Formation of equiaxed crystals by complex inclusions during solidification of advanced high strength steel

Abstract

The effect of Mg-Ti deoxidation on the solidification structure of advanced high strength steel was investigated by observing not only the solidification structure but also the inclusion particles using SEM-EDS. The effect of precipitation of TiN, MgO and 'TiN-MgO' on the formation of fine equiaxed crystals was evaluated. The composition of inclusions was changed as in the order of MgO → 'MgO(core)+TiN(surface)' → Ti2O3 by reaction time, which corresponds to the change of solidification structure as 'columnar → equiaxed → columnar.' This could be understood from the concept of lattice disregistry in between delta iron and MgO (3.97%), TiN (3.91%), and Ti2O3 (18.9%). However, even with very low disregistry between delta iron and MgO, the MgO itself did not work as an effective catalyst, indicating that there is another criterion for determining a good catalyst. The mechanism of the formation of TiN on MgO surface was schematically described. The precipitation of TiN on MgO surface was feasible although the content of Ti and N was lower than the equilibrium solubility product for the formation of TiN. Because disregistry between TiN and MgO is very low (0.05%), the precipitation of TiN on the surface of MgO is energetically more favorable.