Study On Migration Behavior And Control Of Manganese Element During Vacuum Refining Of Manganese-containing Molten Steel

Manganese is known to be a most common alloying element in steel material,which significantly improves the mechanical properties of structural steels.In particular,medium manganese steel with manganese content about 4～12%and high manganese transformation-induced plasticity(TRIP)and twinning-induced plasticity(TWIP)steel with manganese content about 15～33%,as the promising candidates for next generation automotive steels due to their superior combination of high strength,excellent ductility,and high energy absorption capacity.The cost and quality in the smelting process of manganese-containing steel depend on stabilization and high yield control of manganese element.However,a lot of manganese volatilization loss under high temperature steel melt during vacuum treatment,causing unstable manganese composition control in manufacturing process as well as poor final product properties.Additionally,the manganesecontaining volatile phase will also reduce the service life of the refractory.Therefore,in this paper,focusing on the migration behavior and control of manganese element among manganese-containing molten steel/gas phase/refractory during vacuum refining.A series of key scientific issues such as the migration mechanism and volatilization kinetics of manganese element between molten steel and gas phase,the relationship between vacuum denitrification and manganese volatilization,the reaction behavior of manganese-containing volatile phase and refractories,and the characteristics of manganese-containing inclusions have been carried out in this paper.This work has important guiding significance for precise control of manganese composition and quality control in vacuum refining on manganese-containing steel and research and development of key smelting technology for medium/high manganese steel.Based on molecular dynamics calculations,the microstructure properties of Fe-Mn melt were explored.The influencing factors of manganese volatilization from molten steel are analyzed by thermodynamics and kinetics calculations,and the theoretical transition conditions of manganese volatilization are obtained.The influence law of manganese volatilization was revealed by thermal experiments,and the first-order kinetic equation of manganese volatilization rate and the corresponding kinetic parameters were investigated.Furthermore,the migration mechanism of manganese element between molten steel and gas phase was revealed,and the theoretical model for mass transfer of manganese volatilization was established,which is consistent with experimental results.Finally,the limiting link of manganese volatilization was evaluated:At 1600℃,100 Pa,the rate-limiting step changes gradually from mass transfer in the steel melt boundary layer to mass transfer in gas phase with the initial manganese content increased from 2%to 10%.Gas phase mass transfer resistance increases with the increase of manganese content and vacuum pressure.The manganese volatilization at high temperature is determined by the mass transfer in steel melt boundary layer.It is proposed that,the argon is employed to increase the gas phase mass transfer resistance could suppress the manganese volatilization,which provides a theoretical basis for the control of manganese volatilization in medium/high manganese molten steel.The limit of denitrification of molten steel and the relationship between degassing rate and manganese volatilization rate were investigated by thermal experiment.It has been determined the denitrification kinetic model of manganesecontaining molten steel and corresponding denitrification rate limiting link.The interaction mechanism between vacuum denitrification and manganese volatilization was clarified.Simultaneously,the effects of vacuum pressure of 1700～1000 Pa and the slag layer on denitrification and manganese volatilization were systematically studied,which were conducted to explore the migration mechanism of manganese among molten steel/slag/gas phase.Therefore,a synergistic treatment mode for effective control of the manganese composition and nitrogen removal of molten steel under vacuum was further proposed.For molten steel with a manganese content of 5%,slag smelting at 370 Pa for 40 min was performed to remove nitrogen rapidly and effectively reduce manganese evaporation at the same time.Based on characteristics analysis and formation reasons of manganese volatiles,the occurrence states of manganese volatiles were considered to be manganese vapor,liquid manganese and manganese oxides with the decrease of temperature.Moreover,the mechanism model of the interaction between manganese volatile phase and MgO based refractory was established:MgO particles were dissociated by Manganese vapor into small blocks,resulting in destruction of the internal structure of the refractory;It was found that Manganese oxides and liquid manganese can cause a large degree of erosion to refractory materials,which is attributed to the formation of MgO·MnO solid solution.These results would serve as an effectual basis for reducing the damage of refractory materials caused by manganese-containing phase and evaluating the erosion degree of refractory materials.On the basis of effective manganese control,the three-dimensional characteristic difference and distribution of inclusions in steels with different manganese content were systematically studied,considering the manganesecontaining inclusions are mainly MnS-type.The thermal experimental simulation and the precipitation thermodynamic calculation were carried out for the precipitation and growth of MnS with different sulfur content,manganese content and cooling rate.It is found that the manganese content in the steel is greater than 2.09%,and the precipitation amount,precipitation size and morphology of MnS are determined by the sulfur content.The manganese-containing inclusions were transformed into speckle and multipoint MnS-MnO species with sizes less than 5μm under top slag and induction stirring.Ultimately,the transformation mechanism and removal behavior of manganese-containing inclusions were proposed,which provided a theoretical basis for the control of manganese-containing inclusions in medium/high manganese steels.