Tough bainitic heat treatments on steels for tooling

Abstract

The invention relates to a method that has been developed to obtain good toughness and homogeneous properties through heavy sections in tool steels or likely highly alloyed steels. The microstructure attained is mostly bainitic. The method is especially good for hot work tool steels in applications demanding heavy sections and very high toughness. The method consists on the application of a low temperature bainitic transformation to tool steels presenting a low enough martensite transformation temperature (Ms). Additionally or alternatively cementite is replaced from the bainite by other finer carbides, mainly mixed carbides containing elements with stronger affinity for carbon than iron. The method is especially simple if applied to steels with high contents of Si or Al (>1.3% and >0.4% respectively) where cementite growth is impaired. The method works also well for low cost plastic injection moulding and structural steels. Even some higher alloyed tool steels can benefit from, the present method.

1 . A method to manufacture a steel, casting die or tool, comprising providing a steel with both a bainite and a martensite domain of existence, wherein the steel is subjected to a thermal treatment comprising the following steps:
 a) Austenitization,   b) Cooling rapid enough to avoid the formation of more than a 20% stable phases with a transformation temperature higher than bainite,   c) Maintaining the temperature to transform at least 60% vol of remaining austenite between Ms+300° C. and Ms−50° C., wherein Ms is the martensite start of transformation temperature;   wherein at least 70% of the microstructure is bainitic with fine carbide-like constituents with a CVN higher than 8 Joule within at least 20 mm from the surface of the thermally treated steel.   
     
     
         2 . The method according to  claim 1 , wherein the steel has a martensite start (Ms) of transformation equal or lower than 480° C. 
     
     
         3 . The method according to  claim 1 , wherein the steel contains at least 3% carbide formers stronger than iron and the thermal treatment is followed by at least one tempering cycle above 500° C. to separate the alloy cementite, to dissolve the cementite in solid solution, and to separate the carbide formers stronger than iron. 
     
     
         4 . The method according to  claim 1 , wherein the steel contains at least 0.4% Si and/or 0.4% Al to retard the cementite growth. 
     
     
         5 . The method according to  claim 1  wherein, upon formation of the bainite, the steel is tempered with at least one tempering cycle at a temperature above 500° C. to ensure that a significant portion of the cementite is replaced by carbide-like structures containing carbide formers stronger than iron. 
     
     
         6 . The method according to  claim 1 , wherein the steel is a high thermal conductivity steel and that at the least one tempering cycle is carried out at a temperature above 540° C., providing a low scattering structure characterized by a thermal diffusivity higher than 8 mm 2 /s. 
     
     
         7 . The method according to  claim 1 , wherein the steel has the following composition all percentages being indicated in weight percent: 
       
         
           
                 
                 
                 
                 
               
                     
                 
                   % C eq  = 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 wherein,
   % C eq =% C+0.86*% N+1.2*% B, 
   and 
   % Mo+½·% W>2.0.
 
 
       
     
     
         8 . The method according to  claim 1 , wherein the steel has the following composition, all percentages being indicated in weight percent: 
       
         
           
                 
                 
                 
                 
               
                     
                 
                   % C eq  = 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 of iron and trace elements wherein,
   % C eq =% C+0.86*% N+1.2*% B. 
 
       
     
     
         9 . The method according to  claim 1 , wherein the steel has the following composition, all percentages being indicated in weight percent: 
       
         
           
                 
                 
                 
                 
               
                     
                 
                   % C eq  = 0.5-3.0 
                   % C = 0.5-3.0 
                   % N = 0-2.2 
                   % B = 0-2.0 
                 
                   % Cr = 0.0-14 
                   % Ni = 0-6.0 
                   % Si = 0-2.0 
                   % Mn = 0-3 
                 
                   % Al = 0-2.5 
                   % Mo = 0-15 
                   % W = 0-15 
                   % Ti = 0-4 
                 
                   % Ta = 0-4 
                   % Zr = 0-12 
                   % Hf = 0-4 
                   % V = 0-12 
                 
                   % Nb = 0-4 
                   % Cu = 0-2 
                   % Co = 0-6 
                 
                     
                 
             
                
               
               
                
                
                
                
                
                
               
            
           
         
         the rest consisting of iron and trace elements wherein, 
         % C eq =% C+0.86*% N+1.2*% B. 
       
     
     
         10 . The method according to  claim 1 , wherein the steel has the following composition, all percentages being indicated in weight percent: 
       
         
           
                 
                 
                 
                 
               
                     
                 
                   % C eq  = 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 of iron and trace elements wherein,
   % C eq =% C+0.86*% N+1.2*% B, 
 
         characterized in that
   % Si+% Mn+% Ni+% Cr>2.0, 
   or 
   % Mo>1.2, 
   or 
   % B>2 ppm 
 
       
     
     
         11 . The method according to  claim 1 , wherein at least 70% of the bainitic transformation is made at temperature below 400° C., attaining a microstructure of fine bainite characterized by a hardness above 45 HRc without tempering. 
     
     
         12 . The method according to  claim 1 , wherein at least 70% of the bainitic transformation is made at temperatures below 400° C. and/or the thermal treatment includes at least one tempering cycle at a temperature above 500° C. to ensure separation of stronger carbide formers carbides, so that most of the attained microstructure, with the exception of the eventual presence of primary carbides, is characterized by the minimization of rough secondary carbides, in particular at least 60% in volume of the secondary carbides has a size of 250 nm or less, such that a toughness of 10 J CVN or more is attained. 
     
     
         13 . The method according to  claim 1 , wherein the composition and tempering strategy is chosen so that high temperature separation secondary carbide types such as types MC, MC-like type as M4C3, M6C and M2C are formed, in such a manner that a hardness above 47 HRc is obtainable even after holding the material for 2 h at a temperature of 600° C. 
     
     
         14 . The method according to  claim 1 , wherein the steel has a composition within the following range:
   % Cr<3.0     % Si<0.8     wherein     % Mo+½·% W>2.0.
   and the tempering strategy is chosen to minimize carriers scattering, such that a low scattering structure characterized by a diffusivity of 8 mm 2 /s or more is obtainable even for a hardness of 45 HRc or more.   
     
     
         15 . The method according to  claim 1 , wherein the steel has the following composition:
   % Ni>0.8   
     
     
         16 . The method according to  claim 1 , characterized in that the steel presents at least two of the following features:
 it contains retained austenite;   its cementite is not wholly dissolved in the solid solution;   its cementite has not fully coalesced; and   carbide formers stronger than iron are present in the solid solution,   
       so that the hardness of the steel can be raised in an amount of at least 4 HRc upon application of a later thermal treatment below austenitizing temperature.