Precipitation behaviour of TiN in Nb-Ti containing alloyed steel during the solidification process

A transverse crack on the slab corner became a severe defect for Nb-containing steel due to precipitation of NbN particles along the prior austenite grain boundary. By adding Ti, the TiN particles were in priority precipitation than NbN which kept more Nb as solution condition in steel. In the present research, the formation mechanism of TiN was investigated by thermodynamics calculation and experiments. It came to the following conclusions. With an increase in the cooling rate of molten steel, the precipitation location of TiN particles moves from the gamma grains boundary into the matrix. Based on the Ohnaka micro-segregation model, the precipitation behaviour of [Ti] and [N] was investigated under various solid fractions during the solidification process. As the solid fraction is larger than 0.95, the [Ti][N] value was higher than equilibrium value. Furthermore, inclusion of a high melting material such as Al2O3 played an important role in decreasing the nucleation potential barrier of TiN, which enhanced the formation of TiN particles in the molten steel. For micro-segregation models, according to the segregation degree at the solidification front, they can be sorted in the following sequence, Scheil > Ohnaka(2 alpha) > B-F > C-K > Ohnaka(4 alpha) > V-B > Lever.