Patent Description:
From the viewpoints of suppressing the emission of CO<NUM> gas and economic efficiency, there is a demand to improve the power generation efficiency in thermal power plants. Therefore, there is a trend of increasing temperature and pressure of turbine steam in thermal power plants. Heat transfer members that are used in thermal power plants are exposed to high temperature and high pressure steam for long time periods. A heat transfer member is, for example, a boiler pipe. When exposed to high temperature steam for a long time period, oxide scale forms on the surface of the heat transfer member. If the steam oxidation resistance properties of the heat transfer member are insufficient, a large amount of oxide scale will form on the surface of the heat transfer member. The heat transfer member thermally expands and contracts due to starting and stopping of the boiler. Therefore, if a large amount of oxide scale is formed, the oxide scale will peel off and cause a blockage in the pipe. Furthermore, in a case where a large amount of oxide scale is formed, thermal conduction from outside the pipe to inside the pipe is inhibited by the oxide scale. Therefore, in order to maintain the temperature within the pipe at a high temperature, it will be necessary to apply a greater amount of heat from the outside. An increase in the temperature of the pipe causes a reduction in the creep strength. Therefore, high steam oxidation resistance properties are required for heat transfer members that are to be used in equipment such as thermal power boilers, turbines or steam pipes.

For example, a heat resistant austenitic steel and a heat resistant ferritic steel have been developed as materials that meet the demands regarding such properties. A heat resistant austenitic steel is, for example, a heat resistant austenitic steel having a Cr content of <NUM> to <NUM> mass%. A heat resistant ferritic steel is, for example, a heat resistant ferritic steel having a Cr content of <NUM> to <NUM> mass%. A heat resistant ferritic steel is less expensive than a heat resistant austenitic steel. A heat resistant ferritic steel also has a lower coefficient of thermal expansion and a higher thermal conductivity than a heat resistant austenitic steel. Therefore, a heat resistant ferritic steel is suitable as the material for a pipe in a thermal power plant. However, the Cr content of a heat resistant ferritic steel is lower than the Cr content of a heat resistant austenitic steel. Consequently, the steam oxidation resistance properties of the heat resistant ferritic steel are lower than the steam oxidation resistance properties of the heat resistant austenitic steel. Therefore, there is a need for a heat resistant ferritic steel that is excellent in steam oxidation resistance properties.

A heat resistant ferritic steel which inhibits oxide scale from falling off is disclosed, for example, in <CIT> (Patent Literature <NUM>). The heat resistant ferritic steel disclosed in Patent Literature <NUM> is a heat resistant ferritic steel containing a high Cr content that forms an oxide film on the surface during use, in which ultra-fine oxides having a diameter of <NUM> micron or less are formed at the boundary with the oxide film or in the vicinity thereof. Patent Literature <NUM> describes that, as a result, the adhesiveness between the oxide film and the base metal improves.

A method for improving steam oxidation resistance properties by increasing the Cr concentration at the surface of a heat resistant ferritic steel is disclosed, for example, in <CIT> (Patent Literature <NUM>). According to the method disclosed in Patent Literature <NUM>, powder particles containing Cr are caused to be carried at the surface of a heat resistant ferritic steel containing Cr, and a Cr-oxide layer having a high Cr concentration is formed on the ferritic steel surface under a high temperature. Patent Literature <NUM> describes that, according to this method, the (steam) oxidation resistance of a ferritic steel containing Cr can be easily and economically improved.

A method for improving oxidation resistance by forming a Cr-oxide coating on the surface of a heat resistant ferritic steel is disclosed, for example, in <CIT> (Patent Literature <NUM>). An antioxidation treatment method for a heat resistant ferritic steel described in Patent Literature <NUM> includes subjecting a heat resistant ferritic steel containing chromium to a heat treatment under a gas atmosphere with a low oxygen partial pressure that consists of a gaseous mixture of carbon dioxide gas with an inert gas to thereby form an oxide coating that contains chromium on the surface of the heat resistant steel. Patent Literature <NUM> describes that, according to this method, the Cr concentration in the scale is increased, and the oxidation resistance of the heat resistant ferritic steel can be easily and economically improved.

A heat resistant ferritic steel in which steam oxidation resistance properties are improved by depositing Cr on the surface of the heat resistant ferritic steel is disclosed, for example, in <CIT> (Patent Literature <NUM>). The heat resistant ferritic steel disclosed in Patent Literature <NUM> is a heat resistant ferritic steel that is used under a high-temperature and highpressure steam environment, and has on its surface a Cr oxide film which is formed by subjecting Cr that was deposited by a shot peening treatment using a shot material of powdery Cr to a pre-oxidizing treatment. Patent Literature <NUM> describes that, because a protection film of oxides with oxidation resistance is formed on the heat resistant steel prior to use in an oxidation environment, the steam oxidation resistance properties of the heat resistant ferritic steel are improved.

<CIT> discloses a heat resistant ferritic steel including a base material including, by mass percent, C: <NUM> to <NUM>%, Si: <NUM> to <NUM>%, Mn: <NUM> to <NUM>%, P: at most <NUM>%, S: at most <NUM>%, Cr: <NUM> to <NUM>%, sol. Al: at most <NUM>%, and N: <NUM> to <NUM>%, the balance being Fe and impurities, and an oxide film that is formed on the base material and contains <NUM> to <NUM>% of Fe and <NUM> to <NUM>% of Cr. This heat resistant ferritic steel is excellent in photoselective absorptivity and oxidation resistance.

However, even when the aforementioned techniques are used, in some cases the heat transfer characteristics and steam oxidation resistance properties of a heat transfer member cannot be increased sufficiently. As described above, various kinds of studies have been conducted regarding methods for suppressing the formation of oxide scale by forming Cr oxides on the surface of a heat transfer member. However, the thermal conductivity of Cr oxides is low. Therefore, if Cr oxides are formed, even though the steam oxidation resistance properties of the heat transfer member will increase, the heat transfer characteristics will decrease.

An objective of the present invention is to provide a method of producing a ferritic heat transfer member that is excellent in heat transfer characteristics and steam oxidation resistance properties, and a heat resistant ferritic steel capable of realizing the ferritic heat transfer member.

A method of producing heat resistance steel according to the invention is defined in claim <NUM>. A method for producing a ferritic heat transfer member according to the invention is defined in claim <NUM>.

The heat resistant ferritic steel and the ferritic heat transfer member produced by the method according to the present embodiment are excellent in heat transfer characteristics and steam oxidation resistance properties.

The present embodiment is described in detail hereunder while referring to the drawings. Identical or equivalent portions in the drawings are assigned the same reference symbols, and a description of such portions is not duplicated.

The present inventors conducted various studies regarding heat resistant ferritic steels and ferritic heat transfer members. As a result, the present inventors obtained the following findings.

Cr oxides and Mn oxides improve the steam oxidation resistance properties of the base material. However, if the Cr content is too high, the heat transfer characteristics of the oxide film decrease. If the Mn content is too high, the creep strength of the base material decreases. Therefore, the oxidized layer C contains Cr and Mn in a total amount in a range of more than <NUM>% to <NUM> mass%.

When Mo, Ta, W and Re are contained in the oxidized layer C, the thermal conductivity of the oxidized layer C increases. However, if the content of these elements is too high, in some cases the steam oxidation resistance properties of the oxidized layer C decrease. Accordingly, the oxidized layer C contains one or more types of element selected from a group consisting of Mo, Ta, W and Re in a total amount in a range of <NUM> to <NUM> mass%.

Thus, the oxidized layer C exhibits excellent heat transfer characteristics and excellent steam oxidation resistance properties.

(<NUM>) In order to form the oxidized layer B and oxidized layer C under a high-temperature steam environment, it is necessary to form the oxidized layer A on the base material in advance. The chemical composition of the oxidized layer A contains, in mass%, Cr and Mn in a total amount in a range of <NUM> to <NUM>%. The chemical composition of the oxidized layer A contains, in mass%, one or more types of element selected from a group consisting of Mo, Ta, W and Re in a total amount in a range of <NUM> to <NUM>%. When used in a high-temperature steam environment, the oxidized layer A changes to an oxide film including the oxidized layer B and the oxidized layer C. The term "high temperature" refers to, for example, a temperature in the range of <NUM> to <NUM>.

A heat resistant ferritic steel produced by the method according to the present embodiment that was completed based on the above findings includes a base material, and an oxidized layer A on the surface of the base material. The base material has a chemical composition consisting of, in mass%, C: <NUM> to <NUM>%, Si: <NUM> to <NUM>%, Mn: <NUM> to <NUM>%, P: <NUM>% or less, S: <NUM>% or less, Cr: <NUM> to <NUM>%, N: <NUM> to <NUM>%, sol. Al: <NUM> to <NUM>%, one or more types of element selected from a group consisting of Mo: <NUM> to <NUM>%, Ta: <NUM> to <NUM>%, W: <NUM> to <NUM>% and Re: <NUM> to <NUM>%: <NUM> to <NUM>% in total, one or more types of element selected from a group consisting of Cu: <NUM> to <NUM>%, Ni: <NUM> to <NUM>%, and Co: <NUM> to <NUM>%, Ti: <NUM> to <NUM>%, V: <NUM> to <NUM>%, Nb: <NUM> to <NUM>%, Hf: <NUM> to <NUM>%, Ca: <NUM> to <NUM>%, Mg: <NUM> to <NUM>%, Zr: <NUM> to <NUM>%, B: <NUM> to <NUM>%, and rare earth metal: <NUM> to <NUM>%, with the balance being Fe and impurities. The oxidized layer A includes a chemical composition containing, in mass%, <NUM> to <NUM>% in total of Cr and Mn. The oxidized layer A includes a chemical composition excluding oxygen and carbon containing, in mass%, <NUM> to <NUM>% in total of one or more types of element selected from a group consisting of Mo, Ta, W and Re.

The heat resistant ferritic steel produced by the method according to the present embodiment is excellent in heat transfer characteristics and steam oxidation resistance properties.

The chemical composition of the aforementioned base material may contain one or more types of element selected from a group consisting of Ti: <NUM> to <NUM>%, V: <NUM> to <NUM>%, Nb: <NUM> to <NUM>% and Hf: <NUM> to <NUM>%.

The chemical composition of the aforementioned base material may contain one or more types of element selected from a group consisting of Ca: <NUM> to <NUM>%, Mg: <NUM> to <NUM>%, Zr: <NUM> to <NUM>%, B: <NUM> to <NUM>% and rare earth metal: <NUM> to <NUM>%.

A ferritic heat transfer member produced by the method 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 chemical composition described above. The oxide film includes an oxidized layer B and an oxidized layer C. The oxidized layer B contains, in vol%, <NUM>% or more in total of Fe<NUM>O<NUM> and Fe<NUM>O<NUM>. The oxidized layer C is disposed between the oxidized layer B and the base material. The chemical composition excluding oxygen and carbon of the oxidized layer C contains Cr and Mn in a total amount in a range of more than <NUM>% to <NUM> mass%, and contains one or more types of element selected from a group consisting of Mo, Ta, W and Re in a total amount in a range of <NUM> to <NUM> mass%.

The ferritic heat transfer member produced by the method according to the present embodiment is excellent in heat transfer characteristics and steam oxidation resistance properties.

Preferably, the oxidized layer B contains excluding oxygen and carbon Cr and Mn in a total amount of not more than <NUM> mass%.

Preferably, the oxidized layer C contains not more than <NUM> vol% of Cr<NUM>O<NUM>.

In this case, the thermal conductivity of the oxide film is improved by suppressing the amount of precipitated Cr<NUM>O<NUM> that has low thermal conductivity. Therefore, the heat transfer characteristics of the boiler can be enhanced.

Hereunder, the heat resistant ferritic steel and the ferritic heat transfer member produced according to the present invention are described in detail. The symbol "%" in relation to an element means "mass%" unless specifically stated otherwise.

The shape of the heat resistant ferritic steel produced according to the present invention is not particularly limited. The heat resistant ferritic steel is, for example, a steel pipe, a steel bar, or a steel plate. Preferably, the heat resistant ferritic steel is a heat resistant ferritic steel pipe. An oxidation treatment is performed on the base material of the heat resistant ferritic steel according to the present embodiment. An oxidized layer A is formed on the surface of the base material of the heat resistant ferritic steel by the oxidation treatment.

<FIG> is a sectional view of the heat resistant ferritic steel produced according to the present embodiment. Referring to <FIG>, a heat resistant ferritic steel <NUM> includes a base material <NUM> and an oxidized layer A. The heat resistant ferritic steel <NUM> that includes the base material <NUM> and the oxidized layer A is used as a heat transfer member under a high-temperature steam environment. As a result, the oxidized layer A changes to an oxide film <NUM> that includes an oxidized layer B and an oxidized layer C.

The base material <NUM> has the following chemical composition.

Carbon (C) stabilizes austenite. C also increases the creep strength of the base material by solid-solution strengthening. However, if the C content of the base material <NUM> is too high, an excessive amount of carbides precipitate, and the workability and weldability of the base material <NUM> will decrease. Accordingly, the C content is set in a range of <NUM> to <NUM>%. A preferable lower limit of the C content is <NUM>%, and a preferable upper limit of the C content is <NUM>%.

Silicon (Si) deoxidizes the steel. Si also improves the steam oxidation resistance properties of the base material <NUM>. However, if the Si content is too high, the toughness of the base material <NUM> decreases. Accordingly, the Si content is set in a range of <NUM> to <NUM>%. A preferable lower limit of the Si content is <NUM>%, and more preferably is <NUM>%. A preferable upper limit of the Si content is <NUM>%, and more preferably is <NUM>%.

Manganese (Mn) deoxidizes the steel. Mn also combines with S in the base material <NUM> to form MnS, and suppresses grain-boundary segregation of S. Thus, the hot workability of the base material <NUM> improves. However, if the Mn content is too high, the base material <NUM> becomes brittle and, in addition, the creep strength of the base material <NUM> decreases. Accordingly, the Mn content is set in a range of <NUM> to <NUM>%. A preferable lower limit of the Mn content is <NUM>%, and more preferably is <NUM>%. A preferable upper limit of the Mn content is <NUM>%, and more preferably is <NUM>%.

Phosphorus (P) is an impurity. P segregates at crystal grain boundaries of the base material <NUM>, and decreases the hot workability of the base material <NUM>. P also concentrates at the boundary between the oxide film <NUM> and the base material <NUM>, and reduces the adhesiveness of the oxide film <NUM> with respect to the base material <NUM>. Accordingly, the P content is preferably as low as possible. The P content is set to <NUM>% or less, and preferably is <NUM>% or less. A lower limit of the P content is, for example, <NUM>%.

Sulfur (S) is an impurity. S segregates at crystal grain boundaries of the base material <NUM>, and decreases the hot workability of the base material <NUM>. S also concentrates at the boundary between the oxide film <NUM> and the base material <NUM>, and reduces the adhesiveness of the oxide film <NUM> with respect to the base material <NUM>. Accordingly, the S content is preferably as low as possible. The S content is set to <NUM>% or less, and preferably is <NUM>% or less. A lower limit of the S content is, for example, <NUM>%.

Chromium (Cr) improves the steam oxidation resistance properties of the base material <NUM>. Cr is also contained in the oxide film <NUM> as oxides defined by Cr<NUM>O<NUM> and (Fe, Cr)<NUM>O<NUM>. The Cr oxides improve the steam oxidation resistance properties of the base material <NUM>. The Cr oxides also improve the adhesiveness of the oxide film <NUM> with respect to the base material <NUM>. However, if the Cr content is too high, the concentration of Cr<NUM>O<NUM> in the oxide film <NUM> becomes high and the heat transfer characteristics of the oxide film <NUM> will decrease. Accordingly, the Cr content is set in a range of <NUM> to <NUM>%. A preferable lower limit of the Cr content is <NUM>%, and more preferably is <NUM>%. A preferable upper limit of the Cr content is <NUM>%, and more preferably is <NUM>%.

Nitrogen (N) dissolves in the base material <NUM>, and increases the strength of the base material <NUM>. In addition, N forms nitrides with alloy elements in the base material <NUM> and precipitates in the base material <NUM>, thereby increasing the strength of the base material <NUM>. However, if the N content is too high, the nitrides coarsen and the toughness of the base material <NUM> decreases. Accordingly, the N content is set in a range of <NUM> to <NUM>%. A preferable lower limit of the N content is <NUM>%. A preferable upper limit of the N content is <NUM>%.

Aluminum (Al) deoxidizes the steel. However, if the Al content is too high, the hot workability of the base material <NUM> decreases. Accordingly, the Al content is set in a range of <NUM> to <NUM>%. A preferable lower limit of the Al content is <NUM>%, and a preferable upper limit of the Al content is <NUM>%. In the present embodiment, the term "Al content" means the soluble Al (sol.

<NUM> to <NUM>% in total of one or more types of element selected from a group consisting of:.

One or more types of element selected from a group consisting of molybdenum (Mo), tantalum (Ta), tungsten (W) and rhenium (Re) is contained. Hereinafter, these elements are also referred to as "specific oxidized layer forming elements". The specific oxidized layer forming elements form the oxidized layer A on the surface of the base material <NUM>. The specific oxidized layer forming elements also form the oxide film <NUM> including the oxidized layer B and the oxidized layer C under a high-temperature steam environment of <NUM> to <NUM>. These effects are obtained if even one type of these elements is contained. However, if the content of the specific oxidized layer forming elements is too high, the toughness, ductility and workability of the base material <NUM> will decrease. Accordingly, the Mo content is set in a range of <NUM> to <NUM>%, the Ta content is set in a range of <NUM> to <NUM>%, the W content is set in a range of <NUM> to <NUM>% and the Re content is set in a range of <NUM> to <NUM>%. A preferable lower limit of the Mo content is <NUM>%, and more preferably is <NUM>%. A preferable lower limit of the Ta content is <NUM> %, and more preferably is <NUM>%. A preferable lower limit of the W content is <NUM>%, and more preferably is <NUM>%. A preferable lower limit of the Re content is <NUM>%, and more preferably is <NUM>%. A preferable upper limit of the Mo content is <NUM>%, and more preferably is <NUM>%. A preferable upper limit of the Ta content is <NUM>%, and more preferably is <NUM>%. A preferable upper limit of the W content is <NUM>%, and more preferably is <NUM>%. A preferable upper limit of the Re content is <NUM>%, and more preferably is <NUM>%. The total content of the specific oxidized layer forming elements is set in the range of <NUM> to <NUM>%. A preferable lower limit of the total content of the specific oxidized layer forming elements is <NUM>%, and more preferably is <NUM>%. A preferable upper limit of the total content of the specific oxidized layer forming elements is <NUM>%, and more preferably is <NUM>%.

The balance of the base material <NUM> of the heat resistant ferritic steel according to the present embodiment is Fe and impurities. In the present embodiment, the term "impurities" refers to substances which are mixed in from ore or scrap that is utilized as a raw material of the steel or from the environment of the production process or the like, and are substances that are contained within a range that does not adversely affect a heat transfer member <NUM> according to the present embodiment. The impurities include, for example, oxygen (O), arsenic (As), antimony (Sb), thallium (Tl), lead (Pb) and bismuth (Bi).

The base material <NUM> of the heat resistant ferritic steel according to the present embodiment may further contain the following elements in lieu of a part of Fe.

One or more types of element selected from a group consisting of Copper (Cu), nickel (Ni) and cobalt (Co) are contained in an amount of <NUM>% or more. These elements stabilize austenite. By this means, retention of delta ferrite that lowers the shock resistance of the base material <NUM> is suppressed. This effect is obtained if even one type of these elements is contained. However, if the content of these elements is too high, the long-term creep strength of the base material <NUM> will decrease. Accordingly, the Cu content is set in a range of <NUM> to <NUM>%, the Ni content is set in a range of <NUM> to <NUM>%, and the Co content is set in a range of <NUM> to <NUM>%. A preferable upper limit of the Cu content is <NUM>%, and more preferably is <NUM>%. A preferable upper limit of the Ni content is <NUM>%, and more preferably is <NUM>%. A preferable upper limit of the Co content is <NUM>%, and more preferably is <NUM>%.

Titanium (Ti), vanadium (V), niobium (Nb) and hafnium (Hf) are optional elements and need not be contained. If contained, these elements combine with carbon and nitrogen to form carbides, nitrides or carbo-nitrides. These carbides, nitrides and carbo-nitrides act to perform precipitation strengthening of the base material <NUM>. This effect is obtained if even one type of these elements is contained. However, if the content of these elements is too high, the workability of the base material <NUM> will decrease. Accordingly, the Ti content is set in a range of <NUM> to <NUM>%, the V content is set in a range of <NUM> to <NUM>%, the Nb content is set in a range of <NUM> to <NUM>% and the Hf content is set in a range of <NUM> to <NUM>%. A preferable upper limit of the Ti content is <NUM>%, and more preferably is <NUM>%. A preferable upper limit of the V content is <NUM>%, and more preferably is <NUM>%. A preferable upper limit of the Nb content is <NUM>%, and more preferably is <NUM>%. A preferable upper limit of the Hf content is <NUM>%, and more preferably is <NUM>%. A preferable lower limit of the content of each of these elements is <NUM>%.

Calcium (Ca), magnesium (Mg), zirconium (Zr), boron (B) and rare earth metal (REM) are optional elements, and need not be contained. If contained, these elements increase the strength, workability and oxidation resistance of the base material <NUM>. This effect is obtained if even one type of these elements is contained. However, if the content of these elements is too high, the toughness and weldability of the base material <NUM> will decrease. Accordingly, the Ca content is set in a range of <NUM> to <NUM>%, the Mg content is set in a range of <NUM> to <NUM>%, the Zr content is set in a range of <NUM> to <NUM>%, the B content is set in a range of <NUM> to <NUM>% and the REM content is set in a range of <NUM> to <NUM>%. A preferable upper limit of the Ca content is <NUM>%. A preferable upper limit of the Mg content is <NUM>%. A preferable upper limit of the Zr content is <NUM>%. A preferable upper limit of the B content is <NUM>%. A preferable upper limit of the REM content is <NUM>%. A preferable lower limit of the content of each of these elements is <NUM>%. Herein, the term "REM" refers to one or more types of element selected from a group consisting of yttrium (Y) which is the element with atomic number <NUM>, the elements from lanthanum (La) with atomic number <NUM> to lutetium (Lu) with atomic number <NUM> that are lanthanides, and the elements from actinium (Ac) with atomic number <NUM> to lawrencium (Lr) with atomic number <NUM> that are actinides.

An oxidation treatment is performed on the base material <NUM> having the aforementioned chemical composition. The oxidized layer A is formed on the surface of the base material <NUM> by the oxidation treatment. The heat resistant ferritic steel <NUM> having the base material <NUM> and the oxidized layer A on the surface of the base material <NUM> is used under a high-temperature steam environment. Under a high-temperature steam environment, the oxidized layer A changes to the oxide film <NUM> that is excellent in heat transfer characteristics, while maintaining steam oxidation resistance properties. That is, the oxidized layer A is a starting material for forming the oxide film <NUM> that includes the oxidized layer B and the oxidized layer C. Although the mechanism by which the oxidized layer A changes into the oxide film <NUM> is not certain, it is surmised that the oxidized layer A principally contributes to formation of the oxidized layer C.

The thickness of the oxidized layer A is not particularly limited. If the oxidized layer A is formed even slightly, the oxide film <NUM> will be formed. The thickness of the oxidized layer A is preferably not less than <NUM>. In this case, under a high-temperature steam environment, the oxide film <NUM> can be uniformly formed on the surface of the base material <NUM> in a stable manner. Therefore, it is easy to completely cover the base material <NUM> with the oxide film <NUM>. As a result, the thermal conductivity at the surface of the ferritic heat transfer member <NUM> increases. More preferably, the thickness of the oxidized layer A is not less than <NUM>. Although the upper limit of the thickness of the oxidized layer A is not particularly limited, in consideration of mass productivity, the upper limit is preferably not more than <NUM>.

The thickness of the oxidized layer A is determined by the following method. The heat resistant ferritic steel <NUM> that was subjected to an oxidation treatment that is described later is cut perpendicularly to the surface thereof. In a case where the heat resistant ferritic steel <NUM> is a steel pipe, the heat resistant ferritic steel <NUM> is cut perpendicularly to the axial direction of the steel pipe. A cross-section including the surface of the heat resistant ferritic steel <NUM> is observed using a scanning electron microscope (SEM) manufactured by JEOL Ltd. In a case where the heat resistant ferritic steel <NUM> is a steel pipe, SEM is used to observe a cross-section that includes the inner surface of the steel pipe. The observation magnification is <NUM> times. In the observation visual field, the thickness of the oxidized layer on the surface of the heat resistant ferritic steel <NUM> (the inner surface in a case where the heat resistant ferritic steel <NUM> is a steel pipe) is measured. The measurement is made on four different cross-sections of the heat resistant ferritic steel <NUM>. In a case where the heat resistant ferritic steel <NUM> is a steel pipe, measurement is performed at four locations at a pitch of <NUM>°. The average value of the measurement results is adopted as the thickness of the oxidized layer A.

The chemical composition of the oxidized layer A contains a total content of <NUM> to <NUM>% of Cr and Mn. If the total content of Cr and Mn in the oxidized layer A is less than <NUM>%, the total content of Cr and Mn in the oxidized layer C will be <NUM>% or less under a high-temperature steam environment. In this case, the thermal conductivity of the oxidized layer C will be too high. In such case, the steam oxidation resistance properties of the ferritic heat transfer member <NUM> will decrease. On the other hand, if the total content of Cr and Mn in the oxidized layer A is more than <NUM>%, the total content of Cr and Mn in the oxidized layer C will be more than <NUM>% under a high-temperature steam environment. In this case, the thermal conductivity of the oxidized layer C will be too low. As a result, the heat transfer characteristics of the ferritic heat transfer member <NUM> will decrease. Therefore, the chemical composition of the oxidized layer A contains Cr and Mn in a total amount in a range of <NUM> to <NUM>%. A preferable lower limit of the total content of Cr and Mn in the oxidized layer A is <NUM>%. A preferable upper limit of the total content of Cr and Mn in the oxidized layer A is <NUM>%.

The chemical composition of the oxidized layer A further contains a total of <NUM> to <NUM>% of one or more types of element selected from the group consisting of Mo, Ta, W and Re (specific oxidized layer forming elements). If the total content of the specific oxidized layer forming elements of the oxidized layer A is less than <NUM>%, the total content of the specific oxidized layer forming elements of the oxidized layer C will be less than <NUM>% under a high-temperature steam environment. In this case, the thermal conductivity of the oxidized layer C will be too low. As a result, the heat transfer characteristics of the ferritic heat transfer member <NUM> will decrease. On the other hand, if the total content of the specific oxidized layer forming elements of the oxidized layer A is more than <NUM>%, under a high-temperature steam environment the total content of the specific oxidized layer forming elements of the oxidized layer C will be more than <NUM>%. In this case, the thermal conductivity of the oxidized layer C will be too high. As a result, the steam oxidation resistance properties of the ferritic heat transfer member <NUM> will decrease. Therefore, the chemical composition of the oxidized layer A contains the specific oxidized layer forming elements in a total amount that is in a range of <NUM> to <NUM>%. A preferable upper limit of the total content of the specific oxidized layer forming elements is <NUM>%.

The total content of Cr and Mn and the total content of the specific oxidized layer forming elements (Mo, Ta, W and Re) in the oxidized layer A is calculated by the following method. The heat resistant ferritic steel <NUM> that was subjected to an oxidation treatment that is described later is cut perpendicularly to the surface thereof. In a case where the heat resistant ferritic steel <NUM> is a steel pipe, the heat resistant ferritic steel <NUM> is cut perpendicularly to the axial direction of the steel pipe. A cross-section including the surface of the heat resistant ferritic steel <NUM> is observed using a scanning electron microscope (SEM) manufactured by JEOL Ltd. The oxidized layer A that appears with a comparatively white contrast of the surface of the heat resistant ferritic steel <NUM> (inner surface in a case where the heat resistant ferritic steel <NUM> is a steel pipe) is identified. At the center of the thickness of the oxidized layer A, an elemental analysis is performed using a field emission electron probe micro analyzer (FE-EPMA) manufactured by JEOL Ltd. The conditions for the elemental analysis are: detector: <NUM><NUM> SD, accelerating voltage: <NUM> kV, and measurement time period: <NUM> secs. The elemental analysis is made on four different cross-sections of the heat resistant ferritic steel <NUM>. In a case where the heat resistant ferritic steel <NUM> is a steel pipe, elemental analysis is performed at four locations at a pitch of <NUM>°. Among the compositions for the respective elements that are obtained, a composition from which the quantities of oxygen (O) and carbon (C) are excluded is taken as <NUM>%. The proportion (mass%) of the total amount of Cr and Mn is calculated. The proportion (mass%) of the total content of specific oxidized layer forming elements (Mo, Ta, W and Re) is calculated. Average values of the elemental analysis values obtained at the four locations are adopted as the total content (mass%) of Cr and Mn in the oxidized layer A, and the total content (mass%) of the specific oxidized layer forming elements (Mo, Ta, W and Re) in the oxidized layer A.

A method for producing the heat resistant ferritic steel <NUM> according to the present invention includes a preparation process and an oxidation treatment process. In the preparation process, the base material <NUM> having the aforementioned chemical composition is prepared. The base material <NUM> is produced from a starting material having the aforementioned chemical composition. The starting material may be a slab, a bloom or a billet produced by a continuous casting process. The starting material may also be billet produced by an ingot-making process. A heating temperature when producing the starting material is, for example, in a range of <NUM> to <NUM>.

For example, in the case of producing a steel pipe, the prepared starting material is charged into a reheating furnace or a soaking pit and heated. The heated starting material is subjected to hot working to produce the base material <NUM>. The hot working is, for example, the Mannesmann process. The Mannesmann process subjects the starting material to piercing-rolling using a piercing machine to thereby form the starting material into a material pipe. Thereafter, the starting material is subjected to drawing and rolling as well as sizing using a mandrel mill and a sizing mill. The temperature for the hot working is, for example, in a range of <NUM> to <NUM>. By this means, the base material <NUM> is produced as a seamless steel pipe. A process for producing the base material <NUM> is not limited to the Mannesmann process, and the base material <NUM> may be produced by subjecting the starting material to hot extrusion or hot forging. In addition, the base material <NUM> produced by hot working may be subjected to a heat treatment or may be subjected to cold working. The base material <NUM> may also be a steel plate. In the case of producing the base material <NUM> as a steel plate, the starting material is subjected to hot working to produce the base material <NUM> as a steel plate. The steel plate may also be processed into a steel pipe by welding to produce the base material <NUM> as a welded steel pipe.

An oxidation treatment is performed on the aforementioned base material <NUM>. The oxidation treatment is performed by heating the base material <NUM> in a gas atmosphere containing CO, CO<NUM> and N<NUM>. The CO/CO<NUM> ratio of the gas used for the oxidation treatment is <NUM> or more in volume ratio. By making the CO/CO<NUM> ratio <NUM> or more, preferential oxidation of Fe can be suppressed. As a result, the oxidized layer A containing Cr and Mn in a total amount of <NUM> mass% or more and also containing specific oxidized layer forming elements in a total amount of <NUM> mass% or more is formed on the surface of the base material <NUM>. The oxidized layer A changes into the oxide film <NUM> after the steam oxidation treatment that is described later. Although an upper limit of the CO/CO<NUM> ratio is not particularly provided, an upper limit of <NUM> is preferable in consideration of operational practicability.

On the other hand, in the present embodiment, the (CO+CO<NUM>)/N<NUM> ratio of the gas that is used in the oxidation treatment is set as not more than <NUM> in volume ratio. If the (CO+CO<NUM>)/N<NUM> ratio is more than <NUM>, the base material <NUM> will carburize. Therefore, Cr and Mn in the oxidized layer A will form carbides. As a result, the total content of Cr and Mn in the oxidized layer A will be less than <NUM>%. Although a lower limit of the (CO+CO<NUM>)/N<NUM> ratio is not particularly provided, a lower limit of <NUM> is preferable in consideration of operational practicability.

The temperature for the oxidation treatment is in a range of <NUM> to <NUM>. If the oxidation treatment temperature is less than <NUM>, because outward diffusion of specific elements in the base material <NUM> will be slow, the total content of specific oxidized layer forming elements in the oxidized layer A will be too low. In this case, under a high-temperature steam environment, the total content of specific oxidized layer forming elements in the oxidized layer C will be too low. As a result, the thermal conductivity of the oxidized layer C will be too low. Consequently, the thermal conductivity at the surface of the ferritic heat transfer member <NUM> will decrease. Therefore, the heat transfer characteristics of the ferritic heat transfer member <NUM> will decrease. If the oxidation treatment temperature is more than <NUM>, because the diffusion of Cr and Mn will be fast, the total content of Cr and Mn in the oxidized layer A will be more than <NUM>%. As a result, under a high-temperature steam environment, the total content of Cr and Mn in the oxidized layer C will be more than <NUM>%. In this case, the thermal conductivity of the oxidized layer C will be too low. As a result, the heat transfer characteristics of the ferritic heat transfer member <NUM> will decrease. Accordingly, the oxidation treatment temperature is set in the range of <NUM> to <NUM>. A preferable lower limit of the oxidation treatment temperature is <NUM>, and more preferably is <NUM>. A preferable upper limit of the oxidation treatment temperature is <NUM>.

The oxidation treatment time period is in a range of <NUM> minute to <NUM> hour. If the oxidation treatment time period is too short, because concentration of the specific oxidized layer forming elements will occur, the total content of the specific oxidized layer forming elements in the oxidized layer A will be more than <NUM>%. Therefore, under a high-temperature steam environment, the total content of the specific oxidized layer forming elements in the oxidized layer C will be more than <NUM>%. As a result, the thermal conductivity at the surface of the ferritic heat transfer member <NUM> will be too high. On the other hand, if the oxidation treatment time period is too long, productivity will decrease. When taking productivity into consideration, a shorter oxidation treatment time period is preferable. Furthermore, if the oxidation treatment time period is too long, the total content of Cr and Mn in the oxidized layer A will be less than <NUM>% because Fe will preferentially oxidize. Thus, the oxidation treatment time period is set in the range of <NUM> minute to <NUM> hour. Preferably, an upper limit of the oxidation treatment time period is <NUM> minutes, and more preferably is <NUM> minutes. Preferably, a lower limit of the oxidation treatment time period is <NUM> minutes.

A tempering treatment (low-temperature annealing) may be performed after the oxidation treatment. In addition, although the oxidation treatment may be performed on the entire base material <NUM>, the oxidation treatment may also be performed only on a face of the base material <NUM> which comes in contact with high temperature steam (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, degreasing or cleaning or the like may be performed to remove dirt or oil that adhered to the surface of the base material <NUM>. The oxidized layer A will not be affected even if degreasing or cleaning or the like is performed. Even if degreasing or cleaning or the like is performed, it will not affect formation of the oxide film <NUM> thereafter.

The heat resistant ferritic steel <NUM> is produced by the production method described above.

The ferritic heat transfer member <NUM> produced according to the present invention includes a base material <NUM> and an oxide film <NUM>. The base material <NUM> of the ferritic heat transfer member <NUM> is the same as the base material of the heat resistant ferritic steel <NUM> that is described above. Accordingly, the chemical composition of the base material <NUM> of the ferritic heat transfer member <NUM> is the same as the chemical composition of the base material <NUM> of the heat resistant ferritic steel <NUM> that is described above. The shape of the ferritic heat transfer member <NUM> produced according to the present embodiment is not particularly limited. The ferritic heat transfer member <NUM> is, for example, a pipe, a bar or a plate material. In the case where the ferritic heat transfer member <NUM> has a tubular shape, the ferritic heat transfer member <NUM> is used, for example, as a boiler pipe. Accordingly, the ferritic heat transfer member <NUM> is preferably a ferritic heat-transfer pipe.

<FIG> is a sectional view of the ferritic heat transfer member <NUM> produced according to the present embodiment. Referring to <FIG>, the ferritic heat transfer member <NUM> includes the base material <NUM> and the oxide film <NUM>. The oxide film <NUM> includes the oxidized layer B and the oxidized layer C.

The oxide film <NUM> is formed on the surface of the base material <NUM> by performing a steam oxidation treatment on the heat resistant ferritic steel <NUM> having the base material <NUM> and the oxidized layer A. Referring to <FIG>, the oxide film <NUM> is an oxide film including two layers, namely, the oxidized layer B and the oxidized layer C. Because the oxide film <NUM> includes the oxidized layer B, the oxide film <NUM> is excellent in heat transfer characteristics. Because the oxide film <NUM> includes the oxidized layer C, the oxide film <NUM> is excellent in both steam oxidation resistance properties and heat transfer characteristics. That is, the oxide film <NUM> is not just excellent in steam oxidation resistance properties, but is also excellent in heat transfer characteristics. The oxidized layer B is formed as the uppermost layer of the ferritic heat transfer member <NUM>. The oxidized layer C is disposed between the oxidized layer B and the base material <NUM>. In a case where the ferritic heat transfer member <NUM> is a boiler pipe, the oxidized layer B corresponds to the inner surface side of the boiler pipe, and the base material <NUM> corresponds to the outer surface side of the boiler pipe. In this case, the oxidized layer B comes in contact with high temperature steam.

The oxidized layer B contains, in vol%, a total of <NUM>% or more of Fe<NUM>O<NUM> and Fe<NUM>O<NUM>. The thermal conductivity of Fe<NUM>O<NUM> and Fe<NUM>O<NUM> is high. Accordingly, the thermal conductivity of the oxidized layer B is high, and heat imparted from the outside of the ferritic heat transfer member <NUM> is transferred to the inside of the ferritic heat transfer member <NUM> without being significantly decreased. Therefore, the heat transfer characteristics of the boiler can be improved. Preferably, the oxidized layer B contains, in vol%, a total of <NUM>% or more of Fe<NUM>O<NUM> and Fe<NUM>O<NUM>. Preferably, the Fe<NUM>O<NUM> content of the oxidized layer B is less than <NUM> vol%. More preferably, the oxidized layer B is composed of Fe<NUM>O<NUM>.

In some cases a part of Cr and Mn contained in the base material <NUM> forms an oxide and is contained in the oxidized layer B. The thermal conductivity of Cr<NUM>O<NUM>, in particular, is low. Therefore, the Cr<NUM>O<NUM> content of the oxidized layer B is preferably low. Accordingly, the chemical composition of the oxidized layer B preferably contains, in mass%, not more than <NUM>% of Cr and Mn in total. More preferably, the chemical composition of the oxidized layer B contains, in mass%, not more than <NUM>% of Cr and Mn in total.

A preferable thickness of the oxidized layer B is <NUM> to <NUM>.

The oxidized layer C is disposed between the oxidized layer B and the base material <NUM>, and contacts the base material <NUM>.

The chemical composition of the oxidized layer C contains Cr and Mn in a total amount in a range of more than <NUM>% to <NUM>%. In the oxidized layer C, Cr and Mn are present as oxides represented by the chemical formula (Fe, M)<NUM>O<NUM>. In the formula, Cr and Mn are substituted for M. The oxides represented by the chemical formula (Fe, M)<NUM>O<NUM> are oxides that have a so-called spinel crystal structure that is the same as Fe<NUM>O<NUM>, and in which a part of Fe is substituted with Cr and Mn. In a case where the total amount of Cr and Mn contained in the oxidized layer C is <NUM>% or less, the proportion of Fe<NUM>O<NUM> and Fe<NUM>O<NUM> in the oxidized layer C cannot be kept low. In this case, the thermal conductivity of the oxidized layer C becomes too high. Consequently, a large amount of oxide scale arises on the inner surface of the ferritic heat transfer member <NUM>. On the other hand, in a case where the total amount of Cr and Mn contained in the oxidized layer C is greater than <NUM>%, the thermal conductivity of the oxidized layer C becomes too low. In this case, the heat transfer characteristics of the boiler decrease. Accordingly, the content of Cr and Mn in the oxidized layer C is set in a range of more than <NUM>% to <NUM>% in total. By this means, the thermal conductivity of the oxidized layer C can be controlled within an appropriate range while maintaining the steam oxidation resistance properties. A preferable lower limit of the total content of Cr and Mn in the oxidized layer C is <NUM>%, and more preferably is <NUM>%. A preferable upper limit of the total content of Cr and Mn in the oxidized layer C is <NUM>%, and more preferably is <NUM>%.

The oxidized layer C contains one or more types of element selected from a group consisting of Mo, Ta, W and Re in a total amount in a range of <NUM> to <NUM>%. If the total content of the specific oxidized layer forming elements (Mo, Ta, W and Re) of the oxidized layer C is less than <NUM>%, the thermal conductivity of the oxidized layer C will be too low. On the other hand, if the total content of the specific oxidized layer forming elements of the oxidized layer C is more than <NUM>%, the thermal conductivity of the oxidized layer C will be too high. In such case, the steam oxidation resistance properties of the ferritic heat transfer member <NUM> will decrease. Accordingly, the total content of the specific oxidized layer forming elements in the oxidized layer C is in the range of <NUM> to <NUM>%. A preferable upper limit of the total content of the specific oxidized layer forming elements (Mo, Ta, W and Re) in the oxidized layer C is <NUM>%, and more preferably is <NUM>%. A preferable lower limit of the total content of the specific oxidized layer forming elements (Mo, Ta, W and Re) in the oxidized layer C is <NUM>%.

In addition, preferably a major portion of the oxidized layer C is oxides having the aforementioned spinel crystal structure, and the oxidized layer C contains Cr<NUM>O<NUM> in an amount that is not more than <NUM> vol%. By suppressing formation of Cr<NUM>O<NUM> which has low thermal conductivity to an amount that is not more than <NUM> vol% and causing the formation of oxides having a spinel crystal structure, the thermal conductivity of the oxidized layer C can be controlled to be within an appropriate range. The content of Cr<NUM>O<NUM> in the oxidized layer C is preferably <NUM> vol% or less, and more preferably is <NUM> vol% or less.

The thermal conductivity of the oxidized layer C is preferably controlled within a range of <NUM> to <NUM> W·m<NUM>·K-<NUM>. If the thermal conductivity of the oxidized layer C is <NUM> W·m-<NUM>·K-<NUM> or more, thermal conduction from the outside of the ferritic heat transfer member <NUM> to the inside of the ferritic heat transfer member <NUM> is not inhibited, and the heat transfer characteristics of the boiler stably increase. On the other hand, if the thermal conductivity of the oxidized layer C is not more than <NUM> W·m-<NUM>·K-<NUM>, the heat of high temperature steam that is transferred to the surface of the base material <NUM> can be stably controlled. By this means, excessive heating of the surface of the base material <NUM> is suppressed, and an oxidation reaction at the surface of the base material <NUM> is suppressed. Therefore, formation of a large amount of oxide scale at the surface of the base material <NUM> is stably suppressed. As a result, the steam oxidation resistance properties of the ferritic heat transfer member <NUM> stably increase. Accordingly, the thermal conductivity of the oxidized layer C is preferably controlled within the range of <NUM> to <NUM> W·m-<NUM>·K-<NUM>. In this case, it is easy to improve the steam oxidation resistance properties of the ferritic heat transfer member <NUM> without loss of the heat transfer characteristics. In the oxidized layer C, a preferable lower limit of the thermal conductivity is <NUM> W·m-<NUM>·K-<NUM>, and more preferably is <NUM> W·m-<NUM>·K-<NUM>. In the oxidized layer C, a preferable upper limit of the thermal conductivity is <NUM> W·m-<NUM>·K-<NUM>, and more preferably is <NUM> W·m-<NUM>·K-<NUM>.

The volume ratio of Fe<NUM>O<NUM> and Fe<NUM>O<NUM> in the oxidized layer B is measured by the following method. The ferritic heat transfer member <NUM> that has undergone a steam oxidation treatment which is described later is cut perpendicularly to the surface thereof. In a case where the ferritic heat transfer member <NUM> is a pipe, the ferritic heat transfer member <NUM> is cut perpendicularly to the axial direction of the pipe. At a cross-section (observation surface) including the oxidized layer B, a chemical composition analysis of the oxidized layer B is performed using a field emission electron probe micro analyzer (FE-EPMA) manufactured by JEOL Ltd. The conditions for the chemical composition analysis are: detector: <NUM><NUM> SD, accelerating voltage: <NUM> kV, and measurement time period: <NUM> secs. By means of the chemical composition analysis, regions in which Fe and O (oxygen) are detected and Cr is not detected are identified. Next, it is confirmed by means of the chemical composition analysis that all of the identified regions have Fe<NUM>O<NUM> or Fe<NUM>O<NUM>. Next, the strength of Fe in the oxidized layer B of the observation surface is subjected to binarization processing. At this time, the maximum strength of the grayscale extraction objects is set as <NUM>/<NUM> or more. It is confirmed that all regions other than the identified regions (regions confirmed as having Fe<NUM>O<NUM> and Fe<NUM>O<NUM>) are included in black regions after binarization. After the binarization processing, the area fraction of black regions in the oxidized layer B of the observation surface is determined, and the resulting value is subtracted from <NUM>%. The obtained area fraction is taken as the volume ratio of Fe<NUM>O<NUM> and Fe<NUM>O<NUM> in the oxidized layer B.

The volume ratio of Cr<NUM>O<NUM> in the oxidized layer C is measured by the following method. The ferritic heat transfer member <NUM> that has undergone a steam oxidation treatment which is described later is cut perpendicularly to the surface thereof. In a case where the ferritic heat transfer member <NUM> is a pipe, the ferritic heat transfer member <NUM> is cut perpendicularly to the axial direction of the pipe. SEM is used to observe a cross-section (observation surface) including the oxidized layer B and the oxidized layer C, to thereby identify the oxidized layer C. In the SEM observation, the oxidized layer B and the oxidized layer C are distinguished from each other by means of a contrast difference obtained with an SEM backscattered electron image (BSE). The contrast of the oxidized layer B is brighter than the contrast of the oxidized layer C. The volume ratio of Cr<NUM>O<NUM> in the oxidized layer C is determined by a similar method as the method used for determining the volume ratio of Fe<NUM>O<NUM> and Fe<NUM>O<NUM> in the oxidized layer B. That is, at a cross-section (observation surface) including the oxidized layer C, a chemical composition analysis is performed using a field emission electron probe micro analyzer (FE-EPMA) manufactured by JEOL Ltd. The conditions for the chemical composition analysis are: detector: <NUM><NUM> SD, accelerating voltage: <NUM> kV, and measurement time period: <NUM> secs. By means of the chemical composition analysis, regions in which Cr and O (oxygen) are detected and Fe is not detected are identified. Next, it is confirmed by means of the chemical composition analysis that all of the identified regions have Cr<NUM>O<NUM>. Next, the strength of Cr in the oxidized layer C of the observation surface is subjected to binarization processing. At this time, the maximum strength of the grayscale extraction objects is set as <NUM>/<NUM> or more. It is confirmed that all regions other than the identified regions (regions confirmed as having Cr<NUM>O<NUM>) are included in black regions after binarization. The area fraction of black regions after binarization processing of the observation surface is determined, and the resulting value is subtracted from <NUM>%. The obtained area fraction is taken as the volume ratio of Cr<NUM>O<NUM> in the oxidized layer C.

The total content of Cr and Mn and the total content of the specific oxidized layer forming elements (Mo, Ta, W and Re) in the oxidized layer B and the oxidized layer C are determined by a similar method as the method used with respect to the oxidized layer A. In the SEM observation, the oxidized layer B and the oxidized layer C are distinguished from each other by means of a contrast difference obtained with an SEM backscattered electron image (BSE). The contrast of the oxidized layer B is brighter than the contrast of the oxidized layer C. Under the same conditions as used in the case of the oxidized layer A, an elemental analysis is performed at the center of the thickness of the oxidized layer B and the center of the thickness of the oxidized layer C. In a similar manner as in the case of the oxidized layer A, the total content (mass%) of Cr and Mn and the total content (mass%) of the specific oxidized layer forming elements (Mo, Ta, W and Re) are determined based on the compositions of the respective elements that are obtained.

The thermal conductivity of the oxidized layer C is determined by the following method. After mechanically removing the oxidized layer B of the ferritic heat transfer member <NUM>, the bulk density, specific heat and thermal diffusivity of the oxidized layer C including the base material <NUM> are measured. Next, after mechanically removing the oxidized layer C, the bulk density, specific heat and thermal diffusivity of the base material <NUM> are measured in a similar manner. The thermal conductivity κ can be determined by converting the differences between the respective measurement values to measurement values of the oxidized layer C, and substituting the resulting measurement values into the following formula.

Where, the bulk density is substituted for ρ, the specific heat is substituted for Cp, and the thermal diffusivity is substituted for D.

A preferable lower limit of the thickness of the oxidized layer C is <NUM>.

Although the thickness of the oxide film <NUM> is not particularly limited, a thin thickness is preferable. If the oxide film <NUM> is thin, the heat transfer characteristics of the ferritic heat transfer member <NUM> increase. Therefore, the heat transfer characteristics of the boiler can be improved. When the ferritic heat transfer member <NUM> is used for a long time period, the oxide film <NUM> thickens. The oxide film <NUM> also thickens in a case where the temperature for a steam oxidation treatment of the ferritic heat transfer member <NUM> is high. When an oxidation treatment and a steam oxidation treatment that are described later are performed, the oxidized layer B and the oxidized layer C are formed to almost the same thickness. Accordingly, in a case where the oxidized layer C is thin, the oxide film <NUM> will also be thin.

The thicknesses of the oxidized layer B and the oxidized layer C are determined by the same method as the method used for determining the thickness of the oxidized layer A. The ferritic heat transfer member <NUM> that has undergone the steam oxidation treatment which is described later is prepared. The prepared ferritic heat transfer member <NUM> is observed by means of SEM by the same method as the method used for determining the thickness of the oxidized layer A. The oxidized layer B and the oxidized layer C are distinguished from each other by means of a contrast difference obtained with an SEM backscattered electron image. The contrast of the oxidized layer B is darker than the contrast of the oxidized layer C. The respective thicknesses of the oxidized layer B and the oxidized layer C are determined by the same method as the method used for determining the thickness of the oxidized layer A.

A method for producing ferritic heat transfer member <NUM> according to the present invention includes a steam oxidation treatment process.

A steam oxidation treatment is performed on the heat resistant ferritic steel that underwent the aforementioned oxidation treatment. The steam oxidation treatment is performed by exposing the heat resistant ferritic steel to steam at a temperature in a range from <NUM> to <NUM>. An upper limit of the time period of the steam oxidation treatment is not particularly limited as long as the treatment time period is not less than <NUM> hours. By performing the steam oxidation treatment, the oxidized layer A changes to the oxide film <NUM> that includes the oxidized layer B and the oxidized layer C. By this means, the oxide film <NUM> that includes the oxidized layer B and the oxidized layer C is formed on the base material <NUM>.

The ferritic heat transfer member <NUM> is produced by the above processes. A similar effect as the effect obtained in a case of performing the steam oxidation treatment is obtained by exposing the heat resistant ferritic steel <NUM> under a high-temperature steam environment. That is, if the heat resistant ferritic steel <NUM> is exposed under a high-temperature steam environment for not less than <NUM> hours, the ferritic heat transfer member <NUM> can be produced even without performing a steam oxidation treatment, this method of producing the ferritic heat transfer being outside of the claimed invention.

Respective cast pieces having the chemical compositions shown in Table <NUM> were produced, and an oxidation treatment and a steam oxidation treatment were performed under the conditions illustrated in Table <NUM>. Specifically, ingots having the chemical compositions shown in Table <NUM> were prepared. Each of the obtained ingots was subjected to hot rolling and cold rolling to produce a steel plate, which was adopted as the base material. A test specimen was prepared from each of the obtained base materials, and each test specimen was subjected to an oxidation treatment under the conditions shown in Table <NUM>.

The thickness of the oxidized layer A of each test specimen was determined by the method described above. The results are shown in Table <NUM>.

The content of each metallic element in a cross-section of each test specimen was determined by the method described above. For the oxidized 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 <NUM>.

Each test specimen was subjected to a steam oxidation treatment under the conditions in Table <NUM>. Each of the obtained test specimens was subjected to the following measurement tests.

The total volume ratio of Fe<NUM>O<NUM> and Fe<NUM>O<NUM> in a cross-section (that is, a cross-section of the oxidized layer B) of each test specimen was determined by the method described above. Furthermore, the volume ratio of Cr<NUM>O<NUM> in a cross-section of the oxidized layer C was determined. The results are shown in Table <NUM>.

The content of each metallic element in a cross-section of each test specimen was determined by the method described above. With respect to the oxidized layer B, the total content (mass%) of Cr and Mn was determined. The results are shown in Table <NUM>. With respect to the oxidized 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 <NUM>.

The thermal conductivity of the oxidized layer C of each test specimen was determined by the method described above. The results are shown in Table <NUM>.

The thickness of the oxidized layer C of each test specimen was determined by the method described above. The results are shown in Table <NUM>.

Referring to Table <NUM> and Table <NUM>, the production conditions of the steels of Test Nos. <NUM>, <NUM>, <NUM>, <NUM>, <NUM> to <NUM>, <NUM> and <NUM> were appropriate. Therefore, the oxidized layer A of each of these Test Nos. contained Cr and Mn in a total amount in a range of <NUM> to <NUM>%, and contained one or more types of element selected from the group consisting of Mo, Ta, W and Re in a total amount in a range of <NUM> to <NUM>%. As a result, the oxidized layer B formed on the base material after the steam oxidation treatment contained Fe<NUM>O<NUM> and Fe<NUM>O<NUM> in a total amount of <NUM>% or more in vol%. In addition, the total content of Cr+Mn in the oxidized layer C was in a range of more than <NUM>% to <NUM>%, and the total content of the specific oxidized layer forming elements was in a range of <NUM> to <NUM>%. As a result, the thermal conductivity of the oxidized layer C was within the range of <NUM> to <NUM> W·m-<NUM>·K-<NUM>, and thus exhibited excellent thermal conductivity. In addition, the thickness of the oxidized layer C was not more than <NUM>, and thus exhibited excellent steam oxidation resistance properties.

In contrast, in Test No. <NUM>, the oxidation treatment temperature was too high, and consequently the total amount of Cr and Mn in the oxidized layer A was more than <NUM>%. Therefore, the Cr+Mn amount in the oxidized layer C was more than <NUM>%, and the thermal conductivity was less than <NUM> W·m-<NUM>·K-<NUM>.

In Test No. <NUM>, although the chemical composition was appropriate, an oxidation treatment was not performed, and the oxidized layer A was not formed. Consequently, the thermal conductivity of the oxidized layer C was less than <NUM> W·m-<NUM>·K-<NUM>. It is considered that because the total amount of the specific oxidized layer forming elements in the oxidized layer C was less than <NUM>%, the thermal conductivity was decreased.

In Test No. <NUM>, because the oxidation treatment temperature was too low, the total amount of the specific oxidized layer forming elements in the oxidized layer A was <NUM>%, which was too low. Consequently, the total amount of the specific oxidized layer forming elements in the oxidized layer C was less than <NUM>%. As a result, the thermal conductivity of the oxidized layer C was <NUM> W·m-<NUM>·K-<NUM>, which was too low.

In Test No. <NUM>, the CO/CO<NUM> ratio in the oxidation treatment was less than <NUM>. Therefore, the total content of Cr and Mn in the oxidized layer A was less than <NUM>%. Consequently, the total content of Cr and Mn in the oxidized layer C was not more than <NUM>%, and the thermal conductivity of the oxidized layer C was more than <NUM> W·m-<NUM>·K-<NUM>. Further, because the Fe<NUM>O<NUM> volume ratio in the oxidized layer B was less than <NUM>%, the inward flux of oxygen was large and growth of the oxidized layer C was promoted, and the thickness of the oxidized layer C was more than <NUM>.

In Test No. <NUM>, although the chemical composition was appropriate, the oxidation treatment time period was too long. Therefore, the total content of Cr and Mn in the oxidized layer A was <NUM>%, which was too low. Consequently, the total content of Cr and Mn in the oxidized layer C was <NUM>%, which was too low. As a result, the thermal conductivity of the oxidized layer C was <NUM> W·m-<NUM>·K-<NUM>, which was too high. Furthermore, in Test No. <NUM>, the thickness of the oxidized layer C was more than <NUM>. It is considered that this was because the thermal conductivity of the oxidized layer C was too high.

In Test No. <NUM>, the oxidation treatment time period was too short. Therefore, the total content of the specific oxidized layer forming elements in the oxidized layer A was <NUM>%, which was too high. Consequently, the total content of the specific oxidized layer forming elements in the oxidized layer C was <NUM>%, which was too high. As a result, the thermal conductivity of the oxidized layer C was <NUM> W·m-<NUM>·K-<NUM>, which was too high. Furthermore, in Test No. <NUM>, the thickness of the oxidized layer C was more than <NUM>. It is considered that this was because the thermal conductivity of the oxidized layer C was too high.

In Test No. <NUM>, the steel did not contain any of the specific oxidized layer forming elements. Therefore, even though the production method was appropriate, the total content of the specific oxidized layer forming elements in the oxidized layer A was less than <NUM>%, which was too low. Consequently, the total content of the specific oxidized layer forming elements in the oxidized layer C was less than <NUM>%, which was too low. As a result, the thermal conductivity of the oxidized layer C was <NUM> W·m-<NUM>·K-<NUM>, which was too low.

In Test No. <NUM>, the Cr content was too high. Therefore, even though the production method was appropriate, the total content of Cr and Mn in the oxidized layer A was <NUM>%, which was too high. Consequently, the total content of Cr and Mn in the oxidized layer C was <NUM>%, which was too high. As a result, the thermal conductivity of the oxidized layer C was <NUM> W·m-<NUM>·K-<NUM>, which was too low.

In Test No. <NUM>, the Cr content was too low. Therefore, even though the production method was appropriate, the total content of Cr and Mn in the oxidized layer A was <NUM>%, which was too low. Consequently, the total content of Cr and Mn in the oxidized layer C was <NUM>%, which was too low. As a result, the thermal conductivity of the oxidized layer C was <NUM> W·m-<NUM>·K-<NUM>, which was too high. Furthermore, in Test No. <NUM>, the thickness of the oxidized layer C was more than <NUM>. It is considered that this was because the thermal conductivity of the oxidized layer C was too high.

In Test No. <NUM>, the content of the specific oxidized layer forming elements was too high. Therefore, the total content of the specific oxidized layer forming elements in the oxidized layer A was <NUM>%, which was too high. Consequently, the total content of the specific oxidized layer forming elements in the oxidized layer C was <NUM>%, which was too high. As a result, the thermal conductivity of the oxidized layer C was <NUM> W·m-<NUM>·K-<NUM>, which was too high. Furthermore, in Test No. <NUM> the thickness of the oxidized layer C was more than <NUM>. It is considered that this was because the thermal conductivity of the oxidized layer C was too high.

In Test No. <NUM>, the (CO+CO<NUM>)/N<NUM> ratio was more than <NUM>. Therefore, the total content of Cr and Mn in the oxidized layer A was <NUM>%, which was too low. Consequently, the total content of Cr and Mn in the oxidized layer C was <NUM>%, which was too low. As a result, the thermal conductivity of the oxidized layer C was <NUM> W·m-<NUM>·K-<NUM>, which was too high. Furthermore, in Test No. <NUM>, the thickness of the oxidized layer C was more than <NUM>. It is considered that this was because the thermal conductivity of the oxidized layer C was too high.

Claim 1:
A method of producing heat resistant ferritic steel, wherein the heat resistant ferritic stainless steel comprises:
a base material, and
an oxidized layer A on a surface of the base material;
wherein:
the base material has a chemical composition consisting of, in mass%:
C: <NUM> to <NUM>%,
Si: <NUM> to <NUM>%,
Mn: <NUM> to <NUM>%,
P: <NUM>% or less,
S: <NUM>% or less,
Cr: <NUM> to <NUM>%,
N: <NUM> to <NUM>%,
sol. Al: <NUM> to <NUM>%,
one or more types of element selected from a group consisting of Mo: <NUM> to <NUM>%, Ta: <NUM> to <NUM>%, W: <NUM> to <NUM>% and Re: <NUM> to <NUM>%: <NUM> to <NUM>% in total,
one or more types of element selected from a group consisting of Cu: <NUM> to <NUM>%, Ni: <NUM> to <NUM>%, and Co: <NUM> to <NUM>%,
Ti: <NUM> to <NUM>%,
V: <NUM> to <NUM>%,
Nb: <NUM> to <NUM>%,
Hf: <NUM> to <NUM>%,
Ca: <NUM> to <NUM>%,
Mg: <NUM> to <NUM>%,
Zr: <NUM> to <NUM>%,
B: <NUM> to <NUM>%, and
rare earth metal: <NUM> to <NUM>%,
with the balance being Fe and impurities;
wherein the rare earth metal is one or more types of element selected from the elements with atomic numbers <NUM>, <NUM> to <NUM> and <NUM> to <NUM>, and
the oxidized layer A has a chemical composition excluding oxygen and carbon containing, in mass%, as measured according to the method described in the description:
Cr and Mn: <NUM> to <NUM>% in total, and
one or more types of element selected from a group consisting of Mo, Ta, W and Re: <NUM> to <NUM>% in total,
wherein the method includes a preparation process and an oxidation treatment process,
wherein in the preparation process, the base material having the aforementioned chemical composition is prepared, and
the oxidation treatment process is performed on the aforementioned base material prepared in the preparation process by heating the base material prepared in the preparation process in a gas atmosphere containing CO, CO<NUM> and N<NUM>, wherein the CO/CO<NUM> ratio of the gas used for the oxidation treatment is <NUM> or more in volume ratio and the (CO+CO<NUM>)/N<NUM> ratio of the gas used in the oxidation treatment is set at no more than <NUM> in volume ratio; and
the temperature for the oxidation treatment process is in a range of <NUM> to <NUM> and an oxidation treatment time period is in a range of <NUM> minute to <NUM> hour.