Precipitation mechanism and behavior of TiN during Ti-IF steel solidification

Characterization of TiN precipitation during Ti-IF steel solidification was analyzed by thermodynamics and the solidification segregation model. It is found that TiN inclusions cannot precipitate in liquid steel before solidification. In the earlier stage of solidification,TiN precipitates at low temperature in solidified steel. When the solid fraction reaches to 0.64,TiN can precipitate in solid steel around the solidification front at higher temperatures,due to the increased enrichment of Ti and N. However,at the end stage of solidification,TiN can not only precipitate in solidified steel,but also in molten steel,for the segregation of Ti and N is getting serious. TiN distribution in the Ti-IF slab was observed by scanning electron microscopy(SEM). The results show that the number and size of TiN precipitates vary a lot from the slab surface to the center. The TiN size is increasing from the slab surface to the center,from 1-2 μm to 5 μm,and it is maximum in the area of 70 to 80 mm from the slab surface. In the middle of the slab in thickness,the TiN size is large,with an average size of 5 μm; while in the center of the slab,the TiN size is getting smaller again,with an average size of 3 μm. The TiN denseness is high in the surface layer of the slab,but decreases significantly in the center and middle. The precipitation moment,the size and number of TiN precipitates are closely related with the segregation of Ti and N and the solidification rate.