| Mold fluxes are irreplaceable materials in the continuous casting processes of steel-based metals. To maintain a stable casting process and improve the quality of continuous casting billet, various performances must be fulfilled by the employed mold fluxes and these include: protection of steel meniscus from oxidation, providing a thermal insulation to prevent the steel from freezing, absorption of the inclusions, control of the heat transfer, and lubrication of the strand. However, there are two problems need to be settled in the applilate process.One is the steel-slag reaction during continuous casting of high-Al steel. Alloying elements such as Al react with the CaO-SiO2 mold flux used in the typical continuous casting processes, decreasing the SiO2 content and increasing the Al2O3 content in mold flux, thus deteriorating the continuous casting processes and lowering the quality of steel. Another is the fluorine hazard. Mold fluxes usually contain fluoride?such as CaF2? to lower the viscosity and increase the crystallization tendency, ensuring that the mold fluxes meet the requirements. But when the mold fluxes used, fluoride will produce fluorine hazard, which is harm to human health and corroding plant equipment. The removal of fluoride additives from mold fluxes would cause operational problems?such as sticker breakout? or defects?such as longitudinal cracking?, and these problems are particularly salient in middle carbon steels. So, the development of alternative mold flux satisfying not only the requirements for continuous casting but also environment-friendly is a difficult problem being called for urgent solution at present. Because properties of materials are determined by the component and structure, the study of the effects of component?such as Al2O3 and CaF2? on the structure and the properties of mold fluxes is to solve those problems. Hence, systematic studies on the component, structure and properties of mold fluxes containing Al2O3 and CaF2 are very important.In this paper, CaO-SiO2, CaO-SiO2-Al2O3, and CaO-Al2O3 mold fluxes were designed and their structures, as well as the effect of CaF2 on the structure, were studied by Raman spectroscopy and magic-angle spinning nuclear magnetic resonance?MAS-NMR?. The rheological behavior, the solidification and crystallization behavior, and the ion release behavior of mold fluxes were investigated.The aim of this study was to elucidate the relationship of component, structure and properties, beneficial for the design of mold fluxes. Major results and conclusions are as follows:?1? the formation of the molten slag with multicomponent oxides involves the equilibrium and redistribution processes of O2- ions. The network breakers?M2O and MO? offered O2- ions and the network formers?such as SiO2? accepted the O2- ions. The Al3+ holds three distinct coordination environments: IVAl, VAl, and VIAl. The addition of F affects the coordination environment of Al3+ to form AlO3F that accommodates the network structure of slags. The different types of bonds are often classified in terms of the F bonds formed in the mold flux containing Al2O3 and CaF2, namely: i) Free F‘s?FF, bonded to cations and not to IVAl species? like in F–Ca clusters, ii) Non-bridging F‘s?NBF? like IVAl–F–M+, and iii) Bridging F‘s?BF? such as IVAl–F–IVAl bond.?2? the three-dimension network is formed by SiO4, AlO4, and AlO3F tetrahedrons in the mold fluxes containing Al2O3 and CaF2, the cation of network breakers fills the gaps in network to provide charge balance. The structural stability is affect by the polymerization degree and cation to change the rheological behavior.?3? The crystallization characteristics of slags relate to the component and structure. CaO· SiO2, 3CaO·2SiO2, 3CaO·2SiO2· CaF2, and 2CaO· SiO2 are formed in CaO-SiO2 mold fluxes. 2CaO· SiO2 and 2CaO· SiO2·Al2O3 are formed in CaO-SiO2-Al2O3 mold fluxes. The crystal transforming into the 3CaO·Al2O3 and 12 CaO·7Al2O3 in CaO- Al2O3 mold fluxes, and a few of 2CaO· SiO2 is formed at high temperature. The crystallization characteristics?such as crystallization temperature?Tc?, the critical cooling rate?Rc?, and the incubation time? are changed due to the different crystal formed in the mold fluxes.?4? The DHTT?Double Hot Thermocouple Technique? experiment is used to simulate the temperature of mold, this can be used to study the solidification and crystallization behavior under non-isothermal conditions. The solidification and crystallization properties of the mold flux are characterized by the solid fraction of molten slag?Fs?, the crystalline fraction of solid slag film?Fc?, the initial crystal location?Dc?, and the time for recrystallization completion?Trc?. The crystallizing power of CaO-SiO2-Al2O3 mold fluxes at low temperatures is weaker compared to CaO-SiO2 mold fluxes and brings inadequate ability to thermal control in the mold flux. For the CaO-Al2O3 mold fluxes, 3CaO·Al2O3 is formed in liquid slag film and deteriorates the mold lubrication.?5? The ions are released from the mold flux by ion exchange, there are two types of ion exchange occurs at the slag's surface: 1) Cations such as Na+ or Ca2+ go into solution in exchange for H+ ions from the water. 2) Similarly, F- ions can be exchanged forOH- ions, resulting in a F- increase in water. The p H is controlled by the ion exchange processes.?6? In the mold flux with Al2O3 and CaF2, the AlO3F and AlO4 accommodate the network structure of slags. The Al–F bond in AlO3F with higher bond dissociation energy is difficult to be broken in water. This inhibits the anion exchange and reduces the F- pollution. AlO4 requires M+ to provide a balance in the electrical charge, thereby inhibiting the cation exchange. A lot of AlO4 and AlO3F tetrahedrons are formed in the CaO-SiO2-Al2O3, and CaO-Al2O3 mold fluxes, this can inhibit two types of ion exchange. This not only reduces the F- pollution, but also avoids the p H change remarkedly. Therefore, Al atoms can replace Si atoms act as the network formers for the high-Al steel mold fluxes design process, and the Al in the mold flux will hopefully solve the damage caused by ion release. |
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