| During casting high aluminum steel, due to the reaction between aluminum in steel and silica in the traditional lime silica based mold flux, it is a great challenge to achieve good casting performance. This thesis mainly focuses on the physicochemistry of mold flux for high aluminum steel, aiming to provide more references for solving the problem of high aluminum steel casting.In the present work, in order to provide understanding on the effect of dramatic chemical composition change on the physics and chemistry of high aluminum steel mold flux, the base flux system of CaO-SiO2-Al2O3-CaF2 were investigated in detail. Addtionally, the new type of CaO-Al2O3 based mould flux were studied in order to search more ways to develop the suitable mold flux for high aluminum steel continuous casting.The conclusions are summarized as follows:(1) Phase equilibria of CaO-SiO2-Al2O3-CaF2 system at 1523 K have been investigated using quenching method. Isothermal section at 1523 K with Al2O3 being less than 25 mass% and CaO%/SiO2% being between 0.8 and 1.25 was experimentally constructed. The liquid region and seven solid-liquid coexistence regions were determined. The effect of w(Al2O3)/w(SiO2) ratio on the viscosities in CaO-Al2O3-SiO2-CaF2 system were investigated by a rotating cylinder method. It can be concluded that the viscosity first decreases and then increases as w(Al2O3)/w(SiO2) ratio increases from 0.22 to 1.75, and the minimum value of viscosity is presented in Al2O3/SiO2 ratio 0.83. Large change of Al2O3/SiO2 values does not cause a great change of viscosity value.29Si-NMR,19F-NMR,27 Al-NMR and Raman spectra were used to investigate the microstructure of the base system of CaO-Al2O3-SiO2-CaF2. The microstructure was thoroughly analyzed with the change of Al2O3/SiO2. It can be seen that F- mainly forms F-Ca(n) and Al3+ mainly forms [AlO4]. Meanwhile, the complicated network of aluminosilicate formed by [AlO4] and [SiO4] has been clearly analyzed. The viscosity is not only related with the degree of polymerization of flux, but also affected by the main chemical bonds of flux.(2) Dissolution of Al2O3 into molten CaO-Al2O3-CaF2, a base system of mould flux for continuous casting of high Al steel, has been investigated by employing a rotating cylinder method. The dissolution rate of an alumina rod into molten flux increased with increase in rotating speed, temperature and The rate controlling step during the dissolution process of the alumina rod into molten flux was found to be the diffusion of Al2O3 in the flux boundary layer. Meanwile, the dissolution rate is closely related with the formation of intermediate compound.(3) Crystallization behaviors of CaO-AlO3 based mold flux for casting of high-Al steels were investigated by high temperature isothermal crystallization experiments. For the CaO-Al2O3-CaF2 based mold flux, it can be concluded that adding alkali metal oxides and acidic oxides can change the type of precipitated calcium aluminate. The addition of acid oxides can promote the crystallization of CaF2 in all fluxes. Alkali oxide promotes the crystallization of CaAlO4 in CaO-Al2O3-B2O3 based mold flux, while the acidic oxides (B2O3) can inhibit the crystallization ability of CaAlO7 and enhance the crystallization ability of Ca3B2O6. Optical basicity was used to correlate the change of Ca/Al ratio in the generated calcium aluminate crystals. With the increase of optical basicity, the Ca/Al ratio of calcium aluminate crystals was increased in CaO-Al2O7 based mold flux, and the type of calcium aluminate crystals changed from CaAl4O7, CaAl2O4,Ca12Al14O33 to Ca3Al206.(4) Crystallization behaviors of new developed CaO-Al2O3-TiO2 based mould fluxes for high-Al steel casting were investigated. The results showed that the viscosity decreased with the increase of TiO2 content. The sequence of crystal precipitation for TiO2-free mould flux during cooling was MgO, and followed by Ca12Al14O33 during cooling. The sequence of crystal formation for TiO2-bearing mould fluxes during cooling is CaTiO3 to MgO, and then Ca12Al14O33. The crystallization temperatures of CaO-Al2O3 based mould fluxes increased with increasing TiO2 content. The overall crystallization of mould fluxes was enhanced with increasing TiO2 content. The crystallization of CaTiO3 was enhanced and the crystallization of MgO and Ca12Al14O33 varied slightly with increase of TiO2.(5) The properties of a new type of CaO-Al2O3-B2O3 based mould flux were investigated, i) The melting temperatures decreased with the increasing B2O3, Na2O and SiO2 content in CaO-Al2O3-B2O3 based mould flux, while the melting temperature of mould fluxes first increased with increasing CaO/Al2O3 mass ratio, and then decreased with further increasing CaO/Al2O3 ratio.ii) The viscosity decreased with increasing of B2O3, Na2O and CaO/Al2O3. The addition of SiO2 would increase the viscosity. iii) The crystallization of mould fluxes was weakened by the increasing B2O3, Na2O and SiO2 content. The crystallization sequence of all samples has been investigated. It can be concluded that the Ca3Al2O6 firstly crystallized from liquid, the second and the third phases were LiAlO2 crystal and Ca3B2O6 crystal respectively.iv) Analysis of the microstructure showed that Al3+ ions and B3+ main formed [AlO4] and [BO3] into the network of this system. The increase of Na20 and CaO/Al2O3 can decrease the degree of polymerization of [AlO4] network. The increase of B2O3 and SiO2 can increase the degree of polymerization of [AlO4] network. |
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