JP6801712B2 - Ferritic heat-resistant steel and ferrite heat transfer members - Google Patents

Description

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

Thermal power plants are required to improve power generation efficiency from the viewpoint of reducing CO ₂ gas emissions and economic efficiency. Therefore, the turbine steam pressure is being increased in temperature and pressure. 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. Ferritic heat-resistant steels are cheaper than austenitic heat-resistant steels. Ferritic heat-resistant steels also have a lower coefficient of thermal expansion and higher thermal conductivity than austenitic heat-resistant steels. Therefore, ferritic heat-resistant steel is suitable as a piping material for thermal power plants. However, the Cr content of ferritic heat-resistant steel is lower than that of austenitic heat-resistant steel. Therefore, the steam oxidation resistance of ferritic heat-resistant steel is lower than that of austenitic heat-resistant steel. Therefore, a ferritic heat-resistant steel having excellent steam oxidation resistance is required.

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. In Patent Document 2, powder particles containing Cr are supported on the surface of a ferritic heat-resistant steel containing Cr to form a Cr oxide layer having a high Cr concentration on the surface of the ferritic steel at a high temperature. Patent Document 2 describes that this method can easily and economically improve the (water vapor) oxidation resistance of ferritic steel containing Cr.

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.

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

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. As described above, various studies have been conducted on methods for suppressing the formation 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, if Cr oxide is formed, the steam oxidation resistance of the heat transfer member is enhanced, but the heat transfer characteristics are deteriorated.

An object of the present invention is to provide a ferritic heat transfer member having excellent heat transfer characteristics and steam 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 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: 7.0 to 14.0%, N: 0.005 to 0.15%, sol. Group consisting of Al: 0.001 to 0.3%, Mo: 0 to 5.0%, Ta: 0 to 5.0%, W: 0 to 5.0%, and Re: 0 to 5.0%. 1 type or 2 or more types selected from 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. It contains 1%, Zr: 0 to 0.1%, B: 0 to 0.1%, and rare earth elements: 0 to 0.1%, 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 a chemical composition containing 0.5 to 10% in total of one or more selected from the group consisting of Mo, Ta, W and Re in mass%.

The ferrite heat transfer member according to the present embodiment includes a base material and an oxide film on the surface of the base material. The base material has the above chemical composition. The oxide film includes an oxide layer B and an oxide layer C. The oxide layer B contains Fe ₃ O ₄ and Fe ₂ O ₃ in a total volume of 80% or more. The oxide layer C is arranged between the oxide layer B and the base material. The chemical composition of the oxide layer C is, in 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: in total. Contains 1-15%.

The ferritic heat-resistant steel and the ferritic 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 a ferritic heat-resistant steel according to the present embodiment. FIG. 2 is a cross-sectional view of the ferrite 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 ferrite-based heat-resistant steel and ferrite-based heat transfer members. As a result, the following findings were obtained.

(1) The ferritic 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 oxides are, for example, Fe ₃ O ₄ , Fe ₂ O ₃ , and Cr ₂ O ₃ . 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 and Fe ₂ O ₃ are 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 and Fe ₂ O ₃ also have high thermal conductivity. Therefore, if 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 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 and Fe ₂ O ₃ 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) Generally, in a heat transfer member such as a boiler pipe, it is possible to improve the Cr concentration on the inner surface of the pipe and form an oxide film containing a large amount of Cr ₂ O ₃ on the inner surface of the heat transfer member. There are many. 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 containing 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 and Fe ₂ O ₃ having a total volume fraction of 80% or more are 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 and Fe ₂ O ₃ having a total 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 a total of more than 5% to 30% by mass of Cr and Mn, and a total of 1 to 15% by mass of one or more selected from the group consisting of Mo, Ta, W and Re. To do.

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 creep strength of the substrate will decrease. Therefore, the oxide layer C contains Cr and Mn in a total amount of more than 5% to 30% by mass.

When 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 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 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 Mo, Ta, W and Re. When used in a high temperature vapor environment, the oxide layer A changes into an oxide film containing the oxide layer B and the oxide layer C. The high temperature is, for example, 500 to 650 ° C.

The ferrite-based 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: 7.0 to 14.0%, N: 0.005 to 0.15%, sol. From the group consisting of Al: 0.001 to 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 types are 0.5 to 7.0% in total, Cu: 0 to 5.0%, Ni: 0 to 5.0%, Co: 0 to 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 to 0.1%, B: 0 to 0.1%, and rare earth element: 0 to 0.1%, 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 a chemical composition containing 0.5 to 10% in total of one or more selected from the group consisting of Mo, Ta, W and Re in mass%.

The ferritic heat-resistant steel according to this embodiment is excellent in heat transfer characteristics and steam oxidation resistance.

The chemical composition of the base material of the ferritic heat-resistant steel is a group consisting of Cu: 0.005 to 5.0%, Ni: 0.005 to 5.0%, and Co: 0.005 to 5.0%. It may contain one kind or two or more kinds selected from.

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 0.0%.

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

The ferrite-based 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.

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

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

FIG. 1 is a cross-sectional view of a ferritic heat-resistant steel according to the present embodiment. With reference to FIG. 1, the ferrite heat-resistant steel 1 includes a base material 2 and an oxide layer A. The ferrite-based heat-resistant steel 1 provided with the base material 2 and the oxide layer A is used as a heat transfer member in a high-temperature steam environment. As a result, the oxide layer A is transformed into an oxide film 3 containing the oxide layer B and the 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 creep strength of the base material by solid solution strengthening. However, if the C content of the base material 2 is too high, carbides are excessively precipitated, and the processability and weldability of the base material 2 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 creep 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 Phosphorus (P) is an impurity. P segregates at the grain boundaries of the base material 2 to reduce the hot workability of the base material 2. P is 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 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 base material 2 to reduce the hot workability of the base material 2. S is 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 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) enhances the water vapor 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 7.0 to 14.0%. The lower limit of the Cr content is preferably 7.5%, more preferably 8.0%. The preferred upper limit of the Cr content is 12.0%, more preferably 11.0%.

N: 0.005 to 0.15%
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 0.15%. The preferable lower limit of the N content is 0.01%. The preferable upper limit of the N content is 0.10%.

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).

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 7.0% in total
It contains one or more selected from the group consisting of 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 film 3 containing the oxide layer B and the oxide layer C in a high temperature vapor 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 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 lower limit of the Mo content is preferably 0.01%, more preferably 0.1%. The lower limit of the Ta content is preferably 0.01%, more preferably 0.1%. The lower limit of the W content is preferably 0.01%, more preferably 0.1%. The lower limit of the Re content is preferably 0.01%, more preferably 0.1%. The preferred upper limit of the Mo content is 4.0%, more preferably 3.0%. The preferred upper limit of the Ta content is 4.0%, more preferably 3.0%. The preferred upper limit of the W content is 4.0%, more preferably 3.0%. The preferred 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 elements is 0.6%, more preferably 1.0%. The preferable upper limit of the total content of the specific oxide layer forming elements is 6.5%, more preferably 6.0%.

The balance of the base material 2 of the ferritic heat-resistant steel according to the present embodiment is Fe and impurities. In the present embodiment, the impurity means an ore or scrap used as a raw material for steel, or an element mixed from the environment of the manufacturing process, etc., and is 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 ferritic heat-resistant steel according to the present embodiment may further 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 optional elements and may not be contained. When included, these elements stabilize 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 long-term creep strength of the base material 2 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 preferred upper limit of the Cu content is 3.0%, more preferably 2.0%. The preferred upper limit of the Ni content is 3.0%, more preferably 2.0%. The preferred upper limit of the Co content is 3.0%, more preferably 2.0%. The preferred lower limit of the content of these elements is 0.005%, respectively.

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 optional elements and may not 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 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 Ti content is 0.8%, more preferably 0.4%. The preferred upper limit of the V content is 0.8%, more preferably 0.4%. The preferred upper limit of the Nb content is 0.8%, more preferably 0.4%. The preferred upper limit of the Hf content is 0.8%, more preferably 0.4%. 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 optional elements and may not be contained. When contained, these elements enhance the strength, processability and oxidation resistance of the substrate 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 REM content is 0 to 0.1%. The preferred upper limit of the Ca content is 0.05%. The preferable upper limit of the Mg content is 0.05%. The preferred upper limit of the Zr content is 0.05%. The preferred upper limit of the B content is 0.05%. The preferred upper limit of the REM content is 0.05%. 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 ferritic heat-resistant steel 1 provided with the base material 2 and the oxide layer A on the surface of the base material 2 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 the oxide film 3 including the oxide layer B and the oxide layer C. The mechanism by which the oxide layer A changes to the oxide film 3 is not clear, but it is presumed 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. If even a small amount of the oxide layer A is formed, 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 and uniformly formed on the surface of the base material 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 on the surface of the ferrite heat transfer member 4 is increased. 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 is preferably 20 μm or less in consideration of mass productivity.

The thickness of the oxide layer A is determined by the following method. The ferrite heat-resistant steel 1 which has been subjected to the oxidation treatment described later 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 perpendicularly 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 (JEOL Ltd.). When the ferrite heat-resistant steel 1 is a steel pipe, SEM observation is performed on the cross section including the inner surface of the steel pipe. The observation magnification is 2000 times. In the observation field of view, 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 on four different cross sections of the ferritic heat-resistant steel 1. When the ferritic heat-resistant steel 1 is a steel pipe, the measurement is performed at four points at a pitch of 45 °. Let the average value of the measurement results be 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 steam oxidation resistance of the ferrite heat transfer member 4 is lowered. On the other hand, if 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 ferrite 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 preferable lower limit of the total content of Cr and Mn in the oxide layer A is 22%. The 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 more (specific oxide layer forming elements) selected from the group consisting of Mo, Ta, W and Re. If 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 ferrite heat transfer member 4 deteriorate. On the other hand, if the total content of the specific oxide layer forming elements of the oxide layer A exceeds 10%, the total content of the specific oxide layer forming elements of the oxide layer C exceeds 15% in a high temperature vapor environment. In this case, the thermal conductivity of the oxide layer C becomes too high. As a result, the steam oxidation resistance of the ferrite heat transfer member 4 is lowered. 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 elements is 1%. The preferable upper limit of the total content of the specific oxide layer forming elements is 8%.

The total contents of Cr and Mn in the oxide layer A and the specific oxide layer forming elements (Mo, Ta, W and Re) are calculated by the following method. The ferrite heat-resistant steel 1 which has been subjected to the oxidation treatment described later 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 perpendicularly 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 Ltd. (JEOL Ltd.). The oxide layer A that appears with a relatively white contrast on the surface of the ferritic heat-resistant steel 1 (inner surface when the ferritic heat-resistant steel 1 is a steel pipe) is specified. Elemental analysis is performed at the center of the thickness of the oxide layer A using a field emission electron probe microanalyzer (FE-EPMA) manufactured by JEOL Ltd. (JEOL Ltd.). The conditions for elemental analysis are detector: 30 mm ² SD, accelerating voltage: 15 kV, measurement time: 60 seconds. Elemental analysis is performed on four different cross sections of the ferritic heat-resistant steel 1. When the ferrite 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 element obtained, the composition excluding the amounts of oxygen (O) and carbon (C) is defined as 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 the total content (mass%) of Cr and Mn of the oxide layer A and the total content of the specific oxide layer forming elements (Mo, Ta, W and Re) of the oxide layer A. The amount (% by mass).

[Manufacturing method of ferritic heat-resistant steel 1]
The method for producing 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 base material 2 having the above-mentioned chemical composition is prepared. The base material 2 is produced from a material having the above-mentioned chemical composition. 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. The heating temperature in producing the 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 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. The temperature of hot working is, for example, 850 to 1200 ° C. 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 process]
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 containing CO, CO ₂ and N ₂ . 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, preferential oxidation of Fe can be suppressed. As a result, an oxide layer A containing 20% by mass or more of Cr and Mn in total and 0.5% by mass or more of specific oxide layer forming elements in total 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 CO / CO ₂ ratio is not particularly limited, but 2.0 is preferable in consideration of practicality in operation.

On the other hand, in the present 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 volume ratio. When the (CO + CO ₂ ) / N ₂ ratio exceeds 1.0, the base material 2 is carburized. Therefore, 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 lower limit, but is preferably 0.1 in consideration of practicality in operation.

The temperature of the oxidation treatment is 900 to 1130 ° C. If the oxidation treatment temperature is less than 900 ° C., the external diffusion of the specific element of the base material 2 is slow, so that the total content of the specific oxide layer-forming element of the oxide layer A becomes too low. In this case, the total content of the specific oxide layer forming elements in the oxide layer C becomes too low in a high temperature steam environment. 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 ferrite heat transfer member 4 decreases. Therefore, the heat transfer characteristics of the ferrite heat transfer member 4 are deteriorated. When the oxidation treatment temperature exceeds 1130 ° C., the diffusion of Cr and Mn is fast, so that the total content of Cr and Mn in the oxide layer A exceeds 45%. As a result, 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 ferrite heat transfer member 4 deteriorate. Therefore, the oxidation treatment temperature is 900 to 1130 ° C. The lower limit of the oxidation treatment temperature is preferably 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, the specific oxide layer-forming element is concentrated, 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 ferrite heat transfer member 4 becomes too high. On the other hand, 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. If the oxidation treatment time is too long, Fe is preferentially oxidized, so that 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.

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 ferrite-based heat-resistant steel 1 of the present embodiment can be manufactured by the above manufacturing method.

[Ferrite heat transfer member 4]
The ferrite 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 ferrite heat transfer member 4 is the same as the base material of the above-mentioned ferritic heat-resistant steel 1. Therefore, the chemical composition of the base material 2 of the ferrite heat transfer member 4 is the same as the chemical composition of the base material 2 of the ferritic heat-resistant steel 1 described above. The shape of the ferrite heat transfer member 4 according to the present embodiment is not particularly limited. The ferrite heat transfer member 4 is, for example, a pipe, a rod or a plate material. When it has a tubular shape, the ferrite heat transfer member 4 is used, for example, as a boiler pipe or the like. Therefore, preferably, the ferrite heat transfer member 4 is a ferrite heat transfer tube.

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

[Oxide film 3]
By performing steam oxidation treatment on the ferrite heat-resistant steel 1 provided with the base material 2 and the oxide layer A, an oxide film 3 is formed on the surface of the base material 2. With reference to FIG. 2, the oxide film 3 is an oxide film containing two layers, 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 contains an oxide layer C. Therefore, the oxide film 3 is excellent in both steam 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 ferrite heat transfer member 4. The oxide layer C is arranged between the oxide layer B and the base material 2. When the ferrite 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 Fe ₃ O ₄ and Fe ₂ O ₃ in a total volume of 80% or more. 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 being significantly reduced. Therefore, the heat transfer characteristics of the boiler can be improved. Preferably, the oxide layer B contains Fe ₃ O ₄ and Fe ₂ O ₃ in a total volume of 90% or more. Preferably, the Fe ₂ O ₃ content of the oxide layer B is less than 20% by volume. More preferably, the oxide layer B is composed of Fe ₃ O ₄ .

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, 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 is mass% and contains Cr and Mn in a total of 5% or less. More preferably, the chemical composition of the oxide layer B is mass% and contains Cr and Mn in a total of 3% or less.

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

[Oxidized layer C]
The oxide layer C is arranged 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 Cr and Mn in a total amount of more than 5% to 30%. In the oxidation layer C, Cr and Mn are present as oxide represented _{(Fe, M)} of 3 _{O 4} in the chemical formula. In the formula, Cr and Mn are substituted for M. _{(Fe, M)} and the oxide represented by the chemical formula 3 _{O 4,} has the same so-called spinel crystalline structure as _Fe 3 _{O 4,} which is an oxide which is partially substituted with Cr and Mn of Fe. 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. Therefore, a large amount of oxidation scale is generated on the inner surface of the ferrite 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 are deteriorated. Therefore, the total content of Cr and Mn in the oxide layer C is more than 5% to 30%. As a result, the thermal conductivity of the oxide layer C can be controlled within an appropriate range while maintaining the steam oxidation resistance. The preferable lower limit of the total content of Cr and Mn in the oxide layer C is 10%, more preferably 13%. The 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 type or 2 or more types selected from the group consisting of Mo, Ta, W and Re in a total of 1 to 15%. If the total content of the specific oxide layer forming elements (Mo, Ta, W and Re) of 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 steam oxidation resistance of the ferrite heat transfer member 4 is lowered. Therefore, the total content of the specific oxide layer forming element in the oxide layer C is 1 to 15%. The 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 preferable 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%.

Furthermore, most of the oxide layer C is an oxide having the above-mentioned spinel-type crystal structure, and Cr ₂ O ₃ is preferably 5% by volume or less. By suppressing the formation of Cr ₂ O ₃ having a 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 C can be controlled within an appropriate range. 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.

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 . 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 ferrite heat transfer member 4 to the inside of the ferrite heat transfer member 4 is not hindered. The heat transfer characteristics of the boiler are stably enhanced. 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 high-temperature steam transmitted to the surface of the base material 2 can be stably controlled. As a result, excessive heating of the surface of the base material 2 is suppressed, and the oxidation reaction of the surface of the base material 2 is suppressed. Therefore, the formation of a large amount of oxidation scale on the surface of the base material 2 is stably suppressed. As a result, the steam oxidation resistance of the ferrite heat transfer member 4 is stably enhanced. 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 steam oxidation resistance of the ferrite heat transfer member 4 without impairing the heat transfer characteristics. The lower limit of the more preferable thermal conductivity in the oxide layer C 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 ferrite heat transfer member 4 after being subjected to the steam oxidation treatment described later is cut perpendicular to the surface. When the ferritic heat transfer member 4 is a pipe, the ferritic heat transfer member 4 is cut perpendicular to the axial direction of the pipe. The composition of the oxide layer B is analyzed on the cross section (observation surface) including the oxide layer B using a field emission electron probe microanalyzer (FE-EPMA) manufactured by JEOL Ltd. The conditions for composition analysis are: detector: 30 mm ² SD, accelerating voltage: 15 kV, measurement time: 60 seconds. Fe and O (oxygen) are detected by composition analysis, and a region where Cr is not detected is specified. Subsequently, composition analysis confirms that all of the identified regions have Fe ₃ O ₄ or Fe ₂ O ₃ . Next, the intensity of Fe is binarized in the oxide layer B on the observation surface. At this time, the grayscale extraction target is 1/10 or more of the maximum intensity. It is confirmed that the black region after binarization includes all regions other than the specified region (the region confirmed to have Fe ₃ O ₄ and Fe ₂ O ₃ ). The area ratio of the black region in the oxide layer B on the observation surface after the binarization treatment is obtained and subtracted from 100%. The obtained area fraction is taken as the volume fraction 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 ferrite heat transfer member 4 after being subjected to the steam oxidation treatment described later is cut perpendicular to the surface. When the ferritic heat transfer member 4 is a pipe, the ferritic heat transfer member 4 is cut perpendicular to the axial direction of the pipe. SEM observation is performed on the cross section (observation surface) including the oxide layer B and the oxide layer C to identify the oxide layer C. In SEM observation, the oxide layer B and the oxide layer C are distinguished by the contrast difference 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 obtained by the same method as the method of obtaining the volume fractions of Fe ₃ O ₄ and Fe ₂ O ₃ of the oxide layer B. That is, the composition of the cross section (observation surface) including the oxide layer C is analyzed using a field emission electron probe microanalyzer (FE-EPMA) manufactured by JEOL Ltd. (JEOL Ltd.). The conditions for composition analysis are: detector: 30 mm ² SD, accelerating voltage: 15 kV, measurement time: 60 seconds. A region in which Cr and O (oxygen) are detected by composition analysis and Fe is not detected is specified. Subsequently, it is confirmed by composition analysis that all of the identified regions have Cr ₂ O ₃ . Next, the strength of Cr is binarized in the oxide layer C on the observation surface. At this time, the grayscale extraction target is 1/10 or more of the maximum intensity. It is confirmed that the black region after binarization includes all regions other than the specified region (the region confirmed to have Cr ₂ O ₃ ). The area ratio of the black area after the binarization treatment of the observation surface is obtained and subtracted from 100%. The obtained area fraction is taken as the volume fraction 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 the contrast difference 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 element obtained, the total content (mass%) of Cr and Mn and the total content of the specific oxide layer forming elements (Mo, Ta, W and Re) are obtained in the same manner as in the case of the oxide layer A. Obtain the amount (mass%).

The thermal conductivity of the oxide layer C is determined by the following method. After mechanically removing the oxide layer B of the ferrite 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 mechanically removing the oxide layer C, the bulk density, specific heat, and thermal diffusivity of the base material 2 are measured 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 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 ferrite heat transfer member 4 are enhanced. Therefore, the heat transfer characteristics of the boiler can be improved. If the ferrite 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 ferrite 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 is determined by the same method as the method for determining the thickness of the oxide layer A. The ferrite heat transfer member 4 after being subjected to the steam oxidation treatment described later is prepared. SEM observation is performed on the prepared ferrite heat transfer member 4 in the same manner as in the method for determining the thickness of the oxide layer A. The oxide layer B and the oxide layer C are distinguished by the contrast difference obtained from the reflected electron image of the SEM. The oxide layer B has a darker contrast than the oxide layer C. 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.

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

[Steam oxidation treatment process]
The above-mentioned oxidation-treated ferritic heat-resistant steel is subjected to steam oxidation treatment. The steam oxidation treatment is carried out by exposing the ferritic heat-resistant steel to steam at 500 to 650 ° 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 film 3 containing the oxide layer B and the oxide layer C. As a result, the oxide film 3 containing the oxide layer B and the oxide layer C is formed on the base material 2.

By the above steps, the ferrite 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 when the steam oxidation treatment is performed can be obtained. That is, if the ferritic heat-resistant steel 1 of the present embodiment is exposed to a high-temperature steam environment for 100 hours or more, the ferritic heat transfer member 4 can be manufactured without subjecting the steam oxidation treatment.

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.

[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 content of Cr and Mn (mass%) and the total content of 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.

[Volume fraction of Fe ₃ O ₄ and Fe ₂ O ₃ of oxide layer B and Cr ₂ O ₃ volume fraction measurement test of oxide layer C]
The total volume fraction of Fe ₃ O ₄ and Fe ₂ O ₃ was determined for the cross section of each test piece (that is, the cross section of the oxide layer B) by the above method. Further, the volume fraction of Cr ₂ O ₃ was determined with respect to the cross section of 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. The total content (mass%) of Cr and Mn was determined for the oxide layer B. 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.

[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.

[Thickness measurement test of oxide layer C]
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, 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% in total of one or more selected from the group consisting of Mo, Ta, W, and Re. It contained 10%. As a result, the oxide layer B formed on the base material after the steam oxidation treatment contained Fe ₃ O ₄ and Fe ₂ O ₃ in a total volume of 80% or more. 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 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. 2, 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%. Therefore, the amount of Cr + Mn in the oxide layer C exceeded 30%, and the thermal conductivity was less than 1.2 W · m ^-1 · K ^-1 .

In Test No. 4, 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 1.2 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, although the chemical composition was appropriate, the oxidation treatment temperature was too low, so that the total amount of the specific oxide layer forming elements in the oxide layer A was 0.4%, which was too low. Therefore, 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 , which was too low.

In Test No. 7, although the chemical composition was appropriate, 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 . Further, since the volume fraction of Fe ₃ O ₄ in the oxide layer B was less than 80%, the inward flux of oxygen became large, 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%, which was too low. Therefore, the total content of Cr and Mn in the oxide layer C was 3.2%, which was too low. As a result, the thermal conductivity of the oxide layer C was 3.2 W · m ^-1 · K ^-1 , which was too high. In Test No. 8, 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. 16, the chemical composition was appropriate, but the oxidation treatment time was too short. Therefore, 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 , which was too high. In Test No. 16, 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 No. 18 did not contain any specific oxide layer forming element. Therefore, although the production method was appropriate, the total content of the specific oxide layer forming elements in the oxide layer A was less than 0.1%, which was too low. Therefore, the total content of the specific oxide layer forming elements in the oxide layer C was less than 0.1%, which was too low. As a result, the thermal conductivity of the oxide layer C was 1.1 W · m ^-1 · K ^-1 , which was too low.

In test number 19, the Cr content was too high. Therefore, although the production 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 was 0.8 W · m ^-1 · K ^-1 , which was too low.

Test number 20 had a Cr content that was too low. Therefore, although the production 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%, which was too low. As a result, the thermal conductivity of the oxide layer C was 3.3 W · m ^-1 · K ^-1 , which was too high. 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.

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 in 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 , which was too high. In Test No. 21, 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. 22, the (CO + CO ₂ ) / N ₂ ratio exceeded 1.0, although the chemical composition was appropriate. 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%, which was too low. As a result, the thermal conductivity of the oxide layer C was 3.4 W · m ^-1 · K ^-1 , which was too high. In Test No. 22, 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 Ferritic heat-resistant steel 2 Base material 3 Oxidized film 4 Ferritic 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: 7.0-14.0%,
N: 0.005 to 0.15%,
sol. Al: 0.001 to 0.3%,
One or more selected from the group consisting of Mo: 0 to 5.0%, Ta: 0 to 5.0%, W: 0 to 5.0% and Re: 0 to 5.0%: total 0.5-7.0%,
Cu: 0-5.0%,
Ni: 0-5.0%,
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 to 0.1% and
Rare earth element: Contains 0 to 0.1% and has a chemical composition with the balance consisting 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 Mo, Ta, W and Re: 0.9-10% in total,
Ferritic heat-resistant steel containing a chemical composition containing. The ferrite-based heat-resistant steel according to claim 1.
The chemical composition of the base material is
Cu: 0.005-5.0%,
Ni: 0.005 to 5.0% and
Co: A ferritic heat-resistant steel containing one or more selected from the group consisting of 0.005 to 5.0%. The ferrite-based 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: A ferritic heat-resistant steel containing one or more selected from the group consisting of 0.01 to 1.0%. The ferrite-based heat-resistant steel according to any one of claims 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: A ferritic heat-resistant steel containing one or more selected from the group consisting of 0.0015 to 0.1%. A base material having the chemical composition according to any one of claims 1 to 4.
An oxide film is provided on the surface of the base material,
The oxide film is
An oxide layer B containing 80 % or more of Fe ₃ O ₄ by volume and
It contains an oxide layer C arranged between the oxide layer B and the base material.
The chemical composition of the oxide layer C is
By mass%
Cr and Mn: Over 5% to 30% in total, and
One or more selected from the group consisting of Mo, Ta, W and Re: Ferritic heat transfer member containing 1 to 15% in total. The ferrite-based heat transfer member according to claim 5.
The chemical composition of the oxide layer B is
By mass%
Cr and Mn: Ferritic heat transfer members containing 5% or less in total. The ferrite-based heat transfer member according to claim 5 or 6.
The oxide layer C is
A ferrite heat transfer member containing Cr ₂ O ₃ of 5% or less by volume.

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