Nickeloxid in Gießpulvern und deren Schlacken ¿ Auswirkung auf Aufschmelz- und Kristallisationsverhalten

Mold powders play an essential role in the continuous casting process in the steel industry. They are added onto the surface of the liquid steel bath in the mold, where they melt and form a liquid slag layer that is drawn into the gap between the strand and the mold wall. Mold powders fulfil several important functions. They prevent oxidation of the steel and reduce heat loss to the atmosphere. Furthermore, the liquid slag film provides lubrication between strand and mold, while the slag is capable of absorbing inclusions rising from the steel. The solidified slag film at the mold wall regulates heat transfer, whereby both the crystalline fraction and the morphology of the formed crystals are of major importance. Sustainability is becoming increasingly important in the steel industry. In the future, steels are expected to be produced increasingly from secondary raw materials such as steel scrap by remelting in electric arc furnaces. However, the use of scrap also introduces challenges. Variations in scrap quality lead to increased copper contents, which promote hot cracking during casting. To counteract this effect, the steel can be alloyed with nickel, although this approach is associated with high costs. Therefore, the steel strand is intended to be alloyed with nickel only near the surface via the mold slag during continuous casting. In preliminary investigations, different amounts of nickel oxide were added to commercially used mold powders in order to study its influence on the solidification behavior. The results showed that nickel oxide also affects the melting behavior depending on the chemical and mineralogical composition of the mold powders. This master¿s thesis therefore focuses on the melting behavior of two mold powder grades with different CaO/SiO¿ ratios. The first sample exhibits a ratio of 0,7, whereas the second sample has a ratio of 1,36. The investigation was carried out by mineralogical characterization of samples after heat treatment in a furnace preheated to selected temperatures. Samples obtained after stepwise heat treatments were examined using reflected light microscopy, scanning electron microscopy (SEM), and X-ray diffraction (XRD). The results demonstrate that nickel oxide does not participate in phase reactions or the formation of new phases. The only reaction observed is its reduction to metallic nickel within a temperature range of 900¿1100 °C through oxidation of the carbon contained in the mold powder. At higher temperatures, the metallic nickel accumulates as droplets in molten phase regions. In general, only few reactions occur at lower temperatures, whereas most reactions take place within the intermediate temperature range of 700¿1000 °C. These include the decarbonation of carbonates. Some larger particles remain detectable as residual phases within the melt even at elevated temperatures, mainly fluorite and wollastonite as well as decarbonated carbonates that have not yet fully dissolved into the slag. One of the most important reactions within the intermediate temperature range is the formation of cuspidine. Cuspidine is formed mainly through the reaction of wollastonite with fluorite, whereby diffusion of Na¿O into wollastonite also plays an important role. This leads to the formation of a characteristic three-zone structure consisting of wollastonite in the center surrounded by sodium silicate and cuspidine at the outer regions. Furthermore, cuspidine is the only phase that remains solid at high temperatures besides metallic nickel. Subsequently, the influence of nickel on the melting behavior was investigated in greater detail using samples produced by the Single Hot Thermocouple Technique (SHTT). The samples were analysed by reflected light microscopy and SEM. Standard mold powders without nickel oxide addition served as reference materials. The results indicate that metallic