Approaches to Carbon-reduced and Carbon-free Mold Powders in the Continuous Casting of Aluminum-alloyed Steels

Abstract During the continuous casting of steels, a carbon-enriched layer forms on top of the liquid slag. Fluctuations in the mold may cause contact between liquid steel and this layer, leading to its re-carburization. To prevent this, mold powders with reduced or zero carbon content are required. For ultra-low-carbon (ULC) steels, replacing carbon with SiC and applying a two-granule concept—separating components into “basic” and “acidic” fractions—has already shown promising melting behavior. Based on this approach, two mold fluxes for aluminum-alloyed steels were investigated. Both contained 2.2 wt.% Li 2 O in the liquid state but differed in composition: MP 1 (CaO/SiO 2 = 0.6, 7.1 wt.% MnO) and MP 2 (CaO/SiO 2 = 1.2, 29.2 wt.% Al 2 O 3 ). MP 1 was first produced using the two-granule method, heated between 900–1200 °C, quenched, and analyzed. However, due to volatile components, the results were unsatisfactory. CaC 2 was then tested as an alternative additive but proved unsuitable because it generated free carbon during heating. Consequently, SiC was evaluated for both fluxes. Either composition showed melting behavior comparable to standard mold powders. For MP 1, it remained stable up to 1200 °C and positively influenced melting behavior. In contrast, in MP 2, SiC was consumed by reactions at temperatures above 1000 °C, eliminating its beneficial effect. These findings indicate that SiC is advantageous for MP 1 but not for MP 2. Therefore, MP 2 was further studied using the two-granule concept. This approach delayed phase formation at lower temperatures while achieving a similar liquid fraction at higher temperatures compared to the standard flux. In summary, the effectiveness of SiC as a melt-controlling additive strongly depends on composition. It is beneficial for fluxes used for casting of high Al-alloyed steels, if their CaO/SiO 2 ratios are low, while for high-Al 2 O 3 , high-basicity systems, the two-granule concept enables complete elimination of such additives.