Charakterisierung des Kristallisationsverhaltens einer fluorfreien Gießschlacke für den Strangguss von Stahl
Research output: Thesis › Master's Thesis
Mould fluxes are applied in the continuous casting process of steel. These mould fluxes are made from different synthetic and natural raw materials and often show a complex composition. Recently applied mould powders contain some amount of Fluorine (e.g. CaF2). Fluorine is responsible for the formation of Cuspidine during application. Due to the environmentally harmful Fluorine, it needs to be substituted by other species. Therefore, a Fluorine-free mould flux is investigated in this theses. The investigation is realised by Single Hot Thermocouple Technique (SHTT), Double Hot Thermocouple Technique (DHTT) and mineralogical investagations via reflected light and scanning electron microscope (SEM). The SHTT results show one nose at 970°C in the TTT-diagram. The mould slag is fully solidified within 10 seconds for temperatures below 1100°C. This is significantly faster than for conventional applied mould fluxes. Mineralogical main phase is a Na-Ca-Mg-Silicate, accompanied by Merwinite. Above 1070°C additionally Perovskite, Calcium-Titanium-Double-Perovskite and Akermanite are present in minor amounts. The crystal size is increasing with the temperature. For the DHTT results a very similar phase composition could be obtained: Na-Ca-Mg-Silicate is the main phase, but also Merwinite, Perovskite and Calcium-Titanium- Double-Perovskite are formed in smaller quantities for all temperature gradients. The Na-Ca-Mg-Silicate forms dendritic as well as columnar crystals in both, SHTT and DHTT experiments. Merwinite frequently shows a needle or columnar shape, Perovskite always appears fine distributed in between the other phases and Calcium-Titanium-Double-Perovskite, however, is preferring the margins of Na-Ca-Mg-Silicate and Merwinite crystals. After the precipitation of Na-Ca- Mg-Silicate and Merwinite, the TiO2 content is increased in the residual melt which leads to the Calcium-Titanium-Double-Perovskite formation. Perovskite is formed in regions where the SiO2 content is low enough. Noticeable is the 1350/800°C temperature gradient. The crystal growth rate as well as the total crystalline amount and the crystal size are increased in comparison to the other temperature gradients. Whereas for 1350/600°C and 1350/900°C the small crystals liquify near the hot end, for the 1350/800◦C temperature gradient they are transported back to the crystalline region at the cold end, where they finally accumulate. This phenomenon can also be observed for 1350/700°C to a lesser extent. Therefore, the conditions for crystal growth and nucleation are assumed to be beneficial at these temperature gradients. The application of the investigated slag for the continuous casting process is doubtful due the severe deviation of the results compared with Fluorine containing mould slags. However, a modification of the alkalis and the TiO2 content can lead to a more proper crystallization behaviour. Moreover, the results can be used as basis for further investigations.
|Translated title of the contribution||Characterization of the crystallization behaviour of a Fluorine free mould slag for the continuous casting of steel|
|Award date||15 Dec 2017|
|Publication status||Published - 2017|