Heat resistant ferritic steel and method for producing the same

1. A heat resistant ferritic steel comprising:

a base material comprising, by mass percent, C: 0.01 to 0.3%, 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, 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, wherein

the oxide film contains spinel-type oxides and Cr 2 O 3 ,

wherein the following Formulas (1) and (2) are satisfied:

0.010≦ Ic/Is≦ 10  (1)

Ih /( Is+Ic )≦0.05  (2)

where the maximum diffraction peak intensity of Cr 2 O 3 in the oxide film obtained by X-ray diffraction is Ic, and the maximum diffraction peak intensity of spinel-type oxides in the oxide film obtained by the X-ray diffraction is Is and where the maximum diffraction peak intensity of Fe 2 O 3 in the oxide film obtained by the X-ray diffraction is Ih,

wherein the oxide film on the base material is formed by oxidizing the base material at a temperature of 500 to 1150° C. in a gas atmosphere in which an oxygen partial pressure Po 2 (atm) satisfies Formula (3):

Po 2 ≦2.76×10 15 ×exp{−493.6×10 3 /( RT )}  (3)

where R is a gas constant whose unit is J·K −1 ·mol −1 , and T is a temperature whose unit is K.

2. The heat resistant ferritic steel according to claim 1 , wherein

the base material 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%; and

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%.