Comparison of methods for modification of non-metallic inclusions using alkali elements on a laboratory scale

The cleanness of modern steel is of utmost importance as non-metallic inclusions (NMIs) can have detrimental effects on the mechanical properties of the finished product. Hard and non-deformable inclusions can act as crack initiation sites, reducing fatigue strength, notched impact toughness, and formability. Even small populations of such NMIs can impair the properties of cleanness-sensitive grades and may also cause operational problems such as nozzle clogging during production. Adapted deoxidation and refining practices like Silicon- (Si)/ Manganese- (Mn) deoxidation can reduce the amount of Aluminium- (Al) rich NMIs, and slag refining treatments of the melt during secondary metallurgical operations can aid in improving steel cleanness. Nonetheless, complete elimination of NMIs is not feasible, highlighting the need for strategies that directly improve the quality of remaining inclusions. This study investigates the potential of alkali elements, specifically sodium (Na) and potassium (K), to chemically modify inclusions during secondary refining. Experiments were performed using alkali oxide-containing slags and combined slag-carbonate treatments in two steel grades. Manual and automated SEM/EDS analysis were used to evaluate both indirect and direct modification effects. Results indicate that treatment using alkali oxide-containing slags can achieve indirect modification of the inclusion landscape, reflected in a reduced relative amount of sulfides. Above all, it could be demonstrated that a combined slag-carbonate treatment is necessary to enable direct incorporation of Na and K into NMIs. These alkali elements predominantly modify Al-containing phases, including alumina-rich phases within multiphase particles. Thermodynamic calculations using FactSage 8.3 predict a decreased liquidus temperature of Al 2 O 3 in the presence of Na 2 O or K 2 O. Future work will show the impact of this modification on the mechanical properties of the inclusions and the steel.