Evolution Mechanism of Non-metallic Inclusions in Al-Killed Alloyed Steel during Secondary Refining Process

The evolution mechanism of inclusions in Al-killed alloyed steel during secondary refining process was studied by industrial experiments and thermodynamic calculations. It is found that during the tapping process, Al-O deoxidization reaction is very close to equilibrium with the formation of many Al2O3 clusters. With the slag/steel reaction, inclusions vary with the route as Al2O3 inclusions -> MgO-Al2O3 system inclusions -> CaO-MgO-Al2O3 system inclusions, and finally change into globular inclusions surrounded by CaO-Al2O3 outer layer, of which the melting point is lower than liquid steel temperature. Since MgO is less stable than CaO and it is easier to be reduced by Al, dissolved Mg is generated earlier and faster than Ca before LF refining, thus the MgO-Al2O3 system inclusions form at first. The mapping photos of inclusions show that the evolution mechanism of MgO-Al2O3 system inclusions into CaO-MgO-Al2O3 system inclusions is Ca element substitution for Mg element in MgO-Al2O3 inclusions. The line scanning shows that there is also the reaction of Ca element substitution for Al element in the outer CaO-Al2O3 layer without MgO.