THE EFFECT OF OXYGEN-CONTENT ON THE FORMA TION 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 formatio n behavior of sulfides of MnS system and the machinability of resulphu rized 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 r ich with MnO are formed at the early stage of solidification and these oxides facilitate the heterogeneous nucleation (H. N.) on MnS crystal lization. This leads the enlargement of MnS inclusions and decrease of the amount of FeS solution in MnS inclusions. In the case of low oxyg en 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. Ther eby MnS inclusions are small and the amount of FeS solution in MnS inc lusions increases. Increase of oxygen content improves the surface rou ghness of plunge cutting and drill life as formerly reported. The mech anism of effect of oxygen content on the machinability is estimated as follows. The difference in oxygen content affects the morphology, siz e and deformability of MnS inclusions. These changes in MnS inclusions have effects on the behavior of formation and falling of built-up-edg e and the friction between chip and tool, through the deformation beha vior of MnS inclusions in front of the top of tool.