Low temperature hardenable steels with excellent machinability

Abstract

The present invention relates to the application of at least partially bainitic or interstitial martensitic heat treatments on steels, often tool steels or steels that can be used for tools. The first tranche of the heat treatment implying austenitization is applied so that the steel presents a low enough hardness to allow for advantageous shape modification, often trough machining. Thus a steel product is obtained which can be shaped with ease and whose hardness can be raised to a higher working hardness with a simple heat treatment at low temperature (below austenitization temperature).

1 . A steel comprising a bainitic microstructure, comprising retained austenite, and/or carbide formers stronger than iron which are present in the solid solution, wherein the microstructure comprises at least 50% vol. bainite. 
     
     
         2 . The steel according to  claim 1 , wherein the bainite is at least 50% high temperature bainite. 
     
     
         3 . The steel according to  claim 1 , wherein the ferrite and/or perlite content is below 18%. 
     
     
         4 . The steel according to  claim 1 , wherein bainite is at least 76% vol. 
     
     
         5 . The steel according to  claim 1 , wherein the steel has a carbon content, wherein at least 8% of the carbon content thereof is present in the form of carbides not belonging to the bainitic microstructure. 
     
     
         6 . The steel according to  claim 1 , wherein the bainite present comprises tempered bainite. 
     
     
         7 . The steel according to  claim 1 , characterized by a low scattering structure characterized by a thermal diffusivity higher than 8 mm2/s. 
     
     
         8 . The steel according to  claim 1 , comprising the following composition, all percentages being indicated in weight percent: 
       
         
           
                 
                 
                 
                 
               
                     
                 
                   % Ceq = 0.15-3.0 
                   % C = 0.15-3.0 
                   % N = 0-1.6 
                   % B = 0-2.0 
                 
                   % Cr > 4.0 
                   % Ni = 0-6.0 
                   % Si = 0-2.0 
                   % Mn = 0-3 
                 
                   % Al = 0-2.5 
                   % Mo = 0-15 
                   % W = 0-15 
                   % Ti = 0-2 
                 
                   % Ta = 0-3 
                   % Zr = 0-3 
                   % Hf = 0-3 
                   % V = 0-12 
                 
                   % Nb = 0-3 
                   % Cu = 0-2 
                   % Co = 0-6 
                 
                     
                 
             
                
               
               
                
                
                
                
                
                
               
            
           
         
         The rest consisting on iron and trace elements, wherein
   %  Ceq =%  C+ 0.86*%  N+ 10.2*%  B    
 
       
     
     
         9 . The steel according to  claim 8 , wherein % V is above 0.1%. 
     
     
         10 . The steel according to  claim 8 , wherein 0% Cr is more than 8%. 
     
     
         11 . The steel according to  claim 1 , comprising the following composition, all percentages being indicated in weight percent: 
       
         
           
                 
                 
                 
                 
               
                     
                 
                   % Ceq = 0.15-2.0 
                   % C = 0.15-0.9 
                   % N = 0-0.6 
                   % B = 0-0.6 
                 
                   % Cr > 11.0 
                   % Ni = 0-12 
                   % Si = 0-2.4 
                   % Mn = 0-3 
                 
                   % Al = 0-2.5 
                   % Mo = 0-10 
                   % W = 0-10 
                   % Ti = 0-2 
                 
                   % Ta = 0-3 
                   % Zr = 0-3 
                   % Hf = 0-3 
                   % V = 0-12 
                 
                   % Nb = 0-3 
                   % Cu = 0-2 
                   % Co = 0-12 
                 
                     
                 
             
                
               
               
                
                
                
                
                
                
               
            
           
         
         The rest consisting on iron and trace elements, wherein
   %  Ceq =%  C+ 0.86*%  N+ 1.2*%  B    
 
       
     
     
         12 . The steel according to  claim 11 , wherein % Ceq is 0.22% or more. 
     
     
         13 . The steel according to  claim 11  wherein % Cr is above 12%. 
     
     
         14 . The steel according to  claim 1 , comprising the following composition, all percentages being indicated in weight percent: 
       
         
           
                 
                 
                 
                 
               
                     
                 
                   % Ceq = 0.2-0.9 
                   % C = 0.2-0.9 
                   % N = 0-0.6 
                   % B = 0-0.6 
                 
                   % Cr = 0.0-4.0 
                   % Ni = 0-6.0 
                   % Si = 0.2-2.8 
                   % Mn = 0.2-3 
                 
                   % Al = 0-2.5 
                   % Mo = 0-6 
                   % W = 0-8 
                   % Ti = 0-2 
                 
                   % Ta = 0-2 
                   % Zr = 0-2 
                   % Hf = 0-2 
                   % V = 0-4 
                 
                   % Nb = 0-2 
                   % Cu = 0-2 
                   % Co = 0-6 
                 
                     
                 
             
                
               
               
                
                
                
                
                
                
               
            
           
         
         The rest consisting on iron and trace elements, wherein
   %  Ceq =%  C+ 0.86*%  N+ 1.2*%  B    
   Characterized in that 
   %  Si +%  Mn +%  Ni +%  Cr> 2.0; or 
   %  Mo> 1.2; or 
   %  B> 2 ppm 
 
       
     
     
         15 . The steel according to  claim 14 , wherein % Si+% Mn+% Ni+% Mo is above 2%. 
     
     
         16 . The Steel according to  claim 1 , characterized in that the sum of the amounts of those elements having an affinity for carbon higher than iron selected from the group consisting of Cr, W, Mo, V, Ti, Nb, Ta, Zr and Hf is more than 4% in weight. 
     
     
         17 . The steel according to  claim 1 , characterized in that the microstructure comprises less than 70% of alloyed carbides that can be attained with the chosen composition. 
     
     
         18 . The Steel according to  claim 1 , characterized in that according to a tempering graph of the steel, the bainite present a tempering degree which is smaller than that corresponding to a secondary hardness peak, and a hardness of the steel is below a hardness level of the secondary hardness peak of the steel in an amount of at least 4 HRc. 
     
     
         19 . The Steel according to  claim 1 , characterized in that the bainite present comprises less than a 80% of a nominal % C of the steel. 
     
     
         20 . The steel according to  claim 1 , characterized in that the bainite present comprises less than a 80% of a nominal % C of the steel in an untempered state.