| 摘要 |
The invention provides a steel for machine structural use, which is excellent in machinability, comprising, in percent by mass, C: 0.1-0.6%, Si: 0.01-2.0%, Mn: 0.2-2.0%, S: 0.005-0.2%, Al: not more than 0.009%, Ti: not less than 0.001% but less than 0.04%, Ca: 0.0001-0.01%, O (oxygen): 0.001-0.01%, and N: not more than 0.02% and satisfying the following relations (1) to (3):n0/S (%)>=2500 (1)n1/n0<=0.1 (2)n2>=10 (3)where n0: total number of sulfide inclusions not smaller than 1 mum per mm2 of a cross section parallel to the direction of rolling (number/mm2); n1: number of MnS inclusions having not smaller than 1 mum and containing not less than 1.0% of Ca per mm2 of a cross section parallel to the direction of rolling (number/mm2); n2: number, per mm2 of a cross section parallel to the direction of rolling, of oxide inclusions having a specific composition comprising CaO-Al2O3-SiO2-TiO2 and having a diameter of not less than 1 mum (number/mm2).
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