Article ID: ISIJINT-2020-679
The three-dimensional morphology, size, content and composition of large-size inclusions extracted by large sample electrolysis from RH refining to hot rolling were investigated during the tinplate steel industrial test without calcium treatment. The results showed that the large-size inclusions in the RH refining process are Al2O3 inclusions and incompletely modified CaO·2Al2O3 inclusions, while those in tundish are Al2O3, CaO–Al2O3, CaO–SiO2–Al2O3 and CaO–SiO2–Al2O3–MgO. The typical types of large-size inclusion in slab and hot rolling plates are Al2O3, CaO–Al2O3, SiO2–Al2O3, CaO–SiO2–Al2O3, CaO–SiO2–Al2O3–MgO and CaO–SiO2–Al2O3–TiO2. Secondary oxidation was found to occur in molten steel during the pouring process and protective casting should be improved. Large-size inclusions in hot rolling plates are Al2O3 with a mass fraction of 19.8% and incompletely modified CaO·2Al2O3 inclusions with a mass fraction of 45.9% those have not been completely modified, which have high hardness and are difficult to deform. Therefore, it is recommended that calcium treatment should be carried out at the end of RH refining to reduce the Al2O3 and CaO·2Al2O3 contents. And the effect of calcium addition on the inclusion evolution has been studied by a thermodynamic analysis at 1873 K. With the increase of calcium addition in molten steel, the evolution route of equilibrium precipitations is Al2O3 → CaO·6Al2O3 → CaO·2Al2O3 → CaO·Al2O3 → 3CaO·Al2O3 → 12CaO·7Al2O3 → CaO. The critical calcium content for CaS and CaO formation increases with increasing oxygen content. To avoid the precipitation reaction between [Ca] and [S], the mass fraction of calcium addition needs to be controlled below 0.0040%.