| Mould fluxes play important roles in improving the surface quality of continuous casting strands and maintaining smooth during casting process. Tradiditional mould fluxes with higher fluorine can lead to the environmental pollution and equipment corrosion, therefore it is an objective requirement for environment protection to decrease or remove fluorine from fluxes. However, there have been a serial problems in the use of fluorine-free mould fluxes, mainly as follows: the viscosity-temperature properties of traditional mould fluxes for high carbon steel was worsen, that would easily cause the sticking and breakout; it was difficult to harmonize the conflict between heat transfer and lubrication functions of the fluorine-free mould fluxes for casting midium carbon steel with crack sensitive. Firstly, by the investigation on emission behavior of fluoride from mould fluxes and the corrosion of fluoride on SEN (Submerged Entry Nozzle), the upper limit content of fluorine in mould fluxes has been determined. And on the basis of studying the effect of fluoride on the properties and micro-structures of mould fluxes, taking major physicochemical properties of various type of traditional mould fluxes as standard reference, it is researched on the existing zones, crystallization properties and mineralogical phases, inclusion absorption properties of low and free fluorine mould fluxes, and the composition and reasonable physicochemical properties of low and free fluorine mould fluxes which are suitable in casting different steels were obtained.Then according to the laboratory experiments, the plant trial and commercial application have been conducted. The results are shown as follows:①In industrial production, the use of high fluorine mould fluxes will lead to the over standard of F- content in secondary cooling water, and erossion of SEN refractory and correlative equipments. Consequently, the control of fluorine content in mould fluxes less than 2%wt is beneficial to reduce the hazard effects of fluorine emission from mould fluxes to the secondary cooling water and decrease the fluorine content in environmental air. The increase of B2O3 content in mould fluxes is helpful to reduce the emission of fluorine into secondary cooling water.②The varieties and relative quantity of silicate microstructure units change with the increase of CaF2 content, and the extent of the silicate network decreases, therefore, the viscosity of melt reduces. Under this experimental condition, silicate microstructure units in high fluorine mould fluxes was dimer structure, thus crystal was easy to precipitate in molten slag during solidification.While in fluoride-free mould fluxes, it was chain structure, hence, its precipitation ability was weaker than high fluorine mould fluxes. It is found that the characteristic of Raman spectrum lines was the same between quenched glass and molten slag; therefore, determining the molten structure in high temperature could be respected by glass structure in room temperature to some extent.③The solidified sample cross section of mould fluxes for billet casting, bloom casting and slab continuous casting of low carbon steel and high carbon steel was almost glassy, the break temperature in viscosity-temperature curve"Tbr"was low, where the viscosity"ηTbr"was high. For slab continuous casting of low alloy steel, it was glass or small part of crystallization, its Tbr andηTbr were higher. For slab continuous casting of hypo-peritectic steel, it was most of crystallization, and there were highest Tbr and crystallization ratio, but lowerηTbr.④Many existing zones of fluorine free mould fluxes were obtained for billet casting of plain carbon steel, billet casting of high carbon steel, slab casting of low carbon steel and slab casting of low alloy steel in the range of designed mould fluxes systems. However, for slab casting of hypo-peritectic steel which was sensitive to surface longitudinal cracks, there was existing zones of low fluorine, but no existing zones of fluorine free.⑤B2O3,Li2O,Cr2O3 and TiO2 can be used to partly substitute CaF2 to obtain similar solidification and crystallization properties compared with high fluorine mould fluxes. CaF2 can't be substitute by CeO2 to control crystallization properties.⑥The major mineralogical phases precipitated from mould fluxes bearing high fluorine are cuspidine (3CaO·2SiO2·CaF2). In CaO-SiO2-Na2O slag system without fluorine, main crystallized phases are Na2O·2CaO·3SiO2, Na2O·CaO·3SiO2, and et al., and mould fluxes for crack sensitive steel was obtained from this slag system, which crystallizing properties, visicosity-temperature curve and melting point are close to those in high fluorine mould fluxes. For CaO-SiO2-TiO2 slag system with fluorine Free, CaO·SiO2·TiO2 or CaO·TiO2 was the main crystallized phase, there were unstable viscosity-high temperature properties and high Tbr in part of this slag system, which was bad for lubrication between liquid flux film and strand. And when temperature was over the Tbr, the viscosity changed slowly with temperature in some part of this slag system, which was shown that it solidified slowly, that went against slow cooling by solid flux film at meniscus to initial solidification shell and caused surface longitudinal cracks on strand.⑦For low fluorine mould fluxes for billet or bloom, its viscosity-temperature property can be steadied by adjusting the content of trace oxides, such as MnO, B2O3, BaO, and et al., to obtain stability property after absorbing inclusions as traditional high fluorine mould fluxes. Mould fluxes for slab casting especially low fluorine mould fluxes or fluorine free mould fluxes for slab casting of hypo-perictectic steel, because of the high requirement on crystallization property, its performance stability after absorbing inclusions was weak than those of high fluorine mould fluxes.⑧Commercial test showed that the metallurgical effect of fluorine free mould fluxes for billet and bloom casting, slab casting of low carbon steel and low alloy steel was the same as traditional mould fluxes bearing fluorine, while F- concentration in secondary cooling water was lower, and that reached national standard. Also that low fluorine mould fluxes for slab casting of hypo-peritectic steel was successfully applied.At present, for slab casting of hypo-peritectic steel with longitudinal crack sensitivity, it was inclined to use low fluorine mould fluxes. Further research was needed to find fluorine free mould fluxes for this kind of steel, and to search for new mineralogical phases instead of cuspidine. Also the characteristics of interface between steel and mould fluxes with fluorine free and high Na2O and the stability of mould fluxes with fluorine free and high TiO2 need further studying.
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