Physicochemical Properties Of CaO-Al₂O₃-based Continuous Casting Mold Fluxes With Low Reactivity For High-Mn High-Al Steel

With the rapid development of industrial modernization,the demand for high value-added steels has gradually increased.Particularly,high-manganese and high-aluminum steelshave been widely used in many fields such as automotive steel,marine military industry and drones,and become the focus of research and development of steel companies in various countries.In the past,during the continuous casting of high-manganese and high-aluminum steel,the traditional CaO-SiO₂-based mold flux reacted with[Al],which not only resulted in deterioration of the physical and chemical properties of the traditional mold flux but also severely affected the surface quality of the cast slab.This paper abandons the traditional CaO-SiO₂-based mold flux and studies the low-reactivity CaO-Al₂O₃-based fluorine-free mold flux,which will laya theoretical foundation for the stable production of continuous casting mold powder of high-manganese and high-aluminum steel.Firstly,based on the study of reaction thermodynamics and kinetics[Al]and the reactivity of the mold flux components,this paper selected low-reactivity or non-reactive mold flux components and content range,established the activity calculation model of the mold flux according to the theory of molecular ion coexistence,and explored the impact of the activity of the component on the low-reactivity mold flux.Secondly,it established a structural calculation model of the CaO-Al₂O₃-based mold flux based on the first principles,studied the basic physical and chemical properties of the molten slag structure,and applied pull Mann spectroscopy to analyze the influencing law of the melt microstructure characteristics of the mold flux.Finally,the rheological behavior and crystallization properties of the mold flux were analyzed by experiments,and the relationship between the microstructure and the macrophysical and chemical properties of the molten slag was studied.The main research results are as follows:（1）According to the results of steel slag reaction experiments,the average content of[Al]varied at 2.49wt.%,the average content of[Mn]varied at 22.14wt.%,and the average content of[Si]varied at 0.19wt.%.Based on the high manganese content,the composition range of aluminum steel was required.As shown by the experimental results,the composition of[Al],[Mn],[Si]was not unqualified.Besides,the double-film theory was adopted to study the steel slag reaction process.Itwas concluded that[Al]is a reaction limiting link in the double-film mass transfer and diffusion at the interface.At the same time,the steel slag reaction thermodynamics study reveals that when[Al]=2wt.%is the lowest critical value of the reaction.（2）The activity calculation model of the CaO-Al₂O₃-basedmold flux was established based on the theory of molecular ion coexistence,and the equation of the activity model obtained by the Newton downhill method was x^k+1=x^k-α[f（x^k）]^-1f（x^k）.The results showed that when SiO₂≤6wt.%,the activity of the mold powder was significantly weakened,and SiO₂=6wt.%could be used as the critical value for judging whether it was a low-reactivity mold powder.When the component Na₂O=3-9wt.%,the activity gradually increased,causing a decrease in the activity of the composite oxide n Na₂O m SiO₂.When Na₂O=9-12wt.%,the activity of Na₂O and SiO₂ gradually weakened,and the decrease trend of compound activity of n Na₂O m SiO₂ gradually weakened.（3）Based on the first principles,the structure calculation model of CaO-Al₂O₃-based based mold flux was established,and the binding energy was calculated by the structural model:the order of the stability of the crystalline phase structure was Ba Al₂O₄>Ca₃B₂O₆>Na Al SiO₄>Ca₂Al₂SiO₇>Li Al O₂.In addition,the[Al O₆]structure was more stable than the[Al O₅]structure.The tetrahedral[Al O₄]structure had the largestΔG driving force,and the reaction to produce[Al O₅]and[Al O₆]structures had a smallerΔG driving force.Thus,it was easier to form in the slag[Al O₄]structure.（4）When the component SiO₂=6wt.%and CaO/Al₂O₃=2 in the mold flux,NBO_corr/T reached the maximum value of 3.7 and Q_corr reached the minimum value of 0.3.Thus,the slag polymerization degree was the lowest.The island structure Q₀ and the chain structure Q₁ gradually increased,and the layered structure Q₂ gradually decreased,and the strongest peak after Raman de-spectrum shifted to the low peak wave number area.（5）When the ratio of CaO/Al₂O₃ increased from 0.88 to 2,the viscosity gradually decreased.The main reason is that the content of Al₂O₃ decreases and the[Al O₄]tetrahedral structure in the slag decreases.Therefore,the degree of polymerization and shear force of the slag structure decrease.The low melting point crystalline phase 12CaO·7Al₂O₃was precipitated in the slag,and the continuous casting temperature was higher than the melting point temperature of the crystalline phase 12CaO·7Al₂O₃.Thus,the crystalline phase12CaO·7Al₂O₃was beneficial to the heat transfer and lubricating properties of the mold slag.The research work of this paper is based on the low-reactivity CaO-Al₂O₃-based mold flux for high-manganese and high-aluminum steel,and the basic research on physical and chemical properties is conducted.It has the basic theoretical guiding significance for the production practice of continuous casting mold powder of high-manganese and high-aluminum steel.