Steel for machine structural use

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 n1/n0 n2 >/= 10 where n0: total number of sulfide inclusions not smaller than 1 mu m per mm<2> of a cross section parallel to the direction of rolling (number/mm<2>); n1: number of MnS inclusions having not smaller than 1 mu m and containing not less than 1.0% of Ca per mm<2> of a cross section parallel to the direction of rolling (number/mm<2>); n2: number, per mm<2> 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 mu m (number/mm<2>).