Research On Effects Of Transition Metal Oxides On Crystallization Kinetics Of Mold Fluxes

The function of casting strand lubricating and heat diffusion controlling for mold fluxes is depended on the slag film between the strand and the mold. Radiative transfer of glassiness and heat transfer of crystal in the film can be reduced by enhancing its crystalline property; however, the lubrication property might be also deterioritaed.Supported by the project of National Natural Science Foundation of China,"Research on the formed Mechanism of Mold Fluxes film with low infrared Radiation properties"(No.:50474024) and by the project of Natural Science Foundation of Chongqing,"Research on Effects of Transitin Metal Oxides on Crystallization and low infrared Transmition of Mould Fluxes"(No.: 2006BB4200), with equipments in lab, the research results were gotten as following:The effects of transition metal oxides on physicochemical property of mold fluxes:①the capability to reduce the melting point of mold fluxes was arrayed as TiO2>MnO>ZnO>NiO (the content is less than 2wt%);②the viscosity and melting point were reduced by little MnO,TiO2,NiO or ZnO;③with the content of oxides increasing from 0% to 8%, the viscosity of mold fluxes was gradually reduced for MnO or ZnO and firstly reduced and then increased for TiO2 or NiO;④for the same content of the oxides, the crystallization temperature was gradually enhanced with the viscosity increasing.The effects of TiO2 or MnO on crystallization property indicated that:①the crystalline ratio of mold fluxes were greatly reduced by both of them;②from the results of XRD, precipitation of cuspidine was restrained by both of them;③analysis of mineragraphy indicated that both MnO and TiO2 can reduce the amount of cuspidine and the shape become more miniature comparing with original spearhead shape.The effects of TiO2 on crystallization kinetics of mold fluxes:①at the isothermal condition, crystallization rate and optimal nucleation rate increased with the content of TiO2 increasing;②under the condition of nonisothermal process and constant cooling rate, the crystallization rate of mold fluxes was gradually reduced (TiO2=0~4wt%), then became a little quicker (TiO2=6wt%), after the perovskite was precipitated (TiO2=8wt%), the crystallization rate is more quicker at the higher temperature range (perovskite was precipitated) than at lowers'(cuspidine was precipitated);③the research verified that mechanism of nucleation is non-homogeneous, the crystal grain of cuspidine as globularity growths towards three dimensional space comparing with the crystal grain of perovskite as cross shape or arborization growths towards two-dimensional space;④though perovskite was precipitated above 1200℃(TiO2=8wt%), the growth rate was too slow to contribute to accelerate the overall growth rate, comparing with the original samples, the crystallization of mold fluxes was restrained.The effects of MnO on crystallization kinetics of mold fluxes:①at the same cooling rates, the crystallization time was prolonged with the content of MnO increasing;②by Jezinorny model, the mechanism of crystallization is that the grain was non-homogeneously nucleated and then growtht toward two or three dimensional space, the crystallization rate became slower with the content of MnO increasing; by Liu model, at the same relative crystallization x, the crystallization property was arrayed as J4> M3-1> M3-2> M3-3;③at the same relative crystallization, the activation energy of crystallization became higher with the content of MnO increasing, which meant that the crystallization became more difficulty.In conclusion, the melting temperature, viscosity and crystallization time for mold fluxes can be reduced by few transition metal oxides, which, however, might reincrease them with the contents increasing. The crystalline ratio for mold fluxes was greatly reduced by few transition metal oxides and the some result can be gotten by analysis of crystallization kinetics, though the content of TiO2=8wt% can promote perovskite to precipitate, it can't contribute much to the overall crystallization rate and crystalline ratio because perovskite growthed very slow among the temperature of 1200～1350℃. Therefore, both MnO and TiO2 (0~8wt%) can improve the glass property and the lubrication between casting strand and mold; in addition, as the overall crystalline ratio wasn't enhanced, the heat transfer between casting strand and mold can be controlled without enhancing the crystallization property.