Design And Basic Properties Investigation Of Mold Fluxes For Rare Earth-bearing Heat-resistant Steel Continuous Casting

In recent years,the good application effect of rare earth in heat-resistant steel has been unanimously approved at home and abroad.However,there are still many difficulties in the process of rare earth heat-resistant steel continuous casting,which affect the billet quality seriously.The molten steel has strong reducibility because the content of cerium is high.Hence,severe slag-metal interface reactions were induced in the mold when conventional mold fluxes based on silicate system were used.Then,changes of compositions caused by slag-metal interface reaction could lead to deterioration of the properties of the mold flux and surface defects of the casting billet.The conventional mold fluxes can not meet the process and quality requirements of multi-heats continuous casting.Developing new mold fluxes for rare earth heat-resistant steel continuous casting is imperative.Based on summarizing the previous research work,a new low-reactive mold flux based on CaO-Al₂O₃ system was devised.Then,the basic structure and properties of the mold fluxes,such as the melt structure,surface tension,viscosity,crystallization property and melting characteristics were emphatically investigated.And the new mold flux for rare earth heat-resistant steel continuous casting was developed.Under the condition of this paper,the following were concluded.(1)The activity calculation model for CaO-Al₂O₃-Li₂O-B₂O₃-Ce₂O₃ mold flux was built based on molecular ion coexistence theory,and the reactivity of the new mold flux was evaluated.The activities of components with low reactivity such as CaO,Li₂O and Ce₂O₃ were high,and the activities of components with relatively high reactivity such as B₂O₃ were very low.Comparing to the conventional CaO-SiO₂ based mold fluxes,the reactivity of the new mold flux decreased obviously.(2)The melt structure of CaO-Al₂O₃-Li₂O-B₂O₃-Ce₂O₃ mold flux and its change mechanism were investigated.In the new mold flux,AlO45-tetrahedron was the main network former,and AlO69-octahedron and BO33-structural unit were the main network modifier.With adding Ce₂O₃ to the mold flux,the AlO45-tetrahedrons with lower polymerization degree,and the polymerization degree of the mold flux decreased.Adding Li₂O can effectively reduce the polymerization degree of the mold flux.When the content of Li₂O was below 14 percent,the damage of the network caused by the dissociated O2-ions play the leading role;When the content of Li₂O exceeded 14 percent,the AlO₄^5-tetrahedron decreased,the AlO₆^9-octahedron increased,and the polymerization degree of the mold flux decreased consequently.With the increase of the value of w(CaO)/w(Al₂O₃)and the content of B₂O₃,the polymerization degree of the mold flux firstly decreased and then kept stable gradually.(3)The surface tension predition model of CaO-Al₂O₃-Li₂O-B₂O₃-Ce₂O₃ mold flux was developed,and the change of surface tension with the vibration of slag composition was comprehensively analized.The model preditions show that the surface tension of the new flux changed in the range of 0.385?0.516 N·m^-1.With increasing the content of Ce₂O₃ from 0 to 20 percent,the polymerization degree and the total ion moment of the mold fluxdecreased,and the surface tension decreased about 10 percent.The addition of fluxing agents such as Li₂O and B₂O₃ could increase the depolymerization degree of the mold flux and decrease the interaction between the cations and the anions in the surface layer,which could lead the decrease of the total ion moment and the decrease of the surface tension.With increasing the value of w(CaO)/w(Al₂O₃)from 0.6 to 2.0,the surface tension increased about 5.5 percent.(4)The viscous properties and its changes of the CaO-Al₂O₃-Li₂O-B₂O₃-Ce₂O₃ mold flux was investigated,and the effect of different component on viscosity,breaking temperature and viscous activitation energy were emphatically investigated.With increasing the content of Ce₂O₃,the viscosity and the viscous activation energy of the mold flux decreased.When the content of Ce₂O₃ exceeded 10 percent,the viscosity at 1300? maintained at about 0.5 Pa·s.With increasing the value of w(CaO)/w(Al₂O₃),the viscosity decreased.When the value of w(CaO)/w(Al₂O₃)was 1.82,the viscosity decreased to the minnium value of 0.22 Pa-s.With increasing the contents of Li₂O and B₂O₃,the viscosity and breaking temperature of the mold flux both show decreasing trend.(5)The change of the crystalline phase in the continuous cooling process of the CaO-Al₂O₃-Li₂O-B₂O₃-Ce₂O₃ mold flux was investigated,and the relevance of viscosity,melt structure and the crystalline phase was analyzied.The precipitation of CaO can be restained by adding Ca₂O₃ and B₂O₃,which can lead to the decrease of the breaking temperature.After adding certain content of Ce₂O₃,the crystalline phases transformed from CaO and LiAlO₂ to CaCeAlO4.With increasing the value of w(CaO)/w(Al₂O₃)from 1 to 1.5,the crystallization ability of the mold flux was weakended,and the breaking temperature decreased consequently.In the continuous cooling process,LiAlO2 crystallized as following:Li+ions distributed in the gap of the AlO₄^5-tetrahedrons for charge compensation,and LiAlO₂ with tetragonal crystal system precipitated in the cooling process.CaO crystallized as following:the precipitation of phases such as LiAlO₂ led to the partial saturation of Ca ions,and Ca combined with the free oxygen for charge balance to form CaO.CaCeAlO₄ crystallized as following:Ce ions and Ca ions together neutralized the negative charges of AlO₆^9-octahedron,and CaCeAlO₄ cryatallized taking AlO₆^9-octahedron as the skeleton.(6)Comprehensive comparison between the new CaO-Al₂O₃ based mold flux and and the conventional CaO-SiO₂ based mold flux was conducted.The results show that the new mold flux had similar properties to the conventional one.The crystalline phases of the new mold flux were nainlu nold flux werer LiAlO₂ and CaCeAlO₄.After absorbing 15 percent of Ce₂O₃,the melting temperature and breaking temperature of the conventional mold flux increased about 100?,the viscosity at 1300? increased two times,and the crystalline phases transformed from cuspidine(3CaO-2SiO₂·CaF₂)to cuspidine(3CaO-2SiO₂·CaF₂)and CaO-2Ce₂O₃·2SiO₂.The precipitation of rare earth silicates with high melting temperature could lead to the deterioration of the viscous properties and the crystallization characteristics.The melting temperature and viscous properties of the new fluxes remained relatively stable after absorbing 15 percent of Ce₂O₃,and the crystalline phases transformed from LiAlO₂ to CaCeAlO₄.The research work of this paper has important guiding significance for the continuous casting process of rare earth heat-resistant steel and the improvement of the quality of casting billet.