Effect of Cooling Conditions on the Size and Morphology Evolution of MgAl₂O₄ Inclusions in Al-Deoxidized Steel Coupling with Mg Treatment

The experiment of Al-deoxidized steel coupling with Mg treatment is conducted under three cooling methods to observe the size and morphology of MgAl2O4. Most of MgAl2O4 in the water-cooled steel are sphere or ellipsoid-like shape, in the furnace-cooled steel are cubic or octahedral-like shape, and in the air-cooled steel are irregular shape. The quantity of inclusions in the size of 1-3 mu m increases with the increasing cooling rate, whereas the quantity of the other size decreases with the increasing cooling rate. The result shows that the changing of cooling methods has little effect on the critical size and the nucleation rate of MgAl2O4. The growth of MgAl2O4 is effected by diffusion, Ostwald, and collision growth, and the increasing cooling rate leads to the decreasing growth time. The size of MgAl2O4 is decided by the diffusion of Mg and the growth time of MgAl2O4. Four possible morphological transformation paths of MgAl2O4 in steel are found, and the shape of MgAl2O4 can evolve from sphere or ellipsoid to octahedron via truncated octahedron or cubic-like shape. The final morphology of MgAl2O4 is controlled by the transformation path which has a close relationship with the cooling rate.