Study On Mechanism Of Oxygen Transfer In Deoxidization By Molten Slag With External Dc Electric Field

With the progress of science and technology and the development of market, highquality steel product is on demand. It is known that the oxygen level dissolved in steelhas tremendous influence on the quality and performance. Since the traditionaldeoxidization methods could not satisfy some special requirements on the oxygenlevel in steel, it is necessary to look for alternative methods to reduce the oxygencontent in steel. In such a background，Chou put forward the deoxidization technologyof applied external voltage between molten steel and slag. This deoxidization methodcan control conduction direction of oxygen ions in the melt slag system so thatdeoxidization from liquid steel orderly can be carried out. It is observed from thetheoretical analysis and previous results that it is reliable to accelerate the reaction bythis method.The aim of the present work is to further investigate the technology ofdeoxidization. The physicochemical properties of the selected CaO-Al₂O₃based slagsystem were investigated to explore a reasonable slag system for the requirement ofthis method, and the effects of component content on the physicochemical propertieswas investigated. In order to explore a reasonable electrode material for therequirement, The Mo-ZrO₂system cermet was chosen as the electrode material,mainly research on the influence of cermet compositions and sintering processes. Thedeoxidization experiments with Carbon steel, X70steel and IF steel were performedwith and without applied external voltage. And the factors such as the initial oxygencontent in steel, slag composition and the applied electric field potential wereinvestigated. To analyze the experimental results and understand the mechanism ofdeoxidization with applied external voltage, a kinetic model was proposed.Exploration on the possibility to enhance the rate of sulfur removal from metal to slagwith application of external voltage was studied. The major contents of this dissertation are epitomized as follows:It is found experimentally that the density decreases approximately linearlywith an increasing temperature. The viscosity and conductivity decreases withincreasing temperature as expected. It is shown that the properties of selectedCaO-Al₂O₃-MgO slag system with low-melting-point zone, such as melting point,viscosity, surface tension and density the melting temperature, density, surfacetension and viscosity, decreased with increasing the MgO content, and following byan increase with further increasing of the MgO content. The conductivity increaseswith increasing the ratios of the MgO content, and following by a decrease withfurther increasing of the MgO content. The effect of the mass ratio of CaO/Al₂O₃ratio on the properties was similar to those changes with MgO content. Theproperties of the selected CaO-7mass%MgO-Al₂O₃-SiO₂with low silica weremeasured. The results indicated that while increasing the NBO/T ratio, not onlylower the melting temperature of slag, but also increase the conductivity of slag. Thesurface tension decrease first, then increase slowly again. The viscosity increaseswith increasing the ratios of the NBO/T ratio, and following by a decrease withfurther increasing of the NBO/T ratio.The obtained results indicated that Mo-ZrO₂cermet can be successfullyprepared by the present powder metallurgy processes. The electrical conductivity ofcermets increased with decreasing the Mo content, and the high-temperatureelectrical conductivity increased linearly with increasing temperature. The GEMequation agrees well with the measured data. The model was proved to be promisingfor Mo-ZrO₂cermet dependence of ZrO₂volume ratio on conductivity of the cermet.The volume fraction of the ZrO₂phase is0.84, and the exponent for the percolationparameter is2.51. The results indicated that increasing sintering time and enhancingthe sintering temperature can lead to samples more compact in structure, and thusenhance the electrical conductivity of Mo-ZrO₂cermet. The corroded resultsindicated that the corrosion resistance of cermet to molten steel declines with the increase of Mo content. By contrast, the corrosion resistance of cermet toCaO-Al₂O₃-MgO slag declines with the reduction of Mo content. The property of50mol%Mo-ZrO₂is most superior, and can well be applied to the deoxidizationexperiment as electrode material.The deoxidization experiments with CaO-Al₂O₃based slag showed that whenCaO/Al₂O₃ratio in the range from0.9to1.0and the MgO content was less than5%,the selected slag is a good oxygen ion carrier, and the slag could remove oxygenfrom molten steel effectively. The experimental results obtained from Carbon steel,X70steel and IF steel were indicated that application of external electrical voltage tothe slag steel system, not only was there an acceleration in the deoxidization rate, butalso that there was lower oxygen content in the steel melt and FeO content in thefinal slag than that dictated by chemical equilibrium of slag. And the finalcomposition of the slag did not change remarkablely with applied stable electricvoltage. The results show that under reducing atmosphere, the total oxygen contentof X70steel and IF steel with applied external voltage between molten steel and slag,were around₂9ppm and₂5ppm, respectively.The interfacial deoxidization reaction kinetics between molten steel and moltenslag was studied. The CV results indicated that the current of interfacial reactionincrease with increasing scan rate and cycle times. The AC impedance resultsindicated that with rising applied potential, the charge transfer resistance of reaction（Rct） decreased obviously. And the diffusion resistance （Warburg impedance） whichdecreased with increasing applied potential indicated that the diffusivities of oxygenin molten steel increased with applied potential. The Chronoamperometry resultsindicated The interfacial reaction on deoxidization process is [O]+2e=(O^2-). Thecurrent of interfacial reaction decreases approximately linearly with t-1/₂, and thediffusivities of oxygen in molten slag calculated by Cottrell equation increaseddramatically with applied potential.Using the equivalent circuit model, the experimental results are analyzed to understand the reaction process and mechanism of deoxidization, and a kineticmodel was proposed for the method. A reasonable agreement has been reachedbetween experimental data and model calculation.It is observed from the present work that the slag could remove sulfur frommolten steel effectively. On application of electric voltage to the slag steel system,not only was there an acceleration in the desulphurization rate, but also that therewas lower sulfur content in the steel melt and FeO content in the final slag than thatdictated by chemical equilibrium of slag.