Low-carbon free cutting steel

Abstract

The invention provides a low-carbon free cutting steel containing no lead and is at least comparable in machinability to the conventional leaded free cutting steels and composite free cutting steels and furthermore has excellent finished surface characteristics. The steel is a low-carbon free cutting steel which comprises, on the percent by mass basis, C: 0.05 to under 0.20%, Mn: 0.4-2.0%, S: 0.21-1.0%, Ti: 0.002-0.10%, P: 0.001-0.30%, Al: not higher than 0.2%, 0: 0.001-0:03% and N: 0.0005-0.02%, with the balance being Fe and impurities, and which satisfies the relations (a) and (b) given below concerning the inclusions contained in the steel: ( A+B )/ C ≧0.8   (a), N A ≧5   (b), wherein, A: the total area occupied by substantial MnS with Ti carbide and/or Ti carbonitride included therein among the inclusions not smaller than 1 μm in circle-equivalent diameter per mm 2 of a cross section parallel to the direction of rolling; B: the total area occupied by substantial MnS with neither Ti carbide nor Ti carbonitride included therein among the inclusions not smaller than 1 μm in circle-equivalent diameter per mm 2 of a cross section parallel to the direction of rolling; C: the total area occupied by all the inclusions not smaller than 1 μm in circle-equivalent diameter per mm 2 of a cross section parallel to the direction of rolling; N A : the number of substantial MnS inclusions with Ti carbide and/or Ti carbonitride included therein among the inclusions not smaller than 1 μm in circle-equivalent diameter per mm 2 of a cross section parallel to the direction of rolling.

1 . A low-carbon free cutting steel which comprises, on the percent by mass basis, C: 0.05 to under 0.20%, Mn: 0.4-2.0%, S: 0.21-1.0%, Ti: 0.002-0.10%, P: 0.001-0.30%, Al: not higher than 0.2%, O (oxygen): 0.001-0.03% and N: 0.0005-0.02%, with the balance being Fe and impurities, and which satisfies the relations (a) and (b) given below concerning the inclusions contained in the steel:  
         ( A+B )/ C≧ 0.8   (a),  N A ≧5   (b),  
       wherein, in the relations (a) and (b), A, B, C and NA denote as follows: 
 A: the total area occupied by substantial MnS with Ti carbide and/or Ti carbonitride included therein among the inclusions not smaller than 1 μm in circle-equivalent diameter per mm 2  of a cross section parallel to the direction of rolling;  
 B: the total area occupied by substantial MnS with neither Ti carbide nor Ti carbonitride included therein among the inclusions not smaller than 1 μm in circle-equivalent diameter per mm 2  of a cross section parallel to the direction of rolling;  
 C: the total area occupied by all the inclusions not smaller than 1 μm in circle-equivalent diameter per mm 2  of a cross section parallel to the direction of rolling;  
 N A : the number of substantial MnS inclusions with Ti carbide and/or Ti carbonitride included therein among the inclusions not smaller than 1 μm in circle-equivalent diameter per mm 2  of a cross section parallel to the direction of rolling.  
 
     
     
         2 . A low-carbon free cutting steel according to  claim 1 , which further contains at least one element selected from among Se: 0.0005-0.10%, Te: 0.0005-0.10%, Bi: 0.01-0.3%, Sn: 0.01-0.3%, Ca: 0.0001-0.01%, Mg: 0.0001-0.005%, B: 0.0002-0.02% and rare earth elements: 0.0005-0.02% in lieu of part of Fe.  
     
     
         3 . A low-carbon free cutting steel according to  claim 1 , which further contains at least one element selected from among Cu: 0.01-1.0%, Ni: 0.01-2.0%, Mo: 0.01-0.5%, V: 0.005-0.5% and Nb: 0.005-0.5% in lieu of part of Fe.  
     
     
         4 . A low-carbon free cutting steel according to  claim 1 , which further contains at least one element selected from among Se: 0.0005-0.10%, Te: 0.0005-0.10%, Bi: 0.01-0.3%, Sn: 0.01-0.3%, Ca: 0.0001-0.01%, Mg: 0.0001-0.005%, B: 0.0002-0.02% and rare earth elements: 0.0005-0.02% and at least one element selected from among Cu: 0.01-1.0%, Ni: 0.01-2.0%, Mo: 0.01-0.5%, V: 0.005-0.5%, and Nb: 0.005-0.5% in lieu of part of Fe.  
     
     
         5 . A low-carbon free cutting steel according to  claim 1 , which further contains either one or both of Si: 0.1-2.0% and Cr: 0.03-1.0% in lieu of part of Fe.  
     
     
         6 . A low-carbon free cutting steel according to  claim 2 , which further contains either one or both of Si: 0.1-2.0% and Cr: 0.03-1.0% in lieu of part of Fe.  
     
     
         7 . A low-carbon free cutting steel according to  claim 3 , which further contains either one or both of Si: 0.1-2.0% and Cr: 0.03-1.0% in lieu of part of Fe.  
     
     
         8 . A low-carbon free cutting steel according to  claim 4 , which further contains either one or both of Si: 0.1-2.0% and Cr: 0.03-1.0% in lieu of part of Fe.