Method for producing oxidation-resistant high cr ferritic heat resistant steel

Abstract

A high Cr ferritic heat-resistant steel (Cr content of 15 mass % or less), which, at least a region 10 μm in depth as measured from the surface has a worked texture comprising elongated ferritic grains or a fine crystal grain texture comprising ferritic grains of 3 μm or less in diameter, and has a protective coating film on the surface; it is a high Cr ferritic heat-resistant steel improved in oxidation resistance without causing drop in high temperature strength or in toughness.

1 . A steel which is a ferritic heat-resistant steel containing 15 mass % or less of Cr, in which at least the region up to 10 μm defined by surface depth is made of a worked texture composed of extended ferritic grains or superfine texture composed of ferrites 3 μm or less in grain diameter, and having a protective coating film on the surface.  
     
     
         2 . The steel of  claim 1 , wherein the shorter diameter of the extended ferritic grains thereof is 5 μm or less.  
     
     
         3 . The steel of  claim 1  wherein the shorter diameter of the extended ferritic grains is 3 μm or less, or the grain diameter of the ferrites is 1 μm or less.  
     
     
         4 . A production method for a steel of  claim 1  which comprises applying working in the temperature range of from 400 to 800° C. to form a worked texture or superfine ferrite grain texture at least in the region up to 10 μm defined by surface depth, and by applying pre-oxidation treatment to form a protective coating film.  
     
     
         5 . The production method of  claim 4 , wherein the working degree on applying working is 0.7 or higher in true strain.  
     
     
         6 . The production method of  claim 4 , wherein the pre-oxidation treatment is carried out by holding for 30 to 90 minutes in the temperature range of from 400 to 800° C.