High thermal diffusivity and high wear resistance tool steel

Abstract

A tool steel family with outstanding thermal diffusivity, hardness and wear resistance has been developed, also exhibiting good hardenability. Also its mechanical strength, as well as its yield strength, at ambient and high temperature (superior to 600° C.) are high, due to a high alloying level in spite of the high thermal conductivity. Because of its high thermal conductivity and good toughness, steels of this invention have also good resistance to thermal fatigue and thermal shock. This steels are ideal for discontinuous processes where it is interesting to reduce cycle time and that require high hardness and/or wear resistance (plastic injection molding, other plastic forming processes and curing of thermosets, hot forming of sheet . . . ). These tool steels are also appropriate for processes requiring high wear resistance and good resistance to thermal fatigue (forging, hot stamping, light-alloy injection . . . ).

1 . A steel, in particular a hot work tool steel, with the following composition, all percentages being indicated in weight percent: 
       
         
           
                 
                 
                 
                 
               
                     
                 
                   % C eq  = 0.3-0.9 
                   % C = 0.3-0.9 
                   % N = 0-0.6 
                   % B = 0-0.6 
                 
                   % Cr <2.8 
                   % Ni = 0-3.8 
                   % Si = 0-1.4 
                   % Mn = 0-3 
                 
                   % A1 = 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 
                   % S = 0-1 
                 
                   % Se = 0-1 
                   % Te = 0-1 
                   % Bi = 0-1 
                   % As = 0-1 
                 
                   % Sb = 0-1 
                   % Ca = 0-1 
                 
                     
                 
             
                
               
               
                
                
                
                
                
                
                
                
               
            
           
         
       
       the rest consisting of iron and unavoidable impurities, wherein
   % C eq =% C+0.86*% N+1.2*% B, 
   characterized in that 
   % Mo+½·% W>3.0.
 
 
     
     
         2 . A steel according to  claim 1 , wherein:
   when % C eq  is <0.35,then K>0.75,or     when % C eq  is >=0.35,then K>0.84,or     when % C eq  is >=0.35,then % Hf+% Zr+% Ta+% Nb>=0.01,     being:     K=% C eq /(0.4+(% Mo eq(real) −4)*0.04173),and
     % Mo eq(real) =% Mo+0.52*% W.   
     
     
         3 . A steel according to  claim 1 , wherein:
   % Mo eq(real) >3.3%.   
     
     
         4 . A steel according to  claim 1 , wherein:
   % V+% Nb+% Hf+% Zr>0.1.   
     
     
         5 . A steel according to  claim 1 , wherein:
   % V+% Nb+% Hf+% Zr>1.2.   
     
     
         6 . A steel according to  claim 1 , wherein:
   % C eq  is >0.32 and % C>0.32.   
     
     
         7 . A steel according to  claim 1 , wherein:
   % C eq >0.36.   
     
     
         8 . A steel according to  claim 1 , wherein:
   % C>0.4.   
     
     
         9 . A steel according to  claim 1 , wherein:
   % Mo+½·% W<10.0.
   
     
     
         10 . A steel according to  claim 1 , wherein:
   % Mo+½·% W<4.5 with % Mo=0-4.5 and % W=0-9.
   
     
     
         11 . A steel according to  claim 1 , with the proviso that:
   when % C eq <0.35,then % V<1.7.   
     
     
         12 . A steel according to  claim 1 , wherein:
   % V<1.8   
     
     
         13 . A steel according to  claim 1 , wherein:
   % Nb<0.09.   
     
     
         14 . A steel according to  claim 1 , wherein:
   % Ni<2.99.   
     
     
         15 . A steel according to  claim 1 , wherein:
   % Ni<1.0.   
     
     
         16 . A steel according to  claim 1 , wherein:
   when % Cr>2,then % Nb+% Ta+% Zr+% Hf>0.2.   
     
     
         17 . A steel according to claim, wherein:
   % C eq >0.32,     % Mo eq >3.2 and,     % Cr<2.5,with the proviso that:     when C eq <=0.36 then:3.56<% Mo eq /% C eq <11.5 or     when 0.36<C eq <=0.38 then:3.56<% Mo eq /% C eq <14 or     when 0.38<C eq  then:3.56<% Mo eq /% C eq <16.8,     being     % Mo eq =% Mo+½·% W.
   
     
     
         18 . A steel according to  claim 1 , wherein:
   % C eq >=0.33 and K<0.81,     being     K=% C eq /(0.4+(% Mo eq(real) −4)*0.04173),and
     % Mo eq(real) =% Mo+0.52*% W.   
     
     
         19 . A steel according to  claim 1  wherein, when subjected to a martensitic, bainitic or martensitic-bainitic quench with at least one tempering cycle at temperature above 590° C., a hardness above 47 HRc is obtainable with a low scattering structure characterized by a diffusivity of 9 mm 2 /s or more. 
     
     
         20 . A steel according to  claim 1  wherein, when subjected to at least one tempering cycle at temperature 590° C., a hardness of 53 HRc or more is obtainable with a low scattering structure characterized by a thermal diffusivity above 9 mm 2 /s. 
     
     
         21 . A steel according to  claim 1  wherein:
   % C>0.32, 
   % Co>1.3 and 
   % V<2.8. 
 
     
     
         22 . A steel according to  claim 21  wherein, when subjected to at least one tempering cycle at temperature above 660° C., a hardness of 50 HRc or more is obtainable with a low scattering structure characterized by a diffusivity of 5.8 mm 2 /s or more at 600° C. 
     
     
         23 . A die, tool or piece at least partially comprising a tool steel according to  claim 1 . 
     
     
         24 . A process to manufacture a hot work tool steel, characterised in that a steel according to  claim 1  to is subjected to a martensitic, bainitic or martensitic-bainitic quench with at least one tempering cycle at temperature above 590° C., so that a steel having a hardness above 47 HRc with a low scattering structure characterized by a diffusivity of 9 mm 2 /s or more is obtainable. 
     
     
         25 . A process to manufacture a hot work tool steel according to  claim 24 , wherein a steel having a hardness above 53 HRc with a low scattering structure characterized by a diffusivity of 9 mm 2 /s or more is obtainable. 
     
     
         26 . A process to manufacture a hot work tool steel according to  claim 24 , wherein the steel is subjected to at least one tempering cycle at temperature above 660° C., so that a steel having a hardness of 50 HRc or more with a low scattering structure characterized by a diffusivity of 5.8 mm 2 /s or more at 600° C. is obtainable.