Micro-alloyed steels
The invention relates to micro-alloyed pearlitic steels for formed parts preferably obtained by forging at high temperatures. 
     (Percentages indicated in the text are mass %). The effect of a fine titanium nitride dispersion of low concentration on the resistance against grain growth is well known (C. J. Cuddy, J. C. Raley: &#34;Austenite Grain Coarsening in Microalloyed Steels&#34;, Metallurgical Trans. A. Vol. 14 A, October 1983, P. 1989-1995). It is, however, hardly possible to make large-scale technical use of this effect, since at usual solidification speeds the temperature range between liquidus and solidus temperature is passed too slowly. 
     Object of the invention is to provide steels with a high fine grain resistance at temperatures of up to 1300° C. having high strength and toughness also in large-scale production. 
    
    
     Referring to the drawings: 
     FIG. 1 shows the properties of steels with about 0.25% C and 1.5% Mn and additions of Nb, Nb+V and V+Ti. The dimension is 50 mm φ and the sample position is 8 mm beneath the surface. 
     FIG. 2 is the microstructure of Steel G in the core of 50 mm φ after treatment at 1300° C. and 0.5 h/air. 
     FIG. 3 shows the grain size of the steels of claim 4, without and with titanium. The dimension is 50 mm diameter. 
    
    
     The micro-alloyed pearlitic steels according to the invention contain 0.20 to 0.50% C, 0.40 to 1.0% Si, 0,80 to 1,80% Mn, 0.008 to 0.2% S, 0 to 0.7% Cr, 0 to 0.1% Al, 0 to 0.04% N, 0.01 to 0.05% Ti, remainder iron and impurities resulting from the melting process, with mixed sulphides (manganese, titanium carbonitrides, and the like) precipitated at the grain boundaries. The steel may preferably contain an additional content of up to 0.20% V and/or up to 0.10% Nb. 
     Suchlike steels can solidify at a solidification speed within the range of 3 to 25 mm/min whereby finely dispersed sulphides precipitate between liquidus and solidus temperatures on grain boundaries. 
     The fine precipitates in this phase are responsible for the high fine grain resistance and the resulting combination of high strength and high toughness (FIG. 1). 
     A preferred steel composition is 0.20 to 0.35% C, 0.50 to 0.80% Si, 1.00 to 1.70% Mn, 0.01 to 0.09% S, 0.20 to 0.50% Cr, 0.015 to 0.06% Al, 0.015 to 0.030% N, 0.05 to 0.15% V and/or 0.02 to 0.10% Nb, 0.01 to 0.04% Ti, remainder iron including impurities resulting from the melting process. 
     It was found that only steels of the claimed composition have a high fine grain resistance at hot forming or annealing temperatures of up to 1300° C. (see FIGS. 1 and 2). The examples given in tables 2 and 3 clearly show that of all steels (composition see table 1) only the steels F and G which are subject matter of the invention present the excellent combination of high strength and high toughness. These steels have a tensile strength of at least 800 N/mm 2 , a 0.2% yield strength of at least 550 N/mm 2 , at least 15% elongation at rupture (l o  =5 d o ) and at least 45% reduction area. The notch impact values (determined on DVM samples) at room temperature are at least 35 Joule. Although the comparison steels A to E without titanium also have high strength, see table 2, they present an unsufficient toughness of less than 30 Joule, see table 3. 
     Steels of the following range are also preferred: 
     0.35 to 0.45% C, 0.5 to 0.8% Si, 1.0 to 1.7% Mn, 0.01 to 0.09% S, 0.2 to 0.5% Cr, 0.015 to 0.06% Al, 0.015 to 0.030% N, 0.05 to 0.15% V and/or 0.02 to 0.10% Nb, 0.01 to 0.04% Ti, remainder iron including impurities resulting from the melting process. 
     It was found that these steels also have a high fine grain resistance at hot forming or annealing temperatures of up to 1300° C. (see FIG. 3). 
     The examples given in tables 4 and 6 (compositions see table 4) illustrate that only the steel I and J which are subject matter of the invention present the excellent combination of high strength and high toughness. These steels have a tensile strength of at least 850 N/mm 2 , a 0.2% yield strength of at least 600 N/mm 2 , at least 12% rupture elongation (l o  =5 d o ) and at least 40% reduction area rupture and a notch impact work on DVM samples at room temperature of at least 30 Joule. Comparison steel 4 without titanium showed with less than 22 Joule unsufficient toughness. 
     This combination of an even higher strength and good toughness of at least 30 Joule is unusual for micro-alloyed pearlitic steels. 
     Therefore, the steels which are subject matter of the invention are excellently suitable to be used as automobile structural parts. 
     Properties over the whole cross section until the core Dimensions of the steels: 50 mm diameter 
     
                       TABLE 1                                                     
______________________________________                                    
chem. composition in mass %                                               
Steel                                                                     
     C     Si    Mn   P    S    Cr  Al   N    Nb  V   Ti                  
______________________________________                                    
A    .26   .54   1.37 .005 .014 .40 .043 .019 --  --  --                  
B    .24   .51   1.57 .012 .031 .58 .016 .022 .04 --  --                  
C    .26   .61   1.48 .005 .009 .31 .014 .031 .05 --  --                  
D    .25   .53   1.34 .005 .013 .37 .032 .020 --  .09 --                  
E    .25   .64   1.44 .006 .024 .35 .012 .016 .04 .05 --                  
F.sup.(1)                                                                 
     .24   .65   1.59 .007 .028 .37 .022 .021 .02 .09 .016                
G.sup.(1)                                                                 
     .27   .66   1.43 .014 .036 .10 .024 .017 --  .10 .018                
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                       TABLE 2                                                     
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mechan. post-treatment properties                                         
1250° C. 2 h/air                                                   
                   1300° C. 2 h/air                                
     R.sub.p 0,2                                                          
             R.sub.m A.sub.5                                              
                          Z    R.sub.p 0.2                                
                                     R.sub.m                              
                                           A.sub.5                        
                                                Z                         
Steel                                                                     
     N/mm.sup.2                                                           
             N/mm.sup.2                                                   
                     %    %    N/mm.sup.2                                 
                                     N/mm.sup.2                           
                                           %    %                         
______________________________________                                    
A    574     841     15.3 41                                              
     558     830     15.7 44                                              
B    523     857     17.5 38                                              
     539     889     16.3 28                                              
C    504     862     19.7 36                                              
     509     857     18.3 33                                              
D    665     894     17.3 44                                              
     622     883     15.0 49                                              
E    610     899     12.0 36                                              
     657     832     14.0 41                                              
F.sup.(1)                                                                 
     608     859     16.7 62                                              
     565     809     16.7 56                                              
G.sup.(1)                                                                 
     571     811     21.2 64   550   809   22.0 66                        
     562     806     22.0 64   567   806   22.0 64                        
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                       TABLE 3                                                     
______________________________________                                    
notch bar impact work on DVM                                              
 samples after treatment                                                  
        1250° C. 2 h/air                                           
                        1300° C. 2 h/air                           
Steel   A.sub.v in Joule                                                  
                        A.sub.v in Joule                                  
______________________________________                                    
A        9    13      20  27                                              
B       14    16      18  24                                              
C       17    19      19  23                                              
D       17    19      20  25                                              
E        7     8      12  13                                              
F.sup.(1)                                                                 
        35    38      47  60                                              
G.sup.(1)                                                                 
        55    57      59  64    39  44    46  53                          
______________________________________                                    
 .sup.(1) according to the invention                                      
 
    
     Properties over the whole cross section until the core Dimensions of the steels: 50 mm diameter 
     
                       TABLE 4                                                     
______________________________________                                    
chem. composition in mass %                                               
Steel                                                                     
     C     Si    Mn   P    S    Cr  Al   N    Nb  V   Ti                  
______________________________________                                    
H    .35   .66   1.35 .008 .043 .23 .025 .015 .02 .09 --                  
I.sup.(2)                                                                 
     .35   .66   1.36 .008 .045 .23 .023 .016 .02 .09 .013                
J.sup.(2)                                                                 
     .34   .75   1.33 .006 .055 .27 .008 .018 --  .09 .021                
 
    
     
                       TABLE 5                                                     
______________________________________                                    
mechan. post-treatment properties                                         
1250° C. 2 h/air                                                   
                   1300° C. 2 h/air                                
     R.sub.p 0,2                                                          
             R.sub.m A.sub.5                                              
                          Z    R.sub. p 0,2                               
                                     R.sub.m                              
                                           A.sub.5                        
                                                Z                         
Steel                                                                     
     N/mm.sup.2                                                           
             N/mm.sup.2                                                   
                     %    %    N/mm.sup.2                                 
                                     N/mm.sup.2                           
                                           %    %                         
______________________________________                                    
H    628     923     15.8 38   651   944   12.8 22                        
     622     921     16.2 39   624   921   11.4 20                        
I.sup.(2)                                                                 
     606     889     20.8 54   609   894   20.8 54                        
     605     886     20.6 56   603   896   20.2 51                        
J.sup.(2)                                                                 
     612     873     19.8 56   617   851   20.0 56                        
     602     865     20.0 56   605   859   24.6 56                        
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                       TABLE 6                                                     
______________________________________                                    
notch bar impact work on DVM                                              
 samples after treatment                                                  
        1250° C. 2 h/air                                           
                        1300° C. 2 h/air                           
Steel   A.sub.v in Joule                                                  
                        A.sub.v in Joule                                  
______________________________________                                    
H       13    15      17  18    13  16    20  21                          
I.sup.(2)                                                                 
        30    35      38  40    30  31    34  37                          
J.sup.(2)                                                                 
        40    47      49  59    35  38    42  44                          
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 .sup.(2) according to the invention