Since lead contaminates the environment, the various types of shape-controlled-sulfide (SCS) free machining steel, in which sulfide shapes are controlled to be spherical, have been developed to replace the leaded ones. However, in these SCS free machining steels, the effect of aspect ratio, size, and composition of sulfides on the machinability has not been revealed clearly. Authors evaluated the effect of sulfide shape on the machinability and investigated the relationship between the performance of sulfide coating on the tool surface and the sulfide composition. The results are as follows. 1) The tool life, VB100, in turning with carbide tools is almost independent of the aspect ratio and the size of sulfides, while it is remarkably improved with the existence of calcium-manganese sulfide ((Ca, Mn)S) coating which has superior lubrication. 2) The processing efficiency, VL1000, in boring with HSS drills is improved with the decrease in aspect ratio and the increase in size of sulfides. It suggests that the sulfides in the SCS free machining steel should contain (Ca,Mn)S with the lower aspect ratio and the larger size in order to secure the sufficient machinability both in turning and boring.
