STEEL FOR MACHINE STRUCTURAL USE HAVING EXCELLENT CHIP TREATABILITY

摘要

PROBLEM TO BE SOLVED: To improve chip treatability in particular without the addition of detrimental Pb by allowing sulfide groups of a specific length in which sulfides of a specific length alone or several sulfides are continuous in a series form to exist at a specific number of pieces in the steel. SOLUTION: The sulfide groups (long-sized sulfide groups) of the length >=20μm in which the sulfides (long-sized sulfides) of the length >=20μm alone or the several sulfides are continuous in approximately the series form within the visual field of the cross section 1 mm<2> in the rolling direction of the steel for machine structural use are made to exist in the steel at >=30. As a result thereof, these long-sized sulfides (groups) act as stress concentration sources at the time of machining and promote the propagation of microcracks in their longitudinal directions. The narrowing of a chip shearing area, the decrease of the shearing stress of the chips and the drastic decrease in machining resistance are then resulted. S of >=0.015 to <=0.12 mass% is preferably incorporated into the steel in order to form the long-sized sulfides (groups) of such a length and number. The sulfides include MnS, etc.