Austenitic steel alloy having excellent creep strength and resistance to oxidation and corrosion at elevated use temeratures

Abstract

An austenitic steel alloy having excellent creep strength and resistance to oxidation and corrosion at elevated use temperatures up to approximately 750° C. has the following chemical composition (in weight %): 0.02≦C≦0.15%; 0.1≦Si≦2.0%; 25≦Cr≦33%; 22≦Ni≦38%; 1≦Mo≦6%; 0.4≦Nb≦1.5%; B≦0.0120%; 0.01≦N≦0.2%; Mn≦2%; Co≦5%; W≦2%; Al≦0.05%; Cu≦5%; Ti≦0.5%; Ta≦0.5%; V≦0.5%; P≦0.05%; S≦0.05%, Remainder iron with melting related impurities and optional addition of rare earths and reactive elements such as Ce, Hf, La, Re, Sc and/or Y of together 1%.

What is claimed is: 
     
         1 . An austenitic steel alloy with excellent creep strength and oxidation and corrosion resistance at increased operating temperature up to about 750° C. with the following chemical composition (in weight %):
 0.02≦C≦0.15% 
 0.1≦Si≦2.0% 
 25≦Cr≦33% 
 22≦Ni≦38% 
 1≦Mo≦6% 
 0.4≦Nb≦1.5% 
 B≦0.0120% 
 0.01≦N≦0.2% 
 Mn≦2% 
 Co≦5% 
 W≦2% 
 Al≦0.05% 
 Cu≦5% 
 Ti≦0.5% 
 Ta≦0.5% 
 V≦0.05% 
 P≦0.05% 
 S≦0.05% 
 Remainder iron with melting related impurities and optional addition of rare earths and reactive elements such as Ce, Hf, La, Re, Sc and/or Y of together 1%. 
 
     
     
         2 . The steel alloy according to  claim 1 , wherein the steel has the following composition (in weight %):
 Cr 26-30%   Ni 25-35%   B≦0.010%   
     
     
         3 . The steel alloy according to  claim 1 , wherein the steel has the following composition (in weight %):
 C: 0.05-0.12%   Si 0.1-1%   Cr 27-30%   Ni 25-35%   Mo 2-5%   Nb 0.4-1.0%   B max. 0.0090%   N 0.05-012%   
     
     
         4 . The steel alloy according to  claim 1 , wherein a minimal content of B is 0.0010 weight %. 
     
     
         5 . The steel alloy according to  claim 1 , wherein for stabilizing a sigma-phase of the steel the steel satisfies the following condition:
   weight % Mo+weight % Cr+weight % Si≧29.
   
     
     
         6 . The steel alloy according to  claim 1 , wherein for forming a sufficient amount of Nb(C,N) at simultaneous stability of M 23 C 6  the steel satisfies the following condition:
   1.5≦weight % Nb/(weight % N+weight % C)≦10.
   
     
     
         7 . The steel alloy according to  claim 1 , wherein a microstructure of the steel under operating conditions at operating temperature has precipitated phases of M 23 C 6  and further carbides, carbonitrides and nitrides on grain boundaries, and precipitated sigma phase, carbides, carbonitrides and nitrides in a grain of the steel. 
     
     
         8 . A seamless or welded steel pipe, steel sheet or work piece or tool steel produced by forging or casting with excellent creep strength and oxidation and corrosion resistance in particular at operating temperatures above 620° C., produced from the steel alloy according to  claim 1 .