浸入式水口氧化铝结瘤堵塞机制

Mechanism of Alumina Clogging on Submerged Entry Nozzle

浸入式水口(SEN)结瘤堵塞已成为中国高品质钢和高性能特殊钢质量和效率提升的关键问题,至今仍未得到有效解决。本实验研究了SEN壁面和钢液中氧化铝夹杂物荷电性。结果表明:连铸过程中SEN荷负电,并随着拉坯速度的提高荷电量增大;钢液中氧化铝夹杂物荷正电,静电力为氧化铝夹杂物向SEN壁面迁移的重要驱动力;高温模拟试验和实际工况试验均表明氧化铝夹杂物会向阴极移动并附着在Al_(2)O_(3)-C材料上,并随着电场强度的增大,附着物厚度增厚。本工作揭示了SEN氧化铝结瘤新机理,为后续研究提供了方向。

Introduction In the continuous casting of aluminum-killed steel,alumina(Al_(2)O_(3))clogging frequently forms on the submerged entry nozzle(SEN)inner wall,causing SEN clogging.SEN clogging has several negative influences,i.e.,flow field instability,reduced continuous casting efficiency,deteriorated billet quality,decreased quality of produced steel,and even interruption of continuous casting.The SEN clogging problem in the continuous casting of aluminum-killed steels has attracted much attention in metallurgy and refractory materials.Although many anti-clogging measures are proposed,the SEN clogging problem has not been solved yet.The main reason is that the mechanism of SEN clogging is still unclear.The clogging of SEN is mainly caused by the movement and adhesion of inclusions in molten steel to the SEN wall.It is thus necessary to clarify the mechanism of SEN clogging,especially force condition of inclusions in molten steel.Recent studies indicate that there is a correlation between the movement of oxide inclusions in molten metal and an applied electric field.The movement of inclusions under the action of electric field force is an important factor in SEN clogging.An effective way to solve SEN clogging problem is to analyze the effect of inclusions in refractory materials and molten steel from the perspective of electric field.In this paper,the charge of Al_(2)O_(3) inclusions in SEN wall and molten steel during continuous casting was investigated to reveal the mechanism of SEN clogging and blockage,and to develop a technology for preventing alumina clogging in SEN.This work can provide a theoretical and technical support for solving the technical problems of SEN clogging in iron and steel industry.Methods The powerful mixers(Erich Co.,Germany)were used for mixing.The blast drying oven was used to control the volatile matter of the material.The samples were prepared by four-column hydraulic press and isostatic press.The samples were heat treated under the condition of carbon embedded(i.e.,850℃ for 6 h).The wall charge of SEN in the continuous casting process was measured by a digital source meter.The charging properties of Al_(2)O_(3) inclusions was determined based on the movement law of Al_(2)O_(3)inclusions under the action of external electric field.The correctness of the experimental results was verified by applying a reverse electric field on the continuous casting site.The microstructure and morphology of the refractory were determined by a model JSM-700 F scanning electron microscope with a resolution of 3.0 nm,and the micro-area composition was analyzed by a model X-Max50 energy spectrum analyzer.Results and discussion The results of rotating flow experiment indicate that the SEN material is charged when rubbing with the molten steel.The experiments in the continuous casting site show that the SEN wall is negatively charged during the continuous casting process.There are three main reasons:1)since the work function of Fe is 4.5 eV less than that of C is 5.0 eV,when the molten steel contacts with graphite in SEN,the electrons will migrate from the molten steel to SEN,making SEN negatively charged;2)because the Fermi level of Fe(11.1 eV)is more than that of Al_(2)O_(3)(8.7 eV),the electrons will migrate from the molten steel to SEN when the molten steel contacts with the corundum Al_(2)O_(3) in SEN,making SEN negatively charged;3)When the two conductors with different temperatures are in contact,the electrons will migrate from the conductor with a higher temperature to the conductor with a lower temperature.In the continuous casting process,the temperature of the SEN is less than that of molten steel,and the electrons will migrate from the molten steel to the SEN,eventually leading to the negative charge on the SEN wall.Al_(2)O_(3) inclusions in molten steel move and adhere to the negative electrode under the action of electric field.This indicates that Al_(2)O_(3) inclusions in molten steel are positively charged.The main reason is that in the continuous casting process,the molten steel is under high temperature and low oxygen conditions,and the oxygen vacancy in the defects of Al_(2)O_(3) formed by the reaction of[Al]and[O]in the molten steel has the lowest energy.The effect of applied electric field on the SEN nodulation was carried out in the continuous casting site.The results show that the degree of nodulation in the inner cavity of SEN connected to the negative electrode of the power supply is higher than that of the reference SEN without electric field,while there is a slight nodulation in the inner wall of SEN connected to the positive electrode of the power supply.This further indicates that the SEN wall is negatively charged in the continuous casting process,and the Al_(2)O_(3)inclusions in the molten steel are positively charged.SEN clogging can be effectively prevented via applying an external reverse electric field.Conclusions The clogging mechanism of SEN was analyzed based on the perspective of electrochemistry.In high-temperature simulation tests,actual working condition test and field verification,the charging characteristics of SEN wall surface,the charging behavior of Al_(2)O_(3) inclusions in molten steel and their motion behavior under electric field were explored.The charging mechanism of alumina inclusions in molten steel was explained by double electric layer and friction charging theory,point defect theory in ion crystal and first-principle.The SEN was negatively charged in continuous casting,and the charge increased with the increase of casting speed.The Al_(2)O_(3)inclusions in molten steel were positively charged,and the electrostatic force was an important driving force for the migration of Al_(2)O_(3) inclusions to the SEN wall.The high-temperature simulation test and the actual working condition test both showed that the Al_(2)O_(3) inclusions could move to the cathode and adhered to the Al_(2)O_(3)-C material,and the thickness of the attachment increased with the increase of the electric field strength.