| Rare earth elements can be used in alloy heavy rail steel production to obtain high impact toughness,good wear-resisting property,contact fatigue property,and hot plasticity.However,rare earth oxides are easily formed in the continuous casting process of rare earth alloy heavy rail steel while adding rare earth core wire into the steel.An amount of the rare earth oxide enters the mold flux,which deteriorates physical and chemical prperties of the mold flux and influences the continuous casting process of rare earth alloy heavy rail steel.Therefore,the effects of CeO2 on the melting temperature,viscosity,structure and crystallization of the mold flux for rare earth alloy heavy rail steel were systematically studies.In this paper,the present work can provide reference for developing the mold flux for the continuous casting of the rare earth alloy heavy rail steel.Effects of CeO2 on the physical and chemical properties of CaO-SiO2-CaF2-based and CaO-SiO2-B2O3-based mold fluxes were studied firstly,and then that of CaO-Al2O3-based mold fluxes with low reactivity for casting rare earth alloy heavy rail steel.The conclusions are summarized as follows:A hemisphere method and a rotating cylinder viscometer were used to investigate the melting temperature and viscosity of the CaO-SiO2-CaF2-based mold fluxes,respectively.With increasing the CeO2 content,the melting temperature of the flux increased,but the viscosity at high temperature(above break temperature)decreased.It can be concluded that the content of CeO2 in the flux should be lower than 12wt%since the mold flux is no longer suitable for continuous casting production.Raman spectroscopy and X-ray photoelectron spectroscopy(XPS)were employed to study the structure of the water-quenched mold flux.The results reveal that the degree of the polymerization of the melt was destroyed,leading to the decrease of viscosity at high temperature.Crystallization behavior of CaO-SiO2-CaF2-based mold flux was investigated using X-ray diffiraction(XRD)and an electron probe micro analyzer(EPMA).The crystalline phases with high melting temperature,Ca4Si2O7F2(1407?)and Ce4.67(SiO4)3O(1500?),were easier to precipitate in the melt with the increase of CeO2,resulting in the sharp increase of the break temperature of the flux.CaO-SiO2-B2O3-based mold flux was proposed and studied because of the harm of fluorine in the mold flux to both production equipment and environment.It turned out that 10wt%B2O3 could significantly decrease the melting temperature,viscosity and crystallization temperature of the mold flux.Besides,influences of CeO2 on the physical and chemical properties of CaO-SiO2-B2O3-based mold flux were studied.The results showed that the melting temperature of the flux increased,but the viscosity at high temperature of the melts decreased with increasing CeO2.The mold flux with 12wt%CeO2 could be still used in casting the rare earth alloy heavy rail steel.Raman spectroscopy and magic angle spinning nuclear magnetic spectrum(MAS NMR)were used to analyze the structure of mold flux,which showed that CeO2 played the role of modifier in the network structure of CaO-SiO2-B2O3-based mold flux.Due to the weak thermal strength and shrinkage of the solidified shell of rare earth alloy heavy rail steel,excellent heat transfer and lubrication capacity of mold flux are required.A slag film heat flux simulator was employed to measure the heat flux values and obtain the solid slag film of CaO-SiO2_CaF2-based and CaO-SiO2-B2O3-based mold fluxes.The addition of CeO2 reduced the heat transfer capability of both mold fluxes,it has less influence on heat transfer of B2O3-bearing mold flux comparatively.X-ray diffraction analysis showed that CeO2 promoted the precipitation of crystalline phases in the solid slag film and enhanced the crystallization of two kinds of mold fluxes,but the effect on the crystallization of B2O3-bearing mold flux was less.In order to reduce the reaction between rare earth elements and mold flux,CaO-Al2O3 based mold flux was introduced in the present work.The melting temperature increased and the viscosity at high temperature decreased in the flux with the addition of CeO2.Fourier Transform infrared spectroscopy(FTIR)was employed to investigate the structure of the melt.It concluded that the Al-O and B-O network structure were destroyed by increasing the CeO2 content,resulting in the decrease of viscosity.Besides,CeO2 could restrain the precipitation of CaO with a high melting temperature of 2580?which decreased the break temperature of mold flux.Based on the analysis of the influence of CeO2 on the viscosity and the break temperature of mold flux,CaO-Al2O3-based mold flux was more favorable for the continuous casting of rare earth heavy rail steel than CaO-SiO2-based mold flux. |
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