THE EFFECT OF OXYGEN-CONTENT ON THE FORMATION BEHAVIOR OF SULFIDES OF MNS SYSTEM AND THE MACHINABILITY OF LOW-CARBON RESULFURIZED STEEL

Study was made to clarify the effect of oxygen content on the formation behavior of sulfides of MnS system and the machinability of resulphurized free-machining steel and the mechanism of the effects of oxygen. In the case of high oxygen heats about 0.02mass%O, the solid oxides rich with MnO are formed at the early stage of solidification and these oxides facilitate the heterogeneous nucleation (H. N.) on MnS crystallization. This leads the enlargement of MnS inclusions and decrease of the amount of FeS solution in MnS inclusions. In the case of low oxygen heats less than about 0.01mass%O, there aren't so many oxides which act as the site of H. N. that MnS forms at final solidification. Thereby MnS inclusions are small and the amount of FeS solution in MnS inclusions increases. Increase of oxygen content improves the surface roughness of plunge cutting and drill life as formerly reported. The mechanism of effect of oxygen content on the machinability is estimated as follows. The difference in oxygen content affects the morphology, size and deformability of MnS inclusions. These changes in MnS inclusions have effects on the behavior of formation and falling of built-up-edge and the friction between chip and tool, through the deformation behavior of MnS inclusions in front of the top of tool.