Research On Reactivity Control And Basic Structure Properties Of Mold Flux For High-Mn High-Al Steel

The high-Mn high-Al steel represented by TWIP and TRIP steels has ultra-high strength and good ductility,as well as low density,which perfectly meet the requirements of the new generation of automobile steel for passenger safety,lightweight and environmental protection.However,since the high-Mn high-Al steel contains a large amount of reductive alloying elements such as Al and Mn,steel-slag reaction will inevitably occur when using traditional CaO-SiO₂-based mold flux for continuous casting,which leads to a significant change in the composition of mold flux and rapid deterioration of liquid slag performance,seriously affecting the lubrication and heat transfer conditions of slab in the mold,and thus all kinds of problems of smooth casting and slab quality appeared.In order to solve the problems existing in the use of mold flux for high-Mn high-Al steel,the development of low reactivity CaO-Al₂O₃-based mold flux has become the hotspot and mainstream direction,and many progress has been made.However,the current CaO-Al₂O₃-based mold flux still contains a large number of reactive flux?e.g.B₂O₃,Na₂O?and a certain amount of SiO₂.In the actual continuous casting process,it still brought about problems such as large slag ring,strong crystallization,poor lubrication and slab surface cracks,etc.To avoid the above problems induced by the low-reactive mold flux,it is important to develop a new low-reactive or non-reactive mold flux for the continuous casting of high-Mn high-Al steel and the improvement of slab quality.Firstly,the reactivity of common components in mold flux with Al in steel was calculated from thermodynamics to determine the non-reactive components and reactive components,and a new low-reactive slag system without reactive flux was proposed.Based on the theory of molecular ion coexistence,the activity calculation model of that new slag system was established,and the effect of components on the reactivity of slag was analyzed.Secondly,the kinetic model of steel-slag reaction was established by combining the two-film theory and the activity calculation model.The effects of parameters of continuous casting process and steel/slag composition on the reaction were discussed.Then,the property stability of CaO-Al₂O₃-based mold fluxes after absorption of Al₂O₃ inclusion was investigated,and the kinetics of the dissolution process of Al₂O₃ inclusion was studied by rotating cylinder method.On basis of the above research,a non-reactive slag system was proposed,and its composition ranges,in which the basic properties of mold flux meet the requirements of continuous casting,was tested and determined.Besides,a viscosity prediction model for non-reactive slag system was established.Finally,infrared and Raman spectroscopy were applied to analyze the microstructure characteristics of non-reactive slag,and the relationship between microstructures and macroscopic properties was preliminarily discussed.Thermodynamic studies showed that,in the new low-reactive slag system of CaO-BaO-Al₂O₃-SiO₂-CaF₂-Li₂O,when the Al₂O₃/SiO₂ ratio is more than 3.5 or SiO₂ content is below 8wt%,the reactivity of mold flux is obviously weakened.Thus,SiO₂=8wt%can be used as a critical value to determine whether the slag system belongs to a low-reactive mold flux.The steel-slag reaction also confirmed the accuracy of that critical value.In addition,CaF₂ enhances the reactivity of mold flux,whereas Li₂O enhances the reactivity first and then weakens.The kinetics experiment of steel-slag reaction revealed that the reaction rate is mainly determined by the component SiO₂,and the proposed kinetics model has been proved to be reliable.The prediction results conducted by the kinetics model showed that increasing the thickness of the liquid slag layer prolongs the time for the steel-slag reaction to reach equilibrium,but it does not affect the Al₂O₃ increment in the final slag.With the reduction of casting speed and consumption of mold flux,the increment of Al₂O₃ in the final slag and the reaction time to reach equilibrium increase.The content of Al₂O₃ and its mass transfer coefficient do not affect the steel-slag reaction,whereas the mass transfer coefficient of SiO₂ determines the reaction rate.The reduction of SiO₂content does not affect the reaction rate,but it can reduce the increment of Al₂O₃ in the final slag.The experimental results of Al₂O₃ absorption into mold flux revealed that the property stability of the non-reactive slag system has obvious advantages and the restrictive step of the dissolution process is the mass transfer of inclusion in the product layer.Based on the results of thermodynamic calculation and Al₂O₃ absorption,a non-reactive slag system of CaO-BaO-Al₂O₃-CaF₂-Li₂O was proposed,and the compositional regions in which the basic properties of mold flux satisfy the requirements of continuous casting were determined as follows:?1?CaO:22⁴0wt%,BaO:14³4wt%,Al₂O₃:20³4wt%,F=8wt%,Li₂O=8wt%;?2?CaO:23⁴0wt%,BaO:14³4wt%,Al₂O₃:20²8wt%,F=12wt%,Li₂O=8wt%.In addition,the modified NPL model is well capable for predicting the viscosity of non-reactive mold flux.Studies on slag microstructure showed that the main structural units in non-reactive slag system are[AlO4],[AlO6]and[Al2OF6],and[AlO4]tetrahedron consists ofQ²_Al,Q³_AlandQ⁴_Al.The form of the melt network structure is presented as follows:[AlO4]tetrahedron connectes with each other to build a spatial network structure in the form of Q²_Al,Q³_AlandQ⁴_Al,while[Al₂OF₆],[AlO₆],F^-ion,O^2-ion and other cations exist among the network units in the form of free ion clusters.With BaO as a substitute for CaO,the content ofQ³_AlandQ⁴_Alincreases,whereas the content ofQ²_Aldecreases,resulting in the increase of polymerization degree,viscosity and viscous activation energy of mold flux.Increasing the ratio of?CaO+BaO?/Al₂O₃,Q³_Al/Q²_Al?parameter for degree of polymerization?decreases,which leads to simplification of melt structure and reduction of viscosity.With the increase in F content,some[AlO₄]tetrahedrons are converted into free structure units[AlO₆]and[Al₂OF₆],which depolymerizes the structure of molten slag and reduces its viscosity.