Study On The Influence Of Li₂O And MgO On The Interfacial Properties Of CaO-SiO₂-Al₂O₃ Mold Flux

Mold flux plays an important role in the continuous casting process.The functions of mold flux,such as lubricating billet,controlling heat transfer and isolating air,have a huge effect on steel quality.With the development of science and technology,the quality of steel is required to be improved consecutively.The mold flux not only ensures the continuous casting progress proceed smoothly,but also,in some particular situations,has a negative impact on the quality of slab,such as the defect of slag entrapment.Once the content of steel is stable,all these defects are highly related to the interface properties of slags.In the traditional CaO-SiO₂ binary slag system,if the SiO₂ content is properly reduced and the Al₂O₃ content is increased at the same time,a new type of high Al₂O₃content slag,widely recognized as CaO-SiO₂-Al₂O₃ system continuous casting slag,can be obtained.Compared with the traditional mold fluxes,the new ternary mold powder has two kinds of tetrahedron,[AlO₄]and[SiO₄].The research shows that the surface tension of mold flux can be increased by using Al₂O₃ substitute for SiO₂,and then the possibility of surface defects can be reduced.In addition,the increase of Al₂O₃ can also play a role of f-fixation and reduce environmental pollution.,which is pronounced as an environment-friendly slag system to improve the quality of steel.However,the increase of Al₂O₃ will also increase the viscosity of the slag,which will affect the lubrication of the continuous casting.In order to guarantee the interface property and flow performance of the slag at the same time,we try to adjust the relevant performance by changing the typical flux content in the slag.Li₂O and MgO usually exist as network destroy in CaO-SiO₂ slag system,and have a great influence on the flow and solidification performance of slag.Compared with the same group of metal elements,the metallicity of Mgis stronger than that of Ca,and that of Li is weaker than that of Na.However,the electrostatic potentials of Mg²⁺and Li⁺are higher than those of Ca²⁺and Na⁺respectively.Considering that it is the metallicity and the electrostatic potential that determine the interfacial properties of flux together,the influence of Li₂O and MgO on the properties of slag interface should be considered.The phenomenon of"Li anomaly"in the study of glass indicates that its influence on the internal structure of slag is different from that of the same group of elements.Because of the diagonal position of Mgand Li in the periodic table,they have many similar properties.Therefore,it is necessary to study the influence of Li₂O and MgO on the slag structure in the new designed CaO-SiO₂-Al₂O₃ternary slag system for ultra-low carbon steel,and then illustrate their influence on the slag interface property.Some kinds of the steel,such as ultra-low steel,are prone to cause slag entrapment and Hook during continuous casting due to their own characteristics.Therefore,especially for those kinds of steels,it is necessary to increase the surface tension and interface tension of the mold flux in order to obtain superb quality.In this paper,CaO-SiO₂-Al₂O₃ system and CaO-SiO₂ system slags were designed under the same basicity of ultra-low carbon steel slag used in continuous casting right now.The influence of Li₂O and MgO on the surface tension and the contact angle of steel/slag was studied by ring method and SHTT.The existence form and relative proportion of Al₂O₃ in the slag were quantitatively analyzed by infrared spectrum.Based on the influence of Li₂O and MgO on the interface properties(including surface tension,steel/slag contact angle),the relationship between the change of Li₂O and MgO content and microstructure was qualitatively analyzed by infrared spectroscopy.Based on this,the influence mechanism of Li₂O and MgO on the slag structure and the interface property of the slag can be analyzed,which can guide the prediction and design of the slag surface tension.In the CaO-SiO₂-Al₂O₃ slag system with Al₂O₃=5 mol%,the main form of Al₂O₃ is4-coordination,AlO₄^5-.With the augment of Li₂O and MgO content,the coordination number of Al₂O₃ changed from 4-coordination to 6-coordination,AlO₆^9-.The increase of the molar substitution of Li₂O for Na₂O and the content of MgO mainly play the role of network destroy in the slag,and increase the surface tension by increasing the ion bond fraction of the whole slag.The effect of MgO content increasing on surface tension is greater than that of Li₂O equivalently substituting Na₂O.In the CaO-SiO₂-Al₂O₃ slag system with Al₂O₃=13 mol%,the main form of Al₂O₃is 6-Coordinated AlO₆^9-.With the augment of the molar substitution of Li₂O for Na₂O and the content of MgO,the form of Al₂O₃ in the slag changed from six coordination to four coordination for the changes of Li₂O and MgO mainly enhance the charge compensation effect of alumina tetrahedron,promotes the release of O^2-in the process of Al₂O₃ slag transformation,increases the occupation of O^2-on the slag surface which would increase the surface tension.The ability of Li₂O replacing Na₂O is equivalent to that of MgO to increase the surface tension.In the two groups of slag with Al₂O₃=13 mol%and Al₂O₃=5 mol%,the contact angle between slag and solid metal increases with the increase of Li₂O equimolar substitution for Na₂O and MgO.The contact angle of slag with high Al₂O₃content is larger,the wettability between slag and solid metal is reduced,and the possibility of Hook is also reduced.Compared with Li₂O,MgO has a stronger ability to increase the contact angle.According to the experimental results of infrared spectrum,the anion radius corresponding to Al³⁺in Butler equation was modified according to the different forms of Al₂O₃ in Al₂O₃=5 mol%and 13 mol%slag system.Compared with the ring method,the modified Butler equation can predict the surface tension more accurately than the original equation and Mills equation,which turns out to be a more reliable prediction model.