High thermal diffusivity, high toughness and low crack risk during heat treatment tool steel

Abstract

The present invention relates to a tool steel as described in WO2012095532A1 or WO2010112319A1 and its heat treatment where high levels of thermal diffusivity, mechanical resistance, wear resistance and/or toughness are attained while having a low cracking risk during the heat treatment. It is achieved by austenitizing at a temperature, normally above AC 3 , cooling to an intermediate temperature below AC 1 and above the temperature where bainitic (and alike) transformation occurs and finally quenching with a severe cooling media to a temperature below the bainitic transformation nose. Final microstructure is mainly composed of martensite and/or lower bainite, which will generally be afterwards tempered.

1 . A process of manufacturing a tool steel comprising the steps of:
 a) providing a steel material having the following composition:   
       
         
           
                 
                 
                 
                 
               
                     
                 
                   % Ceq = 0.16-1.9 
                   % C = 0.16-1.9 
                   % N = 0-1.0 
                   % B = 0-0.6 
                 
                   % Cr < 3.0 
                   % Ni = 0-6 
                   % Si = 0-1.4 
                   % 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 = 0-4 
                 
                   % Nb = 0-1.5 
                   % Cu = 0-2 
                   % Co = 0-6, 
                 
                     
                 
             
                
               
               
                
                
                
                
                
                
               
            
           
         
       
       the rest consisting of iron and trace elements, where the sum of trace elements is not higher than 2%, and wherein
   %Ceq=%C+0.86*%N+1.2*%B 
 
       and
   %Mo+½%W>2.0,
 
 
       characterized in that the process further comprises the steps of:
 b) applying to the steel material a heat treatment comprising at least partially austenitizing the steel; 
 c) cooling the steel material 
 to a temperature T int  lower than AC 1 , and 
 at a cooling speed S, 
 
       which avoid more than a 20% ferritic/perlitic transformation; and
 d) quenching the steel material with a quenching rate Q rate  rate and to a temperature T low  to ensure that the formation of structures other than martensite or lower bainite is kept below 30%. 
 
     
     
         2 . The process according to  claim 1 , wherein T int  is higher than 470° C. 
     
     
         3 . The process according to  claim 1 , wherein T int  is lower than 700° C. 
     
     
         4 . The process according to  claim 1 , wherein T low  is lower than 450° C. 
     
     
         5 . The process according to  claim 1 , wherein Q rate  is that equivalent to oil quenching or faster. 
     
     
         6 . The process according to  claim 1 , wherein Q rate  is that equivalent to polymer quenching or faster. 
     
     
         7 . The process according to  claim 1 , wherein Q rate  is that equivalent to water quenching or faster. 
     
     
         8 . The process according to  claim 1 , wherein step b) is carried out in a furnace or in a chamber within a furnace, and step c) is carried out in a different furnace or in a different chamber within the same furnace. 
     
     
         9 . The process according to  claim 1 , wherein step c) is carried out by submerging the steel material in a fluid where the temperature is controlled. 
     
     
         10 . The process according to  claim 1 , wherein, when the steel material is at a temperature higher or equal to T int  during at least steps a) and/or b), the material is hold under vacuum or any other kind of oxidation protective atmosphere for more than 60% of the time. 
     
     
         11 . The process according to  claim 1 , wherein the material reaches a hardness prior to tempering of 45 HRc or more and an unnotched resilience of 100 J/cm 2  or more after proper tempering to a hardness in the 40-44 HRc range.