Inclusion Composition Control During LF Refining for SPCC Using FactSage Combined With Industrial Trials

Steel plate cold common (SPCC) is a Al-killed steel with Ca-treatment. The control of Al2O3 indusion into low melting point liquid region is beneficial for indusion removal, cast-ability promotion and defects reduction during rolling. Thus it is essential to understand steel-indusion equilibrium since indusion composition is determined by composition of liquid steel directly through steel-indusion reaction. Thermodynamic calculation software FactSage is performed to understand how to control indusion composition du ring ladle furnace (LF) refining, and industrial trials are carried out to verify calculated results. Firstly, target region for controlling CaO-Al2O3-MgO ternary indusion is analyzed on the basis of the ternary phase diagram and the relationship between activities related to pure solid and activities related to pure liquid was fixed by thermodynamic analysis in order to obtain reliable activities for components of inclusions in the target region by FactSage. In addition, indusions in steel sampies are detected by scanning electron microscopy (SEM) combined with energy dispersive spectroscopy (EDS). It is found that most of Al2O3 inclusions are modified into lower melting point region but a number of them are still located in high melting point region at the end of LF refining after Ca-treatment. Moreover, the composition of liquid steel equilibrating with liquid CaO-Al2O3-MgO inclusion is obtained by steel-inclusion equilibrium calculation when w[Al]s is approximating 0.03% as: a[o] is 1.0 × 10−6 to 4.0 × 10−6, w[Ca] is 20 × 10−6 to 50 × 10−6 and w[Mg] is 0.1 × 10−6 to 3.0 × 10−6. At last, stability diagrams of various calcium aluminates and CaS are established and they show that liquid calcium aluminate inclusions form when w[Ca] is more than 20 × 10−6, but CaS precipitation is difficult to prevent because sufficiently low w[s] (<0.003%) is required.