Austenitic stainless steel

Abstract

The present invention provides an austenitic stainless steel material which has excellent carburization resistance even in a high temperature environment of 650°C or higher. An austenitic stainless steel material according to the present disclosure contains, in mass%, 0.030% or less of C, 0.01% to 1.00% of Si, 0.01% to 2.00% of Mn, 0.040% or less of P, 0.0100% or less of S, 8.00% to 18.00% of Ni, 15.00% to 25.00% of Cr, 0.10% to 5.00% of Mo, 0.20% to 1.00% of Nb, 0.060% to 0.250% of N, more than 2.00% but not more than 4.00% of Cu and 0.0010% to 0.0100% of B, while satisfying formula (1).　(1): Nbs × 0.8 > Nbb In formula (1), the average Nb concentration in mass% in the surface layer spanning from the surface of the austenitic stainless steel material to a depth of 10 µm is used for Nbs, and the average Nb concentration in mass% in the regions of the austenitic stainless steel material excluding the surface layer is used for Nbb.

Description

[Technical Field]

[0001] This disclosure relates to steel materials, and more specifically to austenitic stainless steel materials. [Background technology]

[0002] Steel materials used in chemical plant equipment, such as oil refineries and petrochemical plants, require high-temperature strength. Austenitic stainless steel is used for these chemical plant applications.

[0003] Chemical plant equipment includes multiple devices. Examples of devices found in chemical plant equipment include vacuum distillation units, direct desulfurization units, and catalytic reforming units. These devices include heating furnace tubes, reaction towers, tanks, heat exchangers, and piping. Austenitic stainless steel is used for these devices, taking into consideration strength, corrosion resistance, ease of construction, and cost-effectiveness in high-temperature environments.

[0004] In petrochemical plants, there are sometimes processes that involve further reforming the coke residue obtained after reforming the raw material, petroleum, to produce product gas. Plants that carry out this process are called delayed cokers.

[0005] The average operating temperature of each piece of equipment differs. Hereafter, the average operating temperature will be referred to as the "average operating temperature." For example, the average operating temperature of a vacuum distillation unit is 400-450°C. The average operating temperature of a direct desulfurization unit is 400-450°C. The average operating temperature of a catalytic reforming unit is 420-600°C. Therefore, austenitic stainless steel materials used in heating furnace tubes, reaction towers, tanks, heat exchangers, piping, etc., within these pieces of equipment are maintained at an average operating temperature of around 400-600°C for extended periods. On the other hand, some chemical plant equipment, such as catalytic reforming units including delayed cokers, have average operating temperatures of 650-750°C and design maximum service life of 815°C.

[0006] Austenitic stainless steel materials that can be used in chemical plants with an average operating temperature of 650°C or higher are proposed, for example, in Japanese Patent Publication No. 2021-021093 (Patent Document 1) and Japanese Patent Publication No. 2021-066928 (Patent Document 2).

[0007] The austenitic stainless steel material disclosed in Patent Document 1 has the following composition in mass%, of which C: 0.030% or less, Si: 1.00% or less, Mn: 2.00% or less, P: 0.040% or less, S: 0.0100% or less, Cr: 15.00~25.00%, Ni: 8.00~18.00%, Mo: 0.10~5.00%, Cu: greater than 2.00~4.00%, N: 0.06~0.25%, Nb: 0.2~1.0%, B: 0.0010~0.0100%, Ti: 0~0.50%, Ta: 0~0.50%, V: 0~1.00%, Zr: 0~0.10%, Hf: 0~0. The composition consists of 10% of the following elements: Co: 0-1.00%, W: 0-5.00%, sol.Al: 0-0.100%, Ca: 0-0.0200%, Mg: 0-0.0200%, rare earth elements: 0-0.100%, Sn: 0-0.010%, As: 0-0.010%, Zn: 0-0.010%, Pb: 0-0.010%, Sb: 0-0.010%, and the remainder being Fe and impurities, satisfying formulas (1) and (2), and the Nb content in the residue obtained by the extraction residue method is 0.052% or more by mass, and the Cr content in the residue is 0.245% or less by mass. 0 ≤ B + 0.21Mo - 1.9C ≤ 0.220 (1) 8.8 ≤ Ni + 0.05 Cu - 0.1 × (Mo) 2 ≤13.2 (2) Here, the elemental symbols in equations (1) and (2) are substituted with the content (mass%) of the corresponding element in the aforementioned chemical composition.

[0008] Patent Document 1 describes how the sensitization properties and stress relaxation cracking resistance of austenitic stainless steel are improved by satisfying formulas (1) and (2).

[0009] The austenitic stainless steel material disclosed in Patent Document 2 has the following composition in mass%, C: 0.030% or less, Si: 0.10~1.00%, Mn: 0.2~2.0%, P: 0.01~0.04%, S: 0.0100% or less, Cr: 15.00~25.00%, Ni: 9.00~18.00%, Mo: 1.0~5.0%, Nb: 0.20~2.00%, N: 0.050~0.180%, sol.Al: 0.001~0.080%, B: 0.0005~0.0080%, Cu: The composition is 0-2.00%, V: 0-1.00%, Co: 0-1.0%, Y: 0-1.00%, Zr: 0-1.0%, Hf: 0-0.20%, Ta: 0-0.20%, W: 0-5.0%, Ca: 0-0.0100%, Mg: 0-0.0100%, and rare earth elements other than Y: 0-0.100%, with the remainder being Fe and impurities, and the grain size number is 4.0-9.0, and the concentration of B (mass%) at the austenitic grain boundaries in the austenitic stainless steel is [B GB [B] is defined as [B], and the B concentration (mass%) within the austenite crystal grains is defined as [B] BM When defined as ], equation (1) is satisfied. [B GB ] / [B BM ]≧500 (1)

[0010] Patent Document 2 describes how satisfying formula (1) increases grain boundary strength, thereby improving the creep strength and creep ductility of austenitic stainless steel materials in high-temperature environments. [Prior art documents] [Patent Documents]

[0011] [Patent Document 1] Japanese Patent Publication No. 2021-021093 [Patent Document 2] Japanese Patent Publication No. 2021-066928 [Overview of the project] [Problems that the invention aims to solve]

[0012] By the way, at an average operating temperature of 650°C or higher, carbon contained in raw materials such as petroleum and residual coke penetrates into the interior of austenitic stainless steel materials in the apparatus, resulting in carburization. Due to carburization, carbides are excessively generated inside the austenitic stainless steel materials. As a result, the ductility of the austenitic stainless steel materials is significantly reduced. Therefore, excellent carburization resistance is required for austenitic stainless steel materials.

[0013] In Patent Document 1 and Patent Document 2, carburization in a high-temperature environment of 650°C or higher has not been studied. Therefore, in the austenitic stainless steel materials disclosed in Patent Document 1 and Patent Document 2, carburization in a high-temperature environment of 650°C or higher may not be suppressed.

[0014] An object of the present disclosure is to provide an austenitic stainless steel material having excellent carburization resistance even in a high-temperature environment of 650°C or higher.

Means for Solving the Problems

[0015] The austenitic stainless steel material according to the present disclosure is in mass%, C: 0.030% or less, Si: 0.01 to 1.00%, Mn: 0.01 to​​​​​​​​​​​​​​​​​​​​​​​​​​​Ti: 0~0.50%, Ta: 0~0.50%, Hf: 0~0.10%, Zr: 0~0.10%, Ca: 0~0.0200%, Mg: 0~0.0200%, Rare earth elements: 0-0.100%, and, The remainder consists of Fe and impurities, satisfying equation (1). Nbs × 0.8 > Nbb (1) Here, in equation (1), Nbs is substituted with the average Nb concentration of the surface layer of the austenitic stainless steel material up to a depth of 10 μm from the surface, in mass percent, and Nbb is substituted with the average Nb concentration of the region of the austenitic stainless steel material excluding the surface layer, in mass percent. [Effects of the Invention]

[0016] The austenitic stainless steel material according to this disclosure exhibits excellent carburization resistance even in high-temperature environments of 650°C or higher. [Brief explanation of the drawing]

[0017] [Figure 1] Figure 1 is a graph showing the relationship between the solution treatment conditions and the enrichment of Nb on the surface of austenitic stainless steel. [Modes for carrying out the invention]

[0018] The inventors investigated and studied austenitic stainless steel materials that exhibit excellent carburization resistance even in high-temperature environments of 650°C or higher. As a result, the inventors obtained the following findings.

[0019] To improve carburizing resistance in high-temperature environments above 650°C, it is effective to form a Cr2O3-containing film on the surface of the austenitic stainless steel material during use in high-temperature environments above 650°C. In order to uniformly form a Cr2O3-containing film on the surface of the austenitic stainless steel material during use in high-temperature environments above 650°C, it is effective to facilitate the diffusion of Cr in the austenitic stainless steel material to the surface in high-temperature environments above 650°C.

[0020] Of the elements contained in austenitic stainless steel, chromium (Cr) primarily diffuses preferentially through grain boundaries. Since Cr present near grain boundaries easily diffuses through them, it is effective to maintain a high Cr concentration near grain boundaries.

[0021] On the other hand, in high-temperature environments above 650°C, Cr combines with C in austenitic stainless steel to form Cr carbides at grain boundaries. At this time, Cr near the grain boundaries is consumed as Cr carbides, so if grain boundary Cr carbides are formed, the Cr concentration near the grain boundaries decreases. Here, Cr carbides are formed not only from C that has entered from the outside, but also from C originally contained in the austenitic stainless steel. Therefore, reducing the C content in the austenitic stainless steel is effective in suppressing the decrease in Cr concentration near grain boundaries.

[0022] The inventors hypothesized that reducing the carbon content in austenitic stainless steel would uniformly form a Cr2O3-containing coating, thereby improving the carburizing resistance of the austenitic stainless steel. Therefore, the inventors investigated the chemical composition of austenitic stainless steel with reduced carbon content. As a result, in mass%, the composition was: C: 0.030% or less, Si: 0.01~1.00%, Mn: 0.01~2.00%, P: 0.040% or less, S: 0.0100% or less, Ni: 8.00~18.00%, Cr: 15.00~25.00%, Mo: 0.10~5.00%, Nb: 0.20~1.00%, N: 0.060~0.250%, Cu: greater than 2.00~4.00%, B: 0.0010~0. We found that a chemical composition consisting of 0.100%, sol.Al:0~0.100%, Co:0~1.00%, W:0~5.00%, V:0~1.00%, Ti:0~0.50%, Ta:0~0.50%, Hf:0~0.10%, Zr:0~0.10%, Ca:0~0.0200%, Mg:0~0.0200%, rare earth elements:0~0.100%, and the remainder being Fe and impurities is effective.

[0023] However, it has been found that even austenitic stainless steel materials having the above-mentioned chemical composition may not provide sufficient carburization resistance in high-temperature environments above 650°C. Therefore, the inventors further investigated methods to improve the carburization resistance of austenitic stainless steel materials even in high-temperature environments above 650°C.

[0024] The inventors focused on the average Nb concentration on the surface of austenitic stainless steel and made the following inference: If Nb is concentrated on the surface of austenitic stainless steel, Nb carbonitrides will be formed near the surface of the austenitic stainless steel in a high-temperature environment of 650°C or higher. This consumes the solid-solution C near the surface of the austenitic stainless steel, suppressing the formation of Cr carbides. If the formation of Cr carbides is suppressed, the decrease in Cr concentration near the grain boundaries will be suppressed. As a result, Cr on the surface of the austenitic stainless steel becomes more easily diffused across the grain boundaries to the surface of the austenitic stainless steel, promoting the uniform formation of a film containing Cr2O3.

[0025] Based on the above reasoning, the inventors conducted further verification. As a result, they found that in an austenitic stainless steel material having the above-described chemical composition, it is effective that the average Nb concentration of the surface layer up to a depth of 10 μm from the surface of the austenitic stainless steel material and the average Nb concentration of the region excluding the surface layer satisfy equation (1). Nbs × 0.8 > Nbb (1) Here, in equation (1), Nbs is substituted with the average Nb concentration of the surface layer of the austenitic stainless steel material up to a depth of 10 μm, in mass percent, and Nbb is substituted with the average Nb concentration of the region of the austenitic stainless steel material excluding the surface layer, in mass percent.

[0026] Based on the above findings, the gist of the austenitic stainless steel material according to this embodiment is as follows:

[0027] [1] In mass%, C: 0.030% or less, Si: 0.01~1.00%, Mn: 0.01~2.00%, P: 0.040% or less, S: 0.0100% or less, Ni: 8.00~18.00%, Cr: 15.00~25.00%, Mo: 0.10~5.00%, Nb: 0.20~1.00%, N: 0.060~0.250%, Cu: Over 2.00 to 4.00%, B: 0.0010~0.0100%, sol.Al: 0~0.100%, Co: 0~1.00%, W: 0~5.00%, V: 0~1.00%, Ti: 0~0.50%, Ta: 0~0.50%, Hf: 0~0.10%, Zr: 0~0.10%, Ca: 0~0.0200%, Mg: 0~0.0200%, Rare earth elements: 0-0.100%, and, The remainder consists of Fe and impurities, satisfying formula (1). Austenitic stainless steel material. Nbs × 0.8 > Nbb (1) Here, in equation (1), Nbs is substituted with the average Nb concentration of the surface layer of the austenitic stainless steel material up to a depth of 10 μm from the surface, in mass percent, and Nbb is substituted with the average Nb concentration of the region of the austenitic stainless steel material excluding the surface layer, in mass percent.

[0028] [2] [1] The austenitic stainless steel material described above, In mass%, sol.Al: 0.001~0.100%, Co: 0.01~1.00%, W: 0.01~5.00%, V: 0.01~1.00%, Ti: 0.01~0.50%, Ta: 0.01~0.50%, Hf: 0.01~0.10% Zr: 0.01~0.10%, Ca: 0.0001~0.0200%, Mg: 0.0001~0.0200%, and, Contains one or more elements selected from the group consisting of rare earth elements (REM): 0.001 to 0.100%. Austenitic stainless steel material.

[0029] [3] Austenitic stainless steel material as described in [1] or [2], The oxide scale retention rate on the surface of the austenitic stainless steel material is 5% or less, and the two-dimensional arithmetic mean roughness Ra of the surface of the austenitic stainless steel material is 10 μm or less. Austenitic stainless steel material.

[0030] The austenitic stainless steel material according to this embodiment will be described in detail below. Unless otherwise specified, the "%" for elements refers to mass percentage.

[0031] [Features of the austenitic stainless steel material of this embodiment] The austenitic stainless steel material of this embodiment has the following characteristics. (Feature 1) The content of each element in the chemical composition is within the range of this embodiment (i.e., within the range shown below). (Feature 2) The average Nb concentration Nbs of the surface layer of the austenitic stainless steel material up to a depth of 10 μm from the surface, and the average Nb concentration Nbb of the region of the austenitic stainless steel material excluding the surface layer, satisfy equation (1). Nbs × 0.8 > Nbb (1)

[0032] [Optional features of the austenitic stainless steel material of this embodiment] The austenitic stainless steel material of this embodiment may optionally have the following characteristics. The austenitic stainless steel material of this embodiment may or may not have these optional characteristics. (Optional feature 1) The oxide scale retention rate on the surface of the austenitic stainless steel material is 5% or less, and the two-dimensional arithmetic mean roughness Ra of the surface of the austenitic stainless steel material is 10 μm or less.

[0033] Features 1 and 2, and optional feature 1, are described below.

[0034] [(Feature 1) Regarding chemical composition] The chemical composition of the austenitic stainless steel material of this embodiment contains the following elements:

[0035] C: 0.030% or less Carbon (C) is inevitably present. In other words, the C content is greater than 0%. C is present at the grain boundaries. 23 This process generates C6 type Cr carbides. If the C content exceeds 0.030%, excessive Cr carbides will be generated, even if the content of other elements is within the range of this embodiment. Excessive Cr carbide generation reduces the concentration of Cr near the grain boundaries. In this case, the amount of Cr supplied to the surface of the austenitic stainless steel material through the grain boundaries decreases. As a result, the uniform formation of a film containing Cr2O3 is inhibited. In other words, if the C content exceeds 0.030%, excessive Cr carbides are generated, even if the content of other elements is within the range of this embodiment, reducing the carburizing resistance of the austenitic stainless steel material. Therefore, the C content should be 0.030% or less. The preferred upper limit for the C content is 0.026%, more preferably 0.024%, more preferably 0.022%, more preferably 0.020%, and still more preferably 0.018%. It is preferable to have as low a C content as possible. However, excessive reduction of the C content increases manufacturing costs. Therefore, for industrial production purposes, the preferred lower limit of the C content is 0.001%, and more preferably 0.002%.

[0036] Si: 0.01~1.00% Silicon (Si) deoxidizes austenitic stainless steel during the manufacturing process. If the Si 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. Furthermore, if the Si content is less than 0.01%, the manufacturing cost will increase. On the other hand, if the Si content exceeds 1.00%, the hot workability of the steel will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Si content is 0.01 to 1.00%. The preferred lower limit of the Si content is 0.02%, more preferably 0.03%, more preferably 0.10%, more preferably 0.15%, and more preferably 0.20%. The preferred upper limit of the Si content is 0.90%, more preferably 0.85%, more preferably 0.80%, and more preferably 0.75%.

[0037] Mn: 0.01~2.00% Like silicon, manganese (Mn) deoxidizes austenitic stainless steel during the manufacturing process. Mn further combines with sulfur to form MnS, improving the hot workability of austenitic stainless steel. If the Mn 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 Mn content exceeds 2.00%, a spinel-type oxide containing Cr and Mn (Cr-Mn spinel-type oxide) is formed on the surface of the austenitic stainless steel in a high-temperature environment of 650°C or higher. The Cr-Mn spinel-type oxide inhibits the formation of a film containing Cr2O3 on the surface of the austenitic stainless steel in a high-temperature environment of 650°C or higher. As a result, the carburizing resistance of the austenitic stainless steel is reduced. In other words, if the Mn content exceeds 2.00%, the carburizing resistance of the austenitic stainless steel is reduced, even if the content of other elements is within the range of this embodiment. Therefore, the Mn content is 0.01 to 2.00%. The preferred lower limit of the Mn content is 0.02%, more preferably 0.03%, more preferably 0.05%, more preferably 0.10%, more preferably 0.30%, more preferably 0.40%, and more preferably 0.50%. The preferred upper limit of the Mn content is 1.90%, more preferably 1.80%, more preferably 1.70%, more preferably 1.60%, and more preferably 1.50%.

[0038] P:0.040% or less Phosphorus (P) is an unavoidable impurity. In other words, the P content is greater than 0%. If the P content exceeds 0.040%, the weldability and hot workability of the austenitic stainless steel 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. The preferred upper limit for the P content is 0.035%, and more preferably 0.030%. A lower P content is preferable. However, excessive reduction of the P content increases manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit for the P content is 0.001%, and more preferably 0.003%.

[0039] S: 0.0100% or less Sulfur (S) is an unavoidable impurity. That is, the S content is greater than 0%. If the S content exceeds 0.0100%, the weldability and hot workability of the austenitic stainless steel material will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the S content is 0.0100% or less. The preferred upper limit for the S content is 0.0090%, and more preferably 0.0080%. A lower S content is preferable. However, excessive reduction of the S content increases manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit for the S content is 0.0010%, and more preferably 0.0020%.

[0040] Ni: 8.00~18.00% Nickel (Ni) stabilizes austenite. Ni further enhances the carburizing resistance of austenitic stainless steel materials in high-temperature environments of 650°C or higher. If the Ni content is less than 8.00%, 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 Ni content exceeds 18.00%, even if the content of other elements is within the range of this embodiment, the hot workability of the austenitic stainless steel material decreases, and the raw material cost increases. Therefore, the Ni content is 8.00 to 18.00%. The preferred lower limit of the Ni content is 8.50%, more preferably 9.00%, even more preferably 9.50%, and even more preferably 10.00%. The preferred upper limit of the Ni content is 17.00%, more preferably 16.00%, even more preferably 15.00%, and even more preferably 14.00%.

[0041] Cr: 15.00~25.00% Chromium (Cr) enhances the carburization resistance of austenitic stainless steel materials in high-temperature environments above 650°C. Specifically, in high-temperature environments above 650°C, Cr forms Cr2O3 on the surface of the austenitic stainless steel material. If Cr2O3 is uniformly formed on the surface of the austenitic stainless steel material, the carburization resistance of the austenitic stainless steel material is enhanced. If the Cr content is less than 15.00%, 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 25.00%, the stability of the austenite decreases, even if the content of other elements is within the range of this embodiment. In this case, the creep strength of the austenitic stainless steel material decreases in high-temperature environments above 650°C. Therefore, the Cr content is 15.00 to 25.00%. The preferred lower limit of the Cr content is 15.50%, more preferably 16.00%, and even more preferably 16.50%. The preferred upper limit for the Cr content is 23.00%, more preferably 21.00%, and even more preferably 19.00%.

[0042] Mo: 0.10~5.00% Molybdenum (Mo) dissolves in the austenite matrix, and through solid solution strengthening, it increases the creep strength of austenitic stainless steel materials in high-temperature environments above 650°C. Furthermore, Mo generates fine intermetallic compounds in high-temperature environments above 650°C. These fine intermetallic compounds increase the creep strength of austenitic stainless steel materials in high-temperature environments above 650°C through precipitation strengthening. If the Mo content is less than 0.10%, 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 5.00%, even if the content of other elements is within the range of this embodiment, the diffusion of Cr is suppressed in high-temperature environments above 650°C. As a result, the formation of Cr2O3 on the surface of the austenitic stainless steel material is suppressed, and the carburizing resistance of the austenitic stainless steel material decreases. Therefore, the Mo content is 0.10 to 5.00%. The preferred lower limit of the Mo content is 0.20%, more preferably 0.30%, and even more preferably 0.40%. The preferred upper limit for the Mo content is 4.00%, more preferably 3.00%, even more preferably 2.00%, and even more preferably 1.00%.

[0043] Nb: 0.20~1.00% Niobium (Nb), together with nitrogen (N), forms Nb carbonitrides within the austenite crystal grains. The formation of Nb carbonitrides can reduce the amount of dissolved carbon (C). If the amount of dissolved carbon can be reduced, the M at the grain boundaries can be reduced in high-temperature environments above 650°C. 23The formation of C6 type carbides is suppressed. As a result, the decrease in Cr concentration near grain boundaries is suppressed, and the carburization resistance of the austenitic stainless steel material is improved. If the Nb content is less than 0.20%, the above effect cannot be obtained even if the content of other elements is within the range of this embodiment. On the other hand, if the Nb content exceeds 1.00%, even if the content of other elements is within the range of this embodiment, Nb carbonitrides and Nb carbides are excessively formed within the grains. In this case, the strength within the crystal grains becomes excessively high, and the strength difference between the crystal grains and the crystal grain boundaries becomes large. Therefore, stress concentration occurs at the grain interface, and the high-temperature strength of the austenitic stainless steel material decreases. For this reason, the Nb content is 0.20 to 1.00%. The preferred lower limit of the Nb content is 0.30%, and more preferably 0.40%. The preferred upper limit of the Nb content is 0.90%, more preferably 0.80%, and still more preferably 0.70%.

[0044] N: 0.060~0.250% Nitrogen (N) dissolves in the matrix phase to stabilize the austenite. N further generates Nb carbonitrides in the austenitic stainless steel. The formation of Nb carbonitrides reduces the amount of dissolved carbon (C) in the austenitic stainless steel. Therefore, the formation of Cr carbides can be suppressed in high-temperature environments above 650°C. As a result, the carburizing resistance of the austenitic stainless steel is improved. Nb carbonitrides further enhance the creep strength of the austenitic stainless steel in high-temperature environments above 650°C through precipitation strengthening. If the N content is less than 0.060%, 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 N content exceeds 0.250%, Cr nitrides (Cr2N) are formed at the grain boundaries. In this case, the Cr concentration near the grain boundaries decreases, suppressing Cr diffusion at the grain boundaries. As a result, even if the content of other elements is within the range of this embodiment, the carburizing resistance of the austenitic stainless steel in high-temperature environments above 650°C decreases. Therefore, the N content is 0.060 to 0.250%. The preferred lower limit of the N content is 0.070%, and more preferably 0.080%. The preferred upper limit of the N content is 0.200%, more preferably 0.160%, and still more preferably 0.140%.

[0045] Cu: Over 2.00 ~ 4.00% Copper (Cu) precipitates as a Cu phase in austenitic stainless steel material in high-temperature environments of 650°C or higher. The Cu phase increases the creep strength of the austenitic stainless steel material in high-temperature environments of 650°C or higher. If the Cu content is 2.00% or less, the above effect cannot be sufficiently obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Cu content exceeds 4.00%, even if the content of other elements is within the range of this embodiment, the Cu phase precipitates excessively, reducing the creep ductility of the austenitic stainless steel material in high-temperature environments of 650°C or higher, and further increasing raw material costs. Therefore, the Cu content is between 2.00% and 4.00%. The preferred lower limit of the Cu content is 2.20%, more preferably 2.40%, and even more preferably 2.60%. The preferred upper limit of the Cu content is 3.80%, more preferably 3.60%, and even more preferably 3.40%.

[0046] B: 0.0010~0.0100% Boron (B) segregates at grain boundaries in high-temperature environments above 650°C, thereby increasing grain boundary strength. If the B content is less than 0.0010%, 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 B content exceeds 0.0100%, even if the content of other elements is within the range of this embodiment, it promotes the formation of Cr carbides at the grain boundaries of austenitic stainless steel material in high-temperature environments above 650°C. Therefore, the B content is 0.0010 to 0.0100%. The preferred lower limit of the B content is 0.0012%, more preferably 0.0015%, and even more preferably 0.0020%. The preferred upper limit of the B content is 0.0090%, more preferably 0.0080%, even more preferably 0.0070%, even more preferably 0.0060%, even more preferably 0.0050%, and even more preferably 0.0040%.

[0047] The remainder of the chemical composition of the austenitic stainless steel material in this embodiment consists of Fe and impurities. Here, impurities in the chemical composition refer to substances that are mixed in from the ore, scrap, or manufacturing environment used as raw materials during the industrial production of austenitic stainless steel material, and are acceptable within a range that does not adversely affect the austenitic stainless steel material of this embodiment.

[0048] [Optional Elements] The chemical composition of the austenitic stainless steel material in this embodiment is further modified by substituting a portion of the Fe with: sol.Al: 0~0.100%, Co: 0~1.00%, W: 0~5.00%, V: 0~1.00%, Ti: 0~0.50%, Ta: 0~0.50%, Hf: 0~0.10%, Zr: 0~0.10% Ca: 0~0.0200%, Mg: 0~0.0200%, and, Rare earth elements: 0~0.100%, It may contain one or more elements selected from the group consisting of the following. These optional elements will be described below.

[0049] [Group 1: Regarding Al] The chemical composition of the austenitic stainless steel material in this embodiment may further contain Al instead of some of the Fe.

[0050] sol.Al: 0~0.100% Aluminum (Al) is an optional element and may not be present. In other words, the Al content may be 0%. If Al is present, that is, if the Al content is greater than 0%, Al deoxidizes the austenitic stainless steel material during the manufacturing process. Even if only a small amount of Al is present, the above effect can be obtained to some extent. On the other hand, if the Al content exceeds 0.100%, even if the content of other elements is within the range of this embodiment, the hot workability and creep ductility of the austenitic stainless steel material in high-temperature environments of 650°C or higher will decrease. Therefore, the Al content is 0 to 0.100%. The preferred lower limit of the Al content is 0.001%, more preferably 0.005%, and even more preferably 0.010%. The preferred upper limit of the Al content is 0.080%, more preferably 0.060%, and even more preferably 0.040%. Here, Al content refers to the acid-soluble Al (sol.Al) content.

[0051] [Group 2: Regarding Co and W] The chemical composition of the austenitic stainless steel material in this embodiment may further include one or more elements selected from the group consisting of Co and W in place of a portion of Fe. These elements are all optional and may not be included. If included, these elements increase the creep strength of the austenitic stainless steel material.

[0052] Co: 0~1.00% Cobalt (Co) is an optional element and may not be present. In other words, the Co content may be 0%. When Co is present, that is, when the Co content is greater than 0%, Co stabilizes the structure of the steel and increases the creep strength of austenitic stainless steel in high-temperature environments of 650°C or higher. Even if only a small amount of Co is present, the above effect can be obtained to some extent. On the other hand, if the Co content exceeds 1.00%, the raw material cost increases even if the content of other elements is within the range of this embodiment. Therefore, the Co content is 0 to 1.00%. The preferred lower limit of the Co content is 0.01%, more preferably 0.05%, still more preferably 0.10%, and still more preferably 0.20%. The preferred upper limit of the Co content is 0.90%, more preferably 0.80%, and still more preferably 0.70%.

[0053] W: 0~5.00% Tungsten (W) enhances the creep strength of austenitic stainless steel materials through solid solution strengthening in high-temperature environments of 650°C or higher. The above effect can be obtained even if only a small amount of W is present. On the other hand, if the W content exceeds 5.00%, even if the content of other elements is within the range of this embodiment, the stability of the austenite decreases and the toughness of the austenitic stainless steel material decreases. Therefore, the W content is 0 to 5.00%. The preferred lower limit of the W content is 0.01%, more preferably 0.05%, still more preferably 0.10%, still more preferably 0.20%, and still more preferably 0.30%. The preferred upper limit of the W content is 4.00%, more preferably 3.00%, and still more preferably 2.00%.

[0054] [Group 3: Regarding V, Ti, Ta, Hf, and Zr] The chemical composition of the austenitic stainless steel material in this embodiment may further contain one or more elements selected from the group consisting of V, Ti, Ta, Hf, and Zr in place of a portion of Fe. Any of these elements are optional and may not be included. If included, any of these elements further enhance the carburization resistance of the austenitic stainless steel material.

[0055] V: 0~1.00% V (vanadium) is an optional element and may not be present. In other words, the V content may be 0%. When V is present, that is, when the V content is greater than 0%, in a high-temperature environment of 650°C or higher, V combines with C in the austenitic stainless steel to form V carbides. V carbides suppress the formation of Cr carbides in a high-temperature environment of 650°C or higher. This further promotes the uniform formation of a film containing Cr2O3, and further improves the carburization resistance of the austenitic stainless steel. Even if only a small amount of V is present, the above effect can be obtained to some extent. On the other hand, if the V content exceeds 1.00%, even if the content of other elements is within the range of this embodiment, the V carbides will coarse in a high-temperature environment of 650°C or higher. In this case, the creep ductility of the austenitic stainless steel decreases. Therefore, the V content is 0 to 1.00%. The preferred lower limit of the V content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferred upper limit for the V content is 0.80%, more preferably 0.60%, and even more preferably 0.40%.

[0056] Ti: 0~0.50% Titanium (Ti) is an optional element and may not be present. In other words, the Ti content may be 0%. When Ti is present, that is, when the Ti content is greater than 0%, in a high-temperature environment of 650°C or higher, Ti combines with C in the austenitic stainless steel to form Ti carbides. Ti carbides suppress the formation of Cr carbides in a high-temperature environment of 650°C or higher. This further promotes the uniform formation of a film containing Cr2O3, and further improves the carburization resistance of the austenitic stainless steel. Even if only a small amount of Ti is present, the above effect can be obtained to some extent. On the other hand, if the Ti content exceeds 0.50%, even if the content of other elements is within the range of this embodiment, the Ti carbides become coarser, and the creep ductility of the austenitic stainless steel decreases in a high-temperature environment of 650°C or higher. Therefore, the Ti content is 0 to 0.50%. The preferred lower limit of the Ti content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferred upper limit for the Ti content is 0.45%, more preferably 0.40%, even more preferably 0.35%, and even more preferably 0.30%.

[0057] Ta: 0~0.50% Tantalum (Ta) is an optional element and may not be present. In other words, the Ta content may be 0%. If Ta is present, that is, if the Ta content is greater than 0%, Ta combines with C to form Ta carbides. Ta carbides suppress the formation of Cr carbides in high-temperature environments of 650°C or higher. This further promotes the uniform formation of a film containing Cr2O3, and further improves the carburization resistance of the austenitic stainless steel material. Even if only a small amount of Ta is present, the above effect can be obtained to some extent. On the other hand, if the Ta content exceeds 0.50%, even if the content of other elements is within the range of this embodiment, precipitates will be excessively formed, and the toughness of the austenitic stainless steel material will decrease. Therefore, the Ta content is 0 to 0.50%. The preferred lower limit of the Ta content is 0.01%, more preferably 0.02%, and still more preferably 0.03%. The preferred upper limit of the Ta content is 0.45%, more preferably 0.40%, and still more preferably 0.35%.

[0058] Hf: 0~0.10% Hafnium (Hf) is an optional element and may not be present. In other words, the Hf content may be 0%. When Hf is present, that is, when the Hf content is greater than 0%, Hf combines with C to form Hf carbides. Hf carbides suppress the formation of Cr carbides in high-temperature environments of 650°C or higher. This further promotes the uniform formation of a film containing Cr2O3, and further improves the carburization resistance of the austenitic stainless steel material. Even if only a small amount of Hf is present, the above effect can be obtained to some extent. On the other hand, if the Hf content exceeds 0.10%, even if the content of other elements is within the range of this embodiment, precipitates will be excessively formed, and the toughness of the austenitic stainless steel material will decrease. Therefore, the Hf content is 0 to 0.10%. The preferred lower limit of the Hf content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferred upper limit of the Hf content is 0.08%, and even more preferably 0.06%.

[0059] Zr: 0~0.10% Zirconium (Zr) is an optional element and may not be present. In other words, the Zr content may be 0%. If Zr is present, that is, if the Zr content is greater than 0%, Zr combines with C to form Zr carbides. Zr carbides suppress the formation of Cr carbides in high-temperature environments of 650°C or higher. This further promotes the uniform formation of a film containing Cr2O3, and further enhances the carburization resistance of the austenitic stainless steel material. Even if only a small amount of Zr is present, the above effect can be obtained to some extent. On the other hand, if the Zr content exceeds 0.10%, even if the content of other elements is within the range of this embodiment, precipitates will be excessively formed, and the toughness of the austenitic stainless steel material will decrease. Therefore, the Zr content is 0 to 0.10%. The preferred lower limit of the Zr content is 0.01%, more preferably 0.02%, and still more preferably 0.03%. The preferred upper limit of the Zr content is 0.08%, and still more preferably 0.06%.

[0060] [Group 4: Regarding Ca, Mg, and REM] The chemical composition of the austenitic stainless steel material in this embodiment may further contain one or more elements selected from the group consisting of Ca, Mg, and REM in place of a portion of Fe. These elements are all optional and may not be included. If included, these elements enhance the hot workability of the austenitic stainless steel material.

[0061] Ca: 0~0.0200% Calcium (Ca) is an optional element and may not be present. In other words, the Ca content may be 0%. If Ca is present, that is, if the Ca content is greater than 0%, Ca fixes S as a sulfide, improving the hot workability of the austenitic stainless steel material. Ca also deoxidizes the austenitic stainless steel material. Even if only a small amount of Ca is present, the above effects can be obtained to some extent. On the other hand, if the Ca content exceeds 0.0200%, the toughness and hot workability of the austenitic stainless steel material will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Ca content is 0 to 0.0200%. The preferred lower limit for Ca is 0.0001%, more preferably 0.0005%, even more preferably 0.0010%, even more preferably 0.0020%, and even more preferably 0.0050%. The preferred upper limit for Ca content is 0.0100%.

[0062] Mg: 0~0.0200% Magnesium (Mg) is an optional element and may not be present. In other words, the Mg content may be 0%. When Mg is present, that is, when the Mg content is greater than 0%, Mg fixes S as a sulfide, improving the hot workability of the austenitic stainless steel material. Mg also deoxidizes the austenitic stainless steel material. Even if only a small amount of Mg is present, the above effects can be obtained to some extent. On the other hand, if the Mg content exceeds 0.0200%, the toughness and hot workability of the austenitic stainless steel material will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Mg content is 0 to 0.0200%. The preferred lower limit of Mg is 0.0001%, more preferably 0.0005%, even more preferably 0.0010%, even more preferably 0.0020%, and even more preferably 0.0050%. The preferred upper limit of Mg content is 0.0100%.

[0063] Rare earth elements (REM): 0~0.100% Rare earth elements (REMs) are optional and do not need to be included. In other words, the REM content may be 0%. When REMs are included, that is, when the REM content is greater than 0%, the REMs fix sulfur as sulfides, improving the hot workability of the austenitic stainless steel material. Even if only a small amount of REM is included, the above effect can be obtained to some extent. On the other hand, if the REM content exceeds 0.100%, even if the content of other elements is within the range of this embodiment, the amount of inclusions such as oxides will be excessively high, and the weldability of the austenitic stainless steel material will decrease. Therefore, the REM content is 0 to 0.100%. The preferred lower limit of the REM content is 0.001%, and more preferably 0.002%. The preferred upper limit of the REM content is 0.090%, more preferably 0.080%, more preferably 0.070%, and still more preferably 0.060%. In this disclosure, rare earth elements (REMs) refer to the 15 elements from lanthanum (atomic number 57) to lutetium (atomic number 71), plus scandium (atomic number 21) and yttrium (atomic number 39), for a total of 17 elements. The REM content refers to the total content of these elements.

[0064] [Regarding (Feature 2) Equation (1)] The austenitic stainless steel material of this embodiment satisfies formula (1). Nbs × 0.8 > Nbb (1) Here, in equation (1), Nbs is substituted with the average Nb concentration of the surface layer of the austenitic stainless steel material up to a depth of 10 μm, in mass percent, and Nbb is substituted with the average Nb concentration of the region of the austenitic stainless steel material excluding the surface layer, in mass percent.

[0065] In this specification, the area from the surface of an austenitic stainless steel material to a depth of 10 μm is simply referred to as the surface layer of the austenitic stainless steel material. In this specification, the area of ​​the austenitic stainless steel material excluding the surface layer is simply referred to as the interior of the austenitic stainless steel material.

[0066] Transforming equation (1), we get Nbs > 1.25 × Nbb. In other words, the average Nb concentration Nbs on the surface of austenitic stainless steel is more than 1.25 times the average Nb concentration Nbb inside the austenitic stainless steel. The reason why the carburization resistance of austenitic stainless steel improves when Nb is concentrated on the surface is thought to be as follows: When Nb is concentrated on the surface of austenitic stainless steel, Nb carbonitrides are formed near the surface of the austenitic stainless steel in high-temperature environments of 650°C or higher. This consumes the solid-solution C near the surface of the austenitic stainless steel, suppressing the formation of Cr carbides. If the formation of Cr carbides is suppressed, the decrease in Cr concentration near the grain boundaries is suppressed. As a result, Cr on the surface of the austenitic stainless steel becomes more easily diffused across the grain boundaries to the surface of the austenitic stainless steel, promoting the uniform formation of a film containing Cr2O3.

[0067] [Regarding the measurement method of Nb concentration] The average Nb concentration in the surface layer of austenitic stainless steel up to a depth of 10 μm is called the average Nb concentration (Nbs). The average Nb concentration (Nbs) is determined by the following method: The austenitic stainless steel is cut perpendicular to the surface. If the austenitic stainless steel is a steel pipe, it is cut perpendicular to the axial direction of the pipe. An arbitrary field of view is selected in the cut surface including the surface of the austenitic stainless steel. Line analysis of elemental concentrations is performed using an electron probe microanalyzer (EPMA) in the region from the surface of the austenitic stainless steel to a depth of 10 μm within the selected field of view. The number of measurement points for the line analysis is 100. The beam diameter used for the line analysis is 0.1 μm. The direction of the line analysis is perpendicular to the surface of the austenitic stainless steel. Analysis is performed at 100 points from the surface to a depth of 10 μm in the austenitic stainless steel at a pitch of 0.1 μm. The arithmetic mean of the Nb concentration (mass%) of all measurement points is taken as the average Nb concentration (Nbs).

[0068] The average Nb concentration in the region of an austenitic stainless steel material excluding the surface is called the average Nb concentration Nbb. The average Nb concentration Nbb is determined by the following method: The austenitic stainless steel material is cut perpendicular to the surface. If the austenitic stainless steel material is a steel pipe, it is cut perpendicular to the axial direction of the steel pipe. A chip sample is taken from the surface of the austenitic stainless steel material at a position half the thickness of the austenitic stainless steel material, i.e., the 1 / 2 thickness position. The collected sample is dissolved in a mixed solution of hydrochloric acid and nitric acid prepared according to JIS G1258-2 (2014), and a mixed solution of sulfuric acid and phosphoric acid to obtain a solution. The solution is analyzed by inductively coupled plasma (ICP) analysis to measure the Nb concentration. Chip samples are taken from any three locations on the cut surface of the austenitic stainless steel material. The arithmetic mean of the Nb concentrations (mass%) at the three chip sample collection locations is taken as the average Nb concentration Nbb.

[0069] [Effects of the austenitic stainless steel material of this embodiment] The austenitic stainless steel material of this embodiment has the following characteristics. (Feature 1) The content of each element in the chemical composition is within the range of this embodiment. (Feature 2) The average Nb concentration Nbs of the surface layer of the austenitic stainless steel material up to a depth of 10 μm from the surface, and the average Nb concentration Nbb of the region of the austenitic stainless steel material excluding the surface layer, satisfy equation (1). Nbs × 0.8 > Nbb (1) The austenitic stainless steel material of this embodiment, having features 1 and 2, exhibits excellent carburization resistance even in high-temperature environments of 650°C or higher.

[0070] [(Optional Feature 1) Regarding the remaining oxide scale and surface roughness Ra] In addition to features 1 and 2, the austenitic stainless steel material of this embodiment, when having optional feature 1, exhibits further enhanced carburization resistance in high-temperature environments of 650°C or higher.

[0071] [Regarding the residual rate of oxide scale] In this specification, the oxide scale retention rate on the surface of the austenitic stainless steel material of this embodiment is also referred to simply as the oxide scale retention rate. The oxide scale retention rate on the surface of the austenitic stainless steel material of this embodiment is not particularly limited. However, oxide scale suppresses the formation of a film containing Cr2O3 in high-temperature environments of 650°C or higher. If the oxide scale retention rate is low, the coverage rate of the film containing Cr2O3 on the surface of the austenitic stainless steel material increases. As a result, the carburization resistance of the austenitic stainless steel material is further enhanced in high-temperature environments of 650°C or higher. Specifically, if the oxide scale retention rate is 5% or less, the carburization resistance of the austenitic stainless steel material is further enhanced in high-temperature environments of 650°C or higher. The oxide scale retention rate is more preferably 4% or less, more preferably 3% or less, more preferably 2% or less, and more preferably 1% or less. A lower oxide scale retention rate is preferable. The oxide scale retention rate may even be 0%.

[0072] [Method for measuring the residual rate of oxide scale] The oxide scale retention rate is measured by the following method: A digital camera is used to photograph the surface of the austenitic stainless steel material within a field of view of 30 mm in width and 30 mm in length. The captured image is converted to black and white by image processing to determine the area percentage in which oxide scale remains within the field of view. The photographs are taken at any three locations on the surface of the austenitic stainless steel material. The arithmetic mean of the area percentages obtained from the three photographs is taken as the oxide scale retention rate (%).

[0073] [About the two-dimensional arithmetic mean roughness Ra] In this specification, the two-dimensional arithmetic mean roughness Ra of the austenitic stainless steel material of this embodiment is also referred to simply as surface roughness Ra. The surface roughness Ra of the austenitic stainless steel material of this embodiment is not particularly limited. However, if the surface roughness Ra of the austenitic stainless steel material is low, the surface area of ​​the austenitic stainless steel material will be reduced. As a result, the coverage rate of the Cr2O3-containing film on the surface of the austenitic stainless steel material will increase. Specifically, if the surface roughness Ra of the austenitic stainless steel material is 10 μm or less, the coverage rate of the Cr2O3-containing film on the surface of the austenitic stainless steel material will increase. As a result, the carburizing resistance of the austenitic stainless steel material in high-temperature environments of 650°C or higher will be further improved. The surface roughness Ra of the austenitic stainless steel material is more preferably 8 μm or less, more preferably 5 μm or less, and more preferably 2 μm or less. The lower limit of the surface roughness Ra of the austenitic stainless steel material is not particularly limited, but for example, it is 1 μm.

[0074] [Method for measuring surface roughness Ra] The two-dimensional arithmetic mean roughness Ra is measured by the following method: A test specimen containing the surface of austenitic stainless steel is taken. The test specimen contains the surface of austenitic stainless steel with a width of 10 mm and a length of 10 mm. The test specimen is taken from any three locations on the austenitic stainless steel. The two-dimensional arithmetic mean roughness Ra (μm) is measured on the surface of the austenitic stainless steel contained in the test specimen using a three-dimensional shape measuring instrument. Measurements are taken in two directions: the longitudinal direction on the surface of the austenitic stainless steel and a direction perpendicular to it. The larger value is taken as the two-dimensional arithmetic mean roughness Ra (μm) of the austenitic stainless steel. If the austenitic stainless steel is a steel pipe, measurements are taken in two directions: the axial direction and the circumferential direction of the steel pipe. The larger value is taken as the two-dimensional arithmetic mean roughness Ra (μm) of the austenitic stainless steel. The reference length is 1000 μm. If oxide scale remains on the surface of the austenitic stainless steel, measurements are taken from locations where no oxide scale remains. The arithmetic mean of the measurement results from the three locations is defined as the two-dimensional arithmetic mean roughness Ra (μm).

[0075] [Effects of the austenitic stainless steel material of this embodiment when optional feature 1 is satisfied] If the austenitic stainless steel material of this embodiment has optional feature 1 in addition to features 1 and 2, the carburization resistance of the austenitic stainless steel material in high-temperature environments of 650°C or higher is further enhanced.

[0076] [Shape of austenitic stainless steel material] The shape of the austenitic stainless steel material in this embodiment is not particularly limited. The austenitic stainless steel material may be, for example, a steel pipe or a steel plate. The thickness of the austenitic stainless steel material is not particularly limited, but may be, for example, 1 to 100 mm.

[0077] [Manufacturing method] An example of a method for manufacturing the austenitic stainless steel material of this embodiment will be described. The austenitic stainless steel material having the above-described configuration may be manufactured by methods other than the manufacturing method described below. However, the manufacturing method described below is a preferred example of a method for manufacturing the austenitic stainless steel material of this embodiment.

[0078] The method for manufacturing austenitic stainless steel according to this embodiment includes the following steps. (Process 1) Material preparation process (Process 2) Hot working process (Step 3) Solution treatment process (Step 4) Pickling treatment step

[0079] The method for manufacturing austenitic stainless steel according to this embodiment optionally includes the following steps. (Optional process 1) Cold working process

[0080] [(Process 1) Material preparation process] In the material preparation process, a material having the above-mentioned chemical composition is prepared. The material may be supplied by a third party or manufactured. The material may be an ingot, slab, bloom, or billet. When manufacturing the material, the material is manufactured by the following method: Molten steel having the above-mentioned chemical composition is manufactured. An ingot is manufactured using the manufactured molten steel by the ingot-making method. A slab, bloom, or billet (cylindrical material) may be manufactured using the manufactured molten steel by the continuous casting method. A billet may be manufactured by hot working on the manufactured ingot, slab, or bloom. For example, a cylindrical billet may be manufactured by hot forging on an ingot, and this billet may be used as the material (cylindrical material). In this case, the temperature of the material immediately before the start of hot working is not particularly limited, but for example, it is 900 to 1300°C.

[0081] [(Process 2) Hot working process] In the hot working process, the raw material is subjected to hot working to produce intermediate steel material of a predetermined shape. The intermediate steel material may be, for example, a steel pipe, a steel plate, or a steel bar.

[0082] When the intermediate steel material is a steel pipe, the following processing is carried out in the hot working process: A cylindrical material is prepared as the raw material. A through hole is formed along the central axis of the cylindrical material by machining. Hot extrusion, such as the Eugène Séjournet method, is carried out on the cylindrical material with the through hole to produce the intermediate steel material (steel pipe). Instead of hot extrusion, the steel pipe may be manufactured by perforation rolling using the Mannesmann method. In this case, the material is perforated and rolled using a perforating machine. The perforated and rolled material is further hot-rolled using a mandrel mill, reducer, sizing mill, etc., to produce the intermediate steel material (steel pipe).

[0083] When the intermediate steel material is a steel plate, the following hot rolling is performed in the hot working process: One or more rolling mills equipped with a pair of work rolls are used. The material is hot-rolled using the rolling mill to produce a steel plate.

[0084] When the intermediate steel material is a steel bar, the hot working process includes, for example, a rough rolling process and a finish rolling process. In the rough rolling process, the material is hot-rolled to produce a billet. The rough rolling process uses, for example, a bloc mill. If a continuous rolling mill is installed downstream of the bloc mill, the billet after bloc rolling may be further hot-rolled using the continuous rolling mill to produce smaller billets. In a continuous rolling mill, for example, horizontal stands with a pair of horizontal rolls and vertical stands with a pair of vertical rolls are arranged alternately in a row. In the finish rolling process, the billet after the rough rolling process is reheated to a known temperature (900-1300°C). In the finish rolling process, the heated billet is hot-rolled using a continuous rolling mill to produce a steel bar.

[0085] Furthermore, intermediate steel materials (steel pipes, steel plates, and steel bars) may be manufactured by hot forging as a hot working process. Hot forging is, for example, draw forging.

[0086] [(Process 3) Solution Treatment Process] In the solution treatment process, solution treatment is carried out on the intermediate steel material after the hot working process or after the cold working process. By the solution treatment, precipitates in the intermediate steel material are dissolved. In the solution treatment process, the austenite crystal grains in the steel material are further adjusted.

[0087] The solution treatment is carried out in the following manner. The intermediate steel material after the cold working process is charged into the heat treatment furnace. In the heat treatment furnace, let the solution treatment temperature be T and the holding time at the solution treatment temperature T be t. The intermediate steel material is held at T (°C) and t (minutes) that satisfy the following conditions. Solution treatment temperature T: 950 °C or higher Holding time t of the solution treatment: 2 minutes or longer

[0088] Furthermore, the solution treatment conditions are adjusted so as to satisfy the following formula (2). (T - 940) × (log(t) + 10 2 × [Nbb] - 2 × P O2 ) ≥ 12900 (2) T: Solution treatment temperature (°C) t: Holding time of the solution treatment (minutes) [Nbb]: Average Nb concentration (mass %) inside the austenitic stainless steel material P O2 : Oxygen partial pressure (atm) in the heat treatment furnace <000046I>log(t): Common logarithm of t with base 10

[0089] When the solution treatment temperature T is less than 950 °C, the holding time of the solution treatment is less than 2 minutes, or the solution treatment conditions do not satisfy formula (2), Cr carbides and CrN are not sufficiently dissolved. In this case, the dissolved Cr in the austenitic stainless steel material is reduced, and the carburization resistance of the austenitic stainless steel material is lowered. Furthermore, since the formation of the oxide scale mainly composed of Fe and Cr is suppressed, the enrichment of Nb on the surface layer of the austenitic stainless steel material is suppressed. As a result, the average Nb concentration Nbs is lowered, and the austenitic stainless steel material does not satisfy formula (1).

[0090] F2 = (T - 940) × (log(t) + 10) 2 ×[Nbb]-10 2 ×P O2 ) is defined as follows. Figure 1 is a graph showing the relationship between the solution treatment conditions and the concentration of Nb on the surface of austenitic stainless steel. Figure 1 was created by plotting the results of test numbers 1 to 21 and 24 in the example described later. The horizontal axis of Figure 1 represents F2. The vertical axis of Figure 1 represents the value obtained by dividing the average Nb concentration Nbs on the surface of the austenitic stainless steel by the average Nb concentration Nbb inside the austenitic stainless steel. The larger the value on the vertical axis of Figure 1, the more concentrated Nb is on the surface of the austenitic stainless steel. Referring to Figure 1, it can be seen that as F2 increases, Nb is more likely to be concentrated on the surface of the austenitic stainless steel. If F2 is 12900 or higher, the average Nb concentration Nbs on the surface is more than 1.25 times the average Nb concentration Nbb inside.

[0091] The preferred lower limit of the solution treatment temperature T is 1000°C, more preferably 1010°C, and even more preferably 1020°C. There is no particular upper limit for the solution treatment temperature T, but for example, it is 1350°C. The preferred upper limit for the solution treatment temperature T is 1290°C, more preferably 1280°C, even more preferably 1270°C, and even more preferably 1260°C.

[0092] The preferred lower limit of the holding time t for the solution treatment is 3 minutes, more preferably 4 minutes, and even more preferably 5 minutes. There is no particular upper limit for the holding time t for the solution treatment, but considering economics, it is, for example, 60 minutes.

[0093] [(Step 4) Pickling treatment step] In the pickling process, the intermediate steel material is pickled after solution treatment. The pickling process removes the oxide scale formed on the surface of the intermediate steel material. The oxide scale formed on the surface of the intermediate steel material before the pickling process mainly consists of Fe oxide and / or Fe-Cr spinel oxide and / or Cr oxide. The pickling process is carried out to sufficiently remove the oxide scale from the surface of the intermediate steel material, making the metal surface the outermost surface. By making the metal surface the outermost surface, a film containing Cr2O3, which has excellent carburization resistance, is formed.

[0094] The pickling solution can be any solution capable of removing the oxide scale formed by the solution treatment. The pickling solution is, for example, a mixed solution containing nitric acid and / or hydrofluoric acid. The concentration of nitric acid and / or hydrofluoric acid in the mixed solution is not particularly limited. However, if the total concentration of nitric acid and / or hydrofluoric acid in the mixed solution is 5.0% or more, the oxide scale formed by the solution treatment can be removed more efficiently, and the residual oxide scale rate will decrease. On the other hand, if the concentration of nitric acid and / or hydrofluoric acid in the mixed solution is 15.0% or less, acid corrosion along the grain boundaries on the metal surface of the austenitic stainless steel material can be suppressed, and the increase in surface area can be suppressed. Therefore, preferably, the range of the concentration of nitric acid and / or hydrofluoric acid in the mixed solution is 5.0% to 15.0%. The lower limit of the concentration of nitric acid and / or hydrofluoric acid in the mixed solution is more preferably 5.5%, and even more preferably 6.0%. The upper limit of the concentration of nitric acid and / or hydrofluoric acid in the mixed solution is more preferably 14.5%, and even more preferably 14.0%.

[0095] [(Optional process 1) Cold working process] The cold working process may or may not be performed. If performed, the cold working process is carried out after the hot working process (process 2) and before the solution treatment process (process 3). In the cold working process, the intermediate steel material after hot working is subjected to pickling before cold working is performed. If the intermediate steel material is a steel pipe or bar, the cold working is, for example, cold drawing. If the intermediate steel material is a steel plate, the cold working is, for example, cold rolling. The reduction ratio in the cold working process is not particularly limited, but is, for example, 10 to 90%.

[0096] The austenitic stainless steel material of this embodiment is manufactured through the above manufacturing process. [Examples]

[0097] The effects of the austenitic stainless steel material of this embodiment will be further explained in detail below with reference to examples. The conditions in the following examples are just one example of conditions adopted to confirm the feasibility and effects of the austenitic stainless steel material of this embodiment. Therefore, the austenitic stainless steel material of this embodiment is not limited to this one example of conditions.

[0098] Molten steel having the chemical composition shown in Table 1 was produced using a vacuum melting furnace.

[0099] [Table 1]

[0100] In Table 1, a "-" indicates that the corresponding element content was below the detection limit. A cylindrical ingot was manufactured using the molten steel corresponding to the test number in Table 1. The ingot was heated at 1220°C for 3 hours. After heating, the ingot was hot forged to produce a cylindrical billet. A through hole was formed in the central axis of the cylindrical billet by machining. An intermediate steel material (steel pipe) was manufactured by hot extrusion of the cylindrical billet with the through hole. The temperature of the billet before hot extrusion was 1200°C. After hot extrusion, the intermediate steel material was cold drawn to produce an intermediate steel material (steel pipe) with a diameter of 45 mm and a wall thickness of 9.5 mm.

[0101] The obtained intermediate steel material was subjected to solution treatment at the solution treatment temperature T, holding time t, and oxygen partial pressure (atm) shown in Table 2. The steel material was cooled after solution treatment. After cooling, the intermediate steel material was pickled using a pickling solution to remove the oxide scale from the surface. The pickling solution was a mixed solution containing nitric acid and hydrofluoric acid. The total concentration of nitric acid and hydrofluoric acid in the mixed solution is indicated in the "Pickling Solution Concentration (Volume %)" column in Table 2. Through the above process, austenitic stainless steel material for each test number was manufactured.

[0102] [Table 2]

[0103] [Evaluation Test] The following evaluation tests were performed on the austenitic stainless steel material for each test number.

[0104] [Nb concentration measurement] The average Nb concentration Nbs was determined by the following method: Each austenitic stainless steel pipe (steel pipe) for each test number was cut perpendicular to the surface. Specifically, the austenitic stainless steel pipe (steel pipe) was cut perpendicular to the axial direction. An arbitrary field of view was selected from the cut surface including the inner surface of the austenitic stainless steel pipe (steel pipe). Line analysis of elemental concentrations using EPMA was performed on the region from the surface of the austenitic stainless steel to a depth of 10 μm within the selected field of view. The number of measurement points for the line analysis was 100. The beam diameter used for the line analysis was 0.1 μm. The direction of the line analysis was perpendicular to the surface of the austenitic stainless steel. Analysis was performed at 100 points from the surface of the austenitic stainless steel to a depth of 10 μm, with a pitch of 0.1 μm. The arithmetic mean of the Nb concentrations (mass%) of all measurement points was taken as the average Nb concentration Nbs. The results are shown in the "Nbs (mass%)" column in Table 2.

[0105] The average Nb concentration Nbb was determined by the following method. Austenitic stainless steel material for each test number was cut perpendicular to the surface. Specifically, the austenitic stainless steel material (steel pipe) was cut perpendicular to the axial direction. Chip samples were taken from the surface of the austenitic stainless steel material at a position half the thickness of the material, i.e., at the 1 / 2 thickness position. The collected samples were dissolved in a mixed solution of hydrochloric acid and nitric acid prepared according to JIS G1258-2 (2014), and a mixed solution of sulfuric acid and phosphoric acid to obtain a solution. The solution was analyzed by ICP analysis to measure the Nb concentration. Chip samples were taken from any three locations on the cut surface of the austenitic stainless steel material. The arithmetic mean of the Nb concentration (mass%) at the three chip sample collection locations was defined as the average Nb concentration Nbb. The results are shown in the "Nbb (mass%)" column in Table 2.

[0106] [Measurement of oxide scale retention rate] The oxide scale retention rate was measured using the following method. A digital camera was used to photograph the inner surface of each austenitic stainless steel pipe (steel pipe) for each test number within a field of view of 30 mm in width and 30 mm in length. The captured images were converted to black and white by image processing, and the area percentage of oxide scale remaining within the field of view was determined. The images were taken at three arbitrary locations on the inner surface of the austenitic stainless steel pipe. The arithmetic mean of the area percentages obtained from the three photographs was defined as the oxide scale retention rate (%). The results are shown in the "Oxide Scale Retention Rate" column in Table 2. In Table 2, "E (Excellent)" is indicated if the oxide scale retention rate is 5% or less, and "A (Acceptable)" is indicated if the oxide scale retention rate is greater than 5%. Note that in test number 22, where the oxide scale retention rate was greater than 5%, the oxide scale retention rate was 13% or less.

[0107] [Measurement of two-dimensional arithmetic mean roughness Ra] The two-dimensional arithmetic mean roughness Ra was measured using the following method. A test specimen containing the inner surface of the austenitic stainless steel material was taken for each test number. The test specimen contained the inner surface of the austenitic stainless steel material with a width of 10 mm and a length of 10 mm. The test specimen was taken from three arbitrary locations on the austenitic stainless steel material. The two-dimensional arithmetic mean roughness Ra (μm) was measured on the inner surface of the austenitic stainless steel material contained in the test specimen using a 3D shape measuring instrument VR-3200 manufactured by KEYENCE Corporation. Measurements were taken in two directions: the axial direction of the austenitic stainless steel material (steel pipe) (longitudinal direction of the steel pipe) and the direction perpendicular to it (circumferential direction of the steel pipe). The larger value was taken as the two-dimensional arithmetic mean roughness Ra (μm) of the austenitic stainless steel material. The reference length was set to 1000 μm. If oxide scale remained on the surface of the austenitic stainless steel material, measurements were taken from areas where no oxide scale remained. The arithmetic mean of the measurement results from three locations was defined as the two-dimensional arithmetic mean roughness Ra (μm). The results are shown in the "Surface Roughness Ra" column of Table 2. In Table 2, "E (Excellent)" is indicated if the two-dimensional arithmetic mean roughness Ra is 10 μm or less, and "A (Acceptable)" is indicated if the two-dimensional arithmetic mean roughness Ra is greater than 10 μm.

[0108] [Solid Carburizing Test] The carburizing resistance of austenitic stainless steel materials in high-temperature environments above 650°C was evaluated by the following method. One test specimen, including the inner surface, was taken from each test number of austenitic stainless steel material (steel pipe). The size of the test specimen was approximately 4 mm in the thickness direction (t direction) of the austenitic stainless steel pipe, 35 mm in the longitudinal direction (l direction) of the austenitic stainless steel pipe, and 25 mm in the direction perpendicular to both the thickness direction and the longitudinal direction (w direction). The test specimen was embedded in a solid carburizing agent (KG30.2, manufactured by HEF DURFFERIT Co., Ltd.). The test specimen and solid carburizing agent were placed in an austenitic stainless steel container with an inner diameter of 100 mm and a depth of 120 mm. The austenitic stainless steel container was inserted into a muffle furnace and heated to 850°C and held for 100 hours. After the holding period, the test specimen was removed and cut using a microcutter. The cut specimen was embedded in resin with the tw plane as the observation surface. A sample for cross-sectional analysis was prepared by mirror polishing the observation surface using diamond abrasive grains. Linear analysis of the carbon concentration was performed on the prepared cross-sectional analysis sample using EPMA to a depth of 1000 μm from the surface of the specimen. The analysis pitch was 2 μm and the analysis beam diameter was 0.1 μm. The carbon concentration at each analysis point includes the carbon concentration contained in the austenitic stainless steel material before the solid carburizing test. Therefore, the carbon concentration contained in the molten steel of the austenitic stainless steel material was subtracted from the carbon concentration at each analysis point. The carbon concentration at each analysis point was multiplied by the analysis pitch (2 μm), the obtained values ​​were summed, and then divided by 1000 μm. The obtained value was evaluated as the total amount of carbon (mass%) that penetrated the range from the surface of the austenitic stainless steel material to a depth of 1000 μm. Hereafter, this value will be referred to as the amount of carbon penetration. The results are shown in Table 2. Test numbers with a carbon penetration of 1.0% by mass or more and less than 3.0% by mass were evaluated as having acceptable carburization resistance and were marked "A (Acceptable)". Test numbers with a carbon penetration of less than 1.0% by mass were evaluated as having particularly excellent carburization resistance and were marked "E (Excellent)". On the other hand, test numbers with a carbon penetration of 3.0% by mass or more were evaluated as having poor carburization resistance and were marked "NA (Not Acceptable)".

[0109] [Test Results] Referring to Tables 1 and 2, test numbers 1 to 23 had appropriate chemical compositions and manufacturing conditions. Test numbers 1 to 23 satisfied formula (1). As a result, test numbers 1 to 23 had a carbon penetration amount of less than 3.0% by mass and exhibited excellent carburization resistance in high-temperature environments above 650°C.

[0110] Furthermore, in tests 1-21, where the oxide scale retention rate was 5% or less and the two-dimensional arithmetic mean surface roughness Ra was 10 μm or less, the amount of carbon penetration was less than 1.0 mass%. Compared to tests 22 and 23, tests 1-21 showed even better resistance to carburization in high-temperature environments above 650°C.

[0111] On the other hand, in test number 24, the solution treatment conditions did not satisfy equation (2). Therefore, the average Nb concentration Nbs and average Nb concentration Nbb did not satisfy equation (1). As a result, the amount of carbon penetration was 3.0% by mass or more, resulting in low carburization resistance.

[0112] In test number 25, the solution retention time t was too short. As a result, the average Nb concentration Nbs and average Nb concentration Nbb did not satisfy equation (1). Consequently, the amount of carbon penetration was 3.0% by mass or more, resulting in poor carburization resistance.

[0113] In test number 26, the carbon content in the austenitic stainless steel was too high. As a result, the amount of carbon penetration exceeded 3.0% by mass, leading to poor carburization resistance.

[0114] In test number 27, the chromium content in the austenitic stainless steel was too low. As a result, the amount of carbon penetration was 3.0% by mass or more, leading to poor carburization resistance.

[0115] In test number 28, the Ni content in the austenitic stainless steel was too low. As a result, the amount of carbon penetration was 3.0% by mass or more, leading to poor carburization resistance.

[0116] In test number 29, the Nb content in the austenitic stainless steel was too low. As a result, the amount of carbon penetration was 3.0% by mass or more, leading to poor carburization resistance.

[0117] 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. In mass percent, C: 0.030% or less, Si: 0.01-1.00%, Mn: 0.01 to 2.00%, P: 0.040% or less, S: 0.0100% or less, Ni: 8.00-18.00%, Cr: 15.00-25.00%, Mo: 0.10-5.00%, Nb: 0.20-1.00%, N: 0.060-0.250%, Cu: more than 2.00 to 4.00%, B: 0.0010-0.0100%, Sol. Al: 0-0.100%, Co: 0-1.00%, W: 0-5.00%, V: 0-1.00%, Ti: 0 to 0.50%, Ta: 0 to 0.50%, Hf: 0-0.10%, Zr: 0 to 0.10%, Ca: 0-0.0200%, Mg: 0 to 0.0200%, Rare earth elements: 0-0.100%, and The remainder consists of Fe and impurities, satisfying formula (1). Austenitic stainless steel material. Nbs×0.8>Nbb (1) Here, in equation (1), Nbs is substituted with the average Nb concentration of the surface layer up to a depth of 10 μm from the surface of the austenitic stainless steel material in mass percent, and Nbb is substituted with the average Nb concentration of the region of the austenitic stainless steel material excluding the surface layer in mass percent.

2. The austenitic stainless steel material according to claim 1, In mass percent, Sol. Al: 0.001–0.100%, Co: 0.01 to 1.00%, W: 0.01-5.00%, V: 0.01-1.00%, Ti: 0.01 to 0.50%, Ta: 0.01 to 0.50%, Hf: 0.01-0.10%, Zr: 0.01 to 0.10%, Ca: 0.0001-0.0200%, Mg: 0.0001 to 0.0200%, and, Contains one or more elements selected from the group consisting of rare earth elements (REM): 0.001 to 0.100%. Austenitic stainless steel material.

3. An austenitic stainless steel material according to claim 1 or claim 2, On the surface of the austenitic stainless steel material, the oxide scale retention rate, which is the area ratio of the region where oxide scale remains within the observation field, is 5% or less, and the two-dimensional arithmetic mean roughness Ra of the surface of the austenitic stainless steel material is 10 μm or less. Austenitic stainless steel material.

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