Mechanism Of Slag Composition Change During Electroslag Remelting Process

It is well known that electroslag plays an important role in metal remelting process. CaF2-based slag is commonly used slag system, especially the slag(ANF-6) with the chemical composition of 70%CaF2+30%Al2O3. The characteristics of these slags including phase diagram, activity, electrical conductivity, viscosity, density, surface tension and other physical and chemical properties had been compared and evaluated in 1990s or earlier. However, the mechanism of slag composition changes isn't systematically studied, especially in the process that slag changed from fluid to solid or crystal under interaction of composition and temperature. The slag composition changes will affect slag physical-chemistry and metallurgical properties, and eventually affect the quality of final products. Therefore, it is of practical and theoretical significance to study slag property change with electro slag composition change and the composition change mechanism in remelting process.In this thesis, The weight loss rate and crystallization temperature of CaF2-CaO-Al2O3-SiO2-MgO slag system originated from ANF-6 is studied with CaF2wt%, CaO wt%, Al2O3wt%, SiO2wt%, MgO wt% as variables. The method of quadratic regression orthogonal design was used to determine the effects of W(Cao),W(SiO2),W(Mgo),W(Al2O3) and W(CaF2) on the weight loss rate and crystallization temperature of CaF2-based slag by 449cJup5ter simultaneous thermal analyzer. The electrical conductivity of molten CaF2-CaO-Al2O3-SiO2-MgO slags has also been studied.The thermo gravimetry(TG) experimental results indicated that weight loss rate of CaF2-based slag can be increased by 6% with CaF2 changing from 50% to 65% and also increases with adding dry SiO2, Al2O3 and MgO but decrease with increasing CaO content. Moreover, it is also found that oxides react with CaF2 to generate volatile fluoride near by melting point. These reaction can result in obvious slag chemical composition change, such as increase of CaO content, decrease of CaF2 and Al2O3 content. X-ray structure analysis on solidified slag showed that 2CaOSiO2(2130℃),3Al2O3·2SiO2(1750℃),2CaO·Al2O3·SiO2(1596℃), CaO·6Al2O3(1860℃), 11CaO·7Al2O3·CaF2(1577℃),12CaO·7Al2O3(1455℃)and other phases with high melting point and complexity structure exist in the solidified slags. This indicates that high melting point phases may give priority to precipitate during the slag film forming in ESR process, which cause the composition change of molten slag pool in actual production.The differential scanning calorimetery (DSC) results indicated that crystallization temperature of slag increases with adding SiO2 and decreases with CaO and MgO content increasing. When adding Al2O3 to CaF2 based slag, crystallization temperature firstly incrseases and then decreases.The electrical conductivity of molten CaF2-CaO-Al2O3-SiO2-MgO slags studied by the way of four AC probes. The results showed that electrical conductivity of CaF2-CaO-Al2O3-SiO2-MgO slag system lowers gradually with the increasing mass fractions of Al2O3, SiO2 contents, and rises with increasing mass fractions of CaF2,CaO and MgO content. The effects of CaF2, CaO, Al2O3, SiO2, MgO on electrical conductivity as following the order of CaF2> SiO2>Al2O3> MgO> CaO.