Study On The Basic Properties Of CaO-Al₂O₃ Based Mold Fluxes

As important alloy elements,Mn and Al are widely used in the manufacture of High-Mn high-Al steels,such as wear-resistant steels,non-magnetic steels,high-Mn stainless steels,and resistant steels etc.With excellent mechanical properties,strength and toughness,these steels have great application potential in the defense industry,transportation and automobile industry.At present,the major process for the high-Mn high-Al steel manufacture is mold casting,with complex procedures and low efficiency.Therefore,the“converter-refining-continuous casting”process has great potential and should be taken into consideration.However,the restriction of mold fluxes for high-Mn high-Al steels makes the continuous casting process unstable.The basic properties of CaO-Al₂O₃ based low-reactivity mold fluxes were investigated in this study.Firstly,the activity calculation model of CaO-Al₂O₃-SiO₂-BaO-Na₂O-B₂O₃-CaF₂ slag system was established based on the molecular-ion coexistence theory,and the thermodynamic characteristics of reactions between molten steel and mold fluxes were investigated.Then,the property requirements of mold fluxes for high Mn-high Al-steels were proposed based on the solidification behavior of the steels.The experimental composition of slags was designed by simplicity grid method and the melting and flowing properties were measured.According to the results,the composition range of low-reactivity mold fluxes was proposed.Based on the previous results,laboratory experiments were carried out to simulate the reactions between molten steel and mold fluxes during continuous casting,and the properties of mold fluxes after experiments were also measured.?1?Thermodynamic calculation shows both contents of reaction components in mold fluxes and contents of Al in molten steel would influence the reactivity between steel and slags.With the increase of Al in steel,the reactivity of SiO₂,B₂O₃ and Na₂O in slags would increase;The reactivity became constant when Al₂O₃/SiO₂ increased to 3after replacing SiO₂ with Al₂O₃;Reactivity of SiO₂ and B₂O₃ in CaO-Al₂O₃ based mold fluxes decreased obviously,and the order of reaction capacity is B₂O₃>Na₂O>SiO₂;The composition regions of reaction components in CaO-Al₂O₃ based mold fluxes are:SiO₂:0¹2wt%,Na₂O:0⁶wt%,B₂O₃:0⁴wt%.?2?Based on the solidification behavior of high-Mn high-Al steels,The appropriate basic properties of mold fluxes were proposed:melting temperatures range from 900 to 1150?,viscosities at 1300?from 0.05 to 0.3 Pa?s,and breaking temperatures from 900 to 1150?.According to the measured melting and flowing properties of slag samples,five composition regions of CaO-Al₂O₃ based mold fluxes were found,a typical composition is:CaO+F 39wt%,Li₂O+MgO 5wt%,Na₂O 4wt%,B₂O₃ 8wt%,SiO₂ 4⁸wt%,BaO 8²0wt%,Al₂O₃ 16²8wt%.?3?The results of experiment shows that in CaO-Al₂O₃ based mold fluxes,B₂O₃and Na₂O in slags will react with Al in steel,SiO₂ will not react with Al when the SiO₂content less than 4wt%.The equilibrium experiments indicate the slag-steel reaction mainly occurred in the later 30 minutes,and reaches the equilibrium after 60 minutes of the contact.The basic properties of CaO-SiO₂ based mold fluxes deteriorated obviously after the reaction,which cannot meet the requirements.However,the No.13 flux sample reduced the reactivity effectively,the melting temperature after reaction is 1137?,viscosities at 1300?is 0.123 Pa?s,and breaking temperature is 1080?,the property change could meet the requirements of high-Mn high-Al steel continuous casting.