Study On Properties And Microstructure Of Ultrahigh-basicity Mold Fluxes For Peritectic Steel

During the continuous casting of peritectic steel,the problems about the surface quality of slab frequently occur.The application of mold fluxes with strong crystallization property can effectively reduce longitudinal cracks,but it may lead to the deterioration of lubrication on strand surface and result in the sticking between soldifiying shell and copper piate,and even breakout.In recent years,the ultrahigh basicity mold slag(R=1.65-1.85)has successfully balanced the contradiction between lubrication and heat transfer in the continuous casting of peritectic steel by its characteristics of low initial crystallization temperature and fast crystallization rate.However,there are few studies on the effects of related components on the crystallization properties of the ultrahigh basicity mold slag,and the corresponding structural characteristics are also rarely reported.Therefore,it is of great theoretical and practical significance to further reveal the action mechanism of ultrahigh basicity mold fluxes by exploring the influence of typical components on the crystallization performance and clarifying the differences of micro-action characteristics of typical components on molten slag.In this paper,the influences of typical components,such as Na₂O,MgO,Al₂O₃ and Li₂O,on the basic properties of ultrahigh basicity mold fluxes(R=1.75)were first studied.Then,the crystallization thermodynamics and kinetics,crystalline mineral phase and phase microstructure of ultra-high basicity mold fluxes were studied by means of Factsage thermodynamics software,in house improved full field high temperature confocal microscope,X ray diffraction(XRD)and Scanning electron microscope(SEM).Raman spectroscopy was used to analyze the effect of typical components on the microstructure of the melt,The relationship between the macroscopic properties and the microstructure of the slag was discussed.The measurement of basic properties of ultrahigh basicity mold fluxes shows that the viscosity at 1300?,melting temperature and break temperature all decreased with the increase of Na₂O and Li₂O,increased with the increase of Al₂O₃,first decreased and then increased with the increase of MgO.However,the change of components had little effect on the viscosity,and great effect on the melting temperature and break temperature.Li₂O significantly reduced the melting temperature and break temperature,while Al₂O₃reduced the break temperature and slightly increased the melting temperature.The crystallization measurement of ultrahigh basicity mold flux shows that the initial crystallization temperature would decreases by 18?,15?,11?and 34?respectively,as the individual content of Na₂O,MgO,Li₂O and Al₂O₃ increased by 1%.The change of composition content mainly affected the late crystallization rate and had little effect on the early crystallization rate.In addition,the crystallization rate was found to be negatively correlated with the size of precipitation,that was the larger the size of precipitation,the slower the crystallization rate was;The precipitation of new phase in slag lead to the increased crystallization rate,and the average crystallization rate first decreased and then increased with the increase of Na₂O and MgO content,and decreased with the increase of Al₂O₃ and Li₂O content.It is found that the main precipitated phases in the slag was Ca₄Si₂F₂O₇,and there were NaF,CaF₂ and Ca₃MgSi₂O₈ existed in slags with some certain composition.In terms of slag structure,the polymerization degree decreased with the increase of Na₂O and Li₂O content in slag.With the increase of Al₂O₃,the polymerization degree increased.With the increase of MgO,the polymerization degree first decreased and then increased The Crystallization and break temperature of molten slag were affected by both polymerization degree and undercooling degree.With the increase of Na₂O,MgO and Li₂O,the degree of polymerization slightly varied,but the degree of undercooling decreased obviously,which lead to the decrease of initial crystallization temperature and break temperature.With the increase of Al₂O₃,the melting temperature did not change obviously,but the polymerization degree of slag increased.Resulting in the reduction of break temperature and crystallization temperature.