High chromium ferritic heat resisting steel and method of heat treatment for the same

Abstract

High chromium ferritic steel containing chromium (Cr) in amount of 7˜12 weight %, one or two elements of molybdenum (Mu) and tungsten (W) as a solid-solution strengthening element, and one or more of elements for forming a MX carbonitride(s) is heat-treated at a temperature higher than both solid-solution temperature of the element(s) for forming the MX carbonitride(s) and a precipitation starting temperature of 6 ferrite for 5 seconds or longer, cooled at a rate of 0.5° C./s or faster, and subsequently tempered.

What is claimed is:  
     
         1 . A method of heat treatment for high chromium ferritic heat resisting steel, which comprises the steps of heat-treating high chromium ferritic steel containing chromium (Cr) in amount of 7˜12 weight %, one or two elements of molybdenum (Mo) and tungsten (W) as a solid-solution strengthening element, and one or more of elements for forming a MX carbonitride(s) at a temperature higher than both solid-solution temperature of said element(s) for forming the MX carbonitride(s) and a precipitation starting temperature of δ ferrite for 5 seconds or longer, cooling said steel at a rate of 0.5° C./s or faster, and subsequently tempering the steel.  
     
     
         2 . The method as claimed in  claim 1 , wherein a temperature of the steel before heat-treating is preserved at a temperature range of an austenitic single-phase region for homogenization of an initial structure.  
     
     
         3 . The method as claimed in  claim 1 , wherein a temperature of the steel is preserved at a temperature range of an austenitic single-phase region during cooling.  
     
     
         4 . The method as claimed in  claim 2 , wherein a temperature of the steel is preserved at a temperature range of an austenitic single-phase region during cooling.  
     
     
         5 . The method as claimed in  claim 1 , wherein the steel is heat-treated for 30 seconds or longer.  
     
     
         6 . The method as claimed in  claim 2 , wherein the steel is heat-treated for 30 seconds or longer.  
     
     
         7 . The method as claimed in  claim 3 , wherein the steel is heat-treated for 30 seconds or longer.  
     
     
         8 . High Cr ferritic heat resisting steel, which is characterized in that said steel is heat-treated according to the method as claimed in  claim 1 .  
     
     
         9 . High Cr ferritic heat resisting steel, which is characterized in that said steel is heat-treated according to the method as claimed in  claim 2 .  
     
     
         10 . High Cr ferritic heat resisting steel, which is characterized in that said steel is heat-treated according to the method as claimed in  claim 3 .  
     
     
         11 . High Cr ferritic heat resisting steel, which is characterized in that said steel is heat-treated according to the method as claimed in  claim 4 .  
     
     
         12 . High Cr ferritic heat resisting steel, which is characterized in that said steel is heat-treated according to the method as claimed in  claim 5 .  
     
     
         13 . High Cr ferritic heat resisting steel, which is characterized in that said steel is heat-treated according to the method as claimed in  claim 6 .  
     
     
         14 . High Cr ferritic heat resisting steel, which is characterized in that said steel is heat-treated according to the method as claimed in claim  7 .