easily repassivated and do not become the starting point for SCC (scaling cell crash). Therefore, by incorporating S as a composite inclusion, even if dissolution occurs, the depressions can be kept small, increasing SCC resistance. Furthermore, increasing the number density of composite inclusions allows S to be incorporated into the composite inclusions. As a result, the amount of S available for the formation of high-S inclusions is reduced. Consequently, the number density of high-S inclusions is reduced.

[0023] In inclusions, if Fn1, as defined by formula (1), is 5.0 or higher, the sulfur content in the inclusion is sufficiently low. Such inclusions are considered to be composite inclusions. In the following explanation, inclusions containing sulfur and with an Fn1 of 5.0 or higher will be referred to as "low-sulfur inclusions." Fn1=(Mg+Al+Ca+Ti+V+Mn) / S (1) Here, the elemental symbols in equation (1) are substituted with the mass percentage of the element's content in the corresponding inclusion. If no element is present, "0" is substituted for the corresponding elemental symbol.

[0024] As described above, in austenitic alloy materials, increasing the number density of low-sulfur inclusions can minimize the depressions caused by melting, thereby improving SCC resistance. Furthermore, increasing the number density of low-sulfur inclusions leads to more sulfur being incorporated into them. As a result, the amount of sulfur available for the formation of high-sulfur inclusions is reduced. Consequently, the number density of high-sulfur inclusions can be sufficiently reduced, further improving SCC resistance.

[0025] Based on the above considerations, the inventors attempted to increase the number density of low-sulfur inclusions in austenitic alloy materials. As a result, they found that increasing the number density of low-sulfur inclusions in austenitic alloy materials can sufficiently reduce the number density of high-sulfur inclusions.

[0026] However, even when the number density of low-sulfur inclusions was increased and the number density of high-sulfur inclusions was decreased, sufficient SCC resistance in highly corrosive sour environments was still not obtained in some cases. Therefore, the inventors conducted further investigations. As a result, the following was found.

[0027] In a sour environment, even low-sulfide (S) inclusions are prone to dissolution if they are coarse with an equivalent circle diameter exceeding 10.0 μm. Therefore, by increasing the number density of fine S inclusions with an equivalent circle diameter of 10.0 μm or less, reducing the number density of high-sulfide (S) inclusions, and further reducing the number density of coarse S-inclusions with an equivalent circle diameter exceeding 10.0 μm, the SCC resistance of austenitic alloy materials in a sour environment can be improved.

[0028] Based on the above findings, the inventors conducted further investigations. As a result, in an austenitic alloy material having the above-mentioned chemical composition, the number density ND1 of fine low-S inclusions, which are inclusions containing S, with an Fn1 defined by formula (1) of 5.0 or more and an equivalent circular diameter of 2.0 to 10.0 μm, was found to be 3.0 inclusions / mm². 2 The above conditions are met, and the number density ND2 of coarse, low-S inclusions containing S, with an Fn1 of 5.0 or higher and an equivalent circular diameter greater than 10.0 μm is 2.0 inclusions / mm². 2The following is the number density ND3 of high-S inclusions that contain S, have a Fn1 of less than 5.0, and have an equivalent circle diameter of 2.0 μm or more is 2.0 per mm 2 The inventors have found that excellent SCC resistance can be obtained even in a sour environment if the following conditions are met.

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

[0030] The austenitic alloy material of the first form has a chemical composition in mass%, C: 0.030% or less, Si: 0.10 - 1.00%, Mn: 0.01 - 2.00%, P: 0.040% or less, S: 0.0050% or less, Cr: 19.0 - 28.0%, Ni: 28.0 - 37.0%, Mo: 2.00 - 6.00%, Cu: 0.01 - 2.00%, Nb: 0.001 - 0.100%, V: 0.01 - 0.50%, Co: 0.01 - 2.00%, Sn: 0.001 - 0.100%, Al: 0.010 - 0.500%, Ca: 0.0001 - 0.0100%, N: 0.001 - 0.350%, 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 consists of Fe and impurities. In the austenitic alloy material, it contains S, the Fn1 defined by formula (1) is 5.0 or more, and the number density ND1 of fine low-S inclusions that are inclusions with an equivalent circle diameter of 2.0 - 10.0 μm is 3.0 per mm 2 The above, contains S, has a Fn1 of 5.0 or more, and the number density ND2 of coarse low-S inclusions that are inclusions with an equivalent circle diameter exceeding 10.0 μm is 2.0 per mm 2 The following is the number density ND3 of high-S inclusions that contain S, have a Fn1 of less than 5.0, and have an equivalent circle diameter of 2.0 μm or more is 2.0 per mm 2 The following. Fn1 = (Mg + Al + Ca + Ti + V + Mn) / S (1) Here, the elemental symbols in equation (1) are substituted with the mass percentage of the element's content in the corresponding inclusion. If no element is present, "0" is substituted for the corresponding elemental symbol.

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

[0032] The third form of austenitic alloy material is an austenitic alloy material of the first or second form, which is an alloy tube.

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

[0034] [Characteristics of the austenitic alloy material of this embodiment] The austenitic alloy material of this embodiment satisfies the following features 1 and 2. (Feature 1) The chemical composition, in mass%, is as follows: C: 0.030% or less, Si: 0.10-1.00%, Mn: 0.01-2.00%, P: 0.040% or less, S: 0.0050% or less, Cr: 19.0-28.0%, Ni: 28.0-37.0%, Mo: 2.00-6.00%, Cu: 0.01-2.00%, Nb: 0.001-0.100%, V: 0.01-0.50%, Co: 0.01-2.00%, Sn: 0.001-0.100%, Al: 0.010-0.50%. The composition is 0%, Ca: 0.0001~0.0100%, N: 0.001~0.350%, 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 remainder consists of Fe and impurities.

[0035] (Feature 2) In an austenitic alloy material, the number density ND1 of fine low-S inclusions, which contain S, have an Fn1 of 5.0 or higher as defined by formula (1), and have an equivalent circular diameter of 2.0 to 10.0 μm, is 3.0 inclusions / mm³. 2 The above conditions are met, and the number density ND2 of coarse, low-S inclusions containing S, with an Fn1 of 5.0 or higher and an equivalent circular diameter greater than 10.0 μm is 2.0 inclusions / mm². 2 The following conditions apply to high-sulfide inclusions: they contain sulfur, have an Fn1 of less than 5.0, and have an equivalent circular diameter of 2.0 μm or more; their number density ND3 is 2.0 inclusions / mm³. 2 The following applies: Fn1=(Mg+Al+Ca+Ti+V+Mn) / S (1) Here, the elemental symbols in equation (1) are substituted with the mass percentage of the element's content in the corresponding inclusion. If no element is present, "0" is substituted for the corresponding elemental symbol. Features 1 and 2 are described below.

[0036] [(Feature 1) Regarding 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%, chromium carbides will form at the grain boundaries, even if the content of other elements is within the range of this embodiment. Chromium carbides increase the susceptibility to cracking at the grain boundaries. As a result, the corrosion resistance of the alloy material decreases. Therefore, the C content is 0.030% or less. A low carbon content is preferable. However, excessive reduction of the carbon content significantly increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit for the carbon content is 0.001%, and more preferably 0.003%. The preferred upper limit for the C content is 0.028%, more preferably 0.025%, even more preferably 0.020%, and even more preferably 0.015%.

[0038] Si: 0.10~1.00% Silicon (Si) deoxidizes alloys. If the Si content is less than 0.10%, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Si content exceeds 1.00%, the hot workability of the alloy material will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Si content is 0.10 to 1.00%. The preferred lower limit of the Si content is 0.11%, more preferably 0.12%, and even more preferably 0.20%. The preferred upper limit for the Si content is 0.95%, more preferably 0.90%, even more preferably 0.85%, and even more preferably 0.80%.

[0039] Mn: 0.01~2.00% Manganese (Mn) deoxidizes and desulfurizes alloys. If the Mn content is less than 0.01%, the above effects cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Mn content exceeds 2.00%, the hot workability of the alloy material will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Mn content is 0.01-2.00%. The 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%. The preferred upper limit for the Mn content is 1.95%, more preferably 1.80%, more preferably 1.60%, more preferably 1.50%, and 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 will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the P content is 0.040% or less. A low phosphorus (P) content is preferable. 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 for 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 will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the sulfur content is 0.0050% or less. A low sulfur (S) content is preferable. 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 for the S content is 0.0040%, more preferably 0.0035%, even more preferably 0.0030%, and even more preferably 0.0025%.

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

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

[0044] Mo: 2.00~6.00% Molybdenum (Mo) contributes to stabilizing the corrosion-protective film and enhances the corrosion resistance of the alloy material. Furthermore, Mo increases the strength of the alloy material through solid solution strengthening. If the Mo content is less than 2.00%, the above effects cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Mo content exceeds 6.00%, the hot workability of the alloy material will decrease, even if the content of other elements is within the range of this embodiment. In this case, the manufacturing cost will also increase 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%, more preferably 2.15%, more preferably 2.20%, more preferably 2.40%, more preferably 2.50%, and more preferably 2.70%. The preferred upper limit for the Mo content is 5.90%, more preferably 5.80%, more preferably 5.70%, more preferably 5.60%, more preferably 5.50%, more preferably 5.00%, and more preferably 4.50%.

[0045] Cu: 0.01~2.00% Copper (Cu) contributes to stabilizing 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 obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Cu content exceeds 2.00%, the hot workability of the alloy material will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Cu content is 0.01-2.00%. The preferred lower limit of the Cu content is 0.02%, more preferably 0.05%, more preferably 0.10%, more preferably 0.15%, and still more preferably 0.20%. The preferred upper limit for the Cu content is 1.95%, more preferably 1.90%, more preferably 1.80%, more preferably 1.70%, and more preferably 1.50%.

[0046] Nb: 0.001~0.100% Niobium (Nb) forms carbonitrides and other compounds with carbon and nitrogen, thereby increasing the strength of the alloy material. If the Nb content is less than 0.001%, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Nb content exceeds 0.100%, even if the content of other elements is within the range of this embodiment, carbonitrides and the like will be formed in excess, reducing the ductility of the alloy material. Therefore, the Nb content is between 0.001% and 0.100%. The preferred lower limit of the Nb content is 0.002%, more preferably 0.003%, more preferably 0.005%, more preferably 0.008%, and more preferably 0.010%. The preferred upper limit for the Nb content is 0.095%, more preferably 0.085%, even more preferably 0.080%, and even more preferably 0.075%.

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

[0048] Co: 0.01~2.00% Cobalt (Co) stabilizes austenite in the alloy material. If the Co content is less than 0.01%, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Co content exceeds 2.00%, the manufacturing cost will increase significantly, even if the content of other elements is within the range of this embodiment. Therefore, the Co content is 0.01-2.00%. The preferred lower limit of the Co content is 0.02%, more preferably 0.03%, more preferably 0.05%, more preferably 0.10%, more preferably 0.15%, and more preferably 0.20%. The preferred upper limit for the Co content is 1.95%, more preferably 1.90%, more preferably 1.80%, more preferably 1.70%, more preferably 1.60%, and more preferably 1.50%.

[0049] Sn: 0.001~0.100% Tin (Sn) enhances the corrosion resistance of alloy materials. If the Sn content is less than 0.001%, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Sn content exceeds 0.100%, the hot workability of the alloy material will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Sn content is between 0.001% and 0.100%. The preferred lower limit for 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 for the Sn content is 0.095%, more preferably 0.090%, more preferably 0.080%, more preferably 0.075%, and more preferably 0.070%.

[0050] Al: 0.010~0.500% Aluminum (Al) deoxidizes the alloy. Al further generates oxides to fix oxygen, improving the hot workability of the alloy material. Al further 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 obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Al content exceeds 0.500%, excessive Al oxide is produced. As a result, even if the content of other elements is within the range of this embodiment, the hot workability of the alloy material actually decreases. Therefore, the Al content is between 0.010% and 0.500%. The preferred lower limit of the Al content is 0.015%, more preferably 0.020%, more preferably 0.030%, more preferably 0.040%, and more preferably 0.050%. The preferred upper limit for the Al content is 0.475%, more preferably 0.460%, even more preferably 0.450%, and even more preferably 0.400%. In this specification, the Al content refers to the content of "acid-soluble Al," that is, sol.Al.

[0051] Ca: 0.0001~0.0100% Calcium (Ca) neutralizes sulfur 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 obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Ca content exceeds 0.0100%, coarse oxides are formed in the alloy material. Therefore, even if the content of other elements is within the range of this embodiment, the hot workability of the alloy material will actually decrease. Therefore, the Ca content is between 0.0001% and 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 preferred upper limit for the Ca content is 0.0095%, more preferably 0.0090%, and even more preferably 0.0080%.

[0052] N: 0.001~0.350% Nitrogen (N) enhances the strength of alloy materials through solid solution strengthening. If the N content is less than 0.001%, the above effect cannot be fully obtained, even if the content of other elements is within the range 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 content of other elements is within the range of this embodiment. Therefore, the N content is between 0.001% and 0.350%. The preferred lower limit of the N content is 0.005%, more preferably 0.010%, more preferably 0.015%, more preferably 0.020%, and more preferably 0.030%. The preferred upper limit for the N content is 0.340%, more preferably 0.320%, more preferably 0.300%, more preferably 0.270%, and 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 content of other elements is within the range of this embodiment, the hot workability of the alloy material will decrease. In this case, the corrosion resistance of the alloy material will also decrease. Therefore, the O content is 0.0100% or less. A low oxygen content is preferable. However, excessive reduction of the oxygen content significantly increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the oxygen content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The preferred upper limit for the O content is 0.0090%, more preferably 0.0085%, and even more preferably 0.0080%.

[0054] The remainder of the chemical composition of the austenitic alloy material in this embodiment consists of Fe and impurities. Here, impurities refer to substances that are introduced during the industrial production of the austenitic alloy material from the raw materials such as ore, scrap, or the manufacturing environment, and are acceptable within a range that does not adversely affect the effects of the austenitic alloy material in this embodiment.

[0055] [Regarding arbitrary elements] The chemical composition of the alloy material in 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 arbitrary elements. These arbitrary elements will be described below.

[0056] [Regarding Ta, Ti, and Zr] The chemical composition of the austenitic alloy material in this embodiment may include one or more elements selected from the group consisting of Ta, Ti, and Zr in place of some of the Fe. All of these elements enhance the strength of the alloy material. Ta, Ti, and Zr will be described below.

[0057] Ta: 0~0.100% Tantalum (Ta) is an optional element and does not need to be included. In other words, the Ta content may be 0%. If Ta is present, that is, if the Ta content is greater than 0%, Ta forms precipitates, increasing the strength of the alloy material. Even if only a small amount of Ta is present, 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 content of other elements is within the range of this embodiment. In this case, the toughness of the alloy material decreases. Therefore, the Ta content is between 0 and 0.100%. The preferred lower limit for the Ta content is 0.001%, more preferably 0.002%, and even more preferably 0.005%. The preferred upper limit for the Ta content is 0.096%, more preferably 0.090%, and even more preferably 0.080%.

[0058] Ti: 0~0.400% Titanium (Ti) is an optional element and does not need to be included. In other words, the Ti content may be 0%. If Ti is present, that is, if the Ti content is greater than 0%, Ti forms precipitates, increasing the strength of the alloy material. Even if only a small amount of Ti is present, the above effect can be obtained to some extent. However, if the Ti content exceeds 0.400%, the strength of the alloy material becomes too high, even if the content of other elements is within the range of this embodiment. In this case, the toughness of the alloy material decreases. Therefore, the Ti content is 0-0.400%. 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 for the Ti content is 0.390%, more preferably 0.383%, and even more preferably 0.350%.

[0059] Zr: 0~0.100% Zirconium (Zr) is an optional element and does not need to be included. In other words, the Zr content may be 0%. If Zr is present, that is, if the Zr content is greater than 0%, Zr forms precipitates, increasing the strength of the alloy material. Even if only a small amount of Zr is present, 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 content of other elements is within the range of this embodiment. In this case, the toughness of the alloy material decreases. Therefore, the Zr content is between 0 and 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 for the Zr content is 0.097%, 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 in this embodiment may contain one or more elements selected from the group consisting of W, Zn, Sb, As, and Pb in place of a portion of Fe. All of these elements enhance the SCC resistance of the alloy material. W, Zn, Sb, As, and Pb will be described below.

[0061] W: 0~0.300% Tungsten (W) is an optional element and does not need to be included. In other words, the W content may be 0%. When present, i.e., when the W content is greater than 0%, W enhances the SCC resistance of the alloy material. Even a small amount of W content will provide some degree of the above effect. However, if the W content exceeds 0.300%, the strength of the alloy material becomes too high, even if the content of other elements is within the range of this embodiment. In this case, the toughness of the alloy material decreases. Therefore, the W content is between 0 and 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 for the W content is 0.297%, more preferably 0.250%, and even more preferably 0.150%.

[0062] Zn: 0~0.010% Zinc (Zn) is an optional element and does not need to be included. In other words, the Zn content may be 0%. When Zn is present, i.e., when the Zn content is greater than 0%, Zn enhances the SCC resistance of the alloy material. Even a small amount of Zn can provide some degree of the above effect. However, if the Zn content exceeds 0.010%, the SCC resistance of the alloy material may actually decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Zn content is 0-0.010%. The preferred lower limit for the Zn content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. The preferred upper limit for the Zn content is 0.009%, more preferably 0.008%, and even more preferably 0.007%.

[0063] Sb: 0~0.1000% Antimony (Sb) is an optional element and does not need to be included. In other words, the Sb content may be 0%. When present, i.e., when the Sb content is greater than 0%, Sb enhances the SCC resistance of the alloy material. Even a small amount of Sb content will provide some degree of the above effect. However, if the Sb content exceeds 0.1000%, the manufacturing cost will increase, even if the content of other elements is within the range of this embodiment. Therefore, the Sb content is between 0 and 0.1000%. The preferred lower limit of the Sb content is 0.0001%, more preferably 0.0002%, and even more preferably 0.0010%. The preferred upper limit for the Sb content is 0.0950%, more preferably 0.0800%, and even more preferably 0.0600%.

[0064] As: 0~0.050% Arsenic (As) is an optional element and may not be present. In other words, the As content may be 0%. When present, i.e., when the As content is greater than 0%, As enhances the SCC resistance of the alloy material. Even a small amount of As can provide some degree of the above effect. However, if the As content exceeds 0.050%, the SCC resistance of the alloy material may actually decrease, even if the content of other elements is within the range of this embodiment. Therefore, the As content is 0-0.050%. The preferred lower limit for the As content is 0.001%, more preferably 0.002%, and even more preferably 0.005%. The preferred upper limit for the As content is 0.048%, more preferably 0.040%, and even more preferably 0.035%.

[0065] Pb: 0~0.0100% Lead (Pb) is an optional element and does not need to be included. In other words, the Pb content may be 0%. When Pb is present, i.e., when the Pb content is greater than 0%, Pb enhances the SCC resistance of the alloy material. Even a small amount of Pb can provide some degree of the above effect. However, if the Pb content exceeds 0.0100%, the hot workability of the alloy material will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Pb content is 0-0.0100%. 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 for 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 in 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 enhance the hot workability of the alloy material. B, Mg, and REM will be described below.

[0067] B: 0~0.0200% Boron (B) is an optional element and does not need to be included. In other words, the B content may be 0%. When present, i.e., when the B content is greater than 0%, B suppresses the segregation of S at grain boundaries in the alloy material, thereby improving the hot workability of the alloy material. Even if only a small amount of B is present, the above effect can be obtained to some extent. However, if the B content exceeds 0.0200%, coarse B nitrides will be formed. In this case, even if the content of other elements is within the range of this embodiment, the toughness of the alloy material will decrease. Therefore, the B content is 0-0.0200%. The preferred lower limit for the B content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0007%. The preferred upper limit for the B content is 0.0196%, more preferably 0.0150%, and even more preferably 0.0100%.

[0068] Mg: 0~0.020% Magnesium (Mg) is an optional element and does not need to be included. In other words, the Mg content may be 0%. When Mg is present, that is, when the Mg content is greater than 0%, Mg neutralizes S in the alloy material by fixing it as sulfide, thereby improving the hot workability of the alloy material. Even if only a small amount of Mg is present, the above effect can be obtained to some extent. However, if the Mg content exceeds 0.020%, coarse oxides will be formed. In this case, even if the content of other elements is within the range of this embodiment, the SCC resistance of the alloy material will decrease. Therefore, the Mg content is 0-0.020%. 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 for the Mg content is 0.019%, more preferably 0.017%, and even more preferably 0.014%.

[0069] Rare earth elements (REM): 0~0.200% Rare earth elements (REMs) are optional elements and do not need to be included. In other words, the REM content may be 0%. When REM is present, i.e., when the REM content is greater than 0%, REM neutralizes sulfur in the alloy material by fixing it as sulfide, thereby improving the hot workability of the alloy material. Even a small amount of REM can provide the above effect to some extent. However, if the REM content exceeds 0.200%, the oxides in the alloy material become coarser, even if the content of other elements is within the range of this embodiment. In this case, the toughness of the alloy material decreases. Therefore, the REM content is between 0 and 0.200%. The preferred lower limit for the REM content is 0.001%, more preferably 0.005%, and even more preferably 0.010%. The preferred upper limit for 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 lanthanides from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. In this specification, REM content 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] In this embodiment, Fn1 is defined by formula (1) for S-containing inclusions in the austenitic alloy material. Fn1=(Mg+Al+Ca+Ti+V+Mn) / S (1) Here, the elemental symbols in equation (1) are substituted with the mass percentage of the element's content in the corresponding inclusion. If no element is present, "0" is substituted for the corresponding elemental symbol. Fn1 is the value obtained by rounding the second decimal place of the result to the first decimal place.

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

[0073] At this time, the number density ND1 of fine low-sulfide inclusions in the austenitic alloy material is 3.0 particles / mm³. 2 The above results indicate that the number density ND2 of coarse, low-S inclusions is 2.0 particles / mm³. 2 The following is observed: the number density ND3 of high S inclusions is 2.0 particles / mm³. 2 The following applies:

[0074] Furthermore, in the case of inclusions containing sulfur and having an equivalent circular diameter of less than 2.0 μm, even if they dissolve and form depressions, these depressions quickly repassivate. Therefore, in austenitic alloy materials having the chemical composition of Feature 1, inclusions containing sulfur and having an equivalent circular diameter of less than 2.0 μm do not affect SCC resistance.

[0075] As described above, in the austenitic alloy material of this embodiment, instead of drastically reducing the sulfur content in the chemical composition, a certain amount of sulfur content is allowed, and the number density ND1 of fine, low-sulfur inclusions, which are composite inclusions, is increased, thereby reducing the sulfur content available for the formation of high-sulfur inclusions. This makes it possible to reduce the number density ND3 of high-sulfur inclusions that are easily dissolved in a sour environment.

[0076] Furthermore, among low-sulfur inclusions with an Fn1 of 5.0 or higher, coarse low-sulfur inclusions are coarse even with low sulfur content. Therefore, coarse low-sulfur inclusions tend to dissolve in highly corrosive sour environments, forming large depressions. Such coarse depressions are difficult to repassivate and promote the occurrence of SCC. Therefore, the number density ND1 of fine low-sulfur inclusions is increased, the number density ND3 of high-sulfur inclusions is decreased, and the number density ND2 of coarse low-sulfur inclusions is also decreased.

[0077] [Regarding the number density ND1 of fine, low-S inclusions] The number density (ND1) of fine, low-S inclusions is 3.0 particles / mm³. 2 If the value is less than 2.0, the amount of fine low-sulfide inclusions in the alloy material is insufficient. In this case, the amount of sulfur available for the formation of high-sulfide inclusions in the alloy material increases, and the number density ND3 of high-sulfide inclusions becomes 2.0 particles / mm³. 2 It exceeds this limit. As a result, sufficient resistance to SCC in a sour environment cannot be obtained. Therefore, the number density ND1 of fine low-S inclusions is 3.0 particles / mm³. 2 That concludes this section. The preferred lower limit for number density ND1 is 3.2 pieces / mm². 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 That is the case. The upper limit of the number density ND1 is not particularly limited. However, in an alloy material satisfying feature 1, the upper limit of the number density ND1 is, for example, 25.0 pieces / mm 2 For example, 20.0 pieces / mm 2 That is the case.

[0078] [Regarding the number density ND2 of coarse, low-S inclusions] The number density ND2 of coarse, low-S inclusions is 2.0 pieces / mm³. 2If the value exceeds a certain level, excessive amounts of coarse, low-sulfur inclusions are being generated. In this case, in a highly corrosive sour environment, the coarse, low-sulfur inclusions are likely to dissolve, forming large depressions. These large depressions tend to promote the occurrence of SCC (Steel Chloride Crust). Therefore, the number density ND2 of coarse, low-S inclusions is 2.0 pieces / mm³. 2 The following applies: The preferred upper limit for number density ND2 is 1.9 pieces / mm². 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 That is the case. A number density ND2 should be as low as possible. In other words, a number density ND2 of 0 pieces / mm² is desirable. 2 This is preferable. However, excessively reducing the number density ND2 may increase manufacturing costs. Therefore, a preferred lower limit for the number density ND2 is 0.1 pieces / mm². 2 And more preferably 0.2 pieces / mm 2 That is the case.

[0079] [Regarding the number density ND3 of high S inclusions] As mentioned above, even small high-sulfur inclusions readily dissolve and form depressions in highly corrosive sour environments. These depressions, even small ones, tend to promote the occurrence of SCC (Scaling Chain Corrosion). Therefore, a number density (ND3) of 2.0 high-sulfur inclusions is desirable. 2 If it exceeds this value, sufficient resistance to SCC (Sour Critical Crude Carbide) cannot be obtained in a sour environment. Therefore, the number density ND3 of high S inclusions is 2.0 inclusions / mm³. 2 The following applies: The preferred upper limit for number density ND3 is 1.9 pieces / mm². 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 That is the case. A number density ND3 should be as low as possible. In other words, a number density ND3 of 0 pieces / mm² is desirable. 2 This is preferable. However, excessively reducing the number density ND3 may increase manufacturing costs. Therefore, a preferred lower limit for the number density ND3 is 0.1 pieces / mm². 2 And more preferably 0.2 pieces / mm 2 That is the case.

[0080] [Number density of fine, low-S inclusions ND1 (pieces / mm²)] 2 ), number density of coarse, low-S inclusions ND2 (pieces / mm³) 2 ) and the number density of high S inclusions ND3 (pieces / mm³) 2 ) Measurement method] In this embodiment, the number density ND1 (pieces / mm³) of fine low-S inclusions in the austenitic alloy material is 2 ), number density of coarse, low-S inclusions ND2 (pieces / mm³) 2 ) and the number density of high S inclusions ND3 (pieces / mm³) 2 ) can be found using the following method.

[0081] Test specimens are taken from austenitic alloy materials. When the austenitic alloy material is an alloy tube, a test specimen with an observation surface including the axial direction of the tube and the thickness direction (diameter direction of the tube) is taken from the center of the wall thickness. When the austenitic alloy material is in the form of an alloy sheet, a test specimen having an observation surface that includes the rolling direction and the thickness direction is taken from the center of the sheet thickness. When an austenitic alloy material is an alloy rod with a circular cross-section perpendicular to the axial direction, a test specimen having observation surfaces including the axial and radial directions is taken from the R / 2 portion of the alloy rod. The R / 2 portion refers to the central part of the circular cross-section of the alloy rod perpendicular to the axial direction, where radius R is located.

[0082] The observation surface of the collected test specimen is polished to a mirror finish. An observation field is selected from the mirror-polished observation surface. At this time, at least three observation fields are selected, and the total area of ​​the observation fields is 500 mm². 2The following procedure is followed: Using a scanning electron microscope (SEM-EDS) equipped with compositional analysis capabilities, backscattered electron images of each observation field are obtained at 800x magnification. The obtained backscattered electron images are observed, and particles are identified based on their contrast. The positional coordinates of all identified particles within the observation field are recorded.

[0083] Based on the position coordinates of the identified particles, elemental concentration analysis (EDS analysis) is performed on those particles. Specifically, the identified particles are scanned with an electron beam based on their position coordinates, and the elemental concentration of the particles is analyzed. At this time, the entire particle is scanned with the electron beam, rather than just a part of it. This allows for the analysis of elemental concentrations throughout the entire particle. In EDS analysis, the acceleration voltage is set to 20kV, 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] Of 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 set to 100.0% by mass, particles with a Cl content exceeding 10.0%, particles with a K content exceeding 10.0%, particles with a Na content exceeding 20.0%, particles with a Ca content exceeding 75.0%, and particles with an O content exceeding 70.0% are considered to be dirt or abrasive material that adhered during mirror polishing. Therefore, these particles are judged not to be inclusions and are excluded from the analysis. Furthermore, among the identified particles, if the total content of N, O, Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, V, Mn, Cu, Zr, and Nb is set to 100.0% by mass, then particles with an S content of less than 1.0% are not S-containing inclusions, but rather oxides or nitrides. Therefore, particles with an S content of less than 1.0% are also excluded from consideration. In other words, among the identified particles, if the total content of N, O, Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, V, Mn, Cu, Zr, and Nb is set to 100.0% by mass, then particles other than those with a Cl content of more than 10.0%, a K content of more than 10.0%, a Na content of more than 20.0%, a Ca content of more than 75.0%, an O content of more than 70.0%, and an S content of less than 1.0% are classified as S-containing inclusions.

[0085] For particles identified as sulfur-containing inclusions, the equivalent circle diameter (μm) is determined. The equivalent circle diameter refers to the diameter (μm) of a circle with the same area as the particle. The equivalent circle diameter is rounded to the first decimal place by rounding the second decimal place of the obtained value.

[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 with a circle equivalent diameter of 2.0 to 10.0 μm are identified as "fine low-S inclusions" based on the mass percent content of Mg, Al, Ca, Ti, V, Mn, Cu, Zr, and S when the total content of N, O, Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, V, Mn, Cu, Zr, and Nb is set to 100.0 mass%. Fn1=(Mg+Al+Ca+Ti+V+Mn) / S (1) Here, the elemental symbols in equation (1) are substituted with the mass percentage of the element's content in the corresponding particle (inclusion). If no element is present, "0" is substituted for the corresponding elemental symbol. (B) Coarse low S inclusions Particles with an equivalent circle diameter greater than 10.0 μm and an Fn1 of 5.0 or greater, as defined by formula (1), are identified as "coarse, low-S inclusions" based on the mass percent content of Mg, Al, Ca, Ti, V, Mn, Cu, Zr, and S, assuming a total content of N, O, Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, V, Mn, Cu, Zr, and Nb of 100.0 mass%. (C) High S inclusions Particles with an equivalent circle diameter of 2.0 μm or more and an Fn1 defined by formula (1) of less than 5.0, based on the mass percent content of Mg, Al, Ca, Ti, V, Mn, Cu, Zr, and S when the total content of N, O, Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, V, Mn, Cu, Zr, and Nb is set to 100.0 mass%, are identified as "high S inclusions".

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

[0088] Based on the total number of counted microscopic low-S inclusions across all observation fields and the total area of ​​all observation fields, the number density of microscopic low-S inclusions ND1 (items / mm²) is calculated. 2 ) Based on the total number of counted coarse, low-S inclusions across all observation fields and the total area of ​​all observation fields, the number density of coarse, low-S inclusions ND2 (items / mm²) is calculated. 2 ) Based on the total number of counted high-S inclusions in all observation fields and the total area of ​​all observation fields, the number density of high-S inclusions ND3 (items / mm²) is calculated. 2 )

[0089] Furthermore, the number density of fine low-S inclusions is ND1 (pieces / mm³). 2 ), number density of coarse, low-S inclusions ND2 (pieces / mm³) 2 ), and the number density of high S inclusions ND3 (pieces / mm³). 2 The value obtained is rounded to the first decimal place, with the second decimal place of the result rounded off. Furthermore, as a SEM-EDS instrument, for example, the automated analyzer manufactured by FEI (ASPEX), product name: Metals Quality Analyzer, can be used.

[0090] [Effects of the austenitic alloy material in this embodiment] The austenitic alloy material of this embodiment satisfies features 1 and 2. Therefore, excellent resistance to SCC (Sulfur Chloride Crushing) is obtained in a sour environment.

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

[0092] Two uniaxial tensile test specimens are taken from an austenitic alloy material. The tensile test specimens are as specified in NACE TM0198 (2020), with a parallel section diameter of 3.81 mm and a distance between scoring points of 25.4 mm. When the austenitic alloy material is used in the form of an alloy tube, 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 tube. When the austenitic alloy material is in the form of an alloy sheet, the test specimen is taken from the center of the sheet thickness. In this case, the longitudinal direction of the test specimen is parallel to the rolling direction of the alloy sheet. When the austenitic alloy material is in the form of an alloy rod, a test specimen is taken from the R / 2 section. In this case, the longitudinal direction of the test specimen is parallel to the axial direction of the alloy rod.

[0093] The test specimen is placed inside the test container (autoclave). The test solution, which is a 25% by mass NaCl (sodium chloride) aqueous solution, is transferred to the test container and the test specimen is immersed in the test solution. Furthermore, the temperature inside the autoclave is set to 170°C and H2S gas at a gauge pressure of 100 psi is introduced into the autoclave. After that, 4 × 10 -6 Tensile tests are performed at a strain rate of 1 / second to determine the reduction of area at fracture (%). Tensile tests are performed on two specimens, and the arithmetic mean of the reductions of fracture of the two specimens is taken as the reduction of fracture (%) for the austenitic alloy material.

[0094] Furthermore, the constricted area of ​​the two test specimens is observed with a 10x magnification loupe to determine whether or not a crack different from the primary fracture surface (secondary crack) has occurred. If the observation with the loupe suggests the presence or absence of a secondary crack, the specimen is further observed with a 100x magnification optical microscope to confirm its presence or absence. In the SSRT test, if the reduction in area at fracture is 60.0% or more and no secondary cracks are found in the constricted area of ​​the two test specimens, the specimen is judged to have excellent SCC resistance.

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

[0096] [Method for measuring yield strength] The yield strength of the austenitic alloy material in this embodiment can be determined by the following method. Tensile tests will be conducted in accordance with ASTM E8 / E8M (2022). First, tensile test specimens will be taken from the austenitic alloy material. If the austenitic alloy material is an alloy tube, a round bar-shaped tensile test specimen or an arc-shaped tensile test specimen shall be taken from the center of the wall thickness. In this case, the longitudinal direction of the tensile test specimen shall be parallel to the axial direction of the alloy tube. If the alloy material is an alloy tube and a round bar test specimen cannot be taken from the alloy tube, an arc-shaped tensile test specimen shall be taken from the alloy tube. When the austenitic alloy material is in the form of an alloy sheet, a round bar-shaped tensile test specimen is taken from the center of the sheet thickness. In this case, the longitudinal direction of the tensile test specimen is parallel to the rolling direction of the alloy sheet. When the austenitic alloy material is in the form of an alloy rod, a round tensile test specimen is taken from the R / 2 section. In this case, the longitudinal direction of the tensile test specimen is parallel to the axial direction of the alloy rod.

[0097] For example, a round bar-shaped tensile test specimen has a parallel section diameter of 6 mm and a gauge length of 30 mm. For example, an arc-shaped tensile test specimen has a thickness equal to the total wall thickness, a width of 25.4 mm, and a gauge length of 50.8 mm. A tensile test is performed using a tensile test specimen at room temperature (24±3℃) in air. In this embodiment, the 0.2% offset proof strength 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 the first decimal place.

[0098] [Regarding the microstructure of the austenitic alloy material in this embodiment] The austenitic alloy material of this embodiment has a microstructure consisting of austenite. Here, "microstructure consisting of austenite" means that other structures besides austenite are negligibly small. Other structures besides austenite include, for example, inclusions and precipitates.

[0099] [Shape of austenitic alloy material] The shape of the austenitic alloy material in this embodiment is not particularly limited. The austenitic alloy material may be an alloy tube, an alloy plate, or an alloy rod with a circular cross-section perpendicular to the axial direction. Preferably, the alloy material according to this embodiment is an alloy tube, and more preferably a seamless alloy tube.

[0100] [Manufacturing method] An example of a method for manufacturing the austenitic alloy material of this embodiment having the above-described configuration will be explained. Note that the method for manufacturing the austenitic alloy material of this embodiment is not limited to the method described below. An example of a method for manufacturing the austenitic alloy material of this embodiment includes the following steps. (Process 1) Material preparation process (Process 2) Hot working process (Step 3) Solution treatment process (Process 4) Cold working process The following details each manufacturing process.

[0101] [(Process 1) Material preparation process] In the material preparation process, a molten metal, which is a liquid alloy having a chemical composition satisfying characteristic 1, is produced. The molten metal may be produced in an electric furnace, an Ar-O2 mixed gas bottom-blowing decarburization furnace (AOD furnace), or a vacuum decarburization furnace (VOD furnace).

[0102] Using molten metal, cast slabs, blooms, or billets are manufactured by continuous casting. In continuous casting, molten metal is first poured from a ladle into a tundish. The molten metal, temporarily stored in the tundish, is then guided to the mold through an immersion nozzle. The molten metal gradually solidifies from the cooled outer surface in the mold. The solidified portion is gradually withdrawn from the mold downwards, and the slab is further cooled by applying cooling water to the outer surface of the slab to solidify it. Through this manufacturing process, cast slabs that will become the raw material for alloys are produced.

[0103] In the material preparation process, the bloom may be further subjected to bloc rolling to form billets. In this case, for example, the bloom is heated to 1150-1300°C. After heating, bloc rolling is performed on the bloom to form billets. The above process is used to manufacture austenitic alloy materials (slabs, blooms, or billets).

[0104] In continuous casting during the material preparation process, the following conditions must be met: (Condition 1) The holding time t for the molten metal at a molten metal temperature of 1600-1500°C in the tundish is set to 5-100 minutes. (Condition 2) The vertical distance D from the molten metal surface inside the mold to the center of the discharge port on the outer surface of the immersion nozzle is set to 150 to 400 mm. Conditions 1 and 2 are explained below.

[0105] [Regarding Condition 1] Fine, low-sulfur inclusions are formed when oxides act as nuclei, and sulfides and nitrides aggregate around them. To form such fine, low-sulfur inclusions, it is effective to allow coarse oxides in the molten metal to float to the surface in the tundish, thereby suppressing their inflow into the mold.

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

[0107] On the other hand, if the holding time t exceeds 100 minutes, while coarse oxides in the molten metal float sufficiently, fine oxides also become coarser and float. As a result, the amount of fine oxides that serve as nuclei for fine low-sulfur inclusions is insufficient in the molten metal flowing from the immersion nozzle into the mold. Consequently, in the austenitic alloy material after manufacturing, the number density ND1 of fine low-sulfur inclusions becomes excessively low, while the number density ND3 of high-sulfur inclusions becomes excessive. Therefore, the holding time t is set to 5 to 100 minutes.

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

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

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

[0111] At a distance D, a stirring region DA is formed 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 into the molten metal 10 from the discharge port 22 of the immersion nozzle 20 rises, and the molten metal is stirred vertically. At this time, the temperature of the molten metal gradually decreases, and sulfides and nitrides are formed. If the molten metal flowing out from the discharge port 22 contains a sufficient amount of fine oxides, sulfides and nitrides are formed in the stirring region DA with the fine oxides as nuclei, or sulfides and nitrides generated in the molten metal collide with the fine oxides and aggregate, forming low-S inclusions with an Fn1 of 5.0 or higher. The formation of low-S inclusions suppresses the formation of high-S inclusions. In addition, 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 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-sulfide inclusions remain in the molten metal, the amount of coarse low-sulfide inclusions in the molten metal is reduced.

[0112] As described above, the stirring region DA affects the number density of fine low-sulfide inclusions, coarse low-sulfide inclusions, and high-sulfide inclusions in the austenitic alloy material after manufacturing. Furthermore, the distance D is a factor that determines the size of the stirring region DA.

[0113] If the distance D is less than 150 mm, the stirring area DA is too narrow. In this case, low-sulfide inclusions may not be generated to a sufficient extent to reduce the number density ND3 of high-sulfide inclusions. As a result, the number density ND3 of high-sulfide inclusions may become excessive, or the number density ND1 of fine low-sulfide inclusions may become excessively low.

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

[0115] Furthermore, 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, with electromagnetic stirring, it is difficult to form a stirring region DA in which the molten metal 10 is stirred vertically 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 immersion nozzle 20 to 150 to 400 mm, an appropriate stirring region DA can be formed.

[0116] [(Process 2) Hot working process] In the hot working process, the material (slab, bloom, or billet) manufactured in the material preparation process is hot-worked to produce an intermediate alloy material. In this specification, an intermediate alloy material is a raw tube if the final product is an alloy tube, a plate-shaped alloy material if the final product is an alloy plate, and a rod-shaped alloy material with a circular cross-section perpendicular to the axial direction if the final product is an alloy rod. The hot working may be hot forging, hot extrusion, or hot rolling. The method of hot working is not particularly limited and any well-known method may be used.

[0117] If the intermediate alloy material is a raw tube, hot extrusion such as the Eugène-Séjournet method or the Erhardt-Pushbench method may be performed. Alternatively, hot rolling such as perforation rolling by the Mannesmann method may 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 for example, it is 1100 to 1300°C. After performing the perforation rolling described above on the material extracted from the heating furnace, stretch rolling is performed to produce a raw tube which is the intermediate alloy material.

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

[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 for example, it is 1100 to 1300°C. Hot working is performed on the material extracted from the heating furnace to produce the rod-shaped alloy material that is the intermediate alloy material. Hot working is performed, for example, by bract rolling using a bract rolling mill or by hot rolling using a continuous rolling mill. A continuous rolling mill has alternating horizontal stands with a pair of hole-shaped rolls arranged vertically and vertical stands with a pair of hole-shaped rolls arranged horizontally.

[0120] [(Step 3) Solution treatment process] In the solution treatment process according to this embodiment, the intermediate alloy material is subjected to solution treatment. The method of solution treatment is not particularly limited and any well-known method may be used. For example, the intermediate alloy material is placed in 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) means the temperature (°C) of the heat treatment furnace used to perform the solution treatment. The time for holding at the solution treatment temperature (holding time) means the time (minutes) during which the intermediate alloy material is held at the heat treatment temperature.

[0121] The solution treatment temperature in the solution treatment process is set to 1050-1150°C. If the solution treatment temperature is 1050°C or higher, the precipitates in the intermediate alloy material will dissolve sufficiently during the solution treatment. On the other hand, even if the solution treatment temperature exceeds 1150°C, the effect will saturate.

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

[0123] [(Process 4) Cold working process] In the cold working process, the solution-treated intermediate alloy material is cold-worked to produce the alloy material. The cold working may be cold drawing or cold rolling. In the cold working process, known cold working methods may be carried out under 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 rate (%) is, for example, 5 to 40%. Here, if S0 is the area of ​​the cross-section (transverse plane) perpendicular to the longitudinal direction of the intermediate alloy material before the cold working process, and S1 is the area of ​​the cross-section (transverse plane) perpendicular to the longitudinal direction of the alloy material after the cold working process, then the cold working rate (%) is defined by the following formula. Cold working rate (%) = 100 × (1 - S1 / S0)

[0125] If the cold working rate is between 5% and 40%, the yield strength of the alloy material after manufacturing can be adjusted to a range of 758 to 1034 MPa.

[0126] The austenitic alloy material of this embodiment can be manufactured through the above process. Note that the above-described method for manufacturing the austenitic alloy material is merely an example, and austenitic alloy materials 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. [Examples]

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

[0128] [Table 1A]

[0129] [Table 1B]

[0130] [Table 1C]

[0131] Specifically, molten metal was prepared for each test number, and bloom was produced by continuous casting. In continuous casting, the holding time t (minutes) at a molten metal temperature of 1600-1500°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 immersion nozzle, were as shown in the "Holding time t (minutes)" and "Distance D (mm)" columns in Table 2. The upward inclination angle θ (see Figure 1) of the discharge port of the immersion nozzle was 15°.

[0132] [Table 2]

[0133] The manufactured bloom was subjected to bract rolling to produce round billets. The bloom was heated to 1250°C during bract rolling. A through hole was formed along the central axis of the round billet by machining. Subsequently, hot extrusion using the Eugène Séjournay method was performed on the round billet heated to 1250°C to produce a raw tube (seamless alloy tube) with an outer diameter of 170.0 mm and a wall thickness of 14.5 mm. Furthermore, the raw tube was subjected to solution treatment. The solution treatment temperature (°C) is shown in the "Solution Treatment Temperature (°C)" column of Table 2. The holding time at the solution treatment temperature was 20 minutes for all test numbers. After the holding time, the raw tube was water-cooled. Cold working (cold drawing) was performed on the raw tube after solution treatment. The cold working rate (%) is shown in the "Cold Working Rate (%)" column of Table 2. Through the above process, austenitic alloy materials (seamless alloy tubes) for each test number were manufactured. All of the austenitic alloy materials for each test number possessed a microstructure composed of austenite.

[0134] [Evaluation Test] The following evaluation tests were performed on each austenitic alloy material with each test number produced. (Test 1) Number density of fine low-S inclusions ND1 (pieces / mm³) 2 ), number density of coarse, low-S inclusions ND2 (pieces / mm³) 2 ) and the number density of high S inclusions ND3 (pieces / mm³) 2 ) Measurement test (Test 2) Yield strength measurement test (Test 3) SCC resistance evaluation test The following describes each test.

[0135] [(Test 1) Number density of fine low-S inclusions ND1 (pieces / mm²) 2 ), number density of coarse, low-S inclusions ND2 (pieces / mm³) 2 ) and the number density of high S inclusions ND3 (pieces / mm³) 2 ) Measurement test] The above-mentioned number density of fine, low-S inclusions is ND1 (pieces / mm²). 2 ), number density of coarse, low-S inclusions ND2 (pieces / mm³) 2 ) and the number density of high S inclusions ND3 (pieces / mm³)2 Based on the method described in [Measurement Method], the number density of fine low S inclusions for each test number is ND1 (pieces / mm³). 2 ), number density of coarse, low-S inclusions ND2 (pieces / mm³) 2 ), and the number density of high S inclusions ND3 (pieces / mm³). 2 The following was calculated. The observation field was set to five 10mm x 10mm fields, and the total area of ​​the observation field was 500mm². 2 The measured number density was ND1~ND3 (pieces / mm²). 2 ) refer to the "Number Density ND1 (pieces / mm²)" in Table 3. 2 )", "Number density ND2 (pieces / mm 2 )", "Number density ND3 (pieces / mm 2 )" will be shown in each of these.

[0136] [Table 3]

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

[0138] [(Test 3) SCC resistance evaluation test] Based on the method described in [Evaluation Method for SCC Resistance] above, the SCC resistance of the austenitic alloy material for each test number was evaluated. If the obtained reduction in area at fracture was 60.0% or more and no secondary cracks were observed in either of the two test specimens, "pass" was indicated in the "SSRT Test" column in Table 3. On the other hand, if the obtained reduction in area at fracture was less than 60.0% or if a secondary crack was observed in either of the two test specimens, "fail" was indicated in the "SSRT Test" column in Table 3. If the obtained reduction in area at fracture was 60.0% or more and no secondary cracks were observed in either of the two test specimens, it was evaluated that excellent SCC resistance was obtained.

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

[0140] On the other hand, in tests 22 and 23, the holding time t at molten metal temperature of 1600-1500°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 / mm³. 2 It exceeded that limit. As a result, excellent resistance to SCC (Steel Crushed Crystalline Interaction) was not achieved.

[0141] In tests 24 and 25, the holding time t at a molten metal temperature of 1600-1500°C in the tundish during continuous casting in the material preparation process was too long. As a result, the number density ND1 of fine low-S inclusions was 3.0 pieces / mm³. 2 Furthermore, the number density ND3 of high S inclusions was less than 2.0 particles / mm³. 2 It exceeded that limit. As a result, excellent resistance to SCC (Steel Crushed Crystalline Interaction) was not achieved.

[0142] In tests 26 and 27, during the continuous casting process of the material preparation stage, the distance D from the molten metal surface in the mold to the center of the discharge port on the outer surface of the immersion nozzle was too short. As a result, the number density ND1 of fine low-S inclusions was 3.0 pieces / mm². 2 Furthermore, the number density ND3 of high S inclusions was less than 2.0 particles / mm³. 2 It exceeded that limit. As a result, excellent resistance to SCC (Steel Crushed Crystalline Interaction) was not achieved.

[0143] In tests 28 and 29, the distance D was too short during the continuous casting process in the material preparation stage. As a result, the number density ND3 of high S inclusions was 2.0 pieces / mm². 2 It exceeded that limit. As a result, excellent resistance to SCC (Steel Crushed Crystalline Interaction) was not achieved.

[0144] In tests 30 and 31, the distance D was too long during the continuous casting process in the material preparation stage. As a result, the number density ND2 of coarse, low-S inclusions was 2.0 pieces / mm². 2 It exceeded that limit. As a result, excellent resistance to SCC (Steel Crushed Crystalline Interaction) was not achieved.

[0145] The embodiments of this disclosure have been described above. However, the embodiments described above are merely examples for implementing this disclosure. Therefore, this disclosure is not limited to the embodiments described above, and the embodiments described above can be modified as appropriate without departing from the spirit of this disclosure.

Claims

1. Austenitic alloy material, The chemical composition is expressed in mass percent. C: 0.030% or less, Si: 0.10-1.00%, Mn: 0.01 to 2.00%, P: 0.040% or less, S: more than 0% and less than 0.0050%, Cr: 19.0-28.0%, Ni: 28.0 to 37.0%, Mo: 2.00-6.00%, Cu: 0.01-2.00%, Nb: 0.001 to 0.100%, V: 0.01-0.50%, Co: 0.01-2.00%, Sn: 0.001 to 0.100%, Al: 0.010-0.500%, Ca: 0.0001-0.0100%, N: 0.001-0.350%, O: 0.0100% or less, Ta: 0-0.100%, Ti: 0 to 0.400%, Zr: 0 to 0.100%, W: 0-0.300%, 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: 0-0.200%, and The remainder consists of Fe and impurities. In the austenitic alloy material, The number density ND1 of fine low-S inclusions, which contain sulfur, have an Fn1 of 5.0 or higher as defined by formula (1), and have an equivalent circular diameter of 2.0 to 10.0 μm, is 3.0 inclusions / mm². ２ The above is 25.0 pieces / mm² or less, Coarse, low-S inclusions containing S, with an Fn1 of 5.0 or more and an equivalent circular diameter greater than 10.0 μm, have a number density ND2 of 2.0 inclusions / mm². ２ The following: High-S inclusions that contain S, have an Fn1 of less than 5.0, and have an equivalent circular diameter of 2.0 μm or more have a number density ND3 of 2.0 particles / mm². ２ The following is: Austenitic alloy material. Fn1=(Mg+Al+Ca+Ti+V+Mn) / S (1) Here, the elemental symbols in equation (1) are substituted with the mass percentage of the elemental content in the corresponding inclusion. If an element is not present, the corresponding element symbol is replaced with "0".

2. The austenitic alloy material according to claim 1, The aforementioned chemical composition is Ta: 0.001 to 0.100%, Ti: 0.001 to 0.400%, Zr: 0.001 to 0.100%, W: 0.001-0.300%, Zn: 0.001-0.010%, 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: 0.001-0.020%, and, Contains one or more rare earth elements selected from the group consisting of 0.001 to 0.200%. Austenitic alloy material.

3. An austenitic alloy material according to claim 1 or claim 2, The austenitic alloy material is an alloy tube. Austenitic alloy material.

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