DESCRIPTION Title of Invention: HEAT RESISTANT FERRITIC STEEL AND METHOD FOR PRODUCING THE SAME Technical Field [0001] The present invention relates to a heat resistant steel and a method for producing the steel and, more particularly, to a heat resistant ferritic steel and a method for producing the steel. Background Art [0002] In recent years, to achieve energy saving, the development of highly efficient boilers has been advanced. For example, an ultra supercritical pressure boiler uses higher temperature and pressure of steam than those in a conventional boiler to enhance the energy efficiency. Also, a boiler using wastes or biomass as a fuel other than fossil fuels has been developed. Further, there has been advanced the development of an electric power plant boiler utilizing solar heat has been developed. In particular, a solar thermal power plant boiler has attracted attention from the viewpoints of energy saving and environmental preservation. As a steel material of heat exchangers and the like for these boilers, a heat resistant ferritic steel may be used. The boiler steam temperature is high, and reaches a temperature close to 600°C in some cases. The heat resistant ferritic steel used in such an application is required to have excellent photoselective absorptivity. [0003] The photoselective absorptivity is a property such that absorptivity changes in different wavelength regions. The term of "excellent photoselective absorptivity" means that, for light (electromagnetic wave) of visual to nearinfrared region (wavelength: 0.3 to 1 p , hereinafter referred to as "low wavelength side"), the absorptivity is high, and for light (electromagnetic wave) of mediumto far-infrared region (wavelength: 2.5 to 25 p , hereinafter referred to as "high wavelength side"), the radioactivity is low. In other words, the photoselective absorptivity means that the reflectance of light on the low wavelength side is low, and the reflectance of light on the high wavelength side is high. [0004] To attain excellent photoselective absorptivity, various methods have been proposed so far. JP52-126434A (Patent Document 1) and JP58-195746A (Patent Document 2) disclose methods in which the photoselective absorptivity is enhanced by forming an organic coated film on the surface of steel material. The paint disclosed in Patent Document 1 consists of semiconductor particles having an energy band width of 0.4 to 1.5 eV, a polyvinyl butyral organic binder, and a solvent. The paint for photoselective absorbing film disclosed in Patent Document 2 contains carboxylic acid amide copolymer, oxides, and solvent-based paint. [0005] I JP53-75132A (Patent Document 3), JP60-57157A (Patent Document 4), and JP62-182553A (Patent Document 5) disclose methods in which, to attain the photoselective absorptivity, triiron tetraoxide (FesOl: magnetite) is formed on the surface of steel by chemical treatment or the like. Specifically, in Patent Document 3, a selective absorbing film consisting of magnetite is formed by immersing a base material consisting mainly of iron in a high-temperature alkaline solution. In Patent Document 4, a selective absorbing film consisting of magnetite is formed by electrooxidizing a base material consisting mainly of iron in an acidic solution. In Patent Document 5, a selective absorbing film consisting of magnetite is formed by electrooxidizing a base material consisting mainly of iron in an acidic solution after the surface of base material has been iron-plated. [0006] JP55-77667A (Patent Document 6) discloses a method in which an oxide film consisting mainly of Fe that has a film thickness of 500 to 2000 angstroms and contains 11.00 to 26.00 wt% of Cr is formed by a chemical treatment method or the like method, and the surface of oxide film is mirror-polished. Patent Document 6 describes that the photoselective absorptivity is enhanced by this method. [0007] JP7-325212A (Patent Document 7) discloses a method in which a film consisting of iron oxide is formed on the surface of steel by spraying. Patent Document 7 describes that the photoselective absorptivity is enhanced by this method. Disclosure of the Invention [0008] In recent years, to increase power generation efficiency, the boiler steam temperature in solar power generation is as high as 500 to 600°C, and in the future, it is expected that the boiler steam temperature will become much higher. In such a high-temperature environment, it is difficult to maintain the photoselective absorptivity. Since the coated film described in Patent Documents 1 and 2 is organic, the coated film is less applicable in the above-described high-temperature environment. The oxide film described in Patent Documents 3 to 5 consists of magnetite. Therefore, the radioactivity at high temperatures, that is, the radioactivity on the high wavelength side is high, and the photoselective absorptivity is poor. The oxide film described in Patent Document 6 may have low photoselective absorptivity at high temperatures. The oxide film described in Patent Document 7 may have high radioactivity especially at high temperatures, that is, high radioactivity on the high wavelength side. [0009] An objective of the present invention is to provide a heat resistant ferritic steel excellent in photoselective absorptivity. [OOlO] The heat resistant ferritic steel in accordance with the present invention includes a base material comprising, by mass percent, C: 0.01 to 0.3%, Si: 0.01 to 2%, Mn: 0.01 to 28, P: at most 0.10%, S: at most 0.03%, Cr: 7.5 to 14.0%, sol.Al: at most 0.3%, and N: 0.005 to 0.15%, the balance being Fe and impurities, and an oxide film which is formed on the base material and whose chemical composition excluding oxygen and carbon contains 25 to 97% of Fe and 3 to 75% of Cr. The oxide film contains spinel-type oxides and Cr203. [0011] The heat resistant ferritic steel in accordance with the present invention is excellent in photoselective absorptivity. Best Mode for Carrying Out the Invention [0012] An embodiment of the present invention will now be described in detail. The symbol " % " for the content of each element means "% by mass" unless otherwise noted. [0013] The present inventors conducted researches and studies on the heat resistant ferritic steel excellent in photoselective absorptivity. As a result, the present inventors obtained the following findings. [0014] (1) Among various oxides, triiron tetraoxide (hereinafter, referred to as magnetite), which is an Febased oxide, exhibits excellent absorptivity for light (electromagnetic wave) of visual to near-infrared region (wavelength: 0.3 to 1 p , "low wavelength side"). However, for light (electromagnetic wave) of medium- to far-infrared region (wavelength: 2.5 to 25 p , "high wavelength side"), magnetite has a high radioactivity. That is to say, in a high-temperature environment of 500 to 600°C, magnetite is liable to radiate heat. [0015] (2) If the magnetite is made thin, the radioactivity for the light (electromagnetic wave) on the high wavelength side decreases. However, even if the oxide film consisting of very thin magnetite is formed, in high-temperature environments, Fe diffuses from the base material to the oxide film, and the oxide film grows and becomes thick. If the oxide film becomes thick, the photoselective absorptivity decreases. [0016] (3) In the case where the oxide film contains Febased oxides and Cr-based oxides, or in the case where the Fe-based oxides themselves in the oxide film contain Cr, the radioactivity on the high wavelength side can be restrained. Chromium in the oxide film further restrains the growth of oxide film in high-temperature environments. Therefore, Cr can maintain the photoselective absorptivity of the oxide film for a long period of time. [0017] (4) If the chemical composition of oxide film contains 25 to 97% of Fe and 3 to 75% of Cr, a heat resistant ferritic steel excellent in photoselective absorptivity can be obtained. [0018] (5) Preferably, the oxide film contains spinel-type oxides and Cr203 (chromia) . In this description, the spinel-type oxides include magnetite as well. The spinel-type oxides other than magnetite are oxides that contain, for example, Fe and Cr, and have spinel-type structures. [0019] Chromia (Cr203) enhances the reflectance on the high wavelength side, and restrains the heat radiation of steel. Further, Cr203 enhances the oxidation resistance. Therefore, if the oxide film contains not only spineltype oxides but also Cr203, the heat resistant ferritic steel having that oxide film is excellent in oxidation resistance, and also is excellent in photoselective absorptivity. Specifically, the reflectance of light (electromagnetic wave) on the low wavelength side is low, and the reflectance of light on the high wavelength side is high. [0020] (6) Further preferably, in the case where the maximum diffraction peak intensity of spinel-type oxides obtained by X-ray surface analysis (XRD) is defined as Is, and the maximum diffraction peak intensity of Cr203 is defined as Ic, if Formula (1) is satisfied, the heat resistant ferritic steel attains excellent photoselective absorptivity. This is because, if Formula (1) is satisfied, Cr203 of an amount sufficient to enhance the reflectance on the high wavelength side is contained in the oxide film. 0.010 I Ic/Is I 10 . . . (1) [0021] (7) Still further preferably, Fez03 (hematite) contained in the oxide film is restrained. If a large amount of Fe203 is contained in the oxide film, the reflectance of light (electromagnetic wave) on the low wavelength side of the oxide film is high, and the reflectance of light on the high wavelength side is low. As a result, the photoselective absorptivity decreases. Therefore, the amount of Fe203 (hematite) in the oxide film is preferably smaller. [0022] More specifically, in the case where the maximum diffraction peak intensity of Fe103 is defined as Ih, Formula (2) is preferably satisfled. If the oxide film of the produced heat resistant ferritic steel satisfies Formula (2), since Cr203 of an amount sufficient to enhance the reflectance on the high wavelength side is contained in the oxide film with respect to the content of Fe203, excellent photoselective absorptivity can be attained. Ih/ (Is + Ic) < 0.05 . . . (2) [0023] (8) The oxide film of the heat resistant ferritic steel is formed by oxidation treatment. In the oxidation treatment, if the oxygen partial pressure Po2 (atm) in a gas atmosphere satisfies Formula (3) , Fez03 is restrained effectively. More specifically, if the oxygen partial pressure Po2 (atm) satisfies Formula (3), the formed oxide film satisfies Formula (2) : Po2 i 2.76 x 1015 x expi-493.6 x ~o~/(RT).). . (3) where R is a gas constant whose unit is ~.~-'*mol-'a,n d T is a temperature whose unit is K. [0024] The heat resistant ferritic steel in accordance with this embodiment, completed on the basis of the above findings, and the method for producing the steel are as described below. [0025] The heat resistant ferritic steel includes a base material and an oxide film. The base material comprises, by mass percent, C: 0.01 to 0.35, Si: 0.01 to 2%, Mn: 0.01 to 2%, P: at most 0.10%, S: at most 0.03%, Cr: 7.5 to 14.0%, sol.Al: at most 0.3%, and N: 0.005 to 0.15%, the balance being Fe and impurities. The oxide film is formed on the base material and has a chemical composition, excluding oxygen and carbon in the oxide film, containing 25 to 97% of Fe and 3 to 75% of Cr. The oxide film contains spinel-type oxides and Cr203. [0026] In this case, the heat resistant ferritic steel has excellent photoselective absorptivity. [0027] Preferably, in the case where the maximum diffraction peak intensity of Cr203 obtained by X-ray diffraction is defined as Ic, and the' maximum diffraction peak intensity of spinel-type oxides obtained by the Xray diffraction is defined as IS, the following Formula (1)' is satisfied. 0.010 < Ic/Is < 10 . . . (1) [0028] In this case, excellent photoselective absorptivity can be attained. [0029] The above-described base material of the heat resistant ferritic steel may further comprises one or more elements selected from first to fourth groups in lieu of some of Fe. First group: Cu: at most 5 % , Ni: at most 5%, and Co: at most 5% Second group: Ti: at most 1.0%, V: at most 1.0%, Nb: at most 1.0%, Zr: at most 1.0%, and Hf: at most 1.0% Third group: Mo: at most 5%, Ta: at most 5%, W: at most 5%, and Re: at most 5% Fourth group: Ca: at most 0.1%, Mg: at most 0.1%, B: at most 0.1%, and rare earth metal (REM): at most 0.1% [0030] The method for producing the heat resistant steel in accordance with this embodiment includes a step of preparing the base material having the above-described chemical composition, and a step of forming an oxide film on the base material by oxidizing the base material at a temperature of 500 to 1150°C in a gas atmosphere in which the oxygen partial pressure Po2 (atm) satisfies Formula (3) : Po2 -< 2.76 x 1015 x exp{-493.6 x ~o~/(R)T ). . . (3) where R is a gas constant whose unit is ~.~-'.mol-', and T is a temperature whose unit is K. [ 0 0 3 1 ] The heat resistant ferritic steel produced by this production method has excellent photoselective absorptivity. [0032] In the following, the details of the heat resistant ferritic steel in accordance with this embodiment are explained. [0033] [Configuration of heat resistant ferritic steel] The heat resistant ferritic steel in accordance with this embodiment includes a base material and an oxide film formed on the base material. [0034] [Configuration of base material] The base material has the following chemical composition. [0035] C: 0.01 to 0.3% Carbon (C) is an austenite stabilizing element for making the base material martensitic. Furthermore, C enhances the high-temperature strength of steel by forming carbides. On the other hand, if the C content is too high, carbides precipitate excessively, and therefore the workability and weldability of steel are decreased. Therefore, the C content is set to 0.01 to 0.3%. The lower limit of C content is preferably higher than 0.01%, further preferably 0.03%. The upper limit of C content is preferably lower than 0.3%, further preferably 0.15%. [0036] Si: 0.01 to 2% Silicon (Si) deoxidizes the steel. Furthermore, Si enhances the steam oxidation resistance of steel. On the other hand, if the Si content is too high, the toughness of steel is decreased. Further, since the oxide film contains Si, if the Si content is too high, the steel becomes liable to dissipate heat, and the photoselective absorptivity decreases. Therefore, the Si content is set t o 0.01 t o 2%. The lower l i m i t of S i content is p r e f e r a b l y higher than 0.01%, f u r t h e r p r e f e r a b l y 0.05%, and s t i l l f u r t h e r p r e f e r a b l y 0.1%. The upper l i m i t of S i content is p r e f e r a b l y lower than 2%, f u r t h e r p r e f e r a b l y 1.0%, and s t i l l f u r t h e r p r e f e r a b l y 0.5%. [0037] Mn: 0.01 t o 2% Manganese (Mn) deoxidizes t h e s t e e l . Furthermore, Mn forms MnS by combining with S i n t h e base m a t e r i a l , and t h e r e f o r e enhances t h e hot w o r k a b i l i t y of steel. On t h e o t h e r hand, i f t h e Mn content is too high, t h e steel is e m b r i t t l e d , and a l s o t h e high-temperature s t r e n g t h of steel is decreased. Therefore, t h e Mn content is set t o 0.01 t o 2%. The lower l i m i t of Mn content is p r e f e r a b l y higher than 0.01%, f u r t h e r p r e f e r a b l y 0.05%, and s t i l l f u r t h e r p r e f e r a b l y 0.1%. The upper l i m i t of Mn content is p r e f e r a b l y lower than 2%, f u r t h e r p r e f e r a b l y 1.0%, and s t i l l f u r t h e r p r e f e r a b l y 0.8%. [0038] P: a t most 0.10% S: a t most 0.03% Phosphorus ( P ) and s u l f u r (S) a r e i m p u r i t i e s . P and S s e g r e g a t e a t c r y s t a l g r a i n boundaries i n the base m a t e r i a l , and decrease t h e hot workability of s t e e l . Furthermore, P and S concentrate a t t h e i n t e r f a c e between t h e oxide film and t h e base m a t e r i a l , and decrease t h e adhesiveness of oxide f i l m . Therefore, t h e P content and t h e S content a r e p r e f e r a b l y a s low as p o s s i b l e . The P content is set to at most 0.10%, and the S content is set to at most 0.03%. The P content is preferably at most 0.03%, and the S content is preferably at most 0.015%. [0039] Cr: 7.5 to 14.0% Chromium (Cr) enhances the oxidation resistance of steel. Furthermore, Cr is contained in the oxide film, and enhances the photoselective absorptivity of steel. In particular, Cr enhances the reflectance on the high wavelength side, and contributes to the suppression of heat radiation of steel. Furthermore, Cr enhances the adhesiveness of steel relative to the oxide film. On the other hand, if the Cr content is too high, the amount of delta ferrite is increased, and therefore the strength and toughness of steel are decreased. Furthermore, much Cr203 is contained in the oxide film on the base material by oxidation treatment, and in particular, the light absorption on the low wavelength side is decreased. Therefore, the Cr content is set to 7.5 to 14.0%. The lower limit of Cr content is preferably higher than 7.5%, further preferably 7.7%, and still further preferably 8.0%. The upper limit of Cr content is preferably lower than 14.0%, further preferably 12.0%, and still further preferably 10.0%. [0040] so1.A:: at most 0.3% Aluminum (Al) deoxidizes the steel. On the other hand, if the A1 content is too high, the cleanliness of steel is decreased, and the hot workability of steel is decreased. Therefore, the sol.Al content is set to at most 0.3%. The lower limit of sol.Al content is preferably 0.001%. The upper limit of sol.Al content is preferably lower than 0.3%, further preferably 0.1%. The sol.Al means acid soluble Al. [0041] N: 0.005 to 0.15% Nitrogen (N) solid-solution strengthens the steel. Furthermore, N forms nitrides and/or carbo-nitrides, and therefore precipitation strengthens the steel. On the other hand, if the N content is too high, the nitrides and carbo-nitrides are coarsened, and the toughness of steel is decreased. Therefore, the N content is set to 0.005 to 0.15%. The lower limit of N content is preferably higher than 0.005%, further preferably 0.01%. The upper limit of N content is preferably lower than 0.15%, further preferably 0.10%. [0042] The balance of the base material of the heat resistant ferritic steel in accordance with this embodiment consists of Fe and impurities. The term "impurities" so referred to in this description indicates the elements that are mixed on account of ore or scrap used as a raw material of steel, environments in the process of production, and the like. An impurity is, for example, oxygen ( 0 ) . [0043] Furthermore, the base material of the heat resistant ferritic steel in accordance with this embodiment may contain one or more elements selected from the following first to fourth groups in lieu of some of Fe. First group: Cu: at most 5%, Ni: at most 5%, and Co: at most 5% Second group: Ti: at most 1.0%, V: at most 1.0%, Nb: at most 1.0%, Zr: at most 1.0%, and Hf: at most 1.0% Third group: Mo: at most 5%, Ta: at most 5%, W: at most 5%, and Re: at most 5% Fourth group: Ca: at most 0.1%, Mg: at most 0.1%, B: at most 0.1%, and rare earth metal (REM): at most 0.1% [0044] First group: Cu: at most 5%, Ni: at most 5%, and Co: at most 5% All of copper (Cu), nickel (Ni), and cobalt (Co) are selective elements. These elements are austenite stabilizing elements, and restrain the formation of delta ferrite. If at least one of these elements is contained even a little, the above-described effect can be achieved. On the other hand, if the contents of these elements are too high, the creep strength on the long time side is decreased. Therefore, the Cu content is set to at most 5%, the Ni content is set to at most 5%, and the Co content is set to at most 5%. The lower limit of the content of each of these elements is preferably 0.005%. The upper limit of each of these elements is preferably lower than 5%, further preferably 3%, and still further preferably 1%. [0045] Second group: Ti: at most 1.0%, V: at most 1.0%, Nb: at most 1.0%, Zr: at most 1.0%, and Hf: at most 1.0% All of titanium (Ti), vanadium (V), niobium (Nb), zirconium (Zr) , and Hafnium (Hf) are selective elements. These elements form carbides, nitrides, and carbonitrides, and precipitation strengthen the steel. If at least one of these elements is contained even a little, the above-described effect can be achieved. On the other hand, if the contents of these elements are too high, the workability of steel is decreased. Therefore, the Ti content is set to at most 1.0%, the V content is set to at most 1.0%, the Nb content is set to at most 1.0%, the Zr content is set to at most 1.0%, and the Hf content is set to at most 1.0%. The lower limit of the content of each of these elements is preferably 0.01%. The upper limit of the content of each of these elements is preferably lower than 1.0%, further preferably 0.8%, and still further preferably 0.4%. [0046] Third group: Mo: at most 5%, Ta: at most 5%, W: at most 5%, and Re: at most 5% All of molybdenum (Mo), tantalum (Ta), tungsten (W), and rhenium (Re) are selective elements. All of these elements enhance the strength of steel. If at least one of these elements is contained even a little, the abovedescribed effect can be achieved. On the other hand, if the contents of these elements are too high, the toughness, ductility, and workability of steel are decreased. Therefore, the Mo content is set to at most 5%, the Ta content is set to at most 5%, the W content is set to at most 5%, and the Re content is set to at most 5%. The lower limit of the content of each of these elements is preferably 0.01%, further preferably 0.1%. The upper limit of the content of each of these elements is preferably lower than 5%, further preferably 4%, and still further preferably 3%. [0047] Fourth group: Ca: at most 0.1%, Mg: at most 0.1%, B: at most 0.1%, and rare earth metal (REM): at most 0.1% All of calcium (Ca), magnesium (Mg), boron (B), and rare earth metal (REM) are 'selective elements. All of these elements enhance the strength, workability, and oxidation resistance of steel. If at least one of these elements is contained even a little, the above-described effects can be achieved. On the other hand, if the contents of these elements are too high, the toughness and weldability of steel are decreased. Therefore, the Ca content is set to at most 0.1%, the Mg content is set to at most 0.1%, the B content is set to at most 0.1%, and the REM content is set to at most 0.1%. The lower limit of the content of each of these elements is preferably 0.0015%. The upper limit of the content of each of these elements is preferably lower than 0.1%, further preferably 0.05%. The "REM" is the general term of seventeen elements in which yttrium (Y) and scandium (Sc) are added to the elements ranging from lanthanum (La) of atomic number 57 to lutetium (Lu) of atomic number 71 in the periodic table. [0048] [Oxide film] The oxide film of the heat resistant ferritic steel in accordance with this embodiment is formed on the base material. The heat resistant ferritic steel in accordance with this embodiment has excellent photoselective absorptivity because of having the oxide film explained below. [0049] [Chemical composition of oxide film] The oxide fiim consists of oxides. The chemical composition of oxide film contains 25 to 97% of Fe and 3 to 75% of Cr. The content of chemical composition of oxide film described here is a content excluding oxygen (0) and carbon (C) . Other than Fe and Cr, about 5% or less of an element of Al, Si, Ti, Mn, Nb or the like having a high affinity to oxygen may be contained. The heat resistant ferritic steel can attain excellent oxidation resistance and photoselective absorptivity because the oxide film has the above-described chemical composition, especially because the Cr content meets the condition of the above-described content range. [0050] The chemical composition of oxide film can be measured by subjecting the base material having the oxide film to EDX (energy dispersive X-ray spectroscopy) from the surface thereof. The chemical composition is determined from the detected elements excluding oxygen (0) and carbon (C) as described above. [0051] The preferable chemical composition contains 50 to 95% of Fe and 5 to 50% of Cr. The further preferable chemical composition contains 70 to 95% of Fe and 5 to 30% of Cr. [0052] [Structure of oxide film] The oxide film contains a plurality of oxides. Preferably, the oxide film mainly contains spinel-type oxides and Cr203. The term "mainly" described here means that, in the case where the cross section in the thickness direction of oxide film is microscopically observed, the area ratio of the spinel-type oxides and Cr203 is 60% or more of the whole oxide film. [0053] The oxide film may contain oxides containing Al, Si, Ti, Mn, and Nb in addition to spinel-type oxides and Cr203. If the oxide film contains spinel-type oxides and Cr203, the heat resistant ferritic steel can have excellent photoselective absorptivity. More specifically, by causing the oxide film to contain Cri03, the reflectance on the high wavelength side is further enhanced, and the radiation of heat in high-temperature environments is restrained. [0054] The oxides in the oxide film are identified by XRD (X-ray diffractometry) in which X-rays are applied to the surface of the base material having the oxide film (heat resistant ferritic steel). In the XRD, a Co bulb may be used as an X-ray bulb, or any other bulbs may be used. [0055] Preferably, the heat resistant ferritic steel satisfies Formula (1) : 0.010 I Ic/Is I 10 . . . (1) where Is means the maximum diffraction peak intensity of spinel-type oxides in the oxide film, which is obtained by XRD. The symbol Ic means the maximum diffraction peak intensity of Cr203 in the oxide film. The maximum diffraction peak intensity so referred to in this description corresponds, for spinel-type oxides, to the intensity on the (311) plane, and corresponds, for Crz03, to the intensity on the (104) plane. Generally, the volume ratio of each of oxides is determined from the integration of peak intensities. However, as described above, if the oxide film satisfies Formula (1) defined by the maximum diffraction peak intensity ratio, the heat resistant ferritic steel exhibits excellent photoselective absorptivity. I00561 It is defined that IR1 = Ic/Is. If IR1 is less than 0.010, the ratio of Cr203 in the oxide film is excessively low. Therefore, the photoselective absorptivity decreases. In particular, the reflectance on the high wavelength side decreases. Furthermore, the oxidation resistance of the heat resistant ferritic steel decreases. [ 0 0 5 7 1 On the other hand, if IR1 exceeds 10, the ratio of Cr203 in the oxide film is excessively high. In this case, although the oxidation resistance of the heat resistant ferritic steel increases, the photoselective absorptivity decreases remarkably. [0058] If IR1 satisfies Formula (I), the heat resistant ferritic steel is liable to absorb light, and is less liable to dissipate heat. Specifically, the reflectance on the low wavelength side decreases, and the reflectance on the high wavelength side increases. The lower limit of IR1 is preferably higher than 0.010, further preferably 0.020, and still further preferably 0.050. The upper limit of IR1 is preferably lower than 10, further preferably 7, and still further preferably 5. [0059] For the oxide film in accordance with this embodiment, the content of Fez03 is preferably lower. If the content of Fe203 is high, the reflectance of light (electromagnetic wave) on the low wavelength side of oxide film increases, and the reflectance of light on the high wavelength side decreases. That is to say, the photoselective absorptivity of oxide film decreases. Therefore, the content of Fe203 is preferably lower. [0060] More specifically, the oxide film of the heat resistant ferritic steel preferably satisfies Formula (2) : Ih/ (Is + Ic) 5 0.05 . . . (2) where Ih means the maximum diffraction peak intensity of Fe203 in the oxide film. The maximum diffraction peak intensity so referred to in this description corresponds, for Fe203, to the intensity on the (104) plane. Generally, the volume ratio of each of oxides is determined from the integration of peak intensities. However, as described above, if the oxide film satisfies Formula (2) defined by the maximum diffraction peak intensity ratio, the heat resistant ferritic steel exhibits quite excellent photoselective absorptivity. [0061] It is defined that IRh = Ih/(Is + Ic). If IRh is 0.05 or less, the ratio of Fe203 in the oxide film is sufficiently low. Therefore, the heat resistant ferritic steel is liable to absorb light, and less liable to dissipate heat. Specifically, the reflectance on the low wavelength side decreases, and the reflectance on the high wavelength side increases. The lower limit of ;Rh is preferably lower than 0.05, further preferably 0.010, and still further preferably 0.005. [0062] The oxide film in accordance with this embodiment may contain FeO (wustite). Wustite is less liable to appear on the surface of oxide film because it is formed on the base material side as compared with magnetite, which is a spinel-type oxide. That is to say, wustite is less liable to be formed in the outermost layer of oxide film. Therefore, wustite does not substantially exert an influence on the photoselective absorptivity. Therefore, the oxide film may contain or need not contain wustite. [0063] [Production method] There is explained one example of a method for producing the heat resistant ferritic steel in accordance with this embodiment. [0064] The method for producing the heat resistant ferritic steel in accordance with this embodiment includes a step of preparing the base material (base material preparing step) and a step of oxidizing the prepared base material to form the oxide film on the base material (oxidizing step). In the following, the base material preparing step and the oxidizing step are described in detail. [0065] [Base material preparing step? A starting material having the above-described chemical composition is prepared. The starting material may be a slab, bloom, or billet produced by the continuous casting process (including the round continuous casting). Also, the starting material may be a billet produced by hot-working an ingot produced by the ingot-making process, or may be a billet produced by hotworking a slab or bloom. [0066] The prepared starting material is charged into a heating furnace or a soaking pit, and is heated. The heated starting material is hot-worked to produce the base material. For example, as the hot working, the Mannesmann process is carried out. Specifically, the starting material is piercing-rolled by using a piercing machine to form a material pipe. Successively, the starting material is elongation-rolled and sized by using a mandrel mill and a sizing mill to produce the base material as a seamless steel pipe. As the hot working, the hot-extrusion process or the hot forging process may be carried out to produce the base material. As necessary, the base material produced by hot working may be subjected to heat treatment, or may be subjected to cold working. The cold working is, for example, cold rolling or cold drawing. By the above-described step, the base material as a seamless pipe is produced. [ 0 0 6 7 ] The base material may be a steel plate. In this case, the base material used as a steel plate is produced by hot-working the starting material. Also, the base material used as a bar steel may be produced by hot working. Further, the base material used as a welded steel pipe may be produced by welding a steel plate. [0068] [Oxidizing step] Successively, the oxide film is formed on the produced base material. The oxide film is produced, for example, by the method described below. [0069] The base material is subjected to oxidation treatment. The oxidation treatment is performed in a gas atmosphere of, for example, mixed gas or combustion gas. The preferable oxidation treatment temperature is 1'1500~ or lower, and the preferable oxidation treat time is 3 hours or shorter. [0070] If the oxidation treatment temperature is too high, the ratio of the spinel-type oxides in the oxide film increases excessively, and the ratio of Cr203 decreases excessively. If the oxidation treatment temperature is too low, the oxide film is formed unevenly on the base material, and in some cases, the oxide film cannot cover the base material. For this reason, the photoselective absorptivity decreases. Therefore, the preferable oxidation treatment temperature is 500°C to 1150°C. [0071] Preferably, by controlling the gas atmosphere of oxidation treatment, and by changing the structure of oxide film, an oxide film satisfying Formula (2) can be obtained. More specifically, it is preferable that the oxygen partial pressure Po2 (atm) in the gas atmosphere of oxidation treatment satisfy Formula (3). Po2 < 2.76 x 1015 x expi-493.6 x 103/(RT)) . . . (3) [0072] If Po2 satisfies Formula (3), the oxygen partial pressure in the gas atmosphere thermodynamically becomes lower than the oxygen partial pressure necessary for steady formation of Fe203. Therefore, the formation of Fe203 is restricted. In the case where composition fluctuations caused by a gas flow in the gas atmosphere and composition fluctuations on account of combustion state are considered, further preferably, the oxygen partial pressure Po2 satisfies Formula (4). Po2 5 1.00 x 1014 x expi-493.6 x 103/(RT)) . . . (4) [0073] Concerning the gas atmosphere of oxidation treatment, for example, the air-fuel ratio of combustion gas may be controlled. Specifically, if the air-fuel ratio is controlled, the gas composition in the gas atmosphere changes. Based on the gas composition in the oxidation treatment gas atmosphere, the oxygen partial pressure is determined. Based on the gas composition, the oxygen partial pressure can be calculated by using, for example, the thermodynamic computation software "MALT-2 for WIN". [0074] As a fuel, natural gas, methane, propane, butane, or the like may be used. Also, a mixed gas such as H2-H20 or CO-C02 may be used. Further, an oxidation treatment gas atmosphere in which these gases are mixed may be used. [0075] Oxidation treatment that doubles as normalizing treatment (normalizing) may be performed. In this case, the cold-rolled base material is normalized. The preferable oxidation treatment temperature in this case is 900°C or higher. The oxidation treatment time is preferably 30 minutes or shorter, further preferably 20 minutes or shorter. If the oxidation treatment temperature is too high and if the oxidation treatment time is too long, the oxide film becomes excessively thick. In this case, the adhesiveness between oxide film and base material decreases, and the oxide film sometimes peels off. For this reason, the photosensitive absorptivity of the heat resistant ferritic steel decreases. [0076] Oxidation treatment that doubles as tempering treatment (low-temperature annealing) may be performed. In this case, the normalized base material is subjected to the oxidation treatment that doubles as tempering treatment. In this case, the preferable oxidation treatment temperature is 650 to 850°c,'and the preferable oxidation treatment time is 2 hours or shorter. [0077] The oxidation treatment may be performed after the normalizing treatment and tempering treatment. In this case, it is preferable that the base material structure formed by the normalizing treatment and tempering treatment be not changed in property. For this reason, the preferable oxidation treatment temperature is not higher than the tempering treatment temperature. Since the oxidation treatment temperature is as low as not higher than the tempering treatment temperature, the oxidation rate is low. Therefore, the oxidation treatment time may be long. However, considering the productivity, the preferable oxidation treatment time is 3 hours or shorter. [0078] The oxide film may be formed on the whole surface of base material. However, the oxide film may be formed only on the surface required to be excellent in photoselective absorptivity, such as the outer peripheral surface of a pipe, which is the base material. [0079] The above-described oxidation treatment may be performed one time or a plurality of times. After each step of normalizing treatment, tempering treatment, and oxidation treatmen:, straightening or the like may be performed mechanically. In the case where oil or dirt sticks to the surface of oxide film formed on the base material, even if the treatment of degreasing or cleaning is performed, the properties of oxide film are not changed. [0080] In the above-described oxidation treatment, the composition of oxide film can be changed by controlling the concentration of combustion gas. By following the above-described steps, the heat resistant ferritic steel having the base material and the oxide film of this embodiment can be produced. [0081] In the above-described oxidation treatment step, if Fe203 (hematite) is formed on the outermost layer of oxide film as the result that the oxygen partial pressure Po2 in the gas atmosphere of oxidation treatment does not satisfy Formula (3), the Fe203 (hematite) may be removed by shotblasting treatment. Even in this case, the oxide film containing magnetite, spinel-type oxides, and Cr203 of this embodiment is formed. Example 1 [0082] Heat resistant ferritic steels having various chemical compositions were produced, and the photoselective absorptivity thereof was examined. [0083] [Examination method] Heat resistant ferritic steels of steel Nos. 1 to 9 having the chemical compositions given in Table 1 were melted to produce ingots. [0084] [Table 11 Referring to Table 1, for steels of steel Nos. 1 to 7, the chemical composition of base material was within the range of chemical composition of the present invention. On the other hand, for steels of steel Nos. 8 and 9, the chemical composition of base material was out of the range of chemical composition of the present invention. Specifically, the Cr content of steel No. 8 exceeded the upper limit of Cr content of the base material of the present invention. The Cr content of steel No. 9 was lower than the lower limit of Cr co'ntent of the base material of the present invention. [0086] Each of the produced ingots was hot-rolled and coldrolled to produce a base material. In this example, the base material was a steel plate. The produced base material was subjected to oxidation treatment under various conditions to form an oxide film on the surface of base material. Table 2 gives steel No. used in each of test Nos. and oxidation treatment conditions. [0087] [Table 21 I-' I-' I-' .Ln 'l.,,k~,E r ?:st No . / 1 5 u 3 i i. !I I 2 1.1 : 4 15 el P, u I-' (D Reflectonce I-' Str.,'l Nn. 1 ! 2 -! i 2 4 3 7 4 6 4: 8' 9' 0.5 wn 8 2 4 3 6 4 6 4 6 9 8 8 4 19+ 4 I-' I-' L.C ($1 10 11rn 60 3 3 65 4 6 b2 58 55 4 8 4 6 65 35 37 14' 2 0" lo* * -c>icat->L cviatlon :rrm ringe a a ~ c l f i r di n present invention. Oxidarisn M a x i m d i f f r a c t i o n peak r reat~rent Tn:~p~.saturre t.ine 105OCCx10 n1i.1 10SO°C~10 mi:. 1050"Cx10 a ~ i n 75C*Cx60mia 600°Cx60 min 1050°Cx30 *nln l1)5O"Cx10 rnlrl ?kOoCx4> n~in .i20°C*5 rllln 73O0Cx50 min 107O"CxlO m:n 10bO°Cx10 m-n 116uDCx15 min !9CO"cxlO nrln lOt.u"ix10 min Oxide Chi-l~cal ,.~moositoin ( ?. 1 Fe:50, Cr:46 'i'c:9b, Cr:3 Fe:72, Cr:15 Fe:88,Cr:8 Fc:7Er Cr:22 F?:80, Cr:16 Fe:S.!, Cr: 17 Fe:72, Cr:T5 Fe:10, Cr:iO Fa:27, Cr:70 Fa:96, Cr:4 Fe:96, Cr:3 F?:J8, Cr:l* &:lo, Cr:8bb Fs:99, ir: