JP7538417B2 - Austenitic Stainless Steel - Google Patents

Description

This disclosure relates to austenitic stainless steel materials.

In power plants, such as thermal power plants, there is a demand for improving power generation efficiency from the viewpoint of reducing _CO2 gas emissions and economic efficiency. Therefore, the turbine steam pressure is being increased in temperature and pressure. Heat transfer members used in power plants are exposed to high-temperature and high-pressure steam for a long time. The heat transfer members are, for example, piping used in power plant equipment such as thermal power boilers, turbines, and steam pipes. When exposed to high-temperature steam for a long time, oxide scale is formed on the surface of the heat transfer member. If the heat transfer member does not have sufficient steam oxidation resistance, a large amount of oxide scale is formed on the surface of the heat transfer member. The heat transfer member thermally expands and contracts due to the start and stop of the boiler. Therefore, if a large amount of oxide scale is formed, the oxide scale peels off and causes clogging of the pipe. If a large amount of oxide scale is formed, the oxide scale further inhibits heat conduction from the outside of the pipe to the inside of the pipe. Therefore, in order to maintain a high temperature inside the pipe, it is necessary to provide more heat from the outside. The increase in temperature of the pipe causes a decrease in creep strength. For this reason, heat transfer members used in power plant equipment such as thermal power plant boilers, turbines, and steam pipes are required to have high resistance to steam oxidation.

Austenitic stainless steel materials are used as steel materials for power plant applications. Austenitic stainless steel materials for power plant applications are disclosed, for example, in International Publication No. 2016/204005 (Patent Document 1), JP 2017-14576 A (Patent Document 2), Japanese Patent No. 5000805 A (Patent Document 3), and JP 2005-509751 A (Patent Document 4).

The high Cr austenitic stainless steel disclosed in Patent Document 1 has a composition, in mass%, of C: 0.03-0.12%, Si: 0.10-1.00%, Mn: 0.10-3.00%, P: 0.030% or less, S: 0.020% or less, Cr: 21.50-28.00%, Ni: over 26.00-35.00%, W: over 2.00-5.00%, Co: 0.80% It contains the following: V: 0.01-0.70%, Nb: 0.15-1.00%, Al: 0.001-0.040%, B: 0.0001-0.0100%, N: 0.010-0.400%, with the balance being Fe and impurities. Furthermore, the content of the impurity elements Ta, Nd, and Zr satisfies Ta + 0.8Nd + 0.5Zr: 0.020-0.200%. Patent Document 1 states that by limiting the content of the impurity elements Ta, Nd, and Zr, creep strength in high-temperature environments is increased.

The austenitic heat-resistant alloy disclosed in Patent Document 2 has a chemical composition, in mass %, of C: 0.04-0.14%, Si: 0.05-1%, Mn: 0.5-2.5%, P: 0.03% or less, S: 0.002% or less, Ni: 23-32%, Cr: 20-25%, W: 1-5%, Nb: 0.1-0.6%, V: 0.1-0.6%, N: 0.1-0.3%, B: 0.0005-0.01%, Al: 0.03% or less, O: 0.02% or less, and the sum of one or more of Se, Te, Bi, Sn, Zn, and Pb: 0.001-0.02%, and the balance: Fe and impurities. Patent Document 2 states that creep strength in high-temperature environments is increased by limiting the total content of one or more of Se, Te, Bi, Sn, Zn, and Pb.

The austenitic stainless steel disclosed in Patent Document 3 contains, by mass%, C: 0.04-0.10%, Si: 0.4% or less, Mn: 0.6% or less, Cr: 20-27%, Ni: 22.5-32%, Mo: 0.5% or less, Nb: 0.20-0.60%, W: 0.4-4.0%, N: 0.10-0.30%, B: 0.002-0.008%, Al: 0.003-0.05%, Cu: 2.0-3.5%, Co: 0.5-3%, and the balance is Fe and impurities. Patent Document 3 states that the inclusion of Cu and Co increases creep strength in high-temperature environments.

The austenitic stainless steel disclosed in Patent Document 4 contains, in mass %, Cr: 23.0-30.0%, Ni: 25.0-35.0%, Mo: 2.0-6.0%, Mn: 1.0-6.0%, N: 0-0.4%, C: 0.05% or less, Si: 1.0% or less, S: 0.02% or less, Cu: 3.0% or less, W: 0-6.0%, Mg, Ce, Ca, B, La, Pr, Zr, Ti, and one or more elements of the Nd group: 2.0% or less, and the balance being Fe and impurities. Patent Document 4 states that by making the total content of one or more elements of the Mg, Ce, Ca, B, La, Pr, Zr, Ti, and Nd group 2.0% or less, the grain boundary corrosion resistance can be improved in acidic and alkaline environments.

International Publication No. 2016/204005 JP 2017-14576 A Patent No. 5000805 Special Publication No. 2005-509751

However, as mentioned above, power plants require excellent steam oxidation resistance in high-temperature environments of 650°C or higher. Patent documents 1 to 4 do not discuss steam oxidation resistance in high-temperature environments of 650°C or higher.

The objective of this disclosure is to provide an austenitic stainless steel material that has excellent steam oxidation resistance even in high-temperature environments of 650°C or higher.

The austenitic stainless steel material disclosed herein is
In mass percent,
C: 0.01-0.15%,
Si: 0.01-1.00%,
Mn: 2.00% or less,
P: 0.040% or less,
S: 0.010% or less,
Cr: 20.0-25.0%,
Ni: 23.0-32.0%,
Nb: 0.10-1.00%,
Mo: 0.01 to 2.50%,
W: 2.5-6.0%,
Al: 0.010-0.300%,
V: 0.01-1.00%,
B: 0.0005-0.0500%,
REM: 0.001-0.100%,
N: 0.15-0.35%,
Ti: 0 to 0.100%,
Cu: 0 to 1.00%,
Co: 0 to 1.0%,
Zr: 0 to 0.100%,
Ta: 0-1.0%,
Re: 0-5.0%,
Ca: 0-0.0500%,
Mg: 0 to 0.0500%,
Hf: 0 to 1.0%, and
The balance is Fe and impurities.
The grain size number D of the austenitic stainless steel material measured in accordance with ASTM E112-88 satisfies formula (1),
The oxygen concentration at a depth of 1 μm from the surface of the austenitic stainless steel material is less than 1.00 mass %.
2W/7-Mo+3D/4≧3.00 (1)
Here, the grain size number is substituted for D in formula (1), and the content (mass %) of the corresponding element is substituted for each element symbol in formula (1).

The austenitic stainless steel material disclosed herein has excellent steam oxidation resistance even in high-temperature environments of 650°C or higher.

FIG. 1 is a diagram showing the relationship between the F1 value and the inner oxide scale thickness (μm) obtained in a steam oxidation resistance evaluation test for an austenitic stainless steel material having a chemical composition within the range of this embodiment and an oxygen concentration of less than 1.00 mass % at a depth of 1 μm from the surface.

The present inventors have investigated austenitic stainless steel materials having excellent steam oxidation resistance in high-temperature environments of 650° C. or higher. When improving steam oxidation resistance in high-temperature environments, it is effective to form a coating containing Cr ₂ O ₃ on the steel surface during use in the high-temperature environment. In order to uniformly form Cr ₂ O ₃ on the steel surface during use in the high-temperature environment, it is effective to make Cr in the steel easily diffuse to the surface in the high-temperature environment.

Among the elements contained in steel, W promotes the diffusion of Cr in high-temperature environments of 650°C or higher. On the other hand, Mo suppresses the diffusion of Cr in high-temperature environments of 650°C or higher. Therefore, the inventors investigated chemical compositions that facilitate the diffusion of Cr in high-temperature environments of 650°C or higher. As a result, the following contents were found in mass %: C: 0.01-0.15%, Si: 0.01-1.00%, Mn: 2.00% or less, P: 0.040% or less, S: 0.010% or less, Cr: 20.0-25.0%, Ni: 23.0-32.0%, Nb: 0.10-1.00%, Mo: 0.01-2.50%, W: 2.5-6.0%, Al: 0.010-0.300%, V: 0.01-1.00%, B: 0.0005-0.0500%, REM: 0.001-0. . 100%, N: 0.15-0.35%, Ti: 0-0.100%, Cu: 0-1.00%, Co: 0-1.0%, Zr: 0-0.100%, Ta: 0-1.0%, Re: 0-5.0%, Ca: 0-0.0500%, Mg: 0-0.0500%, Hf: 0-1.0%, and the balance being Fe and impurities, it was thought that there was a possibility that a steel material with a chemical composition consisting of excellent steam oxidation resistance in a high-temperature environment of 650°C or higher could achieve this.

However, it was found that even an austenitic stainless steel material having the above-mentioned chemical composition may not have sufficient steam oxidation resistance in a high-temperature environment of 650°C or higher. Therefore, the present inventors further studied a method for improving steam oxidation resistance. As a result, it was found that it is effective to make the oxygen concentration at a depth of 1 μm from the surface less than 1.00 mass% in an austenitic stainless steel material having the above-mentioned chemical composition. The oxygen concentration at a depth of 1 μm from the surface means the presence or absence of oxide scale on the surface. If the oxygen concentration at a depth of 1 μm from the surface is less than 1.00 mass%, the oxide scale is substantially not present on the surface of the austenitic stainless steel material. In this case, the generation of Cr ₂ O ₃ on the steel surface is not inhibited by the oxide scale in a high-temperature environment of 650°C or higher. Therefore, Cr ₂ O ₃ is easily generated uniformly on the steel surface in a high-temperature environment.

The present inventors further discovered that in an austenitic stainless steel material having the above-mentioned chemical composition and in which the oxygen concentration at a depth of 1 μm from the steel surface is less than 1.00 mass%, if the grain size number D of the austenite grains satisfies formula (1), then the steam oxidation resistance in a high-temperature environment of 650° C. or higher is significantly improved.
2W/7-Mo+3D/4≧3.00 (1)
Here, the grain size number is substituted for D in formula (1), and the content (mass%) of the corresponding element is substituted for each element symbol in formula (1).

F1=2W/7-Mo+3D/4 is an index showing the degree of formation of Cr ₂ O ₃ on the steel surface in a high-temperature environment of 650°C or higher. As described above, W promotes the diffusion of Cr in a high-temperature environment. Mo suppresses the diffusion of Cr in a high-temperature environment. The larger the grain size number D, that is, the finer the crystal grains, the more the diffusion of Cr is promoted in a high-temperature environment. If the crystal grains are fine, the area of the grain boundary increases. It is considered that the grain boundary becomes a diffusion path for Cr. Therefore, it is considered that the larger the grain size number D, the more the diffusion path of Cr increases, the more the diffusion of Cr is promoted, and the more likely Cr ₂ O ₃ is formed on the steel surface in a high-temperature environment of 650°C or higher.

Figure 1 shows the relationship between the F1 value and the inner oxide scale thickness (μm) obtained in the steam oxidation resistance evaluation test described below for an austenitic stainless steel material with an oxygen concentration of less than 1.00 mass% at a depth of 1 μm from the surface. The thinner the inner oxide scale thickness, the higher the steam oxidation resistance in a high-temperature environment of 650°C or higher.

Referring to FIG. 1, even if the F1 value increases, the thickness of the inner oxide scale remains thick until the F1 value reaches 3.00. On the other hand, when the F1 value reaches 3.00 or more, the thickness of the inner oxide scale decreases significantly. Based on the above results, the inventors have found that in an austenitic stainless steel material whose chemical composition is within the above-mentioned range and whose oxygen concentration at a depth of 1 μm from the surface is less than 1.00 mass%, if F1 is set to 3.00 or more, the steam oxidation resistance is significantly improved in a high-temperature environment of 650°C or more.

The austenitic stainless steel material of this embodiment was developed based on the above findings and has the following configuration:

[1]
An austenitic stainless steel material,
In mass percent,
C: 0.01-0.15%,
Si: 0.01-1.00%,
Mn: 2.00% or less,
P: 0.040% or less,
S: 0.010% or less,
Cr: 20.0-25.0%,
Ni: 23.0-32.0%,
Nb: 0.10-1.00%,
Mo: 0.01-2.50%,
W: 2.5-6.0%,
Al: 0.010-0.300%,
V: 0.01-1.00%,
B: 0.0005-0.0500%,
REM: 0.001-0.100%,
N: 0.15-0.35%,
Ti: 0 to 0.100%,
Cu: 0 to 1.00%,
Co: 0-1.0%,
Zr: 0 to 0.100%,
Ta: 0 to 1.0%,
Re: 0-5.0%,
Ca: 0-0.0500%,
Mg: 0 to 0.0500%,
Hf: 0 to 1.0%, and
The balance is Fe and impurities.
The grain size number D of the austenitic stainless steel material measured in accordance with ASTM E112-88 satisfies formula (1),
The oxygen concentration at a depth of 1 μm from the surface of the austenitic stainless steel material is less than 1.00 mass%.
Austenitic stainless steel.
2W/7-Mo+3D/4≧3.00 (1)
Here, the grain size number is substituted for D in formula (1), and the content (mass %) of the corresponding element is substituted for each element symbol in formula (1).

[2]
The austenitic stainless steel material according to [1],
The grain size number D is 2.50 to 8.00;
Austenitic stainless steel.

[3]
The austenitic stainless steel material according to [1] or [2],
The arithmetic mean roughness Ra of the steel surface is 10.00 μm or less.
Austenitic stainless steel.

[4]
An austenitic stainless steel material according to any one of [1] to [3],
Ti: 0.001 to 0.100%,
Cu: 0.01 to 1.00%,
Co: 0.1 to 1.0%,
Zr: 0.001 to 0.100%,
Contains one or more elements selected from the group consisting of Ta: 0.1 to 1.0% and Re: 0.1 to 5.0%;
Austenitic stainless steel.

[5]
An austenitic stainless steel material according to any one of [1] to [4],
Ca: 0.0001 to 0.0500%, and
Mg: Contains one or more elements selected from the group consisting of 0.0001 to 0.0500%;
Austenitic stainless steel.

[6]
An austenitic stainless steel material according to any one of [1] to [5],
Hf: 0.1 to 1.0%;
Austenitic stainless steel.

The austenitic stainless steel material of this embodiment will be described in detail below. "%" for elements means mass % unless otherwise specified.

[Chemical composition]
The chemical composition of the austenitic stainless steel material of this embodiment contains the following elements.

C: 0.01-0.15%
Carbon (C) forms carbides during use of austenitic stainless steel in high temperature environments, enhancing the creep strength of the steel. If the C content is less than 0.01%, the contents of other elements are Even if the content of the other elements is within the range of this embodiment, the above-mentioned effect cannot be sufficiently obtained. However, during use in a high-temperature environment, the carbides become coarse and the creep strength decreases. Therefore, the C content is 0.01 to 0.15%. The preferred lower limit of the C content is 0.02%. The upper limit of the C content is preferably 0.14%, more preferably 0.13%, and even more preferably 0. It is 12%.

Si: 0.01~1.00%
Silicon (Si) deoxidizes steel during the manufacturing process. Furthermore, excessive reduction in the Si content increases manufacturing costs. Therefore, the Si content is 0.01% or more. If the Si content exceeds 1.00%, the hot workability of the steel material decreases even if the contents of other elements are within the range of this embodiment. The lower limit of the Si content is preferably 0.02%, more preferably 0.03%, still more preferably 0.10%, and still more preferably 0.15%. The upper limit of the Si content is preferably 0.90%, more preferably 0.85%, still more preferably 0.80%, and still more preferably 0.75%. %.

Mn: 2.00% or less Manganese (Mn) is inevitably contained. That is, the Mn content is more than 0%. Mn, like Si, deoxidizes the steel in the manufacturing process. Mn further combines with S to form MnS, improving the hot workability of the steel. However, if the Mn content exceeds 2.00%, even if the contents of other elements are within the range of this embodiment, a spinel type oxide containing Cr and Mn (Cr-Mn spinel type oxide) is formed on the surface of the steel during use in a high-temperature environment of the austenitic stainless steel material. The Cr-Mn spinel type oxide inhibits the generation of Cr ₂ O ₃ on the surface of the steel during use in a high-temperature environment, and reduces the steam oxidation resistance of the steel. Therefore, the Mn content is 2.00% or less. The lower limit of the Mn content is preferably 0.01%, more preferably 0.05%, more preferably 0.10%, more preferably 0.30%, more preferably 0.50%, and more preferably 0.70%. The upper limit of the Mn content is preferably 1.90%, more preferably 1.85%, more preferably 1.80%, more preferably 1.75%, and more preferably 1.60%.

P: 0.040% or less Phosphorus (P) is an impurity that is inevitably contained. That is, the P content is more than 0%. P reduces the weldability and hot workability of the steel material. If the P content exceeds 0.040%, the weldability and hot workability of the steel material cannot be sufficiently obtained even if the contents of other elements are within the range of this embodiment. Therefore, the P content is 0.040% or less. The preferred upper limit of the P content is 0.035%, and more preferably 0.030%. Excessive reduction of the P content increases the manufacturing cost of the steel material. Therefore, in consideration of normal industrial production, the preferred lower limit of the P content is 0.001%, and more preferably 0.003%.

S: 0.010% or less Sulfur (S) is an unavoidably contained impurity. That is, the S content is more than 0%. S reduces the weldability and hot workability of the steel material. If the S content exceeds 0.010%, the weldability and hot workability of the steel material cannot be sufficiently obtained even if the contents of other elements are within the range of this embodiment. Therefore, the S content is 0.010% or less. The preferred upper limit of the S content is 0.009%, more preferably 0.008%. Excessive reduction of the S content increases the manufacturing cost of the steel material. Therefore, in consideration of normal industrial production, the preferred lower limit of the S content is 0.001%, more preferably 0.002%.

Cr:20.0~25.0%
Chromium (Cr) enhances the steam oxidation resistance of austenitic stainless steels when used in high-temperature environments. Specifically, when used in high-temperature environments of 650° C. or higher, Cr forms on the surface of the steel. If _Cr2O3 is uniformly formed on the surface of the steel material, the steam oxidation resistance of the steel material will be improved. If the _Cr content is less than _{20.0 %} , other On the other hand, if the Cr content exceeds 25.0%, the above-mentioned effect cannot be sufficiently obtained even if the content of the other elements falls within the range of the present embodiment. Even if the Cr content is within the range of 20.0 to 25.0%, the stability of austenite decreases. In this case, creep strength decreases during use in high temperature environments. Therefore, the Cr content is 20.0 to 25.0%. The lower limit of the amount is preferably 20.5%, more preferably 21.0%, and even more preferably 21.5%. The upper limit of the Cr content is preferably 24.5%, more preferably 24.0%, and further preferably 23.5%.

Ni: 23.0-32.0%
Nickel (Ni) stabilizes austenite. Ni also enhances the steam oxidation resistance of austenitic stainless steel materials during use in high-temperature environments. If the Ni content is less than 23.0%, On the other hand, if the Ni content exceeds 32.0%, the above-mentioned effect cannot be sufficiently obtained even if the contents of the other elements are within the range of this embodiment. Even if the Ni content is within this range, the hot workability of the steel material is reduced. Therefore, the Ni content is 23.0 to 32.0%. The preferable lower limit of the Ni content is 23.5%, and The upper limit of the Ni content is preferably 31.5%, more preferably 31.0%. , more preferably 30.5%, and even more preferably 30.0%.

Nb: 0.10-1.00%
Niobium (Nb) combines with N to form nitrides during use of austenitic stainless steel materials in high-temperature environments. Nb further combines with Ni and Fe to form the Laves phase (Fe ₂ (Nb, These precipitates form intermetallic compounds, typically the γ'' phase ( _Ni3Nb ) and/or the γ'' phase (Ni3Nb) during use in high temperature environments. As a result, the creep strength of the steel material is increased. If the Nb content is less than 0.10%, the above effect is sufficiently obtained even if the contents of other elements are within the range of this embodiment. On the other hand, if the Nb content exceeds 1.00%, even if the contents of other elements are within the ranges of this embodiment, the above-mentioned intermetallic compounds are generated in excessive amounts. The toughness and hot workability of the steel material are reduced. Therefore, the Nb content is 0.10 to 1.00%. The lower limit of the Nb content is preferably 0.12%, more preferably 0.14%, still more preferably 0.18%, and still more preferably 0.20%. is 0.95%, more preferably 0.90%, and even more preferably 0.85%.

Mo: 0.01~2.50%
Molybdenum (Mo) dissolves in the parent austenite phase and increases the creep strength of steel materials used in high-temperature environments through solution strengthening. Mo also forms fine intermetallic compounds in high-temperature environments. Therefore, the creep strength of the steel material during use in a high-temperature environment is improved by precipitation strengthening. If the Mo content is less than 0.01%, the above-mentioned On the other hand, if the Mo content exceeds 2.50%, even if the contents of other elements are within the range of this embodiment, the diffusion of Cr is suppressed in a high-temperature environment, and the effect is not sufficiently obtained. The Mo content is 0.01 to _2.50 %. The Mo content suppresses the formation of _Cr2O3 on the steel surface in the environment. As a result, the steam oxidation resistance of the steel decreases. The lower limit is preferably 0.03%, more preferably 0.05%, and even more preferably 0.10%. The upper limit of the Mo content is preferably 2.45%, more preferably 2.40%, further preferably 2.35%, and further preferably 2.30%.

W: 2.5-6.0%
Tungsten (W) dissolves in the parent austenite phase and increases the creep strength of steel materials used in high-temperature environments through solution strengthening. W also forms Laves phase (Fe ₂ W) etc. in high-temperature environments. W forms fine intermetallic compounds of Cr, which enhances the creep strength of steels in use at high temperatures through precipitation strengthening. W also promotes the diffusion of Cr in high temperature environments, and reduces the amount of Cr on the surface of steels in high temperature environments. As a result, _the steam oxidation resistance of the steel material is improved. If the _W content is less than 2.5%, even if the contents of other elements are within the range of this embodiment, On the other hand, if the W content exceeds 6.0%, the toughness and hot workability of the steel material are deteriorated even if the contents of other elements are within the range of this embodiment. Therefore, the W content is 2.5 to 6.0%. The lower limit of the W content is preferably 2.6%, more preferably 2.7%, and even more preferably 2.8%. %. The upper limit of the W content is preferably 5.8%, more preferably 5.5%, further preferably 5.3%, and further preferably 5.0%.

Al: 0.010-0.300%
Aluminum (Al), like Si and Mn, deoxidizes the steel during the manufacturing process. If the Al content is less than 0.010%, the other element contents can be within the range of this embodiment. On the other hand, if the Al content exceeds 0.300%, a large amount of nonmetallic inclusions is generated even if the contents of other elements are within the range of this embodiment. In this case, the hot workability and creep strength in a high-temperature environment of the steel material are reduced. Therefore, the Al content is 0.010 to 0.300%. The preferable lower limit of the Al content is 0.015%. The upper limit of the Al content is preferably 0.250%, more preferably 0.200%, and even more preferably 0.025%. It is preferably 0.150%, and more preferably 0.100%.

V:0.01~1.00%
Vanadium (V) combines with N to form V nitrides during use of austenitic stainless steel in high-temperature environments. V nitrides increase the creep strength of steel in high-temperature environments. V content If the V content is less than 0.01%, the above-mentioned effect cannot be sufficiently obtained even if the contents of other elements are within the range of this embodiment. Even if the contents of other elements are within the ranges of this embodiment, V nitrides will coarsen in a high temperature environment. In this case, the creep ductility of the steel material will decrease. Therefore, the V content should be 0.01 to 1 The preferred lower limit of the V content is 0.02%, more preferably 0.05%, and even more preferably 0.10%. The preferred upper limit of the V content is 0.90%. %, more preferably 0.80%, and even more preferably 0.70%.

B: 0.0005-0.0500%
Boron (B) segregates at the grain boundaries and promotes fine precipitation of intermetallic compounds, such as the Laves phase, and carbides at the grain boundaries in high-temperature environments. If the B content is more than 0.0500%, the above effect cannot be sufficiently obtained even if the contents of the other elements are within the range of this embodiment. Even within the range of the embodiment, the creep strength and creep ductility of the steel material during use in a high temperature environment are reduced. Therefore, the B content is 0.0005 to 0.0500%. The lower limit is 0.0010%, more preferably 0.0015%, and further preferably 0.0020%. The upper limit of the B content is preferably 0.0400%, more preferably 0.0300%, still more preferably 0.0200%, still more preferably 0.0100%, and still more preferably 0.0070%. %, more preferably 0.0050%, more preferably 0.0048%, more preferably 0.0046%, and even more preferably 0.0045%.

Rare earth elements (REM): 0.001-0.100%
Rare earth elements (REM) fix S as _sulfides and improve the hot workability of steel. REM also improves the adhesion of _Cr2O3 formed on the steel surface to the steel surface. If the REM content is less than 0.001%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the range of this embodiment. Even if the contents of other elements are within the ranges of the present embodiment, the amount of inclusions such as oxides will be excessively large, and the weldability of the steel material will decrease. Therefore, the REM content is set to 0.001 to 0.100. The preferred lower limit of the REM content is 0.003%, more preferably 0.005%, and even more preferably 0.010%. The preferred upper limit of the REM content is 0.090%. The content of C is preferably 0.080%, more preferably 0.070%, and even more preferably 0.050%.

In this specification, REM is a general term for 17 elements in total: Sc, Y, and lanthanides. When the REM contained in the steel material of this embodiment is one of these elements, the REM content means the content of the element contained. When the REM contained in this embodiment is two or more types, the REM content means the total content of those elements. REM is generally contained in misch metal. For example, misch metal may be added to the steel material in the steelmaking process to adjust the REM content to be within the above range.

N: 0.15-0.35%
Nitrogen (N) combines with V or Nb to form nitrides during use of austenitic stainless steels in high-temperature environments. These nitrides increase creep strength during use in high-temperature environments. N also dissolves in the steel material to increase the strength of the steel material. N also stabilizes austenite. If the N content is less than 0.15%, the contents of other elements are not affected by the present invention. Even if the N content is within the range of the above-mentioned form, the above-mentioned effect cannot be sufficiently obtained. On the other hand, if the N content exceeds 0.35%, an excessive amount of nitrides is formed. In this case, Even within the range of this embodiment, the toughness and hot workability of the steel material are reduced. Therefore, the N content is 0.15 to 0.35%. The preferable lower limit of the N content is 0.16 %, more preferably 0.17%, and even more preferably 0.18%. The upper limit of the N content is preferably 0.32%, more preferably 0.31%, and even more preferably is 0.30%.

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

[Regarding optional elements]
The chemical composition of the austenitic stainless steel material of this embodiment may further contain, in place of a portion of Fe, one or more elements selected from the following Groups 1 to 3. Groups 1 to 3 will be described below.
Group 1:
Ti: 0~0.100%
Cu: 0-1.00%
Co: 0 to 1.0%
Zr: 0-0.100%
Ta: 0 to 1.0%
Re: 0 to 5.0%
Group 2:
Ca: 0-0.0500%
Mg: 0-0.0500%
Group 3 Hf: 0-1.0%

[Group 1 (Ti, Cu, Co, Zr, Ta, Re)]
The austenitic stainless steel material of the present embodiment may further contain one or more elements selected from the group consisting of Ti, Cu, Co, Zr, Ta, and Re in place of a portion of Fe. All of the above elements are optional and increase the creep strength of the steel during use in high temperature environments.

Ti: 0~0.100%
Titanium (Ti) is an optional element and may not be contained. In other words, the Ti content may be 0%. If contained, Ti is added to the austenitic stainless steel material during use in a high-temperature environment. In this case, Ti bonds with N to form nitrides. Ti nitrides increase the creep strength of steel materials during use in high-temperature environments. The above effect can be obtained to some extent if even a small amount of Ti is contained. However, If the Ti content exceeds 0.100%, even if the contents of other elements are within the range of this embodiment, Ti nitrides become coarse, and the creep ductility of the steel material is deteriorated during use in a high-temperature environment. Therefore, the Ti content is 0 to 0.100%. The lower limit of the Ti content is preferably 0.001%, more preferably 0.002%, and even more preferably 0.005%. The upper limit of the Ti content is preferably 0.090%, more preferably 0.080%, and further preferably 0.070%.

Cu: 0-1.00%
Copper (Cu) is an optional element and may not be contained. In other words, the Cu content may be 0%. When contained, Cu is added to the steel material during use in a high-temperature environment. Cu precipitates as the Cu phase. The Cu phase increases the creep strength of steel materials used in high-temperature environments. Even if only a small amount of Cu is contained, the above effect can be obtained to some extent. However, if the Cu content exceeds 1.00%, If Cu is added, the raw material cost will be high. Therefore, the Cu content is 0 to 1.00%. The lower limit of the Cu content is preferably 0.01%, more preferably 0.05%, and even more preferably The upper limit of the Cu content is preferably 0.90%, more preferably 0.80%, still more preferably 0.70%, and still more preferably 0.60%. , and more preferably 0.50%.

Co: 0 to 1.0%
Cobalt (Co) is an optional element and may not be contained. That is, the Co content may be 0%. When contained, Co stabilizes the structure of the steel material and increases the creep strength of the steel material during use in a high-temperature environment. If even a small amount of Co is contained, the above effect can be obtained to a certain extent. However, if the Co content exceeds 1.0%, the raw material cost becomes high. Therefore, the Co content is 0 to 1.0%. The preferred lower limit of the Co content is 0.1%, and more preferably 0.2%. The preferred upper limit of the Co content is 0.9%, and more preferably 0.8%, and more preferably 0.7%.

Zr: 0-0.100%
Zirconium (Zr) is an optional element and may not be contained. In other words, the Zr content may be 0%. When contained, Zr combines with N and O to form Zr nitride or Zr oxides are formed. These nitrides and oxides become nuclei for the precipitation of fine carbonitrides, and increase the creep strength of steel materials during use in high-temperature environments. Even a small amount of Zr can achieve the above effects. However, if the Zr content exceeds 0.100%, a large amount of Zr nitrides and Zr oxides are produced even if the contents of other elements are within the ranges of this embodiment. In this case, the hot workability and weldability of the steel material are deteriorated. Therefore, the Zr content is 0 to 0.100%. The lower limit of the Zr content is preferably 0.001%, and more preferably 0.002%. %, and more preferably 0.003%. The upper limit of the Zr content is preferably 0.090%, more preferably 0.080%, still more preferably 0.050%, and still more preferably is 0.040%.

Ta: 0 to 1.0%
Tantalum (Ta) is an optional element and may not be contained. That is, the Ta content may be 0%. When contained, Ta promotes refinement of carbonitrides. As a result, the creep strength of the steel material during use in a high-temperature environment is increased. If even a small amount of Ta is contained, the above effect can be obtained to a certain extent. However, if the Ta content exceeds 1.0%, excessively large amounts of precipitates are generated, and the toughness of the steel material decreases even if the contents of other elements are within the range of this embodiment. Therefore, the Ta content is 0 to 1.0%. The preferred lower limit of the Ta content is 0.1%, more preferably 0.2%, and more preferably 0.3%. The preferred upper limit of the Ta content is 0.9%, more preferably 0.8%, and more preferably 0.7%.

Re: 0 to 5.0%
Rhenium (Re) is an optional element and may not be contained. That is, the Re content may be 0%. When contained, Re dissolves in the steel material and increases the creep strength of the steel material during use in a high-temperature environment. If even a small amount of Re is contained, the above effect can be obtained to a certain extent. However, if the Re content exceeds 5.0%, the hot workability of the steel material decreases even if the contents of other elements are within the range of this embodiment. Therefore, the Re content is 0 to 5.0%. The preferred lower limit of the Re content is 0.1%, more preferably 0.2%, more preferably 0.5%, more preferably 1.0%, more preferably 1.5%, and more preferably 2.0%. The preferred upper limit of Re is 4.8%, more preferably 4.6%.

[Group 2 (Ca, Mg)]
The austenitic stainless steel material of the present embodiment may further contain one or more elements selected from the group consisting of Ca and Mg in place of a portion of Fe. All of these elements are optional elements, Improves the hot workability of steel.

Ca: 0-0.0500%
Calcium (Ca) is an optional element and may not be contained. In other words, the Ca content may be 0%. When contained, Ca fixes S as sulfides and improves the strength of the steel material. Improves hot workability. Ca also deoxidizes the steel. Even if even a small amount of Ca is contained, the above effects can be obtained to some extent. However, if the Ca content exceeds 0.0500%, the other elements Even if the content is within the range of this embodiment, the toughness and hot workability of the steel material are reduced. Therefore, the Ca content is 0 to 0.0500%. The preferable lower limit of Ca is 0.0001%. and more preferably 0.0005%, more preferably 0.0010%, more preferably 0.0020%, more preferably 0.0050%, and even more preferably 0.0100%. The preferred upper limit of the Ca content is 0.0400%.

Mg: 0-0.0500%
Magnesium (Mg) is an optional element and may not be contained. In other words, the Mg content may be 0%. When Mg is contained, Mg fixes S as sulfides and improves the strength of the steel material. The hot workability is improved. If even a small amount of Mg is contained, the above effect can be obtained to some extent. However, if the Mg content exceeds 0.0500%, the contents of other elements within the range of this embodiment are Even if Mg is present, the toughness and hot workability of the steel material will decrease. Therefore, the Mg content is 0 to 0.0500%. The preferable lower limit of Mg is 0.0001%, and more preferably 0.0005%. The upper limit of the Mg content is preferably 0.0400%, more preferably 0.0010%, more preferably 0.0020%, and even more preferably 0.0050%. It is 0.0300%.

[Third Group (Hf)]
The austenitic stainless steel material of this embodiment may further contain Hf in place of a portion of Fe.
Hf: 0 to 1.0%
Hafnium (Hf) is an optional element and may not be contained. That is, the Hf content may be 0%. When contained, Hf enhances the action of Ta, Zr and REM. If even a small amount of Hf is contained, the above effect can be obtained to some extent. However, if the Hf content exceeds 1.0%, a large amount of nonmetallic inclusions is generated. In this case, even if the contents of other elements are within the range of this embodiment, the toughness, hot workability and weldability of the steel material are reduced. Therefore, the Hf content is 0 to 1.0%. The preferable lower limit of the Hf content is 0.1%, more preferably 0.2%, and more preferably 0.3%. The preferable upper limit of the Hf content is 0.9%, and more preferably 0.8%.

[Regarding formula (1)]
Furthermore, in the austenitic stainless steel material of this embodiment, the grain size number D of the austenitic stainless steel material measured in accordance with ASTM E112-88 satisfies formula (1).
2W/7-Mo+3D/4≧3.00 (1)
Here, the grain size number is substituted for D in formula (1), and the content (mass%) of the corresponding element is substituted for each element symbol in formula (1).

It is defined as F1 = 2W/7 - Mo + 3D/4. F1 is an index showing _the degree of _Cr2O3 formation on the steel surface in a high temperature environment. As described above, W promotes the diffusion of Cr in a high temperature environment. Mo suppresses the diffusion of Cr in a high temperature environment. And, the larger the grain size number D, the more the diffusion of Cr is promoted.

If F1 is 3.00 or more, _Cr is sufficiently diffused in a high-temperature environment. Therefore, _Cr2O3 is easily generated uniformly on the steel surface during use of the austenitic stainless steel material in a high-temperature environment. As a result, the steam oxidation resistance of the austenitic stainless steel material in a high-temperature environment is sufficiently improved. The preferable lower limit of F1 is 3.10, more preferably 3.20, more preferably 3.30, more preferably 3.50, and more preferably 3.80. F1 is a value obtained by rounding off the second decimal place of the calculated value (i.e., the value to the second decimal place).

[Regarding grain size number D of austenitic stainless steel material]
The grain size number D of the austenitic stainless steel material of this embodiment can be determined by the following method. Take one sample from the center position of the thickness of the austenitic stainless steel material. If the austenitic stainless steel material is a steel pipe, take a sample from the center position of the wall thickness. If the austenitic stainless steel material is a steel plate, take a sample from the center position of the plate width and the center position of the plate thickness. If the austenitic stainless steel material is a steel bar, take a sample from the center position of a cross section perpendicular to the longitudinal direction.

Of the surfaces of the collected samples, a cross section perpendicular to the longitudinal direction of the austenitic stainless steel material is taken as the observation surface. The observation surface is mirror-polished. The mirror-polished observation surface is corroded using a mixed solution of hydrochloric acid and nitric acid to reveal the austenite grain boundaries on the observation surface. Three arbitrary fields of view on the corroded observation surface are observed, and the grain size number of the austenite grains is determined based on the following formula in accordance with ASTM E112-88. The area of each field is 0.75 ^mm2 . The arithmetic average value of the grain size numbers of the three fields of view is defined as the grain size number D.
Grain size number D = -3.2877-6.6439log ₁₀ L
Here, L is the average line segment length (mm) per crystal grain of the test line crossing the crystal grain.

In order to obtain the grain size number D of the austenitic stainless steel material, it is a prerequisite that the area ratio of the unrecrystallized region in each of the above-mentioned fields (0.75 ^mm2 ) is less than 20%. In other words, in the austenitic stainless steel material of this embodiment, if the area ratio of the unrecrystallized region in each of the above-mentioned three fields is less than 20%, the grain size number can be determined. If the grain size number cannot be measured in even one of the above-mentioned three fields, the grain size number D of the austenitic stainless steel material of this embodiment cannot be determined.

The area ratio of the unrecrystallized region in the microstructure can be measured by the following method. In the visual field (area 0.75 ^mm2 ) used to determine the above-mentioned grain size number D, the unrecrystallized region and the region other than the unrecrystallized region can be clearly distinguished by contrast. Therefore, the area of the unrecrystallized region is calculated in each visual field. The area ratio (%) of the unrecrystallized region is calculated based on the area of the unrecrystallized region and the visual field area (0.75 ^mm2 ).

[Oxygen concentration on the surface of steel material]
Furthermore, in the austenitic stainless steel material of this embodiment, the oxygen concentration at a depth of 1 μm from the surface of the austenitic stainless steel material is less than 1.00 mass %. In other words, in the austenitic stainless steel material of this embodiment, substantially no oxide scale is present on the surface.

When the oxygen concentration at a depth of 1 μm from the surface of an austenitic stainless steel material is 1.00 mass% or more, it means that an oxide scale is present on the surface of the steel material. Here, the oxide scale consists of Fe oxides and/or Fe-Cr spinel oxides. If an oxide scale is present on the surface, the oxide scale inhibits the formation of Cr ₂ O ₃ on the surface of the austenitic stainless steel material during use in a high-temperature environment. In this case, Cr ₂ O ₃ is unlikely to be formed uniformly on the steel material surface. As a result, the steam oxidation resistance of the steel material in a high-temperature environment is reduced.

When the oxygen concentration at a depth of 1 μm from the surface of the austenitic stainless steel material is less than 1.00 mass%, substantially no oxide scale is present on the steel surface. In this case, _Cr2O3 is easily generated uniformly on the surface of the austenitic stainless steel material during use in a high-temperature environment. As a result, the steam oxidation resistance of the steel material in _a high-temperature environment is improved.

[Method of measuring oxygen concentration]
The oxygen concentration at a depth of 1 μm from the surface of the austenitic stainless steel material can be measured by the following method. A sample including the surface of the austenitic stainless steel material is collected. The surface of the collected sample that corresponds to the surface of the austenitic stainless steel material is used as the observation surface. A depth direction analysis is performed on the observation surface in the depth direction by X-ray photoelectron spectroscopy (hereinafter referred to as XPS). At this time, the beam diameter is set to 100 μm, and the depth direction analysis is performed at a pitch of 48 nm in the depth direction. In the depth direction analysis, the oxygen concentration (mass %) is measured. The oxygen concentration (mass %) at a depth of 1 μm from the observation surface is measured. By the above method, the oxygen concentration (mass %) at three points on the surface of the austenitic stainless steel material is determined at a depth of 1 μm (= 1008 nm) from the surface. The arithmetic average value of the determined oxygen concentrations is defined as the oxygen concentration (mass %) at a depth of 1 μm from the surface of the austenitic stainless steel material.

The preferred upper limit of the oxygen concentration at a depth of 1 μm from the surface of the austenitic stainless steel material of this embodiment is 0.80 mass%, more preferably 0.50 mass%, more preferably 0.30 mass%, more preferably 0.10 mass%, more preferably 0.05 mass%, and even more preferably 0.01 mass%.

[Preferred Grain Size Number D]
The grain size number D of the austenitic grains of the austenitic stainless steel material of this embodiment is preferably 2.50 to 8.00. If the grain size number D is 2.50 or more, a path (grain boundary) for Cr to diffuse from the inside of the steel material to the steel material surface in a high temperature environment is sufficiently formed. Therefore, Cr is more easily diffused in a high temperature environment, and Cr ₂ O ₃ is more easily generated uniformly on the steel material surface in a high temperature environment. In addition, if the grain size number D is 8.00 or less, the creep strength in a high temperature environment can be maintained high. Therefore, the preferred grain size number D is 2.50 to 8.00. A more preferred lower limit of the grain size number D is 2.55, more preferably 2.60, more preferably 2.65, and even more preferably 2.70. A more preferred upper limit of the grain size number D is 7.80, and even more preferably 7.50.

[Preferable arithmetic average roughness Ra]
In the austenitic stainless steel material of this embodiment, the arithmetic mean roughness Ra of the steel material surface is preferably 10.00 μm or less.

The roughness of the steel surface corresponds to the surface area on which _Cr2O3 is formed. If the arithmetic mean roughness Ra of the steel surface is _10.00 μm _or less, the surface of the steel is flat and has a small surface area. In this case, the formation of _Cr2O3 becomes easier. As a result, the steam oxidation resistance of the steel in a high-temperature environment is further improved.

[Method of measuring arithmetic mean roughness Ra]
The arithmetic mean roughness Ra of the austenitic stainless steel surface is measured by the arithmetic mean roughness measurement method specified in JIS B 0601 (2013). Specifically, 10 arbitrary locations are measured on the surface of the austenitic stainless steel. At each measurement location, the arithmetic mean roughness Ra is measured over an evaluation length extending in the longitudinal direction of the steel. The evaluation length is 5 times the reference length (cutoff wavelength). The arithmetic mean roughness Ra is measured using a laser measurement type roughness meter, and the measurement speed is 0.5 mm/sec. Of the 10 arithmetic mean roughnesses Ra obtained, the arithmetic mean value of the six arithmetic mean roughnesses Ra excluding the maximum arithmetic mean roughness Ra, the second maximum arithmetic mean roughness Ra, the minimum arithmetic mean roughness Ra, and the second minimum arithmetic mean roughness Ra is defined as the "arithmetic mean roughness Ra" (μm).

The more preferred upper limit of the arithmetic mean roughness Ra of the surface of the austenitic stainless steel material of this embodiment is 9.50 μm, more preferably 9.00 μm, more preferably 8.50 μm, and even more preferably 8.00 μm. The lower limit of the arithmetic mean roughness Ra is not particularly limited. However, taking industrial production into consideration, the preferred lower limit of the arithmetic mean roughness Ra is 0.10 μm, more preferably 0.50 μm, and even more preferably 0.80 μm.

As explained above, the austenitic stainless steel material of this embodiment has the above-mentioned chemical composition, and the grain size number D satisfies formula (1). Furthermore, the oxygen concentration at a depth of 1 μm from the surface is less than 1.00 mass %. Therefore, excellent steam oxidation resistance is obtained in high-temperature environments.

[Production method]
A method for producing the austenitic stainless steel material of the present embodiment described above will be described. The method for producing the austenitic stainless steel material described below is one example of the method for producing the austenitic stainless steel material of the present embodiment. Therefore, the austenitic stainless steel material having the above-mentioned configuration may be produced by a production method other than the production method described below. However, the production method described below is a preferred example of the method for producing the austenitic stainless steel material of the present embodiment.

The manufacturing method of the austenitic stainless steel material of this embodiment includes a process of preparing a material (preparation process), a process of performing hot working on the material to produce an intermediate steel material (hot working process), a process of performing pickling on the intermediate steel material after the hot working process and then performing cold working (cold working process), a process of performing solution treatment on the intermediate steel material after the cold working process (solution treatment process), and a process of performing pickling on the intermediate steel material after the solution treatment process to remove surface scale (pickling treatment process). Each process will be described below.

[Preparation process]
In the preparation step, a material having the above-mentioned chemical composition is prepared. The material may be supplied from a third party or may be manufactured. The material may be an ingot, a slab, a bloom, or a billet. When manufacturing the material, the material is manufactured by the following method. Molten steel having the above-mentioned chemical composition is manufactured. The manufactured molten steel is used to manufacture an ingot by an ingot casting method. The manufactured molten steel may be used to manufacture a slab, a bloom, or a billet (cylindrical material) by a continuous casting method. The manufactured ingot, slab, or bloom may be subjected to hot working to manufacture a billet. For example, hot forging may be performed on an ingot to manufacture a cylindrical billet, 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 is, for example, 900 to 1300°C.

[Hot processing process]
In the hot working step, hot working is performed on the raw material to produce an intermediate steel material having a predetermined shape. The intermediate steel material may be, for example, a steel pipe, a steel plate, or a steel bar.

Before hot working, the material is heated. The heating temperature of the material is not particularly limited, but is, for example, 900 to 1300°C.

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 material. A through hole is formed along the central axis of the cylindrical material by machining. The cylindrical material with the through hole is subjected to hot extrusion, typically the Ugine-Séjournet process, to produce the intermediate steel material (steel pipe). Instead of hot extrusion, piercing and rolling according to the Mannesmann process may be carried out to produce the steel pipe. In this case, the material is pierced and rolled using a piercing machine. The pierced and rolled material is further hot rolled using a mandrel mill, reducer, sizing mill, etc. to produce the intermediate steel material (steel pipe).

When the intermediate steel material is a steel plate, the hot working process involves the following hot rolling: 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 the steel plate.

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. For example, a blooming mill is used for the rough rolling process. If a continuous rolling mill is installed downstream of the blooming mill, the billet after blooming may be further hot rolled using the continuous rolling mill to produce a smaller billet. In the continuous rolling mill, for example, a horizontal stand having a pair of horizontal rolls and a vertical stand having a pair of vertical rolls are alternately arranged in a row. In the finish rolling process, the billet after the rough rolling process is reheated at a known temperature (900 to 1300°C). In the finish rolling process, the heated billet is hot rolled using the continuous rolling mill to produce a steel bar.

Note that hot forging may be carried out as a hot working process to produce intermediate steel products (steel pipes, steel plates, steel bars). Hot forging is, for example, stretch forging.

[Cold working process]
In the cold working step, the intermediate steel material is subjected to pickling treatment and then cold working. When the intermediate steel material is a steel pipe or a steel bar, the cold working is, for example, cold drawing. When the intermediate steel material is a steel plate, the cold working is, for example, cold rolling. The reduction in area in the cold working step is not particularly limited, but is, for example, 10 to 90%.

[Solution treatment process]
In the solution treatment process, the intermediate steel material after the cold working process is subjected to solution treatment. The solution treatment dissolves precipitates in the intermediate steel material. The solution treatment process also adjusts the austenite grains in the steel material.

The solution treatment is carried out by the following method. The intermediate steel material after the cold working process is charged into a heat treatment furnace in which the atmosphere inside the furnace is an air atmosphere. The air atmosphere here means an atmosphere containing 78% or more by volume of nitrogen, which is a gas that constitutes the air, and 20% or more by volume of oxygen. In the furnace in the air atmosphere, the intermediate steel material is heated to the next solution treatment temperature T1, and is held at the solution treatment temperature T1 for a holding time t1.
Solution treatment temperature T1: 1150°C or higher Holding time t1: 1 minute or longer

If the solution treatment temperature T1 is less than 1150 ° C. or the holding time t1 is less than 1 minute, the recrystallization of austenite during heat treatment is insufficient. Therefore, the proportion of unrecrystallized regions in the microstructure increases. In this case, the steam oxidation resistance in a high-temperature environment decreases. In this case, the proportion of unrecrystallized regions in the microstructure increases, so the grain size number D cannot be determined. Therefore, the solution treatment temperature T1 is 1150 ° C. or higher, and the holding time t1 is 1 minute or higher. The upper limit of the solution treatment temperature T1 and the upper limit of the holding time t1 are not particularly limited. However, if the solution treatment temperature T1 is too high or the holding time t1 is too long, the grain size number D may become small (i.e., coarse grains) and may not satisfy formula (1). Therefore, the solution treatment temperature T1 and the holding time t1 may be appropriately adjusted within a range that satisfies formula (1). The preferred lower limit of the solution treatment temperature T1 is 1160°C, more preferably 1170°C, and even more preferably 1180°C. The preferred lower limit of the holding time t is 2 minutes, and even more preferably 3 minutes.

Preferably, the solution treatment temperature T1 and holding time t1 are as follows:
Solution treatment temperature T1: 1150 to 1300°C
Holding time t1: 1 to 30 minutes

If the solution treatment temperature T1 is 1300°C or less and the holding time t1 is 30 minutes or less, the grain size number D will be 2.50 or more. Therefore, the preferred upper limit of the solution treatment temperature T1 is 1300°C, and the preferred upper limit of the holding time t1 is 30 minutes. The preferred upper limit of the solution treatment temperature T1 is 1290°C, and more preferably 1280°C. The preferred upper limit of the holding time t is 28 minutes, more preferably 25 minutes, and even more preferably 22 minutes.

[Pickling treatment process]
In the pickling process, the intermediate steel material after the solution treatment is subjected to pickling. The oxide scale formed on the surface of the intermediate steel material is removed by the pickling process. As described above, the oxide scale formed on the surface of the intermediate steel material before the pickling process is mainly composed of Fe oxides and/or Fe-Cr spinel oxides. The pickling process is performed to sufficiently remove the oxide scale from the surface of the intermediate steel material, so that the oxygen concentration at a depth of 1 μm from the steel material surface is less than 1.00 mass%.

A mixed solution of nitric acid and hydrofluoric acid is used as the pickling solution. The mixed solution is, for example, an aqueous solution containing 5.0 to 8.0% by volume of nitric acid and 5.0 to 8.0% by volume of hydrofluoric acid.

The immersion time t2 in the above-mentioned pickling solution in the pickling treatment (hereinafter referred to as pickling time) is set to the following condition.
Pickling time t2: 1.0 hours or more

If the pickling time t2 is less than 1.0 hour, the oxide scale cannot be sufficiently removed from the steel surface. As a result, the oxygen concentration at a depth of 1 μm from the surface of the austenitic stainless steel material will be 1.00 mass% or more. If the pickling time t2 is 1.0 hour or more, the oxygen concentration at a depth of 1 μm from the surface of the austenitic stainless steel material will be less than 1.00 mass%.

Preferably, in the pickling treatment step, the temperature T2 of the pickling solution (hereinafter referred to as the bath temperature) and the pickling time t2 are set to the following conditions.
Bath temperature T2: Room temperature to 40°C
Pickling time t2: 1.0 to 5.0 hours

If the bath temperature T2 is 40°C or less, the arithmetic mean roughness Ra of the steel surface after pickling treatment will be 10 μm or less. Furthermore, if the pickling time t2 is 1.0 to 5.0 hours, the arithmetic mean roughness Ra of the steel surface after pickling treatment will be 10.00 μm or less. As a result, the steam oxidation resistance of the austenitic stainless steel material is further improved.

The austenitic stainless steel material of this embodiment is manufactured by the above manufacturing process. The austenitic stainless steel material of this embodiment has the above-mentioned chemical composition, and the grain size number D satisfies formula (1). Furthermore, the oxygen concentration at a depth of 1 μm from the surface is less than 1.00 mass %. Therefore, excellent steam oxidation resistance is obtained in a high-temperature environment. If the grain size number D is 2.50 to 8.00, even better steam oxidation resistance is obtained. If the arithmetic mean roughness Ra of the steel surface is 10.00 μm or less, even better steam oxidation resistance is obtained.

The effects of the austenitic stainless steel material of this embodiment will be explained in more detail below using examples. The conditions in the following examples are 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.

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

"-" in Table 1 means that the corresponding element content was below the detection limit. A cylindrical ingot was produced using the molten steel with the test number in Table 1. The ingot was heated at 1220°C for 3 hours. The heated ingot was hot forged to produce a cylindrical billet. A through hole was formed in the central axis of the cylindrical billet by machining. The cylindrical billet with the through hole was hot extruded to produce an intermediate steel material (steel pipe). The temperature of the billet before hot extrusion was 1200°C. The intermediate steel material after hot extrusion 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.

The resulting intermediate steel material was subjected to solution treatment at the solution treatment temperature T1 and holding time t1 shown in Table 2. The intermediate steel material after solution treatment was subjected to pickling treatment at the bath temperature T2 and pickling time t2 shown in Table 2. The pickling solution was an aqueous solution containing 5.0 to 8.0% by volume of nitric acid and 5.0 to 8.0% by volume of hydrofluoric acid. Austenitic stainless steel materials with each test number were manufactured using the above process.

[Evaluation test]
The following evaluation tests were carried out on the austenitic stainless steel materials with each test number.

[Surface oxygen concentration measurement test]
A sample including the surface of the austenitic stainless steel material (steel pipe) of each test number was collected. The surface of the collected sample corresponding to the surface of the austenitic stainless steel material was used as the observation surface. An XPS depth direction analysis was performed on the observation surface in the depth direction. At this time, the beam diameter was set to 100 μm, and the depth direction analysis was performed at a pitch of 48 nm in the depth direction. In the depth direction analysis, the oxygen concentration (mass%) at a depth position of 1 μm from the observation surface was measured. Specifically, the oxygen concentration (mass%) at a depth position of 1 μm from the surface was obtained at three arbitrary points on the surface of the austenitic stainless steel material. The arithmetic average value of the obtained oxygen concentration was defined as the oxygen concentration (mass%) at a depth position of 1 μm from the surface of the austenitic stainless steel material. The obtained oxygen concentration (mass%) is shown in the "Oxygen concentration (mass%)" column in Table 2.

[Grain size number D measurement test]
The grain size number D of the austenitic stainless steel material of each test number was obtained by the following method. One sample was taken from the center position of the wall thickness of the austenitic stainless steel material (steel pipe). Of the surfaces of the taken sample, a cross section perpendicular to the longitudinal direction of the austenitic stainless steel material was used as the observation surface. The observation surface was mirror-polished. The mirror-polished observation surface was corroded using a mixed solution of hydrochloric acid and nitric acid to reveal the austenite grain boundaries on the observation surface. Three arbitrary fields (each field area of 0.75 mm ² ) of the corroded observation surface were observed, and the grain size number of the austenite grains was obtained based on the following formula in accordance with ASTM E112-88. The arithmetic average value of the grain size numbers of the three fields was defined as the grain size number D.
Grain size number D = -3.2877-6.6439log ₁₀ L
Here, L is the average line segment length (mm) per crystal grain of the test line crossing the crystal grain.
The resulting grain size numbers D are shown in Table 2.

[Measurement test of arithmetic mean roughness Ra]
The arithmetic mean roughness Ra of the surface of the austenitic stainless steel material (steel pipe) of each test number was measured by the arithmetic mean roughness measurement method specified in JIS B 0601 (2013). Specifically, 10 arbitrary locations were measured on the surface of the austenitic stainless steel material. At each measurement location, the arithmetic mean roughness Ra was measured over an evaluation length extending in the longitudinal direction of the steel material. The evaluation length was 5 times the reference length (cutoff wavelength). The arithmetic mean roughness Ra was measured using a laser measurement type roughness meter (VR-3000 G2 manufactured by KEYENCE) at a measurement speed of 0.5 mm/sec. Of the ten arithmetic mean roughnesses Ra thus obtained, the maximum arithmetic mean roughness Ra, the second maximum arithmetic mean roughness Ra, the minimum arithmetic mean roughness Ra, and the second minimum arithmetic mean roughness Ra were excluded, and the arithmetic mean of the six arithmetic mean roughnesses Ra was defined as the "arithmetic mean roughness Ra." The obtained arithmetic mean roughnesses Ra (μm) are shown in the "Ra (μm)" column of Table 2.

[Steam oxidation resistance evaluation test]
A test piece including the inner surface (pipe inner surface) of the austenitic stainless steel material (steel pipe) of each test number was taken. The size of the test piece was about 3 mm in the wall thickness direction (t direction), 25 mm in the longitudinal direction of the steel material (l direction), and 10 mm in the direction perpendicular to the wall thickness direction and the longitudinal direction of the steel material (w direction). The test piece was held in a 100% water vapor atmosphere at 650°C for 1000 hours. The dissolved oxygen content of the water vapor atmosphere was set to 100 ppb. The test piece after being held in a 100% water vapor atmosphere at 650°C for 1000 hours was cut perpendicular to the l direction. The cut surface (a surface including the t direction and the w direction, i.e., a cross section perpendicular to the longitudinal direction of the steel pipe) was used as the observation surface. The observation surface was mirror-polished. Among the observation surfaces after mirror polishing, any 10 fields of view near the pipe inner surface were selected. In each field of view, the inner layer oxide scale thickness (inner layer oxide scale depth) from the pipe inner surface was measured. The arithmetic mean value of the inner oxide scale thicknesses in the 10 visual fields was defined as the inner oxide scale thickness (μm) of the austenitic stainless steel material of that test number. The obtained inner oxide scale thicknesses are shown in the "Inner oxide scale thickness (μm)" column in Table 2.

[Test Results]
With reference to Table 2, the chemical compositions and manufacturing conditions were appropriate for test numbers 1 to 18, 26, 27, 30, 31, and 39 to 46. Therefore, the austenitic stainless steel materials of test numbers 1 to 18, 26, 27, 30, 31, and 39 to 46 satisfied formula (1) and had oxygen concentrations of less than 1.00 mass% at a depth of 1 μm from the surface. As a result, the inner oxide scale thicknesses of test numbers 1 to 18, 26, 27, 30, 31, and 39 to 46 were thin, less than 12.00 μm, and had excellent steam oxidation resistance.

Compared to test numbers 39 to 42, test numbers 1 to 18, 26, 27, 30, 31, and 43 to 46 had grain size numbers D in the range of 2.50 to 8.00, or arithmetic mean roughness Ra of 10.00 μm or less. Therefore, test numbers 1 to 18, 26, 27, 30, 31, and 43 to 46 had thinner inner oxide scale thicknesses than test numbers 39 to 42.

In addition, in test numbers 1 to 18, the grain size number D was within the range of 2.50 to 8.00, and the arithmetic mean roughness Ra was 10.00 μm or less. Therefore, the thickness of the inner oxide scale was even thinner than in test numbers 26, 27, 30, 31, and 43 to 46.

On the other hand, in test numbers 19 and 20, the solution treatment temperature T1 was too low. As a result, there were fields where the area ratio of the unrecrystallized region exceeded 20%, and the grain size number D could not be identified. As a result, the thickness of the inner oxide scale was 12.00 μm or more, and the steam oxidation resistance was low.

In test numbers 21 and 22, the solution treatment temperature T1 was high, the grain size number D was small, and F1 did not satisfy formula (1). As a result, the thickness of the inner oxide scale was 12.00 μm or more, and the steam oxidation resistance was low.

In test numbers 23 and 24, the holding time t1 at the solution treatment temperature T1 was too short. As a result, there were fields of view where the area ratio of the unrecrystallized region exceeded 20%, and the grain size number D could not be identified. As a result, the thickness of the inner oxide scale was 12.00 μm or more, and the steam oxidation resistance was low.

In test number 25, the holding time t1 at the solution treatment temperature T1 was long, the grain size number D was small, and F1 did not satisfy formula (1). As a result, the thickness of the inner oxide scale was 12.00 μm or more, and the steam oxidation resistance was low.

In test numbers 28 and 29, the pickling time t2 in the pickling process was too short. As a result, the oxygen concentration at a depth of 1 μm from the surface was 1.00 mass% or more. As a result, the thickness of the inner oxide scale was 12.00 μm or more, and the steam oxidation resistance was low.

In test numbers 32 and 33, the Cr content was too low. As a result, the thickness of the inner oxide scale was 12.00 μm or more, and the steam oxidation resistance was low.

In test numbers 34 and 35, the Mo content was too high. As a result, the thickness of the inner oxide scale was 12.00 μm or more, and the steam oxidation resistance was low.

In test number 36, the W content was too low. As a result, the thickness of the inner oxide scale was 12.00 μm or more, and the steam oxidation resistance was low.

In test numbers 37 and 38, F1 did not satisfy formula (1). As a result, the thickness of the inner oxide scale was 12.00 μm or more, and the steam oxidation resistance was low.

The above describes an embodiment of the present invention. However, the above-described embodiment is merely an example for implementing the present invention. Therefore, the present invention is not limited to the above-described embodiment, and the above-described embodiment can be modified as appropriate within the scope of the spirit of the present invention.

Claims (6)

An austenitic stainless steel material,
In mass percent,
C: 0.01-0.15%,
Si: 0.01-1.00%,
Mn: 2.00% or less,
P: 0.040% or less,
S: 0.010% or less,
Cr: 20.0-25.0%,
Ni: 23.0-32.0%,
Nb: 0.10-1.00%,
Mo: 0.01-2.50%,
W: 2.5-6.0%,
Al: 0.010-0.300%,
V: 0.01-1.00%,
B: 0.0005-0.0500%,
REM: 0.001-0.100%,
N: 0.15-0.35%,
Ti: 0 to 0.100%,
Cu: 0 to 1.00%,
Co: 0-1.0%,
Zr: 0 to 0.100%,
Ta: 0 to 1.0%,
Re: 0-5.0%,
Ca: 0-0.0500%,
Mg: 0 to 0.0500%,
Hf: 0 to 1.0%, and
The balance is Fe and impurities.
The grain size number D of the austenitic stainless steel material measured in accordance with ASTM E112-88 satisfies formula (1),
The oxygen concentration at a depth of 1 μm from the surface of the austenitic stainless steel material is less than 1.00 mass%.
Austenitic stainless steel.
2W/7-Mo+3D/4≧3.00 (1)
Here, the grain size number is substituted for D in formula (1), and the content (mass %) of the corresponding element is substituted for each element symbol in formula (1). 2. The austenitic stainless steel material according to claim 1,
The grain size number D is 2.50 to 8.00;
Austenitic stainless steel. The austenitic stainless steel material according to claim 1 or 2,
The arithmetic mean roughness Ra of the steel surface is 10.00 μm or less.
Austenitic stainless steel. The austenitic stainless steel material according to any one of claims 1 to 3,
Ti: 0.001 to 0.100%,
Cu: 0.01 to 1.00%,
Co: 0.1 to 1.0%,
Zr: 0.001 to 0.100%,
Contains one or more elements selected from the group consisting of Ta: 0.1 to 1.0% and Re: 0.1 to 5.0%;
Austenitic stainless steel. The austenitic stainless steel material according to any one of claims 1 to 4,
Ca: 0.0001 to 0.0500%, and
Mg: Contains one or more elements selected from the group consisting of 0.0001 to 0.0500%;
Austenitic stainless steel. The austenitic stainless steel material according to any one of claims 1 to 5,
Hf: 0.1 to 1.0%;
Austenitic stainless steel.

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