| With excellent organization and performance,high aluminum steel has been applied in many military and civil fields.In recent years,industrial production of TRIP and TWIP steel with high strength and ductility based on steelmaking and continuous casting has become an important technology pursured by large iron and steel enterprises at home and abroad.However,owning to the deficiency of special mould fluxes,serious slab surface crack defect and breakout accident happen during continuous casting of high aluminum steel frequently,so that it is hard to casting high aluminum steel with stability,high quality and high efficiency.Therefore,it has been a research focus of mould fluxes for continuous casting of high aluminum steel.In this paper,to solve the casting problems about reactivity of tradinational mould fluxes and structure and property stability of mould fluxes with low reactivity,a series of related researches were carried out to provide theoretical guidance for research and development of mould fluxes,and promote the development of continuous casting of high aluminum steel.First of all,in view of reaction between molten steel and mould fluxes during continuous casting of high aluminum steel,thermodynamic calculation,laboratory equilibrium experiment and field test were carried out to investigate the reduction of different mold fluxes by the aluminum in molten steel,and consequently obtain control objectives of compositions in mould fluxes with low reactivity.It was found that SiO2 and MnO were easily reduced by Al,Na2 O and B2O3 with high content would react with Al,while CaO,CaF2,MgO and BaO did not.In field test for continuous casting of high aluminum steel with Al content is 1.0 wt%,after the steel/slag reaction reach dynamic equilibrium,the SiO2 content stabilises at about 8 wt%,the Na2 O content stabilises at about 5 wt%,and the B2O3 content stabilises at about 2.5 wt%.The steel/slag reaction resultes in significant increase of melting temperature and viscosity,thus affecting smooth running of continuous casting,and causing sticking breakout probably.Therefore,it is unfavorable to add a large number of Na2 O and B2O3 as main fluxes in mould fluxes with low reactivity.Then,the influence mechanisms of some typical components on Al-O structure were studied by molecular dynmics simulation to provide theoretical guidance for the stability control of microstructure of mould fluxes with low reactivity.The results shown that in CaO-SiO2 and CaO-Al2O3 melts,the [AlO4]5-tetrahedron is not as stable as [SiO4]4-,and there is a linear relation between the natural logarithm of viscosity and conductivity and the structural parameter NBO/T,while the accuracy of simulation is verified since simulation results fit experimental data very well.For Al2O3 based binary system melts,Al-O network structure is complicated with the increase of Al2O3 content,while metal cation and three coordinated oxygen Ot can balance the negative excess charge of [AlO4]5-tetrahedron.The Al-O disorder and polymerization degree of structure are increased with the increase of metal cation electronegativity,that is to say,the effects of the components on stabilizing the Al-O network is sequenced as K2O>BaO>Na2O>CaO>MgO,and the polymerization degree of the structure in different systems follows the order K2O-Al2O3<BaO-Al2O3<Na2O-Al2O3 <CaO-Al2O3 < MgO-Al2O3.Therefore,it can be considered that adding suitable amount of BaO in Al2O3 based mould fluxes with low reactivity can stabilize Al-O network structure and adjust viscosity.The substitution of O by F has a dual effect on structure of CaO-Al2O3-SiO2-CaF2 melts with high Al2O3 content.On the one hand,it promotes the formation of Ca-F network structure,reducing the content of network modifier Ca and then resulting in polymerization of network structure.On the other hand,it promotes the formation of [AlO3F]4-tetrahedron to balance negative excess charge of [AlO4]5-tetrahedron,reducing the proportion of high coordinated Al and three coordinated oxygen Ot and then leading to deploymerization of network structure.Therefore,the effect of F on structure of mould fluxes with low reactivity should be discussed in specific composition range.Afterwards,based on research of steel/slag reaction and melt microstructure,the influences of typical components BaO,Li2 O and F-on macro properties of CaO-SiO2-Al2O3 based and CaO-Al2O3 based mould fluxes have been performed by properties measurements,to obtain composition range of mould flxues whose properties are stable and meet the requirements of continuous casting of high aluminum steel.It was found that substitution of SiO2 by Al2O3 simulating the component changes caused by steel/slag reaction,leads to considerably inrease of melting point and viscosity,which impede the smooth running of continuous casting.Replacement of CaO by BaO has a dual effect on basic properties,while stabilize the solidification temperature and viscous flow activation energy,improve the lubrication,and increase the stability of the slag properties after absorbing Al2O3,and it is can be concluded that BaO content is preferred to be 15 wt% in mould fluxes with low reactivity.Replacement of CaO by Li2 O can effectively reduce the melting point,viscosity and solidification temperature,while increase the viscous flow activation energy.High Li2 O content will promote the precipitation of LiAlO2 mineral phase,and increase crystallization temperature,which does not favor the slag lubrication performance,thus Li2 O content is preferred to control aat about 5 wt% in mould fluxes with low reactivity.Since with increase of F in CaO-Al2O3 based mould fluxes,increase of melting temperature is more obvious than decrease of viscosity,and viscous flow activation energy and solidification temperature will rise which is not conducive to slag lubrication performance,the F-content is preferred to control at around 10 wt%.The main crystallization phases are 12CaO·7Al2O3 and 11CaO·7Al2O3·CaF2 in CaO-Al2O3 based mould fluxes with low reactivity,which is expected to replace cuspidine in traditional CaO-SiO2 based mould fluxes controlling the heat transfer from slab to copper mould.Meanwhile,based on the measured data,combining FactSage thermodynamics software and corresponding database,the simple dynamic reaction model and viscosity model for BaO containing melts were established.It can provide certain methods for research and development of mould fluxes for high aluminum steel.Finally,based on theoretical study,combining properties of high aluminum steel and casting conditions,industrial application test were conducted.The preliminary application of mould fluxes with low reactivity is successful,and the target of smooth running of continuous casting of high aluminum steel slab with no defects has been basically achieved. |
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