JP6805574B2 - Austenitic heat resistant steel and austenitic heat transfer member - Google Patents

Description

The present invention relates to heat-resistant steel and heat transfer members, and more particularly to austenitic heat-resistant steel and austenitic heat transfer members used in a high-temperature steam oxidation environment or the like.

In thermal power plants, improvement of power generation efficiency is required from the viewpoint of CO ₂ gas emission control and economic efficiency, and turbine steam pressure is being increased and increased. Heat transfer members used in thermal power plants are exposed to high temperature and high pressure steam for a long time. The heat transfer member is, for example, a pipe for a boiler. When exposed to high temperature steam for a long time, oxide scale is formed on the surface of the heat transfer member. If the water vapor oxidation resistance of the heat transfer member is not sufficient, a large amount of oxidation scale is generated on the surface of the heat transfer member. By starting and stopping the boiler, the heat transfer member thermally expands and contracts. Therefore, if a large amount of oxide scale is generated, the oxide scale is peeled off and causes clogging of the pipe. When a large amount of oxide scale is generated, the oxide scale further hinders heat conduction from the outside of the pipe to the inside of the pipe. Therefore, it is necessary to apply more heat from the outside in order to keep the temperature inside the pipe high. An increase in the temperature of the pipe causes a decrease in creep strength. Therefore, high steam oxidation resistance is required for heat transfer members used in equipment such as boilers for thermal power generation, turbines, and steam pipes.

For example, austenitic heat-resistant steel and ferritic heat-resistant steel have been developed as materials satisfying such characteristics. The austenitic heat-resistant steel is, for example, an austenitic heat-resistant steel having a Cr content of 18 to 25% by mass. The ferritic heat-resistant steel is, for example, a ferritic heat-resistant steel having a Cr content of 8 to 13% by mass. Austenitic heat-resistant steel has higher high-temperature strength than ferrite-based heat-resistant steel. Furthermore, austenitic heat-resistant steel has higher stability of the matrix structure and excellent high-temperature corrosion resistance as compared with ferritic heat-resistant steel. Therefore, austenitic heat-resistant steel is more suitable for heat transfer members than ferrite-based heat-resistant steel. However, austenitic heat-resistant steel is inferior in heat transfer characteristics to ferrite-based heat-resistant steel. Therefore, if not only the steam oxidation resistance of austenitic heat-resistant steel but also the heat transfer characteristics can be improved, it is possible to further improve the power generation efficiency in the thermal power plant.

A ferrite-based heat-resistant steel that suppresses the shedding of oxide scale is disclosed in, for example, Japanese Patent Application Laid-Open No. 11-92880 (Patent Document 1). The ferritic heat-resistant steel described in Patent Document 1 is a ferritic heat-resistant steel containing a high Cr content in which an oxide film is formed on the surface during use, and has a diameter of 1 micron or less at or near the interface with the oxide film. Very fine oxides are formed. Therefore, it is described in Patent Document 1 that the adhesion between the oxide film and the base material is improved.

For example, Japanese Patent Application Laid-Open No. 2007-39745 (Patent Document 2) discloses a method for improving steam oxidation resistance by increasing the Cr concentration on the surface of a ferrite heat-resistant steel. The method for improving the steam oxidation resistance of ferritic heat-resistant steel described in Patent Document 2 is to support powder particles containing Cr on the surface of a ferritic heat-resistant steel containing Cr, and to have a Cr concentration on the surface of the ferritic steel at a high temperature. A high Cr oxide layer is formed. It is described in Patent Document 2 that the oxidation resistance of a ferrite steel containing Cr can be easily and economically improved by this method.

For example, Japanese Patent Application Laid-Open No. 2013-127103 (Patent Document 3) discloses a method for improving oxidation resistance by forming a Cr oxide film on the surface of a ferrite heat-resistant steel. The oxidation-resistant treatment method for ferritic heat-resistant steel described in Patent Document 3 heat-treats a ferritic heat-resistant steel containing chromium in a gas atmosphere having a low oxygen partial pressure composed of a mixed gas of carbon dioxide gas and an inert gas. Therefore, it is characterized in that an oxide film containing chromium is formed on the surface of the ferritic steel. Patent Document 3 describes that this method can increase the Cr concentration in the scale and easily and economically improve the oxidation resistance characteristics of the ferrite heat-resistant steel.

For example, JP-A-2009-179884 (Patent Document 4) discloses a ferritic heat-resistant steel having improved steam oxidation resistance by adhering Cr to the surface of the ferritic heat-resistant steel. The ferrite-based heat-resistant steel described in Patent Document 4 is a ferrite-based heat-resistant steel used in a high-temperature and high-pressure steam environment, and Cr adhered by a shot peening treatment of a powdered Cr shot material is pre-oxidized. It is characterized by having a Cr oxide film on the surface of the base material. Patent Document 4 states that this ferritic heat-resistant steel has improved water vapor oxidation resistance because a protective film of an oxidation-resistant oxide is formed on the heat-resistant steel before it is used in an oxidizing environment. Is listed.

For example, Japanese Patent Application Laid-Open No. 2003-268503 (Patent Document 5) discloses an austenitic stainless steel pipe having improved water vapor oxidation resistance by forming a fine-grained structure having a uniform size as a whole. The austenitic stainless steel pipe described in Patent Document 5 has a mass% of C: 0.03 to 0.12%, Si: 0.1 to 0.9%, Mn: 0.1 to 2%, Cr: 15 to 22%, Ni: 8 to 15%, Ti: 0.002 to 0.05%, Nb: 0.3 to 1.5%, sol. Al: 0.0005 to 0.03%, N: 0.005 to 0.2%, and O (oxygen): 0.001 to 0.008%, the balance consisting of Fe and impurities, austenite grain size It is characterized by having a fine grain structure having a number of 7 or more. By forming the whole into a fine grain structure, the crystal grain boundaries serve as a diffusion path for Cr, and Cr inside the base metal is easily supplied to the surface layer portion. As a result, a film made of Cr ₂ O ₃ having high protection is formed on the surface, and the steam oxidation resistance is improved, which is described in Patent Document 5.

Japanese Unexamined Patent Publication No. 11-92880 Japanese Unexamined Patent Publication No. 2007-39745 Japanese Unexamined Patent Publication No. 2013-127103 JP-A-2009-179884 Japanese Unexamined Patent Publication No. 2003-268503

However, even if the above technique is used, the heat transfer characteristics and steam oxidation resistance of the heat transfer member may not be sufficiently enhanced.

An object of the present invention is to provide an austenitic heat-resistant steel and an austenitic heat transfer member having excellent heat transfer characteristics and water vapor oxidation resistance.

The austenitic heat-resistant steel according to the present embodiment includes a base material and an oxide layer A on the surface of the base material. The base material is mass%, C: 0.01 to 0.3%, Si: 0.01 to 2.0%, Mn: 0.01 to 2.0%, P: 0.10% or less, S. : 0.03% or less, Cr: 15.0 to 24.0%, Ni: 6.0 to 27.0%, N: 0.005 to 0.3%, sol. Al: 0.001 to 0.3%, Co: 0 to 5.0%, Ti: 0 to 1.0%, V: 0 to 1.0%, Nb: 0 to 1.0%, Hf: 0 ~ 1.0%, Ca: 0 to 0.1%, Mg: 0 to 0.1%, Zr: 0 to 0.1%, B: 0 to 0.1%, rare earth elements: 0 to 0.1 % And Cu: 0-5.0%, Mo: 0-5.0%, Ta: 0-5.0%, W: 0-5.0%, and Re: 0-5.0% A total of 0.5 to 10.0% of one or more selected from the above group is contained, and the balance has a chemical composition of Fe and impurities. The oxide layer A contains a chemical composition containing 20 to 45% in total of Cr and Mn in mass%. The oxide layer A contains, in mass%, a chemical composition containing 0.5 to 10% in total of one or more selected from the group consisting of Cu, Mo, Ta, W, and Re. The oxide layer A has a thickness of 1 μm or more.

The austenitic heat-resistant steel and the austenitic heat transfer member according to the present embodiment are excellent in heat transfer characteristics and steam oxidation resistance.

FIG. 1 is a cross-sectional view of the austenitic heat-resistant steel according to the present embodiment. FIG. 2 is a cross-sectional view of the austenitic heat transfer member according to the present embodiment.

Hereinafter, the present embodiment will be described in detail with reference to the drawings. The same or corresponding parts in the drawings are designated by the same reference numerals, and the description thereof will not be repeated.

The present inventors have conducted various studies on austenitic heat-resistant steel and austenitic heat transfer members. As a result, the following findings were obtained.

(1) The austenitic heat-resistant steel of the present embodiment can be used as a heat transfer member for boiler piping and the like. Heat transfer members such as boiler pipes come into contact with high-temperature steam. When exposed to high temperature water vapor for a long time, oxide scale is formed on the surface of the heat transfer member. Oxidation scale consists of various oxides and impurities. The oxide is, for example, Fe ₃ O ₄ , Cr ₂ O _{3, and the} like. The oxide scale forms an oxide film on the surface of the heat transfer member.

(2) If the thermal conductivity of the oxide film is low, the heat transfer characteristics from the outside of the heat transfer member to the inside of the heat transfer member deteriorate. Therefore, in order to maintain the inside of the heat transfer member at a high temperature, it is necessary to apply a large amount of heat from the outside of the heat transfer member, and the heat transfer characteristics of the boiler are deteriorated. When a large amount of heat is applied from the outside of the heat transfer member, the creep strength of the heat transfer member may further decrease. Therefore, it is preferable that the thermal conductivity of the oxide film is high. However, if the thermal conductivity of the oxide film is too high, the heat of high-temperature steam is transferred to the inner surface of the heat transfer member. Since the transferred heat promotes the oxidation reaction on the inner surface of the heat transfer member, a large amount of oxidation scale is generated on the inner surface of the heat transfer member. A large amount of oxide scale is exfoliated from the inner surface of the heat transfer member. When the heat transfer member is a pipe, the peeled oxide scale causes clogging of the pipe. Therefore, the thermal conductivity of the oxide film needs to be controlled within a certain range.

(3) If the thickness of the oxide scale is too thick, heat conduction from the outside of the heat transfer member to the inside of the heat transfer member is hindered. Therefore, the heat transfer characteristics of the boiler are deteriorated. Therefore, the thickness of the oxide film is preferably as thin as possible.

(4) Of the above-mentioned oxides, Fe ₃ O ₄ is thermodynamically and stably formed in a high-temperature steam oxidation environment (hereinafter, also referred to as a high-temperature steam environment). Fe ₃ O ₄ also has a high thermal conductivity. Therefore, if an oxide film containing a large amount of Fe ₃ O ₄ is formed on the surface of the heat transfer member in contact with high-temperature steam, the thermal efficiency of the boiler is improved. However, the thermal conductivity of the oxide film containing a large amount of Fe ₃ O ₄ is too high. Therefore, with this oxide film alone, as described above, a large amount of oxidation scale is generated on the inner surface of the heat transfer member.

(5) In a heat transfer member such as a boiler pipe, the Cr concentration on the inner surface of the pipe is improved, and an oxide film containing a large amount of Cr ₂ O ₃ is often formed on the inner surface of the heat transfer member. As a result, the formation of a large amount of oxidation scale is suppressed, and the steam oxidation resistance of the heat transfer member is improved. However, the oxide film containing a large amount of Cr ₂ O ₃ has a low thermal conductivity. Therefore, the heat transfer characteristics of the heat transfer member are deteriorated. Therefore, the heat transfer characteristics of the boiler cannot be improved only by this oxide film.

(6) Therefore, in a high-temperature steam environment, an oxide film composed of two layers, an oxide layer having excellent heat transfer characteristics and an oxide layer having both steam oxidation resistance and heat transfer characteristics, is formed as a heat transfer member. Formed on the inner surface. As a result, both excellent heat transfer characteristics and excellent water vapor oxidation resistance can be achieved.

(7) When Fe ₃ O ₄ having a volume fraction of 80% or more is contained, the thermal conductivity of the oxide layer is high. Therefore, the heat transfer characteristics of the boiler can be improved. Therefore, an oxide layer B containing Fe ₃ O ₄ having a volume fraction of 80% or more is formed on the surface of the heat transfer member in contact with high-temperature water vapor.

(8) On the other hand, an oxide layer C is formed between the oxide layer B and the base material as an oxide layer in which both water vapor oxidation resistance and heat transfer characteristics are achieved. The oxide layer C contains Cr and Mn in a total amount of more than 5 to 30% by mass, and one or more selected from the group consisting of Cu, Mo, Ta, W and Re in a total amount of 1 to 15% by mass. contains.

Cr oxide and Mn oxide enhance the steam oxidation resistance of the base material. However, if the Cr content is too high, the heat transfer characteristics of the oxide film deteriorate. If the Mn content is too high, the high temperature strength of the base material decreases. Therefore, the oxide layer C contains Cr and Mn in a total amount of more than 5 to 30% by mass.

When Cu, Mo, Ta, W and Re are contained in the oxide layer C, the thermal conductivity of the oxide layer C is increased. However, if the content of these elements is too high, the water vapor oxidation resistance of the oxide layer C may decrease. Therefore, the oxide layer C contains 1 type or 2 or more types selected from the group consisting of Cu, Mo, Ta, W and Re in a total amount of 1 to 15% by mass.

As described above, the oxide layer C has excellent heat transfer characteristics and excellent water vapor oxidation resistance.

(9) In order to form the oxide layer B and the oxide layer C in a high temperature steam environment, it is necessary to form the oxide layer A on the substrate in advance. The thickness of the oxide layer A is 1 μm or more. The chemical composition of the oxide layer A is mass% and contains 20 to 45% of Cr and Mn in total. The chemical composition of the oxide layer A is, in mass%, containing 0.5 to 10% in total of one or more selected from the group consisting of Cu, Mo, Ta, W, and Re. When used in a high-temperature steam environment, the oxide layer A changes to an oxide film having excellent heat transfer characteristics, which will be described later. The high temperature is, for example, 500 to 650 ° C.

The austenitic heat-resistant steel according to the present embodiment completed based on the above findings includes a base material and an oxide layer A on the surface of the base material. The base material is mass%, C: 0.01 to 0.3%, Si: 0.01 to 2.0%, Mn: 0.01 to 2.0%, P: 0.10% or less, S. : 0.03% or less, Cr: 15.0 to 24.0%, Ni: 6.0 to 27.0%, N: 0.005 to 0.3%, sol. Al: 0.001 to 0.3%, Co: 0 to 5.0%, Ti: 0 to 1.0%, V: 0 to 1.0%, Nb: 0 to 1.0%, Hf: 0 ~ 1.0%, Ca: 0 to 0.1%, Mg: 0 to 0.1%, Zr: 0 to 0.1%, B: 0 to 0.1%, rare earth elements: 0 to 0.1 %, Cu: 0-5.0%, Mo: 0-5.0%, Ta: 0-5.0%, W: 0-5.0% and Re: 0-5.0%. It contains 0.5 to 10.0% in total of one or more selected from the group, and has a chemical composition in which the balance is Fe and impurities. The oxide layer A contains a chemical composition containing 20 to 45% in total of Cr and Mn in mass%. The oxide layer A contains, in mass%, a chemical composition containing 0.5 to 10% in total of one or more selected from the group consisting of Cu, Mo, Ta, W, and Re. The oxide layer A has a thickness of 1 μm or more.

The austenitic heat-resistant steel according to the present embodiment is excellent in heat transfer characteristics and steam oxidation resistance.

The chemical composition of the base material of the austenitic heat-resistant steel may contain Co: 0.005 to 5.0%. The chemical composition of the base material is Ti: 0.01 to 1.0%, V: 0.01 to 1.0%, Nb: 0.01 to 1.0%, and Hf: 0.01 to 1. It may contain one or more selected from the group consisting of 0%. The chemical composition of the base material is Ca: 0.0015 to 0.1%, Mg: 0.0015 to 0.1%, Zr: 0.0015 to 0.1%, B: 0.0015 to 0.1. % And rare earth element: 1 type or 2 or more types selected from the group consisting of 0.0015 to 0.1% may be contained.

The heat transfer member according to the present embodiment includes a base material and an oxide film on the surface of the base material. The oxide film includes an oxide layer B and an oxide layer C. The oxide layer B contains 80% by volume or more of Fe ₃ O ₄ . The oxide layer C is formed between the oxide layer B and the base material. The chemical composition of the oxide layer C is 1 to 30% by mass in total of Cr and Mn, and 1 or 2 or more selected from the group consisting of Cu, Mo, Ta, W and Re. Contains 15% by mass.

The heat transfer member according to the present embodiment is excellent in heat transfer characteristics and steam oxidation resistance.

Preferably, the oxide layer B contains Cr and Mn in a total amount of 5% by mass or less.

Preferably, the oxide layer C contains 5% by volume or less of Cr ₂ O ₃ .

In this case, the thermal conductivity of the oxide film is increased by suppressing the precipitation amount of Cr ₂ O ₃ having a low thermal conductivity. Therefore, the heat transfer characteristics of the boiler can be improved.

Preferably, the thermal conductivity of the oxide layer C is 1.0 to 3.0 W · m ^-1 · K ^-1 .

The austenitic heat-resistant steel and the austenitic heat transfer member according to the present embodiment will be described in detail below. Unless otherwise specified, "%" for an element means mass%.

[Austenitic heat resistant steel]
The shape of the austenitic heat-resistant steel according to this embodiment is not particularly limited. Austenitic heat resistant steels are, for example, steel pipes, steel bars, and steel plates. Oxidation treatment is performed on the austenitic heat-resistant steel according to the present embodiment. The oxide layer A is formed on the surface of the austenitic heat-resistant steel by the oxidation treatment.

FIG. 1 is a cross-sectional view of the austenitic heat-resistant steel according to the present embodiment. With reference to FIG. 1, the austenitic heat-resistant steel 1 includes a base material 2 and an oxide layer A. The austenitic heat-resistant steel 1 provided with the base material 2 and the oxide layer A is used as an austenitic heat transfer member in a high-temperature steam environment. As a result, the oxide layer A changes to an oxide film 3 having excellent heat transfer characteristics, which will be described later.

[Austenitic heat transfer member]
The base material of the austenitic heat transfer member according to the present embodiment is austenitic heat-resistant steel. The shape of the austenitic heat transfer member according to the present embodiment is not particularly limited. The austenitic heat transfer member is, for example, a pipe, a rod or a plate material. When having a tubular shape, the heat transfer member is used, for example, as a boiler pipe or the like.

FIG. 2 is a cross-sectional view of the austenitic heat transfer member according to the present embodiment. With reference to FIG. 2, the heat transfer member 4 includes a base material 2 and an oxide film 3. The oxide film 3 includes an oxide layer B and an oxide layer C.

[Chemical composition of substrate 2]
The base material 2 has the following chemical composition.

C: 0.01-0.3%
Carbon (C) stabilizes austenite. C further enhances the high temperature strength of the base material 2 by solid solution strengthening. However, if the C content of the base material 2 is too high, carbides are excessively precipitated, and the workability and weldability of the base material are lowered. Therefore, the C content is 0.01 to 0.3%. The preferred lower limit of the C content is 0.03%, and the preferred upper limit of the C content is 0.15%.

Si: 0.01-2.0%
Silicon (Si) deoxidizes steel. Si further improves the steam oxidation resistance of the base material 2. However, if the Si content is too high, the toughness of the base material 2 decreases. Therefore, the Si content is 0.01-2.0%. The lower limit of the Si content is preferably 0.05%, more preferably 0.1%. The preferred upper limit of the Si content is 1.0%, more preferably 0.5%.

Mn: 0.01-2.0%
Manganese (Mn) deoxidizes steel. Mn further combines with S in the base material 2 to form MnS and suppresses grain boundary segregation of S. As a result, the hot workability of the base material 2 is improved. However, if the Mn content is too high, the base material 2 becomes brittle and the high temperature strength of the base material 2 decreases. Therefore, the Mn content is 0.01 to 2.0%. The preferred lower limit of the Mn content is 0.05%, more preferably 0.1%. The preferred upper limit of the Mn content is 1.0%, more preferably 0.8%.

P: 0.10% or less S: 0.03% or less Phosphorus (P) and sulfur (S) are impurities. P and S segregate at the grain boundaries of the base material 2 to reduce the hot workability of the base material 2. P and S are further concentrated at the interface between the oxide film 3 and the base material 2 to reduce the adhesion of the oxide film 3 to the base material 2. Therefore, it is preferable that the P content and the S content are as low as possible. The P content is 0.10% or less, preferably 0.03% or less. The S content is 0.03% or less, preferably 0.015% or less.

Cr: 15.0 to 24.0%
Chromium (Cr) enhances the oxidation resistance of the base material 2. Cr is further contained in the oxide film 3 as an oxide defined by Cr ₂ O ₃ and (Fe, Cr) ₃ O ₄ . Cr oxide enhances the steam oxidation resistance of the base material 2. The Cr oxide further enhances the adhesion of the oxide film 3 to the substrate 2. However, if the Cr content is too high, the concentration of Cr ₂ O _{3 in} the oxide film 3 becomes high, and the heat transfer characteristics of the oxide film 3 deteriorate. Therefore, the Cr content is 15.0 to 24.0%. The lower limit of the Cr content is preferably 16.0%, more preferably 17.0%. The preferred upper limit of the Cr content is 23.5%, more preferably 23.0%.

Ni: 6.0-27.0%
Nickel (Ni) stabilizes austenite. Ni further enhances the strength of austenitic heat resistant steels at high temperatures. However, if the Ni content is too high, the thermal conductivity of the heat transfer member will decrease. If Ni is too high, the cost will be even higher. Therefore, the Ni content is 6.0 to 27.0%. The preferred lower limit of the Ni content is 6.5%, more preferably 7.0%. The preferred upper limit of the Ni content is 26.5%, more preferably 26.0%.

N: 0.005-0.3%
Nitrogen (N) dissolves in the base material 2 to increase the strength of the base material 2. Further, N forms a nitride with the alloy component in the base material 2 and precipitates in the base material 2 to increase the strength of the base material 2. However, if the N content is too high, the nitride becomes coarse and the toughness of the base material 2 decreases. Therefore, the N content is 0.005 to 0.3%. The preferred lower limit of the N content is 0.01%, and the preferred upper limit of the N content is 0.27%.

sol. Al: 0.001 to 0.3%
Aluminum (Al) deoxidizes steel. However, if the Al content is too high, the hot workability of the base material 2 is lowered. Therefore, the Al content is 0.001 to 0.3%. The preferable lower limit of the Al content is 0.005%, and the preferable upper limit of the Al content is 0.1%. In the present embodiment, the Al content means acid-soluble Al (sol.Al).

Cu: 0-5.0%,
Mo: 0-5.0%,
Ta: 0-5.0%,
One or more selected from the group consisting of W: 0 to 5.0% and Re: 0 to 5.0%: 0.5 to 10.0% in total
It contains one or more selected from the group consisting of copper (Cu), molybdenum (Mo), tantalum (Ta), tungsten (W) and rhenium (Re). Hereinafter, these elements are also referred to as specific oxide layer forming elements. The specific oxide layer forming element forms the oxide layer A on the surface of the base material 2. The specific oxide layer forming element further forms an oxide layer C having excellent heat transfer characteristics in a high temperature steam environment of 500 to 650 ° C. This effect can be obtained if even one of these elements is contained. However, if the content of the specific oxide layer forming element is too high, the toughness, ductility and processability of the base material 2 are lowered. Therefore, the Cu content is 0 to 5.0%, the Mo content is 0 to 5.0%, the Ta content is 0 to 5.0%, and the W content is 0 to 5.0. The Re content is 0 to 5.0%. The preferable lower limit of the content of each specific oxide layer forming element is 0.01%, and more preferably 0.1%, respectively. The preferable upper limit of the content of each specific oxide layer forming element is 4.0%, and more preferably 3.0%, respectively. The total content of the specific oxide layer forming element is 0.5 to 10.0%. The preferable lower limit of the total content of the specific oxide layer forming elements is 0.6%, more preferably 1.0%. The preferable upper limit of the total content of the specific oxide layer forming elements is 9.0%, more preferably 8.0%.

The rest of the base material 2 according to this embodiment is Fe and impurities. In the present embodiment, the impurities refer to ores and scraps used as raw materials for steel, or elements mixed from the environment of the manufacturing process, etc., and are contained within a range that does not adversely affect the heat transfer member 4 according to the present embodiment. It means what is done. Impurities are, for example, oxygen (O), arsenic (As), antimony (Sb), thallium (Tl), lead (Pb), bismuth (Bi) and the like.

The base material 2 of the heat transfer member 4 according to the present embodiment may further contain the following elements in place of a part of Fe.

Co: 0-5.0%
Cobalt (Co) is optionally contained. Co stabilizes austenite. As a result, residual delta ferrite, which lowers the impact resistance of the base material 2, is suppressed. This effect can be obtained if even one of these elements is contained. However, if the content of these elements is too high, the hot workability of the base material 2 is lowered. Therefore, the Co content is 0 to 5.0%. The preferred upper limit of the Co content is 3.0%, more preferably 2.0%. The preferable lower limit of the Co content is 0.005%.

Ti: 0-1.0%
V: 0-1.0%
Nb: 0-1.0%
Hf: 0 to 1.0%
Titanium (Ti), vanadium (V), niobium (Nb) and hafnium (Hf) are optionally contained. These elements combine with carbon and nitrogen to form carbides, nitrides or carbonitrides. These carbides, nitrides and carbonitrides precipitate and strengthen the base material 2. This effect can be obtained if even one of these elements is contained. However, if the content of these elements is too high, the processability of the base material 2 is lowered. Therefore, the Ti content is 0 to 1.0%, the V content is 0 to 1.0%, the Nb content is 0 to 1.0%, and the Hf content is 0 to 1.0. %. The preferred upper limit of the content of these elements is 0.8%, and more preferably 0.4%, respectively. The preferred lower limit of the content of these elements is 0.01%, respectively.

Ca: 0-0.1%
Mg: 0-0.1%
Zr: 0-0.1%
B: 0-0.1%
Rare earth element: 0-0.1%
Calcium (Ca), magnesium (Mg), zirconium (Zr), boron (B) and rare earth element (REM) are optionally contained. These elements enhance the strength, processability and oxidation resistance of the base material 2. This effect can be obtained if even one of these elements is contained. However, if the content of these elements is too high, the toughness and weldability of the base material 2 will decrease. Therefore, the Ca content is 0 to 0.1%, the Mg content is 0 to 0.1%, the Zr content is 0 to 0.1%, and the B content is 0 to 0.1. The content of REM is 0 to 0.1%. The preferred upper limit of the content of these elements is 0.05%, respectively. The preferred lower limit of the content of these elements is 0.0015%, respectively. Here, REM refers to ittium (Y) having an atomic number of 39, lantern (La) having an atomic number of 57 which is a lanthanoid, lutetium (Lu) having an atomic number of 71, and actinium having an atomic number of 89 which is an actinoid. (Ac) One or more elements selected from the group consisting of lawrencium (Lr) Nos. 103.

[Oxidized layer A]
The base material 2 having the above-mentioned chemical composition is subjected to an oxidation treatment. The oxide layer A is formed on the surface of the base material 2 by the oxidation treatment. The austenitic heat-resistant steel composed of the base material 2 and the oxide layer A is used in a high-temperature steam environment. In a high-temperature steam environment, the oxide layer A changes to an oxide film 3 having excellent heat transfer characteristics while maintaining the steam oxidation resistance. That is, the oxide layer A is a material for forming an oxide film having excellent electric heating characteristics, which will be described later. The mechanism by which the oxide layer A changes to the oxide film 3 is not clear, but the oxide layer A mainly contributes to the formation of the oxide layer C.

The oxide layer A is formed on the surface of the base material 2 with a thickness of 1 μm or more. If the thickness of the oxide layer A is less than 1 μm, the oxide film 3 is not stably formed in a high temperature steam environment. In this case, the heat transfer characteristics are deteriorated. Therefore, the thickness of the oxide layer A is set to 1 μm or more. The upper limit of the thickness of the oxide layer A is not particularly limited, but is preferably 20 μm or less in consideration of mass productivity.

The thickness of the oxide layer A can be determined by, for example, the following method. A test piece is prepared from austenitic heat-resistant steel that has been subjected to the oxidation treatment described later. The obtained test piece is embedded in a resin, and the cross section is observed at a magnification of 2000 times using a scanning electron microscope (SEM). The thickness of the oxide layer A can be obtained by measuring the thickness of the portion corresponding to the oxide layer.

The chemical composition of the oxide layer A contains 20 to 45% of Cr and Mn in total. The chemical composition of the oxide layer A further contains 0.5 to 10% in total of one or more selected from the group consisting of Cu, Mo, Ta, W, and Re. In this case, an oxide film 3 having excellent heat transfer characteristics is formed in a high temperature steam environment. The preferred total content of Cr and Mn is 22-40%. The preferable total content of the specific oxide layer forming element is 1 to 8%.

The contents of Cr and Mn in the oxide layer A can be measured by, for example, the following method. The elemental composition of the cross section of the oxide layer A is analyzed by using the energy dispersive X-ray analysis (EDS) method. Of the obtained composition of each element, the total amount of Cr and Mn is calculated. Further, the ratio (mass%) of the total amount of Cr and Mn is calculated with the composition excluding the amounts of oxygen (O) and carbon (C) as 100%. The total content of the specific oxide layer forming elements can be measured by the same method.

[Oxide film 3]
An oxide film 3 is formed on the surface of the base material 2 by performing an oxidation treatment and a steam oxidation treatment described later on the base material 2 having the above-mentioned chemical composition. With reference to FIG. 2, the oxide film 3 is a two-layer oxide film composed of the oxide layer B and the oxide layer C. The oxide layer B is formed on the uppermost layer of the heat transfer member 4. The oxide layer C is formed between the oxide layer B and the base material 2. When the heat transfer member 4 is a boiler pipe, the oxide layer B corresponds to the inner surface side of the boiler pipe, and the base material 2 corresponds to the outer surface side of the boiler pipe. In this case, the oxide layer B is in contact with high temperature water vapor.

[Oxidized layer B]
The oxide layer B contains 80% by volume or more of Fe ₃ O ₄ . The thermal conductivity of Fe ₃ O ₄ is high. Therefore, the thermal conductivity of the oxide layer B is high, and the heat given from the outside of the heat transfer member is transferred to the inside of the heat transfer member without being significantly reduced. Therefore, the heat transfer characteristics of the boiler can be improved. Preferably, the content of Fe ₃ O ₄ in the oxide layer B is 90% by volume or more. The oxide layer B is composed of Fe ₃ O ₄ , but a part thereof may be Fe ₂ O ₃ . The oxide layer B can contain Fe ₂ O ₃ in an amount of less than 20% by volume.

The oxide layer B may contain a part of Cr and Mn contained in the base material 2 as an oxide. Cr ₂ O ₃ has a particularly low thermal conductivity. Therefore, the total amount of Cr and Mn contained in the oxide layer B is 5% or less. The total amount of Cr and Mn contained in the oxide layer B is preferably 3% or less.

The preferable thickness of the oxide layer B is 10 to 400 μm.

[Oxidized layer C]
The oxide layer C is formed between the oxide layer B and the base material 2, and is in contact with the base material 2.

The chemical composition of the oxide layer C contains a total of more than 5 to 30% of Cr and Mn. In the oxide layer C, Cr and Mn are present as oxides represented by the chemical formula of (Fe, M) ₃ O ₄ . In the formula, Cr and Mn are substituted for M. The oxide represented by the chemical formula of (Fe, M) ₃ O ₄ is an oxide having the same so-called spinel-type crystal structure as Fe ₃ O _4, and a part of Fe is substituted with Cr and Mn. When the total amount of Cr and Mn contained in the oxide layer C exceeds 5%, the ratio of Fe ₃ O ₄ in the oxide layer C can be suppressed. Therefore, the thermal conductivity of the oxide layer C can be controlled within an appropriate range while maintaining the steam oxidation resistance. On the other hand, when the total amount of Cr and Mn contained in the oxide layer C is more than 30%, the thermal conductivity of the oxide layer C becomes too low. In this case, the heat transfer characteristics of the boiler are deteriorated. Therefore, the total content of Cr and Mn in the oxide layer C is more than 5 to 30%. The preferable lower limit of the Cr and Mn contents in the oxide layer C is 10%, more preferably 13%. The preferable upper limit of the Cr and Mn contents in the oxide layer C is 28%, more preferably 25%.

The oxide layer C may contain 1 type or 2 or more types selected from the group consisting of Cu, Mo, Ta, W and Re in a total of 1 to 15%. When the specific oxide layer forming element is contained in the oxide layer C, the thermal conductivity of the oxide layer C is increased. If these elements are contained in an amount of more than 15%, the water vapor oxidation resistance of the oxide layer C may decrease. Therefore, the upper limit of the content is preferably 15%. The preferred upper limit of the total content of these elements in the oxide layer C is 9%. The preferable lower limit of the total content of these elements in the oxide layer B is 1.5%.

The oxide layer C is further an oxide having the above-mentioned spinel-type crystal structure in most of the oxide layer C, and Cr ₂ O ₃ is preferably 5% by volume or less. By suppressing the formation of Cr ₂ O ₃ with low thermal conductivity to 5% by volume or less and forming an oxide having a spinel-type crystal structure, the thermal conductivity of the oxide layer B is 1.0 to 3.0 W. It can be controlled in the range of m ^-1 and K ^-1 . The content of Cr ₂ O ₃ in the oxide layer C is preferably 3% by volume or less.

The thermal conductivity of the oxide layer C is preferably controlled in the range of 1.0 to 3.0 W · m ^-1 · K ^-1 . In this case, the heat of high-temperature steam transmitted to the surface of the base material 2 can be controlled. As a result, the formation of a large amount of oxidation scale on the surface of the base material 2 can be suppressed, and the water vapor oxidation resistance of the heat transfer member 4 can be improved without impairing the heat transfer characteristics. If the thermal conductivity of the oxide layer C is smaller than 1.0 W · m ^-1 · K ^-1 , the heat transfer from the outside of the heat transfer member to the inside of the heat transfer member 4 is hindered, and the heat transfer characteristics of the boiler are improved. descend. If the thermal conductivity of the oxide layer B is larger than 3.0 W · m ^-1 · K ^-1 , the surface of the base material 2 is excessively heated and the oxidation reaction on the surface of the base material 2 is promoted. Therefore, a large amount of oxidation scale is generated on the surface of the base material 2, and the water vapor oxidation resistance of the heat transfer member 4 is lowered. In the oxidation layer C, the lower limit of the preferred thermal conductivity is 1.25W · m ^-1 · K ^-1, more preferably from 1.3W · m ^-1 · K ^-1. The upper limit of the preferable thermal conductivity in the oxide layer B is 2.8 W · m ^-1 · K ^-1 , and more preferably 2.5 W · m ^-1 · K ^-1 .

The volume fraction of the oxide in each oxide layer can be measured by, for example, the following method. First, oxides are identified on the surface of oxide layer B using X-ray diffraction (XRD). Next, the test piece is embedded in a resin, and an electron backscattered electron (BSE) image is taken with respect to the cross section of the test piece using an electron probe microanalyzer (EPMA). The area ratio of each oxide is obtained from the shade of the obtained image and converted into the volume ratio. After removing the oxide layer A, the volume fraction of the oxide of each oxide layer can be obtained by measuring the oxide layer B in the same manner.

The contents of Cr and Mn in each oxide layer can be measured by, for example, the following method. The elemental composition of the surface of the oxide layer B is analyzed using the energy dispersive X-ray analysis (EDS) method. Of the obtained composition of each element, the total amount of Cr and Mn is calculated. Further, the ratio (mass%) of the total amount of Cr and Mn is calculated with the composition excluding the amounts of oxygen (O) and carbon (C) as 100%. After removing the oxide layer B, the content of the element contained in each oxide layer can be determined by measuring the oxide layer B in the same manner. The total content of Cu, Mo, Ta, W and Re can be measured by the same method.

The thermal conductivity of the oxide layer C can be determined by, for example, the following method. After removing the oxide layer B of the heat transfer member 4, the bulk density, specific heat and thermal diffusivity of the oxide layer C including the base material 2 are measured. Next, after removing the oxide layer C, the bulk density, the specific heat and the thermal diffusivity are measured for the base material 2 in the same manner. The thermal conductivity κ can be obtained by converting the difference between the measured values into the measured value of the oxide layer C and substituting it into the following equation.
κ ＝ ρ × C _p × D
Here, the bulk density is substituted for ρ, the specific heat is substituted for C _p , and the thermal diffusivity is substituted for D.

The preferable lower limit of the oxide layer C thickness is 10 μm.

[Thickness of oxide film 3]
The thickness of the oxide film 3 is not particularly limited, but it is preferably thin. When the oxide film 3 is thin, the heat transfer characteristics of the heat transfer member 4 are enhanced. Therefore, the heat transfer characteristics of the boiler can be improved. If the heat transfer member 4 is used for a long time, the oxide film 3 becomes thicker. Even when the temperature of the steam oxidation treatment of the heat transfer member 4 is high, the oxide film 3 becomes thick. By performing the oxidation treatment and the steam oxidation treatment described later, the oxide layer B and the oxide layer C are formed to have almost the same thickness. Therefore, when the oxide layer C is thin, the oxide film 3 is also thin.

The thickness of the oxide layer B and the oxide layer C can be determined by, for example, the following method. A test piece is prepared from the heat transfer member 4 which has been subjected to the oxidation treatment and the steam oxidation treatment described later. The obtained test piece is embedded in a resin, and the cross section is observed at a magnification of 2000 times using a scanning electron microscope (SEM). By measuring the thickness of the portion corresponding to each oxide layer, the thickness of the oxide layer B and the oxide layer C can be obtained.

[Manufacturing process]
The manufacturing process of the heat transfer member according to the present embodiment includes a preparation step, an oxidation treatment step, and a steam oxidation treatment step. In the preparatory step, a material having the above-mentioned chemical composition is prepared. The material may be slabs, blooms and billets produced by the continuous casting method. The material may be a billet produced by the lump formation method. For example, when manufacturing a steel pipe, the prepared material is charged into a heating furnace or a soaking furnace and heated. The heated material is hot-processed to produce the base material 2. Hot working is, for example, the Mannesmann method. In the Mannesmann method, the material is drilled and rolled using a drilling machine to make a raw pipe. Subsequently, it is a method of stretching rolling and standard rolling of a material using a mandrel mill and a sizing mill. As a result, the base material 2 is manufactured as a seamless steel pipe. The method for producing the base material 2 is not limited to the Mannesmann method, and the material may be produced by hot extrusion or hot forging. Further, the base material 2 produced by hot working may be heat-treated or cold-worked. The base material 2 may be a steel plate. When the base material 2 is a steel plate, the material is hot-processed to manufacture the base material 2 as a steel plate. The steel plate may be processed into a steel pipe by welding to manufacture the base material 2 as a welded steel pipe.

[Oxidation treatment]
The above-mentioned base material 2 is subjected to an oxidation treatment. The oxidation treatment is performed by heating the base material 2 in a gas atmosphere such as combustion gas. The CO / CO ₂ ratio of the gas used for the oxidation treatment shall be 0.6 or more in terms of volume ratio. By setting the CO / CO ₂ ratio to 0.6 or more, the oxide layer A is formed on the surface of the base material 2. The oxide layer A changes to an oxide film 3 after the steam oxidation treatment described later. The oxide film 3 is formed on the surface of the base material 2 by two layers composed of an oxide layer B and an oxide layer C. The CO / CO ₂ ratio is not particularly limited, but 2.0 is preferable in consideration of practicality in operation. The gas used for the oxidation treatment may have a CO / CO ₂ ratio of 0.6 or more, and the type of gas is not particularly limited. A mixed gas of CO-CO ₂ may be used, or a combustion gas may be used. When using combustion gas, the CO / CO ₂ ratio can be adjusted to 0.6 or more by adjusting the air-fuel ratio.

In the oxidized gas atmosphere, for example, the air-fuel ratio in the combustion gas is controlled. If the air-fuel ratio is controlled, the gas composition in the oxidation-treated gas atmosphere changes. Natural gas, methane, propane, butane and the like may be used as the fuel. Further, a mixed gas such as CO-CO ₂ may be used. Further, an oxidation-treated gas atmosphere in which these are mixed may be used.

The temperature of the oxidation treatment is 1050 to 1240 ° C. If the oxidation treatment temperature is less than 1050 ° C., the thickness of the oxide layer A is less than 1 μm. In this case, the oxide film 3 cannot be uniformly formed on the surface of the base material 2 in a high temperature steam environment. As a result, the base material 2 cannot be completely covered with the oxide film 3. As a result, the thermal conductivity on the surface of the austenitic heat transfer member 4 decreases. If the oxidation treatment temperature exceeds 1240 ° C., the oxide layer A becomes too thick. In this case, the adhesion between the oxide layer A and the base material 2 is lowered, and the oxide layer A is peeled off in the cooling step after the oxidation treatment. As a result, the formation of the oxide film 3 becomes incomplete in a high-temperature steam environment, and the heat transfer characteristics deteriorate. The thickness of the oxide film 3 formed after the oxidation treatment can be suppressed. Therefore, peeling of the oxide film can be suppressed, and the oxide film 3 can be stably formed after the steam oxidation treatment described later. Therefore, the oxidation treatment temperature is 1050 to 1240 ° C. The lower limit of the oxidation treatment temperature is preferably 1060 ° C, more preferably 1080 ° C. The preferred upper limit of the oxidation treatment temperature is 1230 ° C, more preferably 1210 ° C.

The preferred oxidation treatment time is 1 minute to 1 hour. If the oxidation treatment time is too short, the thickness of the oxide layer A becomes less than 1 μm. In this case, the oxide film 3 cannot be uniformly formed on the surface of the base material 2 in a high temperature steam environment. As a result, the base material 2 cannot be completely covered with the oxide film 3. As a result, the thermal conductivity on the surface of the austenitic heat transfer member 4 decreases. If the oxidation treatment time is too long, the productivity will decrease. In consideration of productivity, it is preferable that the oxidation treatment time is short. Therefore, the oxidation treatment time is more preferably 30 minutes or less, still more preferably 20 minutes or less. A more preferable lower limit of the oxidation treatment time is 3 minutes.

Temper treatment (low temperature annealing) may be carried out after the oxidation treatment. Further, the oxidation treatment may be performed on the entire base material 2, but may be performed only on the surface where the base material 2 is in contact with high-temperature steam (for example, the inner surface of the steel pipe).

The oxidation treatment may be carried out once or multiple times. After the oxidation treatment, degreasing, cleaning, or the like may be performed in order to remove dirt and oil adhering to the surface of the base material 2. Even if degreasing or cleaning is performed, the oxide layer A is not affected. Even if degreasing or cleaning is performed, it does not affect the subsequent formation of the oxide film 3.

The austenitic heat-resistant steel produced by the above production method has excellent heat transfer characteristics in a high-temperature steam environment.

[Steam oxidation treatment]
The austenitic heat-resistant steel subjected to the above-mentioned oxidation treatment is subjected to steam oxidation treatment. The steam oxidation treatment is carried out by exposing the austenitic heat-resistant steel to steam at 550 to 700 ° C. As long as the steam oxidation treatment is 100 hours or more, the upper limit of the treatment time is not particularly limited. By the steam oxidation treatment, the oxide layer A is changed to the oxide layer B and the oxide film 3. As a result, the oxide film 3 composed of the oxide layer B and the oxide layer C is formed on the base material 2.

Through the above steps, the austenitic heat transfer member according to the present embodiment can be manufactured.

Each steel piece having the chemical composition shown in Table 1 was produced and subjected to oxidation treatment and steam oxidation treatment under the conditions shown in Table 2. Specifically, an ingot having the chemical composition shown in Table 1 was melted. Each of the obtained ingots was hot-rolled and cold-rolled to produce a steel sheet, which was used as a base material. A test piece was prepared from each of the obtained base materials, and each test piece was subjected to an oxidation treatment under the conditions shown in Table 2. The thickness of the oxide layer A and the metal element content of the oxide layer A were measured.

[Thickness measurement test of oxide layer A]
The thickness of the oxide layer A of each test piece was determined by the above method. The results are shown in Table 2.

[Measurement test of metal element content of oxide layer A]
The content of each metal element was determined for the cross section of each test piece by the above method. For the oxide layer A, the total amount of Cr and Mn (mass%) and the total amount of Cu, Mo, Ta, W and Re (mass%) were determined. The results are shown in Table 2.

Each test piece was subjected to steam oxidation treatment under the conditions shown in Table 2. The following measurement test was performed on each of the obtained test pieces.

[Oxide volume fraction measurement test]
The volume fraction of the oxide was determined for the cross section of each test piece by the above method. For the oxide layer B, the volume fraction of Fe ₃ O _{4 and} the volume fraction of Fe ₂ O ₃ were determined. The results are shown in Table 2. The volume fraction of Cr ₂ O ₃ was determined for the oxide layer C. The results are shown in Table 2.

[Metal element content measurement test]
The content of each metal element was determined for the cross section of each test piece by the above method. For the oxide layer B, the total amount (mass%) of Cr and Mn was determined. The results are shown in Table 2. For the oxide layer C, the total amount of Cr and Mn (mass%) and the total amount of Cu, Mo, Ta, W and Re (mass%) were determined. The results are shown in Table 2.

[Thermal conductivity measurement test of oxide layer C]
The thermal conductivity of the oxide layer C of each test piece was determined by the above method. The results are shown in Table 2.

[Oxidation layer C thickness measurement test]
The thickness of the oxide layer C of each test piece was determined by the above method. The results are shown in Table 2.

[Evaluation results]
With reference to Tables 1 and 2, the chemical composition and production conditions of the steels of test numbers 1, 2, 4, 8 and 10-14 were appropriate. Therefore, the thickness of the oxide layer A is 1 μm or more. As a result, the oxide layer B contained 80% by volume or more of Fe ₃ O ₄ . The total Cr + Mn content of the oxide layer C was more than 5 to 30%, and the content of the specific oxide layer forming element was 1 to 15%. As a result, the thermal conductivity of the oxide layer C was in the range of 1.0 to 3.0 W · m ^-1 · K ^-1 , and showed excellent thermal conductivity. The oxide layer C further had a thickness of 60 μm or less, and exhibited excellent water vapor oxidation resistance.

On the other hand, in Test No. 3, although the chemical composition was appropriate, the oxidation treatment was not performed and the oxide layer A was not formed. Therefore, the thermal conductivity of the oxide layer C was less than 1.0 W · m ^-1 · K ^-1 . Since the total amount of Cr and Mn in the oxide layer B exceeded 5%, the inward flow velocity of oxygen was suppressed, and as a result, Cr ₂ O ₃ having a low thermal conductivity exceeded 5% by volume in the oxide layer C. It is probable that it was generated. Further, since the total amount of the specific oxide layer forming elements was less than 1%, it is considered that the thermal conductivity was further lowered.

In Test No. 5, although the chemical composition was appropriate, the oxidation treatment temperature was too high, so that the total amount of Cr and Mn in the oxide layer A exceeded 45% by mass. Therefore, the thermal conductivity of the oxide layer C was less than 1.0 W · m ^-1 · K ^-1 . It is considered that Cr ₂ O ₃ having a low thermal conductivity exceeded 5% by volume, and the total amount of Cr and Mn in the oxide layer C exceeded 30% to be formed in the oxide layer C.

In Test No. 6, although the chemical composition was appropriate, the thickness of the oxide layer A was less than 1 μm because the oxidation treatment temperature was too low. Therefore, the thermal conductivity of the oxide layer C was less than 1.0 W · m ^-1 · K ^-1 . It is considered that the total amount of the specific oxide layer forming elements in the oxide layer C was less than 1%.

In Test No. 7, the CO / CO ₂ ratio in the oxidation treatment was less than 0.6, although the chemical composition was appropriate. Therefore, the total amount of Cr and Mn in the oxide layer A was less than 20%, and the total amount of the specific oxide layer forming elements was less than 0.5%. Therefore, the thermal conductivity of the oxide layer C exceeded 3.0 W · m ^-1 · K ^-1 . This is because the volume fraction of Fe ₃ O ₄ in the oxide layer B was less than 80%, so that the inward flux of oxygen became large and the growth of the oxide layer C was also promoted. In Test No. 7, the thickness of the oxide layer C further exceeded 60 μm. It is considered that the thermal conductivity of the oxide layer C was too high.

In Test No. 9, although the chemical composition was appropriate, the oxidation treatment time was too short, so that the total amount of the specific oxide layer forming elements in the oxide layer A exceeded 10%. Therefore, the thermal conductivity of the oxide layer C exceeded 3.0 W · m ^-1 · K ^-1 . It is considered that this is because the total amount of the specific oxide layer forming elements in the oxide layer C exceeded 15%. In Test No. 9, the thickness of the oxide layer C further exceeded 60 μm. It is considered that the thermal conductivity of the oxide layer C was too high.

Test numbers 15 and 16 did not contain any of the specific oxide layer forming elements. Therefore, the total amount of the specific oxide layer forming element in the oxide layer A was less than 0.5%. Therefore, the thermal conductivity of the oxide layer C was less than 1.0 W · m ^-1 · K ^-1 . Since the total amount of the specific oxide layer forming elements in the oxide layer C was less than 1%, it is considered that the thermal conductivity was lowered.

Test number 17 had a Cr content that was too low. Therefore, although the production method was appropriate, the thermal conductivity of the oxide layer C exceeded 3.0 W · m ^-1 · K ^-1 . It is considered that the total amount of Cr and Mn in the oxide layer C was less than 5% because the Cr content was too low. In Test No. 17, the thickness of the oxide layer C further exceeded 60 μm. It is considered that the thermal conductivity of the oxide layer C was too high.

Test number 18 had too high a Cr content. Therefore, although the production method was appropriate, the thermal conductivity of the oxide layer C was less than 1.0 W · m ^-1 · K ^-1 . It is considered that Cr ₂ O ₃ having a low thermal conductivity was formed in the oxide layer C in excess of 5% by volume.

Test number 19 had too high a Ni content. Therefore, although the production method was appropriate, the thermal conductivity of the oxide layer C was less than 1.0 W · m ^-1 · K ^-1 . It is considered that Cr ₂ O ₃ having a low thermal conductivity was formed in the oxide layer C in excess of 5% by volume.

In test number 20, the content of the specific oxide layer forming element was too high. Therefore, the total amount of the specific oxide layer forming elements in the oxide layer A exceeded 10%. Therefore, the thermal conductivity of the oxide layer C exceeded 3.0 W · m ^-1 · K ^-1 . It is considered that this is because the total amount of the specific oxide layer forming elements in the oxide layer C exceeded 15%. In Test No. 20, the thickness of the oxide layer C further exceeded 60 μm. It is considered that the thermal conductivity of the oxide layer C was too high.

The embodiments of the present invention have been described above. However, the embodiments described above are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and the above-described embodiment can be appropriately modified and implemented without departing from the spirit of the present invention.

1 Austenitic heat-resistant steel 2 Base material 3 Oxidation film 4 Austenitic heat transfer member A Oxidized layer A
B Oxidized layer B
C Oxidized layer C

Claims (7)

With the base material
An oxide layer A is provided on the surface of the base material.
The base material is
By mass%
C: 0.01-0.3%,
Si: 0.01-2.0%,
Mn: 0.01-2.0%,
P: 0.10% or less,
S: 0.03% or less,
Cr: 15.0 to 24.0%,
Ni: 6.0-27.0%,
N: 0.005-0.3%,
sol. Al: 0.001 to 0.3%,
Co: 0-5.0%,
Ti: 0-1.0%,
V: 0-1.0%,
Nb: 0-1.0%,
Hf: 0-1.0%,
Ca: 0-0.1%,
Mg: 0-0.1%,
Zr: 0-0.1%,
B: 0-0.1%,
Rare earth elements: 0-0.1% and
Selected from the group consisting of Cu: 0-5.0%, Mo: 0-5.0%, Ta: 0-5.0%, W: 0-5.0% and Re: 0-5.0%. 1 type or 2 or more types: 0.5 to 10.0% in total,
Has a chemical composition in which the balance is composed of Fe and impurities.
The oxide layer A is
By mass%
Cr and Mn: 20-45% in total, and
One or more selected from the group consisting of Cu, Mo, Ta, W, and Re: 0.5-10% in total,
And the chemical composition containing
Austenitic heat-resistant steel containing a thickness of 1 μm or more and 20 μm or less . The austenitic heat-resistant steel according to claim 1.
The chemical composition of the base material is
Austenitic heat-resistant steel containing 0.005 to 5.0% of Co. The austenitic heat-resistant steel according to claim 1 or 2.
The chemical composition of the base material is
Ti: 0.01-1.0%,
V: 0.01-1.0%,
Nb: 0.01 to 1.0% and
Hf: An austenitic heat-resistant steel containing one or more selected from the group consisting of 0.01 to 1.0%. The austenitic heat-resistant steel according to claim 1 to 3.
The chemical composition of the base material is
Ca: 0.0015-0.1%,
Mg: 0.0015-0.1%,
Zr: 0.0015-0.1%,
B: 0.0015 to 0.1%, and
Rare earth element: An austenitic heat-resistant steel containing one or more selected from the group consisting of 0.0015 to 0.1%. A substrate having the chemical composition according to claim 1 to 4.
An oxide film is provided on the surface of the base material,
The oxide film is
Oxidized layer B containing 80% or more of Fe ₃ O ₄ by volume,
It contains an oxide layer C formed between the oxide layer B and the base material.
The chemical composition of the oxide layer C is
By mass%
Cr and Mn: 5 to 30% in total, and
One or more selected from the group consisting of Cu, Mo, Ta, W and Re: an austenitic heat transfer member containing 1 to 15% in total. The austenitic heat transfer member according to claim 5.
The chemical composition of the oxide layer B is
By mass%
Cr and Mn: A heat transfer member containing 5% or less in total. The austenitic heat transfer member according to claim 5 or 6.
The oxide layer C is
The _Cr 2 _{O 3} having 5% or less under free volume%, the heat transfer member.

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