KR102165758B1 - Ferritic heat-resistant steel and ferritic heat transfer member - Google Patents

Abstract

A ferritic heat transfer member 4 having excellent heat transfer characteristics and water vapor oxidation resistance, and a ferritic heat-resistant steel 1 capable of realizing the same are provided. The ferritic heat-resistant steel 1 includes a base material 2 and an oxide layer A on the surface of the base material 2. The base material (2) is in mass%, C: 0.01 to 0.3%, Si: 0.01 to 2.0%, Mn: 0.01 to 2.0%, Cr: 7.0 to 14.0%, N: 0.005 to 0.15%, and sol.Al: It contains 0.001 to 0.3%, and 0.5 to 7.0% in total of one or two or more selected from the group consisting of Mo, Ta, W and Re, and the balance has a chemical composition consisting of Fe and impurities. Oxidation layer A contains, in mass%, Cr and Mn in a total of 20 to 45%, and, Mo, Ta, W, and Re in a total of 0.5 to 10% of one or more selected from the group consisting of Contains chemical composition.

Description

Ferritic heat-resistant steel and ferritic heat transfer member

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

In a thermal power plant, improvement of power generation efficiency is required from the viewpoint of suppression of CO ₂ gas emission and economical efficiency. For this reason, high temperature and high pressure of the turbine steam pressure are in progress. The heat transfer member used in a thermal power plant is exposed to high temperature and high pressure water vapor for a long time. The heat transfer member is, for example, a pipe for a boiler. When exposed to high temperature water vapor for a long time, oxidized scale is generated on the surface of the heat transfer member. When the water vapor oxidation resistance of the heat transfer member is not sufficient, a large amount of oxide 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, when a large amount of oxidized scale is generated, the oxidized scale is peeled off, causing clogging of the pipe. When a large amount of oxidized scale is generated, heat conduction from the outside of the pipe to the inside of the pipe is inhibited by the oxidized scale. Therefore, in order to keep the temperature inside the pipe high, it is necessary to give more heat from the outside. The increase in the temperature of the pipe causes a decrease in creep strength. For this reason, high water vapor oxidation resistance is required for heat transfer members used in devices such as boilers for thermal power generation, turbines, and steam pipes.

As materials satisfying these characteristics, for example, austenitic heat-resistant steel and ferritic heat-resistant steel have been developed. 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. Ferritic heat-resistant steel is cheaper than austenitic heat-resistant steel. The ferritic heat-resistant steel further has a lower coefficient of thermal expansion and a higher thermal conductivity than the austenitic heat-resistant steel. Therefore, ferritic heat-resistant steel is suitable as a piping material for a thermal power plant. However, the Cr content of ferritic heat-resistant steel is lower than that of austenitic heat-resistant steel. Therefore, the water vapor oxidation resistance of the ferritic heat-resistant steel is lower than that of the austenitic heat-resistant steel. For this reason, a ferritic heat-resistant steel excellent in water vapor oxidation resistance is required.

A ferritic heat-resistant steel in which oxidized scale is suppressed from falling off is disclosed in, for example, Japanese Patent Laid-Open No. 11-92880 (Patent Document 1). The ferritic heat-resistant steel described in Patent Document 1 is a high-Cr-containing ferritic heat-resistant steel in which an oxide film is formed on the surface during use, and an extremely fine oxide having a diameter of 1 micron or less is formed at the interface with the oxide film or in the vicinity thereof. For this reason, it is described in Patent Document 1 that the adhesion between the oxide film and the base material is improved.

A method of improving the water vapor oxidation resistance by increasing the Cr concentration on the surface of a ferritic heat-resistant steel is disclosed in, for example, Japanese Patent Laid-Open No. 2007-39745 (Patent Document 2). In Patent Document 2, powder particles containing Cr are supported on the surface of a ferritic heat-resistant steel containing Cr, and a Cr oxide layer having a high Cr concentration is formed on the surface of the ferritic steel under high temperature. It is described in Patent Document 2 that this method can easily and economically improve the oxidation resistance (water vapor) of a ferritic steel containing Cr.

A method of improving oxidation resistance by forming a Cr oxide film on the surface of ferritic heat-resistant steel is disclosed, for example, in Japanese Patent Laid-Open No. 2013-127103 (Patent Document 3). In the oxidation resistance treatment method of ferritic heat-resistant steel described in Patent Document 3, a ferritic heat-resistant steel containing chromium is heat-treated in a gas atmosphere of low oxygen partial pressure consisting of a mixed gas of carbon dioxide gas and an inert gas, and chromium is applied to the surface of the heat-resistant steel It is characterized by forming a containing oxide film. Patent Document 3 describes that by this method, the Cr concentration in the scale can be increased, and the oxidation resistance properties of the ferritic heat-resistant steel can be easily and economically improved.

Ferritic heat-resistant steel having improved water vapor oxidation resistance by attaching Cr to the surface of ferritic heat-resistant steel is disclosed, for example, in Japanese Patent Laid-Open No. 2009-179884 (Patent Document 4). The ferritic heat-resistant steel described in Patent Document 4 is a ferritic heat-resistant steel used in a high-temperature, high-pressure steam environment, and has a Cr oxide film on the surface of the substrate obtained by pre-oxidizing Cr adhered by shot peening treatment of powdered Cr shot material. To do. Patent Document 4 describes that the ferritic heat-resistant steel has improved water vapor oxidation resistance because a protective film of oxidation-resistant oxide is formed on the heat-resistant steel before use in an oxidizing environment.

Japanese Patent Laid-Open No. Hei 11-92880 Japanese Patent Publication No. 2007-39745 Japanese Patent Publication No. 2013-127103 Japanese Patent Publication No. 2009-179884

However, even when the above-described technique is used, the heat transfer characteristics and the water vapor oxidation resistance of the heat transfer member may not be sufficiently improved. As described above, various studies have been conducted on a method of suppressing generation of oxide scale by forming Cr oxide on the surface of the heat transfer member. However, the thermal conductivity of Cr oxide is low. Therefore, when the Cr oxide is formed, the water vapor oxidation resistance of the heat transfer member is increased, but the heat transfer property is deteriorated.

An object of the present invention is to provide a ferritic heat transfer member excellent in heat transfer characteristics and water vapor oxidation resistance, and a ferritic heat-resistant steel capable of realizing the same.

The ferritic 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 substrate is in 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: 7.0 to 14.0%, N: 0.005 -0.15%, sol.Al: 0.001 to 0.3%, Mo: 0 to 5.0%, Ta: 0 to 5.0%, W: 0 to 5.0%, and one selected from the group consisting of Re: 0 to 5.0% or 0.5 to 7.0% in total, Cu: 0 to 5.0%, Ni: 0 to 5.0%, Co: 0 to 5.0%, Ti: 0 to 1.0%, V: 0 to 1.0%, Nb: 0 to 1.0%, Hf: 0 to 1.0%, Ca: 0 to 0.1%, Mg: 0 to 0.1%, Zr: 0 to 0.1%, B: 0 to 0.1%, and rare earth elements: 0 to 0.1%, The balance has a chemical composition consisting of Fe and impurities. The oxide layer A contains a chemical composition containing 20 to 45% of Cr and Mn in total by mass%. The oxide layer A contains a chemical composition containing 0.5 to 10% in total of one or two or more selected from the group consisting of Mo, Ta, W, and Re in mass%.

The ferritic heat transfer member according to the present embodiment includes a substrate and an oxide film on the surface of the substrate. The substrate has the above chemical composition. The oxide film includes an oxide layer B and an oxide layer C. The oxide layer B contains 80% or more of Fe ₃ O ₄ and Fe ₂ O ₃ in total by volume. The oxide layer C is disposed between the oxide layer B and the substrate. The chemical composition of the oxide layer C is mass%, Cr and Mn: more than 5% to 30% in total, and one or more selected from the group consisting of Mo, Ta, W, and Re: 1 to 15 in total %.

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

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

Hereinafter, this embodiment will be described in detail with reference to the drawings. The same reference numerals are assigned to the same or corresponding parts of the drawings, and the description is not repeated.

The present inventors conducted various studies on ferritic heat-resistant steel and ferritic heat transfer member. As a result, the following knowledge was obtained.

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

(2) When 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 decrease. Therefore, in order to keep the inside of the heat transfer member at a high temperature, it is necessary to give a large amount of heat from the outside of the heat transfer member, and the heat transfer characteristic of the boiler is 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 also decrease. Therefore, it is preferable that the thermal conductivity of the oxide film is high. However, when the thermal conductivity of the oxide film is too high, heat of high-temperature water vapor is transmitted to the inner surface of the heat transfer member. Since the transferred heat accelerates the oxidation reaction on the inner surface of the heat transfer member, a large amount of oxidized scale is generated on the inner surface of the heat transfer member. A large amount of oxidized scale peels off from the inner surface of the heat transfer member. In the case where 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) When 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 inhibited. For this reason, the heat transfer characteristics of the boiler deteriorate. Therefore, it is preferable that the thickness of the oxide film is as thin as possible.

(4) Among the oxides described above, Fe ₃ O ₄ and Fe ₂ O ₃ are thermodynamically stable and formed in a high-temperature steam oxidizing environment (hereinafter, also referred to as a high-temperature steam environment). Fe ₃ O ₄ and Fe ₂ O ₃ also have high thermal conductivity. Therefore, when an oxide film containing a large amount of Fe ₃ O ₄ and Fe ₂ O ₃ is formed on the surface of the heat transfer member in contact with hot water vapor, 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 and Fe ₂ O ₃ is too high. Therefore, with only this oxide film, a large amount of oxide scale is generated on the inner surface of the heat transfer member as described above.

(5) In general, in a heat transfer member such as a boiler pipe, the concentration of Cr in 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, generation of a large amount of oxidized scale is suppressed, and the water vapor 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 characteristic of the heat transfer member is deteriorated. Therefore, only this oxide film cannot improve the heat transfer characteristics of the boiler.

(6) Therefore, in a high-temperature steam environment, an oxide film including two layers of an oxide layer having excellent heat transfer properties and an oxide layer having both water vapor oxidation resistance and heat transfer characteristics is formed on the inner surface of the heat transfer member. This makes it possible to achieve both excellent heat transfer characteristics and excellent water vapor oxidation resistance.

(7) When containing 80% or more of Fe ₃ O ₄ and Fe ₂ O ₃ in total by volume ratio, the thermal conductivity of the oxide layer is high. Therefore, the heat transfer characteristics of the boiler can be improved. Thus, an oxide layer B containing Fe ₃ O ₄ and Fe ₂ O ₃ at a total volume ratio of 80% or more is formed on the surface of the heat transfer member in contact with the high temperature water vapor.

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

Cr oxide and Mn oxide increase the water vapor oxidation resistance of the substrate. However, when the Cr content is too high, the heat transfer property of the oxide film is deteriorated. When the Mn content is too high, the creep strength of the substrate decreases. Therefore, the oxide layer C contains more than 5% to 30% by mass of Cr and Mn in total.

When Mo, Ta, W, and Re are contained in the oxide layer C, the thermal conductivity of the oxide layer C increases. However, when the content of these elements is too high, the water vapor oxidation resistance of the oxide layer C may decrease. Accordingly, the oxide layer C contains 1 to 15 mass% in total of one or two or more selected from the group consisting of Mo, Ta, W and Re.

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 chemical composition of the oxide layer A contains 20 to 45% of Cr and Mn in total by mass%. The chemical composition of the oxide layer A contains 0.5 to 10% in total of one or two or more selected from the group consisting of Mo, Ta, W, and Re in mass%. When used in a high-temperature steam environment, the oxide layer A changes to an oxide film comprising an oxide layer B and an oxide layer C. High temperature is 500-650 degreeC, for example.

The ferritic heat-resistant steel according to the present embodiment completed based on the above findings includes a substrate and an oxide layer A on the surface of the substrate. The substrate is in 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: 7.0 to 14.0%, N: 0.005 1 or 2 selected from the group consisting of ~0.15%, sol.Al: 0.001~0.3%, Mo: 0~5.0%, Ta: 0~5.0%, W: 0~5.0%, and Re: 0~5.0% 0.5 to 7.0% in total, Cu: 0 to 5.0%, Ni: 0 to 5.0%, Co: 0 to 5.0%, Ti: 0 to 1.0%, V: 0 to 1.0%, Nb: 0 to 1.0 %, Hf: 0 to 1.0%, Ca: 0 to 0.1%, Mg: 0 to 0.1%, Zr: 0 to 0.1%, B: 0 to 0.1%, and rare earth elements: 0 to 0.1%. It has a chemical composition consisting of added Fe and impurities. The oxide layer A contains a chemical composition containing 20 to 45% of Cr and Mn in total by mass%. The oxide layer A contains a chemical composition containing 0.5 to 10% in total of one or two or more selected from the group consisting of Mo, Ta, W, and Re in mass%.

The ferritic heat-resistant steel according to the present embodiment is excellent in heat transfer characteristics and water vapor oxidation resistance.

The chemical composition of the substrate of the ferritic heat-resistant steel may contain one or two or more selected from the group consisting of Cu: 0.005 to 5.0%, Ni: 0.005 to 5.0%, and Co: 0.005 to 5.0%.

The chemical composition of the above description contains one or two or more selected from the group consisting of Ti: 0.01 to 1.0%, V: 0.01 to 1.0%, Nb: 0.01 to 1.0%, and Hf: 0.01 to 1.0%. You can do it.

The chemical composition of the above description is 1 selected from the group consisting of 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 elements: 0.0015 to 0.1% They may contain species or two or more.

The ferritic heat transfer member according to the present embodiment includes a substrate and an oxide film on the surface of the substrate. The substrate has the above chemical composition. The oxide film includes an oxide layer B and an oxide layer C. The oxide layer B contains 80% or more of Fe ₃ O ₄ and Fe ₂ O ₃ in total by volume %. The oxide layer C is disposed between the oxide layer B and the substrate. The chemical composition of the oxide layer C is more than 5% to 30% by mass in total of Cr and Mn, and 1 to 15% by mass in total of one or more selected from the group consisting of Mo, Ta, W and Re Contains.

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

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

Preferably, the oxide layer C contains 5 vol% 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 low thermal conductivity. For this reason, it is possible to improve the heat transfer characteristics of the boiler.

Hereinafter, the ferritic heat-resistant steel and the ferritic heat transfer member according to the present embodiment will be described in detail. "%" with respect to an element means mass% unless otherwise indicated.

[Ferritic heat-resistant steel]

The shape of the ferritic heat-resistant steel according to the present embodiment is not particularly limited. Ferritic heat-resistant steel is, for example, a steel pipe, a bar, and a steel plate. Preferably, the ferritic heat-resistant steel is a ferritic heat-resistant steel pipe. The substrate of the ferritic heat-resistant steel according to the present embodiment is subjected to oxidation treatment. Oxidation layer A is formed on the surface of the substrate of the ferritic heat-resistant steel by oxidation treatment.

1 is a cross-sectional view of a ferritic heat-resistant steel according to the present embodiment. Referring to FIG. 1, the ferritic heat-resistant steel 1 includes a base material 2 and an oxide layer A. The ferritic heat-resistant steel 1 including the base material 2 and the oxide layer A is used as a heat transfer member in a high-temperature steam environment. Thereby, the oxide layer A changes to the oxide film 3 including the oxide layer B and the oxide layer C.

[Chemical composition of base material (2)]

The base material 2 has the following chemical composition.

C: 0.01~0.3%

Carbon (C) stabilizes austenite. C also increases the creep strength of the substrate by solid solution strengthening. However, when the C content of the substrate 2 is too high, carbides precipitate excessively, and the workability and weldability of the substrate 2 deteriorate. 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 also improves the water vapor oxidation resistance of the substrate 2. However, when the Si content is too high, the toughness of the substrate 2 decreases. Therefore, the Si content is 0.01 to 2.0%. The preferred lower limit of the Si content is 0.05%, more preferably 0.1%. The upper limit of the Si content is preferably 1.0%, and more preferably 0.5%.

Mn：0.01~2.0%

Manganese (Mn) deoxidizes steel. Mn further combines with S in the substrate 2 to form MnS, and suppresses grain boundary segregation of S. Thereby, the hot workability of the base material 2 is improved. However, when the Mn content is too high, the substrate 2 is liable to become brittle, and the creep strength of the substrate 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 upper limit of the Mn content is preferably 1.0%, more preferably 0.8%.

P: 0.10% or less

Phosphorus (P) is an impurity. P segregates at the grain boundaries of the substrate 2 and lowers the hot workability of the substrate 2. P is further concentrated at the interface between the oxide film 3 and the substrate 2 to reduce the adhesion of the oxide film 3 to the substrate 2. Therefore, it is preferable that the P content is as low as possible. The P content is 0.10% or less, preferably 0.03% or less. The lower limit of the P content is, for example, 0.005%.

S: 0.03% or less

Sulfur (S) is an impurity. S segregates at the grain boundaries of the substrate 2 and lowers the hot workability of the substrate 2. S is further concentrated at the interface between the oxide film 3 and the substrate 2, thereby reducing the adhesion of the oxide film 3 to the substrate 2. Therefore, it is preferable that the S content is as low as possible. The S content is 0.03% or less, preferably 0.015% or less. The lower limit of the S content is, for example, 0.0001%.

Cr：7.0~14.0%

Chromium (Cr) increases the water vapor oxidation resistance of the substrate 2. Cr is also 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 water vapor oxidation resistance of the substrate 2. Cr oxide further enhances the adhesion of the oxide film 3 to the substrate 2. However, when the Cr content is too high, the concentration of Cr ₂ O _{3 in} the oxide film 3 becomes high, and the heat transfer property of the oxide film 3 decreases. Therefore, the Cr content is 7.0 to 14.0%. The preferable lower limit of the Cr content is 7.5%, more preferably 8.0%. The upper limit of the Cr content is preferably 12.0%, more preferably 11.0%.

N：0.005~0.15%

Nitrogen (N) is dissolved in the base material 2 to increase the strength of the base material 2. N further forms an alloy component and nitride in the base material 2 and precipitates in the base material 2 to increase the strength of the base material 2. However, when the N content is too high, the nitride becomes coarse, and the toughness of the substrate 2 decreases. Therefore, the N content is 0.005 to 0.15%. The preferred lower limit of the N content is 0.01%. The preferred upper limit of the N content is 0.10%.

sol.Al：0.001~0.3%

Aluminum (Al) deoxidizes steel. However, when the Al content is too high, the hot workability of the substrate 2 is deteriorated. 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 this embodiment, the Al content means acid-soluble Al (sol.Al).

Mo: 0~5.0%,

Ta: 0~5.0%,

W: 0 to 5.0%, and

Re: 1 type or 2 or more selected from the group consisting of 0~5.0%: 0.5~7.0% in total

One or two or more types selected from the group consisting of molybdenum (Mo), tantalum (Ta), tungsten (W) and rhenium (Re) are contained. These elements are hereinafter also referred to as specific oxide layer forming elements. The specific oxide layer forming element forms an oxide layer A on the surface of the substrate 2. The specific oxide layer forming element also forms the oxide film 3 including the oxide layer B and the oxide layer C in a high-temperature steam environment of 500 to 650°C. If even one of these elements is contained, this effect is obtained. However, when the content of the specific oxide layer forming element is too high, the toughness, ductility and workability of the substrate 2 are deteriorated. Therefore, the Mo content is 0 to 5.0%, the Ta content is 0 to 5.0%, the W content is 0 to 5.0%, and the Re content is 0 to 5.0%. The preferred lower limit of the Mo content is 0.01%, more preferably 0.1%. The preferable lower limit of the Ta content is 0.01%, more preferably 0.1%. The preferred lower limit of the W content is 0.01%, more preferably 0.1%. The preferred lower limit of the Re content is 0.01%, more preferably 0.1%. The upper limit of the Mo content is preferably 4.0%, more preferably 3.0%. The upper limit of the Ta content is preferably 4.0%, more preferably 3.0%. The upper limit of the W content is preferably 4.0%, more preferably 3.0%. A preferable upper limit of the Re content is 4.0%, more preferably 3.0%. The total content of the specific oxide layer forming element is 0.5 to 7.0%. The preferable lower limit of the total content of the specific oxide layer forming element is 0.6%, more preferably 1.0%. The preferable upper limit of the total content of the specific oxide layer forming element is 6.5%, more preferably 6.0%.

The balance of the substrate 2 of the ferritic heat-resistant steel according to the present embodiment is Fe and impurities. In the present embodiment, impurities refer to ore or scrap used as a raw material for steel, or an element mixed from the environment of the manufacturing process, and are contained within a range that does not adversely affect the heat transfer member 4 according to the present embodiment. It says to be. Impurities are, for example, oxygen (O), arsenic (As), antimony (Sb), thallium (Tl), lead (Pb), bismuth (Bi), and the like.

The substrate 2 of the ferritic heat-resistant steel according to the present embodiment may also contain the following elements in place of a part of Fe.

Cu: 0~5.0%

Ni：0~5.0%

Co：0~5.0%

Copper (Cu), nickel (Ni), and cobalt (Co) are arbitrary elements and do not need to be contained. When contained, these elements stabilize austenite. Thereby, residual delta ferrite which lowers the impact resistance of the base material 2 is suppressed. If even one of these elements is contained, this effect is obtained. However, when the content of these elements is too high, the creep strength of the substrate 2 for a long time decreases. Therefore, the Cu content is 0 to 5.0%, the Ni content is 0 to 5.0%, and the Co content is 0 to 5.0%. The upper limit of the Cu content is preferably 3.0%, more preferably 2.0%. The upper limit of the Ni content is preferably 3.0%, more preferably 2.0%. The upper limit of the Co content is preferably 3.0%, more preferably 2.0%. The preferable lower limits of the content of these elements are each 0.005%.

Ti：0~1.0%

V：0~1.0%

Nb：0~1.0%

Hf：0~1.0%

Titanium (Ti), vanadium (V), niobium (Nb), and hafnium (Hf) are arbitrary elements and do not need to be contained. When contained, these elements combine with carbon and nitrogen to form carbides, nitrides or carbonitrides. These carbides, nitrides and carbonitrides precipitate and strengthen the substrate 2. If even one of these elements is contained, this effect is obtained. However, when the content of these elements is too high, the workability of the substrate 2 is deteriorated. 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 upper limit of the Ti content is preferably 0.8%, and more preferably 0.4%. The upper limit of the V content is preferably 0.8%, and more preferably 0.4%. The upper limit of the Nb content is preferably 0.8%, more preferably 0.4%. The upper limit of the Hf content is preferably 0.8%, and more preferably 0.4%. The preferable lower limits of the content of these elements are each 0.01%.

Ca: 0~0.1%

Mg: 0~0.1%

Zr：0~0.1%

B：0~0.1%

Rare earth elements: 0 to 0.1%

Calcium (Ca), magnesium (Mg), zirconium (Zr), boron (B), and rare earth elements (REM) are arbitrary elements and do not need to be contained. When contained, these elements increase the strength, workability and oxidation resistance of the substrate 2. If even one of these elements is contained, this effect is obtained. However, when the content of these elements is too high, the toughness and weldability of the substrate 2 deteriorate. 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%, the B content is 0 to 0.1%, and the REM content is 0 to 0.1%. The preferable upper limit of the Ca content is 0.05%. The preferable upper limit of the Mg content is 0.05%. The preferable upper limit of the Zr content is 0.05%. The preferable upper limit of the B content is 0.05%. The preferred upper limit of the REM content is 0.05%. The preferable lower limits of the content of these elements are each 0.0015%. Here, REM means yttrium (Y) of atomic number 39, lanthanum (La) of atomic number 57, which is a lanthanoid, ruthenium (Lu) of atomic number 71, and actinium (Ac) of atomic number 89, which is an actinoid (Ac) to 103 It is one or two or more elements selected from the group consisting of lawrencium (Lr).

[Oxidation layer A]

The substrate 2 having the above-described chemical composition is subjected to oxidation treatment. An oxide layer A is formed on the surface of the substrate 2 by oxidation treatment. The ferritic heat-resistant steel 1 including the substrate 2 and the oxide layer A on the surface of the substrate 2 is used in a high-temperature steam environment. In a high-temperature steam environment, the oxide layer A changes to the oxide film 3 excellent in heat transfer characteristics while maintaining the water vapor oxidation resistance. That is, the oxide layer A becomes a material for forming the oxide film 3 including the oxide layer B and the oxide layer C. Although the structure in which the oxide layer A changes to the oxide film 3 is not certain, it is speculated that the oxide layer A mainly contributes to the formation of the oxide layer C.

The thickness of the oxide layer A is not particularly limited. When the oxide layer A is formed even slightly, the oxide film 3 is formed. The thickness of the oxide layer A is preferably 0.2 μm or more. In this case, the oxide film 3 can be stably formed uniformly on the surface of the substrate 2 in a high-temperature steam environment. Therefore, it becomes easy to completely cover the base material 2 with the oxide film 3. As a result, the thermal conductivity of the surface of the ferritic heat transfer member 4 increases. More preferably, the thickness of the oxide layer A is 1.0 μm or more. The upper limit of the thickness of the oxide layer A is not particularly limited, but in consideration of mass productivity, it is preferably 20 μm or less.

The thickness of the oxide layer A is determined by the following method. The ferritic heat-resistant steel 1 subjected to the oxidation treatment described below is cut perpendicular to the surface. When the ferritic heat-resistant steel 1 is a steel pipe, the ferritic heat-resistant steel 1 is cut perpendicular to the axial direction of the steel pipe. The cross section including the surface of the ferritic heat-resistant steel 1 is observed using a scanning electron microscope (SEM) manufactured by JEOL (Japan Electron Corporation). When the ferritic heat-resistant steel 1 is a steel pipe, SEM observation is performed on a cross section including the inner surface of the steel pipe. The observation magnification is 2000 times. In the observation field, the thickness of the oxide layer on the surface of the ferritic heat-resistant steel 1 (inner surface when the ferritic heat-resistant steel 1 is a steel pipe) is measured. The measurement is performed for four different cross sections of the ferritic heat-resistant steel 1. In the case where the ferritic heat-resistant steel 1 is a steel pipe, it is measured at four locations with a 45° pitch. The average value of the measurement result is taken as the thickness of the oxide layer A.

The chemical composition of the oxide layer A contains 20 to 45% of Cr and Mn in total. When the total content of Cr and Mn in the oxide layer A is less than 20%, the total content of Cr and Mn in the oxide layer C is 5% or less in a high-temperature steam environment. In this case, the thermal conductivity of the oxide layer C becomes too high. In this case, the water vapor oxidation resistance of the ferritic heat transfer member 4 decreases. On the other hand, when the total content of Cr and Mn in the oxide layer A exceeds 45%, the total content of Cr and Mn in the oxide layer C exceeds 30% in a high-temperature steam environment. In this case, the thermal conductivity of the oxide layer C becomes too low. As a result, the heat transfer characteristics of the ferritic heat transfer member 4 deteriorate. Therefore, the chemical composition of the oxide layer A contains 20 to 45% of Cr and Mn in total. The preferred lower limit of the total content of Cr and Mn in the oxide layer A is 22%. A preferable upper limit of the total content of Cr and Mn in the oxide layer A is 40%.

The chemical composition of the oxide layer A further contains 0.5 to 10% in total of one or two or more (specific oxide layer forming elements) selected from the group consisting of Mo, Ta, W and Re. When the total content of the specific oxide layer forming elements in the oxide layer A is less than 0.5%, the total content of the specific oxide layer forming elements in the oxide layer C is less than 1% in a high-temperature steam environment. In this case, the thermal conductivity of the oxide layer C becomes too low. As a result, the heat transfer characteristics of the ferritic heat transfer member 4 deteriorate. On the other hand, when the total content of the specific oxide layer forming elements in the oxide layer A exceeds 10%, the total content of the specific oxide layer forming elements in the oxide layer C exceeds 15% in a high-temperature steam environment. In this case, the thermal conductivity of the oxide layer C becomes too high. As a result, the water vapor oxidation resistance of the ferritic heat transfer member 4 decreases. Therefore, the chemical composition of the oxide layer A contains 0.5 to 10% of the specific oxide layer forming elements in total. The preferable lower limit of the total content of the specific oxide layer forming element is 1%. The preferable upper limit of the total content of the specific oxide layer forming element is 8%.

The total content of Cr and Mn in the oxide layer A and the specific oxide layer forming elements (Mo, Ta, W, and Re) is calculated by the following method. The ferritic heat-resistant steel 1 subjected to the oxidation treatment described below is cut perpendicular to the surface. When the ferritic heat-resistant steel 1 is a steel pipe, the ferritic heat-resistant steel 1 is cut perpendicular to the axial direction of the steel pipe. The cross section including the surface of the ferritic heat-resistant steel 1 is observed using a scanning electron microscope (SEM) manufactured by JEOL (Japan Electron Corporation). The oxide layer A exhibited by a relatively white contrast of the surface of the ferritic heat-resistant steel 1 (the inner surface in the case of the ferritic heat-resistant steel 1 is a steel pipe) is specified. At the center of the thickness of the oxide layer A, elemental analysis is performed using a field emission electron probe microanalyzer (FE-EPMA) manufactured by JEOL (Japan Electron Corporation). The conditions for elemental analysis were: detector: 30 mm ² SD, acceleration voltage: 15 kV, and measurement time: 60 seconds. Elemental analysis is performed on four different cross sections of the ferritic heat-resistant steel 1. When the ferritic heat-resistant steel 1 is a steel pipe, elemental analysis is performed at four locations at a pitch of 45°. Of the composition of each obtained element, the composition excluding the amount of oxygen (O) and carbon (C) is set to 100%. The ratio (mass %) of the total amount of Cr and Mn is calculated. The ratio (mass%) of the total content of the specific oxide layer forming elements (Mo, Ta, W, and Re) is calculated. The average value of the four elemental analysis values is taken as the total content (mass%) of Cr and Mn in the oxide layer A, and the total content (mass%) of the specific oxide layer forming elements (Mo, Ta, W, and Re) in the oxide layer A. .

[Method of manufacturing ferritic heat-resistant steel (1)]

The manufacturing method of the ferritic heat-resistant steel 1 according to the present embodiment includes a preparation step and an oxidation treatment step. In the preparation step, the substrate 2 having the above-described chemical composition is prepared. The substrate 2 is made of a material having the above-described chemical composition. The material may be a slab, bloom, and billet manufactured by a continuous casting method. The material may be a billet manufactured by the ingot method. The heating temperature in manufacturing a material is, for example, 850 to 1200°C.

For example, when manufacturing a steel pipe, the prepared material is charged into a heating furnace or a crack furnace and heated. The heated material is hot-processed to manufacture the substrate 2. Hot working is, for example, the Mannesman method. In the Mannesman method, a material is punch-rolled using a perforator to make a material pipe. Subsequently, it is a method of stretching-rolling and shaping-rolling a material using a mandrel mill and a sizing mill. The temperature of hot working is, for example, 850 to 1200°C. Thereby, the base material 2 is manufactured as a seamless steel pipe. The manufacturing method of the base material 2 is not limited to the Mannesman method, and the raw material may be manufactured by hot extrusion or hot forging. In addition, heat treatment may be performed on the base material 2 manufactured by hot working, or cold working may be performed. The base material 2 may be a steel plate. When the base material 2 is used as a steel sheet, the material is hot-worked to produce the base material 2 as a steel sheet. A steel plate may be processed into a steel pipe by welding, and the base material 2 may be manufactured as a welded steel pipe.

[Oxidation treatment process]

The above-described substrate 2 is subjected to oxidation treatment. The oxidation treatment is performed by heating the substrate ₂ in a gas atmosphere containing CO, CO ₂ and N ₂ . The CO/CO ₂ ratio of the gas used for the oxidation treatment is set to 0.6 or more by volume ratio. By setting the CO/CO ₂ ratio to 0.6 or more, preferential oxidation of Fe can be suppressed. As a result, on the surface of the substrate 2, an oxide layer A containing 20% by mass or more of Cr and Mn in total and 0.5% by mass or more of a specific oxide layer forming element in total is formed. The oxide layer A changes to the oxide film 3 after the steam oxidation treatment described later. There is no particular upper limit for the CO/CO ₂ ratio, but 2.0 is preferable in consideration of operational practicality.

On the other hand, in this embodiment, the (CO + CO ₂ )/N ₂ ratio of the gas used for the oxidation treatment is set to 1.0 or less in terms of the volume ratio. When the (CO+CO ₂ )/N ₂ ratio exceeds 1.0, the base material 2 is carburized. For this reason, Cr and Mn in the oxide layer A form carbides. As a result, the total content of Cr and Mn in the oxide layer A is less than 20%. The (CO+CO ₂ )/N ₂ ratio does not have a special lower limit, but in consideration of operational practicality, 0.1 is preferable.

The temperature of the oxidation treatment is 900 to 1130°C. If the oxidation treatment temperature is less than 900°C, the outward diffusion of the specific element of the substrate 2 is slow, so that the total content of the specific oxide layer forming elements of the oxide layer A becomes too low. In this case, in a high-temperature vapor environment, the total content of the specific oxide layer forming elements in the oxide layer C becomes too low. As a result, the thermal conductivity of the oxide layer C becomes too low. As a result, the thermal conductivity on the surface of the ferritic heat transfer member 4 decreases. For this reason, the heat transfer characteristics of the ferritic heat transfer member 4 deteriorate. When the oxidation treatment temperature exceeds 1130°C, the diffusion of Cr and Mn is rapid, so that the total content of Cr and Mn in the oxide layer A exceeds 45%. As a result, in a high-temperature steam environment, the total content of Cr and Mn in the oxide layer C exceeds 30%. In this case, the thermal conductivity of the oxide layer C becomes too low. As a result, the heat transfer characteristics of the ferritic heat transfer member 4 deteriorate. Therefore, the oxidation treatment temperature is 900 to 1130°C. The preferred lower limit of the oxidation treatment temperature is 920°C, more preferably 950°C. The preferred upper limit of the oxidation treatment temperature is 1120°C.

The oxidation treatment time is 1 minute to 1 hour. If the oxidation treatment time is too short, concentration of the specific oxide layer forming element occurs, so that the total content of the specific oxide layer forming element in the oxide layer A exceeds 10%. Therefore, in a high-temperature steam environment, the total content of the specific oxide layer forming elements in the oxide layer C exceeds 15%. As a result, the thermal conductivity on the surface of the ferritic heat transfer member 4 becomes too high. On the other hand, when the oxidation treatment time is too long, productivity decreases. In consideration of productivity, it is preferable that the oxidation treatment time is short. If the oxidation treatment time is too long, since Fe is preferentially oxidized, the total content of Cr and Mn in the oxide layer A is less than 20%. Therefore, the oxidation treatment time is 1 minute to 1 hour. Preferably, the upper limit of the oxidation treatment time is 30 minutes, more preferably 20 minutes. Preferably, the lower limit of the oxidation treatment time is 3 minutes.

After the oxidation treatment, a tempering treatment (low temperature annealing) may be performed. Further, the oxidation treatment may be performed on the entire substrate 2, but may be performed only on the surface of the substrate 2 in contact with hot water vapor (for example, the inner surface of a steel pipe).

The oxidation treatment may be performed once or may be performed multiple times. After the oxidation treatment, in order to remove dirt or oil adhering to the surface of the substrate 2, degreasing or washing may be performed. Even if degreasing or washing is performed, the oxide layer A is not affected. Even if degreasing or washing is performed, the formation of the oxide film 3 after that is not affected.

The ferritic heat-resistant steel 1 of this embodiment can be manufactured by the above manufacturing method.

[Ferritic heat transfer member (4)]

The ferritic heat transfer member 4 according to the present embodiment includes a base material 2 and an oxide film 3. The base material 2 of the ferritic heat transfer member 4 is the same as the base material of the ferritic heat-resistant steel 1 described above. Therefore, the chemical composition of the substrate 2 of the ferritic heat transfer member 4 is the same as the chemical composition of the substrate 2 of the ferritic heat-resistant steel 1 described above. The shape of the ferritic heat transfer member 4 according to the present embodiment is not particularly limited. The ferritic heat transfer member 4 is, for example, a tube, rod, or plate. In the case of having a tubular shape, the ferritic heat transfer member 4 is used as, for example, a pipe for a boiler. Therefore, preferably, the ferritic heat transfer member 4 is a ferritic heat transfer tube.

2 is a cross-sectional view of the ferritic heat transfer member 4 according to the present embodiment. Referring to FIG. 2, the ferritic 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.

[Oxidation film (3)]

The oxide film 3 is formed on the surface of the substrate 2 by subjecting the ferritic heat-resistant steel 1 including the substrate 2 and the oxide layer A to a steam oxidation treatment. Referring to FIG. 2, the oxide film 3 is an oxide film including two layers of an oxide layer B and an oxide layer C. The oxide film 3 contains an oxide layer B. Therefore, the oxide film 3 is excellent in heat transfer characteristics. The oxide film 3 includes an oxide layer C. Therefore, the oxide film 3 is excellent in both water vapor oxidation resistance and heat transfer characteristics. That is, the oxide film 3 is excellent not only in water vapor oxidation resistance but also in heat transfer characteristics. The oxide layer B is formed on the uppermost layer of the ferritic heat transfer member 4. The oxide layer C is disposed between the oxide layer B and the substrate 2. When the ferritic 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 contacts high-temperature water vapor.

[Oxidation layer B]

The oxide layer B contains 80% or more of Fe ₃ O ₄ and Fe ₂ O ₃ in total by volume %. The thermal conductivity of Fe ₃ O ₄ and Fe ₂ O ₃ is high. Therefore, the thermal conductivity of the oxide layer B is high, and the heat given from the outside of the ferritic heat transfer member 4 is transferred to the inside of the ferritic heat transfer member 4 without significantly reducing it. For this reason, it is possible to improve the heat transfer characteristics of the boiler. Preferably, the oxide layer B contains 90% or more of Fe ₃ O ₄ and Fe ₂ O ₃ in total by volume. Preferably, the content of Fe ₂ O _{3 in the} oxide layer B is less than 20% by volume. More preferably, the oxide layer B is made of Fe ₃ O ₄ .

In the oxide layer B, some of Cr and Mn contained in the base material 2 are sometimes contained as oxides. In particular, Cr ₂ O ₃ has a small thermal conductivity. Therefore, it is preferable that the Cr ₂ O ₃ content of the oxide layer B is low. Therefore, preferably, the chemical composition of the oxide layer B contains 5% or less in total of Cr and Mn in mass%. More preferably, the chemical composition of the oxide layer B contains 3% or less in total of Cr and Mn in mass%.

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

[Oxidation layer C]

The oxide layer C is disposed between the oxide layer B and the substrate 2 and contacts the substrate 2.

The chemical composition of the oxide layer C contains more than 5% to 30% of Cr and Mn in total. In the oxide layer C, Cr and Mn exist as oxides represented by the formula of (Fe, M) ₃ O ₄ . In the formula, Cr and Mn are substituted for M. An oxide represented by the formula of (Fe, M) ₃ O ₄ is an oxide having the same so-called spinel crystal structure as Fe ₃ O ₄ and in which a part of Fe is substituted with Cr and Mn. When the total amount of Cr and Mn contained in the oxide layer C is 5% or less, the ratio of Fe ₃ O ₄ and Fe ₂ O _{3 in} the oxide layer C cannot be suppressed. In this case, the thermal conductivity of the oxide layer C becomes too high. As a result, a large amount of oxidized scale is generated on the inner surface of the ferritic heat transfer member 4. 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 deteriorate. Therefore, the content of Cr and Mn in the oxide layer C is more than 5% to 30% in total. Thereby, the thermal conductivity of the oxide layer C can be controlled within an appropriate range while maintaining the water vapor oxidation resistance. A preferable lower limit of the total content of Cr and Mn in the oxide layer C is 10%, more preferably 13%. A preferable upper limit of the total content of Cr and Mn in the oxide layer C is 28%, more preferably 25%.

The oxide layer C contains 1 to 15% in total of one or two or more selected from the group consisting of Mo, Ta, W and Re. When the total content of the specific oxide layer forming elements (Mo, Ta, W, and Re) in the oxide layer C is less than 1%, the thermal conductivity of the oxide layer C becomes too low. On the other hand, when the total content of the specific oxide layer forming elements in the oxide layer C exceeds 15%, the thermal conductivity of the oxide layer C becomes too high. In this case, the water vapor oxidation resistance of the ferritic heat transfer member 4 decreases. Therefore, the total content of the specific oxide layer forming elements in the oxide layer C is 1 to 15%. A preferable upper limit of the total content of the specific oxide-layer forming elements (Mo, Ta, W, and Re) in the oxide layer C is 10%, more preferably 9%. The preferred lower limit of the total content of the specific oxide layer forming elements (Mo, Ta, W, and Re) in the oxide layer C is 1.5%.

Further, the oxide layer C is an oxide most of which has the spinel crystal structure described above, and it is preferable that Cr ₂ O ₃ is 5 vol% or less. The thermal conductivity of the oxide layer C can be controlled within an appropriate range by suppressing the generation of Cr ₂ O ₃ having a low thermal conductivity to 5 vol% or less and generating an oxide having a spinel crystal structure. The content of Cr ₂ O _{3 in} the oxide layer C is preferably 5% by volume or less, and more preferably 3% by volume or less.

It is preferable that the thermal conductivity of the oxide layer C is controlled in the range of 1.2 to 3.0 W·m ^-1 ·K ^-1 . When the thermal conductivity of the oxide layer C is 1.2 W·m ^-1 ·K ^-1 or more, the heat conduction from the outside of the ferritic heat transfer member 4 to the inside of the ferritic heat transfer member 4 is not impaired, and the heat transfer characteristics of the boiler are reduced. It becomes stable and increases. On the other hand, when the thermal conductivity of the oxide layer C is 3.0 W·m ^-1 ·K ^-1 or less, the heat of the high-temperature water vapor transmitted to the surface of the substrate 2 can be stably controlled. Thereby, excessive heating of the surface of the substrate 2 is suppressed, and an oxidation reaction on the surface of the substrate 2 is suppressed. Therefore, generation of a large amount of oxidized scale on the surface of the substrate 2 is stably suppressed. As a result, the water vapor oxidation resistance of the ferritic heat transfer member 4 is stabilized and improved. Therefore, the thermal conductivity of the oxide layer C is preferably controlled in the range of 1.2 to 3.0 W·m ^-1 ·K ^-1 . In this case, it is easy to improve the water vapor oxidation resistance of the ferritic heat transfer member 4 without impairing the heat transfer characteristics. In the oxide layer C, the lower limit of the more preferable thermal conductivity is 1.3 W·m ^-1 ·K ^-1 , and more preferably 1.4 W·m ^-1 ·K ^-1 . The upper limit of the more preferable thermal conductivity in the oxide layer C is 2.8 W·m ^-1 ·K ^-1 , and more preferably 2.5 W·m ^-1 ·K ^-1 .

The volume fractions of Fe ₃ O ₄ and Fe ₂ O _{3 in} the oxide layer B are measured by the following method. The ferritic heat transfer member 4 after performing the steam oxidation treatment described later is cut perpendicularly to the surface. When the ferritic heat transfer member 4 is a tube, the ferritic heat transfer member 4 is cut perpendicular to the axial direction of the tube. With respect to the cross section (observation surface) including the oxide layer B, a composition analysis of the oxide layer B is performed using a field emission electron probe microanalyzer device (FE-EPMA) manufactured by JEOL (Japan Electronics Co., Ltd.). The conditions of the composition analysis were: detector: 30 mm ² SD, acceleration voltage: 15 kV, and measurement time: 60 seconds. By composition analysis, Fe and O (oxygen) are detected, and a region in which Cr is not detected is specified. Subsequently, it is confirmed by composition analysis that the whole of the specified region has Fe ₃ O ₄ and Fe ₂ O ₃ . Next, the strength of Fe in the oxide layer B on the observation surface is binarized. At this time, the gray scale extraction target is set to 1/10 or more of the maximum intensity. It is confirmed that all areas other than the specified areas (areas confirmed to have Fe ₃ O ₄ and Fe ₂ O ₃ ) are included in the black areas after binarization. The area ratio of the black region in the oxide layer B on the observation surface after the binarization treatment is determined, and is reduced from 100%. The obtained area ratio is taken as the volume ratio of Fe ₃ O ₄ and Fe ₂ O _{3 in} the oxide layer B.

The volume fraction of Cr ₂ O _{3 in} the oxide layer C is measured by the following method. The ferritic heat transfer member 4 after performing the steam oxidation treatment described later is cut perpendicularly to the surface. When the ferritic heat transfer member 4 is a tube, the ferritic heat transfer member 4 is cut perpendicular to the axial direction of the tube. About the cross section (observation surface) including the oxide layer B and the oxide layer C, SEM observation is performed, and the oxide layer C is specified. In SEM observation, the oxide layer B and the oxide layer C are distinguished by a difference in contrast obtained by the backscattered electron image (BSE) of the SEM. The oxide layer B has a brighter contrast than the oxide layer C. For the oxide layer C, the volume fraction of Cr ₂ O ₃ is determined in the same manner as the method of determining the volume fractions of Fe ₃ O ₄ and Fe ₂ O _{3 in} the oxide layer B. That is, the cross section (observation surface) including the oxide layer C is subjected to composition analysis using a field emission electron probe microanalyzer (FE-EPMA) manufactured by JEOL (Japan Electronics Co., Ltd.). The conditions of the composition analysis were: detector: 30 mm ² SD, acceleration voltage: 15 kV, and measurement time: 60 seconds. Cr and O (oxygen) are detected by composition analysis, and a region in which Fe is not detected is specified. Subsequently, it is confirmed by composition analysis that the whole of the specified region has Cr ₂ O ₃ . Next, the strength of Cr in the oxide layer C of the observation surface is binarized. At this time, the gray scale extraction target is set to 1/10 or more of the maximum intensity. It is confirmed that all areas other than the specified area (area confirmed to have Cr ₂ O ₃ ) are included in the black area after binarization. The area ratio of the black area after the binarization treatment of the observation surface was calculated and reduced from 100%. The obtained area ratio is taken as the volume ratio of Cr ₂ O _{3 in} the oxide layer C.

The total content of Cr and Mn in the oxide layer B and the oxide layer C, and the total content of the specific oxide layer forming elements (Mo, Ta, W, and Re) are determined by the same method as for the oxide layer A. In SEM observation, the oxide layer B and the oxide layer C are distinguished by a difference in contrast obtained by the backscattered electron image (BSE) of the SEM. The oxide layer B has a brighter contrast than the oxide layer C. Elemental analysis is performed at the center of the thickness of the oxide layer B and the center of the thickness of the oxide layer C under the same conditions as in the case of the oxide layer A. From the composition of each obtained element, in the same manner as in the case of oxide layer A, the total content (mass%) of Cr and Mn, and the total content (mass%) of specific oxide layer forming elements (Mo, Ta, W, and Re) Seek.

The thermal conductivity of the oxide layer C is determined by the following method. After the oxide layer B of the ferritic heat transfer member 4 is mechanically removed, the bulk density, specific heat, and thermal diffusivity of the oxide layer C including the substrate 2 are measured. Next, after mechanically removing the oxide layer C, the bulk density, specific heat, and thermal diffusivity are measured in the same manner for the substrate 2. By converting the difference between each measured value into the measured value of the oxide layer C and substituting it into the following equation, the thermal conductivity κ can be obtained.

κ=ρ×C _p ×D

Here, bulk density is substituted for ρ, specific heat is substituted for C _p , and thermal diffusivity is substituted for D.

The preferred lower limit of the thickness of the oxide layer C 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 ferritic heat transfer member 4 increase. For this reason, it is possible to improve the heat transfer characteristics of the boiler. When the ferritic heat transfer member 4 is used for a long time, the oxide film 3 becomes thick. Even when the temperature of the steam oxidation treatment of the ferritic heat transfer member 4 is high, the oxide film 3 becomes thick. When the oxidation treatment and steam oxidation treatment described later are performed, the oxide layer B and the oxide layer C are formed to have substantially the same thickness. Therefore, when the oxide layer C is thin, the oxide film 3 also becomes thin.

The thickness of the oxide layer B and the oxide layer C is determined by the same method as the method of determining the thickness of the oxide layer A. The ferritic heat transfer member 4 after performing the steam oxidation treatment described later is prepared. With respect to the prepared ferritic heat transfer member 4, SEM observation is performed in the same manner as the method of determining the thickness of the oxide layer A. The oxide layer B and the oxide layer C are distinguished by a difference in contrast obtained by the reflected electron image of the SEM. The oxide layer B has a darker contrast than the oxide layer C. In the same manner as the method of determining the thickness of the oxide layer A, the thicknesses of the oxide layer B and the oxide layer C are determined.

[Method of manufacturing ferritic heat transfer member 4]

The manufacturing method of the ferritic heat transfer member 4 according to the present embodiment includes a steam oxidation treatment step.

[Water vapor oxidation treatment process]

The ferritic heat-resistant steel subjected to the oxidation treatment described above is subjected to steam oxidation treatment. The steam oxidation treatment is performed by exposing ferritic heat-resistant steel to 500 to 650°C steam. If 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 into the oxide film 3 including the oxide layer B and the oxide layer C. As a result, the oxide film 3 including the oxide layer B and the oxide layer C is formed on the substrate 2.

Through the above steps, the ferritic heat transfer member 4 according to the present embodiment can be manufactured. By exposing the ferritic heat-resistant steel 1 of the present embodiment to a high-temperature steam environment, the same effect as in the case of performing the steam oxidation treatment is obtained. That is, when the ferritic heat-resistant steel 1 of the present embodiment is exposed in a high-temperature steam environment for 100 hours or more, the ferritic heat transfer member 4 can be manufactured without performing steam oxidation treatment.

Example

Each steel piece having the chemical composition shown in Table 1 was prepared, and oxidation treatment and steam oxidation treatment were performed under the conditions shown in Table 2. Specifically, ingots having the chemical composition shown in Table 1 were smelted. Each of the obtained ingots was hot-rolled and cold-rolled to produce a steel sheet, and used as a base material. Test pieces were prepared from each of the obtained substrates, and oxidation treatment was performed for each test piece under the conditions shown in Table 2.

[Table 1]

[Table 2]

[Thickness measurement test of oxide layer A]

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

[Test for measuring the content of metal elements in oxide layer A]

About the cross section of each test piece, the content of each metal element was calculated|required by the method mentioned above. For the oxide layer A, the total content (mass%) of Cr and Mn, and the total content (mass%) of Mo, Ta, W, and Re were determined. The results are shown in Table 2.

For each test piece, steam oxidation treatment was performed under the conditions shown in Table 2. About each obtained test piece, the following measurement test was performed.

[Volume ratio of Fe ₃ O ₄ and Fe ₂ O ₃ of oxide layer B, and Cr ₂ O ₃ volume ratio of oxide layer C]

With respect to the cross section of each test piece (that is, the cross section of the oxide layer B), the total volume fraction of Fe ₃ O ₄ and Fe ₂ O ₃ was determined by the method described above. In addition, the volume fraction of Cr ₂ O ₃ was determined for the cross section of the oxide layer C. The results are shown in Table 2.

[Measurement test of the content of metal elements]

About the cross section of each test piece, the content of each metal element was calculated|required by the method mentioned above. About the oxide layer B, the total content (mass %) of Cr and Mn was calculated|required. The results are shown in Table 2. For the oxide layer C, the total content (mass%) of Cr and Mn, and the total content (mass%) of Mo, Ta, W, and Re were determined. The results are shown in Table 2.

[Test for measuring thermal conductivity of oxide layer C]

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

[Thickness measurement test of oxide layer C]

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

[Evaluation results]

With reference to Table 1 and Table 2, the chemical composition and production conditions of the steels of Test Nos. 1, 3, 6, 9-15, and 17 were appropriate. Therefore, the oxide layer A of these test numbers contains 20 to 45% of Cr and Mn in total, and 0.5 to 10% of one or more selected from the group consisting of Mo, Ta, W, and Re. did. Thereby, the oxide layer B formed on the substrate after the steam oxidation treatment contained 80% or more of Fe ₃ O ₄ and Fe ₂ O ₃ in total by volume. Further, the total content of Cr+Mn in the oxide layer C was more than 5% to 30%, and the total content of the specific oxide layer forming elements was 1 to 15%. As a result, the thermal conductivity of the oxide layer C was in the range of 1.2 to 3.0 W·m ^-1 ·K ^-1 , and excellent thermal conductivity was exhibited. 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. 2, although the chemical composition was appropriate, the oxidation treatment temperature was too high, so the total amount of Cr and Mn in the oxide layer A exceeded 45%. Therefore, the amount of Cr+Mn in the oxide layer C exceeded 30%, and the thermal conductivity became less than 1.2 W·m ^-1 ·K ^-1 .

In Test No. 4, although the chemical composition was appropriate, no oxidation treatment was performed and the oxide layer A was not formed. For this reason, the thermal conductivity of the oxide layer C was less than 1.2 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.

In Test No. 5, the chemical composition was appropriate, but since the oxidation treatment temperature was too low, the total amount of the specific oxide layer forming elements in the oxide layer A was 0.4%, and was too low. For this reason, the total amount of the specific oxide layer forming elements in the oxide layer C was less than 1.0%. As a result, the thermal conductivity of the oxide layer C was 1.0 W·m ^-1 ·K ^-1 and was too low.

In Test No. 7, the chemical composition was appropriate, but the CO/CO ₂ ratio in the oxidation treatment was less than 0.6. Therefore, the total content of Cr and Mn in the oxide layer A was less than 20%. Therefore, the total content of Cr and Mn in the oxide layer C was 5% or less, and the thermal conductivity of the oxide layer C exceeded 3.0 W·m ^-1 ·K ^-1 . In addition, since the volume fraction of Fe ₃ O ₄ in the oxide layer B was less than 80%, the internal flow rate of oxygen increased, the growth of the oxide layer C was promoted, and the thickness of the oxide layer C exceeded 60 μm.

In Test No. 8, the chemical composition was appropriate, but the oxidation treatment time was too long. Therefore, the total content of Cr and Mn in the oxide layer A was 6.5%, and was too low. Therefore, the total content of Cr and Mn in the oxide layer C was 3.2% and was too low. As a result, the thermal conductivity of the oxide layer C became 3.2 W·m ^-1 ·K ^{-1 and} was too high. In Test No. 8, the thickness of the oxide layer C exceeded 60 μm. It is thought that this is because the thermal conductivity of the oxide layer C was too high.

In Test No. 16, the chemical composition was appropriate, but the oxidation treatment time was too short. For this reason, the total content of the specific oxide layer forming elements in the oxide layer A was 12.9%, which was too high. Therefore, the total content of the specific oxide layer-forming elements in the oxide layer C was 17.2%, which was too high. As a result, the thermal conductivity of the oxide layer C was 3.5 W·m ^-1 ·K ^-1 , and it was too high. In Test No. 16, the thickness of the oxide layer C exceeded 60 μm. It is thought that this is because the thermal conductivity of the oxide layer C was too high.

Test No. 18 did not contain any specific oxide layer forming elements. Therefore, although the manufacturing method was appropriate, the total content of the specific oxide layer forming elements of the oxide layer A was less than 0.1% and was too low. Therefore, the total content of the specific oxide layer-forming elements in the oxide layer C was less than 0.1%, and was too low. As a result, the thermal conductivity of the oxide layer C was 1.1 W·m ^-1 ·K ^-1 , and it was too low.

In Test No. 19, the Cr content was too high. Therefore, although the manufacturing method was appropriate, the total content of Cr and Mn in the oxide layer A was 47.6%, which was too high. Therefore, the total content of Cr and Mn in the oxide layer C was 56.7%, which was too high. As a result, the thermal conductivity of the oxide layer C became 0.8 W·m ^-1 ·K ^{-1 and} was too low.

In Test No. 20, the Cr content was too low. Therefore, although the manufacturing method was appropriate, the total content of Cr and Mn in the oxide layer A was 16.3%, which was too low. Therefore, the total content of Cr and Mn in the oxide layer C was 1.3% and was too low. As a result, the thermal conductivity of the oxide layer C was 3.3 W·m ^-1 ·K ^-1 and was too high. In Test No. 20, the thickness of the oxide layer C exceeded 60 μm. It is thought that this is because the thermal conductivity of the oxide layer C was too high.

In Test No. 21, the content of the specific oxide layer forming element was too high. Therefore, the total content of the specific oxide layer-forming elements of the oxide layer A was 13.9%, which was too high. Therefore, the total content of the specific oxide layer-forming elements in the oxide layer C was 18.6%, which was too high. As a result, the thermal conductivity of the oxide layer C was 3.8 W·m ^-1 ·K ^-1 , and it was too high. In Test No. 21, the thickness of the oxide layer C exceeded 60 μm. It is thought that this is because the thermal conductivity of the oxide layer C was too high.

In Test No. 22, the chemical composition was appropriate, but the (CO + CO ₂ )/N ₂ ratio exceeded 1.0. Therefore, the total content of Cr and Mn in the oxide layer A was 10.6%, which was too low. Therefore, the total content of Cr and Mn in the oxide layer C was 4.6% and was too low. As a result, the thermal conductivity of the oxide layer C was 3.4 W·m ^-1 ·K ^-1 and was too high. In Test No. 22, the thickness of the oxide layer C exceeded 60 μm. It is thought that this is because the thermal conductivity of the oxide layer C was too high.

In the above, embodiments of the present invention have been described. However, the above-described embodiment is only an illustration for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately changing the above-described embodiment within a range not departing from the spirit thereof.

1 Ferritic heat-resistant steel
2 description
3 oxide film
4 Ferritic heat transfer member
A oxide layer A
B oxide layer B
C oxide layer C

Claims (12)

Base material and
Providing an oxide layer A on the surface of the substrate,
The above description,
In% by mass,
C: 0.01~0.3%,
Si: 0.01 to 2.0%,
Mn: 0.01 to 2.0%,
P: 0.10% or less,
S: 0.03% or less,
Cr: 7.0-14.0%,
N: 0.005 to 0.15%,
sol.Al: 0.001~0.3%,
Mo: 0 to 5.0%, Ta: 0 to 5.0%, W: 0 to 5.0%, and Re: 0 to 5.0% One or more selected from the group consisting of: 0.5 to 7.0% in total,
Cu: 0 to 5.0%,
Ni: 0 to 5.0%,
Co: 0~5.0%,
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%, and,
Rare-earth element: contains 0 to 0.1%, the balance has a chemical composition consisting of Fe and impurities,
The oxide layer A,
In% by mass,
Cr and Mn: 20 to 45% in total, and,
One or two or more selected from the group consisting of Mo, Ta, W and Re: 0.9 to 10% in total
Ferritic heat-resistant steel containing a chemical composition containing. The method according to claim 1,
The chemical composition of the substrate is,
Cu: 0.005 to 5.0%,
Ni: 0.005 to 5.0%, and,
Co: Ferritic heat-resistant steel containing one or two or more selected from the group consisting of 0.005 to 5.0%. The method according to claim 1,
The chemical composition of the substrate is,
Ti: 0.01 to 1.0%,
V: 0.01 to 1.0%,
Nb: 0.01 to 1.0%, and,
Hf: Ferritic heat-resistant steel containing one or two or more selected from the group consisting of 0.01 to 1.0%. The method according to claim 2,
The chemical composition of the substrate is,
Ti: 0.01 to 1.0%,
V: 0.01 to 1.0%,
Nb: 0.01 to 1.0%, and,
Hf: Ferritic heat-resistant steel containing one or two or more selected from the group consisting of 0.01 to 1.0%. The method according to claim 1,
The chemical composition of the substrate is,
Ca: 0.0015 to 0.1%,
Mg: 0.0015 to 0.1%,
Zr: 0.0015~0.1%,
B: 0.0015 to 0.1%, and,
Rare earth elements: ferritic heat-resistant steel containing one or two or more selected from the group consisting of 0.0015 to 0.1%. The method according to claim 2,
The chemical composition of the substrate is,
Ca: 0.0015 to 0.1%,
Mg: 0.0015 to 0.1%,
Zr: 0.0015~0.1%,
B: 0.0015 to 0.1%, and,
Rare earth elements: ferritic heat-resistant steel containing one or two or more selected from the group consisting of 0.0015 to 0.1%. The method of claim 3,
The chemical composition of the substrate is,
Ca: 0.0015 to 0.1%,
Mg: 0.0015 to 0.1%,
Zr: 0.0015~0.1%,
B: 0.0015 to 0.1%, and,
Rare earth elements: ferritic heat-resistant steel containing one or two or more selected from the group consisting of 0.0015 to 0.1%. The method of claim 4,
The chemical composition of the substrate is,
Ca: 0.0015 to 0.1%,
Mg: 0.0015 to 0.1%,
Zr: 0.0015~0.1%,
B: 0.0015 to 0.1%, and,
Rare earth elements: ferritic heat-resistant steel containing one or two or more selected from the group consisting of 0.0015 to 0.1%. A substrate having the chemical composition according to any one of claims 1 to 8, and
Providing an oxide film on the surface of the substrate,
The oxide film,
An oxide layer B containing 80% or more of Fe ₃ O ₄ and Fe ₂ O ₃ in total by volume%, and
It includes an oxide layer C disposed between the oxide layer B and the substrate,
The chemical composition of the oxide layer C is,
In% by mass,
Cr and Mn: more than 5% to 30% in total, and,
One or two or more selected from the group consisting of Mo, Ta, W and Re: a ferritic heat transfer member containing 1 to 15% in total. The method of claim 9,
The chemical composition of the oxide layer B is,
In% by mass,
Cr and Mn: A ferritic heat transfer member containing 5% or less in total. The method of claim 9,
The oxide layer C,
A ferritic heat transfer member containing 5% or less of Cr ₂ O ₃ by volume %. The method of claim 10,
The oxide layer C,
A ferritic heat transfer member containing 5% or less of Cr ₂ O ₃ by volume %.

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