Evolution Of Non-metallic Inclusions During IF Steel Making Process

IF steel has advanced deep draw behavior and high surface quality, but the presence of inclusions seriously affect IF steel’s property, even lead to increasing product defects and reducing economic benefits. This article takes a steel mill’s IF steel making process as aresearch subject, through tests and sample analysis, oxygen, nitrogen composition and inclusions in converter smelting, RH refining, continuous casting and various processes was studied systematically, the results is a guiding significance to the optimization of IF steel’s making process.The quantitative metallography analysis results showed that, the average diameter, unit area quantity and area ratio of non-metallic inclusions were significantly decreased from deoxidation process to the continuous casting tundish during RH refining process. And unit area quantity decreased from more than900to less than100per square millimeter. The area ratio of non-metallic inclusions decreased0.53%to0.007%. Comparing with the steady-state casting stage, the number and the size of the inclusions significantly increased at the beginning of casting.From3min after deoxidation to the tundish process, the average of total oxygen content in ladle decreased from more than0.04%to less than0.0048%. But the average of total oxygen content was at a higher level, the analysis show that the result was related to the higher oxidation of ladle slag. And optimization of the smelting process should be carried out to further reduce the total oxygen content. The average content of nitrogen decreased from0.0031%to0.0024%during the RH refining. But the content of nitrogen significant increased by0.0010%during the continuous casting process. In order to further reduce the content of nitrogen, the entire smelting process should be controlled better.According to nonmetallic inclusions composition analysis, from the deoxidation process to the casting slabs, non-metallic inclusions change from single cluster inclusions of Al2O3system into the inclusions of Al2O3system and the complex inclusions of Al2O3-TiOx system coexisting. Individual TiN and the complex inclusions of TiN-Al2O3-MnS system was contained in the casting slabs. Thermodynamic calculations show that, the formation of Al2O3-TiOx system could be explained by the theory that Ti2O3and Ti3O5would precipitate on the Al2O3surface. TiN could be formed during solidification due to the decrease of nitrogen solubility. The complex inclusions of TiN-Al2O3-MnS could also precipitate on the Al2O3surface during solidification.