Study On Properties Of Lubrication,Crystallization And Radiation Heat Transfer Of Continuous Casting Mold Fluxes For High-Mn And High-Al Steels

High Mn high Al steels,because of its high strength and high plasticity,have a broad prospect for the application.In addition to being a popular steel material in terms of vehicle lightweight and high safety,this large type of steel are also favored in the machinery,transportation and construction industries.The use of“converter-furnace refining-continuous casting”process to produce high-manganese high-aluminum steel can greatly improve production efficiency,reduce costs and optimize quality.Researchers have overcome difficulties and solved a large number of technical problems in smelting and refining,especially in the field of physical metallurgy such as material forming and subsequent heat treatment.However,the reaction of steel-slag in the mold during the continuous casting process and a series of small problems caused by it have become a fishbone that is currently stuck in the continuous casting of high manganese high alumina steel.The components such as SiO₂ in the slag are reduced by alloying elements such as[Al],[Mn]and[Ti]in the molten steel.The composition of the slag is greatly changed,the performance is deteriorated seriously,and quality defects such as cracks and breakout are likely to occur.As a result,it is difficult to achieve multi-furnace continuous casting.Many researchers are tackling the problem,but the effect is not good.The most important problem is to coordinate the lubrication of the slag and the control of heat transfer on the basis of reducing or avoiding the steel-slag reaction,but the actual situation is often lost.Based on the low-reactive and non-reactive mold fluxes obtained by the research team,this paper further studies the lubrication and heat transfer characteristics,to promote the industrialization application of mold fluxes for the casting of high Mn high Al steels.The paper focuses on CaO-SiO₂-Al₂O₃ based mold fluxes,and the lubrication and heat transfer control functions were studied.Firstly,the contact angles between different kinds of mold fluxes and different steel substrates were tested,and during the liquid slag translate to solid slag,the rheological properties of mold fluxes in all temperature range were studied.The effects of steel-slag reaction on contact angle and rheological properties of CaO-SiO₂-Al₂O₃ based mold fluxes were studied.A lubrication model of slag was established,which took into account multiple factors such as melting point,viscosity,breaking temperature,contact angle,rheological properties and steel-slag reaction.In addition,in order to achieve the coordination of lubrication and heat transfer after steel-slag reaction,the influence of typical components on the crystallization performance was studied,and the range of fluxes composition with low melting point CaF₂ as the main crystalline phase was obtained.In order to compensate for the insufficiency of the control of heat transfer function caused by CaF₂ as the main crystalline phase,the radiation heat transfer of mold fluxes was studied.Mathematical model of radiation heat transfer was established,and the effect of non-reactive transition oxide on radiative heat transfer of CaO-SiO₂-Al₂O₃ based mold flusex was studied.Finally,the reaction of 20Mn23AlV steel with CaO-SiO₂-Al₂O₃ based mold flusex was simulated in the laboratory,and the lubrication and heat transfer control performance were evaluated.The main results of this paper are as follows:?1?The wettability between traditional CaO-SiO₂ based,CaO-SiO₂-Al₂O₃ based,CaO-Al₂O₃ based mold fluxes and solid steel were studied.The contact angle?,critical slag film thickness H_l and adhesion work W were compared,which clearly clarifies the advantages of CaO-SiO₂-Al₂O₃ based mold fluxes in spreading,uniformity and separation.The differences of rheological properties of traditional CaO-SiO₂ based mold fluxes and CaO-SiO₂-Al₂O₃ based mold fluxes were compared.The reasons for the difference in rheological properties of the two system were revealed.The CaO-SiO₂-Al₂O₃ based mold fluxes flow was advantage to lubricate the shell.?2?A lubrication model of slag was established,which took into account multiple factors such as melting point,viscosity,breaking temperature,contact angle,rheological properties and steel-slag reaction.The slag rim morphology,liquid film thickness and longitudinal distribution,and the slag consumption were used to evaluate the lubrication properties of the CaO-SiO₂-Al₂O₃ based mold fluxes.?3?The effects of typical components on the crystallization properties of CaO-SiO₂-Al₂O₃ based mold fluxes were studied.The composition range of mold fluxes with low-melting CaF₂ as the main crystalline phase was obtained,and the influence of each component on the precipitation behavior of CaF₂ was obtained.The method of increasing BaO content and decreasing Al₂O₃ content to inhibit the precipitation of high-melting mineral phase caused by extra Al₂O₃ was proposed.The difference of crystallization properties between CaO-SiO₂-Al₂O₃ based mold fluxes for high Mn and high Al steels and traditional CaO-SiO₂ based mold fluxes for peritectic steel and high carbon steel were compared.The initial crystallization temperature T_c of CaO-SiO₂-Al₂O₃ based mold fluxes after steel-slag reaction was lower than that of mold fluxes for peritectic steel,but higher than that of mold fluxes for high carbon steel.?4?A mathematical model for the radiation heat transfer of the mold fluxes was established.It was found that the CaO-SiO₂-Al₂O₃ based mold fluxes and the CaO-SiO₂based mold fluxes had not significantly different in controlling the radiation heat transfer.However,in order to avoid the failure of some components that regulate radiation heat transfer in the traditional CaO-SiO₂ based mold fluxes to react with high-manganese and high-aluminum steel,the method of adding non-reactive transition oxides ZrO₂ and Y₂O₃ to strengthen the controlling radiation heat transfer of CaO-SiO₂-Al₂O₃ based mold fluxes was put forward.Finally,the reasonable properties of lubrication and control heat transfer of CaO-SiO₂-Al₂O₃ based mold fluxes A2 after steel-slag reaction were confirmed.The?_1300°C,T_m and T_br are 0.24Pa?s,1102?and 1145?,within a reasonable range;?and H_l are 35°and 0.054mm,leading to a good spreadability and uniformity of molten slag;the pseudoplastic characteristic temperature interval is 30°C,and at the 400mm under the meniscus,the thickness of the liquid slag film is 0.14mm-0.38mm,and the consumption is 0.29Kg/m².The thickness of the liquid slag film is reasonable and the consumption is sufficient.In the range of 1°C/s-4°C/s cooling rate,T_c is 1067°C-1136°C,which is lower than that of peritectic steel mold flux;adding 4wt%ZrO₂ and Y₂O₃,can also reduce radiation heat transfer by about 20%.The mold fluxes for high Mn high Al steel studied in the laboratory has met the performance requirements for production test.