It is necessary to investigate the behavior of MnS formation on the solidification of steel quantitatively, in order to improve machinability of resulphurized free-cutting steel. From this viewpoint, a mathematical model analyzing MnS formation on the solidification of steel was developed on the basis of the method for analyzing interdendritic microsegregation. Furthermore, analysis by this mathematical model and directional solidification tests were carried out to investigate this behavior and the effect of cooling rate on it.
The results are as follows:
1) The MnS particles in the low carbon resulphurized free-cutting steel are formed by crystallization and precipitation on cooling.
2) The rate of crystallization of MnS is regulated by the diffusion rate of solute Mn from center of dendrite to interdendrite region.
3) The rate of precipitation of MnS is regulated by the diffusion rate of solute Mn in the region of the order of several μm from the surface of precipitating MnS.
4) The amount of MnS is effectively increased by increase of Mn content and decrease of cooling rate on solidification.