Synopsis Ca treatment on sulfide-containing free machining steel is known as technology to control the sulfide shape. It is also possible to generate the duplex sulfide inclusion which has good tool life improvement effect by adjustment of Ca addition. This duplex inclusion has a core-and-shell construction which consist of (Ca,Mn)S precipitate wrapping CaO•mAl 2 O 3 type oxide as the overlaying layer. A tool life is improved remarkably by duplex inclusion dur to the behavior of generating a calcium-manganese sulfide film on the tool surface. It is, however, necessary to adjust a Ca concentration in shell sulfide

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