STEEL FOR MACHINE STRUCTURAL USE EXCELLENT IN FRIABILITY OF CHIPS

摘要

A steel for machine structural use, which has an alloy composition in wt %: C: 0.05 to 0.8 %, Si: 0.01 to 2.5 %, Mn: 0.1 to 3.5 wt%, S: 0.01 to 0.2 %, Ca or Ca+Mg: 0.0005 to 0.02 %, Ti: 0.002 to 0.010 % and/or Zr: 0.002 to 0.025, O: 0.0005 to 0.010 %, and the balance: Fe and inevitable impurities, and comprises MnS inclusions having an average particle diameter of 1.0 mum or more in an amount of 5 pieces/mm<2> or more per 0.01 % of S content, wherein in a microscopic visual field, the requirement of (area[mum<2>]/aspect ratio) >= 10 is satisfied and the area occupied by Ca-containing sulfide based inclusions containing 1.0 wt % of Ca accounts for 15 to 40 % of the area occupied by all the sulfide based inclusions. The steel for machine structural use is a free-cutting steel excellent in the friability of chips.