Patent ID: 6838048

Claim:
A steel for machine structural use which comprises, on the percent by mass basis, C: 0.1 to 0.6%, Si: 0.01 to 2.0%, Mn: 0.2 to 2.0%, S: 0.005 to 0.20%, P: not more than 0.1%, Ca: 0.0001 to 0.01%, N: 0.001 to 0.02% and Al: not more than 0.1%, with the balance being Fe and impurities, the effective Ca concentration index defined by the formula (1) given below being not more than 5 ppm by mass: [Ca]e=T.[Ca]−(T.[O]/(O) ox )×(Ca) ox (1) in which the symbols are defined as follows: [Ca]e: effective Ca concentration index (ppm by mass); T.[Ca]: Ca content in ppm by mass; T.[O]: O (oxygen) content in ppm by mass; (O) ox : proportion of O (oxygen) contained in oxide inclusions; (Ca) ox : proportion of Ca contained in oxide inclusions.