Hot work tool steel with outstanding toughness and thermal conductivity

Abstract

A hot work tool steel family with exceptional thermal difusivity, toughness (both fracture toughness and notch sensitivity resilience CVN—charpy V-notch) and trough hardenability has been developed. Mechanical resistance and yield strength at room and high temperatures (above 600° C.) are also high, because the tool steels of the present invention present a high alloying level despite the high thermal conductivity. Given the exceptional resistance to thermal fatigue and thermal shock, wear resistance can be severely increased for many applications requiring simultaneously resistance to thermal cracking and wear like is the case for some forging and some parts of die casting dies.

1 . A steel, in particular a hot work tool steel, comprising iron and unavoidable impurities, and having the following composition, all percentages being in weight percent: 
       
         
           
                 
                 
                 
                 
               
                     
                 
                   % Ceq = 0.20-1.2 
                   % C = 0.20-1.2 
                   % N = 0-1 
                   % B = 0-1 
                 
                   % Cr < 1.5 
                   % Ni = 1.0-9 
                   % Si < 0.4 
                   % Mn = 0-3 
                 
                   % Al = 0-2.5 
                   % Mo = 0-10 
                   % W = 0-15 
                   % Ti = 0-3 
                 
                   % Ta = 0-3 
                   % Zr = 0-3 
                   % Hf = 0-3, 
                   % V = 0-4 
                 
                   % Nb = 0-3 
                   % Cu = 0-4 
                   % Co = 0-6, 
                   % S = 0-1 
                 
                   % Se = 0-1 
                   % Te = 0-1 
                   % Bi = 0-1 
                   % As = 0-1 
                 
                   % Sb = 0-1 
                   % Ca = 0-1, 
                 
                     
                 
                   wherein % Ceq = % C + 0.86 * % N + 1.2 * % B, % Mo + ½ · % W > 1.2 and the steel presents a low scattering structure defined by a thermal diffusivity higher than 8 mm 2 /s. 
                 
             
                
               
               
                
                
                
                
                
                
                
                
                
               
            
           
         
       
     
     
         2 . A steel according to  claim 1 , wherein the thermal diffusivity is higher than 11 mm 2 /s. 
     
     
         3 . A steel according to  claim 1 , wherein at least 80% weight of the carbides are carbides of primarily Fe, Mo or W, alone or in combination. 
     
     
         4 . A steel according to  claim 3 , wherein no other single metallic element is present in solid solution within the Fe, Mo and/or W carbides in a concentration higher than 10% weight. 
     
     
         5 . A steel according to any one of  claims 3 , wherein the % C in the carbides is at least partly replaced by % N and/or % B. 
     
     
         6 . A steel according to  claim 1 , wherein no single element is present in solid solution within the Fe metallic matrix embedding the carbides in a concentration higher than 0.5% except % Ni and/or % Mn. 
     
     
         7 . A steel according to  claim 1  wherein:
   0.03<xC eq −AC·[xMo/(3·AMo)+xW/(3·AW)+xV/AV]<0.165
 
 where: 
 xC eq —weight percent Carbon; 
 xMo—weight percent Molybdenum; 
 xW—weight percent Tungsten; 
 xV—weight percent Vanadium; 
 AC—carbon atomic mass (12.0107 u); 
 AMo—molybdenum atomic mass (95.94 u); 
 AW—tungsten atomic mass (183.84 u); 
 AV—vanadium atomic mass (50.9415 u). 
 
     
     
         8 . A steel according to  claim 1  wherein:
   %Ni+9*%Mn+5*%Si<8. 
 
     
     
         9 . A steel according to any one of  claims 1  wherein % Cr<0.1. 
     
     
         10 . A steel according to  claim 1  wherein: 
       
         
           
                 
                 
                 
                 
               
                     
                 
                   % C eq  = 0.26-0.4 
                   % C = 0.26-0.4 
                   % N = 0-0.45 
                   % B = 0-0.3 
                 
                   % Cr < 0.5 
                   % Ni = 2.99-6 
                   % Si < 0.3 
                   % Mo = 2.5-8 
                 
                   % W = 0-5. 
                 
                     
                 
             
                
               
               
                
                
                
                
               
            
           
         
       
     
     
         11 . A steel according to  claim 1  wherein:
   xC eq *(xMo+0,5*xW)/(xCr+xV+xNb)>8 
 wherein: 
 xC eq —weight percent Carbon; 
 xMo—weight percent Molybdenum; 
 xW—weight percent Tungsten; 
 xV—weight percent Vanadium; 
 xNb—weight percent Niobium; 
 where xCr, xV and xNb are the real weight percents even if present at concentrations lower than 0.05%. 
 
     
     
         12 . A steel according to  claim 1  wherein, when subjected to prolonged exposure to 600° C. or more, a hardness of more than 42 HRc is obtainable with a thermal diffusivity higher than 8 mm2/s. 
     
     
         13 . A steel according to  claim 1  characterized in that the carbides are of the type M 3 Fe 3 C. 
     
     
         14 . A die, tool or part comprising at least one steel according to  claim 1 . 
     
     
         15 . A steel, in particular a hot work tool steel, comprising iron and trace elements, and having the following composition, all percentages being in weight percent: 
       
         
           
                 
                 
                 
                 
               
                     
                 
                   % Ceq = 0.20-1.2 
                   % C = 0.20-1.2 
                   % N = 0-1 
                   % B = 0-1 
                 
                   % Cr < 1.5 
                   % Ni = 1.0-9 
                   % Si < 0.4 
                   % Mn = 0-3 
                 
                   % Al = 0-2.5 
                   % Mo = 0-10 
                   % W = 0-15 
                   % Ti = 0-3 
                 
                   % Ta = 0-3 
                   % Zr = 0-3 
                   % Hf = 0-3, 
                   % V = 0-4 
                 
                   % Nb = 0-3 
                   % Cu = 0-4 
                   % Co = 0-6, 
                   % S = 0-1 
                 
                   % Se = 0-1 
                   % Te = 0-1 
                   % Bi = 0-1 
                   % As = 0-1 
                 
                   % Sb = 0-1 
                   % Ca = 0-1, 
                 
                     
                 
                   wherein % Ceq = % C + 0.86 * % N + 1.2 * % B, % Mo + ½ · % W > 1.2 and the steel presents a low scattering structure defined by a thermal diffusivity higher than 8 mm 2 /s.