Concepts for the development of carbon-free mold powders for the continuous casting of steels

Mold powders for the continuous casting of ultra-low-carbon steels already contain low amounts of carbon. Furthermore, various approaches favor carbon burn-off at high temperatures to prevent the formation of a carbon-enriched layer on top of the liquid slag pool in the mold. Nevertheless, re-carburization of the steel still occurs. Therefore, the development of different mold powder compositions to reduce the free carbon content without negatively affecting melting behavior is required. An initial approach involved replacing carbon with SiC, which showed promising results. Subsequently, further laboratory investigations were conducted on different mold powder compositions with reduced SiC contents. However, to further decrease the total carbon content, a new concept is required: a mold powder consisting of basic and acidic granules, which are mixed after granulation. In the next step, melt-controlling additives are completely removed from the samples. For these mold powders, the influence of varying raw material components on melting behavior was additionally investigated. To verify this concept, the standard and newly developed mold powders were introduced into a furnace preheated to temperatures between 900−1200°C to simulate high heating rates. After a dwell time of 10 min followed by quenching to room temperature, the samples were investigated mineralogically. The results reveal that the diffusion path between raw material particles is increased due to the separation of the raw materials into two different granule types (basic and acidic). Furthermore, the formation of an intermediate liquid phase is shifted to higher temperatures. This shift results in a delay in the formation of new (intermediate) phases, such as the equilibrium liquid phase, fluorides, or cuspidine. The developed concept enables the complete removal of melt-controlling additives from the mold powder composition. As a side effect, the CO2 emissions from the mold powder is reduced.