Study on the Formation and Control of TiN Inclusion in Mushy Zone for High Ti Microalloyed Steel

The coarse TiN inclusions can act as potential fracture initiation sites and deteriorate the impact toughness of steels. The experimental observation results indicated that CaS and TiN inclusions precipitated in solidification mushy zone. Meanwhile, it was found that CaS inclusions could act effectively a heterogeneous nucleation substrate for the precipitation of TiN inclusions, and the size of TiN inclusions with CaS core was obviously larger than those TiN inclusions without CaS core. Additionally, the thermodynamic calculation result showed that TiN inclusions began to precipitate at the end of solidification from the dendrites front; moreover, the preexisted CaS inclusions promoted the formation of TiN inclusions. Decreasing sulfur and nitrogen content could significantly reduce precipitation temperature and growth time of CaS and TiN inclusions in mushy zone. Furthermore, TiN inclusions could be prevented from precipitation at the surface of CaS inclusions when the S content of molten steel was below 0.0008 wt%.